Recently, the first molecular nanowheel was synthesized and characterized from Scanning Tunneling Microscope (STM) experiments. It was demonstrated that a specifically designed hydrocarbon molecule (C44H24) could roll on a copper substrate along the [110] surface direction. In this work we report a preliminary theoretical analysis of the isolated molecule and of its rolling processes on different Cu surfaces. We have used ab initio and classical molecular dynamics methods. The simulations showed that the rolling mechanism is only possible for the [110] surface. In this case, the spatial separation among rows of copper atoms is enough to ‘trap’ the molecule and to create the necessary torque to roll it. Other surface orientations ([111] and [100]) are too smooth and cannot provide the necessary torque for the rolling process.

In this study, we report the luminescence quenching by radical cations of aromatic diamines used as a hole transport layer (HTL) in organic electroluminescent (EL) devices. The EL characteristics of green organic EL devices with an electron transport layer (ETL) as an emitter i.e. ITO/TPD HTL/Alq3 ETL/Al is studied. Here, ITO, TPD, and Alq3 are abbreviations for indium-tin-oxide, N,N’-diphenyl-N,N’-bis(3-methylphenyl)-1,1’-biphenyl-4,4’-diamine, and tris (8-hydroxyquinoline) aluminum, respectively. UV-visible absorption and electrochemical data indicate the formation of radical cations in thin film and solution of TPD after chemical oxidation. We find that the EL luminance increases less than linearly with an increase in current for the EL devices studied in this study. The luminance loss in the devices is attributed to quenching of singlet excited states by large excess radical cations of TPD are accumulated in the emission zone due to large overlap between a flourescence spectrum of Alq3 and an absorption spectrum of radical cations of TPD.

We have investigated the barrier height for electron injection at the cathode / polyflu-orene derivatives interface by the internal photoemission (IPE) spectroscopy techniques using the “electron only device” structure consisting of TiO2, electron transporting polyimide inter-layer (IL), and polyfluorene derivatives. We also estimated the barrier height by the current analysis based on the Schottky thermal emission current model, and it coincides well to the threshold energy of IPE result only when the energy is lower than 1.1eV. The measured barrier height obtained by IPE linearly increases with both the work-function of cathode materials. However, the slope parameter becomes less than 1 (~0.6) for poly (9,9-dioctylfluorene) (F8) probably due to the interfacial gap states. On the other hand, the slope parameter becomes very small (~0.18) for the poly (9,9-dioctylfluorene)-co- benzo- thiadiazole) (F8BT) probably due to the electron pinning at the cathode/ acceptor interface.

Both luminous efficiency and lifetime in blue fluorescence organic light emitting devices (OLEDs) have been improved by modified HTMs with higher LUMO energy levels. The LUMO energy levels of HTM were increased by modifying substituent in HTM molecules. Two HTMs containing ortho and meta biphenyl substituent and one HTM containing thiophene substituent were synthesized via palladium catalyzed amine coupling reactions to compare with a para biphenyl substituent HTM-1 as a standard molecule. According to TDDFT calculations, these three modified HTMs showed 0.05-0.15 eV higher LUMO energy levels compared to the para biphenyl substituent HTM-1. The luminous efficiency and the lifetime (LT90) of OLEDs using HTM-2 at 500 cd/m2 have been enhanced up to 20 % and 52 %, respectively, compared to the standard device using HTM-1.

Small size CdS QDs were synthesized by (i) the single source precursor methodology and by (ii) the microwave synthetic route. The consequences of CdS QD direct exposure to air for a period of 7 days were investigated by following the evolution of the photoluminescence (PL) and absortion spectra. For QDs obtained by (i), the excitonic emission band (3.0 ‑ 3.1 eV) decreases in intensity, relatively to the low energy one (2.2 ‑ 2.5 eV) tentatively associated to midgap surface states. This suggests arising of new recombination path(s) associated to degradations during aging, possibly an oxidative formation of a CdO surface layer. On the other hand, no significant change is observed in the absorption spectra. For QDs obtained by (ii), no degradation is revealed by the PL spectra which remain unchanged. On the other hand, the absorption spectra are dominated by an unexplained broad band around 3.6 eV which tends to hide the fundamental excitonic transition one and increases in intensity with aging.

Organic light-emitting diodes (OLEDs) are developing into a competitive alternative to conventional light sources. Nevertheless, OLEDs need further improvement in terms of efficiency and color rendering for lighting applications. Fluorescent blue emitters allow deep blue emission and high stability, while phosphorescent blue emitter still suffer from insufficient stability. The concept of triplet harvesting is the key for achieving internal quantum efficiencies up to 100 % and simultaneously benefiting from the advantages of fluorescent blue emitters. Here, we present a stacked OLED consisting of two units comprising four different emitters in total. The first unit takes advantage of the concept of triplet harvesting and combines the light emission of a fluorescent blue and a phosphorescent red emitter. The second unit emits light from a single emission layer consisting of a matrix doped with phosphorescent green and yellow emitters. With this approach, we reach white color coordinates close to the standard illuminant A and a color rendering index of above 75. The presented devices are characterized by high luminous efficacies of above 30 lm/W on standard glass substrates without outcoupling enhancement.

To enhance the lifetime of large-sized active matrix organic light emitting diodes (AMOLEDs), we developed a liquid desiccant for encapsulation. The liquid desiccant was prepared by mixing nano-sized calcium oxide (CaO) powders and silicone binder including polyalkylalkenylsiloxane, polyalkylhydrogensiloxane and platinum compound. It was confirmed that liquid desiccant had an effect on absorption of penetrated moisture and oxygen through calcium tests. Also, the test cells encapsulated with only epoxy sealant dispensed at the edge of the cell developed dark spots within 100 hrs, which grew larger with time at 85 oC and 85 % R.H. On the other hand, the test cell sealed with epoxy sealant and liquid desiccant showed no dark spots and retained 97% of its initial luminance even after being stored for 800 hrs at 85 oC and 85 % R.H. Furthermore, the accelerating storage lifetimes of 31-inch bottom-emitting AMOLEDs with epoxy sealant and liquid desiccant showed about 1000 hrs. These results suggest that the liquid desiccant can be applied to encapsulation of large-sized AMOLEDs.

In this study we investigate how exposure to ambient air and light during device processing affects the opto-electronic properties of poly-3-hexylthiophene (P3HT) : [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulk heterojunction solar cells (BHJ). The properties of pure P3HT layers prepared in an inert atmosphere, under ambient conditions, and additionally degraded under light in ambient conditions were investigated using photoluminescence (PL) and photoinduced absorption (PIA). It was observed that exposure to air during processing leads to oxygen doping of the polymer. Exposure to air combined with light was found to significantly decrease the PL and PIA signals. The current-voltage (I-V) and external quantum efficiency (EQE) characteristics of solar cells fabricated in an inert atmosphere were compared to solar cells processed under ambient conditions. It was observed that processing in air leads to a reduction in the photocurrent in the devices which is attributed to electron trapping by oxygen in the active layer.