This presentation covers the basics of silicon photovoltaic cells, looking at the photovoltaic effect, the chemical properties of silicon, PN junctions, how photovoltaic cells are constructed, the factors affecting their performance and how they can be tested and evaluated.

1. Silicon Photovoltaic Cells:
An Introduction
Gavin D. J.
Harper
g.harper@glyndwr.ac.uk
@gavindjharper
www.gavindjharper.com
http://orcid.org/0000-0002-4691-6642
Welsh Energy Sector Training (WEST)
OpTIC Glyndwr
Ffordd William Morgan, St Asaph, Wales
October 2014

2. Structure of Silicon
• Matter is composed of atoms.
• Atoms in turn are made up of
• Protons (which are positively charged)
• Electrons, (which are negatively charged)
• Neutrons (which are neither positively nor negatively charged).
• If we took an atom of silicon, and examined how it was
composed, we would see a dense central “core” which is known
as the ‘nucleus’ which consists of Protons and Neutrons; 14 of
each to be exact.
• Around this core are electrons that orbit in a ‘cloud’. Some of
these electrons orbit at different distances from the central
nucleus. We call these distances ‘shells’

3. Structure of Silicon
• If you think of a shell as an imaginary sphere surrounding the
nucleus of an atom; these shells are arranged concentrically.
Each shell only has room for so many electrons, therefore as
each shell fills up, the next shell in line must be filled with
electrons.
• When a molecule has eight electrons in its outer shell (known as
the valence shell), then it is stable; which is to say not very
chemically reactive. The ‘Noble Gases’ which reside on the far
right of the Periodic Table are very stable and as such have little
tendency to participate in chemical reactions – all atoms strive
to fill their outer shells and reach this unreactive state.

5. Structure of Silicon
As we can tell from its number on the periodic table, “14” we can tell that silicon has
14 electrons.
A little knowledge as to the pattern in which electrons are arranged in tells us that
these electrons are arranged in three ‘shells’.
The first two ‘shells’ are completely full with electrons, however, the outermost shell
only has half of the total number of electrons possible.

6. Structure of Silicon
• Silicon is what is known as “tetravalent”, this is to say that there are
four electrons available in the outer shell which can form ‘covalent’
bonds. A covalent bond is one where electrons are “shared” between
pairs of atoms.
• The natural tendency in chemicals is for them to work towards filling
their outer shells with eight electrons by forming bonds with other
chemicals.
• Silicon is no exception, and fills its outer shell by bonding with other
silicon atoms and “sharing” electrons.
• Silicon forms a regular crystalline structure. This is to say that the
forces between the atoms of silicon are such that they arrange
themselves in the most compact pattern that the forces between
atoms will allow.

7. Crystalline
Silicon
• This is what silicon
looks like when
represented in
three dimensions.
• For the purposes of
the rest of this
presentation, we
will represent
silicon in two
dimensions for
simplicity.

8. Metalloids
Silicon is a metalloid. We can classify most chemicals easily into either “metals”
or “non-metals”, however, silicon is one of a small group of elements that
occupy a ‘staircase’ diagonal line on the periodic table, where the elements
chemical properties fit neither description completely. Some metalloids are
semiconductors.

9. Semiconductors
• Silicon is part of a class of materials called ‘semiconductors’ that
is to say, that it isn’t a conductor, and it isn’t an insulator.
• It has electrical resistivity that is somewhere in between a
conductor and an insulator.
• In its regular state, silicon isn’t all that conductive, as none of its
electrons can move (and movement of electrons is essential for
conductivity); the electrons are locked into the crystalline
structure – each atom sharing electrons with its neighbour.

11. Doping Silicon
• We can control and enhance this semiconducting
property by adding other chemicals to change
the way silicon behaves.
• This is known as “doping”, because the silicon is
“doped” with a small quantity of a different
chemical.
• By introducing free electrons, or creating gaps
into which free electrons can go, we can create
useful devices.

14. To give you an example of how doping
affects the silicon atoms, replacing one in
every one-hundred and six silicon atoms
with a gallium atom increases the
conductivity of the solid silicon by a factor
of five million as can replacing one in every
one hundred and six silicon atoms with
Phosphorus.

