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Photovoltaic Cells - The Next TechnologicalRevolution

Bikash Agarwal

Department of Electronics & Communication Engineering

Assam Don Bosco University

[email protected]

Abstract

Solar electricity produced by photovoltaic solar cells is one of the most promising options yet identified for sustainablyproviding the world's future energy requirements. Although the technology has, in the past, been based on the same siliconwafers as used in microelectronics, a transition is in progress to a second generation of a potentially much lower-cost thin-"lmtechnology. Cost reductions from both increased manufacturing volume and such improved technology are expected tocontinue to drive down cell prices over the coming two decades to a level where the cells can provide competitively pricedelectricity on a large scale. The subsidised, urban residential rooftop application of photovoltaics is expected to provide themajor application of the coming decade and to provide the market growth needed to reduce prices. Large centralised solarphotovoltaic power stations able to provide low-cost electricity on a large scale would become increasingly attractiveapproaching 2020 The photovoltaic market is booming with over 30% per annum compounded growth over the last five years.The government-subsidised urbanresidential use of photovoltaics, particularly in Germany and Japan, is driving thissustained growth. Most of the solar cells being supplied to this market are _first generation_ devices based on crystalline ormulti-crystalline silicon wafers. Second generation thin-film solar cells based on amorphous silicon/hydrogen alloys orpolycrystalline compound semiconductors are starting to appear on the market in increasing volume. Bulk crystalline siliconsolar cells have been the workhorse of the photovoltaic industry over the past decades. Recent major investments in newmanufacturing facilities for monocrystalline and multicrystalline wafer-based cells, as well as for closely related silicon ribbonand sheet approaches, ensure this role will continue well into the future. Such investments suggest that the silicon wafer-based

approach has successfully withstood the challenge mounted by thin-film chalcogenide based cells, in the form ofpolycrystalline films of CdTe and CuInSe , as well as that mounted by thin-film cells based on 2 amorphous silicon and itsalloys with germanium. The encumbent now faces a fresh challenge by a new wave of thin-film technologies developed in the1990s, more closely related to the bulk approach and with some advantages over the earlier contenders. One new approach isbased on a stack of two silicon thin-film cells, one cell using amorphous silicon and the other mixed-phase microcrystallinesilicon. The second uses silicon thin-films in polycrystalline form deposited onto glass, even more directly capturing thestrengths of the wafer-based approach.

KEYWORDS: Silicon, Crystalline, wafer, substrate, thin-film

1. INTRODUCTION

Solar cells, or photovoltaic cells, transform light, usuallysunlight, into electric current. Few power-generationtechnologies are as clean as photovoltaics (PV). As itsilently generates electricity, PV produces no air pollutionor hazardous waste. It doesn't require liquid or gaseousfuels to be transported or combusted. And because sunlightis free and abundant, PV systems, especially baseloadSpace Solar Power, may eliminate uncertaintiessurrounding oil, gas, or other energy fuel supplies frompolitically unstable regions. Because PV systems burn nofuel and have no moving parts, they are clean and silent,producing no atmospheric emissions or greenhouse gasesto cause detrimental effects on our water, air, and soil.Compared with electricity generated from fossil fuels, eachPV-produced kilowatt eliminates up to 830 pounds of

nitrogen oxides, 1,500 pounds of sulfur dioxide, and217,000 pounds of carbon dioxide, every year, according to

National Renewable Energy Laboratory (NREL) research 1.

Photovoltaics involves the direct conversion of sunlightinto electricity in thin layers of material known assemiconductors with properties intermediate between thoseof metals and insulators. Silicon, the material ofmicroelectronics and the information age, is the mostcommon semiconductor. In the latter half of the 20thcentury, silicon photovoltaic solar cells started to be usedmainly to generate small amounts of electricity in remoteareas where there was no conventional source of electricity.In the 21st century, photovoltaics will grow to maturity.Almost everyone will be aware of photovoltaics sincephotovoltaic solar cells will be on the roof of their home orthat of their neighbours be they in one of the growing mega


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cities across the globe or in a remote rural village. The bigproblem with solar energy collected on earth is that the sundoesnt shine 24 hours every day. Anywhere, even highlyfavourable locations, such as the Arizona desert, onlyprovide acceptable sunshine for power production about sixhours per day. Most locations have fewer hours of sun andoften clouds, fog, precipitation or haze for long periods, notto mention dust, leaves, hail, and even abuse by animals.High temperatures can also reduce PV power production.Storing massive quantities of electrical energy overnight orfor cloudy days to provide utility-scale baseload power onearth is not cost effective (except for a few price-insensitivebuyers, such as the military or hospitals). Very fewsituations can economically justify the massive storage ofelectrical energy.

