Effect of Bending on the Electrical Characteristics of Flexible Organic Single Crystal-based Field-effect Transistors

Summary

This manuscript describes the bending process of an organic single crystal-based field-effect transistor to maintain a functioning device for electronic property measurement. The results suggest that bending causes changes in the molecular spacing in the crystal and thus in the charge hopping rate, which is important in flexible electronics.

Abstract

The charge transport in an organic semiconductor depends highly on the molecular packing in the crystal, which influences the electronic coupling immensely. However, in soft electronics, in which organic semiconductors play a critical role, the devices will be bent or folded repeatedly. The effect of bending on the crystal packing and thus the charge transport is crucial to the performance of the device. In this manuscript, we describe the protocol to bend a single crystal of 5,7,12,16-tetrachloro-6,13-diazapentacene (TCDAP) in the field-effect transistor configuration and to obtain reproducible I-V characteristics upon bending the crystal. The results show that bending a field-effect transistor prepared on a flexible substrate results in nearly reversible yet opposite trends in charge mobility, depending on the bending direction. The mobility increases when the device is bent toward the top gate/dielectric layer (upward, compressive state) and decreases when bent toward the crystal/substrate side (downward, tensile state). The effect of bending curvature was also observed, with greater mobility change resulting from higher bending curvature. It is suggested that the intermolecular π-π distance changes upon bending, thereby influencing the electronic coupling and the subsequent carrier transport ability.

Introduction

Soft electronic devices, such as sensors, displays, and wearable electronics, are currently being designed and researched more actively, and many have even been launched in the market in recent years^1,2,3,4. Organic semiconducting materials play an important role in these electronic devices due to their inherent advantages, including low development cost, the ability to be prepared in solution or at low temperatures, and, in particular, their flexibility when compared to inorganic semiconductors^5,6. One special consideration for these electronics is that they will be subjected to frequent bending. Bending introduces strain in the components and the materials within the device. A stable and consistent performance is required as such devices are bent. Transistors are a vital component in most of these electronics, and their performance under bending is of interest. A number of studies have addressed this performance issue by bending organic thin film transistors^7,8. While the changes in conductance upon bending may be attributed to the changes in spacing between the grains in a polycrystalline thin film, a more fundamental question to ask is whether the conductance may change within a single crystal upon bending. It is well accepted that charge transport between organic molecules depends strongly on electronic coupling between molecules and the reorganization energy involved in the interconversion between the neutral and charged states⁹. Electronic coupling is highly sensitive to the distance between neighboring molecules and to the overlap of frontier molecular orbitals. The bending of a well-ordered crystal introduces strain and may change the relative position of molecules within the crystal. This can be tested with a single crystal-based field-effect transistor. One report used single crystals of rubrene on a flexible substrate to study the effect of crystal thickness upon bending¹⁰. Devices with copper phthalocyanine nanowire crystals prepared on a flat substrate were shown to have a higher mobility upon bending¹¹. However, the properties for an FET device bent in different directions have not been explored.

The molecule 5,7,12,16-tetrachloro-6,13-diazapentacene (TCDAP) is an n-type semiconductor material¹². The crystal of TCDAP has a monoclinic packing motif with shifted π-π stacking between neighboring molecules along the a axis of the unit cell at a cell length of 3.911 Å. The crystal grows along this packing direction to give long needles. The maximum n-type field-effect mobility measured along this direction reached 3.39 cm²/V·sec. Unlike many organic crystals that are brittle and fragile, TCDAP crystal is found to be highly flexible. In this work, we used TCDAP as the conducting channel and prepared the single crystal field-effect transistor on a flexible substrate of polyethylene terephthalate (PET). Mobility was measured for the crystal on a flat substrate, with the device bent toward the flexible substrate (downward) or bent toward the gate/dielectric side (upward). I-V data were analyzed based on changes in the stacking/coupling distance among the neighboring molecules.

Protocol

1. Preparation of TCDAP¹²

1. Synthesize TCDAP by following literature procedures¹³.
2. Purify the TCDAP product by the temperature-gradient sublimation method, with the three temperature zones set at 340, 270, and 250 °C, respectively, under a vacuum pressure of 10^-6 Torr^12,14.

2. Grow Single Crystals of TCDAP Using a Physical Vapor Transfer (PVT) System¹⁴

1. Put the TCDAP sample at one end of a boat (5 cm long) and load the boat into a glass inner tube (15 cm long with a diameter of 1.2 cm).
2. Load the inner tube into a longer glass tube (83 cm long and 2 cm in diameter) and push it in to about 17 cm from the opening.
3. Load the long glass tube into a copper tube (60 cm long and 2.5 cm in diameter) horizontally fixed on a rack; make sure the boat of TCDAP is located in the middle of the heating area defined by a heating band around the copper tube.
4. Purge the PVT system with helium gas at a flow rate of 30 cc/min, and then turn on the transformer to heat up the heating band to 310 °C; maintain at this temperature for two days.
5. After cooling the system to room temperature, collect the crystals from the inner tube.

