Precisely Controlling the Grain Sizes with an Ammonium Hypophosphite Additive for High-Performance Perovskite Solar Cells

A facile approach to precisely control the perovskite grain sizes is proposed and demonstrated for high-performance photovoltaic (PV) solar cells. With the introduction of various amounts of NH₄H₂PO₂ (AHP) additives into the PbI₂/CH₃NH₃I precursors, the grain scale of CH₃NH₃PbI₃ films can be finely turned from hundreds of nanometer to micrometer scale, allowing evaluating the effects of crystalline grain boundary on trap densities, charge recombination, and PV device performance. The X-ray diffraction and X-ray photoelectron spectroscopy measurements indicate that the formation of intermediates plays a key role in assisting the perovskite crystal growth. The optimized devices show much larger open-circuit voltages (V_OC) up to 1.10 ± 0.02 V and significantly enhance power conversion efficiencies (PCEs) of 16.5 ± 0.7%, as compared to the control devices with PCE of 9.4 ± 1.0% and V_OC of 1.00 ± 0.03 V. Further investigations confirm that the boosted PV performance origins from the decreased defect densities due to enlarged grain sizes. It is also demonstrated that the approach is general and applicable to other perovskite systems, e.g., HC(NH₂)₂PbI₃. The results suggest the promising application of AHP in achieving high-performance perovskite PV devices, and shed light on understanding the grain boundary effects on perovskite optoelectronics.