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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.
State of the art CIGS thin films have been studied by means of the semiconductorelectrolyte junction. They appear as chemically robust allowing to use aggressive electrolyte compositions as for instance more acidic pH, down to 0 in sulfuric acid environment. In these electrolytes, reliable capacitance and photo-current related characterization techniques have been used. It has been shown that a short treatment in a gold (III) solution can facilitate the characterizations, and stabilize the surface composition. These results tend towards settling a standardized electrochemical testing procedure for CIGS layers
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.
The control of the barrier height at the Cu(In,Ga)Se2(CIGS)/CdS interface, via the chemical state of the CIGS surface, is a key to improve solar cell performances. In this paper chemical modifications have been achieved by the electrochemical method, by varying both the pH of the solution and the applied potential. It is shown that the potential for electrochemical oxidation depends on the pH, in good agreement with the thermodynamic predictions. The resulting surface composition of CIGS, analyzed by XPS, also depends on the pH. Contact Potential Differences (CPD) and Surface Photovoltage (SPV) measurements show that the electronic properties of the CIGS surface are modified. The important result is that the oxidation leads to the formation of permanent positive surface charges, which increase the band bending up to 0.20 V in a direction favorable for the solar cell performances.
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