Htay Hlaing

Nanoimprint-Induced Molecular Orientation in Semiconducting Polymer Nanostructures

The morphology and orientation of thin films of the polymer poly-3(hexylthiophene)-important parameters influencing electronic and photovoltaic device performance-have been significantly altered through nanoimprinting with 100 nm spaced grooves. Grazing-incidence small-angle X-ray scattering studies demonstrate the excellent fidelity of the pattern transfer, while wide-angle scattering convincingly shows an imprinting-induced π-π reorientation and polymer backbone alignment along the imprinted grooves. Surprisingly, temperature-dependent scattering measurements indicate that the imprinted induced orientation and alignment remain intact even at temperatures where the imprinted topographical features nearly vanish.

Bilayer order in a polycarbazole-conjugated polymer

One of the best performing semiconducting polymers used in bulk heterojunction devices is PCDTBT, a polycarbazole derivative with solar-conversion efficiencies as high as 7.2%. Here we report the formation of bilayer ordering in PCDTBT, and postulate that this structural motif is a direct consequence of the polymers molecular design. This bilayer motif is composed of a pair of backbones arranged side-to-side where the alkyl tails are on the outer side. This is in stark contrast to the monolayer ordering found in other conjugated polymers. The crystalline bilayer phase forms at elevated temperatures and persists after cooling to room temperature. The existence of bilayer ordering, along with its high-packing fraction of conjugated moieties, may guide the synthesis of new materials with improved optoelectronic properties.

Low-Voltage Organic Electronics Based on a Gate-Tunable Injection Barrier in Vertical graphene-organic Semiconductor Heterostructures

The vertical integration of graphene with inorganic semiconductors, oxide semiconductors, and newly emerging layered materials has recently been demonstrated as a promising route toward novel electronic and optoelectronic devices. Here, we report organic thin film transistors based on vertical heterojunctions of graphene and organic semiconductors. In these thin heterostructure devices, current modulation is accomplished by tuning of the injection barriers at the semiconductor/graphene interface with the application of a gate voltage. N-channel devices fabricated with a thin layer of C60 show a room temperature on/off ratio >10(4) and current density of up to 44 mAcm(-2). Because of the ultrashort channel intrinsic to the vertical structure, the device is fully operational at a driving voltage of 200 mV. A complementary p-channel device is also investigated, and a logic inverter based on two complementary transistors is demonstrated. The vertical integration of graphene with organic semiconductors via simple, scalable, and low-temperature fabrication processes opens up new opportunities to realize flexible, transparent organic electronic, and optoelectronic devices.

Nanostructured surfaces frustrate polymer semiconductor molecular orientation.

Nanostructured grating surfaces with groove widths less than 200 nm impose boundary conditions that frustrate the natural molecular orientational ordering within thin films of blended polymer semiconductor poly(3-hexlythiophene) and phenyl-C61-butyric acid methyl ester, as revealed by grazing incidence X-ray scattering measurements. Polymer interactions with the grating sidewall strongly inhibit the polymer lamellar alignment parallel to the substrate typically found in planar films, in favor of alignment perpendicular to this orientation, resulting in a preferred equilibrium molecular configuration difficult to achieve by other means. Grating surfaces reduce the relative population of the parallel orientation from 30% to less than 5% in a 400 nm thick film. Analysis of in-plane X-ray scattering with respect to grating orientation shows polymer backbones highly oriented to within 10 degrees of parallel to the groove direction.