15. The Photoelectric Effect
• As far back as 1839, Edmond Becquerel discovered
that sunlight could induce an electrical current in
solid material; however, it was many years before this
process became well understood.
• We now have a more complex understanding of the
photoelectric effect through understanding of
Quantum mechanics.
• Einstein won the Nobel Prize for physics, by
theorizing that in each quantum of light (think of a
quantum as a ‘little package’), the frequency of the
light (or it’s colour in simple terms), multiplied by a
constant number (known as Planck’s constant)
determines the energy that little parcel of light has.

16. Photovoltaic
Effect
• Certain materials
can convert
incoming
electromagnetic
radiation (in this
case photons) into
‘free electrons’.

17. Electron Work Function
• Electrons have a property known as the “work function”, which is
the amount of energy they need to stay bound in their place.
• As photons hit the electrons, they may absorb some of their
energy.
• Once an electron garners more energy than it’s work function, it
is ejected from its position in the ‘valence band’, which is to say
the outer shell of the atom, into the higher energy ‘conduction
band’, where the electron can travel and create an electric
current.
• However, if the photon has less energy than the electrons work
function, it will stay in the position.

18. PN Junctions
• The two types of doped silicon have slightly different electrical
properties, and we can take advantage of this by creating a
“junction”.
• We are joining a piece of “P-Type” silicon and a piece of “N-Type”
silicon; so the junction we create is known as a “PN
junction”.
• Those of you who are familiar with electronics, will recognise
that a “PN junction” is effectively a diode – our photovoltaic cell
is effectively a large, planar diode with a big surface area.

20. Changing Irradiance
• If you make the light source more
intense (i.e. increase the irradiance),
then it has the effect of increasing
the number of photons in the light
beam
• Whilst it increases the quantity, it still
does not affect the fundamental energy
that each photon possesses
• This is determined by its frequency (or
colour in simple terms). So, we need the
right wavelength light for a given
electron energy, and making it brighter
increases the number of interactions
that occur.

21. Silicon Photovoltaic Cell
Construction

23. Photovoltaic Cell Construction
• The “PN Junction” is the “sandwich” of silicon.
• We have looked at the chemical properties of this
sandwich already and how it generates electricity.

24. Photovoltaic Cell Construction
• There is a need to make a physical connection between our single
photovoltaic cell, other photovoltaic cells in the circuit, and the
world.
• This is accomplished with an electrode which can harness the
moving electrons that the cell produces on each side of the cell.

25. Photovoltaic Cell Construction
• The rear surface contact can be of relatively simple construction – all it needs to
do is provide a connection with the P type silicon wafer.
• The electrode on the front surface which contacts the N type silicon needs to be a
little bit more sophisticated. In addition to making contact with the N type silicon,
it also needs to permit the passage of light so that photons can hit the PN
junction.

26. Photovoltaic Cell Construction
• There are a number of different types of electrode – one of
the simplest is simply screen-printed on top of the cell.
More advanced electrodes might be ‘grooved’ into the
cell. A good electrode design will make good contact with
the silicon whilst obscuring as little light as possible.

27. Photovoltaic Cell Construction
• The cell is given an anti-reflection coating.
• We’ll see later in this chapter that reflection is one of the
enemies of good photovoltaic cell performance.

28. Measuring PV Performance

29. Standard Test Conditions
In order to make clear comparisons between
different photovoltaic cells we use a standard set
of test conditions.
Standard Test Conditions are taken to be:
• Vertical Irradiance of 1000 W/m2
• Cell Temperature of 25°C +/- 2°C
• A ‘light spectrum’ equivalent to AM = 1.5

30. Testing PV
Cells
We can make a
circuit with a
voltmeter, ammeter
and variable resistor
for a load in order to
test a solar cells
performance.

31. Testing PV Cells
• As we change the load on the solar cell, the voltage and current
readings will also change. We can make a plot of these readings.
• Modern photovoltaic inverters are able to match the
characteristics of the solar cell with the load that they present to
the cell, this helps to extract the most energy from the device.
• This is known as “Maximum Power Point Tracking”.
• We will see the “Maximum Power Point” on the graphs on the
slides to follow.

32. PV Power Curve
• The brown line represents
the plot of current against
voltage, current is read
from the left hand scale.
• Current multiplied by
voltage gives us ‘power’; It’s
useful to plot this at the
same time.
• The purple line is power
against voltage.
• The scale for power can be
read from the right hand
side.

