In this study, the back contact in Cu(In,Ga)(S,Se)2 (CIGS) solar cell was investigated as the origin of the rollover effect, which is characterized as a distortion of current-voltage (JV) characteristics under illumination at forward bias with saturating current at higher voltage. Presence of Schottky barrier between CIGS and Mo(S,Se)2/Mo back contact in the solar cell with rollover JV was elucidated by the composition and the valence band energy measured on the CIGS backside exposed by exfoliation. The device simulation using the obtained values confirmed that the barrier at the back contact is responsible for the rollover. Furthermore, the impedance analysis clearly showed the existence of the back diode in the equivalent circuit of the device. The circuit simulation illuminated that the rollover is described as a distortion of JV curve due to the voltage distribution between the pn junction diode and the back diode. By using JV characteristics of the back contact, a potential JV curve intrinsic to the pn junction was derived. As a result, the cell with rollover JV was found to have similar or even higher potential performance as compared with the one without rollover. It was also found that although the rollover was less apparent in the JV curve, the device possibly holds a performance loss due to the back contact barrier.
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