Max Shtein

Effects of film morphology and gate dielectric surface preparation on the electrical characteristics of organic-vapor-phase-deposited pentacene thin-film transistors

Organic vapor phase deposition was used to grow polycrystalline pentacene channel thin-film transistors. Substrate temperature, chamber pressure during film deposition, and growth rate were used to vary the crystalline grain size of pentacene films on O2-plasma treated SiO2 from 0.2 to 5 μm, leading to room-temperature saturation regime field-effect hole mobilities (μeff) from 0.05±0.02 to 0.5±0.1 cm2/V s, respectively. Surface treatment of SiO2 with octadecyltrichlorosilane (OTS) prior to pentacene deposition resulted in μeff⩽1.6 cm2/V s, and drain current on/off ratios of ⩽108 at room temperature, while dramatically reducing the average grain size. X-ray diffraction studies indicate that the OTS treatment decreases the order of the molecular stacks. This suggests an increased density of flat-lying molecules, accompanying the improvement of the hole mobility at the pentacene/OTS interface.

Transparent and conductive electrodes based on unpatterned, thin metal films

Transparent electrodes composed of ultrathin, unpatterned metal films are investigated in planar heterojunction (PHJ) and bulk heterojunction organic photovoltaic (OPV) cells. Optimal electrode composition and thickness are deduced from electrical and optical models and experiments, enabling a PHJ-OPV cell to be realized using a silver anode, achieving power conversion efficiency parity with an analogous cell that uses an indium tin oxide anode. Beneficial aspects of smooth, unpatterned metal films as transparent electrodes in OPV cells are also discussed in the text.

Flexible conjugated polymer photovoltaic cells with controlled heterojunctions fabricated using nanoimprint lithography

The authors describe conjugated polymer-based photovoltaic devices in which the shape and area of the interface between the electron donor and acceptor layers were controllably varied using nanoimprint lithography. The short circuit current is shown to increase with the interfacial area of the heterojunction, without affecting the open circuit voltage. The fill factor and power conversion efficiency are also shown to increase with donor-acceptor interfacial area.

Nanolithography based on patterned metal transfer and its application to organic electronic devices

We demonstrate a patterning method capable of producing features of submicron scale based on the transfer of a metal film from a stamp to a substrate assisted by cold welding. The patterned metal film can be used as an etch mask to replicate the pattern on the substrate, or the film itself can serve as contact electrodes for a wide range of electronic devices. We demonstrate the versatility of the technique by fabricating a polymer grating on SiO2 with lateral dimensions <80 nm and a pattern resolution approaching 10 nm, and by fabricating organic solar cells and pentacene channel organic thin-film transistors with channel lengths as short as 1 μm.

Fiber based organic photovoltaic devices

A fiber-shaped organic photovoltaic cell is demonstrated, utilizing concentric thin films of small molecular organic compounds. Illuminated at normal incidence to the fiber axis through a thin metal electrode, the cell exhibits 0.5% power conversion efficiency, compared to 0.76% for a planar control device. The fiber device efficiency is nearly independent of illumination angle, increasing its power generation over the planar counterpart for diffuse illumination. Losses due to partial shading of the fiber surface are minimal, while the coated fiber length is limited only by the experimental deposition chamber geometry—factors favoring scale-up to woven energy harvesting textiles.

Material transport regimes and mechanisms for growth of molecular organic thin films using low-pressure organic vapor phase deposition

We determine the physical mechanisms controlling the growth of amorphous organic thin films by the process of low-pressure organic vapor phase deposition (LP-OVPD). In LP-OVPD, multiple host and dopant molecular sources are introduced into a hot wall reactor via several injection barrels using an inert carrier gas, allowing for controlled film growth rates exceeding 10 A/s. The temperature and carrier flow rate for each source can be independently regulated, allowing considerable control over dopant concentration, deposition rate, and thickness uniformity of the thin films. The rate of film deposition is limited either by the rate of condensation on the substrate or by the rate of supply from the source. The source-limited regime can be further classified into equilibrium or kinetically limited evaporation, coupled to convection- or diffusion-limited deposition. Models are developed to relate the rate of film growth to source and substrate temperature, and carrier gas flow rate. These models characterize ...

