Chemical Reaction of FA Cations Enables Efficient and Stable Perovskite Solar Cells.

Organometal halide perovskite solar cells (PSCs) have received great attention owing to a rapid increase in power conversion efficiency (PCE) over the last decade. However, the deficit of long-term stability is a major obstacle to the implementation of PSCs in commercialization. The defects in perovskite films are considered as one of the primary causes. To address this issue, isocyanic acid (HNCO) is introduced as an additive into the perovskite film, in which the added molecules form covalent bonds with FA cations via a chemical reaction. This chemical reaction gives rise to an efficient passivation on the perovskite film, resulting in an improved film quality, a suppressed non-radiation recombination, a facilitated carrier transport, and optimization of energy band levels. As a result, the HNCO-based PSCs achieve a high PCE of 24.41% with excellent storage stability both in an inert atmosphere and in air. Different from conventional passivation methods based on coordination effects, this work presents an alternative chemical reaction for defect passivation, which opens an avenue toward defect-mitigated PSCs showing enhanced performance and stability.

Stable Electron-Transport-Layer-Free Perovskite Solar Cells with over 22% Power Conversion Efficiency.

Due to their low cost and simplified production process, electron-transport-layer-free (ETL-free) perovskite solar cells (PSCs) have attracted great attention recently. However, the performance of ETL-free PSCs is still at a disadvantage compared to cells with a conventional n-i-p structure due to the severe recombination of charge carriers at the perovskite/anode interface. Here, we report a strategy to fabricate stable ETL-free FAPbI3 PSCs by in situ formation of a low dimensional perovskite layer between the FTO and the perovskite. This interlayer gives rise to the energy band bending and reduced defect density in the perovskite film and indirect contact and improved energy level alignment between the anode and perovskite, which facilitates charge carrier transport and collection and suppresses charge carrier recombination. As a result, ETL-free PSCs with a power conversion efficiency (PCE) exceeding 22% are achieved under ambient conditions.

Carbon Nanotube Supported Molybdenum Carbide as Robust Electrocatalyst for Efficient Hydrogen Evolution Reaction.

Molybdenum carbide is considered to be one of the most competitive catalysts for hydrogen evolution reaction (HER) regarding its high catalytic activity and superior corrosion resistance. But the low electrical conductivity and poor interfacial contact with the current collector greatly inhibit its practical application capability. Herein, carbon nanotube (CNT) supported molybdenum carbide was assembled via electrostatic adsorption combined with complex bonding. The N-doped molybdenum carbide nanocrystals were uniformly anchored on the surfaces of amino CNTs, which depressed the agglomeration of nanoparticles while strengthening the migration of electrons. The optimized catalyst (250-800-2h) showed exceptional electrocatalytic performance towards HER under both acidic and alkaline conditions. Especially in 0.5 M H2SO4 solution, the 250-800-2h catalyst exhibited a low overpotential of 136 mV at a current density of 10 mA/cm2 (η10) with the Tafel slope of 49.9 mV dec-1, and the overpotential only increased 8 mV after 20,000 cycles of stability test. The active corrosive experiment revealed that more exposure to high-activity γ-Mo2N promoted the specific mass activity of Mo, thus, maintaining the catalytic durability of the catalyst.

A Universal Approach Toward Intrinsically Flexible All-Inorganic Perovskite-Gel Composites with Full-Color Luminescence.

The combination of all-inorganic perovskites (PVSKs) and polymers allows for free-standing flexible optoelectronic devices. However, solubility difference of the PVSK precursors and concerns over the compatibility between polymer carriers and PVSKs imply a great challenge to incorporate different kinds of PVSKs into polymer matrices by the same manufacturing process. In this work, PVSK precursors are introduced into poly(2-hydroxyethyl acrylate) (PHEA) hydrogels in sequence, in which the PVSK-gel composites are achieved with full-color emissions by simply varying the precursor species. Moreover, it is found that CsBr has a higher interaction energy with the (111) plane of CsPbBr3 than the (110) plane; thus, the CsPbBr3 crystals with a shape of truncated cube and tetragon are observed during the CsPbBr3-Cs4PbBr6 phase transition over time. The PVSK-gel composites feature excellent bendability, elasticity, and stretchable deformation (tensile strain > 500%), which allows for 3D printing emissive customized stereoscopic architectures with shape-memory features.

