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International Journal of Photoenergy
Volume 2011 (2011), Article ID 801292, 10 pages
Progress in Thin Film Solar Cells Based on
School of Engineering System, Queensland University of Technology, Brisbane, QLD 4001, Australia
Received 1 June 2011; Accepted 1 July 2011
Academic Editor: Leonardo Palmisano
Copyright © 2011 Hongxia Wang. 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.
The research in thin film solar cells has been dominated by light absorber materials based on CdTe and Cu(In,Ga)Se2 (CIGS) in the last several decades. The concerns of environment impact of cadmium and the limited availability of indium in those materials have driven the research towards developing new substitute light absorbers made from earth abundant, environment benign materials. Cu2ZnSnS4 (CZTS) semiconductor material has emerged as one of the most promising candidates for this aim and has attracted considerable interest recently. Significant progress in this relatively new research area has been achieved in the last three years. Over 130 papers on CZTS have been published since 2007, and the majority of them are on the preparation of CZTS thin films by different methods. This paper, will review the wide range of techniques that have been used to deposit CZTS semiconductor thin films. The performance of the thin film solar cells using the CZTS material will also be discussed.
Thin film solar cells based on polycrystalline cadmium telluride (CdTe), copper indium diselenide (CIS), and copper indium (gallium) diselenide (CIGS) have reached the commercialization stage; however, restrictions on heavy metal usage for Cd and limitations in supply for In and Te have raised concern about limitations on production capacity of the PV devices. To solve this issue, it is necessary to develop alternative light absorber materials that are both nontoxic and easily available. In this context, Cu2ZnSnS4 (CZTS) quaternary semiconductor compound has emerged as one of the promising candidates. All the constitute elements in CZTS are earth abundant and environment benign. Figure 1 illustrates the earth crust content and the current world trading price of the elements used in CZTS and CIS as well as CdTe light absorbers (logarithmic scale is used for clarity). It is found that the abundance of Zn and Sn in earth’s crust is 1500 times and 45 times greater than that of In, respectively, and the price of In is almost two orders of magnitude higher than that of Zn and Sn.
2. Basic Information on CZTS Compound and Solar Cells
CZTS is a mineral which has been found in nature . It shares similar structure with the chalcopyrite material CuInS2 except that half of the In is replaced with Zn and another half with Sn. Crystallographically speaking, CZTS has two principal structures known as stannite-type (space group I2 m) and kesterite-type (space type I). The two structures are similar except the different arrangement of Cu and Zn atoms . However, CZTS material usually appears in kesterite phase because it is more stable thermodynamically compared to the stannite-type . CZTS can be synthesized through solid state chemical reactions between ZnS, Cu2S, and SnS2. The investigation on the phase diagram of the system has shown that a single-phase CZTS material can be formed only in a very small region (Figure 2) . Impurities including ternary and quaternary compounds are easier to form than CZTS. Therefore, it is very challenging to make pure CZTS crystals.
Limited reports on the assessment of the photovoltaic performance of CZTS-based thin film solar cells normally adopt the device structure shown in Figure 3. It consists of a molybdenum- (Mo-) coated soda lime glass (SLG) as the electrical contact, a thin CZTS light absorber layer which is in contact with an n-type CdS layer to create a p-n junction, and a thin i-ZnO/Al : ZnO layer on top of the CdS layer playing the role of a window layer and electrical contact.
3. Evolution of the Conversion Efficiency of CZTS-Based Thin Film Solar Cells
The recognition of the photovoltaic effect of CZTS material was reported by Ito and Nakazawa in 1988 . They fabricated a heterodiode that consisted of a transparent cadmium-tin-oxide thin film and a CZTS thin film on a stainless steel substrate. An open-circuit voltage of 165 mV was obtained with the device. In 1997, Friedlmeier et al. fabricated thin film solar cells using a CZTS layer as the light absorber in contact with an n-CdS/ZnO window layer. The best energy conversion efficiency produced by the cells was 2.3% . This record was later broken by Katagiri’s group in 1999 who produced a CZTS solar cell with 2.63% power conversion efficiency. In this cell, the CZTS film was deposited on a Mo-coated soda lime glass (SLG) substrate . By optimization of the sulfurization process, the efficiency of the solar cells was increased to 5.45% in 2003 , and then to 6.7% in 2008 . Two review papers regarding the efficiency milestones achieved in CZTS-based thin film solar cells were reported by Katagiri in 2005 and in 2009, respectively [9, 10]. The current world record efficiency of CZTS-based thin film solar cells is 9.6%, which was reported by Todorov et al. in 2010. In this cell, the CZTS thin film was partly selenized to have a broader spectral photonresponse . The evolution of the power conversion efficiency of CZTS-based solar cells is summarized in Figure 4.