By placing large PV arrays in space, however, they can beeven more environmentally benign and storage isunnecessary, since in GeoSynchronous Orbit (GSO) thesun shines over 99% of the year. GSO provides much moreintense sunlight nearly continuously with no clouds,eliminating the storage problem and utilizing thosevaluable photovoltaic cells 10 times more effectively.Someday commercial companies may even manufacturePV and other components in space - from lunar capturedasteroid1.

As we will outline, fabricating the PV we need to buildSSP is chiefly a matter of increasing production levels forspace PV from one hundred kilowatts annual productionupwards to many Gigawatts, and tuning those productionlines to space-qualified thin film. This transition capabilityhas already been demonstrated for amorphous silicon, themost mature space qualified thin film.

2. BRIEF HISTORY

Solar cells have their origins from some of the mostimportant scientific developments of the 20th century,combining the Nobel prize winning work of several of themost important scientists of that century. The Germanscientist, Max Planck, began the century engrossed in theproblem of trying to explain the nature of light emitted byhot bodies, such as the sun. He had to make assumptionsabout energy being restricted to discrete levels to matchtheory and observations. This stimulated Albert Einstein, inhis `miraculous year of 1905 (Stachel, 1998), to postulatethat light was made of small `particles, later calledphotons, each with a tiny amount of energy that depends onthe photon's colour. Blue photons have about twice theenergy of red photons. Infrared photons, invisible to theeye have even less energy. Ultraviolet photons, the cause ofsunburn and skin cancer, are also invisible but carry evenmore energy than the blue ones, accounting for the damagethey can do.

Einstein's radical suggestion led to the formulation anddevelopment of quantum mechanics, culminating in 1926in Edwin Schrodinger's wave equation. Wilson solved thisequation for material in solid form in 1930. This allowedhim to explain the difference between metals, goodconductors of electricity and insulators; also the propertiesof semiconductors with their intermediate electricalproperties. Electrons, the carriers of electrical charge, are

free to move around in metals, allowing electrical currentsto flow readily. In insulators, electrons re locked into thebonds holding the atoms of the insulator together. Theyneed a jolt of energy to free themfrom these bonds, so theycan become mobile. The same applies to semiconductors,except a smaller jolt is needed even the red photons insunlight have enough energy to free an electron in thearchetypical semiconductor, silicon.

Russel Ohl discovered the first silicon solar cell byaccident in 1940. He was surprised to measure a largeelectrical voltage from what he thought was a pure rod ofsilicon when he shone a flashlight on it. Closerinvestigation showed that small concentrations ofimpurities were giving portions of the silicon propertiesdubbed `negative (n-type)3. These properties are nowknown to be due to a surplus of mobile electrons with theirnegative charge. Other regions had `positive (p-type)properties, now known to be due to a deficiency ofelectrons, causing an effect similar to a surplus of positivecharge (something close to a physical demonstration of themathematical adage that two negatives make a positive).

William Shockley worked out the theory of the devicesformed from junctions between `positive and `negativeregions (p-n junctions) in 1949 and soon used this theory todesign the first practical transistors. The semiconductorrevolution of the 1950s followed, which also resulted in thefirst efficient solar cells in 1954.

The first commercial use of the new solar cells was onspacecraft, beginning in 1958. This was the majorcommercial application until the early 1970s, when oilembargoes of that period stimulated a re-examination of thecells' potential closer to home. From small beginnings, aterrestrial solar cell industry took root at this time and hasgrown rapidly, particularly over recent years, to US$1billion per year in sales, by the end of the 20 th century(Perlin, 1999). Increasing international resolve to reducecarbon dioxide emissions as a first step to reigning in the`Greenhouse Effect, combined with decreasing cell costs,sees the industry poised to make increasing impact over thefirst two decades of the new millennium3.

