Relating recombination, density of states, and device performance in an effi cient polymer:fullerene organic solar cell blend

We explore the interrelation between density of states, recombination kinetics, and device performance in effi cient 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 effi ciency (50-67 wt% PC 71 BM). Using transient and steady-state techniques, we fi nd that nongeminate recombination limits the device effi-ciency 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 signifi cant change in the magnitude or energetic separation of the density of localized states. Our comprehensive approach to understanding device performance represents signifi cant progress in understanding what limits these high-effi ciency polymer:fullerene systems.KGaA, Weinheim.