Morphology dependent optical anisotropies in the n-type polymer P(NDI2OD-T2)

TL;DR

This study investigates the optical anisotropies of the n-type polymer P(NDI2OD-T2) films, revealing how different morphologies affect their optical properties and providing insights for optimizing organic electronic devices.

Contribution

The paper introduces momentum-resolved spectroscopic methods to quantify optical anisotropies and morphological differences in P(NDI2OD-T2) films, enhancing understanding of structure-property relationships.

Findings

Edge-on films show larger out-of-plane dipole tilt angles.

Disordered regions are detected in low-temperature face-on films.

Optical anisotropies correlate with film processing conditions.

Abstract

Organic semiconductors tend to self-assemble into highly ordered and oriented morphologies with anisotropic optical properties. Studying these optical anisotropies provides insight into processing-dependent structural properties and informs the photonic design of organic photovoltaic and light-emitting devices. Here, we measure the anisotropic optical properties of spin-cast films of the n-type polymer P(NDI2OD-T2) using momentum-resolved absorption and emission spectroscopies. We quantify differences in the optical anisotropies of films deposited with distinct face-on and edge-on morphologies. In particular, we infer a substantially larger out-of-plane tilt angle of the optical transition dipole moment in high temperature annealed, edge-on films. Measurements of spectral differences between in-plane and out-of-plane dipoles, further indicate regions of disordered polymers in low temperature annealed face-on films that are otherwise obscured in traditional X-ray and optical characterization techniques. The methods and analysis developed in this work provide a way to identify and quantify subtle optical and structural anisotropies in organic semiconductors that are important for understanding and designing highly efficient thin film devices.