Investigation of the Effects of Li+-doping on the Photovoltaic Performance of SnO2-based Quasi-Solid-State Dye-Sensitized Solar Cells

SnO2 has high electron mobility and low photocatalytic action compared to TiO2. However, SnO2-based DSCs have shown limited performance due to their high electron recombination. Recombination can be suppressed to a certain extent using Li+ as an electron screening layer on the SnO2 surface. Li+ ions are adsorbed on to the SnO2 surface as the surface is negatively charged by O2- sites and –OH groups. This will shift the conduction band edge of SnO2 towards more positive direction thus increasing the electron harvesting efficiency of the device. Also, recombination occurring at the FTO/electrolyte interface can be reduced using tert-butylpyridine (TBP) as a gel electrolyte-additive as it covers the uncovered FTO surface. The gel electrolyte showed 4.7 x 10-3 S cm-1 of ionic conductivity and 4.91 x 10-6 cm2 s-1 of diffusivity for triiodide ions. These values agreed with the reported values in literature. In this study, the DSCs were fabricated with the device structure of FTO/Li+-doped SnO2/D358 dye/gel electrolyte/lightly platinized FTO counter electrode. The device containing 1:0.06 molar ratio of SnO2 to Li+ gave an efficiency of 4.3% while undoped device gave 1.6%. Li+-doped system showed 30% increase in current density and 61% increase in VOC.