Development of BiOI thin films for lead-free perovskite solar cells

Lead-free bismuth oxyiodide (BiOI) perovskite solar cells (PSCs) have been successfully fabricated with multilayer structure by successive ionic layer adsorption and reaction (SILAR) technique. The conductivity and microstructure of BiOI thin films under several heat treatment has been studied prior to the device fabrication. From SEM result, flower shaped flakes microstructure is obtained in as deposited BiOI layer and it enlarged as the heat treatment increased up to 350°C, and afterward crushed with higher heat treatment temperature. This is associate with the trend in conductivity measurement where BiOI achieved the optimum conductivity when it was heat treated at 350°C and reduced as the heat treatment exceed 450°C. This could be due to the reduction of grain boundaries as a result of development of the flakes sizes during the heat treatment, material sublimation and particle aggregation that developed higher resistance in the BiOI layers. The device performance under optimum heat treatment has been characterized using I-V measurement under a solar spectrum simulator with AM 1.5
illuminations. BiOI PSCs treated at 350°C showed -6% efficiency. This study provided better understanding on BiOI thin film behaviors under several heat treatments and its potential to be applied as lead-free PSCs.