Buried interfaces: interfacial interactions in materials for polymer-LED applications

Abstract

Chemical interactions at the interface between a precursor polymer for poly(p‐phenylenevinylene), namely poly(p‐xylylene‐α‐tetrahydrothiophene chloride), and an indium–tin–oxide (ITO) transparent electrode, have been studied through polymer films of > 10 nm thick using x‐ray photoelectron spectroscopy. The HCl released in the conversion process interacts with the surface of the ITO substrate at the polymer/substrate interface, leading to the formation of indium chloride, which then diffuses into the polymer. It appears that ITO, or possibly the indium chloride generated at the interface, acts as a catalyst for the thermal elimination reaction. The results show unequivocally that interfacial interactions are of major importance in understanding, and ultimately improving, the performance of polymer‐based electroluminescent devices, and that interfaces buried under > 10 nm of polymer may be studied. Copyright © 1999 John Wiley & Sons, Ltd.

Nanometer-Scale Dielectric Self-assembly Process for Anode Modification in Organic Light-Emitting Diodes. Consequences for Charge Injection and Enhanced Luminous Efficiency

“…The present observed increase in heterostructure quantum efficiency with anode dielectric functionalization is in partial phenomenological agreement with two other recent studies in which the microstructure and contiguity of dielectric structures, grown by non conformal line-of-sight physical vapor deposition techniques were not as well-characterized. For SiO 2 ,13d V turn - on was found to first fall and then rise with apparent increasing dielectric thickness, while for Al 2 O 3 , 15b,c V turn - on rises continuously with apparent increasing dielectric thickness. In both cases, the rationalization for this behavior has been better balance between hole and electron injection into the HTL and ETL, respectively.…”

“…At each metal/semiconductor−organic interface, a barrier to charge injection exists that is poorly understood but doubtless dependent upon, among other factors, atomic scale microstructural details and the energetic alignment of the electrode Fermi levels with the HOMO or LUMO of the hole transport layer (HTL) or electron transport layer (ETL), respectively. ,,,− It has recently been observed that OLED response characteristics (quantum efficiency, turn-on voltage, and device robustness) can be dramatically altered by nanoscale modification of the electrode−organic interface and via mechanisms that are in a great many cases incompletely defined. 3f,5b,, Modifications to the ETL−metal cathode interface are variously postulated to increase the electron injection fluence into the ETL, to prevent migration of cathode metal ions into the ETL, 3f,5b,13e and to provide buffer layers to protect the ETL from sputter damage during cathode deposition 3c. Likewise, the ITO−HTL interface has also been subjected to extensive empirical manipulation.…”

“…Likewise, the ITO−HTL interface has also been subjected to extensive empirical manipulation. Recent reports describe ITO surface modification by oxidative/protic chemical treatment, oxide overlayer physical vapor deposition, metal deposition, and amphiphile/polymer/monolayer chemisorption. 3a,b,, Details of the resulting interface structure(s) (e.g., contiguity, thickness, smoothness, microstructure, cohesion, and pinhole density) and mechanism(s) of action have generally been difficult to characterize; however, altering interfacial electric fields, ,13e balancing electron/hole injection fluences, reducing injected charge backscattering,13i confining electrons in the emissive layer,13k and minimizing anode Fermi level−HTL HOMO energetic discontinuities 13d,j,k have all been proposed. In each case, varying degrees of improved device performance such as lower turn-on voltage, greater thermal stability, and/or higher quantum efficiency have been reported.…”

“…In each case, varying degrees of improved device performance such as lower turn-on voltage, greater thermal stability, and/or higher quantum efficiency have been reported. Recent studies in this laboratory with self-assembled arylamine hole transport/injection monolayers suggests that one major function of these, and by implication, similar ultrathin organic anode derivatization processes, is to stabilize the physical integrity of anode−HTL interfacial cohesion and ohmic contact by moderating the disparate surface energies. 13a,b …”

A robust ultrathin, transparent gold electrode tailored for hole injection into organic light-emitting diodes

“…The surface of ITO glass is, however, notoriously chemically and physically ill-defined, to the detriment of its performance as the hole-injecting electrode in OLEDs. [4][5][6][7][8][9][10][11] Whilst numerous alternatives have been proposed in recent years, [12][13][14] their shortcomings are such that ITO remains the anode of choice.…”