General Information

OFET Fabrication Routine

Field effect mobilities in excess of 0.12 cm2/Vs are recorded using M104 when the active layer is dispensed on OTS-treated silicon oxide dielectric by static spin coating from an optimized high/low boiling point solvent mix.

High hole mobility in conjunction with good solubility and partial air stability make regioregular P3HT a reference material of choice for both fundamental and applied research in organic electronic, physics and chemistry. As one of the most well-studied organic semiconductor, P3HT is often acknowledge to be one of the benchmark against which any new p-type or donor conjugate molecule should be compared and evaluated.

Mobility has previously been found to be positively correlated with increasing region-regularity, slow drying time (achieved using high boiling point solvent), lowering of the surface energy, and molecular weight in excess of 50 kD. These conditions favour p-p stacking parallels to the OFET substrate, which in turn results in improved charge transport across the transistor channel [1-13].

The active layer solution preparation, spin coating, substrate annealing and measurements are performed in a glove box under a nitrogen atmosphere (H2O <0.1 PPM; O2 < 5/8 PPM).

For generic details on the fabrication of OPV devices, please see our written guide and video demonstration.

Active Layer Preparation

High-Regioregular and high molecular weight RR-P3HT (M104) (RR = 96.3%, Mw = 77,500, Mn = 38,700) is dissolved in a mix of high and low boiling point solvent in order to exploit the beneficial effect of long drying time and increase the wettability of low energy surface, respectively.

• 5 mg/ml of M104 dissolved in anhydrous Chloroform:Trichlorobenzene (99:1) mix;
• Vial is placed on hot plate (70°C) with a stirrer bar for 30 minutes;
• Solution cooled down at room temperature and then filtered with a 0.45 µm PTFE (hydrophobic) filter;
• Solution stored overnight on a hot plate at 30°C to prevent excessive aggregation of the P3HT molecules.

Substrate Cleaning

• Substrates loaded on to substrate rack (to keep them in upright position);
• Sonicated in hot Hellmanex III solution (1%) for five minutes;
• Rinsed twice in hot water;
• Sonicated in warm Isopropyl alcohol (70°C) for five minutes;
• Rinsed twice in cold DI water;
• Substrates stored in DI water.

Thermal Deposition of Electrodes and Contact Pads

• Done on Edwards 306 Thermal coater in clean room condition;
• Substrates are blown dry and loaded in a low density evaporation stack with a low density shadow mask to pattern the desired features;
• Secondary mask is added to selectively evaporate the gate and drain/source pads;
• Vacuum chamber pumped down to a vacuum pressure of 5 x 10-6 mbar;
• Chromium adhesion layer: 5 nm, rate 0.05 nm/s;
• Aluminum: 80 nm, rate: 0.4 nm/s;
• Changed secondary mask to deposit electrodes (FET channels);
• Vacuum: 2-3 x 10-6 mbar;
• Chromium adhesion layer: 1 nm, rate 0.05 nm/s;
• Gold: 40 nm; rate 0.05 nm/s.

PFBT Treatment for Au Electrodes (Laminar flow)

• Oxygen plasma treatment, 30 seconds at 100 W;
• Substrates immersed in 2.5 mMol/l solution of PFBT in isopropyl alcohol at room temperature;
• Substrates rinsed twice in pure isopropyl alcohol;
• Substrates are blown with nitrogen gun.

OTS Treatment for SiO2 Dielectric (Laminar flow)

• A solution of OTS (25 microlitres) in cyclohexane (anhydrous grade, 1 ml) prepared in glove box;
• Substrates (pre-loaded on a substrate rack) loaded into the annealing beaker, which is filled with approx. 50 ml of cyclohexane in a fume hood;
• Previously prepared OTS solution quickly added to the cyclohexane and mixed with a pipette tip;
• The glass lid is placed halfway onto the beaker, which is carefully filled with more cyclohexane until it is full and the lid is fully closed;
• The final solution (60 ml) contains OTS at a concentration of 1 mMol/l;
• Substrates kept for 20 minutes in the OTS solution;
• Substrates removed from the OTS solution, quickly rinsed twice in clean cyclohexane, and then are blown dry with nitrogen gun.

Contact Angle Assessment

The water-drop test on the treated silicon is a quick test to qualitatively assess the effect of the OTS on the silicon substrates to ensure that the fabrication has functioned correctly. You can get a good approximation of the contact angle using your eye or a simple digital photo.

Previous quantitative assessments have shown that this routine will produce contact angles between 90 and 110°C (depending on the lab temperature, humidity and other factors). You can quantify that contact angle easily and accurately using the Ossila Contact Angle Goniometer.

P3HT (M104) spin coating (glove box)

• 30 µl of Organic Semi-Conductor (OSC) solution delivered on the middle of the substrate and then spin coated at 1000 rpm for 10 s followed by 60 s at 2000 rpm;
• Cotton swab soaked in chlorobenzene to thoroughly wipe clean the contact pads and the rest of the substrates with the exception of the area around the channel;
• High precision cotton swab to clean between devices to avoid cross-talking and reduce leakage;
• Substrates annealed at 90°C for 30 minutes;
• Cooled down for ten minutes;
• Five devices per substrate are characterised using OFET Test Board for Low-Density OFETs in a glove box;
• Second annealing at 120°C for 20 minutes, slow cooling down at room temperature and measurement;
• Annealing at 150°C for 20 minutes, slow cooling down at room temperature and measurement.