Journal Article all-perovskite tandem solar cells perovskite solar cells SnF2 additives Voc loss carrier management colloidal quantum dots defect passivation electron transfer functional molecules grain boundary passivation hole transport layer-free hysteresis interface passivation lead-free, perovskite solar cells low-bandgap perovskites low-bandgap tin-lead passivation pinhole-free solar cells taurine uniform perovskites