Jeffrey F. Wheeldon

Estimating cell temperature in a concentrating photovoltaic system

A temperature calibrated equivalent circuit model of a high efficiency CPV solar cell is used to simulate a measured six-cell module J-V curve to estimate its average operating temperature. The simulation is based on a two diode equivalent circuit model for each subcell of a representative triple junction cell. Module J-V curves in a real CPV system were measured with a test station that performs continuous voltage sweeps allowing cells to reach a well defined thermal equilibrium during measurement. The average electrical power extracted during measurement is then used to determine the cell temperature when they are operating at their maximum power point. It is shown that the cells would operate at 42 ± 2 C° above ambient (32 ± 2°C abs.) given the ambient conditions during the measurement.

The Effects of Absorption and Recombination on Quantum Dot Multijunction Solar Cell Efficiency

The key characteristics of quantum dot (QD)-enhanced multijunction solar cells (MJSC) are explored theoretically by focusing on the generation and recombination rates throughout the QD layers in the middle subcell. The quantum dots are modeled using an effective medium to describe light absorption, confinement, and recombination properties. We report an 8% increase in the short-circuit current density accompanied by a 3% drop in an open-circuit voltage for a QD- enhanced MJSC at 1 sun illumination (1 kW/m2) compared with a control MJSC without QD. The drop in an open-circuit voltage is due in part to the increased recombination rates in the depletion region, decreased carrier lifetimes in the QDs, and the increased recombination rates resulting from carrier escape and capture. Overall, these contribute to an absolute increase in efficiency of over 1% for the studied QD-enhanced MJSC design for a QD density of 125 QD/μm2.

Multijunction Solar Cell Designs Using Silicon Bottom Subcell and Porous Silicon Compliant Membrane

A novel approach to the design of multijunction solar cells on silicon substrates for 1-sun applications is described. Models for device simulation, including porous silicon layers, are presented. A silicon bottom subcell is formed by diffusion of dopants into a silicon wafer. The top of the wafer is porosified to create a compliant layer, and a III-V buffer layer is then grown epitaxially, followed by middle and top subcells. Because of the resistivity of the porous material, these designs are best suited to high-efficiency 1-sun applications. Numerical simulations of a multijunction solar cell that incorporates a porous silicon-compliant membrane indicate an efficiency of 30.7% under AM1.5G, 1-sun for low-threading dislocation density, decreasing to 23.7% for a TDD of 107 cm-2.

Enhanced Efficiencies for High-Concentration, Multijunction PV Systems by Optimizing Grid Spacing under Nonuniform Illumination

The design of a triple junction solar cell’s front contact grid can significantly affect cell conversion efficiency under high concentration. We consider one aspect of grid design, choosing a linear grid within a distributed resistance cell model to optimize finger spacings at concentrations between 500 and 2500 suns under uniform and nonuniform illumination. Optimization for maximum efficiency under Gaussian irradiance profiles is done by SPICE analysis. Relative to the optimized uniform illumination designs, we find enhancements of 0.5% to 2% in absolute efficiencies for uniform spacing. Efficiency enhancement with nonuniform spacing under nonuniform illumination is also evaluated. Our model suggests that, at lower concentrations (<1000 suns), the penalty for using uniformly spaced fingers instead of nonuniformly spaced fingers is <0.1%. However, at a concentration of 2500 suns the penalty increases to 0.3%. Thus, relative to a uniform irradiance optimization, an absolute efficiency increase of 2.3% can be attained for an optimized nonuniform spacing given the Gaussian irradiance profile under consideration.

AlGaAs tunnel junction for high efficiency multi-junction solar cells: Simulation and measurement of temperature-dependent operation

AlGaAs tunnel junctions are shown to be well-suited to concentrated photovoltaics where temperatures and current densities can be dramatically higher than for 1-sun flat-panel systems. Detailed comparisons of AlGaAs/AlGaAs tunnel junction experimental measurements over a range of temperatures expected during device operation in concentrator systems are presented. Experimental and simulation results are compared in an effort to decouple the tunnel junction from the overall multi-junction solar cell. The tunnel junction resistance is experimentally studied as a function of the temperature to determine its contribution to overall efficiency of the solar cell. The current-voltage behavior of the isolated TJ shows that as the temperature is increased from 25°C to 85°C, the resistance decreases from ~4.7×10-4 ¿·cm2 to ~0.3×10-4 ¿·cm2 for the operational range of a multi-junction solar cell under concentration.

