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International Journal of Photoenergy
Volume 2010, Article ID 468147, 19 pages
http://dx.doi.org/10.1155/2010/468147
Review Article

Progress in Polycrystalline Thin-Film Cu(In,Ga) Solar Cells

School of Electronics Engineering, KIIT University, Campus-3 (Kathjodi), Patia Bhubaneswar 751024, India

Received 7 January 2010; Revised 21 May 2010; Accepted 30 June 2010

Academic Editor: Gaetano Di Marco

Copyright © 2010 Udai P. Singh and Surya P. Patra. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

For some time, the chalcopyrite semiconductor CuInSe2 and its alloy with Ga and/or S [Cu(InGa)Se2 or Cu(InGa)(Se,S)2], commonly referred as CIGS, have been leading thin-film material candidates for incorporation in high-efficiency photovoltaic devices. CuInSe2-based solar cells have shown long-term stability and the highest conversion efficiencies among all thin-film solar cells, reaching 20%. A variety of methods have been reported to prepare CIGS thin film. Efficiency of solar cells depends upon the various deposition methods as they control optoelectronic properties of the layers and interfaces. CIGS thin film grown on glass or flexible (metal foil, polyimide) substrates require p-type absorber layers of optimum optoelectronic properties and n-type wideband gap partner layers to form the p-n junction. Transparent conducting oxide and specific metal layers are used for front and back contacts. Progress made in the field of CIGS solar cell in recent years has been reviewed.

1. Introduction

Current trends suggest solar energy will play an important role in future energy production [1]. Silicon has been and remains the traditional solar cell material of choice. Although silicon is a highly abundant material, it requires an energy intensive process to purify and crystallize. Furthermore, installations of silicon cells require heavy glass protection plates, which reduce residential applications [2].

Recently, commercial interest is beginning to shift towards thin-film cells [3]. Material, manufacturing time, and weight savings are driving the increase in thin-film cells. Cu(In,Ga)Se2 (CIGS) is one of the most promising semiconductors for the absorber-layer of thin-film solar cells [4]. The conversion efficiency of such cells on glass substrates is approaching 20% [5]. Chalcopyrite-based solar modules are uniquely combining advantages of thin-film technology with the efficiency and stability of conventional crystalline silicon cells. Copper indium gallium selenide (CIGS) solar cells have the highest production among thin film technologies. Advances in preparation and efficiency have allowed these cells to be produced rapidly and are approaching market values for carbon-based energy production [6].

The first report on chalcopyrite-based solar cell was published in 1974 [7]. The cell was prepared from a p-type CuInSe2 (CISe) single crystal onto which a CdS film was evaporated in vacuum. This combination of a p-type chalcopyrite absorber and a wide-gap n-type window layer still is the basic concept upon which current cell designs are based. The typical design, first described in 1985 [8] is shown in Figure 1 and a typical cross-section CIGS device structure is shown in Figure 2. The CuInSe2 crystal was replaced by a polycrystalline thin film of the more general composition Cu(In,Ga)(S,Se)2.

468147.fig.001
Figure 1: Schematic cross-section of a chalcopyrite-based thin-film solar cell. Typical materials for the individual parts of the cell are given in brackets.
468147.fig.002
Figure 2: Scanning electron micrograph of the cross-section of a typical chalcopyrite solar cell with Cu(In,Ga)Se2 (CIGSe) absorber (substrate now shown). Taken from [34].

Many groups across the world have developed CIGS solar cells with efficiencies in the range of 15–19%, depending on different growth procedures. Glass is the most commonly used substrate, but now efforts are being made to develop flexible solar cells on polyimide [917] and metal foils [2, 1829]. Highest efficiencies of 14.1% and 17.6% have been reported for CIGS cells on polyimide [30] and metal foils [31], respectively. Recently a slight increase in efficiency of 14.7% and 17.7% has been reported for CIGS cells on polyimide and metal foils [32], respectively.

CIGS solar cells also attract considerable interest for space applications due to their two main advantages. It offers specific power up to 919 W/Kg, the highest for any solar cell [23]. CIGS cells are also superior to GaAs cells in radiation hardness [33]. Moreover, the flexibility of these cells allows for novel storage and deployment options [23].

There are several reviews available dealing with different aspects of CIGS solar cells [3539]. The emphasis of the present paper is placed on the progress made in different aspects of CIGS solar cells in the recent times.

2. CIGS Cell Configuration

The Cu-chalcopyrites exhibit the highest efficiencies among the thin-film solar cells with a record labscale efficiency reaching nearly 20% [5]. Most commonly CIGS solar cells are grown in substrate configuration (see Figure 3). This configuration gives the highest efficiency owing to favourable process conditions and material compatibility. Cell preparation starts with the deposition of back contact, usually Mo, on glass, followed by the p-type CIGS absorber, CdS or other weakly n-type buffer layer, undoped ZnO, n-type transparent conductor (usually doped ZnO or In2O3), metal grid, and antireflection coating. It requires an additional encapsulation layer and/or glass to protect the cells surface.

468147.fig.003
Figure 3: Schematic cross-section of “substrate” and “superstarte” configuration of CIGS solar cell.

The structure of a CIGS solar cell is quite complex because it contains several compounds as stacked films that may react with each other. Fortunately, all detrimental interface reactions are either thermodynamically or kinetically inhibit at ambient temperature [40]. The cover glass used in substrate configuration is not required for the cells grown in the superstrate configuration (see Figure 3). CIS-based superstrate solar cells were investigated by Duchemin et al. [41] using spray pyrolysis deposition, but efficiencies did not exceed 5%. The main reason for this low efficiency in CdS/CIGS superstrate cells is the undesirable interdiffusion of Cd into CIS (or CIGS) during the elevated temperatures required for absorber deposition on CdS buffer layers [42]. To overcome this problem of interdiffusion more stable buffer materials and low-temperature deposition processes such as electrodeposition (ED), low-substrate temperature coevaporation and screen printing were investigated. Nakada and Mise [43] achieved a breakthrough by replacing CdS with undoped ZnO and coevaporating Se during CIGS deposition. With the additional introduction of composition grading in absorber layer, 12.8% efficiency cells were developed [43].

This coevaporation of Se for incorporation of sodium in CIGS is essential for high-efficiency cells, as the ZnO front contact acts as diffusion barrier for Na from the glass substrate and leads to a low net carrier density in CIGS and cells with low open-circuit voltage ( ) and fill factor (FF) [44]. (The influence of Na on CIGS optoelectronic properties is discussed in Section 6) Investigations of the interface between the ZnO buffer layer and CIGS revealed the presence of a thin layer of Ga2O3 which acts as barrier against photocurrent transport [43, 45, 46]. However, Na-free superstrate solar cells with efficiencies up to 11.2% was obtained, but a strong light-soaking treatment was necessary [47].

Preparing a blocking contact in superstrate structure has been difficult. Only small-area cells have been demonstrated so far and even those show limited performance (see Table 1). It is interesting to note that approaches not using buffer layers have resulted in higher efficiency than those using CdS buffers prepared by various methods.

tab1
Table 1: Superstrate Cells.

Another interesting application for superstrate solar cells are tandem solar cells. These solar cells employ two separate solar cell structures for a more efficient conversion of the illumination. Superstrate solar cells are then required as top cell for the short wavelength part of the solar irradiation. Tandem solar cell will not be the part of present discussion. The details can be referred to in the following articles [5356].

3. Back Contact

Molybdenum (Mo) is the most common metal used as a back contact for CIGS solar cells. Several metals, Pt, Au, Ag, Cu, and Mo, have been investigated for using as an electrical contact of CIS- and CIGS-based solar cells [5759]. Mo emerged as the dominant choice for back contact due to its relative stability at the processing temperature, resistance to alloying with Cu and In, and its low contact resistance to CIGS. The typical value of resistivity of Mo is nearly  Ω cm or less. The preferred contact resistivity value is 0.3 cm. Results have been reported in several papers [57, 60, 61] concerning the influence of the mechanical and electrical properties of the Mo films on the performance of the photovoltaic devices. Molybdenum is typically deposited by e-gun evaporation [61, 62] or sputtering [6365] on soda-lime glass which ideally provides an inexpensive, inert, and mechanical durable substrate at temperatures below C. Intrinsic stresses in molybdenum films depend on the experimental deposition parameters [6166], inducing significant changes in the structural and electrical properties. Films with compressive stresses have near bulk like values of the electrical resistivity and a dense microstructure, but films under tensile stresses exhibit altered physical properties and a more open porous structure [6164]. Gross stress may be determined by visual inspection in that highly compressed films tend to buckle up, frequently in zigzag patterns, whereas films under extreme tensile stress develop a system of stress lines that look scratches. Orgassa et al. [67] fabricated CIGS solar cells with different back-contact materials, emphasizing the role of the back contact as an optical reflector. Early results by Russell et al. [68] and Jaegaermann et al. [69] suggested that Mo back contacts for CIS form a Schottky-type barrier with a barrier height of 0.8 eV for the intimate p-doped CIS/Mo contact. The work of Shafarman et al. [70], who analyzed the Mo/CIS interface separately from the cell, shows the contact to be ohmic.

The influence of MoSe2 on the ohmic contact behaviour at the CIGS/Mo interface makes MoSe2 formation an important issue. Fundamental work by Raud and Nicolet [71] on Mo/Se, Mo/In, and Mo/Cu diffusion couples showed Se to react with Mo, forming MoSe2 in very small amounts after annealing at C. Kohara et al. [72] have also shown the formation of nearly ideal ohmic contact between Mo and CIGS that occurs via an intermediate MoSe2 layer. Jones et al. [73] investigated the interface properties of d.c.-sputtered Mo on CIS layers, deposited by coevaporation, and concluded that MoSe2 does not form below C and it might be an artifact of the sputtering process. Similar results have been obtained by Schmid et al. [74] they detected Mo–O and Mo–O–Se compounds, while selenizing the Mo-coated substrate prior to the CIS deposition at C. They concluded that there should be a Schottky-type barrier at the CIS-Mo/MoO2 interface. Wada et al. [75] have also suggested that CIGS/Mo heterocontact including a MoSe2 layer is not Schottky type, but a favorable ohmic type contact. Nishiwaki et al. [76, 77] have also studied the formation of MoSe2 layer at the CIGS/Mo interface during “3-stage” process.

Wada et al. [78] reported the formation of a MoSe2 layer at the Mo/CIGS interface during the second stage of the three-stage process, yet only under (In,Ga)-rich growth and for substrate temperatures higher than C. They found Na to enhance the formation of MoSe2. Assmann et al. [79] have also shown the presence of MoSe2 at the Mo/CIGS interface; they conclude that mechanical stable MoSe2 at the interface gives good adhesion. Recently, Shimizu et al. [80] have studied the variation of Mo thickness from 0.2  m to 0.07  m on the properties of CIGS solar cells. They conclude that there is a tradeoff between the decreased sodium diffusion for thicker Mo layers and decreased fill factor for thin layers. The optimum Mo thickness suggested was 0.2  m. They have also found that water vapour introduced during CIGS growth improve the overall photovoltaic properties.

MoSe2 layers were confirmed also in CuGaSe2-based solar cells by Würz et al. [81]. Contrary to the above results, Ballif et al. [82] could not detect any intermediate compound within the Mo/CIGS interface. Mo back contact for flexible polyimide is also been investigated by Zhang et al. [83].

The properties of molybdenum thin films evaporated onto large area ( ) soda-lime glass substrates at different deposition rates have been investigated by Guillén and Herrero [84]. During the formation of films, Na ion diffuse from the soda lime glass substrate through the Mo back contact into the absorber layer. The diffusion of Na into absorber film depends on the deposition conditions of the Mo back contact [8587]. Nowadays, Mo growth by sputtering or e-beam evaporation is the most commonly used back contact for CIGS solar cells.

Kim et al. [88] have tried Na-doped Mo/Mo bilayer on Alumina substrate and have shown improvement in photovoltaic properties. Nakada [89] has tried transparent conducting oxide as back contact. The TCO back contact deteriorated at high absorber deposition temperature. The formation of Ga2O3 was also reported at the CIGS/ITO and CIGS/ZnO:Al interfaces.

4. CIGS Absorber Layer—Deposition Methods

I–III–VI2 semiconductors, such as CIS or CIGS are often simply referred to as chalcopyrites because of their crystal structure. These materials are easily prepared in a wide range of compositions and the corresponding phase diagrams are well investigated [9092]. For the preparation of solar cells, only slightly Cu-deficient compositions of p-type conductivity are suited [93, 94]. The details of material properties will not be discussed here.

A wide variety of thin-film deposition methods has been used to deposit Cu(In,Ga)Se2 thin films. To determine the most promising technique for the commercial manufacture of modules, the overriding criteria are that the deposition can be completed at low cost while maintaining high deposition or processing rate with high yield and reproducibility. Compositional uniformity over large areas is critical for high yield. Device considerations dictate that the Cu(In,Ga)Se2 layer should be at least 1  m thick and that the relative compositions of the constituents are kept within the bounds determined by the phase diagram.

The most promising deposition methods for the commercial manufacture of modules can be divided into two general approaches that have both been used to demonstrate high device efficiencies and in pilot scale manufacturing. The first approach is vacuum coevaporation in which all the constituents, Cu, In, Ga, and Se, can be simultaneously delivered to a substrate heated at 40 C to C and the Cu(In,Ga)Se2 film is formed in a single growth process. The second approach is a two-step process that separates the delivery of the metals from the reaction to form device-quality films. Typically the Cu, Ga, and In are deposited using low-cost and low-temperature methods that facilitate uniform composition. Then, the films are annealed in a Se atmosphere, also at 40 C to C. The reaction and anneal step often takes longer time than formation of films by coevaporation due to diffusion kinetics, but is amenable to batch processing.

4.1. Coevaporation

The most successful technique for deposition of CIGS absorber layers for highest-efficiency cells is the simultaneous evaporation [95] of the constituent elements from multiple sources in single processes where Se is offered in excess during the whole deposition process. The process uses line-of-sight delivery of the Cu, In, Ga, and Se from Knudsen-type effusion cells or open boat sources to the heated substrate. While a variation of the In-to-Ga ratio during the deposition process leads to only minor changes in the growth kinetics, variation of the Cu content strongly affects the film growth.

The sticking coefficients of Cu, In, and Ga are very high, so the film composition and growth rate are determined simply by the flux distribution and effusion rate from each source. Different deposition variations, using elemental fluxes deliberately varied over time, have been explored using coevaporation. Four different sequences that have been used to fabricate devices with efficiencies greater than 16% are shown in Figure 4.

fig4
Figure 4: Schematic illustration of different coevaporation process. In all cases, a constant Se flux is also supplied.

The first process (Figure 4(a)) is the simplest stationary process in which all fluxes as well as substrate temperature is constant throughout the deposition process [96]. Advanced preparation sequences include a Cu-rich stage during the growth process and end up with an In-rich overall composition in order to combine the large grains of the Cu-rich stage with the otherwise more favourable electronic properties of In-rich composition. The use of this kind of procedure is called “Boeing or bilayer process” (Figure 4(b)) originates from the work of Mickelsen and Chen [97, 98].

This bilayer process yields larger grain sizes compared to the constant rate (single stage) process. This is attributed to the formation of a Se phase during the Cu-rich first stage, which improves the mobility of group III atoms during growth [99101]. Another possibility is the inverted process where first (In,Ga)2Se3 is deposited at a lower temperature (typically ~3 C). Then Cu and Se are evaporated at an elevated temperature until an overall composition close to stoichiometry is reached [102104]. This process leads to smoother film morphology than bilayer process. The so-called “three-stage process” introduced by Gabor et al. [103] from NREL is shown in Figure 4(c).

This method leads, up to now, to the most efficient solar cells. The smoother surface obtained with three-stage process reduces the junction area and thereby is expected to reduce the number of defects at the junction and also facilitates the uniform conformal deposition of a thin buffer layer and prevents ion damage in CIGS during sputter deposition of ZnO/ZnO:Al. Variations of the Ga/In ratio during deposition (Figure 4(d)) allow the design of graded band-gap structures [105].

In one of the other process (shown in Figure 5) is an in-line process in which the flux distribution results from the substrate moving sequentially over the Cu, Ga, and In sources. This was first simulated in a stationary evaporation system [106] and demonstrated by Hanket et al. [107] and has subsequently been implemented by several groups in pilot manufacturing systems.

