Third Generation Solar cells 1

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 3

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 4

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 5

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) 6

The Main Losses in Solar Cells Sub bandgap and Lattice thermalisation losses acount for more than 50% of the total loss Energy Lattice thermalisation loss Junction loss Contact loss Sub bandgap loss qV Recombination 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 Efficiency and cost projections Third Generation First Generation Second Generation ARC Photovoltaics center of Excellence, University of New Soth Wales, Annual Report (2007) 8

Third Generation Solar Cells Band gap engineering using quantum confinment effect Multiple Exciton Generation Hot Carrier Solar Cell Up Conversion Down Conversion Tandem Cells

Band Gap Engineering Excitonc Bohr radius 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 10

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 11

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) 12

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 Eg ½ Eg 13

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 Egap Eg 2 Eg Eg 14

The only proven 3rd generation technique so far 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) 15

Tandem Cells 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 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) 16

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

Tandem Cells 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 18

Summary Objectives in third generation solar cells Quantum confinment 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 19

Acknowledgment I want to sincerely thank my supervisor, professor G. Palasantzas whose kind attentions led me through the difficulties.