Charge transport in intermixed regions of all-polymer solar cells based on a blend of poly(3-hexylthiophene) (P3HT; electron donor) with poly[2,7-(9,9-didodecylfluorene)-alt-5,5-(4',7'-bis(2-thienyl)-2',1',3'-benzothiadiazole)] (PF12TBT; electron acceptor) was studied by conductive atomic force microscopy (C-AFM). For a blend film fabricated from a chlorobenzene solution, intermixed regions were detected between the P3HT-rich and PF12TBT-rich domains. The overall hole current in the intermixed regions remained almost constant, both before and after thermal annealing at 80 °C, but it increased in the P3HT-rich domains. For a blend film fabricated from a chloroform solution, the entire observed area constituted an intermixed region, both before and after thermal annealing. The overall hole current in this film was significantly improved following thermal annealing at 120 °C. These finely mixed structures with efficient charge transport yielded a maximum power conversion efficiency of 3.5%. The local charge-transport properties in the intermixed region, as observed via C-AFM, was found to be closely related to the photovoltaic properties, rather than the bulk-averaged properties or topological features.

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The charge-transport characteristics of phase-separated blend films of poly(3-hexylthiophene) (P3HT; electron donor) and poly[2,7-(9,9-didodecylfluorene)-alt-5,5-(4′,7′-bis(2-thienyl)-2′,1′,3′-benzothiadiazole)] (PF12TBT; electron acceptor) were visualized by conductive atomic force microscopy (C-AFM). The C-AFM hole-current images clearly showed two phasesan electrically conductive region assigned to the P3HT-rich donor domain and a non-conductive region assigned to the PF12TBT-rich acceptor domain. The hole current in the conductive region was small compared with that of a neat P3HT film with similar thickness, indicating that the P3HT-rich domain contained a large fraction of PF12TBT as a minor component. Thermal annealing initially increased the hole current throughout the P3HT-rich domain because of reorganization of the P3HT chains from their as-cast configurations. Further annealing increased the hole current mainly in the middle of the P3HT-rich domain, but it decreased the hole current in the boundary areas close to the PF12TBT-rich domain owing to the presence of an intermixed region with a gradient of the P3HT/PF12TBT composition ratio. After annealing at temperatures above the glass-transition point of PF12TBT, the widths of the intermixed regions decreased to ~30 nm as phase separation proceeded with decomposition of the intermixed region. Such variations in the intermixed region, which were electrically resolved by the C-AFM, accounted for the temperature dependence of the photovoltaic properties of P3HT/PF12TBT blend solar cells.3

We have successfully measured electron transport nanostructures of conjugated polymer thinfilms by conductive atomic force microscopy, using an air-stable electron-injecting electrode coated with ethoxylated polyethyleneimine. Electron-and hole-transport networks in donor/acceptor polymer blends can be selectively observed by using an appropriately coated electrode. This approach enables us to visualize phase-separated nanostructures of donor/acceptor polymer blends for thin-film electronic devices based on their semiconducting properties.