A Solar Cell (The Photovoltaic Cell) Essay

A Solar Cell (The photovoltaic Cell) EssayA solar mobile ph genius or the photovoltaic cell is an electrical artifice that converts light energy photographic filmly into electrical energy. This cell when exposed to light stack arrest and support an electric time period rate without existence attached to any external voltage source. The solar cell personas the photovoltaic inwardness to produce electricity.thw word Photovoltaic comes from the Greek word meaning light, and from the word Volt, which is the the unit of electro-motive force excessively the word volt comes from the last name of the inventor of the battery (electrochemical cell), the Italian physicist Alessandro Volta. then we get the term photo-voltaic.The photovoltaic effect, in brief, is a process in which electric electric current is produced in a material upon exposure to light. The photovoltaic effect being directly related to the photoelectric effect is real a contrasting phenomenon. In photovoltaic effect, the light is incident upon the material grow the negatrons put in in the valance peal of the atom absorb energy from the light and jump to the conduction band (band theory). Now these electrons ar attracted by a positively shipd electrode and thus the circuit is completed and the light energy is converted into electric energy. On the other(a) hand, in photoelectric effect, the electrons ar ejected from a materials surface upon exposure to radiation.Photovoltaic systems are extraordinary and very useful with a huge list of advantages. The reason they are so unique is they abide no moving intermits (in the classical mechanical sense) to wear out. there are no fluids or gases (except in hybrid systems) that advise leak out. The best part about these is that they need no fuel to operate. Having a rapid response, they achieve full output instantly. These cells can operate at moderate temperatures producing no taint while producing electricity, although waste produc ts from their manufacture and toxic gases in the steadyt of catastrophic failure and disposal may be a concern. Solar cells require minuscule maintenance if properly manufactured and installed.Silicon, the second most abundant element in the earths crust can be apply to make these cells. Thus, their production is practicable on a big(p) scale with an added advantage of them being modular, permitting a wide range of solar-electric applications much(prenominal) as1) Small scale for remote applications and residential use.2) Intermediate scale for business and neighborhood supplementary authority.3) Large scale for centralized energy farms of square kilometers size.Solar cells have a sexual relationly exalted conversion efficiency giving the eminentest overall conversion efficiency from sunlight to electricity yet measured. This gives them wide power-handling capabilities, from microwatts to megawatts. Clearly, photovoltaic cells have an appealing range of characteristics. 11. 1 narrativeThe physical phenomenon responsible for converting light to electricity-the photovoltaic effect-was setoff observed in 1839 by a French physicist, Edmund Becquerel. Becquerel noned that a voltage appeared when one of cardinal identical electrodes in a weak conducting solution was illuminated. The Photovoltaic effect was first to be noticed and looked in solids, e.g. selenium, in the 1870s. However, it was in 1883 Charles Fritts built the first solid state photovoltaic cell he coated the semiconductor selenium with an extremely debase layer of gold to form the junctions. The device was except around 1% in effect(p). In 1888 Russian physicist Aleksandr Stoletov built the first photoelectric cell establish on the outer photoelectric effect discover by Heinrich Hertz earlier in 1887.The first practical photovoltaic cell was true in 1954 at Bell Laboratories by Daryl Chapin, Calvin Souther Fuller and Gerald Pearson. A diffused silicon p-n junction was used by them th is junction could reach 6% efficiency, as compared to the selenium cells in which it was difficult to reach 0.5%. At first, cells were developed for toys and other minimal uses, since the cost of the electricity they produced then was very high i.e. a cell that produces 1 watt of electrical power in smart sunlight cost about $250, compared to $2 to $3 per watt for a coal plant. 2In 1958, the U.S. Vanguard space satellite used a baseborn (less than one-watt) array of cells to power its radio. These cells posted so well that the space scientists soon realized the Photovoltaic could be a very effective power source for legion(predicate) another(prenominal) space missions. Technological development of the solar cell has been a part of the space program ever since then. Besides the space programs, another source is the transistor industry has contributed greatly to solar-cell technology. Transistors and PV cells are made from similar materials, and their workings are determined by numerous of the same physical mechanisms. A great join of research and development has been done in improving and developing the ever-useful transistor, and there has been a regular discovery of valuable information in relation to solar cells. This