Lucimara S. Roman

Polymer diodes with high rectification

Polymer diodes made using a bilayer of doped poly(3,4-ethylenedioxythiophene) and a semiconducting polymer in a sandwich structure with two low-work-function metals are reported. The conducting polymer layer acted as a modifier of the injection properties of the low-work-function metal, allowing easy hole injection. Upon insertion of the conducting polymer layer, the contact-limited current flow became bulk limited. With this anode, the fabrication of diodes with a rectification ratio of seven orders of magnitude was possible. We present patterned microdiodes made with crossing of 10 μm lines, showing similar performance as the mm-size diode.

The use of tin oxide thin films as a transparent electrode in PPV based light-emitting diodes

Tin oxide (TO) thin films, nominally undoped, have been used as electrodes in poly(p-phenylene vinylene) (PPV) based organic electroluminescent devices. The evolution of the crystallinity and the electrical resistance of TO films submitted to the PPV thermal conversion conditions, have been investigated. It has been found that the electrical resistance is decreased whereas the crystallinity of the film is increased. It is shown in this work, that the photoluminescence of PPV converted on top of TO substrates is not as quenched as it is when converted on top of indium-tin oxide (ITO) substrates. The quantum efficiency of light-emitting diode is 0.07% at 17 V forward bias. It is also shown that the work function of TO films is very stable to different cleaning procedures, in contrast with previous results obtained for ITO films.

Preparation and characterization of transparent conducting tin oxide thin film electrodes by chemical vapour deposition from reactive thermal evaporation of SnCl2

A chemical vapour deposition technique has been developed for highly transparent conducting tin oxide thin films from the reactive thermal evaporation of SnCl2. We have demonstrated that good quality tin oxide thin films for electrode application in electronic devices can be produced by such a technologically simple and inexpensive way. Best results concerning optical transmittance, electrical conductivity and crystallinity (SnO2) were obtained in this way at substrate temperatures in the range of 500–550°C.

Electrosprayed superhydrophobic PTFE: a non-contaminating surface

This paper reports on the exposure of superhydrophobic polytetrafluoroethylene (PTFE) coatings to common aqueous solutions which are used in biology, biotechnology and chemical sensor applications. Advancing contact angles as high as 173° for aqueous solutions were measured on the PTFE surface. Water drop sliding angles at 2° show a very low contact angle hysteresis. X-ray photoelectron spectroscopy measurements confirm that aqueous solutions can move or stay on the superhydrophobic surface without contamination. Owing to the chemical inertness of the polymer, these results indicate that superhydrophobic PTFE can be used in lab-on-a-chip and multi-sensor devices as well as in biological cultures, where aqueous solutions meet solid surfaces, without contaminating the interface.

Polymer and polymer/metal interface characterization via Fowler–Nordheim tunneling measurements

In this letter we present a procedure used to characterize metal/polymer interfaces via Fowler‐Nordheim tunneling current measurements and use it to obtain the electron affinity of the PPV (poly p‐phenylene vinylene) and to calculate the metal/PPV barrier heights using different Al/PPV/metal (metal: Ga,In,Sn) structures.

Hole mobility effect in the efficiency of bilayer heterojunction polymer/C60 photovoltaic cells

We report here bilayer heterojunction solar cells fabricated by using poly[9,9′-hexyl-fluorene-alt-bithiophene] (LaPPS43) polymer as active layer. The power conversion efficiency (η) displays a sevenfold increase upon annealing at 200 °C, reaching the value of 2.8%. This result is comparable to the highest η reported so far for bulk heterojunction solar cells using the same polymer. Simulation, absorbance spectra, and current versus voltage results indicate that the π–π stacking in solid state is enhanced after annealing with a reduction in traps and thus reflecting in higher hole mobility.

Modeling bilayer polymer/fullerene photovoltaic devices

The electrical transport and charge generation of polymer/fullerene photovoltaic devices were investigated. The polymer/fullerene photodiodes were formed by a heterojunction of fullerene and a semiconducting polymer poly (3,4-ethylenedioxythiophene). The current-voltage characteristic of the devices were measured with variable thickness of the C60 layer, under monochromatic light of different wavelengths. The results show that the values of the electrical photoconductivity are related to the optical absorption coefficient of the fullerene, and thus implying a large contribution of the C60 films to the diode photocurrent.

Development of a Chemiresistor Sensor Based on Polymers-Dye Blend for Detection of Ethanol Vapor

The conductive blend of the poly (3,4-ethylene dioxythiophene) and polystyrene sulfonated acid (PEDOT-PSS) polymers were doped with Methyl Red (MR) dye in the acid form and were used as the basis for a chemiresistor sensor for detection of ethanol vapor. This Au | Polymers-dye blend | Au device was manufactured by chemical vapor deposition and spin-coating, the first for deposition of the metal electrodes onto a glass substrate, and the second for preparation of the organic thin film forming ∼1.0 mm2 of active area. The results obtained are the following: (i) electrical resistance dependence with atmospheres containing ethanol vapor carried by nitrogen gas and humidity; (ii) sensitivity at 1.15 for limit detection of 26.25 ppm analyte and an operating temperature of 25 °C; and (iii) the sensing process is quickly reversible and shows very a low power consumption of 20 μW. The thin film morphology of ∼200 nm thickness was analyzed by Atomic Force Microscopy (AFM), where it was observed to have a peculiarly granulometric surface favorable to adsorption. This work indicates that PEDOT-PSS doped with MR dye to compose blend film shows good performance like resistive sensor.

Electronic structure, molecular orientation, charge transfer dynamics and solar cells performance in donor/acceptor copolymers and fullerene: Experimental and theoretical approaches

By combining experimental and theoretical approaches, the electronic structure, molecular orientation, charge transfer dynamics and solar cell performance in donor/acceptor copolymer poly[2,7-(9,9-bis(2-ethylhexyl)-dibenzosilole)-alt-4,7-bis(thiophen-2-yl)benzo-2,1,3-thiadiazole] (PSiF-DBT) films and blended with 6,6.-phenyl-C 61-butyric acid methyl ester (PSiF-DBT:PCBM) were investigated. Good agreement between experimental and theoretical PSiF-DBT UV-Vis absorption spectrum is observed and the main molecular orbitals contributing to the spectrum were determined using DFT single point calculations. Non-coplanar configuration was determined by geometric optimization calculation in isolated PSiF-DBT pentamer and corroborated by angular variation of the sulphur 1s near-edge X-ray absorption fine structure (NEXAFS) spectra. Edge-on and plane-on molecular orientations were obtained for thiophene and benzothiadiazole units, respectively. A power conversion efficiency up to 1.58%, open circuit voltage of 0.51 V...

Water based, solution-processable, transparent and flexible graphene oxide composite as electrodes in organic solar cell application

In this work we propose an easy method to achieve a conductive, transparent and flexible graphene oxide (GO)-based composite thin film from an aqueous dispersion. We investigated the blend ratio between GO and the conjugated polymer poly(3,4–ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) by comparing the thin film optical transmittance, sheet resistance, morphology and mechanical stability. It was found that reasonable values of transmittance and resistivity coupled with its excellent flexibility – the conductivity remains almost the same even after 1000 bends cycles – make this composite very attracting for flexible optoelectronic applications. Thus, these films were used as transparent electrodes in a bilayer structured organic solar cell and the device architecture PET/GO:PEDOT/F8T2/C60/Al could reach a power conversion efficiency around 1.10%. This result presents a better performance compared with pristine PEDOT produced with similar parameters.