1. Organic Polymer and Electronics Laboratory – Professor Lynn Loo
Tuning the Threshold Voltage in Organic Field-Effect Transistors by Solvent-Vapor Annealing
Abhiram Karuppur
Organic Polymer and Electronics Laboratory – Professor Lynn Loo
09/30/2016

2. Background Transistors are used in everyday electronics
Computers, TVs, Phones
Are analogous to an electrical “switch”
Can control the current amplitude by altering the gate-source voltage

3. Organic field-effect transistors
SiO2 (300 nm) Au
Organic Semiconductor
Si (gate)
Top-down view of
BGTC device L W
Bottom gate, top contact
Field-effect transistors operate through the creation and elimination of charge carriers at the dielectric/active layer interface
Charge carriers travel in the channel (active layer between source and drain electrodes).
Gate voltage is used to alter the conductance of the active channel region.
Lee, Stephanie. (2012). Ph.D. Thesis. Princeton University, USA

4. Goals
How can we control the threshold voltage without affecting other device parameters?
Determine if threshold voltage is correlated with solvent properties
Dipole Moment
Hansen’s Polarity Index
Can threshold voltage be predicted with different conditions:
Mixtures of solvents
Can the threshold voltage be changed?
2-step SVA with Methanol

5. Overview Deposited TES-ADT thin films Annealing Method:
2 wt% in toluene
Spin at 1000 rpm for 60 seconds
Place on hot plate at 100C for 2 minutes
Annealing Method:
Initially placed substrates in sealed chamber with reservoir of different solvents, remove when crystallization complete
Better Method: Vent chamber every 15 seconds
crystallized film
Expose to
1,2-dichloroethane vapor
250 μm
as-spun film
Spin-coat from 2 wt% solution in toluene
250 μm
linearly-fused heteroaromatic molecules  key components in high-performance organic semiconductors
ex. ADTs outperform pentacene due to high mobilities & barrier to oxidation
Triethylsilylethynyl anthradithiophene (TES ADT), though a molecular semiconductor, does not form well-defined crystalline grains in the conventional sense. Its bulky triethylsilylethynyl substituents allow the compound to be readily dissolved in common solvents, making it solution-processable. The presence of these substituents also necessarily weakens the π–π interactions between neighboring anthradithiophene cores; spin-coating results in thin films that exhibit limited order but can crystallize to form macroscopic spherulites on subsequent exposure to solvent vapor. These spherulites continue to grow radially outward in the direction of π-stacking until neighboring spherulites impinge. The interspherulite boundaries (ISBs) are thus qualitatively different from those found in conventional molecular-semiconductor thin films or those found in semicrystalline polymeric semiconductor thin films and provide a unique test case for examining charge transport.
based on Dickey, K. C.; Anthony, J. E.; Loo, Y.-L. Adv. Mater. 2006, 18, 1721

6. DCE

7. SVA with Different Solvents
DCE
DCM
Pentane
Propyl Acetate
Toluene
Cyclohexane
Acetone

8. SVA with Different Solvents
Device
μave (cm2V-1s-1)
VT – V0 (V)
Dipole 𝛿P
Pentane
0.376±
Cyclohexane
0.471± 0.115
+1.3
Toluene
0.513± 0.204
+6.2
0.31
1.4
DCM
0.489± 0.306
+5.3
1.6
6.3
Propyl Acetate
0.489± 0.131
+19.3
1.78
4.3
Acetone
0.608± 0.124
+19.6
2.69
10.4
DCE
0.419±
-0.1
1.8
7.4
Pentane is the reference point

9. Quantifying Trends
Pentane is the reference point

10. Re-testing SVA with Different Solvents
Device
μave (cm2V-1s-1)
VT – V0 (V)
Dipole 𝛿P
Pentane
0.675± 0.112
61:39 Acetone:Pentane
0.261±
+6.9
1.64
6.3
Acetone
0.393± 0.106
+16.1
2.69
10.4
Acetone
x=0.61
Pentane

11. SVA with Methanol
DCE 60

12. Conclusions Different Solvents affect Threshold Voltage
Positive correlation between polarity/dipole moment of the solvent and the threshold voltage
Different solvents result in different spherulite shapes
The kinetics of spherulite formation are different with different solvents
Threshold Voltage Can be Modified Ex-situ
VT can be shifted after treatment with methanol
Preliminary results suggest this is reversible
Exposing these films to nonpolar solvents results in a negative VT shift

13. Acknowledgements