solar 1095 Solar Cells an 46 Solar Cells

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					1095

Solar Cells an
46. Solar Cells and Photovoltaics
Photovoltaic solar cells are gaining wide acceptance for producing clean, renewable electricity. This has been based on more than 40 years of research that has benefited from the revolution in silicon electronics and compound semiconductors in optoelectronics. This chapter gives an introduction into the basic science of photovoltaic solar cells and the challenge of extracting the maximum amount of electrical energy from the available solar energy. In addition to the constraints of the basic physics of these devices, there are considerable challenges in materials synthesis. The latter has become more prominent with the need to reduce the cost of solar module manufacture as it enters mainstream energy production. The chapter is divided into sections dealing with the fundamentals of solar cells and then considering five very different materials systems, from crystalline silicon through to polycrystalline thin films. These materials have been chosen because they are all in production, although some are only in the early stages of production. Many more materials are being considered in research and some of the

46.1 Figures of Merit for Solar Cells .............. 1096 46.2 Crystalline Silicon ................................ 1098 46.3 Amorphous Silicon ............................... 1100 46.4 GaAs Solar Cells ................................... 1101 46.5 CdTe Thin-Film Solar Cells..................... 1102 46.6 CuInGaSe2 (CIGS) Thin-Film Solar Cells .... 1103 46.7 Conclusions ......................................... 1104 References .................................................. 1105
more exciting, polymer and dye-sensitised cells are mentioned in the conclusions. However, there is insufficient space to give these very active areas of research the justice they deserve. I hope the reader will feel sufficiently inspired by this topic to read further and explore one of the most exciting areas of semiconductor science. The need for high-volume production at low cost has taken the researcher along paths not normally considered in semiconductor devices and it is this that provides an exciting challenge.

Part E 46

Photovoltaic (PV) devices are a method of converting radiant solar energy into electrical energy. Most of our energy sources, including fossil fuels, hydroelectric and wind power actually come from solar radiation but are indirect conversions into electricity. Another class of solar energy conversion is the heating of water in solar thermal panels. Although the conversion efficiency can be high, they do not generate the thermal energy necessarily where and when it is needed, so storage is required. Direct generation of electric energy is attractive because it is a versatile energy form, rapidly converted into heat, mechanical or light energy. Photovoltaic energy is the main source of energy in the rapidly expanding satellite market with high-efficiency photovoltaic modules producing more than 1 kW of power. Terrestrial applications are also rapidly growing with an estimated total installed capacity worldwide over 1 GW in 2004, increasing annually by 30–40%. This, how-

ever, is tiny compared with the total amount of electrical energy consumed each year, around 0.1% of primary energy demand. However, solar energy is very attractive as it is completely non-polluting and can help to reduce the amount of fossil fuels that we burn to generate electricity. World CO2 emissions have grown by 8% since 1990. Any contribution from non-fossil-fuel alternatives such as solar energy will help to reduce this annual burden of CO2 emissions that is now a widely accepted cause of global warming. So how much solar energy is available for conversion to electricity? The total solar energy falling on the Earth’s surface each year is huge and 10 000 times the current consumption of energy. We only need to capture a tiny fraction of this to make a major contribution to our electricity supply but this will mean incorporating solar electric panels into most buildings; to achieve this it will need to be much cheaper to compete with current fossil-fuel electricity generation.

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Part E

Novel Materials and Selected Applications

The range of applications outlined here place different demands on the design of the photovoltaic module and hence the materials solutions may well be different. Cost has already been mentioned as paramount for terrestrial power requirements, but for space applications resistance to ionising radiation is important. This section will consider the materials implications of competing technologies and how well they meet the criteria for different applications. The first section will deal with how a photovoltaic cell operates and introduce the

figures of merit that are used to compare photovoltaic cells and relate performance to theoretical performance. Solar-cell technology is promoted as a clean fuel technology which does not introduce CO2 and pollutants into the atmosphere. However, for this to be a truly environmentally friendly technology, the manufacturing and eventual disposal will also have to be environmentally safe and this depends on the materials used and the fabrication technologies. These factors will be considered for each material system.

