Photo-doping is a process to optimize solid-state dye-sensitized solar cells based on 2,2'7,7'-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (spiro-OMeTAD). We investigate the effect of photo-doping on performance for solid-state DSSC based on spiro-OMeTAD and carbon counter electrode. The efficiency is improved to 4.09% by optimizing doping level under standard solar conditions (AM1.5G 100 mW cm(-2)). Results indicate that photo-doping has significant influence on charge transferring properties at TiO2/spiro-OMeTAD and spiro-OMeTAD/carbon interface, which is consistent with the change of photovoltaic performance. (C) 2013 Elsevier Ltd. All rights reserved.