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The Use of Nanostructured Silicon in Photovoltaics

June 2009 – Jan 2010

James Allan MSc Sustainable Energy: Technologies and Management

Brunel University

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Overview

• The Photovoltaic Industry– History, Current State and Future Direction– Research

• The Potential of Nanomaterials – Theory – Improvement of Existing Cells

• My Project – Objectives & Approach– Results– Findings

• Reflection – Challenges Encountered– Was it a Success?

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The Solar Potential

•7500 times more energy than the current global demand falls onto the earth in the form of solar radiation.

•Annual incident solar radiation is a magnitude higher than all the discovered and undiscovered fossil and nuclear resources put together! (WEC 2007 Survey of Energy Resources)

If we were able to covert only 0.1% of this energy at an efficiency of only 10% then we would be producing 4 times the global demand!

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The Photovoltaic Industry

• First Generation∼ 16-22% Efficient • Developed in the 1950’s• High purity crystalline wafers • High efficiency & high material costs

• Second Generation ∼ 5-15% Efficient• Developed in the 1970’s • Use of thin films to reduce high material costs of 1st Gen • Low efficiency & low material costs

• Third Generation ∼ 40-60% Efficient • Enhance the efficiency of thin film technologies

Source: 1. http://construction.com/CE/CE_images/2009/Nov_JM_1.jpg2. http://www.csa.com/discoveryguides/solar/review2.php

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Cost ComparisonForecast of the efficiency-cost trade off for each of the three generations1:

1. Conibeer, G. (2007). Third-generation photovoltaics. Materials Today , 10 (11), 42-50

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Single Junction PV Cell

Incoming light with sufficient energy Because the device only has a single band gap, the efficiency is limited to 30%. Technological advances means that this ceiling is almost being reached

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How can this be Improved? • By stacking p-n junctions of different band gap energies, it is possible to absorb a greater proportion of the incident spectrum•Very expensive and is currently only used in space applications •World Record efficiency stands at 41.1%1

1. Fraunhofer ISE. (2009, 1 14). World Record: 41.1% efficiency reached for multi-junction solar cells at Fraunhofer ISE . Retrieved from http://www.ise.fraunhofer.de.

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What has this got to do with nano?

• Nanomaterials have unique properties compared to the bulk material.

• If made sufficiently small, the nanoparticles have a higher band gap energy than the bulk material.

• This means a high efficiency tandem device can be formed using the same material for each layer of the device.

• This process is known as quantum confinement.

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Quantum Confinement• It is possible to engineer the band gap of a material through a process known as

quantum confinement• In a semiconductor, the distance between a light generated exciton (electron and

hole pair), is know as the excitation Bohr radius. • If the particle is reduced below this, then the energy levels become discrete, as

opposed to continuous, and the band gap energy is increased.

8nm

3 nm

2nm1nm

eBr

eBreBr

eBr

1.1eV 1.3eVBand Gap:

1.5eV 1.7eV

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The All Silicon Tandem •Quantum confinement is possible in silicon and the resulting particles have shown to increase PV output when deposited onto the surface of a cell1

1.Stupca, M., Alsalhi, M., Saud, T. A., Almuhanna, A., & Nayfeh, M. H. (2007). Enhancement of polycrystalline silicon solar cells using ultrathin films of silicon nanoparticles. Appl. Phys. Lett. , 91 (6)

Professor Martin Greens’ conception of an all silicon PV cell2:

2. Conibeer, G., Green, M., Cho, E.-C., König, D., Cho, Y.-H., Fangsuwannarak, T., et al. (2008). Silicon quantum dot nanostructures for tandem photovoltaic cells. Thin Solid Films , 516, 6748-6756.

•Through using an all silicon tandem device the theoretical efficiency is increased to 40%.

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What Quantum Confinement Looks Like

From Kang, Z., Liu, Y., Tsang, C. H., Ma, D. D., Fan, X., Wong, N.-B., et al. (2009). Water-Soluble Silicon Quantum Dots with Wavelength-Tunable Photoluminescence. Adv. Mater , 21, 661–664.

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My Project – Objectives & Approach

1. Demonstrate the ability to reduce the particle size of mass produced silicon nanoparticles

2. Deposit the particles onto the surface of a crystalline solar cell

3. Compare the performance of the cell under a known spectral irradiance

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Nanoparticle Size Reduction

•Hydrofluoric : nitric acid concoction used to gradually etch away the surface of the nanoparticles

•The particles were then recovered by filtration using Buchner apparatus

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Before•The particles were visualised before and after using both scanning and transmission electron microscopy.

Average Size = 115nm

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After

Average Size = 80nm

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Measurement

Before After

115nm 80nm

•Due to the large size of the particles in the pre-prepared nanopowder (>100nm) it was not possible to reduce the size of the nanoparticles to quantum confined levels (<5nm). Without the unique optoelectronic properties there was little reason to deposit them onto the surface of the cell before the method could be optimised.

•The results however show a statistically significant reduction in size and had the prepared nanoparticles been smaller (∼20nm) quantum confinement might be possible.

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Application to the Cell•Plasma enhanced chemical vapour deposition (PECVD) used to coat the cell with nanolayer of SiC • Band Gap ∼ 1.7eV – same as silicon nanoparticles in tandem device.

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Performance 1

•Light box built to generate a parallel beam of uniform irradiance to test isolated areas of the cell

•This is the spectrum of the halogen light source used in the testing. The irradiance over the 7.1cm2 area was 1200W/m2

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Performance 2

•A variable resistance decade box was built (0.01 - 10 Ω) to vary the load across the cell and generate an IV curve.

• The generated current and voltage were measured for each resistance using separate multimeters

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Performance 3

•The short circuit current for the coated cell was reduced. This is indicative of reduced irradiance and suggests that the coating was blocking part of the spectrum and preventing it from reaching the cell.

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Findings• Nanoparticles

– It was not possible to produce particles exhibiting quantum confinement; however size reduction was observed and if the prepared particles were small enough then this could be achieved.

– The method had a number of experimental flaws that need to be corrected before the particles can be deposited onto the surface of the cell.

• Nanocoating– The coating was largely transparent but still prevented a portion of light from

reaching the photosensitive semiconductor of the cell. – It was not possible to test the modified cell under a high energy UV light

source. This needs to be determined as this is the area in which absorption is thought to be improved.

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Reflection

• The project was a huge challenge • Had to be resourceful • Organisation of resources and time

management• Worked with many different departments • Although the results were not groundbreaking

it has set the foundations for others to continue to develop and optimise the method.

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Thanks for Listening

Any Questions