Third Generation Solar cells Hiwa Modarresi 17 th June 2009 "Energy & Nano" - Top Master Symposium in Nanoscience 2009 1

Third Generation Solar cells

Hiwa Modarresi

17th June 2009

"Energy & Nano" - Top Master Symposium in Nanoscience 20091

Outline Sunlight spectrum How a classical solar cell works First generation solar cells Second generation solar cells The main losses in solar cells Third generation solar cells

Band gap engineering Multiple exciton generation Hot carrier solar cells Up conversion Down conversion Tandem cells

Summary


Sunlight Spectrum Sunlight consists of a broad range of spectrum The photon energy depends on the photon wavelength: Ephot = hc/λ

Harnessing the great amount of sunlight energy


Solar Radiation Spectrum

Online: http://www.globalwarmingart.com/wiki/Image:Solar_Spectrum_png

How a Classical Solar Cell Works Photovoltaic cell is a device that converts solar energy into electricity by the

photovoltaic effect Energy of the incident photon should be greater than or equal to the band

gap of the semiconductor If an exciton is created in space charge region, its electron-hole components

would be separated

Electric Field


First Generation Solar Cells Single crystal silicon wafers Dominant in the commercial production of solar cells Consist of a large-area, single layer p-n junction Best crystalline Si solar cell efficiency: ~ 25%

Advantages Broad spectral absorption range High carrier mobility

Disadvantages Most of photon energy is wasted as heat Require expensive manufacturing technologies


Second Generation Solar Cells Thin-film Technologies

Amorphous silicon Polycrystalline silicon Cadmium Telluride (CdTe)

Best large area Si-based solar cell efficiency: ~ 22%

Advantages Low material cost Reduced mass

Disadvantages Toxic material (Cd), Scarce material (Te)


The Main Losses in Solar Cells


qV

Lattice thermalisation loss

Junction loss

Recombination loss

Contact loss

Sub bandgap loss

Energy

Sub bandgap and Lattice thermalisation losses acount for more than 50% of the total loss

Third Generation Solar Cells Solar cells which use concepts that allow for a more efficient utilization of the

sunlight than FG and SG solar cells The biggest challenge is reducing the cost/watt of delivered solar electricity Third generation solar cells pursue

More efficiency More abundant materials Non-toxic material Durability

First Generation Second Generation

Third Generation

ARC Photovoltaics center of Excellence, University of New Soth Wales, Annual Report (2007)

Efficiency and cost projections


Band gap engineering using quantum confinment effect

Multiple Exciton GenerationHot Carrier Solar Cell

Up ConversionDown Conversion

Tandem Cells

Third Generation Solar Cells


Band Gap Engineering Quantum confinement Discrete energy levels Excitonc Bohr radius the size of the band gap is controlled simply by adjusting the size of

the dot. Thin film Si band gap: Eg = 1.12 eV 2 nm QD Si band gap: Eg = 1.7 eV

Enhanced impact ionization (inverse Auger recombination) Greatly enhanced non-linear optical properties


Egap

Multiple Exciton Generation Objective: fighting termalization

In quantum dots, the rate of energy dissipation is significantly reduced

One photon creates more than one exciton via impact ionization Higher photocurrent via impact ionization (inverse Auger process)

Multiple exciton generation evidence PbSe (lead selenide) QDs


Egap Egap

Hot Carrier Solar Cell Objective: fighting thermalization

Energy selective contacts Need to slow carrier cooling Higher photovoltage via hot electron

transport The idea is to suppress the Klemens

transitions ELO>2ELA such that LO→2LA (Klemens

mechanism) is forbidden The LO→TO+LA (Ridley mechanism) can

occur

Hot carrier evidence InN

G.J. Conibeer, D. König et al., “Slowing of carrier cooling in hot carrier solar cells,” Thin Solid Films, 516, 6948-6953, (2008)


Up Conversion Objective: transforming large wavelength photons into

small wavelengh photons

Nearly half of the intensity of sunlight is within the invisible infrared region

Can be implemented by quantum wells and quantum dots The drawback is that it is a non-linear effect

Up conversion evidence: (Erbium) It is far from realization

½ Eg

½ Eg

Eg


Down Conversion Objective: transforming small wavelength photons into large

wavelength photons

Suitable materials must efficiently absorb high energy photons and reemit more than one photon with sufficient energies

can be implemented by quantum wells and quantum dots

Down conversion evidence: Multiple exciton generation

Eg

Eg

2 Eg


Egap Egap

Tandem Cells

The only proven 3rd generation technique so far

Light

Upper cell (absorbs high energy photons)

Middle cell (absorbs medium energy photons)

Lower cell (absorbs low energy photons)


Nanocrystal sizes approaching the excitonic Bohr radius Small nanocrystals of Si embedded in a silicon dielectric matrix Annealing at 1100 oC Multi-layers containing n-type Si QDs on p-type Si wafers

Tandem Cells


G. Conibeer, M. Green et al., “Silicon quantum dot nanostructures for tandem photovoltaic cells,” Thin Solid Films, 516, 6748-6756, (2008)

G. Conibeer, M. Green et al., “Silicon quantum dot nanostructures for tandem photovoltaic cells,” Thin Solid Films, 516, 6748-6756, (2008)

Best device in this respect, was the one with 3 nm QDs with an efficiency of 10.6% This is comparable to a conventional p-n junction crystalline silicon solar

cells with a non-textured surface


Tandem Cells

ARC Photovoltaics center of Excellence, University of New Soth Wales, Annual Report (2007)

What we can also investigate:

Effect of excitonic Bohr radius Si = 4.9 nm Ge = 24.3 nm Sn = 40 nm

The quantum size effects should be more prominent in Tin nanocrystals even for larger sizes of nanocrystals


Tandem Cells

Summary


Objectives in third generation solar cells More efficient Less expensive Readily available Non-toxic

Quantum confinment Band gap engineering

Multiple exciton generation Already seen in QDs but with very low efficiencies

Hot carriers Far from utilization

Up conversion So far has not been realized

Down conversion Can be utilized through the concept of multiple exciton generation

Tandem cells The only proven technique in 3rd generation solar cells

Acknowledgment

I want to sincerely thank my supervisor, professor

G. Palasantzas whose kind attentions led me through the difficulties.


THANK YOU FOR YOUR ATTENTION

Documents

Third Generation Solar cells Hiwa Modarresi 17 th June 2009 "Energy & Nano" - Top Master Symposium in Nanoscience 2009 1