Efficient hybrid mixed-ion perovskite photovoltaics: in situ diagnostics of the roles of cesium and potassium alkali cation addition
Ming Chun Tang, Yuanyuan Fan, Dounya Barrit, Ruipeng Li, Hoang X. Dang, Siyuan Zhang, Timothy J. Magnanelli, Nhan V. Nguyen, Edwin J. Heilweil, Christina A. Hacker, Detlet-M Smilgies, Kui Zhao, Aram Amassian, Thomas D. Anthopoulos
Perovskite photovoltaics have made extraordinary progress in efficiency and stability in the past few years owing to process and formulation developments like antisolvent drip and mixed-cation mixed-halide compositions. Perovskite solar cells performance benefits tremendously from the addition of alkali metal cations, including cesium (Cs+) and potassium (K+), but root reasons are not fully understood yet, which hinders further improvement and processing approaches remain primarily empirical. In this study, we compare the solidification process from 5, 10, to 20% of Cs+- and K+-incorporated perovskite thin films during spin coating using in situ grazing incidence wide-angle X-ray scattering (GIWAXS) method. We found all K+-doped inks tend to form non-perovskite 4H phases from disordered solvate without desired 3C perovskite crystals formation. Besides, for Cs+-doped formulations, 4H and 3C phases are dominant microstructures in 5% Cs+ case, while desired 3C perovskite phase spontaneously dominates in 10% Cs+ ink and perovskite halide segregation occur in 20% Cs+ formulation. The intermediate 4H phases were suppressed and desirable 3C perovskite phase was promoted via thermal annealing. A small amount of Cs+ or K+ in perovskite structure remediates trap states, enhances carrier mobility, and increases carrier lifetime from optoelectronic measurements. Therefore, through control of ink formulation, we demonstrate perovskite solar cells with an average power conversion efficiency of low metallic cation concentration (5% of Cs+ or K+) are ≈5% higher than the high concentration ones (20%). This study provides valuable insights into the crystallization pathway that aims at perovskite phase formation, ultimately resulting in efficient perovskite photovoltaics produced with ease and high reproducibility.