Structure-Dependent Charge Transfer in Molecular Perylene-Based Donor/Acceptor Systems and Role of Side Chains Valentina Belova,* Alexander Hinderhofer,* Clemens Zeiser, Timo Storzer, Jakub Rozbor ̌ il, Jan Hagenlocher, Jir ̌ í Nova ́ k, Alexander Gerlach, Reinhard Scholz, and Frank Schreiber Cite This: J. Phys. Chem. C 2020, 124, 11639-11651 Read Online ACCESS Metrics & More Article Recommendations * sı Supporting Information ABSTRACT: In organic electronics and optoelectronics several crucial physical processes are related to charge transfer (CT) effects. In this work, we investigate mixing behavior and intermolecular coupling of donor and acceptor molecules in thin films prepared by organic molecular beam deposition (OMBD). Diindenoperylene (DIP) and pentacene (PEN) are used as the donor materials, and perylene diimide derivatives PDIR-CN 2 and PDIF-CN 2 as the acceptor materials.. The formation of charge transfer complexes coupled in the electronic excited state vs. noninteracting phase separating components is studied by structural and optical techniques. The CT mechanism and properties are considered in close connection with the thin film microstructure of the D/A blends which can be controlled via a change of the molecule geometry and/or growth temperature. We discuss two key findings for our systems: (1) The CT intensity correlates directly with the possibility of cocrystallization between acceptor and donor. (2) Side chain modification to tune the ground state energy levels has nearly no effect on the energy of the excited state CT, whereas replacement of molecular core modifies the CT energy correspondingly. ■ INTRODUCTION Charge transfer (CT) between a donor (D) and an acceptor (A) is a crucial phenomenon for performance of organic photovoltaic (OPV) devices. 1-4 Since this complex process mediates creation of charge carriers at a D/A interface, and their potential subsequent recombination, its mechanism needs to be understood. Over the past years the most established practical solutions in the field of OPVs were based on polymer/fullerene (or derivatives) combinations. 5-11 How- ever, despite reaching efficiencies of over 10%, fullerene-based solar cells meet a number of limitations, which might be overcome by small molecule semiconductors. 12-17 Small molecule semiconductors provide almost countless possibilities for the tailoring of device properties. 18-20 For example, by choosing different organic compounds, the resulting energy gap (E DA ) between a donor ionization energy (IE) and an acceptor electron affinity (EA) can be tuned. Thus, (i) a larger E DA results in a higher open circuit voltage (V OC ) 21 and smaller nonradiative energy losses (in the absence of any influence of the morphology). 22 (ii) In the case of a narrower E DA , a direct excitation of low-lying CT states leads to broadening of the optical absorption wavelength range in the near-infrared region most favorable for solar energy harvesting. However, the CT states might also serve as efficient recombination channels for excitons. 23-28 This aspect is particularly important and has to be taken into consideration when designing an active layer in a bulk heterojunction (BHJ) configuration. Compared with planar heterojunctions, the BHJ configuration provides a larger interface area between donor and acceptor and would therefore be more advantageous in terms of photon to charge carrier quantum yield. Consequently, for BHJs the morphology of the mixed layer plays a paramount role. 4,29-31 First of all, charge transport suffers from numerous in-gap trap states introduced by lattice disorder. 32 Another factor is the nucleation of one of the pure phases, in particular an acceptor phase, along with the presence of a mixed phase which is considered as a beneficial condition for increasing charge separation rates. 33,34 A higher dielectric constant of the acceptor phase is required to lower the Coulomb exciton binding energy and facilitate exciton dissociation. 35 Furthermore, crystalline domains of pure phases provide percolation pathways for delivering charge carriers to the electrodes. 36 The exciton diffusion length in polycrystalline Received: January 10, 2020 Revised: April 28, 2020 Published: April 29, 2020 Article pubs.acs.org/JPCC © 2020 American Chemical Society 11639 https://dx.doi.org/10.1021/acs.jpcc.0c00230 J. Phys. Chem. C 2020, 124, 11639-11651 Downloaded via UNIV TUEBINGEN on June 3, 2020 at 09:37:29 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.