Organic Vapor Jet Printing

Yiru Sun, Mimi Gupta, and Max Shtein

Fig. 1 Schematic of the OVJP apparatus (courtesy of Max Shtein)

Fig. 2 A 24x32 pixel bitmap image of a bicyclist figure printed by OVJP using Alq₃ (Courtesy of Max Shtein)

Fig. 3 The comparative moving path of the nozzle above the substrate

Introduction

Previous work by the OCM group [1] demonstrated that the solvent-free, high-resolution direct printing of molecular organic semiconductors can be achieved by the Organic Vapor Jet Printing (OVJP). The internal core structure of the OVJP is shown in Figure 1. A center dilution channel, two source cells, and a modular collimating nozzle are all heated from the outside. This design of source cells serves a dual purpose, with one as the source container and the other as the hot valve. For the source cell whose hot valve is open, a hot inert carrier gas enters the apparatus, picks up the organic vapor and ejects the gas mixture through the nozzle. The collimated vapor jet impinges onto a cooled substrate where the organic molecules selectively physisorb, forming a well-defined deposit.
Figure 2 shows the promising patterning function of the OVJP. Alq₃ in the source cell was heated up to 270ºC, and printed according to a 24x32 pixel bitmap image with a dwell-time of 2s above each pixel location, and <0.2s time interval for translation between each pixel. The maximum pattern resolution in this image is 1000 dpi. The local deposition rate was above 1000 Å/s at 270ºC source temperature for Alq₃. The thickness profiles of the Alq₃ dots determined from the interference fringes turned out to be in the Gaussian-shape distribution. The height of each dot is around 2000 Å.

Our first objective is to use the OVJP to write a high performance, electro-phosphorescent OLED at high deposition rate. In order to make OLEDs, we must avoid fluctuation of the surface resulted from the Gaussian-shape distribution of each dot. We controlled the nozzle to move above the substrate smoothly (with no dwell time above each pixel location, and <0.2s time interval for adjusting the separation between the nozzle and the substrate from pixel to pixel). Figure 3 schematically shows the path of the comparative movement of the nozzle above the substrate. By this method NPD was deposited at 250ºC. As we can see from figure 3, under this deposition strategy "Å/s" is no longer a proper unit for measuring the deposition rate. We thus introduce "Å·cm²/s" as the unit of deposition rate for the OVJP. Currently the deposition rate is approximately 1 Å·cm²/s, which is not limited by the flow rate of N₂ or the temperature of source but by the requirement for the thickness (~500 Å) for the thin film.
 

Reference

[1] Max Shtein, Peter Peumans, Jay B. Benziger, Stephen R. Forrest, Direct mask-free patterning of molecular organic semiconductors using organic vapor jet printing (submitted for publication, Advanced Materials)