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5.2.2 Short description

Photovoltaic devices are based on semiconductors. The work principle is the following: the absorption of a photon with energy greater than the semiconductor bandgap promotes an electron across the bandgap into the conduction band, leaving behind a vacancy, or "hole" which acts like a unipositive charge in the valence band. If this electron-hole pair or exciton is formed in the vicinity of a space charge, such as at a p-n junction, charge separation takes place due to the electric field. With the proper provisions for collecting the opposite charges, this generates a potential that can drive an electric circuit.

The range of semiconductors suitable for solar cells devices is the following:

• Silicon (Si): single-crystalline Si, multicrystalline Si, and amorphous Si

• Polycrystalline thin films: copper indium diselenide (CIS), cadmium telluride (CdTe), and thin-film silicon

• Single-crystalline thin films: high-efficiency material such as gallium arsenide (GaAs)

• Dye sensitized thin films

• Organic/Polymer Solar Cell

Silicon based solar cell

These bulk technologies are often referred to as wafer-based manufacturing. Silicon, used to make some of the earliest photovoltaic (PV) devices, is still the most popular material for solar cells. One major advantage is that silicon is the second-most abundant element in the Earth's crust. However, to be useful as a semiconductor material in solar cells, silicon must be refined to a purity of almost 100%.

In single-crystal silicon, the molecular structure is uniform, because the entire structure is grown from the same crystal. This uniformity is ideal for transferring electrons efficiently through the material. To make an effective PV cell, however, silicon has to be doped to create the p-n junction.

Semicrystalline silicon, in contrast, consists of several smaller crystals or grains, which introduce boundaries. These boundaries impede the flow of electrons and encourage them to recombine with holes to reduce the power output of the solar cell. However, semicrystalline silicon is much less expensive to produce than single-crystalline silicon. So researchers are working on other ways to minimize the effects of grain boundaries.

Three major types of silicon solar cell materials appear:

• Single-Crystal Silicon: to create silicon in a single-crystal state, high-purity silicon has to be melted. It has to be then reformed or solidified very slowly in contact with a single crystal "seed". The silicon adapts to the pattern of the single-crystal seed as it cools and gradually solidifies. This process is growing a new rod of single-crystal silicon out of molten silicon. \\\"Ribbon-growth" techniques can also be utilised.

• Multicrystalline Silicon: the most popular commercial methods involve a casting process in which molten silicon is directly cast into a mold and allowed to solidify into an ingot. The starting material can be a refined lower-grade silicon, rather that the higher-grade semiconductor grade required for single-crystal material. The cooling rate is one factor that determines the final size of crystals in the ingot and the distribution of impurities. The mold is usually square, producing an ingot that can be cut and sliced into square cells that fit more compactly into a PV module.

• Amorphous Silicon: in this material, atoms are not arranged in any particular order. They do not form crystalline structures at all, and they contain large numbers of structural and bonding defects, and then display lower efficiency. But they have some economic advantages that make them appealing for use in photovoltaic (PV) systems. It absorbs solar radiation 40 times more efficiently than single-crystal silicon, so a film only about 1 µm thick can absorb 90% of the usable light energy shining on it. This is one of the chief reasons that amorphous silicon could reduce the cost of photovoltaics. Other economic advantages are that it can be produced at lower temperatures and can be deposited on low-cost substrates such as plastic, glass, and metal. Amorphous silicon is the leading thin-film PV material.

Polycrystalline thin film solar cell

The materials used in polycrystalline thin-film cells have properties that are different from those of silicon. It works better to create the electric field with a heterojunction interface between two different semiconductor materials.

The typical polycrystalline thin film has a very thin layer on top called the "window" layer, which role is to absorb light energy from only the high-energy end of the spectrum. It must be thin enough and have a wide enough bandgap to let all available light through the interface to the absorbing layer. The absorbing layer under the window, usually doped p-type must have a high absorptivity for high current and a suitable band gap to provide a good voltage.

The various thin-film technologies currently being developed reduce the amount (or mass) of light absorbing material required in creating a solar cell. This can lead to reduced processing costs but also tends to reduce energy conversion efficiency, although many multi-layer thin films have efficiencies above those of bulk silicon wafers.

CIS/CIGS

Chalcopyrite are semi-conductors materials based on elements from I-III-IV groups [(Cu, Ag)(Al, Ga, In)(S, Se, Te)₂]. These semiconductors are especially attractive for thin film solar cell applications because of their high optical absorption coefficients and versatile optical and electrical characteristics which can in principle be manipulated and tuned for a specific need in a given device. CIS films achieved greater than 14% efficiency. However, manufacturing costs of CIS solar cells are yet high when compared with amorphous silicon solar cells but continuing work is leading to more cost-effective production processes.

