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Combinatorial chemical bath deposition of CdS contacts for chalcogenide photovoltaics Krishnaiah Mokurala, Lauryn L. Baranowski, Francisco Willian de Souza Lucas, Sebastian Siol, Maikel van Hest, Sudhanshu Mallick, Parag Bhargava, and Andriy Zakutayev ACS Comb. Sci., Just Accepted Manuscript • DOI: 10.1021/acscombsci.6b00074 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 2, 2016

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ACS Combinatorial Science

Combinatorial chemical bath deposition of CdS contacts for chalcogenide photovoltaics Krishnaiah Mokurala a, b,*, Lauryn L. Baranowskia,c, Francisco W. de Souza Lucasa,d, Sebastian Siol a, Maikel F. A. M van Hesta , Sudhanshu Mallick b , Parag Bhargava b, Andriy Zakutayeva,* a

National Renewable Energy Laboratory, Golden, Colorado 80401, USA b

Indian Institute of Technology Bombay, Mumbai 400076, India b

d

Colorado School of Mines, Golden, Colorado, 80401, USA

Federal University of Sao Carlos, São Carlos-SP, 13565-905, Brazil

* Corresponding authors: [email protected], [email protected]

Abstract: Contact layers play an important role in thin film solar cells, but new materials development and optimization of its thickness is usually a long and tedious process. High throughput experimental approach has been used to accelerate the rate of research in photovoltaic (PV) light absorbers and transparent conductive electrodes, however the combinatorial research on contact layers is less common. Here, we report on the chemical bath deposition (CBD) of CdS thin films by combinatorial dip coating technique, and apply these contact layers to Cu(In,Ga)Se2 (CIGSe) and Cu2ZnSnSe4 (CZTSe) light absorbers in PV devices. Combinatorial thickness steps of CdS thin films were achieved by removal of the substrate from the chemical bath in steps, at regular intervals of time, and in equal distance increments. The trends in the photoconversation efficiency and in the spectral response of the PV devices as a function of thickness of CdS contacts were explained with the help of optical and morphological characterization of the CdS thin films. The maximum PV efficiency achieved for the combinatorial dip-coating CBD was similar to that for the PV devices processed using conventional CBD. The results of this study lead to the conclusion that combinatorial dip-coating can be used to accelerate the optimization of PV device performance of CdS and other candidate contact layers for a wide range of emerging absorbers. Keywords: Combinatorial synthesis; buffer layer; CIGSe; CZTSe; current-voltage characteristics; external quantum efficiency; solar cells

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Introduction Thin film chalcogenide solar cell based on CdTe, Cu(In,Ga)Se2 (CIGSe) and Cu2ZnSnSe4 (CZTSe) have many attractive features, such as a high efficiencies, low materials utilization, and potential for low overall cost, as compared to other photovoltaic technologies.

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Among

numerious layers of thin film solar cells (absorbers, contacts, electrodes etc),5 the choice and the thickness contact layer (usually 50 nm thick CdS films) plays a vital role in p-n heterojunction formation, and hence extraction of photo-generated electric charge carriers. 5, 6 CdS buffer layer can be prepared by various techniques, such as chemical bath deposition (CBD), sputtering, coevaporation, atomic layer deposition (ALD), ionic layer gas atomic reaction (ILGAR), etc.

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Among these techniques, the highest photo conversion efficiency (PCE) of the chalcogenide solar cells (22.3% for CIGSe 3, 22.1% for CdTe 3, 7, and 12.6% for CZTSe 8) has been achieved by preparation of buffer layer through CBD. The high-throughput combinatorial experimental approach is a rapid and efficient method for discovery and optimization of new materials for a wide range of applications. 9, 10 This approach consists of deposition of sample libraries with varying composition, temperature, or thickness,9 16

followed by spatially-resolved measurements of the relevant properties.11

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16

The

combinatorial approach has been implemented for pharmaceutical, electronic, magnetic, optical, and energy - related materials, 9, 13 as well as in photovoltaics (PV) for earth-abundant non-toxic absorbers 14, 15 and transparent conductive electrodes 16. Recently, we demonstrated development of alternative contact layers for thin film solar cells using combinatorial reactive sputtering17 that lends itself easily to high-throughput experimental approach. However, combinatorial studies of conventional solution processed contact layers, such as CdS prepared by CBD, have not been reported, except in the context of the nanocrystal applications of this important material. 18, 19 One possible approach to combinatorial solution processing is the dip-coating technique. Dip-coating is a thin film deposition process that relies on immersing and extracting a substrate (in this case, soda lime glass) from a vessel containing the precursor liquid (here, chemical bath of CdS precursors). In the past, combinatorial dip-coating with a continuous gradient in the film thickness of several polymers and oxides has been demonstrated20, 21 and applied. 20, 21 Similarly, combinatorial synthesis of the inorganic sulfide buffer layers prepared by chemical bath