46.1 Figures of Merit for Solar Cells
In this section we will consider the operation of a photovoltaic cell and the significant parameters that characterise its performance. The objective of a photovoltaic cell is to capture as much of the solar energy as possible and convert this into electrical energy. The solar energy reaching the Earth’s atmosphere fits a black-body distribution for a body at 5800 K. This spectrum becomes highly structured, particularly in the infrared part of the spectrum, by absorption bands due to atmospheric gases. By the time the solar radiation reaches the Earth’s surface, it no longer fits a black-body distribution [46.1]. The shift in the spectral distribution will obviously affect the efficiency of absorption of the solar radiation, particularly considering that all semiconductor materials will display a cut-off wavelength dictated by the band gap of the semiconductor. It is necessary to specify the atmospheric absorption when quoting efficiency, as the depth of atmosphere that the solar radiation has passed through will affect both the spectral distribution and the total amount of energy. The measurement used is air mass (AM) and is defined as zero for solar radiation outside the atmosphere and 1 for radiation reaching the ground when the sun is at its zenith. For shallower angles the solar radiation has to penetrate a larger depth of atmosphere and the AM is therefore going to be greater than 1. For space applications AM 0 is the appropriate condition and for terrestrial applications (depending on the position on the Earth) AM 1.5 is a typical value quoted, which corresponds to a solar angle of 45◦ . All this assumes that there is no cloud cover, which will further reduce the integrated intensity and modify the solar spectrum. Although cloud cover will reduce the amount of solar energy available for conversion, useful amounts of electricity can still be generated. The available solar energy also decreases from 1353 W/m2 outside the atmosphere (AM 0) to 925 W/m2 for AM 1, when the sun is directly overhead. In practice, for terrestrial applications the available solar energy is considerably less than this where, in general, AM > 1 and is further reduced by cloud cover. A photovoltaic cell is basically a diode with a photogenerated current. A band diagram is shown schematically in Fig. 46.1. Absorption of radiation can occur on both sides of the junction, creating minority carriers that can diffuse towards the junction. A photocurrent is generated if the minority carriers can drift across the junction without recombination. In practice, the junction is shallow and absorption will occur predominantly on one side where there is a greater depth of absorbing material. There are also heterojunction p-n devices where less absorption will occur in the widerband-gap layer, as shown in Fig. 46.2. So, one side of the junction is considered to be the absorber layer and it is the spectral absorption characteristics of this layer that

Part E 46.1

Photon flux n-type layer Fermi level (zero bias) p-type absorber layer

Fig. 46.1 Schematic of an energy-band diagram for a p-n

junction solar cell showing photoabsorption to create an electron–hole pair and diffusion of the electron towards the junction

Solar Cells and Photovoltaics

46.1 Figures of Merit for Solar Cells

1097

n-type window layer Fermi level (zero bias)

Photon flux p-type absorber layer

Larger barrier to prevent leakage of holes

is the applied voltage, kB is Boltzmann’s constant, T is the temperature of the cell and JL is the photogenerated current. In the ideal cell this is equal to the short-circuit current, indicated as Jsc on the J–V curve in Fig. 46.3. Power can be extracted from the device in the +V , −J quadrant of the J–V plot and the load will determine the operating point in this quadrant. The power is determined from the product JV at this operating point, shown graphically in Fig. 46.3. The maximum power will correspond to the operating point that will give the largest JV area on this graph. For the ideal diode characteristics given in (46.1), this maximum power is given by the following (46.2): Pm = Im Vm qVm kB T kB T ln 1 + − q kB T q = IL (E m /q) , = IL Voc −

Fig. 46.2 Band diagram for a heterojunction PV cell show-

ing illumination through the wide-band-gap window layer
J (mA/cm ) 40 30 20 10 0 –10 –20 –30 –40 –0.4 –0.2 0 JSC 0.2 0.4 0.6 0.8 Jm ,V m Volts VOC
2

(46.2)

where E m is the maximum energy that can be extracted per photon and depends on the band parameters for the semiconductor absorber layer, which determine Voc and Vm . These parameters are marked on the J–V plot in Fig. 46.3. We now have two fundamental parameters which will limit the efficiency of the cell:

• •

The fraction of solar photons absorbed in the cell. The electrical energy created per photon.

The first factor can be calculated by integrating over the solar spectrum for the appropriate AM number and including the cut-off wavelength of the semiconductor absorber layer.

Part E 46.1

∞

Fig. 46.3 Ideal J–V characteristic for a photovoltaic cell,

n E (E) dE ηabs =
Eg ∞

according to (46.1) with Js equal to 30 mA/cm2 . The parameters Jsc and Voc are indicated on the graph. The maximum extracted power is shown as the shaded area

. n E (E) dE

(46.3)

0

will determine the maximum absorption possible from the available solar radiation. An ideal diode characteristic for an illuminated cell is shown in Fig. 46.3. In the dark the J–V plot would go through the origin and, as the light intensity increases, the short-circuit current becomes increasingly negative, indicating the presence of a photogenerated current. The equation for the J–V characteristic of this ideal device is: J = Js eqV/kB T − 1 − JL ,
(46.1)

where Js is the saturation current in reverse bias under zero illumination, q is the charge on the carrier, V

