Recently, the first molecular nanowheel was synthesized and characterizedfrom Scanning Tunneling Microscope (STM) experiments. It was demonstratedthat a specifically designed hydrocarbon molecule (C44H24) could roll on a copper substrate along the[110] surface direction. In this work we report a preliminary theoreticalanalysis of the isolated molecule and of its rolling processes on differentCu surfaces. We have used ab initio and classical moleculardynamics methods. The simulations showed that the rolling mechanism is onlypossible for the [110] surface. In this case, the spatial separation amongrows of copper atoms is enough to ‘trap’ the molecule and to create thenecessary torque to roll it. Other surface orientations ([111] and [100])are too smooth and cannot provide the necessary torque for the rollingprocess.

In this study, we report the luminescence quenching by radical cations ofaromatic diamines used as a hole transport layer (HTL) in organicelectroluminescent (EL) devices. The EL characteristics of green organic ELdevices 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 andelectrochemical data indicate the formation of radical cations in thin filmand solution of TPD after chemical oxidation. We find that the EL luminanceincreases less than linearly with an increase in current for the EL devicesstudied in this study. The luminance loss in the devices is attributed toquenching of singlet excited states by large excess radical cations of TPDare accumulated in the emission zone due to large overlap between aflourescence spectrum of Alq3 and an absorption spectrum ofradical cations of TPD.

We have investigated the barrier height for electron injection at thecathode / polyflu-orene derivatives interface by the internal photoemission(IPE) spectroscopy techniques using the “electron only device” structureconsisting of TiO2, electron transporting polyimide inter-layer(IL), and polyfluorene derivatives. We also estimated the barrier height bythe current analysis based on the Schottky thermal emission current model,and it coincides well to the threshold energy of IPE result only when theenergy is lower than 1.1eV. The measured barrier height obtained by IPElinearly 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. Onthe other hand, the slope parameter becomes very small (~0.18) for the poly(9,9-dioctylfluorene)-co- benzo- thiadiazole) (F8BT) probably due to theelectron pinning at the cathode/ acceptor interface.

Both luminous efficiency and lifetime in blue fluorescence organic lightemitting devices (OLEDs) have been improved by modified HTMs with higherLUMO energy levels. The LUMO energy levels of HTM were increased bymodifying substituent in HTM molecules. Two HTMs containing ortho and metabiphenyl substituent and one HTM containing thiophene substituent weresynthesized via palladium catalyzed amine coupling reactions to compare witha para biphenyl substituent HTM-1 as a standard molecule. According to TDDFTcalculations, these three modified HTMs showed 0.05-0.15 eV higher LUMOenergy levels compared to the para biphenyl substituent HTM-1. The luminousefficiency 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 precursormethodology and by (ii) the microwave synthetic route. The consequences ofCdS QD direct exposure to air for a period of 7 days were investigated byfollowing 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 ofnew recombination path(s) associated to degradations during aging, possiblyan oxidative formation of a CdO surface layer. On the other hand, nosignificant change is observed in the absorption spectra. For QDs obtainedby (ii), no degradation is revealed by the PL spectra which remainunchanged. On the other hand, the absorption spectra are dominated by anunexplained broad band around 3.6 eV which tends to hide the fundamentalexcitonic transition one and increases in intensity with aging.

Organic light-emitting diodes (OLEDs) are developing into a competitivealternative to conventional light sources. Nevertheless, OLEDs need furtherimprovement in terms of efficiency and color rendering for lightingapplications. Fluorescent blue emitters allow deep blue emission and highstability, while phosphorescent blue emitter still suffer from insufficientstability. The concept of triplet harvesting is the key for achievinginternal quantum efficiencies up to 100 % and simultaneously benefiting fromthe advantages of fluorescent blue emitters. Here, we present a stacked OLEDconsisting of two units comprising four different emitters in total. Thefirst unit takes advantage of the concept of triplet harvesting and combinesthe light emission of a fluorescent blue and a phosphorescent red emitter.The second unit emits light from a single emission layer consisting of amatrix doped with phosphorescent green and yellow emitters. With thisapproach, we reach white color coordinates close to the standard illuminantA and a color rendering index of above 75. The presented devices arecharacterized by high luminous efficacies of above 30 lm/W on standard glasssubstrates without outcoupling enhancement.

To enhance the lifetime of large-sized active matrix organic light emittingdiodes (AMOLEDs), we developed a liquid desiccant for encapsulation. Theliquid desiccant was prepared by mixing nano-sized calcium oxide (CaO)powders and silicone binder including polyalkylalkenylsiloxane,polyalkylhydrogensiloxane and platinum compound. It was confirmed thatliquid desiccant had an effect on absorption of penetrated moisture andoxygen through calcium tests. Also, the test cells encapsulated with onlyepoxy sealant dispensed at the edge of the cell developed dark spots within100 hrs, which grew larger with time at 85 oC and 85 % R.H. On the other hand, the test cell sealed with epoxysealant and liquid desiccant showed no dark spots and retained 97% of itsinitial luminance even after being stored for 800 hrs at 85 oC and 85 % R.H. Furthermore, the accelerating storage lifetimes of31-inch bottom-emitting AMOLEDs with epoxy sealant and liquid desiccantshowed about 1000 hrs. These results suggest that the liquid desiccant canbe applied to encapsulation of large-sized AMOLEDs.

In this study we investigate how exposure to ambient air and light duringdevice processing affects the opto-electronic properties of poly-3-hexylthiophene (P3HT) : [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) bulkheterojunction solar cells (BHJ). The properties of pure P3HT layersprepared in an inert atmosphere, under ambient conditions, and additionallydegraded under light in ambient conditions were investigated usingphotoluminescence (PL) and photoinduced absorption (PIA). It was observedthat exposure to air during processing leads to oxygen doping of thepolymer. Exposure to air combined with light was found to significantlydecrease the PL and PIA signals. The current-voltage (I-V) and externalquantum efficiency (EQE) characteristics of solar cells fabricated in aninert atmosphere were compared to solar cells processed under ambientconditions. It was observed that processing in air leads to a reduction inthe photocurrent in the devices which is attributed to electron trapping byoxygen in the active layer.