Achieving a Higher Energy Charge-Transfer State and Reduced Voltage Loss for Organic Solar Cells using Nonfullerene Acceptors with Norbornenyl-Functionalized Terminal Groups

Achieving a high-energy charge-transfer state (E-CT) and concurrently reduced energy loss is of vital importance in boosting the open-circuit voltage (V-oc) of organic solar cells (OSCs), but it is difficult to realize. We report herein a novel design tactic to achieve this goal by incorporating a three-dimensional (3D) shape-persistent norbornenyl group into the terminals of acceptor-donor-acceptor-type nonfullerene acceptors (NFAs). Compared with ITIC-based OSCs, norbornenyl-fused 1,1-dicyanomethylene-3-indanone (CBIC) terminals endow IDTT-CBIC-based OSCs with simultaneously higher E-CT and lower radiative and non-radiative voltage loss, hence enhancing V-oc by 90 mV. CBIC also improves the miscibility and modulates the molecular packing structures for efficient charge carrier transport and a better short-circuit current density in IDTT-CBIC-based OSCs. Consequently, the power conversion efficiency is improved by 22%, compared to that of the OSC based on ITIC. Furthermore, the effectiveness of the use of CBIC as the terminals is observed using different electron-donating cores. The utilization of the 3D shape-persistent building blocks represents a breakthrough in the design strategies for terminal groups toward efficient NFA-based OSCs with high V-oc.

Automated summary

By incorporating a three-dimensional norbornenyl group into the terminals of nonfullerene acceptors, a novel design tactic was developed to achieve simultaneously higher energy charge-transfer state and lower energy loss, resulting in a 90 mV enhancement of open-circuit voltage in organic solar cells. The new design also improves charge carrier transport efficiency, short-circuit current density, and overall power conversion efficiency, marking an important breakthrough in the design of nonfullerene acceptor-based OSCs.