and KAPTON® colorless polyimide film (KAPTON) with a thickness of 25 µm from DuPontTM, which show better transparency in comparison with UPILEX.
31st European Photovoltaic Solar Energy Conference and Exhibition
FLEXIBLE CdTe SOLAR CELLS ON POLYIMIDE AND FLEXIBLE GLASS SUBSTRATES Andrei Salavei, Elisa Artegiani1, Fabio Piccinelli2, Simone Di Mare, Daniele Menossi1, Alessio Bosio1, Nicola Romeo1 and Alessandro Romeo LAPS-Laboratory for Applied Physics, Department of Computer Science, University of Verona, Strada Le Grazie 15, 37134 Verona, Italy 1Physics and Earth Science Department, University of Parma, V.le G.P. Usberti, 7A-43124, Italy 2Solid State Chemistry Laboratory, Department of Biotechnology, University of Verona, Strada Le Grazie 15, Italy
ABSTRACT: In this paper flexible CdTe solar cells made by vacuum evaporation on ultra thin glass and on two different polyimides are presented. Ultra-thin glass is a very promising flexible substrate for CdTe solar cells in superstrate configuration because of it superior transparency. Cell preparation was optimized for each different substrate. Efficiencies exceeding 11% on polyimide and 12% on ultra-thin flexible glass have been achieved. Keywords: Thin Film Solar Cell, Flexible Substrate, CdTe, Deposition
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INTRODUCTION
both more transparent than UPILEX, furthermore, Ultrathin glass is more transparent than soda-lime glass, meaning that it will have less parasitic absorbance than usual glass as shown in Fig. 1.
Ongoing increase of production of thin-film solar modules makes this technology more and more market attractive with an increased efficiency that brings the thin films at the same performance level of silicon solar cells [1]. CdTe has shown a remarkable ability in mass production that has made it the market leader of thin film based photovoltaics. Thin films have many advantages compared to crystalline silicon, among these the fact that when deposited on flexible substrate it becomes flexible with a very high power/mass ratio. Flexible CdTe solar cells could be made in substrate [2] or superstrate configurations [3]. World record flexible CdTe device is, at the moment, made by hightemperature process on flexible glass showing efficiency of 14.05% [4]. As a follow up of the previous studies on flexible substrates [5], in this paper we show different solutions for making flexible CdTe solar devices keeping the superstrate configuration, which requires the substrate to be transparent. CdTe devices have been deposited on ultra thin glass by vacuum evaporation for the first time and compared with our polyimide substrate baseline. 2
Figure 1: Transparency of different materials, used as substrates for flexible superstrate CdTe solar cells We used our process developed for production of flexible solar cells on UPILEX [5] but with lower CdCl2 treatment annealing temperature due to further better optimization of the process [7]. Current density-voltage (J–V) characteristics of the finished cells were measured with a Keithley SourceMeter 2420, using a halogen lamp calibrated with a silicon solar cell under an irradiation of 100 mW/cm2.
EXPERIMENTAL DETAILS
In our laboratory we use low temperature physical vapor deposition (PVD) based fabrication process for CdTe solar cells. The low substrate temperature, not exceeding 450°C, allows us to use flexible polyimides as substrate for our devices. We already reported the study of solar cells made on UPILEX-12.5SN high temperature polyimide film (UPILEX) with a thickness of 12.5µm from UBE Europe GmbH [5]. In that study the best flexible cell performed an efficiency of 10% (Voc=796mV, FF=63.8%, Jsc=19.7mA/cm2). This result, compared with solar cell made on soda-lime glass with efficiencies above 14%, showed that the difference in the performance of flexible devices is mainly due to the lower transparency of UPILEX, compared with glass, delivering lower current values. In order to improve the performance of our devices we used 100 µm thick borosilicate glass THIN GLASS D 263® T ECO from SCHOTT AG (Ultra-thin glass) [6] and KAPTON® colorless polyimide film (KAPTON) with a thickness of 25 µm from DuPontTM, which show better transparency in comparison with UPILEX polyimide (see Fig. 1). Ultra-thin glass and KAPTON are
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RESULTS AND DISCUSSION
By means of optic and atomic force microscopes, we studied how the CdCl2 activation treatment changes the morphology of treated CdTe layers in the solar cells deposited on different substrates (hereinafter the phrase “CdTe layer is deposited on x-substrate” means that CdTe is deposited on the stack: x-substrate/ITO/ZnO/CdS). We have already observed that treated CdTe on soda-lime glass and on UPILEX have very similar morphology [5]. As expected, the treated films deposited on Ultra-thin glass and on KAPTON show the same behavior. The typical increase of grain size is observed and with similar structure. Finished devices have been prepared by depositing 300 nm CdS by vacuum evaporation at 100 °C substrate temperature and a stack of 300 nm ITO/100 nm ZnO by Radio Frequency sputtering at 400 °C.
