| Flexible organic solar cells have attracted a lot of attention due to their light weight, low cost, easy processing, and availability for large-scale production. However, the traditional transparent conductive electrodes which have been studied maturely, such as ITO film, cannot meet the requirement as a transparent conducting electrode prepared on the heat-sensitive substrate for flexible organic solar cells, because of its inherent mechanical brittleness and the high processing or heat treatment temperature of preparing high-quality ITO films. On the other hand, compared with traditional inorganic solar cells, the poor stability and low energy conversion efficiency need to be improved. Therefore, more and more researchers are trying to fabricate the electrode, which have better flexibility and more suitable to be transparent conducting films prepared on heat-sensitive polymer substrates. Meanwhile, researchers are trying to optimize the structure and morphology characteristics to further enhance the properties of the electrode, in order to improve the working parameters of the photovoltaic devices, such as power conversion efficiency and stability. The improvement of the transparent conductive electrode will contribute to the development of the flexible photovoltaic devices and provide the experimental and technological support.In this research, the structure, morphology and properties of the electrode were optimized by surface modification, minimal dopant and control of the sputtering parameter using magnetron sputtering. The photoelectric properties, mechanical properties and the photoelectric conversion characteristic were studied with the characterization of the microstructure. In this way, the relations between the structure and morphology and the photoelectric properties were revealed. And the properties of several electrodes were optimized in order to provide technological support to the development of the flexible photovoltaic devices, such as flexible organic solar cell. In this research, we studied: Firstly, ITO-AgOx-ITO (IAOI) electrode using an OMO (oxide-metal-oxide) sandwich structure effectively reduced the thickness of the electrode, which greatly improved the flexibility of the electrode. Here, the traditional OMO structure was applied and the interlayer AgOx was obtained by doping a small amount of02. After the metallic Ag film was extremely slightly oxidized, the optical transmittance of Ag films was greatly improved while maintaining good electrical conductivity. Compared to the traditional single-layer ITO electrode and the ITO-Ag-ITO (IAI) electrode, transmittance had been greatly improved while a relatively good conductivity was kept. Thus, when such an electrode was applied as a transparent conductive electrode of the organic solar cell, the energy conversion efficiency was increased from4.72%using conventional ITO electrode to5.88%based on IAOI electrode. In this way, the power conversion efficiency has been improved by25%. And the bending test result indicated that IAOI film electrode exhibited the similar flexibility as IAI electrodes, which was much better than the traditional single-layer ITO electrodes.Secondly, ZnO was applied as the oxide in OMO structure instead of ITO in order to prepare an indium-free electrode, ZnO-AgOx-ZnO(ZAOZ) electrode. The electrode was more transparent and conductive than ZnO-Ag-ZnO (ZAZ) and single-layer ITO electrode. And it was found that ZAOZ electrode was much more flexible than conventional traditional single-layer ITO electrodes. Since the energy matching between ZnO and the photoactive polymer layer, a ZAOZ electrode was applied to prepare inverted organic solar cell. Due to the good electrical and optical performance of the electrode, the organic solar cell based on ZAOZ electrode exhibited much higher power conversion efficiency (6.34) compared to the organic solar cells based on ITO electrode (5.76) and ZAZ electrode (5.65%). What’s more, the stability of the organic solar cell was improved a lot because of the inverted structure. The power conversion efficiency remained more than85%of the initial value while the solar cell using conventional structure failed after5days.Thirdly, based on ITO-AgOx-ITO electrode, nanoparticle arrays of three-dimensional structure were applied to prepare a three-dimensional ITO-AgOx-ITO nanoparticle array (IAOI-NPA) electrode. As a result, the flexibility has been greatly improved due to the reduced thickness of the continuous film layer between the nanoparticles and because the antireflection of the film could be controlled by adjusting the distance between the nanoparticles. Therefore, greatly reduction of the reflection of incident light directly leaded to increase of the light transmittance. This three-dimensional structure electrode for an organic solar cell, compared to the two-dimensional ITO electrode and ITO-Ag-ITO electrode, not only had superior optical performance, but also increased the contact area of the electrode and the photoactive polymer. In this way, an effective solution was found to the tradeoff between minimizing the thickness of the photoactive layer in order to reduce the transport distance of excitons and increasing the optical thickness of the active layer to increase light absorption. Simultaneously, vertical ITO nano-particles provided direct paths for the transfer and collection of charges. Therefore, power conversion efficiency of the organic solar cells was improved by22%.In this research, the tradeoff between thickness of the metallic interlayer and the photoelectric properties has been resolved. Also, the understanding of mechanism of the transport and the control of the photovoltaic properties of the nano-films was improved. The brittleness of the ITO electrode was improved a lot in order to provide technological support for the fabrication of solar cell. The3D transparent conductive electrode was prepared and it provided a new idea for the improvement of the power efficiency of flexible organic solar cell. |
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