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Near-Infrared to Visible Hybrid Molecule-Nanocrystal Photon Upconversion

Creative Commons 'BY-NC-SA' version 4.0 license

Third generation photovoltaics are inexpensive modules that promise power conversion efficiencies exceeding the thermodynamic Shockley–Queisser limit, perhaps by using up or down-converters, intermediate band solar cells, tandem cells, hot carrier devices, or multiexciton generation. Photon upconversion has attracted enormous attention due to its wide range of applications in biological imaging, photocatalysis, and especially photovoltaics. Here, in the first chapter, the effect of quantum confinement on the efficiency of Dexter energy transfer from PbS and PbSe nanocrystals (NCs) to a rubrene acceptor is studied. A series of experiments exploring the relationship between NC size and the upconversion quantum yield (QY) in this hybrid platform show that energy transfer occurs in the Marcus normal regime. By decreasing the NC diameter from 3.5 to 2.9 nm for PbS and from 3.2 to 2.5 nm for PbSe, the relative upconversion QY is enhanced about 700 and 250‐fold respectively. In addition, the dynamic Stern–Volmer constant (KSV) for the quenching of PbSe NCs by rubrene increases approximately fivefold with a decrease in NC diameter from 3.2 to 2.5 nm to a value of 200 M−1. This work shows that high quality, well‐passivated, small NCs are critical for efficient triplet energy transfer to molecular acceptors. In the second chapter, we report an efficient upconversion of infrared to visible light. Colloidally synthesized core–shell lead sulfide– cadmium sulfide nanocrystals in combination with tetracene derivatives absorb near infrared light and emit visible light at 560 nm with an upconversion quantum yield (QY) of 8.4 ± 1.0%, which is a factor of 4 lower than the maximum upconversion QY possible. The molecular and nanocrystal engineering here paves the way toward utilizing this hybrid upconversion platform in photovoltaics, photodetectors and photocatalysis. In the third chapter, we study the effect of shell composition and thickness on triplet energy transfer (TET) from PbS QDs to rubrene in a hybrid organic–inorganic photon upconverison system and show that defect states introduced by surface adsorbed Zn and Cd result in up to 700 and 325 fold enhancements in the photon upconverison quantum yield (QY). Time-correlated single-photon counting, photoluminescence (PL), and photon upconversion QY measurements on PbS QD light absorbers with submonolayer shells of 0.37–1.60 Å of CdS and 0.04–0.22 Å of ZnS reveal these core–shell QDs have reduced radiative and nonradiative rates relative to the core. Their broadened absorption and PL line widths suggest TET may occur from thermally accessible surface defects, in addition to the dark excitonic states at the band-edge. Finally by study the photophysics and electronic properties of differently sized PbS and PbSe QDs we study the effect of composition in the photon upconveersion process and we demonstrate that appropriate QDs synthesis precursors are essential for gaining high quality QDs that results in efficient upconversion QY of ~12%.

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