ORGANIC SOLAR CONCENTRATORS
ORGANIC SOLAR CONCENTRATORS
Tim Heidel Massachusetts Institute of Technology Department of Electrical Engineering & Computer Science [email protected] http://softsemi.mit.edu
OUTLINE • The need for higher efficiency solar cells • Conventional solar concentrators • Organic Solar Concentrators
(b)
RECENT GROWTH IN PHOTOVOLTAICS Feed‐in tariffs, production tax credits, renewable portfolio standards, and good old fashioned research and development are working!
“2007 WORLD PV INDUSTRY REPORT HIGHLIGHTS: World solar photovoltaic (PV) market installations reached a record high of 2,826 megawatts (MW) in 2007, representing growth of 62% over the 2,826 megawatts (MW) in 2007, representing growth of 62% over the previous year. Germany's PV market reached 1,328 MW in 2007 and now accounts for 47% of the world market. Spain soared by over 480% to 640 MW, while the United of the world market. Spain soared by over 480% to 640 MW, while the United States increased by 57% to 220 MW. It became the world's fourth largest States increased by 57% to 220 MW. It became the world's fourth largest market behind Japan, once the world leader, which declined 23% to 230 MW. World solar cell production reached a consolidated figure of 3,436 MW in 2007, up from 2,204 MW a year earlier.” 2007, up from 2,204 MW a year earlier.” Source: Solar Buzz 6.3MW (Germany, opened 2005)
Source: Sun Power
11 MW (Portugal, opened 2007) Source: Power Technology
Source: Prometheus Institute
20MW (Spain, opened 2007) Source: City Solar AG
COST OF ELECTRICITY WITH PHOTOVOLTAICS Levelized cost of electricity $/kWh
1MW system size 0.25
Deutsche Bank: Solar Photovoltaics, July 2007 0.20 0.15 0.10 0.05 0.00 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Year
Analysts are extrapolating trends to grid parity… But how we will achieve it?
HOW CAN WE INCREASE THE EFFICIENCY OF SOLAR CELLS? Build a tandem, or multijunction… EC Long wavelength 1.8 eV
Short wavelength
1.1 eV
EC
EV recombination interface
High energy gap
EV
Low energy gap
(collects visible light) (collects near Infrared) Example: GaInP
Example: Si
With additional junction, theoretical maximum increases from 30% to 42%. But there is a problem: large gap solar cells are very expensive (e.g. GaInP at > $10k/m2).
CONVENTIONAL SOLAR CONCENTRATORS Gather light by inexpensive ‘collector’, concentrate light on high performance solar cell. Fixed lens or mirror collector
Tracking collectors
Conventional solar concentrators concentrate light up to 1000 times. However, they are expensive and need tracking and/or cooling.
COST MODEL OF SOLAR CONCENTRATORS collector cost 1 PV cost $ Wp = + efficiency × solar flux G efficiency × solar flux G >> 1 Geometric concentration factor (G ) =
area of collector area of solar cell
¾ Concentration factor (G) must be large ¾ Collector should be cheaper than conventional PV cell
AN ALTERNATIVE: THE LUMINESCENT SOLAR CONCENTRATOR solar radiation dye
air
glass
photoluminescence PV
PVair
• Passive system that doesn’t need to track the sun. • Smoothes out non‐uniform optical excitation. • Ability to concentrate diffuse light. • Tolerant of fabrication defects. W. H. Weber and J. Lambe, Applied Optics 15, 2299 (1976) A. Goetzberger, W. Greubel, Applied Physics 14, 123 (1977)
ORGANIC SOLAR CONCENTRATORS (OSCs) Currie, Mapel, Heidel, Goffri & Baldo, Science 321, 226 (2008)
Replace dye‐doped polymer with organic thin film on glass or plastic Allows better control over intermolecular interactions, especially energy transfer
Air Dyes
Solar cell
Glass
Near field energy transfer couples molecules within 3‐5nm radius Increases energy difference between absorbed and emitted photons
SOME EXAMPLES OF LUMINESCENT SOLAR CONCENTRATION
Our lab proto‐types are 10x10x0.1cm. For characterization, we attach a Sunpower Si PV cell to one edge.
