The Impacts of Solvents, Heat Treatments and Hole Injection Layers on the Electroluminescent Lifetime of Organic Light-Emitting Devices

Since their invention over three decades ago, organic light-emitting devices (OLEDs) have attracted tremendous interest for display and solid-state lighting applications and have already been commercialized in smartphones, tablets and television screens. However, the most coveted potential of OLED technology is to enable ultra-low cost, roll-to-roll manufacturing of large-area panels on flexible substrates. To date, commercial OLED products rely on high-cost vacuum deposition techniques and thus fail to realize this potential. In particular, the lifetime, of solution-based (and thus printable) devices remains well below commercially acceptable standards. The significant lifetime limitations of solution-based devices demand a more thorough understanding of the impact of the unique factors involved in the fabrication of these devices. Solution-processable hole injection layers (HILs), solvents, and heat or drying treatments are three such factors that play a crucial role in solution-processed devices. The principle aim of this work is to understand the influence of these factors on OLED lifetime in vacuum-deposited devices, independent of the multitude and variability of other parameters (e.g.: drying conditions, solubility, solution concertation) involved in most solution-processing methods; and to demonstrate proof-of-concept strategies to mitigate potentially adverse effects for application in solution-processed OLEDs. Results show that solution-processed poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) HILs are susceptible to electron-induced degradation, a mechanism that can lead to relatively short OLED electroluminescent (EL) lifetimes. This degradation can be minimized by selecting hole transporting materials and device structures that minimize electron leakage to the HIL, resulting in a lifetime improvement of up to 20x. The effects of solvent and heat treatments on device efficiency and EL lifetime across a variety of hole injection and hole transport materials were found to vary considerably depending on the specific material combination. The extent of the morphological changes induced by the two treatments is highly material-dependent and does not necessarily correlate with device efficiency and EL lifetime. This suggests that additional, material-specific factors should be likely be considered in future correlations of device characteristics to the morphology of corresponding organic films for solution-processed devices. Finally, solvent treatment of carbazole hole transport layers was found to induce substantial aggregation and lead to shorter EL lifetimes and lower device efficiency. The origin of this effect was found to be a decrease in photoluminescence quantum yield resulting from this aggregation. Material intermixing was shown to suppress this aggregation and resulted in improved device efficiency and a 2.5x increase in EL lifetime

Universal high work function flexible anode for simplified ITO-free organic and perovskite light-emitting diodes with ultra-high efficiency

Flexible transparent electrode materials such as conducting polymers, silver nanowires, carbon nanotubes and graphenes are being investigated as possible replacements for conventional brittle inorganic electrodes. However, they have critical drawbacks of low work function (WF), resulting in a high hole injection barrier to an overlying semiconducting layer in simplified organic or organic-inorganic hybrid perovskite light-emitting diodes (OLEDs or PeLEDs). Here, we report a new anode material (AnoHIL) that has multifunction of both an anode and a hole injection layer (HIL) as a single layer. The AnoHIL has easy WF tunability up to 5.8 eV and thus makes ohmic contact without any HIL. We applied our anodes to simplified OLEDs, resulting in very high efficiency (62% ph el(-1) for single and 88% ph el(-1) for tandem). The AnoHIL showed a similar tendency in simplified PeLEDs, implying universal applicability to various optoelectronics. We also demonstrated large-area flexible lightings using our anodes. Our results provide a significant step toward the next generation of high-performance simplified indium tin oxide (ITO)-free light-emitting diodes.

