Science ArticlesCatching Energy From the Sun

How Solar Cells Work

Niraj Lal

Electricity is pretty old - one of the oldest things ever really. The Universe is thought tobe about 13.7 billion years old and electricity - the flow of charge - has been around fornearly all of it. But it is only in the last two centuries that humans have learnt how tomake charges flow around circuits and do things for us; that is, make electricity.

First, a bit about charge. We know threefundamental things about charge:

1. There are two types.

2. Different charges attract each other.

3. Like charges repel each other.

No one really knows why charges behave the way they do; it seems to just be a fact ofour Universe.

Today, we know of eight ways to make charge flow.

Some of the ways are familiar to everyday life, others are less common and are lessintuitive to understand, but they all have one thing in common - they are a way of movingcharge from one place to another - generating electricity.

1. Static Electricity (charges moving when materials rub past each other)

This is the type of electricity that is responsible for lightning and why balloons stick tohair.

2. Chemical Electricity (charges moving when chemical reactions take place)

This type of electricity powers all of our batteries – from the small alkaline batteries intorches to the big lead-acid ones in cars. Nature has invented interesting ways to useelectrochemistry:the electric eel is thought to have evolved about 500 million years ago,and through chemical reactions in special cells in the side of its body the eel can build upenough voltage to send a massive shock through its prey, stunning it for long enough sothat it can kill it.

3. Piezoelectricity (charges moving when certain crystals are squeezed)

This type of electricity is generated when yousqueeze the trigger on a gas lighter. As the piezoelectric crystal is compressed, chargesmove to either end. If you keep squeezing, the voltage can be big enough to ionize airand make mini-lightning – for example, the spark you see when you light a gas stove.

4. Pyroelectricity (charges moving when certain crystals are heated)

Although less commonly known, pyroelectricity happens when crystals similar topiezoelectric crystals are heated up, forcing different charges to opposite ends of thecrystals. Their use isn’t widespread, but they have applications as heat sensors andpotentially in nuclear fusion generators.

5. The thermoelectric effect (charges moving by heating two different metalsconnected to each other).

This type of electricity relies on the intrinsicproperties of metals and their densities. A thermoelectric generator is what is poweringthe Cassini-Huygens spacecraft - still orbiting Saturn. The heat in the Cassini-Huygensgenerator is provided by the radioactive decay of plutonium.

6. Nuclearvoltaics (charges flying off an atom as it radioactively decays)

Really big atoms (such as uranium or plutonium orcaesium - pictured) are unstable. What this means is that after some time, they break upinto smaller parts and some of these parts have charge. No-one has yet figured out away to make any use of nuclear-voltaics to produce electricity due to the dangers ofworking with radioactive atoms.

7. Electromagnetism (charges moving due to moving magnets)

About 95% of the world's electricity is made byelectromagnetism.

One of the fundamental things about our Universe is that when a magnet moves, electricfields flow.

Since the 19th century, we have been using coal, diesel, gas, water and the heat fromnuclear decay, the tides, and the blowing winds to turn magnets inside wires to generateelectricity.

It's a fantastic way of generating electricity, and will almost certainly be with us for a longtime.

8. Photoelectricity (charges moving by the absorption of light).

The photovoltaic effect was discovered in 1839 by a French scientist namedAlexander-Edmond Becquerel.

Becquerel used a platinum sheet covered in silver chloride and found that it generatedelectricity when light was shone on it.

Back then, the efficiency was less than 1% - but we've come a long way since then.

The reason why light can make electricity is a result of three things:

1. Light has energy;

2. Matter has charge;

3. When certain materials absorb light, these charges become excited and can movearound.

Solar cells are devices designed to use these properties to make electricity.

Solar Cells

A solar cell is a little bit like a pool table, where the white ball represents a photon of lightand the other balls are charge carriers.

When light (the white ball) is shone onto the solar cell (the pool table), it makes thecharge carriers (coloured balls) move. And if the light has enough energy, it will excitethe balls so much that they will travel to the edge of the solar cell (we need a specialpool table here – one with pockets all along the outside) where they can be collected tomake electricity.

That's really it.

Of the easiest solar cells to understand arethose made up of two types of material.One material has lots of free negativecharge carriers (electrons). This is calledn-type material.

The second is a material with lots of freepositive charge carriers (called holes – theabsence of electrons), which is calledp-type material. When these two types arenext to each other, an electric field is builtup in the junction between them.

The important thing is that when light isshone on this structure, both positive andnegative charges are formed. Because ofthe electric field between the p-type andn-type material, these charges are pushedto opposite edges of the cell.

So if you connect the two ends of the solarcell to external wires and complete thecircuit then electricity can flow!!!

