Nontoxic (CH3NH3)3Bi2I9 Bismuth based perovskite solar cells : Improved device performance and stability through morphological tailoring

Perovskite solar cells have shown remarkable improvement in certified power conversion efficiency (PCE) [1] of >22%. Nevertheless, many challenges regarding the stability and toxicity of the lead based perovskite material remains at the forefront of current research. The toxicity of lead which is present in a rather water-soluble form in perovskite solar cells remains an environmental concern, that is yet to be resolved. The bismuth based zero-dimensional perovskite shows a high band gap (Eg) ≈ 1.8 eV [2], [3 - 6] which makes it a suitable candidate for application in tandem solar cells. Recently, layered bismuth triiodide (BiI3) has also been used in solar cells as photoactive materials. [7-8] with the highest reported efficiency of 0.3%. [8] However, there are very few attempts to make pure bismuth triiodide based solar cells and also lack in systematic investigation on morphological tailoring, a viable route to fully utilize the potential of this material is to fine-tune the desired composition and properties for CH3NH3I_BiI3 material without any doping. In this direction, we report the vapor assisted solution process (VASP) two-step method to prepare bismuth perovskite samples at different reaction time The samples prepared at an optimum reaction time of 25 minutes exposure of MAI(v) give reproducible power conversion efficiency upto 2 %, (FF = 0.75%, Jsc = 2.9 mA/cm2, VoC = 0.91 V), highest so far reported for methyl amine based bismuth perovskite devices. This work demonstrates the efficacy of the VASP process in producing highly compact films that give improved optoelectronic performance