High performance p-type organic thin film transistors with an intrinsically photopatternable, ultrathin polymer dielectric layer.

We report on the morphological influence of solution-processed zinc oxide (ZnO) semiconductor films on the electrical characteristics of ZnO thin-film transistors (TFTs). Different film morphologies were produced by controlling the spin-coating condition of a precursor solution, and the ZnO films were analyzed using atomic force microscopy, X-ray diffraction, X-ray photoemission spectroscopy, and Hall measurement. It is shown that ZnO TFTs have a superior performance in terms of the threshold voltage and field-effect mobility, when ZnO crystallites are more densely packed in the film. This is attributed to lower electrical resistivity and higher Hall mobility in a densely packed ZnO film. In the results of consecutive TFT operations, a positive shift in the threshold voltage occurred irrespective of the film morphology, but the morphological influence on the variation in the field-effect mobility was evident. The field-effect mobility in TFTs having a densely packed ZnO film increased continuously during consecutive TFT operations, which is in contrast to the mobility decrease observed in the less packed case. An analysis of the field-effect conductivities ascribes these results to the difference in energetic traps, which originate from structural defects in the ZnO films. Consequently, the morphological influence of solution-processed ZnO films on the TFT performance can be understood through the packing property of ZnO crystallites.

Morphological Influence of Solution-Processed Zinc Oxide Films on Electrical Characteristics of Thin-Film Transistors

An organic thin-film transistor (OTFT) memory was fabricated using deoxyribonucleic acid (DNA) as a gate dielectric layer. Since natural DNA contains intrinsic ions and water molecules, the OTFT exhibited low resistivity and a low on/off ratio because of the mobile ions. In order to remove these mobile ions from DNA, it was complexed with a bulky surfactant through an electrostatic interaction. The OTFT fabricated using a DNA–surfactant complex exhibited a relatively high on/off ratio because of the decrease in the off-current. Residual ions that remained in the complex caused a decrease in the cell performance with respect to parameters such as the on/off ratio and long-term memory stability. When poly(methyl methacrylate) (PMMA) was layered on the DNA–surfactant layer, the OTFT exhibited better on-current and long-term memory stability than a typical OTFT. The pentacene layer deposited on the PMMA surface as a semiconductor had a relatively large grain size and relatively high crystallinity. This contributed to an increase in the hole mobility in the pentacene layer and the prevention of the abovementioned adverse effects of the residual ions. Consequently, the performance of the OTFT memory having the DNA complex was considerably improved.

Non-volatile transistor memory fabricated using DNA and eliminating influence of mobile ions on electric properties

Surface modifications of the dielectric and the metal of pentacene-based field effect transistors using self-assembled monolayer (SAM) were studied. First, a low interfacial trap density and pentacene 2D-growth were favored by the nonpolar and low surface energy of octadecyltrichlorosilane-based SAM. This treatment leaded to increased mobility up to 0.4 cm2 V−1 s−1 and no observable hysteresis on transfer curves. Second, reduced hole injection barrier and contact resistance were achieved by fluorinated thiols deposited on gold contacts resulting in an increased mobility up to 0.6 cm2 V−1 s−1. Finally, a high mobility of 2.6 cm2 V−1 s−1 was achieved by cumulative effects of both treatments.

Cumulative effects of electrode and dielectric surface modifications on pentacene-based transistors

Silane nanolayers deposited from the vapor phase onto indium-tin oxide (ITO) coated glass are shown to be an effective means of tuning the work function and stabilizing the surface of this complex ternary oxide. Using this approach a pair of model hole-extracting electrodes have been developed to investigate how the performance of bi-layer organic photovoltaics is impacted by built-in positive space charge in the critical region close to the hole-extracting electrode. The magnitude and spatial distribution of positive space charge resulting from ground-state electron transfer from the donor layer to the ITO electrode upon contact formation, is derived from direct measurements of the interfacial energetics using the Kelvin probe technique. This judiciously designed experiment shows that it is unnecessary to engineer the work function of the hole-extracting electrode to match the ionization potential of the donor layer, rather only to ensure that the former exceeds the latter, thus simplifying an important aspect of device design. In addition, it is shown that silane nanolayers at the ITO electrode surface are remarkably effective at retarding device degradation under continuous illumination.

An Electrode Design Rule for Organic Photovoltaics Elucidated Using Molecular Nanolayers

We describe how the surface energy of a polymeric adhesion layer affects the initial growth of a pentacene film for field-effect transistors (FETs). Three types of adhesion layers having different surface energies and morphologies were produced with varying the composition ratio of two different polyimides. For the pentacene film of 0.4 nm thick, the monolayer coverage of pentacene grains increases from 26.3% to 64.9% with increasing the surface energy of the underlying adhesion layer from 12.9 to 25.8 mJ/m 2 . The surface energy of the adhesion layer is found to play a more dominant role on the initial growth of a pentacene film than the surface roughness. The field-effect mobility in the pentacene FET depends strongly on the monolayer growth of pentacene grains.

Jeong Cheol Noh

Papers

The surface energy-dictated initial growth of a pentacene film on a polymeric adhesion layer for field-effect transistors