Most high-performance non-fullerene acceptors are of the acceptor−donor− acceptor (A−D−A)-type structure. Under photoexcitation, the intramolecular charge transfer effect on the A−D−A framework results in a large dipole moment change, facilitating the efficient generation of charge carriers. Achieving more efficient intramolecular charge transfer by adjusting the molecular structure is one of the current research ideas. Recently, we found that the power conversion efficiency can be improved from 4.41 to 13.13% by tuning the planarity of the non-fused ring electron acceptor backbone through steric hindrance of lateral substituents. We found that the planar backbone can effectively improve the intramolecular charge transfer, which has a great influence on the power conversion efficiency of the device. Our results demonstrate that charge transfer dynamics can be controlled by optimizing steric hindrance, which plays a crucial role in the photovoltaic performance of organic solar cells.

Photoelectrochemical technology provides a promising way to solve the emerging energy crisis, yet it still faces obstacles like weak charge transfer and separation of traditional photoelectrodes. Here, by using single crystalline epitaxial ferroelectric Bi2WO6 thin films, a bifunctional photoelectrode for photoelectrochemical water splitting is demonstrated, which originates from the in-plane ferroelectricity and charged domain walls. The optimized anodic (cathodic) photocurrent density is 0.175 (0.240) mA cm–2 at 1.23 V (0 V) vs RHE in Na2SO4 (pH = 7) electrolyte solution. Manipulations of the in-plane polarization and charged domain walls provide an alternative catalyzing strategy toward efficient and controllable photoelectrochemical water splitting.

The aggregation of perylene diimide (PDI) strongly affects the efficiency of organic optoelectronic devices. Here, a simple N,N′-bis(6-undecyl)perylene-3,4,9,10-bis(dicarboximide) (PDI) was prepared. The steady-state absorption spectrum revealed that PDI showed a monomer state in dimethylformamide (DMF). In contrast, a binary DMF/water (25−50% volume percentage of water) solvent showed an aggregation state due to the hydrophobic effect. Further, we examined the excited state relaxation dynamics of both monomeric and aggregated states of PDI using transient absorption (TA) spectroscopy. We found that the aggregated state of PDI in binary DMF/water solvent with the volume percentage of water at 30% strongly coupled with each other due to the hydrophobic effect, which is favorable to the intermolecular symmetry-breaking charge separation (SB-CS) process. Notably, the TA spectroscopy reveals that the SB-CS state is disfavored for the simple flat structure of monomeric PDI in DMF solvent. Therefore, our work provided the photophysical understanding of PDI aggregation caused by hydrophobicity. Since the hydrophobic effect is significant for the aggregation of PDI, it is possible to change the hydrophobicity by controlling the molecular structure of the PDI derivative, to achieve a more effective charge separation state.

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The performance of a highly efficient nonfused ring electron acceptor PBDB-T/CH 3 -2F system with 12.28% power conversion efficiency (PCE) is degraded significantly to 6.67% when CH 3 -2F is substituted by an SCH 3 -2F acceptor, which has only small side-chain engineering difference. We explored the reasons for this difference and performed transient absorption (TA) spectroscopy to scrutinize the aggregation and charge carrier dynamics in each of the blend films. There are significant differences in their aggregation, which affect exciton generation, exciton dissociation, and carrier excited-state lifetime. We found that the PBDB-T/CH 3 -2F blend film has both face to face and head to tail aggregations, while the PBDB-T/SCH 3 -2F blend film has only head to tail aggregation. Through ultrafast TA spectroscopy and global analysis, it is found that head to tail aggregation is beneficial to produce more excitons, and face to face aggregation is beneficial to the dissociation of more excitons. In addition, the aggregation also affects important factors of the device, such as the electronic energy level and domain size. Our results indicate that both face to face and head to tail aggregations are important factors in achieving high PCE of nonfullerene acceptor solar cells, which can be achieved through small side-chain engineering, so it is worthy of future attention.