468147.fig.005
Figure 5: Flux distributions of different elements for in-line system. A constant Se flux is also supplied (from [35]).
4.2. Sequential Approach—Selenization of Precursor Material

The interest in sequential processes is sparked by its suitability for large-area film deposition with good control of the composition and film thickness. Such processes consist of the deposition of a precursor material, followed by thermal annealing in controlled reactive or inert atmosphere for optimum compound formation via the chalcogenization reaction. This is commonly referred as selenization of stacked metal or alloy layers. The metals and alloys can be deposited by variety of methods which involve vacuum or no vacuum. The most common of vacuum process is sputtering [108114] and thermal evaporation [113, 115131]. The two step process has many variations in both the precursor deposition and the Se reaction step.

4.2.1. Vacuum-Based Approach

This general approach was first demonstrated by Grindle et al. [132] who sputtered Cu/In layers and reacted them in hydrogen sulfide to form CuInS2. This was first adapted to CuInSe2 by Chu et al. [121]. The highest-efficiency Cu(InGa)Se2 cell reported using the reaction in H2Se is 16.2%, on the basis of the active area [133], but there have been less effort at optimizing laboratory-scale cell efficiencies than with coevaporated Cu(InGa)Se2. Using the two-step selenization/sulfurization approach, some groups have reported CIGSS-solar cells with and efficiency above 600 mV [134] and 14%, respectively, [135]. Lately, an efficiency of 13% [136] has been achieved on  cm large modules. Recently 14.3% has been reported on cm large modules [137] and 14.7% on  cm minimodules [138]. In both cases, sulphur as well as selenium is used for absorber preparation.

The precursor films are typically reacted in either H2Se or Se vapor at 40 C to C for 30 to 60 min to form the best device quality material. Poor adhesion [139] and formation of a MoSe2 layer [140] at the Mo/CuInSe2 interface may limit the reaction time and temperature. Reaction in H2Se has the advantage that it can be done at atmospheric pressure and can be precisely controlled, but the gas is highly toxic and requires special precautions for its use. The precursor films can also be reacted in a Se vapor, which might be obtained by thermal evaporation, to form the CuInSe2 film [141]. A third reaction approach is rapid thermal processing (RTP) of either elemental layers, including Se [142, 143] or amorphous evaporated Cu-In-Se layers [144]. Recently, Chen et al. had tried one step sputtering using Cu-In-Ga alloy target followed by selenization [145].

4.2.2. Nonvacuum-Based Approach

Vacuum-evaporated, polycrystalline copper indium gallium diselenide (CIGS) thin films are used as the absorber layers in the highest efficiency thin film photovoltaic (PV) cells reported to date [5, 146]. However, the high cost and low material utilisation of the equipment used to produce these layers may be a barrier to their commercialization and will increase the cost of the electricity generated by CIGS based systems [147]. Nonvacuum techniques for CIGS deposition offer potential reductions in capital cost and many such techniques have been investigated [148]. These techniques generally split CIGS formation into two stages, one in which the precursor is deposited and one in which the precursor is converted into CIGS.

Nonvacuum approaches to CIGS deposition can be divided into the following categories depending on the deposition method and the scale of mixing of the precursor materials:(1)electrochemical process, (2)particulate process,(3)solution based process.

A detailed review on nonvacuum process dealing with above process is recently published [149]. In view of this, the present section will only update the recent work done in this area, so as not to duplicate the recently published work. In the recent development, Kang et al. [150] have prepared CIGS absorber by selenizing electrodeposited precursor with rich Se and poor Se content. The Se-poor electrodeposited precursor had better crystallinity and increased Ga content. The best cell obtained has efficiency of 1.63% only. In another study, Lai et al. [151] investigated the electrodeposition of CIGS using cyclic voltammetry in a DMF-aqueous solution containing citrate as complexing agent. They performed the cyclic voltammetry study on a ternary Cu-In-Se system, a quaternary system Cu-In-Ga-Se and binary Cu-Se, In-Se, and Ga-Se systems.

Nanoparticle-based approach was carried out by Yoon et al. [152] for the formation of CIS solar cells. They concluded that the Se loss can be minimized by using high heating rate and core-shell structure with a binary compound. The highest efficiency reported was 1.11%. Park et al. [153] have synthesized CIGS absorber using a paste of a Cu, In, Ga, and Se with an aim to develop a simpler and lower cost method of fabricating the absorber layer. Kaigawa et al. [154] have also reported the absorber formation using spray and sintering the film using spot welding machine. Recently a nonvacuum process for preparing nanocrystalline CIGS materials involving an open-air solvothermal route has been demonstrated [155]. In continuation of their earlier work [156, 157], that is, hydrazine-based processor approach for the depositing CIGS and related chalcogenide-based absorber layer, Liu et al. have recently reported 12% efficient CuIn(SeS) solar cell [158]. Hou et al. [159] have also reported the formation of hydrazine-based CuIn(SeS) thin film solar cell.

5. Alternative CIS or CIGS Growth Process

The CIS or CIGS compound has been reported using other alternative techniques. In one of the studies, Ahmed et al. [160] have studied the thermal annealing of flash evaporated CIGS thin films. In another approach [161], efficiency as high as 15.4% was achieved using additional deposition of In, Ga, and Se at high temperatures. In spray pyrolysis, metal salts with a chalcogen reactant are sprayed on heated substrate to form CIS layer. However, a subsequent heat treatment in a reducing atmosphere is still required to improve crystallinity and purity [162164]. Different other approaches have also been successfully adopted for the fabrication of CIGS absorber layers. Flash evaporation has also been used to prepare CIGS film [165]. MOCVD has also been investigated [166] for the deposition of CGS layers as part of a tandem structure, but the growth rate and cell efficiency is rather low. CuInGaSe2 thin films have been prepared by a low pressure metalorganic chemical vapor deposition technique using three precursors without additional Se [167]. A plasma-enhanced CVD has also been reported for fabricating stoichiometric CIS film [168]. CIS thin film has also been deposited by atmospheric pressure metal organic chemical vapor deposition (AP- MOCVD) [169, 170]. Brien et al. [171173] have deposited CuInSe2, CuInS2 and CuGaSe2 thin film by low pressure MOCVD. Recently deposition of CuInSe2 thin film on CuGaSe2 thin film and vice versa has been achieved by a low pressure metal organic chemical vapor deposition technique with three precursors without additional Se [174]. No devices have been attempted by the authors.

Few different techniques have also been used to deposit and characterize CIGS thin film. The technique used are closed-spaced vapor transport [175177] electrodeposition of CIS using ethylene glycol at C [178] CIGSS films by sol-gel route [179] electron beam evaporated CIGS film [180185] CuInSe2 thin films prepared using sequential vacuum evaporation of In, Se, and Cu at moderately low substrate temperatures, avoiding any treatment using toxic H2Se gas [186], CIS using hot wall vacuum evaporation [187], MBE grown CIGS, CIS [188, 189] CIGS using ion-beam plasma evaporation in vacuum [190] and CIGS using a two-stage hybrid sputtering/evaporation method [191].

6. Influence of Sodium

The most important effect of the soda lime glass substrate on Cu(InGa)Se2 film growth is that it supplies sodium to the growing chalcopyrite material. It has been clearly shown that this effect is distinct from the thermal expansion match of soda lime glass [192]. The sodium diffuses through the Mo back contact, which also means that it is important to control the properties of the Mo [193]. Na, incorporated into CIGS absorber layers are widely known to have significantly beneficial effects that lead to enhanced CIGS-related photovoltaic cell efficiencies.

The effect of Na include an improvement in p-type conductivity due to an increase in the effective hole carrier density and improved open circuit voltage ( ) and fill factor for solar cells fabricated from Na doped CIGS [194, 195]. In addition to this, the effect of Na on the growth orientation of CIGS films results in an enhancement of (112) texture [194, 196, 197]. Among the various Na effects, variations in the electrical properties have been well discussed. The observed improvement in has been proposed to originate from an increase in the effective acceptor density [198]. Na in polycrystalline CIGS films is considered to act on the grain boundaries rather than in the bulk [199, 200]. Na substituting on a Cu site results in the formation of a stable compound NaInSe2, which has a larger band gap energy and in turn leads to a larger has also been proposed [201].

The correlation between the CIGS grain size and the presence of Na has been occasionally discussed. While some groups [192, 196, 202] have reported an increase of the grain size in CIGS films containing Na, others did not support these observations [203206]. A decreasing grain size was observed for several Na incorporation methods in a direct comparison [200]. This may be due to the fact that the nature of the effects of Na on grain size depends on the CIGS growth method and the Na-doping process.

The CIS compound formation in rapid-thermal-processed layers was found to be delayed in the presence of Na, resulting in CIS growth at a higher mean temperature, which serves as an explanation for the observed increase in grain size [207]. In any case, the grain size of Na-doped CIGS films seems to have no critical role in solar cell performance [208, 209]. Higher doses of Na are shown to lead to smaller grain sizes, porous films and detrimental to the cell performance [204, 205]. The most obvious electronic effect of Na incorporation into CIGS films is a decrease in resistivity by up to two orders of magnitude [210212]. In one of the recent studies [213] NaF was deposited prior to CIGS absorber and it reported that NaF precursors modify the CIGS growth kinetics: a reduction of the grain size and a slightly enhanced Ga-gradient through the absorber layer were observed. and FF (fill factor) increase when the Na content increases at , during the absorber deposition.

In other studies, Erslev et al. [214] have studied the role of sodium in CIGS solar cells using junction capacitance methods. The increase in solar cell efficiency with sodium was attributed to passivation of a defect state near the CdS/CIGS junction. Recently, Ishizuka et al. [215] have studied the variations in the structural, optical, and electrical properties of polycrystalline Cu(In,Ga)Se2 thin films with Na doping level. Na incorporation into CIGS absorber was controlled using alkali-silicate glass thin layers. They found that the Ga composition gradient in CIGS films became larger and the grain size decreased with increasing Na concentration.

7. CdS Buffer Layers

Semiconductor compounds with n-type conductivity and band gaps between 2.0 and 3.6 eV have been applied as buffer for CIGS solar cells. However, CdS remains the most widely investigated buffer layer, as it has continuously yielded high-efficiency cells. CdS for high-efficiency CIGS cells is generally grown by a chemical bath deposition (CBD), which is a low-cost, large-area process. However, incompatibility with in-line vacuum-based production methods is a matter of concern. Physical vapor deposition- (PVD-) grown CdS layers yield lower efficiency cells, as thin layers grown by PVD do not show uniform coverage of CIGS and are ineffective in chemically engineering the interface properties. For a comprehensive review on CBD-deposited CdS see Ortega-Borges and Lincot [216] and Hodes [217].

The recent trend in buffer layers is to substitute CdS with “Cd-free” wide-bandgap semiconductors and to replace the CBD technique with in-line-compatible processes. The first approach has been to omit CdS and form a direct junction between CIGS and ZnO, but the plasma (ions) during ZnO deposition by RF sputtering can damage the CIGS surface and enhance interface recombination. Possible solutions include ZnO deposited by metal organic chemical vapor deposition (MOCVD), atomic layer deposition (ALD) or a novel technique, called ion layer gas reaction (ILGAR) [218220]. CIS and CIGS solar cells have yielded highest efficiencies with CdS buffer layers deposited by chemical bath deposition (CBD). Omitting the buffer layer always resulted in lower efficiencies [221]. Also, as-grown CIGS cells with CdS buffers deposited by physical vapor deposition (PVD) have always shown significantly lower efficiencies than cells with CBD-CdS buffers [222, 223].

The role of the CdS buffer layer is twofold: it affects both the electrical properties of the junction and protects the junction against chemical reactions and mechanical damage. From the electric point of view the CdS layer optimizes the band alignment of the device [224, 225] and builds a sufficiently wide depletion layer that minimizes tunneling and establishes a higher contact potential that allows higher open circuit voltage value [225]. The buffer layer also play a very important role as a “mechanical buffer” because it protects the junction electrically and mechanically against the damage that may otherwise be caused by the oxide deposition (especially by sputtering). Moreover, in large-area devices the electric quality of the CIGS film is not necessarily the same over the entire area, and recombination may be enhanced at grain boundaries or by local shunts. Together with the undoped ZnO layer, CdS enables self-limitation of electric losses by preventing defective parts of the CIGS film from dominating the open circuit voltage of the entire device [226].

The thickness as well as the deposition method of the CdS layer has a large impact on device performance. During the early days, the device structure consisted of a CuInSe2/CdS junction with a thick (about 1–3  m) CdS layer [227229]. The CdS layer of these devices were most often prepared by evaporation at substrate temperatures between RT and about C or in some cases by sputtering [229]. Also CdS film was often doped with either In [229] or Ga [106] and in some cases a CdS bilayer was used [230, 231] consisting of a thinner high-resistive layer, prepared either by evaporation [143] or chemical bath deposition [194, 230, 231] and a thicker low-resistivity layer, doped with 2% In [231] or Ga [194]. Alternatively, evaporated CdS has been used also in combination with the transparent conducting oxide layer [232234]. Nowadays chemical bath deposition (CBD) is used almost exclusively [235, 236].

In contrast to evaporated films [237], CBD films contain high amounts of oxygen-related impurities that originate from the deposition solution; the amount of oxygen in the films can be as high as at 10–15 at % [237, 238]. Most of the oxygen is present as OH– and H2O [237, 238]. Thus, the composition of the CBD-CdS films is more accurately stated as Cd(S,O,OH) [237]. Additional impurities such as C and N containing compounds result from the side reactions of thiourea [238]. The amount and identity of the impurities, and consequently the performance of the solar cell, depend strongly on the CdS deposition conditions [225, 239241]. Negami et al. [242] for instance, reported an increase of conversion efficiency from 17.6 to 18.5% when the CBD-CdS process was improved.

In addition to the CdS film deposition, the chemical bath also modifies the absorber surface region [235, 243]. Thus, the interface between CIGS and CBD-CdS is not abrupt, but the layers are intermixing to some extent [238, 244]. Both Cu and Cd diffusion play a role, and the intermixing is further enhanced during the post deposition air-annealing [226]. According to Nakada and Kunioka [238], Cu was substituted by Cd at the surface region of CIGS. The diffusion depth of Cd atoms was about 10 nm, which may be related to the thickness of the Cu-deficient surface layer (CuIn3Se5) of CIGS [238]. On the other hand, Heske et al. [244] observe diffusion of Se and In from CIGS into CdS and the diffusion of S from CdS into CIGS. The extent of interdiffusion depends on the structure of the absorber: (224/208) oriented CIGS films have been found to allow more Cd atoms to diffuse into the CIGS film [245].

One advantage of the CBD method as compared to evaporation is that a complete, conformal coverage of the CIGS surface can be obtained at very low thicknesses: already 10 nm has been reported to be sufficient [246]. The coverage depends on deposition conditions, particularly on the concentration ratio of the S and Cd precursors, being better with higher S/Cd precursor ratios [240].

Some studies have been conducted on the fundamental properties of CdS films deposited by an ammonia-free CBD process [247] and very few studies have used an ammonia-free buffer layer for the fabrication of a CIGS solar cell [248]. In a recent work, Mann et al. [249] has also deposited CdS by CBD and used optical reflectance-based measurement of the growing film to determine in situ film thickness.

8. Alternative Buffer Layer

As an alternative to CdS, various materials show promising results. The different buffer layer used and the deposition method for the same is tabulated in Table 2.

tab2
Table 2: Alternative buffer layers and their deposition methods.

The Zn-based compounds tend to form a blocking barrier due to the band alignment with CIGS [253]. Using layers of less than 50 nm thickness, the barrier can be crossed by tunneling of charge carriers, but this poses high requirements on the quality of the deposition process and the CIGS surface to obtain a uniform coverage. The band offset can be reduced as well, if impurities such as hydroxides that can be present in a CBD are incorporated in the CIGS/buffer layer interface [281].

9. Front Contact

There are two main requirements for the electric front contact of a CIGS solar cell device: sufficient transparency in order to let enough light through to the underlying parts of the device, and sufficient conductivity to be able to transport the photo-generated current to the external circuit without too much resistance losses. Transparent conducting metal oxides (TCO) are used almost exclusively as the top contacts. Narrow lined metal grids (Ni–Al) are usually deposited on top of the TCO in order to reduce the series resistance. The quality of the front contact is thus a function of the sheet resistance, absorption and reflection of the TCO as well as the spacing of the metal grids [282].

