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Magnesium (Mg) and silver (Ag) alloys are staple electrodes for organic light-emitting diodes (OLEDs). In this study, the Mg:Ag binary phase diagram is used to establish an electrode materials selection guide for both bottom- and top-emitting OLEDs. To achieve low turn-on voltage and high efficiency in bottom-emitting OLEDs, it is found that Mg:Ag cathodes must be selected from AgMg3 Mg phase region which provides the best figure of merits in low work function and high optical reflectance. For top-emitting OLEDs, however, Mg:Ag cathodes in the AgMg3 Mg region show large imaginary refractive index, and thus a stacked Mg:Ag/Ag is found essential to achieve optimal device performance. Furthermore, insertion of a thin LiF layer is found to create an effective charge injection cathode. In order to theoretically compute the devices’ optical characteristics, a database of complex optical constants of various materials including the metals is obtained by using a spectroscopic ellipsometer. This new database is found essential for accurate device optical design and engineering.

In a non-crystalline molecular material, glass-transition temperature Tg is typically defined by an inflection point in the specific volume (or thickness) vs. temperature plot. In this paper, the glass-transitions in thin-film organic semiconductors are investigated by using various temperature spectroscopic ellipsometer. We observed that glass transition temperatures in organic thin films are characterized by a disruptive change in thickness vs. temperature plot. In order to modify the glass transition temperature of an organic semiconductor, we also made composite thin-films by inclusion of another organic molecule with higher Tg. We found that the transition temperature can be significantly improved by inclusion of another organic molecule. The glass-transition processes of these composite semiconductors, however, also proceed in a disruptive manner, in stark contrast to typical polymeric materials. Despite this disruptive glass transition behavior, the experimental Tg of the composite semiconductors are found to follow precisely a simple inverse sum equation of constituent temperatures with their respective weight factors.

High-contrast organic light-emitting diodes (HC-OLEDs) with black phase-changing (PC) cathodes have been designed and fabricated. The black PC-cathode consists of a front thin semitransparent metal layer, an organic PC layer, and a rear thick reflective metal layer. To achieve a targeted high contrast over the full visible spectrum, several organic molecules and their binary and ternary compounds were researched to meet optical design requirement. By combining a design in the complex refractive index and in the thickness of the PC layer, it is demonstrated that the optical reflectance of the HC-OLED can be reduced from ∼80% to below that of the glass substrate (∼5%) by using a ternary organic compound as the PC layer. This black PC-cathode produces extremely high optical contrast, and yet it does not alter the current–voltage characteristics of the device.

Molecular orientation in organic semiconductors plays a critical role in maximizing external quantum efficiencies of organic light-emitting diodes. It was generally believed that the molecular packing of organic semiconductors is either amorphous or liquid-crystal-like with a preferred molecular orientation distributed uniformly throughout the film. In this paper, however, we report that the orientation of organic molecules in physical-vapor deposited films varies drastically depending on thickness. The thermal stability of the molecular network, measured by its characteristic glass transition temperature, also varies as a function of the film thickness. Based on a two-layered film-structure model, we propose a simple function to quantify the molecular dipole orientation S parameter as a function of film thickness. This function describes well experimental data. In addition to contributing to external quantum efficiency, the molecular orientation parameter S is found to have a strong impact on disruptive change in material density after thermal anneal and glass transition.

Nonradiative recombination is one of the key electronic processes in semiconductor devices. In this study, we demonstrate that nonradiative recombination rate of intermolecular charge-transfer (CT) exciton at organic heterojunction increases exponentially with decreasing energy difference between the LUMO of acceptor and the HOMO of donor. In addition, the effect of orbital overlap between the acceptor LUMOs and donor HOMOs on nonradiative recombination is studied using a wide bandgap organic spacer layer. It is found that the rate for nonradiative CT exciton recombination decreases exponentially as the donor–acceptor separation distance increases. Nonradiative CT exciton recombination is an important consideration in selecting materials for organic devices such as organic solar cells, organic light-emitting diodes, and so on.

Connecting electrodes play a crucial role to assist charge injection into the adjacent electroluminescent units in tandem organic light-emitting diodes (OLEDs). In this study, we demonstrate that Mg:Ag alloy is an effective connecting electrode for bottom- and top-emitting tandem OLEDs. Optical cavity design and simulation are also conducted to predict the luminance of tandem OLEDs. It is found that the theoretical luminance of tandem OLEDs is close to but not higher than twofold enhancement over the luminance of a single OLED optimized to the first resonance mode, which is theoretically higher than high-order resonance modes. It is also found that the optical properties of Mg:Ag connecting electrodes, while having relatively small influence on weak microcavity bottom-emitting tandem OLEDs, have large influence on strong microcavity top-emitting tandem OLEDs.

