# Deuteration-enhanced neutron contrasts to probe amorphous domain sizes in organic photovoltaic bulk heterojunction films

## TL;DR

This paper uses neutron scattering with deuteration to reveal hidden structures in organic solar films, improving understanding of how material arrangement affects performance.

## Contribution

The study introduces deuteration-enhanced neutron scattering to detect amorphous domain structures in organic photovoltaic films, revealing new morphological features.

## Key findings

- Deuteration of Y6 increases neutron scattering contrast by nearly one order of magnitude.
- Short-range aggregation of d-Y6 molecules is observed in amorphous regions, aiding charge extraction.
- Molecular dynamics simulations confirm the structural advantage of Y6 in suppressing charge recombination.

## Abstract

An organic photovoltaic bulk heterojunction comprises of a mixture of donor and acceptor materials, forming a semi-crystalline thin film with both crystalline and amorphous domains. Domain sizes critically impact the device performance; however, conventional X-ray scattering techniques cannot detect the contrast between donor and acceptor materials within the amorphous intermixing regions. In this study, we employ neutron scattering and targeted deuteration of acceptor materials to enhance the scattering contrast by nearly one order of magnitude. Remarkably, the PM6:deuterated Y6 system reveals a new length scale, indicating short-range aggregation of Y6 molecules in the amorphous intermixing regions. All-atom molecular dynamics simulations confirm that this short-range aggregation is an inherent morphological advantage of Y6 which effectively assists charge extraction and suppresses charge recombination as shown by capacitance spectroscopy. Our findings uncover the amorphous nanomorphology of organic photovoltaic thin films, providing crucial insights into the morphology-driven device performance.

The use of conventional X-ray scattering techniques is challenging to detect donor-acceptor contrast within amorphous intermixing regions. Here, the authors apply neutron scattering and targeted deuteration to enhance the contrast by one order of magnitude and reveal short-range aggregations of d-Y6.