The photoactive layer of an organic solar cell is a solution-processed thin film consisting of a blend of electron-donor and electron-acceptor molecules, sandwiched between two electrodes. The high absorbance and complementary absorption spectra of donor and acceptor molecules, together with the nano-scale morphology of this bulk heterojunction, are key factors for the high performance organic solar cells, for which record power conversion efficiencies of over 19% have been achieved.

Scanning probe and electron microscopies are common characterisation techniques to image the nanostructure of the blend film, but they lack the chemical contrast to be able to provide a compositional map of the donor/acceptor blend film. Scanning X-ray Transmission Microscopy (STXM) is a valuable synchrotron-based technique to image the donor-acceptor blend films with resolution below 50 nm. However, STXM imaging requires a distinct X-ray absorption resonance for donor and acceptor molecules, to be able to chemically distinguish donor and acceptor molecules in the bulk heterojunction films. This has proven to be challenging for the new non-fullerene acceptors.

In this presentation, we will present the use of AFM-IR microscopy to map the nano-scale composition with low beam damage. The technique combines the high resolution of scanning probe microscopy with the chemical fingerprint of infrared spectroscopy. Using a pulsed, tuneable MIRcat laser, covering a total range of 1000 cm-1, resonant enhanced tapping mode AFM-IR yields compositional maps of donor and acceptor molecules based on their specific IR fingerprints. We demonstrate the presence of a PC70BM network with characteristic length of 20 nm inside micrometer sized phase-separated domains of a donor:fullerene blend, as well as the fine fibrils in the acceptor-rich phase of an all-polymer blend. The chemical mapping by AFM-IR yields new insights in the nanostructure formation in mixed molecular semiconductor films. The results demonstrate the potential of AFM-IR as a high-resolution and non-destructive chemical analysis technique for emerging photovoltaic materials.