flexibility, and tunable optical transparency, which are advantageous for applications in building-integrated photovoltaic (PV) and wearable electronics compared with conventional inorganic cells. [1][2][3][4][5] To achieve high efficiency, it is essential to develop suitable donor (D) and acceptor (A) materials that can be easily processed into a finely phase-separated morphology in active layer with low energy loss (E loss ) in OSCs. [6][7][8][9] Recently, the vibrant progress made on nonfullerene acceptors (NFAs), [10,11] especially the Y-series NFAs, [12] with facilely tailored chemical structures and precisely tuned bandgap, energy levels, and crystallization properties, has enabled OSCs to achieve very high power conversion efficiencies (PCEs) ≈19%. [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32] The bulk-heterojunction (BHJ) architecture-based devices prepared from mixing D and A in solution is the most widely adopted method for depositing photoactive layer and have successfully increased the PCE of single-junction OSCs to 19% recently. [31] Another emerging strategy is to deposit neat D and A materials A record power conversion efficiency (PCE) of over 19% is realized in planarmixed heterojunction (PMHJ) organic solar cells (OSCs) by adopting the asymmetric selenium substitution strategy in making a pseudosymmetric electron acceptor, BS3TSe-4F. The combined molecular asymmetry with more polarizable selenium substitution increases the dielectric constant of the D18/ BS3TSe-4F blend, helping lower the exciton binding energy. On the other hand, dimer packing in BS3TSe-4F is facilitated to enable free charge generation, helping more efficient exciton dissociation and lowering the radiative recombination loss (ΔE 2 ) of OSCs. As a result, PMHJ OSCs based on D18/BS3TSe-4F achieve a PCE of 18.48%. By incorporating another mid-bandgap acceptor Y6-O into D18/BS3TSe-4F to form a ternary PMHJ, a higher open-circuit voltage (V OC ) can be achieved to realize an impressive PCE of 19.03%. The findings of using pseudosymmetric electron acceptors in enhancing device efficiency provides an effective way to develop highly efficient acceptor materials for OSCs.

Solution processed zinc oxide (ZnO) nanoparticles (NPs) with excellent electron transport properties and a low-temperature process is a viable candidate to replace titanium dioxide (TiO2) as electron transport layer to develop high-efficiency perovskite solar cells on flexible substrates. However, the number of reported high-performance perovskite solar cells using ZnO-NPs is still limited. Here we report a detailed investigation on the chemistry and crystal growth of CH3NH3PbI3 perovskite on ZnO-NP thin films. We find that the perovskite films would severely decompose into PbI2 upon thermal annealing on the bare ZnO-NP surface. X-ray photoelectron spectroscopy (XPS) results show that the hydroxide groups on the ZnO-NP surface accelerate the decomposition of the perovskite films. To reduce the decomposition, we introduce a buffer layer in between the ZnO-NPs and perovskite layers. We find that a commonly used buffer layer with small molecule [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) can slow down but cannot completely avoid the decomposition. On the other hand, a polymeric buffer layer using poly(ethylenimine) (PEI) can effectively separate the ZnO-NPs and perovskite, which allows larger crystal formation with thermal annealing. The power conversion efficiencies of perovskite photovoltaic cells are significantly increased from 6.4% to 10.2% by replacing PC61BM with PEI as the buffer layer.

It has been generally believed and assumed that organometal halide perovskites would form type II P–N junctions with fullerene derivatives (C60 or PCBM), and the P–N junctions would provide driving force for exciton dissociation in perovskite‐based solar cell. To the best of our knowledge, there is so far no experiment proof on this assumption. On the other hand, whether photogenerated excitons can intrinsically dissociate into free carrier in the perovskite without any assistance from a P–N junction is still controversial. To address these, the interfacial electronic structures of a vacuum‐deposited perovskite/C60 and a solution‐processed perovskite/PCBM junctions is directly measured by ultraviolet photoelectron spectroscopy. Contrary to the common believes, both junctions are found to be type I N–N junctions with band gap of the perovskites embedded by that of the fullerenes. Meanwhile, device with such a charge inert junction can still effectively functions as a solar cell. These results give direct experimental evidence that excitons are dissociated to free carriers in the perovskite film even without any assistance from a P–N junction.

Self-assembled monolayers (SAMs) have been widely employed as an effective way to modify interfaces of electronic/optoelectronic devices. To achieve a good control of the growth and molecular functionality of SAMs, we develop a co-assembled monolayer (co-SAM) for obtaining efficient hole selection and suppressed recombination at the hole-selective interface in inverted perovskite solar cells (PSCs). By engineering the position of methoxy substituents, an aligned energy level and favorable dipole moment can be obtained in our newly synthesized SAM, ((2,7-dimethoxy-9H-carbazol-9-yl) methyl) phosphonic acid (DC-PA). An alkyl ammonium containing SAM is co-assembled to further optimize the surface functionalization and interaction with perovskite layer on top. A champion device with an excellent power conversion efficiency (PCE) of 23.59 % and improved device stability are achieved. This work demonstrates the advantage of using co-SAM in improving performance and stability of PSCs.

Controlled construction of bimetallic nanostructures with a well-defined heterophase is of great significance for developing highly efficient nanocatalysts and investigating the structure-dependent catalytic performance. Here, a wet-chemical synthesis method is used to prepare Au@Pd core−shell nanorods with a unique fcc-2H-fcc heterophase (fcc: face-centered cubic; 2H: hexagonal close-packed with a stacking sequence of "AB"). The obtained fcc-2H-fcc heterophase Au@Pd core−shell nanorods exhibit superior electrocatalytic ethanol oxidation performance with a mass activity as high as 6.82 A mg Pd −1 , which is 2.44, 6.96, and 6.43 times those of 2H-Pd nanoparticles, fcc-Pd nanoparticles, and commercial Pd/C, respectively. The operando infrared reflection absorption spectroscopy reveals a C2 pathway with fast reaction kinetics for the ethanol oxidation on the prepared heterophase Au@Pd nanorods. Our experimental results together with density functional theory calculations indicate that the enhanced performance of heterophase Au@Pd nanorods can be attributed to the unconventional 2H phase, the 2H/fcc phase boundary, and the lattice expansion of the Pd shell. Moreover, the heterophase Au@Pd nanorods can also serve as an efficient catalyst for the electrochemical oxidation of methanol, ethylene glycol, and glycerol. Our work in the area of phase engineering of nanomaterials (PENs) opens the way for developing highperformance electrocatalysts toward future practical applications.