33. Maximum Power
Point
• The blue dot which
indicates the ‘Maximum
Power Point’ denotes that
at this point the load is
matched well to the output
of the photovoltaic cell, and
as such the photovoltaic
cell is delivering maximum
power into the load. It
should be noted that the
units for the Maximum
Power Point are “Peak
Watts”.

34. Fill Factor
Now, if we draw an imaginary line
from the MPP straight across to the X
and Y axis and enclose a square – then
– draw another two lines from the
short circuit current and open circuit
voltage back until they meet enclosing
another square; we can look at the
difference between the two areas. The
smaller area divided by the larger
gives us what is known as the ‘Fill
Factor’. The fill factor is a number
between 0 and 1 which can be used to
describe the quality of photovoltaic
cells.

35. Short Circuit Current
If you look to the left of the
graph where the brown line
crosses the Y axis; the reading
on the Y axis when voltage is
zero is the “short circuit
current” This is the current that
flows when there is no load but
one terminal of the panel is
connected directly to the other.
The short circuit current will be
in the region of 5-15% higher
than the current that flows at
the maximum power point.
Short circuit Current

36. Short Circuit Current
If you look to the left of the
graph where the brown line
crosses the Y axis; the reading
on the Y axis when voltage is
zero is the “short circuit
current” This is the current that
flows when there is no load but
one terminal of the panel is
connected directly to the other.
The short circuit current will be
in the region of 5-15% higher
than the current that flows at
the maximum power point.
Short circuit Current

37. Open Circuit
Voltage
Now look at the bottom
right of the graph where
the brown and purple
lines cross the X-axis. Here
we can take the reading
for “Open Circuit Voltage”,
which is the potential
difference between the
two terminals of the
photovoltaic cell when no
load is connected and no
circuit is present.
Short circuit Current
Open circuit voltage

38. Factors affecting
photovoltaic cell efficiency

39. Conduction
• Important to ensure that
electrons can move to the
conduction layer and to the
electrodes.
• Also that holes can migrate
to the rear contact of the
photovoltaic cell as smoothly
as possible without
resistance.

40. Absorption
• Photons are absorbed in the p-layer
of a photovoltaic cell.
• It is important that this layer
absorbs as many photons as
possible in order to create ‘free
electrons’.
• In designing photovoltaic cells,
engineers try to maximise
absorption.

41. Reflection
• If light reflects off an element of
the solar cell before it gets the
chance to pass through and do
useful work, then it is a wasted
electron!
• Want to try and minimise
reflection as far as possible in
order to ensure that we absorb as
much light in the photovoltaic cell
as possible.

42. Recombination
• An electron, once free, meets up with a
hole and recombines before it gets the
chance to do useful work and travel
around the circuit.
• This is known as “recombination” and is an
undesirable property that we want to try
to minimise.
• We can control and minimise
recombination, by designing the PN
junction in such a way that the electrons
are ‘freed’ as near to the junction – the
space charge region, as possible

43. Amorphous Silicon
• Amorphous silicon is a ‘thin film’ device.
• It is not a “crystalline” form of silicon.
• Rather than the tetrahedral crystalline structure carrying on over
a long range, it is disordered, there are ‘dangling bonds’.
• These dangling bonds can be “passivated” by bonding hydrogen to
them.
• This is known as a-Si:H, hydrogenated amorphous silicon.
• This hydrogen reduces the number of “dangling bonds” by several
orders of magnitude.
• However, it also leads to light-induced degradation over time.
• This is known as the Staebler-Wronski effect.

45. Amorphous Silicon
• “Amorphous Silicon” solar cells
commonly use a technology called “P-I-N”.
• Here, inbetween the P and N type
silicon, is an intermediate layer of silicon
called “I-Type” which stands for
“Intrinsic-Silicon”.
• This I-type silicon is undoped.
• When light hits the intrinsic region free
electrons and holes are generated
which are the divided by the electrical
field created by the adjacent N and P
type silicon.

46. If you found any of this interesting…
Please stay in touch
Gavin Harper
g.harper@glyndwr.ac.uk
www.gavindharper.com
http://www.cser.org.uk/
https://www.westproject.org.uk/
@gavindjharper
@CSER_PV
@LCRI_WEST