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In perovskite solar cells, the interfaces between the perovskite and charge-transporting layers contain high concentrations of defects (about 100 times that within the perovskite layer), specifically, deep-level defects, which substantially reduce the power conversion efficiency of the devices1–3. Recent efforts to reduce these interfacial defects have focused mainly on surface passivation4–6. However, passivating the perovskite surface that interfaces with the electron-transporting layer is difficult, because the surface-treatment agents on the electron-transporting layer may dissolve while coating the perovskite thin film. Alternatively, interfacial defects may not be a concern if a coherent interface could be formed between the electron-transporting and perovskite layers. Here we report the formation of an interlayer between a SnO2 electron-transporting layer and a halide perovskite light-absorbing layer, achieved by coupling Cl-bonded SnO2 with a Cl-containing perovskite precursor. This interlayer has atomically coherent features, which enhance charge extraction and transport from the perovskite layer, and fewer interfacial defects. The existence of such a coherent interlayer allowed us to fabricate perovskite solar cells with a power conversion efficiency of 25.8 per cent (certified 25.5 per cent)under standard illumination. Furthermore, unencapsulated devices maintained about 90 per cent of their initial efficiency even after continuous light exposure for 500 hours. Our findings provide guidelines for designing defect-minimizing interfaces between metal halide perovskites and electron-transporting layers. An atomically coherent interlayer between the electron-transporting and perovskite layers in perovskite solar cells enhances charge extraction and transport from the perovskite, enabling high power conversion efficiency.

High-performance inorganic-organic lead halide perovskite solar cells (PSCs) are often fabricated with a liquid additive such as dimethyl sulfoxide (DMSO), which retards crystallization and reduces roughness and pinholes in the perovskite layers. However, DMSO can be trapped during perovskite film formation and induce voids and undesired reaction byproducts upon later processing steps. Here, it is shown that the amount of residual DMSO can be reduced in as-spin-coated films significantly through use of preheated substrates, or a so-called hot-casting method. Hot casting increases the perovskite film thickness given the same concentration of solutions, which allows for reducing the perovskite solution concentration. By reducing the amount of DMSO in proportion to the concentration of perovskite precursors and using hot casting, it is possible to fabricate perovskite layers with improved perovskite-substrate interfaces by suppressing the formation of byproducts, which increase trap density and accelerate degradation of the perovskite layers. The best-performing PSCs exhibit a power conversion efficiency (PCE) of 23.4% (23.0% stabilized efficiency) under simulated solar illumination. Furthermore, encapsulated devices show considerably reduced post-burn-in decay, retaining 75% and 90% of their initial and post-burn-in efficiencies after 3000 h of operation with maximum power point tracking (MPPT) under high power of ultraviolet (UV)-containing continuous light exposure.

Summary Cesium lead triiodide (CsPbI3) presents a desirable band gap, does not require the use of mixed halides for Si tandem solar cells, and possesses relatively high thermal stability owing to its inorganic components. However, the power conversion efficiency (PCE) of CsPbI3 is lower than that of organic cation-based halide perovskites with identical band gaps. The main factors that govern the PCE of CsPbI3 are the surface morphology and defect passivation of its thin films on substrates. In this study, we used the sequential dripping of a methylammonium chloride (MACl) solution (SDMS) to obtain highly uniform and pinhole-minimized thin films by controlling the intermediate stages of the crystallization process, followed by surface passivation using octylammonium iodides in ambient air. SDMS accelerated the crystallization process of the CsPbI3 perovskite layer, resulting in the formation of a uniform and dense surface with few pinholes. Consequently, we fabricated CsPbI3 solar cells with excellent PCE (20.37%).

For the fabrication of perovskite solar cells (PSCs) using a solution process, it is essential to understand the characteristics of the perovskite precursor solution to achieve high performance and reproducibility. The colloids (iodoplumbates) in the perovskite precursors under various conditions were investigated by UV-visible absorption, dynamic light scattering, photoluminescence, and total internal reflection fluorescence microscopy techniques. Their local structure was examined by in situ X-ray absorption fine structure studies. Perovskite thin films on a substrate with precursor solutions were characterized by transmission electron microscopy, X-ray diffraction analysis, space-charge-limited current, and Kelvin probe force microscopy. The colloidal properties of the perovskite precursor solutions were found to be directly correlated with the defect concentration and crystallinity of the perovskite film. This work provides guidelines for controlling perovskite films by varying the precursor solution, making it possible to use colloid-engineered lead halide perovskite layers to fabricate efficient PSCs.

Maintaining the bandgap The bandgap of the black α-phase of formamidinium-based lead triiodide (FAPbI 3 ) is near optimal for creating high-efficiency perovskite solar cells. However, this phase is unstable, and the additives normally used to stabilize this phase at ambient temperature—such as methylammonium, caesium, and bromine—widen its bandgap. Min et al. show that doping of the α-FAPbI 3 phase with methylenediammonium dichloride enabled power conversion efficiencies of 23.7%, which were maintained after 600 hours of operation. Unencapsulated devices had high thermal stability and retained >90% efficiency even after annealing for 20 hours at 150°C in air. Science , this issue p. 749

In perovskite solar cells, the interfaces between the perovskite and charge-transporting layers contain high concentrations of defects (about 100 times that within the perovskite layer), specifically, deep-level defects, which substantially reduce the power conversion efficiency of the devices

Relieving unwanted strain Although the α-phase of formamidinium lead iodide (FAPbI 3 ) has a suitable bandgap for use in solar cells, it must be stabilized with additional cations. These compositions can adversely affect the bandgap and produce lattice strain that creates trap sites for charge carriers. Kim et al. found that substituting small, equimolar amounts of cesium and methylenediammonium cations for formamidinium reduced the lattice strain and trap densities. The enhancement in open-circuit voltage led to a certified power conversion efficiency of 24.4%, and encapsulated devices retained 90% of their initial efficiency after 400 hours of maximal power point operating conditions. Science , this issue p. 108

Recently, lead-tin-based alloyed halide perovskite quantum dots (QDs) with improved stability and less toxicity have been introduced. However, the perovskite QDs containing tin are still unstable and exhibit low photoluminescence quantum yields (PLQYs), owing to the presence of defects in the alloyed system. Here, we have attempted to introduce sulfur anions (S2-) into the host lattice (MAPb0.75Sn0.25Br3) as a promising route to stable alloyed perovskite QDs with improved stability and PLQY. In this study, we used elemental sulfur as a sulfur precursor. The successful incorporation of sulfur anions into the host lattice resulted in a highly improved PLQY (>75% at room temperature), which is believed to be due to a reduction in the defect-related non-radiative recombination centers present in the host lattice. Furthermore, we found that the emission property could be tuned between the bright green and cyan-bluish regions without compromising on color quality. This work invigorates the perovskite research community to prepare stable, bright, and color-tunable alloyed inorganic-organic perovskite QDs without compromising on their phases and color quality, which can lead to considerable advances in display technology.