The Application Of Metal Oxide Nanocomposites In Organic Optoelectronic Devices

Organic electronic devices based on organic semiconductors have attracted tremendous academic and industrial attention in the past few decades, due to the low material cost, easy and high-throughput fabrication, compatibility with large-area substrates and high mechanical flexibility of organic materials.Among these devices, organic bistable devices(OBDs) are one of the intensively studied topics, which have simple device architectures and can be integrated with next-generation portable and flexible logic storage circuits. Many studies have been carried out and there has been great progress on developing OBDs for memory applications. Particularly, using nanocomposites or nanoparticle/organic blends as the active layers has become one of the most successful methods to achieve OBDs with high ON/OFF ratio and long operation lifetime. Zinc oxide nanoparticles(ZnO NPs) and polyvinylpyrrolidone(PVP) polymer are widely studied and utilized in memory devices in the past few years, due to their good environmental stability, easy processability and great electrical properties.Recently the power conversion efficiency(PCE) of polymer:fullerene based solar cells has reached 10.6% with an inverted tandem cell structure and a certified PCE of 9.94% for an inverted single device. Many studies have been carried out to further improve the efficiency of organic photovoltaics(OPVs), including the methods to improve the light absorption efficiency and the charge collection efficiency. To increase the active layer thickness is one of the most effective ways to improve the light absorption efficiency, however, the thick active layer will cause a significant charge recombination in OPV devices due to the short exciton diffusion length in organic semiconductors, leading to a low PCE. The introduction of metal nanoparticles in polymer solar cells is one of the effective ways to improve the PCE of OPV devices, where the active layer can be made relatively thin for better charge collection and the metal nanoparticles can help improve the light absorption by the active layers. Particularly,incorporating Au-ZnO nanocomposites into OPVs, where ZnO and Au are covalently bonded, is a promising approach. The Au nanoparticles will enhance the light harvesting in OPV due to the plasmon resonance enhancement effect, whereas the ZnO will effectively passivate Au nanoparticles and increase the device stability.In this work, the ZnO/PVP and Au/ZnO nanocomposites are simply synthesized using an in-suit reaction method. We first applied ZnO/PVP nanocomposites in organic bistable devices, where the trapping the trapping media(ZnO) for storage charges were fully covered with the insulator(PVP). Such bistable device based on the ZnO/PVP nanocomposites showed a higher ON/OFF ratio than that with ZnO NPs, providing a useful and important in the field of designing hybrid memory devices. Second, we demonstrated a successful strategy to incorporate Au/ZnO nanocomposite into inverted solar cells for plasmon-induced light harvesting enhancement, while the stability and interface quenching issues of metal nanostructure can be prevented. The specific content are:(1) ZnO/polyvinylpyrrolidone(PVP) nanocomposite were synthesized using a simple in-situ reaction method and investigated as active layers in bistable devices. From the current-voltage characteristics, we found that the device with the nanocomposites had a two-magnitude higher ON/OFF ratio than that with pure ZnO NPs and the simple blend of the ZnO NPs and PVP, which opens up a great opportunity to achieve high performance memory devices. We attributed the significant enhancement upon the better film morphology of the nanocomposite film and its better charge carrier storage capability as a result of the covalent bonding between the particles and polymers.(2) We demonstrate that PCE and stability of inverted polymer solar cells can be significantly improved with solution-processed Au-ZnO nanocomposites as the electron transfer layer. Due to the surface plasmonic effect, both the devices based on Au/ZnO bilayer structure and Au-ZnO nanocomposites had enhanced device performatio. By optimizing the concentration of the Au-ZnO nanocomposites, the Au-ZnO nanocomposite based devices showed an optimized PCE of 7.82%, which was similar to that of the device based on the Au/ZnO bilayer. Moreover, in the Au-ZnO nanocomposite, the Au NP surface was covered with ZnO, which passivates the Au surfaces and improves the stability of the devices. After 12days exposure to ambient environment without any encapsulation, the PCE of the Au-ZnO nanocomposite based OPV decreased from 7.62% to 6.92%, whereas that based on Au/ZnO bilayer decreased from 7.6% to 6.14%, suggesting that the device stability can be improved by using metal/oxide nanocomposite materials.