This work studies the charge carrier collection efficiency in organic bulk-heterojunction solar cells based on polymer:fullerene blends. An equivalent circuit with a specific recombination term is proposed to describe the behavior of this type of devices. It is experimentally shown that this recombination term determines the slope of the current–voltage characteristic at the short-circuit condition. The variation of this dynamic resistance with the light intensity can be interpreted considering a dominant first-order recombination process. Finally, an analytical model under a constant electric field approximation is presented that can be used to calculate the charge carrier collection efficiency of the device. This model can be also used to estimate an effective mobility–lifetime product, which is characteristic of the quality of the active layer.

Metal capping layer can be used to enhance the physical properties of thin films. We propose a transparent conductor structure made of Al-doped ZnO (AZO) and an oxidized Ni capping layer, the latter with a thickness in proximity of its percolation threshold (2.5 nm). The capping layer inhibits the penetration of oxygen and water into the AZO’s grain boundaries thus significantly increasing the stability of the combined structure, as it is shown by its resistance in damp heat testing at 95 °C and 95% humidity. In addition, the oxidized Ni capping layer increases the performance of AZO transparent anodes in organic light emitting diodes by producing efficiencies as high as those of indium-tin-oxide based devices.

Al-doped ZnO (AZO) transparent electrodes capped with oxidized ultrathin Ni are proposed. The novel structure show enhanced stability in damp heat and also leads to OLED efficiencies as high as those of similar ITO-based devices.

Bilayer Cu-Ni transparent electrodes were room-temperature grown on glass by sputtering technique and used as anodes for polymer light-emitting diodes (PLEDs). The bilayer electrode structure allows combining the low sheet resistance and high transparency of Cu with the excellent stability and high work function of Ni. We demonstrate that Cu-Ni bilayer based PLED devices exhibit comparable efficiency and lifetime decay behavior to indium tin oxide (ITO) based device, with potentially significant advantages, such as easy processing, low cost and mechanical flexibility. In addition, the ductile nature of the metal electrode and its low thickness (< 10 nm) make the proposed structure particularly suitable for flexible organic electronic and optoelectronic devices. (C) 2011 Elsevier B.V. All rights reserved.

Novel transparent electrodes based on a metallic b i ayer structure are developed and their use in OLEDs is investigated. The devices show similar eff iciency compared to those incorporating state-of-ar t indium based ITO. In addition, the proposed transparent el ectrodes are easy to fabricate, inexpensive, and environmentally friendly.

A blend of the polymer poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEH-PPV) and the electron-transport molecule tris-(8-hydroxyquinoline) aluminum (Alq(3)) has been investigated by means of electroluminescence and fluorescence spectroscopy, upon variation of the Alq(3) content in the blend. A decreased interchain emission is observed upon increasing Alq(3) content, due to lower packing of the MEH-PPV chains which leads to a reduction in the interchain interaction, excimer formation, and emission probability. A branching of MEH-PPV interchain and intrachain emissive contributions is clearly time resolved and analyzed as a function of the Alq(3) content. At high doping concentration, direct emission from Alq(3) molecules is observed.

In this work the density-of-states in the gap of pentacene is calculated from the electrical characteristics of thin-film transistors measured at different temperatures. The density-of-states after a thermal annealing in vacuum (110 °C for 1 h) was clearly different from the as deposited state. A deep level located 0.49 eV above the valence band edge completely disappeared after the annealing. This level could be related to water molecules adsorbed onto the gate dielectric during the fabrication process. Besides, a broad Gaussian distribution centered around 0.28 eV from the valence band was resolved after the annealing. These states could be related to oxygen atoms chemically bonded to adjacent pentacene molecules. The thermal treatment also contributes to reduce structural disorder, as deduced from a thinned band tail.

Hybrid organic-inorganic monomode waveguides of conjugated polymers on porous silicon (PS) substrates have been fabricated. Different low refractive index PS substrates, varying from 1.46 down to 1.18 have been studied. Amplified spontaneous emission (ASE) has been observed for all the samples and the ASE threshold has been monitored as a function of the PS refractive index. A decrease in the ASE threshold is detected when the PS refractive index decreases. These results have been analysed in the frame of a four level waveguide amplifier model and the theoretical predictions are in agreement with the experimental data.

We report on the possibility of using a thin Ni layer, instead of ITO, as a semitransparent hole-injecting electrode for bottom polymer LEDs. Thin metal layers of Ni were deposited by a sputtering technique and their electrical and optical properties with different deposition times have been investigated. Both square resistance and transmittance were seen to decrease with deposition time ( thickness). The films showed a transmittance of around 30-40%, which is quite low compared to the 86% of ITO, while their square resistance was higher than that of ITO. Nevertheless, diodes based on a blue emitting polymer, polyfluorene (PFO), showed the same efficiency for either ITO or thin Ni electrodes, although the Ni transmittance is around 2.5 times lower than the ITO transmittance. Such preliminary results definitively suggest that indium-free organic devices can be achieved.

Transparent electrodes based on ultrathin metal films are realized, whose transparency is high compared to ITO in UV and IR and similar in visible region. Performance of an OLED is demonstrated as a potential application.

