Organic electroluminescent devices: enhanced carrier injection using SAM derivatized ITO electrodes

Abstract

Taking as a device model ITO|TPD|Alq 3 |Al (where TPD is N,N'-bis(3-methylphenyl)-N,N'-diphenyl-1,1'biphenyl-4,4'-diamine and Alq 3 is tris(quinolin-8-olato)aluminium) it is shown that control and improvement of carrier injection may be achieved using self-assembled monolayers (SAMs) to manipulate the Schottky energy barrier at the ITO±TPD interface. By using polar adsorbate molecules with the dipole oriented outward from the surface an arti®cial dipolar layer is formed and the work function is increased, and vice versa. With this method the threshold voltage for light emission (turn-on) can be reduced by 4 V and the maximum luminance increased by a factor of 3.5, giving an overall performance superior to that using the more stable Ag/Mg counter electrode. The SAMs effect is con®rmed using a Scanning Kelvin Probe (SKP) to pro®le the relative work function of half-coated ITO samples. Increases in work function in excess of 0.3 eV are observed, in line with predictions using the calculated molecular dipoles of the SAM molecules.

Organic Electronic Devices and Their Functional Interfaces

“…In addition, the use of ITO surface-attached self-assembled monolayers (SAMs) comprising dipolar moieties has been shown to allow adjustment of HIBs towards organic semiconductors, again with the Helmholtz equation as conceptual basis. [53][54][55][56][57][58] A very successful approach in this direction consists of using phosphonic acid derivatives of fully conjugated moieties, which can self-assemble on ITO to form a dense monolayer. Subsequent annealing leads to the formation of covalent bonds from every molecule to ITO, thus forming a highly stable interface.…”

Electronic structure of interfaces with conjugated organic materials

“…(ii) Self-assembled monolayers with dipoles: The use of electrode surface-attached self-assembled monolayers (SAMs) comprising dipolar moieties has been shown to allow adjusting charge injection barriers towards organic semiconductors, having the Helmholtz equation as conceptual basis as well [63][64][65]. SAMs based on alkyl chains with thiol end-groups for covalent bonding to metals are frequently used.…”

Modification of ITO surface using aromatic small molecules with carboxylic acid groups for OLED applications

“…Therefore, a variety of interfacial treatments have been applied in both cathode/organic [7,8] and anode/organic [9] interfaces to modify the ITO surface in order to enhance the efficiency of OLED devices [10]. Self-assembled monolayer technique (SAM) provides various kinds of functionalities for the hydroxyl groups present on ITO surface by using different organic compounds having various functional groups such as phosphonic acids [11] and carboxylic acids [12]. Moreover the other functions of SAMs reported in the literature for OLED applications include the formation of current blocking layer [13] or a moisture penetration blocking layer [14,15], a dipolar surface layer to enhance charge injection [16], changing the work function of ITO [17] and enhancing the adhesion and stability of HTL layer [18].…”