Top Contact Organic Thin Film Research Articles

Comparative analysis of unity gain frequency of top and bottom-contact organic thin film transistors

This work shows that placement of source/drain contacts on top of organic semiconductor film in top-contact organic thin film transistors (OTFT) can offer significant improvement in device speed as compared to bottom-contact devices. 2D numerical simulations and analytical models for unity gain frequency are used to quantify the differences between the top and bottom-contact devices. It is shown that while transconductance is lower in top-contact devices, the gate–drain capacitance is significantly lower as well resulting in improved frequency performance. It is also shown that there is an optimum semiconductor film thickness at which maximum unity gain frequency in top-contact OTFT is obtained.

An analysis of the difference in behavior of top and bottom contact organic thin film transistors using device simulation

This paper presents a systematic analysis of an already reported phenomenon, namely, the difference in device performance of top and bottom contact organic thin film transistors (OTFT) by combining experiments and two-dimensional device simulations. The mobility of the as measured devices in the bottom contact OTFT is found to be lower by two orders of magnitude than the top contact structure, which is generally attributed to the higher metal-semiconductor contact resistance in the bottom contact devices due to lower contact area. However, we found that this large mobility difference exists even after correcting for the metal-semiconductor contact resistance through transfer line method (TLM). This result suggests that structural differences are playing a dominant role in lowering down the performance of bottom contact devices. This effect is then systematically investigated through two-dimensional physics-based numerical simulations by considering several structural inhomogenities around the contacts. The main reason for such an occurrence is attributed to the poor morphology (or comparatively low mobility) of pentacene films around the source/drain electrodes in the bottom contact devices. Finally, we also show a reasonable match between the simulated and experimental device characteristics, enabling calibration of the simulator for further use in design of OTFTs.

Pentaceno[2,3-b]thiophene, a Hexacene Analogue for Organic Thin Film Transistors

Hexacene and larger fused rings remain elusive targets for chemists. Here, we report a hexacene-like molecule containing six linearly fused rings, specifically a pentacene molecule fused with a terminal thiophene ring, pentaceno[2,3-b]thiophene. It can be purified and isolated as a purple-black powder at ambient conditions. This molecule has a low HOMO-LUMO gap of 1.75 eV in o-DCB and an optical band gap of 1.58 eV in thin film. Top contact organic thin film transistors (OTFTs) were made, and atomic force microscopy (AFM) reveals a dendritic thin film growth characteristic of pentacene. An OTFT mobility of 0.574 cm(2)/V s was measured for pentaceno[2,3-b]thiophene under nitrogen.

Study of the characteristics of organic thin film transistors with phenyltrimethoxysilane buffer under low gate modulation voltage

We have fabricated the top-contact organic thin film transistors （OTFTs） with, a 60 nm thick pentacene films as active layer and a 120 nm thermal growth SiO2 as gate insulator. Through using different self-assembled monolayers ,such as octadecyltrichlorosilane （OTS） and phenyltrimethoxysilane （PhTMS）, as a buffer between the organic semiconductor active layer and gate insulator, we studied the effect of different buffers on the performance of OTFTs device. At the same time, we also investigated the field-effect behavior and carrier transport mechanism of OTFTs device with PhTMS buffer under low gate modulation voltage. When |VGS|IDS of OTFTs device keeping balance. But the OTFT device still has good output characteristics, with the field-effect mobility （μEF） of 3.22×10-3 cm2/Vs, （on/off） current ratio of 1.43×102, and threshold voltage （VTH） of -0.66 V.

An Analytic Current–Voltage Equation for Top-Contact Organic Thin Film Transistors Including the Effects of Variable Series Resistance

An analytic current–voltage (I–V) equation for top-contact organic thin film transistors (OTFTs) is derived by analyzing the channel and the overlap region separately. From the analysis on the overlap region, the series resistance of OTFTs is found to be a function of the gate voltage due to the sheet resistance change of the accumulation layer. Using the derived I–V equation, the characteristics of both the channel and the overlap region are well-explained. The I–V equation is verified with fabricated top-contact OTFTs and metal–insulator–semiconductor (MIS) capacitors, and the predicted I–V characteristics from the equation agree well with the measurements. Also, the ratio of the series resistance to the total resistance of the device is up to 60% which shows significant influence of the series resistance on top-contact OTFT performance.

Micropatterning of Electrodes by Microcontact Printing Method and Application to Thin Film Transistor Devices

Electrode micropatterning without etching was successfully achieved by the microcontact printing (µCP) method. We used a conductive polymer poly(3,4-ethylenedioxythiophene)/poly(4-stylenesulfonate) (PEDOT/PSS) as an electrode material. We controlled the surface free energy of poly(dimethylsiloxane) (PDMS) by exposure to vacuum ultraviolet (VUV) light with a wavelength of 172 nm using an excimer lamp. Consequently, we established an inking technique for PEDOT/PSS aqueous dispersion on a PDMS elastomer stamp. The micropatterns of PEDOT/PSS thin films of 1 µm width were fabricated by transferring the PEDOT/PSS aqueous dispersion on the PDMS elastomer stamp to another substrate by the µCP method. Furthermore, we fabricated top contact (TC) organic thin film transistors (OTFTs) with PEDOT/PSS as source and drain electrodes. We demonstrated the successful operation of the fabricated OTFTs with a channel length of 5 µm and an electrode width of 8 µm. Results show that the µCP method is promising for application to various devices that require micropatterning.

Fabrication of vertical channel top contact organic thin film transistors

Vertical channel top contact (TC) organic thin film transistors (OTFTs) have been successfully realized on Si substrates with SiO 2 as gate insulators and P3HT(poly ∼3-hexylthiophene) as organic semiconductors. The active channel region was defined by a steep step through a Si etching method. Source and drain metal contacts were deposited by vacuum evaporation through a shadow mask at a high tilting angle. Top contact transistors with channel lengths 5 μm can be fabricated with a relatively simple and efficient (yield >85%) fabrication process with only two photolithography steps (two photo masks) while no need for high-resolution and precision alignment for channel definition. Measurement results showed that the vertical channel BC (bottom contact) devices have compatible performance with planar BC devices. However, vertical channel TC transistors showed improved performance with double field effect mobilities and three times larger current ON/OFF ratios than vertical channel BC devices.

Intra-Grain and Oligo-Grain Top-Contact Organic Thin film Transistors

AbstractWe describe and demonstrate a micromachined shadowmask that allows the realization of intra-grain and oligo-grain top-contact organic thin film transistors (OTFTs). First experimental results of OTFT's show that for small channel lengths, grain boundary barriers indeed appear to dominate the output characteristics of OTFTs.