Modulating Exciton Dynamics in Composite Nanocrystals for Excitonic Solar Cells.

Quantum dots (QDs) represent one of the most promising materials for third-generation solar cells due to their potential to boost the photoconversion efficiency beyond the Shockley-Queisser limit. Composite nanocrystals can challenge the current scenario by combining broad spectral response and tailored energy levels to favor charge extraction and reduce energy and charge recombination. We synthesized PbS/CdS QDs with different compositions at the surface of TiO2 nanoparticles assembled in a mesoporous film. The ultrafast photoinduced dynamics and the charge injection processes were investigated by pump-probe spectroscopy. We demonstrated good injection of photogenerated electrons from QDs to TiO2 in the PbS/CdS blend and used the QDs to fabricate solar cells. The fine-tuning of chemical composition and size of lead and cadmium chalcogenide QDs led to highly efficient PV devices (3% maximum photoconversion efficiency). This combined study paves the way to the full exploitation of QDs in next-generation photovoltaic (PV) devices.

Diameter-dependent surface photovoltage and surface state density in single semiconductor nanowires.

Based on single-nanowire surface photovoltage measurements and finite-element electrostatic simulations, we determine the surface state density, N(s), in individual n-type ZnO nanowires as a function of nanowire diameter. In general, N(s) increases as the diameter decreases. This identifies an important origin of the recently reported diameter dependence of the surface recombination velocity, which has been commonly considered to be independent of the diameter. Furthermore, through the determination of the surface carrier lifetime, we suggest that the diameter dependence of the surface state density accounts for the rather abrupt transition from bulk-limited to surface-limited carrier transport over a narrow nanowire diameter regime (~30-40 nm). These findings are supported by the comparison between bulk-limited and surface-dependent minority carrier diffusion lengths measured at various diameters.

Quantitative heat dissipation characteristics in current-carrying GaN nanowires probed by combining scanning thermal microscopy and spatially resolved Raman spectroscopy.

Using an approach combining scanning thermal microscopy (SThM) and spatially revolved Raman spectroscopy, we have investigated quantitatively the heat dissipation characteristics in substrate-supported and suspended (with asymmetric type of contacts) current-carrying GaN nanowires with diameters of ∼40-60 nm, where the phonon confinement is expected to play an important role in thermal transport. In particular, this approach allows direct measurements of nanowire-substrate/electrode interface thermal resistances and the nanowire thermal conductivity. On the basis of these results, the nanowire-substrate thermal transfer was suggested to be the main heat dissipation route, counting for ∼80-93% of the total dissipated heat, whereas the nanowire-electrode interface plays a minor role. The relative significance of nanowire-substrate/electrode interfaces in dissipating heat was further demonstrated in suspended nanowire devices. The measured nanowire thermal conductivity (∼40-60 W/mK) is lower than that in bulk GaN, possibly due to the phonon confinement and boundary scattering effects. Besides providing quantitative insight into heat dissipation characteristics, our results also reveal aspects, particularly the topography-related thermal signals and the relative significance of various tip-sample thermal transfer processes, that are important to advancing the applications of SThM technique in nanoscale thermal characterizations.