Ionically self-assembled monolayer (ISAM) films have been shown to spontaneously produce noncentrosymmetric ordering that gives rise to a substantial second order nonlinear optical (NLO) response. Typically, the ISAM films for NLO response are an assemblage of bilayers of oppositely charged polymers whose thickness can be controlled through variation of pH and ionic strength of the immersion solutions. Here, we investigate the effects of replacing the NLO-active polymer layers with layers of monomeric chromophores containing ionic and covalent bonding sites. Films fabricated exclusively using polyelectrolytes contain some fraction of both randomly oriented and anti-parallel oriented chromophores. We have examined the incorporation of monomeric chromophores into ISAM films in order to increase the net polar orientation of the chromophores and reduce bilayer thickness.

The performance of solid-state dye-sensitized solar cells based on spiro-MeOTAD (2,2'7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene) was considerably improved by decreasing charge recombination across the interface of the heterojunction. This was achieved by blending the hole conductor matrix with a combination of 4-tert-butylpyridine (tBP) and Li[CF3SO2]2N. Open circuit voltages (Uoc) over 900mV and short circuit currents (Isc) up to 5.1 mA were obtained, yielding an overall efficiency of 2.56 % at AM1.5 illumination. Further improvement of the device performance was observed when conducting stripes of silver were deposited onto the devices as charge collector. The beneficial effect however could be assigned to the contamination of the dye-sensitized TiO2 film with silver during the dyeing process.

We have investigated the growth of polythiophene from self-assembled monolayers (SAMs) that contain pendant thiophene groups using electrochemistry and atomic force microscopy. The SAMs are formed on indium tin-oxide (ITO) by coadsorption of 11-(3-thienyl)undecyltrichlorosilane (3TUTS) and undecyltrichlorosilane (UTS). By altering the composition of the underlying monolayers we can manipulate the onset of electrochemical polymerization and affect the surface topography of the resultant polythiophene layer. Films made on SAMs that have high loadings of 3TUTS have small, distinct grains, but as the monolayers become enriched in UTS, the grain size increases; however, these films are neither as rough nor as diffuse as films formed on ITO without an underlying SAM. These experiments suggest that the electrochemical growth and structure of the polythiophene layer can be manipulated by tuning the underlying SAM.

Asymmetrically substituted oligo(phenylene ethynylene) (OPE) derivatives have been prepared by a new iterative method and attached to C60 molecules by a 1,3-dipolar cycloaddition of the azomethine ylides generated in situ from the corresponding aldehydes and sarcosine. The molecules obtained are suitable for the fabrication of photovoltaic devices based on interpenetrating networks of donors and acceptors, with a characteristic length scale of the network topology smaller than the exciton diffusion length. We have constructed a number of photovoltaic cells incorporating the C60-OPEs as the active material between two energetically asymmetric electrode contacts. The device performance has been examined and the functional characteristics have been determined too. These seem to depend systematically on the nature of the attached end-group and on the length of the donating moiety.

Dry dye-printing and solvent-enhanced dye diffusion were used to locally dope a previously spin-coated poly(9-vinylcarbazole) (PVK) polymer film with different dyes to fabricate side-by-side RGB OLED pixels. To reduce reverse leakage current and raise efficiency, a blanket tris-8-hydroxyquinoline aluminum (Alq) electron transport layer (ETL) was deposited over the polymer layer after the dye diffusion step, along with a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) hole/exciton blocking layer between the Alq and the PVK to ensure all light emission occurred from the doped polymer and not from the Alq. Devices with this tri-layer structure have an extremely low reverse leakage current (rectification ratio of 106 at +/- 10V) and a higher external quantum efficiency (∼1%) than single layer devices. A three-color passive-matrix test array with 300μm x 1mm RGB subpixels was demonstrated with this structure.