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deposition should be possible by dip coating, allowing for parallel screening of many thicknesses in a single experiment. In this paper, we present a combinatorial study of thickness of the CdS contact layers synthesized by chemical bath deposition for chalcogenide thin film solar cell applications. First, we describe the combinatorial dip-coating process for creating discrete thickness steps of CdS films across the substrate. Next, properties of the chemical bath deposited CdS layers prepared by combinatorial and standard processes are compared. Finally, the photovoltaic device with combinatorialy processed CdS layers are described. This combinatorial dip-coating approach should be applicable to other PV absorber- and contact materials, and other thin film technologies where thickness is an important parameter. Results and Discussion Combinatorial experiments with CdS thin films and PV devices The step changes in thickness of CdS film were achieved with the help of the combinatorial dip coating process, schematically demonstrated in Fig. 1a (see Fig. S1 and Fig. S2 in Supplementary Information for more details). The dip coater was programmed in such a way that the 50x50 mm soda lime glass (SLG) substrate was first immersed into a bath in the reaction beaker with constant high speed, and then removed with the same speed at constant intervals of time (every 5 min), in approximately 10 mm distance increments, to achieve the desired thickness profile with the thickness steps (Fig. 1b). After four such steps, the resulting CdS sample library was placed in deionized water and subjected to 30 s ultra-sonication to remove the loosely attached CdS particles, and then dried with N2 gas. Similarly, CdS films were grown on CIGSe and CZTSe absorber layers, and processed into combinatorial libraries of PV devices, as schematically illustrated in Fig. 1b. Note that this hybrid CBD / dip-coating process differs from the conventional dip coating (Fig. 1a) and from conventional CBD. In the conventional dip-coting process, the deposition is controlled by the speed of the liquid meniscus at the interface with air. Here, the deposition mechanism is more similar to that in CBD, in that CdS continuously deposits onto the substrate as long as it still in the bath. Therefore extraction of the substrate stops deposition on the part


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that is in the air, but continuous deposition on the still submerged part, leading to steps in the resulting film thickness (Fig. 1b). This is different than the conventional CBD, where the entire substrate is submerged for the whole deposition time, and hence no thickness steps are achieved.

Fig. 1 Schematic illustrations of (a) combinatorial dip-coating process for creating thickness steps in the CdS contact layer, and (b) PV device structure with integrated combinatorially deposited CdS contact layer. Also, note that here we chose to use the combinatorial dip coating process in step mode, in order to obtain the sample statistics needed for unambiguous interpretation of the PV device efficiency results, and comparison with the conventional CdS CBD process. However, the more traditional continuous gradient mode of combinatorial experiments can also be used for the combinatorial CBD, and this gradient can be much higher than for the continuous-spread combinatorial sputtering approaches (usually a factor of ~2x in thickness).17 More CBD methods details, as well as characterization information, are described in the Experimental Procedures section. To test the combinatorially deposited CdS thin films, they were integrated in the thin film photovoltaic (PV) devices with CIGSe and CZTSe chalcogenide absorbers synthesized as reported earlier.22, 23 The absorber synthesis (Fig. 2a) was followed by chemical bath deposition of CdS buffer layers via combinatorial dip-coating process (Fig. 2b). Subsequently the front transparent conductive bilayer of ZnO/Al:ZnO was grown by RF sputtering at ambient temperature. Then, the front electrode pads (Al/Ni) were deposited by electron beam evaporation


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(Fig. 2c). Finally, the individual PV devices were isolated by mechanical scribing, and tested for energy conversion efficiency (J-V) and external quantum efficiency (EQE). For comparative studies, similar photovoltaic devices were also fabricated with CdS contact layer prepared by a conventional chemical bath deposition (CBD) 24, without the combinatorial thickness steps.

