Temperature dependent charge transfer state absorption and emission reveal dominant role of dynamic disorder in organic solar cells

TL;DR

This study investigates how temperature influences charge transfer states in organic solar cells, revealing that vibrational effects, rather than static disorder, primarily cause spectral broadening and impact device performance.

Contribution

It provides detailed temperature-dependent spectroscopic analysis of CT states, confirming the role of vibrational effects over static disorder in spectral broadening.

Findings

Temperature-activated vibrational broadening dominates spectral features.

Electro-optical reciprocity relation validated across temperatures.

CT state energies correlate with open circuit voltage at 0 K.

Abstract

The energetic landscape of charge transfer (CT) states at the interface of electron donating and electron accepting domains in organic optoelectronic devices is crucial for their performance. Central questions -- such as the role of static energetic disorder and vibrational effects -- are under ongoing dispute. This study provides an in-depth analysis of temperature dependent broadening of the spectroscopic absorption and emission features of CT states in devices with small molecule-fullerene blends. We confirm the validity of the electro-optical reciprocity relation between the photovoltaic external quantum efficiency ($\mathrm{EQE_{PV}}$) and electroluminescence ($\mathrm{EQE_{EL}}$), enabling us to validate the device temperature during the experiment. The validated temperature allows us to fit our experimental data with several models, and compare extracted CT state energies with the corresponding open circuit voltage limit at $0\mathrm{\,K}$. Our findings unveil that the absorption and emission characteristics are usually not symmetric, and dominated by temperature-activated broadening (vibrational) effects instead of static disorder.