The logic circuit and the memory circuit are the basic cells in normal electronic products. All the developments of these cells are driven by the market requirement for low cost. For the memory cells, the traditional way of achieving low cost per bit is by scaling size or multilevel storage, and great developments have been achieved in these areas. The multibit storage is more and more attracting research attention due to the scaling method limited by photolithography. The future goal of the memory cell is to obtain multibit information from a small cell by generating 2n threshold-voltage levels for n bits per cell.

In this communication we introduce the multibit-storage concept in organic field-effect transistors (OTFTs), and show a possible way to realize it in a single OTFT. A top-contact OTFT was built on a SiO2/Si platform. The source (S) and drain (D) electrodes were produced by high vacuum evaporation of Au. The organic semiconductor pentacene (or copper phthalocyaine) was used as the active layer, and deposited on the polystyrene (PS, or polymethylmethacrylate)-modified SiO2 dielectric layer (see Fig. 1). The devices with the additional PS layer showed a good p-type OFET behavior, with a mobility of 0.52 cm2 V-1 s-1, a threshold voltage of -20 V, and an on/off ratio of 106. In addition, The devices showed excellent multibit storage ability, with 2 bits data storage in a single OTFT. These characteristics are believed to originate from the use of optical and electrical organic semiconductors, from performing the appropriate write and erase programs, and from the charge-storage ability of the polymer (or the surface between the polymer and SiO2). After further investigation and optimization, the OTFTs can be potentially applied in low-cost, lightweight, and high-density-bits storage devices.

Advanced Materials，2009, 21, 1954–1959

Schematic diagram of the device and the current levels as a function of time at different photo-electric processes.