The dipole orientation of a light-emitting molecule dictates the external quantum efficiency (EQE) of an organic light emitting diode (OLED). In this paper, we studied both experimental and theoretical relationships between dipole orientation and measurable optical properties of working OLEDs. Theoretical electroluminescence (EL) spectrum, EQE, and luminance angle distribution are simulated by incorporating the dipole radiation pattern into Fabry- Perot cavity theory with horizontal dipole ratio as a variable parameter. The horizontal ratio is determined by optimizing the fitness of theoretical EL spectra to the experimental data and EL angular distribution. We show that the optical model proposed in this paper describes well the emission dipole dependent device optical data including EL spectra, EQEs, and EL luminance angular distributions. The emission fill factor (EFF), defined as the area ratio of EL angular distribution to the Lambertian curve, is found to follow a linear relationship with horizontal dipole ratio. These results provide a simple guide to deduce dipole orientations in working OLEDs.

Tandem device is an important architecture in fabricating high performance organic light-emitting diodes and organic photovoltaic cells. The key element in making a high performance tandem device is the connecting materials stack, which plays an important role in electric field distribution, charge generation and charge injection. For a tandem organic light-emitting diode (OLED) with a simple Liq/Al/MoO3 stack, we discovered that there is a significant current lateral spreading causing light emission over an extremely large area outside the OLED pixel when the Al thickness exceeds 2 nm. This spread light emission, caused by an inductive electric field over one of the device unit, limits one’s ability to fabricate high performance tandem devices. To resolve this issue, a new connecting materials stack with a C60 fullerene buffer layer is reported. This new structure permits optimization of the Al metal layer in the connecting stack and thus enables us to fabricate an efficient tandem OLED having a high 155.6 cd/A current efficiency and a low roll-off (or droop) in current efficiency.

White organic light emitting diodes (WOLEDs) is becoming a new platform technology for a range of applications such as flat-panel displays, solid-state lightings etc. and are under intensive research. For general solid-state illumination applications, a WOLED’s color rendering index (CRI) and correlated color temperature (CCT) are two crucial parameters. This paper reports that WOLED device structures can be constructed using four stacked emission layers which independently emit lights at blue, green, yellow and red color respectively. The intensity of each emission layer is then engineered by funneling excitons to the targeted emission layer to achieve an ultrahigh 92 CRI at 5000 cd/m2 and to reduce CCT to below 2500 K.

Thermal stability is one of the key technical challenges in developing high brightness organic light-emitting diodes (OLEDs). In this study, thermal catastrophic failure, a process involving temperature-induced abrupt device breakdown, is found to have a high degree of correlation to the glass transition process of the organic molecules. It is also found that C60-organic nano-composites can be used effectively to increase the glass transition temperature of the organic small molecular thin-films leading to the improvement of the thermal stability of OLEDs. In addition, a universal mathematical formula with only one variable, concentration, is discovered to quantify well the glass transition temperatures of all C60 nano-composites. The thermal catastrophic failure temperature of OLEDs with C60 nano-composites as a hole transport layer is found to be proportional to the glass transition temperature of C60 nano-composites.Thermal stability is one of the key technical challenges in developing high brightness organic light-emitting diodes (OLEDs). In this study, thermal catastrophic failure, a process involving temperature-induced abrupt device breakdown, is found to have a high degree of correlation to the glass transition process of the organic molecules. It is also found that C60-organic nano-composites can be used effectively to increase the glass transition temperature of the organic small molecular thin-films leading to the improvement of the thermal stability of OLEDs. In addition, a universal mathematical formula with only one variable, concentration, is discovered to quantify well the glass transition temperatures of all C60 nano-composites. The thermal catastrophic failure temperature of OLEDs with C60 nano-composites as a hole transport layer is found to be proportional to the glass transition temperature of C60 nano-composites.

An efficient organic near-infrared (NIR) to visible upconversion imaging device is obtained by integrating a photoactive buffer layer between two organic light-emitting diodes in a tandem configuration. As both types of photo-carriers (electrons and holes) are harvested to generate visible photons, this tandem device exhibits a significantly higher On/Off switching and has higher photon-photon conversion efficiency than conventional upconversion devices. We show that a high 5% photon-photon conversion rate is possible by optimizing the various functional layers in the tandem structure. We also demonstrate that a pixel-less NIR imaging chip can be made simply by using a large area single tandem organic upconversion device.

Nonradiative Auger recombination and radiative exciplex emissions at organic-organic heterojunctions are found to be mediated by the nature of the majority charge carrier. The Auger recombination is dominant when the heterojunction is electron rich, the exciplex emission is dominant when the heterojunction is hole rich, and both Auger recombination and exciplex emission are present when the heterojunction is charge balanced.