Since majority of the publications in this area have been devoted to the synthesis and characterizations of CZTS thin films, therefore this paper will review the most recent research activities of employing variable techniques for deposition of CZTS thin films. The main electrical and optical properties of the synthesized materials will be briefly discussed as well.
The techniques for synthesis of CZTS thin films can be classified into two categories: vacuum, and nonvacuum-based method. According to the nature of the approach used for the deposition of the thin films, each category has a few subclassifications, the details of which will be shown in the following.
4. Vacuum-Based Deposition Method
Vacuum-based fabrication techniques normally involve deposition of the constitute atoms of the CZTS compound on a substrate by sputtering or evaporation/coevaporation of the target sources under a certain pressure and temperature. These techniques have the advantage of easily controlling the chemical composition and phase profile in the thin films and normally have good reproducibility.
4.1. Vacuum-Based Sputtering Deposition Technique
An atomic beam sputtering technique was employed by Ito and Nakazawa to deposit CZTS thin films in 1988. The band gap energy of the films was determined to be 1.45 eV . In 2003, Seol et al. used Cu2S, ZnS, and SnS2 as target sources to deposit CZTS precursor thin films onto a glass substrate at room temperature by a RF magnetron sputtering technique. The as-deposited films were amorphous. After annealing in Ar + S2(g) atmosphere, a stoichiometric ratio of Cu/Zn/Sn was achieved with the films, but S was deficient. This problem was solved by further annealing the films in a sulphur vapour atmosphere at temperature above 200°C. It was found that the composition of the thin film materials was affected by the RF power in the sputtering .
An integrated vacuum apparatus which combined the RF sputtering technique with the sulfurization process was employed to reduce the effect of moisture from ambient air on the property of CZTS films. Target sources were based on ZnS, and SnS and Cu. Both the quality and reproducibility of the CZTS films were significantly improved by this method. A 5.7% conversion efficiency with the CZTS-based thin film solar cells was achieved . By eliminating the metal oxide impurity in the CZTS films using deionized water, the efficiency of the solar cells was further improved to 6.7% .
Tanaka et al. used a vacuum-based hybrid sputtering approach to deposit the constitute elements Cu, Zn, Sn on a quartz substrate. Specifically, Sn and Zn metal layers were deposited by DC-sputtering and Cu by RF-sputtering. Stoichiometry CZTS thin films were formed at the substrate temperature above 400°C. However, substantial Zn losses were observed when the temperature was above 450°C . Yoo and Kim studied the influence of Cu content on the structure and morphology of CZTS films. Their results indicated that Cu-rich or a stoichiometric Cu content would lead to the formation of in the films, whereas a Cu-poor composition favoured the formation of a smooth film surface . Liu et al. reported CZTS films with a high light absorption coefficient (>105 cm−1). The films were synthesized by a DC reactive magnetron sputtering method using a source target consisting of Cu/Zn/Sn metal mixture with stoichiometric composition (Cu : Zn : Sn = 2 : 1 : 1, molar ratio). Therefore, the precursor films could be made in one step. The CZTS compound was formed after sulfurization of the precursors. However, besides the desired CZTS materials, secondary phases and Cu3SnS4 were also observed in the materials. And further characterization showed that the carrier concentration of the films was in the order of 1018 cm−3 . Similar approach was also explored by Momose et al. for the fabrication of CZTS thin films. Cu, Zn, and Sn metals were simultaneously deposited on a SLG substrate by sputtering to form CZTS precursor films. After a rapid sulfurization (7 min) of the precursor at 590°C, the CZTS films were formed. An efficiency of 3.7% was obtained with the best thin film solar cell . Most synthesized CZTS material is kesterite-type due to a better stability thermodynamically compared to the stannite-type. Nevertheless, stannite phase CZTS films which showed a very low sheet resistivity (0.156 Ohm cm) were synthesized by an ion-beam sputtering technique. The band gap energy of the material was determined to be 1.51 eV .
