Self-biased photodetector using 2D layered bismuth triiodide (BiI₃) prepared using the spin coating method

Layered bismuth triiodide (BiI₃) is a 2D material that has emerged as an ideal choice for optical sensors. Although BiI₃ has been prepared using vacuum-based deposition techniques, there is a dearth of research studies on synthesizing this material using chemical route. The present work uses a facile spin coating method with varying rotation speeds (rpm) to fabricate BiI₃ material thin films for photodetection applications. The structural, optical, and morphological study of BiI₃ thin films prepared at 3000-6000 rpm were investigated. XRD analysis indicates formation of BiI₃ films and revealed that BiI₃ has a rhombohedral crystal structure. FESEM analysis showed that BiI₃ films prepared at different rpm are homogeneous, dense, and free from cracks, flaws, and protrusions. In addition, films show an island-like morphology with grain boundaries having different grain sizes, micro gaps, and the evolution of the granular morphology of BiI₃ particles. The UV spectroscopy and photoluminescence analysis revealed that BiI₃ films strongly absorb light in the visible region of spectra with a high absorption coefficient of ∼10⁴ cm^-1, have an optical band gap of ∼1.51 eV. A photodetector was realised using fabricated BiI₃ film obtained at an optimum spin speed of 4000 rpm. It showed rapid rise and decay times of 0.4 s and 0.5 s, a responsivity of ∼100 μA W^-1, external quantum efficiency of 2.1 × 10^-4%, and detectivity of ∼3.69 × 10⁶ Jones at a bias voltage of 0 V. Our results point towards a new direction for layered 2D BiI₃ materials for the application in self-biased photodetectors.