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

This work aims to define the optimization criteria for Cu(In,Ga)Se₂ (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.