This is shown graphically in Fig. 46.4 for AM 1.5. Any photons with energy less than the band gap will not be absorbed and will not contribute to the photocurrent. The second efficiency factor mentioned above implies that not all the photon energy will be converted into electrical energy, even if one photon absorbed constitutes one minority carrier crossing the junction. The electrical energy per carrier is given by the factor E m in (46.2), so the maximum power of the device is the product of the absorption rate of photons and the mean electrical energy created per photon. This product is represented by the inner shaded area of the solar spectrum shown in Fig. 46.4. The difference between curve 1 and

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Part E

Novel Materials and Selected Applications

Efficiency (%) 40 Maximum power for AM 1.5 30 AM 1.5 solar spectrum
Proportion of spectrum captured by CdTe

20

500

1000

1500

2000

2500 Wavelength nm

10
Si GaAs CGS CdTe

Fig. 46.4 Graphical representation of the maximum energy

that can be extracted from a CdTe solar cell with a band-gap energy of 1.45 eV

Ge

CIS

0

0

0.5

1.0

1.5

2.0

curve 2 is simply the energy lost per photon because not all the photon energy is converted into electrical energy. Different semiconductor materials will have different efficiencies primarily because of different values for the band gap. The ideal value for E m will track the band gap, so for narrower-band-gap materials there will be a larger proportion of photons absorbed but less electrical energy per photon. The function of efficiency for semiconductors with different band gap, taken from curve 2 in Fig. 46.4 is plotted in Fig. 46.5 and shows that the optimum efficiency occurs for semiconductors with a band gap in the near-infrared region, around 1.5 eV. This represents the best compromise between absorption of solar radiation and transferring the optimum amount of energy per photon into electrical energy. The maximum efficiency is predicted for Si, InP, GaAs and CdTe, which are in the region of 30% for AM 1.5 irradiation (1 sun illumination). This means that the very best conversion efficiency for a single-junction cell is approximately one third of the available solar energy. In practice, photovoltaic cells have efficiencies considerably less than this due to optical reflections, poor junction characteristics and carrier recombination. These are materials issues that are often traded off against production costs, e.g. using polycrystalline

2.5 3.0 3.5 Bandgap energy (eV)

Fig. 46.5 Plot of ideal efficiency against band-gap energy for a single-junction cell for AM 1.5 illumination conditions, (after Henry [46.1])

rather than single-crystal material. Conversely, higher conversion efficiencies can be achieved using multiple junctions which are more expensive but attractive for space applications and when used in combination with solar concentrators, so less surface area of the expensive multijunction cell is required. Other factors that can influence the choice of photovoltaic materials include the following:

Part E 46.2

• • • • • •

absorption coefficient, contact resistance, abundance of raw materials, toxicity of materials, stability of materials and junctions, radiation resistance.

These factors will be considered in the following examples of different photovoltaic systems to assess the merits of different materials. It is probably true to say that there is not one ideal material but different applications can make one material more attractive than another.

46.2 Crystalline Silicon
Over 80% of the current world production of solar modules are made from either single-crystal or multigrain silicon. This is the most mature of the photovoltaic materials and has benefited considerably

Solar Cells and Photovoltaics

46.2 Crystalline Silicon

1099

from the size of the silicon semiconductor industry. This has ensured a ready supply of material and processing tools suitable for large-scale production. However, crystalline silicon does suffer from a fundamental disadvantage in that the band gap of silicon is indirect, which means that the absorption coefficient is much lower than a direct-band-gap semiconductor such as CdTe. In practice this means that a much thicker piece of material is needed to absorb all the solar radiation greater than the band-gap energy. This requires wafers of silicon thicker than 100 µm and means that this material is not suitable for thinfilm technology. The highest-performance solar electric modules are made from wafers of single-crystal silicon cut from Czochralski-grown crystals, up to 30 cm in diameter. This was originally developed on the back of the silicon electronics industry, but the rapid increase in production volume of PVs is now driving production of silicon crystals. The conversion efficiency for single-crystal PV modules is around 17% with the potential for increases in the near future to 20%. Multicrystalline silicon involves the relatively cheap path of casting silicon ingots that are not seeded but produce a random grain size of the order of 1 cm across. The ingots are cast in blocks, larger than 100 kg, and then sliced into wafers 300-µm thick and with an area of up to 20 cm2 . The grain boundaries cause these wafers to be mechanically weaker than the single-crystal wafers and they will typically be thicker. This causes some trade off in price. Each cell can be expected to contain grain boundaries, so loss of photogenerated charge at
Mechanical or laser grooved surface Back filling with metallization

