An Investigation For TiO₂/PANI Photovoltaic Cell

Recently, the TiO₂ nanotubes have gained much attention because of their unique microstructures. In comparison with TiO₂ nano-crystallite, the TiO₂ nanotubes possesses higher specific surface area and stronger adsorption capability. Therefore, it exhibits better photo-catalysis capability and higher photoelectric conversion efficiency. The TiO₂ nanotubes array film has been widely applied in the fields of photocatalyst, solar cell, gas sensor, and so on. But the microstructure of TiO₂ nanotubes affects electro-optical property, on which the study is rather rare. In our report, The NH₄F/glycerol/water electrolyte anodic oxidation system for TiO₂ nanotube arrays is optimized by the glycerol ratio. The TiO₂ nanotube arrays were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). TiO₂/ITO photovoltaic cells are determined by the open circuit Voltage (V_oc) and short circuit current density (J_sc) without adding photosensitizer under UV irradiation. Experimental results show that the different ratios of glycerol in the system make different nanotube morphology, crystallinity and photoelectric properties, especially make great effect on short-circuit current density, of which the sample with 90% glycerol by volume is 1.5 times than that of the sample with halves by volume.In recent years, photo-electronic properties of doped PANI make it a promising material as a component of p–n heterojunctions used for photovoltaic cells. Meanwhile, PANI as the counter electrode of dye-sensitized solar cell is an interesting topic for its low cost, high-conductivity, and excellent catalytic activity for I₃^- reduction. Our laboratory has concentrated polyaniline on the research of super capacitors. The doped PANI particles shows obviously three-dimensional reticulation by electrochemical method, has a relatively high specific capacitance, high coulombic efficiency, and a high redox reversibility at high charge–discharge process. Though the TiO₂/ITO cells all owned a comparative value of Voc, but the Jsc was weak due to the great inner-resistance of the cells. In order to decrease inter-resistance of the cells, we fabricated polyaniline film doped with sulfuric acid on ITO glass substrates, as the counter electrode of TiO₂ nanotube array cell. Then we assembled and tested the TiO₂/PANI-ITO photovoltaic cells without dye. The photovoltaic behavior the ITO/PANI/I₃^-+I₂/TiO₂ (TiO₂/PANI-ITO) cell is compared with the device structure ITO/I₃^-+I₂/TiO₂ (TiO₂/ITO) cell, The open circuit potential (V_oc) of the TiO₂/PANI-ITO cell was lower than the TiO₂/ITO cell, while the short-circuit current density (J_sc) obviously increased, up to approximately two times higher than that of TiO₂/ITO cell. It was concluded that the decrease in V_oc is mainly due to the narrow energy gap between the conduction band of TiO₂ and I₂/I₃^- redox potential on the PANI-ITO electrode. The increase in J_sc is due to large active surface area, the small charge-transfer resistance R_CT, and the higher electrocatalytic activity of PANI for I₂/I₃^- redox reaction. Meanwhile, we studied optical properties of polyaniline film in different oxidation states, we found that the polyaniline film in emeraldine had the better V_oc and J_sc than the polyaniline film in reduced state (leucoemeraldine). Because the polyaniline film in emeraldine owns the very good conductivity, the polyaniline film in reduced state is non-conducting , it can conduct the electricity after oxidating by the electrolyte, so J_sc the polyaniline film in emeraldine had the higher value than that in reduced state. The V_oc of the TiO₂/ITO cell is derived from the energy gap between the conduction band level of TiO₂ and the I₂/I₃^- redox potential affected by different contiguous electrode The more negative the I₂/I₃^- redox potential is, the smaller the energy gap between the conduction band of TiO₂ and I₂/I₃^- redox potential is. The smaller energy gap resulted in the smaller V_oc compared that of ITO electrode, the I₂/I₃^- redox potential affected by the polyaniline film in reduced state was more negative than that in emeraldine, so the V_oc was smaller than that in emeraldine.