Effects of bulk heterojunction nanostructure on solar cell performance

Optimized bulk heterojunction solar cells have power conversion efficiencies of 5-6%. Pushing these efficiencies higher requires detailed analysis of the losses in these devices. OPVs generate power through three major processes: exciton generation (absorption), exciton harvesting (the process of excitons migrating to the heterojunction and being split into their constituent charges at the interface), and charge transport. Most analyses assume the overall exciton harvesting efficiency to be very close to 100%. We show that this is not the case in the highly optimized and well studied poly-3-hexylthiophene:[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) solar cell. Specifically we show that only ∼40% of excitons formed in the PCBM phase contribute to the photocurrent. Fixing this problem would result in a 7-8% increase in the photocurrent, suggesting that the efficiency of world record P3HT:PCBM cells could be boosted from 5% to 5.4%. This result has implications for most state of the art organic solar cells, since all of the most efficient devices use fullerenes as electron acceptors. Intercalation of fullerene derivates between the polymer side chains, a phenomena recently shown to occur in some polymer fullerene:blends, is also likely to affect solar cell performance. We demonstrate that intercalation of fullerene derivatives between the side chains of conjugated polymers can be controlled by adjusting the fullerene size and compare the properties of intercalated and nonintercalated poly(2,5-bis(3-hexadecylthiophen-2-yl)thieno[3,2-b]thiophene (pBTTT):fullerene blends. The intercalated blends, which exhibit optimal solar-cell performance at 1:4 polymer:fullerene by weight, have better photoluminescence quenching and lower absorption than the nonintercalated blends, which optimize at 1:1. Understanding how intercalation affects performance will enable more effective design of polymer:fullerene solar cells.