Muhsen Aljada

Simultaneous enhancement of brightness, efficiency, and switching in RGB organic light emitting transistors.

An innovative design strategy for light emitting field effect transistors (LEFETs) to harvest higher luminance and switching is presented. The strategy uses a non-planar electrode geometry in tri-layer LEFETs for simultaneous enhancement of the key parameters of quantum efficiency, brightness, switching, and mobility across the RGB color gamut.

Diluted Magnetic Semiconductor Nanowires Prepared by the Solution-Liquid-Solid Method

Wiry frame: Manganese-doped cadmium selenide (Mn-CdSe) colloidal nanowires (see picture) exhibit ferromagnetism and good conductivity without any changed to their optical properties. The nanowires are synthesized by a novel solution-liquid-solid approach that offers a low-cost route towards magnetically active quantum wires with excellent potential applications in electronics, photonics, and spintronics. (Figure Presented).

High-speed (2.5 Gbps) reconfigurable inter-chip optical interconnects using opto-VLSI processors

Reconfigurablele optical interconnects enable flexible and high-performance communication in multi-chip architectures to be arbitrarily adapted, leading to efficient parallel signal processing. The use of Opto-VLSI processors as beam steerers and multicasters for reconfigurable inter-chip optical interconnection is discussed. We demonstrate, as proof-of-concept, 2.5 Gbps reconfigurable optical interconnects between an 850nm vertical cavity surface emitting lasers (VCSEL) array and a photodiode (PD) array integrated onto a PCB by driving two Opto-VLSI processors with steering and multicasting digital phase holograms. The architecture is experimentally demonstrated through three scenarios showing its flexibility to perform single, multicasting, and parallel reconfigurable optical interconnects. To our knowledge, this is the first reported high-speed reconfigurable N-to-N optical interconnects architecture, which will have a significant impact on the flexibility and efficiency of large shared-memory multiprocessor machines.

Nanostructured, Active Organic-Metal Junctions for Highly Efficient Charge Generation and Extraction in Polymer-Fullerene Solar Cells

A facile one step method for periodic nanostructuring of organic solar cells is presented. The nanostructured metal-organic interface delivers combined enhanced light trapping and improved charge extraction leading to up to a 10% increase in power conversion efficiency of already optimized planar devices.

High quality shadow masks for top contact organic field effect transistors using deep reactive ion etching

In this paper, we demonstrate the fabrication of top-contact silicon shadow masks for organic field effect transistors (OFETs) using plasma deep reactive ion etching (DRIE). Over 50 parallel and interdigitated finger contact masks of 30 µm thickness have been created on a single silicon wafer, with lengths spanning from 6.5 to 60 µm and channel widths varying from 1000 to 50 000 µm. Unlike all other mask fabrication techniques to date, these shadow masks are inexpensive, reusable, have nanoscopically sharp edges and can be made with precise (nanoscale) control over various sizes and shapes. Because a large number of these masks can be made at the same time, they can act as a platform for researchers studying new organic materials and OFET structures. Top contact OFETs have been successfully fabricated using these masks with performances comparable if not superior to those made with standard lithography.

Three-dimensional carbazole-based dendrimers: model structures for studying charge transport in organic semiconductor films