situation has reversed in recent times and much of the research happening in PV is affecting transistor technology.1.2 EFFICIANCY OF SOLAR CELLSToday, photovoltaic systems are fitted of transforming one kilowatt of solar energy falling on one square meter into about a hundred watts of electricity. One hundred watts can power most household appliances like television, stereo, or a lamp and so forth . In fact, on a standard basis a roof covered with solar cells facing the sun in a typical home provides about 8500-kilowatthours of electricity one-yearly, which also is almost equal to a average households annual consumption on electricity. On comparison, a present-day, 200-ton electric-arc steel furnace, demanding electricity worth 50,000 kilowatts, would for a PV power supply, require about a square kilometer of land. trusted grammatical constituents make capturing solar energy difficult. Apart from the suns low illuminating power per square meter, sunlight is discontinuous and is moved(p) by time of day, climate, pollution, and season. Power sources based on photovoltaic require either back-up from other sources or storage for times when the sun is obscured. Moreover, the cost of a photovoltaic system is very high (electricity from PV systems in 1980 cost about 20 times * that from conventional systems powered by fossil fuels). Thus, solar energy for photovoltaic conversion into electricity is abundant, inexhaustible, and somewhat yet, on the other hand it also requires special techniques to gather enough of it effectively.When sunlight is incident on the solar cell, most of the energy is lost even before it can be converted to electricity. Maximal sunlight-to-electricity conversion efficiencies for solar cells range up to 30% (and even higher for some exceedingly conglomerate cell designs), but typical efficiencies are 10%-15%. Most current work on cells is directed at enhancing efficiency while lowering cost. authoritative physical processes limit cell efficiency-some are inherent and cannot be changed many can be improved by proper design.Reflection is the first agentive role that reduces the efficiency of the cell. Normal, untreated silicon reflects 36% (or more) of the sunlight that strikes it. This would be a horrendous loss in terms of efficiency. Fortunately, there are several ship canal of treating cell surfaces to cut reflection drastically. By dint of these methods, reflection can be lowered to a quite manageable 5% or so.The second factor affecting the electricity production and then, in turn, the efficiency of the cell is the intensity of light falling on it. Now, this light can be of two types--Light that is not energetic enough to separate electrons from their atomic b onds.-Light that has extra energy beyond that needed to separate electrons from bonds.Both of the above types of light contribute in reduce the efficiency of the cell. Light entering a solar cell can-a. Go right through it.b. Become absorbed, generating heat in the form of atomic vibrations.c. set out an electron from its atomic bond, producing an electron-hole pair.d. Produce an electron-hole pair but have an excess of energy, which then becomes heat.Only (c) is a near-perfect means of transforming sunlight into electricity.Since the suns spectrum has a wide class of energies and intensities, the key is to match a material and its characteristic band gap energy with the spectrum so that the maximum amount of energy in the suns spectrum falls skillful above the characteristic energy.The third factor that reduces the efficiency of the cell is electron-hole recombination.There are two ways in which recombination of electrons and holes occurs, which can be characterized as direct a nd indirect recombination.-Direct Recombination Direct recombination is relatively rare. It happens when an electron and a hole randomly encounter each other, and the electron falls back into the hole. Thus the materials original bonds are reasserted, and the electrons and holes energies are lost as heat.-Indirect Recombination Indirect recombination can occur in many ways. (Indirect means that the electron and hole do not just depart into each other and combine-the interaction is influenced by further circumstances.) Contrary to what one might expect, indirect recombination is much more of a problem in solar cells than direct recombination.Resistance is a factor which reduced efficiency of almost all bedn electrical appliances and the solar cell is no several(predicate). Resistance losings in solar cells occur predominantly in ternion places in the bulge of the base material, in the narrow top-surface layer typical of many cells and at the interface between the cell and the el ectric strikings leading to an external circuit. Resistance losses lower voltage and enhance the chances of recombination of upsurges, reducing current. Usually it is better to highly dope silicon to reduce resistance as highly doped silicon has numerous free carriers to conduct the current.After considering the various factors discussed, we can actually look forward to see and study the construction of the solar cells with maximum possible efficiency. 