Ambipolar organic transistors are technologically interesting because oftheir potential applications in light-emitting field-effect transistors [1]and complementary-metal-oxide-semiconductor (CMOS) devices by providing easeof design, low cost of fabrication, and flexibility [2]. Although commonorganic semiconductors show either n- or p-type charge transportcharacteristic, organic transistors with ambipolar characteristics have beenreported recently. In this work, we show that ambipolar transport can beachieved within a single transistor channel using LiF gate dielectric in thetransistors with pentacene active layer. This ambipolar behavior can becontrolled by the applied source-drain and gate biases. It was found that atlow source-drain biases multistep hopping is the dominant conductionmechanism, while in high voltage regimes I-V data fits in Fowler-Nordheim(F-N) tunneling model. From the slope of the F-N plots, the dependencybetween field enhancement factor and the transition point in conductionmechanism upon gate bias has been extracted. The transition points show moredependency on gate voltage for negative biases compared to the positivebiases. While sweeping negative gate voltages from -5 to -20 V, thesource-drain voltages change from about 27 to 17 V. On the other hand, forpositive gate voltages from 5 to 20 V, the value of the transition pointstays at approximately 36 V. In order to further understand the transportmechanisms, new structures with an interface layer between dielectric andactive layer have been fabricated and characterized. As expected, asignificant decrease in the amount of the source-drain current has beenobserved after introducing the interface layer.

Inorganic/organic hybrid light-emitting diodes (LEDs) (IO-HyLEDs) composedof p-type GaN/n-type Tris-(8-hydoroxyquinoline) aluminum (Alq3)were fabricated with and without thin MgO electron-blocking layer (EBL) atthe inorganic/organic interface. These LEDs showed clear and stable currentrectifying diode characteristics and electroluminescence (EL) peaked at UVregion at room temperature. For the sample with MgO-EBL, obvious enhancementof green emission from Alq3 layer was observed. This resultsuggests that due to effective suppression of electron transport from Alq3 to p-GaN by MgO-EBL, radiative recombination of electronsand holes in Alq3 layer was enhanced. It was indicated that theband engineering technique can be applied to control the emission propertyof inorganic/organic hybrid LED.

We have investigated the current-voltage characteristics of themulti-layered photovoltaic devices consisting of ITO/oxide /p-type(donor)/fullerene/ bathocuproine (BCP)/ Al structures. We chose variousp-type (donors) small molecules and polymers in order to tune the values ofionization potential (IP) of donor molecules. The open-circuit voltage (Voc)increases with the increment of IP of donor materials. However, VOC was limited at ~0.6-0.7V for the devices without oxidelayer. On the other hand, the VOC increases up to 0.9V for thedevices with NiO and to ~ 1.1V for the devices with MoOX as ahole extraction buffer layer, respectively. We also estimated thework-function differences between Al and the oxide as 0.7, 0.9-1.0, and1.2-1.3 eV for the device without oxide, with NiO, and with MoOX,respectively. We therefore concluded the value of VOC is limitedby the lower part of VOC and energy difference between the LUMOof fullerene and the HOMO of donor ΔE.

We report the observation of an ultrafast (~ 430 fs) charge transfer processat the interface between a single-walled carbon nanotube (SWNT) wrapped by asemi-conducting polymer, poly(3-hexylthiophene) (P3HT), creating freepolarons on both materials. The addition of excess P3HT as a surroundingnetwork allows these free polarons to be long-lived at room temperature. Ourresults suggest that SWNT-P3HT blends incorporating only 1% fractions ofSWNTs can achieve a charge separation efficiency comparable to aconventional 60:40 P3HT-fullerene blend, provided small-diameter tubes areembedded in an excess P3HT matrix.

