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

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Presentation on theme: "Third Generation Solar cells Hiwa Modarresi 17 th June 2009 "Energy & Nano" - Top Master Symposium in Nanoscience 2009 1."— Presentation transcript:

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

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

3 Sunlight Spectrum  Sunlight consists of a broad range of spectrum  The photon energy depends on the photon wavelength: E phot = hc/λ  Harnessing the great amount of sunlight energy "Energy & Nano" - Top Master Symposium in Nanoscience Solar Radiation Spectrum Online:

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

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

6 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) "Energy & Nano" - Top Master Symposium in Nanoscience

7 The Main Losses in Solar Cells "Energy & Nano" - Top Master Symposium in Nanoscience 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

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

9 Band gap engineering using quantum confinment effect Multiple Exciton Generation Hot Carrier Solar Cell Up Conversion Down Conversion Tandem Cells Third Generation Solar Cells "Energy & Nano" - Top Master Symposium in Nanoscience

10 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: E g = 1.12 eV  2 nm QD Si band gap: E g = 1.7 eV  Enhanced impact ionization (inverse Auger recombination)  Greatly enhanced non-linear optical properties "Energy & Nano" - Top Master Symposium in Nanoscience E gap

11 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 "Energy & Nano" - Top Master Symposium in Nanoscience E gap

12 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  E LO >2E LA 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, , (2008) "Energy & Nano" - Top Master Symposium in Nanoscience

13 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 E g EgEg "Energy & Nano" - Top Master Symposium in Nanoscience

14 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 EgEg EgEg 2 E g "Energy & Nano" - Top Master Symposium in Nanoscience E gap

15 Tandem Cells The only proven 3 rd generation technique so far Light Upper cell (absorbs high energy photons) Middle cell (absorbs medium energy photons) Lower cell (absorbs low energy photons) "Energy & Nano" - Top Master Symposium in Nanoscience

16  Nanocrystal sizes approaching the excitonic Bohr radius  Small nanocrystals of Si embedded in a silicon dielectric matrix  Annealing at 1100 o C  Multi-layers containing n-type Si QDs on p-type Si wafers Tandem Cells "Energy & Nano" - Top Master Symposium in Nanoscience G. Conibeer, M. Green et al., “Silicon quantum dot nanostructures for tandem photovoltaic cells,” Thin Solid Films, 516, , (2008)

17  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 "Energy & Nano" - Top Master Symposium in Nanoscience Tandem Cells ARC Photovoltaics center of Excellence, University of New Soth Wales, Annual Report (2007)

18  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 "Energy & Nano" - Top Master Symposium in Nanoscience Tandem Cells

19 Summary "Energy & Nano" - Top Master Symposium in Nanoscience  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 3 rd generation solar cells

20 Acknowledgment  I want to sincerely thank my supervisor, professor G. Palasantzas whose kind attentions led me through the difficulties. "Energy & Nano" - Top Master Symposium in Nanoscience

21 THANK YOU FOR YOUR ATTENTION


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