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Combinatorial Reactive Sputtering of InS as an Alternative Contact Layer for Thin Film Solar Cells Sebastian Siol, Tara P. Dhakal, Ganesh S. Gudavalli, Pravakar P. Rajbhandari, Clay DeHart, Lauryn L. Baranowski, and Andriy Zakutayev ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 13 May 2016 Downloaded from http://pubs.acs.org on May 13, 2016
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Combinatorial Reactive Sputtering of In2S3 as an Alternative Contact Layer for Thin Film Solar Cells
Sebastian Siol*,1, Tara P. Dhakal2, Ganesh S. Gudavalli2, Pravakar P. Rajbhandari2, Clay DeHart1, Lauryn Baranowski1 and Andriy Zakutayev1
1
2
National Renewable Energy Laboratory, Golden, CO 80401
Department of Electrical and Computer Engineering, Binghamton University, Binghamton, NY 13902
Abstract
High-throughput computational and experimental techniques have been used in the past to accelerate the discovery of new promising solar cell materials. An important part of the development of novel thin film solar cell technologies, that is still considered a bottleneck for both theory and experiment, is the search for alternative interfacial contact (buffer) layers. The research and development of contact materials is difficult due to the inherent complexity that arises from its interactions at the interface with the absorber. A promising alternative to the commonly used CdS buffer layer in thin film solar cells that contain absorbers with lower
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electron affinity can be found in β-In2S3. However, the synthesis conditions for the sputter deposition of this material are not well established. Here, In2S3 is investigated as a solar cell contact material utilizing a high throughput combinatorial screening of the temperature-flux parameter space, followed by a number of spatially-resolved characterization techniques. It is demonstrated that by tuning the sulfur partial pressure, phase pure β-In2S3 could be deposited using a broad range of substrate temperatures between 500°C and ambient temperature. Combinatorial photovoltaic device libraries with Al/ZnO/In2S3/Cu2ZnSnS4/Mo/SiO2 structure were built at optimal processing conditions to investigate the feasibility of the sputtered In2S3 buffer layers and for an accelerated optimization of the device structure. The performance of the resulting In2S3/Cu2ZnSnS4 photovoltaic devices is on par with CdS/Cu2ZnSnS4 reference solar cells with similar values for short circuit currents and open circuit voltages, despite the overall quite low efficiency of the devices (~2%). Overall, these results demonstrate how a highthroughput experimental approach can be used to accelerate the development of contact materials and facilitate the optimization of thin film solar cell devices.
Keywords: In2S3 thin films, reactive sputtering, combinatorial device studies, buffer layer, CZTS, thin film solar cells, high throughput
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1. Introduction
Facing the rising global energy demand, the development of novel, non-toxic and earthabundant solar cell technologies is more important than ever. In recent years high throughput methods have been successfully employed for an accelerated screening of promising new candidates for absorber and electrode materials in photovoltaic devices. Using high-throughput computational screening1 as well as combinatorial synthesis combined with spatially resolved characterization2 the discovery of new functional materials and the investigation of their feasibility for photovoltaic applications has been accelerated substantially.
A key challenge in the rapid development of emerging solar cell technologies is the search for alternative contact layers (also called buffer layers) and the development of suitable device structures. For several novel absorber materials, a limiting factor is a deficit of open circuit voltage that can be tracked back to an inferior band alignment at the interface with the contacts. To address this challenge, the same high throughput experiments can be used to quickly screen the deposition parameter space of new, promising contact materials, and to investigate their performance in a given solar cell technology.
As of now, most emerging chalcogenide solar cell technologies are being developed using well-established device structures containing CdS buffer layers. A promising alternative to CdS is In2S3, a III-VI, n-type semiconductor. Besides eliminating the toxicity concerns associated with Cd, In2S3 features a slightly lower electron affinity. This could lead to an optimized conduction band offset for a variety of emerging absorber materials such as SnS3, CuSbSe24, Cu2S5, Sb2Se36 and Cu2ZnSnS4 (CZTS)7–9. It is known that In2S3 can crystallize in three different 3 ACS Paragon Plus Environment
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forms, i.e. α-In2S3, β-In2S3 and γ-In2S3. Among these structures β-In2S3, a defective spinel structure, is the most stable at room temperature with the transition temperatures to α-In2S3 and γ-In2S3 being at 420°C and 750°C, respectively10. In2S3 is used for many technological applications, mainly due to its stability and optoelectronic properties11.
