Liu Yao's research group publishes in Angew. Chem. Int. Ed.: Creating photovoltaic interface materials with a strong built-in electric field to promote the development of highly efficient and stable organic solar cells

Organic solar cells (OSCs) have attracted significant attention from academia and industry due to their advantages of low cost, flexibility, wearability, and solution processability. However, insufficient device stability and difficulties in interface engineering optimization have severely hindered their commercialization. In this context, the development of cathode interface materials with a wide film thickness processing window, high carrier mobility, and excellent interface compatibility has become a key research direction to break through the efficiency bottleneck of OSCs and improve device stability. Among them, small-molecule cathode interlayer materials (SM-CIMs) have been widely studied in recent years due to their unique advantages such as clear molecular structure, high purity, and controllable crystallinity. However, there is still a lack of systematic theoretical guidance and in-depth mechanistic research on how to effectively regulate the built-in electric field (BEF) of SM-CIMs through molecular engineering strategies.

The Liu Yao research group at the Soft Matter Science and Engineering Center proposed a core idea of regulating the donor-acceptor (D-A) structural configuration to enhance BEF, designing and synthesizing a series of imidazole-substituted ternary small molecule interface materials: (TBT) ₂ NDI、(NDI) ₂ TBT and (PDI) ₂ TBT (Figure 1). By transforming the molecular configuration from D-A-D to A-D-A and introducing a stronger electron-withdrawing acceptor unit (PDI), the research team achieved a significant enhancement of BEF, while also obtaining better energy level regulation, higher electron mobility, and film crystallinity, providing a theoretical and material basis for achieving high-performance thick-film interlayers.

Figure 1. Molecular design ideas of SM-CIMs

Among the three SM-CIMs, (PDI) ₂ TBT has the largest traceless quadrupole moment (|Θ|) and molecular polarity index (MPI), reflecting the trend of BEF enhancement with the A-D-A structure. At the same time, (PDI) ₂ TBT exhibits excellent energy level matching ability and interface work function regulation performance. The work function of the Ag electrode can be effectively reduced from 4.74 eV to 4.12 eV, which is conducive to forming ohmic contact and improving the open-circuit voltage and short-circuit current density of the device. In addition, (PDI) ₂ TBT exhibits the highest electron mobility (1.85 × 10 ⁻ ³ cm ²·V ⁻ ¹·s ⁻ ¹) and conductivity, which is more conducive to improving the electron extraction and injection efficiency of the device. Two-dimensional GIWAXS test results show that all three materials exhibit advantageous edge-on orientation, and (PDI) ₂ TBT exhibits stronger crystallinity and tighter π–π stacking. Molecular dynamics simulation results show that (PDI) ₂ TBT forms tighter stacking with the non-fullerene acceptor Y6. Transient absorption spectroscopy shows that (PDI) ₂ TBT-treated PM6:Y6 bilayer films have faster hole transfer rates, longer carrier lifetimes, and lower triplet excited state formation probabilities, which help to improve the open-circuit voltage and reduce energy loss.

Figure 2. Characterization of the optoelectronic, condensed matter properties, interaction with acceptors, and device kinetics of SM-CIMs

In the PM6:Y6 active system, the device modified with (PDI)2TBT achieved a power conversion efficiency (PCE) of 17.85%, and it can still maintain 85% of the peak efficiency when the thickness of the interlayer exceeds 100 nm. In addition, in various active layer systems such as PM6:BTP-eC9, PM6:D18:L8-BO, and PM6:L8-BO:BTP-eC9, (PDI) ₂ TBT shows excellent adaptability and interface regulation ability, superior to the comparative material (TBT) ₂ NDI and PDINN. In particular, in the PM6:L8-BO:BTP-eC9 system, the device achieved a power conversion efficiency of 20.10%, reaching the current advanced level. In terms of stability testing, after continuous operation for 500 hours under ambient light, (PDI) ₂ TBT and (NDI) ₂ TBT-modified devices still maintained 87% and 84% of their initial efficiency, respectively, significantly better than (TBT) ₂ NDI (decreased to 78%). Bare chip thermal stability tests further confirmed that (PDI) ₂ TBT devices maintained 90.6% of their initial efficiency after being placed at high temperature for 225 h, demonstrating excellent thermal stability.

Figure 3. Characterization of material photovoltaic device performance

In summary, this study significantly enhanced the BEF and interface modification ability of SM-CIMs by regulating the D-A configuration and electronic structure, proposing a new molecular design strategy for constructing SM-CIMs with a wide film thickness processing window, providing new ideas for achieving highly efficient and stable organic solar cells.

This achievement, titled "Enhancing the Built-In Electric Field of Thickness-Insensitive Small Molecule Cathode Interlayers for High-Efficiency and Stable Organic Solar Cells," was published in the internationally renowned journal Angewandte Chemie International Edition. The first authors are Yuxing Wang and Junjie Wen, doctoral students at the High-end Research Center of Beijing University of Chemical Technology. The corresponding authors are Associate Professor Wenxu Liu from the School of Chemistry, Beijing University of Chemical Technology, and Professor Yao Liu from the High-end Research Center. Thanks to the important support from Professor Hui Li's research group at the High-end Research Center in theoretical calculations, this research was supported by the National Natural Science Foundation of China.

Article Information:

Yuxing Wang, Junjie Wen, Zhe Shang, Yanyi Zhong, Huixiang Zhang, Wenxu Liu, Wentian Han, Huanhuan Yang, Jiming Liu, Jiangbin Zhang, Hui Li, and Yao Liu. Enhancing the Built-In Electric Field of Thickness-Insensitive Small Molecule Cathode Interlayers for High-Efficiency and Stable Organic Solar Cells. Angew. Chem. Int. Ed., 2025, e20250625. DOI: 10.1002/anie.202506252.

Original Link:

https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.202506252