Flexible CdTe/CdS solar cells on thin glass substrates Won-Oh Seo,1 Donghwan Kim,2 and Jihyun Kim1,* 1

Department of Chemical and Biological Engineering, Korea University, Seoul 136-701, South Korea 2 Department of Materials Science and Engineering, Korea University, Seoul 136-701, South Korea * [email protected]

Abstract: We demonstrate flexible CdTe/CdS thin-film solar cells in a superstrate configuration with a cell conversion efficiency as high as 10.9%. We deposit a CdS window layer and a CdTe absorber layer on a flexible glass substrate using the chemical bath deposition method and close-spaced sublimation method, respectively. The thin and flexible glass substrates were able to tolerate a high growth temperature and post-growth processes. We repeatedly apply a strain of 0.15% to the fabricated CdTe/CdS solar cells, and this was shown to have a negligible effect on their performances. Our proposed thin films-on-compliant substrate structure, which was prepared by replacing a rigid glass with a bendable one, demonstrated flexible CdTe/CdS p-n junction thin-film solar cells without compromising the cell performance. ©2015 Optical Society of America OCIS codes: (350.6050) Solar energy; (250.0250) Optoelectronics; (160.6000) Semiconductor materials.

References and links 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

C. Lungenschmied, G. Dennler, H. Neugebauer, S. N. Sariciftci, M. Glatthaar, T. Meyer, and A. Meyer, “Flexible, long-lived, large-area, organic solar cells,” Sol. Energy Mater. Sol. Cells 91(5), 379–384 (2007). M. Pagliaro, R. Ciriminna, and G. Palmisano, “Flexible Solar Cells,” ChemSusChem 1(11), 880–891 (2008). F. C. Krebs, S. A. Gevorgyan, and J. Alstrup, “A roll-to-roll process to flexible polymer solar cells: model studies, manufacture and operational stability studies,” J. Mater. Chem. 19(30), 5442–5451 (2009). L. Kranz, C. Gretener, J. Perrenoud, R. Schmitt, F. Pianezzi, F. La Mattina, P. Blösch, E. Cheah, A. Chirilă, C. M. Fella, H. Hagendorfer, T. Jäger, S. Nishiwaki, A. R. Uhl, S. Buecheler, and A. N. Tiwari, “Doping of polycrystalline CdTe for high-efficiency solar cells on flexible metal foil,” Nat. Commun. 4, 2306 (2013). A. McEvoy, T. Markvart, and L. Castañer, Solar Cells: Materials, Manufacture and Operation, in CdTe ThinFilm PV Modules 2nd ed. (Elsevier, 2013). J. Poortmans and V. Arkhipov, Thin Film Solar Cells: Fabrication, Characterization and Applications in Cadmium Telluride Thin Film Solar Cells: Characterization, Fabrication, and Modelling (Wiley, 2006). M. A. Green, K. Emery, Y. Hishikawa, W. Warta, and E. D. Dunlop, “Solar cells efficiency tables (Version 45),” Prog. Photovolt. Res. Appl. 23(1), 1–9 (2015). X. Mathew, J. P. Enriquez, A. Romeo, and A. N. Tiwari, “CdTe/CdS solar cells on flexible substrates,” Sol. Energy 77(6), 831–838 (2004). A. N. Tiwari, A. Romeo, D. Baetzner, and H. Zogg, “Flexible CdTe Solar Cells on Polymer Films,” Prog. Photovolt. Res. Appl. 9(3), 211–215 (2001). W. L. Rance, J. M. Burst, D. M. Meysing, C. A. Wolden, M. O. Reese, T. A. Gessert, W. K. Metzger, S. Garner, P. Cimo, and T. M. Barnes, “14%-efficient flexible CdTe solar cells on ultra-thin glass substrates,” Appl. Phys. Lett. 104(14), 143903 (2014). S. Wagner, H. Gleskova, I.-C. Cheng, J. C. Sturm, and Z. Suo, “Mechanics of TFT Technology on Flexible Substrates,” in Flexible Flat Panel Display, edited by G. P. Crawford (John Wiley and Sons, Ltd., West Sussex, England, 2005). W.-O. Seo, Y. Jung, J. Kim, D. Kim, and J. Kim, “Chemical bath deposition of cadmium sulfide on graphenecoated flexible glass substrate,” Appl. Phys. Lett. 104(13), 133902 (2014). W.-O. Seo, Y. H. Koo, B. Kim, B. C. Lee, D. Kim, and J. Kim, “Flexible cadmium telluride thin films grown on electron-beam-irradiated graphene/thin glass substrates,” Appl. Phys. Lett. 105(8), 083902 (2014). K. Durose, P. R. Edwards, and D. P. Halliday, “Materials aspects of CdTe/CdS solar cells,” J. Cryst. Growth 197(3), 733–742 (1999). Y. Jung, G. Yang, S. Chun, D. Kim, and J. Kim, “Post-growth CdCl2 treatment on CdTe thin films grown on graphene layers using a close-spaced sublimation method,” Opt. Express 22(S3), A986–A991 (2014).

