Yichun Luo

Causes of efficiency roll-off in phosphorescent organic light emitting devices: Triplet-triplet annihilation versus triplet-polaron quenching

Delayed electroluminescence measurements are used to probe and differentiate between triplet-triplet-annihilation (TTA) and triplet-polaron-quenching (TPQ) processes and their correlation with efficiency roll-off in fac-tris(2-phenylpyridine) iridium-based phosphorescent organic light emitting devices. Investigations on devices employing 4,4′-bis(9-carbazolyl)-1,1′-biphenyl (CBP) and 4,4′,4″-tris(N-carbazolyl) triphenylamine, two widely used host materials, show that the efficiency roll-off is primarily due to TPQ processes. Guest-guest TTA, on the other hand, is found to play no major role, contrary to speculations, especially at low guest concentrations. Evidence of host-host TTA in certain cases, and its possible contribution to exciton quenching in the case of devices with CBP host, is also reported.

Photodegradation of the organic/metal cathode interface in organic light-emitting devices

We study the photostability of organic light-emitting devices (OLEDs). Irradiating OLEDs by external illumination is found to result in a gradual increase in driving voltage and decrease in electroluminescence (EL) efficiency. This photoinduced degradation in device performance is found to be caused by changes at the organic/metal cathode interface that lead to a deterioration in electron injection. Evidence of photodegradation of the same interface, inherently, by device own EL, is also reported. The results uncover an important degradation mechanism in OLEDs and shed the light on a phenomenon that might limit the stability of other organic optoelectronic and photovoltaic devices.

Degradation mechanisms in organic light-emitting devices: Metal migration model versus unstable tris(8-hydroxyquinoline) aluminum cationic model

To resolve the issue of which of the “indium migration” model and the “unstable AlQ3 cationic” model plays a more important role in luminescence degradation of organic light-emitting devices, we investigated the effect of the device structures on device operational stability. The results show that alterations in device layer structures can significantly affect the device operational stability, although they do not appear to noticeably change the magnitude of indium migrations. This suggests that the indium migration model may not play a dominant role in device degradation. On the other hand, the change in device stability with the alteration in the device structures can be plausibly explained by the unstable AlQ3 cationic model.

Electric-field-induced fluorescence quenching in dye-doped tris(8-hydroxyquinoline) aluminum layers

The authors measured electric-field-induced fluorescence quenching (EFIFQ) in both undoped and fluorescent dye-doped tris(8-hydroxyquinoline)aluminum (AlQ3) layers of organic light-emitting devices. Results show that doped AlQ3 layers demonstrate smaller EFIFQ than undoped ones. The phenomenon is attributed to the narrower energy band gap of the guest molecule relative to that of the host material, which makes it less prone to electric-field-induced dissociation of the excited state. Results also show that increasing the concentration of the guest material or decreasing its band gap leads to a decrease in EFIFQ.

Luminescence degradation in phosphorescent organic light-emitting devices by hole space charges

We studied electroluminescence degradation in phosphorescent organic light-emitting devices (PHOLEDs) and found that two distinctive mechanisms are responsible for device degradation depending on the device structure. For a device without a hole blocking layer (HBL), excess holes penetrate into the electron transport layer (ETL) and lead to the deterioration of the ETL adjacent to the interface of the emitting layer. The lower electron transport capacity of the degraded ETL alters the balance in hole/electron injection into the emitting layer and results in a decrease in the luminescence efficiency of the PHOLEDs. For a device with a HBL, on the other hand, holes accumulate and become trapped in the emitting layer, and result in a decrease in the luminescence efficiency of the PHOLEDs, likely due to their role in acting as exciton quenchers or as nonradiative charge recombination centers.

