A research group led by Prof. GE Ziyi at the Ningbo Institute of Materials Technology and Engineering (NIMTE) of the Chinese Academy of Sciences (CAS), has applied an asymmetric substitution strategy of end-groups in molecular donors, thus achieving high-performance all-small-molecule organic solar cells (OSCs) with power conversion efficiency (PCE) exceeding 16.34%. The study was published in Advanced Materials.

Thanks to the appealing advantages like low cost, lightweight and mechanical flexibility, OSCs have drawn increasing attention. As a promising power source for flexible electronic systems, all-small-molecule OSCs feature definite chemical structure and excellent batch-to-batch reproducibility, except for relatively low PCE performance.

To break this bottleneck, researchers at NIMTE designed and synthesized a series of symmetric and asymmetric small-molecule donors. The difluorothiophene substituted benzodithiophene (BDT) served as a donor moiety, while 2-ethylhexyl cyanoacetate (CA), 2-ethylhexyl rhodanine (Reh) and 1H-indene-1,3(2H)-dione (ID), were selected as end-groups.

Blending with acceptor N3, SM-CA-Reh was substituted with asymmetric end-groups of CA and Reh, thus integrated photovoltaic advantages of high fill factor (FF) of SM-CA and high short-circuit current density (J_sc) of SM-Reh. In this case, the device based on the SM-CA-Reh achieved an increased FF of 77.5% and a highly improved PCE of 16.34%, which was the highest PCE reported for binary all-small-molecule OSCs. By contrast, the devices based on the SM-CA-ID and SM-ID showed relatively low PCE of 8.2% and 2.76%, respectively.

In addition, characterization results indicated that the stacking morphology in blend films is mainly determined by the π–π interaction instead of dipole effect or crystallinity.

This research has revealed that the asymmetric substitution of end-groups in molecular donors can be an efficient method to further improve the all-small-molecule OSCs and illuminated how the end-groups effectively modulate the phase-separated morphology.

The work was financially supported by the National Science Fund for Distinguished Young Scholars (No. 21925506), National Natural Science Foundation of China (Nos. U21A20331 and 81903743), Ningbo S&T Innovation 2025 Major Special Programme (No. 2018B10055), CAS Key Project of Frontier Science Research (No. QYZDB-SSW-SYS030), and Ningbo Natural Science Foundation (No. 2021J192).

Fig. The asymmetric substitution of end-groups in molecular donors to achieve high-performance all-small-molecule OSCs (Image by NIMTE)

Contact
HUANG Ye
Ningbo Institute of Materials Technology and Engineering
E-mail: huangye@nimte.ac.cn