3-101.3 types AND GENERATIONS of solar cellsSolar cells can be of many types as we know them. Todays modern technology has allowed us to be able to study each in detail and help with improving energy output and increasing efficiency.There are three types of solar cell-Amorphous cells,PolycrystallineMonocrystalline.Amorphous, also known as the thin-film solar cells are more commonly seen in devices like toys, calculators etc. Monocrystalline solar cells are cut from one silicon metal bar which is got from a single large silicon crys tal. Polycrystalline cells are cut from an ingot derived from many smaller crystals.Mono cells are made by growing an ingot of the silicon crystal from a smaller crystal, hence the name mono-crystalline or single-crystal. This ingot is then trimmed and sliced into wafers.In case of polycrystalline cells, molten silicon is poured into a square mould allowing it to set. Now silicon cools and sets at diametric rates, that is, the inside cools slower than the outer part and there is no seed crystal to grow the new material. This uneven cooling itself creates multiple crystals within the duck thus giving it the name of poly-crystalline or multi-crystalline. repayable to its multifaceted surface, this type of solar cell is a better performer even in wraithlike light conditions giving greater wattage even from a small surface area.Amorphous cells are made by depositing a thin piece of paper of silicon over a surface like steel. The panel we get is a single piece and individual cells a re not visible. These cells do not have a high efficiency and thus give a lesser investment for our investment.Apart from this solar cells can be divided into three generations, being 1st generation second generation 3rd generationFirst-generation cells are based on expensive silicon wafers and makeup 85% of the current commercial market. Second-generation cells are based on thin films compounds such as amorphous silicon, or copper inch selenide. The materials are relatively cheap, but research is needed to raise the efficiency of these cells if the cost of delivered power is to be reduced. Third-generation cells have shown a dramatic increase in efficiency that maintains the cost advantage of 2nd generation materials. Their design may make use of carrier multiplication, hot electron extraction, multiple junctions, sunlight concentration, or new materials.11First generation solar cellsThese are the dominant type of cells available in the commercial market. A crystalline silicon waf er is used for the production of these cells. They tend to have a large surface area and a single layer p-n junction diode. Being so widely used, these cells have their own advantages and disadvantages. On the pros side, these cells have a across-the-board spectral absorption rate and also have high carrier mobility. But these cells require expensive manufacturing technologies and also growing ingots is a very intensifier process. Another disadvantage we can usually observe in these cells is that it is relatively easy for an electron to encounter a hole and thus that leads to recombination instead of electricity production. Most of the energy from a high energy photon is usually lost as heat.12Second generation solar cellsSecond generation solar cells are mounted on glass substrates. The production costs that were plaguing first generation solar cells find some relief with the second generation. There are many companies who desire to release second-generation thin-film solar cells to the public. The material used in second generation solar cells are normally amorphous silicon, micro-crystalline silicon, cadmium telluride (CadTel) and copper indium selenide/sulfide14.We see a potential for cost advantages in this generation over crystalline silicon because of various reasons. There is a lower material use along with a couple of(prenominal)er and simpler manufacturing steps. These cells also have the perfect band gap for solar energy conversion.1314Third generation solar cellsThe third generation cells are very different from the previously discussed cells. They do not rely on a p-n junction to separate photo-generated charge carriers but are based on a silicon substrate with a coating of nanocrystals. The third generation is the future of solar cells and the cheapest of them all. They are exactly what the sun-powered industry needs for renewable and efficient power sources. As solar cell technology go ons to grow, our solar conversion efficiency will contin ue to rise and production expenditure will continue to drop. The third generation solar cells focus on reducing manufacturing cost and enhancing the performance of 2nd generation solar cell technology. Nanotechnology is one area that is being researched upon by this new generation of cells. Nanotechnology is being used to improve the basic solar cell to have improved electrical performance which also makes it more cost efficient. 