Concerning the stability of hybrid biexcitons in organic polymer aggregates

TL;DR

This paper predicts and experimentally verifies the formation of stable biexciton states in organic polymer aggregates, arising from both attractive and repulsive interactions, challenging traditional views on bound exciton pairs.

Contribution

It introduces a theoretical framework for stable biexcitons from repulsive interactions and confirms these predictions with ultrafast spectroscopy in conjugated polymers.

Findings

JJ-biexcitons from repulsive dipolar interactions with energies above twice the single exciton energy

H-biexcitons from attractive dipolar interactions with energies below twice the single exciton energy

Experimental evidence of both biexciton types in PBTTT material exhibiting J- and H-like behavior

Abstract

Frenkel excitons are the primary photoexcitations in organic semiconductors and are ultimately responsible for the optical properties of such materials. They are also predicted to form \emph{bound} exciton pairs, termed biexcitons, which are consequential intermediates in a wide range of photophysical processes. Generally, we think of bound states as arising from an attractive interaction. However, here we report on our recent theoretical analysis predicting the formation of stable biexciton states in a conjugated polymer material arising from both attractive and repulsive interactions. We show that in J-aggregate systems, JJ-biexcitons can arise from repulsive dipolar interactions with energies $E_{JJ}> 2E_J$ while in H-aggregates, HH-biexciton states $E_{HH} < 2E_H$ corresponding to attractive dipole exciton/exciton interactions. These predictions are corroborated by using ultrafast double-quantum coherence spectroscopy on a PBTTT material that exhibits both J- and H-like excitonic behavior.