7 research outputs found
Time Dependent Study of Multiple Exciton Generation in Nanocrystal Quantum Dots
We study the exciton dynamics in an optically excited nanocrystal quantum
dot. Multiple exciton formation is more efficient in nanocrystal quantum dots
compared to bulk semiconductors due to enhanced Coulomb interactions and the
absence of conservation of momentum. The formation of multiple excitons is
dependent on different excitation parameters and the dissipation. We study this
process within a Lindblad quantum rate equation using the full many-particle
states. We optically excite the system by creating a single high energy exciton
in resonance to a double exciton . With Coulomb
electron-electron interaction, the population can be transferred from the
single exciton to the double exciton state by impact ionisation (inverse Auger
process). The ratio between the recombination processes and the absorbed
photons provide the yield of the structure. We observe a quantum yield of
comparable value to experiment assuming typical experimental conditions for a
nm PbS quantum dot.Comment: 10 pages, 6 figures. Submitted to the conference "Progress in
Nonequilibrium Green's Functions VI Proceedings" at Lund University, Sweden,
August 17th - 21st, 2015. To be published in the Journal of Physics:
Conference Serie
Transport in serial spinful multiple-dot systems: The role of electron-electron interactions and coherences
Quantum dots are nanoscopic systems, where carriers are confined in all three
spatial directions. Such nanoscopic systems are suitable for fundamental
studies of quantum mechanics and are candidates for applications such as
quantum information processing. It was also proposed that linear arrangements
of quantum dots could be used as quantum cascade laser. In this work we study
the impact of electron-electron interactions on transport in a spinful serial
triple quantum dot system weakly coupled to two leads. We find that due to
electron-electron scattering processes the transport is enabled beyond the
common single-particle transmission channels. This shows that the scenario in
the serial quantum dots intrinsically deviates from layered structures such as
quantum cascade lasers, where the presence of well-defined single-particle
resonances between neighboring levels are crucial for device operation.
Additionally, we check the validity of the Pauli master equation by comparing
it with the first-order von Neumann approach. Here we demonstrate that
coherences are of relevance if the energy spacing of the eigenstates is smaller
than the lead transition rate multiplied by .Comment: 12 pages, 7 figure
Optimization Schemes for Efficient Multiple Exciton Generation and Extraction in Colloidal Quantum Dots
Multiple exciton generation is a process in which more than one electron hole
pair is generated per absorbed photon. It allows us to increase the efficiency
of solar energy harvesting. Experimental studies have shown the multiple
exciton generation yield of 1.2 in isolated colloidal quantum dots. However
real photoelectric devices require the extraction of electron hole pairs to
electric contacts. We provide a systematic study of the corresponding quantum
coherent processes including extraction and injection and show that a proper
design of extraction and injection rates enhances the yield significantly up to
values around 1.6.Comment: 5 pages, accepted by The Journal of Chemical Physic
Two-dimensional action spectroscopy of excitonic systems : Explicit simulation using a phase-modulation technique
Two-dimensional (2D) spectroscopy has been intensively used to study electronic and vibronic coherences in biological systems and semiconductors. This technique studies coherent as well as incoherent signals that arise from the nonlinear interaction of a sequence of laser pulses. In this paper we present a direct evaluation of the 2D signal based on elementary quantum kinetics in order to compare with the common approximate diagrammatic approaches. Here we consider incoherent action signals such as fluorescence or photocurrent as the observable, which is easily accessible in a measurement. These observables are calculated by solving the time evolution of the density matrix in the Lindblad form, which can take into account all possible decoherence processes. The phase modulation technique is used to separate the relevant nonlinear signals from the other possible interaction pathways. The approach can be used to calculate 2D spectra of any quantum system. For our model system we find a good agreement for the quantum beating between the coupled states