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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The aim of this work is to provide an insight into the impact of the P1 shunt on the performance of ZnO/CdS/Cu(In,Ga)Se2/Mo modules with monolithic interconnects. The P1 scribe is a pattern that separates the back contact of two adjacent cells and is filled with Cu(In,Ga)Se2 (CIGS). This scribe introduces a shunt that can affect significantly the behavior of the device, especially under weak light conditions. Based on 2D numerical simulations performed with TCAD, we postulate a mechanism that affects the current flow through the P1 shunt. This mechanism is similar to that of a junction field effect transistor device with a p-type channel, in which the current flow can be modulated by varying the thickness of the channel and the doping concentration. The results of these simulations suggest that expanding the space charge region (SCR) into P1 reduces the shunt conductance in this path significantly, thus decreasing the current flow through it. The presented simulations demonstrate that two fabrication parameters have a direct influence on the extension of the SCR, which are the thickness of the absorber layer and its acceptor concentration.