Impact of indium sulphide based buffer layers on the electrical properties of CIGSSe thin film solar cells
Thin film solar cells on CIGSSe basis require the utilization of a buffer layer between the absorber and the front electrode. Besides the task of the formation of the pn-hetero-junction as well as the electrical and optical alignment between the neighbouring layers, the buffer protects the absorber during subsequent process steps. Commonly, this is accomplished by a wet chemical deposition of CdS onto the absorber. However, the inherent toxicity and the process management diminish the attractiveness of this compound for a commercial application. A promising alternative to CdS is the utilization of In2S3. The present work analyses the structural, chemical and optical properties of InxSy buffer layers as a function of their S/In ratio. The observed changes are associated to the electrical properties of solar cells with the help of the respective diode characteristics and discussed, subsequently. It is shown, that the optimal efficiency is reached at an In-content of about 42 at%. Any deviation from this composition results in a degradation of the efficiency towards both, In- and S-richer films. By evaluating the secondary parameters, a model is developed that explains the observed behaviour, ascribing it to the band alignment at the absorber-buffer-interface. An optimal composition is connected to a slight type inversion of the absorber’s surface. Due to a more pronounced crystallinity for S-rich layers, an increased Cu-diffusion from the absorber is induced, which reduces the charge carrier density inside the buffer. This leads to a loss of the inversion and deteriorates the electrical properties of the solar cells because of enhanced interface recombination. For higher In-contents inside the buffer the proportion of In6S7 and InS phases increases, which also reduces the effective charge carrier density and diminishes the inversion. Since the utilization of a pure InxSy buffer leads to a significant discrepancy in the achievable efficiency compared to CdS, a doping process for the buffer layer has been developed in this work, using Na2S. The structural, optical and electrical properties of the improved buffer allow a considerable increase of the absorber’s type-inversion, enabling a complete closure of the observed efficiency gap on a level of up to 14.9 % for a 10x10 cm² module.