3. OPERATING PRINCIPLES

Fig. 1 is a schematic of a solar cell under illumination.Light entering the cell through the gaps between the topcontact metal gives up its energy by temporarily releasing


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electrons from the covalent bonds holding thesemiconductor together; at least this is what happens forthose photons with sufficient energy. The p-n junctionwithin the cell ensures that the now mobile charge carriersof the same polarity all move off in the same direction. Ifan electrical load, such as the lamp shown in Fig. 1, isconnected between the top and rear contacts to the cell,electrons will complete the circuit through this load,

constituting an electrical current in it. Energy in theincoming sunlight is thereby converted into electricalenergy consumed by this load.

Fig. 1: Incoming sunlight is converted to an electrical current flow ina load such as a lamp connected between the cell contacts.

The cell operates as a `quantum device, exchangingphotons for electrons. Ideally, each photon of sufficientenergy striking the cell causes one electron to flow throughthe load. In practice, this ideal is seldom reached. Some ofthe incoming photons are reflected from the cell or getabsorbed by the metal contacts (where they give up their

energy as heat). Some of the electrons excited by thephotons relax back to their bound state before reaching thecell contacts and thereby the load. The electrical powerconsumed by the load is the product of the electricalcurrent supplied by the cell and the voltage across it. Eachcell can supply current at a voltage from 0 V to a maximumin the 0.5-1.0 V range, depending on the particularsemiconductor used for the cell.

Although photovoltaics cells have been used since the1950s in space craft, the interest in their terrestrial use washeightened by the oil embargoes of the early 1970s. Sincethen, a steadily growing terrestrial industry has developed

which, in the past, has supplied cells mainly for remotearea applications where conventional electricity isexpensive. However, the industry is now in an explosiveperiod of growth where the subsidised urbanresidentialuse of photovoltaics is providing the main market. Theindustry has grown at a compounded rate of 30% perannum over the last five years, corresponding to aquadrupling of annual production over this period (Fig. 2)2.

The present healthy state of the industry is stimulating thefabrication of several large new manufacturing facilities

and the commercialisation of new cell technologiesAlthough most of the product over the coming decade willbe first generation silicon wafer based, it is thought likelythat a second generation thin-film technology will makeits mark during this period.

Fig. 2: Evolution of the World annual PV market 2000-2009

4. CELL TECHNOLOGY

4.1 Silicon Wafer

The technology used to make most of the solar cells,fabricated so far, borrows heavily from themicroelectronics industry.

Fig. 3: Growth of a cylindrical silicon crystal.

The silicon source material is extracted from quartz,although sand would also be a suitable material. The siliconis then refined to very high purity and melted. From themelt, a large cylindrical single crystal is drawn (Fig.3),usually of 10-15 cm diameter and 1 m or more in length,weighting several tens of kilograms. The crystal, or `ingot,is then sliced into circular wafers, less than half amillimetre thick, like slicing bread from a loaf. Sometimesthis cylindrical ingot is `squared-off before slicing so thewafers have a `quasi-square shape that allows processedcells to be stacked more closely side by side.

Most of this technology is identical to that used in themuch larger microelectronics industry, benefiting from thecorresponding economies of scale. Since good cells can be


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Fig. 4: (a) Directional solidification of a large block of multicrystallinesilicon from a melt; (b) Sawing of large block into smaller ingots prior

to slicing into multicrystalline wafers.

made from material of lower quality than that used inmicroelectronics, additional economies are obtained byusing òff-specification silicon and òff-specificationsilicon wafers from this industry.

As the photovoltaics industry matures, it will increasinglyuse technology optimised for its own requirements. Anexample is the increasing use of `multicrystalline siliconwafers. The starting ingot is formed simply by solidifyingthe molten silicon slowly in its container (Fig. 4(a)). Thisingot can be massive, weighing several hundreds ofkilograms. It is sawn into pieces of a more manageable size(Fig. 4(b)) and then sliced into wafers. Techniques forgrowing silicon in the form of ribbons from the melt have

also been developed. These have the advantage that noslicing is required.

4.2. From wafers to cells

Some manufacturers make their own wafers while othersbuy them from wafer suppliers. In either case, the first stepin processing a wafer into a cell is to etch the wafer surfacewith chemicals to remove damage from the slicing step.