4.2. Vacuum-Based Evaporation Deposition
The synthesis of CZTS thin films by a thermal evaporation approach was firstly reported by Friedlmeier et al. in 1997. The elements and binary chalcogenides compounds were deposited onto a substrate in high vacuum. They found that the film composition and grain size were largely dependent on the substrate temperature . Moreover, they observed that the electrical resistivity of the films could be varied between 1 and 100 ohm cm by treating the films with a KCN solution . Katagiri et al. used an electron beam evaporation method to deposit a Cu/Sn/Zn stack layer sequentially on a SLG substrate at 150°C in high vacuum. The precursor film was subsequently annealed in a sulphur-containing atmosphere (N2 + 5% H2S) at 500°C to form the CZTS compound. The characterization of the optical and electrical properties of the film showed that the optical band gap energy was 1.45 eV and the light absorption coefficient was in the order of 104 cm−1. The corresponding CZTS thin film solar cells with an architecture of Al/ZnO/CdS/CZTS/Mo/SLG provided an efficiency of 0.66% . The same group later reported that using ZnS to replace Zn as the target source could dramatically enhance the adhesion of CZTS film to the Mo/SLG substrate . An approach based on one-step synthesis of CZTS thin films by coevaporation of precursor sources Cu, Zn, Sn, S simultaneously was reported by Tanaka et al. The CZTS material showed a preferential orientation along plane. The grain size of the film depended on the substrate temperature. Larger grains were obtained at higher substrate temperature . In addition, growth of CZTS crystals on Si (100) wafers using a high vacuum multisource evaporation technique was reported by Oishi et al. A temperature-induced orientation growth was observed in the films .
From commercial application point of view, it is necessary to develop a process by which high-quality CZTS thin films can be produced rapidly and reproducibly. Such process has been explored recently. Wang et al. reported the deposition of Cu, Zn, Sn, and S elements on a SLG substrate simultaneously by a rapid thermal evaporation method. The thickness of the light absorber layer was less than 1.0 μm and the annealing time was only a few minutes. Thin film solar cells with 6.8% conversion efficiency were fabricated. The analysis of the electrical property of the PV device indicated that the performance of the cell was limited by the high series resistance and the high charge recombination . A similar method based on rapid deposition (16 min) of CZTS thin films on Mo/SLG substrate was also reported by Schubert et al. ZnS, Sn, Cu, and S target sources were coevaporated simultaneously onto the substrate at 550°C in vacuum. The composition of the precursor films was copperrich, which resulted in the formation of secondary phases . Nevertheless, stoichiometry CZTS films were obtained by posttreating the films with KCN aqueous solution. An energy conversion efficiency of 4.1% was achieved with the thin film solar cells .
Moriya et al. employed a vacuum-based pulsed laser deposition technique to fabricate CZTS light absorber layer for thin film solar cells and an efficiency of 1.74% was obtained with the devices [26, 27]. The effect of the laser incident energy on the structural, morphological, and optical properties of the CZTS thin films was investigated by Pawar et al. The study revealed that the crystallinity of the films increased with the increase of the laser energy until certain value (e.g., 2.5 J/cm2) . Moreover, the pulsed laser deposition technique can also be used to grow CZTS crystals on a GaP substrate. Well-crystallized CZTS crystals were obtained at substrate temperature of 400°C and the crystals showed a preferential orientation along , .