The feasibility of In2S3 as an alternative contact layer for thin film solar cells has already been demonstrated for Cu(In,Ga)Se2 (CIGSe), where efficiencies in the range of 16 % have been reported12. While the best results for In2S3/CIGSe cells could be achieved with ALD deposited In2S3 buffer layers13, a variety of other deposition methods have been used to prepare thin films of In2S3 including chemical bath deposition (CBD)14, hydrothermal synthesis15, thermal evaporation16, closed space sublimation17, sputtering18,19 and spray pyrolysis20. For the application as an alternative contact layer for thin film solar cells, the main desired criteria are low deposition temperature, suitable optoelectronic properties, and scalability of the process. Magnetron sputtering as a state of the art industrial process would meet all these criteria.
A time-efficient way to investigate the multi-dimensional synthesis parameter space of the sputtering process is a high throughput approach, which includes combinatorial synthesis and spatially-resolved characterization21,22. By applying intentional and well-controlled gradients on the substrate during combinatorial thin film synthesis, several values of deposition parameters can be covered in a single synthesis experiment. The resulting combinatorial sample libraries are then characterized as a function of position with regard to their structural and optoelectronic properties. Usually combinatorial synthesis is utilized for chemical composition screening of multinary materials with a tendency to off-stoichiometry2. In the cases of the line compounds
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(like binary In2S3 or ternary CuSbS2) where no larger composition variations are possible, substrate temperature gradients can be applied for investigation of materials properties23, and thickness gradients can be used for an accelerated device optimization24.
In this work we report on the high throughput study of alternative contact layer materials for solar cells, using the reactive sputter deposition of In2S3 as a prototype model system. A combinatorial approach is used for an in-depth screening of the temperature-flux growth parameter space. The effects of doping with Zr4+ and Sn4+ are investigated, with the goal to improve the conductivity of the films. To demonstrate the feasibility of the sputtered In2S3 buffer layers, thin film photovoltaic devices with CZTS absorbers were fabricated and compared to a baseline device using conventional CdS buffer layers. By applying thickness gradients of the In2S3 contact layers in combinatorial device libraries, an accelerated optimization of the contact layer is demonstrated. This approach can be extended to a rapid development of other alternative contact layers for a wide range of emerging photovoltaic absorber materials and for other deposition techniques, that allow for combinatorial deposition like pulsed laser deposition25 or microreactor chemical bath deposition26.
2. Experiment
For an accelerated study of the In2S3 temperature-flux parameter space during the sputtering, temperature gradients were applied on the substrates to cover a variety of substrate temperatures in a single deposition step (see Figure 1a). A flux gradient is achieved by sputtering at an incident angle of roughly 25°. The temperature gradient is realized by only creating a partial thermal contact of the substrate to the heated sample holder. Several libraries were deposited to 5 ACS Paragon Plus Environment
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cover a broad range of deposition temperatures (see below for deposition conditions). A total of 44 samples were defined on a single library. Those were characterized using automated, spatially resolved measurements, as described in more details below.
Figure 1: a) Schematic illustration of the combinatorial deposition technique: Orthogonal gradients of temperature T and film thickness d were applied on the In2S3 material library. b) Device layout used in this study: The In2S3 film thickness was varied across the device library. The front contact was realized through RF sputtering of an intrinsic/conductive i-ZnO/ZnO:Al (AZO) stack and an Al grid.
