fabrication of perovskite materials provide the feasibility of preparing flexible PSCs (F-PSCs), where their light-weight and bending-flexible properties, make this technology desirable in various occasions such as wearable bioelectronics, portable power equipment, deployable tents, etc. [7] Nowadays, with continuous developments in device structure and materials processing, PCEs of F-PSCs in laboratory have exceeded 21% in small-area (<0.1 cm 2 ) [8,9] and 15% in large-area (100 cm 2 ) [10] solar cell devices, thus showing promises in future flexible PV applications.F-PSCs are fabricated via replacing conventional rigid glass substrates with flexible substrates (e.g., polyethylene terephthalate (PET), and polyethylene naphthalate (PEN)). However, owing to the heat and chemically vulnerable nature of most organic materials, flexible substrate cannot withstand excessively high temperature without losing its elastic properties and therefore presents multiscale challenges that would hinder device performance. First of all, halide perovskites generally have inferior mechanical adhesion with adjacent functional layers or substrates due to their low cohesion energies, [11,12] while the large thermal expansion/contraction of flexible substrates during heat treatment will exacerbate such interfacial adhesion, thus rendering charge transport and mechanical durability in F-PSCs outstanding issues. [13] To achieve low-temperature processable functional layers, SnO 2 has been most frequently used as electron transport layer (ETL) in F-PSCs. [14] However, it presents unsatisfying interfacial electronic compatibility such as surface defects, lattice mismatch and large conduction band offset (ΔE CB ) with perovskite layer. [15,16] In addition, due to the discrepant physical properties (e.g., thermal expansivity) between organiccontaining perovskite and inorganic SnO 2 , perovskite/SnO 2 interface also contributes to phenomenal interfacial residual stress [17,18] and thus the consequent mechanical delamination, [13] once again affecting the PV performance and long-term durability of F-PSCs. To tackle the perovskite/SnO 2 interface problem, previous work included formamidinium iodide (FAI) in SnO 2 layer that consequently formed porous and interpenetrating interface between SnO 2 and perovskite for robust mechanical durability in F-PSCs. [19] While precise control of SnO 2 surface morphology and thickness enabled high-quality perovskite film formation with reduced trap-state density and Halide perovskites have shown superior potentials in flexible photovoltaics due to their soft and high power-to-weight nature. However, interfacial residual stress and lattice mismatch due to the large deformation of flexible substrates have greatly limited the performance of flexible perovskite solar cells (F-PSCs). Here, ammonium formate (HCOONH 4 ) is used as a preburied additive in electron transport layer (ETL) to realize a bottom-up infiltration process for an in situ, integral modification of ETL, perovskite layer, and their interface. The ...

To date, the improvement of open-circuit voltage (V OC ) offers a breakthrough for the performance of perovskite solar cells (PSCs) toward their theoretical limit. Surface modification through organic ammonium halide salts (e.g., phenethylammonium ions PEA + and phenmethylammonium ions PMA + ) is one of the most straightforward strategies to suppress defect density, thereby leading to improved V OC . However, the mechanism underlying the high voltage remains unclear. Here, polar molecular PMA + is applied at the interface between perovskite and hole transporting layer and a remarkably high V OC of 1.175 V is obtained which corresponds to an increase of over 100 mV in comparison to the control device. It is revealed that the equivalent passivation effect of surface dipole effectively improves the splitting of the hole quasi-Fermi level. Ultimately the combined effect of defect suppression and surface dipole equivalent passivation effect leads to an overall increase in significantly enhanced V OC . The resulted PSCs device reaches an efficiency of up to 24.10%. Contributions are identified here by the surface polar molecules to the high V OC in PSCs. A fundamental mechanism is suggested by use of polar molecules which enables further high voltage, leading ways to highly efficient perovskite-based solar cells.