The surface of crystalline wafers is then etched again usinga chemical that etches at different rates in differentdirections through the silicon crystal. This leaves featureson the surface, with the silicon structure that remainsdetermined by crystal directions that etch very slowly. Thesquare-based pyramids apparent in Fig. 5 that are formedby this process are similar in shape, if not in size, to thegreat pyramids of Egypt. These pyramids are very effectivein reducing reflection from the cell surface. (Light reflectedfrom the side of a pyramid will be reflected downwards,getting a second chance to get coupled in).

Fig. 5: Structure of a typical commercial cell with textured surfaceand screen-printed contacts.

The all-important p-n junction is then formed. The impurityrequired to give p-type properties (usually boron) is

introduced during crystal growth, so it is already in thewafer. The n-type impurity (usually phosphorus) is nowallowed to seep into the wafer surface by heating the waferin the presence of a phosphorus source. This gives a thinskin of phosphorus-doped material around the entire wafer.The skin along the wafer edge is removed (that along therear is rendered inactive during the rear contacting step).

Next, the top and bottom contacts are applied using metalparticles (usually silver) suspended in a paste with otheradditives. This paste is `screen-printed onto the cellsurface in the desired pattern using a simple process similarto that used to print patterns onto T-shirts. After printing,

the paste is dried and heated at high temperature, leavingthe metal particles agglomerated together.

A very thin layer of insulating material is sometimes addedto the top cell surface as an antireflection coating, similar tothe coating used on high-quality camera lenses. Suchcoatings are always used for `multicrystalline wafers,since the pyramidal-texturing approach is not effective forsuch wafers and this alternative approach is essential tocontrol reflection.

All the equipment required for this process is available òffthe shelf from the microelectronics industry (the `hybrid

or `thick film industry sector). This, combined with the

small number of processing steps, has made this sequencealmost universally used by solar cell manufacturers.However, the penalty paid for its simplicity andconvenience is lower cell performance than is inherentlyavailable from the staring wafers. The screen-printed silverpastes are also quite expensive. After fabrication, the cellsare soldered together and packaged under a sheet of glassinto a weatherproof package known as a module.


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Generally, 36 cells are packaged into a module, since this isthe required number to generate enough voltage to allowcharging of a lead-acid battery.

4.3. Improved technology

The previous `screen-printed sequence was developed inthe early 1970s and produces cells with performance

typical of this era. As shown in Fig. 6, there has been over50% relative improvement in laboratory silicon cellperformance since that time. Only 20% of the cost ofproducing a standard module is due to cell processingcosts. Both these and the remaining 80% of the costs areable to be reduced per unit power output by increasing cellefficiency (since these costs are related to cell and modulearea). This means that cell processing costs per unit areacan double if this results in 20% improvement in cellperformance (or triple for 40% improvement)3. Given thatthe screen-printing sequence is simple but still reasonablyexpensive, due to the costs of the required metal pastes,such a trade-off is feasible.

Fig. 6: Evolution of silicon laboratory cell e$ciency. The dark squaresshow improvements demonstrated by the author's group.

The most successful commercialisation to data of animproved, higher performance sequence has involved theburied contact cell of Fig. 7. This cell offers 20-30%improvement in output with virtually no increase in cellprocessing costs3. The overall relative cost advantage of thetechnology is therefore close to its relative efficiencyadvantage. The improved performance comes from betterquality surface regions that allow much better response toblue light, absorbed close to the surface, and much lowerelectrical resistance and optical losses due to the improvedtop contacting scheme.

A more recently commercialised approach is theHeterojunction with intrinsic thin layer (HIT) cell of Fig. 7.

This combines crystalline silicon technology with that ofamorphous silicon, discussed below. The HIT cell would

Fig. 7: Buried contact solar cell.

Fig. 8: Heterojunction with intrinsic thin layer (HIT) cell.

be expected to give some of the improvements of theburied contact sequence in the areas mentioned, althoughnot to the same extent. The rear processing of the cell isimproved compared to the buried contact sequence and thecell responds to light from both directions, a feature that

can be used to advantage in some applications.

For the processing of multicrystalline wafers, the use ofsilicon nitride as an antireflection coating has advantagesknown for some time. These arise from the presence ofhydrogen in this layer, arising from its presence in one ofthe source gases (SiH4) used in the deposition process. Thehydrogen diffuses into the silicon and is effective inreducing detrimental activities at the boundaries betweenthe individual grains in multicrystalline material. The use


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of nitride is expected to be more widely adopted in thefuture for such multicrystalline material, in particular.

