Henry Schriemer

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.

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.

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.

Fabrication-Tolerant Higher Order Laterally-Coupled Distributed Feedback Lasers

To avoid the commonly required regrowth steps in conventional distributed feedback laser fabrication, laterally-coupled distributed feedback (LC-DFB) lasers lithographically pattern the grating out of the ridge waveguide. Using higher order gratings increases the lithographic tolerances, resulting in lasers that are more amenable to mass-manufacturing techniques, such as stepper lithography. We have extended the modified coupled-mode theory to a two-dimensional cross-section, and thereby identified grating geometries that are both fabrication-tolerant and provide high performance.

Modal birefringence and power density distribution in strained buried-core square waveguides

The optical properties of engineered devices derive from the flow of energy within the structure, a flow that is governed by the interaction between device architecture and material properties. With evolving device and system sophistication, implementation of manufacturing processes becomes an exercise in global optimization. This task is addressed in the design phase by choosing metrics that correlate with critical operating parameters of the device. We illustrate two complementary metrics, arising from the same physical effect, that manifest at different length scales. By considering the thermoelastic properties of a doped-glass multilayer with an embedded waveguide, we reveal the impact of stress on modal birefringence and optical power distributions. The modal birefringence of a single-mode buried-core square waveguide irreducibly describes a discrete metric whose value derives from global strain contributions. It is an ideal metric for addressing polarization-dependent wavelength shifts in arrayed waveguide grating devices, which are particularly sensitive to material stresses over long length scales. By contrast, the power density distribution in the bound mode is a distributed metric dependent on local properties and is thus more suitable for devices whose operability is dependent upon shorter length scales. These complementary characterizations of stress-optical coupling are numerically assessed by finite element analysis. The material stresses are first found through solution of the structural problem, with subsequent solution of the the full-vector anisotropic Maxwells equations as an eigenvalue problem. We found the modal birefringence to be primarily governed by the properties of the cladding material, the core properties having a negligible effect, while fine control may be achieved by varying the position of the interface between the lower and upper cladding. This demonstrates its suitability as a metric for devices primarily reliant on isolated waveguides. At shorter length scales, through contrast with the isotropic case, we showed that stress-optical coupling suppresses the in-plane symmetry axis of the logarithmic difference in spatial power densities, regardless of the degeneracy of the optical mode, with asymptotic behavior that effectively diverges. This revealed a metric of potential applicability for interferometric and evanescently coupled structures.

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%.

Design and fabrication of a λ/4 phase-shifted 1310 nm laterally-coupled distributed-feedback laser

We describe the design and fabrication of a lambda/4 phase-shifted laterally-coupled distributed-feedback laser. The third-order grating for distributed-feedback is fabricated without regrowth using stepper lithography, a process that is amenable to high-yield, low-cost manufacturing.

Reconstruction of solar spectral resource using limited spectral sampling for concentrating photovoltaic systems

One of the challenges associated with forecasting and evaluating concentrating photovoltaic system (CPV) performance in diverse locations is the lack of high-quality spectral solar resource data. Various local atmospheric conditions such as air mass, aerosols, and atmospheric gases affect daily CPV module operation. A multi-channel filter radiometer (MFCR) can be used to quantify these effects at relatively low cost. The proposed method of selectively sampling the solar spectrum at specific wavelength channels to spectrally reconstruct incident irradiance is described and extensively analyzed. Field spectroradiometer (FSR) measurements at the University of Ottawas CPV testing facility (45.42°N, 75.68°W) are fed into our model to mimic the outputs from the MCFR. The analysis is performed over a two year period (2011-2012), using 46,564 spectra. A recommendation is made to use four aerosols channels at 420, 500, 780, and 1050 nm, one ozone channel at 610 nm and one water vapour channel at 940 nm, all of which can be measured with ubiquitous Si photodiodes. A simulation of this MFCR channel configuration produces an RMS error under 1.5% over 96% of the 350-1830 nm range, when compared with the FSR, for the 2012 data set in Ottawa.

Energy transport through structures with finite electromagnetic stop gaps

The transport of electromagnetic radiation in one-dimensional finite periodic structures is studied via the electromagnetic energy velocity, which is given by the spatially local ratio of the time-averaged power flux to the time-averaged energy density. This energy velocity, which is shown to not exceed the speed of light in vacuum, is determined analytically for the case of continuous waves. Further, the differences between energy and group velocities are enumerated, and propagation both within and outside stop gap regions are discussed. Our rate description of the electromagnetic transport in finite photonic crystals thus eliminates the usual false paradoxes generated by naive interpretation of tunnelling phenomena, and so avoids apparent conflicts with causality.

A novel instrument for cost-effective and reliable measurement of Solar Spectral Irradiance

A next generation solar spectral irradiance meter (SSIM) was installed at the University of Ottawa solar test site in September 2014, for ongoing collection of environmental and spectral data. The instruments performance is compared against a commercial pyrheliometer and a spectroradiometer during an eight month study where ambient temperature fluctuates from -30°C to 30°C. The cumulative solar energy measured by the SSIM over the duration of the experiment agreed to within 0.5% as compared to the Eppley pyrheliometer. A good spectral agreement between the SSIM and the ASD FieldSpec 3 spectroradiometer is observed, with the integral of the spectral irradiance agreeing to within 1% for both instruments. No degradation is observed at any point during this investigation.