In recent years, organic−inorganic hybrid perovskite solar cells (PSCs) have attracted extensive attention due to their high power conversion efficiency (PCE) and simple preparation process. The selection and optimization of the hole transport layer (HTL) are very important for device performance. Compared to other HTLs, nickel oxide (NiO x ) has been widely used in PSCs due to its good chemical stability, high hole mobility, and simple preparation method. This review begins with the application of NiO x HTL in planar PSCs and systematically introduces the influence of the structure and photoelectrical properties of devices by doping and surface modification. The effects of NiO x modification on the power conversion efficiency (PCE), filling factor (FF), open-circuit voltage (V oc ), short-circuit current (J sc ), and stability of PSCs are reviewed in detail from the perspectives of energy-level matching, hole mobility, and crystallinity. Finally, the future of NiO x -based planar PSCs is discussed.

Perovskite solar cells (PSCs) have gained much attention because of their expressive power conversion efficiency (PCE) of up to 25.5%. A good contact and a well-aligned energy level at the buried interfaces between electron transport layers (ETLs) and perovskite films play an essential role in promoting charge-carrier collection and suppressing nonradiative recombination. Currently, low-temperature-processed SnO2 thin films are widely used as the ETLs to achieve efficient and stable planar PSCs. However, fabricating proper SnO2/perovskite interfaces with a good contact and a well-aligned energy level is necessary but implies a great challenge. Herein, we modify the SnO2 ETL using benzylamine hydrochloride (BH), which is expected to facilitate the energy level alignment and to enhance perovskite crystallization. Moreover, the BH interlayer is found to effectively reduce the trap-state density and thereby improve the charge-carrier extraction between the ETL and the perovskite layer. Consequently, the PSC with BH modification yields a higher PCE, a lower hysteresis, and better stability than the device without a BH interlayer. This study highlights the key role of molecule modification of ETLs in designing efficient and stable PSCs.

Organic–inorganic perovskite solar cells (PSCs) have achieved great attention due to their expressive power conversion efficiency (PCE) up to 25.7%. To improve the photovoltaic performance of PSCs, interface engineering between the perovskite and hole transport layer (HTL) is a widely used strategy. Following this concept, benzyl trimethyl ammonium chlorides (BTACls) are used to modify the wet chemical processed perovskite film in this work. The BTACl‐induced low dimensional perovskite is found to have a bilayer structure, which efficiently decreases the trap density and improves the energy level alignment at the perovskite/HTL interface. As a result, the BTACl‐modified PSCs show an improved PCE compared to the control devices. From device modeling, the reduced charge carrier recombination and promoted charge carrier transfer at the perovskite/HTL interface are the cause of the open‐circuit ( V oc ) and fill factor (FF) improvement, respectively. This study gives a deep understanding for surface modification of perovskite films from a perspective of the morphology and the function of enhancing photovoltaic performance.

Ionic liquid (IL) additives proved to have a positive effect on the device efficiency and stability of perovskite solar cells. However, since ILs are small molecules and undergo Coulomb interactions, they can easily aggregate and evaporate over long times, which would cause instabilities during a long-term device operation. To overcome these problems, we polymerize ILs into macromolecules and incorporate them into perovskite films as well as into the corresponding solar cells. Both cations and anions of the used poly­[1-(2-acryloylethyl)-3-methylimidazolium] bis (trifluoromethane) sulfonamides (PAEMI-TFSIs) are designed to coordinate with the Pb and I of PbI6 2– octahedra, respectively, which changes the crystallization behavior of the perovskite films. Importantly, the PAEMI-TFSI efficiently passivates electronic defects on the grain boundaries and thereby enhances the charge-carrier transport in the perovskite film. As a result, PAEMI-TFSI-modified MAPbI3 solar cells show a high power conversion efficiency of 22.4% and an excellent storage stability (92% of the initial efficiency remains after 1200 h operation in a nitrogen atmosphere for nonencapsulated devices).