Matthew Wilkins

Ultrahigh efficiencies in vertical epitaxial heterostructure architectures

Optical to electrical power converting semiconductor devices were achieved with breakthrough performance by designing a Vertical Epitaxial Heterostructure Architecture. The devices are featuring modeled and measured conversion efficiencies greater than 65%. The ultrahigh conversion efficiencies were obtained by monolithically integrating several thin GaAs photovoltaic junctions tailored with submicron absorption thicknesses and grown in a single crystal by epitaxy. The heterostructures that were engineered with a number N of such ultrathin junctions yielded an optimal external quantum efficiencies approaching 100%/N. The heterostructures are capable of output voltages that are multiple times larger than the corresponding photovoltage of the input light. The individual nanoscale junctions are each generating up to ∼1.2 V of output voltage when illuminated in the infrared. We compare the optoelectronic properties of phototransducers prepared with designs having 5 to 12 junctions and that are exhibiting volt...

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.

Five-volt vertically-stacked, single-cell GaAs photonic power converter

The high-efficiency conversion of photonic power into electrical power is of broad-range applicability to many industries due to its electrical isolation from the surrounding environment and immunity to electromagnetic interference which affects the performance and reliability of sensitive electronics. A photonic power converter, or phototransducer, can absorb several watts of infrared laser power transmitted through a multimode fiber and convert this to electrical power for remote use. To convert this power into a useful voltage, we have designed, simulated, and fabricated a photovoltaic phototransducer that generates >5 V using a monolithic, lattice-matched, vertically-stacked, single-cell device that eliminates complex fabrication and assembly steps. Experimental measurements have demonstrated a conversion efficiency of up to 60.1% under illumination of ~11 W/cm2 at a wavelength of 835 nm, while simulations indicate that efficiencies reaching 70% should be realistically achievable using this novel design.

Luminescent coupling in planar opto-electronic devices

Effects of luminescent coupling are observed in monolithic 5 V, five-junction GaAs phototransducers. Power conversion efficiency was measured at 61.6% ± 3% under the continuous, monochromatic illumination for which they were designed. Modeling shows that photon recycling can account for up to 350 mV of photovoltage in these devices. Drift-diffusion based simulations including a luminescent coupling term in the continuity equation show a broadening of the internal quantum efficiency curve which agrees well with experimental measurements. Luminescent coupling is shown to expand the spectral bandwidth of the phototransducer by a factor of at least 3.5 for devices with three or more junctions, even in cases where multiple absorption/emission events are required to transfer excess carriers into the limiting junction. We present a detailed description of the novel luminescent coupling modeling technique used to predict these performance enhancements.

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.

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%.

Performance impact of luminescent coupling on monolithic 12-junction phototransducers for 12 V photonic power systems

A twelve-junction monolithically-integrated GaAs phototransducer device with >60% power conversion efficiency and >14 V open-circuit voltage under monochromatic illumination is presented. Drift-diffusion based simulations including a luminescent coupled generation term are used to study photon recycling and luminescent coupling between each junction. We find that luminescent coupling effectively redistributes any excess generated photocurrent between all junctions leading to reduced wavelength sensitivity. This broadened response is consistent with experimental measurements of devices with high-quality materials exhibiting long carrier lifetimes. Photon recycling is also found to significantly improve the voltage of all junctions, in contrast to multi-junction solar cells which utilize junctions of differing bandgaps and where high-bandgap junctions benefit less from photon recycling.

Advances with vertical epitaxial heterostructure architecture (VEHSA) phototransducers for optical to electrical power conversion efficiencies exceeding 50 percent

A monolithic compound semiconductor phototransducer optimized for narrow-band light sources was designed for and has achieved conversion efficiencies exceeding 50%. The III-V heterostructure was grown by MOCVD, based on the vertical stacking of a number of partially absorbing GaAs n/p junctions connected in series with tunnel junctions. The thicknesses of the p-type base layers of the diodes were engineered for optimal absorption and current matching for an optical input with wavelengths centered in the 830 nm to 850 nm range. The device architecture allows for improved open-circuit voltage in the individual base segments due to efficient carrier extraction while simultaneously maintaining a complete absorption of the input photons with no need for complicated fabrication processes or reflecting layers. Progress for device outputs achieving in excess of 12 V is reviewed in this study.

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.

Modeling nonuniform irradiance and chromatic aberration effects in a four junction solar cell using SPICE

A two-dimensional, distributed resistance model for a four junction solar cell is implemented in SPICE. Efficiency estimates for Gaussian irradiance profiles with different peak-to-average ratios (PAR) are determined via grid optimization at concentrations of 500, 1000 and 2000 suns. Optimizing finger spacing for a PAR of 6 improves cell efficiency by 1.8% (absolute) at 2000 suns compared to that observed from finger spacing optimized for a uniform illumination. To address the impact of chromatic aberration on cell efficiency, a CPV system is modeled in Zemax for a geometric concentration of 1250X. Using a finger spacing optimized for uniform irradiance at the average optical efficiency of 82%, the neglect of chromatic aberration was found to overstate system efficiency by 3% (absolute).