Ion implantation of buried junction

p-type Si substrate

Fig. 46.6 Schematic of buried-contact technology for Si

solar cells

the grain boundary can cause some loss of efficiency. Typical module efficiencies for multicrystalline silicon are currently around 15%. The junction is formed by phosphorus implantation to form a p-n homojunction and contacted by printing of a metal grid, usually of a Ni–Au alloy. Patterning of the surface prior to implantation and contacting has achieved the highest efficiency cells by improving light collection [46.2]. Another recent innovation that has contributed to higher efficiencies is the ‘V’-grove or buried junction. This is shown schematically in Fig. 46.6 and entails the grooving of the p-type silicon substrate, implantation of phosphorus to create a buried junction and filling with metal contact alloy. This helps to improve the collection efficiency of the cell, particularly in the blue part of the spectrum. These manufacturing steps have to handle large volumes and be cheap. Cheaper alternatives to ion implantation involve thermal diffusion from a screen-printed paste or spin-on glass [46.3]. The main disadvantage of silicon is that the absorption coefficient is low because it is an indirect-band-gap semiconductor 2 × 103 cm−1 for Si compared with 1 × 105 cm−1 for CdTe in the green part of the spectrum . This means that the amount of material needed to absorb the solar radiation is greater than for a direct-band-gap semiconductor. The absorption can be improved by having a reflecting back contact that, if it is roughened, will increase the mean path length of the back reflected light in the silicon cell. This is particularly important for thin-film silicon devices where the amount of material is more of an issue, although the thickness of 30–100 µm can be compared with the much thinner layers used for direct-band-gap semiconductors (see CdTe and GaAs). Thin-film polycrystalline silicon is grown by ribbon casting techniques. The pulling speeds are in the range 10–1800 cm/min, and this represents a cheap method for the production of solar cell material but the conversion efficiencies are currently low. One way of increasing the absorption coefficient is to deposit a film of amorphous silicon (a-Si), which has a similar band gap to crystalline silicon but is a direct-band-gap material. Amorphous silicon has led the way in cheap thin-film solar cells but suffers the disadvantages of low efficiency (< 10%) and poor long-term stability. These cells will be discussed in greater detail in the next section. Crystalline silicon solar cells, over the past 10 years, have approximately doubled in efficiency and this has been achieved by a combination of the

Part E 46.2

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Part E

Novel Materials and Selected Applications

following:

• • • •

Improved material quality, leading to improved minority-carrier diffusion length. Improved Voc and fill factor through emitter and base doping and contact optimisation. Improved Jsc through diffusion length improvement using phosphorus gettering, hydrogen passivation and buried contacts. Surface passivation and contact grid optimisation.

All these aspects involve materials issues, either directly associated with the quality of the silicon or with contacting and passivation. It is also important to avoid degradation of the cell and the final encapsulation must avoid exposure of the cell to water. Multicrystalline sil-

icon can be passivated with silicon nitride deposited by plasma-assisted CVD from SiH4 and NH3 to reduce surface recombination [46.4]. Silicon solar modules are becoming cheaper to produce, nontoxic and stability in a non-radiation environment is good. Multicrystalline silicon is not so suitable for space applications because of the combination of thick absorber layers, requiring greater weight per unit area and the sensitivity of the cell to degrade in a high-radiation environment. However, single-crystal silicon competes well with GaAs cells in this market. Single-crystal modules for terrestrial applications produce the highest output powers currently available with a Sharp 1.3 m2 module giving a peak output power of 185 W.

46.3 Amorphous Silicon
Amorphous silicon offers the potential for a cheap production technology for terrestrial photovoltaic solar cells. The amorphous state displays different physical properties to the crystalline with a modified band structure. One consequence of this is that the absorption coefficient in the green part of the visible spectrum is a factor of 10 higher at 2 × 104 cm−1 , which makes it more suitable for thin-film applications. The amorphous structure leaves dangling bonds which pin the Fermi level and would normally prevent doping of the material. This is overcome by the inclusion of hydrogen, which passivates these dangling bonds, so the material is referred to as a-Si:H. The Si can also be alloyed with Ge, C, and N in a glow-discharge evaporator. These alloys are particularly useful for multijunction devices used for increasing the quantum efficiency. In the laboratory single-junction cells have efficiency around 8% with multijunction cells going up to 20%. In production an a-Si module has efficiency around 7%, considerably less than the crystalline materials. The most common method for producing a-Si:H for photovoltaics is by plasma-enhanced CVD from SiH4 mixtures. The films are deposited onto a textured conducting oxide such as indium–tin oxide (ITO) which provides the electrical contact and increases the average light path in the absorber layer to increase absorption. The device structure is a p-i-n with absorption taking place in the middle (insulator) layer, which is only 0.5 µm thick. The p and n layers can be deposited by adding B2 H6 or PH3 respectively to the plasma. One of the major disadvantages of a-Si:H is the instability and long-term deterioration under light illumination. This is caused by the rearrangement of dangling bonds, often associated with rearrangement of hydrogen atoms close to weak or dangling bonds. The energy comes from nonradiative bimolecular reactions and hence depends on the illumination intensity (for further details see the review by Bloss et al. [46.5]). In practice, this causes a downward drift in efficiency with time which can be as much as 2% in 100 h. The current cost is cheaper than crystalline silicon (3 US$/Wp) but with the potential to considerably reduce cost to 0.7 US$/Wp [46.6]. One of the main technical challenges is to maintain stabilised efficiency above 10%. The relatively high cost of PV solar modules compared with more conventional sources of energy can only become attractive if it can operate efficiently over a long period of time. This means that the modules must remain efficient over a period of 20 years. Shorter operating life times would have to be mitigated by much lower production cost. In conclusion, a-Si:H is a low-cost technology for terrestrial applications and is finding its way into low-power applications such as small-scale stand-alone systems. With improved efficiency and stability s-Si has the potential to capture a significant proportion of the terrestrial market but the lower production costs are not sufficient to offset this at present. A typical a-Si module, with an area of 0.8 m2 , will have a peak output power of 40 W. The temperatures used in processing these modules is lower than the high temperatures needed to melt silicon for the crystalline silicon modules which means that the energy payback times could be just a few months.

