Poly(3-hexylthiophene), P3HT, has been widely used in organic electronics as a semiconductor material. It suffers from the low carrier mobility characteristics. This limits P3HT to be employed in applications. Therefore, the blending semiconductor material, carbon nanoparticle (CNP), and P3HT, are developed and examined by inkjet-printing organic field-effect transistor technology in this work. The effective carrier mobility of fabricated OFETs can be enhanced by 8 folds with adding CNP and using O2 plasma treatment. At the same time, the transconductance of fabricated OFETs is also raised by 5 folds. Based on the observations of SEM, XRD, and FTIR, these improvements are contributed to the local field induced by the formation of CNP/P3HT complexes. This observation presents an insight of the development in organic semiconductor materials. Moreover, this work also offers a low-cost and effective semiconductor material for inkjet-printing technology in the development of organic electronics.
Organic electronics have received tremendous interests due to their potential applications in flexible electronics. Moreover, characteristics of organic electronics including low-cost and low-temperature process also promote the value of this research field. Therefore, various organic electronic devices have been proposed and implemented, such as organic thin film transistors (OTFTs) [
On the other hand, the major obstacle of organic electronics, such as OFET, to be overcome is electrical characteristics. For example, the characteristics to be improved are transistor on-off ratio, threshold voltage, and transistor transconductance. To address these, previous research works have been proposed and demonstrated. For instance, self-organized layer [
In this work, poly(3-hexylthiophene) (P3HT) incorporated with carbon nanoparticles (CNPs) is used as the organic semiconductor material. The suspension capability of CNPs gives the feasibility to be implemented with inkjet-printing technologies. Utilizing this developed inkjet-printed CNP/P3HT blending semiconductor material, the mobility and transistor transconductance can be experimentally improved for 10 folds.
The p-type silicon wafer with 200 nm thermal oxide is used as the substrate in this work. To form the source/drain electrodes, 20 nm Cr and 200 nm Au are deposited on the top of the thermal oxide by evaporation. In addition, the oxide on the back side of the wafer is etched by BHF to expose the p-type silicon substrate. And gate contact is formed by deposition of 20 nm Cr and 200 nm Au after BHF etching. On the other hand, the CNP/P3HT semiconductor blending material is made at various concentrations (0, 0.1, 1, and 10 wt.% with respect to P3HT). In specific, CNPs (Qf-Nano Tech. Co. Ltd., Taiwan, model: GF-PHG-1P) with different weights are added to the P3HT (Sigma Aldrich) solution to form the blending material in different concentration. At the same time, the P3HT solution is 0.3 wt.% P3HT dissolved in p-xylene. It should be noted that the CNP/P3HT blending solutions are prepared in nitrogen glove box (H2O < 1 ppm and O2 < 1 ppm) and stirred on the hotplate at 90°C for 1 hour.
Before depositing the CNP/P3HT semiconductor-composite material with different concentration, the substrate with fabricated electrodes are treated by O2 plasma (Harrick plasma, PDC-001, 10.2W) for 2 min. Then CNP/P3HT can be inkjet printed on the top of electrodes to form the OFET structure under atmosphere conditions with relative humidity (RH) maintained at 20%. After printing process, the printed CNP/P3HT semiconductor layer is cured at 70°C for 1 hour and annealed at 150°C for 10 min in nitrogen oven. In the last step to finish the device fabrication, the epoxy (Hisin Han Co. Ltd) is coated on the devices to serve as a protective layer. This protective layer is cured at 130°C for 30 min in a vacuum oven. The schematic and picture of fabricated inkjet-printing device are shown in Figure
(a) Schematic representation of cross section of the inkjet-printed OFETs, (b), (c), and (d) microscope images of the OFETs for pristine P3HT, CNP/P3HT = 0.1 wt.%, and CNP/P3HT = 10 wt.%, respectively.
To evaluate the performance of the fabricated OFETs, the electrical characteristics of the OFETs are measured by using Agilent 4156C semiconductor parameter analyzer under dark-atmosphere conditions with RH < 25%. Moreover, XRD (PANalytical, X’ Pert PRO) and FTIR (Thermo Nicolet, Nexus470) are also performed to analyze CNP/P3HT composite material property.
To clearly demonstrate the transistor performance in different CNP/P3HT concentration, the ID-
The experiment result of different OFETs.
Pristine P3HT (as-pretreated) | Pristine P3HT | CNP/P3HT = 1/1000 | CNP/P3HT = 10/1000 | CNP/P3HT = 100/1000 | |
On/Off ratio | 1.07E4 | 3.41E4 | 8.24E4 | 1.60E4 | 4.76E4 |
Vth (V) | 12 | 8 | 9.5 | 13 | 15 |
Mobility (cm2/Vs) | 0.0007 | 0.0014 | 0.0016 | 0.0017 | 0.0060 |
The ID-
Output characteristics ID-
From Table
For the effect of adding CNP in P3HT, from Table
To further understand the detail of the developed CNP/P3HT blending semiconductor material, scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) measurement are carried out. The SEM picture of different concentration of CNP/P3HT is shown in Figure
The SEM image for various concentration of CNP/P3HT blending solution (a) pristine P3HT; (b) CNP/P3HT = 0.1 wt.%; (c) CNP/P3HT = 1 wt.%; (d) CNPP3HT = 10 wt.%.
At the same time, X-ray diffraction (XRD) measurement is used to study the crystalline behavior of different concentration of CNP/P3HT blending material. This result can be shown in Figure
XRD image of pristine CNP/P3HT and different CNP concentrations.
To understand the material property through the bonding structure, FTIR is used to study the CNP/P3HT complex and shown as Figure
FTIR image of pristine CNP/P3HT and different CNP concentrations.
In general, the disorderliness in crystalline structure leads to a decrease in carrier mobility. In addition, there is no chemical-bonding formation within CNP/P3HT complex. In other words, the direct carrier transportation between CNPs and P3HT cannot be promoted within CNP/P3HT complex. Since there is less alternative conduction path in low concentration of CNP/P3HT, that is, 0.1 wt.% and 1 wt.%, adding CNP should decrease the drain current and effective mobility. However, our experimental result indicates an enhancement in carrier mobility with an increase in CNP concentration. This result can be contributed to the localized field enhancement induced by CNP/P3HT complex [
The experimental transconductance of developed OFETs.
Pristine P3HT (as pretreated) | Pristine P3HT | CNP/P3HT = 1/1000 | CNP/P3HT = 10/1000 | CNP/P3HT = 100/1000 | |
Transconductance (nS) | 2.8 | 4.5 | 11 | 7.2 | 16 |
In summary, we have examined a CNP/P3HT blending semiconductor material for inkjet-printed OFETs. As CNP/P3HT concentration increases, the effective carrier mobility of each fabricated OFET increases. This carrier mobility enhancement is 8 folds for the comparison between pristine P3HT without O2 plasma treatment and CNP/P3HT = 10 wt.%. At the same time, the on-off ratio of fabricated OFETs is kept above 104. Based on the observation of SEM, XRD, and FTIR, this improvement can be contributed to the local field enhancement and the hole-injection lowering by CNP/P3HT complexes. This work offers a low-cost and effective semiconductor material for inkjet-printing technology in the development of organic electronics.
the authors would like to thank Dr. Yi-Jun Lin of Taiwan Textile Research Institute for her suggestion in material experiments. This work is supported by National Science Foundation (Grant no. NCS 98-2218-E-002-042, NSC 98-2221-E-002-139-MY, NSC 98-2627-E-002-003, and NSC99-2911-I-002-001).