Synthesis of Organic Semiconductors
Design criteria
- HOMO/LUMO levels and bandgap
-Controlled by type of conjugated system, electron donating/electron withdrawing groups
- Solid state packing/self-assembly
-Presence and position of substituents
- Solubility
-Introduction of substituents
- Volatility
- Ease of synthesis
HOMO/LUMO level control
- The HOMO increases in energy with increasing conjugation length.
- The LUMO decreases in energy with increasing conjugation length.
- The band gap (Eg) is decreases with increasing conjugation length.
- Polymer is more susceptible to electrophiles because of its higher HOMO. ie. more reactive.
Effect of electron donating and electron withdrawing substituents
Electron donating groups increase the energy levels.
Electron withdrawing groups decrease the energy levels.
Effect of polymer structure
Twists in the structure generally decrease the effective conjugation length and therefore increase the bandgap.
Substituents
Bulky substituents will increase solubility making the material easier to process.
However, in the solid state, bulky substituents will disrupt the packing of molecules/polymers therefore decreasing charge mobility through materials.
The substituent often has to be altered through trial and error to obtain material with the appropriate HOMO/LUMO levels, solubility, and optoelectronic performance.
P-type small molecule/oligomer synthesis
Examples of p-type molecules: Pentacene
Excellent TFT performance Best TFTs give > 5 cm2/(V s), ION/IOFF = 106
Insoluble: Devices fabricated by vacuum sublimation
Pentacene is oxygen and light sensitive
Efforts to solubilize pentacene: Silyl modified pentacene
Solution processed TFTs: > 5 cm2/(V s)
see Anthony 2001[1] see Park 2006 [2]
Soluble precursor approach
Combines best of both worlds by providing material that is soluble, but has good packing once solubilizing group is removed. OTFTs
= 0.1 cm2 / V⋄s
ION / IOFF = 2⋄105
Weidkamp, K. P.; Afzali, A.; Tromp, R. M.; and Hamers, R. J. J. Am. Chem. Soc., 2004, 126, 12740.
Afzali, A.; Dimitrakopoulos, C. D.; Breen, T. L. J. Am. Chem. Soc., 2002, 124, 8812.
Examples of p-type molecules: Oligothiophenes
Introduce substituents to * position to provide solubility
Dihexylsexithiophene Packing aided by liquid crystalline-like behavior of alkyl chains Sparingly soluble in �hot organic solvents
see Lovinger 1998[3]
Soluble precursor route
Precursor is highly soluble in organic solvents Heating burns off the solubilizing groups, anneals thiophenes into terraced structures
OTFTs: = 0.05 cm2 / V⋄s; ION / IOFF = 105 after thermal treatment
see Murphy 2004 [4]
N-type small molecule/oligomer synthesis
N-type materials
Most organic materials are p-type.
Two procedures are generally used to make a material n-type.
-Decrease LUMO level of material by introducing electron withdrawing groups eg. naphthalene derivatives
-Decrease LUMO level by introducing strain eg. C60 derivatives
Examples of n-type molecules: Aromatic bis-imides
One of the early organic n-FET successes.
Katz, H. E.; Lovinger, A. J.; Johnson, J.; Kloc, C.; Slegrist, T.; Li, W.; Lin, Y. Y.; Dodabalapur, A. Nature 2000, 404, 478
F. Würthner; V. Stepanenko; Z. Chen; C. R. Saha-Möller; N. Kocher; D. Stalke J. Org. Chem. 2004, 69, 7933.
Examples of n-type molecules: Fluorinated pentacene
Review of polymers
see Y. Sakamoto; T. Suzukil; M. Kobayashi; Y. Gao; Y. Fukai; Y. Inoue; F. Sato; S. Tokito J. Am. Chem. Soc., 2004, 126, 8138–8140.
P-type polymer synthesis
N-type polymer synthesis
Controlled polymer synthesis
- ↑ J. E. Anthony; J. S. Brooks; D. L. Eaton; S. R. Parkin; J. Am. Chem. Soc. 2001, 123, 9482-9483.
- ↑ S. J. Park; C. C. Kuo; J. E. Anthony; T. N. Jackson; Tech. Dig. − Int. Electron Devices Meet. 2006, 113.
- ↑ A. J. Lovinger; H. E. Katz; A. Dodabalapur Chem. Mater., 1998, 10, 3275.
- ↑ A. R. Murphy; J. M. J. Fréchet; P. Chang; J. Lee; V. Subramanian J. Am. Chem. Soc., 2004, 126, 1596.