Part E 46.3

Solar Cells and Photovoltaics

46.4 GaAs Solar Cells

1101

46.4 GaAs Solar Cells
The III–V semiconductors have advantages over silicon due to their direct-band-gap photon absorption, with an absorption coefficient in the green of 8 × 104 cm−1 . This means that, for example, GaAs can theoretically yield over 30% efficiency (AM 1.5) for an absorber layer thickness of just 1–2 µm, compared with a hundred times this for crystalline silicon. The efficiency, stability and thin-film deposition technology make these cells attractive for space applications. From Fig. 46.5 it is clear that the band gap of GaAs is well matched to the optimum for maximum efficiency. The III–V semiconductors also offer the flexibility of alloying to change the band gap and tune the response of the photovoltaic junction. In addition, heterojunctions can be formed and multijunction solar cells can be produced to convert more of the solar spectrum into electricity and thus exceed the theoretical limits set by the single -junction cells. For example, in the laboratory a GaAs/GaSb tandem solar cell has been reported with 35.6% efficiency [46.8]. Higher efficiencies are possible with three and even four junctions to capture more of the infrared that would otherwise not be absorbed. The Ge substrate, which is closely lattice matched with GaAs, can be used as a narrow-band-gap absorber to capture the radiation that passes through the GaInP2 /GaAs cell [46.7]. The top cell has an absorber of GaInP2 , which has a band gap of 1.9–2 eV, and will capture the visible part of the spectrum without too much voltage loss. The next cell is GaAs, which has a band gap of 1.42 eV, and will capture the red to infrared part of the spectrum. The bottom junction is formed in the Ge substrate, which has a band gap of 0.67 eV and will capture the light further into the infrared. The triple-junction cell, in production, can yield 27% (AM 0) conversion efficiency [46.7]. The layers of these multijunction cells are grown epitaxially onto a single-crystal Ge substrates. As with crystalline silicon, high crystalline quality is needed to obtain high-efficiency cells; polycrystalline GaAs does not work due to grain-boundary conduction that reduces the available photocurrent. This restricts the photovoltaic applications to high-quality epitaxial material and GaAs is not suitable for cheap thin-film photovoltaics on glass substrates. The topic of thin-film polycrystalline photovoltaics will be considered in the next section on CdTe solar cells. Epitaxial growth avoids minority-carrier recombination at grain boundaries, by avoiding their formation, but the cost of the substrates is inherently higher than the glass or ceramic substrates used for thin-film devices. However, defects must also be avoided at the junction and interfaces between layers of different composition and this imposes the constraint that the heterostructures should be lattice matched. This restricts the choice of alloys to those that are either lattice matched or that the thickness and mismatch are such that the film is strained. The simplest structures use AlGaAs, which has a close lattice match to GaAs over the entire composition range, as a window and passivation layer. GaAs is also closely matched to Ge, offering a choice of substrates. The GaAs or Ge substrate is the narrowest-band-gap part of the structure, so the device has to be front-side illuminated with a grid of metal contacts on the top surface to contact the cells. The design of the cell structure needs to have the wider-band-gap layers further from the substrate (last to be grown) to allow the longer wavelengths to penetrate to the narrowband-gap absorber layer. This places further constraints on the design of these epitaxial structures. The Ge and GaAs substrates have a similar lattice parameter to AlGaAs, which is used as a passivation layer for both n-type and p-type GaAs layers. Other III–