Light-emitting textiles: Device architectures, working principles, and applications

E-textiles represent an emerging technology aiming toward the development of fabric with augmented functionalities, enabling the integration of displays, sensors, and other electronic components into textiles. Healthcare, protective clothing, fashion, and sports are a few examples application areas of e-textiles. Light-emitting textiles can have different applications: Sensing, fashion, visual communication, light therapy, etc. Light emission can be integrated with textiles in different ways: Fabricating light-emitting fibers and planar light-emitting textiles or employing side-emitting polymer optical fibers (POFs) coupled with light-emitting diodes (LEDs). Different kinds of technology have been investigated: Alternating current electroluminescent devices (ACELs), inorganic and organic LEDs, and light-emitting electrochemical cells (LECs). The different device working principles and architectures are discussed in this review, highlighting the most relevant aspects and the possible approaches for their integration with textiles. Regarding POFs, the methodology to obtain side emissions and the critical aspects for their integration into textiles are discussed in this review. The main applications of light-emitting fabrics are illustrated, demonstrating that LEDs, alone or coupled with POFs, represent the most robust technology. On the other hand, OLEDs (Organic LEDs) are very promising for the future of light-emitting fabrics, but some issues still need to be addressed

Low-Cost and Green Fabrication of Polymer Electronic Devices by Push-Coating of the Polymer Active Layers

Because of both its easy processability and compatibility with roll-to-roll processes, polymer electronics is considered to be the most promising technology for the future generation of low-cost electronic devices such as light-emitting diodes and solar cells. However, the state-of-the-art deposition technique for polymer electronics (spin-coating) generates a high volume of chlorinated solution wastes during the active layer fabrication. Here, we demonstrate that devices with similar or higher performances can be manufactured using the push-coating technique in which a poly(dimethylsiloxane) (PDMS) layer is simply laid over a very small amount of solution (less than 1μL/covered cm2), which is then left for drying. Using mm thick PDMS provides a means to control the solvent diffusion kinetics (sorption/retention) and removes the necessity for additional applied pressure to generate the desired active layer thickness. Unlike spin-coating, push-coating is a slow drying process that induces a higher degree of crystallinity in the polymer thin film without the necessity for a post-annealing step. The polymer light-emitting diodes and solar cells prepared by push-coating exhibit slightly higher performances with respect to the reference spin-coated devices, whereas at the same time reduce the amounts of active layer materials and chlorinated solvents by 50 and 20 times, respectively. These increased performances can be correlated to the higher polymer crystallinities obtained without applying a post-annealing treatment. As push-coating is a roll-to-roll compatible method, the results presented here open the path to low-cost and eco-friendly fabrication of a wide range of emerging devices based on conjugated polymer materials

Microbial Quality and Pathogen Decontamination Strategies for Locally-Grown, Fresh Produce from West Virginia and Kentucky

This study aimed to evaluate the microbiological quality/safety of fresh produce from farmers\u27 markets (FM) and assess the post-harvest washing practice with antimicrobials to inactivate Salmonella and Listeria monocytogenes on fresh produce. In study I, 212 produce samples were tested for the presence of Salmonella and Listeria spp. using modified FDA-BAM methods. Aerobic plate counts (APCs), total coliforms (TCCs), and yeast/molds were analyzed on petri-films. Among the 212 samples, the APCs, TCCs, and yeast/molds were 3.72-5.63, 3.67-5.47, and 3.07-4.13 log CFU/g, respectively, with spinach containing the highest (P\u3c0.05) populations. Among all tested samples, Salmonella enterica spp. enterica was detected on 18.6% of spinach, 10.9% of tomatoes, 18.5% of peppers, and 56.3% of cantaloupes, which is much higher than previous reported. Only 3.78% of the samples were confirmed for Listeria spp., and 50% of them were identified as L. monocytogenes, based on multiplex PCR results. Due to the high percentage of pathogens detected on the farmers\u27 marker produce, an evaluation of post-harvest produce washing with various antimicrobials was conducted in study II. Specifically, spinach, tomatoes, green peppers and cucumbers were inoculated with S. Typhimurium and Tennessee or L. monocytogenes and washed in tap water, vinegar water (10%), lactic acid (5%), a lactic and citric acid blend (2.5%), and sodium hypochlorite (200 ppm) for 30 sec or unwashed. Vinegar water (10%) showed better (P\u3c0.05) reduction of S. Typhimurium and Tennessee on tomatoes and cucumbers, and L. monocytogenes on tomatoes and peppers than tap water. The three antimicrobials reached an additional reduction level of 0.9 to 2.7 (S. Typhimurium and Tennessee) and 0.2 to 1.4 log CFU/g ( L. monocytogenes) compared to tap water. Lactic acid caused the greatest (P\u3c0.05) reduction of S. Typhimurium and Tennessee on spinach and green peppers, and sodium hypochlorite showed the great (P\u3c0.05) reduction of L. monocytogenes on cucumbers. The results supplied important information for FM vendors to develop post-harvest protocols to control foodborne pathogens

