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CHAPTER 3 SOLAR PHOTOVOLTAICS 6

Chapter 3

Solar Photovoltaics

by Godfrey Boyle

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66 RENEWABLE ENERGY

3.1 Introduction

In Chapter 2 we saw how solar energy can be used to generate electricity by producing high temperature heat to power an engine which thenproduces mechanical work to drive an electrical generator.

This chapter is concerned with a more direct method of generating

electricity from solar radiation, namely photovoltaics: the conversion of solar energy directly into electricity in a solid-state device.

The chapter starts with a brief look at the history and basic principles of photovoltaic energy conversion, concentrating initially on devices usingmonocrystalline silicon. We then review various ways of reducing thecurrently high costs of energy from photovoltaics.

The electrical characteristics of photovoltaic cells and modules are thendescribed, followed by a discussion of the various roles of photovoltaicenergy systems in supplying power in remote locations and in feedingpower into local or national electricity grids.

The concluding sections review the economics and the environmental

impact of photovoltaic electricity, examine the resources available fromphotovoltaics and how it might be increasingly integrated into electricitysupplies, and look at the world market and future prospects forphotovoltaics.

3.2 Introducing photovoltaics

If you were asked to design the ideal energy conversion system, you wouldprobably find it difficult to come up with something better than the solarphotovoltaic (PV) cell.

In this we have a device which harnesses an energy source that is by far

the most abundant of those available on the planet. As earlier chaptershave emphasised, the net solar power input to the earth is more than10000 times humanity’s current rate of use of fossil and nuclear fuels.

The PV cell itself is, in its most common form, made almost entirely fromsilicon, the second most abundant element in the earth’s crust. It has nomoving parts and can therefore in principle, if not yet in practice, operatefor an indefinite period without wearing out. And its output is electricity,probably the most useful of all energy forms.

A brief history of PV

The term ‘photovoltaic’ is derived by combining the Greek word for light,

photos, with volt , the name of the unit of electromotive force – the force thatcauses the motion of electrons (i.e. an electric current). The volt was namedafter the Italian physicist Count Alessandro Volta, the inventor of thebattery.Photovoltaics thus describes the generation of electricity from light.

The discovery of the photovoltaic effect is generally credited to the Frenchphysicist Edmond Becquerel (Figure 3.1), who in 1839 published a paper(Becquerel, 1839) describing his experiments with a ‘wet cell’ battery, in

Figure 3.1 Edmond Becquerel,

who discovered the photovoltaic

effect

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the course of which he found that the battery voltage increased when itssilver plates were exposed to sunlight.

The first report of the PV effect in a solid substance appeared in 1877 whentwo Cambridge scientists, W. G. Adams and R. E. Day, described in a paperto the Royal Society the variations they observed in theelectricalpropertiesof selenium when exposed to light (Adams and Day, 1877).

In 1883 Charles Edgar Fritts, a New York electrician,constructed a seleniumsolar cell that was in some respects similar to the silicon solar cells of today (Figure 3.2). It consisted of a thin wafer of selenium covered with agrid of very thin gold wires and a protective sheet of glass. But his cell wasvery inefficient. The efficiency of a solar cell is defined as the percentageof the solar energy falling on its surface that is converted into electricalenergy. Less than 1% of the solar energy falling on these early cells wasconverted to electricity. Nevertheless, selenium cells eventually came intowidespread use in photographic exposure meters.

The underlying reasons for the inefficiency of these early devices wereonly to become apparent many years later, during the first half of thetwentieth century, when physicists such as Planck and Einstein provided

new insights into the nature of radiation and the fundamental propertiesof materials (see Section 3.3 below).

It was not until the 1950s that the breakthrough occurred that set in motionthe development of modern, high-efficiency solar cells. It took place at theBell Telephone Laboratories (BellLabs) in New Jersey, USA, wherea numberof scientists, including Darryl Chapin, Calvin Fuller and Gerald Pearson(Figure 3.3), were researching the effects of lighton semiconductors. These are non-metallicmaterials, such as germanium and silicon, whoseelectrical characteristics lie between those of conductors, which offer little resistance to theflow of electric current, and insulators, which

block the flow of current almost completely.Hence the term semi conductor.

A few years before, in 1948, two other Bell Labsresearchers, Bardeen and Brattain, had producedanother revolutionary device usingsemiconductors – the transistor. Transistors aremade from semiconductors (usually silicon) inextremely pure crystalline form, into which tinyquantities of carefully selected impurities, suchas boron or phosphorus, have been deliberatelydif fused. This process , known as doping,dramatically alters the electrical behaviour of the

semiconductor in a very useful manner that will be described in detail later.

In 1953 the Chapin–Fuller–Pearson team, building on earlier Bell Labs research on the PVeffect in silicon (Ohl, 1941), produced ‘doped’silicon slices that were much more efficient thanearlier devices in producing electricity from light.

Figure 3.2 Diagram from

Charles Edgar Fritts’ 1884 US

patent application for a solar ce

Figure 3.3 Bell Laboratories’ pioneering PV researchers

Pearson,Chapin and Fuller measure the response of an early

solar cell to light

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68 RENEWABLE ENERGY

By the following year they had produced a paper on their work (Chapin et al ., 1954) and had succeeded in increasing the conversion efficiency of their silicon solar cells to 6%. Bell Labs went on to demonstrate thepracticaluses of solar cells, for example in powering rural telephone amplifiers, butat that time they were too expensive to be an economic source of power inmost applications.

