Relating Recombination, Density of States, and Device Performance in an Efficient Polymer:Fullerene Organic Solar Cell Blend

摘要

We explore the interrelation between density of states, recombination kinetics, and device performance in efficient poly[4,8-bis-(2-ethylhexyloxy)-benzo[1,2-b:4,5-b']dithiophene-2,6-diyl-alt-4-(2-ethylhexyloxy-1-one) thieno[3,4-b]thiophene-2,6-diyl]:[6,6]-phenyl-C_(71)-butyric acid methyl ester (PBDTTT-C:PC_(71)BM) bulk-heterojunction organic solar cells. We modulate the active-layer density of states by varying the polymer:fullerene composition over a small range around the ratio that leads to the maximum solar cell efficiency (50-67 wt％ PC_(71)BM). Using transient and steady-state techniques, we find that nongeminate recombination limits the device efficiency and, moreover, that increasing the PC_(71)BM content simultaneously increases the carrier lifetime and drift mobility in contrast to the behavior expected for Langevin recombination. Changes in electronic properties with fullerene content are accompanied by a significant change in the magnitude or energetic separation of the density of localized states. Our comprehensive approach to understanding device performance represents significant progress in understanding what limits these high-efficiency polymer:fullerene systems.