Fig. 2 Fabrication stages of the combinatorial PV device library, after the deposition of (a) CZTSe absorber, (b) CdS contact with thickness steps, and (c) TCO and metal electrode pads

CdS thin film material properties The morphology of the CdS thin films prepared by CBD process using combinatorial dipcoating on glass substrates is shown in Fig. 3, in comparison with standard CBD process. At the initial stage of growth, the film is non-conformally deposited on the glass (Fig. 3a, 5 min). As time progresses, CdS particles start to cover the substrate uniformly (Fig. 3b, 10 min), but some small voids are present in the film even for later times (Fig. 3c, 15 min). It appears that the CdS film deposited by the standard CBD process (Fig. 3d) is denser than the combinatorial processed film (Fig. 3c), which likely has to do with lower nucleation and growth barriers. The mechanistic origin behind this difference is presently unclear, but it might be related to the orientation of the substrate during the deposition (horizontal in standard CBD, vertical in combinatorial CBD). Also, it is noted that this difference in morphology of CdS films on glass substrate may not necessarily be the same for the deposition on the chalcogenide absorbers.


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Fig. 3 SEM images of CdS film prepared by combinatorial dip-coating chemical bath deposition (CBD) after (a) 5 min, (b) 10 min, (c) 15 min of deposition. Panel (d) shows the results of the conventional CBD of CdS film after 15 min, without combinatorial dip coating. The transmittance spectra of CdS layers deposited on SLG substrate via both combinatorial dip-coating (different times) and standard chemical bath deposition (CBD) technique are shown in Fig. 4a. The transmittance of CdS film decreases with the increase in CdS deposition time, most likely reason for this change is the stronger light absorption in thicker films 25, 26. However, we also observed that the transmittance of the combinatorial CBD film is slightly lower in the long-wavelength region than that of the standard CBD film for the same deposition time (15 min), which may be related to the differences in light scattering due to different film morphologies (Fig. 3).


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Fig. 4 (a) Optical properties (Transmittance) of CdS films prepared by combinatorial and conventional CBD, (b) variation of the CdS thickness with deposition time measured by x-ray fluorescence (XRF) and profilometry techniques. To support our conclusions about the change in the optical transmittance of CdS thin films with changing deposition time, the thickness of CdS film processed through both conventional and dip-coating CBD techniques was measured and is shown in Fig. 4b (see also Table S1 in Supplementary Information). It was not possible to measure the thickness of CdS film at 5 min deposition time due to the rough morphology at the initial growth stages of film. The variation of CdS thickness with deposition time clearly indicates the linear increase. A slight spatial variation in thickness is observed near the gradient steps of combinatorial processed CdS film, evidenced by the changes in color in Fig. 2b. The most likely explanation is the fluctuations of the liquid level in the chemical bath due to stirring and different deposition dynamics at the liquid/atmosphere interface. CIGSe and CZTSe Photovoltaic Device Performance The CdS films grown by both combinatorial and regular CBD processes were used as contact layers in CIGSe and CZTSe solar cells. From the J-V curves (Fig. 5), it appears that for the CIGSe PV devices, the biggest change with changing CdS thickness is in short circuit current, whereas for the CZTSe the short circuit current (Jsc), open circuit voltage (Voc), and fill factor


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(FF) are all changing (also see Table S2 and Table S3 in Supplementary Information). However, for both CIGS and CZTS the device performance parameters increase with increasing deposition time up to certain point (15 min), and then these parameters decrease with increase in deposition time (Fig. S3). This non-monotonic trend derived from the combinatorial experiments is in good agreement with literature results 27 - 30 that used traditional experimentation approaches. Overall, the efficiency of the devices fabricated with combinatorially deposited CdS buffer layer is slightly (~10% relative) lower than that for the CdS contacts deposited using the conventional CBD technique. This small difference in the efficiency is mostly the result of lower short-circuit current, which can be attributed to the lower transmittance (Fig. 4) of the dip-coated CdS film (Fig. 5) due to their slightly higher film thickness (Table S1).

