Current Transport in CuInS 2 Solar Cells Depending on Absorber Preparation
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Current Transport in CuInS2 Solar Cells Depending on Absorber Preparation Joachim Reiß, Jonas Malmström1, Axel Werner, Imke Hengel, Reiner Klenk and Martha Ch. Lux-Steiner Hahn-Meitner-Institut Berlin GmbH, Glienicker Strasse 100, D-14109 Berlin, Germany 1 Ångström Solar Center, Uppsala University, P.O. Box 534, SE-75121 Uppsala, Sweden ABSTRACT According to our model for the current transport in CuInS2/CdS/ZnO solar cells presented previously, the dominant recombination under illumination occurs at the CuInS2/CdS interface. In this contribution we expand this model for cells where the absorber has been prepared in different sequential processes by using results from jV curve measurements performed at varied temperature and illumination. We find that in contrast to CuInSe2 and CuGaSe2 solar cells from the Ångström Solar Center the CuInS2 cells exhibit a qualitative change in the dominant recombination mechanism between the dark and under illumination. The dominant recombination mechanism under illumination appears to be affected by absorber preparation parameters. INTRODUCTION Solar cells with CuInS2 as absorber material have reached total area efficiencies up to 11.4 % [1]. The efficiencies of these devices appear for the most part to be limited by the open circuit voltage Voc. In view of the absorber band gap Eg of 1.5 eV, open circuit voltages of about 750 mV are moderate values. An evaluation of the dominating recombination mechanism is necessary for a better understanding of the difference to the theoretically achievable value [2]. Analysis of the current voltage characteristics as a function of temperature (j(V,T)) and the illumination intensity is a first step in this direction. Results for a specific type of CuInS2 solar cells have already been published [3]. In this contribution we present a more general analysis by comparing CuInS2 solar cells from different preparation methods with state of the art CuInSe2 and CuGaSe2 heterojunctions. EXPERIMENTAL CuInS2/CdS/ZnO solar cells have been prepared by sequential processes, where the Cu/In precursors have been evaporated [4] or sputtered [5] onto a molybdenum back contact. The sulphurization has been performed in sulphur vapor either by rapid thermal processing (RTP-S) [1] or conventionally (CTP) [5], or in a H2S containing atmosphere by a rapid thermal process (RTP-H2S) [6]. The secondary CuxS phase used to promote grain growth has been removed by cyanide etching. The heterojunction is completed by a chemical bath deposited CdS buffer layer and a sputtered two-layer ZnO window [4]. Additionally, for silver incorporation into RTP-S cells, silver has been thermally evaporated onto the back contact before the precursors have been sputtered [7]. CuInSe2 and CuGaSe2 absorbers have been coevaporated onto Mo-coated glass substrates and have been processed to solar cells with the baseline recipe of the Ångström Solar Center [8]. The j(V,T) measurements have been performed in either an evacuated closed-cycle Hecryostat or a N2 cooled cryostat using a source measure unit
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