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 developed two isomerized dimeric acceptors, i.e., 5-IDT and 6-IDT, with different molecular lengths as a third component of the binary organic solar cells (OSCs), achieving highly stable OSCs with the power conversion efficiency (PCE) up to 19.96%.

This work was published in Advanced Materials.

OSCs are of great interest in the field of organic electronic devices because of their light weight, good mechanical flexibility, and translucency. High efficiency and long-term stability are two key requirements for the commercialization of OSCs.

Although the highest PCE of single-junction OSCs has reached 20%, these devices suffer from low operational stability due to the low glass transition temperature of small molecule acceptors.

Researchers at NIMTE designed two isomerized dimeric acceptors, i.e., 5-IDT and 6-IDT. Both dimers tend to be in a U-shaped conformation. The two dimeric acceptors were introduced into the binary OSCs as a third component, respectively, thus effectively balancing efficiency and stability.

The PCEs of the 6-IDT- and 5-IDT-treated  OSCs have been significantly improved to 19.32% and 19.96%, respectively, which are the highest values reported to date for oligomeric acceptors-based OSCs. This can be attributed to the reduced voltage loss and energetic disorder, and the enhanced exciton dissociation and charge transport.

After thermal treatment at 65 and 100 °C for 1000 hours, the 6-IDT- and 5-IDT-treated devices can maintain 32% and 75% of their initial efficiency, respectively. This indicated a remarkable improvement in long-term thermal stability compared with the control device of only 18%.

These results also revealed for the first time that the molecular size of oligomeric acceptors significantly influences the efficiency and stability of the device.

Through the detailed study of the morphology, mechanical robustness, and vertical phase distribution of the active layer before and after aging, the original reason for the improved thermal stability was revealed for the first time. Acting like a lock, the dimer is uniformly distributed in the active layer, thus contributing to the highly robust phase-separated morphology via the polymeric donor and small molecular acceptor.

This work has provided a novel insight into the molecular design of oligomer acceptors and facilitated the commercial application of OSC technology.

Fig. The highly stable and efficient OSCs with U-shaped dimeric acceptors (Image by NIMTE)

Contact
YANG Daobin
Ningbo Institute of Materials Technology and Engineering
E-mail: yangdaobin@nimte.ac.cn