Ambipolar organic transistors are technologically interesting because of their potential applications in light-emitting field-effect transistors [1] and complementary-metal-oxide-semiconductor (CMOS) devices by providing ease of design, low cost of fabrication, and flexibility [2]. Although common organic semiconductors show either n- or p-type charge transport characteristic, organic transistors with ambipolar characteristics have been reported recently. In this work, we show that ambipolar transport can be achieved within a single transistor channel using LiF gate dielectric in the transistors with pentacene active layer. This ambipolar behavior can be controlled by the applied source-drain and gate biases. It was found that at low source-drain biases multistep hopping is the dominant conduction mechanism, 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 dependency between field enhancement factor and the transition point in conduction mechanism upon gate bias has been extracted. The transition points show more dependency on gate voltage for negative biases compared to the positive biases. While sweeping negative gate voltages from -5 to -20 V, the source-drain voltages change from about 27 to 17 V. On the other hand, for positive gate voltages from 5 to 20 V, the value of the transition point stays at approximately 36 V. In order to further understand the transport mechanisms, new structures with an interface layer between dielectric and active layer have been fabricated and characterized. As expected, a significant decrease in the amount of the source-drain current has been observed after introducing the interface layer.

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

We have investigated the current-voltage characteristics of the multi-layered photovoltaic devices consisting of ITO/oxide /p-type (donor)/fullerene/ bathocuproine (BCP)/ Al structures. We chose various p-type (donors) small molecules and polymers in order to tune the values of ionization 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 oxide layer. On the other hand, the VOC increases up to 0.9V for the devices with NiO and to ~ 1.1V for the devices with MoOX as a hole extraction buffer layer, respectively. We also estimated the work-function differences between Al and the oxide as 0.7, 0.9-1.0, and 1.2-1.3 eV for the device without oxide, with NiO, and with MoOX, respectively. We therefore concluded the value of VOC is limited by the lower part of VOC and energy difference between the LUMO of fullerene and the HOMO of donor ΔE.

We report the observation of an ultrafast (~ 430 fs) charge transfer process at the interface between a single-walled carbon nanotube (SWNT) wrapped by a semi-conducting polymer, poly(3-hexylthiophene) (P3HT), creating free polarons on both materials. The addition of excess P3HT as a surrounding network allows these free polarons to be long-lived at room temperature. Our results suggest that SWNT-P3HT blends incorporating only 1% fractions of SWNTs can achieve a charge separation efficiency comparable to a conventional 60:40 P3HT-fullerene blend, provided small-diameter tubes are embedded 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 improve their efficiency and their stability. Few papers have reported the usage of natural dyes in the fabrication of DSSCs. We are most interested in these dyes being easy to extract, low cost and with tunable absorption. For example, Anthocyanin is extracted from red cabbage and is present in a multitude of colors ranging from red to yellow to violet according to the pH. In this study, two industrial types of titanium dioxide powders, the Degussa P 25 and the Crystal 128 with different particle sizes 21 and 200 nm respectively, were used to prepare DSSCs. The dye used is anthocyanin and its color is varied by varying the pH of the medium. The pH effect on the light absorption of anthocyanin in the visible range and the optical properties of titanium dioxide dyed with anthocyanin and coumarin 102 are investigated using UV-Vis spectroscopy. The open-circuit voltage of all the samples is tested and it if found out that it is dependant on the dye color.

Perylene diimides are known as promising n-type semiconductor building blocks. Here we report the synthesis and characterization of a set of three soluble poly(perylene diimide)s and their preliminary characterization in organic photovoltaic cells. These polymers are made through the polycondensation 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 increased solubility in organic solvents and allows the perylene core to assume a conformation that promotes favorable cofacial π–π interactions. Mixtures of these polymers with the hole-transporting polymer, poly(3-hexylthiophene) (P3HT) result in significant fluorescence quenching. However, the phase separation occurs on a scale too large for a bulk heterojunction solar cell. The PPGylated poly(perylene diimide) shows an unusually low free electron concentration (~1.0 × 1012 cm-3) and therefore makes an excellent model system for future doping studies. These new polymers may have promise as stable electron-conductive layers with large light-absorptivities in solution-processable applications of organic electronics.

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

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

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

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

The transient electroluminescence (EL) of phosphorescent organic light-emitting diodes (OLEDs) was investigated. The behaviors of the transient characteristics are analyzed using the triplet-triplet annihilation model. The device exhibited a gradual decrease in quantum current efficiency owing to the triplet-triplet annihilation at a high current density. At a higher current density, the reduced rise and decay times are due to high-density triplet excitons related to the enhanced triplet-triplet annihilation and the increase of the nonradiative process. The modulation speed of the devices is mainly limited by the phosphorescent recombination lifetime.