4.3. Influence of the Stacking Order of the Metallic Layer on the Performance CZTS Solar Cell
Araki et al. studied the effect of the stacking order of Cu, Zn, Sn metal layer on the properties of CZTS thin films and the corresponding solar cell performance. It was found that the films made from the precursor with a stacking order of Mo/Cu/Zn/Sn showed larger grain sizes than that with Mo/Cu/Sn/Zn. The direct contact of Cu and Zn layers should be responsible for the formation of large grains (Figure 5). However, the best cell efficiency (1.79%) was obtained with the precursor where Zn is the bottom layer and Sn is the top layer (stacking order: Mo/Zn/Cu/Sn). This phenomenon was explained by avoiding the formation of voids between Mo and CZTS if Cu layer were in direct contact with the Mo conductive layer .
The properties of CZTS films made from precursors with the stacking order of Mo/Zn/Cu/Sn and Mo/Zn/Sn/Cu deposited by a DC magnetron sputtering was also investigated by Fernandes et al. In contrary to Araki et al.’s conclusion, They concluded that the later stacking sequence was in favour of the CZTS crystal growth because Cu layer on top reduced the loss of Zn and Sn in the annealing process, thus leading to a better composition control and crystallinity. However, CuxS may be formed on the surface of the film when Cu is the top layer . Obviously, a deep understanding of the crystal growth mechanism of this quaternary compound is needed to explain these phenomena.
Weber et al. studied the loss of Sn in CZTS during the annealing process. It was found that desorption of SnS from the CZTS at temperature above 350°C led to the losses of Sn in vacuum. The decomposition process of CZTS could be reduced by addition of an inert gas in the chamber where the sintering was carried out . This issue has been addressed by Redinger et al. as well. They suggested to add extra SnS and S material in the precursor to prevent the decomposition of CZTS at high temperature .
5. Nonvacuum Deposition Method
The vacuum-based deposition techniques generally suffer from relatively slow throughput, low material utilization, and considerable energy consumption. Therefore, nonvacuum based deposition methods have been developed to reduce the production cost. These methods including spray pyrolysis, electrochemical deposition, and spin coating of precursor solutions, have been widely investigated for preparation of semiconductor thin films such as CIGS and CdTe . They have been attempted to synthesize CZTS thin films as well.
5.1. Spray Pyrolysis Deposition
The synthesis of CZTS thin films by deposition of precursor solutions using spray pyrolysis technique in ambient environment was reported by Nakayama and Ito in 1997. A precursor solution consisting of CuCl, ZnCl2, SnCl4, and thiourea at suitable concentrations in deionized water/ethanol solvent mixture was sprayed onto a heated SLG substrate. The as-deposited films were stannite-type . Since then, similar approach has been used by other researchers for the fabrication of CZTS thin films using different precursor solutions. Kamoun et al. studied the effect of the substrate temperature and the spray duration on the crystallinity of the CZTS films. The best crystallinity was obtained at the substrate temperature 613 K and a preferential orientation along  plane was observed with the films . The effect of the substrate temperature and the pH value as well as the compositions of the precursor solution on the crystallinity and the morphology of the CZTS material were also investigated by Kumar et al. Their results showed that the CZTS material with good crystallinity was obtained at the substrate temperature in the range of 643–683 K with the precursor solution . But impurities such as ZnS were observed besides the CZTS compound [37, 38]. Prabhakar and Nagaraju used ultrasonic spray pyrolysis technique to deposit CZTS thin films on a SLG substrate. The films with kesterite structure were obtained at 613 K, consistent with the results reported by others .
5.2. Spin-Coating of CZTS Precursor Solution
It normally involves three steps to prepare a CZTS thin film by spin-coating technique. (1) Preparation of a precursor solution which contains the ions of interest; (2) Spin-coating the precursor solution on a SLG substrate to make the thin film; (3) annealing the thin film at suitable atmosphere to form CZTS materials.
Tanaka et al. synthesized CZTS films by spin-coating a sol-gel precursor solution. The precursor solution contained acetate salts of copper (II) and zinc (II) as well as tin (II) chloride in 2-methoxyethanol solvent. Monoethanolamine was added to the solution to prevent the formation of precipitations. CZTS thin films were formed by annealing the precursor films in H2S containing atmosphere at 500°C . They later reported using non-vacuum methods to fabricate all the other components of thin film solar cells besides the CZTS layer. Specifically, the Zn/ZnO : Al window layer was made by a spin-coating method and the CdS buffer layer was made by a chemical bath deposition approach. The CZTS solar cells fabricated by the non-vacuum methods showed an efficiency of 1.01% () . The effect of the chemical composition of the precursor solution on the morphology and the optical properties of the CZTS thin films were investigated. Large grains were found in the films made from the precursor solution with copper poor (Cu/(Zn+Sn) < 0.8, molar ratio). The band gap energy of the copper-deficient films was larger compared to that of the copper-rich films. The best solar cell showed an efficiency of 2.03% .