Performance Enhancement of Organic Light-Emitting Diodes with an Inorganically Doped Hole Transport Layer

Organic light-emitting diodes (OLEDs) are generally considered as the next generation display and lighting sources owing to their many attractive properties, including low power consumption, wide viewing angle, vibrant color, high contrast ratios and compatibility with flexible substrates. The research and development of OLEDs has attracted considerable interest and has led to significant progress during the last two decades. The use of OLEDs in small-area displays such as cell phone screens, digital cameras, and wearable devices has become a reality. However, the OLED technology is still far from mature, posing a challenge for their widespread acceptance for applications in large-area displays and solid-state lighting. In particular, the lifetime of OLEDs is too short for many commercial applications, and the degradation mechanisms are still under debate. This work aims to improve the OLED device lifetime by doping of organic hole transport materials with inorganic transition metal oxides (TMOs), and to reduce the cost by simplifying the device layer structure and manufacturing procedure.;First, stress tests under continuous wave and pulsed currents were conducted to gain a better understanding of the key factors governing the degradation process of phosphorescent OLEDs. Through comparative studies of the aging behaviors of OLEDs with different hole transport layers (HTLs) under different stressing conditions, we have found that joule heating plays an important role in device degradation when a large energy level misalignment exists at the indium-tin-oxide (ITO) anode/HTL interface. The heating was effectively suppressed by reducing the interfacial energy barrier, leading to a prolonged lifetime of the OLEDs.;P-type doping of hole transport materials with TMOs was then developed as an effective way to reduce the interfacial energy barrier and the operational voltage of OLED devices. A systematical study was carried out on the effects of doping 4,4\u27-Bis(N-carbazolyl)-1,1\u27-biphenyl (CBP), a wide bandgap organic hole transport material, with WO3 and MoO3. The optimal doping conditions including the doping level and doping thickness have been determined by fabricating and characterizing a series of hole-only devices. Integrating the doped HTL into green phosphorescent OLEDs has resulted in a simplified structure, better optoelectronic characteristics, and improved device reliability.;Finally, selective doping of organic materials with the TMOs was developed and the concept of delta doping was applied to OLEDs for the first time. Selective doping was achieved by simple sequential deposition of the organic host and TMO dopant. Hole-only devices with a HTL comprising alternative 0.5 nm TMO-doped/3-10 nm undoped CBP layers exhibited greatly enhanced hole transport and had a turn-on voltage as low as 1.1 V. Simple fluorescent tris-(8-hydroxyquinoline) aluminum (Alq3)-based green OLEDs with a selectively doped CBP HTL showed a lower voltage and longer lifetime under constant-current stressing compared to similar OLEDs with an undoped HTL. Furthermore. delta doping was realized in more thermally stable organic materials, resulting in a marked conductivity increase along the plane of the doped layers by several orders of magnitude. The delta doping effects were explained by hole accumulation in potential wells formed in nanometer-thick doped regions, as revealed by high-resolution secondary ion mass spectrometry (SIMS) measurements

Organic Light-Emitting Diodes (OLEDs) and Optically-Detected Magnetic Resonance (ODMR) studies on organic materials

