A novel TFT‐OLED integration for OLED‐independent pixel programming in amorphous‐Si AMOLED pixels

Abstract— The direct voltage programming of active‐matrix organic light‐emitting‐diode (AMOLED) pixels with n‐channel amorphous‐Si (a‐Si) TFTs requires a contact between the driving TFT and the OLED cathode. Current processing constraints only permit connecting the driving TFT to the OLED anode. Here, a new “inverted” integration technique which makes the direct programming possible by connecting the driver n‐channel a‐Si TFT to the OLED cathode is demonstrated. As a result, the pixel drive current increases by an order of magnitude for the same data voltages and the pixel data voltage for turn‐on drops by several volts. In addition, the pixel drive current becomes independent of the OLED characteristics so that OLED aging does not affect the pixel current. Furthermore, the new integration technique is modified to allow substrate rotation during OLED evaporation to improve the pixel yield and uniformity. The new integration technique is important for realizing active‐matrix OLED displays with a‐Si technology and conventional bottom‐anode OLEDs.     

Reliability of Active-Matrix Organic Light-Emitting-Diode Arrays With Amorphous Silicon Thin-Film Transistor Backplanes on Clear Plastic

We have fabricated active-matrix organic light emitting diode (AMOLED) test arrays on an optically clear high-temperature flexible plastic substrate at process temperatures as high as 285 degC using amorphous silicon thin-film transistors (a-Si TFTs). The substrate transparency allows for the operation of AMOLED pixels as bottom-emission devices, and the improved stability of the a-Si TFTs processed at higher temperatures significantly improves the reliability of the light emission over time.     

Amorphous Silicon Thin-Film Transistor Backplanes Deposited at 200 °C on Clear Plastic for Lamination to Electrophoretic Displays

The transition of thin-film transistor (TFT) backplanes from rigid plate glass to flexible substrates requires the development of a generic TFT backplane technology on a clear plastic substrate. To be sufficiently stable under bias stress, amorphous-silicon (a-Si:H) TFTs must be deposited at elevated temperatures, therefore the substrate must withstand high temperatures. We fabricated a-Si:H TFT backplanes on a clear plastic substrate at 200degC. The measured stability of the TFTs under gate bias stress was superior to TFTs fabricated at 150degC. The substrate was dimensionally stable within the measurement resolution of 1, allowing for well-aligned 8 times 8 and 32 times 32 arrays of pixels. The operation of the backplane is demonstrated with an electrophoretic display. This result is a step toward the drop-in replacement of glass substrates by plastic foil.     

Preparation of quaternary boro-phosphate multifunctional glasses and their structural,optical, switching and antibacterial properties

In this paper, the glass composition of (50-x)P₂O₅-xB₂O₃-30CuO-20Li₂O (x?=?0, 5, 10, 15, and 20?mol%) was prepared and the effect of P₂O₅ substitution by B₂O₃ on their structural, optical, switching, and antibacterial characteristics was studied. FT-IR spectra showed that an increase in the B₂O₃ content leads to gradual erosion of the phosphate characteristic bonds, and the emergence of borate-related ones by creating new linkages between phosphate chains through P–O–B bonds and formation of highly cross-linked P³-O-B⁴ linkages. The incorporation of boron up to 20?mol%, also leads to an overall increase in glass transition temperature together with a decrease in the molar volume which both, implied improvement of glass stability. Optical studies revealed that all glasses are almost transparent in the UV–Vis region with high band gap energy about 3.83?eV, which experiences a red-shift with increase in the B₂O₃ concentration to 15?mol%. By calculating the wavelength-dependent optical parameters, however, it was found that the present glass composition with highest concentration of B₂O₃ shows refractive index near one and very negligible extinction coefficient (and imaginary optical dielectric function) at the visible region. These results support the great potential of the mentioned glass composition as a window layer. The analysis of the high electric field measurements demonstrated a wide range reduction in switching threshold voltage as the B₂O₃ content increases. This hints at their potential application as electrical-induced sensors. The antibacterial activity of x?=?0 and x?=?5 glass compositions has been examined by zone of inhibition measurements and it was found that they have potential applications as antibacterial agent.     

