Solar cell

Solar cell applications can be broadly classified into two types (a) photolytic or photoas-sisted conversion including the oxidation of water to H2 and O2 or decomposition of 

Correlation of optical and electrochemical band gaps for fractions I-IV 

A refers to the peak separation between A1 and Q. Reprinted with permission from reference (138). Copyright 2(X)1, American Chemical Society. 

The surface chemistty of semiconductors in sensor applications has been reviewed by Ellis and co-workers (144). Here, the basic principles behind the use of semiconductor 

The depletion region of the semiconductor is not only an insulating layer but also relatively non-emissive and hence commonly referred to as a dead layer . If we oversimplify the semiconductor interface into a near-surface non-emissive zone and an underlying emissive zone, then the adsorption-induced PL behavior is quantitatively related to changes in the thickness of the dead layer according to 

The solar photovoltaic market has been an area of high growth for the past nine years culminating in a 27% increase in 2005 when the world solar cell market was estimated to be worth around five billion dollars. The world s major solar cell producers include BP Solar, Deutsche Cell/SolarWorld, Isofoton, Kyocera, Mitsubishi Electric, Q-Cell, RWE Short, Sanyo, Sharp, Shell Solar and Suntech Power. It is significant that the world s major oil companies are heavily involved in solar power. In a recent statement Norm Taffe, the Executive Vice-President of the Consumer and Computation Division of Suntech Power s Cypress Semiconductor parent company, predicted that solar power could be competitive with mainstream electricity generation in five years time. 

BP has reported that its solar panels currently produce more thn 100 MW of energy in the US, Spain, India and Australia. This figure is set to double by the end of 2006. The group s BP Solar subsidiary is forecast to generate revenues of 1 billion by 2008. 

In an interesting application, which also involves photovoltaic nanotechnology, Konarka Technologies has developed a form of photovoltaic plastic sheeting, incorporating nanomaterials, which is able 

The use of flexible solar panels on clothing is also the subject of a three nation European development project called H-Alpha Solar whose partners include Akzo Nobel, which has established a pilot plant to produce rolls of silicon cells which are 40 cm wide. Whilst of a similar construction to a conventional solar cell, this type is only 1 pm thick as a result of depositing polymorphous silicon at high pressures and temperatures. 

The largest research project in the history of solar energy is targeted towards the creation of very high efficiency solar cells with reasonable manufacturing costs and efficiencies in excess of 50%. The project is being led by the University of Delaware with snpport from the US Defense Advanced Advanced Research Projects Agency. 

Solar cells, or photovoltaic devices, have been studied for many years [3], Most of the current work is focused on dye-sensitized nanocrystalline solar cells. These provide a technical and economically viable alternative to present-day photovoltaic devices. In contrast to conventional systems, in which the semiconductor assumes both the task of light absorption and charge carrier transport, the two functions are separated in dye-sensitized nanocrystalline solar cells [54] (cf. OPCs). Light is absorbed by the dye sensitizer, which is anchored to the surface of a wide-band-gap semiconductor. Charge separation takes place at the interface via photoinduced electron injection from the dye into the conduction band of the 

The nanocrystalline solids are metal oxides, especially titanium dioxide [54-58], Various dyes are used. Transition metal complexes such as (65) and (66) have broad absorption bands and allow the harvesting of a large fraction of sunlight [54,58], Fluorescent dyes are also used, such as Eosin-Y (67) [57], Dye-sensitized nanocrystalline solar cells are now giving efficiencies in excess of 10% [54,58], compared to just 1 % ten years ago [3], 

Solar cells can be used as electric windows to generate low-cost electricity at peak times [59], 

Solar cells are photodiodes optimized for maximum conversion of photons to electrical current. These are optimized for either terrestrial-based sun or atmosphere-free exposure. Two basic types of ceils are the flat panel (large area) and the concentrator (few mm ). Concentrator cells employ mirrors or lenses to concentrate the sunlight by a factor of 100 to 500 X. 

