The main objective of this study is to show the effect of TiO 2 nanotube length, diameter and intertubular lateral spacings on the performance of back illuminated dye sensitized solar cells (DSSCs). The present study shows that processing short TiO 2 nanotubes with good lateral spacings could significantly improve the performance of back illuminated DSSCs. Vertically aligned, uniform sized diameter TiO 2 nanotube arrays of different tube lengths have been fabricated on Ti plates by a controlled anodization technique at different times of 24, 36, 48 and 72 h using ethylene glycol and ammonium fluoride as an electrolyte medium. Scanning electron microscopy (SEM) showed formation of nanotube arrays spread uniformly over a large area. X-ray diffraction (XRD) of TiO 2 nanotube layer revealed the presence of crystalline anatase phases. By employing the TiO 2 nanotube array anodized at 24 h showing a diameter ∼80 nm and length ∼1•5 μm as the photo-anode for back illuminated DSSCs, a full-sun conversion efficiency (η) of 3•5 % was achieved, the highest value reported for this length of nanotubes.

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In contrast to the conventional DSSC systems, where the dye molecules are used as light harvesting material, here a solid-state absorber was used as a sensitizer in conjunction with the dye. The materials like ZnO and Al 2 O 3 : C, which will show optically stimulated luminescence (OSL) upon irradiation were used as extremely thin absorber layers. This novel architecture allows broader spectral absorption, an increase in photocurrent, and hence, an improved efficiency because of the mobility of the trapped electrons in the absorber material after irradiation, to the TiO 2 conduction band. Nanocrystalline mesoporous TiO 2 photoanodes were fabricated using these solid-state absorber materials and after irradiation, a few number of samples were co-sensitized with N719 dye. On comparing both the dye loaded photoanodes (ZnO/TiO 2 and Al 2 O 3 : C/TiO 2 ), it can be concluded from the present studies that, the Al 2 O 3 : C is superior to ZnO under photon irradiation. Al 2 O 3 : C is more sensitive to photon irradiation than ZnO and hence there can be more trap centres produced in Al 2 O 3 : C.

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