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

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

Charge injection property of organic thin film devices is a key issue to understand the device operation. Displacement current measurement (DCM) is a powerful technique to probe the charge injection behaviors in terms of a change in the apparent capacitance of test devices. However, it requires to suppress actual current flowing through the device for investigating the details 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 of the ILs, the external applied voltage was applied mainly to the organic layer with suppressing the actual current. The DCM curves of Pt wire/IL/α-NPD/ITO structure were measured, and they actually show the signals due to the hole injection from theITO to α-NPD layer and accumulation at the IL/α-NPD.

Among the different recombination mechanisms in organic solar cells the photoluminescence (PL) of charge transfer excitons (CTEs) has been identified has one of the most important, impacting both the open circuit voltage and the short circuit current. Here, we study their recombination dynamics, monitoring the decay of the PL on a time scale spanning three orders of magnitude from nanoseconds to microseconds. As a model system we investigate blends of the conjugated polymer poly(2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene-vinylene) (MDMO-PPV) and the fullerene derivative [6,6]-phenyl C61-butyric acid methyl ester (PCBM). We observe that the dynamics of recombination follows a power-law, which is independent of sample morphology. Upon application of a transient electric field, which is capable of separating the bound charge pairs, we observe different dynamics of recombination only for the separated pairs. Those also follow a power-law and show a strong dependence on the film morphology.

The Coulomb blockade behavior was observed for both C60-PCBM and C70-PCBM at room temperature utilizing a nonvolatile memory cell fabricated through a liquid-transfer process. Room-temperature and low-temperature (10K) electrical characterizations verified the blockade effect was originated from both molecular energy levels and single electron charging energy. Molecular orbital energy was extracted and shown good agreement with the literature [1].The successful integration and operation of this hybrid structure signified a strong potential for molecule-based electronic device design.

The degradation mechanism of CdSe/ZnS quantum dots (QDs) light-emitting diodes (LEDs) was investigated with steady-state and time-resolved photoluminescence measurements. Our study reveals that the degradation is associated with the decreasing quantum efficiency of the CdSe/ZnS QDs in the devices. Two mechanisms that cause the efficiency reduction were verified in the experiments: i.e., thermal instability and luminescence quenching.

Reorganization energy is one of the important factors to decide the rate of electron transfer according to the Marcus theory. Small reorganization energy is highly desirable in design of optoelectronic and electronic devices 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) have been studied theoretically by self-exchange electron transfer theory. By executing the Gaussian 03 calculation we can easily figure out the optimization point which needed for calculation of the inner reorganization energy (λ) of self-exchange electron transfer reaction. Also ionization potential and electron affinities of these molecules can be calculated at the 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 a semi-empirical model for the observed absorption and photoluminescence spectrum.

Solution-based printing and coating processes have the potential to dramatically 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 prohibiting wastage. Here, we report our latest results on the development of solutionprocessed QDLEDs. We have implemented a layer by layer strategy, from a whole evaporated small molecule based OLED to a hybrid QDLED developed by wet deposition techniques for the first layers and by evaporation for the last ones. Intermediate steps are discussed in this paper.

First, we have worked on a poly(3,4-ethylenedioxythiophene poly(styrenesulfonate) (PEDOT:PSS) layer. The PEDOT:PSS formulation for inkjet printing and spin coating were optimised: wettability on an ITO substrate, jettability of the inkjet formulation and baking conditions were studied. Additives as surfactant and ethylene glycol were added to the commercial inkjet grade solution to improve the deposition process. As a consequence to this study, anisotropic conductivity of PEDOT:PSS was observed and is reported here. In particular, ethylene glycol demonstrated a strong ability to increase the parallel conductivity by several orders of magnitude, but not the vertical one.

Then, inkjet-printed and spin-coated device performances are compared to complete this first study. Hybrid devices with an efficacy of 12cd/A at 4V were obtained, with 2.17 % of EQE, and a luminance of 4000 cd/m2 at 4V.

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

Spectroelectrochemical study on a new absorption band of radical cations of 4,4’-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl (α-NPD) as an electron-donor hole-transporting material used in organic electronics is reported in this work. UV-visible spectroscopic and cyclic voltammetric properties for α-NPD in solution are also examined. We find that the results are attributed to quenching process for blue fluorescence from α-NPD by excess α-NPD+ radical cations accumulated in the emission region in the organic light-emitting devices related to a relatively large overlap between the fluorescence spectrum of α-NPD and the absorption spectrum of α-NPD+ radical cations. The band gap energy for α-NPD is calculated from the UV-visible spectroscopic data.