Although conversion efficiency has become the leading measure by which solar cells are judged, this is not necessarily the best yardstick. In fact, slavish adherence to it can be counter-productive.
As Jim Nelson, ceo of U.S. company Solar3D Inc points out, solar efficiency is generally measured under optimum conditions with direct sunlight shining on the cell from the best angle. In the real world, light collection drops off markedly as the sun angle becomes more oblique. Thus, he says a 17% cell acts more like a 5% cell when the sun is at 20° from the side in mornings and evenings. Therefore, “the real deliverable is not efficiency, but the total power the cell generates throughout a day or a year,” Nelson says.
Solar 3D firmly targets this measure with its ‘next generation’ silicon PV cell design that captures more sunlight than conventional cells. This is achieved by giving its cells a three-dimensional topology. In a normal cell, up to 30% of incident sunlight is reflected away and lost. In Solar3D's cell, light that would otherwise escape is trapped inside the 3D micro-PV structure, where photons bounce around until they are converted into electrons. Surface patterning also ensures that sunlight is collected over a greater range of incident angles.
The patent-pending 3D cell is claimed to generate in a day twice the power of a conventional cell, reducing the payback period by some 40%.
As a result of the greater power generated, roof arrays can be more productive or made smaller for the same power. The technology lends itself, says the company, to BIPV solutions such as cells integrated into roof tiles.
Extending the ‘real deliverable’ definition cited above still further to take account of cost, leads to the modified yardstick: ‘Total power generated over a given period per unit cost’. By this measure, thin-film products that might otherwise be dismissed as too inefficient could be worthy of consideration. A PV film that can be produced cheaply in high volumes and easily installed more or less anywhere might out-compete today's conventional products through sheer low-cost ubiquity, despite having relatively low conversion efficiency.
There are a number of flag bearers for this approach. One notable product is PowerFilm, produced by Iowa-headquartered PowerFilm Solar Inc. It is no coincidence that this firm's founders were previously research physicists with the 3M company, famous for its film products.
Over two decades, Dr Frank Jeffrey and Dr Derrick Gimmer have led the development of thin-film solar panel technology along with the industrial process needed to manufacture it in high volumes. Elements of their proprietary solution include the use of a durable, flexible plastic substrate, roll-to-roll manufacturing to minimise handling costs; amorphous silicon to avoid dependence on the silicon wafer market cycle; and printed interconnects to automate the cell connection process.
PowerFilm is paper-thin and ultra flexible. Because it can be combined with metal, fabric, membrane and other materials, it lends itself to integration with buildings and structures. It is equally conformable to flat structures such as standing seam metal roofing or curved elements, and is said to enhance building style in the process. It is also used in items as varied as foldable and roll-up solar chargers, sails on eco-friendly yachts, canvas awnings, caravan/boat/motor home roofs and solar modules for military use. When stuck onto a firm surface with PVA glue, it can be walked on. PowerFilm modules include cables, connectors and blocking diodes.
Admittedly, some thin-film solar is efficient, with up to 20% (on a par with good commercial silicon modules) having been recorded for film utilising copper indium gallium diselenide (CIGS) semiconductor. Texas-based HelioVolt Corporation, for example, claims to have combined the highest thin-film efficiencies available today with low-cost manufacturing in a product that can be applied directly onto conventional construction materials. Using a non-vacuum nanomaterial-based deposition process jointly developed with the National Renewable Energy Laboratory (NREL), the company virtually prints the semiconductor onto film material.
Nanosolar Inc similarly prints CIGS material onto low-cost conductive aluminium foil, at factories in California and Berlin It then integrates these 17% efficient solar cells into nanosolar utility panels at separate panel assembly factories located near points of demand for solar energy, thereby shortening the supply chain and leveraging local resources. The company has several multi-megawatt projects in a number of US states and in France where it has a collaboration with EDF EN.
While examples like these have tempted some pundits to bet against silicon for the solar PV future, doubts may have been raised by the insolvency of Solyndra, another company that banked on CIGS technology. This followed a fall in silicon prices.
Other innovations continue to enhance thin-film prospects. For example, G24 Innovations Ltd has developed a technology that mimics the photosynthesis process used by plants. Gratzel cells combined with dyes and incorporated into extremely flexible thin-film are actually green, just like the plants they imitate. Conversion efficiencies of 12% plus have been achieved.
New Energy Technologies Inc is relying on OPV to realise its vision of producing windows that generate electricity. The Maryland-based company has developed organic solar cells about a quarter the size of a grain of rice and, at one thousandth the thickness of a human hair, they are truly ultra-thin. Spraying a layer of these cells onto glass results in solar glass that can produce electricity. A conductive wiring grid that is practically invisible enables energy to be harvested without interfering with the see-through quality of the panel.
New Energy Technologies has managed to expand the transparencies it can produce to the size of a small window. Further up-scaling to full window size will, the company is convinced, create a revolution in BIPV, given that windows and glass facades account for such a vast surface area in the built environment.
Finally, we should mention efforts being made to produce thin-film substrate materials that are thinner, stronger or cheaper (preferably all three) than the mainly stainless steel or polymer materials currently used.
BioSolar Inc is targeting both cost and environmental credentials by substituting plant-derived biomaterials for the more conventional substrates. By doing so, it expects to halve substrate costs, which can be a substantial proportion of total sheet product cost. It is currently developing a bio-based substrate that thin-film manufacturers can use in roll-to-roll chemical vapour deposition processes. It is also working on a bio-derived superstrate, the glass or polymer protective layer that covers a thin-film solar cell. Both products are based on backsheet technology already developed for silicon solar cells
Bringing matters right up to date, graphene, the wonder material discovered by a team at Manchester University in 2004, could revolutionise solar PV technology. A single layer lattice of carbon atoms that has unique mechanical, electronic and photonic properties, graphene absorbs light of any colour and every photon absorbed generates an electron hole pair that can, in principle, be converted into electric current. This potentially makes the material a substitute for silicon.
However, although graphene-based solar cells have been produced, light absorption overall is at a limited level and there are difficulties making electrical contacts. Researchers at the universities of Manchester and Cambridge might have overcome these drawbacks by combining graphene with certain nanostructures that have an electromagnetic concentration effect. Short of actively carrying out the actual power conversion process, graphene could be used to make transparent electrodes flexible enough for use with organic PV compounds, or as the basis for super-thin, super-strong substrate materials.
Check out our other Solar PV articles here.
In part 1. George Marsh: introduction to PV research
In part 2. George Marsh looks at some other novel technologies under development.
In part 3. Joyce Laird takes up the story, talking to some of the leaders of global solar power R&D
In part 4. Joyce Laird looks at 2 innovative areas of PV technology - Screening Engineered Field Effect PV, and Inverted Organic Solar Cells...
About George Marsh: Engineering roles in high-vacuum physics, electronics, flight testing and radar led George Marsh, via technology PR, to technology journalism. He is a regular contributor to Renewable Energy Focus.