Photoinduced Charge Carrier Dynamics of Metal Chalcogenide Semiconductor Quantum Dot Sensitized TiO₂ Film for Photovoltaic Application

Semiconductor quantum dot (QD) sensitization is one of the most attractive structures to employ QDs for photovoltaic application. The function of QD sensitized solar cells (QDSSC) is controlled by the interfacial charge transfer dynamics. Here we employ transient absorption spectroscopy (TAS) to assess charge transfer dynamics at CdS QD/TiO₂ interface, and correlate their dynamics with their solar cell performance. An electron injection occurs from CdS QD conduction band to TiO₂ on ultrafast time scales, and the time constant decreases from ~10 ps to 1 ps, as the QD size decreases from 4 nm to 1.6 nm. Also, the charge recombination lifetime at the QD/TiO₂ interface increases, as the QD size increases. An absorbed photon to current conversion efficiency (APCE) of the QDSSC increases, as the QD size increases. Therefore, we conclude that the APCE of the CdS QDSSC is controlled by the interfacial charge recombination dynamics competing with dynamics of the hole transfer from the QD valence band to the reduced electrolyte. The optimum CdS QD size is close to or larger than 4 nm, as long as the light harvesting efficiency of the CdS QD sensitized film is sufficiently high.