Synthesis of Organic Semiconductors

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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

Pentacene.png

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
Silyl pentacene2.png

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

see Anthony 2001[1]

see Park 2006 [2]

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

ION / IOFF = 2 x 105


see Weidkamp 2004 [3] see Afzali 2002 [4]

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[5]

Soluble precursor route

Soluble precursor oligo.png
  • 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; ION / IOFF = 105 after thermal treatment

see Murphy 2004 [6]

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

C60


Examples of n-type molecules

Aromatic bis-imides

Aromaticbisamide.png

One of the early organic n-FET successes.

see Katz 2000 [7]

Aromaticbisamide2.png

see Würthner 2004 [8]

Fluorinated pentacene

Fluorinated pentacene synthesis

μ = 0.22 cm2/Vs and Ion/Ioff =105

see Sakamoto 2004 [9].


C60 derivatives

PCBM

Phenyl C60 Butyric Acid Methyl Ester (PCBM)

ThCBM

Thienyl CBM (ThCBM)


See Lacramioara 2006 [10]

Single precursor p & n-type material

Precursor p&n.png

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

P-type OTFT μ = 2 × 10-4 cm2/Vs and Ion/Ioff =104

see Yoon 2007 [11]

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

Polymer mw.png

The Number average molecular weight:

M_n = \frac {\sum_i N_i M_i} {\sum_i N_i}\,\!

The Weight average molecular weight:

M_w = \frac {\sum_i N_i M_i^2} {\sum_i N_i M_i}\,\!

The Polydispersity Index:

PDI = \frac {M_w} {M_n}\,\!


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:

Semeconduct backbone.png

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

Conjugated polymer.png

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

Conjugated polymer common.png

P-type polymer synthesis

Polythiophenes

Polythiophenes history.png

Historical progression of polythiophenes

For comprehensive review on polythiophenes: [12]


Initially, conjugated polymers were synthesized by oxidative coupling reactions.

Polythiophene coupling.png

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

Polythiophene coupling defect.png


Dehalogenation routes were also attempted.

Polythiophene dehalogenation.png


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.

Regiorandom polymer.png


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.

Regioregular.png
Regioregular2.png

see McCullough method: [13]

see Rieke method: [14]


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

P3ht unit.png

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

P3ht lamellar.png

μ = 0.15 cm2 / V⋅s

ION/IOFF = 107

Stable in air for >30d


see Ong 2004 [15]

Fused ring polythiophenes

Fusedring.png

see Heeney 2005 [16]


More p-type polymers: Polyphenylenevinylenes (PPV)

Ppv.png

Synthesized via soluble precursor route

see Wessling 1985 [17]

see Wessling 1972
Wessling, 1968 (?!), US Patent 3,401,152 and 1972, US Patent 3,706,677

Soluble PPV

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

a) 3-(bromomethyl)heptane, KOH, C2H5OH, reflux

b) formaldehyde, conc. HCl, dioxane

c) KOC(CH3)3, THF

See Wudl 1991 [18]

More p-type polymers: Polyfluorenes

Polyfluorene.jpg

Polyfluorene: Originally synthesized in 1989 see Fukuda 1989 [19]

Polyfluorene all.jpg


see [20]

Polyfluorenes: obtained blue polymer for LEDs

Polyfluorene synth.jpg

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.

Polyfluorene spec.jpg

see JACS [21]

Polyfluorene synth2.jpg

see List and Scherf [22]

Polyfluorene synth3.jpg

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

Polyfluorene synth4.jpg

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

Polyfluorene spec2.jpg

see Chan and Holmes [23]

N-type polymer synthesis

N-type polymers

This is 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

Ladder.png

see Usta 2008[24]

see Babel and Jenekhe 2003 [25]

Napthalene based polymers

μ = 0.01 cm2/VS

see JACS [26]

Ambipolar polymers

Ambipolar.png

μh = 10-3 cm2/(V s) μe = 10-2 cm2/(V s)


see Kim and Jenekhe 2009 [27]

Controlled polymer synthesis

Metal catalyzed cross-coupling polymerizations

Crosscoupling.png

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

P3HT synthesis

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


But McCullough and Yokozawa found that the resulting polymer had a narrow PDI and controlled molecular weight, which means that it is likely to be a chain-growth polymerization.

P3HT synthesis

dppp>dppe>dppf>PPh3

Polymerization is heavily ligand dependent and works best if dppp is used as the ligand.

  • Narrow PDI
  • MW proportional to Ni loading

Associated pair

Key step: Ni(0) only adds into the same growing polymer chain resulting in chain-growth polymerization

see McCullough [28]

see Miyakoshi [29]

Externally initiated P3HT synthesis

P3ht synth ext.png
P3ht synth ext2.png


TM = transmetallation RE = reductive elimination OA = oxidative addition

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

see Doubina and Luscombe 2009 [30]




P3ht synth ext ligand.png

see van Leeuwen 2000 [31]

Adapting to ligands other than PPh3

P3ht synth regioregular.png

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.

see Bronstein and Luscombe 2009 [32]

References

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  13. J. Chem. Soc. Chem. Commun. 1992, 70-72
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