Impact of thickness variation of the ZnO:Al window layer on optoelectronic properties of CIGSSe solar cells
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Impact of thickness variation of the ZnO:Al window layer on optoelectronic properties of CIGSSe solar cells Jan Keller1, Martin Knipper1, Jürgen Parisi1, Ingo Riedel1, Thomas Dalibor2, Alejandro Avellan2 1 Energy and Semiconductor Research Laboratory, D-26111 Oldenburg, Germany 2 AVANCIS GmbH & Co. KG, Otto-Hahn-Ring 6, D-81739 Munich, Germany ABSTRACT We studied the thickness variation of equally doped ZnO:Al films used as conductive window layer in Cu(In,Ga)(Se,S)2 (CIGSSe) thin film solar cells. The IV-characteristics of solar cells with window layer thickness of d1=200nm exhibit a strong enhancement of the short-circuit current density JSC (ǻJSC = 3mA/cm2) as compared to samples with module-like ZnO:Al-film thickness (d2=1200nm). Accordingly, the quantum efficiency reveals the spectral regimes where the JSC-gain occurs. Moreover, current-voltage measurements reveal that the cells with thicker ZnO:Al exhibit slightly decreased open circuit voltage VOC. This finding can be assigned to a decreased net-doping density NA, which appears to be introduced by additional heat flux during the longer process time required for deposition of thicker ZnO:Al films. However, the improved efficiency of solar cells with thinner window layer comes along with an increase of the series resistance (RS) by almost a factor of 2, which will have consequences for the series connection of elements in a module. XRD-diffractograms and SEM cross-section imaging suggest that the enhanced RS in cells with thin ZnO:Al is not exclusively related to the thickness but is also due to a reduced (002)-texture and an elongated lateral charge carrier pathway. INTRODUCTION Predicting low production-costs and competitive module efficiencies, thin film technologies enter the photovoltaic market and gain increasing attention. Besides CdTe and thin film silicon, chalcopyrite-based concepts, i.e. Cu(In1-xGax)(Sy,Se1-y)2, are most promising to compete with wafer technologies. The highest lab-scale efficiency reported for co-evaporated CIGSe exceeds 20% [1]. Being likely for thin film technologies, the performance transfer to large areas appears to be difficult as reflected by the reduced module efficiencies (Lj=13-14% champion modules [1-2]). One approach to enhance the module performance is to modify the front-contact, usually made of transparent conducting oxides (TCO). The main requirements for the TCO are a good lateral conductivity and a high optical transmission in the range where the CIGSSe absorbs. The first issue is realized via heavy doping and employment of thick films to achieve sufficiently low sheet resistance. In contrast, the TCO should be thin in order to achieve a high transmission in short wavelength range whereas low doping density is expected to reduce parasitic absorption by free carrier absorption (FCA). Furthermore, reflection losses can be avoided by surface-texturing, for instance by using nanorod-shaped arrays [3] or applying a surface-etching [4]. Most commonly, aluminum-doped ZnO (ZnO:Al) is used as a low-cost TCO in inorganic solar
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