Enhanced stability of organic light-emitting devices fabricated under ultra-high vacuum condition

Fermi Level Shifts of Organic Semiconductor Films in Ambient Air

Here, the Fermi level (EF) shifts of several donor and acceptor materials in different atmospheres are systematically studied by following the work function (WF) changes with Kelvin probe measurements, ultraviolet photoelectron spectroscopy, and near-ambient pressure X-ray photoelectron spectroscopy. Reversible EF shifts are found with the trend of higher WFs measured in ambient air and lower WFs measured in high vacuum compared to the WFs measured in ultrahigh vacuum. The EF shifts are energy level and morphology-dependent, and two mechanisms are proposed: (1) competition between p-doping induced by O2 and H2O/O2 complexes and n-doping induced by H2O; (2) polar H2O molecules preferentially modifying the ionization energy of one of the frontier molecular orbitals over the other. The results provide a deep understanding of the role of the O2 and H2O molecules in organic semiconductors, guiding the way toward air-stable organic electronic devices.

The Blue Problem: OLED Stability and Degradation Mechanisms

OLED technology has revolutionized the display industry and is promising for lighting. Despite its maturity, there remain outstanding device and materials challenges to address. Particularly, achieving stable and highly efficient blue OLEDs is still proving to be difficult; the vast array of degradation mechanisms at play, coupled with the precise balance of device parameters needed for blue high-performance OLEDs, creates a unique set of challenges in the quest for a suitably stable yet high-performance device. Here, we discuss recent progress in the understanding of device degradation pathways and provide an overview of possible strategies to increase device lifetimes without a significant efficiency trade-off. Only careful consideration of all variables that go into OLED development, from the choice of materials to a deep understanding of which degradation mechanisms need to be suppressed for the particular structure, can lead to a meaningful positive change toward commercializable blue devices.

Operando direct observation of spin-states and charge-trappings of blue light-emitting-diode materials in thin-film devices

Spin-states and charge-trappings in blue organic light-emitting diodes (OLEDs) are important issues for developing high-device-performance application such as full-color displays and white illumination. However, they have not yet been completely clarified because of the lack of a study from a microscopic viewpoint. Here, we report operando electron spin resonance (ESR) spectroscopy to investigate the spin-states and charge-trappings in organic semiconductor materials used for blue OLEDs such as a blue light-emitting material 1-bis(2-naphthyl)anthracene (ADN) using metal–insulator–semiconductor (MIS) diodes, hole or electron only devices, and blue OLEDs from the microscopic viewpoint. We have clarified spin-states of electrically accumulated holes and electrons and their charge-trappings in the MIS diodes at the molecular level by directly observing their electrically-induced ESR signals; the spin-states are well reproduced by density functional theory. In contrast to a green light-emitting material, the ADN radical anions largely accumulate in the film, which will cause the large degradation of the molecule and devices. The result will give deeper understanding of blue OLEDs and be useful for developing high-performance and durable devices.

Enhanced Electrical Properties and Air Stability of Amorphous Organic Thin Films by Engineering Film Density

The influences of film density and molecular orientation on the carrier conduction and air stability of vacuum-deposited amorphous organic films of N,N'-di(1-naphthyl)-N,N'-diphenyl-(1,1'-biphenyl)-4,4'-diamine (α-NPD) were investigated. The substrate temperature (T_sub) during vacuum deposition had different effects on the film density and molecular orientation of α-NPD. Film density was a concave function of T_sub; maximum density was attained at T_sub = 270-300 K. α-NPD molecules were randomly oriented at T_sub = 342 K, and their horizontal orientation on the substrate became dominant as T_sub decreased. Hole current and air stability were clearly raised by increasing the film density by 1 to 2%; these effects were, respectively, attributed to enhanced carrier hopping between neighboring α-NPD molecules and suppressed penetration of oxygen and water. These results imply that increasing film density is more effective to enhance the electrical performance of organic thin-film devices with α-NPD films than control of molecular orientation.

Operational stability enhancement in organic light-emitting diodes with ultrathin Liq interlayers

Organic light-emitting diodes (OLEDs) under constant current operation suffer from a decrease of luminance accompanied by an increase of driving voltage. We report a way to greatly improve the stability of OLEDs having a green emitter exhibiting thermally activated delayed fluorescence (TADF), (4s,6s)-2,4,5,6-tetra(9H-carbazol-9-yl) isophthalonitrile (4CzIPN), by introducing ultrathin (1 to 3 nm) interlayers of 8-hydroxyquinolinato lithium (Liq) between hole-blocking layer and its surrounding emissive and electron-transport layers. Under constant current operation starting at a luminescence of 1,000 cd/m², the time to reach 90% of initial luminance (LT₉₀) increased eight times, resulting in LT₉₀ = 1,380 hours after insertion of the interlayers. Combining this new concept and mixed host system, LT₉₅ was further extended to 1315 hours that is 16 times of reference device. This is the best value reported for TADF-based OLEDs and is comparable to the operational lifetimes of well-established phosphorescence-based OLEDs. Thermally stimulated current measurements showed that the number of deep charge traps was reduced with the insertion of the ultrathin Liq interlayer, indicating that reducing the number of deep traps is important for improving the operational lifetime and that exciton-polaron annihilation may be a source of the device degradation.

Solution-processed multilayer small-molecule light-emitting devices with high-efficiency white-light emission

Recent developments in the field of π-conjugated polymers have led to considerable improvements in the performance of solution-processed organic light-emitting devices (OLEDs). However, further improving efficiency is still required to compete with other traditional light sources. Here we demonstrate efficient solution-processed multilayer OLEDs using small molecules. On the basis of estimates from a solvent resistance test of small host molecules, we demonstrate that covalent dimerization or trimerization instead of polymerization can afford conventional small host molecules sufficient resistance to alcohols used for processing upper layers. This allows us to construct multilayer OLEDs through subsequent solution-processing steps, achieving record-high power efficiencies of 36, 52 and 34 lm W(-1) at 100 cd m(-2) for solution-processed blue, green and white OLEDs, respectively, with stable electroluminescence spectra under varying current density. We also show that the composition at the resulting interface of solution-processed layers is a critical factor in determining device performance.

Dry lithography of large-area, thin-film organic semiconductors using frozen CO(2) resists

To address the incompatibility of organic semiconductors with traditional photolithography, an inert, frozen CO(2) resist is demonstrated that forms an in situ shadow mask. Contact with a room-temperature micro-featured stamp is used to pattern the resist. After thin film deposition, the remaining CO(2) is sublimed to lift off unwanted material. Pixel densities of 325 pixels-per-inch are shown.

One pot synthesis of functionalized SBA-15 by using an 8-hydroxyquinoline-5-sulfonamide-modified organosilane as precursor

A novel functionalized SBA-15 mesoporous material was prepared through co-condensation of tetraethylorthosilicate with an 8-hydroxyquinoline-5-sulfonamide-modified organosilane precursor in the presence of P123 as structure-directing agent. After removal of template, the obtained material was characterized by powder X-ray diffraction (XRD), nitrogen adsorption-desorption, Fourier transform infrared (FT-IR), thermal analysis (TGA-DTA), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and elemental analysis. Then, aluminum quinolate complex was attached covalently to this functionalized SBA-15 by using coordinating ability of grafted 8-HQ and its emission spectra showed a slightly blue shift in comparison with AlQ(3) complex.