1. Chimica Fisica dei Materiali Avanzati Part 12 – Plastic electronics
Laurea specialistica in Scienza e Ingegneria dei Materiali
Curriculum Scienza dei Materiali
Chimica Fisica dei Materiali Avanzati Part 12 – Plastic electronics
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

2. Basic questions Is it possible to do electronics with molecules?
What sort of molecules to use? Carbon-based, similar to those used by biology, e.g. for photosynthesis
How will we manipulate and position molecules to create the architectures we want?
Transport molecules in solution (as biology does)
Assemble molecules in correct juxtaposition through use of ‘weak’ intermolecular interactions (e.g., hydrophobic vs. hydrophilic)
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

3. Plastic electronics
Plastics (or, more correctly, polymers), are traditionally used within the electronics industry as ‘passive’ materials, for encapsulation or for their electrically- insulating properties. However, there is now a class of polymers which can behave as semiconductors or as metals.
Our understanding of the semiconductor physics of these materials has enabled us to use them as the active components in a range of devices.
Polymer light-emitting diodes, LEDs, providing full color range and high efficiency as well as solar cells show particular promise.
The electronic behavior of these polymers is very different from inorganic semiconductors such as silicon or gallium arsenide.
Polymer electronic devices require different strategies to make them useful. In some respects, these strategies resemble those already adopted by biology, for example in photosynthesis.
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

4. Large electronic conductivities in organic materials
Charge transfer crystals
E.g. TTF-TCNQ, first metallic conductivity (1973)
Organic superconductors
E.g., (TMTSF)2PF6 (1980)
(BEDT-TTF)2X
Corso CFMA. LS-SIMat

5. Conducting Polymers 1977: First conducting polymer, Poly(acetylene)
Shirakawa, MacDiarmid, Heeger
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

6. Structures of some conjugated polymers
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

7. Electronic structure and charge carriers in conducting polymers
polaron
bipolaron
In conducting polymers, doping is the result of a redox process.
Charges are bound and deep in the gap
A polaron (= radical ion) has both charge (+e) and spin (±1/2)
A bipolaron (dication) has charge (+2e) but no spin
Polarons (A) and bipolarons (B) in PPP
Corso CFMA. LS-SIMat

8. Doping effect on the optical properties: electrochromism
Electrochemical doping of polypyrrole
Bipolaron absorptions (2)
polaron
bipolaron
Interband absorption (3 eV)
Polaron absorptions (3)
Corso CFMA. LS-SIMat

9. Current Uses of Conducting Polymers
Antistatic Coatings and Conducting Films
Electrochromic Displays?
Memory Devices? (HP Labs/Princeton)
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

10. Light Emitting Diodes 1990: Burroughs, Friend (Cambridge)
light emission from undoped semiconducting polymer
2003: full color range possible
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

11. OLEDs Everywhere 2000: first commercial products with OLEDs
Advantage in color spectrum beats solid state materials
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

12. Polymeric Photovoltaics
Solar cell efficiencies of ~ 2% (up to 6% in labs)
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

13. Thin Film Transistors 2004: both p and n-type materials are known
Critical Advances: Crystallinity and purity
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

14. Organic Semiconductors
Molecular Materials:
polycrystalline
vapor deposited
Polymeric Materials:
semi-crystalline
solution processed
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

15. Mobility of organic semiconductors
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

16. Motivations for organic electronics
Organic TFTs show poor performance compared to silicon CMOS
But organic TFTs also show the potential for extremely low cost production (printing)
Organic TFTs are in a stage of development as silicon MOSFETs were 30 years ago
Organic TFT electronics certainly will not replace CMOS
But organic TFT electronics may open new low cost / low performance (but high volume!) markets
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

17. Polymer electronics
Low-end, high volume electronic applications, based on:
Mechanical flexibility
Low-cost
Large area
Potential applications:
Electronic barcodes
Memories
Displays (e-paper)
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

18. Rubber Stamped, Large-Area Plastic Active Matrix Backplanes
10 µm Design Rules, Patterned by Single-Impression Microcontact Printing
PNAS 98(9), (2001).
Science 291, (2001).
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

19. E-paper
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

20. Key feature: solution processing
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

21. Materials and technology
Flexible, all-plastic field effect transistor
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