The use of hybrid devices consisting of organic and inorganic semiconductors provides many advantages for optoelectronic applications, such as combining the high charge carrier mobilities in inorganic polycrystalline thin films and high photosensitivity, intense photoluminescence, and ease of deposition of conjugated molecules. In this paper, thin film hybrid structures based on organic conjugated molecules and II-VI inorganic semiconductors were fabricated using spin-coating, evaporation and pulsed-laser deposition (PLD), respectively. A promising hetero-pairing is CdS and DQP, a diisoquinoline perrylene derivate, with the potential to form sensitive photodiodes and effective solar cells. Varying the deposition techniques alters the quality of the interface between CdS and DQP and results in different efficiencies of charge collection and recombination in the device.

A number of low workfunction metals (samarium, alkali metals) were deposited onto vacuum sublimed thin films of pentacene. The change in the valence electronic structure of the organic material was studied by in situ ultraviolet photoemission spectroscopy (UPS). Alkali metal intercalation leads to the appearance of a new photoemission feature within the pentacene energy gap, due to a charge transfer from the alkali atoms to the lowest unoccupied molecular orbital (LUMO) of the organic material. The energy spacing between this emission feature and the relaxed highest occupied molecular orbital (HOMO) of the pristine molecule is 1 eV. From X-ray photoemission spectroscopy core level analysis, we estimate a concentration ratio of two alkali metal atoms per pentacene molecule at maximum intercalation level, leading to a complete filling of the LUMO. This is consistent with the results from UPS that the new emission is always observed below the Fermi-level. Samarium is found to exhibit a more subtle interaction with pentacene: the molecular orbitals remain almost unperturbed upon Sm deposition. The resulting energy level alignment at this interface seems to be very favorable for the injection of electrons from Sm into pentacene, as the HOMO-onset is found at 1.8 eV below the metal Fermi edge. This value is close to the 2.2 eV HOMO-LUMO gap of pentacene measured by UPS and inverse photoemission spectroscopy, thus corresponding to a small electron injection barrier.

Measurements and theory are presented examining the relationship between mobility and doping in regioregular poly(3-hexylthiophene) (P3HT). Mobility is found to increase super-linearly with doping and is comparable to models reported for other conjugated polymers. Schottky measurements have been used to calculate the doping density and bulk mobility of regioregular P3HT. Aluminium Schottky contacts showed signs of native oxide disrupting current flow through the device. This effect was observed to degrade further with the introduction of dopant into the polymer. Titanium devices show a general shift of the Schottky characteristic to higher current levels with increased dopant. Field effect mobility of P3HT films was also calculated using thin-film transistor (TFT) structures. The field effect mobility values were observed to be more than two orders of magnitude higher than the bulk mobility value. The addition of dopant also increased gate leakage currents in TFT devices. The increased conductivity in doped polymer can increase off currents in the device; this is avoided by using Schottky contacts as the source and drain. Preliminary results on Schottky contact TFTs are also presented as well as a description of the operation of such a device.

The electronic transport and the device properties of pentacene thin film transistors are reported, showing the influence of the dielectric on the structural and transport properties. The structure and morphology of pentacene films on organic and inorganic dielectrics were compared by x-ray diffraction measurements and atomic force microscopy. For the investigated dielectrics we observed a clear correlation between the morphology and the structural properties of the highly polycrystalline films. In the case of inorganic dielectrics the roughness of the dielectric has a distinct influence on the morphology and the structural properties, whereas the films on organic dielectrics bonding between pentacene and the dielectric may have an influence on the growth mechanism. We find that careful control of the deposition conditions give films with similar transport properties on organic and inorganic dielectrics. To study the electronic properties we have realized inverted staggered transistors. The TFTs exhibit mobilities of ∼0.4 cm2/Vs and on/off ratios of 108 on organic and inorganic dielectrics. The influence of the dielectric on the device mobility, threshold voltage and sub-threshold voltage slope are discussed.

A low level of electron injection is one of the major obstacles to achieving high-efficiency organic light-emitting diodes. With the goal of improving injection characteristics, we fabricated and investigated herein multi-layer contacts which included a tunnel-transparent dielectric layer of nanometer thickness. Polymer layers were prepared by the spin-coating method and dielectric and metallic contact layers grown by vacuum deposition. The voltage drop at this layer shifts the metal Fermi level relative to the polymer molecular orbitals responsible for the carrier transport, increasing the injection efficiency. The introduction of a suitably chosen dielectric layer results in an increase in the injection efficiency by up to a factor of several tens.

