1,8-octanedithiol as a processing additive for bulk heterojunction materials: Enhanced photoconductive response

We present measurements of steady state and transient $(\tau <70\mathrm{ps})$ photoconductivity of the low bandgap polymer poly[2,6-(4,4-bis-(2-ethylhexyl)-$4H$ -cyclopenta[2,1-b;3,4-${\mathrm{b}}^{\prime }$ ]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] mixed with [6,6]-phenyl ${\mathrm{C}}_{71}$ -butyric acid methyl ester films processed with and without the 1,8-octanedithiol additive. The data demonstrate enhanced photoconductive response in the bulk heterojunction material processed with the additive. The increase results primarily from enhanced carrier mobility and to a lesser extent from a more efficient photogeneration of mobile carriers and longer mobile carrier lifetimes. The enhanced photoresponse is consistent with the increase in power conversion efficiency obtained from solar cells in which the bulk heterojunction film is processed using 1,8-octanedithiol.