Establishing a microscopic model for nonfullerene organic solar cells: Self-accumulation effect of charges

TL;DR

This paper develops a microscopic model for nonfullerene organic solar cells, revealing how charge self-accumulation due to Coulomb attraction suppresses recombination and enhances performance.

Contribution

It introduces a one-dimensional many-body model accounting for donor-acceptor structure and charge imbalance, explaining charge self-accumulation and ultrafast separation.

Findings

Charges self-accumulate in small regions due to Coulomb attraction.

Charge recombination is suppressed outside the accumulation region.

Electrons spread freely outside the self-accumulation zone, aiding charge separation.

Abstract

A one-dimensional many-body model is established to mimic the charge distribution and dynamics in nonfullerene organic solar cells. Two essential issues are taken into account in the model: The alternating donor and acceptor structure and the local imbalance of the intrinsic electrons and holes. The alternating structure is beneficial for the direct generation of charge transfer state which enhances the local imbalance of intrinsic charges. The most remarkable outcome of the model is that, due to the strong Coulomb attractive potential energy, the intrinsic charges in the cells are self-accumulated in a small spatial region. Outside the self-accumulation region, the charge density vanishes so that the recombination is regarded to be largely suppressed. The photogenerated electrons are subsequently observed to spread freely outside the self-accumulation region implying the Coulomb attraction does not matter in the ultrafast charge separation dynamics. These findings enable an appealing understanding of the high performance of emerging nonfullerene cells, and the designing rules of molecules and devices are then comprehensively discussed.