A Universal Urbach Rule for Disordered Organic Semiconductors

TL;DR

This paper demonstrates that the Urbach energy in disordered organic semiconductors is universally equal to thermal energy and provides a model linking spectral line-shapes to disorder, aiding solar cell optimization.

Contribution

It introduces a simple model explaining absorption line-shapes in disordered organic semiconductors and clarifies the physical meaning of Urbach energy in these materials.

Findings

Urbach energy equals thermal energy in organic semiconductors.

Absorption line-shapes are explained by a convolution of Gaussian states and thermal activation.

A strategy to determine excitonic disorder energy is proposed.

Abstract

In crystalline semiconductors, absorption onset sharpness is characterized by temperature dependent Urbach energies. These energies quantify the static, structural disorder causing localized exponential-tail states, and dynamic disorder from electron-phonon scattering. Applicability of this exponential-tail model to disordered solids has been long debated. Nonetheless, exponential fittings are routinely applied to sub-gap absorption analysis of organic semiconductors. Herein, we elucidate the sub-gap spectral line-shapes of organic semiconductors and their blends by temperature-dependent quantum efficiency measurements. We find that sub-gap absorption due to singlet excitons is universally dominated by thermal broadening at low photon energies and the associated Urbach energy equals the thermal energy, regardless of static disorder. This is consistent with absorptions obtained from a convolution of Gaussian density of excitonic states weighted by Boltzmann-like thermally activated optical transitions. A simple model is presented that explains absorption line-shapes of disordered systems, and we also provide a strategy to determine the excitonic disorder energy. Our findings elaborate the meaning of the Urbach energy in molecular solids and relate the photo-physics to static disorder, crucial for optimizing organic solar cells for which we present a new radiative open-circuit voltage limit.