Fig. 5 Current-Voltage (J-V) curves of (a) CZTSe and (b) CIGSe solar cells with CdS contact layers of different thicknesses prepared by combinatorial dip-coating CBD process. Black lines correspond to the standard CBD process. To get better insight into the effect of CdS film thickness on the device performance we performed external quantum efficiency measurement (EQE), presented in Fig. 6 and in Fig. S4. The EQE of CZTSe and CIGSe solar cells at low wavelength (420 nm) decreases with increase in deposition time from 5 min to 20 min, which is attributed to the light absorption in CdS layer. 24, 27, 28

The variation of the EQE of CZTSe, CIGSe in the visible region (Fig. 6) is due to

changes in the light coupling to the absorber layer (interference effects, Fig. 2b) with the changing CdS film thickness, due to different recombination losses in the bulk of the absorber, 31,


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among other factors. Finally, in the near-infrared region of the spectrum (980 nm), the EQE

shows the same trend as the Jsc, likely in part due to incomplete formation of the heterojunction at the initial stages of growth. We also note that the overall EQE of CZTSe and CIGSe with CdS film deposited by the standard CBD process is higher than with CdS film deposited by combinatorial dip-coating CBD process, which is again consistent with the obtained Jsc values.

Fig. 6 External quantum efficiency (EQE) spectra of (a) CZTSe, (b) CIGSe solar cells with CdS buffer layers deposited by combinatorial dip-coating CBD process. The black lines correspond to PV devices with CdS contact layers prepared by conventional CBD process. Discussion of materials and devices The results of our work demonstrate that the combinatorial dip-coating is a rapid approach to find the optimal thickness of the CdS films prepared by chemical bath deposition. However, one disadvantage of this method compared to the conventional CBD is the somewhat unfavorable morphology of the films (Fig. 3). To address the morphology difference of the films prepared by combinatorial CBD compared to that of the standard CBD process (Fig. 3), the nucleation and growth process can be improved by changing the chemical bath conditions and initial precursor’s concentration.

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It should also be possible to tune the morphology of the CBD grown thin

films by optimizing the dip coating parameters, such as withdrawal speed, solvent evaporation rate, and viscosity of solution of the bath. 20, 21


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We also note that the dip-coating method described here with the substrates being pulled out from the solution is only one possible implementation of the combinatorial approach to solution processing of thin films. Alternatively, the substrates can be lowered into the solution, which would also result in the thickness steps or gradient; however the disadvantage of this lowering method is that the particles start to form in the solution in addition to the film surface, so the resulting coating may be less conformal. Another geometrical implementation of the combinatorial approach to chemical bath deposition or other solution processing would be to place the substrate in the bath with either a tap at its bottom or a fluid pump connected to the bath. Both of these methods would allow changing the level of the fluid in the bath as a function of time, thereby achieving the thickness variation across the sample without moving it. The combinatorial dip-coating approach to contact layers for thin film solar cells, described here for the case of the most commonly used CdS buffer with expected results, may be extended to alternative window layer materials that can be processed using chemical bath deposition. For CIGSe and CZTSe solar cells, 36, 37 some of the CBD examples include ZnS SnO2

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, ZnO

42

materials, as well as Zn(O,S)

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, (Cd,Zn)S

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, In(OH,S)

45

38

In2S3

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ZnSe 40,

and related alloys.

Currently most of these alternative contact materials lead to solar cell performance lower than the state of the art CIGSe and CZTSe with CdS buffer layers, which in part may be due to their non-optimal thicknesses. Optimization of the thickness for these alternative contacts materials using the combinatorial dip-coating approach to chemical bath deposition may address this problem and improve the efficiency of the thin film chalcogenide photovoltaic devices. Beyond the well-studied CIGSe and CZTSe absorbers presented here, the combinatorial approach to chemical bath deposition can be used to quickly determine the optimal thickness of contacts for other emerging absorber materials. For example, it has been shown that changing the contact material from CdS 46 to Zn(O, S) 47 in SnS-based thin film solar cells leads to significant improvement of their performance. The need for alternative contact layer development has also been shown for the CuSbS2 emerging thin film photovoltaic absorber true for the CuSbSe2 analog.

49, 50

Based on the electronic structure

51

48

and similar may hold

, we also expect that the

alternative contact layers would be required to further improve performance of the Sb2Q3 (Q = S, Se) solar cells, where TiO2 52 and CdS 53 are currently used as contacts.


