Alberto Montaigne Ramil

High performance n-channel organic field-effect transistors and ring oscillators based on C60 fullerene films

We report on organic n-channel field-effect transistors and circuits based on C60 films grown by hot wall epitaxy. Electron mobility is found to be dependent strongly on the substrate temperature during film growth and on the type of the gate dielectric employed. Top-contact transistors employing LiF∕Al electrodes and a polymer dielectric exhibit maximum electron mobility of 6cm2∕Vs. When the same films are employed in bottom-contact transistors, using SiO2 as gate dielectric, mobility is reduced to 0.2cm2∕Vs. By integrating several transistors we are able to fabricate high performance unipolar (n-channel) ring oscillators with stage delay of 2.3μs.

Ambipolar organic field effect transistors and inverters with the natural material Tyrian Purple

Ambipolar organic semiconductors enable complementary-like circuits in organic electronics. Here we show promising electron and hole transport properties in the natural pigment Tyrian Purple (6,6’-dibromoindigo). X-ray diffraction of Tyrian Purple films reveals a highly-ordered structure with a single preferential orientation, attributed to intermolecular hydrogen bonding. This material, with a band gap of ∼1.8 eV, demonstrates high hole and electron mobilities of 0.22 cm2/V·s and 0.03 cm2/V·s in transistors, respectively; and air-stable operation. Inverters with gains of 250 in the first and third quadrant show the large potential of Tyrian Purple for the development of integrated organic electronic circuits.

Mobility and photovoltaic performance studies on polymer blends: effects of side chains volume fraction

A 1 : 1 mixture of two thiophene based poly(p-phenylene ethynylene)-alt-poly(p-phenylene vinylene)s denoted DO-PThE1-PPV2 (D1) and MEH-PThE1-PPV2 (D2), consisting of the same conjugated backbone but different types and volume fraction of alkoxy side chains on the phenylene ethynylene unit, has led to enhanced charge carrier mobility (measured using CELIV technique) as compared to the individual polymers. The resulting ternary blend with PC60BM showed better photovoltaic performance as compared to binary blends from the single polymers mixed with PCBM. This is due to the improved active layer nanomorphology in the ternary system as revealed by AFM studies.

Improvement in carrier mobility and photovoltaic performance through random distribution of segments of linear and branched side chains

The random distribution of segments of linear octyloxy side chains and of branched 2-ethylhexyloxy side chains, on the backbone of anthracene containing poly(p-phenylene-ethynylene)-alt-poly(p-phenylene-vinylene) (PPE-PPV) has resulted in a side chain based statistical copolymer, denoted AnE-PVstat, showing optimized features as compared to the well defined homologues whose constitutional units are incorporated into its backbone. Electric field independent charge carrier mobility (µhole) for AnE-PVstat was demonstrated by CELIV and OFET measurements, both methods resulting in similar µhole values of up to 5.43 × 10−4 cm2 V−1 s−1. Upon comparison, our results show that charge carrier mobility as measured by CELIV technique is predominantly an intrachain process and less an interchain one, which is in line with past photoconductivity results from PPE-PPV based materials. The present side chain distribution favors efficient solar cell active layer phase separation. As a result, a smaller amount of PC60BM is needed to achieve relatively high energy conversion efficiencies above 3%. The efficiency of ηAM1.5 ≈ 3.8% obtained for AnE-PVstat:PC60BM blend is presently the state-of-art value for PPV-based materials.

On the potential of porphyrin-spiked triarylamine stars for bulk heterojunction solar cells

A novel porphyrin–triarylamine compound was synthesized to serve as a photosensitizer in bulk heterojunction solar cells (BHJ-OSCs). Based on cyclic voltammetric analysis, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the target porphyrin trimer were determined to be at −5.4 and −3.5 eV versus vacuum, respectively, and compared with those of other materials used for the construction of the BHJ-OSCs. Absorption and emission spectra of the film of the target porphyrin trimer were in good agreement with those of its solution. The charge transfer in a blended film of the porphyrin–triarylamine compound with phenyl-C61-butyric acid methyl ester (PCBM) was proven possible by photoluminescence measurements. The energy conversion efficiency of the BHJ-OSCs based on the target compound depended strongly on the donor : acceptor weight ratio and the thickness of the organic layer. Surface nanomorphologies of the films with different donor : acceptor ratios and thickness were investigated by atomic force microscopy (AFM). The short circuit current density (JSC) up to 2.06 mA cm−2 was obtained from the BHJ-OSC fabricated herein.

Optical characterization of double layers containing epitaxial ZnSe and ZnTe films

In this paper results of the optical characterization of double layers consisting of ZnTe and ZnSe thin films prepared by molecular beam epitaxy onto GaAs single crystal substrates are presented. For this optical characterization the optical method based on combining variable angle spectroscopic ellipsometry and near-normal spectroscopic reflectometry is used in the multi-sample modification applied within the spectral region 230–850 nm. Using this method the spectral dependences of the optical constants of the upper ZnTe thin films are determined within the spectral region mentioned above. Spectral dependences of the optical constants of the lower ZnSe thin films were determined within the spectral region 450–850 nm. Boundary roughness of these double layers and overlayers is respected. RMS values of the heights of the irregularities of the boundaries and thicknesses and optical constants of the overlayers are determined by means of the combined optical method as well. The uppermost boundaries of the double layers are, moreover, analysed using atomic force microscopy because of verification of the RMS values of these boundaries obtained by the optical method. The spectral dependences of the optical constants of the upper ZnTe films and lower ZnSe films determined in this paper are compared with those presented for ZnTe single layers and ZnSe single layers in the literature.

Photo-Fries-based photosensitive polymeric interlayers for patterned organic devices

This work reports on the investigation of the photosensitive polymer poly(diphenyl bicyclo[2.2.1]hept-5-ene-2,3-dicarboxylate) (PPNB), which undergoes the photo-Fries rearrangement upon illumination with UV-light, used as interfacial layers in organic electronic devices. Two cases were investigated: the use of a blend of PPNB with poly-vinylcarbazole (PVK) as an interlayer in para-sexiphenyl (PSP) based organic light emitting diodes (OLEDs) and the use of PPNB as gate dielectric layer in organic field effect transistors (OFETs). The photo-Fries rearrangement reaction causes a change of the polymer chemical structure resulting in a change of its physical and chemical properties. The electroluminescence spectra and emission of the PSP OLEDs are not affected when fabricated with a non-UV-illuminated PPNB:PVK blend. However, the electroluminescence is totally quenched in those OLEDs fabricated with UV-illuminated PPNB:PVK blend. Although the dielectric constant of PPNB increases upon UV-treatment, it is demonstrated that those OFETs built with UV-treated PPNB as gate dielectric have lower performance than those OFETs built with non-UV-treated PPNB. Furthermore, the effect of the UV-illumination of PPNB and PPNB:PVK blend on the growth of the small molecules C60 and PSP has been studied by atomic force microscopy. Using photolithography, this kind of photochemistry can be performed to spatially control and tune the optical and electrical performance of organic electronic devices.