Mechanochemical Alloying of Molecular Dopants in Expanded Analogs of Halide Perovskites

Controlling electronic doping remains a fundamental challenge for halide perovskite semiconductors. Introducing molecular dopants in the bulk of the semiconductor is a promising strategy to achieve reliable doping. Herein, we report on a solid-state alloying method that allows the incorporation of molecular dopants with low-lying lowest-unoccupied molecular orbital (LUMO) energy into the bulk of the semiconductor while enabling the fabrication of films by inkjet printing. First, we report on the synthesis and single-crystal structure of three new expanded analogs of halide perovskite (Hepm)[B₂X₆] (B-X = Pb-Br, Pb-I, Sn-I). The cavities in the expanded perovskite analogs are occupied by Hepm²⁺ cations (1-ethylpyrimidin-1,3-diium). The cations are structurally analogous to the Hepz²⁺ cations (1-ethylpyrazin-1,4-diium) but with higher LUMO energy (>0.5 eV). By alloying two expanded lattices via ball-milling, we obtain (Hepz)_x(Hepm)_1-x[Pb₂Br₆] and (Hepz)_x(Hepm)_1-x[Pb₂Br_6xI_6-6x]. The alloyed lattices contain a mixture of molecular dopants with high (Hepm²⁺) and low (Hepz²⁺) LUMO energy, presenting optical gaps as low as 1.25(5) eV. Thus, the solid-state method allows the introduction of Hepz²⁺ cations in the Pb-I lattice, which remained unattained via solution methods. Finally, we also prepare pinhole-free (Hepm)[Pb₂Br₆] films via inkjet printing, proving that expanded perovskites containing molecular dopants are printable.