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Received 29 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 13 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A316 | OPTICS EXPRESS A316

16. H. R. Moutinho, M. M. Al-Jassim, D. H. Levi, P. C. Dippo, and L. L. Kazmerski, “Effects of CdCl2 treatment on the recrystallization and electro-optical properties of CdTe thin films,” J. Vac. Sci. Technol. A 16(3), 1251–1257 (1998). 17. J. D. Major, R. E. Treharne, L. J. Phillips, and K. Durose, “A low-cost non-toxic post-growth activation step for CdTe solar cells,” Nature 511(7509), 334–337 (2014). 18. J. Lewis, “Material challenge for flexible organic devices,” Mater. Today 9(4), 38–45 (2006).

1. Introduction Flexible photovoltaic technologies have received considerable attention because of their potential applications, including wearable and portable devices that can be used outdoors and by the military [1–3]. Of the commonly used photovoltaic materials such as silicon, GaAs, Cu(In, Ga)Se2, CdTe, and Cu2ZnSn(S, Se)4, CdTe-based thin-film solar cells are commercially competitive and have the shortest energy payback time [4–6]. Recently, there were reports of a record efficiency of 21.0% for CdTe thin-film cells fabricated on a glass substrate [7]. In addition, the module efficiency was 17.5% with an aperture area of 7021 cm2. The bandgap of CdTe, with a high absorption coefficient (>5 × 105 cm−1), is ~1.5 eV, which is ideal for the single-junction conversion of the solar spectrum with the highest theoretical efficiency (~30%) of the different solar cell materials [5, 6]. High-efficiency CdTe solar cells have been fabricated on thick and rigid glass substrates in a “superstrate” configuration, making them highly inflexible. Therefore, there have been considerable efforts to fabricate flexible CdTe thin-film solar cells using metal foil or plastic as the substrate [4, 8, 9]. Because the opaqueness of metal foils renders the superstrate configuration impossible, flexible CdTe-based solar cells on metal foils have been fabricated in a “substrate (inverted)” configuration with record efficiency values of up to 13.6% [4]. The lower conversion efficiencies in CdTe solar cells obtained with the substrate configuration can be attributed to non-optimized post-growth treatments, including CdCl2 activation and the formation of low-resistance back contacts, because technology that is based on metal foil substrates is less mature compared to the traditional glass substrate counterpart. In addition, CdTe solar cells deposited on transparent polyimide substrates in a superstrate configuration suffered from the low optical transmittance characteristics of polyimide films as well as nonoptimal CdTe growth temperature below 450°C [9, 10]. Because the nature of polycrystalline CdTe structure makes all post-process steps interrelated, we believe that the well-established superstrate configuration technology is more suitable for fabricating high-efficiency flexible CdTe solar cells. Replacing the thick rigid glass substrate with a thin flexible one can make the entire structure flexible because the fabricated solar cells are bendable with the “thin-films-oncompliant substrate” configuration [11]. Rance et al. reported flexible CdTe solar cells on Corning Willow glass, where the strain ranged from 0.08% to 0.13% [10]. In particular, flexible glass substrates are applicable to high-throughput roll-to-roll fabrication, which can reduce manufacturing costs. In addition, the thinness of the glass substrate may improve the optical transmittance, which means that more photons are able to reach the absorber layer. In our experiments, we used thin and flexible glass substrates because they are highly transparent, and are thermally stable up to the optimal CdTe growth temperature (~550°C). Then, we fabricated bendable CdTe/CdS p-n junction thin-film solar cells, where the strains were applied up to 0.15%. 2. Experimental details Figure 1 shows a schematic of the fabrication process of “superstrate” CdTe/CdS thin-film solar cells. The indium tin oxide (ITO) thin films, which were 200 nm thick, were deposited on thin glass (borosilicate, Samsung Techno Glass Co.) substrates using the RF-sputtering system. The size and thickness of the thin glass substrate were 2 cm × 3 cm and 300 μm, respectively. The CdS thin films were then deposited on the ITO-coated thin-glass substrates using the chemical bath deposition (CBD) method. Solutions of CdCl2 (2 mM), thiourea (3 mM), and NH4Cl (15 mM) were mixed in a beaker and stirred for 12 h. After adding an NH4OH solution, the CdS layer (300 nm) was deposited on the ITO-coated thin glass sample #233461 - $15.00 USD (C) 2015 OSA

