This review systematically describes the origins, characterization and implications of strain in perovskite solar cells and proposes novel control strategies.

This work summarizes recent progress of low-temperature carbon-based electrodes in perovskite photovoltaic devices and their great up-scalability potential.

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 V_OC are quantified by electrochemical impedance spectroscopy and J_SC–V_OC 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.

Chemical environment and precursor-coordinating molecular interactions within a perovskite precursor solution can lead to important implications in structural defects and crystallization kinetics of a perovskite film. Thus, the opto-electronic quality of such films can be boosted by carefully fine-tuning the coordination chemistry of perovskite precursors via controllable introduction of additives, capable of forming intermediate complexes. In this work, we employed a new type of ligand, namely 1-phenylguanidine (PGua), which coordinates strongly with the PbI₂ complexes in the perovskite precursor, forming new intermediate species. These strong interactions effectively retard the perovskite crystallization process and form homogeneous films with enlarged grain sizes and reduced density of defects. In combination with an interfacial treatment, the resulted champion devices exhibit a 24.6% efficiency with outstanding operational stability. Unprecedently, PGua can be applied in various PSCs with different perovskite compositions and even in both configurations: n–i–p and p–i–n, highlighting the universality of this ligand.

The liquefaction and recrystallization of CH₃NH₃PbI₃ perovskite with methylamine - MA⁰/CH₃NH₂ gas in solar cells have been intensively studied in the past years, yet the exact mechanisms remained hypothesized and unclear due to lack of experimental evidence. The investigation presented in this work connects the interatomic bonding in the methylamine-MAPbI₃ complex during liquefaction and recrystallization with the crystal morphology and the lifetime of photo-generated carriers in planar and mesoporous layer structures. Raman shift analysis and XRD structural investigation have been performed, showing that MA⁰ gas forms a complex with MAPbI₃ via chemisorption of gas molecules to the perovskite crystal surface by replacing I^- atoms in the PbI₆ octahedra and via coordination of MA⁺. Through real-time photoluminescence, the changes in bonding during the liquid-solid transition are related to the radiative charge carrier recombination during the liquefaction and recrystallization. We found that the amount of MA⁰ gas reacting with perovskite determines the crystal morphology, pore-filling and, most importantly, the charge carrier lifetime.