Goldsmiths’ 2013

Solar Energy

Dr Petra Cameron

http://www.bath.ac.uk/chemistry/contacts/academics/petra_cameron/

http://people.bath.ac.uk/chppjc/

Sustainable energy is necessary because of climate change and the need for alternative energy sources as we are running out of fossil fuels.

The lecture looked at photovoltaic essentials and first, second and third generation solar cells.

There is no doubt that greenhouse gases created by humans causes global warming.

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Unfortunately there are some people who don’t accept it and are out to cause trouble and the press are only too happy to give them publicity.

http://en.wikipedia.org/wiki/Climatic_Research_Unit_email_controversy

The Climatic Research Unit email controversy (also known as “Climategate”) began in November 2009 with the hacking of a server at the Climatic Research Unit (CRU) at the University Of East Anglia (UEA) by an external attacker. Several weeks before the Copenhagen Summit on climate change, an unknown individual or group breached CRU’s server and copied thousands of emails and computer files to various locations on the Internet.

Many commentators quoted one email in which Phil Jones said he had used “Mike’s Nature trick” in a 1999 graph for the World Meteorological Organization “to hide the decline” in proxy temperatures derived from tree ring analyses when measured temperatures were actually rising. This ‘decline’ referred to the well-discussed tree ring divergence problem, but these two phrases were taken out of context by climate change sceptics, including US Senator Jim Inhofe and former Governor of Alaska Sarah Palin, as though they referred to some decline in measured global temperatures, even though they were written when temperatures were at a record high.

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Eight committees investigated the allegations and published reports, finding no evidence of fraud or scientific misconduct. Three investigations have cleared the UEA scientists of any charges of tampering with data (not front page headlines). The scientific consensus that global warming is occurring as a result of human activity remained unchanged throughout the investigations.

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In January 2009, a poll of 3146 earth scientists found that 82% answered yes to the question: “Do you think human activity is a significant contributing factor in changing mean global temperatures?”. Of the 77 climatologists actively engaged in research, 75 answered yes (97.4%).

The scientists most likely to answer no were petroleum geologists and meteorologists.

Of course, just because most scientists think something is true does not necessarily mean they are right. But the reason they think the way they do is because of the vast and growing body of evidence. A study in 2004 looked at the abstracts of nearly 1000 scientific papers containing the term “global climate change” published in the previous decade. Not one rejected the consensus position. One critic promptly claimed this study was wrong – but later quietly withdrew the claim.

http://www.newscientist.com/article/dn11654-climate-myths-many-leading-scientists-question-climate-change.html

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The evidence for global warming:

– Satellite measurements of the upper and lower troposphere

– Weather balloons show very similar warming

– Borehole analysis

– Glacial melt observations

– Declining arctic sea ice

– Sea level rise

– Proxy Reconstructions

– Rising ocean temperature

Back to sustainable energy!

We need sustainable and low carbon ways of generating power. These could include solar power and other renewables, nuclear power (although not classed as renewable), biofuels and microbial fuel cells. There are no easy answers this is why research is so important.

Photovoltaics – things to be considered

Solar flux and the solar spectrum

Essential processes common to all photovoltaics

Semi-conductors

p-n and p-i-n junction cells

Dye sensitized solar cells

Organic bulk heterojunction cells

“I’d put my money on the sun and solar energy. What a source of power! I hope we don’t have to wait until oil and coal runs out before we tackle that”.

Thomas Edison, 1910

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Thomas Alva Edison (February 11, 1847 – October 18, 1931) was an American inventor and businessman.

http://en.wikipedia.org/wiki/Thomas_Edison

The solar spectrum

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An efficient device has to maximise the amount of the solar spectrum it can absorb.

The potential for solar energy

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Often quoted – but do we cover all land area in PV? Or do we have arrays in the desert? How do we store and transport the energy?

