Three-dimensional organic field-effect transistors with multiple sub-micrometer channels are developed to exhibit high current density and high switching speed. The sub-micrometer channels are arranged perpendicularly to substrates and are defined by the height of a multi-columnar structure fabricated without using electron-beam-lithography technique. For devices with dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene, extremely high current density exceeding 10 A/cm2 and fast switching within 200 ns are realized with an on-off ratio of 105. The unprecedented performance is beyond general requirements to control organic light-emitting diodes, so that even more extensive applications to higher-speed active-matrices and display-driving circuits can be realized with organic semiconductors.

Gate-voltage dependent Hall coefficient RH is measured in high-mobility field-effect transistors of solution-crystallized and vapor-deposited 2,7-dioctylbenzothieno[3,2-b]benzothiophene. The value of RH evolves with density of accumulated charge Q, precisely satisfying the free-electron formula RH = 1/Q near room temperature. The result indicates that the intrinsic charge transport inside the grains is band-like in the high-mobility organic-semiconductor thin films that are of significant interest in industry. At lower temperature, even Hall-effect mobility averaged over the whole polycrystalline film decreases due to the presence of carrier-trapping levels at the grain boundaries, while the free-electron-like transport is preserved in the grains. With the separated description of the inter- and intra-grain charge transport, it is demonstrated that the reduction of mobility with decreasing temperature often shown in organic thin-film transistors does not necessarily mean mere hopping transport.

Modifications to the p-type semiconductor TIPS-Pentacene can result in elimination of the solid-solid thermal transition at 124 °C. This new material has shown mobility higher than 1 cm2/Vs. Elimination of the solid-solid thermal transition leaves the melting point as the lowest temperature transition at 199 °C.

A quasi-solid-state dye-sensitized solar cell is presented, where the conventional liquid electrolyte is replaced by an electrolyte film, reaching a solar light-to-current conversion efficiency of 3%. Contactless transient photoconductance measurements were performed, revealing decay behavior of photoinduced charge carriers, dependent on external applied potential conditions. The measurements show that the decay is controlled by the injection of electrons into the front contact, hindered or enhanced by the field in the space charge region.

Solution-processable organic bulk-heterojunction photovoltaic devices have made great advances over the past decade. The concept, ultrafast photo induced electron transfer from a conjugated polymer to fullerene derivative molecules in bulk-heterojunction systems, leads to device efficiencies as high as 6%. Light absorption, charge separation and charge transport to electrodes are the most important steps in organic photovoltaic devices. The enhanced light absorption through thicker active layers results in more exciton creation, however, leads to increased recombination due to the relatively short exciton diffusion length. We fabricated poly(3-hexylthiophene)/ [6,6]-phenyl C61 butyric acid methyl ester bulk-heterojunction devices with multiwall carbon nanotubes in the active layer in a bid to address this deficiency. Functionalization of carbon nanotubes allows better dispersion in aromatic solvents, 1,2-dichlorobenzene in this study, and pristine multiwall nanotubes result in poorer dispersions. Organic photovoltaic devices fabricated with pristine multiwall carbon nanotubes in the active layer result in power conversion efficiencies ˜1.4%, which show localized nanotube-rich areas in the active layer. Alternatively, acid functionalized nanotubes in the active layer results in efficiencies as high as 2.2 % with no distinct nanotube-rich sectors. The open circuit voltages of the devices show a dependency on the loading of nanotubes in the active layer. Further, the shunt resistances of the devices with carbon nanotubes decrease, which needs careful selection of the tubes depending on active layer thickness. This work compares the device performances in detail and identifies further improvements to conjugated polymer/fullerene derivative/multiwall carbon nanotubes hybrid photovoltaic systems.

We introduce a novel instrument combining femtosecond pump-probe spectroscopy and confocal microscopy for spatio-temporal imaging of excited-state dynamics of phase-separated polymer blends. Phenomena occurring at interfaces between different materials are crucial for optimizing the device performances, but are poorly understood due to the variety of possible electronic states and processes involved and to their complicated dynamics. Our instrument (with 200-fs temporal resolution and 300-nm spatial resolution) provides new insights into the properties of polymer blends, revealing spatially variable photo-relaxation paths and dynamics and highlighting a peculiar behaviour at the interface between the phase-separated domains.

