What can Materials Science tell us about Solar Energy of the Future?

What can Materials Science tell us about Solar Energy of the Future?

Professor Stuart IrvineCentre for Solar Energy Research

Glyndŵr University

Climate change

Were the conditions in the Pacific in March 2010 exceptional?

• During the winter of 2009-2010 a rare combination of known factors in earth’s climate variability systems ................(AccuWeather.com).

• According to records going back to 1950, this winter saw one of the strongest El Nino events, combined with the most negative Arctic Oscillation (and also with a negative North Atlantic Oscillation) yet seen during a winter.

Can we link this to climate change?

• Yes and no• Individual weather events

cannot be directly attributable to climate change – but

• With the warming of the ocean temperatures we can expect a greater occurrence of extreme weather (Hadley Centre)

Forecast for 2nd April 2010

Evidence for anthropogenic climate change

What can we do to limit carbon dioxide emissions?

• The world is heavily dependent on energy from fossil fuels: coal, gas, oil

• Approximately 80% of the UK electricity grid is still powered by fossil fuel http://www.ecotricity.co.uk/our-green-energy/energy-independence/uk-grid-live

• UK government target to reduce carbon emissions by 80% by 2050 to limit temperature rise to 2o C

• How can we achieve this?

Moving towards a low carbon economy

• More efficient use of energy: transport, heating, electricity

• Replace fossil fuel energy with renewable energy sources: Solar, wind, bio-energy, tidal, wave, hydro

• Build a new generation of nuclear power stations• Fusion power• The task is huge so all of the above have a part to

play!

The sun radiates more than enough energy onto the Earth in just one day to provide enough energy for the

population of 5.9 billion people for 27 years

In Wales enough solar energy radiates onto just 1 square kilometre over a year to supply 10% of our electricity needs

This X-ray image of the Sun, taken by the SOHO satellite, shows numerous active regions in the Sun's atmosphere.

The Sun is by far the largest object in our solar system, containing more than 99% of the total mass.

atmospheric temperature of 5500 oC and a luminosity of 4x1020 megawatts

The sun is composed of 75% hydrogen and 25% helium

The sun’s energy comes from a thermo-nuclear reaction where the nuclei of hydrogen are converted into helium releasing huge amounts of energy

When solar radiation arrives at the Earth it can be converted to heat

Solar radiationheat

But how can we generate electricity from solar radiation?

Our modern understanding of light and colour begins with Isaac Newton (1642-1726) and a series of

experiments that he published in 1672.

It wasn’t until 1901 with the publication of Planck’s black body theory that we started to understand how light interacts with matter

Planck had to assume that light carried “quanta” of energy that we now call “photons”

High energy

photons

Low energy

photons

To make electricity we need a flow of electrons. Einstein was the first to explain how electrons could be released from a metal in a vacuum by light (photons)

beamed at the surfacePeople are also aware of his theories of relativity: the Special Theory of Relativity (published in 1905) and the General Theory of Relativity (published in 1915). What many people do not know is that Einstein was the second person to make a major contribution to the quantum revolution, in a paper also published in 1905 . In fact, this paper won him a Nobel prize.

Only blue light would release electrons and not red light, no matter how intense the red light.

How are photons absorbed in a semiconductor?

energy

Conduction band

Valence band

photon

electron

For absorptionEp > Eg

Silicon cells can now convert up to 20% of the sun’s radiation into electrical energy

Band gap energy

For the electron to become an electric current it must pass across a junction from electron depleted to

electron rich semiconductor materials

Unlike metals where electricity can only be conducted by electrons, semiconductors can conduct electricity with negatively charged electrons and positively charged “holes”

The Sharp silicon PV module factory in Llay is producing around 300 MW of PV panels a year (increasing to 500 MW) this year

CIS tower, Manchester

What are the components of a grid-connected PV system?

To Grid

On-site Load

PV Modules

Inverter

Export Meter

Import Meter

Examples of grid connected silicon PV modules installed by Dulas Ltd

Market price and predicted capacity for PV solar modules

Thin film PV (a-Si, CdTe and CIGS) will be a quarter of the market by 2013

EPIA Report

Lowest Mono- crystalline Module Price $2.17/Wp (€1.69/Wp)

Lowest Multi- crystalline Module Price $1.99/Wp (€1.55/Wp)

Lowest Thin Film Module price $1.07/Wp (€0.83/Wp)

Solar Buzz September ‘10 minimum prices

Potential

to drive

down

cost with

thin film

PV

Materials cost becomes the major cost factor for high volume manufacture

Semiconductor elements

The structure of a CdTe thin film solar cell

p- CdTe

n- CdS

TCO

Glass substrate

Front contact

Back contact

junction

PV modules can be made much cheaper if the semiconductor was just a thin film on a

sheet of glassFirst Solar Inc

Wurth Solar

First solar is leading the way with high volume thin film CdTe PV manufacture

80MW

Enbridge Inc.

Sarnia, Ontario

53MW

juwi Solar GmbH

Lieberose, Germany2005

20062007

20082009

2010*2011*

0

500

1000

1500

2000

2500Manufacturing capacity MW Cost per Watt: $0.77

Output per line 59.6 MWAverage module efficiency: 11.3%Employees: 5,500+

The PV façade at OpTIC Glyndwr Campus, StAsaph demonstrates novel thin film CIGS technology

1000 m2 generating up to 85 kWp of completely clean energy.Largest of its kind outside US

In the first 12 months of operation a total of 65,000 kWh of clean electricity was generated, saving 28 tonnes of carbon emissions from fossil fuelled power stations

Variation of energy output from OpTIC PV facade through the year

Apr-09

May-09

Jun-09Jul-0

9

Aug-09

Sep-09

Oct-09

Nov-09

Dec-09

Jan-10

Feb-10

Mar-10

Apr-10

0

50

100

150

200

250

Average daily output of OpTIC PV wall

kWh/

day

What are the limits to efficiency of PV solar cells?