Contact

Top cell

AR n-AllnP n-GalnP p-GalnP

n+ GaAs

AR

Part E 46.4

p-AlGalnP Wide Eg tunnel junction n-GalnP n-GaAs Middle cell p-GaAs p-GalnP GaAs tunnel junction n-GaAs buffer n-Ge Bottom cell p-Ge substrate Contact

Fig. 46.7 Schematic of a triple-junction GaAs solar cell [46.7], showing the positions of the three junctions

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Part E

Novel Materials and Selected Applications

V compounds that can be used include GaSb, which is sensitive to the near infrared out to 1.8 µm and will therefore absorb the radiation that is transmitted through the GaAs cell, as an alternative to the Gebottom cell. The alloy GaInP also lattice matches for the 50% Ga/In mix and is useful as a wider-band-gap junction. GaSb is also attractive for thermo-photovoltaics where the solar radiation is converted into heat, which is then absorbed by the narrow-band-gap GaSb device. More recently, the narrow-band-gap quaternary GaInNAs has been investigated [46.9] as a narrowband-gap absorber layer and can be inserted between the GaAs layer and the Ge substrate to give a small boost in voltage. The III–V photovoltaic structures are grown by metalorganic vapour-phase epitaxy (MOVPE), which gives excellent control over alloy composition and doping concentration. An example of a multijunction cell is shown

in Fig. 46.7. The structures tend to be complex because a tunnel junction is needed for current to flow between the two (series) photovoltaic cells, otherwise the connection between the two would be a high-impedance reverse-bias junction. It is also important for the device design to match the currents between the two (or more) junctions so that they can both operate at near-optimum conditions. GaAs solar cells are rapidly increasing their share of the space market to power satellites. Their efficiency is high, stability good but the single-crystal substrates increase the cost. There may be toxicity issue for the disposal of solar modules if GaAs was used widely for terrestrial applications. The most attractive terrestrial application would be in concentrators where the solar radiation is optically concentrated onto the PV cells, so the collection area can be much greater than the area of expensive PV modules.

46.5 CdTe Thin-Film Solar Cells
The band gap of CdTe of 1.45 eV at room temperature makes it another semiconductor that is close to the theoretical optimum for efficient conversion of solar radiation (see Fig. 46.5) into electrical power. The absorption coefficient is > 5 × 104 cm−1 for photon energy greater than the band gap, allowing efficient collection for only 2-µm-thick films, similar to GaAs. CdTe, like GaAs is also a direct-band-gap semiconductor but is from the II–VI family of semiconductor compounds. Although the theoretical efficiency for the CdS/CdTe photovoltaic cell is ≈ 30% at AM 1.5, the highest values reported are nearly half this maximum and are less than 10% at AM 1.5 for modules in production. The attraction of CdTe compared with GaAs is that these cells can be made from polycrystalline thin films on glass substrates, thus avoiding the need for expensive single-crystal substrates. A further advantage in using glass substrates is that illumination of the photovoltaic cell occurs through the substrate rather than from the top face, so the substrate becomes the window for the cell. The front contact is made with a transparent conducting oxide (TCO) such as ITO, as shown in Fig. 46.8, and this avoids the need for an opaque grid of metal contacts. This approach of using a TCO on glass substrate as the window is called a superstrate. The TCO has to have high optical transmission as well as a metal-like electrical conductivity. A typical spreading resistance would be 10 Ω per square. The CdTe solar cell is made from a heterojunction between the wider-band-gap CdS and the CdTe absorber layer. CdS is often referred to as the window layer; it has a band gap of 2.4 eV allowing most of the visible spectrum to pass with very little absorption. The