15,161.4 POLYMER SOLAR CELL AND ITS DEVELOPMENTOne of the unique 3rd generation solar cells we know today is the polymeric solar cell. usually populate of an electron- or hole-blocking layer on top of an indium tin oxide (ITO) conductive glass followed by electron presenter and an electron acceptor (in the case of bulk heterojunction solar cells), a hole or electron blocking layer, and metal electrode on top.During the last 30 years the polymer solar cell has developed from an inefficient light-harvesting device with almost no lifetime to a device that ma y be introduced to the commercial market within a short span of years. Today scientists are working with a lot of dierent types of polymer solar cells and since it will be too comprehensive to deal with all of them, only one type will be treated in this report. The type of solar cell which will be treated is a polymer/fullerene bulk hetero-junction solar cell This type of polymer solar cell consist of 6 layers Glass, ITO, PEDOTPSS, active layer, calcium and aluminum.The glass serves as a supporting layer for the solar cell and the only demand glass has to fulll is that it does not absorb light in the visible area, since the solar cell uses this light to generate power. Other and more exible types of supporting layers, like transparent polymers, can also be used. The focus of this report will not lie on the supporting layer and therefore the use of other types of supporting layers will not be discussed any further.18ITO (indium tin oxide) and aluminum serves as the electrodes in the solar cell. Beyond that, the ITO and Aluminium are also used to generate a underlying electric held caused by the difference in the metals work locks. This electric field is used dissociate the excitons, which are generated when the active layer absorbs light, and afterwards to pull the charge carriers out from the active layer. Like glass the ITO layer is transparent in the visible area.PEDOTPSS (poly3,4-(ethylenedioxy)-thiophenepoly(styrene sulfonate)) and calcium are two materials which are introduced into the solar cell in order to increase the built-in electric eld and thereby improve the performance of the solar cell. The active layer in this polymer solar cell consists of a blend between the conjugated polymer MEH-PPV ((poly2-methoxy-5-(2-ethylhexyloxy)- 1,4-phenylenevinylene)) and the modied fullerene PCBM (1-(3-Methoxycarbonylpropyl)-1-phenyl-6.6C61). MEH-PPV is the absorbing part of the active layer and PCBM is introduced into the layer to make the disassociation of the excitons more eective.In bulk heterojunction polymer solar cells, light generates excitons with subsequent insularity of charges in the interface between an electron donor and acceptor blend within the devices active layer. These charges then transport to the devices electrodes where these charges flow outside the cell, perform work and then re-enter the device on the opposite side. The cells efficiency is limited by several factors especially non-geminate recombination. Hole mobility leads to rapid conduction across the active layer.2930By simply blending polymers (electron donors) with fullerene (electron acceptor) in organic upshots, a self assembling interpenetrating network can be obtained utilize various coating technologies ranging from laboratory-scale spin coating or spray coating to large-scale fabrication technologies such as inkjet printing20,21, doctor blading 17 , gravure23 , slot-die coating24 and exographic printing25 .In the last few years, several effective met hods have been developed to optimize the interpenetrating network formed by the electron donor and acceptor, including solving annealing (or slow-growth) 25 , thermal annealing 26-28 and syllable structure temper using mixed solvent mixtures 29 or additives 30 in the solutions of donor/acceptor blends. Poly (3-hexylthiophene)(P3HT) in particular has been subject to increasing interest in the polymer research community, but signi flip progress has also been made in developing new active-layer polymer materials 19,30-37 . Since around 2008, the efficiency of PSCs has risen to 6% using new conjugated polymers as electron donors 34.Although progress has been impressive, there is still much to do before the realization of practical applications of PSCs. Many factors need to be borrown into account in efficiently converting sunlight into electricity.Figure 2 Shows the energy levels in a polymer solar cell. ITO(indium tin oxide) is used as the high work function electrode and Al is use d as the low work function electrode. (a) displays the energy levels before the polymer solar cell is assembled. (b and c) shows the energy levels after assembling. In (b) the polymer is an isolator and therefore the electric field changes linearly through the cell. The polymer used in (c) is a hole conducting polymer and therefore a Schottky junction will be formed between the polymer and the low work function electrode.The absorption range, the photon-electron conversion rate and the carrier mobilities of the light-harvesting polymers are among the crucial parameters for achieving high-efficiency solar cells. Furthermore, fabricating large area devices without signicantly losing efficiency while maintaining long lifetime of the device cadaver challenging.38 39Therefore, a major challenge lies in fabricating polymer solar cells, in which free-charge-carrier generation is a critical step. Fortunately, it has been found that efficient charge transfer can take place between materials , that