Phonon-assisted nonradiative energy transfer from colloidal quantum dots to monocrystalline bulk silicon

Abstract

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][5][6][7][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.In this work, we used bulk monocrystalline p-type Si (100) substrate as the acceptor and core/shell CdSe/ZnS QDs as the donor. To match the absorption spectrum of silicon, QD peak emission wavelength was set to 580 nm. Our Si substrates possessed approximately 1.65 nm thick native oxide on the top, as verified by the ellipsometry measurement. We deposited Al 2 O 3 thin film on pre-cleaned Si substrates as a dielectric spacer using atomic layer deposition(ALD) technique. The film thicknesses of Al 2 O 3 were carefully set to 0, 1.0, 2.0 and 4.0 nm by ALD. The QDs were spin-coated over these Al 2 O 3 /SiO 2 /Si structures, resulting in formation of approximately 17 QD monolayers as measured by ellipsometry. Fluorescence decays of these QDs, which are furnished on the samples, were recorded by time resolved fluorescence spectroscopy with a closed cycle He cryostat at different temperatures varying between 22 and 290 K. QD coated sapphire substrate was also utilized as the reference sample. As a result of the lifetime analysis using decaying exponential components, temperature-dependent exciton lifetimes of core/shell CdSe/ZnS QDs on Al 2 O 3 /SiO 2 /Si and sapphire substrates were obtained. Figure 1 presents (in red circles) amplitude averaged exciton lifetimes of CdSe/ZnS QDs deposited on Al 2 O 3 /SiO 2 /Si substrates for various Al 2 O 3 thicknesses. Black squares are exciton lifetimes obtained for the QDs on the sapphire reference sample. These data were corrected for the difference in refractive index for sa...