Triiodide-Induced Band-Edge Reconstruction of a Lead-Free Perovskite-Derivative Hybrid for Strong Light Absorption

Bismuth-based hybrid perovskites have recently attracted great attention for their environmentally friendly processing, chemical stability, and photoresponsive properties. However, most of the known lead-free hybrids show wide band gaps (E g > 1.9 eV) that afford weak visible-light absorption. Here, we show a newly conceptual design strategy of intercalating triiodide to decrease the prototypic band gap by ca. ∼0.44 eV. A new hybrid semiconducting material of (4-methylpiperidinium) 4 I 3 BiI 6 (MP-T-BiI 6 ), adopting the zerodimensional (0D) perovskite-like framework, was reconstructed from its prototype of (4-methylpiperidinium) 3 Bi 2 I 9 (MP-Bi 2 I 9 ). It is noteworthy that MP-T-BiI 6 has a narrow band gap of 1.58 eV, almost comparable to that of CH 3 NH 3 PbI 3 (∼1.5 eV), which reveals its potential as the highly efficient light absorber. Importantly, its semiconducting properties were solidly confirmed by notable hole mobility (∼12.8 cm 2 V -1 s -1 ), charge-trap density (∼1.13 × 10 10 cm -3 ), and photoconductive behaviors. Moreover, theoretical calculations further disclose that the I-5p orbitals of triiodide induce the band-edge reconstruction and behave as a new conduction-band minimum at the Brillouin zone center. This work paves a potential pathway for the large and diverse family of lead-free hybrids to compete with lead absorbers.