Topic
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: Efficient organic-inorganic perovskite light-emitting diodes were made with nanograin crystals that lack metallic lead, which helped to confine excitons and avoid their quenching.
Abstract: Organic-inorganic hybrid perovskites are emerging low-cost emitters with very high color purity, but their low luminescent efficiency is a critical drawback. We boosted the current efficiency (CE) of perovskite light-emitting diodes with a simple bilayer structure to 42.9 candela per ampere, similar to the CE of phosphorescent organic light-emitting diodes, with two modifications: We prevented the formation of metallic lead (Pb) atoms that cause strong exciton quenching through a small increase in methylammonium bromide (MABr) molar proportion, and we spatially confined the exciton in uniform MAPbBr3 nanograins (average diameter = 99.7 nanometers) formed by a nanocrystal pinning process and concomitant reduction of exciton diffusion length to 67 nanometers. These changes caused substantial increases in steady-state photoluminescence intensity and efficiency of MAPbBr3 nanograin layers.
2,295 citations
TL;DR: In this paper, the authors showed that the photogenerated excitons are usually not split by the built-in electric field, which arises from differences in the electrode work functions.
Abstract: Conjugated polymers are attractive semiconductors for photovoltaic cells because they are strong absorbers and can be deposited on flexible substrates at low cost. Cells made with a single polymer and two electrodes tend to be inefficient because the photogenerated excitons are usually not split by the built-in electric field, which arises from differences in the electrode work functions. The efficiency can be increased by splitting the excitons at an interface between two semiconductors with offset energy levels. Power conversion efficiencies of almost 4% have been achieved by blending polymers with electron-accepting materials such as C60 derivatives, cadmium selenide, and titanium dioxide. We predict that efficiencies higher than 10% can be achieved by optimizing the cell's architecture to promote efficient exciton splitting and charge transport and by reducing the band gap of the polymer so that a larger fraction of the solar spectrum can be absorbed.
2,132 citations
TL;DR: In this paper, the authors reported that the band structure of few-layer black phosphorous is anisotropic and the band gap increases with the decrease in number of staking layers.
Abstract: The authors report that the band structure of few-layer black phosphorous is anisotropic and the band gap increases with the decrease in number of staking layers. The optical absorption and excitonic effects are also anisotropic.
2,021 citations
TL;DR: It is demonstrated for the first time that impact ionization (II) (the inverse of Auger recombination) occurs with very high efficiency in semiconductor nanocrystals (NCs) and can be used to considerably increase the power conversion efficiency of NC-based solar cells.
Abstract: We demonstrate for the first time that impact ionization (II) (the inverse of Auger recombination) occurs with very high efficiency in semiconductor nanocrystals (NCs). Interband optical excitation of PbSe NCs at low pump intensities, for which less than one exciton is initially generated per NC on average, results in the formation of two or more excitons (carrier multiplication) when pump photon energies are more than 3 times the NC band gap energy. The generation of multiexcitons from a single photon absorption event is observed to take place on an ultrafast (picosecond) time scale and occurs with up to 100% efficiency depending upon the excess energy of the absorbed photon. Efficient II in NCs can be used to considerably increase the power conversion efficiency of NC-based solar cells.
1,732 citations
TL;DR: Detailed calculations of the shift of exciton peaks are presented including (i) exact solutions for single particles in infinite wells, (ii) tunneling resonance calculations for finite wells, and (iii) variational calculations ofexciton binding energy in a field.
Abstract: We report experiments and theory on the effects of electric fields on the optical absorption near the band edge in GaAs/AlGaAs quantum-well structures. We find distinct physical effects for fields parallel and perpendicular to the quantum-well layers. In both cases, we observe large changes in the absorption near the exciton peaks. In the parallel-field case, the excitons broaden with field, disappearing at fields \ensuremath{\sim}${10}^{4}$ V/cm; this behavior is in qualitative agreement with previous theory and in order-of-magnitude agreement with direct theoretical calculations of field ionization rates reported in this paper. This behavior is also qualitatively similar to that seen with three-dimensional semiconductors. For the perpendicular-field case, we see shifts of the exciton peaks to lower energies by up to 2.5 times the zero-field binding energy with the excitons remaining resolved at up to \ensuremath{\sim}${10}^{5}$ V/cm: This behavior is qualitatively different from that of bulk semiconductors and is explained through a mechanism previously briefly described by us [D. A. B. Miller et al., Phys. Rev. Lett. 53, 2173 (1984)] called the quantum-confined Stark effect. In this mechanism the quantum confinement of carriers inhibits the exciton field ionization. To support this mechanism we present detailed calculations of the shift of exciton peaks including (i) exact solutions for single particles in infinite wells, (ii) tunneling resonance calculations for finite wells, and (iii) variational calculations of exciton binding energy in a field. We also calculate the tunneling lifetimes of particles in the wells to check the inhibition of field ionization. The calculations are performed using both the 85:15 split of band-gap discontinuity between conduction and valence bands and the recently proposed 57:43 split. Although the detailed calculations differ in the two cases, the overall shift of the exciton peaks is not very sensitive to split ratio. We find excellent agreement with experiment with no fitted parameters.
1,731 citations