Charge Injection In Organic Transistors Research Articles

Contact engineering for efficient charge injection in organic transistors with low-cost metal electrodes

Controlling charge injection at the metal-semiconductor interface is very crucial for organic electronic devices in general as it can significantly influence the overall device performance. Herein, we report a facile, yet efficient contact modification approach, to enhance the hole injection efficiency through the incorporation of a high vacuum deposited TPD [N,N′-Bis(3-methylphenyl)-N,N′-diphenylbenzidine] interlayer between the electrodes and the active semiconducting layer. The device performance parameters such as mobility and on/off ratio improved significantly after the inclusion of the TPD buffer layer, and more interestingly, the devices with cost effective Ag and Cu electrodes were able to exhibit a superior device performance than the typically used Au source-drain devices. We have also observed that this contact modification technique can be even more effective than commonly used metal oxide interface modifying layers. Our investigations demonstrate the efficacy of the TPD interlayer in effectively reducing the interfacial contact resistance through the modification of pentacene energy levels, which consequently results in the substantial improvement in the device performances.

Organic Transistors: Improving the Charge Injection in Organic Transistors by Covalently Linked Graphene Oxide/Metal Electrodes (Adv. Electron. Mater. 4/2016)

Development of an appropriate agent and method for electrode modification is highly important for organic field-effect transistors (OFETs) and related fields. In article number 1500409, L. Q. Li and co-workers demonstrate a facile strategy for the covalent modification of a Au electrode with graphene oxide (GO). OFETs with improved charge injection are constructed with the covalently linked GO-Au electrode.

Improving the Charge Injection in Organic Transistors by Covalently Linked Graphene Oxide/Metal Electrodes

Electrodes, one of the key components of organic field‐effect transistors (OFETs), exert great influence on the device performance as well as circuit fabrication. Conventional metal electrodes generally show poor contact quality with organic semiconductors, especially in bottom‐contact geometry. Development of appropriate modification materials and methods for metal electrodes is an efficient way to improve OFET performance, which is, however, quite a challenging task. In this work, a facile strategy is developed to modify the metal surface with graphene oxide (GO) via covalent bonds for application in OFETs, which has not been reported before. This selective covalent modification strategy is compatible with diverse patterning techniques, and the covalently linked GO‐Au electrode exhibits strong robustness against solvent treatment. Remarkably, the GO‐Au electrode shows very good generality with both p‐type and n‐type organic semiconductors, which contributes to the realization of p‐/n‐type OFETs with significantly improved performance compared with the pristine Au electrode. The facile and low temperature modification method, compatibility with diverse patterning techniques, robustness against solvent treatment, good generality with organic semiconductors, and high OFET performance enable the strategy to be very promising for application in the field of organic electronics.

Metal evaporation dependent charge injection in organic transistors

To illuminate a long-term remaining issue on how contact metallization (metal and speed) affects charge injection, we investigated top-contact pentacene transistors using two categories of metals deposited at various rates. Differing from previous studies such as those devoted to morphological influences by microscopy, in this work we concentrated on their electrical characteristics in particular combining the low-frequency noise which provided a direct quantity of trap density and its evolution with respect to contact metal and deposition rate. It turns out that the transistors with noble metal (Au) suffer from metal-diffusion related charge trapping in the pentacene bulk close to the Au/pentacene interface, and this diffusion-limited injection is greatly tuned from bulk to interface by speeding Au deposition which leads to a Schottky-like injection due to the severe thermal damage to the upper pentacene layer. Applying a conventional contacting metal (Cu), however, Ohmic contacts with much fewer traps are always observed regardless of metallization speed. This is attributed to an ultra-thin interlayer of CuxO that guarantees stable Ohmic injection by introducing gap states and protecting the pentacene film so that those transistors appear to be free from Cu metallization. Our results quantitatively show the limiting factors of charge injection for different metals and at various evaporation rates.

Scaling limits of organic digital circuits

The effect of device scaling on organic circuits’ performance was studied. Particularly, the influence of contact resistance on the static and the dynamic behavior of the circuits was investigated. For that purpose, an analytical model describing the voltage transfer characteristics (VTCs) and the propagation delay was developed. Using the model, it was shown that for OTFTs with channel lengths of less than 10μm the contact resistance has negative influence on both, the static noise margin (SNM) and the propagation delay. Moreover, the model is in a good agreement with experimentally measured data. Scaling the lateral dimensions of the transistors down to few μm limits the circuit performance due to contact effects, and the 1–10MHz frequency range operation required by some applications can only be achieved by reducing the specific contact resistance, ρc, 10–100 times. This need for ρc reduction highlights the importance of improving charge injection in organic transistors that can usually be achieved by contact doping like in inorganic electronics.