5. THIN FILM SOLAR CELLS

5.1. Thin-Film advantages

The potential for on-going cost reductions is the key reason

for confidence in a significant role for photovoltaics in thefuture.

Rather than the wafer-based technology of the previoussection, the future belongs to `thin-Film technology. In hisapproach (Fig. 9), thin layers of semiconductor material aredeposited onto a supporting substrate, or superstrate, suchas a large sheet of glass. Typically, less than a micronthickness of semiconductor material is required, 100-1000times less than the thickness of silicon wafer4.

Fig. 9: Thin-Film approach.

Reduced material use with associated reduced costs is a keyadvantage. Another is that the unit of production, instead ofbeing a relatively small silicon wafer, becomes muchlarger, for example, as large as a conveniently handledsheet of glass might be. This reduces manufacturing costs.

Silicon is one of the few semiconductors inexpensiveenough to be used to make solar cells from self-supportingwafers. However, in thin-"lm form, due to the reducedmaterial requirements, virtually any semiconductor can beused. Since semiconductors can be formed not only byelemental atoms such as silicon, but also from compounds

and alloys involving multiple elements, there is essentiallyan infinite number of semiconductors from which tochoose.

At present, solar cells made from five different thin-Filmtechnologies are either available commercially, or close tobeing so. Over the coming decade, one of these is expectedto establish its superiority and attract investment in majormanufacturing facilities that will sustain the downwardpressure on cell prices. As each of these thin-"lm

technologies has its own strengths and weaknesses, thelikely outcome is not clear at present3, 4.

5.2. Amorphous silicon alloy cells

5.2.1. Properties

Given its success in wafer form, silicon is an obvious

choice for development as a thin-"lm cell. Early attempts tomake thin-film polycrystalline silicon cells did not meetwith much success. However, starting from the mid-1970s,very rapid progress was made with silicon in amorphousform. In amorphous silicon, the atoms are connected toneighbours in much the same way as in the crystallinematerial but accumulation of small deviations fromperfection means that the perfect ordering over largedistances is no longer possible. Amorphous material hasmuch lower electronic quality, as a consequence, andoriginally was not thought suitable for solar cells.However, producing amorphous silicon by decomposingthe gas, silane (SiH4), at low temperature, changed thisopinion. It was found that hydrogen from the source wasincorporated into the cell in large quantities (about 10% byvolume), improving the material quality4. Hydrogenatedamorphous silicon cells very quickly found use in smallconsumer products such as solar calculators and digitalwatches, their main use so far.

The problem with outdoor use is that some of the beneficialeffect of hydrogen becomes undone under bright sunshineand the cell performance degrades. Initially, there was hopethat some simple material-related solution could be found.When this did not happen, the only alternative was todesign around it. Cells had to be developed that could workwell with material of degraded quality, rather than of thestarting quality.

5.2.2. Amorphous cell design

Since the amorphous silicon quality is much poorer thancrystalline silicon, a different cell design approach isrequired. The most active part of a p-n junction solar cell isright at the junction between the p- and n-type region of thecell (Fig. 1). This is due to the presence of an electric fieldat this junction.

With amorphous silicon cell design, the aim is to stretchout the extent of this junction region as far as possible soalmost all the cell is junction. This is done by having the p-and n-type doped regions very thin, with an undoped regionbetween them. The strength of the electric field establishedin this undoped region is nearly constant and depends onthis region's width. The poorer the quality of amorphoussilicon, the stronger the field needs to be for the device towork well and hence the thinner the device needs to be. Fordegraded material, it turns out that the cell needs to bethinner than the thickness required to absorb all the useable


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incident sunlight. The way around this is to stack severalcells on top of one another so that light not absorbed by anupper one passes through to an underlying cellThe best commercial amorphous silicon cells presently usethree cells stacked on top of one another, withprogressively more germanium in the bottom two5. Eachcell is very thin, only 100-200 nm thick. This ensuresreasonable stability (only about 15% degradation in output

when exposed to bright sunlight). However, the stabilisedefficiency is quite poor, only 6-7% for the best commercialmodules, according to manufacturers' data sheets.

