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Metal halide perovskites of the general formula ABX3—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 photovoltaics1–5. Among a large number of compositions investigated, the cubic α-phase of formamidinium lead triiodide (FAPbI3) has emerged as the most promising semiconductor for highly efficient and stable perovskite solar cells6–9, 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.

CsPbX3(X = I, Br, Cl) perovskite nanocrystals (NCs) have recently emerged as emitting materials for optoelectronic and display applications owing to their easily tunable emissions, high photoluminescence quantum yield (PLQY), and vivid color purity (full width at half maximum of approximately 20 nm). However, the lagging quantum yields of blue-emitting perovskite NCs have resulted in low efficiency compared to green or red perovskite light-emitting diodes (PeLEDs); moreover, the long insulating ligands (such as oleylamine and oleic acid) inhibit charge carrier injection. In this study, we demonstrated a facile ligand-mediated post-treatment (LMPT) method for high-quality perovskite NCs with changing optical properties to allow fine-tuning of the target emission wavelength. This method involves the use of a mixed halide ion-pair ligand, di-dodecyl dimethyl ammonium bromide, and chloride, which can induce a reconstruction through a self-anion exchange. Using the LMPT method, the PLQY of the surface-passivated blue-emitting NCs was dramatically enhanced to over 70% within the 485 nm blue emission region and 50% within the 467 nm deep-blue emission region. Through this treatment, we achieved highly efficient blue-PeLED maximum external quantum efficiencies of 0.44 and 0.86% within the 470 and 480 ± 2 nm electroluminescence emission regions, respectively.

We demonstrate cesium-doping in methylammonium lead iodide perovskites (CsxMA1−xPbI3) light absorbers to improve the performance of inverted-type perovskite/fullerene planar heterojunction hybrid solar cells. CsxMA1−xPbI3perovskite devices with an optimized 10% Cs doping concentration exhibit remarkable improvement in device efficiency from 5.51% to 7.68% due to increases in short-circuit current density and open-circuit voltage via increased light absorption at optimum device thickness, improved film morphology and a widening of the energy difference between the valence band of the perovskite and lowest unoccupied molecular orbital level of PCBM.

Formamidinium lead triiodide (FAPbI3) has recently been considered as the most promising candidate to achieve highly efficient perovskite solar cells (PSCs). Excitingly, the state-of-the-art highest efficiency of FAPbI3based PSCs have reached over 25%. However, their device stability still lags behind other compositions of mixed-cation and mixed-halide perovskites. Interfacial engineering is a very powerful method to address this issue and passivation agents have been intensively developed, however there is a lack of in-depth understanding regarding the solvent selection during post-treatment. Here, we employed cyclohexylmethylammonium iodide (CMAI) as passivation agent, which is investigated using either isopropanol (IPA) or chloroform (CF) as carrier mediator to study the solvent influence on the stabilization of FAPbI3. We observed a suppressed-defect perovskite surface toward distinguished composition with 2D CMA2PbI4domain and CMAI domain induced by IPA and CF, respectively. Remarkably, post-treatment with solution of CMAI in CF creates a strain-free environment on the perovskite surface, leading to an improved efficiency of approaching 24% and concurrently an extraordinarily stable α-phase FAPbI3PSCs under operation condition, retaining 95% of its initial efficiency after 1050-hour aging. Our resulting device stability is one of the most stable FAPbI3based PSCs reported in literature.