The objective of this research is to obtain uniform vacuum-deposition triclinic phase II crystal of titanyl phthalocyanine (α-TiOPc) films from various TiOPc crystal forms. The crystal structure and morphology of vacuum-deposited TiOPc films can be manipulated by deposition rate and substrate temperature. Crystal structure was determined by X-ray diffraction (XRD). Thin film morphology was analyzed by scanning electron microscope (SEM). Highly ordered α-TiOPc film with an edge-on molecular orientation was deposited on octadecyltrichlorosilane (OTS) treated Si/SiO2 surface. All TiOPc crystal forms, such as amorphous, α and γ phases, provided the triclinic phase II crystal of TiOPc. The full width at half maximum (FWHM) of the peak at 7.5 degree in XRD spectra was 0.23, 0.27 and 0.29 for γ, α and amorphous powder when substrate temperature maintained at 180°C, respectively. The FWHM of the 7.5 degree peak can be achieved 0.22 deposited from all crystal forms at elevated temperature higher than 220°C. The α-TiOPc deposition film exhibited an excellent p-type semiconducting behavior in air with dense packing structure due to the close π–π molecular packing. The devices, field-effect mobility range from 0.02 to 0.26 cm2/V s depending on various process parameters. The on/off current ratio (Ion/Ioff) is over 105. The TiOPc OTFTs will be applied as multi-parameter gas sensor in the near future.

Photo-induced electron transfer and recombination was investigated for prototype molecular absorbers covalently bound to metal-oxide films. Two perylene derivatives with different anchor/bridge groups, a propionic and an acrylic acid, were adsorbed on both bare ZnO nanorods and TiO2 coated nanorods and the dynamics of the systems compared.

Ambipolar organic thin-film transistors (OTFTs) have been used to study the transport of charge carriers in bulk heterojunction (BHJ) organic photovoltaic devices. Active layers of phase separated blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), have been chosen due to their use in performance BHJ organic photovoltaic devices as well as ease of device fabrication. A method for determining recombination rate after exciton dissociation and measurement of excess carrier lifetime has been reported by studying drain current behavior which yields carrier mobility, conductivity, and carrier concentration both in dark and AM1.5g illumination. Channel-length dependent measurements of the photocurrent show that significant recombination of separated charge carriers begins to occur at lengths greater than 10 μm. A recombination rate of cm-3 s-1 and a carrier lifetime of ≥ 8.8 ms has been calculated.

Copper phthalocyanine (CuPc) belongs to a class of small molecules offering particularly interesting advantages when employed in organic electronic devices. Because of its advantageous attributes like high thermal stability, inertness when exposed to acids or alkalis, relatively high electron conductivity, color and light fastness it has been employed in polymer photovoltaic devices as a unipolar dopants complementing the buckminsterfullerene (C60) acceptors and as a conductive buffer. Other organic applications include ambipolar OFETs and non-linear optics structures. X-ray photoelectron spectroscopy (XPS) has been commonly employed to monitor the quality of thin CuPc films. Although XPS analyses of CuPc have been done for over forty years there has not yet been agreement regarding interpretation of the major C1s signal, particularly in the case of non-stoichometric CuPc composition. This work presents systematic studies of the C1s signal of thin film deposits, fabricated using commercially available CuPc materials. It was found that composite C1s CuPc signal consists of five components: two related to the principal C positions within the CuPc macrocycle (C-C in 6-membered ring, C-C-N in 5-membered ring), two associated with shake-up transitions accompanying principal C transitions, and one due to mostly aliphatic impurities. Detailed analysis showed that the magnitude of shake-up peaks was approximately equal 10% to 12% of their principal transitions, in agreement with the theoretical calculations. Correspondingly, the C1s signal originating from the non-CuPc impurities quantitatively agreed with the IR attenuated total reflectance (ATR-IR) measurement of the C-H aliphatic vibrations originating from these impurities present within the CuPc layer. The proposed C1s interpretation has been successfully tested for a large number of commercial CuPc materials and provides a guideline for a routine XPS analysis of the CuPc in organic photovoltaic devices.

In an attempt to produce low cost and high quality polymer/nanoparticle blends for use in hybrid organic/inorganic photovoltaic devices we prepared blends of dihexylsexithiophene and tetragonal barium titanate particles. These polymer nanoparticle blends were deposited as films by spin coating and sublimation. The films were characterised and compared using a wide range of techniques; The electrical photoconductivity analysis of these structures carried out using an HP4140B picoammeter and a solar simulator after aluminium gap cell electrodes had been deposited on the films by sublimation, spectroscopic studies (FTIR and UV-VIS) were carried out to understand the photoconductivity measurements and ellipsometry was used to determine the thickness of the films. The photoconductivity of the spin coated films was the highest reaching 8.5x 10-10A at 20 V, the sublimed films reached ~4 x 10-10A at 40V. This is thought to be due to the thinness of the sublimed films combined with the inhomogeneous distribution of nanoparticles compared with the spin coated film. Sublimed films have been shown by others to be better structured than spin coated films, if this property can be utilized with further optimization of the sublimation process then this technique offers the potential to produce very thin high quality films for use in organic and hybrid photovoltaic devices.

