M. C. A. York

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...

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.

High-photovoltage GaAs vertical epitaxial monolithic heterostructures with 20 thin p/n junctions and a conversion efficiency of 60%

Photovoltaic power converting III–V semiconductor devices based on the Vertical Epitaxial HeteroStructure Architecture (VEHSA) design have been achieved with up to 20 thin p/n junctions (PT20). Open circuit photovoltages in excess of 23 V are measured for a continuous wave monochromatic optical input power of ∼1 W tuned in the 750 nm–875 nm wavelength range. Conversion efficiencies greater than 60% are demonstrated when the PT20 devices are measured near the peak of their spectral response. Noticeably, the PT20 structure is implemented with its narrowest ultrathin base having a thickness of only 24 nm. In the present study, the spectral response of the PT20 peaks at external quantum efficiency (EQE) of 89%/20 for an input wavelength of 841 nm. We also performed a detailed analysis of the EQE dependence with temperature and for VEHSA structures realised with a varied number of p/n junctions. The systematic study reveals the correlations between the measured conversion efficiencies, the EQE behavior, and th...

Enhanced photocarrier extraction mechanisms in ultra-thin photovoltaic GaAs n/p junctions

PV devices with active areas of ~3:4 mm2 were fabricated and tested with top electrodes having different emitter gridline spacings with active area shadowing values between 0% and 1.8%. As expected, the thicker n/p junctions exhibit hindered photocarrier extraction, with low fill factor (FF) values, for devices prepared with sparse gridline designs. However, this study clearly demonstrates that for thin n/p junctions photocarrier extraction can still be efficient (FF > 80%) even for devices with no gridlines, which we explain using a TCAD model. The electric field profiles of devices with and without hindered photocarrier extraction are also discussed.

Computing with networks of nonlinear mechanical oscillators

As it is getting increasingly difficult to achieve gains in the density and power efficiency of microelectronic computing devices because of lithographic techniques reaching fundamental physical limits, new approaches are required to maximize the benefits of distributed sensors, micro-robots or smart materials. Biologically-inspired devices, such as artificial neural networks, can process information with a high level of parallelism to efficiently solve difficult problems, even when implemented using conventional microelectronic technologies. We describe a mechanical device, which operates in a manner similar to artificial neural networks, to solve efficiently two difficult benchmark problems (computing the parity of a bit stream, and classifying spoken words). The device consists in a network of masses coupled by linear springs and attached to a substrate by non-linear springs, thus forming a network of anharmonic oscillators. As the masses can directly couple to forces applied on the device, this approach combines sensing and computing functions in a single power-efficient device with compact dimensions.

Ultra-efficient N-junction photovoltaic cells with V OC > 14V at high optical input powers

GaAs phototransducers with 5 to 12 p/n junctions are shown to demonstrate breakthrough performance in optical conversion efficiency ranging from 65% to just under 70%. In particular, for a current-matched 12 junction device we have recorded an efficiency of 66.5% at Voc = 14.46V (open circuit voltage) with an approximately 650 suns / 841 nm monochromatic source. Our simulations reproduce the PV characteristics of the physical devices with good (~ %) accuracy for Voc and Isc (short circuit current), which allows us to numerically study the influence of experimental conditions and design factors on device performance. Moreover, our simulations suggest that measured efficiencies are very close to the theoretical limits of the heterostructures under consideration. Our focus in these proceedings is twofold: a) to present an overview of the results of these experiments as well as b) study the factors which limit simulated device performance with the objective of attaining monochromatic conversions efficiencies exceeding 70%.

Pushing the limits of a hybrid thermal / PV system

We study the constraints on the electrical and thermal efficiency of an integrated photovoltaic and solar heating system over a large range of possible design parameters. These include the geometry of the set-up, solar concentration ratios, the properties of the materials used (emissivities, thermal conductivities, etc.) and flow rates. Our model quantifies the relative influence exhibited by each of these parameters on overall efficiency as well as exterior surface temperatures. In effect, we are able to assess and exclude a range of possible configurations which are anticipated to operate at temperatures prohibitively high for photovoltaic cells. We assess the modifications required to render a strictly thermal system suitable for photovoltaic integration, and, for a viable configuration, estimate its total thermal and electrical power output.