Metal halide perovskites of the general formula ABX 3 -where A is a monovalent cation such as caesium, methylammonium or formamidinium; B is divalent lead, tin or germanium; and X is a halide anion-have shown great potential as light harvesters for thin-film photovoltaics [1][2][3][4][5] . Among a large number of compositions investigated, the cubic α-phase of formamidinium lead triiodide (FAPbI 3 ) has emerged as the most promising semiconductor for highly efficient and stable perovskite solar cells [6][7][8][9] , and maximizing the performance of this material in such devices is of vital importance for the perovskite research community. Here we introduce an anion engineering concept that uses the pseudo-halide anion formate (HCOO − ) to suppress anion-vacancy defects that are present at grain boundaries and at the surface of the perovskite films and to augment the crystallinity of the films. The resulting solar cell devices attain a power conversion efficiency of 25.6 per cent (certified 25.2 per cent), have long-term operational stability (450 hours) and show intense electroluminescence with external quantum efficiencies of more than 10 per cent. Our findings provide a direct route to eliminate the most abundant and deleterious lattice defects present in metal halide perovskites, providing a facile access to solution-processable films with improved optoelectronic performance.Perovskite solar cells (PSCs) have attracted much attention since their first demonstration in 2009 [1][2][3][4][5] . The rapid expansion of research into PSCs has been driven by their low-cost solution processing and attractive optoelectronic properties, including a tunable bandgap 6 , high absorption coefficient 10 , low recombination rate 11 and high mobility of charge carriers 12 . Within a decade, the power conversion efficiency (PCE) of single-junction PSCs progressed from 3% to a certified value of 25.5% 13 , the highest value obtained for thin-film photovoltaics. Moreover, through the use of additive and interface engineering strategies, the long-term operational stability of PSCs now exceeds 1,000 hours in full sunlight 14,15 . PSCs therefore show great promise for deployment as the next generation of photovoltaics.Compositional engineering plays a key part in achieving highly efficient and stable PSCs. In particular, mixtures of methylammonium lead triiodide (MAPbI 3 ) with formamidinium lead triiodide (FAPbI 3 ) have been extensively studied 5,7 . Compared to MAPbI 3 , FAPbI 3 is thermally more stable and has a bandgap closer to the Shockley-Queisser limit 6 , rendering FAPbI 3 the most attractive perovskite layer for single-junction PSCs.Unfortunately, thin FAPbI 3 films undergo a phase transition from the black α-phase to a photoinactive yellow δ-phase below a temperature of 150 °C. Previous approaches to overcome this problem have included mixing FAPbI 3 with a combination of methylammonium (MA + ), caesium (Cs + ) and bromide (Br − ) ions; however, this comes at the cost of blue-shifted absorbance and phase segregation under...

Improvements to perovskite solar cells (PSCs) have focused on increasing their power conversion efficiency (PCE) and operational stability and maintaining high performance upon scale-up to module sizes. We report that replacing the commonly used mesoporous–titanium dioxide electron transport layer (ETL) with a thin layer of polyacrylic acid–stabilized tin(IV) oxide quantum dots (paa-QD-SnO 2 ) on the compact–titanium dioxide enhanced light capture and largely suppressed nonradiative recombination at the ETL–perovskite interface. The use of paa-QD-SnO 2 as electron-selective contact enabled PSCs (0.08 square centimeters) with a PCE of 25.7% (certified 25.4%) and high operational stability and facilitated the scale-up of the PSCs to larger areas. PCEs of 23.3, 21.7, and 20.6% were achieved for PSCs with active areas of 1, 20, and 64 square centimeters, respectively.

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