BRIEF DESCRIPTION OF THE DRAWINGS
These and other aspects of the present invention will now be further described, by way of example only, with reference to the accompanying figures in which:
FIG. 1 shows a vertical cross section through an example of an OLED device;
FIG. 2 shows a view from above of a portion of a three color pixelated OLED display;
FIGS. 3
a and 3b show a view from above and a cross-sectional view respectively of a passive matrix OLED display;
FIGS. 4
a and 4b show a simplified cross section of a well of an OLED display substrate filled with, respectively, dissolved material, and dry material;
FIGS. 5
a and 5b show examples of filling a small pixel and a large pixel respectively with droplets of dissolved OLED material;
FIGS. 6
a to 6d show examples of well-filling according to embodiments of the invention; and
FIGS. 7
a to 7e show, respectively, an illustrating of surface forces for a liquid droplet on a solid surface, and a set of figures showing effects of progressively increasing the angle a bank face makes with a substrate.
DETAILED DESCRIPTION
Referring now to FIG. 6a, this shows a simplified vertical cross-section through a well 608 of a substrate 600 according to an embodiment of the present invention. The substrate includes an anode layer 606 on which are formed banks 610, faces 610a of the banks defining wall of well 608. As can be seen the faces 610a of banks 610 overhang the base of the well 608. In the substrate 600 of FIG. 6a the bank angle is approximately 135°, that is −45° and height of a bank is approximately 0.6 μm. In FIG. 6a the well is filled with a solution 602 of OLED material which, in this example, brims over the top of the well and forms a contact angle of approximately 35° with the top surface of a bank. FIG. 6b shows the same substrate and well after the solvent has evaporated to leave a dry film 604 with the material together with small deposits on the tops of the banks adjacent the well-defining faces.
As can be seen from FIGS. 6a and 6b the capillary force around the pixel well edge pulls the ink 602 into the edges of the well, also giving good wetting into the well corners (not shown in FIGS. 6a and 6b) albeit with a slight ink overspill. Furthermore inaccurately placed droplets which land across the bank tend to be drawn into the well rather than drying on the bank. These effects are achieved with bank angles which are greater than the contact angle of the inkjet droplet. In practice this means angles of 40° or more, that is steep “positive” angles and “negative” angles. The degree to which fluid is pulled towards to the edges of a well depends upon the angle of the bank, the viscosity of the fluid, and the contact angle of the fluid with the bank. A suitable angle can be determined by routine experiment, fabricating a range of wells with different bank face angles to see which results in the optimum performance. Generally it is desirable to obtain a substantially flat dry film 604 without too much material being drawn towards the edges of a well, thinning the film in the middle. The choice of a suitable bank face angle is described further below with reference to FIG. 7.
Referring back to FIG. 5b it can be seen that a drop 514 of dissolved material formed from a plurality of smaller droplets, once it grows to touch the sides of a well, will tend to be drawn into the corners. This allows molecular electronic material to be deposited into a well by means of an incomplete filling method, that is where droplets are deposited so that they incompletely fill the well, and are then distributed to fill the well by capillary action.
To fabricate the undercut banks shown in FIGS. 6a and 6b a variety of techniques may be employed. Preferably a photodefinable polymer or photoresist such as polyimide or an acrylic photoresist is lithographically patterned using a mask or reticle and then developed to produce a desired bank face angle. Either a positive or a negative photoresist may be employed (for example there are image reversal methods which may be employed to reverse an image in a positive resist). To obtain an undercut photoresist the photoresist may be under-(or over-) exposed and overdeveloped; optionally an undercut profile may be assisted by soaking in a solvent prior to development. The skilled person will be aware that there are many variations of the basic spin, expose, bake, develop, and rinse procedure used in photolithography (see, for example, A. Reiser, Photoreactive Polymers, Wiley, New York, 1989, page 39, hereby incorporated by reference). Some particularly suitable resist materials are available from Zeon Corporation of Japan, who supply materials adapted for the fabrication of organic electroluminescent displays (negative resist materials in the ELX series, and positive resist materials in the WIX series).
The height of the resist banks 610 is preferably less than or equal to 1.2 μm, more preferably in the range 0.5 to 1.0 μm, although lower height banks, for example down to 0.45 μm or even less may be employed.
