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Center for Organic Photonics and Electronics

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Low-voltage pentacene organic field-effect transistors with high-kappa HfO₂ gate dielectrics and high stability under bias stress

(Georgia Institute of Technology, 2009-12) Zhang, Xiaohong ; Tiwari, Shree Prakash ; Kim, Sung-Jin ; Kippelen, Bernard

Low-voltage pentacene organic field-effect transistors are demonstrated (operating voltage of −3 V) with high-κ hafnium dioxide gate dielectrics grown by atomic layer deposition at 200 °C. A high hole mobility of 0.39 cm²/V s with low threshold voltage (<−0.5 V) and low subthreshold slope of 120 mV/dec is achieved with a HfO₂ dielectric layer modified with a phosphonic acid based treatment. A high value of 94.8 nF/V s is obtained for the product of mobility and capacitance density. The devices show excellent bias stress stability with or without the phosphonic acid at the HfO₂ gate dielectric surface.
Low-voltage solution-processed n-channel organic field-effect transistors with high-k HfO₂ gate dielectrics grown by atomic layer deposition

(Georgia Institute of Technology, 2009-12) Tiwari, Shree Prakash ; Zhang, Xiaohong ; Potscavage, William J., Jr. ; Kippelen, Bernard

High performance solution-processed n-channel organic field-effect transistors based on [6,6]-phenyl C61 butyric acid methyl ester with low operating voltages (3 V) are demonstrated using a high-k hafnium dioxide gate dielectric grown by atomic layer deposition. Devices exhibit excellent n-channel performance with electron mobility values up to 0.14 cm²/V s, threshold voltages of ∼ 0.3 V, current on/off ratios >10⁵, and very low values of subthreshold slope ( ∼ 140 mV/decade).
Area-scaling of organic solar cells

(Georgia Institute of Technology, 2009-09) Choi, Seungkeun ; Potscavage, William J., Jr. ; Kippelen, Bernard

We report on the performance of organic solar cells based on pentacene/ C₆₀ heterojunctions as a function of active area. Devices with areas of 0.13 and 7 cm² were fabricated on indium-tin-oxide (ITO) coated glass. Degradation of the performance with increased area is observed and analyzed in terms of the power loss density concept. The various power loss contributions to the total series resistance (RSA) are measured independently and compared to the values of the series resistance extracted from the current-voltage characteristics using a Shockley equivalent circuit model. The limited sheet resistance of ITO is found to be one of the major limiting factors when the area of the cell is increased. To reduce the effects of series resistance, thick, electroplated, metal grid electrodes were integrated with ITO in large-area cells. The metal grids were fabricated directly onto ITO and passivated with an insulator to prevent electrical shorts during the deposition of the top Al electrode. By integrating metal grids onto ITO, the series resistance could be reduced significantly yielding improved performance. Design guidelines for metal grids are described and tradeoffs are discussed.
Low-voltage InGaZnO thin-film transistors with Al₂O₃ gate grown by atomic layer deposition

(Georgia Institute of Technology, 2009-06) Kim, Jungbae ; Fuentes-Hernandez, Canek ; Potscavage, William J., Jr. ; Zhang, Xiaohong ; Kippelen, Bernard

We report on low-voltage, high-performance amorphous indium gallium zinc oxide n-channel thin-film transistors fabricated using 100-nm-thick Al₂O₃ grown by atomic layer deposition as the gate dielectric layer. The Al₂O₃ gate dielectric shows very small current densities and has a capacitance density of 81±1 nFcm². Due to a very small contact resistance, transistors with channel lengths ranging from 100 μm down to 5 μm yield a channel-independent, field-effect mobility of 8±1 cm² V s, subthreshold slopes of 0.1±0.01 Vdecade, low threshold voltages of 0.4±0.1 V, and high on-off current ratios up to 6 x10⁷ (WL=4005 μm) at 5 V.
Fabrication of a blue M x N pixel organic light-emitting diode video display incorporating a thermally stable emitter

(Georgia Institute of Technology, 2009-04) Haldi, Andreas ; Kim, Jung B. ; Domercq, Benoit ; Kulkarni, Abhishek P. ; Barlow, Stephen ; Gifford, Angela P. ; Jenekhe, Samson A. ; Marder, Seth R. ; Kippelen, Bernard

A 7x11 pixel blue OLED display was fabricated using a patterned indium-tin-oxide (ITO) substrate. The fabrication process for an M x N pixel organic light-emitting diode (OLED) video display including an electrical insulating layer and a physical pixel separator layer is presented. An efficient and thermally stable blue fluorescent organic material, 6, 6'-bis((2-p-biphenyl)-4-phenylquinoline) (B2PPQ), was used in combination with an evaporated hole-transport small molecule with a high ionization potential.
Variable-ratio power splitters using computer-generated planar holograms on multimode interference couplers

