Identification of the nature of excitons in PPDT2FBT using electroabsorption spectroscopy

TL;DR

This study uses electroabsorption spectroscopy and Gaussian fitting to analyze exciton types in PPDT2FBT, revealing mostly mixed excitons with charge transfer character, indicating its potential for photovoltaic applications.

Contribution

It introduces a Gaussian fitting method to deconstruct EA spectra for identifying exciton nature in organic semiconductors, applied to PPDT2FBT.

Findings

Most absorption bands in PPDT2FBT are mixed excitons with charge transfer contributions.

The feature at 3.07 eV is strongly Frenkel-like.

PPDT2FBT's exciton profile suggests suitability for photovoltaic devices.

Abstract

Electroabsorption (EA) measurements can be used to identify the type of excitons contributing to the absorption spectra of semiconductors which have applications in optoelectronics. However, the inferences from the EA measurement greatly depend on the method of fitting and extraction of parameters from the measured spectra. We deconstruct the absorption spectrum by fitting multiple Gaussians and obtain the relative contribution of first and second derivative of each absorption band in EA spectrum, which gives indication of the Frenkel, charge transfer or mixed nature of the excitons involved. We check the applicability of the method for pentacene which is widely used and well studied organic semiconductor. We report EA measurements of poly[(2,5-bis(2-hexyldecyloxy)-phenylene)-alt-(5,6-difluoro-4,7-di(thiophen-2-yl)benzo[c]-[1,2,5]-thiadiazole)] (PPDT2FBT). Our analysis shows that besides the feature around 3.07 eV, which is strongly Frenkel-like, most of the absorption bands for PPDT2FBT are mixed states, having relatively high charge transfer contributions. Since charge transfer excitons have higher dissociation efficiencies, we infer PPDT2FBT to be a promising candidate for photovoltaic applications.