Publications

Photo-conductive AFM spectroscopy ('pcAFMs') is proposed as a high-resolution approach for investigating nanostructured photovoltaics, uniquely providing nanoscale maps of photovoltaic (PV) performance parameters such as the short circuit current, open circuit voltage, maximum power, or fill factor. The method is demonstrated with a stack of 21 images acquired during in situ illumination of micropatterned polycrystalline CdTe/CdS, providing more than 42 000 I/V curves spatially separated by ~5 nm. For these CdTe/CdS microcells, the calculated photoconduction ranges from 0 to 700 picoSiemens (pS) upon illumination with ~1.6 suns, depending on location and biasing conditions. Mean short circuit currents of 2 pA, maximum powers of 0.5 pW, and fill factors of 30% are determined. The mean voltage at which the detected photocurrent is zero is determined to be 0.7 V. Significantly, enhancements and reductions in these more commonly macroscopic PV performance metrics are observed to correlate with certain grains and grain boundaries, and are confirmed to be independent of topography. These results demonstrate the benefits of nanoscale resolved PV functional measurements, reiterate the importance of microstructural control down to the nanoscale for 'PV devices, and provide a widely applicable new approach for directly investigating PV materials.

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Cd1-xZnxTe/CdS solar cells are currently limited by material defects. While nano-structuring promises further defect reductions, the materials synthesis and characterization become more challenging. Molecular dynamics models capable of growth simulations enable defects to be explored without assumptions, and can therefore guide nanoscale experiments. Such models are difficult to develop, and are not routinely available in literature for semiconductor compounds. To fill this gap, we have developed growth simulation enabling Stillinger-Weber and bond-order potentials. These new models begin to enable molecular dynamics to be used to explore nano-structured Cd1-xZnxTe/CdS solar cells with reduced defects.

Microsystems Enabled Photovoltaics (MEPV) is a relatively new field that uses microsystems tools and manufacturing techniques familiar to the semiconductor industry to produce microscale photovoltaic cells. The miniaturization of these PV cells creates new possibilities in system designs that can be used to reduce costs, enhance functionality, improve reliability, or some combination of all three. In this article, we introduce analytical tools and techniques to estimate the costs associated with a hybrid concentrating photovoltaic system that uses multi-junction microscale photovoltaic cells and miniaturized concentrating optics for harnessing direct sunlight, and an active c-Si substrate for collecting diffuse sunlight. The overall model comprises components representing costs and profit margin associated with the PV cells, concentrating optics, balance of systems, installation, and operation. This article concludes with an analysis of the component costs with particular emphasis on the microscale PV cell costs and the associated tradeoffs between cost and performance for the hybrid CPV design.

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We demonstrate numerically that two-dimensional arrays of ultrathin CdTe nano-cylinders on Ag can serve as an effective broadband anti-reflection structure for solar cell applications. Such devices exhibit strong absorption properties, mainly in the CdTe semiconductor regions, and can produce short-circuit current densities of 23.4 mA/cm2, a remarkable number in the context of solar cells given the ultrathin dimensions of our nano-cylinders. The strong absorption is enabled via excitation of surface plasmon polaritons (SPPs) under plane wave incidence. In particular, we identified the key absorption mechanism as enhanced fields of the SPP standing waves residing at the interface of CdTe nano-cylinders and Ag. We compare the performance of Ag, Au, and Al substrates, and observe significant improvement when using Ag, highlighting the importance of using low-loss metals. Although we use CdTe here, the proposed approach is applicable to other solar cell materials with similar absorption properties. © 2014 Optical Society of America.

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Microsystems-enabled photovoltaics (MEPV) can potentially meet increasing demands for light-weight, portable, photovoltaic solutions with high power density and efficiency. The study in this report examines failure analysis techniques to perform defect localization and evaluate MEPV modules. CMOS failure analysis techniques, including electroluminescence, light-induced voltage alteration, thermally-induced voltage alteration, optical beam induced current, and Seabeck effect imaging were successfully adapted to characterize MEPV modules. The relative advantages of each approach are reported. In addition, the effects of exposure to reverse bias and light stress are explored. MEPV was found to have good resistance to both kinds of stressors. The results form a basis for further development of failure analysis techniques for MEPVs of different materials systems or multijunction MEPVs. The incorporation of additional stress factors could be used to develop a reliability model to generate lifetime predictions for MEPVs as well as uncover opportunities for future design improvements.