2.1 Combinatorial thin film deposition methods The deposition of the In2S3 films was carried out through magnetron sputtering from an In2S3 target (99.9 % purity) in a UHV system with a base pressure of 2 × 10-7 mbar. Either pure Argon (99.99 % purity) or a mix of 98% Argon and 2% H2S (± 2% compositional accuracy) were used for the deposition. The changes in sulfur partial pressure were achieved by adjusting the total process pressure during the deposition. The process pressure was kept at 2.0 × 10-3 mbar for depositions in pure Ar and varied in a range of 1.5 × 10-3 mbar to 2.5 × 10-3 mbar for the Ar/H2S mix. The depositions were carried out using a sputter power of P = 40 W on Corning Eagle - XG 6 ACS Paragon Plus Environment
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glass (EXG) substrates. Due to the incident angle of the sputter deposition, the deposition rates vary over the coarse of one library. The highest deposition rates occur for minimal target substrate distance, and they also vary somewhat depending of the substrate temperatures (i.e. TSub=290°C: d = 7.5 nm/min – 3.3 nm/min; TSub=370°C: d = 5 nm/min – 2.5 nm/min). A more detailed description of the combinatorial deposition methods can be found in the supporting information.
2.2 Device fabrication To investigate the feasibility of the sputtered In2S3 layers for contact applications, they were deposited on CZTS absorber layers to create combinatorial device libraries. The CZTS absorber layers were provided by Binghamton University; more details regarding the depositions parameters and absorber layer properties can be found in27. The devices were built in a substrate configuration on Molybdenum coated soda lime glass (SiO2) substrates. The In2S3 was deposited using optimized synthesis conditions at an incident angle of approximately 25° with respect to the normal, to achieve a thickness gradient across the library. In addition different CZTS/Mo/SiO2 substrate temperatures were used for the In2S3 deposition. As a reference sample a CdS layer of 50nm thickness was fabricated using CBD instead of sputtered In2S3, using the procedure described in [28]. The front contacts were realized through RF sputtering of an intrinsic/conductive i-ZnO/ZnO:Al (AZO) stack, and e-beam evaporation of Al metal through a shadow mask. Individual device isolation was done by gentle razor blade scribing all the way down to the Mo back contact, resulting in 22 PV devices, each with an area of A = 0.24 cm2. A schematic illustration of the combinatorial device library structure is given in Figure 1b.
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Additional information regarding the device preparation and characterization can be found in the supporting information.
2.3 Spatially-resolved characterization techniques The In2S3 combinatorial thin film sample libraries were investigated with regard to their structural and optoelectronic properties using spatially-resolved x-ray diffraction (XRD), x-ray fluorescence (XRF), UV-Vis optical transmission/reflection spectroscopy, and 4-point probe measurements. The XRD measurements were carried out in a Bruker D8 Discover x-ray diffractometer in 2Θ geometry using Cu Kα radiation. The composition and thickness were mapped using x-ray fluorescence (XRF; Fischerscope XDV-SDD). Optical UV-Vis transmission and reflection measurements were taken in a custom setup covering a wavelength range from 300nm to 1100nm. The resistivity measurements were performed using spatially resolved 4 point probe mapping in dark at room temperature. The JV-characterization of the photovoltaic combinatorial device libraries was performed on a water-cooled stage (20°C) using a solar simulator with AM1.5 G compliant radiation. The external quantum efficiency (EQE) was measured using an ORIEL IQE 200 at the specific points of interest on the combinatorial PV device library. All combinatorial data sets were evaluated with Igor Pro software using customized routines.
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3. Results and Discussion
3.1 Sputtering in pure argon atmosphere To screen the parameter space for the sputter deposition, In2S3 layers were deposited from the compound target in pure Ar atmosphere. The films were deposited using temperature gradients to cover a wide variety of substrate temperatures between room temperature and 550 °C. Figure 2a shows θ-2θ XRD measurements of thin In2S3 films for a variety of substrate temperatures. Comparison of the XRD patterns with references from literature29 shows a strong dependence of the structural properties based on the substrate temperature used. Above 450 °C the films consist of phase pure β-In2S3. Between 450 °C and 300 °C the formation of the sulfur deficient phase In6S7 can be observed. Below 300 °C the films appear to be amorphous. This is in good agreement with XRF measurements of the composition, where for lower substrate temperatures a decrease in sulfur concentration can be observed (see Figure S5 in supporting information).