Pawar et al. studied the effect of the complexing agent trisodium citrate in the precursor solution on the structure, morphology and composition of the CZTS thin films. The results showed that the crystallinity of the films was improved by using the complexing agent. However, the surface of the precursor films were uneven compared to the films deposited from the electrolyte without the complexing agent. Such effect was reduced by postsulfurization treatment of the films . Similarly, Liu et al. employed additive such as ethylxanthate molecule to facilitate the formation of a homogeneous CZTS precursor solution which was used to deposit CZTS films .
Fischereder et al. deposited CZTS films by spin coating a precursor solution consisting of metal salts of copper (I), tin (IV), and zinc (II) and thioacetamide in pyridine. They found that the formation of CZTS compound occurred at temperature at low as 105°C in vacuum. The band gap energy of the films varied between 1.41 and 1.81 eV by changing the annealing temperature point . This is probably due to the coexistence of secondary phases in the film under low annealing temperature.
Todorov et al. prepared a CZTS precursor which was based on a hydrazine-based particle-solution slurry consisting of Cu, Zn, Sn, and S elements. The CZTS films or partly selenized CZTS (CZTSSe) thin films were obtained by annealing the precursor films in sulphur or sulfoselenide containing atmosphere at 540°C. The films showed large grains and uniform composition (Figure 6(a)). The best cell efficiency was 9.66% which was obtained with the CZTSSe-based solar cells. The onset of the photon response in the IPCE spectrum of the thin film solar cells was close to 1200 nm (Figure 6(b)), indicating a lower band gap energy of the materials .
5.3. Electrochemical Deposition
Electrochemical deposition is considered to be a promising technique for the low-cost preparation of semiconductor thin films and has been employed by BP and CISEL for the commercial deposition of CdTe and CIGS thin film PV modules [46, 47]. The key for using this method is to find suitable electrochemical potential at which the metal cations can be reduced efficiently while unwanted reactions will not occur. CZTS films made by an electrodeposition method were firstly reported by Scragg et al. A stacking metal layer of Cu/Sn/Zn was deposited sequentially on a Mo/SLG substrate using a three-electrode configuration where Ag/AgCl was used as reference electrode. Cu and Sn was deposited at −1.14 V and −1.21 V, respectively using suitable alkaline solutions, and Zn was deposited at −1.20 V in an acidic environment (pH = 3). CZTS thin films were formed after annealing the precursor in a sulphur atmosphere at 500°C. The doping density of the films was in the order of 1016 cm−3 . Further investigation of the photovoltaic performance of the corresponding thin film solar cells indicated that the device performed better under low illumination intensity. At high illumination intensity (equivalent to 1 sun), the recombination in the space charge region was increased, leading to the decrease of the performance . An energy conversion efficiency of 3.2% was obtained with a full-structured thin film solar cell. In this cell, the morphology and uniformity of the CZTS thin films were optimized through modification of the stacking order of the electrodeposited metallic layers . Ennaoui et al. electrodeposited Cu-Zn-Sn precursor thin films on a Mo/SLG substrate using an electrolyte containing copper (II), zinc (II), and tin (IV) cations together with complexing agents and additives. The best thin film solar cell showed an efficiency of 3.4% () which was Cu-poor in the CZTS material. Their study also revealed that secondary phases such as ZnS for the Zn-rich material and Cu2SnS3 for the Zn-poor film existed at the interface of CZTS/Mo-SLG . Scragg et al. also compared the morphology of the electrodeposited CZTS films which were annealed in different gaseous atmosphere. They found that H2S containing atmosphere benefited the enhanced crystallinity of the films compared to the sulphur containing environment . Zhang et al. used electrochemical method to grow the atomic layer of Cu, Zn, Sn, and S layer by layer on a Ag substrate. A  preferential orientation was observed with the as-deposited CZTS thin film .
