Analysis of light harvest in terms of current per mole of dye in dye-sensitized solar cells made with opaque and transparent photoanodes

摘要

Light harvest and Ⅰ-Ⅴ characteristics of dye-sensitized solar cells (DSSCs) made from transparent TiO_2 photoanodes of different thicknesses with and without light scattering layer (LSL) have been studied. The results obtained for transparent TiO_2 films, composed of smaller aggregates, with and without LSL were compared with that of opaque films, which were composed of larger aggregates. The light absorption efficiency of the photoanodes has been expressed in terms of current per mole of the dye. Although short circuit photocurrent density (J_(sc)) increased with thickness/dye loading for cells made with transparent single (～4 μm), double (~7 μm) and triple layer (10 μm) films, current per mole of dye for these films decreased. This was ascribed to presence of a fraction of dye molecules, which does not undergo photoexcitation and thus, does not contribute to current. And, this fraction of dye molecules increases with thickness/dye loading. Current per mole of dye was determined to be highest for transparent single layer film with LSL and hence, it can be considered as the most efficient light harvest. Open circuit voltage and dark current in the cells remained similar while J_(sc) changed with enhancement in light harvest. LSL on the opaque TiO_2 film did not improve light harvest and J_(sc) in the cell while transparent single layer (4 μm) and double layer (～ 7 μm) films with LSL exhibited enhancement in J_(sc) by about 20% and 24% respectively. Moreover, opaque TiO_2 film (～ 10 μm) having slightly more amount of dye than the TiO_2 transparent single layer film (～4 μm) showed lower current per mole of dye than transparent single layer film with and without LSL This was because of larger aggregates present in the opaque TiO_2 film, which scattered the light within the first few layers of the film such that little light reached the TiO_2 layers away from the substrate and thus, a large fraction of dye molecules in these distant layers remained in ground state.