How solar cells work

Semiconductor that has been doped to produce two different regions separated by a p-n junction. Across this junction,

Electrons and holes – are able to cross. In doing so, they deplete the region from which they came and transfer their charge to the new region.

Migration of charge results in a potential gradient

sunlight strikes a solar cell, atoms are bombarded with particles of light called photons, and give up electrons.

When an electron is kicked out of an atom, it leaves behind a hole, which has an equal and opposite (positive) charge.

If either carrier wanders across the junction, the field and the nature of the semiconductor material discourage it from recrossing.

A proportion of carriers that cross the junction can be harvested by completing a circuit from a grid on the cell's surface to a collector on the backplane. In the cell, the light "pumps" electrons out one side of the cell, through the circuit, and back to the other side, energizing any electrical device that is connected along the way.

The current generated in the semiconductor is extracted by contacts at the top and bottom of the cell. The top contact structure, which must allow light to pass

Simplified operation of a solar cell. Image source: US Dept of Energy

through, is made of thin, widely-spaced metal strips (usually called fingers) that supply current to a larger bus bar. The cell is covered with a thin layer of dielectric material – the anti-reflection coating – to minimize light reflection from the top surface.

Characteristics of a solar cell

The usable voltage that a solar cell produces depends on what semiconductor material it's made from. In the case of silicon-based cells, the output is approximately 0.5 V. Although the current increases with increasing luminosity, the terminal voltage is only weakly dependent on the amount of light falling on the cell. A 100 cm2 silicon cell generates a maximum current of about 2 A when radiated by 1000 W/m2.

Different types of solar cell

There are three main types of solar cells, which are distinguished by the type of crystal used in them. They are monocrystalline, polycrystalline, and amorphous. To produce a monocrystalline silicon cell, absolutely pure semiconducting material is necessary. Monocrystalline rods are extracted from melted silicon and then sawed into thin plates. This production process guarantees a relatively high level of efficiency.

The production of polycrystalline cells is more cost-efficient. In this process, liquid silicon is poured into blocks that are subsequently sawed into plates. During solidification of the material, crystal structures of varying sizes are formed, at whose borders defects emerge. As a result of this crystal defect, the solar cell is less efficient.

If a silicon film is deposited on glass or another substrate material, the result is a so-called amorphous or thin-layer cell. The layer thickness amounts to less than 1µm – the thickness of a human hair for comparison is 50-100 µm. The production costs of this type are lower because of the lower material costs. However, the efficiency of amorphous cells is much lower than that of the other two cell types. As a result, they are used mainly in low power equipment, such as watches and pocket calculators, or as facade elements.

From cells to modules

In order to provide suitable voltages and outputs for different applications, solar cells are connected together to form larger units. Cells connected in series have a higher voltage, while those connected in parallel produce more current. The interconnected solar cells are usually embedded in transparent ethylene vinyl

acetate, fitted with an aluminum or stainless steel frame, and covered with transparent glass on the front side to make a solar module.

Typical peak power ratings of such solar modules range from 10 W to 100 W. The characteristic data refer to the standard test conditions of 1000 W/m2 solar radiation at a cell temperature of 25° C (77° F). The manufacturer's standard warranty of 10 or more years is quite long and shows the high quality standards and life expectancy of today's products.

Monocrystalline vs Polycrystalline (Multicrystalline)

Mono crystalline cells are cut from a chunk of silicon that has been grown from a single

crystal.

high efficiencies - which are typically around 15%. 

A polycrystalline cell is cut from multifaceted silicon crystal.

More surface area is required due to inherent flaws

with average efficiencies of around 12%.

Market Share: 78 - 80%

MaterialEfficiency in lab (%)

Efficiency of production cell (%)

monocrystalline silicon

about 24 14-17

polycrystalline silicon

about 18 13-15

amorphous silicon about 13 5-7

Thin film solar panels

It's applied in such a way that flexible panels can be made

Thin film panels are also less efficient that polycrystalline and monocrystalline panels.

Larger surface area is required. Again, performance in thin film technology is constantly

improving in the area of efficiency. Given the processes to create thin film, 

Cheaper alternatives to silicon can also be used, such as cadmium telluride; although

cadmium is frowned upon by many as it's a heavy metal.

Market Share: 18 - 20%

The most common materials -Amorphous silicon polycrystalline materials: cadmium

telluride (CdTe) and copper indium (gallium) diselenide (CIS or CIGS).

8% efficiency at 1cm2 scale

Each c-Si cell generates about 0.5V, so 36 cells are usually soldered together in series to produce a module with an output to charge a 12V battery.

Thin Film Photovoltaics Advantages over Crystalline Silicon Photovoltaics .

Lower cost of production. Lower production facility cost per watt -

CapEx

Uses as little as 1/500 of the amount used in standard silicon cells

Lower energy payback – amount of time until the product produces more energy than was utilized in its manufacture.

Produces more power/watt

Superior performance in hot and cloudy climates

Integrates seemlessly in homes and buildings

Developing Technologies: Concentrators

More efficiently under concentrated light.

Using mirrors or lenses to focus light and use heat sinks, or active cooling of the cells, to dissipate the large amount of heat that is generated.

Concentrator systems require direct sunlight (clear skies) and will not operate under cloudy conditions.

Follow the sun's path using single-axis tracking. Two-axis tracking is sometimes used for in change in season.

