|
We present a theoretical and experimental analysis of operation and degradation of model fluorescent blue bilayer polymer organic light emitting diodes (P-OLED). Optical and electrical simulations of bilayer P-OLEDs are used to highlight the key material and device parameters required for efficient recombination and outcoupling of excitons. Mobility data for a model interlayer material poly (9,9-dioctylfluorene-N-(4-(2-butyl)phenyl)-diphenylamine) (TFB) and
a model fluorescent blue light emitting material poly-(9,9'- dioctylfluorene-co-bis-N, N'-(4-butylphenyl)-bis-N,N'- phenyl-1,4-phenylenediamine) (95:5 mol%) (F8-PFB random copoloymer), is shown to satisfy the key charge transport characteristics required to ensure exciton formation at the optimum location for efficient extraction of the light where μh (LEP) < μe (iL) < μe (LEP) < μh (iL). A method to measure the photon generation zone profile and dipole orientation is presented and shown to follow the expected behavior. The efficiency drop of P-OLEDs during device operation is a
known issue, the understanding and prevention of which is key for the commercial success of P-OLED technology. We present a detailed degradation study of devices containing model materials, and highlight the generation of fluorescence quenching sites as the key factor limiting the operational stability. A striking feature of this degradation is its partial (~50%) reversibility upon baking above the LEP glass transition temperature. Some reversibility is also observed in the conductivity, suggesting a common origin to the optical and electrical degradation. We also show that the species
responsible for the generation of the reversible PL quenching sites are the excitons themselves, and that optically excited excitons can also generate many of the features characteristic of electrical stressing. Finally we demonstrate that materials with a dramatically improved lifetime also suffer from a similar, although slowed down, degradation mechanism, where the reversible component is increased to almost all (>90%) of the quenching sites produced. This
highlights the importance of understanding these reversible phenomena in improving P-OLED lifetime and commercial adoption of the technology.
|
