Morphology Effectively Controls Singlet-Triplet Exciton Relaxation and Charge Transport in Organic Semiconductors

TL;DR

This study investigates how molecular morphology influences exciton relaxation and charge transport in organic semiconductors, highlighting the roles of crystallinity and purity in photogeneration processes relevant for photovoltaic device design.

Contribution

It provides a comparative analysis of ultrafast photo-conversion dynamics in tetracene and pentacene crystals and films, revealing the effects of morphology on exciton behavior and charge trapping.

Findings

Singlet-triplet exciton fission dominates in Tc and Pc crystals.

Electron trapping suppresses exciton decay in Pc films.

Crystallinity and purity are crucial for photogeneration efficiency.

Abstract

We present a comparative study of ultrafast photo-conversion dynamics in tetracene (Tc) and pentacene (Pc) single crystals and Pc films using optical pump-probe spectroscopy. Photo-induced absorption in Tc and Pc crystals is activated and temperature-independent respectively, demonstrating dominant singlet-triplet exciton fission. In Pc films (as well as C$_{60}$-doped films) this decay channel is suppressed by electron trapping. These results demonstrate the central role of crystallinity and purity in photogeneration processes and will constrain the design of future photovoltaic devices.