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With record power conversion efficiencies (PCE) close to 24%, perovskite solar cells (PSCs) hold great promise to combine high photovoltaic performance with low processing cost. [1][2][3][4] This combination can be ascribed to several factors, such as the remarkable optoelectronic properties of perovskites-evidenced by a sharp absorption onset, a small Urbach energy, and a low exciton binding energy-as well as the fact that perovskites use earth-abundant elements. [5][6][7] The organic-inorganic metal halide perovskites used in such devices feature the characteristic AMX 3 structure, where A is an organic or inorganic cation, most often methylammonium (CH 3 NH 3 + , MA + ), formamidinium (HC(NH 2 ) 2 + , FA + ), or cesium (Cs + ), M is a metal cation, such as Pb 2+ or Sn 2+ , and X is a monovalent anion (halide ion Cl, Br, or I). [8][9][10] Various deposition techniques have been developed to obtain polycrystalline perovskite films, including spin-, dip-, and blade-coating, as well as vacuumbased processes. [11,12] To date, one-step spin coating remains the most popular and facile implementation to fabricate high-quality hybrid perovskite thin films from solution. The device efficiency and stability are strongly dictated by the electronic properties and trap state densities in the perovskite film. These, in turn, are influenced by the film crystallization, its morphology, and the presence of defects and imperfections. [13,14] However, despite the simplicity of the one-step deposition, the exact mechanisms underlying the growth process, from precursor solution to solid-state film, and the crystallization mechanism within the sol-gel state, are still poorly understood. This lack of information is partially caused by the heavy reliance on post-deposition ex situ characterization methods to date. [15,16] Recently, in situ time-resolved grazing-incidence wide-angle X-ray scattering (GIWAXS) has emerged as a powerful technique to study the microstructural evolution from the solution-precursor to the solid state, revealing the crystallization behavior and formation of crystalline intermediates and byproducts. [17,18] Using in situ GIWAXS methods, the classic MAPbI 3 perovskite material has already been investigated; Gong and coworkers identified the Perovskite solar cells increasingly feature mixed-halide mixed-cation compounds (FA 1−x−y MA x Cs y PbI 3−z Br z ) as photovoltaic absorbers, as they enable easier processing and improved stability. Here, the underlying reasons for ease of processing are revealed. It is found that halide and cation engineering leads to a systematic widening of the anti-solvent processing window for the fabrication of high-quality films and efficient solar cells. This window widens from seconds, in the case of single cation/ halide systems (e.g., MAPbI 3 , FAPbI 3 , and FAPbBr 3 ), to several minutes for mixed systems. In situ X-ray diffraction studies reveal that the processing window is closely related to the crystallization of the disordered sol-gel and to the number of crystalline byprodu...

Hole-transporting layers (HTLs) are an essential component in inverted, p-i-n perovskite solar cells (PSCs) where they play a decisive role in extraction and transport of holes, surface passivation, perovskite crystallization, device stability, and cost. Currently, the exploration of efficient, stable, highly transparent and low-cost HTLs is of vital importance for propelling p-i-n PSCs toward commercialization. Compared to their inorganic counterparts, organic HTLs offer multiple advantages such as a tunable bandgap and energy level, easy synthesis and purification, solution processability, and overall low cost. Here, recent progress of organic HTLs, including conductive polymers, small molecules, and self-assembled monolayers, as utilized in inverted PSCs is systematically reviewed and summarized. Their molecular structure, hole-transport properties, energy levels, and relevant device properties and resulting performances are presented and analyzed. A summary of design principles and a future outlook toward highly efficient organic HTLs in inverted PSCs is proposed. This review aims to inspire further innovative development of novel organic HTLs for more efficient, stable, and scalable inverted PSCs.

Defects at the surface and grain boundaries of metal–halide perovskite films lead to performance losses of perovskite solar cells (PSCs). Here, organic cyano‐based π‐conjugated molecules composed of indacenodithieno[3,2‐b]thiophene (IDTT) are reported and it is found that their cyano group can effectively passivate such defects. To achieve a homogeneous distribution, these molecules are dissolved in the antisolvent, used to initiate the perovskite crystallization. It is found that these molecules are self‐anchored at the grain boundaries due to their strong binding to undercoordinated Pb2+. On a device level, this passivation scheme enhances the charge separation and transport at the grain boundaries due to the well‐matched energetic levels between the passivant and the perovskite. Consequently, these benefits contribute directly to the achievement of power conversion efficiencies as high as 21.2%, as well as the improved environmental and thermal stability of the PSCs. The surface treatment provides a new strategy to simultaneously passivate defects and enhance charge extraction/transport at the device interface by manipulating the anchoring groups of the molecules.

Producing high efficiency solar cells without high-temperature processing or use of additives still remains a challenge with the two-step process. Here, the solution processing of MAPbI 3 from PbI 2 films in N,N-dimethylformamide (DMF) is investigated. In-situ grazing incidence wide-angle X-ray scattering (GIWAXS) measurements reveal a sol-gel process involving three PbI 2 -DMF solvate complexes-disordered (P 0 ) and ordered (P 1 , P 2 )-prior to PbI 2 formation. When the appropriate solvated state of PbI 2 is exposed to MAI (methylammonium Iodide), it can lead to rapid and complete room temperature conversion into MAPbI 3 with higher quality films and improved solar cell performance. Complementary in-situ optical reflectance, absorbance, and quartz crystal microbalance with dissipation (QCM-D) measurements show that dry PbI 2 can take up only one third of the MAI taken up by the solvated-crystalline P 2 phase of PbI 2 , requiring additional annealing and yet still underperforming. The perovskite solar cells fabricated from the ordered P 2 precursor show higher power conversion efficiency (PCE) and reproducibility than devices fabricated from other cases. The average PCE of the solar cells is greatly improved from 13.2(±0.53)% (from annealed PbI 2 ) to 15.7(±0.35)% (from P 2 ) reaching up to 16.2%. This work demonstrates the importance of controlling the solvation of PbI 2 as an effective strategy for the growth of high-quality perovskite films and their application in high efficiency and reproducible solar cells.