1095
Solar Cells and
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
46.2 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.
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.
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-
46.1 Figures of Merit for Solar Cells .............. 1096
1096
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
Part E 46.1
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 over-
head. 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
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
n-type window
layer
Fermi level
(zero bias)
Photon flux
p-type absorber layer
Larger barrier to
prevent leakage
of holes
Pm = Im Vm
qVm
kB T
kB T
ln 1 +
−
= IL Voc −
q
kB T
q
(46.2)
= IL (E m /q) ,
ing illumination through the wide-band-gap window layer
40
30
20
VOC
10
Volts
0
–10
–20
–30
JSC
–0.2
0
0.2
0.4
0.6
0.8
Jm ,Vm
Fig. 46.3 Ideal J–V characteristic for a photovoltaic cell,
30 mA/cm2 .
The
according to (46.1) with Js equal to
parameters Jsc and Voc are indicated on the graph. The
maximum extracted power is shown as the shaded area
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:
(46.1)
J = Js eqV/kB T − 1 − JL ,
where Js is the saturation current in reverse bias under zero illumination, q is the charge on the carrier, V
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.
∞
ηabs =
n E (E) dE
Eg
∞
.
(46.3)
n E (E) dE
0
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
Part E 46.1
–40
–0.4
1097
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):
Fig. 46.2 Band diagram for a heterojunction PV cell show-
J (mA/cm2)
46.1 Figures of Merit for Solar Cells
1098
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
500
20
1000
1500
2000
2500
Wavelength nm
10
Si
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
Part E 46.2
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
Ge
0
0
0.5
CIS
1.0
GaAs
CGS
CdTe
1.5
2.0
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:
•
•
•
•
•
•
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
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
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
1099
Part E 46.2
Mechanical or
laser grooved
surface
46.2 Crystalline Silicon
1100
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
Part E 46.3
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 il-
lumination. 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.
Solar Cells and Photovoltaics
46.4 GaAs Solar Cells
1101
46.4 GaAs Solar Cells
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
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
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
1102
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
Part E 46.5
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
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
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.
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
1103
past 10 years with impressive improvements in conversion efficiency. The highest efficiency reported for
laboratory solar cells is 19.2%, which is currently the
Part E 46.6
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.
46.6 CuInGaSe2 (CIGS) Thin-Film Solar Cells
1104
Part E
Novel Materials and Selected Applications
Part E 46.7
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.
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
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 appli-
References
1105
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modules into buildings. The equivalent of a gigawatt
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107 m2 of solar modules. In reality it would have to be
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installation would have approximately 100 m2 of solar
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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
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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.
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Part E 46