The control over the precursor concentration is used to fabricate sensitized and thin-film perovskite solar cells. The dominating capacitance contributions in these devices reveal the main processes determining the response of perovskite solar cells, which enables the development of an impedance spectroscopy equivalent circuit for perovskite solar cells derived from the well-known circuit for sensitized devices. This tool can provide essential information for the photovoltaic performance in terms of transport and recombination processes.

Interface engineering and passivating contacts are key enablers to reach the highest efficiencies in photovoltaic devices. While printed carbon–graphite back electrodes for hole‐transporting material (HTM)‐free perovskite solar cells (PSCs) are appealing for fast commercialization of PSCs due to low processing costs and extraordinary stability, this device architecture so far suffers from severe performance losses at the back electrode interface. Herein, a 2D perovskite passivation layer as an electron blocking layer (EBL) at this interface to substantially reduce interfacial recombination losses is introduced. The formation of the 2D perovskite EBL is confirmed through X‐ray diffraction, photoemission spectroscopy, and an advanced spectrally resolved photoluminescence microscopy mapping technique. Reduced losses that lead to an enhanced fill factor and VOC are quantified by electrochemical impedance spectroscopy and JSC–VOC measurements. This enables reaching one of the highest reported efficiencies of 18.5% for HTM‐free PSCs using 2D perovskite as an EBL with a significantly improved device stability.

By two-step sequential Pb 2+ adsorption and reaction with methylammonium-iodide (MAI) or -bromide (MABr) at a low concentration level of 0.06-0.10 m over mesoporous TiO 2 or ZrO 2 film, a well-defined nanoscale CH 3 NH 3 PbI 3 (MAPbI 3 ) photosensitizer or CH 3 NH 3 PbBr 3 (MAPbBr 3 ) light emitter could be prepared in situ, respectively in a reproducible and atom-economical way. The as-prepared nanoscale perovskites are compared with their thin film counterparts in terms of light absorption/emission, crystallinity, surface morphology, and energy-conversion efficiency. The nanoscale perovskite-decorated films display more transparency than the bulky film due to the much lower amount deposited, while blueshifted and overwhelmingly brighter photoluminescence is observed in the "nano" relative to the "bulk" due to quantum size confinement. Transmission electron microscopy images also clearly show that a few nanometer-sized perovskite dots are deposited homogeneously over the surface of TiO 2 -or ZrO 2 -particulate film in the course of the current preparative route. When the nano-MAPbI 3 is tested as a photosensitizer in a solid-state dye-sensitized solar cell configuration with a very thin (≈650 nm) TiO 2 mesoporous film, it has a promising initial power conversion efficiency of 6.23%, which outperformed the result of 2.28% from a typical organic molecular dye coded as MK-2.

Three benzodipyrrole (BDP)‐based organic small molecules with substituted 4‐methoxyphenyl (CB‐1), 3‐fluorophenyl (CB‐2), and 3‐trifluoromethylphenyl (CB‐3) are designed, synthesized, and used as a hole‐transporting material (HTM) for perovskite solar cells (PSCs). The electrochemical, optical, thermal, electronic, and optoelectronic properties of the HTMs are characterized to verify their suitability for PSCs. The terminal functional groups of the HTMs having different heteroatoms mainly target effective defect passivation of perovskites. Photoluminescence studies and molecular dynamic simulations reveal that fluorine atoms within CB‐2 and CB‐3 can contribute to the defect passivation via interaction with Pb of the perovskite. In particular, a highly planar conformation of CB‐2 on the perovskite surface can facilitate more efficient hole transfer at the interface. Thus, the PSCs employing CB‐2 achieve the highest power conversion efficiency (PCE) of 18.23% while the devices using CB‐1 and CB‐3 exhibit a lower PCE of 16.78% and 16.74%, respectively. PSCs with the BDP‐based HTMs demonstrate excellent long‐term storage stability without degradation in their PCEs over 6 months. The highly planar geometry, defect passivation effect, and hydrophobicity of CB‐2 show its great potential as an HTM for efficient and stable PSCs.