Recently, interest also has developed in using solar cells optimized for longer wavelengths to be used with conventional nuclear or gas-burning thermal engines. Such cells are known as Thermal Photovoltaics (TPVs). Most research has focused on anti- 

Organic solar cells exhibit tremendous potential in the world s energy strategy due to their predominant advantages such as low cost, light weight, and large-area fabrication on flexible substrates (121). 

Small molecule semiconductors for bulk heterojunction organic solar cells are attractive because of their advantages over their polymer counterparts, which include well-defined molecular structure, definite molecular weight, and high purity without batch-to-batch variations. 

The use of solar cells is rapidly expanding because they provide a sustainable energy resource. Solar cells can t5rpically be categorized into two types based on the light absorbing material used, i.e., bulk or wafer-based solar cells and thin film solar cells. 

Wafer based solar cells often comprise a series of about 180 and about 240 jim thick self-supporting wafers that are soldered together. 

Such a panel of solar cells is encapsulated by pol5rmeric encap-sulants to form a solar cell assembly. This assembly may be further sandwiched between two protective outer layers to form a weather resistant module. The protective outer layers are formed from sufficiently transparent material to allow photons to reach the solar cells. In modules, where PVB is used as the encapsulant material, it has been foimd that the PVB tends to discolor over time, when in contact with an oxidizable metal component. 

Formulations that contain a chelating agent and an UV absorber have been developed to overcome these difficulties (33). Suitable chelating agents and an ultraviolet absorbers are listed in Table 2.3. 

Ethylenediamine tetraacetic acid (EDTA) is preferably used as chelating agent. EDTA effects a greatly reduced yellowness index. 

A schematic drawing of a prototype solar cell containing doped 

Schematic of the structure of a typical solar cell. Electrons and holes created by the photons are swept by the internal field in the depletion region toward the negative and positive electrodes respectively, thus creating a source of current flow. 

Equivalent drcuit of a solar cell as a current source in parallel with a forward-biased diode. 

G is the rate per volume at which photons produce electron-hole pairs Lp and Lh are the diffusion lengths of the minority carriers. 

In the absence of a load, the photocurrent increases the voltage across the cell until it is balanced by the current flowing through the forward-biased diode. The open circuit voltage 

The short-circuit current is obtained by setting y = 0 in Equation 21.22, which gives 

The efficiency of the amorphous silicon solar cell has reached about 8%, and the cost of the electricity generated is estimated to become comparable with that by fossil fuels in near future. In spite of this situation, the production of a light-weighing 

Ink formulations that can be used for the fabrication of solar cells have been described in detail (37). These are a silver back electrode ink, a zinc oxide nanoparticle ink, a polymeric ink from poly(3-hex-ylthiophen-2,5-diyl) and a fuUerene derivative, a PEDOT-KS, and a silver grid front electrode ink formulation. 

The inlqet printer used for deposition has several control mechanisms, such as pressure control, temperature control, drop observation camera, and others, governed by a computer. Details of the equipment have been given (38). 

It has been found that these solar devices prepared by inkjet printing show a comparable power conversion efficiency to those fabricated by spin-coating techniques (38). 

Organic solar cells free from indium tin oxide have been produced with inlqet printed current collecting grids (39). These grids are sintered by a thermal treatment or by photonic flash sintering, which is a faster alternative method for production. 

The electrical potentials and the electric currents in the devices with different sintering conditions have been modeled. 

Alloys are also employed in multijunction amorphous (Si,Ge,C)-based solar cells (see Chapter 8) and other devices. The most popular solar cells, however, are based on pure elemental Si crystals. These are not the highest performance devices but are very easy to manufacture and produce the largest amount of power per unit cost. As with many technologies, it is hard to beat Si. 

