Role of surface defects on the adsorption of poly(9-vinylcarbazole) on TiO2 using the monomer as a donor:acceptor model

Abstract

In this report, we studied the interaction between 9-vinylcarbazole (VK) –i.e. the monomer of poly(9-vinylcarbazole) (PVK)- and a TiO2 (101) surface in order to shed some light in the interface phenomena of this promising fullerene-free donor:acceptor system as active layer for solar cells. Since the role of the TiO2 surface defects is the main concern of our study, we proceeded to simulate the adsorption of the molecule on TiO2 with oxygen or titanium atom vacancies and without any defect. All these systems were studied using First Principles calculations, utilizing Density Functional Theory (DFT). The adsorption energy of the VK monomer-TiO2 systems was favorable in all cases. The presence of vacancies in the TiO2 surface yielded modifications in the electronic properties: the difference between the LUMO level of VK and the conduction band of TiO2-O surface is only ~0.2 eV, thus allowing to infer a decrease in the energy barrier associated to the injection of electrons from VK monomer towards the TiO2-O surface. The charge density differences in all systems also revealed that the donor:acceptor model is plausible and concurs with previous results, suggesting promising features for potential application as a donor:acceptor systems in fullerene-free solar cells.