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Figure 2: θ-2θ XRD measurements of thin In2S3 films as a function of substrate temperature displayed as false color plots. (a) For sputtering in pure Ar atmosphere, substrate temperatures of 450°C are required to deposit phase pure In2S3. (b) Reactive sputtering facilitates In2S3 deposition over a broad temperature range. The increase in sulfur deficiency for lower deposition temperatures appears to be counter intuitive due to the difference in vapor pressures of In and S, as well as the increased chemical potential of sulfur for lower substrate temperatures. An explanation for the formation of sulfur deficient phases for low temperatures may be related to the surface kinetics during the nucleation process on the substrate. The physical vapor deposition of In2S3 is known to be a dissociative process17, with In and S independently arriving at the surface of the growing film. The reduced surface mobility of adsorbing In and S atoms during the nucleation process can favor the formation of In clusters which will result in sulfur deficient phases and incorporation of In precipitates in the films which is in good agreement with UV-Vis transmission measurements. For layers deposited in pure Ar atmosphere a significant amount of sub-band gap absorption was 10 ACS Paragon Plus Environment
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observed (see Figure S3 in supporting information). This increase in sub-gap absorption is often associated with scattering at impurities or metal precipitates in the bulk of thin-films30,31.
3.2 Reactive sputtering with H2S A feasible way to avoid deposition of sulfur deficient In2S3 phases is the reactive sputtering with H2S. While the use of process pressures as low as 1.5 × 10-3 mbar led to amorphous films for lower substrate temperatures similar to the films deposited in pure Argon atmosphere, increasing the process pressure to 2.0 × 10-3 mbar was sufficient to deposit crystalline, phase pure In2S3 films at temperatures as low as room temperature. Attempts to increase the sulfur partial pressure further by increasing the process pressure led to a change in morphology and increased surface roughness. These films were porous with poor adhesion on the substrates making them unsuitable for solar cell applications (see Figure 3).
The XRD patterns for films deposited using 2.0 × 10-3 mbar process pressure are shown in Figure 2b. While for all deposition temperatures from room temperature up to 500°C phase pure β-In2S3 could be deposited, a decrease in crystallinity for lower substrate temperatures could be observed, especially for the room temperature depositions. The film composition is found to be consistent with the In2S3 formula for a broad range of temperatures. This improvement can be explained with the additional supply of reactive S2- in the process gas, which makes the formation of In clusters less likely. This hypothesis is supported by a significant decrease in sub band gap absorption (see Figure S3 in supporting information). In contrast to the composition vs. temperature trend for the sputtering in 100% Ar, a slight decrease in sulfur concentration in the 0.1% range can be observed for elevated temperatures (Figure S5). This trend has been reported
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for the thermal evaporation of In2S3 and can be attributed to the difference in vapor pressures of In and S and the resulting discrepancy in sticking coefficients32.
Figure 3: SEM images of In2S3 layers sputtered reactively using different process pressures. Higher process pressures (b) lead to a substantial increase in roughness of the films compared to the low process pressures (a).
3.3 Optical properties To investigate the optical properties of the In2S3 layers, UV-Vis transmission and reflection spectroscopy measurements were carried out. Due to the narrow temperature range to produce phase-pure In2S3 for the sputtering in pure Ar atmosphere, the following discussion of the optical properties is focused on the properties of the reactively sputtered In2S3 films.
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Figure 4: Tauc plots for determination of the optical band gap of the reactively sputtered In2S3 films of approximately 600 nm thickness. Using linear extrapolation of the leading edge in the Tauc plots, (a) the direct band gap (using (αE)2) and (b) indirect (using (αE)1/2) band gap can be determined to be (2.65 ± 0.1) eV and (2.15 ± 0.1) eV, respectively. Also displayed in the inset of (a) is the absorption coefficient on the logarithmic scale versus photon energy.