Use of Glycerol Carbonate in an Efficient, One-Pot and Solvent Free Synthesis of 1,3-sn-Diglycerides

An efficient solvent-free synthesis of a variety of highly pure 1,3-sn-diglycerides (1,3-sn-diacylglycerols) in a two-step one pot process is described. Heating glycerol carbonate (4-hydroxymethyl-1,3-dioxolan-2-one) with fatty acid anhydrides 2a–d affords 1:1 mixtures of glycerol carbonate fatty esters 3a–3d and the corresponding fatty acids. Further heating the reaction mixtures in the presence of catalytic amounts of 1,4-diazabicyclo[2.2.2]octane (DABCO) at 195–200 °C yields highly pure 1,3-sn-diglycerides 4a–4d.     

High mobility poly-Ge thin-film transistors fabricated on flexible plastic substrates at temperatures below 130°C

Depletion-mode poly-Ge thin-film transistors (TFTs) with an effective hole mobility of 110 cm²/Vs and an ON/OFF ratio of 10⁴ have been fabricated on flexible polyethylene therephtalate (PET) substrates, taking advantage of a novel stress-assisted crystallization technique. Proper manipulation of an otherwise destructive mechanical stress leads to a drastic drop of crystallization temperature from 400°C to 130°C. External compressive stress is transferred to the Ge/PET interface by bending the flexible substrate inward, during the thermal post-treatment. Proper patterning of the a-Ge layer before thermomechanical post-treatment leads to a minimal crack density in the processed poly-Ge layer. Reduction in the crack density plays a crucial role in alleviating the stress-induced gate leakage current emanated from the crack traces propagating from the channel into the gate oxide.     

Amorphous-Silicon Thin-Film Transistors Fabricated at 300 °C on a Free-Standing Foil Substrate of Clear Plastic

We have made hydrogenated amorphous-silicon thin-film transistors (TFTs) at a process temperature of 300^degC on free-standing clear-plastic foil substrates. The key to the achievement of flat and smooth samples was to design the mechanical stresses in the substrate passivation and transistor layers, allowing us to obtain functional transistors over the entire active surface. Back-channel-passivated TFTs made at 300 ^degC on glass substrates and plastic substrates have identical electrical characteristics and gate-bias-stress stability. These results suggest that free-standing clear-plastic foil can replace display glass as a substrate from the points of process temperature, substrate and device integrity, and TFT performance and stability.     

Improving the performance of perovskite solar cells via addition of poly(3-dodecylthiophene)-grafted multi-walled carbon nanotubes

Multi-walled carbon nanotubes grafted with poly(3-dodecylthiophene) (MWCNT-g-PDDT) and pure MWCNT were embedded into the active layer to investigate the optical, morphological, and photovoltaic properties of CH₃NH₃PbI₃ perovskite solar cells. Using an optimized concentration of MWCNT-g-PDDT (0.01%) improved photoresponse of perovskite structure in the wavelength range of 400?800 nm. Field emission scanning electron microscopy revealed that addition of 0.01% MWCNT-g-PDDT reduced the pin-holes and empty spaces. The grafted MWCNT enhanced the J_sc as well as PCE by reduction in series resistance and impressive charge transfer from perovskite active layer to the contact transport layers.

     

Low-temperature non-metal-induced crystallization of germanium for fabrication of thin-film transistors

Device-quality poly-Ge layers were grown at temperatures as low as 200 °C by successive hydrogenation and annealing steps, with no need to any metal incorporation. Hydrogenation is performed in a PECVD apparatus with 150 W RF hydrogen plasma and annealing is carried out in the same system in N₂ ambient. As a result, grains of the order of 100 nm are formed in the Ge layer. It has been observed that hydrogenation at high temperatures may be destructive to the Ge layer. Successive hydrogenation and annealing at respective temperatures of 150 and 200 °C would result in a device-quality poly crystalline Ge layer which has been employed for fabrication of depletion-mode thin-film transistors. These TFTs show a field-effect mobility of 80 cm²/Vs for holes and an ON/OFF ratio of more than 10³, indicating the feasibility of this technique for applications in large-area electronics.