Structural order promotes efficient separation of delocalized charges at molecular heterojunctions

TL;DR

This study demonstrates that structural ordering at molecular heterojunctions enhances charge separation efficiency in organic solar cells by promoting delocalized charge-transfer states, leading to higher fill factors and improved device performance.

Contribution

It provides experimental and computational evidence that ordered molecular packing facilitates delocalized charge-transfer states, improving charge separation in organic photovoltaic interfaces.

Findings

Ordered molecular packing improves charge separation efficiency.

Higher order correlates with more delocalized charge-transfer states.

Radiative recombination efficiency is independent of charge-transfer state binding.

Abstract

The energetic landscape at the interface between electron donating and accepting molecular materials favors efficient conversion of intermolecular charge-transfer states (CTS) into free charge carriers in high-performance organic solar cells. Here, we elucidate how interfacial energetics, charge generation and radiative recombination are affected by structural ordering. We experimentally determine the CTS binding energy of a series of model, small molecule donor-acceptor blends, where the used acceptors (B2PYMPM, B3PYMPM and B4PYMPM) differ only in the nitrogen position of their lateral pyridine rings. We find that the formation of an ordered, face-on molecular packing in B4PYMPM is beneficial to efficient, field-independent charge separation, leading to fill factors over 70% in photovoltaic devices. This is rationalized by a comprehensive computational protocol showing that, compared to the more amorphous and isotropically oriented B2PYMPM, the higher order of the B4PYMPM molecules provides more delocalized CTS. Furthermore, we find no correlation between the quantum efficiency of radiative free charge carrier recombination and the bound or unbound nature of the CTS. This work highlights the importance of structural ordering at donor-acceptor interfaces for efficient free carrier generation and shows that more ordering and less bound CT states do not preclude efficient radiative recombination.