Scanning Probe IR Microscopy and Soft X-Ray Spectroscopy Reveal Local Composition and Molecular Orientation in Solution Processed Films for Organic Photovoltaics

Solution-processed organic solar cells (OSC), a low-cost renewable energy technology, have shown record power conversion efficiencies approaching 20%. The recent leap in performance of OSC can be ascribed to the development of new electron acceptors, such as the Y-series of small molecules, that efficiently contribute to photogeneration of charges in the visible range. These molecules provide, however, new challenges for film processing due to their limited solubility and tendency to aggregate. The presentation will cover two examples where morphological analysis by means of advanced characterization techniques have provided in new insights in the molecular organization in spin-coated films. First, we present a study of the influence of the processing solvent on the orientation and packing of Y-series acceptor molecules in pure acceptor films by angle-resolved Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy. Secondly, we demonstrate the use of AFM-IR spectromicroscopy to map the nano-scale composition of spin-coated donor-acceptor bulk heterojunction films. AFM-IR spectromicroscopy combines the high resolution of scanning probe microscopy with the precise chemical fingerprint of infrared spectroscopy. Using a pulsed, tuneable MIRcat laser, resonant enhanced tapping mode AFM-IR yields compositional maps of donor and acceptor molecules with low beam damage. This chemical mapping yields new insights in the nanostructure formation in molecular semiconductor films for emerging photovoltaic materials.