Chemical design rules for non-fullerene acceptors in organic solar cells

TL;DR

This paper establishes chemical design rules for non-fullerene acceptors in organic solar cells by analyzing electrostatic fields and molecular architecture, leading to improved charge separation and efficiency.

Contribution

It introduces a new understanding of how molecular quadrupole moments and interface alignment influence NFA performance in organic solar cells.

Findings

Molecular quadrupole moment of ~75 Debye A balances voltage and charge generation.

Parallel molecular alignment promotes energy level bending and charge dissociation.

Design rules improve efficiency of donor-NFA combinations.

Abstract

Efficiencies of organic solar cells have practically doubled since the development of non-fullerene acceptors (NFAs). However, generic chemical design rules for donor-NFA combinations are still needed. Such rules are proposed by analyzing inhomogeneous electrostatic fields at the donor-acceptor interface. It is shown that an acceptor-donor-acceptor molecular architecture, and molecular alignment parallel to the interface, results in energy level bending that destabilizes the charge transfer state, thus promoting its dissociation into free charges. By analyzing a series of PCE10:NFA solar cells, with NFAs including Y6, IEICO, and ITIC, as well as their halogenated derivatives, it is suggested that the molecular quadrupole moment of ca 75 Debye A balances the losses in the open circuit voltage and gains in charge generation efficiency.