David F. Zeigler

Flexible and twistable non-volatile memory cell array with all-organic one diode–one resistor architecture

Flexible organic memory devices are one of the integral components for future flexible organic electronics. However, high-density all-organic memory cell arrays on malleable substrates without cross-talk have not been demonstrated because of difficulties in their fabrication and relatively poor performances to date. Here we demonstrate the first flexible all-organic 64-bit memory cell array possessing one diode-one resistor architectures. Our all-organic one diode-one resistor cell exhibits excellent rewritable switching characteristics, even during and after harsh physical stresses. The write-read-erase-read output sequence of the cells perfectly correspond to the external pulse signal regardless of substrate deformation. The one diode-one resistor cell array is clearly addressed at the specified cells and encoded letters based on the standard ASCII character code. Our study on integrated organic memory cell arrays suggests that the all-organic one diode-one resistor cell architecture is suitable for high-density flexible organic memory applications in the future.

All‐Organic Photopatterned One Diode‐One Resistor Cell Array for Advanced Organic Nonvolatile Memory Applications

ONVM is one emerging technology that has been explored as the next generation data storage media. In literature, there are numerous reports regarding the switching mechanisms for ONVM devices, [ 15–18 ] as well as the development of new organic materials [ 15 , 18–21 ] and device architectures [ 22–27 ] for ONVM applications. Research on novel device architectures can potentially generate more reliable organic memory devices with higher density and improved performance, while simultaneously mitigating misreading (cross-talk) issues. For these reasons, organic one diode-one resistor (1D–1R) type devices have been demonstrated. [ 23,24 ] However, these devices exhibit irreversible switching behavior and are nonpatternable, which strongly limits their application in systems that require both an array architecture and rewritable memory capability. Recently, we have reported a hybrid type 1T–1R and 1D–1R device architecture, which is composed of a hybrid ONVM device with Si-based transistors or Si schottky diodes. [ 25,26 ]

n-Doping of thermally polymerizable fullerenes as an electron transporting layer for inverted polymer solar cells

A novel [6,6]-phenyl-C61-butyric acid methyl styryl ester (PCBM-S) was synthesized and employed as an electron transporting interfacial layer for bulk heterojunction polymer solar cells with an inverted device configuration. After the deposition of PCBM-S film from solution, the styryl groups of PCBM-S were polymerized by post-thermal treatment to form a robust film which is resistive to common organic solvents. This allows the solution processing of upper bulk heterojunction film without eroding the PCBM-S layer. Additionally, the PCBM-S was n-doped with decamethylcobaltocene (DMC) to increase the conductivity of the film, which resulted in a significantly improved power conversion efficiency from 1.24% to 2.33%. The improved device performance is due to the decrease of series resistance and improved electron extraction property of the n-doped PCBM-S film.

Influence of fluorine substituents on the film dielectric constant and open-circuit voltage in organic photovoltaics

Conjugated polymers with fluorine substituents on their backbone have exhibited improved performance over their un-fluorinated analogues by lowering the polymer HOMO level, thereby increasing the open-circuit voltage (VOC). To further investigate how fluorine substituents improve device performance, three polymers with the same donor and acceptor co-monomers, but differing by the number of fluorine atoms on the acceptor unit, were synthesized. Although the HOMO levels of the mono-(P1F) and di-fluorinated (P2F) polymers are essentially the same, an increase in VOC was still observed in the OPV device incorporating P2F. This implies that correlating the VOC to the donor polymer HOMO level is inadequate to fully explain the improvement in VOC. By calculating the charge transfer exciton binding energy from the measured film dielectric constant, it was found that the increase in VOC in going from P1F to P2F matches the decrease in charge transfer exciton binding energy.

Identifying effects of TiO2 nanowires inside bulk heterojunction organic photovoltaics on charge diffusion and recombination

The charge carrier dynamics of poly(3-hexylthiophene) (P3HT):[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) organic bulk-heterojunction photovoltaics with and without TiO2 nanowires are studied. Using inorganic nanowires as electron transport pathways improves the charge transit time and electron diffusion coefficient, which is the origin of fill factor and power conversion efficiency improvement observed in these devices. Through further comparison of devices with surface-modified nanowires (PCB–TiO2-NW), it is found that under AM 1.5 light illumination, charge recombination is dominant in the organic layer rather than at the TiO2 nanowire surface.