A print technique of an electrode on a plastic substrate is one of the most important techniques for developing a printed large area device. Especially, preparation of a metal electrode with low work function by printing is very important to develop printed active devices such as diode and transistors. Work function values of electrodes strongly affect device properties (1,2), because the work function difference between a semiconductor and an electrode is related to charge injection from the electrode to the semiconductor. Recently, many solution-processable n-type semiconductors have been developed (3), because these are required to fabricate printable EL devices, printable CMOS devices, and so on. Low-work function metals are necessary to inject electrons to n-type semiconductor. In spite of this fact, very few printable low-work function metal inks have been developed, because low-work function metals are easily oxidized during print process owing to high temperature annealing treatment and their conductivities are lost.
In this study, we have examined to develop a new annealing technique on a printed electrode to reduce the process temperature during metal printing. We have newly developed a mechanical sintering technique in which mechanical forces is applied on a printed metal pattern. Control of the direction balance of applied mechanical force was effective to reduce resistivity of the printed metal without any destruction of plastic substrate. Furthermore, distribution control of metal particle in the metal ink was also effective to reduce resistivity. By using this technique, we have succeeded in the preparation of an aluminum, zinc, copper and tin electrode on a plastic substrate. On the other hand, we have tried to prepare a metal alloy ink to control the work function of printed electrode. Metal alloy ink was composed of two kinds of metal particles. Work function of the electrode was controlled by changing composition of these metal contents in an alloy ink. By applying our developed mechanical sintering technique on the printed alloy pattern, printed electrode with various work functions from 3.5eV to 5eV could be prepared on a plastic substrate. These printed alloys were effective to improve the performance of printed diode and transistors.