New Applications of Old Materials From Paint to Solar Cells

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1 New Applications of Old Materials From Paint to Solar Cells Peter Peumans Integrated Circuits Lab, Stanford University Sponsored by NSF

2 Solar Energy At earth s surface average solar energy is ~ 4 x J / year Global energy consumption (2001) was ~ 4 x J / year (increasing ~ 2% annually) 23.2% 23.9% 6.73% 6.48% 0.748% Petroleum Natural Gas Coal Hydroelectricity Nuclear Solar/Wind/Wood/Waste 38.9% In US, average power requirement is 3.3 TW. With 10% efficient cells we would need 1.7% of land area devoted to PV (~ area occupied by interstate highways) Source: DOE

3 Solar Energy Land Requirements 230x230 mile 10% ~ 3TW N. Lewis (Caltech) DOE

4 Electrical Energy Production Cost N. Lewis (Caltech)

5 Cost/Efficiency of Photovoltaic Technology $0.30/W Costs are modules per peak W; installed is $5-10/W; $0.35-$1.5/kW-hr M. Green (UNSW)

6 (1) Substrate preparation Cheap: Roll-to-Roll Processing (2) Organic deposition (3) Metal deposition and Metal transfer Solar Fiber Auxillary conductor Barrier coating Photoactive layer Nylon core Outer conductor Inner conductor (1) (2) (3) 1µm 30cm chamber size Roll-to-roll coater (Applied Films) M. Shtein, unpublished P. Peumans, unpublished

7 Efficient? Progress in Organic Solar Cells Power conversion efficiency (%) 10 1 α-si Polymer: solution processed Small molecule: vacuum α-si data from C. Wronski approx. AM mW/cm 2 How? Efficiency limits? Low-cost? Stable? Non-toxic? Abundant?

8 Phthalocyanines Abundant: BASF makes 70,000 tons/year Non-toxic Low-cost: ~1$/g 17 /m 2 Stable

9 Historical Perspective pn-diode PV cell p n Ag PTCBI (500Å) CuPc (300Å) ITO Glass Current [ma/cm 2 ] V OC= 450mV FF = 0.65 η = 0.95% Voltage [V] C.W. Tang, Appl. Phys. Lett. 48, 183 (1986).

10 30 Nanostructured Solar Cells ~2 fold increase in quantum efficiency bulk CuPc/PTCBI 20 η EQE [%] 10 bilayer Wavelength [nm] P. Peumans, S. Uchida and S.R. Forrest, Nature (2003)

11 Unexplained Losses Usually attributed to low mobilities Current density [ma/cm 2 ] mW/cm Voltage [V] P. Peumans, et al., Nature 425, 158 (2003).

12 Ratio of Charge Carrier Mobilities It is not mobility that counts, but mobility ratio! Efficiency or Fill Factor η ED =0.85 1E E µ Acceptor / µ Donor η IQE (V=0) Fill Factor

13 Toy Model Low Mobility Ratio High Mobility Ratio

14 Toy Model Low Mobility Ratio High Mobility Ratio

15 Toy Model Low Mobility Ratio High Mobility Ratio

16 Toy Model Low Mobility Ratio High Mobility Ratio

17 Toy Model Low Mobility Ratio High Mobility Ratio

18 Ordered Structures J (ma/cm 2 ) Peumans, 225mW, Planar, VTE Peumans, 105mW, BHJ, Annealed Yang, 125mW, BHJ, OVPD CuPc/PTCBI Voltage (V) F. Yang, et al, Nature Materials, 4, 37-41, 2005

19 What s Next? Multijunction Cells Balanced Power Conversion Efficiency, η P [%] (30%) 6 (27%) 4 (25%) 2 (19%) AM1.5, 1000W/m 2 multijunction cells # junctions (η P ) single junction (15%) Short Circuit Photocurrent, J SC [A/m 2 ]

20 Cumulative Photon Flux [10 21 sec -1 m -2 ] Material Requirements E G6 =0.49eV E G5 =0.63eV E G4 =0.76eV E G3 =1.08eV E G2 =1.24eV 0.3eV E G1 =1.80eV Photon Energy [ev] AM1.5, 1000W/m 2 organic/inorganic subcells organic subcells hν

21 Nanoparticle Technology Semiconducting absorbers Metallic recombination centers From: Henry, JAP 51, 4494 (1980). PbSe Ge 100nm PV cell 1 ITO Ag nanoclusters Si PV cell 2 10% 27% CdSe Organic semiconductors Ag

22 Manufacturing Technology to pump to pump multilayer deposition substrate heater substrate pyrex sleeve mixing with organic molecules nanoparticle aerosol Carrier gas (N2, Ar, ) ultrasonic atomizer evaporate solvent organic sources Carrier gas (N2, Ar, ) atomized suspension

23 Conclusions Organic pigments are promising materials for large-scale photovoltaics Low-cost, stable, non-toxic, abundant, efficient A detailed understanding of the underlying physics can result in large gains in efficiency Multijunction organic solar cells have an efficiency potential >20% Efforts required: Energy level engineered molecules Multijunction cells Manufacturing technology Packaging technology

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25 Vacuum Deposition is a Low-Cost Technology Paper metallization for food packaging Examples from Applied Films ~50µm Al, 2.45m wide, 1.8m diameter, 14m/s