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.

Currently, photovoltaic/electroluminescent (PV/EL) perovskite bifunctional devices (PBDs) exhibit poor performance due to defects and interfacial misalignment of the energy band. Interfacial energy‐band engineering between the perovskite and hole‐transport layer (HTL) is introduced to reduce energy loss, through adding corrosion‐free 3,3′‐(2,7‐dibromo‐9H‐fluorene‐9,9‐diyl) bis(n,n‐dimethylpropan‐1‐amine) (FN‐Br) into a HTL free of lithium salt. This strategy can turn the n‐type surface of perovskite into p‐type and thus correct the misalignment to form a well‐defined N–I–P heterojunction. The tailored PBD achieves a high PV efficiency of up to 21.54% (certified 20.24%) and 4.3% EL external quantum efficiency. Free of destructive additives, the unencapsulated devices maintain >92% of their initial PV performance for 500 h at maximum power point under standard air mass 1.5G illumination. This strategy can serve as a general guideline to enhance PV and EL performance of perovskite devices while ensuring excellent stability.

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Poly(3,4‐ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) can be roll‐to‐roll deposited on the substrate facilely in the electronics, but its acidity and mismatched energy level limit the performance and stability. Herein, different metal salts are incorporated into PEDOT:PSS solution to prepare PEDOT:PSS‐AxOy (metal oxide) composite hole transport layer and it is found that the performance of inverted perovskite solar cells (PSCs) can be greatly enhanced. PSC using PEDOT:PSS‐MoOx has achieved much higher power conversion efficiency (PCE) (19.64%) than that of pristine PEDOT:PSS (12.19%). Two key factors are important for the performance enhancement. First, the increased surface free energy of PEDOT:PSS‐AxOy is beneficial for the formation of large crystal size and pinhole‐free film, leading to reduced nonradiative recombination. Second, the work function of PEDOT:PSS can be tuned to match the energy level of photoactive layer with small amount incorporation, which greatly enhances the photovoltage by a factor of 1.1. Besides, the devices based on PEDOT:PSS‐AxOy exhibit improved long‐term stability. Unencapsulated PSCs with PEDOT:PSS‐MoOx retain over 90% and 80% of their initial PCEs in N2 for 45 d and in ambient air for 20 d, respectively. The modified PEDOT:PSS solutions overcome the intrinsic imperfection and can be potentially employed for large‐scale production in the electronic devices.