In 1958, however, solar cells were used to power a small radio transmitterin the second US space satellite, Vanguard I. Following this first successfuldemonstration, the use of PV as a power source for spacecraft has becomealmost universal (Figure 3.4).

Rapid progress in increasing the efficiency andreducing the cost of PV cells has been made overthe past few decades. Their terrestrial uses are nowwidespread, particularly in providing power fortelecommunications, lighting and other electricalappliances in remote locations where a moreconventional electricity supply would be toocostly.

A singleconventional PV cell produces only about

1.5 watts, so to obtain more power, groups of cellsare normally connected together to formrectangular modules. To obtain evenmore power,modules are in turn mounted side by side andconnected together to form arrays.

A growing number of domestic, commercial andindustrialbuildings now have PV arrays providinga substantial proportion of their energy needs.And a number of large, megawatt-sized PV powerstations connected to electricity grids are now inoperation in the USA, Germany, Italy, Spain andSwitzerland.

The efficiency of the best single-junction siliconsolar cells has now reached 24% in laboratory testconditions (see Box 3.1 and Box 3.3). The best

silicon PV modules now available commercially have an efficiency of over17%, and it is expected that in about 10 years’ time module efficiencieswill have risen to over 20% (Appleyard, 2003). Over the decade to 2002,the total installed capacity of PV systems increased approximately ten-fold, module costs dropped to below $4 per peak watt and overall systemcosts fell to around $7 per peak watt (see Figures 3.5 and 3.6). As we shallsee, improvements in the cost-effectiveness of PV are likely to continue.

3.3 PV in silicon: basic principlesSemiconductors and ‘doping’

PV cells consist, in essence, of a junction between two thin layers of dissimilar semiconducting materials, known respectively as ‘p’ (positive)-type semiconductors, and ‘n’ (negative)-type semiconductors. Thesesemiconductors are usually made from silicon, so for simplicity we shall

Figure 3.4 The International Space Station is powered by

large arrays of photovoltaic panels with a total output of 110

kilowatts

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Figure 3.5 Cumulative PV power installed by type of application in IEA reporting

countries,1992–2002 (source: International Energy Agency, 2003). Note:data in this figure

are from 20 International Energy Agency reporting countries,mainly ‘developed’ nations.

There is additional significant PV module production and installed capacity in a number of

other countries,particularly in the ‘developing’ world

20022001200019991998199719961995199419931992

grid-connected centralised

grid-connected distributed

off-grid non-domestic

off-grid domestic

1400

1200

1000

800

600

400

200

0

c u m u l a t i v e i n s t a l l e d c a p a c i t y / m e

g a w a t t s

Figure 3.6 PV system and module price trends in three selected IEA reporting

countries (source: International Energy Agency, 2003)

20022001200019991998199719961995199419931992

country 1 modules

country 2 modules

country 3 modules

country 1 systems

country 2 systems

country 3 systems

p r i c e / $ U S p e r w a t t

30

25

20

15

10

5

0

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70 RENEWABLE ENERGY

BOX 3.1 Standard test conditions for PV cells and modules

There is widespread international agreement that theperformance of PV cells and modules should bemeasured under a set of standard test conditions.

Essentially, these specify that the temperature of thecell or module should be 25 °C and that the solar

radiation incident on the cell should have a totalpower density of 1000 watts per square metre (W m−2),with a spectral power distribution known as Air Mass1.5 (AM 1.5).

The spectral power distribution is a graph describingthe way in which the power contained in the solarradiation varies across the spectrum of wavelengths.

The concept of ‘Air Mass’ describes the way in whichthe spectral power distribution of radiation from thesun is affected by the distance the sun’s rays have totravel though the atmosphere before reaching anobserver (or a PV module).

Just outside the earth’s atmosphere, the sun’s radiationhas a power density of approximately 1365 W m−2.The characteristic spectral power distribution of solarradiation as measured before it enters the atmosphereis described as the Air Mass 0 (AM0) distribution.

At the earth’s surface, the various gases of which theatmosphere is composed (oxygen, nitrogen, ozone,water vapour, carbon dioxide, etc.) attenuate the solarradiation selectively at different wavelengths. Thisattenuation increases as the distance over which thesun’s rays have to travel through the atmosphereincreases.

When the sun is at its zenith (i.e.

directly overhead), the distanceover which the sun’s rays haveto travel through the atmosphereto a PV module is at a minimum.The characteristic spectralpower distribution of solarradiation that is observed underthese conditions is known as theAir Mass 1 (AM1) distribution.

When the sun is at a given angleθ to the zenith (as perceived byan observer at sea level), the AirMass is defined as the ratio of

the path length of the sun’s raysunder these conditions to thepath length when the sun is atits zenith. By simpletrigonometry (see Figure 3.7),this leads to the definition:

Air Mass ≈

An Air Mass distribution of 1.5, as specified in thestandard test conditions, therefore corresponds to thespectral power distribution observed when the sun’sradiation is coming from an angle to overhead of about48 degrees, since cos 48° = 0.67 and the reciprocal of this is 1.5.

