Feray Ünlü1 Senol Öz1 Sanjay Mathur1

1, University of Cologne, Cologne, , Germany

Recently, low dimensional bismuth halide perovskites emerged as a promising alternative to lead based 3D hybrid perovskites in the field of optoelectronics, including solution processable thin film solar cells. Lead based metal halide perovskite solar cells have already achieved 22.7% certified solar to electric power conversion efficiency for small area devices (<1 cm2). However, one of the major problems encountered with this new technology, apart from structural and chemical stability, is the toxicity associated with heavy metal lead.
Potentially less toxic bismuth halide perovskites semiconductors possess promising optoelectronic properties including a high absorption coefficient on the order of 104 cm-1 and can be processed from solution using a variety of wet chemical deposition techniques and additives. Achieving full surface coverage along with precise control over nucleation sites has proven to be difficult by employing techniques, derived from the lead halide perovskite counterpart.
In this study, we investigated the influence of bismuth xanthate precursor and various solvents for the single-step deposition of methylammonium bismuth iodide perovskite (CH3NH3)3Bi2I9 thin-films with full surface coverage. The use of acetonitrile solvent in the perovskite synthesis led to pinhole-free absorber layers. Solar cell in n-i-p stack configuration (FTO/c-TiO2/mp-TiO2/(CH3NH3)3Bi2I9/Spiro-OMeTAD/Au) achieved a remarkably high fill factor of 0.74-0.77 with negligible hysteresis in the current-voltage sweep. Bismuth ethyl xanthate was used as a non-halide precursor additive for the deposition of (CH3NH3)3Bi2I9 films. Additionally, a small amount of cesium iodide was incorporated in a multi cation approach to benefit from its known effects such as improvement of stability and phase-pure crystallization. The addition of bismuth ethyl xanthate led to solar cells with improved current density (0.7 mA/cm2), fill factor (0.63) and power conversion efficiency (0.26%) compared to devices fabricated without bismuth ethyl xanthate (JSC= 0.5 mA/cm2, FF= 0.58, PCE= 0.17%).