Energy payback times – one of the great myths

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Data: EPIA Sustainability Working Group Fact Sheet 2011, Graph: PSE AG 2012

Source: Photovoltaics report from the Frauenhofer Institute for Solar Energy Systems December 2012

See also NREL website:

http://www.nrel.gov/solar/

PV payback leaflet 2004

http://www.nrel.gov/docs/fy04osti/35489.pdf

The graph above seems to be evidence for the popular belief that PV systems cannot ‘pay back’ their energy investment. Therefore, it is important to investigate this issue on the basis of solid data.

The term “energy payback time” in this case is to do with getting back the cost of production. If the payback term was a year then it would take a year for the solar cell to produce enough electricity to match the cost of making it.

http://www.ecotopia.com/apollo2/pvepbtne.htm

Solar cells go through accelerated testing but they are expected to last thirty years.

Commercial technologies

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Myth 2 – it’s all very well for Egypt, but it will never work in the UK

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The above map actually shows that photovoltaics are viable in northern Europe. In the UK the payback time is 2.8 years.

Key terms

http://en.wikipedia.org/wiki/Photovoltaics

http://en.wikipedia.org/wiki/Photovoltaic_system

Photovoltaics (PV) is a method of generating electrical power by converting solar radiation into direct current electricity using semiconductors that exhibit the photovoltaic effect.

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One of the defining terms in the overall behaviour of a solar cell is the fill factor (FF). This is the available power at the maximum power point (Pm) divided by the open circuit voltage (VOC) and the short circuit current (ISC):

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The fill factor is directly affected by the values of the cell’s series and shunt resistances. Increasing the shunt resistance (Rsh) and decreasing the series resistance (Rs) lead to a higher fill factor, thus resulting in greater efficiency, and bringing the cell’s output power closer to its theoretical maximum.

Energy conversion efficiency is measured by dividing the electrical power produced by the cell by the light power falling on the cell.

http://en.wikipedia.org/wiki/Solar_cell_efficiency

IV curve with power curve

http://resources.solmetric.com/get/Guide%20to%20Interpreting%20I-V%20Curves.pdf

The I-V (current-voltage) describes its energy conversion capability at the existing conditions of irradiance (light level) and temperature. The span of the I-V curve ranges from the short circuit current (Isc) at zero volts, to zero current at the open circuit voltage (Voc). At the ‘knee’ of a normal I-V curve is the maximum power point (Imp, Vmp), the point at which the array generates maximum electrical power.

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6 – 8% efficient

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A solar simulator is used to test solar cells.

Common confusion

You sometime see quotes of solar cells where the solar energy conversion is very high e.g. 50-90%

Multi junction solar cells can approach efficiencies of 45% for research lab cells, single junction solar cells are governed by the Shockley–Queisser limit (~30%)

http://en.wikipedia.org/wiki/Shockley%E2%80%93Queisser_limit

In physics, the Shockley–Queisser limit or detailed balance limit refers to the maximum theoretical efficiency of a solar cell using a p-n junction to collect power from the cell.

Capture  is the solar conversion efficiency at AM (air mass) of 1.5 when the cell is exposed to the whole solar spectrum

The EQE (external quantum efficiency) also called the IPCE (incident photon to electron conversion efficiency) is the conversion efficiency at a single wavelength and can be >90%.

http://en.wikipedia.org/wiki/External_quantum_efficiency

The graph below left is of external quantum efficiency against wavelength. The quantum efficiency (QE), or incident photon to converted electron (IPCE) ratio, of a photosensitive device or a charge-coupled device (CCD) is the percentage of photons hitting the device’s photoreactive surface that produce charge carriers. It is a measurement of a device’s electrical sensitivity to light. A solar cell’s quantum efficiency value indicates the amount of current that the cell will produce when irradiated by photons of a particular wavelength.

External Quantum Efficiency (EQE) is the ratio of the number of charge carriers collected by the solar cell to the number of photons of a given energy shining on the solar cell from outside (incident photons).

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The external quantum efficiency depends on both the absorption of light and the collection of charges. The bottom right graph shows the peak solar spectrum corresponds with the peak EQE.