Organic semiconductors have evolved rapidly over the last decades and currently are considered as the next-generation technology for many applications, such as organic light-emitting diodes (OLEDs) in flat-panel displays (FPDs) and solid state lighting (SSL), and organic solar cells (OSCs) in clean renewable energy. This dissertation focuses mainly on OLEDs. Although the commercialization of the OLED technology in FPDs is growing and appears to be just around the corner for SSL, there are still several key issues that need to be addressed: (1) the cost of OLEDs is very high, largely due to the costly current manufacturing process; (2) the efficiency of OLEDs needs to be improved. This is vital to the success of OLEDs in the FPD and SSL industries; (3) the lifetime of OLEDs, especially blue OLEDs, is the biggest technical challenge. All these issues raise the demand for new organic materials, new device structures, and continued lower-cost fabrication methods. In an attempt to address these issues, we used solution-processing methods to fabricate highly efficient small molecule OLEDs (SMOLEDs); this approach is cost-effective in comparison to the more common thermal vacuum evaporation. We also successfully made efficient indium tin oxide (ITO)-free SMOLEDs to further improve the efficiency of the OLEDs. We employed the spin-dependent optically-detected magnetic resonance (ODMR) technique to study the luminescence quenching processes in OLEDs and organic materials in order to understand the intrinsic degradation mechanisms. We also fabricated polymer LEDs (PLEDs) based on a new electron-accepting blue-emitting polymer and studied the effect of molecular weight on the efficiency of PLEDs. All these studies helped us to better understand the underlying relationship between the organic semiconductor materials and the OLEDs\u27 performance, and will subsequently assist in further enhancing the efficiency of OLEDs. With strongly improved device performance (in addition to other OLEDs\u27 attributes such as mechanical flexibility and potential low cost), the OLED technology is promising to successfully compete with current technologies, such as LCDs and inorganic LEDs

Development of hybrid inorganic-organic light-emitting devices with metal oxide charge transport layers

Organic light emitting diodes (OLEDs) are currently being considered as the next generation technology in flat panel displays and solid state lighting applications. Among which, phosphorescent organic light emitting diodes (PhOLEDs) with nearly 100% internal quantum efficiency including other properties such as self emitting, high luminescence efficiency, broad wavelength range, wide viewing angle, high contrast, low power consumption, low weight, and large emitting area are gaining popularity in both academic and industrial research. Although development and commercialization of OLED technology is growing, there are still several key issues that need to be addressed---the external quantum efficiency (EQE) needs to be improved and the biggest technical challenge is to increase the device operational lifetime. Balanced charge injection and transport is vital for improving the device efficiency which demands for selection of better charge injection and transport materials. In addition imbalanced charge injection also degrades the device via joule\u27s heating and charge accumulation thereby limiting the device lifetime. Sensitivity of organic materials to the ambient atmosphere, particularly oxygen and moisture impedes the device performance.;This thesis work attempts to address these issues in the PhOLEDs through selection of proper charge injection and transport material as well as device structure optimization. At first we prepared thin films of thermally evaporated zinc-tin oxide (ZTO) with various ZnO and SnO2 compositions and studied its optical, electrical and morphological properties. After optimization of transparency and conductivity, these ZTO films showed promising materials for alternate transparent conducting oxides and electron transport layer (ETL) functions. Similarly, thin films of thermally evaporated tungsten oxide (WO 3) were prepared and their optical and electrical properties were studied and evaluated as a hole transport layer (HTL) material. We then fabricated and characterized various hybrid light emitting diode (HyLED) structures comprising of---ZTO as an ETL, WO3 as a HTL, and MoO3 as a hole injecting layer (HIL). The device structures were optimized for better performance in terms of efficiency and operational lifetime. Significant enhancement in EQE and operational lifetime were obtained in HyLEDs having WO3 as a HTL than of PhOLEDs with organic HTL. This is because WO3 improved hole injection as well as enabled facile hole transport thereby maintaining the balance of charge injection into the device. Finally, we also prepared inverted HyLEDs using WO3 as HTL and several metals including Ca, Ca/LiF, and Al/LiF as a cathode and their electron injecting capability were studied. Balanced charge injection was observed when a nanometer thick Ca was used as a cathode and WO3 as a HTL. As a result, inverted HyLED with better EQE and operational lifetime were fabricated