Received 29 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 13 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A316 | OPTICS EXPRESS A317

at 75°C under stirring at 350 rpm for 1 h. The sheet resistance (Rsheet) of the ITO layer was obtained using a four-point probe equipment (Desk 205, MS Tech Co.) connected to a source meter (2400, Keithley). Any CdS deposited on the back side of the sample was removed using a dilute HCl solution. The polycrystalline CdTe thin films were deposited on the CBD-CdS layer using a close-spaced sublimation (CSS) method under an argon environment for 10 min. The temperatures of the substrate and source were 540°C and 600°C, respectively. The CdTe layer was deposited from the CdTe powder source (99.999%, Johnson Matthey Co.) using the CSS method. Details of the CdS and CdTe growth conditions can be found elsewhere [12, 13]. The optical properties of both the CdS and CdTe layers were characterized by microphotoluminescence (micro-PL) spectroscopy at room temperature and a spectrophotometer (Cary 5000, Varian). For the micro-PL measurements, the excitation wavelengths for CdS and CdTe were 325 nm from a HeCd laser (Kimmon Co.) and 532 nm from a diode-pumped solid-state laser (Omicron), respectively.

Fig. 1. Schematic of the fabrication procedure illustrating the individual steps. (a) ITO thin films (200 nm)/glass (300 μm), (b) CdS (300 nm)/ITO/thin glass, (c) CdTe (10 μm)/CdS/ITO/thin glass, and (d) Cu/Au (30 nm/120 nm) electrodes/CdTe/CdS/ITO/thin glass.

Then, the post-growth procedure followed. The sample (CdTe/CdS/ITO/glass) was dipped into the saturated CdCl2 solution and annealed in a furnace (air environment) at ~380°C for 30 min. The effects of the CdCl2 treatment on the CdTe thin films have been investigated by numerous groups [14–16]. The oxide that formed on the CdTe was etched in a mixture of nitric acid and phosphoric acid (Sigma-Aldrich) because the oxide film that forms during the annealing process in air can affect the cell performance. In addition, the Te-rich layer may be exposed by this etch step, which is important for the formation of a good ohmic contact to CdTe with high electron affinity (~4.28 eV). The Cu/Au (30 nm/120 nm) back-contact electrodes were deposited with a shadow mask using the electron-beam evaporation method; the sample was then annealed in N2 ambient at 300°C for 10 min. The structural properties of the CdTe layer were analyzed by X-ray diffraction (XRD, Cu target, 2-Θ mode, ATX-G, Rigaku). The performance of the fabricated solar cells was measured using a solar simulator. 3. Results and discussion Figure 2(a) shows the room temperature PL spectrum of the CdS window layer deposited on the ITO/thin glass using the CBD method, which is consistent with the bandgap of CdS (~2.4 eV) [5, 6]. The peak at ~3 eV is attributed to the background luminescence of the glass substrate. The PL spectrum of CdTe at room temperature (Fig. 2(b)) was obtained after CdTe was deposited on top of the CdS thin film. The peak at ~1.5 eV corresponds to the bandgap reported for CdTe [5, 6]. Both of the PL spectra confirm the high quality of the CdS and CdTe thin films used in our experiments ((Figs. 2(a) and 2(b)), respectively). In Fig. 2(c), the optical transmittance of the ITO/glass substrate is slightly lower than that of the glass alone because of the ITO (200 nm thick), which is a transparent conductive layer. After the deposition of the window layer (the CdS layer), the optical transmittance decreased further, as shown in Fig. 2(c). Another advantage of using the thin glass substrate is its higher optical transmittance relative to that of a thick glass substrate. The thickness of the CdTe thin film deposited by the CSS method, with an area of 1 cm × 1 cm, was 10 μm, which is considerably thicker than the ideal thickness, as the high absorption coefficient of CdTe ensures that a few microns of CdTe are sufficient to absorb most of the incident photons. However, the polycrystalline nature of CdTe makes pin-holes and grain boundaries inevitable in a CdTe layer; therefore, we used a #233461 - $15.00 USD (C) 2015 OSA

Received 29 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 13 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A316 | OPTICS EXPRESS A318

thick CdTe layer to avoid the formation of pin-holes in polycrystalline CdTe. We believe that the conversion efficiency will be further improved by using thin CdTe layers if the growth conditions on the flexible glass substrate are optimized.