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Microsystems-enabled photovoltaics (MEPV) has great potential to meet the increasing demands for light-weight, photovoltaic solutions with high power density and efficiency. This paper describes effective failure analysis techniques to localize and characterize nonfunctional or underperforming MEPV cells. The defect localization methods such as electroluminescence under forward and reverse bias, as well as optical beam induced current using wavelengths above and below the device band gap, are presented. The current results also show that the MEPV has good resilience against degradation caused by reverse bias stresses. © 2013 IEEE.

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Microsystems-enabled photovoltaic (MEPV) technology is a promising approach to lower the cost of solar energy to competitive levels. This paper describes current development efforts to leverage existing silicon integrated circuit (IC) failure analysis (FA) techniques to study MEPV devices. Various FA techniques such as light emission microscopy and laser-based fault localization were used to identify and characterize primary failure modes after fabrication and packaging. The FA results provide crucial information used in provide corrective actions and improve existing MEPV fabrication techniques. © 2013 IEEE.

Microsystems-enabled photovoltaic (MEPV) technology is a promising approach to lower the cost of solar energy to competitive levels. This paper describes current development efforts to leverage existing silicon integrated circuit (IC) failure analysis (FA) techniques to study MEPV devices. Various FA techniques such as light emission microscopy and laser-based fault localization were used to identify and characterize primary failure modes after fabrication and packaging. The FA results provide crucial information used in provide corrective actions and improve existing MEPV fabrication techniques. © 2013 IEEE.

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Back-contacted, ultrathin (<10 μm), and submillimeter-sized solar cells made with microsystem tools are a new type of cell that has not been optimized for performance. The literature reports efficiencies up to 15% using thicknesses of 14 μm and cell sizes of 250 μm. In this paper, we present the design, conditions, and fabrication parameters necessary to optimize these devices. The optimization was performed using commercial simulation tools from the microsystems arena. A systematic variation of the different parameters that influence the performance of the cell was accomplished. The researched parameters were resistance, Shockley-Read-Hall (SRH) lifetime, contact separation, implant characteristics (size, dosage, energy, and ratio between the species), contact size, substrate thickness, surface recombination, and light concentration. The performance of the cell was measured with efficiency, open-circuit voltage, and short-circuit current. Among all the parameters investigated, surface recombination and SRH lifetime proved to be the most important. Through completing the simulations, an optimized concept solar cell design was introduced for two scenarios: high and low quality materials/passivation. Simulated efficiencies up to 23.4% (1 sun) and 26.7% (100 suns) were attained for 20-μm-thick devices. Copyright © 2012 John Wiley & Sons, Ltd. Back-contacted, ultrathin (<10 μm), and submillimeter-sized solar cells made with microsystem tools are a new type of cell that has not been optimized for performance. In this paper, we present the design conditions and fabrication parameters necessary to optimize these devices via simulations. Through completing the simulations, an optimized concept solar cell design was introduced for two scenarios: high and low quality materials/passivation. Simulated efficiencies up to 23.4% (1 sun) and 26.7% (100 suns) were attained for 20-μm-thick devices. Copyright © 2012 John Wiley & Sons, Ltd.

