Efficient charge generation and collection in amorphous polymer-based solar cells

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-C ₆₁-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 η_CD 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 collection efficiency η_CC is as high as >0.9 in the thin device but decreases to <0.8 in the thick device, suggesting that bimolecular recombination loss is not negligible in the thick device. Furthermore, the origin of the efficient device performance is discussed in terms of the field dependence and the charge delocalization.