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31st European Photovoltaic Solar Energy Conference and Exhibition
On Fig. 2 J-V characteristics of our best flexible cells are shown. The cells grown on Ultra-thin glass and on KAPTON show higher efficiencies than the solar cells made on UPILEX. The main difference in the efficiencies is due to the expected increase of current value, because of the higher transparency of new substrates compared with UPILEX (see Fig. 1). The improvement of other parameters (FF and Jsc) is, probably, due to the choice of the more efficient temperature for CdCl2 activation treatment (mentioned before), which results in forming of a better p-n junction in these cells. Further improvements are expected, since there is no particular limitation in efficiency of solar cells deposited on Ultra-thin glass compared to the ones on standard soda-lime glass (more than 14%).
Figure 3: CdTe solar cells on (left to right) glass, ultrathin glass and polyimide 4
CONCLUSIONS
Flexible CdTe solar cells made by low-temperature vacuum evaporation on ultra thin glass are presented for the first time and are compared with devices formed on KAPTON, UPILEX and on soda-lime glass (see Fig. 3). At the moment our best flexible solar cells have efficiencies of 10% on UPILEX, 11.2% on KAPTON and 12.2% on Ultra-thin glass. Optimization of the ITO+ZnO front contact would probably result in higher efficiency solar cells for Ultra-thin substrate glass. Flexible solar cells will weight almost 200 times less than devices on rigid glass (with same efficiency) because of much higher P/M ratio. Figure 2: J-V characteristics of the best flexible solar cells
ACKNOLEDGMENTS
One of the main advantages of flexible solar cells is the very low substrate weight, compared with usual 3 mm thick glass. Solar cells deposited on polymer or lightweight glass will have high power/mass (P/M) ratio. This parameter taking into account best efficiencies of our devices for different substrates is shown in Table I. Also world record efficiency of 21.5 % [1] was inserted for reference. The P/M ratio of the 12.2 % efficient device made on Ultra-thin glass is 31 times higher than P/M ratio of the 14.7 % efficient device made on 3 mm soda lime glass, and devices on KAPTON and UPILEX have P/M ratio that is 90 and 132 times higher the P/M ratio of the 3mm glass cell. Moreover, the P/M ratio of the device on Ultra-thin glass is more than 20 times higher than the P/M ratio of the cell with record efficiency and when the UPILEX substrate is used the P/M ratio is almost 200 times higher. The solar cells made on polymers and even on special glasses can be hundred times lighter than the standard devices, generating the same power.
SCHOTT AG, UBE Europe GmbH and DuPont de Nemours (Luxembourg) s.àr.l. are thankfully acknowledged for providing the substrate samples. REFERENCES [1] F. Solar, “First Solar Achieves World Record 18.6 % Thin Film Module Conversion Efficiency,” 2015. [2] 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, Nat. Commun., vol. 4, Aug. 2013. [3] J. Perrenoud, B. Schaffner, S. Buecheler, and a. N. Tiwari, Sol. Energy Mater. Sol. Cells, vol. 95, no. SUPPL. 1, pp. S8–S12, May 2011. [4] 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, Appl. Phys. Lett., vol. 104, no. 14, 2014. [5] A. Salavei, I. Rimmaudo, F. Piccinelli, D. Menossi, N. Romeo, A. Bosio, R. Dharmadasa, and A. Romeo, 27th European Photovoltaic Solar Energy Conference and Exhibition, 2012, pp. 2825–2827. [6] S. Ag, “Advanced Materials SCHOTT AG,” pp. 1– 11, 2009. [7] B. L. Xu, I. Rimmaudo, A. Salavei, F. Piccinelli, M. Barbato, M. Meneghini, S. Di Mare, D. Menossi, A. Bosio, N. Romeo, and A. Romeo, 2014, pp. 1826– 1828. [8] “Influence of proton irradiation and development of flexible CdTe solar cells on polyimide A. Romeo, D.L. Bätzner, H. Zogg and A.N. Tiwari,” Mater. Res., vol. 668, pp. 1–6, 2001.
Table I: Power/weight ratio of different materials Sample Eff., Power Weight Power/ % for of 1m2, g weight, 1m2 Watt/gram UPILEX 10 100 50. 4 1.98 KAPTON 11.2 112 82.4 1.36 Ultra-thin 12.2 122 262.4 0.465 glass Glass 14.7 147 9227 0.015 Glass 21.5 215 9227 0.023 Device weight is very important for space applications, where CdTe solar cells have strong potential [8].
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