Correlation of Device Performance and Fermi Level Shift in the Emitting Layer of Organic Light-Emitting Diodes with Amine-Based Electron Injection Layers

We investigate three amine-based polymers, polyethylenimine and two amino-functionalized polyfluorenes, as electron injection layers (EILs) in organic light-emitting diodes (OLEDs) and find correlations between the molecular structure of the polymers, the electronic alignment at the emitter/EIL interface, and the resulting device performance. X-ray photoelectron spectroscopy measurements of the emitter/EIL interface indicate that all three EIL polymers induce an upward shift of the Fermi level in the emitting layer close to the interface similar to n-type doping. The absolute value of this Fermi level shift, which can be explained by an electron transfer from the EIL polymers into the emitting layer, correlates with the number of nitrogen-containing groups in the side chains of the polymers. Whereas polyethylenimine (PEI) and one of the investigated polyfluorenes (PFCON-C) have six such groups per monomer unit, the second investigated polyfluorene (PFN) only possesses two. Consequently, we measure Fermi level shifts of 0.5–0.7 eV for PEI and PFCON-C and only 0.2 eV for PFN. As a result of these Fermi level shifts, the energetic barrier for electron injection is significantly lowered and OLEDs which comprise PEI or PFCON-C as an EIL exhibit a more than twofold higher luminous efficacy than OLEDs with PFN

Inkjet-Printed Dual-Mode Electrochromic and Electroluminescent Displays Incorporating Ecofriendly Materials

Displays and indicators are an integral component of everyday electronics. However, the short lifecycle of most applications is currently contributing to the unsustainable growth of electronic waste. In this work, we utilize ecofriendly materials in combination with sustainable processing techniques to fabricate inkjet-printed, ecofriendly dual-mode displays (DMDs). These displays can be used in a reflective mode or an emissive mode by changing between DC and AC operation due to the combination of an electrochromic (EC) and electrochemiluminescent (ECL) layer in a single device. The EC polymer poly­(3,4-ethylenedioxythiophene):poly­(styrene sulfonate) (PEDOT:PSS) serves as the reflective layer, while an ECL gel made of dimethylsulfoxide (DMSO), poly­(lactic-co-glycolic acid) (PLGA), 1-butyl-3-methylimidazoliumbis­(oxalato)­borate (BMIMBOB), and tris­(bipyridine)­ruthenium­(II) chloride (Ru2+(bpy)3Cl2) enables the emissive mode. The final dual-mode devices exhibited their maximum optical power output of 52 mcd/m2 at 4 V and 40 Hz and achieved an EC contrast of 45% and a coloration efficiency of 244 cm2/C at a wavelength of 690 nm. The fabricated devices showed clear readability in dark and light conditions when operated in reflective or emissive modes. This work demonstrates the applicability of ecofriendly and potentially biodegradable materials to reduce the amount of hazardous components in versatile display technologies

Degradation Mechanisms in Organic Light-Emitting Diodes with Polyethylenimine as a Solution-Processed Electron Injection Layer

In this work, we investigate the performance and operational stability of solution-processed organic light-emitting diodes (OLEDs), which comprise polyethylenimine (PEI) as an electron injection layer (EIL). We show that the primary degradation mechanism in these OLEDs depends on the cathode metal that is used in contact with the EIL. In the case of Al, the deterioration in OLED performance during electrical driving is mainly caused by excitons which reach and subsequently degrade the emitter/PEI interface. In contrast, in the case of Ag, device performance degradation occurs due to an additional mechanism: hole accumulation at the emitter/PEI interface and a consequent drop in the emitter quantum yield. As a result, the operational lifetime of OLEDs that use PEI as EIL can vary significantly with the cathode material, and at a current density of 20 mA cm^–2, LT50 lifetimes of ∼200 h and <10 h are obtained for Al and Ag, respectively. Finally, we show that the first degradation mechanism can be significantly slowed by using a mixture of PEI and ZnO nanoparticles as EIL. As a result, the operational lifetime of OLEDs with an Al cathode is increased to more than 1000 h, without adversely affecting device performance. This lifetime is significantly longer than that of a LiF/Al reference OLED

Plasmonic Photosensitization of a Wide Band Gap Semiconductor: Converting Plasmons to Charge Carriers

A fruitful paradigm in the development of low-cost and efficient photovoltaics is to dope or otherwise photosensitize wide band gap semiconductors in order to improve their light harvesting ability for light with sub-band-gap photon energies.1–8 Here, we report significant photosensitization of TiO2 due to the direct injection by quantum tunneling of hot electrons produced in the decay of localized surface-plasmon polaritons excited in gold nanoparticles (AuNPs) embedded in the semiconductor (TiO2). Surface plasmon decay produces electron–hole pairs in the gold.9–15 We propose that a significant fraction of these electrons tunnel into the semiconductor’s conduction band resulting in a significant electron current in the TiO2 even when the device is illuminated with light with photon energies well below the semiconductor’s band gap. Devices fabricated with (nonpercolating) multilayers of AuNPs in a TiO2 film produced over 1000-fold increase in photoconductance when illuminated at 600 nm over what TiO2 films devoid of AuNPs produced. The overall current resulting from illumination with visible light is ∼50% of the device current measured with UV (ℏω > Eg band gap) illumination. The above observations suggest that plasmonic nanostructures (which can be fabricated with absorption properties that cover the full solar spectrum) can function as a viable alternative to organic photosensitizers for photovoltaic and photodetection applications

