Efficient Charge Generation and Collection in Amorphous Polymer-Based Solar Cells

Abstract

The charge generation and recombination dynamics in amorphous blend films of poly[2,7-(9,9-dioctylfluorene)-alt-5,5-(5′,8′-di-2-thienyl-2′,3′-diphenylquinoxaline)] (N-P7) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) were comprehensively studied in order to address the origin of the relatively high performance for amorphous polymer-based solar cells. Upon photoexcitation, N-P7 singlet excitons are promptly converted to the interfacial charge transfer (CT) state, which is a Coulombically bound pair of the N-P7 polaron and PCBM radical anion, with 100% efficiency in a picosecond. More than half of the N-P7 polarons in the CT state are dissociated into free carriers, and the rest of them recombine to the ground state in a nanosecond. The dissociation efficiency ηCD is estimated to be 0.65 under the open-circuit condition and is slightly enhanced up to 0.7–0.8 under the short-circuit condition. Such highly efficient dissociation is well explained by considering charge delocalization. The charge collec...