Pressure-assisted fabrication of organic light emitting diodes with MoO3 hole-injection layer materials

• Published in: Journal of applied physics 2014-06, Vol.115 (23)
• Main Authors: Du, J., Anye, V. C., Vodah, E. O., Tong, T., Zebaze Kana, M. G., Soboyejo, W. O.
• Format: Article
• Language: English
• Subjects: Adhesion tests; Atomic force microscopy; Blistering; Contact pressure; Electronic devices; Finite element method; Fracture mechanics; Mathematical analysis; Microscopy; Modulus of elasticity; Molybdenum oxides; Molybdenum trioxide; Nanoindentation; Organic light emitting diodes; Polystyrene resins; Pressure effects; Spin coating
• ISSN: 0021-8979; 1089-7550
• DOI: 10.1063/1.4881780

In this study, pressures of ∼5 to ∼8 MPa were applied to organic light emitting diodes containing either evaporated molybdenum trioxide (MoO3) or spin-coated poly(3,4-ethylene dioxythiophene) doped with poly(styrene sulphonate) (PEDOT:PSS) hole-injection layers (HILs). The threshold voltages for both devices were reduced by about half, after the application of pressure. Furthermore, in an effort to understand the effects of pressure treatment, finite element simulations were used to study the evolution of surface contact between the HIL and emissive layer (EML) under pressure. The blister area due to interfacial impurities was also calculated. This was shown to reduce by about half, when the applied pressures were between ∼5 and 8 MPa. The finite element simulations used Young's modulus measurements of MoO3 that were measured using the nanoindentation technique. They also incorporated measurements of the adhesion energy between the HIL and EML (measured by force microscopy during atomic force microscopy). Within a fracture mechanics framework, the implications of the results are then discussed for the pressure-assisted fabrication of robust organic electronic devices.