Nanoscale Domain Imaging of All-Polymer Organic Solar Cells by Photo-Induced Force Microscopy

Abstract

Rapid nanoscale imaging of the bulk heterojunction layer in organic solar cells is essential to the continued development of high-performance devices. Unfortunately, commonly used imaging techniques such as tunneling electron microscopy (TEM) and atomic force microscopy (AFM) suffer from significant drawbacks. For instance, assuming domain identity from phase contrast or topographical features can lead to inaccurate morphological conclusions. Here we demonstrate a technique known as photo-induced force microscopy (PiFM) for imaging organic solar cell bulk heterojunctions with nanoscale chemical specificity. PiFM is a relatively recent scanning probe microscopy technique that combines an AFM tip with a tunable infrared laser to induce a dipole for chemical imaging. Coupling the nanometer resolution of AFM with the chemical specificity of a tuned IR laser, we are able to spatially map the donor and acceptor domains in a model all-polymer bulk heterojunction with resolution approaching 10 nm. Domain size from PiFM images is compared to bulk-averaged results from resonant soft X-ray scattering, indicating excellent quantitative agreement. Further, we demonstrate that in our all-polymer system, the AFM topography, AFM phase, and PiFM show poor correlation, highlighting the need to move beyond standard AFM for morphology characterization of bulk heterojunctions.

Understanding the Effect of the Third Component PC71BM on Nanoscale Morphology and Photovoltaic Properties of Ternary Organic Solar Cells

“…As discussed previously, accurate nanoscale imaging is of great importance to the research and development of organic electronic field, especially for OSCs . Given the truth that the different physical properties of polymer and small molecule, different molecular aggregation behavior of them could be expected.…”

Near-field nanoprobing using Si tip-Au nanoparticle photoinduced force microscopy with 120:1 signal-to-noise ratio, sub-6-nm resolution

“…Two photon-induced luminescence (TPL) microscopy has also been used to map the resonant plasmonic nanostructures; however, the spatial resolution for this system is lower as compared to the tip-based scanning probe microscopy techniques. 14,15 measurement of laterally induced forces at nanoscale, 26,27 mapping nanoscale refractive index contrast, 28 nanoscale imaging of block copolymers 29 and all-polymer organic solar cells 30 at IR frequencies, and near field mapping of plasmonic nanostructures 31 and bimetallic heterodimers. 32 The detection of the photoinduced force in the PiFM system is dependent on a special probe-tip that has sufficiently high electromagnetic response.…”

Infrared Spectroscopy and Imaging at Nanometer Scale

“…The dipole–dipole attractive force is transduced as a change in tapping signal of the cantilever (Adapted with permission from Ref. . Copyright 2018, American Chemical Society).…”

“…Therefore, it is free from far‐field scattering background encountered in IR‐sSNOM, which does not generate gradient force. As in the case of IR‐sSNOM, the technique can be combined with ultrafast laser sources to provide temporal resolutions, and the technique has been successfully applied to chemical imaging of block copolymers and bulk heterojunction polymers . Figure shows one such example.…”

“…As in the case of IR-sSNOM, the technique can be combined with ultrafast laser sources to provide temporal resolutions, 44 and the technique has been successfully applied to chemical imaging of block copolymers 42 and bulk heterojunction polymers. 39 Figure 6 shows one such example. The topography of polystyrene-block-poly(methyl methacrylate) (PSb-PMMA) sample surface ( Figure 6(c)) only partially reveal the polymer domains, whereas vibrationally resonant PiFM images (Figure 6(a) and (b)) at two different IR wavelengths most clearly distinguishes two polymer domains.…”