Dynamics in next-generation solar cells: time-resolved surface photovoltage measurements of quantum dots chemically linked to ZnO (101¯0)

Abstract

The charge dynamics at the surface of the transparent conducting oxide and photoanode material ZnO are investigated, in the presence and absence of light harvesting colloidal quantum dots (QDs). The time-resolved change in the surface potential upon photoexcitation has been measured in m-plane ZnO (101�0) using a laser pump-synchrotron X-ray probe methodology. By varying the oxygen annealing conditions, and hence the oxygen vacancy
concentration of the sample, we find that dark carrier lifetimes at the ZnO surface vary from hundreds of μs to ms timescales, i.e. a persistent photoconductivity (PPC) is observed. The highly-controlled nature of our experiments under ultra-high vacuum (UHV), and the use of band-gap and sub-band-gap photoexcitation, allow us to demonstrate that defect states ca. 340 meV above the valence band edge are directly associated with the PPC, and that PPC mediated by these defects dominates over the oxygen photodesorption mechanism. These observations are consistent with the hypothesis that ionized oxygen vacancy states are responsible for PPC in ZnO. The effect of chemically linking two colloidal QD systems (type I PbS and type II CdS/ZnSe) to the surface has also been investigated. Upon deposition of QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting direct Dynamics in next-generation solar cells: timeresolved surface photovoltage measurements of quantum dots chemically linked to ZnO (���� �) - ResearchGate. Available from: http://www.researchgate.net/publication/260869200_Dynamics_in_nextgeneration_solar_cells_timeresolved_surface_photovoltage_measurements_of_quantum_dots_chemically_linked_to_ZnO_(_) [accessed Apr 20, 2015].