Solar cell 1
absorption, emission
‘MULTIJUNCTION’ ORGANIC SOLAR CONCENTRATORS abs.
emission Band gap 1
Solar cell 2
absorption, emission
Wavelength [λ] absorption
emission Band gap 2
Solar cell 3
absorption, emission
Wavelength [λ] absorption
emission Band gap 3
Wavelength [λ]
Solar cells pumped ~monochromatically at band edge…minimal heating.
APPLICATIONS: HIGH EFFICIENCY THIN FILM PV a) Tandem OSC‐PV:
Must achieve large concentration factor to reduce effective costs of PV cells.
OTHER APPLICATIONS: BUILDING INTEGRATED (WINDOWS & SKYLIGHTS) Better aesthetics (color tunability and image transmission). No need for transparent contacts. Can be fabricated on flexible plastic for easier installation. AUTOMOTIVE SUNROOFS (PV cells power AC and/or fan, reduce engine load on cold starts)
Polycarbonate sunroofs reduce center of mass (fewer rollovers) Lighter weight increases MPG Aesthetic advantages (No visible electrodes)
EFFICIENCY GAINS WITH TANDEM OSC Power conversion Bottom cell efficiency
efficiency at
OSC
G = 3, 50
Tandem (2 layer) OSC
6.8%, 6.1%
Tandem OSC-CdTe PV
9.6%
11.9%, 11.1%
Tandem OSC-CIGS PV
13.1%
14.5%, 13.8%
Calculated, based on lab results
PROJECTED EFFICIENCY GAINS WITH OPTIMIZED OSC With concentrator Base Solar Cell
Initial efficiency
8%, G=250, GaInP
High efficiency Si
20%
22%
Average Si
14%
18%
CdTe
10%
15%
Baseline CdTe and CIGS cell performance is 9.6% and 13.1%, respectively. S. H. Demtsu, J. R. Sites, paper presented at the IEEE Photovoltaic Specialists Conf. 2005. J. Palm et al., Thin Solid Films 451-52, 544 (2004)
CONCLUSIONS • Separate the optical and electrical functions of solar cells • Use molecules for large area light absorption function ‐‐ cheap, easy to fabricate • Use high performance semiconductors for electrical function ‐‐ expensive, but concentrator reduces cost to (1/100th)
• Organic Solar Concentrators • Do not need to track the sun. • Smoothed out non‐uniform optical (solar) excitation. • Can concentrate diffuse light. • Tolerant of fabrication defects. • Unsolved issues • Quantum efficiency can still be doubled (100% is possible) • Broader spectral coverage will require new IR dyes.
Cost and efficiency roadmap US$0.10/W
100
US$0.20/W
US$0.50/W
Thermodynamic limit
Efficiency (%)
80 3G
60
US$1.00/W
40
?
Printed solar cells?