Light

Part E 46.5

Glass substrate

TCO n-CdS Junction p-CdTe Front contact Back contact

Fig. 46.8 Schematic of a CdTe device structure based on

a glass superstrate

Solar Cells and Photovoltaics

46.6 CuInGaSe2 (CIGS) Thin-Film Solar Cells

1103

most common method for depositing CdS is by chemical bath deposition (CBD). The layer is n-type, forming a junction with the p-type CdTe. The thickness is kept to a minimum (around 100–200 nm) in order to minimise absorption at the blue end of the spectrum. A schematic of the CdTe PV structure is shown in Fig. 46.9. Deposition techniques for CdTe (electrodeposition or close-space sublimation (CSS)) require postdeposition treatment for doping, grain growth and resistivity reduction, at temperatures above 400 ◦ C. The exact processes are not clear but improve the efficiency from a few % to > 10% if this anneal is carried out in air in the presence of CdCl2 [46.10]. There are variations in this process in terms of temperature, time and ambient gas but all involve the diffusion of Cl into the CdTe and some interdiffusion of the CdS/CdTe interface. This process improves grain size to ≈ 1 µm if the starting material has a much smaller grain size. This is further complicated by the variations in grain size from the CdS interface up to the back surface, where the junction region displays smaller grains. The best efficiency in the laboratory is 16.5% reported by Wu [46.11]. Productionscale processes achieve somewhat lower efficiencies, typically 8–10% [46.12]. A typical CdTe module with an area of 0.94 m2 has a peak output power of 70 W; this compares favourably with other thin-film photovoltaics but is still much lower than the crystalline silicon modules [46.13]. However, the cost advantage and long-term stability make CdTe modules an attractive candidate for terrestrial applications. Future prospects for CdTe can be seen in current research activities with encouraging results for CdTe solar cells on flexible plastic substrates [46.14]. Using these substrates places tighter constraints on deposition and annealing temperatures and alternatives to the CdCl2 anneal would be attractive. One prospect is for in situ doping of CdTe with arsenic to make it p-type using metalorganic chemical vapour deposition (MOCVD). This technique has been successfully used in the past to deposit films that were given a conventional anneal treatment. MOCVD offers greater flexibility over deposition conditions but would also need to compete with CSS and electrodeposition on cost of deposition.

Light

Front contact

ZnO n-CdS ClGS Mo back contact Glass substrate

Fig. 46.9 Schematic of CIGS device structure based on a glass substrate

The abundance figures look low but up to 5000 tons of Te could be produced each year from copper ore and 20 000 tons of Cd is produced each year as a byproduct of zinc production. An important environmental and resource issue may be the recycling of these materials. It is important to note that although Cd is a very toxic metal it is relatively benign in the form of CdTe as it is a very stable compound and is not soluble in water. The use of Cd in solar cells has the added benefit of making use of the Cd that is produced as a by-product of zinc production in an environmentally friendly way. Jager-Waldau [46.6] states that “Every energy source or product may present some environmental, health and safety hazard, and those of CdTe should by no means be considered barriers to technology scaling”. This technology is less well developed than those in the other case studies but the thin-film technology on glass combined with good stability makes this a very attractive proposition for both terrestrial and space applications.

Part E 46.6

46.6 CuInGaSe2 (CIGS) Thin-Film Solar Cells
This is an alternative material system for achieving an optimum band gap for solar energy conversion and one that has attracted a lot of research effort in the past 10 years with impressive improvements in conversion efficiency. The highest efficiency reported for laboratory solar cells is 19.2%, which is currently the

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Part E

Novel Materials and Selected Applications

best of the thin-film PV technologies [46.15]. In common with other thin-film PV technologies, these films can be deposited onto cheap substrates, at relatively low temperature, and the potential for processing in large volumes. The absorber layer is based on the Chalcopyrite-phase CuInSe2 (CIS). This has a band gap of 1.04 eV, which is lower than the optimum band gap for optimum efficiency. However, cells made from CIS have achieved > 10% efficiency. Alloying with Ga to form CuIn1−x Gax Se2 (CIGS) increases the band gap to 1.7 eV for x = 1. This gives a range of alloy composition across the important, optimum, range for efficient conversion. An increase in band gap will increase the voltage of the cell (Voc ) but decrease the number of absorbed photons, thus decreasing Jsc . In practice the alloy is not uniform and thus a greater proportion of the photon flux is absorbed by the single-junction device than would occur for a single wide-band-gap absorber layer [46.16]. The other benefit of grading of the alloy composition is that the resultant band-gap grading creates a built-in electric field that can drift the electrons towards the junction. Unlike a-Si and CdTe, the preferred arrangement is not to illuminate through the substrate but illuminate from the top surface as with the crystalline GaAs cells. The typical layer structure is shown in Fig. 46.9. The substrate is soda-lime glass with a sputtered coating of molybdenum which acts as the back contact. The next layer is the CuIn1−x Gax Se2 alloy, which is the p-type absorber layer. The junction is formed with a thin layer of CdS (as with CdTe). The front contact is formed by aluminium-doped ZnO, which is highly conducting but transparent, allowing solar radiation to pass without significant absorption. One disadvantage of the substrate approach is that the top surface is protected with another glass sheet when it is fabricated into a solar module. This step is not necessary in the superstrate approach. However, the substrate configuration has shown much higher maximum efficiencies than the superstrate approach and offers the opportunity to deposit the CIGS layers onto flexible metal foils. The structure for CIGS on metal foil substrates is similar to those on the Mo/glass substrates except for a barrier layer of SiO2 between the metal foil and the Mo back contact. This is needed to reduce the diffusion of impurities into the active semiconductor

