Transient photoconductivity in polymer bulk heterojunction solar cells: Competition between sweep-out and recombination

Transient photoconductivity measurements carried out on bulk heterojunction (BHJ) solar cells demonstrate the competition between carrier sweep-out by the internal field and the loss of photogenerated carriers by recombination. The transient photoconductance data imply the existence of a well-defined internal field; carrier sweep-out is proportional to the magnitude of the internal field and limited by the carrier mobility. At external voltages near open circuit where the internal field approaches zero, the photocurrent decays because of the recombination of photogenerated mobile carriers. Mobility and recombination lifetimes are evaluated for carriers in poly[3-hexylthiophene] (P3HT): [6,6]-phenyl-C${}_{61}$-butyric acid methyl ester (PC${}_{60}$BM) and poly[$N$-9\ensuremath{'}\ensuremath{'}-hepta-decanyl-2,7-carbazole-alt-5,5-(4\ensuremath{'},7\ensuremath{'}-di-2-thienyl-2\ensuremath{'},1\ensuremath{'},3\ensuremath{'}-benzothiadiazole)] (PCDTBT): [6,6]-phenyl-C${}_{71}$-butyric acid methyl ester (PC${}_{71}$BM) solar cells.