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.

Despite the rapid increase of efficiency, perovskite solar cells (PSCs) still face some challenges, one of which is the current-voltage hysteresis. Herein, it is reported that yttrium-doped tin dioxide (Y-SnO ) electron selective layer (ESL) synthesized by an in situ hydrothermal growth process at 95 °C can significantly reduce the hysteresis and improve the performance of PSCs. Comparison studies reveal two main effects of Y doping of SnO ESLs: (1) it promotes the formation of well-aligned and more homogeneous distribution of SnO nanosheet arrays (NSAs), which allows better perovskite infiltration, better contacts of perovskite with SnO nanosheets, and improves electron transfer from perovskite to ESL; (2) it enlarges the band gap and upshifts the band energy levels, resulting in better energy level alignment with perovskite and reduced charge recombination at NSA/perovskite interfaces. As a result, PSCs using Y-SnO NSA ESLs exhibit much less hysteresis and better performance compared with the cells using pristine SnO NSA ESLs. The champion cell using Y-SnO NSA ESL achieves a photovoltaic conversion efficiency of 17.29% (16.97%) when measured under reverse (forward) voltage scanning and a steady-state efficiency of 16.25%. The results suggest that low-temperature hydrothermal-synthesized Y-SnO NSA is a promising ESL for fabricating efficient and hysteresis-less PSC.

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.

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 .

film deposition process to achieve desired morphology and microstructure. [2-6] Most of the high-efficiency PSCs were produced by either one-step or two-step fabrication methods. The very first MAPbI 3-based PSC was fabricated via a one-step spincoating process developed by Kojima et al. in 2009. [7] However, the one-step fabricated perovskite film typically exhibited a dendritic morphology with poor coverage. To address the morphology issue, a two-step spin-coating process was developed by Xiao et al. [8] and Im et al. [9] in 2014, which consisted of sequential depositions of an inorganic PbI 2 layer and an organic salt MAI. This two-step method gave rise to compact and pinhole-free MAPbI 3 perovskite films, significantly increasing the efficiency of MA-based PSCs to ≈17%. [9] In 2015, the record PCE received another boost through the development of a simpler antisolvent-assisted one-step method, which could form a very uniform perovskite thin film by promoting the crystallization process. [4,10] Meanwhile, MA cations were replaced by CH(NH 2) 2 + (FA) cations to improve the light Two-step-fabricated FAPbI 3-based perovskites have attracted increasing attention because of their excellent film quality and reproducibility. However, the underlying film formation mechanism remains mysterious. Here, the crystallization kinetics of a benchmark FAPbI 3-based perovskite film with sequential A-site doping of Cs + and GA + is revealed by in situ X-ray scattering and first-principles calculations. Incorporating Cs + in the first step induces an alternative pathway from δ-CsPbI 3 to perovskite α-phase, which is energetically more favorable than the conventional pathways from PbI 2. However, pinholes are formed due to the nonuniform nucleation with sparse δ-CsPbI 3 crystals. Fortunately, incorporating GA + in the second step can not only promote the phase transition from δ-CsPbI 3 to the perovskite α-phase, but also eliminate pinholes via Ostwald ripening and enhanced grain boundary migration, thus boosting efficiencies of perovskite solar cells over 23%. This work demonstrates the unprecedented advantage of the two-step process over the one-step process, allowing a precise control of the perovskite crystallization kinetics by decoupling the crystal nucleation and growth process.