Bita Jan Feshan
Charge Carrier Transfer Dynamics at the ZnO Nanowire/Quantum Dot Interface
Heterojunctions of colloidal Quantum Dots (QD) with wide-bandgap metal oxide semiconductors have been utilized in a number of electronic devices like solar cells, solar fuels, LEDs, and CO2 reduction. The crucial role of metal oxide/QD heterojunction is effective electron and hole separation at the junction. Ideally, the charge carrier separation should outcompete the loss mechanisms such as surface trapping or Auger recombination, in order to have an efficient working device. Zinc oxide (ZnO) nanowires (NW) are wide-bandgap metal oxide with high surface-to-volume ratio. The ZnO NW/QD heterojunction has been employed in QD-sensitized solar cells (QDSSC). Studies on charge carrier transfer dynamics at the ZnO NW/QD interface provide insight into factors hindering the device efficiency. Time resolved photoluminescence (TRPL) is an effective tool for carrier dynamics investigation at the QD/metal oxide heterojunctions. Electron transfer from photo-excited QDs to metal oxides has been characterized by TRPL through probing dynamics of the QD band edge bleach recovery.
In this study, PL decay lifetime at ZnO NW/QD interface was investigated using the time correlated single-photon counting (TCSPC) method with LED lighting as the excitation source at room temperature. Core/shell cadmium selenide/zinc sulfide (CdSe/ZnS) QDs capped with octadecylamine (ODA) or oleic acid (OA) ligands were employed as a sensitizer on the ZnO NW structure. Since a redox electrolyte is used as the hole scavenging agent in the architecture of QDSSCs, the effect of liquid redox electrolyte on the electron/hole transfer rate was also studied. The experimental and theoretical studies show that the QD photo-charging effect is the mechanism behind profound emission quenching of the ZnO NW/QD/electrolyte structure. Charge build-up in the electrolyte in contact with an insulator promotes photo-charging probability and consequently, excited charge carriers relax through the fast non-radiative Auger mechanism. Furthermore, the decay lifetime measurements revealed that the hole transfer rate is slower than that of the electron. This slow hole transfer rate should be considered to improve the performance of devices fabricated based on this structure. Passivation of non-radiative recombination sites has great importance in improving the device efficiency.