The direct conversion of light energy to electrical energy can be accomplished in photovoltaic cells, also called solar cells. These consist basically of two layers of silicon, Si, one doped with about 1 atom per million of arsenic. As, and the other doped with about 1 atom per million of boron, B. To understand what happens, consider the Lewis symbols of the three elements involved  

Photovoltaic solar power conversion was the first major application proposed for a-Si H and to date is the largest in production. The first devices were reported by Carlson and Wronski in 1976 and had an efficiency of only 2-3%. Some of the early devices were Schottky barrier cells, but were quickly discarded in favour of p-i-n cells. Since the first report, there has been a remarkable increase in the efficiency of the cells, increasing by roughly 1 % conversion efficiency per year, to a present value of 14%, as is shown in Fig. 10.17. The increase has resulted from a variety of innovations in the design, materials, and structure of the cells. The electronic properties of the solar cell are described next and then these innovations are outlined more or less in the order in which they occurred. 

The short circuit current is the product of the photon flux of the incident solar spectrum, S(X), and the wavelength-dependent collection efficiency, Q(X) 

The function Q(X) is illustrated in Fig. 10.4. The atmospheric solar spectrum peaks at 5000 A, drops rapidly below 4000 A but extends far into the infra-red. There are collection losses in the solar cells at both ends of the visible spectrum. Most of the loss is at long wavelength where the collection efficiency is imavoidably reduced by the optical absorption edge and by the limited thickness of the cell. On the short wavelength side of the spectrum there are collection losses due to 

The open circuit voltage is related to the built-in potential, and to the electrical quality of the junction. varies with the op erating voltage, V, of the cell and is given by (see Fig. 10.1), 

1 is also related to the current-voltage characteristics of the p-i-n device. The illumination generates electron-hole pairs which result in a reverse current equal to A simple model assumes that as the operating voltage increases, the forward current offsets the photocurrent and that these balance at V. Thus from the diode characteristics of Eqs. (9.12) and (9.14), 

These are essentially large area semicondnctor diodes that absorb sunlight to generate free electrical charges. The photovoltaic effect relies on the in-built electric field that is produced at the junction between two dissimilar semiconducting layers the size of this field determines the voltage that may be produced by the solar cell and it is 

Brownian motion random motion of small particles, such as dust or smoke particles, suspended in a gas or liquid is caused by collisions of the particle with gas or solvent molecules, which transfer momentum to the particle and cause it to move 

in principle, describes the essentials of a solar cell. The following portions of the article deal with each part of the solar cell in more detail, present a quantitative description of its performance, indicate performance limitations (called the efficiency of the solar cell), and give a variety of solar cell materials with comparative performance. 

The ra-junction can be easily understood in the band model with the conduction band populated by free electrons and the valence band populated by free holes. Without fight, these carriers are created in thermodynamic equihbrium by donors and acceptors respectively. Mathematically their concentration is given by the Fermi-function (Eq.l) 

The photovoltaic characteristic is consequently given by (see, e.g., Fahrenbruch and Bube 1983)  

The net carrier transport in a solar cell can be pictured as shown in figure 5, with light (hv) coming from the left and generating (go) electron hole pairs in the p-type front layer (of thickness d). The electrons move towards and through the jtmction, the holes in opposite direction. When both are shown as electron current (the arrow head is inverted for holes) one visualizes the continuity of the carrier flow and its btiilding up with increasing depth of the front layer. 

FIGURE 16.13 Simplified version of a so-called Sanyo heterojnnction with 22% efficiency. 

Solar cells of this type can be used on the roof of a house. Coupled in a series, a high voltage over the end electrodes may be obtained. They are comparatively cheap to produce. In the future, it may become possible to harvest sunlight in deserts— where there is no photosynthetic activity anyway—on a very large scale. 

In a two-layer organic photovoltaic cell based on PMT/vacuum-evaporated rhodamine B (RB), the dedoping of PMT improves the open-circuit photovolt- 

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While hopes are high, heterogeneous photochemical systems seem not yet to have found major practical application. The photovoltaic cell or solar cell is the only system with important (although specialized) commercial use (see Ref. 343). 

Kamat P V 1993 Photochemistry on non-reaotive and reaotive (semioonduotor) surfaoes Chem. Rev. 93 267 Gerisoher H 1990 The impaot of semioonduotors on the oonoepts of eleotroohemistry Eleotroohim. Aota 35 1677 Chandra S 1985 Photoeleotroohemioal Solar Cells (New York Gordon and Breaoh)... 