3. Device Fabrication

1. Put a 200-µm-thick, transparent, pre-cut PET substrate (2 cm x 1 cm) into a vial and clean it by sonication in detergent solution, deionized water, and acetone, in sequence, for 30 min each. Dry the substrate by nitrogen flow.
2. Place double-sided tape on the PET substrate.
3. Examine the crystals under a stereomicroscope. Select good quality, shining crystals with a dimension of ~5 mm x ~0.03 mm for device fabrication. Place a needle-like TCDAP crystal parallel with the length of the PET substrate on the double-sided tape and fix it securely.
4. Under a stereomicroscope, apply water-based colloidal graphite through a microliter syringe needle in a line (several mm) that extends from the two ends of the crystal acting as the source and drain. Wait for about 30 min for the colloidal graphite to dry and measure the distance between the two graphite spots under an optical microscope to determine the exact channel length (keep it at 0.6-1 mm).
5. Use carbon conductive tape to fix the PET substrate on a microscopic slide. Place the slide near the end of the pyrolysis tube of the deposition chamber.
6. Weigh 0.5 g of the precursor of the dielectric insulator, [2.2]paracyclophane, and place it near the inlet of the pyrolysis tube.
7. Pump down the system to a vacuum of 10^-2 Torr. Pre-heat the pyrolysis zone near the center of the tube up to a pre-set temperature of 700 °C and maintain at this temperature.
8. Heat up the [2.2]paracyclophane sample to 150 °C. The vapors of the precursor will pass through the pyrolysis zone to give the monomers, which will condense near the end of the pyrolysis tube to polymerize.
9. Let the pyrolysis/polymerization reaction continue for 2 hr.
10. Cool down the system and take out the samples from the pyrolysis tube.
11. Determine the thickness of the deposited dielectric layer by measuring the step height of the layer and substrate using a profilometer according to the manufacturer's instructions.
12. Apply isopropanol-based colloidal graphite through a microliter syringe needle in a line on the back of the dielectric layer above the crystal to serve as the gate electrode.

4. Measure the Performance of the Device

1. Use the scalpel to cut a hole through the polymeric dielectric film above the source/drain electrode area in order to expose the electrodes underneath for connection.
2. With the help of a stand and clamps, bring the electrode probes from the Parameter Analyzer into contact with the source/drain/gate electrodes. Record the I-V characteristics at different gate potentials according to the manufacturer's instructions.
   Note: Here, the gate potentials are set from -60 V to 60 V at 15 V steps.

5. Bending Experiments

1. To measure the properties in the tensile state, wrap the backside of the flexible PET substrate around cylinders of different radii (14.0 mm, 12.4 mm, 8.0 mm, and 5.8 mm) and fix the PET substrate to the cylinder on four sides with vacuum tape.
2. Connect the probes to the source/drain/gate electrodes and measure the I-V characteristics at different gate potentials as described in 4.2.
3. To measure in the compressive state, wrap half of the front side of the PET substrate around the end of a cylinder, such that the crystal/source/drain/gate electrodes are facing the cylinder and yet are still exposed. Fix the PET substrate on the cylinder with vacuum tape (see Fig. 5).
4. Connect the probes to the source/drain/gate electrodes and measure the I-V characteristics at different gate potentials as described in 4.2.
   NOTE: A cross-sectional illustration of the device structure is shown in Fig. 1.

Results

The single crystal XRD analysis reveals that TCDAP is an extended π system with molecules packing along the a axis. Fig. 2 shows the scan pattern by powder XRD for a TCDAP crystal. A series of sharp peaks are observed, corresponding only to the family of (0,k,ℓ) planes, by comparing with the powder diffraction pattern of the crystal. This would imply that the crystal structure is oriented as shown in Fig. 3.

Discussion

In this experiment, a number of parameters affect the successful measurement of the field-effect mobility. Firstly, the single crystal should be large enough to be fabricated into a field-effect device for property measurement. The physical vapor transfer (PVT) method is the one that allows larger crystals to be grown. By adjusting the temperature and the flow rate of the carrier gas, crystals of sizes up to half a centimeter can be obtained. Secondly, the choice of a single crystal is important. An apparent single cryst...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Colloidal Graphite (water-based)	TED PELLA,INC	NO.16053
Colloidal Graphite (IPA-based)	TED PELLA,INC	NO.16051
[2.2]Paracyclophane, 99%	Alfa Aesar	1633-22-3
polyethylene terephthalate	Uni-Onward
Mini-Mite 1,100 °C Tube Furnaces (Single Zone)	Thermo Scientific	TF55030A
Agilent 4156C Precision Semiconductor Parameter	Keysight	HP4156