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, 5-IDT and 6-IDT, which differ in molecular length. These acceptors serve as a third component in binary organic solar cells (OSCs), resulting in highly stable OSCs with the power conversion efficiency (PCE) of up to 19.96%. This work was published in Advanced Materials.

OSCs are widely employed in organic electronic devices due to their lightweight nature, mechanical flexibility, and translucency. For OSCs to achieve commercialization, high efficiency and long-term stability are essential to meet practical application demands.

Although single-junction OSCs have achieved PCEs of up to 20%, they face challenges in maintaining operational stability. This instability is largely attributed to the low glass transition temperature of small molecule acceptors.

To address this issue, the researchers designed two isomerized dimeric acceptors, 5-IDT and 6-IDT, both of which adopt a U-shaped conformation. These dimeric acceptors were introduced into binary OSCs as a third component, thereby achieving a balance between efficiency and stability.

The PCEs of the 6-IDT- and 5-IDT-treated OSCs have reached 19.32% and 19.96%, respectively, representing the highest values reported to date for oligomeric acceptor-based OSCs. This achievement can be attributed to reduced voltage loss and energetic disorder, as well as enhanced exciton dissociation and charge transport.

After undergoing thermal treatment at 65 °C and 100 °C for 1000 hours, the 6-IDT- and 5-IDT-treated devices retained 32% and 75% of their initial efficiency, respectively. In comparison, the control device retained only 18% of its original efficiency. This demonstrates a significant improvement in long-term thermal stability.  

This study is the first to reveal that the molecular size of oligomeric acceptors has a significant impact on both the efficiency and stability of organic solar cells.

By conducting a detailed study of the active layer's morphology, mechanical robustness, and vertical phase distribution before and after aging, the underlying mechanism for improved thermal stability was identified. The dimers act as stabilizing agents, uniformly distributed within the active layer, which reinforces the robust phase-separated morphology between the polymeric donor and small molecular acceptor.  

This work provides new insights into the molecular design of oligomer acceptors, demonstrating their potential to simultaneously enhance efficiency and thermal stability. These findings represent a step toward the practical commercialization of OSC technology.