Kinetic Studies of Geminate Polaron Pair Recombination, Dissociation, and Efficient Triplet Exciton Formation in PC:PCBM Organic Photovoltaic Blends

The recombination and dissociation of geminate polaron pairs (GPPs) in bulk-heterojunction blends of the dye molecule 1,4,8,11,15,18,22,25-octabutoxy-29H,31H-phthalocyanine (PC) with the electron acceptor [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is studied using femtosecond transient absorption spectroscopy. The kinetics of the GPP state is dominated by efficient recombination to the triplet state of the PC, which competes with GPP dissociation and acts to fundamentally reduce the total number of free polarons generated in the blend. Both recombination and dissociation are strongly dependent on the blend morphology, with increased free polaron yields obtained by creating percolation pathways for polarons to move away from interfacial regions. Under such conditions, however, rapid exciton decay within PC domains occurs before the onset of electron transfer and acts to further reduce the free polaron yield. The consequences of these factors on the operation of solar cells using ternary blends for near-infrared sensitization are discussed.