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The photovoltaic effect whereby solar energy is converted to electrical energy was first discovered by Becquerel in a liquid electrolyte.
All photovoltaic materials have discontinuities or marked changes which create an electrical potential in them, causing charge carriers to be generated by incident light. This is how solar cells generate power.
Silicon is the material that is used most commonly in these cells in two different forms - pure and amorphous silicon.
Pure silicon is used in monocrystalline and polycrystalline forms, of which monocrystalline silicon is preferred due to its solar capture efficiency of about 25-30%,
Broad Issues with PV Cell Development
Material Availability and Expense
Cost-effective manufacture, efficiency of solar energy capture, reliability of performance, and stability for at least 20 years are among the chief features required for a PV cell to be commercially viable. These ensure that less energy is used to produce the cell than it produces over its operating life.
The materials used to create semiconductors in PV cells must have a low energy band gap for efficient energy capture and to be able to regulate the energy conversion processes. These criteria are met only by a small number of materials, like Si, GaAs, InP, CdTe, and CuInSe2 in the single crystal cell, and hydrogenated amorphous silicon (a-Si: H), CdTe, and CuInSe2 for thin films.
Availability and low cost of processing are important factors in material selection. An increasing volume of production has caused concerns about material availability.
Printing technologies could cut down the cost while keeping quality high.
These are first-generation devices.
Monocrystalline GaAs may be combined with a back-contact monocrystalline Si for better efficiency of about 31%. The use of monocrystalline silicon PV cells is limited by the high cost of processing silicon by the expensive and energy-intensive Czochralski method.
Polycrystalline silicon cells are much easier and less expensive to manufacture, but recombination losses bring their efficiency down to about 17%.
Silicon solar cells also face absorption problems as long-wavelength photons are poorly absorbed.
While it would be difficult to change the material properties of silicon such as absorption and grain boundaries, other suggested methods to improve the efficiency of silicon cells include:
- Modification of the Czochralski method for monocrystalline silicon
- Adopt wafer technology to use less silicon
- Use directional solidification methods for the growth of polycrystalline silicon which is much less energy-intensive, at about 12 kWh/kg of electricity versus about 100 kWh/kg for the traditional process
- Use of thick film technology
- Screen printing for easily automated production of aluminum and silver electrical contacts
- Introduction of more holes in the bottom of the PV cell to produce unidirectional movement of the generated holes towards the upper layers, avoiding recombinational losses – a costly approach
- Use newer antireflective coatings and glass materials, to reduce reflectance
Thin-film silicon devices are second-generation PV cells. They are mechanically improved, with greater flexibility, but less efficient.
The use of hydrogenated amorphous silicon in thin-film PV cells brings down production costs. On the other hand, drift collection reduces voltage-dependent current collection, thus lowering the efficiency. It also has a lower lifetime as a minority carrier, breakdown with long-term illumination, doping issues and low quality of alloys. Therefore thin film silicon cells are mostly constructed from monocrystalline silicon to maximize their efficiency.
Other materials which can be used as thin films, such as cadmium telluride, copper-indium:diselenide (CIS) and copper indium gallium selenide (CIGS), are being explored.
CdTe has high light absorption and is easy to produce by a variety of methods, but its doping is difficult, and its stability low, besides the toxicity of waste Cd.
CIS and CIGS in the form of polycrystalline thin films have a capture efficiency of about 19 percent. However, these are currently too unstable for use. Indium is also a costly and limited metal.
Third-generation PV materials are being explored to improve solar cell performance. Many reports show attractive concepts to achieve low operating and material costs. Concepts like multiple exciton generation (MEG), carrier multiplication (CM), hot carrier extraction, and intermediate band solar cell must first be replicated on the PV device itself, which involves a junction barrier. Currently, the III-V multijunction cells are the only third-generation devices to practically exceed the maximum efficiency of a silicon cell.
Multijunction cells exploit the fact that efficient solar-to-electrical energy capture occurs without thermalization and absorption issues when the light wavelength is identical to the bandgap. Different materials, typically III-V materials, are used with bandgaps varying from high to low as one moves from the top to the bottom of the device.