A vertical hybrid microcavity is fabricated by sandwiching a polymer layer between distributed Bragg reflectors (DBRs) composed of porous silicon photonic crystals. The DBRs are made by electrochemical etching of Si and consist of alternating porous Si layers of high and low porosity, the top DBR being a freestanding film. The hybrid microcavity demonstrates a deep microcavity mode placed within a 200 nm wide photonic band gap, and reveals a many-fold enhancement of the third-order nonlinearity of the microcavity layer. The fabrication technique employed is rather simple, enabling the use of a variety of functional materials as the microcavity spacer.

N-type organic thin-film transistors based on N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide have been fabricated by thermal evaporation at different substrate temperatures. The best device was obtained at 120 degrees C with a field-effect mobility of 0.12 cm(2)/V.s and threshold voltage around 46 V. In this work, the microstructure of the films is correlated with the device performance. In particular, the dependence of the activation energy for the channel conductance on gate voltages has been related to the properties of the layers. (C) 2009 Elsevier B.V. All rights reserved.

Thin film transistors based on polyarylamine poly(N,N′-diphenyl-N,N′bis(4-hexylphenyl)-[1,1′biphenyl]-4,4′-diamine (pTPD) were fabricated using spin coating in order to measure the mobility of pTPD upon oxidation. Partially oxidized pTPD with a molecular magnetic cluster showed an increase in mobility of over two orders of magnitude. A transition in the mobility of pTPD upon doping could also be observed by the presence of a maximum obtained for a given oxidant ratio and subsequent decrease for a higher ratio. Such result agrees well with a previously reported model based on the combined effect of dipolar broadening of the density of states and transport manifold filling.

The fabrication and optical characterization of a hybrid waveguide formed by a MDMO-PPV semiconductor layer deposited on a porous silicon cladding is reported. Evidence of amplified spontaneous emission is observed when the intensity of the excitation pump pulses is higher than a certain threshold. A net gain coefficient of about 50 +/- 8 cm(1) is estimated, which is similar to those reported for other PPV derivate conjugated polymers. The demonstration of positive optical gain in these hetero-structures is very promising because new photonic structures could be envisaged due to the versatility of the porous silicon cladding. (C) 2008 Elsevier B.V. All rights reserved.

The solid state thermal, one pot, efficient chemical reaction between Zn and S or Se elements in a closed reactor at 650 degrees C/60 min under their autogenic pressure in an inert atmosphere yielded luminescent ZnS and ZnSe semiconducting nanopowders (NPs). Scanning and Transmission electron microscopy measurements confirmed the size and shape of the as formed ZnS and ZnSe NPs. The wide size distributions of ZnS and ZnSe NPs are confirmed by UV-vis and TEM measurements. The crystalline wurtzite phase of ZnS and face centered cubic phase of ZnSe NPs is revealed from XRD and HR-TEM measurements. The obtained Raman scattering bands also supports the formation of pure ZnS and ZnSe phases. At room temperature, a strong visible green emission centered at approximately 525 nm is measured for ZnS, while ZnSe NPs showed a broad red emission band extending from 550 to 760 nm. The putative reaction mechanism is based on the low melting and boiling points of reactants (Zn, S and Se) under their autogenic pressure in an inert atmosphere.

We report a compact light source that incorporates a semiconductor light-emitting diode, nanostructured distributed feedback (DFB) Bragg grating and spin-coated thin conjugated polymer film. With this hybrid structure, we transferred electrically generated 390 nm ultraviolet light to an organic polymer via optical pumping and out-couple green luminescence to air through a second-order DFB grating. We demonstrate the feasibility of electrically driven, hybrid, compact light-emitting devices and lasers in the visible range.

We investigate defect states in pentacene thin films (PTFs) prepared by two different deposition techniques: standard thermal evaporation in high vacuum and the Langmuir–Blodgett (LB) technique. Time domain capacitance–voltage (C–V) and charge deep-level transient spectroscopy (DLTS) techniques were employed. C–V measurements indicate that Schottky barrier structures on both types of PTFs exhibit a common behavior: the thinner the PTF, the closer the measured C–V curve to the ideal shape for the Schottky barrier. Charge DLTS measurements show a peak with an activation energy of about 0.70eV above Ev and an attempt-to-escape frequency of 3×1014s−1 for PTFs deposited by thermal evaporation. For PTFs prepared by LB technique two deep-levels with activation energies of 0.20eV and 0.21eV, and an attempt-to-escape frequencies of 2×107s−1 and 2×106s−1, respectively, are found. The amplitude of the charge DLTS signals increases with decreasing PTF sample thickness for both deposition techniques. The observed defects can not be removed by annealing in vacuum. The origin of defects responsible for the measured charge DLTS spectra is discussed.

Thin-film transistors with fullerene as n-type organic semiconductor have been fabricated. A polymeric gate dielectric, polymethyl methacrylate, has been used as an alternative to usual inorganic dielectrics. No significant differences in the microstructure of fullerene thin-films grown on polymethyl methacrylate were observed. Devices with either gold or aluminium top electrodes have been fabricated. Although the lower work-function of aluminium compared to gold should favour electron injection, similar field-effect mobilities in the range of 10(-2) Cm-1 V-1 s(-1) were achieved in both cases. Actually, the output characteristics indicate that organic thin-film transistors behave more linearly with gold than with aluminiurn electrodes. These results confirm that not only energy barriers determine carrier injection at metal/organic interfaces, but also chemical interactions. (c) 2006 Elsevier B.V. All rights reserved.