Single junction limit 20
US$3.50/W
2G 0
100
200
300
400
500
Cost (US$/m2) Martin Green UNSW
Best research‐cell efficiencies 36 32
Spectrolab Japan Energy
Crystalline Si Cells Single crystal Multicrystalline Thin Si
28
Efficiency (%)
Spectrolab
Multijunction Concentrators Three-junction (2-terminal, monolithic) Two-junction (2-terminal, monolithic)
NREL NREL
Thin Film Technologies Cu(In,Ga)Se2 CdTe Amorphous Si:H (stabilized)
24 20
Emerging PV Organic cells
Spire ARCO
Stanford
Georgia Tech
Varian
Kodak
Sharp
Georgia Tech
ARCO
8
Monosolar
Kodak Boeing
4 0 1975
RCA
Boeing
AstroPower
RCA
Solarex
NREL Cu(In,Ga)Se2 14x concentration
Boeing
NREL
AstroPower
Boeing
NREL
United Solar United Solar
Photon Energy
University California Berkeley University Konstanz
1985
NREL
NREL
Euro-CIS
University RCA of Maine RCA RCA RCA RCA
1980
UNSW
NREL
AMETEK Masushita
UNSW
NREL
University So. Florida Solarex
UNSW
UNSW
UNSW
No. Carolina State University Boeing
UNSW
Spire
Westinghouse
16 12
NREL/ Spectrolab
1990
1995
Princeton NREL
2000
Si cutoff
GaInP cutoff
REALISTIC POTENTIAL EFFICIENCY OF A GaInP-Si TANDEM
35
33%
Efficiency [%]
30 25 20 15 10 5 0
400
500
600
700
800
900
1000 1100
Wavelength [nm] Si: VOC = 0.68V, FF=0.801 (e.g. Sunpower) GaInP: VOC = 1.4V, FF=0.8 But there is a problem: GaInP costs > $10,000/m2
CAN WE USE ORGANIC DYES IN SOLAR APPLICATIONS? EXAMPLE: PERYLENE DIIMIDE DYES Material Advantages 9 Abundant: 1,500,000 kg/yr; multiple suppliers 9 Non Toxic 9 Low cost: $50/kg (if 0.2 g/m2 $0.01/m2) 9 Stable
O
O
R
R
O
O
‘Toreador red’
compare semiconductor grade silicon production: ~35,000,000 kg/yr (2005)
Reflection
Light
+ ‐ Light absorption at dye
Reflection, Transmission losses
Emission
Photon Thermal loss conversion
Waveguided transmission
Facial emission
Re‐ absoprtion
PV conversion
Recombination losses
(i) Refractive index is hard to increase
1
0.8 0.7
Standard glass/plastic
Trapping efficiency
0.9
0.6 0.5 0.4 0.3 0.2 0.1 0
1
1.2 1.4 1.6 1.8
2
2.2 2.4 2.6 2.8
Refractive index Need n > 2.3 to get trapping efficiency > 90% No low cost solutions?
3
THE OPERATION OF LSCs 2. Transport losses
1. Confinement losses air
glass
air
Non‐zero overlap between absorption and emission can lead to re‐absorption
Non waveguided emission Refractive index n = 1.5 n = 1.7 n = 2.1
Loss 25% 20% 12%
Feeds back in
Re‐absorption losses have limited LSCs to concentration factors of 100,000 hours in OLEDs [Mark Thompson, MRS Bulletin, 32, 694 (2007)] Our preliminary stability measurement in solar concentrators: 8% drop in 3 months of solar exposure
EFFICIENCY OF TANDEM WAVEGUIDES (photons in/photons out) at G=3
Rubrene: DCJTB Pt(TPBP)
GaInP
GaAs
Calculated power efficiency: 6.8%
Optical Quantum Efficiency
0.7 0.6 0.5 0.4 0.3 0.2
30% Rubrene, 1% DCJTB 2% DCJTB, 4% Pt(TPBP) Sum
0.1 0
400 450 500 550 600 650 700
Wavelength (nm)
EFFICIENCY OF SINGLE WAVEGUIDES (photons in/photons out) at G=3
Optical Quantum Efficiency
0.7 0.6
Calculated power efficiencies
0.5
DCJTB‐based OSC with GaInP: Rubrene‐based OSC with GaInP: Pt(TPBP)‐based OSC with GaAs:
0.4 0.3 0.2 0.1
30% Rubrene, 1% DCJTB 2% DCJTB, 4% Pt(TPBP) 2% DCJTB
0 400 450 500 550 600 650
Wavelength (nm) GaInP: C. Baur et al., Journal of Solar Energy Engineering-Transactions of the ASME 129, 258 (2007). GaAs: R. P. Gale et al., paper presented at the 21st IEEE Photovoltaic Specialists Conference, Kissimimee 1990
5.9% 5.5% 4.1%
EFFICIENCY AT HIGHER OPTICAL CONCENTRATIONS Previous maximum 1
Tim Heidel Massachusetts Institute of Technology Department of Electrical Engineering & Computer Science [email protected] http://softsemi.mit.edu
OUTLINE • The need for higher efficiency solar cells • Conventional solar concentrators • Organic Solar Concentrators
(b)
RECENT GROWTH IN PHOTOVOLTAICS Feed‐in tariffs, production tax credits, renewable portfolio standards, and good old fashioned research and development are working!
