The Bottlenecks of Cs<sub>2</sub>AgBiBr<sub>6</sub> Solar Cells: How Contacts and Slow Transients Limit the Performance
Maximilian T. Sirtl, Firouzeh Ebadi, Bas T. van Gorkom, Patrick Ganswindt, René A. J. Janssen, Thomas Bein, Wolfgang Tress
Abstract
Abstract Cs 2 AgBiBr 6 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 Cs 2 AgBiBr 6 ‐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.