Polarization Resolved Optical Excitation of Charge-Transfer Excitons in PEN:PFP Co-Crystalline Films: Limits of Non-Periodic Modelling

TL;DR

This study combines experimental spectroscopy and first-principles calculations to investigate charge-transfer excitons in crystalline PEN:PFP films, revealing the limitations of non-periodic models in capturing long-range interactions.

Contribution

It demonstrates the importance of periodic boundary conditions in accurately modeling charge-transfer excitons in organic crystalline systems.

Findings

Polarization of CTX determined experimentally.

First-principles calculations confirm the CTX as the lowest-energy excitation.

Cluster models fail to reproduce the CTX due to lack of long-range interactions.

Abstract

Charge-transfer excitons (CTX) at organic donor/acceptor interfaces are considered important intermediates for charge separation in photovoltaic devices. Crystalline model systems provide microscopic insights into the nature of such states as they enable microscopic structure-property investigations. Here, we use angular-resolved UV/Vis absorption spectroscopy to characterize the CTX of crystalline pentacene:perfluoro-pentacene (PEN:PFP) films allowing to determine the polarization of this state. This analysis is complemented by first-principles many-body calculations, performed on the three-dimensional PEN:PFP co-crystal, which confirm that the lowest-energy excitation is a CTX. Analogous simulations performed on bimolecular clusters are unable to reproduce this state. We ascribe this failure to the lack of long-range interactions and wave-function periodicity in these cluster calculations, which appear to remain a valid tool for modelling properties of organic materials ruled by local intermolecular couplings.