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This research group
is well known for work on the characterisation of efficiency limiting
effects in thin film solar cells. You can find out more by looking at
our publications page, searching the published litertature or by contacting
'ken.durose@durham.ac.uk , the solar cell materials group leader.
The material below is
intended as an introductory tutorial for some of the concepts of thin
film solar cells and was written by Paul Edwards.
Background to photovoltaics
Faced with ever-increasing
demand, the earth's sources of non-renewable energy are not expected to
last long. Among the many contenders vying to replace fossil fuels, photovoltaic
solar cells offer many advantages, including needing little maintenance
and being relatively "environmentally-friendly"; the major drawback to date
has been cost. In order for photovoltaics to be viable for large-scale energy
conversion, their efficiency must be improved whilst making them cheaper.
Principle of p-n junction
solar cell
In its simplest form, the
solar cell consists of a junction formed between n-type and p-type semiconductors,
either of the same material (homojunction) or different materials (heterojunction).
The bandstructure of the two differently doped sides with respect to their
Fermi levels can be seen in Figure 1.
Figure 1: Band structure
of differently-doped semiconductors
When the two halves are
brought together, the Fermi levels on either side are forced in to coincidence,
causing the valence and conduction bands to bend (Figure 2).
Figure 2: Heterojunction
band-bending
These bent bands represent
a built-in electric field over what is referred to as the depletion region.
When a photon, with an energy greater than the bandgap of the semiconductor,
passes through the solar cell, it may be absorbed by the material. This
absorption takes the form of a band-to-band electronic transition, so
an electon/hole pair is produced. If these carriers can diffuse to the
depletion region before they recombine, then they are separated by the
electric field, causing one quantum of charge to flow through an external
load. This is the origin of the solar cell's photocurrent, and is shown
in Figure 3.
Figure 3: Principle
of photovoltaic device
The CdS/CdTe solar
cell
Advantages of CdS/CdTe
Currently, the semiconductor
most widely used in solar cells is single-crystal silicon. Because of the
cost involved in producing the bulk material, cells produced by this method
are prohibitively expensive for all but the smallest scale or most specialised
applications (such as on calculators and satellites). Higher efficiencies
have been produced by using single-crystal III-V semiconductors and more
elaborate constructions (e.g. multi-quantum wells), but this advantage has
always been more than offset by the resultant increase in cost.
The thin-film cadmium telluride
/ cadmium sulphide solar cell has for several years been considered to be
a promising alternative to the more widely used silicon devices. It has
several features which make it especially attractive:
- The cell is produced
from polycrystalline materials and glass, which is a potentially much
cheaper construction than bulk silicon.
- The chemical and physical
properties of the semiconductors are such that the polysilicon thin-films
can be deposited using a variety of different techniques (see below).
- CdTe has a bandgap
which is very close to the theoretically-calculated optimum value for
solar cells under unconcentrated AM1.5 sunlight.
- CdTe has a high absorption
coefficient, so that approximately 99% of the incident light is absorbed
by a layer thickness of only 1µm (compared with around 10µm
for Si), cutting down the quantity of semiconductor required.
A concern often expressed
about CdS/CdTe solar cells is the effect on health and the environment of
the cadmium used. However, the thinness of the films means that the amount
of active material used is relatively small; it has been estimated that
even if CdTe solar cells were to provide more than 10% of the world's energy
requirements, this would still only account for less than a tenth of the
world's cadmium usage . To put the risk into perspective, B.P. Solar modules
have been reported to have passed the appropriate U.S. Environmental Protection
Agency tests, whereas fluorescent tubes (containing mercury) and computer
screens (containing lead) do not. Cell
construction
The CdTe/CdS solar cell is
based around the heterojunction formed between n-type CdS and p-type CdTe.
The basic composition of the cell can be seen in Figure 4.