This low efficiency, even with the sophisticated cell designinvolved, is expected to make it difficult for thistechnology to be competitive in the long term. However,the low temperatures involved in making these cells meanthat they can be deposited onto low-temperature substratessuch as plastics. This makes them especially suitable forconsumer products.

5.3. Thin-Film,polycrystalline compound semiconductors

Many semiconductors made from compounds can absorblight more strongly than the elemental semiconductors,silicon and germanium, for reasons that are wellunderstood but quite subtle7 (silicon and germanium are`indirect rather than `direct bandgap materials). Thismeans compound semiconductor cells can be thin but stilloperate efficiently. Most compound semiconductors, whenformed in polycrystalline form, have poor electronicproperties due to highly deleterious activity at grainboundaries between individual crystalline grains in thematerial. A small number maintain good performance inpolycrystalline form for reasons that are not usually wellunderstood. These are the candidates for thin-"lmpolycrystalline compound semiconductor solar cells.

One such semiconductor is the compound cadmiumtelluride (CdTe). Technically, it is an ideal material, givingproperties suitable for making reasonable solar cells evenwith relatively crude material deposition approaches (suchas electrodeposition, chemical spraying, and so on). The

junction in these cells is again between p and n-typematerial, but for the latter, a different compoundsemiconductor, cadmium sulphide, gives best results. CdTecells have been used mainly in pocket calculators to date,but large area, moderate performance modules have alsobeen demonstrated7. The main concern with thistechnology is the toxicity of the materials involved, eventhough very small amounts are used in the modules. At thevery least, this would mean that modules would have to becarefully disposed of or, preferably, recycled after theiruseful life was finished. However, there may be someproblems in gaining market acceptance in what is likely tobe mainly a `green market over coming years.

There are also only limited known resources of tellurium(Zweibel and Green, 2000). If all identified reserves wereconverted into cells of the present designs overnight, theycould generate 10% of the world's present electricity use (asteadily decreasing percentage indefinitely, if recycled atend of life).

An even more promising technology at the moment, in the

author's opinion, is one based on the ternary compound,copper indium diselenide (CuInSe2). As if three elementswere not enough, this compound is often alloyed withcopper gallium diselenide (CuGaSe2) and copper indiumdisulphide (CuInS2), giving material with up to fiveelements involved6. The n type layer in these devicesconsists of a layer of cadmium sulphide, as in the previouscadmium telluride cells. An alternative for this layer isbeing sought, to eliminate the toxic cadmium.

Small area laboratory cells have demonstrated efficiencyclose to 19%, despite the fine-grained polycrystallinematerial used6. Modules of this material are nowcommercially available in small volumes with efficiency upto 12% demonstrated in pilot production. This is not farbehind what is achieved with standard crystalline siliconwafer modules.

Apart from the use of cadmium and even more limitedknown resources of indium than tellurium, an often quotedlimitation of this technology is `manufacturability. This isoften interpreted as meaning it is difficult to diagnoseproblems in production with this material, since thedifference between good and bad material is notsufficiently well understood to allow differentiation andcontrol during the various manufacturing steps.

5.4. Thin-Film polycrystalline silicon cells

As previously mentioned, silicon is a weak absorber ofsunlight compared to some compound semiconductors andeven to hydrogenated amorphous silicon. Early attempts todevelop thin-"lm solar cells based on the polycrystallinesilicon did not give encouraging results since the siliconlayers had to be quite thick to absorb most of the availablelight.

However, in early 1980s, understanding of how effectivelya semiconductor can trap weakly absorbed light into itsvolume greatly increased8. Due to the optical properties ofsemiconductors, particularly their high refractive index,cells can trap light very effectively if the light direction israndomised, such as by striking a rough surface, once it isinside the cell. Optically a cell can appear about 50 timesthicker than its actual thickness if this occurs. Such `lighttrapping removes the weak absorption disadvantage ofsilicon.


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Work on polycrystalline thin-Film solar cells is proceedingin two areas. A variety of `high temperature approachessuch as suggested by Fig. 10 are being explored. Theregenerally involve either high-temperature deposition ofsilicon onto a substrate or melting the silicon after

Fig. 10: One approach for high-temperature preparation of siliconthin-film solar cells.

deposition, to obtain large grain size in the "nal "lm.Although preparation details are sketchy, the thin-"lmsilicon product available from the US company,Astropower, is the most developed representative of thisclass of approach. In this case, the silicon is deposited ontoan expansion-matched ceramic substrate. The final materialconsists of millimetre sized grains and is similar in

appearance to multicrystalline silicon wafers. Small areacell performance in the 16-17% range has beendemonstrated, similar to that from such cells on moderate-quality multicrystalline silicon. The performance of largearrays of such cells has also been similar.

