Charge carrier transport properties of twin domains in halide perovskites

The past decade has seen the unprecedentedly rapid emergence of a new class of solar cells based on mixed organic-inorganic halide perovskites. The power conversion efficiency (PCE) of halide perovskite solar cells since then has quickly risen above 25% in single-junction devices and 30% in tandem devices. Twin domains within polycrystalline grains have been recently reported in this material, nevertheless, their roles associated with both ionic and charge carrier transport properties are still to be fully understood. Here, combining molecular dynamic (MD) simulations with nanoscale scanning probe microscopy investigations, we reveal unique properties of the twin domains that exhibit vital channels for ion migration and influence charge separation and collection. Our nanoscale elemental analysis using photo-induced force microscopy reveals that the domain structure possesses an alternating chemical compositional variation, rich and poor in cations for low topography domains (LTDs) and high topography domains (HTDs) respectively. Also, Kelvin probe force microscopy (KPFM) measurements confirm that LTDs provide a confined tunnel for cation vacancy migration. This phenomenon is supported by the MD simulation which suggests the presence of the twin domain wall causes a reduction in the crystal symmetry and appearance of a strain field. Lastly, KPFM and conductive AFM (c-AFM) under illumination show that both photovoltage and photocurrent are higher at LTDs due to the effective charge collection by ion accumulation. This work highlights important elements of the nanoscale intragrain feature that may pave the way to high-efficiency perovskite solar cells.