Overview of Photovoltaic Energy Conversion
Topics Solar Energy Economics Photovoltaic Technologies Challenges and Opportunities II-VI Solar Cells
November 20, 2006
U.S. Energy Overview (Quadrillion BTU) http://www.eia.doe.gov/emeu/mer/pdf/pages/sec1.pdf
http://www.eia.doe.gov/oiaf/forecasting.html
CO 2 Emissions US ~ 20 metric tons / capita / year An average person generates 0.33 metric tons per year through respiration Metric tons per capita per year China ~ 2.2 metric tons / capita / year Source: NY Times
2005 U.S. Energy Consumption http://www.eia.doe.gov/cneaf/solar.renewables/page/rea_data/rea.pdf
U.S. Energy Consumption Overview Year 2002 2003 2004 2005 2006 2006(%) Total (Quadrillion BTU) 97.927 98.28 100.413 100.756 99.96 100.00% Fossil Fuels 83.994 84.386 86.191 86.451 85.307 85.34% Coal 21.904 22.321 22.466 22.785 22.511 22.52% Coal Coke Net Imports 0.061 0.051 0.138 0.044 0.061 0.06% Natural Gas 23.628 22.967 22.993 22.886 22.518 22.53% Petroleum 38.401 39.047 40.594 40.735 40.217 40.23% Electricity Net Imports 0.072 0.022 0.039 0.084 0.06 0.06% Nuclear Electric Power 8.143 7.959 8.222 8.16 8.208 8.21% Renewable Energy 5.893 6.151 6.261 6.404 6.844 6.85% Biomass 2.706 2.817 3.023 3.114 3.277 3.28% Biofuels 0.309 0.414 0.513 0.594 0.758 0.76% Waste 0.402 0.401 0.389 0.403 0.404 0.40% Wood Derived Fuels 1.995 2.002 2.121 2.116 2.114 2.11% Geothermal Energy 0.328 0.331 0.341 0.343 0.349 0.35% Hydroelectric Conventional 2.689 2.825 2.69 2.703 2.89 2.89% Solar/ PV Energy 0.064 0.064 0.064 0.066 0.07 0.07% Wind Energy 0.105 0.115 0.142 0.178 0.258 0.26% http://www.eia.doe.gov/cneaf/solar.renewables/page/prelim_trends/pretrends.pdf
U.S. Electric Power Net Generation, 2006 Total = 4,065 Billion KWh (10% from Renewables and Hydroelectric) 45.6% 42.4% 38.3% 72.6% 89.6% Capacity factor is highest for nuclear power plant http://www.eia.doe.gov/cneaf/electricity/epa/figes1.html
U.S. Electricity Net Generation in thousand kwh: Total (All Sectors), 1996-2006 Note: Solar/PV includes solar thermal http://www.eia.doe.gov/emeu/aer/elect.html
Solar Energy Production Factor Contour Map in KWh / KW-year ~ yearly hours of sunshine Rochester 4.1 hr/day 6.59 hr/day Tucson
Global PV Production and Production Capacity Trend http://www.iea-pvps.org/isr/index.htm
2005 PV Production (MW) by country http://www.iea-pvps.org/isr/index.htm
http://www.iea-pvps.org/isr/index.htm 2005 PV Production (Cells)
2006 PV Production and Production Capacity in U.S. http://www.iea-pvps.org/countries/usa/06usansr.pdf
PV Module and System Price http://www.iea-pvps.org/products/download/rep1_15.pdf
Source: solarbuzz.com
PV Industries in U.S. 2007*(+50%) 309 Peak MW Total including 2007 = 927 Peak MW http://www.eia.doe.gov/cneaf/solar.renewables/page/solarreport/solar.pdf
U.S. PV Cell and Module Revenues 2006 Thin-film PV market share ~ 30% http://www.eia.doe.gov/cneaf/solar.renewables/page/solarreport/solar.pdf
http://www.eia.doe.gov/emeu/aer/elect.html
Renewable Porfolio Standard Charge Delivery Charge Supply Charge Total electric utility charge = $62.55 Tang s household data (averaged over 38 months): Usage: 7284 kwh per year (607kWh / month) Charge: $795 per year ($66 / month) Average cost of electricity = $0.109 per kwh
Incentives (2006) Federal Tax Credit: $2000 NYS incentives (NYSERDA) 100% Tax exemption $4.00 per watt for grid-connected homes, $4.50 per watt for energy-star homes Up to 60% of total cost 4 kw PV system @ $6/W & 30 year life: = 41.8 cents / kwh (before incentives) = 13.2 cents / kwh (after incentives) Vs 10.9 cents / kwh from RGE
Solar Cell Technologies Si-based solar cells Crystalline Si Poly-Si Thin Si Thin-film solar cells Cu(In,Ga)Se 2 CdS/CdTe Emerging solar cells TiO 2 -dye sensitized cells Organic cells PV concentrators
Solar spectrum and max. conversion for single-junction solar cells Si GaAs a-si AM1.5 = 1 kw/m 2
Solar Cell 101 Equivalent circuit I d I = I o [exp(qv/nkt)-1] - I ph V oc = n(kt/q) ln[(i sc /I o )+1] I sc = I ph P max = V oc x I sc x FF
Single-junction limit DOE_BES report - http://www.sc.doe.gov/bes/reports/files/seu_rpt.pdf.