Potential for 30% efficient cells based on single junction PV

The optimum efficiency is a compromise between the proportion of the solar spectrum that can be absorbed and the amount of energy captured per photon absorbed

For greater than 30% efficiency need to go to multi-junction cells

The cost/ performance trade-off

• The highest performance solar cells (triple junction gallium arsenide are over 30% efficient) are too expensive for building integrated PV but used for powering satellites.

• Very low cost dye-sensitised solar cells (DSC) may be suitable for large areas such as industrial roofs (Tata- Dyesol piloting DSC onto sheet steel (approx 5% efficient)

• Crystalline silicon is still a good compromise between efficiency and cost (15-20% efficient)

• Thin film silicon, cadmium telluride and CIGS are moving towards crystalline silicon but with inherently lower cost.

Concentrators might just prove to be a winner for terrestrial triple junction cells

Whitfield Solar – trough type concentrators

Circadian Solar – plastic Fesnel lens concentrators

• concentration up to 500x the amount of expensive solar cell material can be reduced• but the array has to track the sun so not suitable for building facades

The opportunity for the UK to generate substantial amounts of solar electricity is by

incorporating into the fabric of buildings (BIPV)• Thin film PV offers the cost advantage but how can we

get higher efficiency without the cost going through the roof?

• Thin film can be either on rigid surfaces such as glass or on flexible surfaces such as steel or even on plastic.

• Opportunity for designing or even disguising PV in buildings.

Solar glazing

Polysolar partially transmitting thin film silicon modules

Solar tiles

Solar Century solar tiles to replace roof slates and tiles

Solar facades

Examples of PV facades from Green Coast Solar

What do we know from our current knowledge of materials science that can improve on these solar energy materials?

• Improve light capture – if it reflects we are losing energy!

• Need to work with a wider range of materials to integrate PV into buildings

• Improve the efficiency of low cost PV such as thin film and organic

• Photon management to capture more of the spectrum

• Hybrid solar cells

Crystalline silicon cells – textured surface improves light capture

Poly-c Si(x1.0k) grain boundary

Mono-c Si(x2.0k) mounted at 45 degrees

Research in the CSER lab at the OpTIC campus of Glyndwr University, applying materials science to develop new thin film PV technology

Development of a research thin film deposition process to be compatible with production processes

• From single batch to continuous process• Batch process flows chemical vapour over the

substrate• In-line process flows the chemical vapours onto the

surface that moves underneath the injector

SPARC inline process outline (15×15 cm2 ) for experimental PV modules

TCO Buffer CdS CdTe CdTe p+ CdCl2

Exhaust

Loading & pre-heating

zone

Annealing & Cooling zone

Thin film PV materials are complex and uniformity is everything!

Scanning electron microscope (SEM) image of plan view of cadmium telluride thin film PV cell

Scanning electron microscope (SEM) image of a cross section of the cell

New laser scanning method to understand defects in PV cells - Micro-LBIC

Blue red infrared

Areas of thin CdZnS window layer

Plasmonic down conversion to enhance short wavelength response

Wavelength / nm

400 500 600 700 800 900

EQ

E

0.0

0.2

0.4

0.6

0.8

1.0

PMMA (blank)1 dye LDS layer2 dye LDS layerSpline FitSpline FitSpline Fit

Comparison of external quantum efficiency plot (EQE) of CdTe cell (Glyndwr) with a PMMA blank luminescence down shifting (LDS) layer, a single dye and a two dye mixture LDS layer. An inset of a simplified structure of the LDS + cell structure is shown. Observed increased EQE efficiencies are for the single dye ~9.8% and two dye ~ 12.5%

CSER in collaboration with Markvart and Lefteris, Southampton University

Nano-materials for down conversion

Polymer film Polymer + nano-material

Blue laser on polymer

Blue laser on nano-material film

Conclusions• Solar energy has enormous potential but we have to

improve ways of capturing it• Capturing more of the solar spectrum can be very

challenging and expensive!• Solar electric modules in the future will become part of

the fabric of a building – so you might not even recognise them

• Materials Innovation needed at all levels of PV module manufacture – improve efficiency and reduce cost.

• Will we be able to reduce our carbon emissions in time?• What will the climatic conditions be like in 2050?

Acknowledgements• Members of the CSER team • Pilkington Group for supply of

NSG TEC glass• Financial support from the

EPSRC energy programme, funding through PV21 –SUPERGEN consortium.

• Financial support from the Low Carbon Research Institute (LCRI) EU Convergence programme

http://www.cser.org.uk

CSER Team• Dr Vincent Barrioz• Dr Dan Lamb• Dr Louise Jones• Dr Andy Clayton• Dr Giray Kartopu• Dr Sarah Rugen-Hankey• Dr Graham Sparey-

Taylor• Garth Lautenbach

• Eurig Jones• William Brooks• Steve Jones• Simon Hodgson• Peter Siderfin• Fraser Hogg• Emma Dawson