Poly(lactic-<i>co</i>-glycolic acid) (PLGA) as Ion-Conducting Polymer for Biodegradable Light-Emitting Electrochemical Cells

The use of biocompatible and biodegradable materials in optoelectronics will enable the development of promising applications in the field of healthcare and environmental sensors as well as a more sustainable production of technology. Here, we present light-emitting electrochemical cells which utilize the biodegradable polymer poly­(lactic-<i>co</i>-glycolic acid) (PLGA) to promote ionic conductivity in the active layer of light-emitting electrochemical cells. The device performance was analyzed in terms of the volume fraction of PLGA in the active layer blend as well as with respect to three different lactic:glycolic monomer ratios (85:15, 75:25, 65:35). In all three cases, adding PLGA to the active layer leads to an improvement of the turn-on voltage of up to 2 V compared to reference devices without PLGA. This can be attributed to an increase in ionic conductivity, which was determined by impedance spectroscopy. Increasing the relative amount of PLGA in the active layer shows that the improvement is ultimately limited by poor intermixing with the polymeric emitter as observed by fluorescent microscopy. The best devices achieved turn-on voltages of 4.1 V and a maximum luminance of 3800 cd m^–2 at 7.1 V

Photo-Cross-Linkable Polyfluorene–Triarylamine (PF–PTAA) Copolymer Based on the [2 + 2] Cycloaddition Reaction and Its Use as Hole-Transport Layer in OLEDs

We report the synthesis and characterization of a cross-linkable, cinnamic acid functionalized, hole-transporting polyfluorene–triarylamine (PF–PTAA) copolymer. Irradiation with light induces [2 + 2] cycloaddition and renders thin films of this polymer insoluble. Spin-coated films of the polymer and their light-induced cross-linking were investigated by atomic force and electron microscopy. In a proof-of-principle multilayer OLED device the polymer was applied as hole-transport layer (HTL) with commercially available <b>F8BT</b> as emitting layer (EML). Compared to the reference device without HTL we observe a significant increase in OLED performance. These results promise progress in cost-effective large area fabrication of polymer-based multilayer OLEDs with superior performance

Inkjet-Printed Triple Cation Perovskite Solar Cells

Noncontact inkjet printing offers rapid and digital deposition combined with excellent control over the layer formation for printed perovskite solar cells. In this work, inkjet printing is used to deposit triple cation perovskite layers with 10% cesium in a mixed formamidinium/methylammonium lead iodide/bromide composite for solar cells with high temperature and moisture stability. A reliable process control over a wide range of perovskite layer thickness from 175 to 780 nm and corresponding grain sizes is achieved by adjusting the drop spacing of the inkjet printer cartridge. A continuous power output at constant voltage, resulting in a power conversion efficiency of 12.9%, is demonstrated, representing a major improvement from previously reported inkjet-printed methylammonium lead triiodide perovskite solar cells. Moreover, this work highlights the extended resistance of triple cation perovskite solar cells against heat and moisture for our ambient inkjet printing approach. The presented results are a proof of concept for the processability of high efficiency perovskite solar cells using digital inkjet printing for next generation photovoltaic applications

Printing PPEs: Fundamental Structure–Property Relationships

A series of differentially alkyl- and alkoxy-substituted poly­(para-pheneyleneethynylene)­s of different molecular weight were prepared and their rheological properties investigated. It was found that the side chain structure of the PPEs of roughly equal molecular weight and degree of polymerization has a significant influence on the rheology and printing behavior of the PPEs. Introduction of branched alkoxy or alkyl substituents improve the printing behavior of the PPEs dramatically

Emissive Polyelectrolytes As Interlayer for Color Tuning and Electron Injection in Solution-Processed Light-Emitting Devices

Herein we present a solution-processed hybrid device architecture combining organic light-emitting diodes (OLEDs) and light-emitting electrochemical cells (LECs) in a bilayer architecture. The LEC interlayer promotes the charge injection from an air-stable Ag cathode as well as permits the color tuning of the device emission. To this end, we used an alcohol-soluble anionic polyfluorene derivative, the properties of which were investigated by absorption and photoluminescence spectroscopy as well as by cyclic voltammetry. The bilayer device exhibited operating voltages ∼6 V and a color tuning of the emission spectrum dependent on the LEC interlayer thickness. The hybrid devices presented a color emission ranging from the yellow (<i>x</i> = 0.39, <i>y</i> = 0.47) toward the green region (<i>x</i> = 0.29, <i>y</i> = 0.4) of the Commission Internationale de I’Eclairage (CIE) 1931 chromaticity diagram