Imaging Local Trap Formation in Conjugated Polymer Solar Cells: A Comparison of Time-Resolved Electrostatic Force Microscopy and Scanning Kelvin Probe Imaging

We study local photooxidation and trap formation in all-polymer bulk-heterojunction organic photovoltaics (OPVs) using both time-resolved electrostatic force microscopy (trEFM) and conventional scanning Kelvin probe microscopy (SKPM). We create electron-trapping defects at known locations by locally photooxidizing blends of poly[(9,9'-dioctylfluorene-alt-(bis(N,N'-(4-butylphenyl))-bis(N,N'-phenyl-1,4-phenylenediamine)] and poly[9,9'-dioctylfluorene-alt-1,4-benzothiadiazole]. We then compare the local surface photovoltage shifts measured via SKPM and the changes in local photoinduced charging rates measured via trEFM with changes in the performance of macroscopic photodiodes that have been exposed to similar photooxidation. We find that the trEFM charging rate images can identify local photooxidation and trap formation with much better sensitivity than conventional SKPM images. In addition, the changes in the trEFM charging rates correlate well with the external quantum efficiencies of the macroscopic photodiodes. In contrast, the SKPM images not only are less sensitive to trap formation but also show a more complicated response. We conclude that trEFM is well suited to studying local trap formation in organic solar cells and caution that SKPM data by itself can be difficult to interpret on OPV films, especially when materials have been exposed to photooxidation.