Many controversial results have been reported on the optical band gap of In2S3. Most literature report, that the absorption edge of β-In2S3 is mainly due to a direct transition in the range of 2.0 eV – 2.9 eV33–35. However, studies on β-In2S3 single crystals as well as on sputtered thin films have found the optical transition to be a combination of both indirect band gap in the range of 1.8 eV – 2.2 eV and a direct band gap in the range of 2.4 eV - 2.6 eV19,36. The latter results were confirmed for the films characterized in this study. Figure 4 shows Tauc plots for both indirect and direct optical transitions of selected, representative samples. For deposition temperatures of up to 290°C the direct and indirect optical transitions were determined to be 2.6 eV and 2.1 eV, respectively. For higher deposition temperatures the apparent band gap energies increased by 13 ACS Paragon Plus Environment
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0.15 eV. This behavior has been previously reported for In2S3 films grown by thermal evaporation and chemical bath deposition, where annealing of the films led to an increase of the apparent band gaps in the order of 10%14,37. A possible explanation for this increase could be tensile stress during the cooling process of the substrate after the deposition. Also the increase in crystallinity, as observed for the films deposited at higher temperatures, could have contributed to a more well-defined band edge, and hence the change in the apparent band gap. Other groups have reported similar changes of the band gap for different film thicknesses due to the quantum size effect17. We can rule out this effect, since the detailed analysis was performed on selected samples of similar film thickness (600 nm ± 10 nm), which also had similar grain sizes.
3.4 Electrical properties The resistivity of the reactively sputtered films was measured at room temperature, as a function of the substrate temperature during the deposition. The results are depicted in Figure 5. The resistivities reach values of up to 106 Ωcm for depositions without intentional heating. For higher temperatures resistivities as low as 5 Ωcm can be achieved, which is in good agreement with literature 17 and suitable for buffer layer applications38.
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Table 1: Hall effect measurements of reactively sputtered In2S3 films for selected deposition temperatures. For elevated temperatures a strong increase in carrier concentration can be observed. TSub (°C)
170
220
290
370
µH (cm²/Vs)
28.1
22.9
19.4
24.6
n (1/cm³)
5.65×1014
6.57×1014
1.56×1016
1.15×1016
σ (S/cm)
2.51×10-3
2.41×10-3
4.86×10-2
4.52×10-2
To further investigate the origin of the increased conductivity, Hall measurements were carried out on selected samples. Table 1 shows Hall mobilities and carrier concentrations for different deposition temperatures. All films were found to be n-type and the sheet resistivities were in good agreement with the high throughput 4 point mapping measurements. While no significant changes in Hall mobility could be observed, the carrier concentration increases over two decades with increased deposition temperatures. As suggested in literature39 the charge carrier concentration in In2S3 single crystals is strongly dependent on compositional deviations from the stoichiometric compound. In particular, a sulfur deficiency favors the formation of interstitial indium defects, leading to increased n-type conduction. For the reactively sputtered In2S3 films reported here, higher deposition temperatures lead to a slight decrease in sulfur concentration according to XRF measurements (see Figure S5 in supporting information), which could explain the increase in carrier concentration (and therefore conductivity) for higher temperatures.
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Figure 5: Sheet resistivity of the In2S3 layers for different deposition temperatures. An increase in resistivity can be observed for lower deposition temperatures.
3.4.1 Doping studies for increased conductivity In order to achieve lower resistivities while maintaining low deposition temperatures, doping studies with Sn and Zr were attempted. For this purpose, combinatorial libraries with orthogonal temperature and composition gradients were synthesized. In the past, the Sn doping of In2S3 has been shown to successfully reduce sheet resistances without altering other parameters such as crystallinity or band gap40. However, our efforts to dope In2S3 with Sn via co-sputtering from SnS and In2S3 targets led to very high resistivities beyond the measurement range of the 4 point probe setup, even for low doping concentrations of Sn (3.1 at.%). This could be due to the presence of both β-In2S3 and SnS in the synthesized films, in particular if SnS is coating the relatively small β-In2S3 grains. Our XRD measurements show no peak shift of the In2S3 reflexes with increasing Sn-concentration, and a decrease in crystallinity of the In2S3 could be observed for higher Sn concentrations. This leads to the conclusion, that the dopant is likely incorporated as an amorphous secondary phase, rather than incorporated in the lattice. Both the incorporation of secondary phases in the grain boundaries of the material and a decrease in crystallinity can 16 ACS Paragon Plus Environment
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lead to an increase in charger carrier scattering and therefore lower carrier mobilities. Better results could possibly be achieved using an SnS2 target supplying Sn4+ rather than Sn2+ ions.
