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The organic-inorganic hybrid perovskite solar cells have achieved power conversion efficiency on par with that of commercial silicon solar cells. The main challenge towards commercial application is its instability. Quasi-2D perovskite has higher stability and easier tunability compared to 3D perovskite. Tuning the phase distribution according to device architecture is of importance. To better understand the structure-function relationship of perovskite materials, we utilize femtosecond laser spectroscopy to study photophysics of different structures. For example, we manipulate the phase purity and vertical distribution of quasi-2D perovskite, verified by femtosecond transient absorption spectroscopy. We find that solar cell performance is more sensitive to phase purity than vertical phase distribution. In another example, to improve both efficiency and stability simultaneously, a small amount of hydrophobic cation of large size is added into 3D perovskite. We find that the propylammonium is the most effective by comparing a family of different cations. The cations of large size preferentially segregate at the grain boundaries and surface, verified by transient absorption and reflection spectroscopy. Such passivation enhances device efficiency up to 20.1% and improves both device and precursor stabilities.
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