The ability to confine electrons and holes in semiconductor quantum dots (QDs) in the form of excitons creates an electronic structure which is both novel and potentially useful for a variety of applications. Upon optical excitation of the dot, the initial excitonic state may be electronically hot. The relaxation dynamics of this hot exciton is the primary event which controls key processes such as optical gain, hot carrier extraction, and multiple exciton generation. Here, we describe femtosecond state-resolved pump/probe experiments on colloidal CdSe quantum dots that provide the first quantitative measure of excitonic state-to-state transition rates. The measurements and modeling here reveal that there are multiple paths by which hot electrons and hot holes relax. The immediate result is that there is no phonon bottleneck for electrons or holes for excitons in quantum dots. This absence of phonon-based relaxation is confirmed by independent measurements of weak exciton–phonon coupling between the vario...

Hot Exciton Relaxation Dynamics in Semiconductor Quantum Dots: Radiationless Transitions on the Nanoscale

Nanochemistry and nanomaterials provide numerous opportunities for a new generation of photovoltaics with high solar energy conversion efficiencies at low fabrication cost. Quantum-confined nanomaterials and polymer–inorganic nanocomposites can be tailored to harvest sun light over a broad range of the spectrum, while plasmonic structures offer effective ways to reduce the thickness of light-absorbing layers. Multiple exciton generation, singlet exciton fission, photon down-conversion, and photon up-conversion realized in nanostructures, create significant interest for harvesting underutilized ultraviolet and currently unutilized infrared photons. Nanochemical interface engineering of nanoparticle surfaces and junction-interfaces enable enhanced charge separation and collection. In this review, we survey these recent advances employed to introduce new concepts for improving the solar energy conversion efficiency, and reduce the device fabrication cost in photovoltaic technologies. The review concludes with a summary of contributions already made by nanochemistry. It then describes the challenges and opportunities in photovoltaics where the chemical community can play a vital role.

Nanochemistry and nanomaterials for photovoltaics.

Using a photoluminescence-based carrier multiplication mechanism recently proposed for closely spaced silicon nanocrystals in SiO2, scientists demonstrate that adjacent nanocrystals are excited directly upon absorption of a single high-energy photon. They also demonstrate efficient carrier multiplication with an onset close to the energy conservation threshold of twice the bandgap energy.

Direct generation of multiple excitons in adjacent silicon nanocrystals revealed by induced absorption

Thin films of strongly coupled PbSe quantum dots exhibit light-induced charge generation with a yield of one electron and one hole per photon over a wide temperature range.

Unity quantum yield of photogenerated charges and band-like transport in quantum-dot solids

Quantum dot-sensitized solar cells (QDSCs) present promising cost-effective alternatives to conventional silicon solar cells due to their distinctive properties such as simplicity in fabrication, possibility to realize light absorption in wide solar spectrum regions, and theoretical conversion efficiency up to 44%. This review highlights recent developments in critical materials including quantum dots, photoanodes, counter electrodes (CEs), and electrolytes for QDSC applications. Among them, electron recombination at the photoanode/electrolyte interface limits the evolution of high-efficiency QDSCs, therefore the optimized construction of quantum dots, the various microtopographies of wide bandgap semiconductors (TiO2, ZnO) as well as emerging CEs having good electrocatalytic activity are elaborated in this paper. We argue that these key factors can provide design guidelines for future successful applications and significantly promote the development of QDSCs. Liquid, quasi-solid-state, and solid-state electrolytes for QDSCs are summarized, aiming at enhancing the long-term stability of QDSCs. This review presented below gives a succinct summary of materials for QDSC applications, with a conclusion and future prospects section.

Recent advances in critical materials for quantum dot-sensitized solar cells: a review

Multiple exciton generation (MEG) in PbSe quantum dots (QDs), PbSexS1−x alloy QDs, PbSe/PbS core/shell QDs, and PbSe/PbSeyS1−y core/alloy-shell QDs was studied with time-resolved optical pump and probe spectroscopy. The optical absorption exhibits a red-shift upon the introduction of a shell around a PbSe core, which increases with the thickness of the shell. According to electronic structure calculations this can be attributed to charge delocalization into the shell. Remarkably, the measured quantum yield of MEG, the hot exciton cooling rate, and the Auger recombination rate of biexcitons are similar for pure PbSe QDs and core/shell QDs with the same core size and varying shell thickness. The higher density of states in the alloy and core/shell QDs provide a faster exciton cooling channel that likely competes with the fast MEG process due to a higher biexciton density of states. Calculations reveal only a minor asymmetric delocalization of holes and electrons over the entire core/shell volume, which may ...

Gal Grinbom

Papers

Anomalous Independence of Multiple Exciton Generation on Different Group IV−VI Quantum Dot Architectures