Since their re-introduction as new generation solar cells in 1991,dye-sensitized solar cells (DSSCs) have been studied extensively to improvetheir efficiency and their stability. Few papers have reported the usage ofnatural dyes in the fabrication of DSSCs. We are most interested in thesedyes being easy to extract, low cost and with tunable absorption. Forexample, Anthocyanin is extracted from red cabbage and is present in amultitude of colors ranging from red to yellow to violet according to thepH. In this study, two industrial types of titanium dioxide powders, theDegussa P 25 and the Crystal 128 with different particle sizes 21 and 200 nmrespectively, were used to prepare DSSCs. The dye used is anthocyanin andits color is varied by varying the pH of the medium. The pH effect on thelight absorption of anthocyanin in the visible range and the opticalproperties of titanium dioxide dyed with anthocyanin and coumarin 102 areinvestigated using UV-Vis spectroscopy. The open-circuit voltage of all thesamples is tested and it if found out that it is dependant on the dyecolor.

Perylene diimides are known as promising n-type semiconductor buildingblocks. Here we report the synthesis and characterization of a set of threesoluble poly(perylene diimide)s and their preliminary characterization inorganic photovoltaic cells. These polymers are made through thepolycondensation of perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA)with a variety of poly(ethylene glycol) (PEG)- or poly(propylene glycol)(PPG)-based diamine comonomers. The flexible spacer offers increasedsolubility in organic solvents and allows the perylene core to assume aconformation that promotes favorable cofacial π–π interactions. Mixtures ofthese polymers with the hole-transporting polymer, poly(3-hexylthiophene)(P3HT) result in significant fluorescence quenching. However, the phaseseparation occurs on a scale too large for a bulk heterojunction solar cell.The PPGylated poly(perylene diimide) shows an unusually low free electronconcentration (~1.0 × 1012 cm-3) and therefore makesan excellent model system for future doping studies. These new polymers mayhave promise as stable electron-conductive layers with largelight-absorptivities in solution-processable applications of organicelectronics.

Classical molecular dynamics (MD) simulations in conjunction with opticalabsorption and AFM/nano-Raman experiments are employed to relate themolecular-scale arrangement and conjugation of poly-3-hexylthiophene (P3HT)adsorbed onto single-walled carbon nanotubes (SWNTs) and multi-walled carbonnanotubes (MWNTs). Taken together our results demonstrate the templatingrole of carbon nanotubes in increasing the π-conjugation length of the P3HTat the P3HT/carbon nanotube interface. The MD simulations show that SWNTsand MWNTs, due to their inherent 1-dimensional (1D) cylindrical shape andπ-conjugation, planarize the P3HT molecules adsorbed at their surface andthus quench their torsional disorder, regardless of the P3HT conformationand nanotube chirality. This effect is more significant for higher SWNTweight fractions in the sample (since it is an interface effect). Weinvestigated this effect experimentally by acquiring nano-Raman spectra inregions of high-MWNT/low-P3HT content in addition to optical absorptionspectra of P3HT-SWNT composites with different SWNT concentrations . Theincrease in the P3HT conjugation is confirmed by a shift of a P3HT featurein the Raman spectrum when going from P3HT-rich to SWNT-rich areas in themixture. The significance of this work for charge transfer at the P3HT-SWNTinterface in bulk-heterojunction solar cells is discussed.

We investigated temperature dependence of the electrical conductance ofsingle oligothiophene molecular wires with the length of 2.2 nm (5-mer), 5.6nm (14-mer) and 6.7 nm (17-mer) by using the scanning tunneling microscopybreak junction method. Results show that the dominant charge carriertransport for 5-mer molecule is tunneling while for 17-mer molecule ishopping. The carrier transport mechanism of 14-mer are tunneling transport(T ≤ 350 K) and hopping transport (T > 350 K) indicating that hopping andtunnelling transport are competitive process in the molecular junction.

We presented in this paper a photo-assisted ligand exchange approach wherebylight will be introduced to facilitate the replacement of oleic acid (OA)ligand molecules over PbSe quantum dots (QDs). The ligand-exchanged QDs wereused to fabricate quantum dot light-emitting-diodes (QD-LEDs), whichoutperform the devices comprising the QDs without ligand-replacement.

