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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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Journal ArticleDOI
TL;DR: A series of transient absorption and photocurrent experiments as a function of excitation wavelength and temperature for two low-band-gap polymer/fullerene blends to study the mechanism of charge separation at the donor/acceptor interface provide key evidence for the role of excess energy in driving charge separation.
Abstract: The energetic driving force required to drive charge separation across donor/acceptor heterojunctions is a key consideration for organic optoelectronic devices. Herein we report a series of transient absorption and photocurrent experiments as a function of excitation wavelength and temperature for two low-band-gap polymer/fullerene blends to study the mechanism of charge separation at the donor/acceptor interface. For the blend that exhibits the smallest donor/acceptor LUMO energy level offset, the photocurrent quantum yield falls as the photon excitation energy is reduced toward the band gap, but the yield of bound, interfacial charge transfer states rises. This interplay between bound and free charge generation as a function of initial exciton energy provides key evidence for the role of excess energy in driving charge separation of direct relevance to the development of low-band-gap polymers for enhanced solar light harvesting.

189 citations

Journal ArticleDOI
TL;DR: In this article, the electronic structure and size scaling of spectroscopic observables in conjugated polymers are investigated using time-dependent density functional theory, and it is shown that local density approximations and gradient-corrected functionals do not have an effective attractive Coulomb interaction between photoexcited electron-hole pairs to form bound states and therefore do not reproduce finite exciton sizes.
Abstract: The electronic structure and size scaling of spectroscopic observables in conjugated polymers are investigated using time-dependent density functional theory. We show that local density approximations and gradient-corrected functionals do not have an effective attractive Coulomb interaction between photoexcited electron-hole pairs to form bound states and therefore do not reproduce finite exciton sizes. Long-range nonlocal and nonadiabatic density functional corrections (such as hybrid mixing with an exact Hartree-Fock exchange) are necessary to capture correct delocalization of photoexcitations in one-dimensional polymeric chains.

189 citations

Journal ArticleDOI
TL;DR: In this article, a two-step vapor phase transport method on sapphire was used to obtain high energy excitonic emission at low temperatures close to the band-edge which was assigned to the surface exciton in ZnO at $\ensuremath{\sim}3.366\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$.
Abstract: We report ZnO nanowire/nanowall growth using a two-step vapor phase transport method on $a$-plane sapphire. X-ray diffraction and scanning electron microscopy data establish that the nanostructures are vertically well aligned with the $c$ axis normal to the substrate and have a very low rocking curve width. Photoluminescence data at low temperatures demonstrate the exceptionally high optical quality of these structures, with intense emission and narrow bound exciton linewidths. We observe a high energy excitonic emission at low temperatures close to the band-edge which we assign to the surface exciton in ZnO at $\ensuremath{\sim}3.366\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. This assignment is consistent with the large surface to volume ratio of the nanowire systems and indicates that this large ratio has a significant effect on the luminescence even at low temperatures. The band-edge intensity decays rapidly with increasing temperature compared to bulk single crystal material, indicating a strong temperature-activated nonradiative mechanism peculiar to the nanostructures. No evidence is seen of the free exciton emission due to exciton delocalization in the nanostructures with increased temperature, unlike the behavior in bulk material. The use of such nanostructures in room temperature optoelectronic devices appears to be dependent on the control or elimination of such surface effects.

188 citations

Journal ArticleDOI
TL;DR: In this article, the notion and usefulness of an effective dielectric constant are analyzed using a self-consistent linear screening calculation, and the binding energy of hydrogenic impurities is defined and calculated, and it is shown why these are always ionized in porous silicon.
Abstract: Consequences of the modified dielectric properties of semiconductor crystallites are explored. The notion and usefulness of an effective dielectric constant are analyzed using a self-consistent linear screening calculation. The binding energy of hydrogenic impurities is defined and calculated, and it is shown why these are always ionized in porous silicon. Self-energy terms associated with the surface polarization charge are discussed in the context of Coulomb charging effects. Their contribution to exciton binding energies is also determined. Consequences of charging effects on carrier injection in porous silicon are finally considered and shown to be important.

188 citations

Journal ArticleDOI
TL;DR: In this paper, the authors investigated carrier multiplication in colloidal InAs/CdSe/ZnSe core−shell quantum dots (QDs) using terahertz time-domain spectroscopy, time-resolved transient absorption, and quasi-continuous wave excitation spectra.
Abstract: Carrier (exciton) multiplication in colloidal InAs/CdSe/ZnSe core−shell quantum dots (QDs) is investigated using terahertz time-domain spectroscopy, time-resolved transient absorption, and quasi-continuous wave excitation spectroscopy. For excitation by high-energy photons (∼2.7 times the band gap energy), highly efficient carrier multiplication (CM) results in the appearance of multi-excitons, amounting to ∼1.6 excitons per absorbed photon. Multi-exciton recombination occurs within tens of picoseconds via Auger-type processes. Photodoping (i.e., photoinjection of an exciton) of the QDs prior to excitation results in a reduction of the CM efficiency to ∼1.3. This exciton-induced reduction of CM efficiency can be explained by the twofold degeneracy of the lowest conduction band energy level. We discuss the implications of our findings for the potential application of InAs QDs as light absorbers in solar cells.

188 citations


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Performance
Metrics
No. of papers in the topic in previous years
YearPapers
20231,269
20222,623
20211,045
20201,157
20191,096
20181,057