Anna H. Trojnar

Quantum interference in exciton-Mn spin interactions in a CdTe semiconductor quantum dot.

We show theoretically and experimentally the existence of a new quantum-interference effect between the electron-hole interactions and the scattering by a single Mn impurity. The theoretical model, including electron-valence-hole correlations, the short- and long-range exchange interaction of a Mn ion with the heavy hole and with electron and anisotropy of the quantum dot, is compared with photoluminescence spectroscopy of CdTe dots with single magnetic ions. We show how the design of the electronic levels of a quantum dot enables the design of an exciton, control of the quantum interference, and hence engineering of light-Mn interaction.

Effects of luminescent coupling in single- and 4-junction dilute nitride solar cells

A novel method for incorporating the effects of luminescent coupling and photon recycling in numerical simulations of planar devices is described. The carrier generation is incorporated directly in the device simulator as an additional term in the continuity equation, so that no additional iterations are required. The method is applied to single- and four-junction solar cells containing ~1.0 eV dilute nitride material. We find that luminescent coupling increases the short-circuit current (JSC) of the 1-junction dilute nitride cell by 2.4% due to coupling with the Al0.05Ga0.95As filter. In the 4-junction design, there is significant photon recycling within the GaAs and GaInP sub-cells, providing a 60 mV increase in open-circuit voltage. There is a 1.9% relative increase in calculated efficiency to 44.4%.

4 Junction dilute nitride solar cell optimization: Comparing current matching approaches in detailed balance algorithms

The optimization of quadruple junction solar cell designs in the detailed balance limit via an equivalent circuit model for each sub-cell is explored using spectral sharing from three perspectives: (i) current matching at short circuit current (as per etaOpt software), (ii) corrected current matching at short circuit current and (iii) unconstrained short-circuit currents. At 1-sun illumination (968 W/m2), we report an efficiency increase of 1.3% absolute for the second current matching approach over the first, solely due to an increase in fill factor. For concentrated illumination at 1000 suns, the efficiency increase becomes 1.6% absolute.

Optical properties of charged quantum dots doped with a single magnetic impurity

We present a microscopic theory of the optical properties of self-assembled quantum dots doped with a single magnetic manganese (Mn) impurity and containing a controlled number of electrons. The single-particle electron and heavy-hole electronic shells are described by two-dimensional harmonic oscillators. The electron-electron, electron-hole Coulomb as well as the short-range electron spin-Mn spin and hole spin-Mn spin contact exchange interactions are included. The electronic states of the photo-excited electron-hole-Mn complex and of the final electron-Mn complex are expanded in a finite number of configurations and the full interacting Hamiltonian is diagonalized numerically. The emission spectrum is predicted as a function of photon energy for a given number of electrons and different number of confined electronic quantum dot shells. We show how emission spectra allow to identify the number of electronic shells, the number of electrons populating these shells and, most importantly, their spin. We show that electrons not interacting directly with the spin of Mn ion do so via electron-electron interactions. This indirect interaction is a strong effect even when Mn impurity is away from the quantum dot center.

Optimization of GaAs nanowire solar cell efficiency via optoelectronic modeling

We have developed a fully-coupled optoelectronic device model integrating two commercial software packages: COMSOL Multiphysics and Synopsys TCAD Sentaurus. This model is used to optimize GaAs nanowire solar cells containing a vertical p-n junction with passivating shell. We investigate the impact of the nanowire diameter, height, emitter thickness, square-array periodicity, and doping on the cell performance. This model allows quantitative optimization of device parameters. Under normal-incidence 1-sun AM1.5D illumination and with attainable material parameters, a design capable of reaching efficiency over 22.2% is reported.

