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Confocal fluorescence microscopy provides a means to map charge carrier density within the semiconductor layer in an active organic thin film transistor (OTFT). This method exploits the inverse relationship between charge carrier density and photoluminescence (PL) intensity in OTFTs, originating from exciton quenching following exciton-charge energy transfer. This work demonstrates that confocal microscopy can be a simple yet effective approach to gain insight into doping and de-doping processes in OTFT sensors. Specifically, the mechanisms of hydrogen peroxide sensitivity are studied in low-voltage hygroscopic insulator field effect transistors (HIFETs). While the sensitivity of HIFETs to hydrogen peroxide is well known, the underlying mechanisms remain poorly understood. Using confocal microscopy, new light is shed on these mechanisms. Two distinct doping processes are discerned: one that occurs throughout the semiconductor film, independent of applied voltages; and a stronger doping effect occurring near the source electrode, when acting as an anode with respect to a negatively polarized drain electrode. These insights offer important guidance to future studies and the optimization of HIFET-based sensors. More importantly, the methods reported here are broadly applicable to the study of a range of OTFT-based sensors. This work demonstrates that confocal microscopy can be an effective research tool in this field.

Generalized Brewster effect is a phenomenon where light of both TE (S-) and TM (P-) polarization transmit through a surface with no reflection for a particular incident angle. Generalized Brewster angle (GBA) in visible and near-infrared (NIR) wavelength region is very useful in many scientific and technical areas of applications. However, it is very rare to find a material having this effect as it demands both dielectric and magnetic response in that wavelength range and usually magnetic response is extremely weak in the optical wavelengths. Here we demonstrate the GBA effect of an anisotropic material composed of highly ordered high aspect ratio aluminium doped zinc oxide (AZO) nanopillar arrays. Along with the experimental demonstration, we also provide a proper numerical analysis to investigate the origin of this effect in the pillar array system which will be useful for many conventional as well as new applications in photonics including protein sensing.

New all-optical poling (AOP) experiments conducted in multi-core dye-doped poly(methyl methacrylate) optical fibers show that self-sustained and faster rise time second harmonic generation is possible in polymers.

Self-sustained all-optical poling second-harmonic generation (SHG) experiments are conducted in single-core and multicore dye-doped poly(methyl methacrylate) optical fibers. By tuning the polarization of the fundamental beam, the SHG signal is degraded and is reconstructed spontaneously up to its initial level. We found a new situation in which the photo-induced self-organization of azo polymers creates a well-ordered periodic structure.

The influence of thermal annealing at different fabrication stages on rubrene–C60 heterojunction organic solar cells is presented. Devices grown at room temperature showed s-shaped current density–voltage (J–V) characteristic. A pre- or post-fabrication thermal treatment step is found effective in elimination of s-shaped J–V. An exclusive gain in open-circuit voltage was measured when rubrene–C60 layer was fabricated at elevated substrate temperature of 80 °C. The results indicate that introduction of temperature at various stage of device fabrication allows selective control over organic layer growth and hence organic–organic interface formation. The 133 mV increase in open-circuit voltage in solar cells fabricated at substrate temperature is due to temperature induced molecular conformation change at the light in-coupling ITO/PEDOT interface.

We review experimental studies of nonlinear optics in large-core dye doped plastic optical fibres. In particular second-order nonlinearity in all-optical poled optical fiber is promising for application to electro-optic switching, second-harmonic generati

Loss of performance in large area devices is a major concern for organic solar cells. We present studies highlighting up-scaling loss mechanisms caused by interfacial resistance at organic-metal interface in pentacene/C60 heterojunction solar cells. Devices with different active areas and electrodes are investigated, revealing the origin of higher series resistance in large area devices. We also simulate influence of spatial non-uniformity and anisotropic charge collection in active layer through selective coverage using customized electrode geometries and identify grain boundaries as the dominating link behind loss in photocurrent generation and collection.