The performance of dye-sensitized solar cells (DSSCs) is limited by the back-reaction of photogenerated electrons from the porous titanium oxide (TiO2) nanoparticles back into the electrolyte solution, which occurs almost exclusively through the interface. This and the fact that DSSCs have a very large interfacial area makes their performance greatly dependant on the density and activity of TiO2 surface states. Thus, effectively engineering the TiO2/dye/electrolyte interface to reduce carrier losses is critically important for improving the photovoltaic performance of the solar cell. Atomic layer deposition (ALD), which uses high purity gas precursors that can rapidly diffuse through the porous network, was used to grow a conformal and controllable aluminum oxide (Al2O3) and hafnium oxide (HfO2) ultra thin layer on the TiO2 surface. The effects of this interfacial treatment on the DSSC performance was studied with dark and illuminated current-voltage and electrochemical impedance spectroscopy (EIS) measurements.

In this study, the dark current-voltage characteristics of electron-only and hole-only poly(3-hexyl thiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) as a function of regioregularity (RR) and annealing time is investigated using the mobility edge (ME) model. This model is used to analyze the degradation of electron and hole mobilities as a function of annealing time for 93%-RR and 98%-RR P3HT:PCBM devices. The hole mobility is almost unchanged by the RR nature of P3HT and thermal annealing. The electron mobility, however, behaves differently after annealing. The electron mobility of 98%-RR devices, which is initially higher than that of the 93%-RR devices, experiences a steep decline with annealing. Based on ME analysis, this is due to an increase in trap states in the exponential tail caused by phase segregation of solid state blends of 98%-RR polymer and PCBM. The electron mobility of 93%-RR devices increases with annealing due to an optimization of nano-phase separated morphology.

We present a performance comparison of polythiophene/fullerene derivative bulk heterojunction solar cells fabricated on fluorinated tin oxide (FTO) and indium tin oxide (ITO) in the presence and absence of the commonly used poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) hole extraction layer. From a potential commercial perspective the performance of cheaper and more readily available FTO compares well with the more expensive ITO in terms of measured device efficiency (FTO:2.8 % and ITO:3.1%). The devices show similar fill factors (FTO:63% and ITO:64%) with the same open circuit voltage of 0.6 V. The short circuit current density is lower for FTO devices at 7.5 mA/cm2 which compares with 8.0 mA/cm2 for ITO; a behaviour that is mainly attributed to the reduced optical transmission of the FTO layer. Importantly, these devices were part fabricated and wholly characterized under atmospheric conditions. The quoted device performance is the best reported for FTO based bulk heterojunction systems in the absence of the highly acidic PEDOT:PSS hole extraction layer, which is believed to degrade conductive oxides.

Comparison of the IR spectra of the Cu-phthalocyanine (CuPc) nanocrystals obtained using surface sensitive attenuated total reflectance (ATR) and bulk sensitive transmittance sample configurations revealed small but measurable changes of some vibrational frequencies of the molecules at the surface of nanocrystals with the outermost part of the surface CuPc molecules being the most affected. These changes are caused by electrostatic interactions between the polar components of the molecules on the surface of nanocrystals and external polar molecular species vicinal to the nanocrystals. The external polar species can be either chemically bonded to the CuPc nanocrystal's surface or they can reside in its vicinity without forming a chemical bond with the nanocrystal. Molecular modeling (DMOL3 - Materials Studio and Gaussian calculations) of the impact of selected external polar species vicinal to a CuPc molecule on the CuPc molecular vibrations confirmed experimentally observed changes in the vibrational frequencies of the selected CuPc molecular bonding configurations and provided detailed information on the forces involved in these interactions. The population of external polar species vicinal to the CuPc surface can be modified by washing the nanocrystals or by introducing polar molecular additives miscible with the CuPc nanocrystals. Reduction in the number of external polar additives was accomplished by either centrifuging the aqueous dispersion of the nanocrystals or by organic solvent-based Soxhlet extraction, while their number was increased by soaking (followed by drying) the nanocrystals in high and low pH aqueous solutions containing SO3- and OH- ions. These quantitative and qualitative modifications of the population of external polar species surrounding CuPc nanocrystals were reflected in the corresponding changes of the selected vibrational frequencies of the CuPc surface molecules providing an effective tool for not only recognizing the molecular species vicinal to a nanocrystal but also quantifying their concentration. Some of these modifications can also be observed with a naked eye in the form of noticeable color changes of the CuPc nancrystalline powder. This is due to the extremely high visible extinction coefficient of the CuPc nanocrystals causing that the impinging light is mostly absorbed/reflected within the surface region of the nanocrystals. Changes of the electronic structure within this region, caused by the interactions with the vicinal polar species, shift the vis absorption/reflection spectra changing the observed color of the nanocrystalline powder. Similar results were obtained for other molecular nanocrystals, including yellow chromophore molecules. Preliminary data indicate that the described analytical method of analyzing the molecular polar species vicinal to a molecular nanocrystal could find variety of applications ranging from molecular device fabrication to pharmaceutical materials.