Further sophistication of the injecting contacts consists in creating and additional intermediate thin metallic layer playing the role of the third, base electrode, similar to hot-electron transistors with metallic base. Additional bias applied to the base electrode permits variable injection efficiency and quantum yield over a wide range.

We have fabricated phosphorescent light emitting devices using mixed ligand bisorthometalated Ir(III) complexes, chlorobis-(2-phenylpyridinato-N,C 2 ')pyridineiridium(III) [Ir(Cl)(ppy)2(py)] and chlorobis-(2-phenylpyridinato-N,C 2')triphenylphosphine iridium(III) [Ir(Cl)(ppy)2P(Ph)3] chlorobis-(2-phenylpyridinato-N,C 2')tri-n-butylphosphine iridium(III) [Ir(Cl)(ppy)2P(n-Bu)3], where ppy is the orthometalating ligand. These complexes vary in their HOMO (highest occupied molecular orbital), LUMO (lowest unoccupied molecular orbital) and emissive states in accordance with the φ-accepting abilities of the non-orthometalating ligands. The lifetimes and EL spectrum of the devices were studied and compared.

The technique of vacuum evaporation has been applied to deposit organic photovoltaic active layers. The five-ring PPV-type oligomer 2-methoxy-5-(2'-ethylhexyloxy)-1,4-bis((4',4”-bisstyryl)styrylbenzene) (MEH-OPV5) and C60 act as respectively donor and acceptor materials in planar heterojunction (MEH-OPV5/C60) and bulk heterojunction (MEH-OPV5:C60) devices. These devices were fabricated with ITO/PEDOT:PSS bottom electrodes and Al top contacts. The performance of both solar cell configurations has been compared. It was found that under AM1.5 illumination the MEH-OPV5/C60 cells exhibit a higher open-circuit voltage (∼ 1.00 V) than the MEH-OPV5:C60 devices (∼ 0.92 V). On the other hand, the limited exciton diffusion length in these materials was reflected in the lower short-circuit current density of the planar heterojunction cells as compared to the bulk heterojunction structures. Overall AM1.5 power conversion efficiencies reaching 2 % are reported. Also the influence of the organic layer thickness and the substrate temperature during deposition on the device performance has been addressed. Thick organic films generally induce a high series resistance that limits both the short-circuit current density and the fill factor. An elevated substrate temperature during deposition of the MEH-OPV5:C60 layers onto ITO/PEDOT:PSS led to the formation of nucleated islands of 100 - 150 nm diameter with holes in between. As a result, no reliable photovoltaic devices could be realized with such organic films. AFM analysis and spectral response measurements supported these findings.

White electroluminescence (EL) spectra are carefully investigated in terms of the multilayer structures and a rule for designing the white luminescence is discussed. For a simple structure which consists of tris(8-hydroxyquinoline)-aluminum (Alq) and 1,2,3,4,5-pentaphenyl-1,3-cyclopendadiene (PPCP) emitting green and blue luminescence, respectively, as luminescent and electron transport layers, white luminescence appeared if the thickness of each layer was appropriately chosen. The luminescence efficiency was higher in PPCP, and therefore one of the structures effectively emitting white luminescence was designed as PPCP(10 nm)/Alq(40 nm). The 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) was then added to the PPCP/Alq structure intending to enhance red luminescence. We propose the structure where red luminescence is from the DCM as photoluminescence excited by the green and blue luminescence from Alq and PPCP, which was effective, compared to the PPCP/Alq/DCM stacking structure, in avoiding increase of the operating voltage. The importance of designing structures of organic white EL devices based on fundamental optical properties of the constituent layers and on luminescence dynamics is highlighted.

