This paper presents optimization studies on the formation of cadmium free buffer layers for high efficiency copper indium diselenide (CIGS) thin film solar cells using a vapor phase route. Indium sulfide layers have been deposited on CIGS substrates by Atomic Layer Deposition (ALD) at substrate temperatures between 140 and 260 °C using indium acetylacetonate and hydrogen sulfide precursors. The parametric study of the deposition temperature shows an optimal value at about 220°C, leading to an efficiency of 16.4 % which is a technological breakthrough. The analysis of the device shows that indium sulfide layers give an improvement of the blue response of the cells as compared a standard CdS processed cell, due to a high apparent band gap (2.7-2.8 eV), higher open circuit voltages (up to 665 mV) and fill factor (78 %) . This denotes high interface quality of the system. Atomic diffusion processes of sodium and copper in the buffer layer are evidenced.
Chemistry of co-evaporated CIGS surfaces submitted to chemical treatments relevant to fabrication steps were investigated by XPS and admittance spectroscopy. A Se XPS signal specific of the CIGS surfaces was identified. Surface states seen by Admittance and surface chemistry are seen to change significantly during the elaboration steps. Consequences for device elaboration are briefly discussed.
Native point defects in undoped samples of CuInSe2 (CIS) have been identified by a multiple-edge refinement of the extended x-ray absorption fine structure of the copper, indium and selenium absorption edges. Ab initio theoretical models for the pure compound and for various defect structures were constructed and carrier-type statistics were predicted by simultaneously fitting multiple absorption sites to these models. As expected, a model of our measurements based on pure compounds with no defects does not yield a good fit to the data. We find that a best fit requires a significant population of defects. Preliminary quantitative analysis suggests a 15 % vacancy in Cu, a 2-12% population of Cu-Se anti-sites and 15% In-Se anti-sites.
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