Fig. 11: Prototype thin-"lm polycrystalline silicon-on-glass module(photograph courtesy of Paci"c Solar Pty. Ltd., Sydney).

The other type of approach is a `low-temperatureapproach, generally based on amorphous silicon

technology. One approach is to deposit the silicon inamorphous form and then crystallise it by heating forprolonged periods at intermediate temperatures. This`solid-phase crystallisation approach has produced cells ofquite reasonable performance9. Another approach hasinvolved changing the amorphous silicon depositionconditions, to produce a nano crystalline phase of silicon.The potential of this approach was highlighted by early

results with the `micromorph solar cell9. More recently,cell efficiency above 10% has been confirmed with thisapproach. There are plans to use such cells as the lower cellin a tandem configuration with an amorphous silicon uppercell, with commercial product targeted for 2002. Alsotargeted for commercialisation in the same timeframe ispolycrystalline silicon on glass product shown in Fig. 11,based on an amorphous silicon precursor.

6. CONCLUSION

Crystalline or polycrystalline silicon cells are expected tosatisfy most of the rapidly growing demand for solarphotovoltaic product over the coming decade. Cells areexpected to become thinner and higher performing, withpossibly a shift in preferred wafer type from boron- tophosphorus-doped. Silicon directly grown in the form ofself-supporting, multicrystalline ribbons is expected to alsoappear in larger volume on the market. It appears likelythat, by the end of the decade, polycrystalline silicon thin-film product deposited directly onto glass or anothersupporting substrate may also have market impact. As weenter the new millennium, photovoltaics are poised to makea more significant impact upon energy use.

Although still dominating the marketplace, first generationa technology based on silicon wafers is starting to bechallenged by `second generation thin-film technology.This has the advantage of much lower material costs and ofbeing better suited to high-volume manufacture. There isalso scope for a third generation of technology, based onprinciples not yet fully developed, offering prospects forsignificantly enhanced efficiency at some stage further intothe future. Accompanying the rapidly growing demand forphotovoltaics and such on-going improvements intechnology, photovoltaic costs have been steadilydecreasing and are expected to do so well into the future.

The application driving market growth at the present is inurban residential rooftop systems. This is expected tocontinue to be the most important commercial applicationover the coming decade, although dependent on subsidiesfor its viability. The required level of subsidy is expected todecreases over the coming decade, with this applicationfully economic by the end of the decade. Policies thatsupport this market pull mechanism while encouraging thedevelopment and commercialisation of improvedphotovoltaic technology are believed to be the mostappropriate.


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7. REFERENCES

1. Herig, C., Assessing Rooftop Solar-ElectricDistributed Energy Resources for the California LocalGovernment Commission, October, 2000 seehttp://www.eere.energy.gov/solar/to_you.html#environmental_impact

2. European Photovoltaic Industry Association,GlobalMaarket Outlookfor PhotovoltaicsUntill 2014,May,1020 update.

3. M. A. Green,Photovoltaics: technology overview,Energy Policy, Vol. 28,pp 989-998, 2000.

4. Dominique S., Roland E., Silicon feedstock for themulti-crystalline photovoltaic industry Solar EnergyMaterials & Solar Cells,Vol.72, pp 2740, 2002

5. M. A. Green, Crystalline Silicon Photovoltaic Cells,Adv. Material, Vol. 13, pp 12-13, July 4,2001.

6. R. M. Swanson, A Vision for Crystalline SiliconPhotovoltaics, Progress in Photovoltaic: Research andApplications, Vol. 14,pp 443453,2006.

7. A. G. Aberle,Fabrication and characterisation ofcrystalline silicon thin-film materials for solar cells,Thin Solid Films, Vol. 512 ,pp 2634,2006.

8. M. A. Green, Recent Developments inPhotovoltaics, Solar Energy, Vol. 76,pp 38, 2004.

9. M. A. Green,Crystalline and thin-film silicon solarcells: state of the art and future potential, Solar

Energy,Vol. 74,pp 181192,2003.

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