We report the synthesis and charge transport properties of a series of three-dimensional dendrimers up to the third generation that have a 9,9′-spirobifluorene core, carbazole-based dendrons and di-n-propylfluorene surface groups. The dendrimers can all be spin-coated to form good quality amorphous films. The charge carrier mobility of the dendrimers was measured by two different methods; in an organic field-effect transistor (OFET) architecture, and by Charge Extraction by Linearly Increasing Voltage (CELIV). In the OFET configuration the first generation dendrimer had a maximum mobility of 4.1 × 10−4 cm2 V−1 s−1 and an ON/OFF ratio of 1.1 × 105. Unexpectedly, in spite of the third generation dendrimer having a volume approximately six times that of the first generation, the mobility was found to decrease by only an order of magnitude. A similar trend in mobility was seen in the CELIV results. Photoluminescence (PL) measurements in solution showed that the first generation dendrimer was comprised of non-interacting chromophores, while the second and third generation dendrimers had substantial intra-dendrimer interchromophore interactions. In the solid-state, PL measurements showed that for the first generation dendrimer there were clear inter-dendrimer interchromophore interactions with little change for the second and third generations. Comparison of the dendrimer molecular volumes in solution and the solid-state showed that in the latter, the dendrimers took up a smaller volume suggesting that there was interdigitation of the dendrons. For the first generation dendrimer the interdigitation leads to trap sites for charge transport, with the small decrease in mobility in moving from the first to the second and third generation being due to the extra intra-dendrimer interchromophore interactions. Model dendritic systems such as these can be used to gain significant insight into the subtly of charge transport phenomena in solution processable macromolecular organic semiconductors, since they offer a level of molecular control that is difficult to achieve with polymers.

Field-effect transistors fabricated from diluted magnetic semiconductor colloidal nanowires

Field-effect transistors (FETs) fabricated from undoped and Co(2+)-doped CdSe colloidal nanowires show typical n-channel transistor behaviour with gate effect. Exposed to microscope light, a 10 times current enhancement is observed in the doped nanowire-based devices due to the significant modification of the electronic structure of CdSe nanowires induced by Co(2+)-doping, which is revealed by theoretical calculations from spin-polarized plane-wave density functional theory.

A solution processable fluorene-benzothiadiazole small molecule for n-type organic field-effect transistors

We report an n-type organic semiconductor [2-({7-(9,9-di-n-propyl-9H-fluoren-2-yl}benzo[c][1,2,5]thiadiazol-4-yl)methylene]malononitrile (herein referred to as K12) for use in organic field-effect transistors (OFETs). K12 can be processed by spin-coating from solution or by vacuum deposition, organizing into highly orientated microcrystalline structures at modest (75 °C) annealing temperatures. OFETs with n-octyltrichlorosilane or hexamethyldisilazane monolayers, or poly(propylene-co-1-butene) (PPCB) modified dielectric surfaces were prepared. The mobility, ON/OFF ratio, threshold voltage, and current hysteresis were found to be dependent on the thermal history of the film and surface onto which it was deposited. The highest OFET mobility achieved was 2.4×10−3 cm2/V s, for spin-coated films with a PPCB modified silicon dioxide dielectric.

The effect of dendronisation of arylamine centred chromophores on field effect transistor performance

We report the design and synthesis of new dendronised hole transport materials and their first use in organic field effect transistors (OFETs) as solution processed amorphous channels. The dendrimers were comprised of a triphenylamine centre, core chromophores containing one or two thiophene units per arm, and first generation biphenyl dendrons with 2-ethylhexyloxy surface groups attached. The dendronised materials were found to be more easily processed than their non-dendronised equivalents. Top contact OFETS were fabricated and found to have hole mobilities in the saturated regime of 1.7 × 10−6 cm2 V−1 s−1 and 1.1 × 10−5 cm2 V−1 s−1 for dendrimers with one and two thiophene units in each arm of the chromophore, respectively. The devices had threshold voltages of around 10 V and ON/OFF ratios in the range 102 to 103. The OFET results demonstrate that for amorphous films it is important that the chromophore is as large as possible to allow for maximal intermolecular interactions.

Opto-VLSI-based correlator architecture for multiwavelength optical header recognition

A novel optical correlator employing an opto-very-large-scale-integration (VLSI) processor to construct the routing lookup table, in conjunction with an array of fiber Bragg gratings (FBGs) for multiwavelength optical header recognition is demonstrated. The FBG array provides wavelength-dependent time delays, whereas the opto-VLSI processor generates wavelength intensity profiles that match arbitrary bit patterns. The recognition of 4-b optical patterns is experimentally demonstrated at 2.2 Gb/s by showing that the correlator produces an autocorrelation waveform of high peak whenever the input bit pattern matches the wavelength intensity profile.