Tunnel-Junction-Limited Multijunction Solar Cell Performance Over Concentration

The simulation of tunnel junctions is performed by using nonlocal band-to-band and trap assisted tunneling models that are capable of reproducing the experimental current-voltage characteristics of p⁺⁺AlGaAs/ n⁺⁺AlGaAs and p⁺⁺AlGaAs/ n⁺⁺GaAs based devices. These simulated characteristics are then implemented within a lattice matched InGaP/(In)GaAs/Ge multijunction solar cell (MJSC) to assess the performance as a function of tunnel junction layer doping in the regime where the TJ limits the performance of the MJSC. At 500 suns, a 4.6% absolute drop in simulated efficiency is observed for an AlGaAs/GaAs bottom TJ corresponding to a degenerately p-doped layer of 2.5 × 10¹⁹ cm^-3 compared to a TJ with a doping of 4×10²⁰ cm^-3. A minimum p⁺⁺ doping level of 3.3 × 10 ¹⁹ cm^-3 is required in order to avoid bottom TJ limitation up to 1000 suns concentration for an n⁺⁺ doping of 2 × 10¹⁹ cm^-3 based on the calibrated models. Furthermore, the effects of the peak and valley current densities are shown to have a strong influence on the efficiency over concentration within the TJ limiting regime.

Wyckoff positions and the expression of polarization singularities in photonic crystals

We reveal the fundamental relation between linear photonic crystal symmetries and the local polarization states of its Bloch modes, in particular the location and nature of polarization singularities as established by rigorous group theoretic analysis, encompassing the full system symmetry. This is illustrated with the fundamental transverse electric mode of a two-dimensional hexagonal photonic crystal, in the vanishing contrast limit and at the K point. For general Wyckoff positions within the fundamental domain, the transformation of a local polarization state is determined by the nature of the symmetry operations that map to members of its crystallographic orbit. In particular, the site symmetries that correspond to specific Wyckoff positions constrain the local polarization state to singular character--circular, linear or disclination. Moreover, through the application of a local symmetry transformation relation, and the groups character table, the precise natures of the singularities may be determined from self-consistency arguments.

Temperature dependent external quantum efficiency simulations and experimental measurement of lattice matched quantum dot enhanced multi-junction solar cells

The external quantum efficiency (EQE) of a high efficiency lattice matched multi-junction solar cell (MJSC) and a quantum dot enhanced MJSC are numerically simulated. An effective medium is developed and integrated into the model to simulate the absorption characteristics of the quantum dots in the latter device. A calibration of the model is carried out using room temperature EQE measurements of both MJSC designs. The numerical model is further generalized through the development of a novel temperature dependent absorption model based on the Varshni relation for bandgap narrowing due to temperature. Integrating this model into the numerical simulation environment accurately reproduced the experimentally observed shifts in the EQE edge of each sub-cell as a function of temperature, including the shift in the quantum dot peak. The current — voltage characteristics are discussed under the AM1.5D spectrum for concentrated illumination and realistic temperatures in concentrator systems. The development of this temperature dependent absorption model is an important addition to the set of design tools used to optimize high efficiency MJSC under realistic temperatures and spectral conditions experienced in concentrated photovoltaic systems.

Efficiency Measurements and Simulations of GaInP/InGaAs/Ge Quantum Dot Enhanced Solar Cells at up to 1000‐Suns Under Flash and Continuous Concentration

Quantum dot (QD) enhanced GaInP/InGaAs/Ge solar cells are presented and characterized under flash and continuous solar simulators. InAs QD within the middle sub‐cell increase the carrier generation due to absorption in the range 900–940 nm. These QD‐enhanced solar cells routinely achieve production efficiencies of ∼40%, and this set of research samples obtain a peak efficiency of >38% under flash solar simulators. Continuous solar simulator testing is performed to test the QD‐enhanced solar cells under thermal loads similar to concentrated photovoltaic systems, in which cells demonstrate excellent reliability. Numerical simulations of the QD‐enhanced solar cells are performed using an effective medium to model the additional absorption due to the QD layers. Temperature dependence of the QD‐enhanced solar cells are modeled, in which temperature‐dependent bandgap narrowing changes the dark current and the semiconductor absorption profiles. Comparison between the experimental results and numerical model show...

Measurement of high efficiency 1 cm 2 AlGaInP/InGaAs/Ge solar cells with embedded InAs quantum dots at up to 1000 suns continuous concentration

Large commercial-grade 1 cm2 quantum dot enhanced triple-junction AlGaInP/InGaAs/Ge solar cells were characterized using high-concentration flash and continuous-illumination solar simulators. Cyrium Technologies Incorporated (Cyrium™) routinely achieves >40% efficiency under ∼500 suns flash illumination at 25°C using its QDEC™ product line based on this design. For this research project, Cyrium used its Application-Specific Concentrator Cell (ASCC) program to design and manufacture CPV cells with such quantum dot layers in the middle sub-cell of a triple-junction configuration. The high quality of the dislocation-free quantum dot layers used in such structures has been confirmed by photoluminescence, transmission electron microscopy, and quantum efficiency measurements. Receiver devices have been successfully tested up to ∼950 suns of continuous illumination, producing currents >13 A from a 1 cm2 cell. Continuous-illumination testing produced temperatures reaching >90°C above ambient at solar concentrations of >800 suns under some thermal coupling conditions. As a result, ASCC cells that achieved >38% efficiency at standard test conditions of 25°C under flash solar simulators measured 34–37% at high operating temperature under continuous illumination of up to 800 suns with the thermal resistance of the assembly used. These results show that it is essential to develop rigorous thermal management in a real-world concentrator system, for which continuous solar simulators are invaluable tools for testing prior to field deployment.