The maximum efficiency reachable by a photovoltaic device based on a single absorber is thermodynamically limited to ~33%; the Shockley−Queisser (SQ) limit. To a large extent, this efficiency limit is determined by waste heat in the absorber; waste heat which is generated by the thermalization of “hot” charge carriers generated in the material following the absorption of above-bandgap high-energy photons. Several approaches have been proposed in order to bypass thermal losses in solar cell devices, among them, hot carrier solar cells (HCSCs) are distinctly the most promising concept when considering photo-conversion efficiency limits (reaching ~74%). However, practical implementation of operational HCSCs prototypes remains a big challenge; this is in part due to the difficulties on finding/engineering systems where hot carriers are efficiently collected at room temperature.

In this communication, by employing time resolved THz spectroscopy (TRTS), we demonstrate highly-efficient room-temperature hot electron transfer (HET) at QD/mesoporous oxide interfaces. The emergence of HET is directly apparent from photon-energy dependent TRTS measurements. When the samples are irradiated with photon energies matching the QD bandgap, the ET dynamics are monophasic and defined by a time constant of ~10ps. When the samples are irradiated with above QD bandgap photon energies, the ET dynamics become bi-phasic with characteristic time constants of ~10ps and <1ps respectively (representing cold and hot ET components respectively). For even higher photon energies (~3eV photons onto ~1eV bandgap PbS QDs) the ET dynamics become again monophasic with sub-ps time constants (≤0.1ps, limited by the TRST setup resolution). In this case, the HET collection efficiency for photo-generated carries reaches unity quantum yield. Finally, from temperature dependent analysis of interfacial QD-oxide dynamics, we resolve that HET rates (and hence efficiency) are substantially enhaced as the temperature of the system is reduced. This observation is fully consistent with the “a pirori” expectation that HET efficiency is determined by kinetic competition between QD-to-oxide HET rate and hot electron cooling rate within the QD.

Our results reveal the effect and interplay of key parameters governing hot electron transfer at QD-oxide interfaces. The foreseen potential and constraints for the analyzed systems for the realization of hot carrier solar cells prototypes will be briefly discussed.