Silicio nanocristallino: potenzialita’ e promesse L. Pavesi L. Pavesi 28-11-10
Nanoscience Laboratory Oleksiy Anopchenko Minhaz Hossein(*) Stefano Prezioso (*) Zhizhong Yuan (Ryan) (*) Fabrizio Sgrignuoli Alessandro Marconi APP FBK Georg Pucker Yoann Jestin MTLab FBK Pierluigi Bellutti Lorenza Ferrario L. Pavesi 28-11-10
Outline • Silicon photovoltaics: the continuous evolution • Nanocrystalline silicon as a tool to implement third generation PV • Three examples of our research – Downshifter – Cell with internal gain – Towards tandem cells • Conclusions L. Pavesi 28-11-10
Outline • Silicon photovoltaics: the continuous evolution • Nanocrystalline silicon as a tool to implement third generation PV • Three examples of our research – Downshifter – Cell with internal gain – Towards tandem cells • Conclusions L. Pavesi 28-11-10
L. Pavesi “Courtesy of the National Renewable Energy Laboratory, Golden, Colorado. 28-11-10
Solar Cells Market Estimate Solar Cells 2010 Market Share Estimate 50% >90% 40% Market Share 30% 20% 10% 0% Type L. Pavesi SEMI PV Group March 2009 from source Yole Development 28-11-10
Various PV generations L. Pavesi 28-11-10
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 L. Pavesi 8 28-11-10
Various PV generations L. Pavesi 28-11-10
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) L. Pavesi 10 28-11-10
L. Pavesi 28-11-10
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 L. Pavesi 28-11-10
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 Photon electron energy 32.9% conversion Unabsorbed energy loss 18.7% Heat loss 46.8% Other losses 1.6% L. Pavesi 13 28-11-10
Various PV generations L. Pavesi 28-11-10
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 L. Pavesi 15 28-11-10
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 L. Pavesi 16 28-11-10
Third Generation Solar Cells Band gap engineering using quantum confinement effect Multiple Exciton Generation Hot Carrier Solar Cell Up Conversion Down Conversion Tandem Cells L. Pavesi 17 "Energy & Nano" - Top Master Symposium in Nanoscience 2009 28-11-10
Third Generation Solar Cells Band gap engineering using quantum confinement effects Multiple Exciton Generation Hot Carrier Solar Cell Up Conversion Down Conversion Tandem Cells L. Pavesi 18 "Energy & Nano" - Top Master Symposium in Nanoscience 2009 28-11-10
Outline • Silicon photovoltaics: the continuous evolution • Nanocrystalline silicon as a tool to implement third generation PV • Three examples of our research – Downshifter – Cell with internal gain – Towards tandem cells • Conclusions L. Pavesi 28-11-10
Silicon nanocrystals L. Pavesi 28-11-10
Silicon quantum dots 2 2 Si E E gap gap 2 m L Increase the emission energy L. Pavesi 28-11-10
Silicon quantum dots L. Pavesi 28-11-10
Properties of Si-nc 1. Abundant and nontoxic 2. CMOS fabrication compatible 3. Band gap adjustable and higher than that of Si L. Pavesi 28-11-10
Optical properties of Si-nc 10 PL intensity (a.u.) (a) (b) PDS-1 PDS-2 Monitoring PL band at 800 nm PL intensity (a.u.) 1 Absorbance (%) (c) Si 2E g 50 25 0 550 650 750 850 950 400 500 600 700 Wavelength (nm) Stokes shift between absorption and emission L. Pavesi 28-11-10
Significant size dispersion L. Pavesi F. Iacona et al. CNR Catania 28-11-10
Reduce the size dispersion L. Pavesi M. Zacharias et al. MPI Halle 28-11-10
Current? n -Si p -Si n -Si SiO 2 D Si-nc F-N tunneling BH: Barrier height Direct tunneling BH + Electron p -Si Hole IDD - 1.Reduce D IDD: Inter-dot distance 2.Improve overlapping of wave function 1. reduce IDD of Si-nc 2. reduce BH 3. improve overlapping of wave function of Si-nc L. Pavesi 28-11-10