A hindered phenol type radical scavenger (Irganox 1010®) was used to improvethe resistance to photo-oxidation in PPV-type polymers. The degradation wasperformed under blue light (460 nm), and the effect of the added compoundwas investigated following the intensity variation of absorbance andphotoluminescence spectra. The results showed that the addition of thecompound brought about a significant enhancement in photodegrationresistance without interfering in the electronic properties of the polymers.These results suggest that the free radical scavenger attenuates thedecrease of the polymer conjugation length induced by the radiation. FTIRmeasurements confirm this evidence.

The transient electroluminescence (EL) of phosphorescent organiclight-emitting diodes (OLEDs) was investigated. The behaviors of thetransient characteristics are analyzed using the triplet-tripletannihilation model. The device exhibited a gradual decrease in quantumcurrent efficiency owing to the triplet-triplet annihilation at a highcurrent density. At a higher current density, the reduced rise and decaytimes are due to high-density triplet excitons related to the enhancedtriplet-triplet annihilation and the increase of the nonradiative process.The modulation speed of the devices is mainly limited by the phosphorescentrecombination lifetime.

In this work we investigate theoretically the effects imposed by plasmonexcitations in spherical metallic nanoparticles (MNPs) on the rate of energytransfer in peridinin-chlorophyll-protein (PCP) complex reconstituted withboth chlorophyll a (Chl a) and chlorophyll b (Chl b). This light-harvestingcomplex is unique since it features efficient energy transfer both fromhigher-lying Chl b to lower-lying Chl a aswell as in the opposite, less energy-favorable direction. The results ofcalculations show that the Förster energy transfer rate decreases with aMNP-PCP distance changing from 2 to 144nm, while the energytransfer from Chl a to Chl b remains lessefficient at all distances. We conclude that plasmon excitations allow forcontrolling the energy transfer between Chls, as well as the excitationdistribution between two spectrally distinguishable Chls within thereconstituted PCP complex.

We describe the fabrication and characterization of solution processedorganic light emitting diodes (OLEDs) based on novel near-infrared emittingerbium(III) complexes, consisting of three perfluoroalkyl-β-diketone ligandsand 5-NO2-1,10-phenanthroline as chelating N,N-donor molecule.The function of N,N molecule is to saturate the coordination sphere of theerbium ion and to harvest excitation light that can be transferred to theexcited states of the erbium ion. The devices have been fabricated byspin-coating, using 1 %wt methanol precursor solutions. These Er-complexesform very uniform thin films. The OLED structure is glass/indium–tinoxide(ITO) / poly(3,4-ethylenedioxythiophene) / poly(4-styrenesulfonate)(PEDOT:PSS)/Er-complex/Ca/Al. The good electrical response, with lowthreshold voltages (a few volts), together with the very uniform thin filmsformed, made these complexes promising for IR emitting displays.

Charge injection property of organic thin film devices is a key issue tounderstand the device operation. Displacement current measurement (DCM) is apowerful technique to probe the charge injection behaviors in terms of achange in the apparent capacitance of test devices. However, it requires tosuppress actual current flowing through the device for investigating thedetails of interface phenomena. We propose here the use of ionic liquids(ILs) as a top contact insulator in organic metal-insulator-semiconductor(MIS) structures. Because of the high stability and dielectric constant ofthe ILs, the external applied voltage was applied mainly to the organiclayer with suppressing the actual current. The DCM curves of Ptwire/IL/α-NPD/ITO structure were measured, and they actually show thesignals due to the hole injection from theITO to α-NPD layer andaccumulation at the IL/α-NPD.

Among the different recombination mechanisms in organic solar cells thephotoluminescence (PL) of charge transfer excitons (CTEs) has beenidentified has one of the most important, impacting both the open circuitvoltage and the short circuit current. Here, we study their recombinationdynamics, monitoring the decay of the PL on a time scale spanning threeorders of magnitude from nanoseconds to microseconds. As a model system weinvestigate blends of the conjugated polymerpoly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV)and the fullerene derivative [6,6]-phenyl C61-butyric acid methylester (PCBM). We observe that the dynamics of recombination follows apower-law, which is independent of sample morphology. Upon application of atransient electric field, which is capable of separating the bound chargepairs, we observe different dynamics of recombination only for the separatedpairs. Those also follow a power-law and show a strong dependence on thefilm morphology.

