First-Principles Investigations of Electronically Excited States in Organic Semiconductors

Abstract

In this chapter, we review our theoretical investigations of the electronically excited states in organic semiconductor materials. Detailed insights into the electronically excited states are essential for understanding the charge photogeneration process in organic solar cells. However, despite recent developments of quantum chemistry and ab initio software packages, it is still challenging to predict the electronic states of disordered organic materials, such as donor/acceptor interfaces. Here, we present first-principle studies of the organic semiconductors based on large-scale electronic structure calculations, highlighting the effects of intermolecular interactions and molecular aggregations on the excited states. We first outline the effects of molecular aggregations on the electronic states, polarization and delocalization effects. Next, we summarize the recent developments in large-scale electronic structure calculations based on the many-body Green’s function and fragment molecular orbital method. In particular, the advantages of the many-body Green’s function method within the GW approximation are discussed in terms of the exciton binding energy. As an application, we investigate the electronic states of pentacene clusters and illustrate the roles of polarization and delocalization effects. Moreover, we explore the interfacial charge-transfer states in a pentacene/C60 interface structure and discuss the role of induced polarization in the electron-hole pair separation. Finally, we briefly address future challenges for first-principle studies.