During the early days of CIS and CIGS substrate cell development, a bilayer of undoped and doped CdS served as a buffer and front contact, respectively [283, 284]. High conductivity in doped CdS was achieved either by controlling the density of donor type defects or by extrinsic doping with Al or In [283, 284]. Spectral absorption loss in the conducting CdS layer was reduced by increasing the bandgap, alloying with ZnS or later replacing it with TCOs with bandgaps of above 3 eV [283]. Transmission spectra of various TCOs are shown in Figure 6.

468147.fig.006
Figure 6: Optical transmission of different front contacts and buffer layers (from [38]).

Today, CIGS solar cells employ either tin doped In2O3 (In2O3: Sn, ITO) [285287] or, more frequently, RF-sputtered Al-doped ZnO. A combination of an intrinsic and a doped ZnO layer is commonly used, although this double layer yields consistently higher efficiencies, the beneficial effect of intrinsic ZnO is still under discussion [226]. It has been shown that device performance increases due to the increase in by 20–40 mV [226]. It has been discussed that resistive oxide layer provides, together with buffer, a series resistance that protects the device from local electrical loses that may originate from inhomogeneties of the absorber [226]. Doping of the conducting ZnO layer is achieved by group III elements, particularly with aluminum [194, 288298]. However, investigations show boron to be a feasible alternative, as it yields a high mobility of charge carriers [290, 299304] and a higher transmission in the long-wavelength spectral region, giving rise to higher currents [305]. For high-efficiency cells the TCO deposition temperature should be lower than C in order to avoid the detrimental interdiffusion across CdS/CIGS interface.

There had been some recent studies of i-ZnO and doped ZnO. Yu et al. [306] have studied the Ni and Al codoped ZnO grown by dc magnetron cosputtering. A comparative study of i-ZnO and ZnO:Al using rf magnetron sputtering and electrodeposition done by Wellings et al. [307] to be used for CIGS solar cells. Pawar et al. [308] have studied the Boron doped ZnO using spray pyrolysis. Few [309, 310] have reported the deposition of ZnO:Al on polymide substrate. Recently, Calnan and Tiwari have discussed in detail regarding High Mobility Transparent Conductor Oxide (HMTCO) [311].

10. Conclusion

Remarkable progress has been made in the development of high efficiency CIGS solar cells. CIGS PV modules have the potential to reach cost-effective PV-generated electricity. Transition from lab to manufacturing has been much more difficult than anticipated.

Each component of the solar cell structure and its manufacturing requires further investigation to simplify the processing and to have more efficient solar cell with lower cost. Few of the key issues related to development of CIGS solar cells are: higher module efficiency, columnar CIGS structures deposited by alternative process for high efficiency cells and modules, thinner absorber layer ( 1 ), and CIGS absorber film stoichiometry and uniformity over large areas.

There is also a need to develop a robust and low temperature (~400 C) deposition process for CIGS for the flexible substrate (polyimide) to facilitate roll to roll manufacturing and to extend the application for space market.

Acknowledgments

The authors would like to thank Mr. Abdul Khader and Ms. Krishna Dalai, for their support during the work. This work has been supported by the Ministry of New & Renewable Energy (31/12/2009/PVSE), New Delhi, Department of Science & Technology (DST), New Delhi (SR/S2/CMP-30/2003), and AICTE, New Delhi (8023/BOR/RID/RPS-78/2007-08).

