The electrical properties of flexible nonvolatile organic bistable devices (OBDs) fabricated with graphene sandwiched between two insulating poly(methyl methacrylate) (PMMA) polymer layers were investigated. Current-voltage (I-V) measurements on the Al/PMMA/graphene/PMMA/indium-tin-oxide/poly(ethylene terephthalate) devices at 300 K showed a current bistability due to the existence of the graphene, indicative of charge storage in the graphene. The maximum ON/OFF ratio of the current bistability for the fabricated OBDs was as large as 1 x 10(7), and the endurance number of ON/OFF switchings was 1.5 x 10(5) cycles, and an ON/OFF ratio of 4.4 x 10(6) was maintained for retention times larger than 1 x 10(5) s. No interference effect was observed for the scaled-down OBDs containing a graphene layer. The memory characteristics of the OBDs maintained similar device efficiencies after bending and were stable during repetitive bendings of the OBDs. The mechanisms for these characteristics of the fabricated OBDs are described on the basis of the I-V results.

ABSTRACT:We report on the nonvolatile memory characteristics of a bistable organic memory (BOM) device with Au nanoparticles (NPs) embedded in a conducting poly-(N-vinylcarbazole) (PVK) colloids hybrid layer deposited on flexible poly(ethyleneterephthalate) (PET) substrates. Transmission electron microscopy (TEM) images show the Au nanoparticles distributed isotropically around the surface of a PVK colloid. The average induced charge on Au nanoparticles, estimated using the C-V hysteresis curve, was large, as much as 5 holes/NP at a sweeping voltage of (3 V. The maximum ON/OFF ratio of the current bistability in the BOM devices was as large as 1 Â 10 5 . The cycling endurance tests of the ON/OFF switching exhibited a high endurance of above 1.5 Â 10 5 cycles, and a high ON/OFF ratio of ∼10 5 could be achieved consistently even after quite a long retention time of more than 1 Â 10 6 s. To clarify the memory mechanism of the hole-mediated bistable organic memory device, the interactions between Au nanoparticles and poly(Nvinylcarbazole) colloids was studied by estimating the density of states and projected density of state calculations using density functional theory. Au atom interactions with a PVK unit decreased the band gap by 2.96 eV with the new induced gap states at 5.11 eV (HOMO, E 0 ) and LUMO 4.30 eV and relaxed the HOMO level by 0.5 eV (E 1 ). E 1 at ∼6.2 eV is very close to the pristine HOMO, and thus the trapped hole in E 1 could move to the HOMO of pristine PVK. From the experimental data and theoretical calculation, it was revealed that a low-conductivity state resulted from a hole trapping at E o and E 1 states and subsequent hole transportation through Fowler-Nordheim tunneling from E 1 state to Au NPs and/or interface trap states leads to a high conductivity state.

Lead sulfide (PbS) quantum dots (QDs) have great potential in optoelectronic applications because of their desirable characteristics as a light absorber for near-infrared (NIR) photodetection. However, most PbS-based NIR photodetectors are two-terminal devices, which require an integrated pixel circuit to be practical photosensors. Here we report on PbS QD/indium gallium zinc oxide (InGaZnO, IGZO) metal oxide semiconductor hybrid phototransistors with a photodetection capability between 700 and 1400 nm, a range that neither conventional Si nor InGaAs photodetectors can cover. The new hybrid phototransistor exhibits excellent photoresponsivity of over 10 6 A W − 1 and a specific detectivity in the order of 10 13 Jones for NIR (1000 nm) light. Furthermore, we demonstrate an NIR (1300 nm) imager using photogating inverter pixels based on PbS/IGZO phototransistors at an imaging frequency of 1 Hz with a high output voltage photogain of~4.9 V (~99%). To the best of our knowledge, this report demonstrates the first QD/metal oxide hybrid phototransistor-based flat panel NIR imager. Our hybrid approach using QD/metal oxide paves the way for the development of gate-tunable and highly sensitive flat panel NIR sensors/ imagers that can be easily integrated.

Colloidal quantum dots (QDs) are an emerging class of new materials due to their unique physical properties. In particular, colloidal QD based light emitting diodes (QDLEDs) have been extensively studied and developed for the next generation displays and solid-state lighting. Among a number of approaches to improve performance of the QDLEDs, the most practical one is optimization of charge transport and charge balance in the recombination region. Here, we suggest a polyethylenimine ethoxylated (PEIE) modified ZnO nanoparticles (NPs) as electron injection and transport layer for inverted structure red CdSe-ZnS based QDLED. The PEIE surface modifier, incorporated on the top of the ZnO NPs film, facilitates the enhancement of both electron injection into the CdSe-ZnS QD emissive layer by lowering the workfunction of ZnO from 3.58 eV to 2.87 eV and charge balance on the QD emitter. As a result, this device exhibits a low turn-on voltage of 2.0–2.5 V and has maximum luminance and current efficiency values of 8600 cd/m2 and current efficiency of 1.53 cd/A, respectively. The same scheme with ZnO NPs/PEIE layer has also been used to successfully fabricate green, blue, and white QDLEDs.