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The external quantum efficiency therefore depends on both the absorption of light and the collection of charges. Once a photon has been absorbed and has generated an electron-hole pair, these charges must be separated and collected at the junction. A “good” material avoids charge recombination. Charge recombination causes a drop in the external quantum efficiency.

Photovoltaic essentials

The electron (hole) must find an interface before it recombines. The free electron (hole) diffusion length is a critical parameter. The interface is used to separate the hole from the electron. A photo-induced separation occurs here.

1st generation solar cells: silicon p-n junction

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A p–n junction is a boundary or interface between two types of semiconductor material, p-type and n-type, inside a single crystal of semiconductor. It is created by doping, for example by ion implantation, diffusion of dopants, or by epitaxy (growing a layer of crystal doped with one type of dopant on top of a layer of crystal doped with another type of dopant). If two separate pieces of material were used, this would introduce a grain boundary between the semiconductors that severely inhibits its utility by scattering the electrons and holes.

p–n junctions are elementary “building blocks” of most semiconductor electronic devices such as diodes, transistors, solar cells, LEDs, and integrated circuits; they are the active sites where the electronic action of the device takes place.

Selectivity is not built into silicon.

http://en.wikipedia.org/wiki/P%E2%80%93n_junction

Electronic Equilibrium between Phases

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http://en.wikipedia.org/wiki/Phase_(matter)

In the physical sciences, a phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform.

Left to equilibration, many compositions will form a uniform single phase, but depending on external conditions even a single substance may separate into two or more distinct phases. Within each phase, the properties are uniform but between the two phases properties differ. In the above case the two phases have different charges.

The Contact Potential Difference

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The contact potential difference is an electrostatic potential that exists between samples of two different electrically conductive materials (metals or semiconductors with different electron work functions) that have been brought into thermal equilibrium with each other, usually through a physical contact.

Although normally measured between two surfaces which are not in contact, this potential is called the contact potential difference. Initially it is expected that mobile charge carriers (electrons or holes) will migrate from one sample to the other. If there is a net flow of electrons from one material (A) to another (B) B will become negatively charged and material A will become positively charged, assuming that they were originally neutral. This process is self-limiting because a potential difference between the two samples will develop due to the charge separation and will grow to a value sufficient to stop further motion of the electrons from A to B.

http://encyclopedia2.thefreedictionary.com/contact+potential+difference

The free energy of electrons is identical in the two phases.

The Fermi level equilibrates to an electric field at the interface.

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http://en.wikipedia.org/wiki/Fermi_level

The Fermi level is the total chemical potential for electrons (or electrochemical potential for electrons) and is usually denoted by µ or EF. The Fermi level of a body is a thermodynamic quantity, and its significance is the thermodynamic work required to add one electron to the body (not counting the work required to remove the electron from wherever it came from). A precise understanding of the Fermi level—how it relates to electronic band structure in determining electronic properties, how it relates to the voltage and flow of charge in an electronic circuit—is essential to an understanding of solid-state physics.

The diagram below left is showing that p-n junction has holes going “uphill”. The p-i-n junction has a big phase charge region.

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In solid-state physics, a band gap, also called an energy gap or bandgap, is an energy range in a solid where no electron states can exist. In graphs of the electronic band structure of solids, the band gap generally refers to the energy difference (in electron volts) between the top of the valence band and the bottom of the conduction band in insulators and semiconductors. This is equivalent to the energy required to free an outer shell electron from its orbit about the nucleus to become a mobile charge carrier, able to move freely within the solid material, so the band gap is a major factor determining the electrical conductivity of a solid. Substances with large band gaps are generally insulators, those with smaller band gaps are semiconductors, while conductors either have very small band gaps or none, because the valence and conduction bands overlap.

The bandgap can be anywhere but charge separation can only occur at the interphase.

http://en.wikipedia.org/wiki/Band_gap

Myth:

Electrons and holes flow to the contacts down an electric field.

In a p-n junction there is a field at the interface that acts to separate the charges – typically few hundred nm in a c-Si p-n junction, possibly up to 1 μm in a p-i-n cell.

