Design and Performance of Electrospun sp2-Carbon Based Active Layer and Flexible Gas Barrier Film in Photovoltaic Devices

Abstract

This thesis described the design and performance of next generation flexible photovoltaic devices based on sp2-carbon materials for high power conversion efficiency and stability. Photovoltaic devices are capable of converting solar energy into electrical energy, and act as future-oriented energy harvesting devices. Organic photovoltaic devices based on sp2-carbon materials including nanocarbon and conjugated polymers are preferred over conventional photovoltaic devices due to their solution-processable nature, low cost, and flexibility. However, the organic photovoltaic devices are accompanied by issues in efficiency and lifetime. Therefore, it is highly important to develop organic photovoltaic devices with enhanced power conversion efficiency and long stability. The aim of this study is to identify the factors that can be varied in order to achieve these requirements by examining the theoretical considerations, and then to design the photovoltaic devices for high efficiency and long lifetime. Part I provides a general introduction regarding sp2-carbon materials and organic photovoltaic devices. Using theoretical considerations and state of the art analysis, the factors for photovoltaic devices with high efficient and long lifetime are extracted. The aims of the present work are introduced on the basis of these derived factors. In Part II, with the aim of enhancing power conversion efficiency, organic photovoltaic cell based on one-dimensional conjugated polymers is prepared. Electrospinning techniques are used to prepare conjugated polymer nanofibers and photovoltaic devices in ambient air, regardless of relative humidity condition. The diameters of the conjugated polymer nanofibers are approximately twice the exciton diffusion length, 20nm. The short circuit current and power conversion efficiency of the device increased to 11.54 mA/cm2 and 5.82 %, respectively. The suggested method can be applied to the fabrication, in ambient air, of large-area active layers composed of other new conjugated polymers to yield high-performance OPV devices. Part III discusses the preparation and properties of reduced graphene oxide-based gas barrier films for the enhancement of the stability of organic photovoltaic devices. The simple and direct spin-casting of a graphene oxide suspension on an aluminum electrode is performed to encapsulate the OPV device with reduced graphene oxide film. The lifetime of the OPV device after the reduction process was increased by 100 times compared to an unencapsulated OPV device. Furthermore, the gas barrier property is enhanced more by controlling the thermal reduction condition. The gas barrier property is closely related to the surface roughness, dispersibility and reduction condition of graphene oxide. Flexible photovoltaic devices encapsulated by the reduced graphene oxide-based gas barrier film are prepared. The reduced graphene oxide allows a long lifespan while maintaining flexibility. Further investigations of the preparation and reduction of the graphene oxide film would improve the performance.