We present experimental studies of all-optical poling (AOP) in large-core dye-doped poly(methyl methacrylate) plastic optical fibers. We show that the confinement of light within the fiber core permits us to reach the upper limit of second harmonic generation achievable by AOP using repeated trans-cis isomerization. Quantification of the output power indicates self-seeding that can counteract relaxation of the orientation during readout of the induced nonlinearity.

Effects of different cathode metals, such as aluminum, calcium, and silver, and difference in their reflectivity on the photocurrent generation in pentacene-C60 heterojunction solar cell are presented. Using optical transfer matrix calculations, we find that metal reflectivity has a profound impact on the electrical field confinement within the multilayer device structures. Silver as cathode offers better optical-field confinement close to the pentacene-C60 interface over generally preferred aluminum cathode. External quantum efficiency measurements confirm higher exciton dissociation efficiency and high photocurrent generation ability of silver over aluminum cathode making the choice of cathode metal an important parameter in device optimization.

Electromagnetic study of organic photovoltaic cells design shows that electrical parameters depend drastically on the active area geometry: we theoretically show that electrical parameters are altered when the cell length becomes greater than one centimeter. Experimental verification is provided with simple molecular heterojunction cells with areas from 0.03 to 0.78 cm2.

Organic field-effect transistors were fabricated with pentacene as the active material and different polymeric dielectrics such as cyanoethylpullulan, poly(4-vinyl phenol), polymethylmethacrylate, parylene-C and polyimide as the gate material. The performance of the devices is compared with respect to the parameters, field-effect mobility, threshold voltage, subthreshold swing, and the maximum possible value of interface traps.

Organic semiconductors generate a tremendous interest in the development of light weight and flexible electronic devices owing to their ease of fabrication and suitability to large area applications. However, there are many possibilities left unexplored for these conjugated materials. We discuss here a new structure that permits efficient integration of both light and current generation functions from a rubrene/fullerene heterostructure into an efficient Organic Dual Device (ODD)6'7. The solar power conversion efficiency reaches 3 % with 5.3 mA/cm2 short-circuit current density and almost 1 V open circuit voltage under AM 1.5 illumination. Surprisingly, the EL turn-on voltage is below IV, about half of the rubrene band gap (2.2 eV), that cannot be explained using current models of charge injection into organic semiconductors. A physical interpretation is proposed in terms of the so-called Auger fountain mechanism that we could implement into our molecular heterojunction.

Hybrid organic solar cells using nano-porous TiO2 infiltrated with MEH-PPV were made and characterized under standard AM 1.5 G solar illumination. Reverse biased annealing was performed on the fabricated cells as a post treatment and origin of the counter-diode was investigated by illuminating the cells through a UV-blocking filter. Treated devices deliver increased open circuit voltage and short circuit current density values, leading to significant increase in cell performances.

Apart from their ease of fabrication and suitability to large area applications, there are many unexplored fields related to the peculiar physics of conjugated materials.1 Of particular interest in that context is the recent discovery of the exceptionally low-threshold injection characteristics of rubrene crystals.2 We implement here a resonant scheme that permits to drive electroluminescence (EL) with an extremely low turn-on voltage. The heterostructure device behaves like compound semiconductors, in which the EL turn-on voltage is about the same as the open circuit voltage for the photovoltaic effect. Surprisingly, however, this places the EL turn-on voltage at about half of the rubrene band gap (2.2eV). The physical interpretation can be found into the so-called Auger mechanism3 that we could implement in our molecular heterojunction diode.

Molecular blends of pentacene: N, N′-ditridecylperylene-3, 4, 9, 10-tetracarboxylic diimide (PTCDI-C13H27) permit to cover the visible part of the solar spectrum with an absorption onset at 730nm. Although charge mobilities of pentacene and PTCDI are rather large, the efficiency of pentacene:PTCDI-C13H27 blended devices is still lower than that of poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester devices. Comparisons between experimental results and optical modeling indicate that more than 30% of the photocurrent is lost in the blend while the short circuit current should reach 13mAcm−2 in the absence of recombination processes.

