Organic solar cells (OSCs) are an emerging renewable energy technology for the country's "dual carbon" strategy, and have broad application prospects in fields such as photovoltaic buildings and wearable devices. However, their power conversion efficiencies (PCEs) still lag behind those of silicon and perovskite solar cells, which is mainly limited by lower open-circuit voltage and higher energy loss. According to the energy gap law, the current high-performance fused ring electron acceptors (FREAs) have the characteristics of narrow band gap and near-infrared absorption, which leads to accelerated vibrational relaxation from the excited state to the ground state (Figure 1a). This strong electron-phonon interaction (i.e., exciton-vibration coupling) will cause large non-radiative recombination losses, limiting the further improvement of OSCs efficiency.

It should be noted that this type of electron acceptor material is composed of a highly condensed central core and an electron-withdrawing end group connected by a C-C rotatable single bond, in which the intramolecular S‧‧‧O non-covalent interaction can effectively maintain the molecular planarity of the condensed ring electron acceptor (Figure 1b). However, the exciton-vibration coupling mentioned above is closely related to molecular rigidity, so further enhancing the molecular rigidity of the condensed ring electron acceptor will help reduce non-radiative recombination losses. Earlier, Professor Huang Hui's team at the University of the Chinese Academy of Sciences proposed a descriptor for measuring the strength of intramolecular non-covalent interactions by combining theoretical calculations with experiments (Figure 1c), which is of great guiding significance for the development of condensed ring electron acceptor materials with high rigidity and low energy loss.

Based on this, the research group published a research paper titled "Suppressing Exciton–Vibration Coupling via Intramolecular Noncovalent Interactions for Low-Energy-Loss Organic Solar Cells" in the internationally renowned journal Angew. Chem. Int. Ed. This work systematically regulated the molecular rigidity of the condensed ring electron acceptor by gradually introducing stronger intramolecular Se‧‧‧O non-covalent interactions, effectively suppressing exciton-vibration coupling and non-radiative energy loss. Y-SeSe-based binary and ternary photovoltaic devices with dual intramolecular Se‧‧‧O non-covalent interactions achieved photoelectric conversion efficiencies of 19.49% and 20.51%, respectively, breaking the current efficiency records of selenium-substituted photovoltaic materials and ternary photovoltaic devices.

Figure 1. a) Exciton-vibration coupling and molecular rigidity; b) Rotatable C-C single bond in fused-ring electron acceptors; c) Descriptor (S) used to measure the strength of non-covalent interactions within a molecule; d) Novel fused-ring electron acceptors designed in this work

This work is based on the idea of enhancing the rigidity of fused-ring electron acceptor molecules to reduce exciton-vibration coupling. The construction of intramolecular Se‧‧‧O non-covalent interactions is cleverly achieved through positional isomerization of selenium atoms, and three new fused-ring electron acceptors are designed and synthesized (Figure 1d).

Figure 2. Synthesis route of new condensed ring electron acceptors

In order to obtain a fused-ring electron acceptor with isomerized selenium atom position, the authors first designed and synthesized two new 2-butyloctyl (BO)-substituted thieno-selenophene building blocks (9a-b), and further obtained three key intermediates (11a-c) through a one-pot Stille cross-coupling reaction. Finally, three new fused-ring electron acceptors (Y-SS, Y-SSe and Y-SeSe) with double intramolecular S‧‧‧O, S‧‧‧O+Se‧‧‧O, and double Se‧‧‧O non-covalent interactions were obtained.

Figure 3. DFT theoretical calculations and spectroscopy studies

DFT计算得到分子内S‧‧‧O和Se‧‧‧O非共价相互作用的S值为1.386和0.513，证明分子内Se‧‧‧O非共价相互作用强于分子内S‧‧‧O非共价相互作用，因此具有更大的旋转势垒。进一步证明了逐步引入更强的分子内Se‧‧‧O非共价相互作用能够有效降低重组能并抑制振动弛豫，获得更小的Stokes位移，表明分子刚性得到了显著增强，因此有效抑制了稠环电子受体的激子-振动耦合。

Figure 4. Single crystal structure and stacking pattern

Single crystal XRD diffraction analysis results show that the intramolecular S‧‧‧O and Se‧‧‧O non-covalent interactions can both maintain the coplanarity of the condensed ring electron acceptor, but the introduction of stronger Se‧‧‧O NoCLs can induce multiple intermolecular stacking modes, thereby obtaining a tighter molecular stacking, which is conducive to efficient charge transfer.

Figure 4. Photovoltaic performance and device physics research

The device results show that the Y-SeSe-based binary device with dual intramolecular Se‧‧‧O non-covalent interactions achieved a PCE of 19.49% and extremely low energy loss (0.184 eV), setting a new efficiency record for selenium-substituted photovoltaic materials. In addition, in order to further explore the potential of Y-SeSe, the author prepared a ternary blend device of D18:Y-SeSe:Z19 that achieved an energy conversion efficiency of 20.51% (SIMIT certified efficiency of 19.92%), which is the highest efficiency of ternary OSCs at present.

Through rational molecular design, systematic theoretical calculations and experimental characterization, this work fully demonstrates the huge application potential of intramolecular non-covalent interactions in constructing low energy loss and high-performance condensed-ring electron acceptors, providing new ideas for further achieving efficiency breakthroughs in OSCs.

Gu Xiaobin, a doctoral student in the research group, is the first author of this paper. Wei Yanan, a postdoctoral fellow, and Zeng Rui, a doctoral student at Shanghai Jiao Tong University, are co-first authors. Professor Huang Hui and Associate Professor Zhang Xin of the University of Chinese Academy of Sciences are co-corresponding authors of the paper. The authors thank the National Natural Science Foundation of China, the Chinese Academy of Sciences, and other related projects for their funding.

Link：https://doi.org/10.1002/anie.202418926