*Tel.:#61-2-9385-4018;fax:#61-2-9662-4240.E-mail address:m.green @.au (M.A.Green).Energy Policy 28(2000)989}998Photovoltaics:technology overviewM.A.Green *Centre for Photo v oltaic Engineering,Uni v ersity of New South Wales,Sydney,NSW 2052,AustraliaReceived 24May 2000AbstractSolar electricity produced by photovoltaic solar cells is one of the most promising options yet identi "ed for sustainably providing the world 's future energy requirements.Although the technology has,in the past,been based on the same silicon wafers as used in microelectronics,a transition is in progress to a second generation of a potentially much lower-cost thin-"lm technology.Cost reductions from both increased manufacturing volume and such improved technology are expected to continue to drive down cell prices over the coming two decades to a level where the cells can provide competitively priced electricity on a large scale.The subsidised,urban residential rooftop application of photovoltaics is expected to provide the major application of the coming decade and to provide the market growth needed to reduce rge centralised solar photovoltaic power stations able to provide low-cost electricity on a large scale would become increasingly attractive approaching 2020. 2000Elsevier Science Ltd.All rights reserved.1.IntroductionPhotovoltaics involves the direct conversion of sun-light into electricity in thin layers of material known as semiconductors with properties intermediate between those of metals and insulators.Silicon,the material of microelec-tronics and the information age,is the most common semiconductor.In the latter half of the 20th century,silicon photovoltaic solar cells started to be used mainly to gener-ate small amounts of electricity in remote areas where there was no conventional source of electricity.In the 21st cen-tury,photovoltaics will grow to maturity.Almost everyone will be aware of photovoltaics since photovoltaic solar cells will be on the roof of their home or that of their neighbours *be they in one of the growing megacities across the globe or in a remote rural village (Green,2000).This paper reviews the current state of solar cell tech-nology,outlines developments expected over the coming few decades and considers implications for energy policy.2.Brief historySolar cells have their origins from some of the most important scienti "c developments of the 20th century,combining the Nobel prize winning work of several of the most important scientists of that century.The German scientist,Max Planck,began the century engrossed in the problem of trying to explain the nature of light emitted by hot bodies,such as the sun.He had to make assump-tions about energy being restricted to discrete levels to match theory and observations.This stimulated Albert Einstein,in his `miraculous year a of 1905(Stachel,1998),to postulate that light was made of small `particles a ,later called photons,each with a tiny amount of energy that depends on the photon 's colour.Blue photons have about twice the energy of red photons.Infrared photons,invisible to the eye have even less energy.Ultraviolet photons,the cause of sunburn and skin cancer,are also invisible but carry even more energy than the blue ones,accounting for the damage they can do.Einstein 's radical suggestion led to the formulation and development of quantum mechanics,culminating in 1926in Edwin Schro dinger 's wave equation.Wilson sol-ved this equation for material in solid form in 1930.This allowed him to explain the di !erence between metals,good conductors of electricity and insulators;also the properties of semiconductors with their intermediate electrical properties.Electrons,the carriers of electrical charge,are free to move around in metals,allowing electrical currents to #ow readily.In insulators,electrons are locked into the bonds holding the atoms of the insulator together.They need a jolt of energy to free them0301-4215/00/$-see front matter 2000Elsevier Science Ltd.All rights reserved.PII:S 0301-4215(00)00086-0Fig.1.Incoming sunlight is converted to an electrical current #ow in a load such as a lamp connected between the cell contacts.from these bonds,so they can become mobile.The same applies to semiconductors,except a smaller jolt is needed *even the red photons in sunlight have enough energy to free an electron in the archetypical semiconductor,silicon.Russel Ohl discovered the "rst silicon solar cell by accident in 1940(Riordan and Hoddeson,1997).He was surprised to measure a large electrical voltage from what he thought was a pure rod of silicon when he shone a #ashlight on it.Closer investigation showed that small concentrations of impurities were giving portions of the silicon properties dubbed `negative a (n-type).These properties are now known to be due to a surplus of mobile electrons with their negative charge.Other re-gions had `positive a (p-type)properties,now known to be due to a de "ciency of electrons,causing an e !ect similar to a surplus of positive charge (something close to a physical demonstration of the mathematical adage that two negatives make a positive).William Shockley worked out the theory of the devices formed from junctions between `positive a and `negative a regions (p }n junctions)in 1949and soon used this theory to design the "rst practical transistors.The semiconduc-tor revolution of the 1950s followed,which also resulted in the "rst e $cient solar cells in 1954.This caused enor-mous excitement and attracted front-page headlines at the time (Riordan and Hoddeson,1997).The "rst commercial use of the new solar cells was on spacecraft,beginning in 1958.This was the major com-mercial application until the early 1970s,when oil em-bargoes of that period stimulated a re-examination of the cells 'potential closer to home.From small beginnings,a terrestrial solar cell industry took root at this time and has grown rapidly,particularly over recent years,to US $1billion per year in sales,by the end of the 20th century (Perlin,1999).Increasing international resolve to reduce carbon dioxide emissions as a "rst step to reigning in the `Greenhouse E !ect a ,combined with decreasing cell costs,sees the industry poised to make increasing impact over the "rst two decades of the new millennium.3.Operating principlesFig.1is a schematic of a solar cell under illumination.Light entering the cell through the gaps between the top contact metal gives up its energy by temporarily releasing electrons from the covalent bonds holding the semicon-ductor together;at least this is what happens for those photons with su $cient energy.The p }n junction within the cell ensures that the now mobile charge carriers of the same polarity all move o !in the same direction.If an electrical load,such as the lamp shown in Fig.1,is connected 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 electrical energy consumed by this load.The cell operates as a `quantum device a ,exchanging photons for electrons.Ideally,each photon of su $cient energy striking the cell causes one electron to #ow through the load.In practice,this ideal is seldom reached.Some of the incoming photons are re #ected from the cell or get absorbed by the metal contacts (where they give up their energy as heat).Some of the electrons excited by the photons relax back to their bound state before reaching the cell contacts and thereby the load.The electrical power consumed by the load is the product of the electrical current supplied by the cell and the voltage across it.Each cell can supply current at a voltage from 0V to a maximum in the 0.5}1.0V range,depending on the particular semiconductor used for the cell.4.Cell technology 4.1.Silicon wafersThe technology used to make most of the solar cells,fabricated so far,borrows heavily from the microelec-tronics industry (Green,1982).The silicon source material is extracted from quartz,although sand would also be a suitable material.The silicon is then re "ned to very high purity and melted.From the melt,a large cylindrical single crystal is drawn (Fig.2),usually of 10}15cm diameter and 1m or more in length,weighting several tens of kilograms.The crystal,or `ingot a ,is then sliced into circular wafers,less than half a millimetre thick,like slicing bread from a loaf.Sometimes this cylindrical ingot is `squared-o !a before slicing so the wafers have a `quasi-square a shape that allows processed cells to be stacked more closely side-by-side.Most of this technology is identical to that used in the much larger microelectronics industry,bene "ting from the corresponding economies of scale.Since good cells990M.A.Green /Energy Policy 28(2000)989}998Fig.2.Growth of a cylindrical siliconcrystal.Fig.4.Structure of a typical commercial cell with textured surface and screen-printedcontacts.Fig.3.(a)Directional solidi "cation of a large block of multicrystalline silicon from a melt;(b)Sawing of large block into smaller ingots prior to slicing into multicrystalline wafers.can be made from material of lower quality than that used in microelectronics,additional economies are ob-tained by using `o !-speci "cation a silicon and `o !