The approximate spectral power distributions for AirMasses 0 and 1.5 are shown in Figure 3.8.

In practice, the power rating in peak watts (Wp) of a cellor module is determined by measuring the maximumpower it will supply when exposed to radiation fromlamps designed to reproduce the AM1.5 spectraldistribution at a total power density of 1000 W m−2 .

Figure 3.7 Air Mass is the ratio of the path length of the sun’s rays

through the atmosphere when the sun is at a given angle (θ) to the

zenith, to the path length when the sun is at its zenith

sun at zenith

S

θ

O

Z

sun at angle θ

to zenith

a t m o s

p h e r

e e ar t h ’ s s u r f a c e Air Mass =

SOZO

Figure 3.8 The spectral power distributions of solar radiation corresponding to Air Mass 0 and

Air Mass 1.5. Also shown is the theoretical spectral power distribution that would be expected, in

space,if the sun were a perfect radiator (a ‘black body’) at 6000 °C

e n e r g y d i s t r i b u t i o n / k W

m – 2 µ m

– 1

0

2.5

2.01.81.61.41.21.00.80.40.20 0.6

2.0

1.5

1.0

0.5

6000 K black body

AM0 radiation

AM1.5 radiation

wavelength/µm

1

cosθ

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initially consider only silicon-based semiconductors – although,as we shallsee later, PV cells can be made from other materials.

n-type semiconductors are made from crystalline silicon that has been‘doped’ with tiny quantities of an impurity (usually phosphorus) in such away that the doped material possesses a surplus of free electrons. Electronsare sub-atomic particles with a negative electrical charge, so silicon doped

in this way is known as an n (negative)-type semiconductor.p-type semiconductors arealsomadefromcrystalline silicon, but are dopedwith very small amounts of a different impurity (usually boron) whichcauses the material to have a deficit of free electrons. These ‘missing’electrons are called holes. Since theabsence of a negatively charged electroncan be consideredequivalent to a positively charged particle, silicon dopedin this way is known as a p (positive)-type semiconductor (see Box 3.2).

The p–n junction

We can create what is known as a p–n junction by joining these dissimilarsemiconductors. This sets up an electric field in the region of the junction.

This electric field is like the electrostatic field you can generate by rubbinga plastic comb against a sweater. It will cause negatively charged particlesto move in one direction, and positively charged particles to move in theopposite direction. However, a p–n junction in practice is not a simplemechanical junction: the characteristics change from ‘p’ to ‘n’ gradually,not abruptly, across the junction.

The PV effect

What happens when light falls on the p–n junction at the heart of a solar cell?

Light can be considered to consist of a stream of tiny particles of energy,called photons. Whenphotons from light of a suitable wavelength fall within

the p–n junction, they can transfer their energy to some of the electrons inthe material, so ‘promoting’ them to a higher energy level. Normally, theseelectrons help to hold the material together by forming so-called ‘valence’

bonds with adjoining atoms, and cannot move. In their ‘excited’ state,however, the electrons become free to conduct electric current by movingthrough the material. In addition, when electrons move they leave behindholes in the material, which can also move (Figure 3.9 in Box 3.2). The ‘carparking’ analogy shown in Figure 3.10 may be helpful in visualizing theprocesses involved.

When the p–n junction is formed, some of the electrons in the immediatevicinity of the junction are attracted from the n-side to combine with holeson the nearby p-side. Similarly, holes on the p-side near the junction are

attracted to combine with electrons on the nearby n-side.The net effect of this is to set up around the junction a layer on the n-sidethat is more positively charged than it would otherwise be, and, on the p-side, a layer that is more negatively charged than it would otherwise be. Ineffect, this means that a reverse electric field is set up around the junction:negative on the p-side and positive on the n-side. The region around thejunction is also depleted of charge carriers (electrons and holes) and istherefore known as the depletion region.

Figure 3.10 ‘Car parking’

analogy of conduction processes

in a semiconductor. (a)The

ground floor of the car park is

full – the cars there (representin

electrons in the ‘valence band’)

cannot move around. The first

floor is empty. (b)A car

(electron) is ‘promoted’ to the

first floor (representing the

‘conduction band’), where it can

move around freely. This leaves

behind a ‘hole’ that also allows

cars on the ground floor (valenc

band) to move around (source:

Green, 1982)

(b)

(a)

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72 RENEWABLE ENERGY

BOX 3.2 Crystalline silicon, doping, p–n junctions and PV cells

Figure 3.9 (a) Crystal of pure silicon has a cubic

structure, shown here in two dimensions for

simplicity. The silicon atom has four ‘valence’

electrons. Each atom is firmly held in the crystal

lattice by sharing two electrons (small dots) with

each of four neighbours at equal distances from it.

Occasionally thermal vibrations or a photon of light will spontaneously provide enough energy to

promote one of the electrons into the energy

level known as the conduction band, where the

electron (small dot with arrow) is free to travel

through the crystal and conduct electricity. When

the electron moves from its bonding site, it leaves

a ‘hole’ (small open circle), a local region of net

positive charge

(b) Crystal of n-type silicon can be created by

doping the silicon with trace amounts of

phosphorus. Each phosphorus atom (large

coloured circle) has five valence electrons, so that

not all of them are taken up in the crystal lattice.

