Optical and electrical properties of In-doped Cu2ZnSnSe4

• Morphological, compositional, optical and electrical properties of In-doped CZTSe.
• In is effectively introduced into CZTSe layer at doping level.
• High In concentrations reduce the RSH and F.F. of solar cell devices.
• We present a phenomenological model explaining the grain boundaries deterioration.

Cu2ZnSnSe4 (CZTSe) is a very promising material as absorber layer for low cost and earth abundant thin film solar cells. Currently in this technology we can find indium in different layers, including the transparent window layer In2O3:SnO2 (ITO), and the In2S3/CdS double emitter, both used in high efficiency devices. Therefore, during devices fabrication processes, In could be a potential contaminant and consequently, it is very interesting to investigate its possible impact in the solar cells performance. Besides this, a key factor in the control of material properties lies in the doping. Extrinsic doping has been barely studied for CZTSe and among the possible doping elements, In is one of the most interesting candidates, because it has the possibility to occupy either Sn or Cu/Zn positions.In this work we investigate the indium doping of CZTSe thin films. For this purpose, CZTSe was synthesized by a sequential process with different nominal In concentrations ranging from 0 to 2.6×1020 cm−3. We demonstrate that In is uniformly introduced in CZTSe, not affecting the main elements distribution, but impacting in the Na quantity at the surface. Drastic changes on the morphology are observed, where the increasing indium concentration leads to the formation of a bilayer structure. Efficiencies in the range of 7–7.5% or 8.5–9.2% were obtained for In concentrations below 2.6×1019 cm−3 for pure CZTSe and CZTSe:Ge respectively, decreasing for further doping levels mainly due to the deterioration of the fill factor, while the other optoelectronic parameters are less affected. We propose a phenomenological model supported by the complete electrical characterization of the material and devices, showing that a conductive phase deteriorates the properties of the system that we associate to the possible presence of mixed Sn-oxides and In-oxides.

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