References

  1. M. Kaelin, D. Rudmann, and A. N. Tiwari, “Low cost processing of CIGS thin film solar cells,” Solar Energy, vol. 77, no. 6, pp. 749–756, 2004. View at Publisher · View at Google Scholar · View at Scopus
  2. F. Kessler and D. Rudmann, “Technological aspects of flexible CIGS solar cells and modules,” Solar Energy, vol. 77, no. 6, pp. 685–695, 2004. View at Publisher · View at Google Scholar · View at Scopus
  3. D. Graham-Rowe, “Solar cells get flexible,” Nature Photonics, vol. 1, no. 8, pp. 433–435, 2007. View at Publisher · View at Google Scholar · View at Scopus
  4. M. A. Green, K. Emery, D. L. King, S. Igari, and W. Warta, “Solar cell efficiency tables (Version 22),” Progress in Photovoltaics: Research and Applications, vol. 11, no. 5, pp. 347–352, 2003. View at Publisher · View at Google Scholar · View at Scopus
  5. I. Repins, M. A. Contreras, B. Egaas et al., “19.9%-efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor,” Progress in Photovoltaics: Research and Applications, vol. 16, no. 3, pp. 235–239, 2008. View at Publisher · View at Google Scholar · View at Scopus
  6. B. Dimmler and R. Wächter, “Manufacturing and application of CIS solar modules,” Thin Solid Films, vol. 515, no. 15, pp. 5973–5978, 2007. View at Publisher · View at Google Scholar · View at Scopus
  7. S. Wagner, J. L. Shay, P. Migliorato, and H. M. Kasper, “CuInSe2/CdS heterojunction photovoltaic detectors,” Applied Physics Letters, vol. 25, no. 8, pp. 434–435, 1974. View at Publisher · View at Google Scholar · View at Scopus
  8. R. R. Potter, C. Eberspacher, and L. B. Fabick, “Device analysis of CuInSe2/(Cd,Zn)S/ZnO solar cells,” in Proceedings of the Conference Record of the 18th IEEE Photovoltaic Specialists Conference, pp. 1659–1664, 1985. View at Scopus
  9. B. M. Başol, V. K. Kapur, C. R. Leidholm, A. Halani, and K. Gledhill, “Flexible and light weight copper indium diselenide solar cells on polyimide substrates,” Solar Energy Materials and Solar Cells, vol. 43, no. 1, pp. 93–98, 1996. View at Publisher · View at Google Scholar · View at Scopus
  10. A. N. Tiwari, M. Krejci, F.-J. Haug, and H. Zogg, “12.8% efficiency Cu(In,Ga)Se2 solar cell on a flexible polymer sheet,” Progress in Photovoltaics: Research and Applications, vol. 7, no. 5, pp. 393–397, 1999. View at Publisher · View at Google Scholar · View at Scopus
  11. G. M. Hanket, U. P. Singh, E. Eser, W. N. Shafarman, and R. W. Birkmire, “Pilot-scale manufacture of Cu(InGa)Se2 films on a flexible polymer substrate,” in Proceedings of the 29th IEEE Photovoltaic Specialists Conference, pp. 567–569, May 2002. View at Scopus
  12. R. Birkmire, E. Eser, S. Fields, and W. Shafarman, “Cu(InGa)Se2 solar cells on a flexible polymer web,” Progress in Photovoltaics: Research and Applications, vol. 13, no. 2, pp. 141–148, 2005. View at Publisher · View at Google Scholar · View at Scopus
  13. S. Ishizuka, H. Hommoto, N. Kido, K. Hashimoto, A. Yamada, and S. Niki, “Efficiency enhancement of Cu(In,Ga)Se2 solar cells fabricated on flexible polyimide substrates using alkali-silicate glass thin layers,” Applied Physics Express, vol. 1, no. 9, Article ID 092303, pp. 1–3, 2008. View at Publisher · View at Google Scholar · View at Scopus
  14. P. Gečys, G. Račiukaitis, M. Gedvilas, and A. Selskis, “Laser structuring of thin-film solar cells on polymers,” EPJ Applied Physics, vol. 46, no. 1, Article ID 12508, 2009. View at Publisher · View at Google Scholar · View at Scopus
  15. L. Zhang, Q. He, W.-L. Jiang, C.-J. Li, and Y. Sun, “Flexible Cu(In, Ga)Se2 thin-film solar cells on polyimide substrate by low-temperature deposition process,” Chinese Physics Letters, vol. 25, no. 2, pp. 734–736, 2008. View at Publisher · View at Google Scholar · View at Scopus
  16. R. Caballero, C. A. Kaufmann, T. Eisenbarth et al., “The influence of Na on low temperature growth of CIGS thin film solar cells on polyimide substrates,” Thin Solid Films, vol. 517, no. 7, pp. 2187–2190, 2009. View at Publisher · View at Google Scholar · View at Scopus
  17. H. Zachmann, S. Heinker, A. Braun et al., “Characterisation of Cu(In,Ga)Se2-based thin film solar cells on polyimide,” Thin Solid Films, vol. 517, no. 7, pp. 2209–2212, 2009. View at Publisher · View at Google Scholar · View at Scopus
  18. T. Satoh, Y. Hashimoto, S. Shimakawa, S. Hayashi, and T. Negami, “Cigs solar cells on flexible stainless steel substrates,” in Proceedings of the Conference Record of the 28th IEEE Photovoltaic Specialists Conference, p. 567, 2000.
  19. K. Herz, F. Kessler, R. Wächter et al., “Dielectric barriers for flexible CIGS solar modules,” Thin Solid Films, vol. 403-404, pp. 384–389, 2002. View at Publisher · View at Google Scholar · View at Scopus
  20. D. Herrmann, F. Kessler, K. Herz et al., “High-performance barrier layers for flexible CIGS thin-film solar cells on metal foils,” in Proceedings of the Materials Research Society Symposiumml: Compound Semiconductor Photovoltaics, vol. 763, pp. 287–292, San Francisco, Calif, USA, 2003. View at Scopus
  21. D. R. Hollars, R. Dorn, P. D. Paulson, J. Titus, and R. Zubeck, “Large area Cu(In,Ga)Se2 films and devices on flexible substrates made by sputtering,” in Proceedings of the Materials Research Society Spring Meeting, pp. 477–482, April 2005, F14.34.1. View at Scopus
  22. K. Otte, L. Makhova, A. Braun, and I. Konovalov, “Flexible Cu(In,Ga)Se2 thin-film solar cells for space application,” Thin Solid Films, vol. 511-512, pp. 613–622, 2006. View at Publisher · View at Google Scholar · View at Scopus
  23. S. Ishizuka, A. Yamada, P. Fons, and S. Niki, “Flexible Cu(In,Ga)Se2 solar cells fabricated using alkali-silicate glass thin layers as an alkali source material,” Journal of Renewable Sustainable Energy, vol. 1, Article ID 013102, 8 pages, 2008. View at Publisher · View at Google Scholar
  24. R. Wuerz, A. Eicke, M. Frankenfeld et al., “CIGS thin-film solar cells on steel substrates,” Thin Solid Films, vol. 517, no. 7, pp. 2415–2418, 2009. View at Publisher · View at Google Scholar · View at Scopus
  25. T. Yagioka and T. Nakada, “Cd-free flexible Cu(In,Ga)Se2 thin film solar cells with ZnS(O,OH) buffer layers on Ti foils,” Applied Physics Express, vol. 2, no. 7, Article ID 072201, 3 pages, 2009. View at Publisher · View at Google Scholar · View at Scopus
  26. D. Brémaud, D. Rudmann, M. Kaelin et al., “Flexible Cu(In,Ga)Se2 on Al foils and the effects of Al during chemical bath deposition,” Thin Solid Films, vol. 515, no. 15, pp. 5857–5861, 2007. View at Publisher · View at Google Scholar · View at Scopus
  27. S. Ishizuka, A. Yamada, K. Matsubara, P. Fons, K. Sakurai, and S. Niki, “Development of high-efficiency flexible Cu(In,Ga)Se2 solar cells: a study of alkali doping effects on CIS, CIGS, and CGS using alkali-silicate glass thin layers,” Current Applied Physics, vol. 20, no. 2, supplement 1, pp. S154–S156, 2009. View at Publisher · View at Google Scholar · View at Scopus
  28. C. Y. Shi, Y. Sun, Q. He, F. Y. Li, and J. C. Zhao, “Cu(In,Ga)Se2 solar cells on stainless-steel substrates covered with ZnO diffusion barriers,” Solar Energy Materials and Solar Cells, vol. 93, no. 5, pp. 654–656, 2009. View at Publisher · View at Google Scholar · View at Scopus
  29. M. S. Kim, J. H. Yun, K. H. Yoon, and B. T. Ahn, “Fabrication of flexible CIGS solar cell on stainless steel substrate by co-evaporation process,” Diffusion and Defect Data B, vol. 124-126, pp. 73–76, 2007. View at Google Scholar · View at Scopus
  30. D. Brémaud, D. Rudmann, G. Bilger, H. Zogg, and A. N. Tiwari, “Towards the development of flexible CIGS solar cells on polymer films with efficiency exceeding 15%,” in Proceedings of the 31st IEEE Photovoltaic Specialists Conference, pp. 223–226, January 2005. View at Scopus
  31. J. R. Tuttle, A. Szalaj, and J. Keane, “A 15.2 % AM0 / 1433 W/kg thin-film Cu(In,Ga)Se2 solar cell for space applications,” in Proceedings of the 28th IEEE Photovoltaic Specialists Conference, pp. 1042–1045, Anchorage, Alaska, 2000.
  32. S. Ishizuka, “Flexible CIGS photovoltaic cell with energy conversion efficiency of 17.7%—enabling development of a sticker-type high-performance solar cell,” AIST Today, vol. 8, no. 10, p. 20, 2008. View at Google Scholar
  33. J. F. Guillemoles, “The puzzle of Cu(In,Ga)Se2 (CIGS) solar cells stability,” Thin Solid Films, vol. 403-404, pp. 405–409, 2002. View at Publisher · View at Google Scholar · View at Scopus
  34. J. Poortmans and V. Archipov, Thin Film Solar Cells, John Wiley & Sons, New York, NY, USA, 2006.
  35. A. Luque and S. Hegeds, Eds., Handbook of Photovoltaic Science & Engineering, John Wiley & Sons, New York, NY, USA, 2003.
  36. T. Markvant and L. Castarier, Eds., Practical handbook of Photovoltaics: Fundamentals and Applications, Elsevier, Amsterdam, The Netherlands, 2003.
  37. M. D. Archer and R. Hill, Eds., Clean Electricity from Photovoltaics, vol. 1, chapter 7 of Series on Photoconversion of Solar Energy, 2001.
  38. A. Romeo, M. Terheggen, D. Abou-Ras et al., “Development of thin-film Cu(In,Ga)Se2 and CdTe solar cells,” Progress in Photovoltaics: Research and Applications, vol. 12, no. 2-3, pp. 93–111, 2004. View at Google Scholar · View at Scopus
  39. M. Kemell, M. Ritala, and M. Leskelä, “Thin film deposition methods for CuInSe2 solar cells,” Critical Reviews in Solid State and Materials Sciences, vol. 30, no. 1, pp. 1–31, 2005. View at Publisher · View at Google Scholar · View at Scopus
  40. J.-F. Guillemoles, L. Kronik, D. Cahen, U. Rau, A. Jasenek, and H.-W. Schock, “Stability issues of Cu(In,Ga)Se2-based solar cells,” Journal of Physical Chemistry B, vol. 104, no. 20, pp. 4849–4862, 2000. View at Google Scholar · View at Scopus
  41. S. Duchemin, V. Chen, J. C. Yoyotte, J. Bougnot, and M. Savelli, “Backwall CdS(n)-CuInSe2(p) sprayed solar cells,” in Proceedings of the 8th European Photovoltaic Solar Energy Conference, pp. 1038–1042, Florence, Italy, 1988.
  42. T. Nakada, N. Okano, Y. Tanaka, H. Fukuda, and A. Kunioka, “Superstrate-type CuInSe2 with chemically deposited CdS window layers,” in Proceedings of the IEEE 1st World Conference on Photovoltaic Energy Conversion, pp. 95–98, Hawaii, USA, 1994.
  43. T. Nakada and T. Mise, “High-efficiency superstrate type cigs thin film solar cells with graded bandgap absorber layers,” in Proceedings of the 17th European Photovoltaic Solar Energy Conference, pp. 1027–1030, Munich, Germany, 2001.
  44. F.-J. Haug, D. Rudmann, G. Bilger, H. Zogg, and A. N. Tiwari, “Comparison of structural and electrical properties of Cu(In, Ga)Se2 for substrate and superstrate solar cells,” Thin Solid Films, vol. 403-404, pp. 293–296, 2002. View at Publisher · View at Google Scholar · View at Scopus
  45. F.-J. Haug, M. Krejci, H. Zogg, A. N. Tiwari, M. Kirsch, and S. Siebentritt, “Characterization of CuGaxSey/ZnO for superstrate solar cells,” Thin Solid Films, vol. 361, pp. 239–242, 2000. View at Publisher · View at Google Scholar · View at Scopus
  46. M. Terheggen, H. Heinrich, G. Kostorz, F.-J. Haug, H. Zogg, and A. N. Tiwari, “Ga2O3 segregation in Cu(In, Ga)Se2/ZnO superstrate solar cells and its impact on their photovoltaic properties,” Thin Solid Films, vol. 403-404, pp. 212–215, 2002. View at Publisher · View at Google Scholar · View at Scopus
  47. F.-J. Haug, D. Rudmann, H. Zogg, and A. N. Tiwari, “Light soaking effects in Cu(In,Ga)Se2 superstrate solar cells,” Thin Solid Films, vol. 431-432, pp. 431–435, 2003. View at Publisher · View at Google Scholar · View at Scopus
  48. R. Klenk, R. Mauch, R. Schäffler, D. Schmid, and H. W. Schock, “Progress in CuGaSe2 based thin film solar cells,” in Proceedings of the 22nd IEEE Photovoltaic Specialists Conference, pp. 1071–1076, October 1991. View at Scopus
  49. A. Kampmann, A. Abken, G. Leimkühler et al., “Cadmium-free CuInSe2 superstrate solar cell fabricated by electrodeposition using a ITO/In2Se3/CuInSe2/Au structure,” Progress in Photovoltaics: Research and Applications, vol. 7, no. 2, pp. 129–135, 1999. View at Publisher · View at Google Scholar · View at Scopus
  50. T. Yoshida and R. W. Birkmire, “Fabrication of CuInSe2 solar cells in a superstrate configuration,” in Proceedings of the 11th EC Photovoltaic Solar Energy Conference, p. 811, 1992.
  51. T. Negami, M. Nishitani, M. Ikeda, and T. Wada, “Preparation of CuInSe2 films on large grain CdS films for superstrate-type solar cells,” Solar Energy Materials and Solar Cells, vol. 35, no. C, pp. 215–222, 1994. View at Google Scholar · View at Scopus
  52. T. Nakada, Y. Hirabayashi, T. Tokado, D. Ohmori, and T. Mise, “Novel device structure for Cu(In,Ga)Se2 thin film solar cells using transparent conducting oxide back and front contacts,” Solar Energy, vol. 77, no. 6, pp. 739–747, 2004. View at Publisher · View at Google Scholar · View at Scopus
  53. S. Nishiwaki, S. Siebentritt, P. Walk, and M. Ch. Lux-Steiner, “A stacked chalcopyrite thin-film tandem solar cell with 1.2 V open-circuit voltage,” Progress in Photovoltaics: Research and Applications, vol. 11, no. 4, pp. 243–248, 2003. View at Publisher · View at Google Scholar · View at Scopus
  54. S. A. Alagappan and S. Mitra, “Optimizing the design of CIGS-based solar cells: a computational approach,” Materials Science and Engineering B, vol. 116, no. 3, pp. 293–296, 2005. View at Publisher · View at Google Scholar · View at Scopus
  55. T. Nakada, S. Kijima, Y. Kuromiya et al., “Chalcopyrite thin-film tandem solar cells with 1.5 V open-circuit-voltage,” in Proceedings of the IEEE 4th World Conference on Photovoltaic Energy Conversion (WCPEC '06), vol. 1, pp. 400–403, May 2006. View at Publisher · View at Google Scholar · View at Scopus
  56. M. W. Wang, “Novel CdSe-based PV structure for high efficiency CdSe/CIGS tandem solar cells,” in Proceedings of the 34th IEEE Photovoltaic Specialists Conference (PVSC '09), pp. 489–493, Philadelphia, Pa, USA, June 2009. View at Publisher · View at Google Scholar
  57. J. H. Scofield, A. Duda, D. Albin, B. L. Ballard, and P. K. Predecki, “Sputtered molybdenum bilayer back contact for copper indium diselenide-based polycrystalline thin-film solar cells,” Thin Solid Films, vol. 260, no. 1, pp. 26–31, 1995. View at Google Scholar · View at Scopus
  58. R. J. Matson, O. Jamjoum, A. D. Buonaquisti et al., “Metal contacts to CuInSe2,” Solar Cells, vol. 11, no. 3, pp. 301–305, 1984. View at Google Scholar · View at Scopus
  59. E. Moons, T. Engelhard, and D. Cahen, “Ohmic contacts to p-CuInSe2 crystals,” Journal of Electronic Materials, vol. 22, no. 3, pp. 275–280, 1993. View at Publisher · View at Google Scholar · View at Scopus
  60. M. Powalla and B. Dimmler, “Scaling up issues of CIGS solar cells,” Thin Solid Films, vol. 361, pp. 540–546, 2000. View at Publisher · View at Google Scholar · View at Scopus
  61. R. Menner, E. Gross, A. Eicke et al., in Proceedings of the 13th European Photovoltaic Solar Energy Conference, p. 2067, Nice, France, 1995.
  62. R. A. Hoffman, J. C. Lin, and J. P. Chambers, “The effect of ion bombardment on the microstructure and properties of molybdenum films,” Thin Solid Films, vol. 206, no. 1-2, pp. 230–235, 1991. View at Google Scholar · View at Scopus
  63. K. Granath, A. Rockett, M. Bodegard, C. Nender, and L. Stolt, “Mechanical issues of Mo back contacts for Cu(In,Ga)Se2 devices,” in Proceedings of the 13th European Photovoltaic Solar Energy Conference, pp. 1983–1986, Nice, France, 1995.
  64. G. Gordillo, M. Grizález, and L. C. Hernandez, “Structural and electrical properties of DC sputtered molybdenum films,” Solar Energy Materials and Solar Cells, vol. 51, no. 3-4, pp. 327–337, 1998. View at Google Scholar · View at Scopus
  65. M. A. Martínez and C. Guillén, “Effect of r.f.-sputtered Mo substrate on the microstructure of electrodeposited CuInSe2 thin films,” Surface and Coatings Technology, vol. 110, no. 1-2, pp. 62–67, 1998. View at Google Scholar · View at Scopus
