Compositional and morphological engineering of mixed cation perovskite films for highly efficient planar and flexible solar cells with reduced hysteresis

We report on compositional and morphological engineering of mixed methylammonium (MA) and formamidinium (FA) lead triiodide (MA(1-x)FA(x)PbI(3)) perovskite absorber layers to produce highly efficient planar and flexible perovskite solar cells (PVSCs) with reduced hysteresis. Incorporation of FA into the MAPbI(3) extends the absorption edge of the perovskite to longer wavelengths, leading to enhanced photocurrent of the resultant PVSCs. Moreover, adding a small amount of lead thiocyanate (Pb(SCN)(2)) additive into mixed perovskite precursor solutions significantly enlarges the grain size and prolongs the carrier lifetime, leading to improved device performance. With optimal compositional and morphological engineering, the average power conversion efficiency (PCE) improves from 15.74 +/- 0.74% for pure MAPbI(3) PVSCs to 19.40 +/- 0.32% for MA(0.7)FA(0.3)PbI(3) PVSCs with 3% Pb(SCN)(2) additive, exhibiting a high reproducibility and small hysteretic behavior. The best PVSC achieves a PCE of 20.10 (19.85)% measured under reverse (forward) voltage scan. Furthermore, the compositional and morphological engineering allowed the fabrication of efficient flexible PVSCs on indiumdoped SnO2 (ITO)/polyethylene terephthalate (PET) substrates, with the best PCE of 17.96 (16.10)% with a V-OC of 1.076 (1.020) V, a J(SC) of 22.23 (22.23) mA/cm(2) and a FF of 75.10 (71.02)% when measured under reverse (forward) voltage scan. Our approach provides an effective pathway to fabricate highly efficient and reproducible planar PVSCs.