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Microsystems Enabled Photovoltaics (MEPV) is a relatively new field that uses microsystems tools and manufacturing techniques familiar to the semiconductor industry to produce microscale photovoltaic cells. The miniaturization of these PV cells creates new possibilities in system designs that may be able to achieve the US Department of Energy (DOE) price target of $1/Wp by 2020 for utility-scale electricity generation. In this article, we introduce analytical tools and techniques to estimate the costs associated with a concentrating photovoltaic system that uses microscale photovoltaic cells and miniaturized optics. The overall model comprises the component costs associated with the PV cells, concentrating optics, balance of systems, installation, and operation. Estimates include profit margin and are discussed in the context of current and projected prices for non-concentrating and concentrating photovoltaics. Our analysis indicates that cells with a width of between 100 and 300 μm will minimize the module costs of the initial design within the range of concentration ratios considered. To achieve the DOE price target of $1/Wp by 2020, module efficiencies over 35% will likely be necessary. © 2013 IEEE.

Microsystems-enabled photovoltaics (MEPVs) are microfabricated arrays of thin and efficient solar cells. The scaling effects enabled by this technique results in great potential to meet increasing demands for light-weight photovoltaic solutions with high power density. This paper covers failure analysis techniques used to support the development of MEPVs with a focus on the laser beam-based methods of LIVA, TIVA, OBIC, and SEI. Each FA technique is useful in different situations, and the examples in this paper show the relative advantages of each method for the failure analysis of MEPVs. Copyright © 2013 ASM International® All rights reserved.

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Microsystems Enabled Photovoltaics (MEPV) is a relatively new field that uses microsystems tools and manufacturing techniques familiar to the semiconductor industry to produce microscale photovoltaic cells. The miniaturization of these PV cells creates new possibilities in system designs that may be able to achieve the US Department of Energy (DOE) price target of $1/Wp by 2020 for utility-scale electricity generation. In this article, we introduce analytical tools and techniques to estimate the costs associated with a concentrating photovoltaic system that uses microscale photovoltaic cells and miniaturized optics. The overall model comprises the component costs associated with the PV cells, concentrating optics, balance of systems, installation, and operation. Estimates include profit margin and are discussed in the context of current and projected prices for non-concentrating and concentrating photovoltaics. Our analysis indicates that cells with a width of between 100 and 300 μm will minimize the module costs of the initial design within the range of concentration ratios considered. To achieve the DOE price target of $1/Wp by 2020, module efficiencies over 35% will likely be necessary. © 2013 IEEE.

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In this paper we propose a stacked multi-junction solar cell design that allows the intimate contact of the individual cells while maintaining low resistive losses. The cell design is presented using an InGaP and GaAs multi-junction cell as an illustrative example. However, the methodologies presented in this paper can be applied to other III-V cell types including InGaAs and InGaAsP cells. The main benefits of the design come from making small cells, on the order of 2×10-3 cm2. Simulations showed that series resistances should be kept to less than 5 ω for devices up to 400 μm in diameter to keep resistance power losses to less than 1%. Low resistance AuBe/Ni/Au ohmic contacts to n-type InGaP are also demonstrated with contact resistivity of 5×10-6 ωcm-2 when annealed at 420°C. © 2013 IEEE.

Microsystem technologies have the potential to significantly improve the performance, reduce the cost, and extend the capabilities of solar power systems. These benefits are possible due to a number of significant beneficial scaling effects within solar cells, modules, and systems that are manifested as the size of solar cells decrease to the sub-millimeter range. To exploit these benefits, we are using advanced fabrication techniques to create solar cells from a variety of compound semiconductors and silicon that have lateral dimensions of 250 - 1000 μm and are 1 - 20 μm thick. These fabrication techniques come out of relatively mature microsystem technologies such as integrated circuits (IC) and microelectromechanical systems (MEMS) which provide added supply chain and scale-up benefits compared to even incumbent PV technologies. © The Electrochemical Society.

We calculate voltage-matching considerations for stacked independent cells. The calculations show that designs using independent junctions that are voltage matched can achieve better efficiency across temperature, spectrum, and a yearly metric compared to traditional monolithic cells. Voltage matching is shown to be relatively insensitive to temperature and spectrum, but dependent on open circuit voltage as a measure of cell efficiency. Voltage matching can usually yield yearly efficiencies of 98%-99% of the efficiency of a system with each junction operating at its own maximum power point. © 2013 IEEE.

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