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International Journal of Photoenergy
Volume 2012 (2012), Article ID 879261, 6 pages
Effect of Sheet Resistance and Morphology of ITO Thin Films on Polymer Solar Cell Characteristics
1Materials Science Programme, Indian Institute of Technology Kanpur, Kanpur 208016, India
2Department of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
Received 8 October 2011; Accepted 21 December 2011
Academic Editor: Bhushan Sopori
Copyright © 2012 Ram Narayan Chauhan et al. 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.
Solar cell fabrication on flexible thin plastic sheets needs deposition of transparent conducting anode layers at low temperatures. ITO thin films are deposited on glass by RF sputtering at substrate temperature of and compare their phase, morphology, optical, and electrical properties with commercial ITO. The films contain smaller nanocrystallites in (222) preferred orientation and exhibit comparable optical transmittance (~95%) in the wavelength range of 550–650 nm, but high sheet resistance of ~103 Ω/□ (the value being ~36 Ω/□ for commercial ITO).The polymer solar cells with PEDOT: PSS and P3HT: PCBM layers realized on RF sputtered vis-a-vis commercial ITO thin films are shown to display a marginal difference in power conversion efficiency, low fill factor, and low open-circuit voltage but increased short-circuit current density. The decrease in fill factor, open-circuit voltage is compensated by increased short-circuit current. Detailed study is made of increased short-circuit current density.
Indium tin oxide (ITO) is -type degenerate semiconductor with a wide band gap ( eV). It has excellent photoelectrolytic properties, namely, electrical conductivity (103–104 ohm−1 cm−1), transparency (80–95%) in the visible range and reflectance better than 80% in the infrared region [1–5]. It has found numerous applications in transparent electrode, antireflection coating, heat reflecting mirror, transparent electromagnetic shield, display, optoelectronic and photovoltaic devices, and so forth . ITO is of particular interest to high efficiency heterojunction solar cells  and organic light-emitting diode (OLED) [8, 9]. Its thin films can be prepared by a variety of techniques like plasma-enhanced metal organic chemical vapour deposition (PEMOCVD) , ion-assisted deposition (ISD) , pulsed laser deposition (PLD) [12–14], dip coating , ion beam sputtering , RF magnetron sputtering [17, 18], and reactive thermal evaporation . However, majority of the methods require elevated substrate temperatures to obtain thin films of reasonably high conductivity and good optical transmittance. But the amorphous silicon photovoltaic and flexible electrooptical devices demand the deposition of ITO at low temperatures (<200°C). RF sputtering allows preparation at low temperatures and on large areas . Therefore, an attempt has been made here to fabricate solar cells using (i) ITO thin films deposited by RF sputtering and (ii) commercial low sheet resistance ITO and undertake comparative study of their characteristics.
2. Experimental Details
ITO thin films of thickness ~120 nm were deposited on a glass plate (size 30 mm × 30 mm) through shadow mask with 8 mm wide × 30 mm long slits at substrate temperature of 70°C in pure argon by radiofrequency (RF) sputtering using In2O3 : SnO2 (90 : 10 wt%) target of 99.99% purity. The system was evacuated upto ~10−6 mbar prior to deposition. The film thickness, sheet resistance, and optical transmittance were measured by surface Profilometer (Tencoralpha-step X-100), four probe sheet resistance measurement system, and a fiber optic spectrometer (Ocean optic model USB 2000), respectively. The crystalline phase was determined by X-ray diffraction (Thermo Electron ARL X’TRA X-ray diffractometer), whereas the morphology was observed in scanning electron microscopes FEI, (Quanta 200, and carl zeiss SUPRA 40 VP). Further, the work function of RF sputtered and commercial ITO films was measured by Kelvin probe model 7.
For solar cell fabrication, polyethylenedioxythiophene : polystyrenesulfonic acid (PEDOT : PSS) was spin-coated at 2500 rpm on the patterned ITO-glass substrate and annealed at 120°C for 1 h in vacuum ~ mbar. A polymer active layer was then spin-coated at 1100 rpm over the dried PEDOT : PSS layers using the solutions of P3HT (poly 3-hexylthiophene) and PCBM ([6,6]-phenyl-C61-butyric acid methyl ester) 20 mg/cc each in an organic solvent mixed in volume ration 1 : 1 and annealed at 110°C for 20 min in vacuum. Finally, bilayer metal cathode of LiF and Al (thickness 0.8 nm and 200 nm, resp.) was realized on the active layer by thermal evaporation method. The device structure glass/ITO/PEDOT : PSS/P3HT : PCBM/LiF/Al thus fabricated is shown schematically in Figure 1. Similar devices were also made in the same batch on the commercial (36 ohm/sq, 150 nm thickness) ITO coated glass substrates. The characteristics of solar cells were obtained by Keithley source measure unit (SMU) model 236 and a data acquisition system.
