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| Organic Semiconductor Conference 2005: summary of Day One | |
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Cambridge Display Technology: Jonathan Halls The LEP value chain - building and sustaining a competitive advantage Jonathan Halls spoke about CDT's progress in commercializing P-OLED technology. CDT was founded in 1992 and has grown to 110 employees. It owns the fundamental P-OLED IP and over 160 other patent families. CDT sees the markets of lighting, indicators/signage and flat panel displays to be open to P-OLED technology with CDT concentrating on flat panel displays. OLEDs offer advantages for the consumer - high contrast, wide viewing angle, vivid colours - and for the manufacturer - cheap manufacturing costs. CDT uses inkjet printing to deposit its polymer inks and has a prototype manufacturing line operational. CDT reported efficiencies of 3.7 lm/W for red fluorescent LEDs, 17 lm/W for green and 7.1 lm/W for blue (although these numbers are not necessarily for long-lifetime devices and materials). A focus of CDT's research at present is extending the lifetime of blue fluorescent materials. This year CDT has demonstrated 100,000 hrs at 100 cd/m2 with 120,000 hrs projected for the end of the year. The main progress has been achieved through controlling damage at the polymer-hole transporting layer (PEDOT) interface. A polymer buffer layer is inserted between these layers, which stops electron injection into the PEDOT layer. The lifetimes for other colours are longer with 310,000 hours for red, 255,000 hours for green and 290,000 hours for yellow. A common cathode is used for all colours. From a manufacturing perspective, improvements to inkjet printing technology are important. The flatness of polymer films is important for uniform pixel emission and jetting frequencies of up to 10kHz have been used. Printers currently used in the LCD industry can be adapted to print polymer inks - requirements include high precision and automated maintenance and alignment. Bowing of the printer heads causing misalignment is a problem. The largest substrate size used at the moment is 1.8 x 2.2m. A range of P-OLED devices are already on the market, including the Phillips Magic Mirror 639 mobile 'phone and monochrome OSRAM integrated displays on large pieces of electronics such as mixing desks. Casio and Epson having announced development of large screen TV manufacturing to begin in 2007. P-OLEDs are predicted to account for 38% of the $5,300m OLED market in 2008. Phillips Lighting: Dr Dietrich Bertram OLEDs for lighting applications In 2005 OLEDs occupy niche markets such as mobile phone and digital camera screens. OLEDs need to show higher efficiencies and lower cost before they are applied in the general lighting market. Each distinct sector within the lighting market has different requirements for lifetime and efficiency. Since lighting is not a new market, OLEDs must be cheaper than the current technology in order to be adopted. The decorative lighting market has the least stringent requirements and requires a brightness of 50-500 cd/m2 at an efficiency of > 10 lm/W for a lifetime of at least 10,000 hours. The market for general illumination requires much larger panels (up to 5000cm) at high brightness (5000 cd/m2) and very high efficiencies of > 50 lm/W. Of these requirements, the efficiency target will be the hardest to achieve with OLEDs. Projections show OLEDs achieving energy saving lamp performance (3 $/Mlmh) in 2012 and fluorescent tube performance (1 $/Mlmh) by 2015. White OLEDs can be made either from a naturally white emitting layer or from combinations of colours. Using fewer layers allows cheaper manufacturing but single layers suffer from changing CID coordinates with brightness and lifetime. Multiple thin emitting layers are difficult to fabricate and control but have good CID coordinates and high efficiencies. More complex OLEDs can be made with stacked layers of different coloured emitting layers with intermediate contacts. This system allows the lighting colour to be changed dynamically but is complex to fabricate. However, it will have a niche market. A different approach is to use a blue emitting layer and a white phosphor. This is simple to manufacture but relies on the lifetime of the blue component and produces a harsh white. The efficiency of an OLED is limited since only 20-40% of the light produced emits from the front of the device. Phillips have textured the top surface of the LED with micron scale patterns (including pyramids, lenses and