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Exciton
About: Exciton is a research topic. Over the lifetime, 31603 publications have been published within this topic receiving 810642 citations.
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TL;DR: MEG is found in relatively large Si NCs such that the confinement energy is not large enough to produce a large blue-shift of the band gap, but the Coulomb interaction is sufficiently enhanced to produce efficient MEG.
Abstract: Multiple exciton generation (MEG) is a process whereby multiple electron-hole pairs, or excitons, are produced upon absorption of a single photon in semiconductor nanocrystals (NCs) and represents a promising route to increased solar conversion efficiencies in single-junction photovoltaic cells. We report for the first time MEG yields in colloidal Si NCs using ultrafast transient absorption spectroscopy. We find the threshold photon energy for MEG in 9.5 nm diameter Si NCs (effective band gap E g ) 1.20 eV) to be 2.4 ± 0.1Eg and find an excitonproduction quantum yield of 2.6 ± 0.2 excitons per absorbed photon at 3.4Eg. While MEG has been previously reported in direct-gap semiconductor NCs of PbSe, PbS, PbTe, CdSe, and InAs, this represents the first report of MEG within indirect-gap semiconductor NCs. Furthermore, MEG is found in relatively large Si NCs (diameter equal to about twice the Bohr radius) such that the confinement energy is not large enough to produce a large blue-shift of the band gap (only 80 meV), but the Coulomb interaction is sufficiently enhanced to produce efficient MEG. Our findings are of particular importance because Si dominates the photovoltaic solar cell industry, presents no problems regarding abundance and accessibility within the Earth’s crust, and poses no significant environmental problems regarding toxicity.
774 citations
TL;DR: In this article, an organic semiconductor microcavity that operates in the strong-coupling regime was shown to have characteristic mixing of the exciton and photon modes (anti-crossing), and a room-temperature vacuum Rabi splitting.
Abstract: The modification and control of exciton–photon interactions in semiconductors is of both fundamental1,2,3,4 and practical interest, being of direct relevance to the design of improved light-emitting diodes, photodetectors and lasers5,6,7. In a semiconductor microcavity, the confined electromagnetic field modifies the optical transitions of the material. Two distinct types of interaction are possible: weak and strong coupling1,2,3,4. In the former perturbative regime, the spectral and spatial distribution of the emission is modified but exciton dynamics are little altered. In the latter case, however, mixing of exciton and photon states occurs leading to strongly modified dynamics. Both types of effect have been observed in planar microcavity structures in inorganic semiconductor quantum wells and bulk layers1,2,3,4,5,6,7,8. But organic semiconductor microcavities have been studied only in the weak-coupling regime9,10,11,12,13,14,15,16,17,18. Here we report an organic semiconductor microcavity that operates in the strong-coupling regime. We see characteristic mixing of the exciton and photon modes (anti-crossing), and a room-temperature vacuum Rabi splitting (an indicator of interaction strength) that is an order of magnitude larger than the previously reported highest values for inorganic semiconductors. Our results may lead to new structures and device concepts incorporating hybrid states of organic and inorganic excitons19, and suggest that polariton lasing20,21,22 may be possible.
770 citations
TL;DR: A photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO2 single crystals is used to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon, which has implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers.
Abstract: Multiple exciton generation, the creation of two electron-hole pairs from one high-energy photon, is well established in bulk semiconductors, but assessments of the efficiency of this effect remain controversial in quantum-confined systems like semiconductor nanocrystals. We used a photoelectrochemical system composed of PbS nanocrystals chemically bound to TiO 2 single crystals to demonstrate the collection of photocurrents with quantum yields greater than one electron per photon. The strong electronic coupling and favorable energy level alignment between PbS nanocrystals and bulk TiO 2 facilitate extraction of multiple excitons more quickly than they recombine, as well as collection of hot electrons from higher quantum dot excited states. Our results have implications for increasing the efficiency of photovoltaic devices by avoiding losses resulting from the thermalization of photogenerated carriers.
770 citations
TL;DR: The results imply that the exciton binding energy is weak, or that singlet bound states are formed with higher probability than triplets, in polymer light-emitting diodes, attained by blending electron transport materials with the conjugated polymer to improve the injection of electrons.
Abstract: Some conjugated polymers have luminescence properties that are potentially useful for applications such as light-emitting diodes, whose performance is ultimately limited by the maximum quantum efficiency theoretically attainable for electroluminescence1, 2,. If the lowest-energy excited states are strongly bound excitons (electron–hole pairs in singlet or triplet spin states), this theoretical upper limit is only 25% of the corresponding quantum efficiency for photoluminescence: an electron in the π*-band and a hole (or missing electron) in the π-band can form a triplet with spin multiplicity of three, or a singlet with spin multiplicity of one, but only the singlet will decay radiatively3. But if the electron–hole binding energy is sufficiently weak, the ratio of the maximum quantum efficiencies for electroluminescence and photoluminescence can theoretically approach unity. Here we report a value of ∼50% for the ratio of these efficiencies (electroluminescence:photoluminescence) in polymer light-emitting diodes, attained by blending electron transport materials with the conjugated polymer to improve the injection of electrons. This value significantly exceeds the theoretical limit for strongly bound singlet and triplet excitons, assuming they comprise the lowest-energy excited states. Our results imply that the exciton binding energy is weak, or that singlet bound states are formed with higher probability than triplets.
761 citations
TL;DR: A phase transition from a classical thermal mixed state to a quantum-mechanical pure state of exciton polaritons is observed in a GaAs multiple quantum-well microcavity from the decrease of the second-order coherence function.
Abstract: A phase transition from a classical thermal mixed state to a quantum-mechanical pure state of exciton polaritons is observed in a GaAs multiple quantum-well microcavity from the decrease of the second-order coherence function. Supporting evidence is obtained from the observation of a nonlinear threshold behavior in the pump-intensity dependence of the emission, a polariton-like dispersion relation above threshold, and a decrease of the relaxation time into the lower polariton state. The condensation of microcavity exciton polaritons is confirmed.
759 citations