-speci-"cation a silicon wafers from this industry.As the photovoltaics industry matures,it will increas-ingly use technology optimised for its own requirements.An example is the increasing use of `multicrystalline a silicon wafers.The starting ingot is formed simply by solidifying the molten silicon slowly in its container (Fig.3(a)).This ingot can be massive,weighing several hun-dreds of kilograms.It is sawn into pieces of a more manageable size (Fig.3(b))and then sliced into wafers.Techniques for growing silicon in the form of ribbons from the melt have also been developed.These have the advantage that no slicing is required (Green,1982).4.2.From wafers to cellsSome manufacturers make their own wafers while others buy them from wafer suppliers.In either case,the "rst step in processing a wafer into a cell is to etch the wafer surface with chemicals to remove damage from the slicing step.The surface of crystalline wafers is then etched again using a chemical that etches at di !erent rates in di !erent directions through the silicon crystal.This leaves featureson the surface,with the silicon structure that remains determined by crystal directions that etch very slowly.The square-based pyramids apparent in Fig.4that are formed by this process are similar in shape,if not in size,to the great pyramids of Egypt.These pyramids are very e !ective in reducing re #ection from the cell surface.(Light re #ected from the side of a pyramid will be re #ec-ted downwards,getting a second chance to get coupled in).The all-important p }n junction is then formed.The impurity required to give p-type properties (usually boron)is introduced during crystal growth,so it is al-ready in the wafer.The n-type impurity (usually phos-phorus)is now allowed to seep into the wafer surface by heating the wafer in the presence of a phosphorus source.This gives a thin skin of phosphorus-doped material around the entire wafer.The skin along the wafer edge is removed (that along the rear is rendered inactive during the rear contacting step).Next,the top and bottom contacts are applied using metal particles (usually silver)suspended in a paste with other additives.This paste is `screen-printed a onto the cell surface in the desired pattern using a simple process similar to that used to print patterns onto T-shirts.After printing,the paste is dried and heated at high temper-ature,leaving the metal particles agglomerated together.A very thin layer of insulating material is sometimes added to the top cell surface as an antire #ection coating,similar to the coating used on high-quality camera lenses.Such coatings are always used for `multicrystalline a wa-fers,since the pyramidal-texturing approach is not e !ec-tive for such wafers and this alternative approach is essential to control re #ection.All the equipment required for this process is available `o !the shelf a from the microelectronics industry (the `hybrid a or `thick "lm a industry sector).This,combinedM.A.Green /Energy Policy 28(2000)989}998991Fig.5.Evolution of silicon laboratory cell e$ciency.The dark squares show improvements demonstrated by the author'sgroup.Fig.6.Buried contact solarcell.Fig.7.Heterojunction with intrinsic thin layer(HIT)cell.with the small number of processing steps,has made this sequence almost universally used by solar cell manufac-turers.However,the penalty paid for its simplicity and convenience is lower cell performance than is inherently available from the staring wafers.The screen-printed silver pastes are also quite expensive.After fabrication,the cells are soldered together and packaged under a sheet of glass into a weatherproof package known as a module.Generally,36cells are packaged into a module,since this is the required number to generate enough voltage to allow charging of a lead-acid battery.4.3.Impro v ed technologyThe previous`screen-printed a sequence was developed in the early1970s and produces cells with performance typical of this era.As shown in Fig.5,there has been over 50%relative improvement in laboratory silicon cell per-formance since that time.Only20%of the cost of produ-cing a standard module is due to cell processing costs. Both these and the remaining80%of the costs are able to be reduced per unit power output by increasing cell e$ciency(since these costs are related to cell and module area).This means that cell processing costs per unit area can double if this results in20%improvement in cell performance(or triple for40%improvement).Given that the screen-printing sequence is simple but still reasonably expensive,due to the costs of the required metal pastes, such a trade-o!is feasible.The most successful commercialisation to data of an improved,higher performance sequence has involved the buried contact cell of Fig.6.This cell o!ers20}30% improvement in output with virtually no increase in cell processing costs(Bruton et al.,1997).The overall relativecost advantage of the technology is therefore close to its relative e$ciency advantage.The improved performance comes from better quality surface regions that allow much better response to blue light,absorbed close