Covalently Bound Hole-Injecting Nanostructures. Systematics of Molecular Architecture, Thickness, Saturation, and Electron-Blocking Characteristics on Organic Light-Emitting Diode Luminance, Turn-on Voltage, and Quantum Efficiency

Abstract

Hole transporting materials are widely used in multilayer organic and polymer light-emitting diodes (OLEDs, PLEDs, respectively) and are indispensable if device electroluminescent response and durability are to be truly optimized. This contribution analyzes the relative effects of tin-doped indium oxide (ITO) anode-hole transporting layer (HTL) contact versus the intrinsic HTL materials properties on OLED performance. Two siloxane-based HTL materials, N,N'-bis(p-trichlorosilylpropyl)-naphthalen-1-yl)-N,N'-diphenyl-biphenyl-4,4'-diamine (NPB-Si(2)) and 4,4'-bis[(p-trichlorosilylpropylphenyl)phenylamino]biphenyl (TPD-Si(2)), are designed and synthesized. They have the same hole transporting triarylamine cores as conventional HTL materials such as 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) and N,N-diphenyl-N,N-bis(3-methylphenyl)-1,1-biphenyl)-4,4-diamine (TPD), respectively. However, they covalently bind to the ITO anode, forming anode-HTL contacts that are intrinsically different from those of the anode to TPD and NPB. Applied to archetypical tris(8-hydroxyquinolato)aluminum(III) (Alq)-based OLEDs as (1) the sole HTLs or (2) anode-NPB HTL interlayers, NPB-Si(2) and TPD-Si(2) enhance device electroluminescent response significantly versus comparable devices based on NPB alone. In the first case, OLEDs with 36 000 cd/m(2) luminance, 1.6% forward external quantum efficiency (eta(ext)), and 5 V turn-on voltages are achieved, affording a 250% increase in luminance and approximately 50% reduction in turn-on voltage, as compared to NPB-based devices. In the second case, even more dramatic enhancement is observed (64 000 cd/m(2) luminance; 2.3% eta(ext); turn-on voltages as low as 3.5 V). The importance of the anode-HTL material contact is further explored by replacing NPB with saturated hydrocarbon siloxane monolayers that covalently bind to the anode, without sacrificing device performance (30 000 cd/m(2) luminance; 2.0% eta(ext); 4.0 V turn-on voltage). These results suggest new strategies for developing OLED hole transporting structures.

Thin-film metal oxides in organic semiconductor devices: their electronic structures, work functions and interfaces

“…Common surface treatments include halogenation, [16][17][18][19][20] plasma reactions, [21][22][23][24][25][26][27] UV-ozone [28][29][30] and controlled air exposure. [31][32][33][34][35][36] Buffer layers include vapor-deposited small molecules, [37][38][39] spincast polymers, [40][41][42][43][44][45][46] inorganic salts, 12,47-52 covalently bound selfassembled monolayers [53][54][55][56][57][58] and thin metal oxide layers. This review concentrates on the use of transition metal oxides as buffer layers in organic electronics.…”

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

“…ITO surface has been modified with SAM technique using suitable materials for hole injection to improve the interfacial compatibility [19] with charge transfer at the interface between ITO and organic hole-transport layer (HTL) [20]. To enhance such charge transfers in an OLED device, thereby improving turn-on voltage, brightness, and external quantum efficiency, coating of triarylamine-based molecules with trichlorosilane-binding groups on ITO as nanoscale layers were performed in the literature [21][22][23]. But, it is a very well-known fact that such kind of silane derivatives and trialkoxy silanes as well have a tendency to self-condensation in solution [24,25] causing the formation of uniform monolayers difficult.…”

p -Type semiconducting nickel oxide as an efficiency-enhancing anode interfacial layer in polymer bulk-heterojunction solar cells

“…Nanoscale ''engineering'' of the anode-organic interface has been successfully implemented in organic light-emitting diodes (OLEDs) for enhancing electrode-organic interfacial physical and electrical contact, resulting in reduced turn-on voltage, blocking of misdirected carriers, enhanced thermal durability, and increased current/power efficiency (20)(21)(22)(23)(24). In BHJ OPVs, interfacial effects probably limit realization of the maximum theoretical open-circuit voltage (V oc ).…”

“…Furthermore, many researchers find that PE-DOT:PSS depositions yield inconsistent film morphologies and electrical properties in accord with the demonstrated electrical inhomogeneity of these films (32,33). Finally, polymer light-emitting diode results show that PEDOT:PSS is an inefficient electron-blocking layer, reducing device current efficiency due to electron leakage to the anode (21,22,24,30). This combination of limitations motivates replacement of PEDOT:PSS by a more suitable material for optimum OPV performance.…”