Photoimaging with Color-Sensing Abilities by Tuning the Band Gap of a Single-Crystal Lead-Free Perovskite

Photodetectors (PDs) composed of lead-free metal halide perovskites have been a shining topic in optoelectronics. However, it is debatable whether perovskites are an n-type or p-type semiconductor with a direct or indirect band gap. Furthermore, to date, little research has been conducted on lead-free metal halide perovskites with color-sensing abilities. Herein, for the first time, single-crystal MA 3 Bi 2 I 3 x Br 9–3 x ( x = 0, 1, 2, and 3) perovskites were systematically studied, and the results showed that MA 3 Bi 2 I 9 is a p-type direct-band-gap semiconductor, whereas MA 3 Bi 2 Br 9 is an n-type indirect-band-gap semiconductor. Furthermore, the band gap of MA 3 Bi 2 I 3 x Br 9–3 x ( x = 0, 1, 2, and 3) perovskites can be systematically tuned from 2.06 to 2.55 eV, affording it with color-sensing abilities from 450 to 580 nm, respectively. The representative Au-MA 3 Bi 2 I 9 -ITO (ITO = indium tin oxide) PD exhibits a superior self-powered photodetecting performance with a high responsivity (15.8 mA W –1; 580 nm, 1.0 mW cm –2 ), detectivity (8.1 × 10 11 Jones), an on/off ratio (4231), LDR (72.5 dB) and a fast response speed (rise time of 2 μs and decay time of 29 μs). This study not only facilitates the theoretical understanding of the band gap of perovskite materials but also sheds light on the application of lead-free perovskites in object interaction and color perception.