Lipid composition dictates membrane thickness, which in turn can influence membrane protein activity. Lipid composition also determines whether a membrane demixes into coexisting liquid-crystalline phases. Previous direct measurements of demixed lipid membranes have always found a liquid-ordered phase that is thicker than the liquid-disordered phase. Here we investigated non-canonical ternary lipid mixtures designed to produce bilayers with thicker disordered phases than ordered phases. The membranes were comprised of short, saturated (ordered) lipids; long, unsaturated (disordered) lipids; and cholesterol. We found that few of these systems yield coexisting liquid phases above 10 °C. For membranes that do demix into two liquid phases, we measured the thickness mismatch between the phases by atomic force microscopy and found that not one of the systems yields thicker disordered than ordered phases under standard experimental conditions. We found no monotonic relationship between demixing temperatures of these ternary systems and either estimated thickness mismatches between the liquid phases or the physical parameters of single-component membranes comprised of the individual lipids. These results highlight the robustness of a membrane’s liquid-ordered phase to be thicker than the liquid-disordered phase, regardless of the membrane’s lipid composition.

From hybrid perovskites to semiconducting polymer/fullerene blends for organic photovoltaics, many new materials being explored for energy harvesting and storage exhibit performance characteristics that depend sensitively on their nanoscale morphology. At the same time, rapid advances in the capability and accessibility of scanning probe microscopy methods over the past decade have made it possible to study processing/structure/function relationships ranging from photocurrent collection to photocarrier lifetimes with resolutions on the scale of tens of nanometers or better. Importantly, such scanning probe methods offer the potential to combine measurements of local structure with local function, and they can be implemented to study materials in situ or devices in operando to better understand how materials evolve in time in response to an external stimulus or environmental perturbation. This Account highlights recent advances in the development and application of scanning probe microscopy methods that can help address such questions while filling key gaps between the capabilities of conventional electron microscopy and newer super-resolution optical methods. Focusing on semiconductor materials for solar energy applications, we highlight a range of electrical and optoelectronic scanning probe microscopy methods that exploit the local dynamics of an atomic force microscope tip to probe key properties of the solar cell material or device structure. We discuss how it is possible to extract relevant device properties using noncontact scanning probe methods as well as how these properties guide materials development. Specifically, we discuss intensity-modulated scanning Kelvin probe microscopy (IM-SKPM), time-resolved electrostatic force microscopy (trEFM), frequency-modulated electrostatic force microscopy (FM-EFM), and cantilever ringdown imaging. We explain these developments in the context of classic atomic force microscopy (AFM) methods that exploit the physics of cantilever motion and photocarrier generation to provide robust, nanoscale measurements of materials physics that are correlated with device operation. We predict that the multidimensional data sets made possible by these types of methods will become increasingly important as advances in data science expand capabilities and opportunities for image correlation and discovery.

We use frequency-modulated electrostatic force microscopy to track changes in cantilever quality factor (Q) as a function of photochemical damage in a model organic photovoltaic system poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b']dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7) and 3'H-cyclopropa[8,25][5,6]fullerene-C71-D5h(6)-3'-butanoic acid, 3'-phenyl-, methyl ester (PC71BM). We correlate local Q factor imaging with macroscopic device performance and show that, for this system, changes in cantilever Q correlate well with changes in external quantum efficiency and can thus be used to monitor local photochemical damage over the entire functional lifetime of a PTB7:PC71BM solar cell. We explore how Q imaging is affected by the choice of cantilever resonance frequency. Finally, we use Q imaging to elucidate the differences in the evolution of nanoscale structure in the photochemical damage occurring in PTB7:PC71BM solar cells processed with and without the solvent additive 1,8-diiodooctane (DIO). We show that processing with DIO not only yields a preferable morphology for uniform performance across the surface of the device but also enhances the stability of PTB7:PC71BM solar cells-an effect that can be predicted based on the local Q images.

We demonstrate nanoscale imaging of charge transfer state photoexcitations in polymer/fullerene bulk heterojunction solar cells using time-resolved electrostatic force microscopy (trEFM). We compare local trEFM charging rates and external quantum efficiencies (EQE) for both above-gap and below-gap excitation of the model system poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). We show that the local trEFM charging rate correlates with device EQE for both above-gap and below-gap photoexcitation, demonstrating that EFM methods have sufficient sensitivity to detect the low EQEs associated with CT state formation, a result that could be useful for probing weak subgap excitations in nanostructured materials such as quantum dot and organometal halide perovskite solar cells. Further, we use trEFM to map spatial variations in EQE arising from subgap CT excitation in organic photovoltaics (OPVs) and find that the local distribution of photocurrent arising from these states is nearly identical to the spatial variation in EQE from above-gap singlet excitation. These results are consistent with recent work showing that both above-gap and below-gap excitation have similar internal quantum efficiency.

Micron-scale coexisting Lo and Ld liquid phases can appear in lipid bilayers composed of a ternary mixture of a low-melting temperature lipid, a high-melting temperature lipid, and cholesterol. A priori, temperatures at which membranes demix, Tmix, are not simply related to differences in thicknesses, Δh, between Lo and Ld phases. Here, we use fluorescence microscopy to measure Tmix and we use atomic force microscopy at 22°C to measure Δh for a series of bilayers composed of different ratios of the three components. Our data illustrate cases in which a change in Tmix or Δh does not result in a change in the other parameter. The data provide a context in which to evaluate recent reports of a correlation between Tmix and Δh.