is, donor and acceptor molecules, with suitable energy level offsets. The strong electric field at the molecular interface of two materials with different electrochemical potentials is up to(p) of separating the excitons into weakly bounded Coulombic pairs, and thereafter separated charge carriers. In cases where the donor and acceptor molecules form an intimate contact in blend films, efficient charge transfer takes place with an efficiency approaching 100%.The short exciton diffusion length which is much smaller than the necessary film thickness for effective visual absorption, has limited the external quantum efficiency (EQE) and hampers the efficient utilization of the photogenerated excitons in organic photovoltaics.A major breakthrough was achieved with the bulk heterojunction (BHJ) concept, where the nanoscale phase separation creates donor/acceptor interfaces for exciton dissociation via efficient charge transfer from donor to acceptor throughout the film. The conce pts of donor/acceptor and BHJs, thus, establish the cornerstones of polymer solar cells.Diagram of a polymer-fullerene bulk heterojunction.The bulk-heterojunction concept. After absorption of light by the photoactive material, charge transfer can easily occur due to the nano-scopic mixing of the donor and acceptor (solid and dashed area). Subsequently, the photo generated charges are transported and collected at the electrodes. Here highest meshed molecular orbital is abbreviated as HOMO and the lowest unoccupied molecular orbital as LUMO.Despite the high attainable EQE, overall power conversion efficiencies (PCE) reported are still low, due to the inferior charge-transport properties and limited spectral absorption range of the polymer active layer. On one hand, endeavors in implication and development of unexampled low-band-gap polymers are being carried out to harvest the major part of the solar spectrum. 40-46On the other hand, film-growth dynamics of polymer blends via solut ion processes has become one of the central topics to derive maximal efficiency from bulk-heterojunction structures. Meanwhile, precise efficiency measurements provide solutions to the spectral mismatch between the solar spectrum and polymer absorption, offering accurate evaluation of novel photoactive materials.High internal quantum efficiencies can be expected, provided that efficient donor-to-acceptor charge transfer and transport in the bulk heterojunctions occurs. A suitable energy-level alignment between the donor and acceptor to provide the driving force morphology plays a decisive role linking the optoelectronic properties and device performance to the fabrication processes. In addition to experimental results, simulation techniques have also been utilize to predict the optimal morphology, yielding results that are consistent with the experimental conclusion that a nanoscale phase separation with a bi-continuous pathway toward the electrode is desired. assemblage parameter s such as solvent selection and annealing treatment are the most critical factors in film morphology.However, additive incorporation also showed crucial benefits toward improving device performance. The overall effects of morphology manipulation assist in forming an interpenetrating network of donor and acceptor molecules, facilitating both charge transfer and carrier transport. squint-eyed phase separation has been observed and well-understood in several systems. Beyond that, the ingredient distribution of the donor and acceptor molecules along the cross-section of blend films, that is, vertical phase separation has been observed recently in the nanoscale film morphology, which intuitively governs the charge transport and collection. Thus, an ideal morphology consists of phase separation laterally and vertically, which should both be optimized for lusty device performance.47,50-52A variety of post-treatment methods can alter the optoelectronic properties of the polymer-blend fil ms. Annealing processes in polymer solar cells can be divided into two categories thermal annealing 53,57,58 and solvent annealing.48,59-61 Both techniques concentrate on improving the nano scale lateral phase separation of both the crystalline P3HT aggregates and PCBM domains. Thermal annealing can be applied either on the final device (post-annealing) or on the polymer film only (pre-annealing). The annealing temperature and time are the two most critical parameters in this approach. However, the selection of solvent as well as metal electrodes could also affect the ultimate device performance.Solution processing has many advantages over other film fabrication technologies, which usually require complicated instruments as well as costly and time-consuming procedures. Therefore, solution processing has developed into the most elevate methodology for fabricating organic optoelectronic devices. Solution processing also allows the freedom to control phase separation and molecular sel f-organization during solvent evaporation and/or film treatment. The solvent establishes the film evolution environment, and thus has foreseeable impact on the final film morphology. Selection and combination of solvents have been shown to be critical for the morphology in polymer-blend films, and are well-documented in the literature. 