particles) to solve this problem. This leads to a 40% increase in the light output. Modelling work has shown that it should be possible to extract up to 80% of the light. The Phillips 'warm white' OLED product shows 20 lm/W efficiencies at 1000 cd/m2 with a CRI of 85. The 'standard white' product based on a blue phosphorescent emitter has an efficiency of 25 lm/W at 1000 cd/m2 but the active layer has lifetime problems at present. OLLA Project: Peter Visser The OLLA project: Europe's route towards OLED lighting The OLLA project is a European venture involving 24 partners, which started in October 2004. The goal is to create OLED lighting products. In order to be competitive in the lighting sector, OLEDs must show: high luminance, long lifetime, high efficiency (at least 30 lm/W), good CRI (above 80) and good homogeneity. The project's goal is to produce a demonstrator 15cm x 15cm tile with an efficiency of 50 lm/W at 1,000 cd/m2 with a lifetime of 10,000 hrs at a CRI of over 70. OLLA results so far were modest, with a white emitter based on a two colour blue and orange triplet emitters running at 6 lm/W at 1000 cd/m2 with a low CRI of 58 and a very low lifetime of 50 hours. Using blue and yellow singlet emitters the efficiency was 7.8 lm/W with a low CRI of 65 but a lifetime of 1500 hours. However, the first lifetime of 10 lm/W at 1000 cd/m2 and a 2000 hour lifetime should be met in six months. OLLA has a report, available to download from their website, describing how a high CRI (> 95) is possible from broadband emitting OLEDs. Work has been done on device homogeneity through modelling and a 35 cm2 white panel built and tested. Progress has also been made on outcoupling. A cheap process, where an array of micro lenses are embossed into the LED substrate, was shown to allow 80% of the light entering the substrate to be extracted. Results will be published shortly. Polymer Vision: Dr Gerwin Gelinck Rollable QVGA displays Polymer Vision showed a prototype of a rollable display using E-paper (which should be on the market in three years under the name Readius™). Gerwin underlined the advantages of using the E-paper: very flexible, unbreakable, lightweight, and low power. The flexible paper could solve the display size dilemma for mobile phones, offering a large viewing screen only when used and being integrated in the phone. Polymer Vision has a dedicated pilot line with a capacity of 5,000 displays per year. Polymer Vision uses E Ink's electrophoretic paper and AM backplane made with their TFTs. The electrophoretic laminate offers high contrast and paper-like appearance on a flexible sheet of 100m thickness, without the need for polarizers or for alignment layers. The E Ink paper is black and white with four grey levels achieved at a frame rate of 50Hz. Casting from solution at low temperatures offers low-cost manufacturing for larger areas. Gerwin showed pentacene devices (spin coated from a precursor) with mobility of 0.02 cm2/Vs, on-off ratio greater than 106, and leakage current of 10pA, which meet the display specifications. The field isolation of devices was achieved by using the second insulator layer that could however limit the mechanical flexibility of the display. With the top gate biased at -15/+15 V, the devices showed threshold voltage shifts, but the off current stayed lower than 30pA. The reliability of Polymer Vision's displays is remarkable. Their performance was tested for 168 hrs at RT. They were also stored at RT for six months without a significant degradation. An outstanding feature of these devices is the retention of the images after the power has been switched off, allowing reduced power consumption (biasing needed only for refreshing the display). Gerwin spoke about the work in progress at Polymer Vision. They target TFTs with mobility as high as 0.35cm2/Vs, and want to integrate row drivers via dynamic shift registers (a working 240 stage shift register was already shown at OSC-05). Plastic Logic: Dr Catherine Ramsdale Advances in plastic electronics The name 'Plastic Logic' is synonymous with 'plastic electronics', that is electronic devices manufactured by printing polymer solutions on flexible substrates. Catherine underlined the key drivers of Plastic Logic's technology strategy for flexible electronics: solution processing and direct-write fabrication techniques, avoiding mask alignment and vacuum steps, low process temperatures to permit use of cheap flexible substrates, and scalable for volume manufacturing. Based on different flexibility requirements, Catherine sees a wide range of product applications