  66. K. H. Yoon, S. K. Kim, R. B. V. Chalapathy et al., “Characterization of a molybdenum electrode deposited by sputtering and its effect on Cu(In,Ga)Se2 solar cells,” Journal of the Korean Physical Society, vol. 45, no. 4, pp. 1114–1118, 2004. View at Google Scholar · View at Scopus
  67. K. Orgassa, H. W. Schock, and J. H. Werner, “Alternative back contact materials for thin film Cu(In,Ga)Se2 solar cells,” Thin Solid Films, vol. 431-432, pp. 387–391, 2003. View at Publisher · View at Google Scholar · View at Scopus
  68. P. E. Russell, O. Jamjoum, R. K. Ahrenkiel, L. L. Kazmerski, R. A. Mickelsen, and W. S. Chen, “Properties of the Mo-CuInSe2 interface,” Applied Physics Letters, vol. 40, no. 11, pp. 995–997, 1982. View at Publisher · View at Google Scholar · View at Scopus
  69. W. Jaegaermann, T. Löher, and C. Pettenkofer, “Surface properties of chalcopyrite semiconductors,” Crystal Research and Crystal Technology, vol. 31, p. 273, 1996. View at Google Scholar
  70. W. N. Shafarman and J. E. Phillips, “Direct current-voltage measurements of the Mo/CuInSe2 contact on operating solar cells,” in Proceedings of the 25th IEEE Photovoltaic Specialists Conference, pp. 917–919, Washington, DC, USA, May 1996. View at Scopus
  71. S. Raud and M.-A. Nicolet, “Study of the CuInSe2/Mo thin film contact stability,” Thin Solid Films, vol. 201, no. 2, pp. 361–371, 1991. View at Google Scholar · View at Scopus
  72. N. Kohara, S. Nishiwaki, Y. Hashimoto, T. Negami, and T. Wada, “Electrical properties of the Cu(In,Ga)Se2/MoSe2/Mo structure,” Solar Energy Materials and Solar Cells, vol. 67, no. 1–4, pp. 209–215, 2001. View at Publisher · View at Google Scholar · View at Scopus
  73. K. M. Jones, L. L. Kazmerski, and B. G. Yacobi, “Transmission electron microscopy and X-ray photoelectron spectroscopy investigations of the MoCuInSe2 interface,” Thin Solid Films, vol. 116, no. 1–3, pp. L59–L62, 1984. View at Google Scholar · View at Scopus
  74. D. Schmid, M. Ruckh, and H. W. Schock, “A comprehensive characterization of the interfaces in Mo/CIS/CdS/ZnO solar cell structures,” Solar Energy Materials and Solar Cells, vol. 41-42, pp. 281–294, 1996. View at Publisher · View at Google Scholar · View at Scopus
  75. T. Wada, N. Kohara, S. Nishiwaki, and T. Negami, “Characterization of the Cu(In,Ga)Se2/Mo interface in CIGS solar cells,” Thin Solid Films, vol. 387, no. 1-2, pp. 118–122, 2001. View at Publisher · View at Google Scholar · View at Scopus
  76. S. Nishiwaki, N. Kohara, T. Negami, M. Nishitani, and T. Wada, “Characterization of Cu(In,Ga)Se2/Mo interface in CIGS solar cells,” in Materials Research Society Symposium Proceedings, vol. 485, p. 139, 1997.
  77. S. Nishiwaki, N. Kohara, T. Negami, and T. Wada, “MoSe2 layer formation at Cu(In,Ga)Se2/Mo interfaces in high efficiency Cu(In1-xGax)Se2 Solar Cells,” Japanese Journal of Applied Physics. Part 2, vol. 37, no. 1, pp. L71–L73, 1998. View at Google Scholar · View at Scopus
  78. T. Wada, N. Kohara, T. Negami, and M. Nishitani, “Chemical and structural characterization of Cu(In, Ga)Se2/Mo interface in Cu(In, Ga)Se2 solar cells,” Japanese Journal of Applied Physics, vol. 35, no. 10, pp. L1253–L1256, 1996. View at Google Scholar · View at Scopus
  79. L. Assmann, J. C. Bernède, A. Drici, C. Amory, E. Halgand, and M. Morsli, “Study of the Mo thin films and Mo/CIGS interface properties,” Applied Surface Science, vol. 246, no. 1–3, pp. 159–166, 2005. View at Publisher · View at Google Scholar · View at Scopus
  80. Y. K. Shimizu, S. Shimada, M. Watanabe et al., “Effects of Mo back contact thickness on the properties of CIGS solar cells,” Physica Status Solidi A, vol. 206, no. 5, pp. 1063–1066, 2009. View at Publisher · View at Google Scholar · View at Scopus
  81. R. Würz, D. Fuertes Marrón, A. Meeder et al., “Formation of an interfacial MoSe2 layer in CVD grown CuGaSe2 based thin film solar cells,” Thin Solid Films, vol. 431-432, pp. 398–402, 2003. View at Publisher · View at Google Scholar · View at Scopus
  82. C. Ballif, H. R. Moutinho, F. S. Hasoon, R. G. Dhere, and M. M. Al-Jassim, “Cross-sectional atomic force microscopy imaging of polycrystalline thin films,” Ultramicroscopy, vol. 85, no. 2, pp. 61–71, 2000. View at Publisher · View at Google Scholar · View at Scopus
  83. L. Zhang, Q. He, W.-L. Jiang, F.-F. Liu, C.-J. Li, and Y. Sun, “Mo back contact for flexible polyimide substrate Cu(In, Ga)Se2 thin-film solar cells,” Chinese Physics Letters, vol. 25, no. 9, pp. 3452–3454, 2008. View at Publisher · View at Google Scholar · View at Scopus
  84. C. Guillén and J. Herrero, “Low-resistivity Mo thin films prepared by evaporation onto 30 cm × 30 cm glass substrates,” Journal of Materials Processing Technology, vol. 143-144, no. 1, pp. 144–147, 2003. View at Publisher · View at Google Scholar · View at Scopus
  85. B. M. Basol, V. K. Kapur, C. R. Leidholm, A. Minnickand, and A. Halani, “Studies on substrates and contacts for CIS films and devices,” in Proceedings of the IEEE 1st World Conference on Photovoltaic Energy Conversion, pp. 148–151, Hawaii, USA, 1994.
  86. D. C. Fishor, I. L. Repins, J. Schafor et al., “The effect of Mo morphology on the performance of Cu(In,Ga)Se2 thin films,” in Proceedings of the 31st IEEE Photovoltaic Specialists Conference (PVSEC '05), p. 371, 2005.
  87. V. Mohanakrishnaswamy, H. Sankaranarayanan, S. Pethe, C. S. Ferekides, and D. L. Morel, “The effect of Mo deposition conditions on defect formation and device performance for CIGS solar cells,” in Proceedings of the 31st IEEE Photovoltaic Specialists Conference (PVSEC '05), pp. 422–425, Tampa, Fla, USA, 2005.
  88. J. H. Y., K. H. Kim, Y. T. Ahn, and K. H. Yoon, “Effect of Na-doped Mo/Mo bilayer on CIGS cells and its photovoltaic properties,” in Proceedings of the IEEE 4th World Conference on Photovoltaic Energy Conversion (WCPEC '06), pp. 509–511, May 2006. View at Publisher · View at Google Scholar · View at Scopus
  89. T. Nakada, “Microstructural and diffusion properties of CIGS thin film solar cells fabricated using transparent conducting oxide back contacts,” Thin Solid Films, vol. 480-481, pp. 419–425, 2005. View at Publisher · View at Google Scholar · View at Scopus
  90. T. Haalboom, T. Godecke, F. Ernst, M. Ruhle, R. Herberholz, and H. W. Schock, “Phase relations and microstructure in bulk materials and thin films of the ternary system Cu-In-Se,” in Proceedings of the 11th International Conference on Ternary and Multinary Compounds, pp. 249–252, Salford, UK, 1998.
  91. J. C. Mikkelsen, “Ternary phase relations of the chalcopyrite compound CuGaSe2,” Journal of Electronic Materials, vol. 10, no. 3, pp. 541–558, 1981. View at Publisher · View at Google Scholar · View at Scopus
  92. H. Jitsukawa, H. Matsushita, and T. Takizawa, “Phase diagrams of the (Cu2Se, CuSe)-CuGaSe2 system and the crystal growth of CuGaSe2 by the solution method,” Journal of Crystal Growth, vol. 186, no. 4, pp. 587–593, 1998. View at Google Scholar · View at Scopus
  93. U. Rau, M. Schmitt, J. Parisi, W. Riedl, and F. Karg, “Persistent photoconductivity in Cu(In,Ga)Se2 heterojunctions and thin films prepared by sequential deposition,” Applied Physics Letters, vol. 73, no. 2, pp. 223–225, 1998. View at Publisher · View at Google Scholar · View at Scopus
  94. V. Nadenau, D. Hariskos, H.-W. Schock et al., “Microstructural study of the CdS/CuGaSe2 interfacial region in CuGaSe2 thin film solar cells,” Journal of Applied Physics, vol. 85, no. 1, pp. 534–542, 1999. View at Publisher · View at Google Scholar · View at Scopus
  95. D. Mattox, Handbook of Physical Vapor Deposition (PVD) Processing, Noyes, Park Ridge, NJ, USA, 1998.
  96. W. N. Shafarman and J. Zhu, “Effect of substrate temperature and deposition profile on evaporated Cu(InGa)Se2 films and devices,” Thin Solid Films, vol. 361, pp. 473–477, 2000. View at Publisher · View at Google Scholar · View at Scopus
  97. R. A. Mickelsen and W. S. Chen, “High photocurrent polycrystalline thin-film CdS/CuInSe2 solar cella,” Applied Physics Letters, vol. 36, no. 5, pp. 371–373, 1980. View at Publisher · View at Google Scholar · View at Scopus
  98. R. A. Mickelsen and W. S. Chen, “Development of a 9. 4% efficient thin-film CuInSe//2/CdS solar cell,” in Proceedings of the 15th IEEE Photovoltaic Specialists Conference, pp. 800–804, 1981. View at Scopus
  99. R. Klenk, T. Walter, H.-W. Schock, and D. Cahen, “A model for the successful growth of polycrystalline films of CuInSe2 by multisource physical vacuum evaporation,” Advanced Materials, vol. 5, no. 2, pp. 114–119, 1993. View at Google Scholar · View at Scopus
  100. J. R. Tuttle, M. Contreras, A. Tennant, D. Albin, and R. Noufi, “High efficiency thin-film Cu (In, Ga) Se2-based photovoltaic devices: progress towards a universal approach to absorber fabrication,” in Proceedings of the 23rd IEEE Photovoltaic Specialists Conference, pp. 415–421, New York, NY, USA, May 1993. View at Scopus
  101. J. S. Park, Z. Dong, S. Kim, and J. H. Perepezko, “CulnSe2 phase formation during Cu2Se/In2Se3 interdiffusion reaction,” Journal of Applied Physics, vol. 87, no. 8, pp. 3683–3690, 2000. View at Google Scholar · View at Scopus
  102. J. Kessler, K. O. Velthaus, M. Ruekh et al., in Proceedings of the 6th Photovoltaic Science and Engineering Conference (PVSEC '92), pp. 1005–1010, New Delhi, India, 1992.
  103. A. Gabor, J. Tuttle, D. Albin et al., “High efficiency polycrystalline Cu(In,Ga)Se2-based solar cells,” in Proceedings of the 12th NREL Photovoltaic Program Review, pp. 59–66, Denver, Colo, USA, 1993.
  104. S. Zweigart, T. Walter, C. Koble, S. M. Sun, U. Ruhle, and H. W. Schock, “Sequential deposition of Cu(In,Ga)(S,Se)2,” in Proceedings of the IEEE 1st World Conference on Photovoltaic Energy Conversion, vol. 1, pp. 60–67, Hawaii, USA, 1994.
  105. A. M. Gabor, J. R. Tuttle, M. H. Bode et al., “Band-gap engineering in Cu(In,Ga) Se2 thin films grown from (In,Ga)2Se3 precursors,” Solar Energy Materials and Solar Cells, vol. 41-42, pp. 247–260, 1996. View at Publisher · View at Google Scholar · View at Scopus
  106. L. Stolt, J. Hedström, J. Kessler, M. Ruckh, K.-O. Velthaus, and H.-W. Schock, “ZnO/CdS/CuInSe2 thin-film solar cells with improved performance,” Applied Physics Letters, vol. 62, no. 6, pp. 597–599, 1993. View at Publisher · View at Google Scholar · View at Scopus
  107. G. M. Hanket, P. D. Paulson, U. P. Singh et al., “Fabrication of graded Cu(InGa)Se2 films by inline evaporation,” in Proceedings of the IEEE Photovoltaic Specialists Conference (PVSC '00), p. 499, 2000.
  108. M. Marudachalam, H. Hichri, R. Klenk, R. W. Birkmire, W. N. Shafarman, and J. M. Schultz, “Preparation of homogeneous Cu(InGa)Se2 films by selenization of metal precursors in H2Se atmosphere,” Applied Physics Letters, vol. 67, p. 3978, 1995. View at Publisher · View at Google Scholar · View at Scopus
  109. A. M. Hermann, C. Gonzalez, P. A. Ramakrishnan et al., “Fundamental studies on large area Cu(In,Ga)Se2 films for high efficiency solar cells,” Solar Energy Materials and Solar Cells, vol. 70, no. 3, pp. 345–361, 2001. View at Publisher · View at Google Scholar · View at Scopus
  110. J. Zank, M. Mehlin, and H. P. Fritz, “Electrochemical codeposition of indium and gallium for chalcopyrite solar cells,” Thin Solid Films, vol. 286, no. 1-2, pp. 259–263, 1996. View at Google Scholar · View at Scopus
  111. K. T. Ramakrishna Reddy, P. K. Datta, and M. J. Carter, “Detection of crystalline phases in CuInSe2 films grown by selenization process,” Physica Status Solidi A, vol. 182, no. 2, pp. 679–685, 2000. View at Publisher · View at Google Scholar · View at Scopus
  112. F. O. Adurodija, J. Song, S. D. Kim et al., “Growth of CuInSe2 thin films by high vapour Se treatment of co-sputtered Cu-In alloy in a graphite container,” Thin Solid Films, vol. 338, no. 1-2, pp. 13–19, 1999. View at Google Scholar · View at Scopus
  113. M. Marudachalam, R. W. Birkmire, H. Hichri, J. M. Schultz, A. Swartzlander, and M. M. Al-Jassim, “Phases, morphology, and diffusion in Cu(In1-xGax)Se2 thin films,” Journal of Applied Physics, vol. 82, no. 6, pp. 2896–2905, 1997. View at Google Scholar · View at Scopus
  114. C. Guillén, M. A. Martínez, and J. Herrero, “CuInSe2 thin films obtained by a novel electrodeposition and sputtering combined method,” Vacuum, vol. 58, no. 4, pp. 594–601, 2000. View at Google Scholar · View at Scopus
  115. R. Caballero and C. Guillén, “Comparative studies between Cu-Ga-Se and Cu-In-Se thin film systems,” Thin Solid Films, vol. 403-404, pp. 107–111, 2002. View at Publisher · View at Google Scholar · View at Scopus
  116. Ö. F. Yüksel, B. M. Basol, H. Safak, and H. Karabiyik, “Optical characterisation of CuInSe2 thin films prepared by two-stage process,” Applied Physics A, vol. 73, no. 3, pp. 387–389, 2001. View at Google Scholar · View at Scopus
  117. A. G. Chowles, J. H. Neethling, H. Van Niekerk, J. A. A. Engelbrecht, and V. J. Watters, “Deposition and characterization of CuInSe2,” Renewable Energy, vol. 6, no. 5-6, pp. 613–618, 1995. View at Google Scholar · View at Scopus
  118. S. Bandyopadhyaya, S. Roy, S. Chaudhuri, and A. K. Pal, “CuIn(SxSe1-x)2 films prepared by graphite box annealing of In/Cu stacked elemental layers,” Vacuum, vol. 62, no. 1, pp. 61–73, 2001. View at Publisher · View at Google Scholar · View at Scopus
  119. C. Guillén and J. Herrero, “Structure, morphology and photoelectrochemical activity of CuInSe2 thin films as determined by the characteristics of evaporated metallic precursors,” Solar Energy Materials and Solar Cells, vol. 73, no. 2, pp. 141–149, 2002. View at Publisher · View at Google Scholar · View at Scopus
  120. M. S. Sadigov, M. Özkan, E. Bacaksiz, M. Altunbaş, and A. I. Kopya, “Production of CuInSe2 thin films by a sequential processes of evaporations and selenization,” Journal of Materials Science, vol. 34, no. 18, pp. 4579–4584, 1999. View at Publisher · View at Google Scholar · View at Scopus
  121. T. L. Chu, S. S. Chu, S. C. Lin, and J. Yue, “Large grain copoer indium diselenide films,” Journal of the Electrochemical Society, vol. 131, no. 9, pp. 2182–2185, 1984. View at Google Scholar · View at Scopus
  122. J. Bekker, V. Alberts, and M. J. Witcomb, “Influence of selenization techniques on the reaction kinetics of chalcopyrite thin films,” Thin Solid Films, vol. 387, no. 1-2, pp. 40–43, 2001. View at Publisher · View at Google Scholar · View at Scopus
  123. S. F. Chichibu, M. Sugiyama, M. Ohbasami et al., “Use of diethylselenide as a less-hazardous source for preparation of CuInSe2 photo-absorbers by selenization of metal precursors,” Journal of Crystal Growth, vol. 243, no. 3-4, pp. 404–409, 2002. View at Publisher · View at Google Scholar · View at Scopus
  124. A. Garg, K. S. Balakrishnan, and A. C. Rastogi, “Structural properties and growth mechanism of copper and indium selenide films prepared by electrochemical selenization of metal layers,” Journal of the Electrochemical Society, vol. 141, no. 6, pp. 1566–1572, 1994. View at Google Scholar · View at Scopus
  125. A. C. Rastogi, K. S. Balakrishnan, R. K. Sharma, and K. Jain, “Growth phases during electrochemical selenization of vacuum deposited CuIn metal layers for the formation of semiconducting CuInSe2 films,” Thin Solid Films, vol. 357, no. 2, pp. 179–188, 1999. View at Publisher · View at Google Scholar · View at Scopus
  126. C. Guillén and J. Herrero, “New approaches to obtain CuIn1-xGaxSe2 thin films by combining electrodeposited and evaporated precursors,” Thin Solid Films, vol. 323, no. 1-2, pp. 93–98, 1998. View at Google Scholar · View at Scopus
  127. V. Alberts, M. Klenk, and E. Bucher, “Phase separation and compositional changes in two-stage processed chalcopyrite thin films,” Thin Solid Films, vol. 387, no. 1-2, pp. 44–46, 2001. View at Publisher · View at Google Scholar · View at Scopus
