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 conjugation length determines homo lumo levels.

• 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

Silyl modified pentacene

Solution processed TFTs: > 5 cm2/(V s)

see Anthony 2001

see Park 2006

Soluble precursor approach

Soluble precursor approach

Combines best of both worlds by providing material that is soluble, but has good packing once solubilizing group is removed.

OTFTs

μ = 0.1 cm2 / V⋄s

I_ON / I_OFF = 2 x 10⁵

see Weidkamp 2004 see Afzali 2002

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

Soluble precursor route

• Precursor is highly soluble in organic solvents
• Heating burns off the solubilizing groups, anneals thiophenes into terraced structures

OTFTs: μ= 0.05 cm2 / V⋄s; I_ON / I_OFF = 10⁵ after thermal treatment

see Murphy 2004

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

naphthalene derivatives

-Decrease LUMO level by introducing strain eg. C60 derivatives

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C60

Examples of n-type molecules

Aromatic bis-imides

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One of the early organic n-FET successes.

see Katz 2000

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see Würthner 2004

Fluorinated pentacene

Fluorinated pentacene synthesis

μ = 0.22 cm²/Vs and I_on/I_off =10⁵

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.

C60 derivatives

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PCBM

Phenyl C60 Butyric Acid Methyl Ester (PCBM)

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ThCBM

Thienyl CBM (ThCBM)

See Lacramioara 2006

Single precursor p & n-type material

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N-type OTFT μ = 0.08 cm²/Vs and Ion/I_off =10⁶

P-type OTFT μ = 2 × 10^-4 cm2/Vs and I_on/I_off =10⁴

see Yoon 2007

Review of polymers

Step growth vs. Chain growth polymerizations

Step growth

Step growth

Broad molecular weights

• Molecular weight is heavily dependent on the purity of the monomer
• Leads to batch-to-batch variability
• Optoelectronic properties vary which means fluctuating electronic device performance

Chain growth

Chain growth

• One monomer at a time adds to the growing polymer chain.
• Under certain conditions, the polymerization can be controlled to produce specific molecular weights with narrow polydispersities (living polymerization)

Molecular weights of polymers

The Number average molecular weight:

<math>M_n = \frac {\sum_i N_i M_i} {\sum_i N_i}\,\!</math>

The Weight average molecular weight:

<math>M_w = \frac {\sum_i N_i M_i^2} {\sum_i N_i M_i}\,\!</math>

The Polydispersity Index:

<math>PDI = \frac {M_w} {M_n}\,\!</math>

Small molecule vs. Polymer semiconductors

• Small molecules have well-defined molecular weights which lends itself better to provide crystalline packing.

• Polymers generally contain amorphous domains which reduces charge transport.

• Polymers are more amenable to room temperature solution processing. Although both small molecules and polymers can be solubilized, polymers tend to make smoother, more continuous films.

Semiconducting polymers

Semiconductivity in polymers can be achieved in two ways:

1) By having pendant small molecule semiconductors attached onto an insulating polymer backbone.

2) By having a conjugated polymer.

• Polymers with pendant groups tend to show poorer charge mobility because it is difficult to organize the polymer such that the pendant groups stack well.

• But, easier to perform a controlled polymer synthesis on polymers with pendant groups using, for example, ATRP, ROMP, and NMRP.

Common conjugated polymers

P-type polymer synthesis

Polythiophenes

For comprehensive review on polythiophenes: R. D. McCullough, Adv. Mater. 1998, 10, 93

Historical progression of polythiophenes

Initially, conjugated polymers were synthesized by oxidative coupling reactions.

But oxidative coupling can lead to defects. Eg. instead of the required 2,2 coupling, 2,3 coupling can also take place.

Dehalogenation routes were also attempted.

Better than oxidative coupling because 2,3 coupling can be avoided.

However, both routes still suffer when solubilizing groups are added.

Regiorandom polymers end up being synthesized when a regioregular HT-HT polymer is desirable.