Modeling intermediate band solar cells: a roadmap to high efficiency

Intermediate band (IB) photovoltaics have the potential to be highly efficient and cost effective solar cells. When the IB concept was proposed in 1997, there were no known intermediate band materials. In recent years, great progress has been made in developing materials with intermediate bands, though power conversion efficiencies have remained low. To understand the material requirements to increase IB device efficiencies, we must develop good models for their behavior under bias and illumination. To evaluate potential IB materials, we present a figure of merit, consisting of parameters that can be measured without solar cell fabrication. We present a new model for IB devices, including the behavior of their junctions with n- and p-type semiconductors. Using a depletion approximation, we present analytic approximations for the boundary conditions of the minority carrier diffusion equations. We compare the analytic results to Synopsys Sentaurus device models. We use this model to find the optimal thickness of the IB region based on material parameters. For sufficiently poor IB materials, the optimal thickness is zero – i.e., the device is more efficient without the IB material at all. We show the minimum value of the figure of merit required for an IB to improve the efficiency of a device, providing a clear goal for the quality of future IB materials.

Numerical modeling of silicon nanocrystal down-shifting layers for enhanced CIGS solar cell performance

The performance effects of silicon nanocrystals (SiNC) embedded in a silicon dioxide matrix to act as a downshifting (DS) layer mounted on the top surface of a polycrystalline Cu(In, Ga)Se2 solar cell are explored numerically. The DS layers are modeled by modifying the incident AM1.5G spectrum based on the absorption and emission properties of the SiNC. The effects of the DS layers as an anti-reflection coating leads to an 11.4% relative improvement in short-circuit current density under 1-sun illumination (0.1 W/cm2). Comparatively, the effect of down-shifting high-energy photons to lower energy photons showed a 4% relative short-circuit current density improvement, albeit for an optical conversion efficiency of 80%.

Optimizations of GaAs Nanowire Solar Cells

The efficiency of GaAs nanowire (NW) solar cells can be significantly improved with no new processing steps or material requirements. We report coupled optoelectronic simulations of a GaAs NW solar cell with a vertical p-i-n junction and a high-bandgap AlInP passivating shell. Our frequency-dependent model facilitates calculation of quantum efficiency for the first time in NW solar cells. For passivated NWs, we find that short-wavelength photons can be most effectively harnessed by using a thin emitter, while long-wavelength photons are best utilized by extending the intrinsic region to the NW/substrate interface and using the substrate as a base. These two easily implemented changes, coupled with the increase of NW height to 3.5 μm with realistic surface recombination in the presence of a passivation shell, result in an NW solar cell with greater than 19% efficiency.

Optimization of anti-reflection coatings for bifacial solar cells with upconversion layers

Upconversion is an optical process in which two or more low-energy photons are converted to one high-energy photon. Placing layers capable of performing this process at the rear face of a standard bifacial solar cell allows for utilization of sub-bandgap photons that would be otherwise lost. Optimization of antireflection coatings for devices using upconversion is particularly important as it needs to take into account the normally neglected sub-bandgap part of the spectrum. In this paper we study the optimization of antireflection coatings for bifacial silicon solar cells for upconversion applications. A 3- layer stack of MgF2/Si3N4/TiO2 shows the lowest weighted reflectance with 88.6% transmission at 1520 nm as compared to 78% transmission for 1-layer ARC. Minimizing reflection losses results in greater number of photons reaching the upconverter layer resulting in a potential enhancement of 28% in the short-circuit current due to upconversion.

Dynamical magnetic and nuclear polarization in complex spin systems: semi-magnetic II?VI quantum dots

Dynamical magnetic and nuclear polarization in complex spin systems is discussed on the example of transfer of spin from exciton to the central spin of magnetic impurity in a quantum dot in the presence of a finite number of nuclear spins. The exciton is described in terms of electron and heavy-hole spins interacting via exchange interaction with magnetic impurity, via hyperfine interaction with a finite number of nuclear spins and via dipole interaction with photons. The time evolution of the exciton, magnetic impurity and nuclear spins is calculated exactly between quantum jumps corresponding to exciton radiative recombination. The collapse of the wavefunction and the refilling of the quantum dot with a new spin-polarized exciton is shown to lead to the build up of magnetization of the magnetic impurity as well as nuclear spin polarization. The competition between electron spin transfer to magnetic impurity and to nuclear spins simultaneous with the creation of dark excitons is elucidated. The technique presented here opens up the possibility of studying optically induced dynamical magnetic and nuclear polarization in complex spin systems.