Mapping Nanoscale Variations in Photochemical Damage of Polymer/Fullerene Solar Cells with Dissipation Imaging

We use frequency-modulated electrostatic force microscopy to track changes in cantilever quality factor (<i>Q</i>) as a function of photochemical damage in a model organic photovoltaic system poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-<i>b</i>:4,5-<i>b</i>′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-<i>b</i>]thiophenediyl]] (PTB7) and 3′<i>H</i>-cyclopropa[8,25][5,6]fullerene-C71-D5h(6)-3′-butanoic acid, 3′-phenyl-, methyl ester (PC₇₁BM). We correlate local <i>Q</i> factor imaging with macroscopic device performance and show that, for this system, changes in cantilever <i>Q</i> correlate well with changes in external quantum efficiency and can thus be used to monitor local photochemical damage over the entire functional lifetime of a PTB7:PC₇₁BM solar cell. We explore how <i>Q</i> imaging is affected by the choice of cantilever resonance frequency. Finally, we use <i>Q</i> imaging to elucidate the differences in the evolution of nanoscale structure in the photochemical damage occurring in PTB7:PC₇₁BM solar cells processed with and without the solvent additive 1,8-diiodooctane (DIO). We show that processing with DIO not only yields a preferable morphology for uniform performance across the surface of the device but also enhances the stability of PTB7:PC₇₁BM solar cellsan effect that can be predicted based on the local <i>Q</i> images