By differentiating the two ends of the substituted thiophene, which can be done cleanly, it is possible to do a cross coupling reaction and thereby synthesize truly regioregular polyalkylthiophenes.

McCullough method: J. Chem. Soc. Chem. Commun. 1992, 70-72 Rieke method: J. Am. Chem. Soc. 1992, 114, 10087

Improving on regioregular poly(3-hexylthiophene) (P3HT)

Poor TFT performance when devices are fabricated in air. (IOFF high due to O2 doping)

B. S. Ong; Y. Wu.; P. Liu; S. Gardner, J. Am. Chem. Soc., 2004, 126, 3378.

Fused ring polythiophenes

M. Heeney; C. Bailey; K. Genevicius; M. Shkunov; D. Sparrowe; S. Tierney; I. McCulloch J. Am. Chem. Soc., 2005, 127, 1078-1079.

Synthesized via soluble precursor route Wessling, J. Polym. Sci. Polym. Symp. 1985, 72, 55 Wessling, 1968 (?!), US Patent 3,401,152 and 1972, US Patent 3,706,677

Soluble PPV

3-(bromomethyl)heptane, KOH, C2H5OH, reflux formaldehyde, conc. HCl, dioxane KOC(CH3)3, THF

Wudl et al. ACS Symp. Ser, 1991, 455; US Patent 1990, 5,189,136

More p-type polymers: Polyfluorenes

Polyfluorene: Originally synthesized in 1989 Fukuda et al. Jpn. J. Appl. Phys. 28, L1433, 1989

Adv. Funct. Mater. 15, 981, 2005

Polyfluorenes: obtained blue polymer for LEDs

Originally, the emission at approx. 550 nm was thought to be a results of aggregation. Bulky substituents were added to polyfluorene to reduce green emission and create “blue” polymer.

J. Am. Chem. Soc., 123, 6965, 2001

List and Scherf et al. Adv. Mater. 14, 374, 2002

But it was realized that the red-shifted emission was due to keto defects within the polymer.

Polymer design was altered so that there would be a silicon bridge rather than a carbon bridge to prevent keto defects from forming.

Chan and Holmes et al. J. Am. Chem. Soc. 127, 7662, 2005.

N-type polymer synthesis

N-type polymers

Rare but growing area of research. Highly ordered Lamellar packing μ = 0.10−0.16 cm2/(V s), Ion/Ioff = 107 Devices stable in air for >5 months

H. Usta; A. Facchetti; T. J. Marks J. Am. Chem. Soc., 2008, 130, 8580.

=== A. Babel and S. A. Jenekhe J. Am. Chem. Soc., 2003, 125, 13656. Napthalene based polymers ===

JACS, 2009, 8-9.

Ambipolar polymers

F. S. Kim and S. A. Jenekhe et al. Adv. Mater., Vol. 21, P. 1-5, 2009

Controlled polymer synthesis

Metal catalyzed cross-coupling polymerizations

The majority of conjugated polymers are synthesized via metal catalyzed cross-coupling reactions eg. Ni mediated reactions is shown below.

P3HT synthesis

P3HT synthesis was originally thought to occur via a step-growth polymerization.

When Ni(0) reductively eliminates, it can in theory reinsert into any Ar-Br bond. If this were to occur, this would be a step-growth polymerization

Externally initiated P3HT synthesis

TM = transmetallation RE = reductive elimination OA = oxidative addition

Restricted to only being able to use PPh3 as a ligand. dppp gives H/Br polymer.

Doubina and Luscombe, Macromolecules, 2009, 42, 7670

adapting to ligands other than PPh3 A novel method for the external initiated polymerizations of P3HT has developed by CMDITR researchers. The method produces a polymer with a well-defined molecular weight, narrow polydispersity index (PDI), 100% initiation efficiency, 100% regioregularity. This work represents the most control achieved for the synthesis of P3HT.

H. Bronstein, C. K. Luscombe, J. Am. Chem. Soc., 2009, 131, 12894