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

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

Domed Silica Microcylinders Coated with Oleophilic Polypeptides and Their Behavior in Lyotropic Cholesteric Liquid Crystals of the Same Polypeptide

Liquid crystals can organize dispersed particles into useful and exotic structures. In the case of lyotropic cholesteric polypeptide liquid crystals, polypeptide-coated particles are appealing because the surface chemistry matches that of the polymeric mesogen, which permits a tighter focus on factors such as extended particle shape. The colloidal particles developed here consist of a magnetic and fluorescent cylindrically symmetric silica core with one rounded, almost hemispherical end. Functionalized with helical poly­(γ-stearyl-l-glutamate) (PSLG), the particles were dispersed at different concentrations in cholesteric liquid crystals (ChLC) of the same polymer in tetrahydrofuran (THF). Defects introduced by the particles to the director field of the bulk PSLG/THF host led to a variety of phases. In fresh mixtures, the cholesteric mesophase of the PSLG matrix was distorted, as reflected in the absence of the characteristic fingerprint pattern. Over time, the fingerprint pattern returned, more quickly when the concentration of the PSLG-coated particles was low. At low particle concentration the particles were “guided” by the PSLG liquid crystal to organize into patterns similar to that of the re-formed bulk chiral nematic phase. When their concentration increased, the well-dispersed PSLG-coated particles seemed to map onto the distortions in the bulk host’s local director field. The particles located near the glass vial–ChLC interfaces were stacked lengthwise into architectures with apparent two-dimensional hexagonal symmetry. The size of these “crystalline” structures increased with particle concentration. They displayed remarkable stability toward an external magnetic field; hydrophobic interactions between the PSLG polymers in the shell and those in the bulk LC matrix may be responsible. The results show that bio-inspired LCs can assemble suitable colloidal particles into soft crystalline structures

Polypeptide Composite Particle-Assisted Organization of π‑Conjugated Polymers into Highly Crystalline “Coffee Stains”

We demonstrate that homopolypeptides covalently tethered to anisotropically shaped silica particles induce crystalline ordering of representative semiconducting polymers. Films drop-cast from chloroform dispersions of poly­(γ-stearyl-l-glutamate) (PSLG) composite particles and poly­(3-hexythiophene) (P3HT) led to highly ordered crystalline structures of P3HT. Hydrophobic–hydrophobic interactions between the alkyl side chains of P3HT and PSLG were the main driving force for P3HT chain ordering into the crystalline assemblies. It was found that the orientation of rigid P3HT fibrils on the substrate adopted the directionality of the evaporating front. Regardless of the PSLG-coated particle dimensions used, the drop-cast films displayed patterns that were shaped by the coffee ring and Marangoni effects. PSLG-coated particles of high axial ratio (4.2) were more efficient in enhancing the electronic performance of P3HT than low axial ratio (2.6) homologues. Devices fabricated from the ordered assemblies displayed improved charge-carrier transport performance when compared to devices fabricated from P3HT alone. These results suggest that PSLG can favorably mediate the organization of semiconducting polymers

All-Small-Molecule Nonfullerene Organic Solar Cells with High Fill Factor and High Efficiency over 10%

We synthesized two wide bandgap A–D–A structured <i>p</i>-type organic semiconductor (<i>p</i>-OS) small molecules with weak electron-withdrawing ester end groups: <b>SM1</b> with cyano group (CN) on the ester group and <b>SM2</b> without the CN group. <b>SM1</b> showed stronger absorption, lower-lying HOMO energy level, and higher hole mobility in comparison with that of <b>SM2</b> without the CN groups. The all-small-molecule organic solar cell (SM-OSC) with <b>SM1</b> as donor and a narrow bandgap <i>n</i>-OS IDIC as acceptor demonstrated a high power conversion efficiency (PCE) of 10.11% and a high fill factor (FF) of 73.55%, while the PCE of the device based on <b>SM2</b>:IDIC is only 5.32% under the same device fabrication condition. The PCE of 10.11% and FF of 73.55% for the <b>SM1</b>-based device are the highest values for the nonfullerene SM-OSCs reported in the literature so far. The results indicate that the cyano substitution in <b>SM1</b> plays an important role in improving the photovoltaic performance of the <i>p</i>-OS donors in the nonfullerene SM-OSC. In addition, the photoinduced force microscopy (PiFM) was first used in OSCs to characterize the morphology of its donor/acceptor blend active layer