Hence n-type crystal has an excess of free

electrons (small dots with arrows)

(c) Crystal of p-type silicon can be created by

doping the silicon with trace amounts of boron.

Each boron atom (large coloured circle) has only

three valence electrons, so that it shares two

electrons with three of its silicon neighbours and

one electron with the fourth. Hence the p-type

crystal contains more holes than conduction

electrons

(d)A silicon solar cell is a wafer of p-type silicon

with a thin layer of n-type silicon on one side.

When a photon of light with the appropriate

amount of energy (labelled a) penetrates the cell

near the junction of the two types of crystal and

encounters a silicon atom, it dislodges one of theelectrons, which leaves behind a hole. The energy

required to promote the electron into the

conduction band is known as the band gap. The

electron thus promoted tends to migrate into the

layer of n-type silicon, and the hole tends to

migrate into the layer of p-type silicon. The

electron then travels to a current collector on

the front surface of the cell, generates an electric

current in the external circuit and then reappears

in the layer of p-type silicon, where it can

recombine with waiting holes.If a photon with an

amount of energy greater than the band gap

(labelled b) strikes a silicon atom,it again gives

rise to an electron–hole pair, and the excess

energy is converted into heat. A photon with anamount of energy smaller than the band gap (c)

will pass right through the cell, so that it gives up

virtually no energy along the way. Moreover, some

photons (d) are reflected from the front surface of

the cell even when it has an antireflection coating.

Still other photons are lost because they are

blocked from reaching the crystal by the current

collectors that cover part of the front surface

(source for text and figures:Chalmers,1976)

(d)

a bcd

currentflows inexternalcircuit

front metalcontacts

antireflectioncoating

electron-holepairs formed

rear metalcontact ammeter

holes driftto p-region(back contact)


currentcollector

p-typecrystal

electronsdrift ton-region(frontcontacts)n-type

crystal

(a)

(b)

(c)

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74 RENEWABLE ENERGY

by the internal electric field set up at the p–n junction. As we shall see,a single crystalline silicon PV cell typically produces a voltage of about0.5 V at a current of up to around 3 A – that is, a peak power of about 1.5 W.(Depending on their detailed design, some PV cells produce more powerthan this, some less.)

BOX 3.3 Band gaps and PV cell efficiency

According to the quantum theory of matter, thequantity of energy possessed by any given electron in amaterial will lie within one of several levels or ‘bands’.Those electrons that normally hold the atoms of amaterial together (by being ‘shared’ between adjoiningatoms, as we saw in Figure 3.9) are described byphysicists as occupying a lower-energy state known asthe valence band.

In certain circumstances, some electrons may acquireenough energy to move into a higher-energy state,known as the conduction band, in which they canmove around within the material and thus conductelectricity (Figure 3.11). There is a so-called energygap or band gap between these bands, the magnitudeof which varies from material to material, and which ismeasured using an extremely small energy unit: theelectron volt (eV) (see Appendix A2).

Metals, which conduct electricity well, have many

electrons in the conduction band. Insulators, whichhardly conduct electricity at all, have virtually noelectrons in the conduction band. Pure (or ‘intrinsic’)semiconductors have some electrons in the conduction

band, but not as many as in a metal. But ‘doping’ puresemiconductors with very small quantities of certainimpurities can greatly improve their conductivity.

If a photon incident on a doped, n-type semiconductorin a PV cell is to succeed in transferring its energy toan electron and ‘exciting’ it from the valence band tothe conduction band, it must possess an energy at leastequal to the band gap. Photons with energy less thanthe band gap do not excite valence electrons to enter

the conduction band and are ‘wasted’. Photons withenergies significantly greater than the band gap dosucceed in ‘promoting’ an electron into the conduction

band, but any excess energy is dissipated as heat. Thiswasted energy is one of the reasons why PV cells arenot 100% efficient in converting solar radiation intoelectricity. (Another is that not all photons incident ona cell are absorbed: a small proportion are reflected.)

Because the energy of a photon is directly proportionalto the frequency of the light associated with it, photonsassociated with shorter wavelengths (i.e. higherfrequencies) of light, near the blue end of the visiblespectrum, have a greater energy than those of longerwavelength near the red end of the visible spectrum.

The spectral distribution of sunlight variesconsiderably according to weather conditions and the

elevation of the sun in the sky (see Box 3.1). Formaximum efficiency of conversion of light into electricpower, it is clearly important that the band gap energyof the material used for a PV cell is reasonably wellmatched to the spectrum of the light incident upon it.For example, if the majority of the energy in theincoming solar spectrum is in the yellow–green range(corresponding to photons with energy of around1.5 eV), then a semiconductor with a band gap of around 1.5 eV will be most efficient. In general,semiconductor materials with band gaps between 1.0and 1.5 eV are reasonably well suited to PV use.Silicon has a band gap of 1.1 eV.

