Nanoscale charge transport and internal structure of bulk heterojunction conjugated polymer/fullerene solar cells by scanning probe microscopy

Tapping mode atomic force microscopy (AFM) was used to probe the surface and internal structures of bulk heterojunction poly(3-hexylthiophene) and [6,6]-phenyl C61-butyric acid methyl ester (P3HT:PCBM) films. AFM images obtained from the surface of the P3HT:PCBM film and from the cross section reveal for the first time the nanoscale three-dimensional P3HT and PCBM networks. Two different measurement modes of conducting atomic force microscopy (C-AFM) were used to study the nanoscale charge transport in P3HT: PCBM films. In C-AFM, the local current map at a constant bias and the surface topography are obtained simultaneously. Hole and electron current images can be used to determine the composition of the phase segregation domains. Alternatively, local current-voltage curves can be measured that can be used to calculate charge carrier mobilities. Hole current images and mobilities were obtained using Pt-coated silicon probes, whereas Mg-coated silicon probes were used to measure nanoscale electron mobilities. Variations in electrical properties and surface topography were examined. Comparison against films of neat P3HT shows that blending P3HT with PCBM decreases the polymer hole mobility. In all cases, nanoscale hole and electron mobilities increase upon annealing, consistent With increased internal order. A direct correlation between the C-AFM-determined nanoscale properties and solar cell power conversion efficiencies is observed.