A promising alternative for Sn as a tetravalent dopant in In2S3 can be found in Zr4+. The mismatch between the ionic radii of In3+ and Zr4+ is smaller than for In3+ and Sn4+. For lattice sites with a coordination number (CN) of 6 the ionic radii are 0.80 Å for In3+, 0.69 Å for Sn4+ and 0.72 Å for Zr4+. For lattice sites with a CN of 4 the ionic radii are 0.62 Å for In3+, 0.55 Å for Sn4+ and 0.59 Å for Zr4+ 41. This should make the incorporation in the β-In2S3 lattice easier. To test this hypothesis, In2S3:Zr films were deposited using co-sputtering from In2S3 and ZrS2 targets. No changes in crystallinity or peak position could be observed for Zr concentrations of up to 8%. Additionally, 4 point probe measurements showed elevated resistivities with increasing Zr at. %. This leads to the conclusion that, similar to the attempted doping with Sn, no incorporation of Zr in the In2S3 lattice takes place. While the In2S3:Zr resistivities are much lower than for the Sn doped In2S3 layers the process is still unsuitable for PV applications. The results of the 4 point probe mapping as well as the XRD patterns for the doping experiments can be found in the supporting information.
3.5 Device studies on In2S3/CZTS solar cells Reactive sputtering of pure In2S3 with H2S provides a robust and scalable deposition technique for In2S3. To investigate the feasibility of sputtered In2S3 layers for thin film solar cells, combinatorial PV device libraries were built on top of CZTS/Mo/SiO2 stacks (a detailed report of the results of this study can be found in Table S1 in the supporting information). The In2S3 layers
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were deposited using three different substrate temperatures: 370°C, 290°C and without intentional heating. In2S3/CZTS devices using In2S3 buffer layers deposited at 290°C showed to be on par with CdS/CZTS reference devices with similar values for short circuit currents as well as open circuit voltages. Films deposited without heating lead to cells with little to no PV response, likely due to the high resistivities in the MΩ range. The deposition at temperatures above 290°C increases the conductivity, as well as the optical band gap of the In2S3 films. However, devices fabricated with In2S3 layers deposited at 370°C showed low efficiencies of η < 0.4% independent of the buffer layer thickness (see Figure 6a). The poor performance of these devices can be explained by increased interface recombination caused by inter-diffusion during the buffer layer deposition.
Figure 6: Results of the In2S3/CZTS device studies. (a) Device efficiencies for different buffer layer thicknesses. (b) VOC and JSC for devices with buffer layers deposited at 290°C. The dashed lines represent the CdS/CZTS reference cell with the highest efficiency. The best results could be
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achieved for In2S3 deposition temperatures of 290°C. The optimal buffer layer thickness is between 60 nm and 65 nm. For those cells the fill factors are in the order of 48 %.
To determine the ideal buffer layer thickness, photovoltaic device libraries with intentional In2S3 thickness gradients were fabricated and characterized. The influence of the buffer layer thickness on VOC as well as JSC is shown in Figure 6b. A buffer layer thickness in the range of 60 nm to 65 nm provides an ideal tradeoff between increased series resistance and a likelihood of shunting. This results in open circuit voltages in the order of VOC = 400 mV and short circuit currents of JSC =12 mA/cm2. Lower film thicknesses promote shunting due to an incomplete In2S3 layer formation and insufficient coverage of the absorber material. For higher buffer layer thicknesses open circuit voltages of up to VOC = 440 mV could be achieved while compromising the photocurrent of the device. This indicates that an optimization of the transport properties of the In2S3 films might be required in order to further increase the buffer layer thickness to fully exploit the potential of the more suitable band alignment on the front contact between CZTS and In2S3.