48,49. Spin-coating from single-solvent solutions results in thin films, which possess optoelectronic properties determined by the solution parameters and the spin-coating process, for example concentration, blending ratio, spin speed and time, etc. Meanwhile, solvent properties, such as boiling point, vapor pressure, solubility, and polarity, also have considerable impact on the final film morphology. 62-751.5 live function of solar cells1.5.1 Work function of materialThe work function is the minimum energy needed to move an electron from the Fermi energy level, EF, to vacuum energy, Evac.The work function varies by using different materials and als o by doping. It is lower for n-type semiconductor than for p-type because Fermi levels within the band gap of a semiconductor depends upon doping. Where are work functions of the n-type and p-type materials respectively.Junctions having different work functions give way to an electrostatic field.1.5.2 Metal-semiconductor junctionMetal-semiconductor junction is the simplest type of charge separating junction.If we have an n-type semiconductor of work function metal of work function, such that, it is called a Schottky barrier.When metal and semiconductor are separate from each other, the Fermi levels will look like in fig. 5(a). But when they are in contact (electronic), these levels will line up. The exchange of charge carriers across the junction results in this, with the consequence that the layers approach the equilibrium (thermal). The energy at the conduction band butt on at the interface between semiconductor and metal is higher than in the bulk of the semiconductor. The elect rostatic potential energy is shown in fig. 5(b) by the change in Evac.The space charge region or depletion region is the region where there is a net charge.As Evac changes, so must the conduction and valence band energies, and that too by the same amount (proportionality). This happens because the electron affinity and band gap are invariant in the semiconductor, and is called band bending.761.5.3 p-n heterojunctionA heterojunction consists of two different materials with different band gaps and these can also be either p-n or p-i-n junctions. Devices based on heterojunctions can improve carrier collection and thus efficiency. Due to change in the band gap, a discontinuity exists in the conduction and valence band at the junction.The potential step will affect the effective field for the two carrier types in different ways. Usually, one carrier type is assisted by the field change, while the other is opposed. In fig. 6(b), the field that drives electrons to the n side is increased, while the field driving holes towards the p side is decreased. 76We know that the standard form of an organic photovoltaic cell is based upon sandwiching a thin semiconducting organic layer(s) between two conducting layers having different work functions here we have higher work function conductors typically made of gold or ITO and lower work function conductors typically made of aluminum or calcium.We have already discussed in section 1.2 how the efficiency of solar cell can be improved, here we will consider mathematical expression of efficiency. Efficiency is defined as measures the amount of energy converted to electric current relative to the total energy incident upon the cell, it is designated with Greek letter , . The formula for calculating efficiency is = Jsc X Voc X FF,where Jsc is the short-circuit current (when there is maximum current flowing and no voltage difference across the circuit),Voc is the open-circuit voltage (when there is no current flowing), and FF is the fill-factor (the actual power relative to the theoretical power produced by the cell).To increase the efficiency of Polymer Solar cells, we need to improve these 3 factors. Jsc is primarily affected by band-gap, carrier mobility, and film formation properties of the active layer. Voc is primarily affected by the material band-gap and the device structure. FF, is particularly difficult to predict and design, but seems related to the relative motilities of the electrons and holes.77-801.6 Inverted Polymer Solar CellsThe regular device structure for polymer solar cells is indium tin oxide (ITO), where a p-type layer is used for anode contact, and a low-work-function metal as the cathode. Both the p-type layer and the low-work-function metal cathode are known to degrade the device lifetime. 106-108 The p-type layer is potentially detrimental to the polymer active layer due to its acidic nature, which etches the ITO and causes interface instability through indium diffusion into the polym er active layer.Low work- function metals, such as calcium and lithium, are easily oxidized, increasing the series resistance at the metal/BHJ interface and degrading device performance. In principle, ITO is capable of collecting either holes or electrons, since its work function (4.5 to 4.7 eV) lies between the typical highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) value of common organic photovoltaic materials. The polarity of the ITO electrode depends mainly on the contact