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thinfilm solarcells
  Solar Cells RICHARD CORKISH University of New South WalesSydney, New South Wales, Australia 1. Introduction2. How Solar Cells Work3. Efficiency4. Types of Solar Cells5. Conclusions Glossary diffuse insolation Solar radiation received from the sky,excluding the solar disk. direct (beam) insolation Solar radiation received directlyfrom the visible solar disk. efficiency Fraction of incident light power converted toelectrical power by a solar cell or module. global insolation The sum of direct and diffuse insolationcomponents. insolation Flux of radiant energy from the sun, measuredas power per unit area, whose intensity and spectralcontent varies at the earth’s surface due to time of day,season, cloud cover, moisture content of the air, andother factors, but is constant outside the atmosphere at1353W m À 2 . peak watt (Wp) Rating for a cell, module, or system thatproduces 1 W electrical output under standard peakinsolation conditions and at a standard cell or moduletemperature, usually 1000W m À 2 with global AM1.5spectrum at 25 1 C for terrestrial applications, intendedto simulate midday conditions at middle latitudes. photovoltaic effect Generation of an electromotive force(voltage) due to incident photon radiation. solar module Protectively packaged set of solar cells,electrically interconnected (usually several cells con-nected in series) to provide useful voltage and currentoutput. solar spectra Sunlight power content as a function of lightcolor (of radiation frequency) and for which there existseveral ‘‘standard’’ representations to aid calculationsand comparisons, particularly the air-mass 1.5 (AM1.5)spectrum, which models transmittance through 1.5atmospheric thicknesses for terrestrial modeling, air-mass zero (AM0), which models extraterrestrial sun-light, and ideal black-body spectra that model the sun asa thermodynamic black body at temperatures of 6000 Kor less. Solar cells are semiconductor devices that exploit thephotovoltaic effect to directly create electric currentand voltage from the collection of photons (quantaof light). They convert sunlight to electricity silentlyand without moving parts, require little mainte-nance, and are reliable. They are sold with warran-ties of up to 30 years, generate no greenhouse gasesin operation, and are modular, rapidly deployable,and particularly suited to urban rooftops andfac       ¸ ades. Solar cells power anything from lanterns inremote villages; to watches, calculators, solar homesystems, and grid-connected rooftop arrays; tosatellites and exploration vehicles on Mars. 1. INTRODUCTION  According to the review of the field edited by Archerand Hill, solar cells of 15% efficiency covering anarea equivalent to just 0.25% of the global areaunder crops and permanent pasture could meet allthe world’s primary energy requirements, when mostor all of that area would be otherwise alienated land.However, solar cells remain an expensive option formost power-generation requirements relative to fossiland nuclear sources, especially when the naturalenvironment is attributed little or no value, and tosome other sustainable options such as wind energyor energy efficiency enhancement.There is a wide range of solar cells materials andapproaches in production, under development, orhaving been proposed. One way to understand therange is to see the field as comprising threegenerations of technology. The first includes ‘‘bulk’’or ‘‘wafer’’ cells, made from layers of semiconductormaterial thick enough to be self-supporting, usuallyup to 0.5mm. This generation, which dominates Encyclopedia of Energy, Volume 5. r 2004 Elsevier Inc. All rights reserved. 545  markets, is exemplified by single and multicrystallinesilicon cells and commercial production cells, whichcurrently have efficiencies of 10 to 15%. Costreductions for such cells will eventually be limitedby the costs of the wafers. The second generationaims for lower cost at the expense of efficiency byusing thin layers of active material. A rigid substrateor superstrate provides mechanical support for thesemiconductor, which is deposited rapidly over largeareas. It is hoped that eventually the relatively lowmaterial cost of the substrate or superstrate willdominate the cost of the complete device. Unlike thewafer-based cells described earlier, which are me-chanically and electrically connected together toform weatherproof solar modules that produceconvenient voltages and currents, the thin filmmaterials are deposited on large areas of sub/super-strate and subsequently patterned to form multiplecells. Hence, the basic production unit is the module,rather than the individual cell. The four importantthin-film technologies are amorphous silicon, poly-crystalline silicon, cadmium telluride, and copperindium diselenide. Current production efficienciesare 4 to 9%. Thin-film cells have been expected toovertake the first-generation cells’ dominance fordecades, but this has not yet occurred. In fact, theevolution may be faltering since one major cellmanufacturer ceased production of its thin-filmproducts late in 2002 and others struggle forfinancial viability. Third-generation approaches in-tend to eventually