46
Gavin D. J. Harper Silicon Photovoltaic Cells: An Introduction Welsh Energy Sector Training (WEST) OpTIC Glyndwr Ffordd William Morgan, St Asaph, Wales October 2014 [email protected] @gavindjharpe www.gavindjharper.co http://orcid.org/0000-0002-4691-664

Silicon Photovoltaic Basics

Embed Size (px)

DESCRIPTION

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.

Citation preview

Page 1: Silicon Photovoltaic Basics

Gavin D. J. Harper

Silicon Photovoltaic Cells:An Introduction

Welsh Energy Sector Training (WEST) OpTIC Glyndwr

Ffordd William Morgan, St Asaph, WalesOctober 2014

[email protected]@gavindjharper

www.gavindjharper.comhttp://orcid.org/0000-0002-4691-6642

Page 2: Silicon Photovoltaic Basics

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’

Page 3: Silicon Photovoltaic Basics

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.

Page 4: Silicon Photovoltaic Basics
Page 5: Silicon Photovoltaic Basics

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.

Structure of Silicon

Page 6: Silicon Photovoltaic Basics

• 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.

Structure of Silicon

Page 7: Silicon Photovoltaic Basics

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.

Page 8: Silicon Photovoltaic Basics

MetalloidsSilicon 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.

Page 9: Silicon Photovoltaic Basics

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.

Page 10: Silicon Photovoltaic Basics
Page 11: Silicon Photovoltaic Basics

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.

Page 12: Silicon Photovoltaic Basics
Page 13: Silicon Photovoltaic Basics
Page 14: Silicon Photovoltaic Basics

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.

Page 15: Silicon Photovoltaic Basics

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.

Page 16: Silicon Photovoltaic Basics

Photovoltaic Effect

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

Page 17: Silicon Photovoltaic Basics

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.

Page 18: Silicon Photovoltaic Basics

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.

Page 19: Silicon Photovoltaic Basics
Page 20: Silicon Photovoltaic Basics

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.

Page 21: Silicon Photovoltaic Basics

Silicon Photovoltaic Cell Construction

Page 22: Silicon Photovoltaic Basics
Page 23: Silicon Photovoltaic Basics

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.

Page 24: Silicon Photovoltaic Basics

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.

Page 25: Silicon Photovoltaic Basics

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.

Page 26: Silicon Photovoltaic Basics

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.

Page 27: Silicon Photovoltaic Basics

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.

Page 28: Silicon Photovoltaic Basics

Measuring PV Performance

Page 29: Silicon Photovoltaic Basics

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

Page 30: Silicon Photovoltaic Basics

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.

Page 31: Silicon Photovoltaic Basics

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.

Page 32: Silicon Photovoltaic Basics

• 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.

PV Power Curve

Page 33: Silicon Photovoltaic Basics

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”.

Page 34: Silicon Photovoltaic Basics

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.

Fill Factor

Page 35: Silicon Photovoltaic Basics

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

Page 36: Silicon Photovoltaic Basics

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

Page 37: Silicon Photovoltaic Basics

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

Page 38: Silicon Photovoltaic Basics

Factors affectingphotovoltaic cell efficiency

Page 39: Silicon Photovoltaic Basics

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.

Page 40: Silicon Photovoltaic Basics

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.

Page 41: Silicon Photovoltaic Basics

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.

Page 42: Silicon Photovoltaic Basics

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

Page 43: Silicon Photovoltaic Basics

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.

Page 44: Silicon Photovoltaic Basics
Page 45: Silicon Photovoltaic Basics

• “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.

Amorphous Silicon

Page 46: Silicon Photovoltaic Basics

Gavin [email protected]

http://www.cser.org.uk/

https://www.westproject.org.uk/

@gavindjharper

@CSER_PV

@LCRI_WEST

If you found any of this interesting…Please stay in touch