A c-Si cell itself is hundreds of μm thick. Away from the interface electrons and holes diffuse down a concentration gradient created by extraction at the contacts.

Maximum predicted efficiency for a SINGLE JUNCTION solar cell

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Shockley–Queisser limit for a p-n junction

Predicted limit for a silicon (Eg ~ 1.1eV) solar cell is ~ 34% if it is “perfect”

Maximum experimental efficiency is ~ 28%

Silicon has an indistinct band gap which is why the solar cells have to be thick. A 300nm intrinsic space charge region is required.

There is always some loses due to recombination, heat, or the material not absorbing the full useable spectrum.

Power Losses in Solar Cells

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There are physical constraints that create power losses in real solar cells. This diagram shows these constraints. For 100mW of power coming from the sun are coming in, 21 mW is the below the band gap, so it’s not enough energy to excite an electron. 31mW of it is excess of the band gap so it’s lost through heat. So already, more than half of your power is lost just because of the limitations of a single bandgap.

The Voc available in silicon is 1.1V and because of all the above losses, all you have left are 44 mAmps. Recombination losses, which are electrons smashing into holes, drops the 1.1V to approximately 0.6V. As you can see, recombination cuts the maximum voltage almost in half. Now the current, already being cut down earlier, will be cut down once again due to collection efficiency and incomplete absorption. Collection efficiency is when some of the photons cannot excite electron-hole pairs. Incomplete absorption means some of the photons pass through the cell. This demonstrates one of the technical challenges of designing solar cells: If you make your cell thicker, it’s easier to absorb photons but harder to collect electrons because they have to diffuse over longer distances. However, if you make the solar cell thinner, it would be easier to collect electrons but you will lose a lot of photons that do not absorb. In addition, there is top surface reflection on a cell and no absorption where the metal is located due to shadowing. The current of 44milliAmps is now decreased to 21 milliAmps. The fill factor is reduced from 1 to .7. The final power is about 14 mW, which means this solar cell is 14% efficient.

Recombination

This is the opposite of carrier generation, where the electron-hole pair is annihilated.

Most common at:

Impurities;

Defects of crystal structure;

Surface of semiconductor.

They reduce both voltage and current and are major issues in PV,

Now we will go through each one of these losses in detail. Recombination is when an electron-hole pair is created and then is smashed back into each other. Recombination usually happens when there is an impurity. For example, if you have a perfect silicon lattice except for the addition of a few sodium atoms. The sodium atoms create defects in the gap which means you end up with nothing instead of an electron-hole pair. There can also be defects in the crystal structure. No matter how perfect the bulk material is, on the surface of a semiconductor there are silicon atoms that are not fully bonded – which then cause surface states. The more surface states you have on the silicon solar cell, such as is created at grain boundaries in a polycrystalline silicon solar cell, the worse off you are. This explains why polycrystalline silicon solar cells do not have as high efficiencies as seen for single crystal silicon solar cells.

Solar PV Materials: Crystalline & Polycrystalline Silicon

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There are some advantages and disadvantages of crystalline and polycrystalline silicon. Advantages include a high efficiency (14-22%), the fact that it is a very established technology and it is stable. The technology for the crystalline silicon is the same technology that drives the semiconductor industry. The disadvantages for crystalline technology include expensive production, low absorption (so you need a lot of material), and a large amount of highly purified feedstock, which of course is expensive. Polycrystalline silicon is cheaper but the electric properties are not as good so the efficiency is lower.

Amorphous Silicon

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The other kind of silicon solar cell material is amorphous silicon. It has a very high absorption coefficient (you don’t need a lot of it) because it acts like a direct band gap semiconductor instead of an indirect gap like crystalline silicon. It is an established technology. It can also come like a sticker, such as the solar cell in the image, where all you do is peel off and stick it to the building (easily integrated into the building) and it lasts virtually forever (30 year warranty). It also does not need any building materials to put it up and it is cheaper than glass, metal or plastic it is deposited on. It has an excellent ecological balance sheet. The disadvantages include an efficiency of only 7-10% and it degrades when light hits it – this is known as the Staebler-Wronski effect or SWE. All amorphous silicon solar cells sold now have warrantees for the degraded steady state – where SWE has stopped. For example you might buy a solar cell with a warrantee of 10% efficient – when you first install it your will be happy to find that it will produce electricity at 11% efficient. After a few months it will degrade down to 10% and then stay there the rest of its lifetime.

