We report the development of Cd-free buffers by atomic layer deposition for chalcopyrite-based solar cells. Zn(O,S) buffer layers were prepared by atomic layer deposition on sequentially grown Cu(In,Ga)(Se,S) 2 absorbers from Bosch Solar CISTech GmbH. An externally certified efficiency of 16.1% together with an open circuit voltage of 612 mV were achieved on laboratory scale devices. Stability tests show that the behavior of the ALD-Zn(O,S)-buffered devices can be characterized as stable only showing a minor drift of the open circuit voltage and the fill factor.

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In this paper, the impact of the back contact barrier on the performance of Cu (In, Ga) Se2 solar cells is addressed. This effect is clearly visible at lower temperatures, but it also influences the fundamental parameters of a solar cell, such as open-circuit voltage, fill factor and the efficiency at normal operation conditions. A phototransistor model was proposed in previous works and could satisfactorily explain specific effects associated with the back contact barrier, such as the dependence of the saturated current in the forward bias on the illumination level. The effect of this contribution is also studied in this research in the context of metastable parameter drift, typical for Cu (In, Ga) Se2 thin-film solar cells, as a consequence of different bias or light soaking treatments under high-temperature conditions. The impact of the back contact barrier on Cu (In, Ga) Se2 thin-film solar cells is analyzed based on experimental measurements as well as numerical simulations with Technology Computer-Aided Design (TCAD). A barrier-lowering model for the molybdenum/Cu (In, Ga) Se2 Schottky interface was proposed to reach a better agreement between the simulations and the experimental results. Thus, in this work, the phototransistor behavior is discussed further in the context of metastabilities supported by numerical simulations.

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In this contribution, metastabilities of Cu(In,Ga)Se2 (CIGS)/CdS thin films are discussed based on photoluminescence (PL) imaging and complementary vibrating Kelvin probe (VKP) measurements. It has been shown that thermal treatment of CIGS/CdS thin films leads to the PL intensity quenching and a change in a surface band‐bending as deduced from VKP measurements under illumination. The impact of white light soaking has the opposite effect as compared to heat treatment: elevated temperatures enhance surface band‐bending whereas illumination leads to at least a partial band‐flattening. Furthermore, it has been found that etching away the CdS buffer layer reduces the previously measured band‐bending significantly pointing out the importance of the buffer layer deposition process. Moreover, the investigation of P1 scribes revealed that surface potentials in the vicinity of the patterning lines differ from the surface potentials measured on cells what is also reflected in the PL images as differences in the intensity between the bulk of the cells and P1 scribes. This observation suggests that the influence of P1 scribing parameters on electrical characteristics of CIGS solar cells has to be studied additionally as they can significantly affect the overall efficiency of the cell.