Transient photocurrent measurements of PCDTBT:PC_(70)BM and PCPDTBT:PC_(70)BM Solar Cells: Evidence for charge trapping in efficient polymer/fullerene blends

摘要

We report measurements of the turn-on and turn-off photocurrent dynamics as a function of applied voltage for efficient polymer/fullerene bulk heterojunction solar cells composed of poly[N-9″-hepta-decanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole) (PCDTBT): [6,6]-phenyl C_(71)-butyric acid methyl ester (PC_(70)BM) and poly[2,6-(4,4-bis-(2-ethylhexyl)-4H/- cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDTBT):PC_(70)BM blends. In particular we present evidence for charge trapping that facilitates recombination in these systems. For the PCDTBT:PC_(70)BM system, an initial transient photocurrent peak 5-10 μs after turn-on is observed for operating voltages between 0.5 V and open-circuit. Furthermore, a long photocurrent tail is observed in the decay dynamics of PCDTBT:PC_(70)BM devices with charge still being extracted hundreds of microseconds after turn-off. These features in the PCDTBT:PC_(70)BM device are attributed to trapping and detrapping of charge on the microsecond time scale, with charge trapping facilitating recombination either through trap-assisted recombination or space-charge effects. For the PCPDTBT:PC_(70)BM system, evidence for charge trapping is also observed albeit on a faster time scale. No initial transient photocurrent peak is observed, however the faster PCPDTBT:PC_(70)BM decay dynamics show only a weak voltage dependence consistent with rapid trapping and recombination of charge. For both systems the amount of extracted charge as a function of applied voltage follows a similar form to the measured current-voltage curves providing evidence that photocurrent is hampered by the extraction, and not just the separation, of charge in these systems. The origin of charge trapping and the nature of recombination is discussed, along with the influence of additives on charge transport in the PCPDTBT:PC_(70)BM system.