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...

Formamidinium lead iodide perovskites are promising light-harvesting materials, yet stabilizing them under operating conditions without compromising optimal optoelectronic properties remains challenging. We report a multimodal host–guest complexation strategy to overcome this challenge using a crown ether, dibenzo-21-crown-7, which acts as a vehicle that assembles at the interface and delivers Cs+ ions into the interior while modulating the material. This provides a local gradient of doping at the nanoscale that assists in photoinduced charge separation while passivating surface and bulk defects, stabilizing the perovskite phase through a synergistic effect of the host, guest, and host–guest complex. The resulting solar cells show power conversion efficiencies exceeding 24% and enhanced operational stability, maintaining over 95% of their performance without encapsulation for 500 h under continuous operation. Moreover, the host contributes to binding lead ions, reducing their environmental impact. This supramolecular strategy illustrates the broad implications of host–guest chemistry in photovoltaics.

Perovskite light emitting diodes (PeLEDs) have drawn considerable attention for their favorable optoelectronic properties. Perovskite light emitting electrochemical cells (PeLECs)-devices that utilize mobile ions -have recently been reported but have yet to reach the performance of the best PeLEDs. We leveraged a poly(ethylene oxide) electrolyte and lithium dopant in CsPbBr3 thin films to produce PeLECs of improved brightness and efficiency. In particular, we found that a single layer PeLEC from CsPbBr3:PEO:LiPF6 with 0.5% wt. LiPF6 produced highly efficient (22 cd/A) and bright (~15000 cd/m 2 ) electroluminescence. To understand this improved performance among PeLECs, we characterized these perovskite thin films with photoluminescence (PL) spectroscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD).These studies revealed that this optimal LiPF6 concentration improves electrical double layer formation, reduces the occurrence of voids, charge traps, and pinholes, and increases grain size and packing density. TOC GRAPHICSPerovskite light-emitting diodes (PeLEDs) based on inorgano−organometallic halide perovskites, such as CH3NH3PbX3 and CsPbX3 (X = Cl, Br, or I), have attracted much attention due to their low-temperature solution processability, high color purity with narrow spectral width (FWHM of 20 nm), band gap tunability and large charge carrier mobility. [1][2][3][4] To date, devices based on these perovskites have achieved high luminance in excess of 10000 cd/m 2 with high efficiencies (EQE ~10%), comparable to organic LEDs and quantum dot (QD) LEDs. [1][2][3][4][5][6][7] Interestingly, effects such as hysteresis and high capacitance in perovskite semiconductor devices suggest that ion motion can largely influence device operation. In this vein, researchers have recently been investigating perovskite materials in light-emitting electrochemical cell (LEC) architectures instead of traditional LEDs. [8][9][10][11] These LEC devices (PeLEC leverage ion redistribution to achieve balanced and high carrier injection, resulting in high electroluminescence efficiency. Due to this mechanism, LEC devices can be prepared from a simple architecture consisting of a single semiconducting composite layer sandwiched between two electrodes. In addition, they can operate at low voltages below the bandgap, yielding highly efficient devices.Recently, perovskite LECs (PeLECs) have been reported and show promise as electroluminescent devices. [8][9][10][11] However, these PeLECs are generally limited to luminance maxima of 1000 cd/m 2 or lower, below what has been typically observed in PeLEDs. This disparity suggests that further understanding and refinement of PeLEC materials and devices could produce significant improvements of brightness, efficiency, and other performance metrics. To this end, we fabricated a highly efficient (22 cd/A) and bright (~15000 cd/m 2 ) single layer LEC based on a cesium lead halide perovskite, CsPbBr3. To achieve...

Hybrid perovskites are emerging as highly efficient materials for optoelectronic applications, however, their operational lifetime has remained a limiting factor for their continued progress. In this work, perovskite light emitting electrochemical cells (PeLECs) utilizing an optimized fraction of LiPF 6 salt additive exhibit enhanced stability. At 0.5 wt% LiPF 6 , devices exhibit 100 h operation at high brightness in excess of 800 cd m −2 under constant current driving, achieving a maximum luminance of 3260 cd m −2 and power efficiency of 9.1 Lm W −1 . This performance extrapolates to a 6700 h luminance half-life from 100 cd m −2 , a 5.6-fold improvement over devices with no LiPF 6 . Analysis under constant voltage driving reveals three current regimes, with lithium addition strongly enhancing current in the second and third regimes. The third regime correlates lower rates of luminance with lowered current flow. These losses are mitigated by LiPF 6 addition, an effect postulated to arise from preservation of perovskite structure. Electrochemical impedance spectroscopy with equivalent circuit modeling revealed that electrical double layer widths are minimized at 0.5 wt% LiPF 6 and inversely correlate with efficient performance. These results demonstrate that an optimal LiPF 6 concentration improves stability and efficiency through improved double layer formation and retention of perovskite structure.

We propose an intermediate-phase engineering strategy to achieve the robust interfacial contact by utilizing volatile organic salts. The introduction of organic cations (such as methylammonium and formamidinium) leads to the formation of an organic-inorganic hybrid perovskite intermediate phase in the initial film and promotes the high-quality interfacial contact of all-inorganic perovskite/metal oxide. A champion CsPb(I 0.75 Br 0.25 ) 3 -based device with a power conversion efficiency of 17.0% and an open-circuit voltage of 1.34 V was realized.