The maximum theoretical conversion efficiencyattainable in a single-junction silicon PV cell has beencalculated to be about 30%, if full advantage is takenof ‘light trapping’ techniques to ensure that as many of the photons as possible are usefully absorbed (Green,1993). However, multi-junction cells have also been

designed in which each junction is tailored to absorb aparticular portion of the incident spectrum.Theoretically, such cells should have a much higherefficiency, possibly as high as 66% for an infinitenumber of junctions – though the efficiencies so farachieved by multi-junction cells in practice have beenvery much lower than this (see Section 3.6).

In practice, the highest efficiency achieved incommercially available single-junctionmonocrystalline silicon PV modules (as distinct fromindividual PV cells) is currently around 17%. Theefficiency of PV modules is usually lower than thatachieved by cells in the laboratory because:

■ it is difficult to achieve as high an efficiencyconsistently in mass-produced devices as in one-off laboratory cells under optimum conditions;

■ laboratory cells are not usually glazed orencapsulated;

■ in a PV module there are usually inactive areas, both between cells and due to the surroundingmodule frame, that are not available to producepower;

■ there are small resistive losses in the wiring between cells and in the diodes used to protect cellsfrom short circuiting;

■

there are losses due to mismatching between cellsof slightly differing electrical characteristicsconnected in series.

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Monocrystalline silicon cells

Until fairly recently, the majority of solar cells were made from extremelypure monocrystalline silicon – that is, silicon with a single, continuouscrystal lattice structure (Figure 3.9) having virtually no defects or impurities.Monocrystalline silicon is usually grown from a small seed crystal that isslowly pulled out of a molten mass, or ‘melt’, of polycrystalline silicon

(see below), in the sophisticated but expensive Czochralski processdeveloped initially for the electronics industry. The entire process of

Figure 3.12 The overall process of monocrystalline silicon solar cell and module

production

polished wafers

dissolvein HCl

doping to form p–n junction

interconnection,testing,encapsulationand assemblyinto modules

high puritytrichlorosilane

metallurgicalgrade silicon

distillation

chlorosilanes

polycrystallinesilicon

heat(1500 °C)Czochralskiprocess

silicon crystal

diamond sawing

silicon wafers

chem/mechpolishing

formation of front contact


assembly of modules intoarray

sand (SiO2)

2 (900 °C)reduction withH

testing

coke reduction

arc furnace

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76 RENEWABLE ENERGY

monocrystalline silicon solar cell and module production is summarizedin Figure 3.12. Many of the most efficient single-junction monocrystallinePV modules currently available use a ‘laser-grooved buried-grid’ cellstructure as shown in Figure 3.13.

3.4 Crystalline PV: reducing costsand raising efficiency

Although the latest monocrystalline silicon PV modules are highly efficient,they are also expensive because the manufacturing processes are slow,require highly skilled operators, and are labour- and energy-intensive.Another reason for their high cost is that most high-efficiency cells arefabricated fromextremely pure ‘electronic-grade’ silicon.However, PV cellscan be made from slightly less pure, but less costly, ‘solar-grade’ silicon,with only a small reduction in conversion efficiency.

A number of approaches to reducing the cost of crystalline PV cells andmodules, or increasing their efficiency, have been under developmentduring the past 20 years or so. These include cells using polycrystallinerather than single-crystal material; growing silicon in ribbon or sheet form;

and the use of other crystalline PV materials such as gallium arsenide.

Polycrystalline silicon

Polycrystalline silicon essentially consists of small grains of monocrystalline silicon (Figure 3.14). Solar cell wafers can be madedirectlyfrom polycrystalline silicon in various ways. These include the controlledcasting of molten polycrystalline silicon into cube-shaped ingots which

Figure 3.13 Main features of the ‘laser-grooved buried-grid’ monocrystalline PV cell

structure, developed at the University of New SouthWales (Green, 1993) and used in

many high-efficiency PV modules. A pyramid-shaped texture on the top surface increases

the amount of light ‘trapped’.Buried electrical contacts give very low electrical resistance

whilst minimizing losses due to overshadowing. The symbols p+ and n+ denote heavily

doped layers that reduce electrical resistance in the contact areas

p layer

plated metalfront contacts

(in laser-cutgrooves)

metal back contact

oxide

n+ layer

p+ layer

Figure 3.14 Polycrystalline

silicon consists of randomly

packed ‘grains’ of monocrystalline

silicon

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are then cut, using fine wire saws, into thin square wafers and fabricatedinto complete cells in the same way as monocrystalline cells.

Polycrystalline PV cells are easier and cheaper to manufacture than theirmonocrystalline counterparts. But they tend to be less efficient becauselight-generated charge carriers (i.e. electrons and holes) can recombine atthe boundaries between the grains within polycrystalline silicon. However,

it has been found that by processing the material in such a way that thegrains are relatively large in size, and oriented in a top-to-bottom directionto allow light to penetrate deeply into each grain, their efficiency can besubstantially increased. These and other improvements have enabledcommercially available polycrystalline PV modules (sometimes called‘multi-crystalline’ or ‘semi-crystalline’) to reach efficiencies of over 14%.

Companies such as Pacific Solar in Australia and Astropower in the USAare developing ways of depositing polycrystalline films on to ceramic orglass substrates, forming PV modules with an efficiency of around 10%.The silicon films used are somewhat thicker than in other ‘thin film’ PVcells (see below), so cells made in this way are sometimes known as ‘thick film’ polycrystalline cells.

