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 (Q) as a function of photochemical damage in a model organic photovoltaic system poly-[[4,8-bis[(2-ethylhexl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diy1]- [3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno(3,4-b]thiophenediyl]] (PTB7) and 3'H-cyclopropa[8,25][5,6]fullerene-C71-D5h(6)-3'-butanoic acid, 3'-phenyl-, methyl ester (Pc71BM). We correlate local Q factor imaging with macroscopic device performance and show that, for this system, changes in cantilever Q 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 Q imaging is affected by the choice of cantilever resonance frequency. Finally, we use Q 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 Q images.