Calculation of level alignments at dye-semiconductor interfaces: the role of exact exchange in density functional theory

In this work, density functional theory is applied to several models as representative parts in dyesensitized solar cells (DSSCs). The performance of flexible hybrid exchange-correlation functionals is evaluated for the calculation of groundand excited-state properties pertaining to DSSC model systems. The electronic structure of isolated dyes, TiO ₂ clusters, and dyes anchored on a sizeable (TiO ₂ )38 cluster are calculated with hybrid exchange-correlation functionals based on different mathematical forms to incorporate exact exchange. This enables the analysis of level alignments at the dye-TiO ₂ interface within the TDDFT framework, with respect to the role of the exact exchange in various hybrid functionals. Particular emphasis is placed on the assessment of a recently introduced local hybrid functional with range separation which has been shown to improve energies for charge-transfer excitations. Our results indicate that the HOMO-LUMO gap of TiO ₂ clusters that are frequently used to represent the surface of the nanoparticles is significantly larger than the experimental band gap of bulk anatase TiO ₂ (3.2 eV). It is also demonstrated that exchange-correlation functionals with long-range exact exchange perform well for the simulation of UV-Vis spectra of excited organic donor-acceptor dyes and the calculation of band gaps of TiO ₂ clusters, but yield unphysical level alignments at dye-semiconductor interfaces with TDDFT. We further highlight the importance to allow excitations from all occupied orbitals for the calculation of UV-Vis spectra with linear-response TDDFT of organic dyes adsorbed on TiO2 clusters.