Not yet achieved widespread application in photovoltaic, but widely used in solar thermal electricity generation technology where the generated heat is used to power a turbine.

Expected future efficiencies are nearly 50%

PV is advantageous because the solar collector is less expensive than an equivalent area of solar cells.

targeted to be priced well under 3 USD/Watt

CPV could reach grid parity in 2011.

Developing Technologies: Electrochemical PV cells

Electrochemical solar cells have their active component in a liquid phase.

They use a dye sensitizer to absorb the light and create electron-hole pairs in a nanocrystalline titanium dioxide semiconductor layer. This is sandwiched in between a tin oxide coated glass sheet (the front contact of the cell) and a rear carbon contact layer, with a glass or foil backing sheet.

lower manufacturing costs in the future because of their simplicity and use of cheap materials.

Prototypes of small devices powered by dye-sensitised nanocrystalline electrochemical PV cells are now appearing (120cm2 cells with an efficiency of 7%). 

Polymer solar cells

Polymer solar cells are a type of flexible solar cell. They can come in many forms including: organic solar cell (also called plastic solar cell).

That produce electricity from sunlight using polymers.

thin-film semiconductors that can be deposited on different types of polymers to create solar cells.

polymer solar cells are lightweight (which is important for small autonomous sensors)

potentially disposable and inexpensive to fabricate, flexible, and customizable on the molecular level, and they have lower potential for negative environmental impact..

The disadvantages of polymer solar cells are also serious: they offer about 1/3 of the efficiency of hard materials, and they are relatively unstable toward photochemical degradation. For these reasons, despite continuing advances in semiconducting polymers, the vast majority of solar cells rely on inorganic materials.

Device physics

Organic photovoltaics are made of electron donor and electron acceptor materials rather than semiconductor p-n junctions.

The molecules forming the electron donor region of organic PV cells, where exciting electron-hole pairs are generated,

These electrons can be excited by light in or near the visible part of the spectrum from the molecule's highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO), denoted by a electron transition.

The energy bandgap between these orbitals determines which wavelength of light can be absorbed.

Excitons(electrostatic force b/w electrons and holes) in organic photovoltaics are strongly bound with an energy between 0.1 and 1.4 eV.

This strong binding occurs because electronic wavefunctions in organic molecules are more localized, and electrostatic attraction can thus keep the electron and hole together as an exciton.

Organic photovoltaics can be fabricated with an active polymer and a fullerene-based electron acceptor. The illumination of this system by visible light leads to electron

transfer from the polymer chain to a fullerene molecule. As a result, fullerene becomes an ion-radical are highly mobile along the length of the polymer chain and can diffuse away.

Architectures.

The simplest architecture that may be used for an organic PV device is a planar heterojunction.

A film of active polymer (donor) and a film of electron acceptor are sandwiched between contacts in a planar configuration.

Excitons created in the donor region may diffuse to the junction and separate, with the hole remaining behind and the electron passing into the acceptor.

planar heterojunctions are inherently inefficient; because charge carriers have diffusion lengths of just 3-10 nm in typical organic semiconductors, planar cells must be thin to enable successful diffusion to contacts, but the thinner the cell, the less light it can absorb.

Bulk heterojunctions (BHJs) address this shortcoming. the electron donor and acceptor materials are blended together and cast as a mixture that then phase-separates.

Regions of each material in the device are separated by only several nanometers, a distance optimized for carrier diffusion. Although devices based on BHJs are a significant improvement over planar designs, BHJs require sensitive control over materials morphology on the nanoscale. A great number of variables, including choice of materials, solvents, and the donor-acceptor weight ratio can dramatically affect the BHJ structure that results. These factors can make rationally optimizing BHJs difficult.

The next logical step beyond BHJs are ordered nanomaterials for solar cells, or ordered heterojunctions (OHJs). This paradigm eliminates much of the variability associated with BHJs. OHJs are generally hybrids of ordered inorganic materials and organic active regions. For example, a photovoltaic polymer can be deposited into pores in a ceramic such as TiO2. Holes still must diffuse along the length of the pore through the polymer to a contact, so OHJs do have thickness limitations. Mitigating the hole mobility bottleneck will thus be key to further enhancing OHJ device performance, but controlling morphology inside the confines of the pores is challenging.

Engineers at the University of California, San Diego (UCSD) have employed "nanowires" to boost the efficiency of organic solar cells [2].

Conclusion

The present efficiency of polymer solar cells lies near 10 percent, much below the value for silicon cells.

Polymer solar cells also suffer from environmental degradation. Good protective coatings are still to be developed.

Work remains to be done to further improve their performance.

Novel molecular chemistries and materials offer hope for revolutionary, rather than evolutionary, breakthroughs in device efficiencies in the future.

Current commercial status

Polymer solar cells are not generally produced commercially today. One exception is the company Konarka Technologies, which in 2008 opened a factory with

the capacity to produce a gigawatt's worth of polymer-fullerene solar cells each year.

The initial cells from the factory are 3-5% efficient, and only last a couple years, but the company has stated that it would eventually be able to improve both the efficiency and durability.

The company expects to initially sell the cells in for number of niche applications: For example, in laptop-recharging briefcases, put into tents, umbrellas, and awnings, and as window tinting (since the cells can be made semi-transparent).