Quantitative insights into the phase behaviour and miscibility of organic photovoltaic active layers from the perspective of neutron spectroscopy

TL;DR

This paper introduces a neutron spectroscopy method to quantitatively analyze the phase behavior and miscibility of organic photovoltaic blends, providing insights into their structural dynamics and component interactions.

Contribution

The study demonstrates a novel neutron spectroscopy approach using inelastic and quasi-elastic measurements, enhanced by deuteration, to probe miscibility limits in polymer-fullerene blends.

Findings

Neutron spectroscopy effectively distinguishes phase behavior in photovoltaic blends.

Deuteration enhances contrast between polymer and fullerene phases.

Method is applicable to non-fullerene acceptors with similar structures.

Abstract

We present a neutron spectroscopy based method to study quantitatively the partial miscibility and phase behaviour of an organic photovoltaic active layer made of conjugated polymer:small molecule blends, presently illustrated with the regio-random poly(3-hexylthiophene-2,5-diyl) and fullerene [6,6]-Phenyl C$_{61}$ butyric acid methyl ester (RRa-P3HT:PCBM) system. We perform both inelastic neutron scattering and quasi-elastic neutron scattering measurements to study the structural dynamics of blends of different compositions enabling us to resolve the phase behaviour. The difference of neutron cross sections between RRa-P3HT and PCBM, and the use of deuteration technique, offer a unique opportunity to probe the miscibility limit of fullerene in the amorphous polymer-rich phase and to tune the contrast between the polymer and the fullerene phases, respectively. Therefore, the proposed approach should be universal and relevant to study new non-fullerene acceptors that are closely related - in terms of chemical structures - to the polymer, where other conventional imaging and spectroscopic techniques present a poor contrast between the blend components.