Organic Solar Cells Green River Project
Silicon Cells Silicon semiconductors Advantages: Efficiencies Lifetimes Disadvantages: High manufacturing costs Inflexible http://en.wikipedia.org
Organic semiconductors Georgia Institute of Technology Advantages: Light, thin, and flexible Potentially cheap manufacturing Current Disadvantages: Efficiencies Lifetimes
Organic Photovoltaic Devices Konarka Plextronics Home Power Generation San Francisco bus shelters
How does the Organic Photovoltaic Device work? Photons incident on the clear glass results in the formation of excitons. The excitons separate and the electrons are transported to the cathode and holes to the anode giving rise to a voltage and therefore a current when the circuit is completed. PEDOT-PSS helps to lower the energy barrier between the ITO and the P3HT-PCBM layer. Hole transporter P3HT Electron transporter PCBM (soluble derivative of PCBM) http://depts.washington.edu/cmditr/mediawiki/index.php?title=organic_solar_cells
Two approaches to OPVs Form a donor-acceptor bilayer using vacuum deposition. Bulk Heterojunction (BHJ) Maximize interface between donor and acceptor. Polymer-based, solution processable Low cost, light-weight, flexible Process active layer in a single step (much easier than vacuum deposition!) Inkjet printing/spin Coating/Roller casting
Bulk Heterojunction (BHJ) Janssen, R.A.J.; Hummelen, J.C.; Sariciftci, N.S. MRS Bulletin. 2004, 30, 33.
Basic requirements for a good OPV Device must be good at absorbing light (absorption coefficient) Device must generate the greatest number of charge carriers with minimum concomitant loss of energy! Device must be capable of transporting these charge carriers (holes and electrons) to the respective electrodes at a maximum rate. (charge carrier mobility) Morphology (how do PCBM and P3HT interface with each other?)
Energy conversion in Excitonic Solar Cells General Mechanism Polymer-Fullerene Composite Solar Cells, Thompson and Frechet, Angew. Chem. Int. Ed. 2008, 47, 58-77
Factors that affect performance Choice of components in the active layer Determines the electronic interactions that cause the formation of the excitons, diffusion, dissociation and charge transport. Morphology (the AFM tells us this) Determines the physical interactions between the components of the active layer i.e. can the exciton diffuse far enough before it recombines? How well can the excitons contribute to the current?
Choice of materials P3HT-PCBM in 25mg:15mg ratio dissolved in 1mL of dichlorobenzene Electron Acceptor Fullerene [6, 6] phenyl-c61-butyric acid methyl O ester (PCBM) High electron affinity Transports charge easily. Electron Donor P3HT [Poly (3 hexylthiophene)] PCBM Electron Acceptor Typical efficiencies of 5% (this is the HIGH end) OMe H S S P3HT Electron Donor Br n/2
Optimum donor material for PCBM Need to balance donor LUMO acceptor LUMO levels Maximize donor HOMO and acceptor LUMO difference for high V OC Ideal donor: LUMO=3.9 ev & HOMO =5.4 ev Model predicts 20% PCE Dennler & Brabec, Adv. Mater. 2009, 21, 1323
Characterization of OPVs Solar power conversion efficiency (%) = Electrical output power Sun light power = FF x Voc x Jsc Sun light power = Maximum { J (ma/cm2 ) x V (v)} 100 (mw/cm 2 ) ( = 1 kw/m 2, A.M 1.5) Current density J (ma/cm 2 ) = Current (ma) / Active area (cm 2 ) by Solar Simulator Voc V ( v ) Maximum { J x V } Jsc Jsc Voc Fill Factor (FF) = Maximum { J x V } Voc x Jsc J (ma/cm 2 )
Anode: Clean and cut ITO coated glass and anneal at 140 0 C for 10 minutes. Then Spin coat with PEDOT/PSS (2000rpm for 30 seconds). Anneal at 140 0 C for 15 minutes. Spin coat with the active layer (P3HT-PCBM) and annealed all but four samples again at 140 0 C for 10 minutes. Cathode: Apply a small quantity of GaIn on one corner and encircle with thin ring of epoxy. Finally, invert the cathode and gently place on the anode. Need offset to attach alligator clips! Previously unannealed samples are now annealed. ITO glass with Polymer Layer Epoxy ring GaIn Eutectic GaIn Eutectic Optical Adhesive (we used 5 minute epoxy) ITO coated glass ITO coated glass
Finished device Same anode but use a liquid Metal alloy (GaIn eutectic) Work function ~ 4.2eV as the cathode. Note: same work function as the Al electrode but NO VACUUM DEPOSITION!! Epoxy (sealant) Photon Absorbing Layer [PCBM-P3HT] Gallium Indium Eutectic (Cathode) (PEDOT-PSS Conducting Layer ) Anode (ITO) Substrate (glass) Substrate [Glass] ITO http://www.ece.ncsu.edu/oleg/files-wiki/c/cc/modulethreeprocedures.pdf
How do we increase efficiency? Fine tune the electronic interactions between the polymeric donor (P3HT) and the fullerene acceptor (PCBM). Device Architecture Morphology Processing
Role of the Active Layer Governs the mechanism for light absorption The exciton diffusion The charge transport Charge collection occurs in the interface between the eutectic and the active layer. Performance depends on Choice of Active layer components Morphology of the sample (controls the physical interaction between the donor and the accepteor)
Electronic interactions & Morphology Other electronic schemes engineering the LUMO-LUMO difference and the HOMO(donor) and LUMO (acceptor) levels etc. Mean domain size should be approximately the diffusion length (5 10nm) Phase segregation for effective charge pathways.
Intrinsic Morphology Factors that are inherent to the fullerene and the polymer and how they interact. Extrinsic Solvent choice (toluene vs. dichlorobenzene) Ratio of P3HT and PCBM Thickness of active layer (spin speeds) Spin-coating/roller casting/ink-jet printing Solvent evaporation rate annealing times
Anode: Clean and cut ITO coated glass. Spin coat with PEDOT/PSS (4000rpm for 30 seconds). Anneal at 140 0 C for 15 minutes. Spin coat with the active layer (P3HT-PCBM - 4000rpm for 60 seconds) and annealed again at 140 0 C for 10 minutes. Cathode: Apply a small quantity of GaIn on one corner of cleaned ITO glass substrate and encircle with thin ring of epoxy. Finally, invert the cathode and gently place on the anode. Need offset to attach alligator clips! ITO glass with PEDOT-PSS and Active Layer Epoxy ring GaIn Eutectic GaIn Eutectic Optical Adhesive (we used 5 minute epoxy) ITO coated glass ITO coated glass
Goal of GRCC projects Pick one parameter and optimize the organic solar cell for maximum efficiency (as measured using J-V data). AFM data to study the morphology of these samples and correlate it to performance would be a nice thing to do (currently a challenge), Optical characteristics of these cells.
Organization Team of three students Broad division of tasks Fabrication of device (typically takes 2 hours one day and 2 hours the next day to make 8 devices) Voltage vs. Current density measurements: this currently takes an hour per sample. We would like to amplify the current and expedite this process. Calculation of efficiency (1 hour) Optical Properties (still being developed)
Timeline Proposals Who is in your team and what is the team going to do? (April 23) Include the times when you are available to work on the project outside of class time. Indicate preference for an early start or late start (depends on what you are doing also). Progress Report 1 one week after you start work. Progress Report 2 two weeks after you start work (this could be your final report)
Timeline Final write-up due last week of the quarter (TBD) Short presentation (show and tell) last week of quarter. Content Post test and CURE post-survey + supplemental survey last week of quarter.