Methylammonium-free wide-bandgap metal halide perovskites for tandem photovoltaics

Over the past decade, the performance of solar cells based on metal halide perovskite semiconductors has skyrocketed, now rivalling established technologies such as crystalline silicon. The most promising implementation of these perovskites is in tandem solar cells, which have delivered far higher power conversion efficiencies than single-junction devices alone. Given their tunable bandgap, perovskites are uniquely placed to act as both narrow-bandgap and wide-bandgap absorbers in tandem devices. Methylammonium-free wide-bandgap perovskites perform particularly poorly in tandem devices compared with their methylammonium-containing and narrower bandgap counterparts, illustrating considerable scope for improvement. In this Review, we highlight the challenges related to methylammonium-free perovskites, including the energy-loss pathways that currently constrain their open-circuit voltage and efficiency well below their thermodynamic limits. We discuss recent progress in their material development and performance in tandem photovoltaics, and we highlight research trends that seem particularly promising. Finally, we suggest future avenues to expedite the development of wide-bandgap perovskites and, in turn, the deployment of tandem solar cells based on these materials.