Identification of Ultrafast Relaxation Processes As a Major Reason for Inefficient Exciton Diffusion in Perylene-Based Organic Semiconductors Volker Settels, † Alexander Schubert, †,⊥ Maxim Tafipolski, † Wenlan Liu, †,# Vera Stehr, ‡ Anna K. Topczak, ‡,§ Jens Pflaum, ‡,§ Carsten Deibel, ‡ Reinhold F. Fink, †,∥ Volker Engel, † and Bernd Engels* ,† † Institut fü r Physikalische und Theoretische Chemie, Universitä t Wü rzburg, Emil-Fischer-Str. 42, 97074 Wü rzburg, Germany ‡ Lehrstuhl fü r Experimentelle Physik VI, Universitä t Wü rzburg, Am Hubland, 97074 Wü rzburg, Germany § ZAE Bayern e.V., Am Galgenberg 87, 97074 Wü rzburg, Germany * S Supporting Information ABSTRACT: The exciton diffusion length (L D ) is a key parameter for the efficiency of organic optoelectronic devices. Its limitation to the nm length scale causes the need of complex bulk-heterojunction solar cells incorporating difficulties in long-term stability and reproducibility. A comprehensive model providing an atomistic understanding of processes that limit exciton trasport is therefore highly desirable and will be proposed here for perylene-based materials. Our model is based on simulations with a hybrid approach which combines high-level ab initio computations for the part of the system directly involved in the described processes with a force field to include environmental effects. The adequacy of the model is shown by detailed comparison with available experimental results. The model indicates that the short exciton diffusion lengths of α-perylene tetracarbox- ylicdianhydride (PTCDA) are due to ultrafast relaxation processes of the optical excitation via intermolecular motions leading to a state from which further exciton diffusion is hampered. As the efficiency of this mechanism depends strongly on molecular arrangement and environment, the model explains the strong dependence of L D on the morphology of the materials, for example, the differences between α-PTCDA and diindenoperylene. Our findings indicate how relaxation processes can be diminished in perylene-based materials. This model can be generalized to other organic compounds. ■ INTRODUCTION Organic semiconductors are promising materials for thin-film electronic devices such as organic solar cells. Especially for the latter, however, their efficiencies are strongly limited due to small exciton diffusion lengths (L D ). 1,2 Organic solar cells have been tremendously improved in recent years, often by trial-and- error variations of materials and device architecture, 3−6 but further optimization requires a better understanding of the underlying microscopic and atomistic power conversion processes. 7,8 More efficient exciton diffusion is desirable since this improves device efficiencies and allows to use less complex device architectures. 7 In order to design compounds with extended exciton diffusion lengths (L D ), a detailed knowledge about possible loss processes such as exciton trapping is needed. Trapping processes which shorten L D were carefully investigated by various experiments, but a comprehensive model providing an atomistic understanding of these processes 8,9 and their influence on L D 10,11 is still missing. In the case of perylene-based materials, absorption and emission spectra of aggregates, crystals, and thin films pointed toward population transfers from locally excited Frenkel states to spatially separated charge-transfer (CT) states. 11,12 However, these explanations were questioned by recent experiments on neat organic semiconductors, which indicated that the CT states lie energetically above their Frenkel counterparts. 5,6,13,14 Theoretical descriptions supporting the transfer to CT states by simulations based on empirical Hamiltonians 15,16 were furthermore challenged by high-level ab initio calculations which support the recent experiments. 17−19 For perylene tetracarboxylic bisimide (PBI) thin-films transient absorption measurements identified a fast relaxation (∼100 ps) of the exciton to an intermolecular, long-lived (∼20 ns), immobile state which exhibits a red-shifted emission spectrum. 20 Time- dependent spectroscopy on PBI-doped films indicated that dimer states can constitute efficient exciton traps. 21 Raman spectroscopic measurements of α-perylene single crystals indicated that exciton self-trapping is triggered by motions of two monomers relative to each other. 10 A corresponding atomistic model was provided in a recent ab initio-based simulation of PBI aggregates which revealed an efficient self- Received: January 2, 2014 Published: June 9, 2014 Article pubs.acs.org/JACS © 2014 American Chemical Society 9327 dx.doi.org/10.1021/ja413115h | J. Am. Chem. Soc. 2014, 136, 9327−9337