for their technology: conformal, bendable and rollable displays. Using E Ink's electrophoretic paper, Plastic Logic developed a multilayer pixel architecture embedded as a flexible AM display backplane. They successfully integrated the TFT and the pixel capacitor without compromising the aperture ratio (> 80%) onto a 2-inch diagonal PET substrate with a resolution of 50ppi. Their TFTs have mobilities of 0.04 cm2/Vs and on-off ratios of 5 x 105, performance that is enough for driving an A5 100ppi electronic paper display. By patterning the OSC (organic semiconductor) layer into islands, they suppressed the leakage currents that could otherwise degrade the pixel contrast significantly. Without any encapsulation, the TFTs showed no degradation after a continuous stress of 4 x 107 switch cycles. Bending the flexible substrates at the record curvature of 0.3cm showed no degradation in device performance either. With a new production line already available for manufacturing, Plastic Logic intends to produce A5 e-paper displays by the end of the year moving to A4 in 2006. It is looking at achieving resolutions as high as 150ppi, with a line capacity of 100 A4 panels per week. PolyIC: Dr Andreas Ullmann Integrated circuits based on polymer transistors PolyIC, the Erlangen based company, is a joint venture between Siemens and Kurz, both of Germany. They print electronic circuits for thin, flexible, low cost and high volume applications as intelligent packages or RFID tags. Andreas acknowledged that the market for RFIDs is cost driven and noted that the market size increases exponentially as the price per tag decreases. PolyIC selected and optimised materials that would enable printing of circuits. They showed all-polymer transistors based on polythiophene with mobility of 0.02 cm2/Vs, and on-off ratio of 105, which showed no degradation in performance after being stored under ambient conditions for ten months. The polythiophene transistors have a very high yield, with 97.8% of their 1269 manufactured PFETs devices being good enough for circuits. Andreas showed a 192kHz seven-stage ring oscillator and a completely printed integrated circuit. RFID tags comprise of an antenna, a rectifying stage and a transponder stage. The latter two can be made from printed semiconducting polymer. Rectifying performance at 13.56MHz was demonstrated, yielding 6.2V of output. PolyIC wants to move towards full polymer RFID tags, which are expected to gain some market penetrating post 2007, but full standards compliance is unlikely to be obtained before 2010. OrganicID: Klaus Dimmler Fully printable RFID tags OrganicID, a venture financed company, was founded in December 2003 with the mission to develop low-cost electronic process technology to produce printable RFID tags for bar code replacement. Typical prices of the cheapest tags in the current market are $0.20, and will never get to $0.05. Estimates using fully printable methods could yield $0.01, which is the price needed for 'item level tracking'. Klaus believes in Organic CMOS Technology and looks at optimizing the n-type and p-type transistor performance in collaboration with the University of Texas at Austin and Northwestern University. Transmitting at 13.56MHz, the RFID tags need 192 bits of memory and a processor clock frequency of 106kHz. Klaus showed the micrograph of a working rectifier designed with a four-transistor NQS (non quasi-static) stage. The tag power specifications were met by a p-type design. However, CMOS technology gives 10 times less power for the same frequency of operation and is faster than PMOS or NMOS stages for the same mobility. Klaus presented a CMOS ring oscillator (polythiophene/DFHCO-4T) with a speed of 300kHz and similar mobilities for the two transistor types (0.02 cm2/Vs). The devices shared the dielectric and the source/drain material. OrganicID develops new dielectrics that would improve the speed of the CMOS transistors. They look at SAMs (self-assembled mono-layers) and CPBs (cross-linked polymer blends) with breakdown fields of 5-10 MV/cm. Klaus compared the performance of TFTs made with DH-6T (0.08 cm2/Vs) and showed that a SAM nanodielectric of 3.2nm increases the gate capacitance to 710 nF/cm2 and gives an excellent response at voltages less than 1V. He also showed the smallest reported channel length n-type transistor (10-20nm) with a PDI-8CN2 semiconductor and SiO2 dielectric. Future developments at OrganicID will focus on printing technology improvements (pinhole free dielectric deposition, thickness range, resolution), and manufacturing process considerations (surface treatments for electrode interface or to align semiconductor, controlled cooling cycles). Konarka: Dr Jens Hauch Flexible organic solar cells Konarka aim to use organic photovoltaic materials to build a flexible 'power plastic' that can be used to provide power where it is needed. This material has to be lightweight, flexible and very low cost, manufactured using a roll-to-roll process. The current market for PV cells is dominated (99%+) by silicon cells. Half the market (51.6%) is taken by cells based on poly-Si with single crystal Si next (36.4%), and thin-film silicon having under 5% of the market. Alternative technologies such as CdT (Cadmium Telluride) based cells have not been able to scale up to sufficient manufacturing scale to compete. Whereas the silicon industry can now only achieve incremental cost savings, plastic photovoltaic cells can undercut silicon by doing away with expensive vacuum deposition batch-processing and instead using roll-to-roll manufacturing. Cost, cell lifetime and efficiency are the most important parameters for any new PV technology. Other factors such as weight and flexibility do not provide a compelling enough competitive advantage to compete in the established markets. In the short-term silicon has no competitor for efficiency or lifetime. There are several PV technologies that are compatible with roll-to-roll processing. These are solid-state dye-sensitised solar cells, hybrid PV systems and organic photovoltaics. Konarka is concentrating on organic systems based on blends of the polymer P3HT and the C60 derivative PCBM. Light is absorbed mostly in the polymer but also on the PCBM, especially above 3eV. Absorption of light forms an exciton state. The exciton is separated into charges, the hole lying on the P3HT and the electron on the PCBM. The charges then diffuse to the electrodes to form a useful current. Konarka has achieved power conversion efficiencies of 4-5% with several material classes. The EQE (external quantum efficiency), which represents the efficiency of turning light into charges, is at a very high level. To improve the power conversion further the open-circuit voltage and fill-factor need to be improved. For actual deployment multiple PV cells are connected together in series to form PV modules. The voltage supplied by a module is the sum of the open-circuit voltage for each cell minus module losses. Konarka are focusing on flexible modules. A barrier layer is required on the flexible plastic substrate to reduce moisture and oxygen exposure of the active layer. Konarka have achieved 3-4% efficiency with three-cell modules which have a total supply voltage of 1.6V. 10% power conversion efficiency should be achievable with this technology in the future. IMEC: Dr Tom Aernouts Development of organic PV modules by screen printing IMEC are developing plastic solar cell manufacturing processes based on large-scale film casting techniques already well established in the photographic industry. The system being used is a blend of MEH-PPV (an conjugated polymer) and PCBM (a C60 derivative). Screen-printing is a large area, low cost casting technique suitable for depositing soluble systems. It has the advantages of low material loss, high throughput and the capability of using large substrates. Patterning is also possible since the screen may be configured as desired. Screen-printing of organic PVs consists of three stages. The front side contact is screen- printed first, followed by the active layer screen printed on top. The backside contact is currently deposited in vacuum, but a low-cost deposition technique is needed. The screen-printing technique works as follows. The polymer ink paste is placed at the edge of the screen (mask) and pushed across the screen using a squeegee. The pattern of the screen determines where ink is deposited on the substrate. The composition of the paste, shape and pressure of the squeegee and manufacture of the screen are parameters that can be easily controlled. The rheology (flow and deformation) of the MEH-PPV solution has been investigated. The viscosity increases with decreasing temperature and increasing concentration. The shear rate of the paste demonstrated pseudo-plastic behaviour. Adding PCBM to the solution decreases the viscosity but does not alter the flow or shear behaviour. High print speeds form better, more homogeneous films. Changing the pressure and snap-off distance of the squeegee could control the film thickness. Screen-printed PV cells demonstrate power conversion efficiencies of 1.25%. Modules have been printed on 5cm