Templating and Charge Injection from Copper Electrodes into Solution-Processed Organic Field-Effect Transistors

Solution-processed organic field-effect transistors (OFETs) using chemically modified copper electrodes are reported. The purpose of this study is to shed light on the use of inexpensive copper electrodes in bottom-contact OFETs, which is consistent with the major goal of organic electronics: the realization of low-cost electronics. 6,13-Bis(triisopropylsilylethynyl)pentacene was used for solution-processed hole-transporting molecular films and pentafluorobenzenethiol was used to form self-assembled monolayers (SAMs) on the contact metals. We conducted a comparative study on copper and gold contacts and realized that, under the same performance improvement schemes, via SAM treatment and controlled crystal growth, the copper electrode device experienced a more significant enhancement than the gold electrode device. We attribute the beneficial effects of SAMs to the improved charge injection and transport properties, which are critical double effects from the fluorinated aromatic SAM structure. Grazing-incidence wide-angle X-ray scattering (GIWAXS) measurements showed that templating property of SAMs promotes the crystallization of TIPS-pentacene films at the metal/organic interface. The presented result indicates that copper can be regarded as a promising candidate for reducing the use of gold in organic-based circuits and systems, where the cost-effective production is an important issue.

Enhanced charge collection in confined bulk heterojunction organic solar cells

Confining blended poly(3-hexylthiophene) and [6,6]-phenyl-C61-butyric acid methyl ester organic solar cell active layers within nanometer-scale cylindrical pores nearly double the supported short-circuit photocurrent density compared to equivalent unconfined volumes of the same blend and increases the poly(3-hexylthiophene) hole mobility in the blend by nearly 500 times. Grazing incidence x-ray diffraction measurements show that the confinement changes the polymer orientation distribution, suppressing low charge conductivity orientations while simultaneously disrupting polymer ordering.

Strongly Correlated Alignment of Fluorinated 5,11‐Bis(triethylgermylethynyl)anthradithiophene Crystallites in Solution‐Processed Field‐Effect Transistors

The crystallinity of an organic semiconductor film determines the efficiency of charge transport in electronic devices. This report presents a micro-to-nanoscale investigation on the crystal growth of fluorinated 5,11-bis(triethylgermylethynyl)anthradithiophene (diF-TEG-ADT) and its implication for the electrical behavior of organic field-effect transistors (OFETs). diF-TEG-ADT exhibits remarkable self-assembly through spin-cast preparation, with highly aligned edge-on stacking creating a fast hole-conducting channel for OFETs.

Difluorinated 6,13-Bis(triisopropylsilylethynyl)pentacene: Synthesis, Crystallinity, and Charge-Transport Properties

Fluorination has been demonstrated to improve stability and processing in thiophene-containing small-molecule semiconductors. Here, the impact of partial fluorination on these parameters in a pentacene derivative is examined. Although the improvement in photostability is not as dramatic, there is a clear improvement in the stability of the chromophore upon fluorination. The improvement in processability is more dramatic; devices formed by spin-coating with the fluorinated derivative perform substantially better than those formed from the nonfluorinated compound.

Sequential lateral solidification of silicon thin films on low-k dielectrics for low temperature integration

We present the excimer laser crystallization of amorphous silicon on a low dielectric constant (low-k) insulator for very large scale integration monolithic 3D integration and demonstrate that low dielectric constant materials are suitable substrates for 3D integration through laser crystallization of silicon thin films. We crystallized 100 nm amorphous silicon on top of SiO2 and SiCOH (low-k) dielectrics, at different material thicknesses (1 μm, 0.75 μm, and 0.5 μm). The amorphous silicon crystallization on low-k dielectric requires 35% less laser energy than on an SiO2 dielectric. This difference is related to the thermal conductivity of the two materials, in agreement with one dimensional simulations of the crystallization process. We analyzed the morphology of the material through defect-enhanced microscopy, Raman spectroscopy, and X-ray diffraction analysis. SEM micrographs show that polycrystalline silicon is characterized by micron-long grains with an average width of 543 nm for the SiO2 sample and...

Implementing Nanometer-Scale Confinement in Organic Semiconductor Bulk Heterojunction Solar Cells

We discuss fabrication processes for implementing nanometer scale confinement in an organic bulk heterojunction device architecture, including formation and integration of the confining self-assembled template. Such confinement has a beneficial influence on the electrical properties of blended poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester organic solar cell active layers. Crystallization of the blend upon annealing is inhibited by the confining template, which we understand through analysis of x-ray scattering measurements.