Carbon-Based Solar Cells Chabot College Guest Lecture Michael Vosgueritchian PhD Candidate Prof. Zhenan Bao’s Group

Research Overview  Carbon and Organic Electronics 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) 3 Source: cleantech.org

Sustainable Energy  Wind Energy  Solar Energy  Ocean Energy  Geothermal Energy  Biofuel  In ~1 hr we get enough solar power to power the earth for a year! 4 Source: Sandia National Lab

Solar Radiation and Market  Enough  <1% of landmass enough to provide energy demand 5

Solar Cells  Technologies  Crystalline Si – 27.6%  Thin-Film CIGS – 20.4% CdTE – 18.3% α- Si %  OPVs – 11.1%  Nanotechnology Quantum Dots – 7.0% Carbon based PVs (CPVs) – 1.2%* (~0.5%)  Other: GaAs, dye-sensitized, etc. 6 NREL.com GE Konerka

Best Cell Efficiencies 7

Solar Cell Uses and Considerations  Applications  Industrial  Commercial  Home  Portable  Considerations  Cost/efficiency  Materials  Lifetime  Niche applications 8 NREL.com

Portable Solar Cells  Uses  Power portable electronic devices  Lighting  Transportation  Lighting Africa Project  Main failure due to cracks in the solar cells 9 Krebs et al. Energy Environ. Sci., 2010,3,

Transparent Electrodes (TEs)  Materials that offer high conductivity and high transparency, usually in thin film form 10 Displays Sony.com Solar Cells LEDs Touch Screens Energy Storage Sensors Transistors Konarka.com

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  First demonstration of all-C solar Cell  Stability  Chemical/Environmental: water/O 2, heat, etc.  Physical: strains, flexible/stretchable devices  Potential for cheap solar cells  Solution processable  Roll-to-roll fabrication  Lightweight  Near-infrared absorption  Tandem cells 12

Carbon Nanomaterials 13 Carbon Nanotubes (CNTs) – 1D Discovered in 1991 Single and multi-walled Semiconducting or Metallic Fullerenes – 0D Discovered in 1985 (C60) C60, C70, C84 Films – n-type semiconducting Graphene – 2D Discovered in Nobel Prize Metallic/transparent

Solar Cell Operation 14  Short Circuit Current (J sc )  High absorption  Low recombination  Open circuit voltage (V oc )  Optimum band gap  Fill factor (FF)  Reduce parasitic resistances

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 15 M. Vosgueritchian et al. ACS Nano, 2012, 6 (11), pp 10384–10395

Film Fabrication 16 Spray-Coating Spin-Coating Roll-to-roll Coater Konerka.com

Sorting of SC-SWNTs Lee, H. W. et al. Nature Communication 2011, 2,  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)

Active Layer  Bilayer of sorted SWNTs and C60  SWNT spin coated from solution  C60 evaporated in vacuum 18 M. Vosgueritchian et al. ACS Nano, 2012, 6 (11), pp 10384–10395 Absorption Spectrum

Anode – Graphene  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

Reduced Graphene Oxide 20 Oxidation Reduced Graphene Oxide (rGO) thermal reduction Deposit on Surface by spin-coating rGO– 2D Solution Processable Ω/ □ at ~80% T Cheap H. Becerril et al. ACS Nano, 2008, 2 (3), pp 463–470

Cathode – n-doped SWNT TE 21  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 M. Vosgueritchian et al. Nature Nanotech, 2008, 2, pp

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  Smoothen films: roughness/aggregates can cause leakage/shorting  Contact Issues  Better contact between films: better charge transport, decrease recombination  Active Layer Materials  Use variety of SWNTs: increase absorption  Heterojunctions  Thicker films 23 Heterojunction  Electrodes  Improve conductivity  Long Term  Introduce flexibility  Test stability  All solution-processable

 SWNTs absorb mostly in the infrared  Film thickness only about 5 nm  Different deposition process Absorption Issues 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