Systematic investigation of the effects of organic film structure on light emitting diode performance

Abstract

We present a systematic investigation of the effects of organic film structure on light emitting diode (LED) performance. Metal/organic film/metal LEDs were fabricated using a five ring, poly(phenylene vinylene) related oligomer as the active layer. The structure of the vacuum evaporated oligomer films was varied from amorphous to polycrystalline by changing the substrate temperature during deposition. The intrinsic properties of the oligomer films and the LED performance were measured. The measured intrinsic film properties include: optical absorption, photoluminescence (PL) spectra, PL lifetime, PL efficiency, and effective carrier mobility. The measured device characteristics include current–voltage, capacitance–voltage, electroluminescence (EL) efficiency, and the contact metal/organic film Schottky barrier heights. The optical absorption and PL properties of the films are weakly dependent on film structure but the effective carrier mobility decreases with increasing crystallinity. The EL quantum efficiency decreases by more than one order of magnitude, the drive voltage at a fixed current increases, and the electron Schottky barrier height increases as the crystallinity of the film is increased. The diode current–voltage characteristic is determined by the dominant hole current and the electroluminescence efficiency is controlled by the contact limited electron injection. These results demonstrate significant effects of organic film structure on the performance of organic LEDs.

Electroluminescence from Well-Defined Tetrahedral Oligophenylenevinylene Tetramers

“…Compounds such as C( t BuSSB) 4 ) display glass transition temperatures, instead of melting transitions and are strongly emissive in the solid. [3] Despite these encouraging indicators, [4] nothing is known on the utility of compounds based on the ªtetrahedral strategyº in device applications.In this contribution we report that molecules based on the tetrakis(4-styryldistyrylbenzene)methane framework also give amorphous films and that these can be used in the fabrication of light-emitting diodes (LEDs) with small turn-on voltages.We also present characterization studies that indicate varying levels of interchromophore contact within the resulting amorphous films.As shown in Scheme 1, the reaction of tetrakis(4-iodophenyl)methane with excess 2,5-dioctyloxy-1-styryl-4-(4¢-vinylstyryl)benzene (3) under palladium-catalyzed Heck coupling [5] conditions affords tetrakis((4-(2¢,5¢-dioctyloxy-4¢-styryl)styryl)stilbenylmethane (T-4R-OC 8 H 17 ) in 72 % yield. [6] The olefin 3 is prepared by reaction of 2,5-dioctyloxy-4-(4¢-vinyl)styrylbenzaldehyde (2) and benzyltriphenylphosphonium bromide with an 82 % yield.…”

“…These ªmolecular glassesº embody the beneficial properties of small molecules, namely purity and well-defined structures, and of polymeric materials, in particular their resistance to crystallization and film-forming ability. Specific examples include 4,4¢,4²-tris(N-(3-methylphenyl)-N-phenylamino)triphenylamine (MTDATA), [1] 2,2¢,7,7¢-tetrakis(biphenyl-4-yl)-9,9¢-spirofluorene (TBS), [2] and tetrakis(4-tert-butylstyrylstilbenyl)methane (C( t BuSSB) 4 ). In these molecules, electroactive or photoactive groups are attached to a point of convergence that endows the molecular framework with a shape that discourages crystallization.…”

“…The ªstarburstº TDATA and the spiro-shaped TBS have been used in the fabrication of devices. Compounds such as C( t BuSSB) 4 ) display glass transition temperatures, instead of melting transitions and are strongly emissive in the solid. [3] Despite these encouraging indicators, [4] nothing is known on the utility of compounds based on the ªtetrahedral strategyº in device applications.…”

X‐Shaped Electroactive Molecular Materials Based on Oligothiophene Architectures: Facile Synthesis and Photophysical and Electrochemical Properties

“…Aggregation of linear p-conjugated materials (especially H-type aggregates) is often observed and will result in low quantum luminescence efficiencies, [17] so few fluorescent polythiophenes have been reported. Here, our discussion is focused on the photophysical properties of the new electroactive polymer.…”

Anthracene‐Containing Binaphthol Chromophores for Light‐Emitting Diode (LED) Fabrication

“…[4] Additionally, aggregate formation in crystalline samples leads to luminescence quenching, and therefore has detrimental effects on the device stability. [5] In response to the complications mentioned above, efforts have been directed towards the design of amorphous ªmolecu-lar glassesº that have well-defined molecular components. [6] Desirable properties include the ability to form films directly from solution and to be rigorously purified.…”