Besides the conventional electrodeposition approach, a method based on photoelectrochemical deposition was reported by Moriya et al. for the fabrication of CZTS films using an aqueous solution containing the sulphate of Cu2+, Zn2+, and Sn2+ as well as sodium thiosulfate. They reported that the pH value of the precursor solution had little influence on the composition of the film .
5.4. CZTS Nanocrystal by Hot-Injection Method
Hot injection solution chemical method is an established approach for the synthesis of materials with well-defined size and shape (nanocrystals). It has been widely used in preparation of semiconductor nanocrystals. In 2009, three papers on synthesis of CZTS nanocrystals by hot-injection method appeared in Journal of the American Chemical Society. The sizes of the synthesized nanocrystal were less 20 nm. The band gap energy of the nanocrystals was around 1.5 eV, which is comparable to that of the bulk material [55–57]. Kameyama et al. studied the effect of the reaction temperature in the hot-injection process on the formation of CZTS nanocrystals. They found that a single-phase CZTS were formed at temperature over 240°C whereas secondary phase such as CuS was formed at low reaction temperature. The valence band and the conduction band of the as-synthesized films was 0.3 eV and −1.2 eV versus Ag/AgCl, respectively . By tuning the composition of the hot chemical solution, the band gap energy of the nanocrystals was adjusted from 3.48 eV (ZnS) to 1.23 eV (CZTS, sphalerite type). The application of the CZTS nanocrystals in dye-sensitized solar cells was investigated, but the efficiency was very low (0.03%) . In contrast, CZTS nanocrystals which was annealed in Se vapour at 500°C for 15 min showed an enhanced grain growth. The thin film solar cells using the CZTSSe light absorber showed an energy conversion efficiency of 7.2%, and the cell did not show significant degradation in a one-month light soaking test .
Traditionally, CZTS appears in either kesterite or stannite phase with tetragonal structure. Recently, CZTS nanocrystals with wurtzite phase were synthesized by Lu et al. using hot-injection method. The new structure has a hexagonal structure and showed a band gap energy of 1.4 eV .
5.5. Other Nonvacuum Synthesis Methods
Wangperawong et al. reported a novel synthesis method for CZTS precursor films. Firstly, SnS and ZnS materials were coated on a Mo/SLG substrate by chemical bath deposition, and Cu ion was then incorporated into the precursor films via ion exchange mechanism. CZTS films with good crystallinity were formed by annealing the precursor films in a H2S containing atmosphere at 500°C . Another chemical solution-based non-vacuum method for the synthesis CZTS nanocrystals was based on a dissolution-reprecipitation process which involved the use of KSCN as molten salt to finely tune the supersaturation condition of the precursor solution. The successful preparation of the discrete nanocrystals relied on the precursor properties such as ultrafine size of the aggregates and a high short-range order in the nanocrystals according to the authors .
Hydrothermal method has been widely employed for synthesis of nanomaterials. It is thought very difficult to synthesize single-phase CZTS material, however, owing to the complexity of the compound and the narrow stable thermodynamic region where the material can exist. Cao and Shen reported the synthesis of CZTS materials at 150°C in an autoclave using a precursor solution containing CuCl2, (C2H3O2)2Zn, SnCl4, and S in ethylenediamine. Though CZTS target material was obtained, ZnS impurity was also formed .
Zhou et al. reported using a wet ball milling combined with screen-printing technique for the preparation of CZTS thin films. Primary elements copper, zinc, tin, and sulphur were homogeneously mixed under vigorous milling. CZTS powder was formed by sintering the precursor material in an inert gas at 500°C. A screen printable paste made from the CZTS powder was deposited on a Mo-coated polyimide substrate for fabrication of thin film solar cells. The sheet resistance of the thin film was ohm and the carrier density was cm−3. Hall mobility of the CZTS material was 12.61 cm2 V−1 . The best conversion efficiency of the thin film solar cells was 0.49% .
