LAUSR

DOI: 10.1002/aenm.201803135 For a two-terminal monolithic tandem solar cell, the open-circuit voltage (VOC) is the sum of the VOCs of the wide-bandgap and low-bandgap subcells while the shortcircuit current density (JSC) is limited by the lower JSC of the subcells. Therefore, achieving high VOCs for both subcells while maintaining a sufficiently high JSC for current matching is critical for realizing high-efficiency two-terminal tandem solar cells. For all-perovskite tandem solar cells, the commonly used wide-bandgap (wide-Eg) subcell (≈1.7–1.9 eV) is based on FA1−xCsxPb(I1−yBry)3 perovskite absorbers (0 < x, y < 1) (FA = formamidinium, Cs = cesium, I = iodide, Br = bromide), whereas the low-bandgap (low-Eg) (≈1.1– 1.3 eV) subcell is based on tin (Sn)–lead (Pb) halide perovskite absorbers.[12–15] While wide-Eg PSCs have achieved remarkable improvement in performance via composition tuning/engineering, annealing engineering, and interface engineering,[16–19] the performances of low-Eg PSCs reported in the literature are still not satisfactory. Significant efforts have been made to improve the performance of low-Eg PSCs.[6,11,20–27] Kanatzidis and coworkers have first reported the bowing effect in mixed Sn–Pb perovskites, revealing the opportunity of bandgap tuning via compositional engineering.[20] McGehee and co–workers have studied the effect of lattice contraction and octahedral tilting on bandgap tuning in low-Eg mixed Sn–Pb perovskites.[21] Hayase and co-workers have introduced an n-type “spike structure” interface to improve the charge flow at the interface of the absorber and electron transport layer (ETL).[28] Our group has boosted the efficiency to a certified value of 17% for relatively thick low-Eg (1.25 eV) PSCs, beneficial for all-perovskite tandem solar cells.[6,23,24,29] Recently, Jen and coworkers have incorporated 20% Br into low-Eg MASn0.5Pb0.5I3 (MA = methylammonium) perovskite to obtain an optimal bandgap (1.35 eV) for single-junction PSC applications, and a VOC of 0.9 V was obtained.[26] However, the relatively large Eg limits its potential for applications as the low-Eg bottom subcells for tandem devices.[13] So far, many reported lowEg mixed Sn–Pb PSCs show relatively large VOC deficits (Eg/q-VOC, where q is the unit charge) and/or low fill factors The unsatisfactory performance of low-bandgap mixed tin (Sn)–lead (Pb) halide perovskite subcells has been one of the major obstacles hindering the progress of the power conversion efficiencies (PCEs) of all-perovskite tandem solar cells. By analyzing dark-current density and distribution, it is identified that charge recombination at grain boundaries is a key factor limiting the performance of low-bandgap mixed Sn–Pb halide perovskite subcells. It is further found that bromine (Br) incorporation can effectively passivate grain boundaries and lower the dark current density by two–three orders of magnitude. By optimizing the Br concentration, low-bandgap (1.272 eV) mixed Sn–Pb halide perovskite solar cells are fabricated with open-circuit voltage deficits as low as 0.384 V and fill factors as high as 75%. The bestperforming device demonstrates a PCE of >19%. The results suggest an important direction for improving the performance of low-bandgap mixed Sn–Pb halide perovskite solar cells.