Post-growth annealing effect on the performance of Cu 2 ZnSnSe 4 monograin layer solar cells M. Kauk-Kuusik ⁎, M. Altosaar, K. Muska, M. Pilvet, J. Raudoja, K. Timmo, T. Varema, M. Grossberg, E. Mellikov, O. Volobujeva Tallinn University of Technology, Ehitajate tee 5, Tallinn 19086, Estonia abstract article info Available online 6 December 2012 Keywords: Cu2ZnSnSe4 Annealing Solar cell In this work, we investigated the effect of annealing of absorber powder on the conversion efficiency of Cu 2 ZnSnSe 4 (CZTSe) monograin layer solar cells. CZTSe powders were synthesized from binary compounds and elemental Se in the liquid phase of KI in evacuated quartz ampoules at 740 °C. In order to study the effect of post-treatments of the absorber material different annealing parameters such as Se and/or SnSe 2 vapor pressure, annealing time and temperature were varied with the aim to gain uniform, good quality absorber materials for monograin layer solar cells. The annealing temperature was found to be crucial for the perfor- mance of CZTSe monograin layer solar cells. The conversion efficiency of solar cells improved significantly after the heat treatment. The effect can be attributed to the change of the absorber material composition and the crystals surface properties suitable for the effective p–n junction formation. The best CZTSe monograin layer solar cell showed conversation efficiency of 4.4%. © 2012 Elsevier B.V. All rights reserved. 1. Introduction The Cu 2 ZnSnSe 4 (CZTSe) compound has attracted a considerable attention as an alternative absorber in thin film solar cells because CZTSe is a p-type semiconductor with a band gap of 1.0 eV and has an absorption coefficient that is larger than 10 4 cm -1 , which matches the prerequisites for a solar absorber material [1]. Recently, the record efficiency of liquid-processed CZTSe solar cell yielded 10.1% [2]. However, although device performance was greatly improved, basic researches on CZTSe material itself are insufficient. For example, fabrication of compositionally uniform CZTSe film is still a hard task due to Sn-loss during annealing process. Usually, a high temperature annealing in chalcogen atmosphere is essential to form a well-crystallized and single phase CZTSe. It is well known that CZTSe quickly decomposes at temperatures higher than 400 °C [3–5]. The decomposition rate, however, depends drastically on the experimental conditions such as temperature, total pressure inside the annealing container and partial pressures of all involved volatile species. As monograin powder synthesis is an isothermal process, the Sn loss in the synthesis process is not a problem. The problem rises in the post-treatment step. In our previous studies [6], it was shown that due to the distribution of material between the liquid flux and the solid CZTSe crystals during the synthesis process, some part of material is dissolved in molten flux at growth temperature. In the cooling period, the dissolved part precipitates on the solid crystal surfaces. Therefore, as-grown monograins need some chemi- cal etching and annealing before the formation of p–n junction. It has been found that Br 2 –methanol treatment followed by immersion in an aqueous solution of KCN [7] and annealing in SnSe 2 or Se vapors heals crystals' surfaces to the device quality. Considering that high efficiency CZTSe solar cell can be realized just with Cu-poor and Zn-rich CZTSe [8], the proper adjustment of chemical composition of CZTSe is a prerequisite for well working solar cells. Hence, systematic annealing experiments under controlled temperature and atmosphere come to be significant. Here, we report the effect of post-growth annealing and sequen- tial etching of CZTSe monograin absorbers with different composi- tions on the performance of CZTSe monograin layer (MGL) solar cells. 2. Experimental details The CZTSe absorber materials, used for MGL solar cells in this study were synthesized by isothermal recrystallization method in molten flux. The details about monograin growth process could be found in [9]. After the removal of flux, the post-treatments were carried out in closed quartz ampoules using two-temperature zone arrangement. Elemental Se or SnSe 2 pellets were placed into the lower temperature zone of the ampoules. CZTSe powder was heated in the higher tem- perature zone. The temperatures of both zones were regulated and controlled independently. The lowest temperature in the ampoule determined the vapor pressure of Se or SnSe 2 . After annealing, the ampoules were taken out of the furnace and cooled down on a ceramic plate at room temperature. Thin Solid Films 535 (2013) 18–21 ⁎ Corresponding author. Tel.: +372 6203362. E-mail address: marit.kauk@ttu.ee (M. Kauk-Kuusik). 0040-6090/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.tsf.2012.11.075 Contents lists available at SciVerse ScienceDirect Thin Solid Films journal homepage: www.elsevier.com/locate/tsf