Challenges in Solar-to to-electric Energy Conversion: an Introduction Eray S. Aydil Chemical Engineering and Materials Science Department Acknowledgements: National Science Foundation Minnesota Initiative for Renewable Energy and the Environment (IREE)
United States Department of Energy Report on the Basic Energy Sciences Workshop on Solar Energy Utilization by N. S. Lewis et al. (2005). www.er.doe.gov/bes/reports/files/seu_rpt.pdf
The Energy Challenge Find ways to provide clean energy to ~ 10 billion people. 7 6 5 4 3 2 1 0 5.3 4.2 3.5 0.9 0.9 0.07 0.03 0.01 0.004 oil coal gas hydroelectric nuclear wind ethanol geothermal photovoltaic Energy Consumption (TW)
Solar power World demand ~ 15 TW Sun ~ 120,000 TW Covering 0.125 % of earth s surface with 10% efficient solar cells would produce enough energy to supply the annual global demand.
Global solar PV production ~30-50% growth At 35% growth rate we will reach 1 TW in ~ 20 Years
State-of of-the-art in solar cells Module efficiency ~ 0.5-0.8 lab efficiencies United States Department of Energy Report on the Basic Energy Sciences Workshop on Solar Energy Utilization by N. S. Lewis et al. (2005) and from J. Crystal Growth 275, 292 (2005) by T. Surek.
2007 market share for various technologies CdTe (4.7%) thin film Si (5.2%) CIGS (0.5%) c-si (89.6%)
~ 5 more expensive than other sources 30 20-40 Cost ( /kw-hr) 25 20 15 10 1-4 2-5 6-8 5-7 6-7 5 0 Coal Gas Oil Wind Nuclear Solar Residential ~ 40 Commercial ~ 30 Industrial ~ 22
Three generations of solar cells I. Crystalline Si solar cells ($ 8/W ~ 40 /kwh) II. Thin film solar cells III. Advanced future structures Installed PV System Cost = Module Cost + Balance of System (BOS) 100 $ 0.2/W $ 0.5/W 80 Efficiency (%) 60 40 III 20 II I 0 0 200 400 600 Cost $/m 2 $ 1/W Shockley- Quessier Limit $ 4/W M. Green Third Generation Photovoltaics Advanced Solar Energy Conversion, Springer Verlag, Berlin (2004).
Future projections for existing technologies United States Department of Energy Report on the Basic Energy Sciences Workshop on Solar Energy Utilization by N. S. Lewis et al. (2005) and from J. Crystal Growth 275, 292 (2005) by T. Surek.
What do you have to do to convert photons to current? e - h + or A C B e - h + A B Separate photogenerated +ve and ve charges Minimize recombination Material and interfacial properties control rates
p-n junction solar cell Challenge is in cost reduction New ways of making c-si, thinner, cheaper, solar grade DoE/NREL, www.pv.unsw.edu.au
Thin film amorphous silicon solar cell www.semiconductor-sanyo.com
CuIn 1-x Ga x Se 2 (CIGS) solar cells Record 19.9% efficiency achieved through empirically derived deposition Interfaces not well understood Relation between microstructure, composition and performance not well understood Large area production difficult In is scarce
Organic solar cells
Solar cell figures of merit Power P max I V Fill Factor FF = I P V sc max oc Overall Efficiency V oc η = P = I max sc oc S FF I I S V I sc P max V Is A = I ( λ) dλ 1000 W / m 0 AM1.5 2 solar
Shockley-Quessier Limit 100 $ 0.2/W $ 0.5/W 80 Efficiency (%) 60 40 III 20 II I 0 0 200 400 600 Cost $/m 2 $ 1/W Shockley- Quessier Limit $ 4/W M. Green Third Generation Photovoltaics Advanced Solar Energy Conversion, Springer Verlag, Berlin (2004).
Shockley-Quessier Limit Shockley-Queisser limit ~ 33% Increasing Energy band gap,e g conduction band e e e (3) (2) (1) h h valence band energy lost to heat light
Surpassing Shockley-Quessier limit with multijunction solar cells www1.eere.energy.gov
Novel methods for concentrating Design and synthesis of dyes or inorganic particles that can absorb diffuse light and reemit anisotropicaly and efficiently http://web.mit.edu/newsoffice/2008/solarcells-0710.html Baldo et al. Nature 403, 750 (2000)
What do you have to do to convert photons to current? e - h + or A C B e - h + A B Separate photogenerated +ve and ve charges Minimize recombination Material and interfacial properties control rates
Nanostructured materials are emerging as potential solar cell architectures C A B Large surface and interfacial areas found in nanostructured materials present significant advantages both for light absorption and for charge separation, the two critical steps in solar-to-electric energy conversion.
Challenge C A B Find A, B and C with appropriate electronic and optical properties Provide means to separate charge at the A-B-C interface Maximize optical absorption Assemble into high interfacial area nanostructured film Minimize premature charge recombination Cost 0
Heterojunctions between nanostructured materials C B A B A Some architectures that have emerged so far Nanoparticle based dye sensitized solar cells (Gratzel, 1991) Bulk heterojunction solar cells (Heeger, 1995 and Alivisatos, 2002) Nanowire based dye sensitized solar cells (Baxter, 2005; Law 2005) Nanoparticle Quantum dot sensitized solar cells (Vogel, 1990; Nozik 2003) Nanowire quantum dot sensitized solar cells (Leschkies, 2007) Quantum dot solar cells (Nozik, 2007, 2008)
Dye Sensitized Solar Cells Nanocrystalline, mesoporous TiO 2 photoelectrode on TCO. TiO 2 is photosensitized with a monolayer of dye. Efficient light harvesting with large dyed surface area: ~ 1000 flat film O Regan & Grätzel, Nature 353, 737 (1991). Grätzel, Nature 414, 338 (2001).
Bulk heterojunction solar cells Fulerenes blended (80 wt%) with conjugated polymer host Polymer absorbs light donates electron Fullerenes are electron acceptors Bicontinuous, interpenetrating D-A heterojunction Heeger. Science 270, 1995. MEH-PPV: poly[(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene vinylene]
Hybrid Nanorod-Polymer Solar Cell Alivisatos. Science 295, (2002). metal Transparent electrode CdSe nanorods replace fullerenes semiconductor also absorbs light increased aspect ratio gives better transport P3HT: poly-(3-hexylthiphene) ; PEDOT: Polyethylenedioxythiophene
Challenges Can we design appropriate donors and acceptors from first principles? molecular structure to maximize charge transport molecular features to phase separate at right length scales appropriate energy level alignments for exciton dissociation Understand and determine energy level alignments at the D-A interface What is the ideal interface structure that minimizes charge recombination?
Multiple exciton generation in quantum dots conduction band e e e Electron states Hole states Eg QD ħω h h h valence band hν = 2E g Colloidal QDs can generate multiple electron-hole pairs per absorbed photon. Nozik (2003) - multiple exciton generation (MEG) in quantum dots may occur with high probability Klimov (2005) - first demonstration of MEG in PbSe quantum dots
Challenges in making nanostructured solar cells Challenges create high surface area nanostructured materials that enable efficient light absorption, charge separation and charge transport establish the fundamental scientific principles that will enable novel solar cell architectures based on nanostructured materials Design Synthesis Solar cell assembly Understanding the materials & the device physics