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Figure 7: (a) JV and (b) EQE for CZTS-solar cells using In2S3 buffer layers of different thicknesses deposited at 290°C. An increase in VOC can be observed for higher film thicknesses compromising the JSC due to parasitic absorption.
In addition to the J-V characteristics, the influence of the In2S3 buffer layer thickness on the spectral response of the PV devices was investigated. The results of the EQE measurements are shown in Figure 7. As expected, the spectral response is slightly decreased for higher buffer layer film thicknesses and photon energies above the direct band gap of In2S3. Remarkable is a significant increase of the EQE in this spectral region as compared to the CdS reference device. A similar behavior has previously reported for In2S3/CIGSe solar cells18. The increase in EQE indicates lower parasitic absorption in the buffer layer. This can be explained by the lower absorption coefficient of In2S3 for photon energies below the direct band gap of 2.6 eV which is slightly larger than the band gap of CdS (2.4 eV10). This decrease in parasitic absorption makes In2S3 an even more promising alternative to CdS besides the previously mentioned advantages. The overall efficiency for both CdS and In2S3 buffer layers falls short of current state of the art cells produced at SUNY Binghamton27. This can be explained at least partly due to degradation of the absorber layers during shipping and exposure to atmosphere. Also the CBD process of the CdS buffer layers has not been optimized for this particular batch of absorber materials. Nevertheless, since all devices are produced on absorber layers of the same batch, the trends observed can give insight, whether or not In2S3 buffer layers are suitable for this type of absorber material. The combinatorial screening narrows down the feasible deposition parameter space (i.e. deposition temperatures or film thicknesses) for the buffer layer deposition significantly. This
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facilitates further optimization of the devices, potentially leading to In2S3/CZTS devices that are competitive with current record cells.
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4. Conclusions
The reactive sputter deposition of In2S3 was investigated using a high throughput approach with combinatorial temperature gradients. It was found that sputtering from a ceramic In2S3 target in pure Ar atmosphere requires elevated substrate temperatures, which can be problematic during photovoltaic device processing. In contrast, using reactive sputtering with H2S, phase pure In2S3 could be deposited over a broad temperature range, down to ambient temperature. However, the transport properties were found to be strongly dependent on the substrate temperature used, with decreased conductivity corresponding to lower substrate temperature. Doping with Zr4+ and Sn4+ with the intent of improving the conductivity of the films for lower substrate temperatures did not lead to the desired results. However, the feasibility of the sputtered In2S3 buffer layers as a function of their thickness was demonstrated for In2S3/CZTS devices. For the optimal In2S3 thickness, the In2S3/CZTS devices with buffer layers deposited at 290 °C showed the performance on par with the CdS/CZTS reference cells. Further optimization of the In2S3 transport properties would facilitate the use of thicker buffer layers possibly leading to an additional improvement in the open circuit voltage. The differences in the device performance for different contact layer thicknesses and deposition temperatures emphasize the benefits of a combinatorial approach for an accelerated optimization of the contact layer deposition parameters in a given device structure. The deposition process for In2S3, as presented in this work, can be applied to other absorber materials, for which a lower electron affinity of the buffer layer is desired. Overall, the utilization of thickness and temperature gradients in combinatorial device libraries represents a powerful
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tool for an accelerated investigation of alternative buffer layers and device structures for the rapid development of emerging solar cell technologies.
Acknowledgements This work was supported by the U.S. Department of Energy, Office of Energy Efficiency and Renewable Energy, under Contract No. De-AC36-08-GO28308 with the National Renewable Energy Laboratory, as a part of a Non-Proprietary Partnering Opportunity Project with SUNY Binghamton. In addition, Tara P. Dhakal acknowledges support from the Office of Naval Research (Grant No. N00014-11-1-0658).
Corresponding Author Correspondence to Sebastian Siol (
[email protected])
Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
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Supporting information Experimental details of the combinatorial procedures for combinatorial synthesis and device fabrication; XRF data for the In2S3 compositional analysis; additional XRD and 4pp data for In2S3:Zr and In2S3:Sn doping studies; device parameters for the combinatorial device study; detailed information regarding the characterization of the devices.
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