Abstract
Recent progress in the improvement of organic solar cells lead to a power conversion efficiency to over 16%. One of the key factors for this improvement is a more favorable energy level alignment between donor and acceptor materials, which demonstrates that the properties of interfaces between donor and acceptor regions are of paramount importance. Recent investigations showed a significant dependence of the energy levels of organic semiconductors upon admixture of different materials, but its origin is presently not well understood. Here, we use multiscale simulation protocols to investigate the molecular origin of the mixing induced energy level shifts and show that electrostatic properties, in particular higher-order multipole moments and polarizability determine the strength of the effect. The findings of this study may guide future material-design efforts in order to improve device performance by systematic modification of molecular properties.
Highlights
We study the effect of varying type and composition of the molecular environment and its influence on the ionization potential (IP) and HOMO level position in mixed donor-acceptor systems
We find that the non-polarized calculations of the IP energy as a function of the C60 content show the same trend as the experimental data as well as the simulations including polarization effects, which leads to the conclusion that differences in the electrostatic properties of the donor molecules lead to the observed energy shifts rather than differences in polarizability of the molecules
We have shown in this work that multiscale simulations can quantitatively predict the shifts of energy levels such as the ionization potential of a materials as a function of the material mixture, an effect that was experimentally observed by Graham et al.[16]
Summary
We use multiscale simulation protocols to investigate the molecular origin of the mixing induced energy level shifts and show that electrostatic properties, in particular higher-order multipole moments and polarizability determine the strength of the effect. Www.nature.com/scientificreports the polarizability of molecules was shown to be the main reason for energy level differences between molecules in vacuum and bulk materials[22,23] while electrostatic properties such as quadrupole moments dominate energy level shifts at ordered interfaces[20].
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