It is has been observed that at the lower end of the preferred thickness range, with undercut banks, the edge of a bank tends to turn up slightly to form a lip, as shown in FIGS. 6c and 6d, which may improve containment of ink within a well. The formation of such a lip may be related to stress relief within the bank structure.
As stated above, deposition into wells according to the methods of this invention provides improved well-filling and film drying. Each of these advantages are described in more detail below with respect to FIG. 7.
Well Filling
FIG. 7
a illustrates some of the forces which act at the edge of an interface between a solid 700 and a drop of liquid 702. The edge of the drop of liquid makes an angle θ with the surface of the solid and this angle is related to the surface tension of the liquid σst and to the solid (-vapour) surface energy (energy per unit area) σs and solid-liquid surface energy σsl by the equation
σst cos θ+σsl=σs Equation 1
This equation is helpful in understanding FIGS. 7b to 7e described below.
FIGS. 7
b to 7e (not to scale) show effects of progressive increase bank face steepness; like elements of FIG. 6 are indicated by like reference numerals. For each figure the left hand diagram illustrates a vertical cross section through a bank face forming the edge of a well containing dissolved molecular material 602. The centre diagram depicts the configuration of a drop straddling the bank edge, that is half on the bank face and half on the underlying anode.
Referring first to FIG. 7b, this shows a bank having an angle of approximately 15° to the underlying substrate, the liquid drop contacting the face of the bank at approximately 35°. Where a drop straddles the bank edge, one of the factors affecting the extent to which the drop is drawn into the well is the angle of the bank to the substrate. At shallow bank angles, the contact area between the bank face and the drop edge is relatively small. Consequently, there is a relatively small driving force for driving the drop from the low surface energy bank material and onto the higher surface energy well base.
As the steepness of the bank face increases there is an increase in the contact surface area between the bank face and the drop edge, and consequently an increase in the driving force for drawing material off the bank and into the well. This is illustrated by the central diagram in FIGS. 7d and 7e.
FIG. 7
e shows a bank 610 with an undercut or overhanging face. This arrangement provides a particularly high contact area for the drop edge on bank face and in consequence substantially all of a drop straddling the bank and base of the well is drawn into the well. In the example shown in FIG. 7e the face is at an angle of −35°, that is tilted away from the vertical by an angle substantially equal to the contact angle of the solvent, although it will be appreciated that other negative or undercut angles give similar effects.
Film Drying
The effect of bank angle on film drying for a drop located within a well is illustrated by the right hand diagrams of FIGS. 7b-7e.
As shown in FIG. 7b, a shallow positive bank angle results in thickness of the dry film diminishing towards the bank. The inventors have found that this edge thickness may diminish to zero, resulting in a short between the anode and the cathode and a dim or missing pixel.
FIG. 7
c shows a bank with a face at substantially the same angle as the contact angle of the solvent 602, resulting in a substantially flat film as can been seen from the thickness-distance graph. The effect of gradually increasing the angle of the bank face is shown by the dashed lines, this tending to pull the solvent up adjacent the bank face resulting in an increase in dry film thickness adjacent the face and a decrease elsewhere.
FIG. 7
d illustrates a bank face that is at 90° to the substrate. Here a significant volume of the dissolved material is drawn up adjacent to the bank face.
Thus the dry film thickness depends upon the height of the bank, the bank angle, the solvent evaporation (drying stage) conditions and the extent of any coffee-ring effect (also affected by the ink formulation, for example the solid content and molecular weight) and can be determined by experiment (for example by preparing films under a range of conditions thickness-distance graphs using an interferometer, for example from Zygo Corporation of Connecticut, USA. Referring to the centre diagram of FIG. 7e it can also be seen that there is a significant tendency for solvent carrying dissolved material to be drawn out from the sides of the drop along the undercut of the bank face, which is useful for obtaining substantially complete well-filling from incomplete or partial filling of a well by droplet deposition. Referring to equation 1 above and to FIG. 7a, broadly speaking in the “ears”. of the droplet θ is reduced so that cos θ increases, effectively reducing the surface tension pulling the drop towards a more rounded shape.
The skilled person will recognized that the above described techniques are not limited to use in the fabrication of organic light emitting diodes (small molecules or polymer) but may be employed in the fabrication of any type of molecular electronic device in which material is dissolved in a solvent and deposited by a droplet deposition technique. No doubt many effective alternatives will occur to the skilled person and it will be understand that the invention is not limited to the described embodiments encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.
Patent Application
20080095981
