1. Prof. Dr. Gerhard Gobsch
Optical, Electronic and Structural Properties of Semiconductor Nanostructures and Optoelectronic Devices
Inorganic Semi-conductors & Devices
(Compounds of III-V, I-III-VI2, II-IV-V2)
Organic Semi-conductors & Devices
(Polymers & Functional Polymers)
Solar Components & Systems
(Photovoltaic
und Solar Thermal)
Department of Experimental Physics I

2. Organic Optoelectronics
An Introduction
Materials, Processing, Concepts and Devices
ACOPhys, St. Petersburg, Sept 2006

3. Molecular Energy Levels and Charges Materials and Processing Devices
Organic Optoelectronics - Outline
Introduction
Molecular Energy Levels and Charges
Materials and Processing
Devices
Summary

4. Organic Optoelectronics
Organic Optoelectronics deals with semiconductor devices in which the semiconductor is an organic material.
There are light emitting diodes (OLED), thin film field effect transistors (OFET), solar cells, lasers, detectors, sensors...
Beyond the scope of the present lecture is the field of “Organic Electronics ↔ Polymer Electronics“.
Theoretical basics of electronic properties of organic materials were given in the lecture of Prof. Runge.

5. Organic Optoelectronics
                     
The Nobel Prize in Chemistry 2000
"for the discovery and development of conductive polymers"
                               
Alan J. Heeger
Alan G. MacDiarmid
Hideki Shirakawa
    1/3 of the prize
USA
USA and New Zealand
Japan
University of California Santa Barbara, CA, USA
University of Pennsylvania Philadelphia, PA, USA
University of Tsukuba Tokyo, Japan
b. 1936
b (in Masterton, New Zealand)
“for the discovery and developement of conductive polymers“
A new material class!

6. “In creating and expanding the 4th generation of polymers, we attempted to understand nature with sufficient depth that we could achieve materials with novel and unique properties, that are not otherwise available.
This was (and is) an elegant and somewhat dangerous exercise; elegant because it requires the synthesis of knowledge from chemistry, physics and materials science, and dangerous because when working on the boundary of three disciplines, one is always pushing beyond the knowledge and experience of this background.
To our research in this interdisciplinary field has had sufficient impact on chemistry to be recognized by the Nobel Prize gives us, therefore, particular satisfaction.”
Alan J. Heeger (on occasion of giving him the Nobel Prize for Chemistry in 2000)

7. Pros and Cons of polymers for electronic applications
Advantages
Metallic and semiconducting properties by doping
Combination of plastic with electronic properties
Property engineering
Solubility in organic solvents, variable processibility
Use of printing technologies
No vacuum and no high temperature processes
 Low-cost production
Disadvantages
Low integrated devices and circuits (in the very near future)
Degeneration in O2- and H2O-atmosphere
 Long-term stability is still a critical point

8. Application of Functional Polymers
Organic field-effect transistor (OFET)
Polymerelektronik
made by semiconducting and dielectric polymers
Integrated polymerelectronic circuits (IPC)
Polymer actors
for microsystem technology
Organic solar cells
Photovoltaic & optoelectronic devices made by polymer photoconductors
Polymer batteries
Functional polymers
with special / selected electronic properties
Organic light emitting diodes (OLED) and lasers
Fuel cells
Super capacities
Polymer sensors
Optical polymers with optimized spectral transparency
Special elektronic polymers for
Planar antennas
Temperature sensors
Humidity sensors
Chemo- & biosensors
Pressure / force sensors

9. Organic Semiconductors
More conjugated
polymers...

10. Conductivity of materials

11. Charge carrier mobilities
comparably small (FET) mobilities*:
low mobilities & large absorption coefficients  thin absorber
*C. D. Dimitrakopoulos and D. J. Mascaro IBM J. Res. & Dev. 45 (1), 2001

12. Charge carrier mobilities of thin films

13. Absorption overlap with solar spectrum
comparably small absorption range:
 only a small fraction of the sunlight is used today!
Silicon

14. Solar Spectrum and Absorption

15. Molecular Energy Levels and Charges Materials and Processing Devices
Organic Optoelectronics
Introduction
Molecular Energy Levels and Charges
Materials and Processing
Devices
Summary

16. -bands in conjugated polymers

17. Molecular Energy Levels
Optical properties and transitions:
Absorption Emission
LUMO
HOMO
absorption
fluorescence
phosphorescence
luminescence (photo-, or electro-)

18. Molecular Energy Levels
For oligomers a shift of HOMO or LUMO levels towards smaller band-gaps is observed upon increasing the repeat unit. Compare with quantum-mechanical particle-in-a-box problem.
longer chain/wider box
 decrease in „band-gap“

19. Organic Semiconductors: Charges
Charge carriers on Polyacetylene: H H H
Negative
Soliton: C C C C H H H H
(degenerate
groundstate) H H H
Positive
Soliton: C H H H H

20. Charge transport in conjugated polymers
Polymer chain with characteristic defects (a);
schematic energy diagram for positive polarons without electric field (b) and
charge transport under applied electric field (c).