to the surface,and much lower electrical resistance and optical losses due to the improved top contacting scheme.A more recently commercialised approach is the Het-erojunction with intrinsic thin layer(HIT)cell of Fig.7. This combines crystalline silicon technology with that of amorphous silicon,discussed below.The HIT cell would be expected to give some of the improvements of the buried contact sequence in the areas mentioned,al-though not to the same extent.The rear processing of the cell is improved compared to the buried contact sequence and the cell responds to light from both directions,a fea-ture that can be used to advantage in some applications. For the processing of multicrystalline wafers,the use of silicon nitride as an antire#ection coating has advantages known for some time.These arise from the presence of hydrogen in this layer,arising from its presence in one of the source gases(SiH )used in the deposition process. The hydrogen di!uses into the silicon and is e!ective in reducing detrimental activities at the boundaries between992M.A.Green/Energy Policy28(2000)989}998Fig.8.Thin-"lm approach.the individual grains in multicrystalline material.The use of nitride is expected to be more widely adopted in the future for such multicrystalline material,in particular.5.Thin-5lm solar cells 5.1.Thin-x lm ad v antagesThe potential for on-going cost reductions is the key reason for con "dence in a signi "cant role for photovol-taics in the future.Rather than the wafer-based technology of the previous section,the future belongs to `thin-"lm a technology.In this approach (Fig.8),thin layers of semiconductor material are deposited onto a supporting substrate,or superstrate,such as a large sheet of glass.Typically,less than a micron thickness of semiconductor material is required,100}1000times less than the thick-ness of a silicon wafer.Reduced material use with associated reduced costs is a key advantage.Another is that the unit of production,instead of being a relatively small silicon wafer,becomes much larger,for example,as large as a conveniently handled sheet of glass might be.This reduces manufac-turing costs.Silicon is one of the few semiconductors inexpensive enough to be used to make solar cells from self-support-ing wafers.However,in thin-"lm form,due to the reduced material requirements,virtually any semicon-ductor can be used.Since semiconductors can be formed not only by elemental atoms such as silicon,but also from compounds and alloys involving multiple elements,there is essentially an in "nite number of semiconductors from which to choose (Berger,1997).At present,solar cells made from "ve di !erent thin-"lm technologies are either available commercially,or close to being so.Over the coming decade,one of these is expected to establish its superiority and attract invest-ment in major manufacturing facilities that will sustain the downward pressure on cell prices.As each of these thin-"lm technologies has its own strengths and weak-nesses,the likely outcome is not clear at present (Zweibel and Green,2000;Kazmerski,1997;Partain,1995).5.2.Amorphous silicon alloy cells5.2.1.PropertiesGiven its success in wafer form,silicon is an obvious choice for development as a thin-"lm cell.Early attempts to make thin-"lm polycrystalline silicon cells did not meet with much success.However,starting from the mid-1970s,very rapid progress was made with silicon in `amorphous a form.In amorphous silicon,the atoms are connected to neighbours in much the same way as in the crystalline material but accumulation of small deviationsfrom perfection means that the perfect ordering over large distances is no longer possible.Amorphous mater-ial has much lower electronic quality,as a consequence,and originally was not thought suitable for solar cells.However,producing amorphous silicon by decomposing the gas,silane (SiH),at low temperature,changed thisopinion.It was found that hydrogen from the source was incorporated into the cell in large quantities (about 10%by volume),improving the material quality.Hydro-genated amorphous silicon cells very quickly found use in small consumer products such as solar calculators and digital watches,their main use so far.The problem with outdoor use is that some of the bene "cial e !ect of hydrogen becomes undone under bright sunshine and the cell performance degrades.Ini-tially,there was hope that some simple material-related solution could be found.When this did not happen,the only alternative was to design around it.Cells had to be developed that could work well with material of de-graded quality,rather than of the starting quality.5.2.2.Amorphous cell designSince the amorphous silicon quality is much poorer than crystalline silicon,a di !erent cell