Fig. 2. (a) PL spectrum of CdS, (b) PL spectrum of CdTe, (c) transmission spectra of the thin glass substrate (black), ITO/thin glass (blue), and CdS/ITO/thin glass (red), and (d) XRD data of a CdTe thin film after the CdCl2 activation process (inset: photograph of the fabricated CdTe/CdS solar cells).

The CdTe layer was then activated using the CdCl2 treatment. The effects of the CdCl2 treatment have been intensively investigated because it is considered a critical process in the achievement of high conversion efficiencies in CdTe solar cells [14–16]. Durose et al. summarized the metallurgical and optoelectronic effects of CdCl2 treatment on CdTe, as it can cause grain growth, passivation of grain boundaries, interface alloying between CdS and CdTe layers, and recrystallization, although its effect is not yet clearly understood [14]. Recently, it was reported that a non-toxic MgCl2 treatment was also effective in CdTe solar cells [17]. Figure 2(d) shows the XRD data after CdCl2 treatment on the CdTe with a cubic zinc-blend structure. The inset of Fig. 2(d) is a photograph of the completed CdTe/CdS cell after the post-growth processing including the activation step and nitric-phosphoric etch step.

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Received 29 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 13 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A316 | OPTICS EXPRESS A319

Fig. 3. (a)The maximum cell efficiency results, (b) ITO sheet resistance before and after a repeated strain test at a tensile strain of 0.15% (inset: photograph of the ITO/glass under a strain of 0.15%), and (c) device efficiencies after the repeated tensile strain (inset: photograph of the CdS/CdTe solar cells under a strain of 0.15%).

Our flexible CdTe/CdS solar cells showed that the highest cell efficiency was 10.9% (open circuit voltage (VOC): 717.6 mV, short-circuit current (JSC): 25.8 mA/cm2, and fill factor (FF): 58.7%). Shallow slopes of I-V curves near VOC suggest the presence of a diode opposing the main p-n diode (CdS/CdTe), which is attributed to the non-ideal Ohmic contact to p-CdTe. One of the CdTe/CdS cells was investigated after repeatedly applying a 0.15% tensile strain using the setup shown in the inset of Fig. 3(c). The strain (S) was calculated using the equation: S = tglass/(2 × R), where tglass is the glass thickness and R is the radius of the curvature [18]. The cell efficiency was monitored after applying the strain each time. The Rsheet of the ITO was ~22.5 ohm/, and was not significantly affected by the strain condition (at 0.15% tensile strain), as shown in Fig. 3(b). Both the parallel (shunt) and the series

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Received 29 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 13 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A316 | OPTICS EXPRESS A320

resistances were barely affected by the repeated strains, where the series resistance ranged from 423 to 456 Ω. The parallel resistance ranged from 1.92 × 104 to 2.06 × 104 Ω during the strain experiments. To ensure long-term stability under a strained condition, the values of the failure strain and Young’s modulus of the substrate, ITO, CdS, CdTe, and the metal contacts need to be further investigated. As previously reported, the superstrate configuration is considered more suitable for achieving a higher conversion efficiency than the substrate configuration owing to the issues in the post-growth processing. 4. Conclusion We fabricated flexible CdTe/CdS thin-film solar cells with a superstrate configuration using a thin glass substrate, and we achieved a cell efficiency of up to 10.9%. The integration of a flexible glass substrate with CdTe/CdS thin films allows high-quality film growth (of CdS and CdTe) and post-growth processes at elevated temperatures. The tensile strain (0.15%) was repeatedly applied to the flexible CdTe solar cells, which resulted in a nominal change in their photovoltaic performances. Our results are expected to pave the way for high-efficiency flexible CdTe/CdS thin-film solar cells. Acknowledgments This research was supported by the National Nuclear and Radiation Technology R&D program (2013M2A2A6043608) through the National Research Foundation of Korea (NRF), which is funded by the Ministry of Science, ICT, and Future Planning, and the Human Resources Development program (20124030200120) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant, which is funded by the Korea Government Ministry of Trade, Industry, and Energy.

#233461 - $15.00 USD (C) 2015 OSA

Received 29 Jan 2015; revised 9 Mar 2015; accepted 9 Mar 2015; published 13 Mar 2015 6 Apr 2015 | Vol. 23, No. 7 | DOI:10.1364/OE.23.00A316 | OPTICS EXPRESS A321

CdS solar cells on thin glass substrates.

We demonstrate flexible CdTe/CdS thin-film solar cells in a superstrate configuration with a cell conversion efficiency as high as 10.9%. We deposit a...
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