Charge-transfer states and optical transitions at the pentacene-TiO₂interface

Abstract

Pentacene molecules have recently been observed to form a well-ordered monolayer on the (110) surface of rutile TiO 2 , with the molecules adsorbed lying flat, head to tail. With the geometry favorable for direct optical excitation and given its ordered character, this interface seems to provide an intriguing model to study charge-transfer excitations where the optically excited electrons and holes reside on different sides of the organic-inorganic interface. In this work, we theoretically investigate the structural and electronic properties of this system by means of ab initio calculations and compute its excitonic absorption spectrum. Molecular states appear in the band gap of the clean TiO 2 surface, which enables charge-transfer excitations directly from the molecular HOMO to the TiO 2 conduction band. The calculated optical spectrum shows a strong polarization dependence and displays excitonic resonances corresponding to the charge-transfer states, which could stimulate new experimental work on the optical response of this interface.In many dye-sensitized solar-cells, organic molecules with good light-absorbing properties are placed in contact with porous TiO 2 [3,13,14], with the effect that a photoabsorption process can transfer an electron from the molecule to the TiO 2 conduction band. Pentacene is a π-conjugated molecule, and it acts as an electron donor in bulk-heterojuction cells [24] when combined with fullerenes or their functionalized derivatives, such as PCBM [24]. Also its use in dye-sensitized solar cells has been reported [17]. When pentacene adsorbs at semiconductor or insulator surfaces, such as SiO 2 , it is usually almost upright [25], making the π-orbitals pointing parallel to the surface. However, recently pentacene has been shown to adsorb lying down on a rutile TiO 2 (110) surface for a coverage up to 3 monolayers, as was deduced by STM and x-ray absorption measurements [26]. In this configuration, the overlap between the π-orbitals of the molecule and the substrate states is increased, which may increase the interaction strength between the carriers of different constituents. Consequently, the pentacene monolayer on TiO 2 might provide a prototypical system where charge-transfer excitations between the molecule and the surface can take place directly upon optical excitation, enabling indepth studies of these excitations from the related spectra. Furthermore, experiments [26] indicate that pentacene molecules form a very well-ordered wetting layer on rutile TiO 2 (110), providing a well-defined organic-inorganic interface, making it an ideal candidate for theoretical studies, in contrast to other similar interfaces that show very complex and disordered structures.In modeling molecules adsorbed on surfaces, density functional theory (DFT) often produces geometries and ground state electronic properties with sufficient accuracy. As extensions, many-body techniques such as the GW approximation together with the Bethe-Salpeter equation (BSE) [27,28] can access the quasiparticle band energie...

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