x 5cm substrates with cell active areas of 0.6cm2. Traditional front side contacts such as ITO may not be compatible with roll-to-roll processing and are also increasingly expensive. ITO may be replaced with a conducting dispersion of PEDOT/PSS. However, the conductivity is not high enough for use as an electrode so the PEDOT is supplemented with a fine silver grid. This can be patterned using diffusion transfer reversal - a standard photographic technique, which can pattern silver lines with a line width of 40µm and a spacing of 400µm. The combined electrode may be deposited on a flexible PET substrate. Research is being conducted into also depositing the backside contact using metal pastes and screen-printing. NanoIdent Technologies: Dipl-Ing Franz Padinger A new class of sensors: Organic photonic sensors NanoIdent Technologies aims to make photonic sensors based on organic semiconductors for high-volume markets in the Industrial, Life Science and Security fields. Organic sensors are well suited to large-area applications and have mechanical properties - flexibility, low weight - that allow new applications. Printing as a deposition technique provides flexibility and very fast turnaround. The first prototype products from a novel design should be available within a few days of receiving the plans. Large sensor arrays are manufactured in a multilayer process. The substrate is patterned with lines of ITO electrode. The active layer is deposited across the substrate. The top electrode is deposited in rows perpendicular to the ITO so that the crossing points form sensor devices. These arrays are easily integrated into products due to wide range of substrates on which they can be made. Typical pixel sizes are 100µm x 100µm with current densities of 100 uA/mm2 at 20 mW/cm2. On-off ratios are higher than 105. An application in the life sciences is to integrate the sensors onto fluorescent biochips. Typical biochip cost is $100 and the organic sensor may add $5 to the total cost. Integrated detection avoids the need for a conventional biochip reader, which typically costs $100,000. Markets for these chips will be in medical care, the food industry, pollution control, and homeland security. Another application is cheap sensors for dental and medical X-rays. X-ray sensors also have applications in security and non-destructive materials testing. In the security field, organic fingerprint sensors could be integrated into passports and credit cards and provide instant confirmation of a client's identity. A fingerprint sensor would have an integrated OLED light source and would measure the backscattering of photons from a finger incident on an organic photo sensor array to build a unique picture of the user's fingerprint. University of Tokyo: Dr Takao Someya Organic transistor integrated circuits for large-area electronics Dr Someya discussed large-area arrays of integrated sensors for E-skins (human-like skins) and pocket scanners. The former application uses pressure sensors, where as the latter is based on light sensors. The devices have to be produced at low costs to allow for large area integration (sensitive skins could incorporate 1,000-1M pressure sensors), and have to be thin and light weight, resistant to shocks and mechanically flexible. The artificial skin systems comprise Cr/Au gates, smooth polyimide dielectric, pentacene semiconductor and pressure sensitive rubber where the Cu electrodes are mounted. Dr Someya modelled the behavior of the sencels using the Match Level 1 SPICE MOS model with 200k Ω resistor and showed a very good correlation between measured and simulated output curves of the 0.4-1.4 cm2/Vs pentacene TFTs. To move toward human-like skins thermal sensors (OFET + organic diode) need to be incorporated and the array should take a net-shaped, stretchable structure that allows a simultaneous measurement of the temperature and pressure. Dr Someya presented the developments on the sheet image scanner, which could be conformable to the bent page of a book or could take an image of a label on a bottle of wine. He showed a 5 x 5cm2 IC sheet of 72 x 72 cells with a resolution of 36dpi. The cells use top gate OFETs and photodiodes with patterned Ag paste to laminate the frame and diodes. Imaging could also be obtained with an organic diode matrix only. Dr Someya investigates 3D organic transistor ICs with double word-line and bit-line structure that would reduce the delay by a factor of five, and decrease the power by a factor of seven. The performance of the pentacene OFETs is remarkable. They were mechanically flexible