Hybrid organic solar cells using MEH-PPV and nano-porous TiO2 were made and characterized under standard AM 1.5 G solar illumination. Current-voltage characteristics of as fabricated devices show strong signature of counter diode formation and were less efficient (0.001%). Reverse biased annealing was performed on the fabricated cells as a post treatment. origin of the counter diode was investigated by illuminating the cells through UV cut filter (475 nm). Treated devices deliver increased open circuit voltage and short circuit current density values leading to overall increase in cell performance (0.13%). Cells illuminated in combination with UV-cut filter show better performance with no to little appearance of counter diode behavior.

Light and current generation functions from a rubrene/fullerene heterostructure are integrated into an efficient organic dual device (see figure). Electroluminescence, with the characteristic rubrene color, has a turn-on voltage less than 1 V. Solar-power conversion efficiency reaches 3 % with a 5.3 mA cm–2 short-circuit current density and almost 1 V open- circuit voltage under AM 1.5 illumination. The half-bandgap electroluminescence turn-on voltage is interpreted as an Auger fountain effect.

Flexible thin film solar cells based on a pentacene/C 60 C60 heterojunction on indium tin oxide coated polyethylene terephthalate (PET) substrates are described. Devices grown on PET are compared to those on glass substrates. The nature of the organic grains grown on PET and glass substrates is shown to have a significant impact on the device short circuit current. Device performance under mechanical strain and elevated temperature is reported. Devices deliver an 8.8 mA cm−2 short circuit current density with a 1.6% power conversion efficiency under an AM1.5 simulated solar intensity of 80 mW cm−2. The relatively low 300mV open circuit voltage appears to limit device efficiency.

Solar cells were fabricated with a photoactive layer consisting of a vacuum co-evaporated film of pentacene and a perylene derivative: N, N′-ditridecylperylene-3, 4, 9, 10-tetracarboxylic diimide. The device characteristics were evaluated in an oxygen-free environment. Maximum power conversion efficiency of 0.54% under standard AM 1.5 illumination (100 mW/cm2) is obtained with a short circuit current density in excess of 5 mA/cm2. Photocurrent action spectrum shows that such composite film may permit to cover the entire spectrum of visible light with an EQE greater than 10%.

Organic field-effect transistors were fabricated where the active semiconductor layer consisted of a co-evaporated film of pentacene and a perylene derivative, N,N′-ditridecylperylene-3,4,9,10-tetracarboxylic diimide. The device characteristics were evaluated in an oxygen-free environment. The field-effect transistor showed excellent ambipolar operation with field-effect hole mobility of 0.09 cm2 V−1 s−1 and field-effect electron mobility of 9.3 × 10−3 cm2 V−1 s−1. The threshold voltage for p-channel operation was −18 V and the same for n-channel operation was 15 V. This ambipolar device could be a building block to form flexible integrated circuits with low-power consumption and ease of design.

Solar cells exhibiting efficient photon harvesting are built from molecular blends of pentacene: PTCDI-C13H27. Absorption of the composition spans the whole visible spectrum with an onset at 730 nm. External quantum efficiency approaches unity at the peak absorption of pentacene. A 2.0% power conversion efficiency is obtained under 80 mW cm-2 AM 1.5 illumination, with 8.6 mA cm-2 short-circuit current. The comparison with bilayer devices suggests directions of improvement in conversion efficiency such as controlled growth of the blend layers.

Organic field-effect transistors were fabricated using N,N′-ditridecylperylene-3, 4, 9, 10-tetracarboxylic diimide (PTCDI-C13H27) as the active material and a polymeric insulator polymethylmethacrylate (PMMA) as the gate dielectric. A mobility value of 1.7 × 10−2 cm2 V−1 s−1 was estimated from the transistor characteristics. The threshold voltage was 0.2 V and the inverse subthreshold slope was 7.6 V/decade. At low gate and high drain voltages, the device exhibits injection of holes from the Al drain electrode. This was rectified by using LiF as a buffer layer between Al and (PTCDI-C13H27). Such modified devices exhibited mobility 1.2 × 10−2 cm2 V−1 s−1, threshold voltage −0.3 V, and inverse subthreshold swing of 5 V/decade.