Characterisation of CuInS2 Films for Solar Cell Applications by Raman Spectroscopy

Abstract

[eng] CuInS2 is a good candidate as photovoltaic absorber for the fabrication of high efficient and low cost solar cells. In this work, the vibrational properties of CuInS2 films have been studied by means of Raman Spectroscopy, and other techniques such as XRD, TED, SEM and AES. Moreover, the CuInS2 absorber films used for the fabrication of solar cell devices have been characterised, and the sulphurisation reaction of the metallic precursors for the preparation of baseline absorbers investigated. Epitaxial and polycrystalline films were grown by a coevaporation process in a vacuum chamber containing sulphur vapour. The structural analysis of the epitaxial samples revealed that in these films two different crystallographic orderings coexist. These two polytypes of the CuInS2 compound correspond to the chalcopyrite equilibrium phase, and the "Cu-Au" metastable phase. It was found, the later to be responsible for the appearance of characteristic bands in the Raman spectra at 305 cm-1 and 60 cm-1, which had not been identified before this work. This assignment is based in both experimental results and theoretical calculations of the phonon spectra of both polytypes performed using a VFF model. On the other hand, polycrystalline films grown at different conditions were prepared in order to investigate the influence of the growth parameters upon the structural properties of the films. Cu excess conditions and growth temperatures above 470ºC lead to good quality absorbers. Moreover, the Cu excess segregates at the surface of the film as CuS, and can be easily removed by a chemical treatment in KCN. The good quality of the CuInS2 films obtained by this recipe and the wide window process (Cu/In between 1.2 and 1.8) justify this additional step for the preparation of CuInS2 films. This constitutes an important advantage with respect to the processing of CuInSe2 absorbers, for which the effect of the stoichiometric deviations is critical. The efficiency of the cells fabricated from absorbers grown under these conditions is in the range of 12%. On the contrary, films grown under Cu poor stoichiometry and/or lower temperatures present very bad crystalline quality, and lead to photovoltaic devices with very low efficiencies. It was proved that these conditions favour the formation of Cu-Au ordered domains in the films, thus deteriorating the crystalline quality of the chalcopyrite phase. Furthermore, Cu-poor conditions favour the formation of precipitates of CuIn5S8 at the bulk of the films. Baseline sequentially processed films are obtained by the sulphurisation reaction of the metallic precursors, which are previously sputtered onto the Mo-coated glass substrate. Cu excess conditions and substrate temperatures around 500ºC are selected for the growth of these baseline absorbers. The crystalline quality of these films is similar to that of the coevaporated films grown under Cu excess conditions. Furthermore, the effect of the incorporation of Ga in the metallic precursors was also studied. The aim of introducing Ga is to increase the VOC of the final device. However, the sulphurisation of the Ga-containing precursors leads to films presenting a bilayer structure, characterised by the existence of an In rich Cu(In,Ga)S2 quaternary at the surface region of the sample, and a Ga rich Cu(In,Ga)S2 quaternary at the back region of the film. Although the homogeneity of the films could be improved by using higher sulphurisation temperatures and a Rapid Thermal Process in H2S, it was not possible to avoid the accumulation of Ga close to back region of the films. The explanation for the structure of these films was provided by the analysis of samples quenched at different stages of the sulphurisation reaction, which allow establishing the fundamental steps of the sulphurisation reaction of the metallic layers.