Browsing by Subject "Dipole-dipole couplings"

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Open Access
Phonon-assisted nonradiative energy transfer from colloidal quantum dots to monocrystalline bulk silicon
(IEEE, 2012) Yeltik, Aydan; Güzeltürk, Burak; Hernandez-Martinez, Pedro L.; Demir, Volkan Demir
Silicon is one of the most dominant materials in photovoltaics. To increase optical absorption of silicon solar cells, colloidal quantum dots (QDs) have been proposed as a good sensitizer candidate owing to their favorably high absorption cross-section and tunable emission and absorption properties. To this end, QD sensitization of silicon has previously been studied by mostly facilitating radiative energy transfer (RET) [1,2]. Although RET based sensitization has achieved a considerable increase in conversion efficiencies in silicon photovoltaics, RET is fundamentally limited due to the effective coupling problem of emitted photons to silicon. Alternatively, nonradiative energy transfer (NRET), which relies on near field dipole-dipole coupling [3], has been shown to be feasible in sensitizer-silicon hybrid systems [4-8]. Although colloidal QDs as a sensitizer have been used to facilitate NRET into silicon, the detailed mechanisms of NRET to an indirect bandgap nonluminecent material, together with the role of phonon assistance and temperature activation, have not been fully understood to date. In this study, we propose a QD-silicon nanostructure hybrid platform to study the NRET dynamics as a function of temperature for distinct separation thicknesses between the donor QDs and the acceptor silicon plane. Here, we show NRET from colloidal QDs to bulk Si using phonon assisted absorption, developing its physical model to explain temperature-dependent lifetime dynamics of NRET in these QD-Si hybrids. © 2012 IEEE.
Open Access
Singlet and Triplet Exciton Harvesting in the Thin Films of Colloidal Quantum Dots Interfacing Phosphorescent Small Organic Molecules
(American Chemical Society, 2014) Guzelturk, B.; Hernandez Martinez P.L.; Zhao, D.; Sun X.W.; Demir, Hilmi Volkan
Efficient nonradiative energy transfer is reported in an inorganic/organic thin film that consists of a CdSe/ZnS core/shell colloidal quantum dot (QD) layer interfaced with a phosphorescent small organic molecule (FIrpic) codoped fluorescent host (TCTA) layer. The nonradiative energy transfer in these thin films is revealed to have a cascaded energy transfer nature: first from the fluorescent host TCTA to phosphorescent FIrpic and then to QDs. The nonradiative energy transfer in these films enables very efficient singlet and triplet state harvesting by the QDs with a concomitant fluorescence enhancement factor up to 2.5-fold, while overall nonradiative energy transfer efficiency is as high as 95%. The experimental results are successfully supported by the theoretical energy transfer model developed here, which considers exciton diffusion assisted Förster-type near-field dipole-dipole coupling within the hybrid films. © 2014 American Chemical Society.