Towards Thermally Stable Wide Bandgap Perovskites by Vacuum Deposition Methods

Wide bandgap perovskites are very relevant semiconductors in view of their potential for applications in tandem devices, combined with narrow bandgap absorbers such as silicon, CIGS, or a complementary perovskite. Vacuum deposition methods are increasingly applied to the preparation of perovskite films and devices, in view of the possibility to prepare multilayer structures, common to all tandem architectures. However, vacuum-deposited, wide-bandgap solar cells based on mixed-cation and mixed-anion perovskites have been scarcely reported. Here we present multi-component wide bandgap perovskites obtained by using several thermal sources in co-sublimation processes. We review processes to deposit material formulations of increasing complexity, from double to triple cation/mixed halide perovskites. Homogeneous films with bandgap up to 1.8 eV can be readily obtained, with performance on par with similar solution-processed materials. Apart from efficiency, we focus on the development of thermally stable perovskite films and devices, that can be obtained using even more complex stoichiometry. By adding guanidinium (GA) to the material formulation, we develop CsMAFAGA quadruple-cation perovskite solar cells with enhanced thermal stability. In spite of the benefit (efficiency, thermal stability) of such complex formulations, their vacuum processing can be challenging, because one needs to simultaneously control several thermal sources during the deposition. Hence we show a simplified dual-source vacuum deposition method to obtain wide bandgap perovskite film and solar cells, with similar or even larger efficiency as those including multiple A-cations. Vacuum deposited MA-based perovskites are highly thermally stable, with lifetime up to 3500 hours at 85 °C, and record efficiency >19% for bandgap of 1.64 eV.