In Situ Surface Reconstruction toward Planar Heterojunction for Efficient and Stable FAPbI3 Quantum Dot Solar Cells.

Pure-phase α-FAPbI3 quantum dots (QDs) are the focus of an increasing interest in photovoltaics due to their superior ambient stability, large absorption coefficient, and long charge-carrier lifetime. However, the trap states induced by the ligand-exchange process limit the photovoltaic performances. Here, a simple post treatment using methylamine thiocyanate is developed to reconstruct the FAPbI3 -QD film surface, in which a MAPbI3 capping layer with a thickness of 6.2 nm is formed on the film top. This planar perovskite heterojunction leads to a reduced density of trap-states, a decreased band gap, and a facilitated charge carrier transport. As a result, a record high power conversion efficiency (PCE) of 16.23% with negligible hysteresis is achieved for the FAPbI3 QD solar cell, and it retains over 90% of the initial PCE after being stored in ambient environment for 1000 h.

Honeycomb-Type TiO2 Films Toward a High Tolerance to Optical Paths for Perovskite Solar Cells.

Given the advantages of high power conversion efficiencies (PCEs), antisolvent-step free production, and suitability for device production in ambient conditions, perovskite solar cells (PSCs) based on ionic-liquid solvents have attained particular research interest. To further improve device performance, light management could be optimized to increase light harvesting in the perovskite layer. Here, ordered honeycomb-like TiO2 (Hc-TiO2 ) structures with a periodicity of around 450 nm were fabricated through a sacrificial template method. With this photonic crystal structure, the control to light flow and the confinement effect for perovskite growth were achieved simultaneously in the Hc-TiO2 , leading to improved light absorption as well as preferred crystal orientation. Furthermore, a reduced trap-state density and a well-aligned energy level induced by the perovskite/pore interlayer facilitated the charge-carrier extraction from the perovskite layer to electron transport layer. As a result, the structured devices performed better than the planar cells. And the angular dependent J-V sweeps show that the structured device reserved 76 % of its initial short circuit current density (Jsc ), whereas the planar cell showed more than a half loss under the incident light of 40°, demonstrating a reduced downward trend in Jsc with the presence of photonic crystal structures. This occurrence also suggests that the structured PSCs in this work have a high tolerance to optical path changes.

Improved Power Conversion Efficiency and Stability of Perovskite Solar Cells Induced by Molecular Interaction with Poly(ionic liquid) Additives.

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 PbI62- 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).

SnO2 Passivation and Enhanced Perovskite Charge Extraction with a Benzylamine Hydrochloric Interlayer.

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.

Designing Ionic Liquids as the Solvent for Efficient and Stable Perovskite Solar Cells.

As a green solvent, ionic liquids (ILs) are considered as a promising alternative to conventional polar aprotic solvents for the production of efficient and stable perovskite solar cells (PSCs). Moreover, with the use of IL solvents, perovskite films can be prepared without antisolvent treatments in an ambient environment instead of in a glovebox with inert gases, which simplifies the film manufacturing process and is favorable for industrialization production. However, the type of IL solvents that have been studied is limited, and the influence of IL molecular structures on the perovskite-film crystallization and device performance is not completely understood. In this work, four different ILs, methylammonium formate (MAF), methylammonium acetate (MAAc), methylammonium propionate (MAP), and mthylammonium isobutyrate (MAIB), are synthesized as the perovskite precursor solvents. The interaction between the functional groups of the synthesized solvents and Pb2+ in the precursor solution is studied, which has a direct impact on the morphology and crystallization of the deposited perovskite film. It is found that MAP solvent gives a high-quality perovskite film, which leads to the best photovoltaic performance with a champion PCE of 20.56% compared to the devices based on the other IL solvents. Moreover, the MAP-based device maintains 88% of its original PCE after 1000 h of storage in a N2 atmosphere, demonstrating excellent device stability. Therefore, it is concluded that MAP is the most suitable solvent for MAPbI3 films with respect to photovoltaic applications as compared to the other ILs.