combine high efficiency and lowcost. One type of third-generation cells, tandem cells,are already commercially available as both low-cost,low-efficiency amorphous silicon-germanium and, atthe other performance extreme, ultra-high efficiencycells made from elements of Groups III and V of thePeriodic Table (known simply as III-V cells).Market surveys estimated world production to bearound 560MW p in 2002, an estimated 39%increase over the previous year. Ten companiesaccounted for 84% of global production in 2002.The three largest were Sharp, BP Solar, and Kyocera,in order of production volume. Sharp’s rapid expan-sion during 2002 gave it nearly twice the capacity of its next largest rival. Figures 1 and 2 indicate thehistory of regional and global production levels andthe end uses.One figure of merit on which solar cells andmodules are compared is their efficiency. Recordefficiencies for research cells and modules usingdifferent technologies are published regularly by thejournal Progress in Photovoltaics . Another morecommercially important but less precisely definedfigure of merit is the cost per W p . This recognizesthat efficiency is only of prime importance in someapplications, and for many potential users, thecapital cost and the cost of the resultant electricityare more useful measures on which to base decisions.Hence, technologies that are less efficient but cheaperper unit area are able to compete in the market. 2. HOW SOLAR CELLS WORK  Solar cells use semiconducting materials to achievetwo basic tasks: (1) the absorption of light energy togenerate free charge carriers within the material and(2) the separation of the negative and positive charge 0100200300400500600 USA Japan EuropeRest of World Year    2   0   0   2   2   0   0  1  1   9   9   9  1   9   9   8  1   9   9   7  1   9   9  6  1   9   9   5  1   9   9  4  1   9   9  3  1   9   9   2  1   9   9  1  1   9   9   0  1   9   8   9  1   9   8   8   2   0   0   0      M     W   p  FIGURE 1 World solar cell production, 1988–2002, for thethree main production zones and for the rest of the world (ROW).Data from Photon International  and Renewable Energy World  . 050100150200250300350400 Central > 100KWGrid-connectedresidentia/commercialPV-diesel, commercialCommunications and signalWorld off-grid ruralUS off-grid residentialConsumer products      M     W   p    2   0   0  1   2   0   0   0  1   9   9   9  1   9   9   8  1   9   9   7  1   9   9   6  1   9   9  3  1   9   9   0  FIGURE 2 World PV market by application, 1990–2001. Datafrom Renewable Energy World. 546 Solar Cells  carriers to produce unidirectional electrical currentthrough terminals that have a voltage differencebetween them. The separation function is usuallyachieved by a p-n junction, the interface of regions of material that have been ‘‘doped’’ with differentimpurities to give an excess of free electrons (n-type)on one side of the junction and a dearth of them (p-type) on the other. The interface region therefore hasa built-in electrostatic field that sweeps electrons oneway and holes the other. In other words, a solar cellis an illuminated, large-area diode.Figure 3 indicates the main processes. Photonsenter the cell volume through the front surface. High-energy (blue or violet) photons are absorbed stronglyclose to the cell surface, but those from the other endof the visible spectrum are weakly absorbed andpenetrate more deeply. Each absorption of a photonannihilates the photon and transfers its energy to anelectron in the semiconductor, freeing the electronfrom its parent atom and leaving behind a positivelycharged vacancy, or ‘‘hole.’’ In some materials, knownas direct bandgap semiconductors, the absorptioninvolves just the photon and the created carrier pair,but in others, indirect bandgap semiconductors, theprocess also needs one or more phonons (quanta of crystal atomic vibration) to take part. Absorption isweaker in the latter semiconductors. Gallium arsenideand silicon are examples of direct and indirectbandgap semiconductors. The electrons and holesare mobile within the cell and will move in responseto an electric field (‘‘drift’’) or by diffusion to regionsof lower concentration. They are also prone torecombination with each other. Cell theory usuallyconcentrates on minority carriers (electrons in p-typematerial and holes in n-type) since their flows(currents) principally determine the performance.Electron-hole pairs generated near the junction aresplit apart by the strong electric field there. Minoritycarriers are swept across the junction to becomemajority carriers and each crossing of the junction is acontribution to the cell’s output current. Minoritycarriers that are generated too far from the junction tobe immediately affected by the junction field can betransported to the junction by diffusion due to theminority carrier population being reduced near thejunction by the field action there. Metal contacts atthe front and rear of the cell allow connection of thegenerated current to a load. The front contact isnormally in the form of a fine metallic grid to reduceblockage of the light’s access to the semiconductor.However, survival of a minority carrier is tenuoussince it is always prone to recombination with