Plastic Organic Electronics

Presented by : Dagmawi Belaineh

Shuvan Prashant TuragaAs part of PC5212 Physics of Nanostructures Coursework

Outline

Introduction Organic LEDs Organic Photovoltaics

State of the art Conclusion

The use of π-conjugated organic materials in the production of electronic devices

Light weight

Flexible

Low-cost production

http://www.youtube.com/watch?v=TDuP8PtDJbE&feature=related

Motivation : Why Organic ?

XNobel Chemistry 2000 goes to

For their work on conductive polymers…

Electronic structure

ethylene allyl butadiene pentadienyl hexatriene benzene

Electronic structure

Van der Waals forces instead of covalent bonding between polymers• narrow bands with low delocalization

Conjugated molecules tend to change their geometry upon charging• electron-phonon coupling

Electron-phonon coupling leading to a deformation of the lattice structure of the semiconductor

•Lattice•Electron•Electron-phonon couplingHamiltonian

composed of components

for

Charge transport Energy

Sir Richard FriendTan Chin Tuan Centennial Professor

Polymer Light-Emitting Diodes

http://www.lti.unikarlsruhe.de/rd_download/polymer2004/Plastic_Electronic_WS0405_7.pdf

PLED BASICS

Heterostructures

Light emission process

rst is the fraction of singlet excitons, q is the efficiency of radiative decay of these singlet excitons

Understanding Efficiency

circuit external in the flowing electrons # device he within tevents formation exciton #

Organic Photovoltaics

Source : Konarka C. W. Tang, Appl. Phys. Lett. 1985, 48, 183

Major Milestone : Tang reported single heterojunction device OPV in 1985 with a power efficiency of 1%

High optical absorption coefficients of organic molecules thinner solar cells

Photon Absorption ~1fs ηa

Exciton Formation

~100fs to 1ns

Exciton Diffusion ~ 1ns ηED

Exciton Breakup ~ 100 fs

Charge transport ηCT

Charge Collection ~1 us

ηCC

OPVs : Device Physics

ANOD E

CATHODE

-

+

LUMO

HOMO

Light

External Quantum Efficiency(EQE) = ηa ηED ηEB ηCT ηCC

DONOR ACCEPTOR

CharacterizationIV Characteristicsη = maximum deliverable electrical power(VM ×JM) to the incident light power(Pinc)FF = Fill FactorVOC = Open Circuit Voltage JSC= Short Circuit Current

Materials,

• Printing – Screen Printing – Stamping

• Spraying• Spin Coating• Vaporization

Acceptors

PCPDTBT, MEH-PPV,

P3HT, PTB1

Donors

PCDTBT, PCBM, PSBTBT

Electrode

Al, Ca, Ag

Transparent Electrode

PEDOT:PSS, ITO

Krebs F. C., Solar Energy Materials & Solar Cells 93 (2009) 394–412

Processes

Organic Solar Cell ArchitecturesPlanar HeterojunctionTypically exciton diffusion length = 10 nm Layer width limitedBut atleast should be 100 nm to absorb light

completely

GlassTransparent Electrode

Donor LayerAcceptor LayerMetal Electrode Bulk Heterojunction

Ordered Heterojunction

Now, actual Solar Cells can be complex…

Stacking improves efficiency

Adding more layers for hole injection and electron injection adds to the cells efficiency

Tandem Cell is better

Tandem CellJsc = 7.8 mA/cm2, Voc = 1.24 V, FF =

0.67, and ηe = 6.5%

www.sciencemag.org SCIENCE VOL 317 13 JULY 2007 pp. 223-225

Progress in Solar Cell Research

5.4%

NREL Database

State of the art : • Spin coating

– Can coat large areas with high speed

– Redissolution– Patterning not possible as

• Vapour Evaporation – Layer by layer without

chemical interaction in vacuum

– Non uniform, requires UHV, expensive, contamination, LOS

• Vapour Deposition– Better control through gas

flow rate and temperature– Uniformity and not

contaminated

State of the art : Photocrosslinking

Photolysis of FPA gives singlet nitrene which inserts into C-H bonds of polymers to form crosslinksEffective crosslinker conc is low !!