Silicon ribbons and sheets

This approach involves drawing thin ‘ribbons’or ‘sheets’of multicrystallinesilicon from a silicon ‘melt’. One of the main processes is known as edge-defined, film-fed growth (EFG), and was originally developed by the UScompany Mobil Solar. It is described in Figure 3.15. EFG cells aremanufactured by the German company RWE Schott Solar (under the brandname ASE) and silicon ribbon cells by Evergreen Solar in the USA.

Figure 3.15 Edge-defined, film-fed growth process for PV production.Thin, hollow polygonal tubes of polycrystalline silicon upto 6 m long are slowly pulled through a die from a ‘melt’ of pure silicon, then cut by laser into individual cells

silicon meltat 1400 °C

nine-sideddie

nonagon tubepulled from

melt

nonagon tubecut by laser

doping and

processing

finishedsilicon cell

crystallinesilicon

1 3

die

die

siliconmelt

2

Gallium arsenide

Silicon is not the only crystalline material suitable for PV. Another is galliumarsenide (GaAs), a so-called compound semiconductor. GaAs has a crystalstructure similar to that of silicon (see Figure 3.9), but consisting of

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78 RENEWABLE ENERGY

alternating gallium and arsenic atoms. In principle itis highly suitable for use in PV applications because ithas a high light absorption coefficient, so only a thinlayer of material is required. GaAs cells also have a

band gap wider than that of silicon, one close to thetheoretical optimum for absorbing the energy in theterrestrial solar spectrum (seeBox 3.3). Cells made from

GaAs are therefore more efficient than those made frommonocrystalline silicon. They can also operate atrelatively high temperatures without the appreciablereduction in efficiency that affects PV cells made fromsilicon when their temperatures rise. This makes themwell suited to use in concentrating PV systems (seeSection 3.6 below).

On the other hand, cel ls made from GaAs aresubstantially more expensive than silicon cells, partly

because the production process is no t so welldeveloped, and partly because gallium and arsenic arenot abundant materials. GaAs cells have often been

used when very high efficiency, regardless of cost, isrequired – as in many space applications. They havealso powered many of the winning cars in solar carraces (see Figure 3.16).

3.5 Thin film PV

Amorphous silicon

Solar cells can be made from very thin films of siliconina form known as amorphous silicon (a-Si), in which

the silicon atoms are much less ordered than in thecrystalline forms described above. In a-Si, not everysilicon atom is fully bonded to its neighbours, whichleaves so-called ‘dangling bonds’ that can absorb anyadditional electrons introduced by doping, so renderingany p–n junction ineffective.

However, this problem is largely overcome in theprocess by which a-Si cells are normally manufactured.

A gas containing silicon and hydrogen (such as silane, SiH4), and a smallquantity of dopant (such as boron), is decomposed electrically in such away that it deposits a thin film of amorphous silicon on a suitable substrate(backing material). The hydrogen in the gas has the effect of providing

additional electrons that combine with the dangling silicon bonds to form,in effect, an alloy of silicon and hydrogen. The dopant that is also presentin the gas can then have its usual effect of contributing charge carriers toenhance the conductivity of the material.

Solar cells using a-Si have a somewhat different form of junction betweenthe p- and the n-type material. A so-called ‘p–i–n’ junction is usuallyformed, consisting of an extremely thin layer of p-type a-Si on top, followed

by a thicker ‘intrinsic’ (i) layer made of undoped a-Si, and then a very thinlayer of n-type a-Si. The structure is as shown in Figure 3.17. The operation

Figure 3.16 The Dutch Nuna solar car, winner of the

2001World Solar Challenge, arriving at Adelaide, South

Australia after completing the 3000 km journey from

Darwin in just over 32 hours at an average speed

(during daylight hours) of 91 km per hour. The ultra-

lightweight car was produced by students from the

universities of Delft and Rotterdam, sponsored by theutility Nuon. It is driven by electric motors powered by

arrays of high-efficiency, dual-junction and triple-junction

gallium arsenide PV cells developed by the European

Space Agency for satellite use. (The 2003 solar challenge

winner was Nuna II, also from the Netherlands, which

completed the journey at an average speed of 97 km per

hour)

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of the PV effect in a-Si is generally similar to that in crystalline silicon,except that in a-Si the band gap,although wider, is less clearly defined.

Amorphous silicon cells are muchcheaper to produce than thosemadefrom crystalline silicon. a-Si is also a much better absorber of light, somuch thinner (and therefore cheaper) films can be used. Themanufacturing process operates at a much lower temperature than

that for crystalline silicon, so less energy is required; it is suited tocontinuous production; and it allows quite large areas of cell to bedeposited on to a wide variety of both rigid and flexible substrates,including steel, glass and plastics.

But a-Si cells are currently much less efficient than their single-crystalor polycrystalline silicon counterparts:maximumefficiencies achievedwith small, single-junction cells in the laboratory arecurrently around12%. Moreover, the efficiency of many currently available single-junction a-Si modules degrades, within a few months of exposure tosunlight, from an initial 6–10%, stabilizing at around 4–8%.

