Effect of High Dipole Moment Cation on Layered 2D Organic–Inorganic Halide Perovskite Solar Cells

Layered 2D organic–inorganic hybrid perovskite is appearing as a rising star in the photovoltaic field, thanks to its superior moisture resistance by the organic spacer cations. Unfortunately, these cations lead to high exciton binding energy in the 2D perovskites, which suffers from lower efficiency in the devices. It thus requires a clear criterion to select/design appropriate organic spacer cations to improve the device efficiency based on this class of materials. Here, 2,2,2-trifluoroethylamine (F₃EA⁺) is introduced to combine with butylammonium (BA⁺) cations as mixed spacers. While BA⁺ enables self-assembly of 2D perovskite crystals by van der Waals interaction, the introduction of F₃EA⁺ spacers with a high dipole moment suppress nonradiative recombination and promote separation of photogenerated electron–hole pairs by taking the advantage of electronegativity of fluorine. The resultant solar cells based on [(BA)_{1– x}(F₃EA)_x]₂(MA)₃Pb₄I₁₃ exhibit substantially increased open circuit voltage and fill factor compared with that of (BA)₂(MA)₃Pb₄I₁₃. The champion [(BA)_0.94(F₃EA)_0.06]₂(MA)₃Pb₄I₁₃ solar cell yields a power conversion efficiency of 12.51%, which is among the best performances so far. These findings suggest an effective strategy to design organic spacer cations in layered perovskite for solar cells and other optoelectronic applications.