nc-Si/SiO 2 Multilayer LED • Confined growth of nanocrystals • Better oxide quality • Control over the oxide thickness L. Pavesi 28-11-10
nc-Si/SiO 2 Multilayer LED • Less destructive Energy Fowler-Nordheim Tunneling Direct Tunneling • More efficient <3V >3V Position nc-Si Oxide Position L. Pavesi 28-11-10
nc-Si/SiO 2 Multilayer LED L. Pavesi 28-11-10
Single layer vs Multilayer LED Single layer 0,01 Multilayer 1E-3 2 ) 1E-4 Current Density (A/cm 1E-5 1E-6 1E-7 1E-8 1E-9 1E-10 0 1 2 3 Electric Field (MV/cm) Single layer = large current Multilayer= large field Larger Electric Field to achieve the same Current Density, i.e. reduced the leakage current L. Pavesi 28-11-10
Single layer vs Multilayer LED Single layer 5 10 Multilayer Elettroluminescence density (a.u.) 4 10 Same injected current 3 10 2 10 1 10 0 10 0 1 2 3 Electric Field (MV/cm) Increase of EL due to more effective injection into the Si-nc L. Pavesi 32 28-11-10
L. Pavesi 28-11-10
pHotonics ELectronics functional Integration on CMOS (2 nm SiO 2 / 3 nm SRO) 2 ) 1 Optical power density ( W / cm Graded energy gap (2 nm SiO 2 / 4 nm SRO) 0.1 0.2 Power efficiency (%) 0.1 + - 0.01 0.0 n-type -3 -2 -1 1 10 10 10 p-type poly- silicon silicon 2 ) Active Current density (mA / cm 100 wafer Si-NC nm -3 -2 -1 1 1 10 10 10 10 2 ) Current density (mA / cm L. Pavesi 28-11-10
Key issues: Control the current • Different Matrixes: different barrier heights Bulk band alignments between crystallinc silicon and its carbide, nitride and oxide. L. Pavesi 28-11-10
Wave function -overlap of the wave function can enhance the tunneling between adjacent Si-ncs. Si Si Si Si Si Si SiO 2 Si 3 N 4 SiC The wave function of an electron confined to a spherical dot penetrates into the surrounding materials, decaying approximately as exp(-r/L d )/r, where r is the distance from the centre of the dot. Matrix SiO 2 Si 3 N 4 SiC 0 . 1952 nm L d ∆ E(Si-Matrix) 3.2 eV 1.9 eV 0.5 eV L d , decay length. * m E / m m 0 0.86 0.05-0.13 0.24 0 Inter-dot distance for significant wavefunction overlap: 1-2 nm for SiO 2 and 4 nm fro SiC L. Pavesi Eun-Chel Cho, et al., Advances in Optoelectronics. 2007, 1-11 28-11-10
Photoresponsivity-superlattice 0.01 Photoresponsivity (A/W) 1E-3 Q2-SRO/SiO 2 =3/1 Q7-SRN/SiO 2 =3/1 Q8-SRO/Si 3 N 4 =3/1 Q9-SRN/Si 3 N 4 =3/1 1E-4 400 500 600 700 800 Wavelength (nm) L. Pavesi 28-11-10
Third Generation Solar Cells Band gap engineering using quantum confinement effects Multiple Exciton Generation Hot Carrier Solar Cell Up Conversion Down Conversion Tandem Cells L. Pavesi 38 "Energy & Nano" - Top Master Symposium in Nanoscience 2009 28-11-10
A. J. Nozik L. Pavesi 28-11-10
Third Generation Solar Cells Band gap engineering using quantum confinement effects Multiple Exciton Generation Hot Carrier Solar Cell Up Conversion Down Conversion Tandem Cells L. Pavesi 40 "Energy & Nano" - Top Master Symposium in Nanoscience 2009 28-11-10
Hot carrier solar cell-to increase V oc L. Pavesi G. Conibeer, et al., Thin Solid Films,511-512, 654 (2006) 28-11-10
Third Generation Solar Cells Band gap engineering using quantum confinement effects Multiple Exciton Generation Hot Carrier Solar Cell Up Conversion Down Conversion Tandem Cells L. Pavesi 42 "Energy & Nano" - Top Master Symposium in Nanoscience 2009 28-11-10
Mechanism of tandem solar cell Cell stacking Solar cell Sunlight Decreasing band gap L. Pavesi 28-11-10
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