A Zero-Dimensional Mixed-Anion Hybrid Halogenobismuthate(III) Semiconductor: Structural, Optical, and Photovoltaic...

In this contribution, we present the synthesis and characterization of the mixed-anion halogenobismuthate(III) (CH₃NH₃)₆BiI_5.22Cl_3.78 (MBIC) as an alternative lead-free perovskite-type semiconductor, and discuss its optical, electronic, and photovoltaic properties in comparison to the methylammonium bismuth iodide (CH₃NH₃)₃Bi₂I₉ (MBI) compound. The exchange of iodide with chloride during synthesis leads to the formation of an orthorhombic A₆BX₉-type crystal structure ( Cmma, No. 67) with isolated BiX₆ octahedra and methylammonium chloride interlayers. The experimentally found optical indirect band gap of 2.25 eV is in good agreement with the calculated value of 2.50 eV derived from DFT simulations. The valence band maximum and the conduction band minimum were determined to be at -6.2 eV and -4.0 eV vs vacuum. Similar to MBI, thin films of MBIC are composed of microcrystalline platelets. Time-resolved photoluminescence measurements show electron transfer of MBIC to mesoporous TiO₂. The photovoltaic behavior of both compounds is compared in solar cells with the following device architecture: glass/ITO/compact TiO₂/mesoporous TiO₂/MBIC or MBI/spiro-OMeTAD/Au. Despite the zero-dimensional structure of MBIC, a maximum power conversion efficiency of 0.18% and a high fill factor of almost 60% were obtained with this material as absorber layer. When stored under inert conditions, these solar cells show an excellent long-term stability over the investigated period of more than 700 days.