3. Results and Discussion
The XRD patterns of the commercial and RF-sputtered ITO thin films are shown in Figure 2. The commercial ITO sample exhibits five peaks of indices 211, 222, 400, 440, and 622, which match well with the bixbyite tin substituted In2O3 structure having lattice parameter Å . The thermalized sputtered species are known to orient as (222) plane while the species with higher energies prefer growth planes as (400) and (440) . However, RF-sputtered ITO thin films exhibit (222) preferred orientation (Figure 2(b)). Obviously, the glass substrate is either placed in thermalized region or the above observation does not conform to the cited findings . The grain size () has been deduced with the 222 diffraction peak data and using the Scherrer relation where is the corrected full width at half maxima (FWHM) at Bragg angle and is the X-ray wavelength [23–25]. The values of () for the RF-sputtered and commercial ITO samples are ~7 and ~12 nm, respectively.
The morphologies of the thin films are shown in Figure 3. These reveal granular structure with smaller average grain size (~7 nm) for RF-sputtered ITO thin films vis-a-vis commercial sample. This is observation consistent with the XRD data and results due to low deposition rate (0.7 Ås−1) used [26, 27]. The optical transmittance spectra of ITO thin films in the wavelength range of 300–800 nm are shown in Figure 4. Accordingly, the RF sputtered thin films display somewhat poor transmittance than the commercial ITO below 550 nm and above 650 nm, possibly because of substantial scattering from the grain boundary regions. Nevertheless, transmittance level of ITO thin films prepared by RF sputtering is as high as 95% in the wavelength range of 550–650 nm.
The sheet resistance () values of RF sputtered and commercial ITO thin films are 103 and 36 Ω/□, respectively. The values of electrical conductivity () of RF and com ITO thin films, obtained from their sheet resistance () are 8.1 × 102 and 1.9 × 103 ohm−1-cm−1, respectively. Slater theory tells that smaller grain size will have a higher energy barrier. The carriers have to cross the grain boundary, when they travel from one grain to the other. They need additional energy to surmount the grain boundary barrier . The lower electrical conductivity in RF-sputtered ITO thin film is associated with larger barrier height , formed by the smaller grain size (). Here, is the barrier width related to the number of disorder atom layers, and is a fraction of the order of 1/15 to 1/50.
The solar cells fabricated on the commercial and RF-sputtered ITO thin films had areas of 0.05 and 0.08 cm2, respectively. Their characteristics are presented in Figure 5. The fill factor (FF) and conversion efficiency () are deduced from [29–31] where represents the maximum power that can be extracted from the cell and , , and stand for the short-circuit current density (A cm−2), open-circuit voltage (V) and incident photon flux (W cm−2), respectively. Clearly, solar cell based on RF-sputtered ITO generates more current density ( ~ 7.6 mA cm−2) due to more light absorption (Figure 6). More light is scattered from the grain boundaries of RF-sputtered ITO, leading to better absorption. The scattering light intensity () from the grain boundaries is expressed as  where is the intensity of incident light, RL is reflection loss, is the thickness of film, and is the grain boundary scattering coefficient. Here, the light scattering from the pore has been neglected because of film being compact. The scattering coefficient () depends on the wavelength () of incident light, grain density (), grain size (), absolute refractive index difference ( ~ refractive index difference of ITO and glass) and is given by The linear refractive index () of thin films can be deduced from  where is the energy band gap of the film. The of a direct band gap semiconductor can be estimated from the relation  where is a constant, represents the absorption coefficient, and stand for photon energy. The values of the energy band gap of the RF-sputtered and commercial ITO thin films have been determined from the plot versus photon energy () by extrapolating linear portion of the spectrum to the abscissa (-axis) as shown in Figure 7; the values obtained are 3.56 and 3.91 eV for RF sputtered and commercial ITO thin films, respectively. Their refractive index values obtained from (6) are 2.26 and 2.19, respectively. The corresponding absolute refractive index difference () values turn out to 0.76 and 0.69, respectively, by taking the “” value of glass substrate as 1.5. Since the RF-sputtered films have small grain size, their number density () will be larger than the commercial ITO samples. So, RF-sputtered ITO films should exhibit larger grain boundary regions hence the more scattered light towards the active layer for enhancing the short-circuit current density.