design approach is required.The most active part of a p }n junction solar cell is right at the junction between the p-and n-type region of the cell (Fig.1).This is due to the presence of an electric "eld at this junction.With amorphous silicon cell design,the aim is to stretch out the extent of this junction region as far as possible so almost all the cell is junction.This is done by having the p-and n-type doped regions very thin,with an undoped region between them.The strength of the elec-tric "eld established in this undoped region is nearly constant and depends on this region 's width.The poorer the quality of amorphous silicon,the stronger the "eld needs to be for the device to work well and hence the thinner the device needs to be.For degraded material,it turns out that the cell needs to be thinner than the thickness required to absorb all the useable incident sunlight.The way around this is to stack several cells on top of one another so that light notM.A.Green /Energy Policy 28(2000)989}998993absorbed by an upper one passes through to an underly-ing cell.As discussed in Section6,this works best if the mater-ial in the underlying cells is varied so that each responds progressively better to the redder light that is transmitted to it.By alloying silicon with germanium,a material chemically similar to silicon but much scarcer,this is readily achieved.The best commercial amorphous silicon cells presently use three cells stacked on top of one another,with pro-gressively more germanium in the bottom two(Zweibel and Green,2000;Kazmerski,1997;Pantain,1995).Each cell is very thin,only100}200nm thick.This ensures reasonable stability(only about15%degradation in output when exposed to bright sunlight).However,the stabilised e$cien-cy is quite poor,only6}7%for the best commercial mod-ules,according to manufacturers'data sheets.This low e$ciency,even with the sophisticated cell design involved,is expected to make it di$cult for this technology to be competitive in the long term.However, the low temperatures involved in making these cells mean that they can be deposited onto low-temperature substrates such as plastics.This makes them especially suitable for consumer products.5.3.Thin-x lm,polycrystalline compound semiconductors Many semiconductors made from compounds can ab-sorb light more strongly than the elemental semiconduc-tors,silicon and germanium,for reasons that are well understood but quite subtle(Kazmerski,1997)(silicon and germanium are`indirect a rather than`direct a ban-dgap materials).This means compound semiconductor cells can be thin but still operate e$ciently.Most com-pound semiconductors,when formed in polycrystalline form,have poor electronic properties due to highly del-eterious activity at grain boundaries between individual crystalline grains in the material.A small number main-tain good performance in polycrystalline form for rea-sons that are not usually well understood.These are the candidates for thin-"lm polycrystalline compound semiconductor solar cells.One such semiconductor is the compound cadmium telluride(CdTe).Technically,it is an ideal material,giv-ing properties suitable for making reasonable solar cells even with relatively crude material deposition ap-proaches(such as electrodeposition,chemical spraying, and so on).The junction in these cells is again between p-and n-type material,but for the latter,a di!erent com-pound semiconductor,cadmium sulphide,gives best re-sults.CdTe cells have been used mainly in pocket calculators to date,but large area,moderate performance modules have also been demonstrated(Kazmerski,1997). The main concern with this technology is the toxicity of the materials involved,even though very small amounts are used in the modules.At the very least,this would mean that modules would have to be carefully disposedof or,preferably,recycled after their useful life was"nish-ed.However,there may be some problems in gainingmarket acceptance in what is likely to be mainlya`green a market over coming years.There are also only limited known resources of tellu-rium(Zweibel and Green,2000).If all identi"ed reserveswere converted into cells of the present designs overnight,they could generate10%of the world's present electricityuse(a steadily decreasing percentage inde"nitely,if re-cycled at end of life).An even more promising technology at the moment,inthe author's opinion,is one based on the ternary com-pound,copper indium diselenide(CuInSe ).As if three elements were not enough,this compound is oftenalloyed with copper gallium diselenide(CuGaSe )and copper indium disulphide(CuInS ),giving material with up to"ve elements involved(Kazmerski,1997).The n-type layer in these devices consists of a layer of cadmium sulphide,as in