22. Technology
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

23. Operation of the polymer transistor
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

24. Light emitting diode
Organic light emitting diode consists of a thin film ( nm) of an emitting organic compound sandwiched between appropriate anode and cathode layers. A relatively modest voltage (typically Volts) applied across the material will cause it to emit light in a process called electroluminescence.
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

25. Steps of the electroluminescence process
Charge (electrons and holes) injection
Charge transport
Charge recombination and exciton formation
Exciton radiative relaxation
Friend, R.H.; Gymer, R.W.; Holmes, A.B.; Burroughes, J.H.; Marks, R.N.; Taliani, C.; Bradley, D.D.C.; Dos Santos, D.A.; Brédas, J.L.; Logdlund, M.; Salaneck, W.R. Nature, 1999, 397, 121.
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

26. Mechanism of electroluminescence in organic semiconductors
1. Charge (electrons and holes) injection
Negative polaron = radical anion
Positive polaron = radical cation
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

27. Mechanism of electroluminescence in organic semiconductors (cont’d)
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

28. Some common electroluminescent polymers: poly(phenylenevinylene)s (PPVs)
Murray, M.M.; Holmes, A.B. in “Semiconducting Polymers, Chemistry, Physics and Engineering” Hadziioannou G and van Hutten, P.F. Eds. Wiley-VCH 1999, pp1-32Murray, M.M.; Holmes, A.B. in “Semiconducting Polymers, Chemistry, Physics and Engineering” Hadziioannou G and van Hutten, P.F. Eds. Wiley-VCH 1999, pp1-32
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

29. Light emitting metal chelates
Mitschke, U.; Bauerle, P. J. Mater. Chem. 2000, 10, 1471
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

30. Electroluminescence efficiency
Adachi, C.; Baldo, M.A.; Thompson, M.E.; Forrest S.R. J. Appl. Phys. 2001, 90, 5048
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

31. PHOSPHORESCENT OLEDS (PHOLED)s
The internal quantum efficiency of the phosphorescent OLEDs can be in principle increased to 100%, because both singlet and triplet excitons can emit radiatively. OLEDs prepared with these heavy metal complexes are the most efficient OLEDs reported to date, with internal quantum efficiencies > 75% and external efficiencies > 20%.
Baldo, M.A.; O’Brien, D.F.; You, Y.; Shoutstikov, A.; Silbey, S.; Thompson, M.E.; Forrest, S.R. Nature, 1998, 395, 151
Baldo, M.A.; Lamansky, S.; Burrows, P.E.; Thompson, M.E.; Forrest, S.R. Appl. Phys. Lett., 1999, 75, 4
Zhang, Q.; Zhou, Q.; Cheng, Y.; Wang, L.; Ma, D.; Jing, X.; Wang, F. Adv. Mater., 2004, 16, 432
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

32. Working principle of polymer photovoltaic cells (OPV)
1. Absorption of incident light by the active layer
2. Generation of charge carriers
3. Collection of separated charge carriers at contacts
Separation of positive and negative charge carriers by an asymmetry (junction)
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

33. Large area printed devices
Active area of a single stripe: 10 cm2
Isc: > 10 mA/cm2 (under 100 mW/cm² simulated AM1.5)
Voc: ~ 0.6 V
FF: < 0.5 (limited by serial resistivity of the substrate)
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

34. Working principle of a bulk heterojunction
1. Incoming photons are absorbed
Creation of excitons on the Donor /Acceptor
2. Exciton is separated at the donor /acceptor interface
Creation of charge carriers
3. Charge carriers within drift distance reach electrodes
Creation of short circuit current ISC
1. The “photodoping” leads to splitting of Fermi levels
Creation of open circuit voltage VOC
2. Charge transport properties, module geometry
Fill factor FF
Pel,max = VOC x ISC x FF
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

35. Correlation between morphology and transport
Fullerene traps e-
e- and h+ are able to go through
h+ are blocked
[Fullerene] < 17% (no Percolation !) [Fullerene] > 17% [Fullerene] >> 17%
µh,bulk < µh polymer
µe,bulk ~ µe polymer
µh,bulk ~ µh polymer
µe,bulk < µe polymer
µh,bulk ~ µh polymer
µe,bulk > µe polymer
• Upon blending of materials, macroscopic transport properties of single components may change significantly
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

36. Integrated Circuits (IC) based on organics
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

37. Block diagram of an identification tag
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat

38. Design of organic identification tags
The 48 bit identification IC
Corso CFMA. LS-SIMat
Corso CFMA. LS-SIMat