Space charge effects on the electroluminescence efficiency and stability of organic light-emitting devices with mixed emitting layers

In organic light-emitting devices (OLEDs), the decay rate of triplet state population in the electron/hole recombination zone is found to be highly sensitive to space charge densities, providing an avenue for inferring variations in their formation. In OLEDs containing mixtures of N,N′-Bis(naphthalen-1-yl)-N′-bis(phenyl)benzidine (NPB) and tris(8-hydroxyquinoline) aluminum (AlQ3) in the emitting layer, optimizing the NPB/AlQ3 is found to reduce hole space charges, and leads to an increase in electroluminescence stability. Conversely, electroluminescence efficiency is found to be only weakly dependent on the mixture composition, suggesting that hole space charges are not effective quenchers of AlQ3 singlet excitons in mixed emitting layer OLEDs.

Probing triplet-triplet annihilation zone and determining triplet exciton diffusion length by using delayed electroluminescence

The literature shows that triplet-triplet annihilation (TTA) can provide a substantial contribution to the electroluminescence (EL) of fluorescent organic light-emitting devices (OLEDs). In this study, we utilized delayed EL measurements to probe the TTA emission zone of archetypical 8-hydroxyquinoline aluminum (Alq3) based OLEDs. The results demonstrate that the TTA emission zone of these devices is much larger than the prompt emission zone of singlet states that are formed in the electron-hole recombination. The larger TTA emission zone is attributed to the longer diffusion length of the Alq3 triplet states (60 nm) than that of Alq3 singlet states (20 nm).

Improving the stability of organic light-emitting devices by using a thin Mg anode buffer layer

Introducing a thin Mg layer at the hole injection contact of organic light-emitting devices remarkably improves their operational stability. Devices in which a ∼2.5nm thick Mg layer is inserted between the indium tin oxide anode and a tetrafluoro-tetracyanoquinodimethane-doped hole transport material layer exhibit a significantly longer lifetime compared to similar devices without the Mg layer. After 600h of operation at a current density of 62.5mA∕cm2 with a 50% duty cycle, the luminance of devices containing the Mg layer decreases by only ∼10% of the initial value. The stability enhancement resulting from using the Mg layer is attributed to improved balance in charge injection at the anode and cathode contacts.

Evidence of intermolecular species formation with electrical aging in anthracene-based blue organic light-emitting devices

Electrical aging mechanism in blue emitting organic light-emitting devices (OLEDs) based on 9,10-bis (2-naphthyl)-2-t-butyl anthracene (TBADN) fluorescent emitter is investigated using a number of techniques, including delayed electroluminescence measurements. The studies reveal that electrical aging is associated with an increasing concentration of an intermolecular species with a weak characteristic luminescence at around 535 nm. This species is capable of charge trapping, and thus plays a role as an electron-hole recombination center with prolonged electrical driving. Weak green luminescence from this species leads to an increased green/blue emission ratio, and causes the color purity loss in aged devices. The results also suggest that this species is also efficient in dissipating excitation energy nonradiatively, hence is capable of quenching TBADN singlet excitons, contributing to the observed efficiency loss with electrical aging.

Improving the stability of organic light-emitting devices by using a hole-injection-tunable-anode-buffer-layer

Introducing a hole-injection-tunable-anode-buffer-layer (HITABL) at the indium tin oxide anode contact of an organic light-emitting device can finely tune hole injection to establish proper charge balance, thus remarkably improves its operational stability. The HITABL consists of two sublayers: (i) an ∼2.5nm thick metal (e.g., Ca, Mg, or Ag) sublayer and (ii) an ∼10nm thick tetrafluorotetracyanoquinodimethane (F4TCNQ) doped N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine sublayer. Hole injection can be tuned by changing (i) the metal in the first sublayer and/or (ii) the concentration of the F4TCNQ dopant in the second sublayer. The choice of the metal used in the first sublayer and/or the concentration of F4TCNQ in the second sublayer affect the hole-injection efficiency. Therefore, by using the HITABL, one can make the necessary diminutive adjustments to the hole injection of a device and achieve proper charge balance, resulting in a significant improvement in operational stability.