(559a) Controlling Phase Polydispersity and Crystal Orientation of Ruddlesden-Popper Perovskites for Efficient and Stable Solar Cells

Two-dimensional Ruddlesden-Popper perovskites (2D-RPPs) have risen to prominence as stable and efficient photovoltaic materials because of their structural diversity, rich photophysics and low moisture ingression. This is not only applicable to Pb-based perovskites but also to their Sn counterparts. However, thin films processed from stoichiometric precursor solutions possess a broad phase distribution of different number of inorganic layers with random crystal orientation, crippling device performance. In this talk, I will present our recent efforts in controlling phase polydispersity and crystal orientation for both Pb- and Sn-based 2D-RPPs. Our general strategy is to select additives that can form intermediates with metal ions (Pb²⁺ or Sn²⁺) or to use pseudo-halide anions that can break the stacking of large cations in precursor and can form strong coordination with metal ions. We show that the parasitic n ≤ 2 2D phases and 3D-like phases are effectively supressed. The additives and pseudo-halide anions also passivate surface defects, leading to reduced trap state density and increased and more balanced charge carrier mobility and diffusion length. We probe the phase distribution and crystal orientation across the thin films using GIWAXS with different incident angles. We also investigate the charge decay dynamics and charge transfer across phases using femtosecond transient absorption spectroscopy. We show that the arising (PEA)₂MA₄Pb₅I₁₆ thin films enable efficient p-i-n solar cells with an efficiency of 14.34%, and a high V_oc of 1.20 V, retaining 96% initial efficiency after 1440 h under ambient conditions (RH = 50-60%) without encapsulation. Similarly, a prominent efficiency up to 9% is achieved for 2D tin perovskite solar cells containing PEA₂FA₄Sn₅I₁₆ active layer, which retain 80% of their initial PCEs after storage in nitrogen environment for 400 h without encapsulation. Our work has contributed toward understanding crystal growth and accurate control of phase distribution of two-dimensional perovskites. These insights have the translatability to a wide range of efficient optoelectronic devices with long-term stability.