Top-contact Organic Thin-film Transistors Research Articles

Improving the performance of organic thin-film transistor with a doped interlayer

Top contact organic thin-film transistors (TC OTFTs) based on pentacene are fabricated. For improving the contact characteristics between the organic semiconductor thin-film and gold electrodes, we doped the starburst molecular 4,4′,4″-tris{N,(3-methylpheny)-N-phenylamino}-triphenylamine) (m-MTDATA), which is an excellent hole injection material for the organic light-emitting devices (OLEDs), into the interlayer contact with the electrodes. Compared with conventional TC OTFT, the performances of the organic transistor with the doped interlayer are improved. The field-effect mobility increases from 0.16 to 0.51cm2/Vs, and threshold voltage downshifts from –11 to –2.8V for the linear region. The on/off current ratio is more than 104 when the gate voltage varies from 0 to –20V. We ascribe the improvements to the doped interlayer for which the contact resistance is reduced and the hole injection is enhanced.

ディスプレイ―ＩＤＷ／ＡＤ’０５を中心に―ボトムコンタクト型有機ＴＦＴの特性改善とそれを用いたフレキシブル液晶表示パネルの試作

We developed an organic thin-film transistor (OTFT) -driven 16×16 polymer-dispersed liquid crystal (PDLC) display panel on a flexible plastic film. We fabricated low-voltage-operation bottom-contact (BC) OTFT with Ta2O5 gate insulator, using photolithography method. This transistor is suitable for both scaling down and increasing the number of pixels, however its performance is inferior to that of top-contact OTFT because of contamination and defects on the surface of the insulator, and leakage-current of channel region. We have reduced the current of the OTFT by optimizing the pentacene deposition area. We used O2 plasma, UV cleaning and HMDS as surface treatments suitable for Ta2O5 gate insulator, to improve the characteristics. The OTFT showed a good carrier mobility of 0.3 cm2/Vs, a high current on/off ratio of over 106 and a low operating drain voltage of -6V. We also successively confirmed display operation of the fabricated 16×16 PDLC display panel with a low driving voltage of 13 volts.

Numerical simulations of contact resistance in organic thin-film transistors

The origin of the source/drain contact resistance reported in studies of pentacene-based organic thin-film transistors (OTFTs) has been investigated using numerical device simulations. Quantitative agreement with published contact resistance values is obtained, using reasonable values for the physical parameters describing both the semiconductor material and the metal/organic interfaces. In particular, the difference in contact resistance measured in top and bottom contact OTFTs has been reproduced.

Synthesis, Characterization, and Field-Effect Transistor Performance of Carboxylate-Functionalized Polythiophenes with Increased Air Stability

Synthetic strategies to access both regiorandom and regioregular polythiophenes containing electron-withdrawing carboxylate substituents have been developed. Although these polymers have extended conjugation lengths, they provide better oxidative doping stability than conventional polythiophenes due to the lowering of the HOMO energy levels by approximately 0.5 eV. These materials are highly crystalline and exhibit very small π−π-stacking distances in the solid state. High charge mobilities are observed as a result of the close ordering of the polymer chains, and top-contact organic thin-film transistors (OTFTs) fabricated entirely in air had measured mobilities averaging 0.06 cm2/V·s with on/off ratios >105. Off currents in these devices remained low over a period of months, demonstrating the low propensity of these materials toward p-doping by molecular oxygen.

Humidity effect on electrical performance of organic thin-film transistors

Humidity dependence of electrical performance of p-channel organic thin-film transistors (OTFTs) with various semiconductor compounds has been investigated. All devices showed decreased current output and mobility as the relative humidity (RH) was increased. The moisture sensitivity of the OTFT saturation current depends on the device geometry (bottom or top contact device) and channel length. The OTFT configuration with a short channel length and bottom contact was most affected by humidity compared to the top contact and larger channel length OTFT structures. The degradation of electrical performance under high RH is attributed to charge trapping at grain boundaries by polar water molecules reducing the rate of charge transport.

Interface Characteristics between Au Electrode and Pentacene Layer in Organic Thin-film Transistors

In order to investigate interfacial characteristics between top-Au and pentacene layers, devices with the structure of bottom-Au/pentacene/top-Au were fabricated on glass substrates at various deposition rates of top-Au of about 1.0, 3.0 and 15.0 Å/s. It was observed that electrical characteristics could be improved by increasing the deposition rate of top-Au, and the highest electrical conductivity value, 1.5×10-6 S/cm, was obtained for the device with top-Au deposited at 15.0 Å/s. Auger electron spectroscopy (AES) results showed that the atomic content of Au is substantially increased with the deposition rate of top-Au, but there is no marked difference in the depth profile of Au atoms regardless of the deposition rate of top-Au. Likewise, field-emission scanning electron microscopy (FE-SEM) images did not reveal structural defects that might cause short circuit. We also fabricated top-contact organic thin-film transistors (OTFTs) at various deposition rates of the Au layer for source/drain electrodes. As a result, the performance of the OTFTs can be modified by the deposition rate of Au. These results will be discussed.

Organic thin-film transistors in sandwich configuration

Organic thin-film transistors (OTFTs) having source/drain electrodes sandwiched between copper phthalocyanine (CuPc) and cobalt phthalocyanine (CoPc) layers, CuPc/CoPc SC OTFTs, are investigated. Comparing their properties with that of CuPc-based top-contact OTFT, field-effect mobility increases from 0.04 to 0.11 cm2/Vs, threshold voltage shifts from −13.8 to −8.9 V, and the current on/off ratio maintains at a level of 105. A top-contact OTFT with a layer of CuPc and a layer of CoPc (10%)–CuPc mixture reveals that the combination of CuPc and CoPc enhances charge injection from the source electrode into the active layer and increases the off-state current. The sandwich configuration increases the field-effect mobility, reduce the threshold voltage, and improve the on/off ratio at the same time. Our results indicate that using a double-layer of active organic materials in sandwich configuration is an effective way to improve OTFT performance.

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.