All-perovskite–based polycrystalline thin-film tandem solar cells have the potential to deliver efficiencies of >30%. However, the performance of all-perovskite–based tandem devices has been limited by the lack of high-efficiency, low–band gap tin-lead (Sn-Pb) mixed-perovskite solar cells (PSCs). We found that the addition of guanidinium thiocyanate (GuaSCN) resulted in marked improvements in the structural and optoelectronic properties of Sn-Pb mixed, low–band gap (~1.25 electron volt) perovskite films. The films have defect densities that are lower by a factor of 10, leading to carrier lifetimes of greater than 1 microsecond and diffusion lengths of 2.5 micrometers. These improved properties enable our demonstration of >20% efficient low–band gap PSCs. When combined with wider–band gap PSCs, we achieve 25% efficient four-terminal and 23.1% efficient two-terminal all-perovskite–based polycrystalline thin-film tandem solar cells.

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The carrier concentration of the electron-selective layer (ESL) and hole-selective layer can significantly affect the performance of organic-inorganic lead halide perovskite solar cells (PSCs). Herein, a facile yet effective two-step method, i.e., room-temperature colloidal synthesis and low-temperature removal of additive (thiourea), to control the carrier concentration of SnO quantum dot (QD) ESLs to achieve high-performance PSCs is developed. By optimizing the electron density of SnO QD ESLs, a champion stabilized power output of 20.32% for the planar PSCs using triple cation perovskite absorber and 19.73% for those using CH NH PbI absorber is achieved. The superior uniformity of low-temperature processed SnO QD ESLs also enables the fabrication of ≈19% efficiency PSCs with an aperture area of 1.0 cm and 16.97% efficiency flexible device. The results demonstrate the promise of carrier-concentration-controlled SnO QD ESLs for fabricating stable, efficient, reproducible, large-scale, and flexible planar PSCs.

Reducing the energy loss and retarding the carrier recombination at the interface are crucial to improve the performance of the perovskite solar cell (PSCs). However, little is known about the recombination mechanism at the interface of anode and SnO2 electron transfer layer (ETL). In this work, an ultrathin wide bandgap dielectric MgO nanolayer is incorporated between SnO2:F (FTO) electrode and SnO2 ETL of planar PSCs, realizing enhanced electron transporting and hole blocking properties. With the use of this electrode modifier, a power conversion efficiency of 18.23% is demonstrated, an 11% increment compared with that without MgO modifier. These improvements are attributed to the better properties of MgO‐modified FTO/SnO2 as compared to FTO/SnO2, such as smoother surface, less FTO surface defects due to MgO passivation, and suppressed electron–hole recombinations. Also, MgO nanolayer with lower valance band minimum level played a better role in hole blocking. When FTO is replaced with Sn‐doped In2O3 (ITO), a higher power conversion efficiency of 18.82% is demonstrated. As a result, the device with the MgO hole‐blocking layer exhibits a remarkable improvement of all J–V parameters. This work presents a new direction to improve the performance of the PSCs based on SnO2 ETL by transparent conductive electrode surface modification.

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Planar perovskite solar cells (PSCs) based on low-temperature-processed (LTP) SnO 2 have demonstrated excellent photovoltaic properties duo to the high electron mobility, wide bandgap, and suitable band energy alignment of LTP SnO 2 . However, planar PSCs or mesoporous (mp) PSCs based on hightemperature-processed (HTP) SnO 2 show much degraded performance. Here, a new strategy with fully HTP Mg-doped quantum dot SnO 2 as blocking layer (bl) and a quite thin SnO 2 nanoparticle as mp layer are developed. The performances of both planar and mp PSCs has been greatly improved. The use of Mg-SnO 2 in planar PSCs yields a high-stabilized power conversion efficiency (PCE) of close to 17%. The champion of mp cells exhibits hysteresis free and stable performance with a high-stabilized PCE of 19.12%. The inclusion of thin mp SnO 2 in PSCs not only plays a role of an energy bridge, facilitating electrons transfer from perovskite to SnO 2 bl, but also enhances the contact area of SnO 2 with perovskite absorber. Impedance analysis suggests that the thin mp layer is an "active scaffold" selectively collecting electrons from perovskite and can eliminate hysteresis and effectively suppress recombination. This is an inspiring advance toward high-performance PSCs with HTP mp SnO 2 .

Recently, considerable progress is achieved in lab prototype perovskite solar cells (PSCs); however, the stability of outdoor applications of PSCs remains a challenge due to the high sensitivity of perovskite material under moist and ultraviolet (UV) light conditions. In this work, the UV photostability of PSC devices is improved by incorporating a photon downshifting layer—SrAl2O4: Eu2+, Dy3+ (SAED)—prepared using the pulsed laser deposition approach. Light‐induced deep trap states in the photoactive layer are depressed, and UV light‐induced device degradation is inhibited after the SAED modification. Optimized power conversion efficiency (PCE) of 17.8% is obtained through the enhanced light harvesting and reduced carrier recombination provided by SAED. More importantly, a solar energy storage effect due to the long‐persistent luminescence of SAED is obtained after light illumination is turned off. The introduction of downconverting material with long‐persistent luminescence in PSCs not only represents a new strategy to improve PCE and light stability by photoconversion from UV to visible light but also provides a new paradigm for solar energy storage.

The advent of the two-dimensional (2D) family of halide perovskites and their demonstration in 2D/three-dimensional (3D) hierarchical film structures broke new ground toward high device performance and good stability. The 2D Dion−Jacobson (DJ) phase halide perovskites are especially attractive in solar cells because of their superior charge transport properties. Here, we report on 2D DJ phase perovskites using a 3-(aminomethyl)piperidinium (3AMP) organic spacer for the fabrication of mixed Pb/ Sn-based perovskites, exhibiting a narrow bandgap of 1.27 eV and a long carrier lifetime of 657.7 ns. Consequently, solar cells employing mixed 2D DJ 3AMP-based and 3D MA 0.5 FA 0.5 Pb 0.5 Sn 0.5 I 3 (MA = methylammonium, FA = formamidinium) perovskite composites as light absorbers achieve enhanced efficiency and stability, giving a power conversion efficiency of 20.09% with a high open-circuit voltage of 0.88 V, a fill factor of 79.74%, and a short-circuit current density of 28.63 mA cm −2 . The results provide an effective strategy to improve the performance of single-junction narrow-bandgap solar cells and, potentially, to give a highly efficient alternative to bottom solar cells in tandem devices.