Revealing full molecular orientation distributions in organic thin films by nonlinear polarimetry

TL;DR

This paper introduces a nonlinear polarimetry technique combined with the Maximum Entropy Method to accurately reconstruct full molecular orientation distributions in organic thin films, surpassing traditional methods that only measure low-order moments.

Contribution

It presents a novel approach to fully characterize molecular orientation distributions, revealing complex features like asymmetry and bimodality that are invisible to conventional techniques.

Findings

Successfully reconstructs detailed molecular orientation distributions.

Reveals limitations of molecular dynamics simulations in capturing complex distributions.

Establishes a new standard for validating predictive models in material design.

Abstract

The performance of organic optoelectronic devices is critically dependent on how molecules orient within organic thin films. Yet, standard characterization techniques only reveal the first and second moments of the molecular orientation distribution. This limitation obscures the true molecular arrangement, as diverse distributions can yield identical low-order averages while exhibiting distinct functional properties. Here, we bridge this gap by combining multi-harmonic nonlinear polarimetry (second, third, and fourth harmonic) with the Maximum Entropy Method to reconstruct the probability distribution without any \textit{a priori} assumptions. This allows us to resolve features in the distribution such as asymmetry and bimodality, that remain invisible to conventional probes. Furthermore, we use this method to benchmark molecular dynamics simulations, revealing that these simulations often fail to capture the complex distribution despite correctly predicting the first and second moments. This work transforms molecular orientation from an inferred average into a precise observable, establishing essential validation standards for predictive material design.