Paul E. Sims

Feedstock for crystalline silicon solar cells

A mandatory requirement for the development of a large solar photovoltaic power industry is the development a thin silicon layer structure or a cheaper silicon feedstock that does not have the high purity requirements of the semiconductor wafer industry. In the latter case the solar grade silicon feedstock supply must be decoupled from the semiconductor silicon feedstock industry. If one of these these events do not occur, then solar energy will remain a small and specialized industry that will start to shrink as the world power grid capacity increases.

Thin silicon solar cells

The silicon-film design achieves high performance by using a dun silicon layer and incorporating light trapping. Optimally designed thin crystalline solar cells (<50 microns thick) have performance advantages over conventional thick devices. The high-performance silicon-film design employs a metallurgical barrier between the low-cost substrate and the thin silicon layer. Light trapping properties of silicon-film on ceramic solar cells are presented and analyzed. Recent advances in process development are described here.

Non-traditional light sources for solar cell and module testing

Solar cell and module performance testing is a critical component of high volume manufacturing. Indoor testing of solar cells and modules has historically utilized filtered xenon arc lamps as a sunlight simulator. There are a number of problems with are lamps in this environment including high cost, extensive maintenance, area limitations and electrical noise. In this work the use of low-cost, reliable, quartz-tungsten-halide (QTH) lamps as an alternative light source is analyzed. This analysis is focused on evaluating spectral mismatch errors due to testing solar cells with varying spectral response. When combined with outdoor calibration standards, QTH lamps are found to introduce acceptable levels of error (<2%) for testing both single crystal and polycrystalline solar cells and modules.

Thin silicon-on-ceramic solar cells

AstroPower is developing an advanced photovoltaic module product based on thin silicon-on-ceramic substrates. This paper reviews motivations, technical approaches and electrical results for single device cells that benefit the development of a monolithically interconnected solar array product. A short circuit current density of 25.8 mA/cm/sup 2/ was achieved in a single cell device that demonstrates significant light trapping and excellent back surface passivation. A 9.1% efficient thin film silicon-on-ceramic solar cell was demonstrated using a thermal spray technique. These accomplishments form a basis for the continued research and development towards a new class of thin-silicon solar cell products.

High-performance monolithic AlGaAs/GaAs photovoltaic arrays coupled to scintillating fibers for UGS application

This paper reports progress in the development of a miniature, monolithic AlGaAs/GaAs photovoltaic solar cell array, used in combination with scintillating fibers, and its application to recharging batteries of unattended ground sensors. Improvements in the design and processing of these photovoltaic arrays resulted in significant increases in both voltage and current. Moreover, new polycarbonate scintillating fibers that are more durable and flexible than polystyrene ones were tested. Output electric power density as high as 73 mW/cm2 for total area and 105 mW/cm2 for active area was measured outdoors for a 6-cell array integrated with a 1 foot long fiber bundle.

AlGaAs photovoltaic arrays spectrally matched to photoluminescent fibers for UGS application

This paper reports progress in the development of a miniature photovoltaic (PV) arrays consisting of monolithically series connected AlGaAs/GaAs PV cells used in combination with polymeric photoluminescent fibers to recharge batteries of unattended ground sensors (UGS). Outdoor tests of the arrays showed feasibility of this approach. Optimization of the fibers design (material used, diameter, coupling, etc.) is discussed. Better optical matching of the fibers and PV cells was achieved through replacing of GaAs photoactive layers by AlGaAs ones having a higher bandgap.

IIb-3 – Thin Silicon Solar Cells

Publisher Summary This chapter reviews the issues and technical achievements that motivate the current interest in thin silicon solar cells. Thin silicon solar cells are an important class of photovoltaics that are currently the subject of intense research, development, and commercialization efforts. The potential cost reductions realized by manufacturing solar cells in a thin device configuration are highly compelling. This chapter discusses the subject of light trapping in thin silicon solar cells, as well as methods of implementing light trapping. Random texturing, geometrical/regular structuring, external optical elements, and assessment of light-trapping effects are discussed in detail. The chapter also discusses short-circuit current analysis of light trapping, and extended spectral response analysis of light trapping. Minority carrier recombination issues in thin silicon solar cells are described. Silicon deposition and crystal growth for thin solar cells is discussed in detail. The chapter also elaborates concepts related to substrate considerations, high-temperature silicon deposition methods, melt growth techniques, recrystallization of silicon, and high-temperature silicon chemical vapor deposition.