21. Electron transfer in a conjugated polymer after optical excitation calculated within the td-DFT by means of the GAUSSIAN quantum chemistry package

22. Charge carrier mobilities
comparably small (FET) mobilities*:
low mobilities & large absorption coefficients  thin absorber
*C. D. Dimitrakopoulos and D. J. Mascaro IBM J. Res. & Dev. 45 (1), 2001

23. Molecular Energy Levels and Charges Materials and Processing Devices
Organic Optoelectronics
Introduction
Molecular Energy Levels and Charges
Materials and Processing
Devices
Summary

24. Organic Semiconductors: Processing
Solution processing Evaporation
(polymers): (small molecules):

25. Molecular Materials: Pigments & Fullerenes
„p-type“: e.g. phthalocyanines
ZnPc
„n-type“: perylenes & fullerenes
used for evaporation
Me-Ptcdi
C60

26. Conjugated Polymers/ substituted Fullerenes
Solution processing:
„p-type“ conjugated polymers:
MDMO-PPV P3HT PFB
„n-type“ conjugated polymers/fullerenes:
Spin coating
CN-MEH-PPV PCBM F8BT
Doctor blading

27. Molecular Energy Levels and Charges Materials and Processing Devices
Organic Optoelectronics
Introduction
Molecular Energy Levels and Charges
Materials and Processing
Devices
Summary

28. MIM picture of device function
Very thin (low mobility) absorbers of ~ 50 – 300 nm  MIM
a.) short circuit condition: solar cell
b.) open circuit condition (VOC): current = 0
c.) reverse bias: photodiode
d.) forward bias: light emitting diode
(a) (b)
(c) (d)
ITO Al
+ –
– +
VOC
Strong (up to 105 V/cm) internal electric fields drive charge transport

29. Organic Semiconductor Devices
Field effect transistor: bottom gate (usually Si/SiO2)

30. Organic Semiconductor Devices: OFET
Field effect transistor: top gate (polymeric insulator)
W. Fix, A. Ullmann, J. Ficker, and W. Clemens, Applied Physics Letters 81, 9, p (2002)

31. Organic Semiconductor Devices: OFET
Influence of molecular order on charge carrier mobility:
*C. D. Dimitrakopoulos and D. J. Mascaro IBM J. Res. & Dev. 45 (1), 2001

32. Organic Field Effect Transistors: RF ID tags
OFET: Applications and Products
Organic Field Effect Transistors: RF ID tags

33. Organic Semiconductor Devices: OLED
Organic light emitting diode (OLED):
single layer device
1.) charge injection
2.) charge transport
3.) charge recombination
 exciton formation
4.) light emission

34. Organic Semiconductor Devices: OLED
OLED display structure:
electrode bars one pixel = three devices

35. Organic Semiconductor Devices: OLED
Comparison: advantages of OLED versus LCD (STN/TFT)
full color (24 bit)
high contrast (3000:1)
wide viewing angle (170°C)
lower power consumption
faster response time
less complicated architecture
( low cost, thinner device)

36. Organic Light Emitting Displays:
OLED: Applications and Products
Organic Light Emitting Displays:
(2002, polymer)
(2005, molecule)

37. Organic Light Emitting Displays:
OLED: Applications and Products
Organic Light Emitting Displays:
(2003)
(2004)

38. Wide viewing angle! Organic Light Emitting Displays:
OLED: Applications and Products
Organic Light Emitting Displays:
                                                         
Wide viewing angle!

39. Organic Semiconductor Devices: OSC
Organic Solar Cell: device structure (“plastic solar cell“):
Aluminum
Evaporation
Spin coating,
doctor blading,
printing
Etching, laser-etching
LiF (6 Å)
Active layer
ITO
PEDOT:PSS
Glass
active layer: conjugated polymer / fullerene blend
selective contacts:
electrons: Al/LiF
holes: ITO/ PEDOT:PSS
(poly[3,4-(ethylenedioxy) thiophene] : poly(styrene sulfonate))
or flexible
plastic
substrates

40. Organic Semiconductor Devices: OSC
Photoinduced Charge Transfer, dissociates the exciton: E 
Evac
LUMO
Charge separated state!
LUMO
HOMO
HOMO
PPV C60
*N. S. Sariciftci, L. Smilowitz, A. J. Heeger, F. Wudl, Science 258, 1474 (1992)

41. Organic Semiconductor Devices: OSC
The power conversion efficiency h is a function of:
State of the art:
h ~ 5%
Where:
1.) VOC is the “open circuit voltage“
2.) ISC is the “short circuit current“
3.) FF is the “fill factor“

42. Organic (plastic) solar cell

43. OSC: Applications and Products
Organic Solar Cells
                               

44. OSC: Sensor applications
Photodiode (arrays)  sensors
Combination of OLED illumination and photodiode array detection for personal identification.

45. Organic Semiconductor Devices: Laser
Organic Laser (optically pumped):
From left to right: Organic laser emitting red light; structure of the first organic laser using external optical excitation; typical laser emission spectra spanning the visible from the blue at a wavelength of 450nm, to the infrared at 700nm.

46. Molecular Energy Levels and Charges Materials and Processing Devices
Organic Optoelectronics
Introduction
Molecular Energy Levels and Charges
Materials and Processing
Devices
Summary

47. Organic Optoelectronics
Summary
Why “ORGANIC“? Disadvantages:
light weight
flexible
low cost
large area
“tailor-made“ properties
more colors
printing production
enviromental instability - requires encapsulation
lower performance
(charge carrier mobility)
The challenge of today!