Hyperpure silicon can be doped with boron, gallium, phosphorus, or arsenic to produce silicon for use in transistors, solar cells, rectifiers, and other solid-state devices which are used extensively in the electronics and space-age industries. 

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

As described in the chapter on band structures, these calculations reproduce the electronic structure of inhnite solids. This is important for a number of types of studies, such as modeling compounds for use in solar cells, in which it is important to know whether the band gap is a direct or indirect gap. Band structure calculations are ideal for modeling an inhnite regular crystal, but not for modeling surface chemistry or defect sites. 

SILICON AND SILICON ALLOYS - PURE SILICON] (Vol 21) -in solar cells PHOTOVOLTAIC CELLS] (Vol 18)... 

It is used as a fluorinating reagent in semiconductor doping, to synthesi2e some hexafluoroarsenate compounds, and in the manufacture of graphite intercalated compounds (10) (see Semiconductors). AsF has been used to achieve >8% total area simulated air-mass 1 power conversion efficiencies in Si p-n junction solar cells (11) (see Solarenergy). It is commercially produced, but usage is estimated to be less than 100 kg/yr. 

Germane is used, along with silane, SiH, to make amorphous or crystalline siUcon solar cells having an extended solar energy absorption range to increase conversion efficiency. 

Mercury Telluride. Compounds of mercury with tellurium have gained importance as semiconductors with appHcations in infrared detection (9) and solar cells (10). The ratio of the components is varied, and other elements such as cadmium, zinc, and indium are added to modify the electronic characteristics. 

A photovoltaic (PV) solar power system is a complete electrical source that uses solar cells to directly convert light energy into electricity. The system can be self-contained and completely autonomous or it can work in tandem with other conventional fuel-based sources of power to offer robust power availabihty. 

Solar cells have been used extensively and successfully to power sateUites in space since the late 1950s, where their high power-to-weight ratio and demonstrated rehabiUty are especially desirable characteristics. On earth, where electrical systems typically provide large amounts of power at reasonable costs, three principal technical limitations have thus far impeded the widespread use of photovoltaic products solar cells are expensive, sunlight has a relatively low power density, and commercially available solar cells convert sunlight to electricity with limited efficiency. Clearly, terrestrial solar cells must be reasonably efficient, affordable, and durable. International efforts are dedicated to obtaining such devices, and a number of these activities have been reviewed (1). 

When sunlight falls on a p—n junction solar cell while it is short-circuited, the magnitude of remains essentially the same as it was in darkness. Because the diffusion of majority current only varies with lA, the majority current does not change. However, additional minority carriers are formed by... 

Under both short-circuit and open-circuit conditions, a solar cell produces no electric power, the power is consumed internally in the cell and is dissipated as heat. When a resistive load is connected to a cell in sunlight, a photogenerated voltage, F, is induced across the load and a current flows through it. The existence of requites that the flow of majority carriers be reduced from that in the open-circuit condition there must be a higher battier potential than in the open-circuit case (Fig. 2d). This higher barrier potential (V6 — ) indicates a smaller reduction from Since the photogenerated... 

The photogenerated current is in the same direction as /, but is always less than because the battier potential under load conditions is always less than F, which results in a larger flow of majority carriers than that in a short-circuited cell. Thus, when a solar cell is under load, the current and voltage are always less than and lU, respectively this condition is the curve-factor loss. Depending on the characteristics of the particularp—n junction and on the cell operating conditions, there is an optimal load resistance that maximizes the power output of the cell, ie, the product of its current and voltage. 

When the temperature of a solar cell rises, cell conversion efficiency decreases because the additional thermal energy increases the thermally generated minority (dark-drift) current. This increase in dark-drift current is balanced in the cell by lowering the built-in barrier potential, lU, to boost the majority diffusion current. The drop in F causes a decrease in and F. Therefore, a cell s output, ie, the product of F and decreases with increasing cell temperature. is less sensitive to temperature changes than F and actually increases with temperature. 

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