The values of the open-circuit voltage achieved in devices on commercial and RF-sputtered ITO glass substrate are 0.63 and 0.55 V, respectively. The open-circuit voltage is mainly governed by the junction between two organic materials . The open-circuit voltage () is given by energy difference between the LUMO of acceptor and HOMO of the donor with a reduction of 0.2 V arising due to band bending at contact following accumulation of charges when the contact is ohmic in nature. However, in case of a nonohmic contact, is given by the difference of the work functions [35, 36]. The values of work function for the RF-sputtered and commercial ITO measured with Kelvin Probe are eV and eV, respectively. The difference in work function may arise due to size effects and additional defects present in RF-sputtered films vis-a-vis commercial ITO. These features may trap/accumulate charges at the anode interface and cause reduction in the work function. Incidentally, the of RF-sputtered ITO device is lower than that of commercial ITO-based device by about 0.08 V, which is exactly the difference in work functions. This corresponds to situation mentioned above.
The fill factor (FF) of the solar cells fabricated on commercial and RF-sputtered ITO thin films are 0.44 and 0.28, respectively at a light intensity of 95 mW/cm2 under AM 1.5 illumination at room temperature. Since FF is totally dependent on the quality of characteristics. For this, it is crucial to have high carrier mobility, low trap density, and negligible space charge effect. In other ways, effective collection of free charge carriers results better output power . The conversion efficiencies of the above solar cell devices are 1.5 and 1.2%, respectively, that is, lower for the RF-sputtered ITO case. The inferior fill factor and lower conversion efficiency of the solar cell fabricated on the RF-sputtered ITO films is due to higher sheet resistance, that is, poor conductivity ( Ω−1 cm−1). The parasitic resistive power loss also causes reduction in FF and depends mainly on series and shunt resistances. For an efficient solar cell, series resistance should be low whereas shunt resistance should be high. The series resistance () originates from the bulk of organic layer (P3HT : PCBM blends and PEDOT : PSS), electrodes (anode ITO and cathode metals), and the contact resistance between the active layer and the electrode. On the other hand, the shunt or parallel resistance () arises from the charge recombination at the donor/acceptor interface, through the cell and around the edges of the device [38, 39]. The and have been determined from the inverse slopes of curve at bias voltage of over (i.e., ), and zero, respectively . The series resistance of identical device on commercial and RF-sputtered ITO are 55.7 and 64.3 Ω-cm2, respectively. The corresponding shunt resistance values are 325.4 and 110 Ω-cm2, respectively. The higher value of shunt resistance in commercial ITO results in negligible leakage current and so leads to higher conductivity. It may be pointed out that the commercial ITO is normally annealed at a high temperature for attaining low sheet resistance value, that is, high conductivity. The present experiment is directed towards RF sputtered ITO films for solar cells and OLED on polythene sheets, high temperature annealing is intentionally avoided.
The ITO thin films can be prepared by RF sputtering at a substrate temperature of 70°C on glass with (222) preferred orientation, grain size of 7 nm, high optical transparency of 95% in the wavelength range of 550–650 nm, and sheet resistance of 103 Ω/□ (higher than 36 Ω/□ of commercial ITO). The short-circuit current density () and open-circuit voltage () of the device fabricated on the RF sputtered are 7.6 mA-cm−2 and 0.55 V, respectively. The corresponding values of and are 5.2 mA-cm−2 and 0.63 V for solar cells based on commercial ITO. The fill factor (0.28) and conversion efficiency (1.2%) of the solar cell with RF-sputtered ITO layer are smaller than those fabricated with commercial ITO (values being 0.44 and 1.5%, resp.). The substantial increase in short-circuit current density of the solar cell can be attributed to smaller nanocrystallites which cause dominant light scattering at the grain boundaries towards the active layer for better light absorption. The open-circuit voltage is limited by the size effects as well as defects, responsible for trapping/accumulation of charge carriers at the anode interface, of ITO. This emphasizes the importance of the anode (ITO) surface morphology in polymer solar cells.
The financial support from DRDO and DST is duly acknowledged.
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