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Summary and Conclusions Combinatorial dip-coating approach to photovoltaic contact layers has been demonstrated on the example of chemical bath deposited CdS thin film. Optical transmittance properties of the CdS thin film deposited through combinatorial and conventional CBD processes are similar, but the combinatorial CdS thin films had more voids at early stages of deposition. Nevertheless, the dip-coated CdS films have been successfully applied as buffer layers in combinatorial CIGSe and CZTSe solar cell libraries, showing relative ~90% of the efficiency of the devices made using conventional CBD process. Overall, the results of this study show the promise of rapid screening of novel contact layer for emerging absorber materials using combinatorial dip-coating CBD and related solution processing techniques. Experimental Procedures The CdS combinatorial thin films libraries were grown by chemical bath deposition (CBD) on SLG substrate for physical properties measurements, and on CIGSe and CZTSe for PV device performance characterization (see Supplementary Information for more details). After the deposition, the CdS thin films on the SLG substrate were subjected to physical properties measurements. Optical properties of the CdS films were measured using UV - Vis - NIR Spectrometer (Agilent technologies) in the wavelength range from 300 to 1100 nm. Surface morphology of as prepared CdS films on SLG substrate was studied using field emission scanning electron microscope (FEG - SEM, JSM - 7600F). The thickness of CdS film was measured by X-ray Fluorescence (XRF, Helmut Fischer GMbH) and a surface profilometer (VEECO Dektak 8). The current density - voltage (J-V) characteristics of the combinatorial PV device libraries were tested using a Keithley 2420 source meter (Keithley Instruments, Inc.) under AM1.5G simulated solar irradiation (1000 W/cm2), as a function of position on the library (see Supplementary Information for more details). For selected samples on the combinatorial PV device libraries, the external quantum efficiency of CIGSe and CZTSe solar cells was recorded in the range from 300 – 1200 nm and 300 – 1500 nm respectively, using Bentham PVE300 instrument. Acknowledgments


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This work was supported by the Solar Energy Research Institute for India and the U.S. (SERIIUS) funded jointly by the U.S. Department of Energy subcontract DE AC36-08G028308 (Office of Science, Office of Basic Energy Sciences, and Energy Efficiency and Renewable Energy, Solar Energy Technology Program, with support from the Office of International Affairs) and the Government of India subcontract IUSSTF/JCERDC-SERIIUS/2012. The student exchange of KM was financially supported by MAGEEP – SERIIUS visiting scholars program. F.W.S.L was funded by the Sao Paulo Research Foundation (FAPESP), grant 2014/12166-3. L.L.B. was supported by the Department of Defense through the National Defense Science and Engineering Graduate Fellowship Program. AZ acknowledges funding from US Department of Energy, Office of Energy Efficiency and Renewable Energy. The authors would like to thank to Stephen Glynn, Carolyn Beall, James Burst and Clay DeHart for their support during device fabrication and characterization. We would also like to acknowledge the support from NCPRE, DST, IIT Bombay, and SAIF, CEN and IRCC for the analysis. Supporting Information Methods detailing CdS CBD process and PV device characterization; Tables summarizing CdS film thickness and PV device performance; figures showing pictures of the combinatorial dip-coating CBD set up and providing the flow chart for the deposition process; figures quantifying the JV and EQE changes as a function of CdS thickness References [1] Razykov, T. M.; Ferekides, C.S.; Morel, D.; Stefanakos, E., Ullal, H.S.; Upadhyaya, H.M. Solar photovoltaic electricity: Current status and future prospects. Sol Energy. 2011, 85, 1580 – 1608 [2] Bagnall, D. M.; Boreland, M. Photovoltaic technologies. Energ Policy. 2008, 36, 4390 – 4396 [3] http://www.solar-frontier.com/eng/news/2015/C051171.html [4] Green, M. A.; Emery, K.; Hishikawa, Y.; Warta, W.; and Dunlop, E. D. Solar cell efficiency tables (version 47). Prog. Photovolt. Res. Appl. 2016, 24, 3 – 11 [5] Repins, I.; Glynn, S; Duenow, J.; Coutts, T.J.; Metzger, W.; and Contreras; M.A. Required materials properties for high-efficiency CIGS modules preprint. Conference paper NREL/CP-520 - 46235 July 2009 [6] Mughal, M. A.; Engelken, R.; Sharma, R. Progress in indium (III) sulfide (In2S3) buffer layer deposition techniques for CIS, CIGS, and CdTe-based thin film solar cells. Sol Energy. 2015, 120, 131 – 146


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Combinatorial Chemical Bath Deposition of CdS ... - ACS Publications

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