All cells mentioned on the chart below were measured using a certified solar simulator: 1 Sun and AM 1.5. If the efficiency is above 30% it is because multiple solar cells were used. A leaf can renew its dye but solar cells can’t.

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Doping or mixing of electron rich and electron poor materials

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Organic photovoltaics

An organic solar cell or plastic solar cell is a type of polymer solar cell that uses organic electronics, a branch of electronics that deals with conductive organic polymers or small organic molecules, for light absorption and charge transport to produce electricity from sunlight by the photovoltaic effect.

http://en.wikipedia.org/wiki/Organic_solar_cell

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http://en.wikipedia.org/wiki/Polymer_solar_cell

http://en.wikipedia.org/wiki/Heterojunction

A heterojunction is the interface that occurs between two layers or regions of dissimilar crystalline semiconductors. These semiconducting materials have unequal band gaps.

http://en.wikipedia.org/wiki/Exciton

A bulk heterojunction solar cell has a short exciton (a bound state of an electron and an electron hole which are attracted to each other by the electrostatic coulomb force) diffusion length. This means recombination is a big problem. It needs to be fully encapsulated so that no air or water can get to it. It stops working if there is any break in the organic materials.

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Now we have donors and acceptors .Typical donors are conjugated polymers such as polythiophenes. They absorb visible light. Acceptors tend to be fullerenes (molecular solids), can also be perylenes, porphryns etc.

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PCBM and P3HT fullerines need more than one junction. Mixing them together allows you to coat any material.

Dye Sensitised Solar Cell

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A dye-sensitized solar cell (DSSC, DSC or DYSC) is a low-cost solar cell belonging to the group of thin film solar cells. It is based on a semiconductor formed between a photo-sensitized anode and an electrolyte, a photoelectrochemical system.

https://en.wikipedia.org/wiki/Dye-sensitized_solar_cell

The majority carriers are electrons in TiO2

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Multi junction cells

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Schematic diagram of state-of-the-art a-Si:H based substrate n-i-p triple junction cell structure.

There are several cells. The top cell has large bandgap, the middle cell has a mid eV bandgap and the bottom cell has a small bandgap.

This is another method to increase the efficiencies of solar cells (the tandem concept pictured above) where several cells are stacked on top of each other. In the example here there is a triple junction amorphous silicon solar cell. In order to fabricate a tandem cell you do the following. First, you have stainless steel substrate covered with silver as a back reflector on the bottom. Here when light comes into the cell it is reflected and goes back through for another chance at absorption. Also the back is textured so that the light will scatter and must take a longer path through the material. Next is the red cell (which absorbs red light). It is a p-i-n structure. On top of that is green and then blue cells, which each absorb their respective colour of light. Both of those also have the p-i-n structure. Finally, the top surface is made up of a textured transparent conductive layer. The green and red cells are actually silicon germanium. Germanium has a smaller band gap than silicon, so that when you combine those, you get a small gap, then bigger, and bigger, increasing on the way up. The blue cell is on top of the red because blue has a larger band gap it will absorb the highest energy photons and let the photons with less energy (green and red) pass through it to be absorbed by the lower cells. If the tandem cell was turned upside down, the green and blue energy of the photon would get lost because red has the smallest band gap and would absorb them but lose a large percentage of the energy to waste heat.

Acknowledgements

EPSRC for Funding

Collaborators: Ioannis Ieropoulos, John Greenman, Paolo Bombelli, Adrian Fisher.

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Students and PDRAs: Becky Thorne, Huaining Hu, Kenneth Schneider, Rupert Cape, Ellie Johnson, Shane McDonald, Thomas Risbridger, Kathryn Wills, Nikte Gomez…..

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