Nora Eliasson: Quantum Dot-Molecular Hybrid Systems for Solar Energy Conversion: Mechanistic Studies to Guide Rational Design

Abstract

Quantum dots (QDs) are strong light absorbers with tunable electronic properties. Decoration of QD surfaces with molecular structures offers a versatile approach to directing the behaviour of photogenerated carriers. The studies presented herein are based on semiconductor-molecule hybrid systems tailored for solar energy conversion, where light-harvesting colloidal QDs are combined with molecular compounds on the QD surface. This thesis summary primarily focuses on the dynamics of charge carriers during the initial timescales following photoexcitation, as investigated using femtosecond transient absorption spectroscopy (fs-TA).

Fs-TA studies of CuInS2 QDs allowed for the identification of a localized hole contribution to the state-filling dynamics in Cu-deficient QDs. Two well-known hydrogen evolving metal complexes based on abundant FeFe and Co were shown to bind strongly to these QDs when mixed in solution. This enabled ultrafast electron transfer (sub-ps to a few ps), monitored directly from reduced catalyst signatures in the visible and mid-infrared region.  The strong binding of the complexes, following a Poisson distribution over the QDs, provided a rationale for their high H2 turnover numbers (~8000 per catalyst). The results reassess the necessity of advanced linking chemistry to overcome diffusion-limited electron transfer to molecular catalysts.

Further studies demonstrated the potential of rational QD capping ligand design in addressing product selectivity challenges in aqueous photocatalytic CO2 reduction. ZnSe QDs were functionalized with imidazolium-based ligands that promoted CO2 reduction while suppressing H2 evolution. Fs-TA studies and DFT calculations revealed that the strongly bound ligands passivate surface Zn sites, while further stabilizing surface adsorbed CO2-reduction intermediates. 

Photoelectrical and fs-TA studies of PbS QD solar cells fabricated from different QD sizes revealed that the device performance was limited by band-tail states at the interface between the QD layer and the electron transport material. Interfacial trap states had a greater adverse impact on devices made from larger QDs due to an unfavourable energetic alignment between the layers. Smaller-sized QDs with stronger inter-QD electronic coupling performed better, despite greater energetic disorder within the primary absorption layer. Overall, this thesis work underscores the potential of QD-molecular hybrid systems in solar energy conversion and highlights the potential of ultrafast spectroscopic studies to elucidate the underlying mechanisms and limitations.