Solvent Dopant-Regulated Grain Formation for Bismuth Iodide Thin-Film Photodetectors

Simple solution-based processes to generate pinhole-free bismuth iodide (BiI 3 ) thin films with large, interconnected crystal grains have remained elusive. Here, a survey of the solvent systems for BiI 3 is conducted. Using a novel binary solvent doping approach to carefully regulate the crystallization kinetics, we generate high-quality BiI 3 thin films. Our method circumvents the current requirement for unreliable postprocessing techniques such as solvent vapor annealing or antisolvent dripping. Investigations reveal that 2-methoxyethanol doped with small amounts of N -methyl-2-pyrrolidone (NMP) is the ideal combination of parent and dopant solvent providing high vapor pressure and the intermediate coordination strength necessary to successfully achieve large grain formation in a single step. To evaluate optoelectronic performance, we integrated our optimized thin films into lithographically patterned lateral photodetectors (PDs). Our PD devices show fast millisecond response times, record responsivities of ∼1.61 A/W at moderate device bias (5 V), and excellent sensitivity, with detectivity reaching 1.9 × 10 12 cm Hz 1/2 W –1 (Jones). The high-quality BiI 3 semiconductor thin films outlined here, coupled with their ease of fabrication, enhance the development of BiI 3 optoelectronic devices and also serve as excellent template films for the fabrication of related bismuth perovskite materials.