AlxGa1−xSb window layers for InGaAsSb/GaSb thermophotovoltaic cells

We report results of a comparative study of AlSb-based window layers for InGaAsSb/GaSb TPV cells made by liquid-phase epitaxy (LPE). Previous work has shown that an AlGaAsSb window layer significantly improves the performance of InGaAsSb/GaSb TPV cells. As expected, the window layer enhances short-wavelength spectral response and increases the open-circuit voltage by reducing the reverse-saturation current of the diode. We present results for a simpler alternative window layer based on the ternary AlGaSb alloy. We fabricated, characterized, and compared InGaAsAsSb TPV of three types: 1. with an AlGaAsSb window layer, 2. with an AlAsSb window layer, and 3. with no window layer. Both p-on-n (p-type InGaAsSb emitter; n-type InGaAsSb base) and n-on-p (n-type InGaAsSb emitter; p-type InGaAsSb base) homojunction cell configurations were investigated. The InGaAsSb layers have a ∼0.55-ev bandgap and are lattice-matched to a GaSb substrate. As anticipated, both AlGaSb and AlGaAsSb passivated TPV cells were superio...

IIb-3 – Thin silicon solar cells

Publisher Summary This chapter provides an overview of thin silicon solar cells. Thin silicon solar cells can greatly benefit from light-trapping effects, which can offset the relatively weak absorption near-bandgap energy photons by increasing the optical path length of light within a solar cell structure. Three types of reflective surfaces can be employed—random texture, geometric or regular structuring, and the use of optical elements external to the silicon solar cell structure—to implement light trapping in silicon solar cells. Light trapping has been incorporated in structures with thickness ranging from less than 1 micron to 400 microns with varying degrees of success. The factors that influence the open-circuit voltage of a silicon solar cell are the same irrespective of the fact whether the device is thin or thick. These include doping levels, various bulk recombination mechanisms, and surface recombination.

Small power systems for law enforcement applications

Recent events have increased interest in the use of sensors by law enforcement and homeland defense related organizations. Autonomous sensors such as those under development for the Unattended Ground Sensor (UGS) program are suitable for some of these applications. The operational lifetime of a UGS depends on the power consumption of the package and the space allocated for batteries. We survey and assess options for powering these devices ina long-term scenario. These alternatives are in various stages of development, and range from conventional batteries and solar cells that are ready for deployment and are now commercially available; to technologies developed for other applications (e.g., power for deep-space probes, man portable power for soldiers, or for sensors in oil drilling bore holes) that would need to be adapted to UGSs; to new and often speculative concepts that are in the laboratory or are still on the drawing board. Ideally, unattended ground sensors do not require servicing, re- energizing or refueling; and are capable of autonomous operation for weeks or even years. Further, UGSs may need to be used covertly, which restricts schemes that would provide a detectable signature. Reliability, ruggedness, cost, weight, size, camouflaging, use of toxic materials and other safety or disposal aspects, restrictions on their deployment (e.g., whether UGSs can be dropped form the air or whether they need to be uprighted or favorably oriented), storage and inventorying considerations, temperature ranges of operation, and complexity of associated electronics are also important issues. In this paper, we will limit the discussion to systems where operating power does not exceed 5 watts since larger systems are commercially available. Some subjectivity in comparisons is perhaps inevitable, but despite the disparate physics upon which these devices are based, a few common criteria can be invoked for discussing their suitability for energy storage and powering UGSs. Metrics can be developed to assess and compare options, but since most of the options are in very different stages of development, one is sometimes forced to use performance specifications that are predicted, rather than demonstrated. Thus, in some cases the comparisons are tentative or speculative.