  128. F. B. Dejene, “The structural and material properties of CuInSe2 and Cu(In,Ga)Se2 prepared by selenization of stacks of metal and compound precursors by Se vapor for solar cell applications,” Solar Energy Materials and Solar Cells, vol. 93, no. 5, pp. 577–582, 2009. View at Publisher · View at Google Scholar · View at Scopus
  129. F. B. Dejene, “Material and device properties of Cu(In,Ga)Se2 absorber films prepared by thermal reaction of InSe/Cu/GaSe alloys to elemental Se vapor,” Current Applied Physics, vol. 10, no. 1, pp. 36–40, 2010. View at Publisher · View at Google Scholar · View at Scopus
  130. N. M. Shah, C. J. Panchal, V. A. Kheraj, J. R. Ray, and M. S. Desai, “Growth, structural and optical properties of copper indium diselenide thin films deposited by thermal evaporation method,” Solar Energy, vol. 83, no. 5, pp. 753–760, 2009. View at Publisher · View at Google Scholar · View at Scopus
  131. N. M. Shah, J. R. Ray, K. J. Patel et al., “Structural, electrical, and optical properties of copper indium diselenide thin film prepared by thermal evaporation method,” Thin Solid Films, vol. 517, no. 13, pp. 3639–3644, 2009. View at Publisher · View at Google Scholar · View at Scopus
  132. S. P. Grindle, C. W. Smith, and S. D. Mittleman, “Preparation and properties of CuInS2 thin films produced by exposing sputtered Cu-In films to an H2S atmosphere,” Applied Physics Letters, vol. 35, no. 1, pp. 24–26, 1979. View at Publisher · View at Google Scholar · View at Scopus
  133. R. Gay et al., “Efficiency and process improvements in CuInSe2-based modules,” in Proceedings of the 12th European Photovoltaic Solar Energy Conference, pp. 935–938, 1994.
  134. V. Alberts, J. Titus, and R. W. Birkmire, “Material and device properties of single-phase Cu(In,Ga)(Se,S)2 alloys prepared by selenization/sulfurization of metallic alloys,” Thin Solid Films, vol. 451-452, pp. 207–211, 2004. View at Publisher · View at Google Scholar · View at Scopus
  135. K. Kushiya, M. Tachiyuki, T. Kase et al., “Fabrication of graded band-gap Cu(InGa)Se2 thin-film mini-modules with a Zn(O,S,OH)x buffer layer,” Solar Energy Materials and Solar Cells, vol. 49, no. 1–4, pp. 277–283, 1997. View at Google Scholar · View at Scopus
  136. Y. Tanaka, N. Akema, T. Morishita, D. Okumura, and K. Kushiya, “Improvement of Voc upward of 600 mV/cell with CIGS-based absorber prepared by Selenization/Sulfurization,” in Proceedings of the 17th European Photovoltaic Solar Energy Conference, pp. 989–994, Munich, Germany, 2001.
  137. Y. Goushi, H. Hakuma, K. Tabuchi, S. Kijima, and K. Kushiya, “Fabrication of pentanary Cu(InGa)(SeS)2 absorbers by selenization and sulfurization,” Solar Energy Materials and Solar Cells, vol. 93, no. 8, pp. 1318–1320, 2009. View at Publisher · View at Google Scholar · View at Scopus
  138. V. Probst, W. Stetter, W. Riedl et al., “Rapid CIS-process for high efficiency PV-modules: development towards large area processing,” Thin Solid Films, vol. 387, no. 1-2, pp. 262–267, 2001. View at Publisher · View at Google Scholar · View at Scopus
  139. V. K. Kapur, B. M. Basol, and E. S. Tseng, “Low cost methods for the production of semiconductor films for CuInSe2/CdS solar cells,” Solar Cells, vol. 21, no. 1–4, pp. 65–72, 1987. View at Google Scholar · View at Scopus
  140. H. Sato et al., “Fabrication of high efficiency CuIn(Ga)Se2 thin film solar cell by selenization with H2Se,” in Proceedings of the 23rd IEEE Photovoltaic Specialist Conference, p. 521, 1993.
  141. J. Kessler, H. Dittrich, F. Grunwald, and H. Schock, in Proceedings of the 10th European Conference on Photovoltaic Solar Energy Conversion, p. 879, 1991.
  142. H. Oumous, in Proceedings of the 9th European Conference on Photovoltaic Solar Energy Conversion, p. 153, 1992.
  143. F. Karg et al., “Novel rapid-thermal-processing for CIS thin-film solar cells,” in Proceedings of the 23rd IEEE Photovoltaic Specialists Conference, pp. 441–446, 1993.
  144. G. D. Mooney, A. M. Hermann, J. R. Tuttle, D. S. Albin, and R. Noufi, “Formation of CuInSe2 thin films by rapid thermal recrystallization,” Applied Physics Letters, vol. 58, no. 23, pp. 2678–2680, 1991. View at Publisher · View at Google Scholar · View at Scopus
  145. G. S. Chen, J. C. Yang, Y. C. Chan, L. C. Yang, and W. Huang, “Another route to fabricate single-phase chalcogenides by post-selenization of Cu-In-Ga precursors sputter deposited from a single ternary target,” Solar Energy Materials and Solar Cells, vol. 93, no. 8, pp. 1351–1355, 2009. View at Publisher · View at Google Scholar · View at Scopus
  146. M. A. Green, K. Emery, Y. Hishikawa, and W. Warta, “Solar cell efficiency tables (version 31),” Progress in Photovoltaics: Research and Applications, vol. 16, no. 1, pp. 61–67, 2008. View at Publisher · View at Google Scholar · View at Scopus
  147. K. Zweibel, “Issues in thin film PV manufacturing cost reduction,” Solar Energy Materials and Solar Cells, vol. 59, no. 1, pp. 1–18, 1999. View at Publisher · View at Google Scholar · View at Scopus
  148. M. Kaelin, D. Rudmann, and A. N. Tiwari, “Low cost processing of CIGS thin film solar cells,” Solar Energy, vol. 77, no. 6, pp. 749–756, 2004. View at Publisher · View at Google Scholar · View at Scopus
  149. C. J. Hibberd, E. Chassaing, W. Liu, D. B. Mitzi, D. Lincot, and A. N. Tiwari, “Non-vacuum methods for formation of Cu(In, Ga)(Se, S)2 thin film photovoltaic absorbers,” Progress in Photovoltaics: Research and Applications. View at Publisher · View at Google Scholar
  150. F. Kang, J. Ao, G. Sun, Q. He, and Y. Sun, “Properties of CuIn x Ga1- x Se2 thin films grown from electrodeposited precursors with different levels of selenium content,” Current Applied Physics, vol. 10, no. 3, pp. 886–888, 2010. View at Publisher · View at Google Scholar · View at Scopus
  151. Y. Lai, F. Liu, Z. Zhang et al., “Cyclic voltammetry study of electrodeposition of Cu(In,Ga)Se2 thin films,” Electrochimica Acta, vol. 54, no. 11, pp. 3004–3010, 2009. View at Publisher · View at Google Scholar · View at Scopus
  152. S. Yoon, T. Yoon, K.-S. Lee, S. Yoon, J. M. Ha, and S. Choe, “Nanoparticle-based approach for the formation of CIS solar cells,” Solar Energy Materials and Solar Cells, vol. 93, no. 6-7, pp. 783–788, 2009. View at Publisher · View at Google Scholar · View at Scopus
  153. J. W. Park, Y. W. Choi, E. Lee, O. S. Joo, S. Yoon, and B. K. Min, “Synthesis of CIGS absorber layers via a paste coating,” Journal of Crystal Growth, vol. 311, no. 9, pp. 2621–2625, 2009. View at Publisher · View at Google Scholar · View at Scopus
  154. R. Kaigawa, T. Uesugi, T. Yoshida, S. Merdes, and R. Klenk, “Instantaneous preparation of CuInSe2 films from elemental In, Cu, Se particles precursor films in a non-vacuum process,” Thin Solid Films, vol. 517, no. 7, pp. 2184–2186, 2009. View at Publisher · View at Google Scholar · View at Scopus
  155. J. Olejníček, C. A. Kamler, A. Mirasano et al., “A non-vacuum process for preparing nanocrystalline CuIn1-xGaxSe2 materials involving an open-air solvothermal reaction,” Solar Energy Materials and Solar Cells, vol. 94, no. 1, pp. 8–11, 2010. View at Publisher · View at Google Scholar · View at Scopus
  156. D. B. Mitzi, M. Yuan, W. Liu et al., “A high-efficiency solution-deposited thin-film photovoltaic device,” Advanced Materials, vol. 20, no. 19, pp. 3657–3662, 2008. View at Publisher · View at Google Scholar · View at Scopus
  157. D. B. Mitzi, M. Yuan, W. Liu et al., “Hydrazine-based deposition route for device-quality CIGS films,” Thin Solid Films, vol. 517, no. 7, pp. 2158–2162, 2009. View at Publisher · View at Google Scholar · View at Scopus
  158. W. Liu, D. B. Mitzi, M. Yuan, A. J. Kellock, S. J. Chey, and O. Gunawan, “12% efficiency CuIn(Se,S)2 photovoltaic device prepared using a hydrazine solution process,” Chemistry of Materials, vol. 22, no. 3, pp. 1010–1014, 2010. View at Publisher · View at Google Scholar
  159. W. W. Hou, B. Bob, S.-h. Li, and Y. Yang, “Low-temperature processing of a solution-deposited CuInSSe thin-film solar cell,” Thin Solid Films, vol. 517, no. 24, pp. 6853–6856, 2009. View at Publisher · View at Google Scholar · View at Scopus
  160. E. Ahmed, A. Zegadi, A. E. Hill, R. D. Pilkington, R. D. Tomlinson, and W. Ahmed, “Thermal annealing of flash evaporated Cu(In, Ga)Se2 thin films,” Journal of Materials Processing Technology, vol. 300, no. 3-4, pp. 260–265, 1998. View at Google Scholar · View at Scopus
  161. R. N. Bhattacharya, J. F. Hiltner, W. Batchelor, M. A. Contreras, R. N. Noufi, and J. R. Sites, “15.4% CuIn1-xGaxSe2-based photovoltaic cells from solution-based precursor films,” Thin Solid Films, vol. 361, pp. 396–399, 2000. View at Publisher · View at Google Scholar · View at Scopus
  162. M. Krunks, O. Kijatkina, H. Rebane, I. Oja, V. Mikli, and A. Mere, “Composition of CuInS2 thin films prepared by spray pyrolysis,” Thin Solid Films, vol. 403-404, pp. 71–75, 2002. View at Publisher · View at Google Scholar · View at Scopus
  163. A. Katerski, A. Mere, V. Kazlauskiene et al., “Surface analysis of spray deposited copper indium disulfide films,” Thin Solid Films, vol. 516, no. 20, pp. 7110–7115, 2008. View at Publisher · View at Google Scholar · View at Scopus
  164. K. T. R. Reddy and R. W. Miles, “Surface characterization of sprayed CuGaxIn1-xSe2 layers,” Journal of Materials Science: Materials in Electronics, vol. 14, no. 9, pp. 529–532, 2003. View at Publisher · View at Google Scholar · View at Scopus
  165. N. Romeo, V. Canevari, G. Sberveglieri, O. Vigil, and L. Zanotti, “CuGaxIn1-xSe2 thin films for solar cells prepared By flash-evaporation,” Solar Energy Materials, vol. 3, no. 3, pp. 367–370, 1980. View at Publisher · View at Google Scholar · View at Scopus
  166. A. Bauknecht, S. Siebentritt, A. Gerhard et al., “Defects in CuGaSe2 thin films grown by MOCVD,” Thin Solid Films, vol. 361, pp. 426–431, 2000. View at Publisher · View at Google Scholar · View at Scopus
  167. I. H. Choi and D. H. Lee, “Preparation of CuIn1-xGaxSe2 films by metalorganic chemical vapor deposition using three precursors,” Thin Solid Films, vol. 515, no. 11, pp. 4778–4782, 2007. View at Publisher · View at Google Scholar · View at Scopus
  168. P. A. Jones, A. D. Jackson, P. D. Lickiss, R. D. Pilkington, and R. D. Tomlinson, “The plasma enhanced chemical vapour deposition of CuInSe2,” Thin Solid Films, vol. 238, no. 1, pp. 4–7, 1994. View at Google Scholar · View at Scopus
  169. S. Duchemin, M. C. Artaud, F. Ouchen, and J. Bougnot, “Growth of CuInSe2 by metallorganic chemical vapour deposition (MOCVD): new copper precursor,” Journal of Materials Science: Materials in Electronics, vol. 7, no. 3, pp. 201–205, 1996. View at Google Scholar · View at Scopus
  170. M. C. Artaud, F. Ouchen, L. Martin, and S. Duchemin, “CuInSe2 thin films grown by MOCVD: characterization, first devices,” Thin Solid Films, vol. 324, no. 1-2, pp. 115–123, 1998. View at Google Scholar · View at Scopus
  171. J. M. Aleese, P. O. Brien, and D. J. Otway, Chemical Vapor Deposition, vol. 4, p. 94, 1998.
  172. J.-H. Park, M. Afzaal, M. Kemmler et al., “The deposition of thin films of CuME2 by CVD techniques (M = In, Ga and E = S, Se),” Journal of Materials Chemistry, vol. 13, no. 8, pp. 1942–1949, 2003. View at Publisher · View at Google Scholar · View at Scopus
  173. M. Afazaal, D. J. Crouch, P. O. Brien et al., “The synthesis, X-ray structures and Cvd studies of some group 11 complexes of imino-bis(diisopropylphosphine Selenides) and their use in the deposition of I/iii/vi photovoltaic materials,” Journal of Materials Chemistry, vol. 14, pp. 233–237, 2004. View at Google Scholar
  174. I.-H. Choi and P. Y. Yu, “Preparation of CuInSe2/CuGaSe2 two layers absorber film by metal-organic chemical vapor deposition,” Current Applied Physics, vol. 9, no. 1, pp. 151–154, 2009. View at Publisher · View at Google Scholar · View at Scopus
  175. A. Bouloufa, K. Djessas, and D. Todorovic, “Structural and optical properties of Cu(In,Ga)Se2 grown by close-spaced vapor transport technique,” Materials Science in Semiconductor Processing, vol. 12, no. 1-2, pp. 82–87, 2009. View at Google Scholar
  176. G. W. El Haj Moussa, Ariswan, A. Khoury, F. Guastavino, and C. Llinarés, “Fabrication and study of photovoltaic material CuIn1-xGaxSe2 bulk and thin films obtained by the technique of close-spaced vapor transport,” Solid State Communications, vol. 122, no. 3-4, pp. 195–199, 2002. View at Publisher · View at Google Scholar · View at Scopus
  177. G. Massé, K. Djessas, C. Monty, and F. Sibieude, “Morphology of Cu(In,Ga)Se2 thin films grown by close-spaced vapor transport from sources with different grain sizes,” Thin Solid Films, vol. 414, no. 2, pp. 192–198, 2002. View at Publisher · View at Google Scholar · View at Scopus
  178. J. S. Wellings, A. P. Samantilleke, S. N. Heavens, P. Warren, and I. M. Dharmadasa, “Electrodeposition of CuInSe2 from ethylene glycol at 150 °C,” Solar Energy Materials and Solar Cells, vol. 93, no. 9, pp. 1518–1523, 2009. View at Publisher · View at Google Scholar · View at Scopus
  179. L. Oliveira, T. Todorov, E. Chassaing, D. Lincot, J. Carda, and P. Escribano, “CIGSS films prepared by sol-gel route,” Thin Solid Films, vol. 517, no. 7, pp. 2272–2276, 2009. View at Publisher · View at Google Scholar · View at Scopus
  180. B. M. Basol, V. K. Kapur, A. Halani et al., “Cu(In,Ga)Se2 thin films and solar cells prepared by selenization of metallic precursors,” The Journal of Vacuum Science and Technology A, vol. 14, pp. 2251–2256, 1996. View at Google Scholar
  181. R. Caballero, C. Guillén, M. T. Gutiérrez, and C. A. Kaufmann, “CuIn1-xGaxSe2-based thin-film solar cells by the selenization of sequentially evaporated metallic layers,” Progress in Photovoltaics: Research and Applications, vol. 14, no. 2, pp. 145–153, 2006. View at Publisher · View at Google Scholar · View at Scopus
  182. R. Caballero and C. Guillen, in Proceedings of the International Conference on Advances in Materials and Processing Technologies, p. 583, Madrid, Spain, 2001.
  183. R. Caballero and C. Guillén, “CuInSe2 Formation by selenization of sequentially evaporated metallic layers,” Solar Energy Materials and Solar Cells, vol. 86, no. 1, pp. 1–10, 2005. View at Publisher · View at Google Scholar · View at Scopus
  184. H. Sato, T. Hama, E. Niemi, Y. Ichikawa, and H. Sakai, Japanese Journal of Applied Physics, vol. 32, p. 50, 1993.
  185. M. Venkatachalam, M. D. Kannan, N. Muthukumarasamy et al., “Investigations on electron beam evaporated Cu(In0.85Ga0.15)Se2 thin film solar cells,” Solar Energy, vol. 83, no. 9, pp. 1652–1655, 2009. View at Publisher · View at Google Scholar · View at Scopus
  186. K. G. Deepa, R. Jayakrishnan, K. P. Vijayakumar, C. Sudha Kartha, and V. Ganesan, “Sub-micrometer thick CuInSe2 films for solar cells using sequential elemental evaporation,” Solar Energy, vol. 83, no. 7, pp. 964–968, 2009. View at Publisher · View at Google Scholar · View at Scopus
  187. S. Agilan, D. Mangalaraj, SA. K. Narayandass, S. Velumani, and A. Ignatiev, “Structural and optical characterization of CuInSe2 films deposited by hot wall vacuum evaporation method,” Vacuum, vol. 81, no. 7, pp. 813–818, 2007. View at Publisher · View at Google Scholar · View at Scopus
  188. M. M. Islam, T. Sakurai, S. Ishizuka et al., “Effect of Se/(Ga+In) ratio on MBE grown Cu(In,Ga)Se2 thin film solar cell,” Journal of Crystal Growth, vol. 311, no. 7, pp. 2212–2214, 2009. View at Publisher · View at Google Scholar · View at Scopus
  189. S. Niki, P. J. Fons, A. Yamada et al., “High quality CuInSe2 epitaxial films—molecular beam epitaxial growth and intrinsic properties,” Institute of Physics Conference Series, vol. 152E, p. 221, 1994. View at Google Scholar
  190. E. P. Zaretskaya, V. F. Gremenok, V. B. Zalesski, K. Bente, S. Schorr, and S. Zukotynski, “Properties of Cu(In,Ga)(S,Se)2 thin films prepared by selenization/sulfurization of metallic alloys,” Thin Solid Films, vol. 515, no. 15, pp. 5848–5851, 2007. View at Publisher · View at Google Scholar · View at Scopus
