Mark D. Yandt

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.

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.

Dynamic Real-Time I–V Curve Measurement System for Indoor/Outdoor Characterization of Photovoltaic Cells and Modules

A test method that measures the current-voltage I-V curve of a photovoltaic (PV) cell or module in real time is presented as a means of characterizing and understanding the inherently variable nature of performance under field conditions. Temperature, incident light intensity, orientation to the light source, incident spectrum, the uniformity of illumination, as well as a diverse set of failure mechanisms, both catastrophic and otherwise, have characteristic effects on the I-V curve. Seeing the I-V curve change dynamically with these influences allows visual correlation to real-time events. With a live I-V curve generated by performing forward and reversed bias sweeps repeatedly, the effect of parasitic inductance and bias sweep rate on the measurement can be demonstrated directly. This technique also ensures that the device junction is held in quasi-thermal equilibrium during the measurement. The relative alignment of optics in a concentrating photovoltaic module is analyzed to demonstrate the value of the live I-V curve.

Collection and storage of direct spectral irradiance and DNI datasets with high temporal resolution for CPV energy yield assessments

We describe the acquisition and database architecture for direct normal irradiance (DNI) and direct spectral irradiance datasets collected at an outdoor test site at the University of Ottawa. The objective was to build a database of local measurements made across a range of air mass values and atmospheric conditions for energy yield analyses applicable to Concentrated Photovoltaics (CPV) systems. Approximately 422,000 spectra and concurrent meteorological data have been recorded in the SUNLAB database for the two year operation period as of March 2013. Energy yield comparisons using measured spectra and those derived from Canadian Weather for Energy Calculations (CWECS) Typical Meteorological Year (TMY) and Canadian Weather Energy and Engineering Data Sets (CWEEDS) datasets demonstrate significant year-to-year variations, with TMY estimates underestimating 20-year historical assessments by ~7%.

Identifying representative air masses for multi-junction solar cell bandgap optimization to maximize annual energy yield

Annual energy yield is calculated for multijunction solar cells with ideal bandgap combinations optimized for spectra between AM0.7d and AM3.1d. A “representative spectrum” can thus be defined, within the assumptions of the study, as that which corresponds to the air mass that resulted in the highest annual energy yield. Results show that the air mass of this representative spectrum does not correspond to the 50% cumulative energy air mass which has traditionally been used as a point of reference. Further, the optimal design point is shown to be different for cells with different numbers of junctions. Bandgap combinations that maximize clear sky annual energy yield in Boulder, USA and Ottawa, Canada are presented.

Shutter Technique for Noninvasive Individual Cell Characterization in Sealed CPV Modules

An optical shutter technique for quality control and degradation analysis of concentrating photovoltaic (CPV) modules is presented. Shuttering configurations are described and used to determine individual component degradation or failure in field-exposed commercial modules. Cells are shuttered in specific patterns, intentionally engaging internal bypass diodes, and current-voltage (I-V) curves are measured at the housing terminals of the sealed module. Individual cell characteristics may be qualitatively determined by contrasting module I-V responses to different shutter configurations and quantitatively determined by fitting an extended equivalent circuit model to the shutter data.

Shutter array technique for real-time non-invasive extraction of individual channel responses in multi-channel CPV modules

Concentrator photovoltaic (CPV) solar energy systems use optics to concentrate direct normal incidence (DNI) sunlight onto multi-junction photovoltaic (MJPV) cells fabricated from III-V compound semiconductors on germanium substrates. The MJPV receiver, which integrates cell and bypass diode, is then mated with its concentrating optic to form a channel, and several such channels form a CPV module, in which the receivers are connected electrically in series. The two ends of the module receiver string are brought out to a single pair of electrical connections, at which point the lightcurrent- voltage (L-I-V) response of the entire module can be tested. With commercial CPV modules commonly sealed against outdoor exposure, there are no other accessible test points, and field installation on trackers further complicates access to performance data. There are many physical phenomena influencing module performance, and in early development and commercialization some of these may not yet be completely under control. Unambiguous diagnosis of such phenomena from one full-module L-I-V curve is problematic. Simple, fast test methods are needed to develop more detailed information from full-module on-tracker testing, without opening up modules in the field. We describe a test protocol, using a simple optical shutter array constructed to fit mechanically over the module. When module L-I-V curves are recorded for each of various combinations of open and closed shutters, the information can be used to identify one or more anomalous channels, and to further identify the kind of anomaly present, such as optical misalignment, conductor failure, series or shunt resistance, and so on. Simulated results from anomaly models can be compared with the measured results to identify the anomalous behaviour. Results herein are compared with direct single-channel measurements to verify the technique. The L-I-V response curves were obtained in continuous real time, an approach found to be more helpful than single-shot capture in understanding field response. A triangular wave function generator is used to drive the DC power supply, and a four-channel digital sampling oscilloscope displays and stores the real time response. Where modules exhibit unstable or intermittent response under certain conditions, this is immediately obvious in real-time display.

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.