Low Temperature Growth of CIGS Thin Films for Flexible Solar Cells
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Low Temperature Growth of CIGS Thin Films for Flexible Solar Cells D. Rudmann, F.-J. Haug, M. Kaelin, H. Zogg, A.N. Tiwari* and G. Bilger1 Laboratory for Solid State Physics, Thin Film Physics Group, ETH Zürich, Technopark, Technoparkstr. 1, CH- 8005 Zürich, Switzerland. 1 Institute of Physical Electronics, University of Stuttgart, Pfaffenwaldring 47, D-70569 Stuttgart, Germany. * Corresponding author ABSTRACT Low substrate temperatures have to be used for polymer substrates. Therefore, using sodalime glass (SLG) substrates with and without an alkali barrier (Al2O3), a three-step CIGS coevaporation process for a substrate temperature of 450 °C has been developed and compared to film deposition with constant evaporation rates. The three-step process was found to enhance grain nucleation. An efficiency of 14.0 % has been achieved with this process for solar cells on SLG. Since polymers in general do not contain Na, a way of Na addition to the absorber is needed. It is shown that NaF coevaporation can be used to control the Na content in CIGS. Also incorporation of Na in CIGS by diffusion from a NaCl layer through a polyimide is demonstrated. With such SLG/NaCl/polyimide structures flexible solar cells can be obtained using a lift-off process. A cell efficiency of 11.6 % (0.99 cm2 area) has been achieved.
INTRODUCTION High efficiency Cu(In,Ga)Se2 (CIGS) solar cells require absorber deposition using coevaporation methods at substrate temperatures around 550 – 600 °C and soda-lime glass (SLG) as substrates. Polymer substrates can be used for flexible and lightweight solar cells or modules, but they generally do neither withstand such high temperatures nor contain alkali metals. Incorporation of Na into the absorber is known to have several beneficial effects on the properties of absorber layers and on solar cell performance, such as enhanced grain growth and orientation, better film conductivity, enhanced efficiencies. Thus, for high efficiency cells on polymers the control of the Na content in CIGS is needed and absorber deposition at low temperatures has to be optimised. For CIGS layers grown by coevaporation methods on Na-free substrates, Na is often incorporated by deposition of a Na-containing precursor layer of typically 20 - 50 nm thickness, deposited onto the Mo layer prior to CIGS deposition. Among the used compounds are NaF [1, 2, 3], Na2Se [4, 5], Na2S [4] and NaxO [6]. Since all these compounds are hygroscopic, peel-off problems in CBD are encountered when the precursor layer is too thick to be completely incorporated into the CIGS film. An alternative is coevaporation of the Na compound during the CIGS deposition process. To our knowledge, coevaporation processes have up to now only been reported for Na2Se [7, 8], Na2S [8] and metallic Na [9]. In this paper we present a low-temperature CIGS deposition process (substrate temperature approximately 450 °C) which is compared to growth with constant evaporation rates. NaF coevaporation is shown to be a technique to control Na addition and effects of different Na
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