Cantilever Ringdown Dissipation Imaging for the Study of Loss Processes in Polymer/Fullerene Solar Cells

We use dissipation imaging to probe local changes in electronic properties of nanostructured semiconductor films due to local photochemistry. We make quantitative maps of electrostatic dissipation due to photogenerated carriers by measuring the ringdown time of an oscillating atomic force microscope cantilever. Using organic photovoltaic materials as a testbed, we study macroscopic device degradation as a function of photooxidation for three different film morphologies comprising the conjugated polymer poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and the fullerene derivative [6,6]-phenyl-C-71 butyric acid methyl ester (PC71BM). We find that, judged by device performance, the stability of the macroscopic devices is sensitive to processing conditions, with films processed with the solvent additive 1,8-diiodooctane being the most stable. At the microscopic level, we compare the evolution of cantilever power dissipation as a function of photochemical degradation for three different polymer/fullerene blend morphologies and show that the changes in local power dissipation correlate with device stability. Using ringdown imaging to look at local dissipation in a highly phase-separated PTB7:PC71BM film morphology, we show that cantilever power dissipation increases more rapidly over large fullerene aggregates than in well-mixed polymer/fullerene regions, suggesting that local photochemistry on the fullerene contributes strongly to the dissipation signal.