Microcontact printing of self-assembled monolayers to pattern the light-emission of polymeric light-emitting diodes

Abstract

By patterning a self-assembled monolayer (SAM) of thiolated molecules with opposing dipole moments on a gold anode of a polymer light-emitting diode (PLED), the charge injection and, therefore, the light-emission of the device can be controlled with a micrometer-scale resolution. Gold surfaces were modified with SAMs based on alkanethiols and perfluorinated alkanethiols, applied by microcontact printing, and their work functions have been measured. The molecules form a chemisorbed monolayer of only ∼1.5 nm on the gold surface, thereby locally changing the work function of the metal. Kelvin probe measurements show that the local work function can be tuned from 4.3 to 5.5 eV, which implies that this anode can be used as a hole blocking electrode or as a hole injecting electrode, respectively, in PLEDs based on poly(p-phenylene vinylene) (PPV) derivatives. By microcontact printing of SAMs with opposing dipole moments, the work function was locally modified and the charge injection in the PLED could be controlled down to the micrometer length scale. Consequently, the local lightemission exhibits a high contrast. Microcontact printing of SAMs is a simple and inexpensive method to pattern, with micrometer resolution, the light-emission for low-end applications like static displays.

Spatially Modulating Interfacial Properties of Transparent Conductive Oxides: Patterning Work Function with Phosphonic Acid Self‐Assembled Monolayers

“…[20][21][22][23][24][25][26] Given the strong influence SAM modifiers can have on the performance of organic electronic devices, the ability to microcontact print SAMs with large work function contrast is both scientifically interesting from the standpoint of creating model systems to explore the role of barriers and energy level offsets on charge injection in OLEDs, and technologically useful in the context of applications including low-cost illuminated signs and displays. [3,6] Although a limited amount of work has been performed in this area, notably by microcontact printing thiols on gold, [3,6] silanes on hydroxyl-terminated surfaces, [2,11,12] or phosphoryl chlorides on indium tin oxide (ITO), [27] these functional group/substrate combinations are not necessarily ideal for integration into OPV and OLED applications. First, transparent conductive oxides are more commonly used than gold as the anode in OPVs and OLEDs.…”

“…Blom and co-workers also observed lower-than-expected CPD contrast from an analogous microcontact printed thiol-SAM on gold system, reporting that their microcontact printed SAMs imparted a change in work function that was only 10% of the shift in work function achieved by solution deposition of the SAM. [3] Here, we overcome these difficulties using patterns of pentafluorobenzyl phosphonic acid (F 5 BnPA) deposited via microcontact printing to locally modify the ITO work function. We measure CPD variations of ∼500 mV between the modified and unmodified areas of the ITO surface from microcontact printed F 5 BnPA.…”

“…The ability to pattern phosphonic acids with low-cost printing may find use in applications including low-cost LED signage. [3,6] Given the robust nature of the microcontact printed SAM patterns, it should be possible to use backfilling of a second SAM to tailor the local work function and wettability contrast on both the patterned and unpatterned regions. [39] The microcontact printing of phosphonic acid modified catalysts for in situ poly mer growth [9,40] is also an area of significant future interest.…”

“…Self-assembled monolayers (SAMs) have been used to modify the surface chemistry of polymer, [1,2] metal, [3][4][5][6][7][8][9] and metal oxide [10][11][12][13][14][15][16] electrodes in order to control properties including wettability, work function, and charge transfer, by using many functional group/substrate combinations. Examples range from thiols on gold, [3][4][5][6][7][8] to silanes on hydroxyl-terminated surfaces, [11,12,15,17] to mixtures of SAMs containing different terminal functional groups on metal oxide buffer layers. [13] A number of groups have incorporated SAMs into OLEDs, [1][2][3][4]6,[10][11][12]18,19] with many focusing on the use of phosphonic acid SAMs to modulate interfacial properties and improve device performance.…”

Functionalization and patterning of monolayers on silicon(111) and polydicyclopentadiene

“…The patterning of monolayers has been demonstrated on gold, including micro contact printing of thiols. [78][79][80][81][82][83][84] Monolayers on Glass and Silica Monolayers on glass and silica are made by reacting surface hydroxyl groups with reactive species, normally to yield Si-O-Si-C bond. [85][86][87][88][89] Typical attachments involve reacting molecules with trichlorosilanes, Si-O-CH 2 R or Si-CH 2 R. These can polymerize in solution and the resulting attachment may be a single site linking a long polymer chain rather than individual molecules attaching to a surface.…”