one of the surrounding majority carriers. Recombinationcan be radiative, the inverse of the optical generationprocess that produced the carrier pair, in which thecarrier energy is lost in the production of a newphoton, or nonradiative, in which the energy isdissipated as heat in the cell. Crystal defects,accidental impurities, and surfaces are all strongnonradiative recombination sites. Most nonradiativerecombination processes are, in principle, avoidable,but radiative recombination is fundamental.The result of these processes is a light-generatedflow of current in what is normally thought of as the ElectronHoleSolar cellElectrical loadSunlightPhotonsn-typep-type  FIGURE 3 Solar cell operation. Adapted from M. A. Green and J. Hansen (2003). Catalog of Solar Cell Drawings .University of New South Wales. Solar Cells 547  reverse direction in diode theory (i.e., electrons flowout of the cell into the circuit from the n-type contactand back into the cell through the p-type contact)even though the voltage across it is in the forwardbias direction (i.e., positive at the p-type contact).The light-generated current is approximately inde-pendent of the voltage across the cell. Unfortunately,the normal diode current, which would remain evenin the dark at the same contact voltage, may bethought of as simultaneously flowing in the oppositedirection to the light-generated current. That darkcurrent (solid line in Fig. 4) increases strongly withthe voltage and at some voltage, known as the opencircuit voltage (V oc ), completely cancels the light-generated current. As a result, we see the typicalcurrent-voltage characteristic shown as a dashed linein Fig. 4. The (forward) dark current is added to the(reverse) light-generated current to make the resul-tant current negative for a range of positive voltage.In that range, the cell generates power. Power is zeroat zero volts and at V oc with maximum power beingproduced near the so-called knee of the curve. Theelectrical load connected to the cell should be chosento keep the operating point close to the optimumknee point during normal operation. 3. EFFICIENCY  In that brief description of cell operation weconsidered the main processes occurring afterphotons enter into the semiconductor volume.However, reflection from the surface must first bereduced to allow efficient absorption. A polishedsemiconductor surface will reflect a significantfraction of incident photons from the sun, around30% for silicon. Additional antireflection layers of other materials can be applied to the front surface. Inaddition, texturing of the front surface can assistwith reflection reduction by deflecting rays of light sothat they strike the semiconductor surface again.Texturing also redirects photons into oblique pathswithin the cell, giving longer paths for weaklyabsorbed (red) photons and more opportunity forabsorption (light trapping).The bandgap energy of the semiconductor fromwhich a cell is made sets the fundamental upper limitof the cell’s conversion efficiency. The choice of a lowbandgap material allows the threshold energy to beexceeded by a large fraction of the photons insunlight, allowing a potentially high current. How-ever, a solar cell can extract from each photon onlyan amount of energy slightly smaller than thebandgap energy, with the rest lost as heat. On theother hand, a semiconductor is transparent tophotons with energy less than its bandgap and cannotcapture their energy. These, thermalization andtransparency, are two of the largest loss mechanismsin conventional cells. The best solar cell materialshave bandgaps giving a compromise between thesetwo effects. Solar cell performance is limited by thelaws of thermodynamics. Thermodynamic methodsand the related principle of detailed balance can beused to calculate the limits. Shockley and Queissershowed in 1961, by use of the balance betweenincident and escaping photons and extracted elec-trons, that the efficiency limit for a single-materialcell is around 31% for an optimal bandgap of around1.1eV. That work assumed that the only unavoidablelosses from the cell are the emission of photonsproduced by radiative recombination.For such limit calculations, the sun is commonlyassumed to behave as a thermodynamic ‘‘blackbody,’’ or perfect radiator at 6000K, although thisis only a rough approximation to the solar spectrumat the earth. The cell is usually assumed to be anotherperfect radiator at 300K (27 1 C). Other losses areassumed to be, in principle, avoidable or controllableand are neglected. Within that framework, a range of different solar cell concepts produces a range of efficiency limits (Table I) from 31% to 86.8% formaximum concentration, which refers to focusingonly the solar disk onto the cell, with a concentrationof around 46,000 times.The Carnot limit for solar conversion under thepreceding temperatures is 95%, but this could beachieved only if the cell were to produce infinitesimal I L I L I L Current, IDarkVoltage, VV oc MaximumpowerIlluminated  FIGURE 4 Typical current-voltage characteristic. Adaptedfrom M. A. Green and J. Hansen (2003). Catalog of Solar Cell Drawings. University of New South Wales. 548 Solar Cells
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