  191. A. F. da Cunha, F. Kurdesau, D. Rudmann, and P. M. P. Salomé, “Performance comparison of hybrid sputtering/evaporation CuIn1-xGaxSe2 solar cells with different transparent conducting oxide window layers,” Journal of Non-Crystalline Solids, vol. 352, no. 9–20, pp. 1976–1980, 2006. View at Publisher · View at Google Scholar · View at Scopus
  192. M. Bodegard, L. Stolt, and J. Hedstrom, “The influence of sodium on the grain structure of CuInSe2 films for photovoltaic applications,” in Proceedings of the 12th European Conference on Photovoltaic Solar Energy Conversion, pp. 1743–1746, 1994.
  193. M. Bodegård, K. Granath, L. Stolt, and A. Rockett, “Behaviour of Na implanted into Mo thin films during annealing,” Solar Energy Materials and Solar Cells, vol. 58, no. 2, pp. 199–208, 1999. View at Publisher · View at Google Scholar · View at Scopus
  194. J. Hedstrom, H. Ohlsen, M. Bodegard et al., “ZnO/CdS/Cu(In,Ga)Se2 thin film solar cells with improved performance,” in Proceedings of the 23rd IEEE Photovoltaic Specialists Conference, pp. 364–371, New York, NY, USA, May 1993. View at Scopus
  195. M. Ruckh, D. Schmid, M. Kaiser, R. Schaffler, T. Walter, and H. W. Schock, “Influence of substrates on the electrical properties of Cu(In,Ga)Se2 thin films,” in Proceedings of the Conference Record of the IEEE 1st World Conference on Photovoltaic Energy Conversion, pp. 156–159, Hawaii, USA, 1994. View at Publisher · View at Google Scholar
  196. M. Bodeg Ård, K. Granath, and L. Stolt, “Growth of Cu(In,Ga)Se2 thin films by coevaporation using alkaline precursors,” Thin Solid Films, vol. 361, pp. 9–16, 2000. View at Publisher · View at Google Scholar · View at Scopus
  197. D. Rudmann, G. Bilger, M. Kaelin, F.-J. Haug, H. Zogg, and A. N. Tiwari, “Effects of NaF coevaporation on structural properties of Cu(In,Ga)Se2 thin films,” Thin Solid Films, vol. 431-432, pp. 37–40, 2003. View at Publisher · View at Google Scholar · View at Scopus
  198. T. Nakada, D. Iga, H. Ohbo, and A. Kunioka, “Effects of sodium on Cu(In, Ga)Se2-based thin films and solar cells,” Japanese Journal of Applied Physics. Part 1, vol. 36, no. 2, pp. 732–737, 1997. View at Google Scholar · View at Scopus
  199. D. W. Niles, M. Al-Jassim, and K. Ramanathan, “Direct observation of Na and O impurities at grain surfaces of CuInSe2 thin films,” Journal of Vacuum Science and Technology A, vol. 17, no. 1, pp. 291–296, 1999. View at Google Scholar · View at Scopus
  200. D. Rudmann, A. F. Da Cunha, M. Kaelin et al., “Efficiency enhancement of Cu(In,Ga)Se2 solar cells due to post-deposition Na incorporation,” Applied Physics Letters, vol. 84, no. 7, pp. 1129–1131, 2004. View at Publisher · View at Google Scholar · View at Scopus
  201. S.-H. Wei, S. B. Zhang, and A. Zunger, “Effects of Na on the electrical and structural properties of CuInSe2,” Journal of Applied Physics, vol. 85, no. 10, pp. 7214–7218, 1999. View at Google Scholar · View at Scopus
  202. V. Probst, J. Rimmasch, W. Riedl et al., “The impact of controlled sodium incorporation on rapid thermal processed Cu(ln,Ga)Se2-thin films and devices,” in Proceedings of the Conference Record of the IEEE 1st World Conference on Photovoltaic Energy onversion, p. 144, New York, NY, USA, 1994.
  203. A. Rockett, J. S. Britt, T. Gillespie et al., “Na in selenized Cu(In,Ga)Se2 on Na-containing and Na-free glasses: distribution, grain structure, and device performances,” Thin Solid Films, vol. 372, no. 1, pp. 212–217, 2000. View at Publisher · View at Google Scholar · View at Scopus
  204. J. E. Granata and J. R. Sites, “Impact of sodium in the bulk and in grain boundaries of CuInSe2,” in Proceedings of the 2nd World Conference on Photovoltaic Solar Energy Conversion, vol. 1, pp. 604–607, Vienna, Austria, July 1998.
  205. R. J. Matson, J. E. Granata, S. E. Asher, and M. R. Young, “Effects of substrate and Na concentration on device properties, junction formation, and film microstructure in CuInSe2 PV devices,” in Proceedings of the 15th NCPV Photovoltaic Program Review, vol. 462, pp. 542–552, Denver, Colo, USA, March 1999. View at Publisher · View at Google Scholar
  206. T. Nakada, H. Ohbo, M. Fukuda, and A. Kunioka, “Improved compositional flexibility of Cu(In,Ga)Se2-based thin film solar cells by sodium control technique,” Solar Energy Materials and Solar Cells, vol. 49, no. 1–4, pp. 261–267, 1997. View at Google Scholar · View at Scopus
  207. D. Wolf, G. Muller, W. Stetter, and F. Karg, “In-situ investigation of Cu-In-Se reactions: impact of Na on CIS formation,” in Proceedings of the 2nd World Conference on Photovoltaic Solar Energy Conversion, pp. 2426–2430, Vienna, Austria, 1998.
  208. W. N. Shafarman and J. Zhu, “Effect of substrate temperature and deposition profile on evaporated Cu(InGa)Se2 films and devices,” Thin Solid Films, vol. 361, pp. 473–477, 2000. View at Publisher · View at Google Scholar · View at Scopus
  209. S. Ishizuka, H. Shibata, A. Yamada et al., “Growth of polycrystalline Cu (In,Ga) Se2 thin films using a radio frequency-cracked Se-radical beam source and application for photovoltaic devices,” Applied Physics Letters, vol. 91, no. 4, Article ID 041902, 2007. View at Publisher · View at Google Scholar · View at Scopus
  210. J. Holz, F. Karg, and H. V. Philipsborn, “The effect of substrate impurities on the electronic conductivity in CIS thin films,” in Proceedings of the 12th European Photovoltaic Solar Energy Conference, pp. 1592–1595, Amsterdam, The Netherlands, 1994.
  211. R. Kirmura, T. Mouri, T. Nakada et al., “Photoluminescence properties of sodium incorporated in CuInSe2 thin films,” Japanese Journal of Applied Physics, vol. 38, pp. L289–L291, 1999. View at Publisher · View at Google Scholar
  212. M. Lammer, U. Klemm, and M. Powalla, “Sodium co-evaporation for low temperature Cu(In,Ga)Se2 deposition,” Thin Solid Films, vol. 387, no. 1-2, pp. 33–36, 2001. View at Publisher · View at Google Scholar · View at Scopus
  213. R. Caballero, C. A. Kaufmann, T. Eisenbarth et al., “The influence of Na on low temperature growth of CIGS thin film solar cells on polyimide substrates,” Thin Solid Films, vol. 517, no. 7, pp. 2187–2190, 2009. View at Publisher · View at Google Scholar · View at Scopus
  214. P. T. Erslev, J. W. Lee, W. N. Shafarman, and J. D. Cohen, “The influence of Na on metastable defect kinetics in CIGS materials,” Thin Solid Films, vol. 517, no. 7, pp. 2277–2281, 2009. View at Publisher · View at Google Scholar · View at Scopus
  215. S. Ishizuka, A. Yamada, M. M. Islam et al., “Na-induced variations in the structural, optical, and electrical properties of Cu (In,Ga) Se2 thin films,” Journal of Applied Physics, vol. 106, no. 3, Article ID 034908, 2009. View at Publisher · View at Google Scholar · View at Scopus
  216. R. Ortega-Borges and D. Lincot, “Mechanism of chemical bath deposition of cadmium sulfide thin films in the ammonia-thiourea system,” Journal of the Electrochemical Society, vol. 140, no. 12, pp. 3464–3473, 1993. View at Google Scholar · View at Scopus
  217. G. Hodes, Chemical Solution Deposition of Semiconductor Films, Marcel Dekker, New York, NY, USA, 2002.
  218. L. Olsen, P. Eschbach, and S. Kundu, “Role of buffer layers in CIS based solar cells,” in Proceedings of the 29th IEEE Photovoltaic Specialists Conference, p. 652, New York, NY, USA, 2002.
  219. S. Chaisitsak, A. Yamada, and M. Konagai, “Comprehensive study of light-soaking effect in ZnO/Cu(InGa)Se2 Solar Cells with Zn-Based Buffer Layers,” in Proceedings of the Materials Research Society Spring Meeting, San Francisco, Calif, USA, 2001, v.668:H9.10.1.5.
  220. M. Bär, Ch.-H. Fischer, and H.-J. Muffier, in Proceedings of the 29th IEEE Photovoltaic Specialists Conference, p. 636, New Orleans, La, USA, 2002.
  221. J. Kessler, M. Ruckh, D. Hariskos, U. Ruhle, R. Menner, and H. W. Schock, “Interface engineering between CuInSe2 and ZnO,” in Proceedings of the 23rd IEEE Photovoltaic Specialists Conference, pp. 447–452, Louisville, Ky, USA, May 1993. View at Scopus
  222. A. Romeo, R. Gysel, and S. Buzzi, “Properties of CIGS solar cells developed with evaporated II–VI buffer layers,” in Proceedings of the Technical Digest of the 14th International Photovoltaic Science and Engineering Conference, p. 705, Bangkok, Thailand, 2004.
  223. M. Rusu, T. Glatzel, C. A. Kaufmann et al., “High-efficient ZnO/PVD-CdS/Cu(In,Ga)Se2 thin film solar cells: formation of the buffer-absorber interface and transport properties,” in Proceedings of the Materials Research Society Symposiumml: Thin-Film Compound Semiconductor Photovoltaics, vol. 865, pp. F14.25.1–F14.25.7, San Francisco, Calif, USA, April 2005. View at Scopus
  224. D. Schmid, M. Ruckh, and H. W. Schock, “A comprehensive characterization of the interfaces in Mo/CIS/CdS/ZnO solar cell structures,” Solar Energy Materials and Solar Cells, vol. 41-42, pp. 281–294, 1996. View at Publisher · View at Google Scholar · View at Scopus
  225. M. A. Contreras, M. J. Romero, B. To et al., “Optimization of CBD CdS process in high-efficiency Cu(In,Ga)Se2-based solar cells,” Thin Solid Films, vol. 403-404, pp. 204–211, 2002. View at Publisher · View at Google Scholar · View at Scopus
  226. U. Rau and M. Schmidt, “Electronic properties of ZnO/CdS/Cu(In,Ga)Se2 solar cells—aspects of heterojunction formation,” Thin Solid Films, vol. 387, no. 1-2, pp. 141–146, 2001. View at Publisher · View at Google Scholar · View at Scopus
  227. P. Garg, A. Garg, A. C. Rastogi, and J. C. Garg, “Growth and characterization of electrodeposited CuInSe2 thin films from seleno-sulphate solution,” Journal of Physics D, vol. 24, no. 11, pp. 2026–2031, 1991. View at Google Scholar · View at Scopus
  228. S. N. Sahu, R. D. L. Kristensen, and D. Haneman, “Electrodeposition of CuInSe2 thin films from aqueous solution,” Solar Energy Materials, vol. 18, no. 6, pp. 385–397, 1989. View at Google Scholar · View at Scopus
  229. C. X. Qiu and I. Shih, “Investigation of electrodeposited CuInSe2 films,” Canadian journal of physics, vol. 65, no. 8, pp. 1011–1014, 1987. View at Google Scholar
  230. V. K. Kapur, B. M. Basol, and E. S. Tseng, “Low cost methods for the production of semiconductor films for CuInSe2/CdS solar cells,” Solar Cells, vol. 21, no. 1–4, pp. 65–72, 1987. View at Google Scholar · View at Scopus
  231. S. N. Qiu, L. Li, C. X. Qiu, I. Shih, and C. H. Champness, “Study of CuInSe2 thin films prepared by electrodeposition,” Solar Energy Materials and Solar Cells, vol. 37, no. 3-4, pp. 389–393, 1995. View at Google Scholar · View at Scopus
  232. A. Rockett and R. W. Birkmire, “CuInSe2 for photovoltaic applications,” Journal of Applied Physics, vol. 70, no. 7, pp. R81–R97, 1991. View at Publisher · View at Google Scholar · View at Scopus
  233. D. Schmid, M. Ruckh, F. Grunwald, and H. W. Schock, “Chalcopyrite/defect chalcopyrite heterojunctions on the basis of CuInSe2,” Journal of Applied Physics, vol. 73, no. 6, pp. 2902–2909, 1993. View at Publisher · View at Google Scholar · View at Scopus
  234. Y. Sudo, S. Endo, and T. Irie, “Preparation and characterization of electrodeposited CuInSe2 thin films,” Japanese Journal of Applied Physics. Part 1, vol. 32, no. 4, pp. 1562–1567, 1993. View at Google Scholar · View at Scopus
  235. R. W. Birkmire and E. Eser, “Polycrystalline thin film solar cells: present status and future potential,” Annual Review of Materials Science, vol. 27, no. 1, pp. 625–653, 1997. View at Google Scholar · View at Scopus
  236. H.-W. Schock and R. Noufi, “CIGS-based solar cells for the next millennium,” Progress in Photovoltaics: Research and Applications, vol. 8, no. 1, pp. 151–160, 2000. View at Publisher · View at Google Scholar · View at Scopus
  237. A. Kylner, A. Rockett, and L. Stolt, “Oxygen in solution grown CdS films for thin film solar cells,” Diffusion and Defect Data B, vol. 51-52, pp. 533–540, 1996. View at Google Scholar · View at Scopus
  238. T. Nakada and A. Kunioka, “Direct evidence of Cd diffusion into Cu(In,Ga)Se2 thin films during chemical-bath deposition process of CdS films,” Applied Physics Letters, vol. 74, no. 17, pp. 2444–2446, 1999. View at Google Scholar · View at Scopus
  239. Y. Hashimoto, N. Kohara, T. Negami, N. Nishitani, and T. Wada, “Chemical bath deposition of CdS buffer layer for CIGS solar cells,” Solar Energy Materials and Solar Cells, vol. 50, no. 1–4, pp. 71–77, 1998. View at Google Scholar · View at Scopus
  240. C. Guillén, M. A. Martínez, C. Maffiotte, and J. Herrero, “Chemistry of CdS/CuInSe2 structures as controlled by the CdS deposition bath,” Journal of the Electrochemical Society, vol. 148, no. 11, pp. G602–G606, 2001. View at Publisher · View at Google Scholar · View at Scopus
  241. A. Kylner, “Effect of impurities in the CdS buffer layer on the performance of the Cu(In, Ga)Se2 thin film solar cell,” Journal of Applied Physics, vol. 85, no. 9, pp. 6858–6865, 1999. View at Google Scholar · View at Scopus
  242. T. Negami, Y. Hashimoto, and S. Nishiwaki, “Cu(In,Ga)Se2 thin-film solar cells with an efficiency of 18%,” Solar Energy Materials and Solar Cells, vol. 67, no. 1–4, pp. 331–335, 2001. View at Publisher · View at Google Scholar · View at Scopus
  243. L. Kronik, U. Rau, J.-F. Guillemoles, D. Braunger, H.-W. Schock, and D. Cahen, “Interface redox engineering of Cu(In,Ga)Se2-based solar cells: oxygen, sodium, and chemical bath effects,” Thin Solid Films, vol. 361, pp. 353–359, 2000. View at Publisher · View at Google Scholar · View at Scopus
  244. C. Heske, D. Eich, R. Fink et al., “Observation of intermixing at the buried CdS/Cu(In, Ga)Se2 thin film solar cell heterojunction,” Applied Physics Letters, vol. 74, no. 10, pp. 1451–1453, 1999. View at Google Scholar · View at Scopus
  245. S. Chaisitsak, A. Yamada, and M. Konagai, “Preferred orientation control of Cu(In1-xGax)Se2 (x 0.28) thin films and its influence on solar cell characteristics,” Japanese Journal of Applied Physics. Part 1, vol. 41, no. 2, pp. 507–513, 2002. View at Google Scholar · View at Scopus
  246. T. M. Friedlmeier, D. Braunger, D. Hariskos, M. Kaiser, H. N. Wanka, and H. W. Schock, “Nucleation and growth of the CdS buffer layer on Cu(In,Ga)Se2 thin films,” in Proceedings of the 25th IEEE Photovoltaic Specialists Conference, pp. 845–848, May 1996. View at Scopus
  247. M. G. Sandoval-Paz, M. Sotelo-Lerma, A. Mendoza-Galvan, and R. Ramírez-Bon, “Optical properties and layer microstructure of CdS films obtained from an ammonia-free chemical bath deposition process,” Thin Solid Films, vol. 515, no. 7-8, pp. 3356–3362, 2007. View at Publisher · View at Google Scholar · View at Scopus
  248. H. Komaki, A. Yamada, K. Sakurai et al., “CIGS solar cell with CdS buffer layer deposited by ammonia-free process,” Physica Status Solidi A, vol. 206, no. 5, pp. 1072–1075, 2009. View at Publisher · View at Google Scholar · View at Scopus
  249. J. R. Mann, N. Vora, and I. L. Repins, “In Situ thickness measurements of chemical bath-deposited CdS,” Solar Energy Materials and Solar Cells, vol. 94, no. 2, pp. 333–337, 2010. View at Publisher · View at Google Scholar · View at Scopus
  250. N. Naghavi, C. Hubert, O. Kerrec et al., in Proceedings of the 22nd European Photovoltaic Solar Energy Conference, p. 2304, 2007.
  251. M. Powalla, G. Voorwinden, D. Hariskos, P. Jackson, and R. Kniese, “Highly efficient CIS solar cells and modules made by the co-evaporation process,” Thin Solid Films, vol. 517, no. 7, pp. 2111–2114, 2009. View at Publisher · View at Google Scholar · View at Scopus
  252. T. Nakada and M. Mizutani, “18% efficiency Cd-free Cu(In, Ga)Se2 thin-film solar cells fabricated using chemical bath deposition (CBD)-ZnS buffer layers,” Japanese Journal of Applied Physics. Part 2, vol. 41, no. 2 B, pp. L165–L167, 2002. View at Google Scholar · View at Scopus
