Carbon-Based Solar Cells Chabot College Guest Lecture Michael Vosgueritchian PhD Candidate Prof. Zhenan Bao’s Group 2-19-2013 1 Research Overview Carbon and Organic Electronics • Silver or • PDMS / unsorted CNT IT IR light Cathode C60 Exciton Electron Sc-CNT hole Anode Substrate CNT P3DDT • ITO / PEDOT or • Graphene 2 Current Energy World demand is 15 TW (15 trillion Watts) Enough power for 15 billion 100W light bulbs US 26% (even though 5% of population) Source: cleantech.org 3 Sustainable Energy Wind Energy Solar Energy Ocean Energy Geothermal Energy Biofuel Source: Sandia National Lab In ~1 hr we get enough solar power to power the earth for a year! 4 Solar Radiation and Market Enough <1% of landmass enough to provide energy demand 5 Solar Cells Technologies NREL.com Crystalline Si – 27.6% Thin-Film • CIGS – 20.4% • CdTE – 18.3% Konerka • α- Si - 13.4% OPVs – 11.1% Nanotechnology • Quantum Dots – 7.0% • Carbon based PVs (CPVs) – 1.2%* (~0.5%) Other: GaAs, dye-sensitized, etc. GE 6 Best Cell Efficiencies 7 Solar Cell Uses and Considerations Applications Industrial Commercial Home Portable Considerations Cost/efficiency Materials Lifetime Niche applications NREL.com 8 Portable Solar Cells Uses Power portable electronic devices Lighting Transportation Lighting Africa Project Main failure due to cracks in the solar cells Krebs et al. Energy Environ. Sci., 2010,3, 512-525 9 Transparent Electrodes (TEs) Materials that offer high conductivity and high transparency, usually in thin film form Displays Solar Cells • LEDs • Touch Screens • Energy Storage • Sensors • Transistors Sony.com Konarka.com 10 Why do we Need New Alternative Electrodes? Replace ITO Enable flexible (stretchable) organic electronics Images from Google 11 Carbon PVs (CPVs) New class of solar cells Stability Chemical/Environmental: water/O2, heat, etc. Physical: strains, flexible/stretchable devices Potential for cheap solar cells First demonstration of all-C solar Cell Solution processable Roll-to-roll fabrication Lightweight Near-infrared absorption Tandem cells 12 Carbon Nanomaterials Fullerenes – 0D • Discovered in 1985 (C60) • C60, C70, C84 • Films – n-type semiconducting Carbon Nanotubes (CNTs) – 1D • Discovered in 1991 • Single and multi-walled • Semiconducting or Metallic Graphene – 2D • Discovered in 2004 • 2010 Nobel Prize • Metallic/transparent 13 Solar Cell Operation PCE Short Circuit Current (Jsc) J sc Voc FF Pin High absorption Low recombination Fill factor (FF) Reduce parasitic resistances Open circuit voltage (Voc) Optimum band gap 14 CPV Structure Design of first demonstration of all-Carbon solar cell Bilayer active layer: P3DDT sorted CNTs, C60 Electrodes • Anode: ITO/PEDOT reduced graphene oxide (rGO) • Cathode: Ag n-doped CNTs • Silver or • PDMS / unsorted CNT IR light Graphene Exciton Electron Sc-CNT hole Anode Substrate -3.5 -3.8 e CNT -4.7 P3DDT • ITO / PEDOT or • Graphene CNT PEDOT Sc-CNT C60 ITO Cathode C60 P3DDT -5.3 -5 -5.1 -5.3 Anode M. Vosgueritchian et al. ACS Nano, 2012, 6 (11), pp 10384–10395 -4 Ag -4 -4.25 -6.2 LUMO HOMO Active layer Cathode 15 Film Fabrication Spin-Coating Spray-Coating Roll-to-roll Coater Konerka.com 16 Sorting of SC-SWNTs Solution based method to selective sort SWNTs Semiconducting selectivity by P3DDT Can be solution deposited: spin-coating, spray coating, etc. Absorbs in the infrared (IR) Lee, H. W. et al. Nature Communication 2011, 2, 541 17 a) 300 Sheet Resistance sq) 85 150 80 100 75 50 70 1 2 3 IR light n-doped SWNT C60 Exciton Electron hole Sc-CNT Reduced GO 4 Number of Layers Quartz Substrate SWNT spin coated from solution C60 evaporated in vacuum Absorption Spectrum 35 30 Transmission (%) 90 200 c) Bilayer of sorted SWNTs and C60 40 95 % Transmittance (at 550nm) Active Layer b) 100 Batch 1 Batch 2 250 Drop casting, thin area Spin coating 5X Spin coating 3X Spin coating 1X 25 20 15 10 5 0 400 600 800 1000 1200 1400 1600 Wavelength (nm) M. Vosgueritchian et al. ACS Nano, 2012, 6 (11), pp 10384–10395 18 a) 300 95 90 200 85 150 Anode – Graphene 80 100 75 50 1 % Transmittance (at 550nm) Sheet Resistance sq) b) 100 Batch 1 Batch 2 250 c) IR light n-doped SWNT C60 Exciton Electron hole 70 2 3 Sc-CNT Reduced GO 4 Number of Layers Quartz Substrate Can make large area electrodes Smooth (2D) structure Can be made highly conductive (30 ohms/sq at 90%) Bae et al., Nature Nanotechnology 5, 574–578 (2010) 19 a) 300 Sheet Resistance sq) 90 200 85 150 % Transmittance (at 550nm) 95 c) b) 100 Batch 1 Batch 2 250 n-doped SWNT Reduced Graphene Oxide 80 100 75 50 70 1 2 3 Number of Layers IR light C60 Exciton Electron hole Sc-CNT Reduced GO 4 Quartz Substrate Oxidation thermal reduction Reduced Graphene Oxide (rGO) rGO– 2D • Solution Processable • 102-103 Ω/□ at ~80% T • Cheap Deposit on Surface by spin-coating H. Becerril et al. ACS Nano, 2008, 2 (3), pp 463–470 20 a) 300 Sheet Resistance sq) 90 200 85 150 % Transmittance (at 550nm) 95 c) b) 100 Batch 1 Batch 2 250 n-doped SWNT Cathode – n-doped SWNT TE 80 100 75 50 IR light C60 Exciton Electron hole 70 1 2 3 Reduced GO 4 Number of Layers Sc-CNT Quartz Substrate Use stretchable SWNT films on PDMS as the cathode for all-carbon solar cells instead of metal Need n-doping: DMBI organic dopant Previously used as electrodes in pressure an strain sensors Spray-coated from solution As-deposited Biaxially stretched 1 μm M. Vosgueritchian et al. Nature Nanotech, 2008, 2 , pp 788-792 1 μm 21 Device Performance With traditional electrodes • ~0.5% Efficiency for full spectrum • ~0.2% Efficiency in the IR With carbon electrodes • ~0.01% Efficiency full and IR Improving Performance Theoretical Efficiency of ~9-10% Morphological Issues Contact Issues Smoothen films: roughness/aggregates can cause leakage/shorting Better contact between films: better charge transport, decrease recombination Electrodes Improve conductivity Long Term Introduce flexibility Test stability All solution-processable Heterojunction Active Layer Materials Use variety of SWNTs: increase absorption Heterojunctions Thicker films 23 Absorption Issues 0.18 0.16 0.25 Light intensity with filter Absorbance semiconducting SWNT 0.20 0.14 0.12 0.15 0.10 0.08 0.10 0.06 0.04 Absorbance (a.u.) 2 SWNTs absorb mostly in the infrared Film thickness only about 5 nm Different deposition process Optical power intensity (mW/cm ) 0.05 0.02 0.00 800 1000 1200 1400 Wavelength (nm) 1600 0.00 1800 24 Summary First demonstration of all-carbon Solar Cell Sorted-SWNTs used as light absorber C60 used to separate excitons Carbon electrodes replace traditional ITO/metal electrodes Lots of work needs to be done! Acknowledgments Prof. Zhenan Bao Dr. Marc Ramuz Dr. Ghada Koleilat Evan Wang Ben Naab 25 QUESTIONS? 26