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

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 http://www.sigmaaldrich.com/content/dam/sigma-aldrich/product9/140/msc_f0007.eps/_jcr_content/renditions/msc_f0007-medium.jpg http://www.pixuffle.net/wp-content/uploads/2014/01/computers2.jpeg

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

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

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

DCE

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

SVA with Different Solvents Device μave (cm2V-1s-1) VT – V0 (V) Dipole 𝛿P Pentane 0.376± 0.0487 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± 0.0834 -0.1 1.8 7.4 Pentane is the reference point

Quantifying Trends Pentane is the reference point

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± 0.0515 +6.9 1.64 6.3 Acetone 0.393± 0.106 +16.1 2.69 10.4 Acetone x=0.61 Pentane

SVA with Methanol DCE 60

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

Acknowledgements