The electronic character of PTCDA thin films in comparison to other perylene-based organic semi-conductors: ab initio-, TD-DFT and semi-empirical computations of the opto-electronic properties of large aggregates

Abstract

Perylene-based compounds are promising materials for opto-electronic thin film devices but despite intense investigations, important details of their electronic structure are still under debate. For perylene-3,4,9,10-tetracarboxylic dianhydrid (PTCDA), the theoretical models predict a different relative energetic order of Frenkel and Charge Transfer (CT) states. Additionally, while one model indicates strong differences between PTCDA on one hand and other perylene-based compounds on the other, recent ab initio computations indicate electronic properties of all perylene-based compounds to resemble each other. Finally, the models disagree about the amount of mixing between CT and Frenkel states. Definitive answers to these questions are difficult because the approaches use various approximations. Up to date, the ab initio based methods employ rather small model systems and neglect environmental effects. In the present work, we improve our former approach by analyzing the effects of the various simplifications. In more detail, we increase the size of the model systems, include environmental effects and investigate the influence of exciton-phonon couplings on the absorption spectrum. The computations for larger aggregates were performed with the ZINDO/S approach, because benchmark computations show that it provides accurate vertical excitation energies for Frenkel, as well as CT states.

The dimer-approach to characterize opto-electronic properties of and exciton trapping and diffusion in organic semiconductor aggregates and crystals

“…trimers and tetramers of PTCDA. 55 In this study, we also could show that even ZINDO/S gives very reasonable excitation energies for perylene-based compounds. However, it has to be coupled with an appropriate density functional or wave function based approach because it is not able to provide reliable information about the ground state energy surface.…”

“…110,111 Such a delocalization leads to an increase in the number of Frenkel-and CT states to be taken into account and increases the energy difference between the highest and the lowest Frenkel state. 26,55 The increase in the number of states can only be accounted for by oligomer computations which failed due to hard-and software limitations. Hence, we only included the increase in the energy difference between upper and lower Frenkel state via the Apéry's constant.…”

“…50 This failure persuaded us that, for the description of the emission spectroscopy of many organic aggregates, a more comprehensive approach has to be developed. 29,33,34,[51][52][53][54][55] This approach, which we coined as 'dimer approach', is the main topic of this perspective. However, to clarify the differences to standard approaches that are based on monomers as central units, we briefly summarize their model Hamiltonians in Section 2.…”

QM/MM calculations combined with the dimer approach on the static disorder at organic‐organic interfaces of thin‐film organic solar cells composed of small molecules

“…[49] Some recent investigations show its accuracy for the description of absorption and emission spectra of exciton diffusion in PBI aggregates [50][51][52] and in DIP and PTCDA single crystals. [39,44] A comparison with monomer-based approaches showing advantages and disadvantages of both approaches is presented in a recent review. [53] FIGURE 1 Used p-type molecular semiconductors.…”

“…[39,40] Ground-state geometries and energies, excitation energies, and ionization potentials were calculated with the ωB97X-D functional [41] that was shown to provide very accurate ground-state geometries, excitation energies, and ionization potentials. [42][43][44] Furthermore, it yields reliable intermolecular potential energies for dimers composed of molecular semiconductors. [45,46] In well-ordered but also in amorphous systems, excitons and charges are expected to be delocalized over several units.…”