Gallium can be substituted for some of the indium in CIS. The material is then a solid mixture of the semiconductors CuInSe2 and CuGaSe2 (CuInxGa(1-x)Se₂) called CIGS. The use of gallium increases the optical bandgap of the CIGS layer as compared to pure CIS, thus increasing the open-circuit voltage. In another point of view, gallium is added to replace as much indium as possible due to gallium's relative availability compared to indium one. Indeed, approximately 70% of indium currently produced is used by the flat-screen monitor industry. Some investors in solar technology worry that production of CIGS cells will be limited by the availability of indium.

The most common material for the top or window layer in CIS devices is cadmium sulfide (CdS), although zinc is sometimes added to improve transparency.

CdTe

Cadmium telluride is another prominent polycrystalline thin-film material. With a nearly ideal bandgap of 1.44 eV, CdTe is also an efficient light-absorbing material for thin-film solar cells. Although CdTe is most often used in PV devices without being alloyed, it is easily alloyed with zinc, mercury, and a few other elements to vary its properties. Like CIS, films of CdTe can be manufactured using low-cost techniques.

The best CdTe cells employ a heterojunction interface, with cadmium sulfide (CdS) acting as a thin window layer. Tin oxide is used as a transparent conducting oxide and antireflection coating. One problem with CdTe is that p-type CdTe films tend to be highly resistive electrically, which leads to large internal resistance losses. A solution is to allow the CdTe layer to be intrinsic, and add a layer of p-type zinc telluride (ZnTe) between the CdTe and the back electrical contact. Although the n-type CdS and the p-type ZnTe are separated, they still form an electrical field that extends right through the intrinsic CdTe. Compared to other thin-film materials, CdTe is easier to deposit and more suitable for large-scale production.

Single crystalline thin film solar cell

GaAs is especially suitable for use in multijunction and high-efficiency solar cells, for several reasons:

GaAs bandgap is 1.43 eV, nearly ideal for single-junction solar cells.

GaAs has an absorptivity so high it requires a cell only a few microns thick to absorb sunlight. (Crystalline silicon requires a layer 100 microns or more thick.)

Unlike silicon cells, GaAs cells are relatively insensitive to heat. Cell temperatures can often be quite high, especially in concentrator applications.

Alloys made from GaAs and aluminum, phosphorus, antimony, or indium have characteristics that are complementary to those of gallium arsenide, allowing great flexibility in cell design.

GaAs is highly resistant to radiation damage. This, along with its high efficiency, makes GaAs desirable for space applications.

One of the greatest advantages of gallium arsenide and its alloys as PV cell materials is that it is amenable to a wide range of designs. A cell with a GaAs base can have several layers of slightly different compositions; this allows a cell designer to precisely control the generation and collection of electrons and holes. To accomplish the same thing, silicon cells have been limited to variations in the level of doping.

This degree of control allows cell designers to push efficiencies closer and closer to theoretical levels. For example, one of the most common GaAs cell structures has a very thin window layer made of aluminum gallium arsenide. This thin layer allows electrons and holes to be created close to the electric field at the junction.

GaAs based multijunction devices are the most efficient solar cells to date, reaching a record high of 40.7% efficiency under solar concentration and laboratory conditions.

 

Tandem solar cells based on monolithic, series connected, gallium indium phosphide (GaInP), gallium arsenide GaAs, and germanium Ge p-n junctions, are seeing demand rapidly rise. A triple-junction cell, for example, may consist of the semiconductors: GaAs, Ge, and GaInP₂. Each type of semiconductor will have a characteristic band gap energy which, loosely speaking, causes it to absorb light most efficiently at a certain color, or more precisely, to absorb electromagnetic radiation over a portion of the spectrum. The semiconductors are carefully chosen to absorb nearly the entire solar spectrum, thus generating electricity from as much of the solar energy as possible.

High-efficiency multijunction cells were originally developed for special applications such as satellites and space exploration, but at present, their use in terrestrial concentrators might be the lowest cost alternative in terms of $/kWh and $/W.

Light-absorbing dyes (DSSC)

Dye-sensitized solar cells separate the two functions provided by silicon in a traditional cell design. In the dye-sensitized solar cell, the bulk of the semiconductor is used solely for charge transport, the photoelectrons are provided from a separate photosensitive dye. Charge separation occurs at the semiconductor-dye-electrolyte-interface. Those cells use titan dioxide nanoparticles doped with dye molecules.