Figure 4: CdS/CdTe
solar cell (not to scale)
The functions of the different
layers are as follows:
- Glass The solar
cell is produced on a substrate of ordinary window glass, because it
is transparent, strong and cheap. Typically around 2-4 mm thick, this
protects the active layers from the environment, and provides all the
device's mechanical strength. The outer face of the pane often has an
anti-reflective coating to enhance its optical properties.
- Transparent conducting
oxide Usually of tin oxide or indium tin oxide (ITO), this acts
as the front contact to the device. It is needed to reduce the series
resistance of the device, which would otherwise arise from the thinness
of the CdS layer.
- Cadmium sulphide
The polycrystalline CdS layer is n-type doped (as CdS invariably is),
and therefore provides one half of the p-n junction. Being a wide band
gap material (Eg ~ 2.4 eV at 300K) it is transparent down
to wavelengths of around 515 nm, and so is referred toas the window
layer. Below that wavelength, some of the light will still pass through
to the CdTe, due the thinness of the CdS layer (~ 100 nm).
- Cadmium telluride
The CdTe layer is, like the CdS, polycrystalline, but is p-type doped.
Its energy gap (1.5 eV) is ideally suited to the solar spectrum, and
it has a high absorption coefficient for energies above this value.
It acts as an efficient absorber and is used as the p side of the junction.
Because it is less highly doped than the CdS, the depletion region is
mostly within the CdTe layer. This is therefore the active region of
the solar cell, where most of both the carrier generation and collection
occur. The thickness of this layer is typically around 10 µm.
- Back contact
Usually of gold or aluminium, the back contact proves a low resistance
electrical connection to the CdTe. P-type CdTe is a notoriously difficult
material on which to produce an ohmic contact, and so the junction will
inevitably display some Schottky diode (rectifying) characteristics.
Due to its high conductivity, the metal layer needs only be a few tens
of nanometres in thickness.
Since the active layers of
the device are those on top of the glass substrate, this construction is
referred to as a superstrate configuration. Deposition
techniques
The polycrystalline layers
of CdS and CdTe can deposited by a number of different methods, including,
amongst others, those outlined below:
- Physical vapour
deposition (PVD) (or evaporation) involves the vaporisation in a
vacuum of a source of either the compound (CdS or CdTe) or the separate
elements (Cd + S or Cd + Te). The resulting vapours recombine on the
surface of the substrate (which can be heated, but is still much cooler
than the source) to deposit the required polycrystalline material. The
stoichiometry of the deposited layer is difficult to control accurately,
as it depends strongly on the equilibrium vapour pressures of the elements,
as well as the stoichiometry of the source material .
- Close-space sublimation
(CSS), which has been used to produce the highest efficiency cells
so far , is based on the reversible dissociation of the materials at
high temperatures e.g.
2CdTe(s) = Cd(g) + Te2(g)
The source is of a large
area and is positioned close to the substrate. The substrate is maintained
at a high temperature (but below that of the source) such that the elemental
vapours will not become deposited on the substrate but the compound
form will, due to its lower equilibrium vapour pressure.
- Chemical vapour
deposition (CVD) can also be used to deposit both semiconductors.
It involves chemical reactions between vapours to produce the required
species which then condense on the substrate to form the compound. One
variation of this method, Metal-Organic CVD (MOCVD), uses metallo-organic
precursors: this is an especially widespread technique, as it produces
thin films with very good optical and electronic properties.
- Chemical bath deposition
is sometimes used for depositing CdS films, and involves producing the
required ions in a solution by chemical means, which combine and precipitate
out onto the substrate if the required equilibrium conditions are met.
For example, cadmium ions can be produced by the hydrolysis of Cd(OH)2:
Cd(OH)2 = Cd2+
+ 2(OH)-
and sulphide ions from
an alkaline aqueous solution of thiourea:
(NH2)2CS
+ OH- = CH2N2 + H2O +
HS-
HS- + OH-
= H2O + S2-
However, this method
can not used for CdTe, due to the difficulty of synthesising tellurides.
- Electrodeposition
may
also be used to deposit many semiconductor materials at low temperature
from solution.
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