when bent to 0.5mm and showed a fast recovery after 105 bending cycles. The devices were thermally stable, displaying no change after annealing at 140°C. The DC bias stress induced a change of only 2% after 12 hours. Manchester University: Dr Aimin Song MHz organic semiconductor devices made by single-step lithography Dr Song introduced a novel method of processing organic diodes and transistors, which he named 'self-switching' electronic devices. The single-step patterning technique introduces fast circuits that could operate at MHz frequencies. He acknowledges the need for a drastic reduction of the price/tag down to $0.05-0.01, and the move from silicon to all organic RFIDs. Nanodevices made by the conventional nanoimprint (using stamps) have a continuous, high throughput, but the process requires a multi-step alignment to be made, which could also introduce unwanted contact resistances. Single-step lithography however generates planar devices (low contact resistance) with low capacitance and high speed, which are appropriate for low cost, roll printing with high throughput. Etching trenches of different thicknesses (200-300nm) produces the self-switching devices, with the distance between the trenches defining the channel length. The devices work in depletion with the depletion region width depending on the bias polarity. The channel is closed when the depleted areas touch each other. Dr Song showed working InGaAs-InAlAs based SSDs (self-switching diodes), whose threshold voltage was tuned by changing the channel width. Circuits made by 'writing' do not use interconnects, and Dr Song presented circuits (ring oscillator, OR gate) made with SSDs and SSTs (self-switching transistors) with a very simple and elegant IC architecture. The circuits could operate at THz frequencies. The single-step lithography made possible the manufacturing of the simplest P3HT organic transistors on Mica substrates (150-200nm wide trenches, 300-100nm wide channels). Given a mobility of 0.1 cm2/Vs, the OFETs could operate at 2-20MHz, where as a mobility of 1 cm2/Vs could increase the speed to 20-200MHz. Having successfully managed to manufacture planar OFETs, the Manchester Group will look at integrating the devices in logic/analog circuits, then moving to RF rectifiers, pressure sensors, and RFIDs. Linköping University: Professor Olle Inganäs Large area organic electronics - from printed electronics to plastic solar cells Professor Inganäs presented work on 'macroelectronics' - large-scale polymer electronics deposited using conventional printing methods. Electroactive polymers are used which can switch between semiconducting and metallic states. Electrochemical transistors can be made from two patterned layers of PEDOT/PSS and a solid electrolyte. The transistor works by combined electronic and ionic transport via redox reaction at the PEDOT/electrolyte interface. A full set of logic gates has been built with this technology and a very slow inverter (switching every 30 seconds) realised. The redox reaction leads to a colour change of the PEDOT polymer, which, in combination to integrated electrochemical transistors, provides the basis for a very cheap electrochromic display. For organic photovoltaics, several new polyfluorene based polymers have been developed with broad absorptions in the green and red parts of the visible spectrum. These are appropriate for collecting a large part of the solar spectrum. PCBM is a commonly used electron acceptor but work has been going on using other C60 and C70 derivates. Device efficiency limitations are mainly due to transport rather than charge separation efficiency. For LED applications, the work function of electrodes can be modified by adsorption of a chemical with a strong dipole. This work function engineering allows the creation of transparent polymer anodes and cathodes which act as replacements for common unstable low-work function metals. Linköping University also has a spin-off company (Thin Film Electronics) developing polymeric ferroelectric RAM. This RAM is non-volatile and solution processable. Megabit densities have been achieved and there are large markets for low density, integrated RAM in areas such as identification, toys, RFID and e-paper. Read the summary for Day Two of OSC-05 Conference proceedings  You can purchase the full proceedings from OSC-05. The proceedings are provided on CD-ROM and consist of the slides (over 800) from all 23 presentations.
Format: CD-ROM, in PDF format with 20-user licence The proceedings cost £100.00 +VAT. 
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