The Coulomb blockade behavior was observed for both C60-PCBM and C70-PCBM at room temperature utilizing a nonvolatile memorycell fabricated through a liquid-transfer process. Room-temperature andlow-temperature (10K) electrical characterizations verified the blockadeeffect was originated from both molecular energy levels and single electroncharging energy. Molecular orbital energy was extracted and shown goodagreement with the literature [1].The successful integration and operationof this hybrid structure signified a strong potential for molecule-basedelectronic device design.

The degradation mechanism of CdSe/ZnS quantum dots (QDs) light-emittingdiodes (LEDs) was investigated with steady-state and time-resolvedphotoluminescence measurements. Our study reveals that the degradation isassociated with the decreasing quantum efficiency of the CdSe/ZnS QDs in thedevices. Two mechanisms that cause the efficiency reduction were verified inthe experiments: i.e., thermal instability and luminescence quenching.

Reorganization energy is one of the important factors to decide the rate ofelectron transfer according to the Marcus theory. Small reorganizationenergy is highly desirable in design of optoelectronic and electronicdevices like as organic light emitting diode. For this reason,reorganization energy of aromatic diamine derivatives, N,N′-diphenyl-N,N′-bis(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine (TPD) and 4,4′-diphenyl-N,N,N′,N′-tetraphenylbenzidine (DTPB) havebeen studied theoretically by self-exchange electron transfer theory. Byexecuting the Gaussian 03 calculation we can easily figure out theoptimization point which needed for calculation of the inner reorganizationenergy (λ) of self-exchange electron transfer reaction. Also ionizationpotential and electron affinities of these molecules can be calculated atthe density functional theory level with basis set 6-31G** and 6-31G* using Gaussian 03 software on the basis of ab initio method. It gives possibility to develop asemi-empirical model for the observed absorption and photoluminescencespectrum.

Solution-based printing and coating processes have the potential todramatically reduce the production costs of Organic Light Emitting Diodes.This is particularly true for Quantum Dots Light Emitting Diode (QDLEDs),the newborn in the field of LEDs, due to quantum dots price prohibitingwastage. Here, we report our latest results on the development ofsolutionprocessed QDLEDs. We have implemented a layer by layer strategy,from a whole evaporated small molecule based OLED to a hybrid QDLEDdeveloped by wet deposition techniques for the first layers and byevaporation for the last ones. Intermediate steps are discussed in thispaper.

First, we have worked on a poly(3,4-ethylenedioxythiophenepoly(styrenesulfonate) (PEDOT:PSS) layer. The PEDOT:PSS formulation forinkjet printing and spin coating were optimised: wettability on an ITOsubstrate, jettability of the inkjet formulation and baking conditions werestudied. Additives as surfactant and ethylene glycol were added to thecommercial inkjet grade solution to improve the deposition process. As aconsequence to this study, anisotropic conductivity of PEDOT:PSS wasobserved and is reported here. In particular, ethylene glycol demonstrated astrong ability to increase the parallel conductivity by several orders ofmagnitude, but not the vertical one.

Then, inkjet-printed and spin-coated device performances are compared tocomplete this first study. Hybrid devices with an efficacy of 12cd/A at 4Vwere obtained, with 2.17 % of EQE, and a luminance of 4000 cd/m2at 4V.

Finally, we succeeded in the development of our first QDLED based on CdSecore/ CdSZnS shell quantum dots emitting at a wavelength of 600nm. Quantumdots were inkjet printed, in order to waste as little as possible this veryexpensive material.

Spectroelectrochemical study on a new absorption band of radical cations of4,4’-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (α-NPD) as anelectron-donor hole-transporting material used in organic electronics isreported in this work. UV-visible spectroscopic and cyclic voltammetricproperties for α-NPD in solution are also examined. We find that the resultsare attributed to quenching process for blue fluorescence from α-NPD byexcess α-NPD+ radical cations accumulated in the emission regionin the organic light-emitting devices related to a relatively large overlapbetween the fluorescence spectrum of α-NPD and the absorption spectrum of α-NPD+ radical cations. The band gap energy for α-NPD iscalculated from the UV-visible spectroscopic data.