Hot charge carriers in quantum dot-based electron transfer systems for application in photovoltaics

• Applicant: Professor Dr. Josef Wachtveitl
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry ; Experimental Condensed Matter Physics ; Physical Chemistry of Solids and Surfaces, Material Characterisation
• Term: from 2015 to 2023
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 262584021

Low-cost solar cells as an alternative to the widely used silicon-based photovoltaic (PV) devices have attracted immense interest during the past decades. In this field the dye sensitized solar cell (DSSC), with a molecular dye as light absorber, reaches a conversion efficiency of 11.9% with an electron transfer (ET) time on the femtosecond time scale. Inspired by the DSSC the quantum dot solar cell (QDSC) has been developed where the light harvesting is achieved by semiconductor quantum dots (QD). QD exhibit robustness against photodegradation, large extinction coefficients down to the near infrared and the mechanism of carrier multiplication. Main obstacles towards higher conversion efficiencies of QDSCs are sub-band gap states acting as charge traps and nonradiative surface recombination processes because of a large surface-to-volume ratio of QD. An alternative to the semiconductor QD are metal QD (Au, Ag, Cu) performing a plasmon-induced ET to TiO2.The previous project aimed for charge and energy transfer processes in QD-molecular acceptor assemblies with focus on charge separating heterostructures and hot charge carriers. Furthermore, the photoinduced ET and coherent phenomena at the dye/semiconductor interface have been successfully examined. In line with the first funding period we will investigate charge separating heterostructures and focus on the separation of multiexcitons with particular emphasis on the shell effect. Furthermore, the QD intrinsic charge separation dynamics of holes (core) and electrons (shell) will be manipulated by a spacing, large band gap intershell (onion-like QD). By variation of the intershell thickness the electronic coupling between core and shell should be modified leading to different charge separation dynamics. We already implemented a NIR transient absorption set-up, successfully synthesized colloidal PbS QD which are particularly interesting for PV applications and started the spectroscopic characterization. We will test the ET dynamics of systems containing PbS QD and molecular electron acceptors, and quickly move on to the investigation of PbS QD directly grown on the surface of metal oxide thin films. To avoid toxic elements of the standard semiconductor QD, metal QD can serve as an alternative. We will investigate surface plasmon-induced ET in assemblies containing colloidal Cu QD and molecular acceptors and study the photodynamics of Cu QD directly grown on metal oxide films. The effect of pump photon energy on the transfer mechanism in metal QD/TiO2 (QD as electron donor or acceptor) will be explored.We plan to extend our research on QD-based energy transfer systems and want to include the process of two-photon absorption. In an ideal case a large band gap QD (ZnSe) will act as two-photon absorber and subsequently transfer the excitation energy to trigger a certain functionality. This process could e.g. boost the two-photon cross-section of established phototriggers.

• DFG Programme: Research Grants