“2007 WORLD PV INDUSTRY REPORT HIGHLIGHTS: World solar photovoltaic (PV) market installations reached a record high of 2,826 megawatts (MW) in 2007, representing growth of 62% over the 2,826 megawatts (MW) in 2007, representing growth of 62% over the previous year. Germany's PV market reached 1,328 MW in 2007 and now accounts for 47% of the world market. Spain soared by over 480% to 640 MW, while the United of the world market. Spain soared by over 480% to 640 MW, while the United States increased by 57% to 220 MW. It became the world's fourth largest States increased by 57% to 220 MW. It became the world's fourth largest market behind Japan, once the world leader, which declined 23% to 230 MW. World solar cell production reached a consolidated figure of 3,436 MW in 2007, up from 2,204 MW a year earlier.” 2007, up from 2,204 MW a year earlier.” Source: Solar Buzz 6.3MW (Germany, opened 2005)
Source: Sun Power
11 MW (Portugal, opened 2007) Source: Power Technology
Source: Prometheus Institute
20MW (Spain, opened 2007) Source: City Solar AG
COST OF ELECTRICITY WITH PHOTOVOLTAICS Levelized cost of electricity $/kWh
1MW system size 0.25
Deutsche Bank: Solar Photovoltaics, July 2007 0.20 0.15 0.10 0.05 0.00 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 2020
Year
Analysts are extrapolating trends to grid parity… But how we will achieve it?
HOW CAN WE INCREASE THE EFFICIENCY OF SOLAR CELLS? Build a tandem, or multijunction… EC Long wavelength 1.8 eV
Short wavelength
1.1 eV
EC
EV recombination interface
High energy gap
EV
Low energy gap
(collects visible light) (collects near Infrared) Example: GaInP
Example: Si
With additional junction, theoretical maximum increases from 30% to 42%. But there is a problem: large gap solar cells are very expensive (e.g. GaInP at > $10k/m2).
CONVENTIONAL SOLAR CONCENTRATORS Gather light by inexpensive ‘collector’, concentrate light on high performance solar cell. Fixed lens or mirror collector
Tracking collectors
Conventional solar concentrators concentrate light up to 1000 times. However, they are expensive and need tracking and/or cooling.
COST MODEL OF SOLAR CONCENTRATORS collector cost 1 PV cost $ Wp = + efficiency × solar flux G efficiency × solar flux G >> 1 Geometric concentration factor (G ) =
area of collector area of solar cell
¾ Concentration factor (G) must be large ¾ Collector should be cheaper than conventional PV cell
AN ALTERNATIVE: THE LUMINESCENT SOLAR CONCENTRATOR solar radiation dye
air
glass
photoluminescence PV
PVair
• Passive system that doesn’t need to track the sun. • Smoothes out non‐uniform optical excitation. • Ability to concentrate diffuse light. • Tolerant of fabrication defects. W. H. Weber and J. Lambe, Applied Optics 15, 2299 (1976) A. Goetzberger, W. Greubel, Applied Physics 14, 123 (1977)
ORGANIC SOLAR CONCENTRATORS (OSCs) Currie, Mapel, Heidel, Goffri & Baldo, Science 321, 226 (2008)
Replace dye‐doped polymer with organic thin film on glass or plastic Allows better control over intermolecular interactions, especially energy transfer
Air Dyes
Solar cell
Glass
Near field energy transfer couples molecules within 3‐5nm radius Increases energy difference between absorbed and emitted photons
SOME EXAMPLES OF LUMINESCENT SOLAR CONCENTRATION
Our lab proto‐types are 10x10x0.1cm. For characterization, we attach a Sunpower Si PV cell to one edge.
