Olivier Thériault

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.

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.

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.

The Dependence of Multijunction Solar Cell Performance on the Number of Quantum Dot Layers

The performance improvements of adding InAs quantum dots (QDs) in the middle subcell of a lattice matched triple-junction InGaP/InGaAs/Ge photovoltaic device are studied using the simulated external quantum efficiency, photocurrent, open circuit voltage, fill factor, and efficiency under standard testing conditions. The QDs and wetting layer are modeled using an effective medium consisting of trap states for the former and low confinement potentials for the latter. Although the efficiency stabilizes for more than 100 layers of QDs for the structure studied, the efficiency achieves an absolute efficiency of 31.1% under one sun illumination for 140 layers of QDs. This corresponds to a relative increase of 1.3% compared with a control structure with no QD layers. The performance of the device depends intricately on the magnitude of the confinement potentials representing the wetting layer.

Effect of Dot-Height Truncation on the Device Performance of Multilayer InAs/GaAs Quantum Dot Solar Cells

The effect of dot-height truncation on the device performance of multilayer InAs/GaAs quantum dot solar cells is investigated. The different structures were grown by chemical beam epitaxy, and an indium-flush process is used to control the dot height. A series of ten-layer samples with dots truncated at a height of 5 and 2.5 nm, respectively, are studied. Luminescence, atomic force microscopy, and high-resolution scanning transmission electron microscopy results indicate that the quantum dot properties are preserved up to the tenth layer for both structures. Under 1-sun illumination, the truncation of the dot height to 2.5 nm increased the short-circuit current density by 0.7 mA/cm2 and the open-circuit voltage by 31 mV. From the external quantum efficiency curves, limited to wavelengths> 500 nm, a 1.46-mA/cm2 current density enhancement is found over a GaAs reference cell. At least 45% of this enhancement has been attributed uniquely to the presence of quantum dots in the structure.

Carrier Dynamics in Quantum-Dot Multijunction Solar Cells Under Concentration

The key performance metrics of quantum-dot (QD)- lattice-matched multijunction solar cells (MJSCs) composed of InGaP/(In)GaAs/Ge with InAs/GaAs QDs are explored under high-concentration illumination with a focus on the carrier dynamics in the QD layers of the middle subcell. An effective medium approach is used to describe generation and recombination in the QD system, including carrier escape and capture from the weakly confining quantum well and the QD states. At a concentration of 1000 suns, simulations indicate that the specific QD MJSC studied outperforms a standard MJSC by 1.1% in relative efficiency operating at 25 °C. However, this gain in efficiency is highly dependent on the confinement potentials of the wetting layer, as well as the resulting current mismatch between the top and middle subcells when carrier escape rates from within the wetting layer confinements are reduced.

Temperature‐Dependent Quantum Efficiency of Quantum Dot Enhanced Multi‐Junction Solar Cells

The external quantum efficiency of a commercial quantum dot enhanced multi‐junction solar cell is measured over a range of temperatures (15 °C to 75 °C). A complete numerical model of the cell is built and calibrated based on the experimental data. The short circuit current density is calculated over different temperatures under standard AM1.5D illumination; the measurements compare well to simulated results. The current ratio between the top and middle sub‐cell is studied over temperature and air mass. It is shown that the current ratio and hence the optimal AM value for which the cell should be designed increase with increasing temperature.