Study of Kesterite phase formation by selenization of electrodeposited Cu-Sn-Zn thin films

The process of electrodeposition of stacked metal layers followed by chalcogenization to form Cu(In,Ga)(S,Se)2 (CIGSSe) absorbers for photovoltaics is emerging as an attractive industrial process, with reported solar cell power conversion efficiencies as high as 16% [1]. Similarly, the synthesis of Cu2ZnSn(S,Se)4 (CZTSSe) is currently being investigated using electrodeposition of Cu/Sn/Zn metallic stacks and selenization/sulfurization. This process already reached efficiencies up to 7.3% (total cell area) [2] and 8.0 % (active area) [3]. The detrimental effects of secondary phases in the CZTSe system on device performance are known in the literature [4]. For example, the presence of ZnSe secondary phase at the interface between CZTSe and CdS has been related to a current blocking phenomenon [5]. Our group has demonstrated that this phenomenon is directly related to a reduction of the device short circuit current [6]. In this work, compositional optimizations aimed at reducing the amount of ZnSe secondary phase have allowed the achievement of a 6% efficiency (total area) solar cell. However, in order to improve the solar cell device performance even further, it is crucial to better understand when and where the Kesterite and the secondary phases appear in the films during annealing. By understanding the phase formation, routes may be designed to maximize kesterite formation, and to minimize secondary phase formation. To this end, an investigation of the mechanism and kinetics of Kesterite phase formation was realized. This is based on the phase evolution monitoring of part of the time/temperature field, established by ex-situ analysis of the films after annealing in the presence of Se and SnSe powders. A rapid thermal processing (RTP) furnace is employed for the study, thus allowing fast ramps of heating (11°C/s) and cooling (0.45°C/s). The film in-depth and surface phase composition was assessed by Secondary ion mass spectrometry (SIMS) depth profile, X-ray diffraction (XRD), Energy dispersive X-ray spectroscopy (EDX) and Raman spectroscopy. Two types of precursors were employed with and without a selenium surface layer deposited by chemical bath deposition, in order to gain better insights into the role of the selenium location on the phase evolution. After just 30 s at 550°C the film obtained by processing the Se-free precursor is composed of ZnSe, CuxSe and CZTSe. EDX analysis shows a strong loss of Sn after 30s annealing, which explains the presence of secondary phases, followed by an increase of Sn for longer annealing times. This phenomenon can be explained by the loss of SnSe at early annealing stages, and then reincorporation of Sn coming from SnSe powder to form CZTSe [7]: although SnSe solid is added to the annealing chamber, a certain time is required before enough partial pressure of SnSe is established. In order to observe the first steps of Kesterite formation, the phase evolution was monitored at lower processing time (400 °C). The results show that even at 400 °C the Kesterite formation is relatively fast. After 1s, Kesterite is present at the surface together with ZnSe, while the rest of the layer is still composed of alloys of Cu, Sn, Zn. The precursor film containing the selenium surface layer displays a meaningful increase of the Kesterite formation rate. Consequently, using precursors with a selenium cap reduces the thermal annealing budget. This work allows a better understanding of the mechanism and kinetics of formation of Kesterite and secondary phases, which is essential to optimize the annealing process and decrease as much as possible the presence of secondary phases in the final Kesterite absorber films. Acknowledgements: The research leading to these results has received funding from the European Union’s Seventh Framework Programme FP7/2007-2013 under grant agreement nº 284486. [1] Bermudez, V. in 5th International Workshop on CIGS Solar Cell Technology. 2014. Berlin. [2] Ahmed, S., et al., Advanced Energy Materials, 2011. 2(2): p. 253-259. [3] Jiang, F., et al., Advanced Energy Materials, 4 (2013) p. n/a-n/a. [4] S. Siebentritt, Thin Solid Films, 535 (2013) 1-4. [5] J.T. Wätjen, et al., Applied Physics Letters, 100 (2012) 173510-173511 173510-173513. [6] D. Colombara, et al., Solar Energy Materials and Solar Cells, 123 (2014) 220-227. [7] A. Redinger, et al., Journal of the American Chemical Society, 133 (2011) 3320-3323.