6. Defect Physics/Chemistry of CZTS
Defect plays an important role in determining the electrical property of semiconductor materials. Chen et al. have studied the defect mechanism of CZTS material. They employed DFT-based first principal calculation to study the thermodynamic stability of the quaternary compound. The results have revealed that the stable chemical potential region for the formation of a stoichiometry CZTS is very narrow, consistent with the previous experimental observations. Theoretical calculation has also predicted that Cu-rich/Zn-poor conditions are necessary for the growth of a single-phase CZTS crystal. The intrinsic defect in CZTS material is p-type antisite (Cu at Zn site) under such growth condition. Nevertheless, the authors have argued that defect is not optimal for solar cell applications due to the relatively deep acceptor levels. Instead, copper vacancy is preferred for high efficiency solar cells. Moreover, there are self-compensated defect pair complexes such as  for Cu-rich/Zn-poor growth condition and  for Cu-poor/Zn-rich condition in CZTS . These electrically neutral defect complexes can remarkably passivate the deep levels in the band gap, reducing the recombination in the PV devices . Similar results have also been reported by Nagoya et al. and Maeda et al. [68, 69]. These authors have claimed that ZnS should be the main competing phase under the prevalent Cu-poor and Zn-rich growth condition, and Cu at Zn sites is the most stable defect in the entire stability range of CZTS . Chory et al. have used electron spin resonance (ESR) measurement to investigate the defects of CZTS materials made by a sol-gel method. The study has indicated the existence of Cu(II) in the compound. This phenomenon was explained due to the charge transfer between Sn(IV) and Cu(I), leading to the formation of Sn(II) and Cu(II) .
The recent significant progress in Cu2ZnSnS4 has demonstrated the feasibility of developing high efficiency PV devices with low cost and low environment impact materials. A broad range of techniques including vacuum or non-vacuum approaches have been explored to deposit CZTS films. The vacuum-based sputtering method has produced CZTS films for thin film solar cells with an efficiency of 6.77%, which is comparable to the performance of the films synthesized by a rapid thermal evaporation. From application point of view, the rapid vacuum thermal evaporation approach is more promising for large scale production of the thin film materials.
Among the non-vacuum preparation methods, hydrazine-based particle-solution method has demonstrated its capability in fabrication of high quality CZTS materials. This claim is justified by the thin film solar cells with 9.66% energy conversion efficiency using hydrazine method. It seems that hot-injection method is a promising synthesis technique for CZTS material as well. The thin film solar cells with the selenized CZTS nanocrystals have showed 7.2% conversion efficiency. Though electrodeposition has been successfully applied for the deposition of CdTe and CIGS thin films commercially, this method is yet able to compete with other techniques for deposition of high quality CZTS films currently. The best solar cell with the electrodeposited CZTS film has shown 3.4% conversion efficiency. Table 1 summarizes the characteristic parameters of the state-of-the-art thin film solar cells made by different methods. Clearly, the two most high efficiency cells are based on the selenized CZTS light absorber materials (CZTSSe). The short circuit current density of the cells is nearly twofold that of the rest of the cells which are based on CZTS materials. This is attributed to the broader spectral response of the CZTSSe light absorber owing to the lower band gap energy. The open circuit voltage of all the CZTS-based solar cells is around 0.6 eV, which is much lower than the theoretical value of 1.5 eV. A high charge recombination in those devices is probably responsible for the low . A detailed investigation of the electrical properties and kinetics of charge transport in the device is necessary in order to disclose the exact physical reasons for the low .
In order to improve the efficiency of CZTS-based thin film solar cells, a deeper understanding of the fundamental properties of CZTS, particularly the nature of the defects as well as their impact on the properties of CZTS material is important. CZTS material for thin film solar cells which have produced good efficiency normally always shows Cu-poor/Zn-rich in the composition. Therefore, secondary phase(s) should exist in the light absorbers. It is necessary to identify those secondary phases and their effects in order to optimize the fabrication process to make CZTS thin films with desired properties.
H. Wang thanks Queensland University of Technology for funding this project via Vice-Chancellor Research Fellowship Scheme.
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