Strenuous attempts have been made by manufacturers to improve theefficiency of a-Si cells and to solve the degradation problem. One

approach involves the development of multiple-junction a-Si devices(see Section 3.6 below.)

Amorphous silicon cells are already widely used as power sourcesfor a variety of consumer products such as calculators, where therequirement is not so much for high efficiency as for low cost.

Other thin film PV technologies

Amorphous silicon is by no means the only material suited to thinfilm PV, however. Among the many other possible thin filmtechnologies some of the most promising are thosebasedon compoundsemiconductors, in particular copper indium diselenide (CuInSe2,

usually abbreviated to CIS), copper indium gallium diselenide (CIGS)and cadmium telluride (CdTe). Modules based on all of thesetechnologies have reached the production stage, but productionvolumes are small.

Thin film CIGS cells have attained the highest laboratory efficienciesof all thin film devices, around 17%, and CIGS modules with stableefficiencies of around 10% are produced by Shell Solar in the USAand Würth Solar in Germany.

Cadmium telluride modules can be made using a relatively simpleand inexpensive electroplating-type process. The band gap of CdTe isclose to the optimum, andmodule efficiencies ofover 10% are claimed,

without the initial performance degradation that occurs in a-Si cells.However, the modules contain cadmium, a highly toxic substance, sostringent precautions need to be taken during their manufacture, useand eventual disposal (see Section 3.11). BP Solar, a subsidiary of theoil company BP, was involved in CdTe module manufacture butwithdrew from production in 2002, explaining that although it didnot consider the presence of cadmium in its modules to be a problem,customers appeared to believe otherwise.TheJapanese firm Matsushitaalso withdrew from CdTe production in 2002, but First Solar Inc. inthe USA continues to produce CdTe modules.

Figure 3.17 Structure of an

amorphous silicon cell. The top

electrical contact is made of an

electrically conducting,but transparentlayer of tin oxide deposited on the

glass. Silicon dioxide forms a thin

‘barrier layer’ between the glass and

the tin oxide. The bottom contact is

made of aluminium.In between are

layers of p-type, intrinsic and n-type

amorphous silicon

back contact

light

s i l i c o n d i o

x i d e ( S i O 2 )

p - t y p e

a m o r p h o u

s s i l i c o n

i n t r i n s i c

n - t y p e

a l u m i n i u m

t o p c o n d u

c t i n g l a y e r

t i n o x i d e

( S n O 2 )

g l a s s

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80 RENEWABLE ENERGY

3.6 Other innovative PV technologies

Multi-junction PV cells

One way of improving the overall conversion efficiency of PV cellsand modules is the ‘stacked’ or multi-junction approach, in which

two (or more) PV junctions are layered one on top of the other, eachlayer extracting energy from a particular portion of the spectrum of the incoming light. A cell with two layers is often called a ‘tandem’device.

The band gap of amorphous silicon, for example, can be increased byalloying the material with carbon, so that the resulting materialresponds better to light at the blue end of the spectrum. Alloying withgermanium, on the other hand, decreases the band gap so the materialresponds better to light at the red end of the spectrum.

Typically, a wide band gap a-Si junction would be on top, absorbingthe higher-energy photons at the blue end of the spectrum, followed

by other thin film a-Si junctions, each having a band gap designed to

absorb a portion of the lower light frequencies, nearer the red end of the spectrum (Figure 3.18). Multi-junction modules using amorphoussilicon are available with stable efficiencies of around 8% fromcompanies such as Unisolar and RWE.

Cells of different types can also be used, as in the Sanyo ‘Hybrid HIT’module, in which a thin monocrystalline layer is sandwiched betweentwo amorphous silicon layers. This enables very high conversionefficiencies to be achieved with low materialand manufacturingenergyrequirements. Another example is the tandem module produced byKaneka of Japan, which has a layer of amorphous silicon on top of athin layer of ‘microcrystalline’ silicon (i.e. silicon in the form of extremely small crystals less than one micrometre in diameter).

Concentrating PV systems

Another way of getting more energy out of a given number of PV cellsis to use mirrors or lenses to concentrate the incoming solar radiationon to the cells, an approach similar to that described in Chapter 2,Section 2.10, on solar thermal engines. This has the advantage thatsubstantially fewer cells are required – to an extent depending on theconcentration ratio, which can vary from as little as two to severalhundred or even thousand times. The concentrating system must havean aperture equal to that of an equivalent flat plate array to collect thesame amount of incoming energy. In concentrating PV systems thecells usually need to be cooled, either passively or actively, to preventoverheating.

Systems with the highest concentration ratios use complex sensors,motors and controls to allow them to track the sun on two axes –azimuth (horizontal orientation) and elevation (tilt) – ensuring thatthe cells always receive the maximum amount of solar radiation.Systems with lower concentration ratios often track the sun on onlyone axis and can have simpler tracking mechanisms.