It’s a remarkable device, and a truly amazing invention – for the interested reader whowould like to learn more, the PVCDROM website is a great resource:www.pveducation.org/pvcdrom. For the reader who would like to learn more about theunderlying physics of solar cell materials, the texts by Kittel “Introduction to Solid StatePhysics” and Sze “Semiconductor Devices” are excellent introductions.

Efficiency

Probably the most important measure of solar cells is efficiency.

In this context, what is usually meant by efficiency is the amount of electricity put out bythe cell, compared to the amount of sunlight shining on it.

To be more precise, efficiency is defined as the ratio of electrical energy that comes outof the cell per second (the power) to the amount of light energy that hits the cell persecond (incident energy).

η= (V x I)/(Incident Energy)

The symbol ‘η’ represents efficiency, V is voltage and I is current. The product of voltageand current (V x I) is the output power. For the structure discussed above – a single p-njunction, the maximum efficiency that anyone could ever get by placing it in directsunlight is 31%.

So on a bright, sunny day, the theoretical limit to the amount of power you could getfrom a square metre of single p-n junction solar cells would be about 300 Watts –enough to power a microwave. The efficiency limitation is due to the colour of the sunand its distance from Earth.

Below is a diagram showing how much sunlight energy reaches the Earth. It shows theintensity (W) that falls on each square metre of earth (m-2), per wavelength of light(nanometre-1).

Now the thing is, somewhere on thisdiagram a vertical line has to be drawn. This line represents the band-gap of our solarcell material – the width of the pool table. The band-gap is the amount of energy acharge needs to absorb before it can become properly excited. It is called “the solarspectrum”.

For light of energy higher than the band-gap (light to the left of our line), our charges willget excited and our solar cell will put out current. This is a good thing – we need currentfrom our cell so that there are enough charges to power our light bulbs and toasters andhair dryers.

Light of energy less than the band-gap (tothe right of the line) either doesn’t get absorbed by the cell – passing right through, orget’s absorbed in a not-useful way – generating heat. So to increase the amount ofcurrent from the cell, we need to move our band-gap line to the right – so that there ismore light to the left of our line to absorb.

But, the power output of the cell is given by:

Power (W) = Voltage (V) x Current (I)

This is important because it means that even if the current was very, very high, if wehave a cell with low output voltage, then the output power will be low too. Now thevoltage of the cell is directly related to the band-gap of the cell, increasing as theband-gap goes higher in energy – to the left in the above diagram.

So we have a trade-off:

And because efficiency η is given by:

η=(Voltage x Current)/(Incident Energy)

We have a maximum ‘best-position’ place on the spectrum for our line. For sunlight onEarth, the ‘best-position’ line is at about 1200nm, corresponding to a maximumefficiency of about 31%.

Third Generation Photovoltaics

Now, we’d like to do better than 31%, and there are a couple of ways of doing it.

The first and most easy way is by concentrating light onto our solar cell. So if we can geta big magnifying glass, or a big mirror and focus it on our cell, it looks to the solar cell asif the Sun has got brighter – or a bit closer. This is because we’re collecting light from alarge area, and concentrating it on a smaller area. In this way, we can get a maximumefficiency of about 40%.

But we can go even better than this, and there are many, many ideas out there, rangingfrom up-converters and down-converters, to multi-exciton generation and quantum dots,to thermophotovoltaics, to selective contacts and hot-carrier cells.

In the last bit of this article, we’ll discuss only two of these ideas: Multi-junction cells –the cells that currently hold the world record of efficiency, and Plasmonics – thebeginnings of the next generation of nanostructured photovoltaics.

Multi Junction cells

Multi-junction cells broke the 40% efficiency barrier in 2009 and it’s likely that 50%efficiency will be reached before 2020. Multi-junction cells are used on the Mars Roversand the majority of the satellites orbiting the Earth right now to generate power.

How they get beyond 31% is by having morethan one band-gap line on the solar spectrum. They do this by having a stack of cells,each with a band-gap slightly lower than the cell above it. In this way, the highest energylight (the purple) gets absorbed by the first cell – which is transparent to all light of lowerenergy (the blues, greens, yellows and reds). Then the second cell absorbs the nexthighest energy light after the purple (the blue) whilst being transparent to light of lowerenergy than this (the greens, yellows and reds), and so on.

The theoretical maximum efficiency for this type of cell is about 86%. This is the Carnotlimit for sunlight energy conversion on Earth. There’s no way of ever going beyond thislimit on Earth – it’s a fundamental limit. But we can keep getting closer.

Nanostructured Photovoltaics

This is going to be the next revolution of solar cell design. There’s still no way of goingbeyond the fundamental efficiency limits, but there are going to be some new andexciting ways of getting closer to them.

