Dye-sensitized solar cells (DSC) could become low cost alternatives to inorganic photovoltaic devices. The DSC studied here use the hole transporting material 2,2'7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-MeOTAD). The effect of t-butyl pyridine (t-BP) on the current-voltage characteristics of thin layer cells were investigated using complete cells and cells consisting only of the TiO2 blocking layer. In addition, absorption spectroscopy was used to study the effect t-BP has on the doping density of spiro-MeOTAD. The results are correlated with device performance. (c) 2006 Elsevier B.V. All rights reserved.