Sunpower c-si solar module Module efficiency ~ 18%!
Triple-junction a-si solar cells on stainless steel foil @ 1000 w/m 2 Area = 1.12 m 2 Module efficiency = 6%
Thin-Film PV CdS/CdTe fastest growth First Solar CdS/CdTe thin-film PV Module efficiency ~ 9% Production capacity: 75MW in Ohio 100MW in Germany (under construction) 100MW in Malaysia (announced)
DSC Modules from STI (an Australian company, setup in 2001) Flexible DSC from Konarka Module efficiency < 5%
Compact TiO2 Porous TiO2 TiO2-DSC Cells Graetzel Cells Hole-transport Acc. Chem. Res. 2000, 33, 269-277
Best DSC performance
Single-junction limit 5.5% DOE_BES report - http://www.sc.doe.gov/bes/reports/files/seu_rpt.pdf.
Organic Bi-layer Photovoltaic Cell C.W. Tang, Appl. Phys. Lett. 48, 183 (1986) Ag 600 A 200 A FF=0.65 ITO AM2 EFF= 1% 10-3 A/cm^2 10-5 10-7 0.1 0.3 0.5 Volt
Organic heterojunction photovoltaic structure light Efficiency is limited by short exciton diffusion length ~ 10-20 nm
D: CuPc Organic Heterojunction 5.2eV 3.5eV 6.1eV 4.6eV Vacuum Level 1.7eV E A LUMO A: PTCBI IP 0.9eV 1.7eV HOMO DONOR ACCEPTOR
Organic Heterojunction DONOR ACCEPTOR
(1) η A > 50% Organic Heterojunction Optical excitation (~10-15 s) η A Optical absorption length ~ 1000Å
Organic Heterojunction Exciton relaxation (10-13 -10-9 s) (1) η A > 50% η RELAX Lattice polarization: E B = 0.1-1eV Necessity for heterojunction
Organic Heterojunction Exciton diffusion (10-9 s) (1) η A > 50% η ED (2) η ED ~ 10% Exciton diffusion length ~ 50-200Å
Organic Heterojunction Exciton dissociation by charge-transfer (~10-13 s) (1) η A > 50% (2) η ED ~ 10% η CT (3) η CT ~ 100% Ultrafast process (~100fs) Very efficient cf. MDMO-PPV/PCBM: 50fs, Zerza, et al., Synth. Metals 119, 637 (2001)
Charge collection: overcome Coulomb attraction barrier (10-6 s) (1) η A > 50% Organic Heterojunction (2) η ED ~ 10% η CC (3) η CT ~ 100% (4) η CC ~ 100% r th r th P. Peumans and S.R. Forrest, submitted.
(1) η A > 50% Organic Heterojunction Exciton diffusion is limiting step (2) η ED ~ 10% η CC (3) η CT ~ 100% (4) η CC ~ 100% η EQE = η A. η ED. η CT. η CC ~ 10%
CuPc PTCBI PTCBI PL 2 Exciton Diffusion Length PL 1 PL 1 /PL 2 Photoluminescence quenching Assume continuum, isotropic diffusion 1.0 0.8 0.6 0.4 L D =(30±3)Å 0.2 experiment theory 0.0 0 40 80 120 160 Thickness [Å] Peumans et al., J. Appl. Phys. 93, 3693 (2003).
Bulk Heterojunction OPV 0.93% 2.5% Brabec, Solar Energy Materials & Solar Cells 83 (2004) 273 292
Tandem Organic Photovoltaic Cell Tang, unpublished work 0.0 Volt 0.5 1.0 1.5 5 cells 1.0 2 cells ma/cm^2 1 Cell 2.0 * Cells connected electrically in series and optically parallel
Solar Spectrum 1.8 2.04 ev 1.55 ev 1.18 ev 3-stack tandem OSC 1.6 1.4 Cathode Solar Power Flux W /m^2/nm 1.2 1 0.8 0.6 0.4 Long λ Unit Medium λ Unit Short λ Unit Interconnects 0.2 S M L Anode 0 300 400 500 600 700 800 900 1000 1100 1200 1300 1400 Wavelength (nm) Spectral Range, nm S (300-610) M (610-800) L (800-1050) ma/cm 2 13.72 13.75 13.73 Min Photon energy, ev 2.04 1.55 1.18 Voc, v 0.82 0.62 0.47 Eff. w/w; FF = 0.65 7.29% 5.53% 4.21% Eff. Total 17.03% n i p Single-unit p-i-n OSC
20th European Photovoltaic Solar Energy Conference, 6 10 June 2005, Barcelona, Spain PATHS TO ULTRA-HIGH EFFICIENCY (>50% EFFICIENT) PHOTOVOLTAIC DEVICES C. B. Honsberg and A.M. Barnett DARPA sponsored VHESC project ($52M)
http://www.sc.doe.gov/bes/reports/files/seu_rpt.pdf.
Grand Challenge: 1TW photovoltaic power generation by 2050? Will need: Much lower cost Higher efficiency Shorter energy payback Longer life Recyclable / disposable New technologies?
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