November 29-December 4, 2015 | Boston
Meeting Chairs: T. John Balk, Ram Devanathan, George G. Malliaras, Larry A. Nagahara, Luisa Torsi
The attractiveness of e-textile and wearable devices are their ease in collecting biological information in day-to-day lives and exercises. However, even with conductive fibers or threads, the conventional weaving method was not good enough to pattern fine electrodes and wires. This is why more simplified process, such as printing, to directly pattern durable and conductive materials on a fabric was widely awaited. We have developed novel ink that functions electronically, and succeeded in printing through one printing process a tough elastic conductor. This printable conductor keeps its high conductivity even when it is expanded more than 3 times in size. We printed elastic wires and electrodes with this ink on a fabric and achieved a textile electromyograph sensor. This easy to print textile bio-info sensor can be used in sports, healthcare and medicine.
In organic/metal hybrid materials, the thermal boundary conductance across the metal/organic interface plays a significant role in overall thermal conductivity of the film. The conductivity of the organic or hybrid thin film not only plays a critical role in the thermal stability of the organic devices, but also governs the heat transfer mechanisms. Here by embedding metal nanoparticles into organic semiconductor, we have successively developed a thermistor for direct temperature sensing. By integrating the thermistor with the active matrix organic transistor array, we fabricated a large area 16 × 16 temperature sensor which can be directly used for temperature mapping of objects with various shape. Simultaneously, we apply 3-omega; method to measure the effective conductivity of the thin film and the results are compared with the finite element modeling. By carefully controlling the concentration of the silver nanoparticles, we can modify the sensitivity of different temperature sensors. For a thin layer of Ag and intermixed with DNTT (10% volume ratio), the thermal conductivity decrease from 0.363W/m-K (pure DNTT) to 0.305W/m-K which shows the importance of the thermal boundary conductance. In the integrated temperature sensor array, the hyrbid thermistors are connected in series to the drain contacts and the whole array is developed on flexible substrate. By optimizing the anodization growth of the alumium oxide (AlOx) dielectric, the tempearture array can be powered under 5V with dynamic range higher than 10 bits, which clealy shows their capability in portable temperature sensing applications. The low voltage flexible thermal sensor array is suitable for portable electronic devices and potentially scale up for electronic skin applications. Other application directions such as health monitoring or use as surgery tools can be achieved.
Photodiodes with high specific detectivity, which entails high external quantum efficiency (EQE) and low dark current, and large pixel sizes enable optical systems capable of imaging lower light intensities. Additionally, the ability of a photodiode to operate under high electric fields at reverse bias increases the amount of photogenerated charge that may be capacitively stored during a single integration period, which is known as the well capacity. Using only the highly scalable printing techniques of blade coating and screen printing to deposit the layers on plastic, flexible organic photodiode arrays are reported with average specific detectivities of 3.45×1013 cmmiddot;Hz0.5middot;W-1 at a bias of -5 V. Polyethylenimine is blade coated over PEDOT:PSS to form the bottom cathode on these inverted devices, which exhibits excellent uniformity in work function modification on the microscale (20 meV standard deviation) as well as over centimetric areas. Furthermore, it is found that the polyethylenimine interlayer is not only essentially for lowering the work function of the electrode to increase EQE but also serves as a hole blocking layer to decrease the dark current density to an average of 150 pA/cm2. Photodiodes fabricated with a screen printed PEDOT:PSS top anode exhibit dark current shunt resistances an order of magnitude higher than devices fabricated using thermally evaporated top electrodes as a result of the creation of defects in the active layer which serve as leakage paths. This results in a lower dark current at high reverse biases for devices with a screen printed top anode than the devices with thermally evaporated metal electrodes. Additionally, these devices show excellent bias stress stability under high applied fields (88 kV/cm) and low variability, with a coefficient of variation of 15% in specific detectivity for 24 pixels across an array with perfect yield. Integration of these photodiodes with organic thin film transistor arrays and charge integrators will also be demonstrated.
Colloidal quantum dots are highly versatile optoelectronic components that combine size- and shape-tunable properties with the attractiveness of cost-effective solution-phase processing. As such, they are ideal building blocks for the formation of artificial solids in which the colloidal assembly profits from the carefully engineered properties of the individual quantum dots. A variety of quantum-dot-based optoelectronic devices have been reported recently, including light-emitting diodes1 and lasers with tunable emission across the entire visible range,2 as well as demonstrations of efficient solar cells3 and photodetectors.4 It is interesting to consider complementing the nanoscale-structure of the quantum dot assembly with wavelength-scale patterning. This would allow for the creation of hierarchical photonic structures.