The effect of boric acid concentration on the TiO2 compact layer by liquid-phase deposition for dye-sensitized solar cell

Abstract In this study, we prepared titanium oxide (TiO2) compact-layer thin films (thickness: 50 nm) on an indium tin oxide (ITO)/glass substrate by liquid-phase deposition (LPD) for the dye-sensitized solar cell (DSSC). We used a deposition solution of ammonium hexafluoro-titanate and boric acid (H3BO3) for the TiO2 film. We varied the concentration of H3BO3 from 0.4 to 0.7 M to control the deposition rate, surface roughness, and refractive index of the TiO2 film. Based on the results of X-ray diffraction and transmission electron microscopy, the TiO2 films all possessed the polycrystalline feature with (101), (004), (200), (105), and (211) planes. After the LPD deposition, we can observe that the formation of the transition-layer existed in the TiO2 film. The transition-layer is analyzed as In and Sn co-doped TiO2 film, which is caused by the diffusions of In3+ and Sn4+ ions from the ITO substrate. From the depth profiles of X-ray photoelectron spectroscopy, the thickness of the transition-layer increased with a decrease of the boric acid concentration. At the optimum H3BO3 concentration of 0.5 M, we obtained the following photovoltaic properties: a short circuit current density (Jsc) of 11.41 mA/cm2, an open circuit voltage (Voc) of 0.73 V, a fill factor (ff) of 0.63, and an efficiency (η) of 5.24%. The LPD-TiO2 compact-layer between the ITO and the DSSC photo-electrode can reduce the number of carrier recombinations and improve transmittance in the visible light region. The TiO2 compact-layer prevents electron transport to the electrolyte and results in a higher Jsc. Moreover, the LPD-TiO2 compact-layer can enhance interface adhesion and improve the series resistance.