Imaging the Evolution of Nanoscale Photocurrent Collection and Transport Networks during Annealing of Polythiophene/Fullerene Solar Cells

We use photoconductive atomic force microscopy to image nanoscale spatial variations in photocurrent across the surfaces of photovoltaic cells made from blends of the conjugated polymer regioregular poly(3-hexylthiopene) (P3HT) with phenyl-C-61-butyric acid methyl ester (PCBM). We study how the spatial variations In photocurrent evolve with thermal annealing, and we correlate these changes with the evolution of macroscopic film and device properties such as external quantum efficiency and carrier mobility. We use conductive atomic force microscopy to examine the development of injection and transport networks for both electrons and holes as a function of annealing. We find that the hole transport, electron transport, and photocurrent collection networks become increasingly heterogeneous with thermal annealing and remain heterogeneous on the 10-100 nm length scale even in the most efficient P3HT/PCBM devices. After annealing, the regions of the greatest dark hole currents, greatest dark electron currents, and greatest photocurrents are each associated with different regions of the nanostructured films. These results suggest spatial heterogeneity can contribute to the imperfect internal quantum efficiency even in relatively efficient organic photovoltaics and that fully 3D modeling is needed to describe the devices physics of polymer blend solar cells.