By coupling optical and electronic device simulation, we explore the optimization of organic photovoltaic devices. We use optical simulation via FDTD and transmission matrix solution of Maxwell's equations in order to calculate distribution of light intensity, including the reflectivity and transmissivity of transparent anode layers. We derive simple formulas for the conductivity requirements as function of pattern structure of potential replacements for ITO as transparent anode material, and explore the optimization of thickness of active layer and its relationship with the peaks of optical generation inside the active layer.

A novel ZnO/TiO2 assorted photoelectrode for dye-sensitized solar cells (DSSCs) is proposed. The impacts of the ZnO/TiO2 assorted photoelectrode on the photovoltaic performance of dye-sensitized solar cells (DSSCs) were investigated. The measurements of the light transmission spectra showed the higher transmittance through ZnO/FTO than through FTO during the effective wavelength region of 536nm˜800nm for DSSCs, indicating that ZnO/TiO2 assorted photoelectrode is beneficial for the photovoltaic performance of DSSCs. The measurements on the photovoltaic characteristics of the DSSC cell indicate that the inserted ZnO layer can cause the increased open circuit voltage (Voc) more than 70 mV and fill factor (FF) but the decreased short circuit current. The enhanced Voc and FF could be attributed to the suppressed the recombination of photon-generated carriers between the ZnO/TiO2 assorted photoelectrode and electrolyte (dye) compared to TiO2 photoelectrode. However, the additional series resistance of inserted ZnO layer causes the reduced short circuit current. The optimized conversion efficiency can be achieved in the DSSC with ZnO/TiO2 assorted photoelectrode by using low series resistance of ZnO layer.

The critical factor that limits the efficiencies of organic electronic devices is the low charge carrier mobility which is attributed to disorder in organic films. In this work we study the effects of active film morphology on the charge transport in Organic Field Effect Transistors (OFETs). We fabricated the OFETs using different substrate temperature to grow different morphologies of C60 films by Hot Wall Epitaxy. Atomic Force Microscopy images and XRD results showed increasing grain size with increasing substrate temperature. An increase in field effect mobility was observed for different OFETs with increasing grain size in C60 films. The temperature dependence of charge carrier mobility in these devices followed the empirical relation named as Meyer-Neldel Rule and showed different activation energies for films with different degree of disorder. A shift in characteristic Meyer-Neldel energy was observed with changing C60 morphology which can be considered as an energetic disorder parameter.

Conductive nanofibers with the average diameters in the range of 60 nm - 2 μm were fabricated by electrospinning of a mixture of poly(3-hexylthiophene) (P3HT) and polyvinylpyrrolidone (PVP) in a mixed solvent of chlorobenzene and methanol. Beaded fibers and/or uniform, smooth-surface fibers were successfully fabricated. The average diameter of the as-spun fibers decreased and the color of as-spun fibers changed with decreasing the concentration of P3HT or PVP. After the removal of PVP from as-spun fibers by Soxhlet extraction, pure P3HT fibers were obtained as a spindle-like with groove-like morphological appearance which may be widely applicable for some specific applications, such as photovoltaic cells, thin film transistors, and light emitting diodes.

Beaded fibers and/or uniform, smooth-surface fibers of conductive polymers with the average diameters ranging in nanometers to sub-micrometers were fabricated by electrospinning of a mixture of poly(3-hexylthiophene) (P3HT) and polyvinylpyrrolidone (PVP) in a mixed solvent of chlorobenzene and methanol. After the removal of PVP from as-spun fibers by Soxhlet extraction, pure P3HT fibers were obtained as a spindle-like with groove-like morphological appearance which may be widely applicable for some specific applications, such as photovoltaic cells, thin film transistors, and light emitting diodes. Optical properties, including UV absorption and photoluminescence (PL) of fibers were investigated. As-spun fibers showed relatively higher conjugation length and different chain distribution, in comparison with the cast film.