Avoiding Fill Factor Losses in Cu(In,Ga)Se₂ as a Bottom Cell in a Tandem Structure

ABSTRACT This work aims to define the optimization criteria for Cu(In,Ga)Se 2 (CIGS) as a bottom cell in a tandem structure, and to emphasize the differences from optimizing the CIGS when operating alone. Reproducing the single‐cell recipes and only lowering the band gap is insufficient to optimize the bottom cell. We identified that the lack of high‐energy photons, which are absorbed by the top cell, can cause a severe fill factor (FF) loss, and thus diminish the photovoltaic performance. With nonoptimized buffer layers (CdS and ZnMgO), S‐shaped current‐density‐voltage (JV) characteristics leading to a low FF and poor performance can be observed. The S shape can be eliminated within seconds of white‐light exposure and does not return for hours. Therefore, this does not pose a significant problem for single‐cell operation. In the bottom‐cell application, as only the low‐energy part of the spectrum is available, the properties of the buffer layer(s) become crucial and additional optimization is necessary. Filtered JV measurements after white‐light exposure could lead to overseeing important optimization steps. We discuss the causes for an S‐shaped curve under filtered illumination, pinpoint the bottlenecks in the bottom‐cell performance, and present a way to mitigate the losses.