III-V materials show direct bandgaps in many cases. They are fabricated efficiently and when used for thinner cells, use less material. Low current matching and high material cost may limit their use, as with the Germanium/GaAs/InGaP cell.
Other methods such as inverted stacks, wafer bonding, dilute nitride and metamorphic growth, are being explored to achieve optimal multijunction cell architecture.
Ultrathin cells reduce the cost and time of production by epitaxial growth. This also caters to the complex arrangements required for current concepts, which would make it difficult to grow cells once the crystallinity exceeds 100 nm. Ultrathin cells use only enough to absorb most incident photons and generate excitons.
In the case of III-V semiconductors, the required thickness is only a few microns. Because of higher absorption in this limited thickness, efficiency is higher.
In intermediate band solar cells (IBSCs) the two-photon absorption and other similarly non-linear processes are also enhanced, while recombination losses are limited.
Hot-carrier solar cells (HCSCs) must be thin to minimize extraction time and keep thermalization low. However, numerous practical problems remain even at the experimental stage.
Tandem cell architecture consists of two distinct PV devices which soak up the light not absorbed by the other, having different band gaps. This is designed to maximize efficiency.
A perovskite is any material that has a crystal structure identical to that of calcium titanium oxide, (XIIA2+VIB4+X2−3) with the oxygen in the edge center.
Perovskite PV devices are cheap and easy to produce, have many potential structures and can use a diversity of materials. They have very good optoelectronic properties and are gaining better power conversion efficiencies. However, they show hysteresis in I-V curves, are unstable on exposure to moisture, light, and heat, offer environmental toxicity concerns and thus need more work on these aspects before they can become viable for broader application. Passivation of the interface with careful engineering of the morphology may help to overcome these issues.
Nanoparticles in PV
Nanoparticles offer customization of the band gap since varying shapes and sizes of nanoparticles absorb light across the spectrum. Control of production, better impact ionization, and multiple charge carrier generation from one photon of light are other theoretical advantages.
While an efficiency of 44 percent has been reported in the laboratory, the applicability of such materials to real PV devices is still to be proved.
Organic, Dye-Sensitized and Hybrid Cells
Organic PV (OPV) and dye-sensitized cells are being intensively researched. These depend on one or more organic molecules or semiconductors like PCBM (phenyl-C61-butyric acid methyl ester) for the PV effect.
Dye-sensitized solar cells (DSCs) are easy to produce at low cost and have high theoretical efficiency. In hybrid cells, organic or metallo-organic dyes are used.
Both DSCs and hybrid cells are tunable by using different dyes, but long-wavelength photon capture remains a difficult task as such dyes are difficult and expensive to create. Toxicity and aging remain vexing issues which limit the development of organic PV cells.
Problems with OPV include degradation reactions, instability and lowered performance. Lower electron mobility, poor electrode contact, and defects in the material are responsible for low efficiency.
Quantum Dot Solar Cells
Printable colloidal quantum dot solar cells exploit the smallness of the material, which causes the quantum confinement effect to become active, leading to different optoelectronic properties like size-dependent emission and optical absorption bands.
Specialized polymer- and fullerene-based cells are being explored. However, material changes affect the morphology and hence the efficiency. To combat this, cross-linkable molecules or polymers are being studied to prevent any shift in morphology.
Material aging and deterioration on exposure to light are other issues which make the technology inapplicable on a broad scale. Both structure materials and intrinsic stability are involved in this aspect.
Life Cycle Issues
The life cycle may also offer limiting constraints caused by the toxicity and recycling potential of the materials, including supply chain and environmental problems.
Once the volume of application in the power sector becomes large enough to increase the number and variety of situations of PV use, it becomes a challenge to integrate the novel PV system to the global power scenario and to store the generated energy, along with the architectural and aesthetic aspects.
Efficiencies of most PV cells are still substandard in practice, though theoretical energy conversions limits are high for PV devices.
In conclusion, major issues remain to be resolved in the production of third-generation PV devices beyond current bulk and thin film semiconductors. At present, reduction in the size and improvements in manufacturing processes of thin-film PV cells are responsible for the continuing attractiveness of solar cells for power generation.
Sources and Further Reading
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