The sun blesses the earth with enough energy in one hour—4.3×1020 joules—to fulfill all of our energy needs for a year (4.1×1020 joules). Its giant bombardment of free and non-carbon-emitting energy has impelled scientists for decades to develop photon-electron converter solar cells that catch sunlight and convert it directly into electricity.
Silicon solar cells are in use since the beginning of the space programme and now dominate the industry, with worldwide shipments of solar products expected to have totaled $41.9 billion in 2016. Silicon solar cell technology has captured 90 per cent of the solar market, but scientists around the world are working to find a way to make them more efficient, affordable, stable and reliable.
The typical silicon wafer solar panel is power rated using sunlight at zenith (climax or high point), which provides a radiance of just over one kilowatt per square metre at sea level. This radiant energy comprises 445 watts of visible light component, 527 watts of IR radiation and 32 watts of UV radiation. Wafer-type silicon solar cells available today can process only visible sunlight. Thus 56 per cent of the available energy to be converted into electricity is unutilised with silicon solar cells.
The pursuit of lower-cost and more efficient solar cells has led to the search of new materials. In the past several years, several emerging photovoltaic technologies were the focus of R&D effort in the academia and industry. These third-generation solar cells do not require custom-made, high-priced, complicated processing equipment, as also expensive silicon or rare earth elements such as indium.
According to National Renewable Energy Laboratory (NREL), which is an international organisation for certification of solar cell efficiency, multicrystalline silicon cells are 21 per cent efficient and expensive, while single-crystal silicon cells have efficiency of around 25 per cent. The highest efficiency of 46 per cent has been recorded for gallium arsenide-based multijunction solar cells but at excessively high cost. The bright CIGS photovoltaic cells are 22.6 per cent efficient. One of the potential application of these solar cells was envisaged for window glasses. Thin films of semitransparent photovoltaic modules used in place of window glasses on a building will allow people inside to see through the window while appearing as tinted glass from the outside.
The promising emerging photovoltaic technologies, called third generation PV, are organic photovoltaics, dye-sensitised solar cells, perovskite photovoltaics and inorganic quantum-dot solar cells. Unlike traditional silicon solar cells, these represent transformative technologies with great potential for extremely high-throughput manufacturing at very low cost, low environmental impact, mechanical flexibility and molecular tailorability while using non-toxic, earth-abundant materials with short energy payback times. This makes them suitable for applications beyond rooftop and solar-farm panels. The fundamental advantage of these thin-film technologies comes in the form of the amount of material required for fabrication.
However, the lower price advantage of these emerging photovoltaics is offset by their lower efficiency. The power conversion efficiency (a ratio of light energy in to electrical energy out) of dye-sensitised solar cells, organic photovoltaics and quantum-dot cells today sits at roughly 12 per cent. Hence these emerging photovoltaic technologies are applicable only in niche markets that require light weight, flexibility and variable-angle performance, such as in consumer electronics. Some are still being developed by technology incubators and start-up companies. Thus today photovoltaics market is dominated by silicon-photovoltaics.
The two metrics for evaluating and comparing different photovoltaic technologies, in conjunction with their power conversion efficiency, include:
1. Levelised cost of energy (LCOE)=Total life-cycle cost/energy produced
2. Energy payback time (EPBT) =Energy consumed over the life of the module/Energy generated over its lifetime
Fortunately, these two metrics predict bright future for the new emerging technologies.
Conventional silicon cells require ultra-high-purity silicon—of the order of 99.999 per cent pure. These cells are made using energy-intensive crystal growth and vapour deposition methods. One of the drawbacks of silicon technology that further increases its cost and limits applications is that silicon is indirect band semiconductor, so it doesn’t absorb sunlight strongly. Therefore relatively thick layer of silicon is used in cells, which become brittle and hence have to be supported on a rigid, heavy piece of glass. A silicon solar cell requires 14 steps to manufacture, including preparations requiring the use of high-heat, expensive automation and clean rooms.
In comparison, the new emerging photovoltaic cells are made from low-cost materials, including organic polymers, small molecules and various types of inorganic compounds, and can be fabricated on flexible supports via inexpensive solution-phase techniques common in plastics manufacturing, such as high-speed roll-to-roll printing. In addition, these PV cells use 1000 times less, light-absorbing material than silicon solar cells, which further reduces their cost.
Third-generation solar cells
In organic photovoltaics, bulk heterojunction structure (a mixture of donor and acceptor materials in a bulk device) is most commonly used due to its high efficiency and compatibility with low-cost manufacturing techniques such as inkjet printing, spin coating and roller casting. These PVs are renowned as high-PCE polymer-fullerene solar cells.
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