Photoinduced electron transfer from semiconductor quantum dots to metal oxide nanoparticles

Abstract

Quantum dot-metal oxide junctions are an integral part of next-generation solar cells, light emitting diodes, and nanostructured electronic arrays. Here we present a comprehensive examination of electron transfer at these junctions, using a series of CdSe quantum dot donors (sizes 2.8, 3.3, 4.0, and 4.2 nm in diameter) and metal oxide nanoparticle acceptors (SnO(2), TiO(2), and ZnO). Apparent electron transfer rate constants showed strong dependence on change in system free energy, exhibiting a sharp rise at small driving forces followed by a modest rise further away from the characteristic reorganization energy. The observed trend mimics the predicted behavior of electron transfer from a single quantum state to a continuum of electron accepting states, such as those present in the conduction band of a metal oxide nanoparticle. In contrast with dye-sensitized metal oxide electron transfer studies, our systems did not exhibit unthermalized hot-electron injection due to relatively large ratios of electron cooling rate to electron transfer rate. To investigate the implications of these findings in photovoltaic cells, quantum dot-metal oxide working electrodes were constructed in an identical fashion to the films used for the electron transfer portion of the study. Interestingly, the films which exhibited the fastest electron transfer rates (SnO(2)) were not the same as those which showed the highest photocurrent (TiO(2)). These findings suggest that, in addition to electron transfer at the quantum dot-metal oxide interface, other electron transfer reactions play key roles in the determination of overall device efficiency.

Fig. 1.

Diagram of the relative electronic energy differences between CdSe donating species and MO accepting species for all CdSe–MO combinations used in this study.

Fig. 2.

UV-visible (A) and transient absorption spectral (B–E) traces of 4.2-nm-diameter CdSe quantum dots in toluene (A) and attached to SiO₂ (B), SnO₂ (C), TiO₂ (D), and ZnO (E). Transient spectra shown at pump-probe delay times of 0 (black), 1 (red), 10 (blue), 100 (cyan), and 1,000 ps (pink). Also, transient absorption kinetic traces (F) of 4.2-nm-diameter CdSe quantum dots attached to each MO substrate at the 1S_3/2-1S_e transition. Error bars are representative of standard deviation of eight measurements collected at four unique spots on each CdSe-Mo film.

Fig. 3.

Global plot of all CdSe (donor) to MO (acceptor) electron transfer data and trace of Eq. 1 with λ = 10 meV and Δ = 50 meV.

Fig. 4.

Schematic diagram of hot versus cold electron injection and its effect on maximum obtainable open circuit voltage (V_oc,MAX) in a photovoltaic device (A). Depiction (B) and ultrafast transient rise times (C) of multiple sizes of CdSe quantum dot excited at various heights above the band edge utilizing a single pump energy. Rise time of transient versus energy differential between quantum dot bandgap and pump pulse (D) and demonstration of a lack of hot-electron injection in CdSe-SnO₂, CdSe-TiO₂, and CdSe-ZnO systems (E). Note, C and D collected from CdSe in toluene solution.

Fig. 5.

Photovoltaic characteristics of films prepared in an identical manor as those described in the spectroscopic portion of the study. Transient current (A) and incident photon-to-carrier generation efficiency (IPCE) (B) measurements both show improved performance in the CdSe-TiO₂ working electrode. Electron transfer reaction cycle in liquid junction QDSSC (C). Processes which result in loss of performance are depicted as arrows leading away from the center of the cycle.