Investigation On Cathode Modification Of Small Molecule Organic Solar Cells

In recent years, investigation on organic photovoltaic materials and devices developed rapidly. Many kinds of organic materials for photovoltaic diodes have been developed and are on their way for commercialization. Organic semiconductor solar cell has been shown to have the advantages of comparatively simple procedure and low cost, so it has the potential superiority even though the conversion efficiency is low. In order to improve the properties of organic solar cells, a lot of different studies have been carried on.The main purposes of these studies are to seek new materials and new devices.In this paper, basic properties, structures, theory, research status as well as defects and causes of defects of organic solar cells are introduced, and the trends in the development of such devices are also presented.The working mechanism of both organic light-emitting devices and organic solar cells are compared and analyzed. Moreover, organic solar cell I-V curve was analyzed by utilizing the energy band diagram so that we understood its mechanism deeply.In this thesis, the emphasis is focused on the study of ITO/ZnPc/C60/buffer layer/Al double-layer heterojunction structure, in which using ZnPc as the donor, using C60 as the acceptor. LiF, Alq3 and ZnPc are used as cathode modification materials. The preparation processes include the design of ITO electrodes, the pretreatment of the ITO surface, vacuum evaporation. And then we test the photoelectric properties.The light intensity distribution inside ITO/ZnPc/C60/Al structure is disscused by the optical transfer matrix method in 450nm and 630nm which are the peak wavelengthes of quantum efficiency. The relationship between the excitons production and the thicknesses of ZnPc and C60 is obtained by intensity distribution calculation method. It has been shown the optimum structure is ITO(220nm)/ZnPc(30nm)/C60(40nm)/Al(100nm). By amount of experiments to verify the theoretical results, the theoretical and experimental results are consistent.The paper is focused on investigation on the cathode-modified layers on the performances of ZnPc/C60 organic solar cells. It has been found the devices with the appropriate thicknesses of buffer layers show better performances than that of the unmodified devices. From the optical and electrical aspects, the mechanisms of the devices by using the modified layer are analysised. The phenomenon of vacuum level shift has explained that the LiF modified layer can lower Al work function, so the organic layer and the electrode form good ohmic contact, reduce the series resistance and benefite the charge injection to the cathode, and thereby enhance the device performances. The devices with Alq3 modified layer influenced the optical properties of the devices a little, but ZnPc modified layer didn't influenced the optical properties. The role of submonolayer of Alq3 and ZnPc has been suggested to be that of protecting the organic layer during Al deposition, and introducing a dipole at the cathode interface, affecting charge injection. The thicknesses of modified layer have a greater impact on device characteristics. An account of experiments shows 1nm of LiF, 2.5nm of Alq3 and 3nm of ZnPc are the optimal thickness of modified layer. If the buffer layers are too thin, they couldn't block the excitons effectively. But instead, they would block the electrons if they are too thick. Different modified materials showed different functions. Fill factor of LiF-modified device has been increased by 44%, the conversion efficiency of Alq3-modified device has been enhanced by more than 5 times, and ZnPc-modified device showed the highest open-circuit voltage. Mechanisms behind the phenomena were discussed.The stability of the cells with and without modified layer has also been investigated. After being placed in the clean air for a short period of time, an ultra-thin Al2O3 is formed between C60 and Al and the device performances will be improved a little. With time going on, Al2O3 layer becomes gradually thick, the performances of the cells increase first and then decrease. In ITO/ZnPc/C60/buffer layer/Al cell structure, the device performances with LiF have changed with time and showed a decreasing trend. Since LiF is an insulation material, the organic layer and metal contacts are not ideal, the attenuation is the most serious; Alq3 and ZnPc are relatively stable and are good films, they can more effectively delay the rate of the oxygen and water vaporing into the active layer in the test environment. The result is to improve the stability of the devices.The experiments have indicated that different modification layers improve different characteristics of the devices. The next step could be to find a more suitable material or device structure to put these improvements into one device, to improve the performances of small organic molecules. The results of this paper and related analysis may give some guidance on the future work.