The Bottlenecks of Cs₂AgBiBr₆ Solar Cells: How Contacts and Slow Transients Limit the Performance

Cs₂AgBiBr₆ has attracted much interest as a potential lead-free alternative for perovskite solar cells. Although this material offers encouraging optoelectronic features, severe bottlenecks limit the performance of the resulting solar cells to a power conversion efficiency of below 3%. Here, the performance-limiting factors of this material are investigated in full solar cells featuring various architectures. It is found that the photovoltaic parameters of Cs₂AgBiBr₆-based solar cells strongly depend on the scan speed of the J/V measurements, suggesting a strong impact of ionic conductivity in the material. Moreover, a sign change of the photocurrent for bias voltages above 0.9 V during the measurement of the external quantum efficiency (EQE) is revealed, which can be explained by non-selective contacts. The radiative loss of the V_OC from sensitive subgap-EQE measurements is calculated and it is revealed that the loss is caused by a low external luminescence yield and therefore a high non-radiative recombination, supported by the first report of a strongly red shifted electroluminescence signal between 800 and 1000 nm. Altogether, these results point to a poor selectivity of the contacts and charge transport layers, caused by poor energy level alignment that can be overcome by optimizing the architecture of the solar cell.