Solar cell 1
absorption, emission
‘MULTIJUNCTION’ ORGANIC SOLAR CONCENTRATORS abs.
emission Band gap 1
Solar cell 2
absorption, emission
Wavelength [λ] absorption
emission Band gap 2
Solar cell 3
absorption, emission
Wavelength [λ] absorption
emission Band gap 3
Wavelength [λ]
Solar cells pumped ~monochromatically at band edge…minimal heating.
APPLICATIONS: HIGH EFFICIENCY THIN FILM PV a) Tandem OSC‐PV:
Must achieve large concentration factor to reduce effective costs of PV cells.
OTHER APPLICATIONS: BUILDING INTEGRATED (WINDOWS & SKYLIGHTS) Better aesthetics (color tunability and image transmission). No need for transparent contacts. Can be fabricated on flexible plastic for easier installation. AUTOMOTIVE SUNROOFS (PV cells power AC and/or fan, reduce engine load on cold starts)
Polycarbonate sunroofs reduce center of mass (fewer rollovers) Lighter weight increases MPG Aesthetic advantages (No visible electrodes)
EFFICIENCY GAINS WITH TANDEM OSC Power conversion Bottom cell efficiency
efficiency at
OSC
G = 3, 50
Tandem (2 layer) OSC
6.8%, 6.1%
Tandem OSC-CdTe PV
9.6%
11.9%, 11.1%
Tandem OSC-CIGS PV
13.1%
14.5%, 13.8%
Calculated, based on lab results
PROJECTED EFFICIENCY GAINS WITH OPTIMIZED OSC With concentrator Base Solar Cell
Initial efficiency
8%, G=250, GaInP
High efficiency Si
20%
22%
Average Si
14%
18%
CdTe
10%
15%
Baseline CdTe and CIGS cell performance is 9.6% and 13.1%, respectively. S. H. Demtsu, J. R. Sites, paper presented at the IEEE Photovoltaic Specialists Conf. 2005. J. Palm et al., Thin Solid Films 451-52, 544 (2004)
CONCLUSIONS • Separate the optical and electrical functions of solar cells • Use molecules for large area light absorption function ‐‐ cheap, easy to fabricate • Use high performance semiconductors for electrical function ‐‐ expensive, but concentrator reduces cost to (1/100th)
• Organic Solar Concentrators • Do not need to track the sun. • Smoothed out non‐uniform optical (solar) excitation. • Can concentrate diffuse light. • Tolerant of fabrication defects. • Unsolved issues • Quantum efficiency can still be doubled (100% is possible) • Broader spectral coverage will require new IR dyes.
Cost and efficiency roadmap US$0.10/W
100
US$0.20/W
US$0.50/W
Thermodynamic limit
Efficiency (%)
80 3G
60
US$1.00/W
40
?
Printed solar cells?
