Ehsan Najafabadi

A Universal Method to Produce Low–Work Function Electrodes for Organic Electronics

A Sturdy Electrode Coating To operate efficiently, organic devices—such as light-emitting diodes—require electrodes that emit or take up electrons at low applied voltages (that is, have low work functions). Often these electrodes are metals, such as calcium, that are not stable in air or water vapor and have to be protected from environmental damage. Zhou et al. (p. 327; see the Perspective by Helander) report that a coating polymer containing aliphatic amine groups can lower the work functions of various types of electrodes by up to 1.7 electron volts and can be used in a variety of devices. Air-stable, physisorbed polymers containing aliphatic amine groups can improve the efficiency of organic electronic devices. Organic and printed electronics technologies require conductors with a work function that is sufficiently low to facilitate the transport of electrons in and out of various optoelectronic devices. We show that surface modifiers based on polymers containing simple aliphatic amine groups substantially reduce the work function of conductors including metals, transparent conductive metal oxides, conducting polymers, and graphene. The reduction arises from physisorption of the neutral polymer, which turns the modified conductors into efficient electron-selective electrodes in organic optoelectronic devices. These polymer surface modifiers are processed in air from solution, providing an appealing alternative to chemically reactive low–work function metals. Their use can pave the way to simplified manufacturing of low-cost and large-area organic electronic technologies.

Highly efficient inverted top-emitting green phosphorescent organic light-emitting diodes on glass and flexible substrates

Green phosphorescent inverted top-emitting organic light-emitting diodes with high current efficacy and luminance are demonstrated on glass and polyethersulfone (PES) substrates coated with polyethylene dioxythiophene-polystyrene sulfonate (PEDOT:PSS). The bottom cathode is an aluminum/lithium fluoride bilayer that injects electrons efficiently into an electron transport layer of 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene (TpPyPB). The cathode is found to be highly sensitive to the exposure of trace amounts of O2 and H2O. A high current efficacy of 96.3 cd/A is achieved at a luminance of 1387 cd/m2 when an optical outcoupling layer of N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) is deposited on the anode.

Inverted top-emitting blue electrophosphorescent organic light-emitting diodes with high current efficacy

Two different types of inverted top-emitting blue electrophosphorescent organic light-emitting diodes (OLEDs) are demonstrated that differ only in the choice of high electron mobility transport layers. The electron transport layer consists of either 1,3,5-tri(p-pyrid-3-yl-phenyl)benzene (TpPyPB) or 1,3,5-tri(m-pyrid-3-yl-phenyl)benzene) (TmPyPB). Devices with TpPyPB exhibit a current efficacy of 5.1 cd/A at 1259 cd/m2. OLEDs with TmPyPB show higher performance with a current efficacy of 33.6 cd/A at 1126 cd/m2. The difference in performance of OLEDs with TmPyPB is due to a combination of TmPyPBs higher triplet energy that decreases exciton transfer to the ETL and altered charge balance.

Efficient organic light-emitting diodes fabricated on cellulose nanocrystal substrates

Organic light-emitting diodes (OLEDs) fabricated on recyclable and biodegradable substrates are a step towards the realization of a sustainable OLED technology. We report on efficient OLEDs with an inverted top-emitting architecture on recyclable cellulose nanocrystal (CNC) substrates. The OLEDs have a bottom cathode of Al/LiF deposited on a 400 nm thick N,N′-Di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4,4′-diamine (α-NPD) layer and a top anode of Au/MoO3. They achieve a maximum luminance of 74 591 cd/m2 with a current efficacy of 53.7 cd/A at a luminance of 100 cd/m2 and 41.7 cd/A at 1000 cd/m2. It is shown that the α-NPD layer on the CNC substrate is necessary for achieving high performance OLEDs. The electroluminescent spectra of the OLEDs as a function of viewing angle are presented and show that the OLED spectra are subject to microcavity effects.

Stacked inverted top-emitting white organic light-emitting diodes composed of orange and blue light-emitting units

Stacked inverted top-emitting white electrophosphorescent organic light-emitting diodes (OLEDs) are demonstrated. The OLEDs consist of orange and blue light-emitting units interconnected with a connecting unit of 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile/Al/LiF. These OLEDs combine the features of having inverted electrode positions, top-emission, and a stacked architecture. They exhibit an average current efficacy of 26.5 cd/A at a luminance of 100 cd/m2. Single-unit inverted top-emitting OLEDs based on the constituent orange and blue light-emitting units are also characterized for comparison. The current efficacies of the orange and blue OLEDs are 21.2 cd/A and 32.6 cd/A, respectively, at a luminance of 100 cd/m2.

Recent advances in stacked inverted top-emitting organic electrophosphorescent diodes (presentation video)

In this talk, we will discuss recent advances in green and white electrophosphorescent stacked organic light-emitting diodes (OLEDs) with inverted top-emitting structures. These devices combine the advantages of having inverted electrode positions, a top-emissive design, and a stacked architecture. We will also demonstrate OLEDs that are fabricated on cellulose nanocrystal substrates and discuss how the use of such naturally-derived materials can reduce the environmental footprint of organic electronic devices such as OLEDs.

Efficient Green and Blue Electrophosphorescent Light-Emitting Diodes using a Combination of Solution and Vacuum-Processed Materials

The performance of organic light-emitting diode devices with a spin-coated hole-transporting layer and a thermally deposited emissive layer consisting of a bis-sulfone small molecule, as a host for the blue phosphorescent emitter will be presented.