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Abstract
Compositional engineering of recently arising methylammonium (MA) lead (Pb) halide
based perovskites is an essential approach for finding better perovskite compositions
to resolve still remaining issues of toxic Pb, long-term instability, etc. In this
work, we carried out crystallographic, morphological, optical, and photovoltaic characterization
of compositional MASn0.6Pb0.4I3-xBrx by gradually introducing bromine (Br) into parental
Pb-Sn binary perovskite (MASn0.6Pb0.4I3) to elucidate its function in Sn-rich (Sn:Pb
= 6:4) perovskites. We found significant advances in crystallinity and dense coverage
of the perovskite films by inserting the Br into Sn-rich perovskite lattice. Furthermore,
light-intensity-dependent open circuit voltage (Voc) measurement revealed much suppressed
trap-assisted recombination for a proper Br-added (x = 0.4) device. These contributed
to attaining the unprecedented power conversion efficiency of 12.1% and Voc of 0.78
V, which are, to the best of our knowledge, the highest performance in the Sn-rich
(≥60%) perovskite solar cells reported so far. In addition, impressive enhancement
of photocurrent-output stability and little hysteresis were found, which paves the
way for the development of environmentally benign (Pb reduction), stable monolithic
tandem cells using the developed low band gap (1.24-1.26 eV) MASn0.6Pb0.4I3-xBrx with
suggested composition (x = 0.2-0.4).