We have synthesized a new tris-orthometalated Ir(mpp)3 and several bis-orthometalated Ir(III) complexes of 3-methyl-2-phenylpyridine(Hmpp). The influence of the methyl substituent in Hmpp and different ligand environments on the optical, electrochemical and electroluminescence properties of the orthometalated complexes are examined by varying the donor acceptor properties. This includes several uninegative bidentate ligands consisting of different organic functional groups. Absorption, excitation and emission spectra in solution and solid state of these complexes agree with a strong electronic excitation to 1MLCT in Ir(mpp)3 and preferably to 3MLCT in other complexes. These complexes demonstrate strong phosphorescence at room temperature with a lifetime of 1-2 μs. Multilayer organic light emitting devices using these complexes as the triplet emitter are studied using PVK (poly(9-vinyl carbazole)), CBP (4,4'-N,N'-dicarbazole-biphenyl) and TAZ (3-phenyl-4-(1'-napthyl)-5-phenyl-1,2,4-triazole) as the host materials. Ir(mpp)3exhibits quite different behavior in different host materials.

A 2,3-dibutoxy-1,4-phenylenevinylene comonomer was incorporated into a distyrylbenzene derivative 11. Novel 1,2-disubstituted-3,6-dibromobenzene comonomers 15 and 18 were prepared by directed metallation. Copolymerization of 11with a 9,9-dioctylfluorene-2,7-diboronate ester 1 yielded a green fluorescent polymer while copolymerization of 15 and 18 with the 9,9-dihexylfluorene-2,7-diboronate 22 afforded promising blue fluorescent polymers 23 and 24 respectively.

We report on the valence orbital structure of poly(para-phenylenevinylene) (PPV)-like oligomers. We studied these molecules as isolated oligomers in the gas phase, as well as in thin films deposited on metal substrates. We use a simple model based on a previously reported Hamiltonian that accurately describes the development of the low lying electronic excitations as a function of the number of repeating units. In the study on the thin organic films we report on the energy level alignment at metal / organic and organic / organic interfaces, where the organic layer is either a PPV-like oligomer or C60. The results are important for understanding organic photovoltaic devices.

Royal Philips Electronics has recently launched the first commercially available polymer Light Emitting Diode (LED). In this paper, physical properties and phenomena will be discussed that must be taken into account during the selection of a polymer material and the choice of the device structure; for example, color, electron and hole mobility, polymer thickness, lifetime, leakage current and failure.

In this paper, we report strategies for electrodepositing and patterning ultrathin films of conjugated polymers on flat electrode surfaces using the precursor polymer approach. This involves a rational synthesis design of the precursor polymer followed by careful electrodeposition and characterization of ultrathin films on conducting substrates. This has resulted in the preparation of smooth, high optical quality films, which should be important for applications involving flat electrode surfaces in devices. Characterization was made using surface sensitive spectroscopic and microscopic techniques. Copolymerization with monomers, polymer backbone design, and grafting on modified surfaces are key points in this strategy. Novel methods of in-situ characterization techniques have also been developed combing electrochemistry and surface plasmon resonance techniques.

We synthesized novel cross-linkable fluorinated copoly(arylene ether sulfide)s for optical waveguide applications, which have high thermal stability, high optical transparency in the infrared communication region, and much smaller birefringence than other thermally stable fluorinated polyimides. The refractive index of the material can be easily controlled from 1.515 to 1.587 by changing the copolymer composition in the materials. The birefringence of the cured polymers were 0.0031∼0.0039 at the wavelength of 1.55 μm. This is much lower than those of fluorinated polyimides for optical waveguides. The refractive index of fluorinated poly(arylene ether sulfide) (FPAESI) after being stored at 100 °C for 1000 hr remains almost constant demonstrating the thermal stability. The propagation loss of the channel waveguides fabricated using reactive ion etching was less than 0.4 dB/cm at the wavelength of 1.55 μm.

The phase characteristics of transmitted optical pulses in three-dimensional photonic crystals were investigated in the frequency and time domain by using the spectrally resolved cross-correlation technique. The temporal evolution of femtosecond pulses passing through polystyrene colloidal crystals exhibits a large phase distortion near the stop bands. The phase discontinuity around the band gap was observed in the frequency-domain. The phase of the transmitted pulses is found to change by φ across the band gap. The dispersion curve estimated from the phase shift shows good correspondence with those calculated from a photonic band calculation. The group velocity significantly slows down near the stop bands. A large change of the group velocity dispersion is also observed near the band edges. These results are in good agreement with the band theory.