A Study on Optical and Electronic Devices fabricated utilizing Hybrid Organic/Inorganic Nanocomposites

Abstract

Quantum structures containing nanoparticles have attracted much attention because of their promising potential applications in electronic and optoelectronic devices operating at lower currents and higher temperatures. Firstly, we demonstrate the hybrid polymer-quantum dot based multi-functional device (Organic bistable devices, Light-emitting diode, and Photovoltaic cell) with a single active-layer structure consisting of CdSe/ZnS semiconductor quantum-dots (QDs) dispersed in a poly N-vinylcarbazole (PVK) and 1,3,5-tirs- (N-phenylbenzimidazol-2-yl) benzene (TPBi) fabricated on indium-tin-oxide (ITO)/glass substrate by using a simple spin coating technique. The multi-functionality of the device as Organic bistable device (OBD), Light Emitting Diode (LED), and Photovoltaic cell can be successfully achieved by adding an electron transport layer (ETL) TPBi to OBD for attaining the functions of LED and Photovoltaic cell in which the lowest unoccupied molecular orbital (LUMO) level of TPBi is positioned at the energy level between the conduction band of CdSe/ZnS and LiF/Al electrode (band-gap engineering). Through transmission electron microscopy (TEM) study, the active layer of the device has a p-i-n structure of a consolidated core-shell structure in which semiconductor QDs are uniformly and isotropically adsorbed on the surface of a p-type polymer core and the n-type small molecular organic materials surround the semiconductor QDs. Through more studies of the OBDs containing nanomaterials (quantum dots or nanoparticles), Firstly, we have introduced carrier transport in flexible organic bistable devices of ZnO nanoparticles embedded in an insulating poly(methyl methacrylate) polymer layer. The carrier transport mechanism of the bistable behavior for the fabricated organic bistable device (OBD) structures is described on the basis of the I –V results by analyzing the effect of space charge. Secondly, we report on the non-volatile 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 polyethylene terephthalate (PET) substrates. From the experimental data and theoretical calculation, it was revealed that a low-conductivity state resulted from a hole trapping at Eo and E1 states and subsequent hole transportation through Fowler-Nordheim tunneling from E1 state to Au NPs and/or interface trap states leads to a high conductivity state. Thirdly, we have introduced the electrical bistabilities, the memory stabilities, and the memory mechanisms of nonvolatile organic memory devices fabricated utilizing ZnO QDs blended with the PMMA polymer formed by using a simple spin-coating technique. Transmission electron microscopy (TEM) images showed that colloidal ZnO quantum dots (QDs) were distributed around the surface of a polymethylmethacrylate (PMMA) polymer. Current-voltage (I-V) measurements on the Al/colloidal ZnO QDs blended with a PMMA polymer layer/indium-tin-oxide/glass devices at 300 K showed a current bistability due to the existence of the ZnO QDs, indicative of carrier storage in the ZnO QDs. The maximum ON/OFF ratio of the current bistability for the organic bistable devices (OBDs) was as large as 5 × 104, and the cycling endurance time of the ON/OFF switching for the OBDs was above 105. The memory mechanisms of the fabricated OBDs are described on the basis of the I-V results. Through more studies of the LEDs containing nanomaterials (quantum dots or nanoparticles), Firstly, we have introduced WLEDs were fabricated by using a spin-coating technique with hybrid ZnO QD-polymer composites. The ZnO QDs adsorbed at the surface of the PVK polymer, acting as a hole transport layer, dominantly contributed to the EL process of the WLEDs. The luminescence mechanisms of the WLEDs are described on the basis of the experimental results and the energy band diagram. Secondly, we report solution-processed white light-emitting diode utilizing hybrid polymer and red-green-blue quantum dots. Spectrally clean white-light emission having a center at around 474 (Blue), 544 (Green), and 629 (Red) nm with CIE coordinates, (0.44, 0.32) from this polymer-RGB QDs hybrid WLEDs is revealed to directly relate with exciton originated from a narrow electron-hole recombination zone with uniformly dispersed CdSe/ZnS QDs between the PVK interface. These results indicate that the nanomaterials based multi-functional devices are easy to fabricate by simple spin-coating method and hold a promise for potential applications in future transparent opto-electronic devices based on hybrid polymer-QDs.