Quantum-chemical calculations of dye-sensitized semiconductor nanocrystals

Abstract

Quantum chemical calculations providing detailed information of dye-sensitized semiconductor nanocrystals are presented. The calculations are used to elucidate both structural and electronic properties of photoelectrochemical devices, such as environmentally friendly Dye-Sensitized Solar Cells (DSSCs), at the molecular level. Quantum chemical calculations have recently been performed on both organic and organometallic dye molecules attached to titanium dioxide (TiO 2 ) nanocrystals via different anchor and spacer groups. Strategies to make accurate quantum chemical calculations, e.g. at the DFT level of theory, on increasingly realistic models of such dye-sensitized semiconductor interfaces are presented. The ability of different anchor and spacer groups to act as mediators of ultrafast photo-induced electron injection from the dye molecules into the semiconductor nanocrystals is, in particular, discussed in terms of calculated electronic coupling strengths, and direct comparisons with experimental information are made whenever possible. Progress in the development of multi-scale simulation techniques using so called reactive force fields is illustrated for dye-sensitized solar cell systems.

Spacer and Anchor Effects on the Electronic Coupling in Ruthenium-bis-Terpyridine Dye-Sensitized TiO2 Nanocrystals Studied by DFT

“…We have successfully used this method combination in previous studies. 25,27,30,33,34,40,59,64 In the analysis of the electronic structure, an effective density of states (DOS) was constructed from the discrete one-electron molecular orbital (MO) levels through a Gaussian broadening of the individual orbital contributions by an arbitrary factor of 0.3 eV. Moreover, the five lowest excitations were calculated using TD-B3LYP/VD(T)Z for the sensitized system.…”

“…65 The estimated electron-transfer time was obtained from the lifetime broadening through τ (fs) ) 658/pΓ (meV). This approach has previously been used by our group 30,37,40 and is described in more detail in refs 30 and 37.…”

“…27 In addition, so-called spacer groups can be introduced between the chromophore and the anchor group in order to spatially separate the chromophore and semiconductor and to control the electronic interactions between the dye and the nanoparticle. [28][29][30] Rigid spacers are, in this context, of particular interest, as they provide the possibility to tune the electronic properties with full control over the distance dependence of the surface electron transfer (ET). 31,32 Computer simulations are now applicable to increasingly realistic model systems, providing information that can help to understand and optimize both the structure and the electronic properties of dye-TiO 2 interfaces and thus complement experimental work.…”

“…[35][36][37] Such large cluster models have also been used to investigate the interfacial properties of several dyes bound to TiO 2 nanocrystals. 30,[38][39][40] In this paper, we use a cluster model containing 46 TiO 2 units that we have used previously to study the adsorption of phosphonic and carboxylic acid adsorbed to an anatase (101) type surface section. 37 For photosensitization of TiO 2 , the position of the excitedstate energy level of the dye relative to the substrate conduction band (CB) edge is an important factor because the electron injection from the excited dye molecule to the CB of the semiconductor requires that the first excited-state energy level of the dye is higher than the edge of the CB.…”

“…4 However, the energetic position of the CB edge is not fixed, but can shift in both directions when the environment around the semiconductor is changed, for example, by adsorption of different cations on the TiO 2 surface and thus influence the injection process. 41 A second critical factor for efficient photoinduced surface ET is the interfacial electronic coupling, which can be estimated using the mixing of dye and semiconductor orbitals as a simple measure of the effective lifetime broadening according to the Newns-Anderson model 30,33,34 as long as the excited state of the dye is sufficiently high above the CB edge to be in the socalled wide-band limit. 42 In this case, the excited dye donor state is effectively in constant resonance with a quasicontinuum of substrate band acceptor states.…”