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

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 surface potential upon photoexcitation has been measured in the m-plane ZnO (101[combining macron]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 the 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 the 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 the QDs onto the surface, the dark carrier lifetime and the surface photovoltage are reduced, suggesting a direct injection of charge carriers into the ZnO conduction band. The results are discussed in the context of the development of next-generation solar cells.

Chemically-specific time-resolved surface photovoltage spectroscopy: Carrier dynamics at the interface of quantum dots attached to a metal oxide

“…This oxygen annealing period controls the concentration of oxygen vacancies, which is critical in controlling the final conductivity of the surface and the timescales for PPC. A change in the length of the oxygen anneal (for example from 10 to 20 min) changes the dark carrier lifetime substantially [10,19]. The SPV at the ZnO surface was monitored using the Zn 3d core level.…”

“…The non-polar m-plane ZnO (1010) surface was prepared according to an established recipe as has been detailed elsewhere [10,19]. The final part of the cleaning procedure includes a period of annealing in 10 − 7 mbar O 2 at 700 K, which is required to heal an excess of oxygen vacancies created by previous argon ion sputtering and electron bombardment annealing cycles.…”

“…For the case of colloidal QDs chemically linked to a semiconductor, this technique then provides a way to monitor the carrier dynamics in the semiconductor substrate upon photoexcitation of the QDs, i.e., as additional carriers are injected into the semiconductor conduction band upon laser illumination of the QDs [19]. Moreover, photoemission allows chemical specificity because the substrate and QDs can be monitored separately or in some cases simultaneously, simply by monitoring different core level photoemission lines.…”

“…The key advantage of this system is the access this affords to long monitoring periods, from μs to ms, s and beyond. This is important for the SPV at the ZnO (1010) surface where ms dynamics are exhibited, due to PPC controlled by trapping at band gap states associated with ionized oxygen vacancies [10,19,22,23].…”

Multiple Exciton Generation and Dynamics in InP/CdS Colloidal Quantum Dots

“…28,29 Observation respectively, consistent with the addition of a CdS shell to the InP core. Two S 2p doublets were observed in the InP/CdS sample, one characteristic of CdS at 161.7 eV BE, 30 and a second at 162.9 eV BE attributed to the 3-MPA ligand bound to the CQD surface. The latter was also observed in the InP sample, which was also capped with 3-MPA.…”