(Georgia Institute of Technology, 2009-02) Tseng, Shuo-Yen ; Choi, Seungkeun ; Kippelen, Bernard

Variable-ratio power splitters using computer-generated planar holograms on multimode interference couplers are analyzed. The coherent wave at the device input is transformed to the desired output using numerically calculated refractive index perturbations on multimode channel waveguides at half the beat length. Devices are fabricated on the silicon-on-insulator platform and characterized at a wavelength of 1.55 μm . Power-splitting ratios are varied by changing the hologram etch depth and the hologram length.
SPICE Optimization of Organic FET Models Using Charge Transport Elements

(Georgia Institute of Technology, 2009-01) Vaidya, Vaibhav ; Kim, Jungbae ; Haddock, Joshua N. ; Kippelen, Bernard ; Wilson, Denise

We report on a modeling technique that uses charge transport equations to calculate channel current in organic field effect transistors (OFETs) by numerical solution in. the SPICE simulation program. SPICE is also used to optimize the model and achieve a fit to measured characteristics within 5% error. The overall modeling technique is a bridge between physical models of charge transport and a SPICE model useful in circuit simulation without requiring a closed-form drain-current equation. The automatic optimization of the simulation to measured curves will also allow, in the future, the empirical weighing of various charge transport effects in search of physical device operation, given sufficient empirical data. This modeling technique was applied to the measured characteristics of an OFET using pentacene in which the mobility was dependent on the voltage in the channel. The accuracy of the fit was better than 5% for 40 V > V-DS > 7 V and better than 20% for VDS < 7 V. Simulation was completed within 3 min for this optimization on a modern personal computer.
Tailoring the work function of indium tin oxide electrodes in electrophosphorescent organic light-emitting diodes

(Georgia Institute of Technology, 2009) Sharma, Asha ; Hotchkiss, Peter J. ; Marder, Seth R. ; Kippelen, Bernard

We investigate the use of organic surface modifiers based on phosphonic acid anchoring groups that react with the surface of indium tin oxide (ITO) in order to modify its work function, surface energy, and the charge injection in organic multilayer electrophosphorescent devices. The phosphonic acid surface modifiers, possessing different substituting groups, are found to tune the work function of ITO in the range of 4.40–5.40 eV. These surface modifiers have been tested as an interfacial layer between the ITO anode and hole transport layers HTL that are either processed from the vapor phase or from solution. The use of this interfacial layer with a solution-processible HTL results in high quantum and luminous efficiencies of 20.6% and 68 cd/A at 100 cd/m² (17.5% and 60 cd/A at 1000 cd/m²). The enhanced performance of the devices incorporating phosphonic acid modifiers could be associated with an improved charge injection and a better compatibility with the hydrophobic nature of the organic layer. The performance of these devices is also compared to that of devices in which ITO is modified with other well-known techniques such as air plasma treatment or the use of a layer of poly (3,4-ethylenedioxythiophene)/poly (styrenesulfonate).
Effect of phosphonic acid surface modifiers on the work function of indium tin oxide and on the charge injection barrier into organic single-layer diodes

(Georgia Institute of Technology, 2009) Sharma, Asha ; Haldi, Andreas ; Hotchkiss, Peter J. ; Marder, Seth R. ; Kippelen, Bernard

We investigate the use of several phosphonic acid surface modifiers in order to increase the indium tin oxide (ITO) work function in the range of 4.90–5.40 eV. Single-layer diodes consisting of ITO/modifier/N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′ biphenyl-4,4″ diamine (α-NPD)/Al and ITO/modifier/pentacene/Al were fabricated to see the influence of the modified ITO substrates with different work functions on the charge injection. To calculate the charge injection barrier with different surface modifiers, the experimentally measured current density-voltage (J-V) characteristics at different temperatures are fitted using an equivalent circuit model that assumes thermionic emission across the barrier between the ITO work function and the highest occupied molecular orbital of the organic material. The charge injection barrier height extracted from the model for various surface modifier-based diodes is independent of the ITO work function within the range of changes achieved through modifiers for both α-NPD and pentacene-based single-layer diodes.
Low-voltage flexible organic complementary inverters with high noise margin and high dc gain

(Georgia Institute of Technology, 2009-01) Zhang, Xiaohong ; Potscavage, William J., Jr. ; Choi, Seungkeun ; Kippelen, Bernard

We report on flexible organic complementary inverters using pentacene and C₆₀ as active semiconductors fabricated on a plastic substrate. Individual transistors as well as inverters show good operational stability with negligible hysteresis. The threshold voltages are comparable for p-channel pentacene and n-channel C₆₀ organic field-effect transistors, and noise margins larger than 80% of the maximum theoretical values were obtained at a supply voltage VDD as low as 3 V. A high dc gain of 180 was achieved at VDD=5 V. The inverters demonstrated good mechanical stability when tested after bending under both tensile and compressive stress.