Recently, researchers have focused on the design of highly efficient flexible perovskite solar cells (PVSCs), which enables the implementation of portable and roll-to-roll fabrication in large scale. While NiOx is a promising material for hole transport layer (HTL) candidate for fabricating efficient PVSCs on a rigid substrate, the reported NiOx HTLs are formed using different multistep treatments (such as 300-500 °C annealing, O2-plasma, UVO, etc.), which hinders the development of flexible PVSCs based on NiOx. Meanwhile, the features of nanostructured morphology and flawless film quality are very important for the film to function as highly effective HTL of PVSCs. However, it is difficult to have the two features coexist natively, particularly in a solution process that flawless film will usually come with smooth morphology. Here, we demonstrate the flawless and surface-nanostructured NiOx film from a simple and controllable room-temperature solution process for achieving high performance flexible PVSCs with good stability and reproducibility. The power conversion efficiency (PCE) can reaches a promising value of 14.53% with no obvious hysteresis (and a high PCE of 17.60% for PVSC on ITO glass). Furthermore, the NiOx-based PVSCs show markedly improved air stability. Regarding the performance improvement, the flawless and surface-nanostructured NiOx film can make the interfacial recombination and monomolecular Shockley-Read-Hall recombination of PVSC reduce. In addition, the formation of an intimate junction of large interfacial area at NiOx film/the perovskite layer improve the hole extraction and thus PVSC performances. This work contributes to the evolution of flexible PVSCs with simple fabrication process and high device performances.

In the early stage, Marks and co-workers [ 13 ] reported a striking performance improvement of OSCs by replacing PEDOT:PSS with NiO x fi lm using a pulsed-laser deposition technology. From then on, NiO x HTLs have been reported for organic optoelectronics by various preparation methods, such as thermal evaporation, [ 14 ] sputtering, [ 9a ] and solution process. [ 9d , 15 ] Among them, solution process method is desirable for low-cost, large-scale and roll-to-roll production. Olson and co-workers [ 16 ] proposed a solution-processed NiO x fi lm as highly effi cient HTL in OSCs. The functional NiO x HTL was fabricated through annealing the precursor fi lm at a temperature of 275 °C. So and co-workers [ 17 ] also presented a NiO x fi lm by using monoethanolamine (MEA) to react with Ni in ethanol solution followed by thermally converting (275 °C) coordination complexes ions [Ni(MEA) 2 (OAc)]+ into high-quality NiO x . Meanwhile, solution-processed NiO x at 150 °C has also been realized. Ma and co-workers [ 18 ] reported a solution-processed NiO x fi lm for OSCs using oxygen-plasma treatment and annealing treatment simultaneously. Zhang et. al. reported that the colloidal NiO nanoparticles are used as the anode buffer layer in OSCs without high temperature post-annealing to induce decomposition and crystallization. [ 9f ] For a long period, the studies of NiO x HTLs were focused on utilizing sol-gel methods with thermally converting the precursor solution to NiO x thin fi lms. In the process of device fabrications, thermal annealing process and oxygen-plasma treatment may be simultaneously required, which hinders the applications of NiO x in fl exible optoelectronic devices. Instead of precursor method, an approach to signifi cantly reduce the processing temperature of TMO HTLs is to directly use high-quality colloidal nanoparticles (NPs). Jin and co-workers demonstrated a facile and general strategy based on ligand protection for the synthesis of unstable colloidal NiO nanocrystals. [ 19 ] Fattakhova-Rohlfi ng and co-workers described the preparation of ultrasmall, crystalline, and dispersible NiO nanoparticles, which are promising candidates as catalysts for electrochemical oxygen generation. [ 9e ] Herein, we will demonstrate a facile chemical precipitation method which is robust and simple for direct preparation of high-quality non-stoichiometric NiO x NPs. Remarkably, by using this method, NiO x HTL fi lm can be formed through a room-temperature solution process without any post-treatments during device fabrication. Interestingly, our results show that the NiO x NPs fi lm can function as effective HTLs over a wide range of annealing temperatures from room temperature to 150 °C. Very good optoelectronic performances utilizing the NiO x NPs fi lm as HTLs have been demonstrated in both OSCs and OLEDs. High power conversion effi ciency (PCE) of 9.16% (best 9.28%) was achieved in OSCs using NiO x

Entirely low‐temperature solution‐processed (≤100 °C) planar p‐i‐n perovskite solar cells (PSCs) offer great potential for commercialization of roll‐to‐roll fabricated photovoltaic devices. However, the stable inorganic hole‐transporting layer (HTL) in PSCs is usually processed at high temperature (200–500 °C), which is far beyond the tolerant temperature (≤150 °C) of roll‐to‐roll fabrication. In this context, inorganic NiOx nanoparticles (NPs) are an excellent candidate to serve as the HTL in PSCs, owing to their excellent solution processability at room temperature. However, the low‐temperature processing condition is usually accompanied with defect formation, which deteriorates the film quality and device efficiency to a large extent. To suppress this setback, we used a series of benzoic acid selfassembled monolayers (SAMs) to passivate the surface defects of the NiOx NPs and found that 4‐bromobenzoic acid could effectively play the role of the surface passivation. This SAM layer reduces the trap‐assisted recombination, minimizes the energy offset between the NiOx NPs and perovskite, and changes the HTL surface wettability, thus enhancing the perovskite crystallization, resulting in more stable PSCs with enhanced power conversion efficiency (PCE) of 18.4 %, exceeding the control device PCE (15.5 %). Also, we incorporated the above‐mentioned SAMs into flexible PSCs (F‐PSCs) and achieved one of the highest PCE of 16.2 % on a polyethylene terephthalate (PET) substrate with a remarkable power‐per‐weight of 26.9 W g−1. This facile interfacial engineering method offers great potential for the large‐scale manufacturing and commercialization of PSCs.

A new, all room-temperature solution process is developed to fabricate efficient, low-cost, and stable perovskite solar cells (PVSCs). The PVSCs show high efficiency of 17.10% and 14.19%, with no hysteresis on rigid and flexible substrates, respectively, which are the best efficiencies reported to date for PVSCs fabricated by room-temperature solution-processed techniques. The flexible PVSCs show a remarkable power-per-weight of 23.26 W g .