Predicting the Rate Constant of Electron Tunneling Reactions at the CdSe–TiO₂ Interface

Abstract

Current interest in quantum dot solar cells (QDSCs) motivates an understanding of the electron transfer dynamics at the quantum dot (QD)-metal oxide (MO) interface. Employing transient absorption spectroscopy, we have monitored the electron transfer rate (ket) at this interface as a function of the bridge molecules that link QDs to TiO2. Using mercaptoacetic acid, 3-mercaptopropionic acid, 8-mercaptooctanoic acid, and 16-mercaptohexadecanoic acid, we observe an exponential attenuation of ket with increasing linker length, and attribute this to the tunneling of the electron through the insulating linker molecule. We model the electron transfer reaction using both rectangular and trapezoidal barrier models that have been discussed in the literature. The one-electron reduction potential (equivalent to the lowest unoccupied molecular orbital) of each molecule as determined by cyclic voltammetry (CV) was used to estimate the effective barrier height presented by each ligand at the CdSe-TiO2 interface. The electron transfer rate (ket) calculated for each CdSe-ligand-TiO2 interface using both models showed the results in agreement with the experimentally determined trend. This demonstrates that electron transfer between CdSe and TiO2 can be viewed as electron tunneling through a layer of linking molecules and provides a useful method for predicting electron transfer rate constants.

Nonadiabatic molecular dynamics with decoherence and detailed balance under a density matrix ensemble formalism

“…59 In our simulation the diameter of the Si QD is only ∼1.3 nm, which is much smaller than 2.4 nm. In this regard, our resulting (sub-)10 ps charge transfer time should be reasonable when compared with experiments 59,60 .…”

“…Experimentally, the charge transfer time between quantum dots is found to be sensitive to the QD size. 59,60 For instance, in CdSe/TiO 2 systems, the charge transfer time decreases by three orders of magnitude (from ∼100 ns to ∼0.1 ns) when the size of CdSe QD decrease from 7.5 nm to 2.4 nm. 59 In our simulation the diameter of the Si QD is only ∼1.3 nm, which is much smaller than 2.4 nm.…”

Shell effects on hole-coupled electron transfer dynamics from CdSe/CdS quantum dots to methyl viologen

“…Hines et al obtained a smaller value of β = 0.08 Å −1 in CdSe/TiO 2 bridged by organic molecules with an increasing length from 3 to 21 Å. 45 Importantly, we have determined the actual number of ET-active MV 2+ acceptor molecules on the QDs and used this number in the calculation of the intrinsic ET rates, instead of using the number of added MV 2+ that was assumed in Dworak's work. As discussed above (see Fig.…”

“…The values obtained lie in the range 10 8 -10 10 s −1 in good agreement with recent published results for other QD-acceptor pairs. 17,18,21,45 Fig. 7 shows an exponential fit k ET = k 0 e −βt , to the measured intrinsic ET rate k ET1 from 1S e 1S 3/2 with varying shell thickness t, yielding an attenuation factor β of 0.13 ± 0.05 Å −1 .…”

“…where ΔG is the change in the Gibbs free energy, λ E is the reorganisation energy of the complex system, R is the electron donor-to-acceptor distance, and |H(R)| 2 describes the wavefunction overlap between the electron level and the molecular orbital of the adsorbed acceptors. Based on this model, the inorganic shell affects the ET process via three distinct mechanisms: (i) by separating the adsorbed electron acceptors away from the QDs in space; 45 (ii) by serving as a tunnelling barrier that slows the electron transfer; 48 and (iii) by directly modifying the excited state carrier wavefunction. 17 In the three batches of CdSe/CdS QDs studied here, the energy levels are marginally affected by the CdS shell, as indicated by the red shift of ∼85 meV of the band-edge absorption peak from 1.2 to 5.7 MLs (see Fig.…”