Dredging photocarrier trapping pathways via charge bridge driven exciton-phonon decoupling enables efficient and photothermal stable quaternary organic solar cells

The operational stability of high-performance organic solar cells (OSCs) based on Y-series small-molecule acceptors is hindered by strong photocarrier trapping processes governed by thermodynamic relaxation in the mixed morphological domains. Herein, the signature of photocarrier trapping associated with exciton-phonon coupling is identified by investigating the pump fluence-dependent exciton diffusion and annihilation dynamics. The kinetic fittings with the exciton-exciton annihilation model demonstrate the intensified exciton-phonon coupling under continuous photothermal stress. Then, we show that a feasible charge bridge approach, which enables the decoupling of exciton-phonon interactions of bulk heterojunctions, brings about an enhancement in device efficiency and photothermal stability by dredging non-radiative photocarrier trapping pathways. Experimental proof is provided by the design and fabrication of quaternary OSCs with reduced energetic disorder and the generation of trap states, where a charge bridge for charge transfer and transport through the interfacial region is constructed by regulating the energy landscape with the introduced polymer donor and acceptor. The revealed intensified exciton-phonon interactions after photothermal aging are suppressed by the proposed charge bridge strategy, giving rise to less exciton and charge trapping resulting from improved exciton-phonon decoupling. This work emphasizes the importance of tailoring exciton-phonon coupling behaviors, providing a design pathway for developing efficient and stable non-fullerene OSCs.

Automated summary

The operational stability of high-performance organic solar cells based on Y-series small-molecule acceptors is hindered by strong photocarrier trapping processes governed by thermodynamic relaxation in the mixed morphological domains. Exciton-phonon coupling is identified as the cause of the photocarrier trapping, and a charge bridge approach is proposed to enhance device efficiency and photothermal stability by decoupling exciton-phonon interactions. Experimental proof is provided through the design and fabrication of quaternary OSCs with reduced energetic disorder and trap states. This work highlights the importance of tailoring exciton-phonon coupling behaviors for developing efficient and stable non-fullerene OSCs.