  253. A. J. Nelson, C. R. Schwerdtfeger, S.-H. Wei et al., “Theoretical and experimental studies of the ZnSe/CuInSe2 heterojunction band offset,” Applied Physics Letters, vol. 62, no. 20, pp. 2557–2559, 1993. View at Publisher · View at Google Scholar · View at Scopus
  254. M. M. Islam, S. Ishizuka, A. Yamada et al., “CIGS solar cell with MBE-grown ZnS buffer layer,” Solar Energy Materials and Solar Cells, vol. 93, no. 6-7, pp. 970–972, 2009. View at Publisher · View at Google Scholar · View at Scopus
  255. S. Siebentritt, T. Kampschulte, A. Bauknecht et al., “Cd-free buffer layers for CIGS solar cells prepared by a dry process,” Solar Energy Materials and Solar Cells, vol. 70, no. 4, pp. 447–457, 2002. View at Publisher · View at Google Scholar · View at Scopus
  256. F. Engelhardt, M. Schmidt, Th. Meyer, O. Seifert, and J. Parisi, in Proceedings of the 2nd World Conference on PV Solar Energy Conversion, 1998.
  257. Y. Othake, K. Kushiya, A. Yamada, and M. Konagai, “Development of ZnO/ZnSe/CuIn1-xGaxSe2 thin-film solar cells with band gap of 1.3 to 1.5 eV,” in Proceedings of the 1st World Conference on Photovoltaic Solar Energy Conversion, p. 218, 1994.
  258. Y. Ohtake, K. Kushiya, M. Ichikawa, A. Yamada, and M. Konagai, “Polycrystalline Cu(InGa)Se2 thin-film solar cells with ZnSe buffer layers,” Japanese Journal of Applied Physics. Part 1, vol. 34, no. 11, pp. 5949–5955, 1995. View at Google Scholar · View at Scopus
  259. A. Ennaoui, S. Siebentritt, M. Ch. Lux-Steiner, W. Riedl, and F. Karg, “High-efficiency Cd-free CIGSS thin-film solar cells with solution grown zinc compound buffer layers,” Solar Energy Materials and Solar Cells, vol. 67, no. 1–4, pp. 31–40, 2001. View at Publisher · View at Google Scholar · View at Scopus
  260. Y. Ohtake, S. Chaisitsak, A. Yamada, and M. Konagai, “Characterization of ZnInxSey, thin films as a buffer layer for high efficiency Cu(InGa)Se2 thin-film solar cells,” Japanese Journal of Applied Physics. Part 1, vol. 37, no. 6, pp. 3220–3225, 1998. View at Google Scholar · View at Scopus
  261. Y. Ohtake, T. Okamoto, A. Yamada, M. Konagai, and K. Saito, “Improved performance of Cu(InGa)Se2 thin-film solar cells using evaporated Cd-free buffer layers,” Solar Energy Materials and Solar Cells, vol. 49, no. 1–4, pp. 269–275, 1997. View at Google Scholar · View at Scopus
  262. T. Negami, T. Aoyagi, T. Satoh et al., “Cd free CIGS solar cells fabricated by dry processes,” in Proceedings of the 29th IEEE Photovoltaic Specialists Conference, p. 656, 2002.
  263. D. Hariskos, B. Fuchs, R. Menner et al., “The Zn(S,O,OH)/ZnMgO buffer in thin-film Cu(In,Ga)(Se,S)2-based solar cells part II: magnetron sputtering of the ZnMgO buffer layer for in-line co-evaporated Cu(In,Ga)Se2 solar cells,” Progress in Photovoltaics: Research and Applications, vol. 17, no. 7, pp. 479–488, 2009. View at Publisher · View at Google Scholar · View at Scopus
  264. E. B. Yousfi, B. Weinberger, F. Donsanti, P. Cowache, and D. Lincot, “Atomic layer deposition of zinc oxide and indium sulfide layers for Cu(In,Ga)Se2 thin-film solar cells,” Thin Solid Films, vol. 387, no. 1-2, pp. 29–32, 2001. View at Publisher · View at Google Scholar · View at Scopus
  265. S. Spiering, D. Hariskos, M. Powalla, N. Naghavi, and D. Lincot, “CD-free Cu(In,Ga)Se2 thin-film solar modules with In2S3 buffer layer by ALCVD,” Thin Solid Films, vol. 431-432, pp. 359–363, 2003. View at Publisher · View at Google Scholar · View at Scopus
  266. J.-F. Guillemoles, B. Canava, E. B. Yousfi et al., “Indium-based interface chemical engineering by electrochemistry and atomic layer deposition for copper indium diselenide solar cells,” Japanese Journal of Applied Physics. Part 1, vol. 40, no. 10, pp. 6065–6068, 2001. View at Google Scholar · View at Scopus
  267. S. Spiering, L. Bürkert, D. Hariskos et al., “MOCVD indium sulphide for application as a buffer layer in CIGS solar cells,” Thin Solid Films, vol. 517, no. 7, pp. 2328–2331, 2009. View at Publisher · View at Google Scholar · View at Scopus
  268. F. Couzinié-Devy, N. Barreau, and J. Kessler, “Influence of absorber copper concentration on the Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS based solar cells performance,” Thin Solid Films, vol. 517, no. 7, pp. 2407–2410, 2009. View at Publisher · View at Google Scholar · View at Scopus
  269. A. Darga, D. Mencaraglia, Z. Djebbour et al., “Comparative study of Cu(In,Ga)Se2/(PVD)In2S3 and Cu(In,Ga)Se2/(CBD)CdS heterojunction based solar cells by admittance spectroscopy, current-voltage and spectral response measurements,” Thin Solid Films, vol. 517, no. 7, pp. 2423–2426, 2009. View at Publisher · View at Google Scholar · View at Scopus
  270. S. Buecheler, D. Corica, D. Guettler et al., “Ultrasonically sprayed indium sulfide buffer layers for Cu(In,Ga)(S,Se)2 thin-film solar cells,” Thin Solid Films, vol. 517, no. 7, pp. 2312–2315, 2009. View at Publisher · View at Google Scholar · View at Scopus
  271. A. Eicke, S. Spiering, A. Dresel, and M. Powalla, “Chemical characterisation of evaporated In2Sx buffer layers in Cu(In,Ga)Se2 thin-film solar cells with SNMS and SIMS,” Surface and Interface Analysis, vol. 40, no. 3-4, pp. 830–833, 2008. View at Publisher · View at Google Scholar · View at Scopus
  272. D. Abou-Ras, G. Kostorz, D. Hariskos et al., “Structural and chemical analyses of sputtered InxSy buffer layers in Cu(In,Ga)Se2 thin-film solar cells,” Thin Solid Films, vol. 517, no. 8, pp. 2792–2798, 2009. View at Publisher · View at Google Scholar · View at Scopus
  273. Y. Ohtake, M. Ichikawa, T. Okamoto, A. Yamada, M. Konagai, and K. Saito, “Cu(InGa)Se2 thin-film solar cells with continuously evaporated Cd-free buffer layers,” in Proceedings of the 25th IEEE Photovoltaic Specialists Conference, pp. 793–796, May 1996. View at Scopus
  274. N. Barreau, A. Mokrani, F. Couzinié-Devy, and J. Kessler, “Bandgap properties of the indium sulfide thin-films grown by co-evaporation,” Thin Solid Films, vol. 517, no. 7, pp. 2316–2319, 2009. View at Publisher · View at Google Scholar · View at Scopus
  275. D. Hariskos, R. Herberholz, M. Ruckh et al., in Proceedings of the 3th European Photovoltaic Solar Energy Conference, p. 1995, France, 1995.
  276. A. Ennaoui, U. Blieske, and M. CH. Lux-Steiner, “13.7%-efficient Zn(Se,OH)x/ Cu(In,Ga)(S,Se)2 thin-film solar cell,” Progress in Photovoltaics: Research and Applications, vol. 6, no. 6, pp. 447–451, 1998. View at Google Scholar · View at Scopus
  277. C. Hubert, N. Naghavi, O. Roussel et al., “The Zn(S,O,OH)/ZnMgO buffer in thin film Cu(In,Ga)(S,Se)2-based solar cells part I: fast chemical bath deposition of Zn(S,O,OH) buffer layers for industrial application on co-evaporated Cu(In,Ga)Se2 and electrodeposited cuIn(S,Se)2 solar cells,” Progress in Photovoltaics: Research and Applications, vol. 17, no. 7, pp. 470–478, 2009. View at Publisher · View at Google Scholar · View at Scopus
  278. R. Sáez-Araoz, A. Ennaoui, T. Kropp, E. Veryaeva, T. P. Niesen, and M. CH. Lux-Steiner, “Use of different Zn precursors for the deposition of Zn(S,O) buffer layers by chemical bath for chalcopyrite based Cd-free thin-film solar cells,” Physica Status Solidi A, vol. 205, no. 10, pp. 2330–2334, 2008. View at Publisher · View at Google Scholar · View at Scopus
  279. A. Hultqvist, C. Platzer-Björkman, J. Pettersson, T. Törndahl, and M. Edoff, “CuGaSe2 solar cells using atomic layer deposited Zn(O,S) and (Zn,Mg)O buffer layers,” Thin Solid Films, vol. 517, no. 7, pp. 2305–2308, 2009. View at Publisher · View at Google Scholar · View at Scopus
  280. R. Sáez-Araoz, D. Abou-Ras, T. P. Niesen et al., “In situ monitoring the growth of thin-film ZnS/Zn(S,O) bilayer on Cu-chalcopyrite for high performance thin film solar cells,” Thin Solid Films, vol. 517, no. 7, pp. 2300–2304, 2009. View at Publisher · View at Google Scholar · View at Scopus
  281. T. Nakada, M. Hongo, and E. Hayashi, “Band offset of high efficiency CBD-ZnS/CIGS thin film solar cells,” Thin Solid Films, vol. 431-432, pp. 242–248, 2003. View at Publisher · View at Google Scholar · View at Scopus
  282. W. E. Devaney, W. S. Chen, J. M. Stewart, and R. A. Mickelsen, “Structure and properties of high efficiency ZnO/CdZnS/CuInGaSe2 solar cells,” IEEE Transactions on Electron Devices, vol. 37, no. 2, pp. 428–433, 1990. View at Publisher · View at Google Scholar · View at Scopus
  283. Rothwarf, “A p-i-n heterojunction model for the thin-film CuInSe2/CdS solar cell,” IEEE Transactions on Electron Devices, vol. 29, no. 10, pp. 1513–1515, 1982. View at Google Scholar · View at Scopus
  284. N. Romeo, A. Bosio, and V. Canevari, “R.F. sputtered CuInSe2 thin films for photovoltaic applications,” in Proceedings of the 8th European Photovoltaic Solar Energy Conference, vol. 2, pp. 1092–1096, Florence, Italy, 1988.
  285. S. H. Kwon, S. C. Park, B. T. Ahn, K. H. Yoon, and J. Song, “Effect of CuIn3Se5 layer thickness on CuInSe2 thin films and devices,” Solar Energy, vol. 64, no. 1–3, pp. 55–60, 1998. View at Publisher · View at Google Scholar · View at Scopus
  286. T. Negami, Y. Hashimoto, and S. Nishiwaki, “Cu(In,Ga)Se2 thin-film solar cells with an efficiency of 18%,” Solar Energy Materials and Solar Cells, vol. 67, no. 1–4, pp. 331–335, 2001. View at Publisher · View at Google Scholar · View at Scopus
  287. T. Negami, T. Satoh, Y. Hashimoto et al., “Production technology for CIGS thin film solar cells,” Thin Solid Films, vol. 403-404, pp. 197–203, 2002. View at Publisher · View at Google Scholar · View at Scopus
  288. T. Minami, H. Sato, H. Nanto, and S. Takata, “Group III impurity doped zinc oxide thin films prepared by RF magnetron sputtering,” Japanese Journal of Applied Physics. Part 2, vol. 24, no. 10, pp. L781–L784, 1985. View at Publisher · View at Google Scholar
  289. M. A. Contreras, B. Egaas, K. Ramanathan et al., “Progress toward 20% efficiency in Cu(In,Ga)Se2 polycrystalline thin-film solar cells,” Progress in Photovoltaics: Research and Applications, vol. 7, no. 4, pp. 311–316, 1999. View at Publisher · View at Google Scholar · View at Scopus
  290. Y. Hagiwara, T. Nakada, and A. Kunioka, “Improved Jsc in CIGS thin film solar cells using a transparent conducting ZnO:B window layer,” Solar Energy Materials and Solar Cells, vol. 67, no. 1–4, pp. 267–271, 2001. View at Publisher · View at Google Scholar · View at Scopus
  291. M. A. Contreras, J. Tuttle, A. Gabor et al., “High efficiency Cu(In,Ga)Se2 based solar cells: processing of novel absorber structures,” in Proceedings 1st World conference on Photovoltaic Solar Energy Convesion, pp. 68–75, IEEE, Hawaii, USA, 1994.
  292. K. Granath, M. Bodegård, and L. Stolt, “Effect of NaF on Cu(In, Ga)Se2 thin film solar cells,” Solar Energy Materials and Solar Cells, vol. 60, no. 3, pp. 279–293, 2000. View at Publisher · View at Google Scholar · View at Scopus
  293. M. Klenk, O. Schenker, V. Alberts, and E. Bucher, “Preparation of device quality chalcopyrite thin films by thermal evaporation of compound materials,” Semiconductor Science and Technology, vol. 17, no. 5, pp. 435–439, 2002. View at Publisher · View at Google Scholar · View at Scopus
  294. M. Powalla and B. Dimmler, “Scaling up issues of CIGS solar cells,” Thin Solid Films, vol. 361, pp. 540–546, 2000. View at Publisher · View at Google Scholar · View at Scopus
  295. F. H. Karg, “Development and manufacturing of CIS thin film solar modules,” Solar Energy Materials and Solar Cells, vol. 66, no. 1–4, pp. 645–653, 2001. View at Publisher · View at Google Scholar · View at Scopus
  296. C. Eberspacher, C. Fredric, K. Pauls, and J. Serra, “Thin-film CIS alloy PV materials fabricated using non-vacuum, particles-based techniques,” Thin Solid Films, vol. 387, no. 1-2, pp. 18–22, 2001. View at Publisher · View at Google Scholar · View at Scopus
  297. S. Siebentritt, T. Kampschulte, A. Bauknecht et al., “Cd-free buffer layers for CIGS solar cells prepared by a dry process,” Solar Energy Materials and Solar Cells, vol. 70, no. 4, pp. 447–457, 2002. View at Publisher · View at Google Scholar · View at Scopus
  298. Z. A. Wang, J. B. Chu, H. B. Zhu, Z. Sun, Y. W. Chen, and S. M. Huang, “Growth of ZnO:Al films by RF sputtering at room temperature for solar cell applications,” Solid-State Electronics, vol. 53, no. 11, pp. 1149–1153, 2009. View at Publisher · View at Google Scholar · View at Scopus
  299. T. Nakada, N. Murakami, and A. Kunioka, “Transparent conducting Al-, AIB12-, and B-doped ZnO films for solar cells by dc magnetron sputtering,” in Proceedings of the 12th European Photovoltaic Solar Energy Conference, pp. 1507–1610, Amsterdam, The Netherlands, 1994.
  300. S. Chaisitsak, A. Yamada, and M. Konagai, “Preferred orientation control of Cu(In1-xGax)Se2(x0.28) thin films and its influence on solar cell characteristics,” Japanese Journal of Applied Physics, vol. 41, pp. 507–513, 2002. View at Publisher · View at Google Scholar
  301. V. Probst, W. Stetter, W. Riedl et al., “Rapid CIS-process for high efficiency PV-modules: development towards large area processing,” Thin Solid Films, vol. 387, no. 1-2, pp. 262–267, 2001. View at Publisher · View at Google Scholar · View at Scopus
  302. B. M. Basol, V. K. Kapur, G. Norsworthy, A. Halani, C. R. Leidholm, and R. Roe, “Efficient CuInSe2 solar cells fabricated by a novel ink coating approach,” Electrochemical and Solid-State Letters, vol. 1, no. 6, pp. 252–254, 1998. View at Google Scholar · View at Scopus
  303. T. Sugiyama, S. Chaisitsak, A. Yamada et al., “Formation of pn homojunction in Cu(InGa)Se2 thin film solar cells by Zn doping,” Japanese Journal of Applied Physics. Part 1, vol. 39, no. 8, pp. 4816–4819, 2000. View at Google Scholar · View at Scopus
  304. N. F. Cooray, K. Kushiya, A. Fujimaki et al., “Large area ZnO films optimized for graded band-gap Cu(InGa)Se2-based thin-film mini-modules,” Solar Energy Materials and Solar Cells, vol. 49, no. 1–4, pp. 291–297, 1997. View at Google Scholar · View at Scopus
  305. Y. Hagiwara, T. Nakada, and A. Kunioka, “Improved Jsc in CIGS thin film solar cells using a transparent conducting ZnO:B window layer,” Solar Energy Materials and Solar Cells, vol. 67, no. 1–4, pp. 267–271, 2001. View at Publisher · View at Google Scholar · View at Scopus
  306. M. Yu, H. Qiu, X. Chen, H. Liu, and M. Wang, “Structural and physical properties of Ni and Al co-doped ZnO films grown on glass by direct current magnetron co-sputtering,” Physica B, vol. 404, no. 12-13, pp. 1829–1834, 2009. View at Publisher · View at Google Scholar · View at Scopus
  307. J. S. Wellings, A. P. Samantilleke, P. Warren, S. N. Heavens, and I. M. Dharmadasa, “Comparison of electrodeposited and sputtered intrinsic and aluminium-doped zinc oxide thin films,” Semiconductor Science and Technology, vol. 23, no. 12, Article ID 125003, 2008. View at Publisher · View at Google Scholar · View at Scopus
  308. B. N. Pawar, G. Cai, D. Ham et al., “Preparation of transparent and conducting boron-doped ZnO electrode for its application in dye-sensitized solar cells,” Solar Energy Materials and Solar Cells, vol. 93, no. 4, pp. 524–527, 2009. View at Publisher · View at Google Scholar · View at Scopus
  309. X.-T. Hao, J. Ma, D.-H. Zhang et al., “Comparison of the properties for ZnO:Al films deposited on polyimide and glass substrates,” Materials Science and Engineering B, vol. 90, no. 1-2, pp. 50–54, 2002. View at Publisher · View at Google Scholar · View at Scopus
  310. S. Fernández, A. Martínez-Steele, J. J. Gandía, and F. B. Naranjo, “Radio frequency sputter deposition of high-quality conductive and transparent ZnO:Al films on polymer substrates for thin film solar cells applications,” Thin Solid Films, vol. 517, no. 10, pp. 3152–3156, 2009. View at Publisher · View at Google Scholar · View at Scopus
  311. S. Calnan and A. N. Tiwari, “High mobility transparent conducting oxides for thin film solar cells,” Thin Solid Films, vol. 518, no. 7, pp. 1839–1849, 2010. View at Publisher · View at Google Scholar