layer. However, one impurity, sodium, has to be put back in to give the desired doping properties for the CIGS absorber layer [46.17]. This impurity occurs naturally when cheap soda-lime substrates are used, as the sodium will diffuse from the glass substrate into the CIGS layer during film deposition and annealing. Using metal foil substrates makes the PV cells flexible and enables their manufacture in a continuous reel-to-reel process. There are various deposition methods that can be used for the deposition of CIGS and this can lead to lowcost production routes. The early results were obtained by co-evaporation from separate elemental sources. More recently a range of techniques have been used from e-beam evaporation of the metal sources and subsequent selenisation and annealing to electrodeposition. The characteristic of these approaches is a separation of the deposition process and alloy formation, allowing cheap and potentially high-throughput techniques to be used. This also enables control of the alloy composition and stoichiometry that will affect the doping [46.18]. The subsequently deposited junction layer, CdS, can be deposited by either chemical bath deposition, sputtering or CVD. The transparent contact layer, ZnO, is typically deposited by sputtering. All these techniques can be operated on a large industrial scale and lend themselves to cheap production methodology. CIGS solar cells have moved rapidly into production and a number of manufacturers are offering modules with efficiencies in excess of 10%. For example, Shell Solar produce CIGS panel with a peak output power of 40 W for a module area of 0.42 m2 . The potential price reduction is similar to other thin-film technologies and with a potential for breaking the critical 1 $/W barrier, along with a-Si and CdTe. The current production cost is three to four times this value. As with CdTe solar cells, the abundance of the elements In and Se appears low, but for a thin-film photovoltaic, where each module only needs a few grams of material, the total amount needed is not huge. However, the current price of In is high as it is not readily produced as a by-product of other mining processes. The cell stability for terrestrial applications appears to be good and has potential for space applications but more needs to be understood about the effects of high-energy γ fluxes on long-term stability.

Part E 46.7

46.7 Conclusions
This chapter has introduced some basic concepts about photovoltaic solar cells and examples of some of the more common materials being used for solar-cell production. The area of research is huge with a number

Solar Cells and Photovoltaics

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of specialist journals dedicated to solar energy and large international conferences held annually. The drive to find cheaper solutions to the conversion of solar energy has taken research down some interesting and unusual avenues. One of the more successful of the alternative approaches is the dye-sensitised cell, known as the Gratzel cell after its inventor Michael Gratzel [46.19]. This uses organic dyes to absorb the solar radiation and transfer electrons to a porous TiO2 surface, which is effectively the junction. Other approaches have looked at conducting polymers and polymer blends to form photovoltaic junctions. These approaches could enable very cheap photovoltaic solar cells to become a reality but over a much longer timescale than for the materials considered in this chapter. All the materials considered here are in production and look set to make a major contribution to the production of solar energy over the next 10 years. The largest and most mature production facilities are based on crystalline or multicrystalline silicon. These modules are also the most efficient for single-junction cells. The most efficient modules are made from GaAs on Ge and are the most complex and most expensive. Most of this production is for the space market but increasingly is showing potential for use with concentrators for terrestrial appliReferences
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cations. This is acceptable for stand-alone solar power generation but the largest challenge for large-scale terrestrial power generation is to integrate photovoltaic modules into buildings. The equivalent of a gigawatt power station would require, at least, of the order of 107 m2 of solar modules. In reality it would have to be much larger to account for the average power production being much less than the peak production. A modest installation would have approximately 100 m2 of solar panels so we would need more than 105 buildings. So, the goal has to be that every house and building has a PV facade or roof. ¸ Large-scale implementation will require the modules to be architecturally acceptable, in appearance and size, and probably serve multiple functions such as keeping the rain out and heat in etc. This need is opening the opportunity for a range of materials and designs and ultimately flexible panels. On top of this the cost per watt of electricity produced needs to come down, probably by a factor of five based on current installed costs. The thin-film technologies are inherently cheaper but much of this cost benefit is not realised at present because the efficiencies and production volumes are low. The current growth in PV solar energy after decades of research has become very rapid and this expansion is set to continue well into the future.

46.11 46.12

46.4 46.5 46.6 46.7

46.13

46.8 46.9 46.10

46.14 46.15

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I. M. Kotschau, G. Bilger, H. W. Schock: Mater. Res. Soc. Symp. Proc. 763, 263 (2003) D. Hermann, F. Kessler, K. Hertz, M. Powalla, A. Schulz, J. Schneider, U. Schumacher: Mater. Res. Soc. Symp. Proc. 763, 287 (2003)

46.18

46.19

S. Hishikawa, T. Satoh, S. Hayashi, Y. Hashimoto, S. Shimakawa, T. Megami, T. Wada: Sol. Energy Mater. Sol. Cells 67, 217 (2001) M. Gratzel: MRS Bull. 30, 23 (2005)

Part E 46


				
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