Catherine Ramsdale

The origin of the open-circuit voltage in polyfluorene-based photovoltaic devices

The influence of device structure on the open-circuit voltage of polyfluorene-based photovoltaic devices has been investigated. Bilayers of hole- and electron-accepting polyfluorenes have been fabricated using an aqueous “float-off” lamination technique and subsequently incorporated into organic photovoltaic devices with a range of cathodes and anodes. A scaling of the open-circuit voltage with electrode work function difference has been observed with an additional intensity- dependent contribution from the active layer within the device. This additional contribution is attributed to photoinduced generation of carriers, whereby accumulation of charge at the polymer–polymer heterojunction results in a dipole across the interface and gives rise to a diffusion current that must be counterbalanced by a drift current at open circuit.

Raman microscopy determination of phase composition in polyfluorene composites

Confocal Raman spectroscopy with a spatial resolution of ⩽1 μm is used to determine the composition of binary polyfluorene composites with micro- and mesoscale phase separation. The phases are found to contain significant proportions of both constituents, implying that exciton dynamics such as charge and energy transfer may occur within a particular phase. The results presented here provide an insight into thin-film phase separation of conjugated polymer blends of interest for optoelectronic device applications. In particular, in this letter the high degree of intraphase mixing is discussed in relation to the relatively high efficiency of photovoltaic devices fabricated from these blends.

Printing of polymer thin-film transistors for active-matrix-display applications

We present a process for active-matrix flat-panel-display manufacture based on solution processing and printing of polymer thin-film transistors. In this process, transistors are fabricated using soluble semiconducting, conducting, and dielectric polymer materials. Accurate definition of the transistor channel and other circuit components are achieved by direct ink-jet printing combined with surface-energy patterning. We have used this process to create 4800-pixel 50-dpi active-matrix backplanes. These backplanes were combined with polymer-dispersed liquid crystal to create the first ink-jet-printed active-matrix displays. Our process is, in principle, environmentally friendly, low temperature, compatible with flexible substrates, cost effective, and advantageous for short-run length and large display sizes. As well as polymer-dispersed liquid crystal, this technology is applicable to conventional liquid-crystal and electrophoretic display effects.

A scalable manufacturing process for flexible active-matrix e-paper displays

— A scalable manufacturing process for fabricating active-matrix backplanes on low-cost flexible substrates, a key enabler for electronic-paper displays, is presented. This process is based on solution processing, ink-jet printing, and laser patterning. A multilayer architecture is employed to enable high aperture ratio and array performance. These backplanes were combined with E Ink electrophoretic media to create high-performance displays that have high contrast, are bistable, and can be flexed repeatedly to a radius of curvature of 5 mm.

7.4: A Flexible Plastic SVGA e‐Paper Display

We have developed a scalable manufacturing process for active matrix displays compatible with low temperature plastic substrates and have applied this technology to the fabrication of flexible SVGA display backplanes. We combined these backplanes with E Ink® Imaging Film to produce 100 PPI SVGA (800×600 pixel) displays exhibiting grey scale and with a high aperture ratio. Power consumption is zero in between image changes. Flexible high information content e-paper displays will change the way in which information is conveyed by enabling lightweight, robust e-reader devices.

ESEM imaging of polyfluorene blend cross-sections for organic devices

Abstract We report the use of environmental scanning electron microscopy (ESEM) to determine the phase separation in the cross-section of a 200 nm thick polyfluorene blend film, of the type used in polymer photovoltaic devices and LEDs. The micron and sub-micron surface phases are found to penetrate through the film to the underlying substrate, whilst smaller surface features do not necessarily propagate through the film. The observed cross-sectional structure helps to explain the optoelectronic response of these blends and shows that ESEM is an effective tool in the characterisation of polymer blend cross-sections.

Polyfluorenes as organic semiconductors for polymeric field effect transistors

Well-characterized F8T2 polyfluorene (Dow Chemical) has been prepared with weight average molecular weights (Mw) ranging from about 20,000 to 120,000. This semiconducting polymer has been used by Plastic Logic to fabricate arrays of 4,800 thin film transistors (TFTs) with 50 dpi, to be used as backplanes for active matrix displays. In this paper, the effects that molecular weight and thermal treatment have on the electrical characteristics of F8T2-based TFTs are reported. First, transistor performance improves with increasing molecular weight, with maximum values of TFT mobility approaching 1x 10-2 cm2 /V-s. Consistently higher mobilities are obtained when the F8T2 semiconductor makes contact with PEDOT/PSS versus gold electrodes. Alignment of F8T2 on a rubbed polyimide substrate is maintained after quenching, as determined by measurement of the dichroic ratios. Early-stage results on the development of inks based on F8T2 polyfluorene are also reported.

3.4: Flexible Active‐Matrix Displays

Plastic Logic has developed a process for organic TFT backplane deposition that is fully compatible with low glass transition temperature, inexpensive plastic substrates. We have combined backplanes made with this process with microencapsulated electrophoretic imaging films to make 50 and 100PPI displays that are flexible down to a radius of curvature of 5mm. This process is scalable and is being implemented in a pilot line for substrate sizes up to 350mm × 350mm.

Charge transport and efficiency in photovoltaic devices based on polyfluorene blends

Polymer blends allow control of microstructure in donor-acceptor photovoltaic devices. Here we present measurements of devices containing polyfluorene blend layers of different thicknesses, and we are able to extract characteristic transport lengths for electrons and holes. We also present analytical and numerical modeling of single-layer and bilayer photovoltaic devices, which demonstrates the importance of bound polaron pairs formed after the initial electron transfer from donor to acceptor. Field-assisted dissociation of these polaron pairs is a critical process in determining device performance.