Figure 3. 18 Structure of a multi-

junction amorphous silicon cell

back contact

light

s i l i c o n d i o

x i d e ( S i O 2 )

p - t y p e

a m o r p h o u

s s i l i c o n

t o p c o n d u

c t i n g l a y e r

t i n o x i d e

g l a s s

p - t y p e

a m o r p h o u s s i l i c

o n

( i n t r i n s i c ) a

l l o y e d w i t h

c a r b o n

i n t r i n s i c

n - t y p e

n - t y p e

p - t y p e

a m o r p h o u

s s i l i c o n

( i n t r i n s i c ) a l l o

y e d

w i t h g e r m a n i u m

n - t y p e

i n d i u m - t i n

- o x i d e

s i l v e r

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Most concentrators can only utilize direct solar radiation. This is a problemin countries like the UK where nearly half the solar radiation is diffuse.However, some unconventional designs of concentrator allow some of thediffuse radiation, as well as direct radiation, to be concentrated (see Boesand Luque, 1993).

Silicon spheresThe US firm Texas Instruments has developed an ingenious way of makingPV cells using tiny, millimetre-sized, spheres of polycrystalline siliconembedded at regular intervals between thin sheets of aluminium foil.Among the advantages of this approach are that impurities in the silicontend to diffuse out to the surface of the spheres, where they can be ‘groundoff’ as part of the manufacturing process. This allows relatively cheap,low-grade silicon to be used as a starting material. The resulting sheets of PV material are very flexible (see Figure 3.19), which can be an advantagein some applications.

The technology is being commercialized by Automation Tooling SystemsInc. in Canada, wherea 20 MWp(megawatts-peak) plant is under construction.

Photoelectrochemical cells

A radically different, photoelectrochemical, approach to producing cheapelectricity from solar energy has been pioneered by researchers at the SwissFederal Institute of Technology in Lausanne. (Strictly, photoelectrochemicaldevices are not photovoltaic: this term implies a solid-state device.Photoelectrochemical devices,however, use liquids.) The idea of harnessingphotoelectrochemical effects to produce electricity from sunlight is notnew – indeed, Becquerel’s pioneering PV experiments were with liquid-

based devices.

The Swiss researchers, led by Professor Michael Grätzel, have achieved

much higher efficiencies than before, in a device that could be extremelycheap to manufacture.

It consists essentially of two thin glass plates, both covered with a thin,transparent, electrically conducting tin oxide layer (Figure 3.20, overleaf).To one plate is added a thin layer of titanium dioxide (TiO2), which is asemiconductor. The surface of the TiO2 has been treated to give itexceptionally high roughness, enhancing its light-absorbing properties.

Immediately next to the roughened surface of the TiO2 is a layer of ‘sensitizer’ dye only one molecule thick, made of a proprietary ‘transitionmetal complex’ based on ruthenium or osmium. Between this ‘sensitized’TiO2 and the other glass plate is a thicker layer of iodine-based electrolyte.

On absorption of a photon of suitable wavelength, the sensitizer layer injectsan electron into the conduction band of the titanium dioxide. Electrons sogenerated then move to the bottomelectrically conducting layer (electrode)and pass out into an external circuit where they can do work. They thenre-enter through the top electrode, where they drive a reduction–oxidationprocess in the iodine solution. This then supplies electrons to the sensitizedTiO2 layer in order to allow the process to continue.

Figure 3.19 ‘Silicon spheres’ PV

technology

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82 RENEWABLE ENERGY

The Swiss team claims to have achieved efficiencies of 10% in full (AM1.5) sunlight and that its cells are stable over long periods, though someresearchers are not fully convincedof this. PV cells based on this technologyarebeing manufactured on a small scaleby STI in Australia andby GreatcellSolar SA in Switzerland. (See Grätzel et al ., 1989,Grätzel, 2001 and O’Reganet al., 1991, 1993.)

‘Third generation’ PV cells

Still at the frontiers of research is a range of new photovoltaic technologiesknown as ‘third generation’ PV (crystalline PV is considered ‘firstgeneration’ and thin film PV ‘second generation’). These devices are

generally based on nanotechnology – that is, technology which aims tomanipulate molecules and atoms at extremely small scales, measured in billionths of a metre,or nanometres (nm).These tiny particles and structuresare called ‘nanoparticles’ and ‘nanostructures’; crystals of such sizes aretermed ‘nanocrystals’. Nanoparticles consisting of extremely smallcollections of atoms of semiconducting material are called ‘quantum dots’.

As Dresselhaus and Thomas (2001) observe, a whole new class of materialsis emerging,

such as films containing nanocrystalline structures, quantum dotsand nanostructured conducting polymers. These films typicallycontain nanoparticles with a size distribution showing a wide rangeof electronic band gaps (it is this gap that determines which

wavelengths can be absorbed) so that much of the solar spectrumcan be absorbed by the cell. If such films can be made cheaply,with an optimised distribution of nanoparticle diameters, and can

be properly aligned, one might have an ideal solar collector. Thisfield is still very young, and is moving very rapidly.

Eventually, if research on third generation PV proves successful, it couldlead to PV cells made, for example, from extremely thin stacked plasticsheets, converting solar energy to electricity with very high efficiency atvery low cost.

Figure 3.20 Principles of operation of the photoelectrochemical ‘Grätzel Cell’,

developed at the Swiss Federal Institute ofTechnology, Lausanne

iodine (I)-based electrolyte

current flows

I−

+ I oxidationreduction +Ie−

light

glass

sensitiser layer

tin oxide

glass

titanium dioxide

tin oxide e−

e−

e−

e− e−

e−

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