the previous cadmium telluride cells.An alternative for this layer is being sought,to eliminate the toxic cadmium.Small area laboratory cells have demonstrated e$cien-cy close to19%,despite the"ne-grained polycrystallinematerial used(Zweibel and Green,2000;Kazmerski,1997;Partain,1995).Modules of this material are nowcommercially available in small volumes with e$ciencyup to12%demonstrated in pilot production.This is notfar behind what is achieved with standard crystallinesilicon wafer modules.Apart from the use of cadmium and even more limitedknown resources of indium than tellurium,an oftenquoted limitation of this technology is`manufacturabil-ity a.This is often interpreted as meaning it is di$cult todiagnose problems in production with this material,sincethe di!erence between good and bad material is notsu$ciently well understood to allow di!erentiation andcontrol during the various manufacturing steps.5.4.Thin-x lm polycrystalline silicon cellsAs previously mentioned,silicon is a weak absorber ofsunlight compared to some compound semiconductorsand even to hydrogenated amorphous silicon.Early at-tempts to develop thin-"lm solar cells based on the poly-crystalline silicon did not give encouraging results sincethe silicon layers had to be quite thick to absorb most ofthe available light.However,in early1980s,understanding of how e!ec-tively a semiconductor can trap weakly absorbed lightinto its volume greatly increased(Green,1995).Due tothe optical properties of semiconductors,particularlytheir high refractive index,cells can trap light very e!ec-tively if the light direction is randomised,such as bystriking a rough surface,once it is inside the cell.Optical-ly a cell can appear about50times thicker than its actual994M.A.Green/Energy Policy28(2000)989}998Fig.9.One approach for high-temperature preparation of silicon thin-"lm solarcells.Fig.10.Prototype thin-"lm polycrystalline silicon-on-glass module (photograph courtesy of Paci "c Solar Pty.Ltd.,Sydney).Fig.11.Nanocrystalline dye sensitised solar cell.thickness if this occurs.Such `light trapping a removes the weak absorption disadvantage of silicon.Work on polycrystalline thin-"lm solar cells is pro-ceeding in two areas.A variety of `high temperature a approaches such as suggested by Fig.9are being ex-plored.There generally involve either high-temperature deposition of silicon onto a substrate or melting the silicon after deposition,to obtain large grain size in the "nal "lm.Although preparation details are sketchy,the thin-"lm silicon product available from the US company,Astropower,is the most developed representative of this class of approach.In this case,the silicon is deposited onto an expansion-matched ceramic substrate.The "nal material consists of millimetre sized grains and is similar in appearance to multicrystalline silicon wafers.Small area cell performance in the 16}17%range has been demonstrated (Kazmerski,1997),similar to that from such cells on moderate-quality multicrystalline silicon.The performance of large arrays of such cells has also been similar.The other type of approach is a `low-temperature a approach,generally based on amorphous silicon techno-logy.One approach is to deposit the silicon in amorph-ous form and then crystallise it by heating for prolonged periods at intermediate temperatures.This `solid-phase crystallisation a approach has produced cells of quite reasonable performance (Baba et al .,1995).Another ap-proach has involved changing the amorphous silicon deposition conditions,to produce a nanocrystalline phase of silicon.The potential of this approach was highlighted by early results with the `micromorph a solar cell (Wyrsch et al .,1998).More recently,cell e $ciency above 10%has been con "rmed with this approach (Yamamoto et al .,1999).There are plans to use such cells as the lower cell in a tandem con "guration with an amorphous silicon upper cell,with commercial product targeted for 2002(Yamamoto et al .,1999).Also targeted for commercialisation in the same timeframe is a poly-crystalline silicon on glass product shown in Fig.10,based on an amorphous silicon precursor (Paci "c Solar,1999).5.5.Nanocrystalline dye cellsA completely di !erent thin-"lm approach is based on the use of ruthenium-based organic dyes [8,9].Dye mol-ecules are coated onto a porous network to titanium dioxide particles and immersed in an electrolyte (Fig.11).In a process bearing some relationship to photosynthesis,light absorbed by the dye photoexcites a electron into the titanium dioxide which completes the circuit through the external load and the electrolyte.Interestingly,the dye only absorbs a band of photon energies,rather than all photons of energy above theM.A.Green /Energy Policy 28(2000)989}998995。