Single junction limit 20
US$3.50/W
2G 0
100
200
300
400
500
Cost (US$/m2) Martin Green UNSW
Best research‐cell efficiencies 36 32
Spectrolab Japan Energy
Crystalline Si Cells Single crystal Multicrystalline Thin Si
28
Efficiency (%)
Spectrolab
Multijunction Concentrators Three-junction (2-terminal, monolithic) Two-junction (2-terminal, monolithic)
NREL NREL
Thin Film Technologies Cu(In,Ga)Se2 CdTe Amorphous Si:H (stabilized)
24 20
Emerging PV Organic cells
Spire ARCO
Stanford
Georgia Tech
Varian
Kodak
Sharp
Georgia Tech
ARCO
8
Monosolar
Kodak Boeing
4 0 1975
RCA
Boeing
AstroPower
RCA
Solarex
NREL Cu(In,Ga)Se2 14x concentration
Boeing
NREL
AstroPower
Boeing
NREL
United Solar United Solar
Photon Energy
University California Berkeley University Konstanz
1985
NREL
NREL
Euro-CIS
University RCA of Maine RCA RCA RCA RCA
1980
UNSW
NREL
AMETEK Masushita
UNSW
NREL
University So. Florida Solarex
UNSW
UNSW
UNSW
No. Carolina State University Boeing
UNSW
Spire
Westinghouse
16 12
NREL/ Spectrolab
1990
1995
Princeton NREL
2000
Si cutoff
GaInP cutoff
REALISTIC POTENTIAL EFFICIENCY OF A GaInP-Si TANDEM
35
33%
Efficiency [%]
30 25 20 15 10 5 0
400
500
600
700
800
900
1000 1100
Wavelength [nm] Si: VOC = 0.68V, FF=0.801 (e.g. Sunpower) GaInP: VOC = 1.4V, FF=0.8 But there is a problem: GaInP costs > $10,000/m2
CAN WE USE ORGANIC DYES IN SOLAR APPLICATIONS? EXAMPLE: PERYLENE DIIMIDE DYES Material Advantages 9 Abundant: 1,500,000 kg/yr; multiple suppliers 9 Non Toxic 9 Low cost: $50/kg (if 0.2 g/m2 $0.01/m2) 9 Stable
O
O
R
R
O
O
‘Toreador red’
compare semiconductor grade silicon production: ~35,000,000 kg/yr (2005)
Reflection
Light
+ ‐ Light absorption at dye
Reflection, Transmission losses
Emission
Photon Thermal loss conversion
Waveguided transmission
Facial emission
Re‐ absoprtion
PV conversion
Recombination losses
(i) Refractive index is hard to increase
1
0.8 0.7
Standard glass/plastic
Trapping efficiency
0.9
0.6 0.5 0.4 0.3 0.2 0.1 0
1
1.2 1.4 1.6 1.8
2
2.2 2.4 2.6 2.8
Refractive index Need n > 2.3 to get trapping efficiency > 90% No low cost solutions?
3
THE OPERATION OF LSCs 2. Transport losses
1. Confinement losses air
glass
air
Non‐zero overlap between absorption and emission can lead to re‐absorption
Non waveguided emission Refractive index n = 1.5 n = 1.7 n = 2.1
Loss 25% 20% 12%
Feeds back in
Re‐absorption losses have limited LSCs to concentration factors of 100,000 hours in OLEDs [Mark Thompson, MRS Bulletin, 32, 694 (2007)] Our preliminary stability measurement in solar concentrators: 8% drop in 3 months of solar exposure
EFFICIENCY OF TANDEM WAVEGUIDES (photons in/photons out) at G=3
Rubrene: DCJTB Pt(TPBP)
GaInP
GaAs
Calculated power efficiency: 6.8%
Optical Quantum Efficiency
0.7 0.6 0.5 0.4 0.3 0.2
30% Rubrene, 1% DCJTB 2% DCJTB, 4% Pt(TPBP) Sum
0.1 0
400 450 500 550 600 650 700
Wavelength (nm)
EFFICIENCY OF SINGLE WAVEGUIDES (photons in/photons out) at G=3
Optical Quantum Efficiency
0.7 0.6
Calculated power efficiencies
0.5
DCJTB‐based OSC with GaInP: Rubrene‐based OSC with GaInP: Pt(TPBP)‐based OSC with GaAs:
0.4 0.3 0.2 0.1
30% Rubrene, 1% DCJTB 2% DCJTB, 4% Pt(TPBP) 2% DCJTB
0 400 450 500 550 600 650
Wavelength (nm) GaInP: C. Baur et al., Journal of Solar Energy Engineering-Transactions of the ASME 129, 258 (2007). GaAs: R. P. Gale et al., paper presented at the 21st IEEE Photovoltaic Specialists Conference, Kissimimee 1990
5.9% 5.5% 4.1%
EFFICIENCY AT HIGHER OPTICAL CONCENTRATIONS Previous maximum 1