FPA Methodology

Roll 2 Roll Manufacturing

DUV

Solvent Wash

Spin Coating/ Ink Jet Printing

DUV

Solvent Wash

Key Advantage : Continuity for e and h conduction paths is guaranteed.

Contiguous Interpenetrating Structure for PVs

Using Photocrosslinking for PVs

A factor of 4.5 improvement is achieved internal recombination bottleneck overcome

Photocrosslinking possible without significant loss of device properties

4.5% external photons per injected electron

Similar life time

Similar results for other types of PPV

Can we make heterostructures?

PLEDs using PHOTOCROSSLINKING

ITOTFB the hole-transporting and

electron-blocking interlayer

F8BT the electron-transport and light-emitting layer

Can we make heterostructures?

For solution-processed polymer OSCs, however, this is a considerable challenge because of redissolution, and the difficulty of fixing a p-i-n profile -> photocrosslinking!!!

For molecular organic semiconductors (OSCs), doping is readily achieved by coevaporation of the dopant with the transport layer (eg. by CVD)

Observation of p-doping with sodium naphthalenide (Na+Np−) from transmission spectra: the band at 2.7 eV bleaches while a sub-gap polaron transition emerges at 1.8 eV

Observation of n-doping with nitronium hexafluoroantimonate (NO2

+SbF6−)from transmission

spectra: the band bleaches while a different subgap polaron transition emerges at 2.1 eV

DOPING

Sivaramakrishnan et al, APL 2009

Efficient bipolar injection Greatly improved external electroluminescence efficiency compared to control devices without the p-i-n structure

photocrosslinking of the first layer

bulk p-doping by diffusion of solution-state dopant,

deposition of an intrinsic polymer layer

solid-state surface n-doping to limit the doping depth in this layer

Methodology

Organic PV Road Ahead…

Presented at Large Area, Organic & Printed Electronics Convention, 2010

Bright Future of OLEDs

Source : http://www.oled-display.net

Challenges

Material Level• OPV: Low bandgap polymers with absorption edge at 1eV, large absorption

coefficients, higher charge mobilities• OLED: Low work function cathodes with higher stability• Minimise Energy Loss at junctions

Process Level • To be able to create nanostructures with appropriate domain size and dimensions

Device Level• Increase power conversion efficiency• Increase stability• Reduce costs • Develop a technology for large scale production and reproducibility• Ruggedness FlexibilityTransparent

Summary

• Organic Electronics has tremendous potential of replacing the present rigid electronics.

• However, there are issues of cost-efficiency tradeoff which need to be dealt with.

• Competitive research among the companies is empowering the progress of organics.

• Dynamic and highly interdisciplinary field: physicists, chemist, material scientists, electrical engineers … truly Nano!

Sources

1. http://parsleyspics.blogspot.com/2011/02/stop-glenn-beck-new-petition-letter.html2. http://courses.chem.psu.edu/chem210/mol-gallery/pi-systems/pisystems.html3. http://www.blazedisplay.com/LCD_Knowledge.asp?Id=484. J.H. van Lienden, Charge transport in trans-polyacetylene, Thesis, Rijksuniversiteit Groningen (2006)5. K. Walzer, B. Maennig, M. Pfeiffer, and K. Leo, Highly Efficient Organic Devices Based on Electrically

Doped Transport6. Layers, Chem. Rev. 107, 1233-1271 (2007)7. M. Pfeiffer, S.R. Forrest, K. Leo, M.E. Thompson, Electrophloroscent p-i-n OLEDS for very high efficiency

flat-panel displays, Adv. Mat. , 14, 22, (2002)8. www.wikipedia.org