In operation, sunlight enters the cell through the transparent top contact, striking the dye on the surface of the nanoparticles. Photons striking the dye with enough energy to be absorbed will create an excited state of the dye, from which an electron can be "injected" directly into the conduction band of the nanoparticles, and from there it moves by diffusion (as a result of an electron concentration gradient) to the clear anode on top. Meanwhile, the dye molecule has lost an electron and the molecule will decompose if another electron is not provided. The dye strips one from the iodide electrolyte below the nanoparticles, oxidizing it into triiodide. This reaction occurs quite quickly compared to the time that it takes for the injected electron to recombine with the oxidized dye molecule, preventing this recombination reaction that would effectively short-circuit the solar cell. The missing electron of triiodide is recovered by mechanically diffusing to the bottom of the cell, where the counter electrode re-introduces the electrons after flowing through the external circuit.

Organic/polymer solar cells

Organic solar cells and Polymer solar cells are built from thin films (typically 100 nm) of organic semiconductors such as polymers and small-molecule compounds like polyphenylene vinylene, copper phthalocyanine (a blue or green organic pigment) and carbon fullerenes. Energy conversion efficiencies achieved to date using conductive polymers are low compared to inorganic materials, with the highest reported efficiency of 6.5% for a tandem cell architecture. However, these cells could be beneficial for some applications where mechanical flexibility and disposability are important.

These devices differ from inorganic semiconductor solar cells in that they do not rely on the large built in electric field of a p-n junction to separate the electrons and holes created when photons are absorbed. The active region of an organic device consists of two materials, one which acts as an electron donor and the other as an acceptor. When a photon is converted into an electron hole pair, typically in the donor material, the charges tend to remain bound in the form of an exciton, and are separated when the exciton diffuses to the donor-acceptor interface. The short exciton diffusion lengths of most polymer systems tend to limit the efficiency of such devices. Nanostructured interfaces, sometimes in the form of bulk heterojunctions, can improve performance.

Typically a ruthenium metalorganic dye (Ru-centered) is used as a monolayer of light-absorbing material. The dye-sensitized solar cell depends on a mesoporous layer of nanoparticulate titanium dioxide to greatly amplify the surface area (200-300 m²/g TiO₂, as compared to approximately 10 m²/g of flat single crystal). The photogenerated electrons from the light absorbing dye are passed on to the n-type TiO₂, and the holes are passed to an electrolyte on the other side of the dye. The circuit is completed by a redox couple in the electrolyte, which can be liquid or solid. This type of cell allows a more flexible use of materials, and is typically manufactured by screen printing, with the potential for lower processing costs than those used for bulk solar cells. However, the dyes in these cells also suffer from degradation under heat and UV light, and the cell casing is difficult to seal due to the solvents used in assembly. In spite of the above, this is a popular emerging technology with some commercial impact forecast within this decade.

Advantages of dye solar cells are cheap manufacturing processes through screen printing, application possibilities even at diffuse incidence of light (e.g. for internal application) as well as the transparency and color design possibilities of the cells, which open up interesting architectural application perspectives. What is disadvantageous is that, in case of leakages, the applied chemically-reactive liquid electrolytes can reach the environment and show relatively low efficiencies of below 10%. Reproductible prototypes have been implemented and fabrication processes have been well described by Dr. Grätzel team from EPFL. [i]

Quantum Dots Solar Cells

Quantum dots are nanoscale clusters of semiconductor compounds with great optoelectronic properties, which are modifiable due to quantum physical effects in dependence of the cluster size. Applications in solar cells are interesting, since on the one hand, several electron-hole pairs per photon can be produced by quantum dots (it seems to be possible that quantum dots of lead selenide could produce up to seven electrons per photon when exposed to high-energy ultraviolet light) (TechReview). On the other hand, the absorption bands can be optimally adjusted to the wavelengths of the irradiating light. Some quantum dots indeed have the ability to collect light at multiple wavelengths. This tuning capability is control by the size of the quantum dots [ii]

On the laboratory scale, three-dimensional grids of quantum dots or even other structures like nanowires are possible which would be interesting for the application in solar cells. With such cells, conversion efficiencies of over 60% are theoretically feasible. However, the current state of research is still far from this, and up to now it has not been possible to experimentally show a functioning model of a quantum dot solar cell.

Conversion efficiencies records

Efficiencies of the different solar cells system previously described are given on the next figure.

The most efficient solar cells devices developed to date are the one based on multijunctions. Crystalline Si cells are now approaching the theoretical limit. As organic cells technology is quite new, it is difficult to predict their development but they seem promising. Concerning the dye sensitized cell, the performances are limited to the record which has only been obtained by Dr. Grätzel team.

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[i] Fabrication of thin film dye sensitized solar cells with solar to electric power conversion efficiency over 10%; S. Ito, T.N. Murakami, P. Comte, P. Liska, C. Grätzel, M.K. Nazeeruddin, M. Grätzel: Thin Solid Films 516, 4613 (2008)

[ii] Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe-TiO2 Architecture; A. Kongkanand, K. Tvrdy, K. Takechi, M. Kuno, & P.V. Kamat; Journal of the American Chemical Society 130, 4007 (2008)