Morphology control and material mixing by high-temperature organic vapor-phase deposition and its application to thin-film solar cells

Organic vapor-phase deposition is used to grow photovoltaic cells consisting of molecular compounds that require high-evaporation temperatures. The organic crystal size and film morphology are controlled by adjusting the source evaporation temperature, substrate temperature, carrier gas flow rate, and chamber pressure. The resulting surface morphology produces a highly folded donor–acceptor interface with a fourfold increase in area compared with a planar heterojunction, leading to an increased exciton diffusion efficiency, and hence an 80% increase in photovoltaic cell power conversion efficiency. Mixed organic layers with controlled constituent ratios and layer thicknesses are grown by the simultaneous codeposition of multiple source materials. Photovoltaic cells comprised of a copper phthalocyanine:C60 mixed layer show comparable performance to analogous vacuum-deposited cells.

Dynamic kirigami structures for integrated solar tracking

Optical tracking is often combined with conventional flat panel solar cells to maximize electrical power generation over the course of a day. However, conventional trackers are complex and often require costly and cumbersome structural components to support system weight. Here we use kirigami (the art of paper cutting) to realize novel solar cells where tracking is integral to the structure at the substrate level. Specifically, an elegant cut pattern is made in thin-film gallium arsenide solar cells, which are then stretched to produce an array of tilted surface elements which can be controlled to within ±1°. We analyze the combined optical and mechanical properties of the tracking system, and demonstrate a mechanically robust system with optical tracking efficiencies matching conventional trackers. This design suggests a pathway towards enabling new applications for solar tracking, as well as inspiring a broader range of optoelectronic and mechanical devices.

Micropatterning of small molecular weight organic semiconductor thin films using organic vapor phase deposition

Using both analytical and experimental methods, we show that micron scale patterned growth of small molecular weight organic semiconductor thin films can be achieved by the recently demonstrated process of organic vapor phase deposition (OVPD). In contrast to the conventional process of vacuum thermal evaporation, the background gas pressure during OVPD is typically 0.1–10 Torr, resulting in a molecular mean free path (mfp) of from 100 to 1 μm, respectively. Monte Carlo simulations of film growth through apertures at these gas densities indicate that when the mfp is on the order of the mask-to-substrate separation, deposit edges can become diffuse. The simulations and deposition experiments discussed here indicate that the deposited feature shape is controlled by the mfp, the aperture geometry, and the mask-to-substrate separation. Carefully selected process conditions and mask geometries can result in features as small as 1 μm. Furthermore, based on continuum and stochastic models of molecular transport in confined geometries, we propose the in situ direct patterning growth technique of organic vapor jet printing. The high pattern definition obtained by OVPD makes this process attractive for the growth of a wide range of structures employed in modern organic electronic devices.

Direct mask-free patterning of molecular organic semiconductors using organic vapor jet printing

We demonstrate the solvent-free, high-resolution direct printing of molecular organic semiconductors for use in low cost optoelectronic applications. In this method, called organic vapor jet printing, hot inert carrier gas picks up the molecular organic vapor and expands it through a microscopic nozzle, resulting in physisorption of the molecules onto a cooled substrate. Pattern resolution and printing speed are determined by the nozzle shape, nozzle-to-substrate distance, downstream pressure, and molecular mass of the carrier gas. Quantitative models are developed using a combination of scaling analysis, direct simulation Monte Carlo modeling, and printing experiments. Pattern resolutions of up to 1000dpi and local deposition rates exceeding 2300A∕s are achieved. Pentacene channel thin film transistors are printed at a local deposition rate of 700A∕s at both low and atmospheric pressures, resulting in a field-effect mobility of 0.25cm2∕Vs and a current on/off ratio of 7×105 for devices grown at a backgro...