Organic Solar Cells Green River Project Silicon Cells Silicon - - PowerPoint PPT Presentation

Organic Solar Cells

Green River Project

Silicon Cells

• Silicon semiconductors

http://en.wikipedia.org Advantages:

• Efficiencies
• Lifetimes

Disadvantages:

• High manufacturing costs
• Inflexible

Organic semiconductors

Georgia Institute of T echnology 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?

http://depts.washington.edu/cmditr/mediawiki/index.php?title=Organic_Solar_Cells

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. Hole transporter – P3HT Electron transporter – PCBM (soluble derivative of PCBM) PEDOT-PSS helps to lower the energy barrier between the ITO and the P3HT-PCBM layer.

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 ester (PCBM)

• High electron affinity
• Transports charge easily.
• Electron Donor

P3HT [Poly (3 – hexylthiophene)] Typical efficiencies of 5% (this is the HIGH end)

OMe O S S H Br n/2 PCBM Electron Acceptor P3HT Electron Donor

Optimum donor material for PCBM

Model predicts 20% PCE Ideal donor: LUMO=3.9 eV & HOMO =5.4 eV

Dennler & Brabec, Adv. Mater. 2009, 21, 1323

• Need to balance donor LUMO – acceptor

LUMO levels

• Maximize donor HOMO and acceptor

LUMO difference for high VOC

Solar power conversion efficiency (%) = Electrical output power Sun light power = Maximum { J (mA/cm2) x V (v)} 100 (mW/cm2) ( = 1 kW/m2, A.M 1.5)

J (mA/cm2) V ( v ) Voc Jsc

Maximum { J x V }

Voc Jsc Fill Factor (FF)

= Maximum { J x V }

Voc x Jsc

Current density J (mA/cm2) = Current (mA) / Active area (cm2)

by Solar Simulator

FF x Voc x Jsc Sun light power =

Characterization of OPVs

GaIn Eutectic Optical Adhesive (we used 5 minute epoxy) ITO coated glass

ITO glass with Polymer Layer

GaIn Eutectic Epoxy ring ITO coated glass

Anode: Clean and cut ITO coated glass and anneal at 1400C for 10 minutes. Then Spin coat with PEDOT/PSS (2000rpm for 30 seconds). Anneal at 1400C for 15 minutes. Spin coat with the active layer (P3HT-PCBM) and annealed all but four samples again at 1400C 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.

Finished device

Substrate (glass)

(PEDOT-PSS Conducting Layer )

Anode (ITO) Photon Absorbing Layer [PCBM-P3HT] Substrate [Glass] ITO Gallium Indium Eutectic (Cathode) Epoxy (sealant) 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!!

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.

Morphology

• Intrinsic

– 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

GaIn Eutectic Optical Adhesive (we used 5 minute epoxy) ITO coated glass

ITO glass with PEDOT-PSS and Active Layer

GaIn Eutectic Epoxy ring ITO coated glass

Anode: Clean and cut ITO coated glass. Spin coat with PEDOT/PSS (4000rpm for 30 seconds). Anneal at 1400C for 15 minutes. Spin coat with the active layer (P3HT-PCBM - 4000rpm for 60 seconds) and annealed again at 1400C 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!

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

• n 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
• f quarter.
• Content Post test and CURE post-survey +

supplemental survey – last week of quarter.