One idea is plasmonics. A "plasmon" is the name given for a particular way that chargeand light can resonate together. When the right wavelengths of light shine onto certainmetals, the electrons inside the metal move back and forwards like a slinky spring. Thismoving backwards and forwards in turn sets up a moving electric field – and thiscoupling is called a plasmon. By structuring metals on the nanoscale these resonancescan be tuned. Two of the ideas being applied to solar cells are those of nano-particleplasmonics and nanovoid plasmonics.

The idea is to use the electric field hotspots to increase absorption in exactly the regionthat you want to in a solar cell. In this way, light and charge can be guided to the optimalplaces in solar cells, increasing current, voltage and efficiency all in one structure.

Research into plasmonic photovoltaics is happening all over the world, including in theNanoPhotonics Centre in the Department of Physics at the University of Cambridge,where I am making solar cells on plasmonic nanovoids and seeing them shine.

How Semiconductors work

Now, for the reader who would like to know moreabout the underlying physics of how the p-n junction works, we need a brief introductionto semiconductor physics.

The most common solar cell material is silicon, and it serves as a good example to usein discussing semiconductors. Silicon has four valence electrons which it likes to sharewith other silicons when it’s in a crystal.

The little electron and hole in the diagram shows an electron becoming excited, leavingbehind a ‘hole’ in the gap where the crystal would like it to be. In this case the requiredenergy for exciting the electron has been given by the heat of the crystal – somethingthat happens even at room temperature.

The interesting thing is that if a neighbouring electron popped into that hole – it wouldleave behind a hole at the place it just came from – physicists think of this as the holeitself moving.

The previous diagram shows pure ‘intrinsic’silicon. When you introduce another atom into the structure, such as phosphorus with anextra electron in the valence band, the amount of free electrons changes.

In the second diagram, the extra electron that was attached to the phosphorous atomhas broken free (again due to the heat energy supplied at room temperature), and isnow able to wander around in the crystal lattice. The material above is now called n-typesilicon.

If, instead of phosphorous, we introduce boron into the structure, the amount of freeholes increase. This is because boron has one less electron in its valence band.

In the third diagram, there is now a free hole that is able to wander about. The abovematerial is then called p-type silicon.

The p-n structure is formed when these two types of material, p-type and n-type, comeinto contact with each other.

When they are first brought into contact theelectrons and holes swim across the junction to recombine with their opposite chargeson the other side.

As soon as they have finished recombining, there exists a region in the middle of thestructure without any free charges.

This region without any charge is called “the depletion region”, and is what gives rise tothe electric field in the middle of a solar cell.

That’s the basic physics and it helps us to understand what’s going on, but for realdevices the actual method of making the p-n junction isn’t quite done by sandwiching thetwo types together. The typical method is to add some boron atoms in the mix while theintrinsic silicon is crystallising – thus making p-type silicon. Later some phosphorous isdriven into the wafer using gas deposition at high temperatures to make the top surfacen-type, thus creating the junction.

The p-type and n-type comeinto contact

The electrons and holesrecombine with each other

Once recombined, there areno longer any free charges

As an aside, this is the same field that exists inside a diode – allowing charge to floweasily in one direction (forward bias) and hardly at all in the other direction (reversebias).

It is a truly remarkable device – and again the interested reader is referred to the texts ofKittel and Sze for a more thorough introduction.

MaterialsThe reader who has gotten this far might be interested in the different materials thatsolar cells can be made of.

Lots of different materials are used, including inorganic semiconductors, organicsemiconductors, and electrochemical solutions.

Within each of these sub-types there is still further variety in the type of materials beingresearched – within the inorganic semiconductors the range extends from silicon togermanium to compound semiconductors such as gallium-arsenide andcadmium-telluride, to name just a few. There also is expanding research in the field ofquantum-dot and quantum well solar cells – cells made up of small particles of materialembedded within a semiconductor which change their electric properties as theirdimensions are varied.

Organic semiconductors work in a slightly differently way to inorganic semiconductors.Instead of having free charge carriers within a crystal lattice – charge carriers areassociated more with individual molecules and polymers. This gives rise to differentelectrical properties, but the underlying physics of light-absorption remains the same. Itis a relatively new field, and much research is being done on the particular organicmaterials that might work best.

Dye-sensitised solar cells are different again – in these cells a special dye absorbs lightand rapidly transfers an excited charge into a solar cell substrate, the dye is thenreplenished via a conducting electrolyte solution. This too is a relatively recent invention.

Each of these device types are incredible inventions with their own particular advantagesand disadvantages. Each material and device type has many researchers working on it –the interested reader is referred to the book by Martin Green “Third GenerationPhotovoltaics” (2003) for an introduction to the next generation of solar cell design.

As to which one is the best? We’ll find out. But it’s a promising field, with a bright future.