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 ...

Lead‐free double perovskites have been demonstrated as promising alternatives to solve the toxicity and stability issues in conventional lead trihalide perovskites. However, different solubility of components in the precursors hinders fabrication of double perovskite films with commonly used solution procedures. Here, for the first time, the authors successfully prepared double perovskite Cs2AgBiBr6 thin films throughout a sequential‐vapor‐deposition procedure. The obtained thin films with pure double perovskite phase show large grain sizes, uniform, and smooth surface properties. In addition, the high‐quality vapor‐deposited Cs2AgBiBr6 films exhibit a photoluminescence (PL) lifetime of 117 ns, indicative of significant potential in photovoltaic applications. The resulting solar cells with planar device structure show an optimized power conversion efficiency of 1.37%, which can be maintained at 90% after 240 h of storage under ambient condition. Our results demonstrate the feasibility of employing vapor deposition technique to fabricate high‐quality double perovskite thin films, which paves the way for further development of various optoelectronic devices based on these promising lead‐free semiconductors.

Current popular and efficient strategies to improve the long-term stability regarding protection against moisture in the field of PSCs.

Organic-inorganic lead halide perovskite compounds are very promising materials for high-efficiency perovskite solar cells. But how to fabricate high-quality perovksite films under controlled humidity conditions is still an important issue due to their sensitivity to moisture. In this study, we investigated the influence of ambient humidity on crystallization and surface morphology of one-step spin-coated perovskite films, as well as the performance of solar cells based on these perovskite films. On the basis of experimental analyses and thin film growth theory, we conclude that the influence of ambient humidity on nucleation at spin-coating stage is quite different from that on crystal growth at annealing stage. At the spin-coating stage, high nucleation density induced by high supersaturation prefers to appear under anhydrous circumstances, resulting in layer growth and high coverage of perovskite films. But at the annealing stage, the modest supersaturation benefits formation of perovskite films with good crystallinity. The films spin-coated under low relative humidity (RH) followed by annealing under high RH show an increase of crystallinity and improved performance of devices. Therefore, a mechanism of fast nucleation followed by modest crystal growth (high supersaturation at spin-coating stage and modest supersaturation at annealing stage) is suggested in the formation of high-quality perovskite films.

Two-dimensional (2D) Sn-based lead-free perovskites have attracted extensive attention because of their nontoxicity and wide light absorption. It has been proven that the introduced organic spacer cations in the perovskite crystal prevent Sn2+ from oxidation to Sn4+. However, the effects of the alkyl chain length of these cations on the perovskite properties are unclear. Here, we investigate the impacts of chain length on crystal orientation and phase distribution of 2D Sn-based perovskite films by employing different alkylamines spacer cations (butylamine, octylamine, and dodecylamine). With the increase of alkyl chain length, the phase distribution of 2D Sn-based perovskite crystals become disordered and less oriented. Therefore, benefiting from application of a short alkyl chain in the organic spacer cation (e.g., BA), we manage to retard the oxidation process of Sn2+ for better device performance. Our work provides systematic understanding of configuration and size of organic spacer cations, which will further contribute to highly stable and efficient lead-free perovskite solar cells.

Perovskite/silicon tandem solar cells are promising avenues for achieving high‐performance photovoltaics with low costs. However, the highest certified efficiency of perovskite/silicon tandem devices based on economically matured silicon heterojunction technology (SHJ) with fully textured wafer is only 25.2% due to incompatibility between the limitation of fabrication technology which is not compatible with the production‐line silicon wafer. Here, a molecular‐level nanotechnology is developed by designing NiOx/2PACz ([2‐(9H‐carbazol‐9‐yl) ethyl]phosphonic acid) as an ultrathin hybrid hole transport layer (HTL) above indium tin oxide (ITO) recombination junction, to serve as a vital pivot for achieving a conformal deposition of high‐quality perovskite layer on top. The NiOx interlayer facilitates a uniform self‐assembly of 2PACz molecules onto the fully textured surface, thus avoiding direct contact between ITO and perovskite top‐cell for a minimal shunt loss. As a result of such interfacial engineering, the fully textured perovskite/silicon tandem cells obtain a certified efficiency of 28.84% on a 1.2‐cm2 masked area, which is the highest performance to date based on the fully textured, production‐line compatible SHJ. This work advances commercially promising photovoltaics with high performance and low costs by adopting a meticulously designed HTL/perovskite interface.

Summary Additive engineering has become increasingly important for making high-quality perovskite solar cells (PSCs), with a recent example involving acid during fabrication of cesium-based perovskites. Lately, it has been suggested that this process would introduce dimethylammonium ((CH 3 ) 2 NH 2 + , DMA + ) through hydrolysis of the organic solvent. However, material composition of the hydrolyzed product and its effect on the device performance remain to be understood. Here, we present an in-depth investigation of the hydrolysis-derived material (i.e., DMAPbI 3 ) and detailed analysis of its role in producing high-quality PSCs. By varying the ratio of CsI/DMAPbI 3 in the precursor, we achieve high-quality Cs x DMA 1-x PbI 3 perovskite films with uniform morphology, low density of trap states, and good stability, leading to optimized power conversion efficiency up to 14.3%, with over 85% of the initial efficiency retained after ∼20 days in air without encapsulation. Our findings offer new insights into producing high-quality Cs-based perovskite materials.

Inorganic CsPbI3 perovskites have shown promising potential for achieving all-inorganic photovoltaic (PV) devices. However, the black perovskite polymorph (α-phase) of CsPbI3 easily converts into yellow colour (δ-phase) in an ambient environment and it is only stable at high temperature (above 320 °C), which limits its practical application. Here we tailor the three-dimensional CsPbI3 perovskite into quasi-two-dimension through adding a large radius cation phenylethylammonium (PEA+). The incorporation of PEA+ into the CsPbI3 perovskite significantly improves the film morphology as well as the phase stability. An optimal CsxPEA1-xPbI3 perovskite film remains stable in the α-phase from room temperature to 250 °C in air and yields a power conversion efficiency of 5.7% for its solar device. The concept of using large radius cations in the 3D perovskite system provides a new perspective to further enhance the phase stability while retaining the device performance.