Thin film solar cells based on bismuth oxyiodide

Perovskite solar cells (PSCs) have drawn a lot of attention among scientific community because their power conversion efficiency (PCE) is comparable to crystal silicon solar cells. The conventional PSCs are using Pb-based halide perovskites, but the presence of Pb in these devices has caused some concerns due to the high perceived toxicity and hinder the pace of commercialization. Therefore, in this work we developed a Pb-free perovskite material based on bismuth oxyiodide (BiOI) thin films to replace Pb-halide PSCs. BiOI thin film has been grown by successive ionic layer adsorption and reaction (SILAR) dip coating. The films were heat-treated at 250°C, 350°C, 450°C and 550°C. Microstructure and electrical conductivity of BiOI were studied prior the device development through scanning electron microscope (SEM) and I-V measurement. The BiOI was then applied as an active layer in PSCs with the device structure glass/1TO/Ti02/Bi01/spiro-OMeTAD/Au and heat treated at 250°C, 350°C, 450°C and 550°C. All the fabricated devices were measured using I-V measurement under a solar spectrum simulator with AM 1.5 illuminations. The SEM image shows the agglomerations of flakes for as-deposited sample. These agglomerations and smaller flakes increase from ~0.8 µm to ~3.2 µm after annealed at 350°C. The electrical conductivity of BiOI film increased from 2.68x10⁶ to 1.63x10⁴ Smˉ¹ and achieved the highest peak after annealed at 350°C due to the improvement of crystallinity, the enlargement of the flakes size and the reduction of defects which promote the higher mobility of electrons. PSC device heat treated at 350°C demonstrated the best performance with Jsc = ~16.8 mAcmˉ², Voc = 0.62 V and efficiency 6.1% compare to others.