Formamidinium Incorporation into Compact Lead Iodide for Low Band Gap Perovskite Solar Cells with Open-Circuit Voltage Approaching the Radiative Limit.

To bring hybrid lead halide perovskite solar cells toward the Shockley-Queisser limit requires lowering the band gap while simultaneously increasing the open-circuit voltage. This, to some extent divergent objective, may demand the use of large cations to obtain a perovskite with larger lattice parameter together with a large crystal size to minimize interface nonradiative recombination. When applying the two-step method for a better crystal control, it is rather challenging to fabricate perovskites with FA+ cations, given the small penetration depth of such large ions into a compact PbI2 film. In here, to successfully incorporate such large cations, we used a high-concentration solution of the organic precursor containing small Cl- anions achieving, via a solvent annealing-controlled dissolution-recrystallization, larger than 1 µm perovskite crystals in a solar cell. This solar cell, with a largely increased fluorescence quantum yield, exhibited an open-circuit voltage equivalent to 93% of the corresponding radiative limit one. This, together with the low band gap achieved (1.53 eV), makes the fabricated perovskite cell one of the closest to the Shockley-Queisser optimum.