Preparation of Single-Phase Films of CH₃NH₃Pb(I_1–<i>x</i>Br_<i>x</i>)₃ with Sharp Optical Band Edges

Abstract

Organometallic lead-halide perovskite-based solar cells now approach 18% efficiency. Introducing a mixture of bromide and iodide in the halide composition allows tuning of the optical bandgap. We prepare mixed bromide–iodide lead perovskite films CH₃NH₃Pb­(I_1–<i>x</i>Br_<i>x</i>)₃ (0 ≤ <i>x</i> ≤ 1) by spin-coating from solution and obtain films with monotonically varying bandgaps across the full composition range. Photothermal deflection spectroscopy, photoluminescence, and X-ray diffraction show that following suitable fabrication protocols these mixed lead-halide perovskite films form a single phase. The optical absorption edge of the pure triiodide and tribromide perovskites is sharp with Urbach energies of 15 and 23 meV, respectively, and reaches a maximum of 90 meV for CH₃NH₃PbI_1.2Br_1.8. We demonstrate a bromide–iodide lead perovskite film (CH₃NH₃PbI_1.2Br_1.8) with an optical bandgap of 1.94 eV, which is optimal for tandem cells of these materials with crystalline silicon devices