Towards a unified description of the charge transport mechanisms in conductive atomic force microscopy studies of semiconducting polymers

Correlation between morphology and local mechanical and electrical properties of van der Waals heterostructures

Properties of van der Waals (vdW) heterostructures strongly depend on the quality of the interface between two dimensional (2D) layers. Instead of having atomically flat, clean, and chemically inert interfaces without dangling bonds, top-down vdW heterostructures are associated with bubbles and intercalated layers (ILs) which trap contaminations appeared during fabrication process. We investigate their influence on local electrical and mechanical properties of MoS₂/WS₂heterostructures using atomic force microscopy (AFM) based methods. It is demonstrated that domains containing bubbles and ILs are locally softer, with increased friction and energy dissipation. Since they prevent sharp interfaces and efficient charge transfer between 2D layers, electrical current and contact potential difference are strongly decreased. In order to reestablish a close contact between MoS₂and WS₂layers, vdW heterostructures were locally flattened by scanning with AFM tip in contact mode or just locally pressed with an increased normal load. Subsequent electrical measurements reveal that the contact potential difference between two layers strongly increases due to enabled charge transfer, while localI/Vcurves exhibit increased conductivity without undesired potential barriers.

Morphological design strategies to tailor out-of-plane charge transport in conjugated polymer systems for device applications

The transport of charge carriers throughout an active conjugated polymer (CP) host, characterized by a heterogeneous morphology of locally varying degrees of order and disorder, profoundly influences the performance of CP-based electronic devices, including diodes, photovoltaics, sensors, and supercapacitors. Out-of-plane charge carrier mobilities (μ_out-of-plane) across the bulk of the active material host and in-plane mobilities (μ_in-plane) parallel to a substrate are highly sensitive to local morphological features along their migration pathways. In general, the magnitudes of μ_out-of-plane and μ_in-plane are very different, in part because these carriers experience different morphological environments along their migration pathways. Suppressing the impact of variations in the morphological order/disorder on carrier migration remains an important challenge. While much is known about μ_in-plane and its optimization for devices, the current challenges are associated with μ_out-of-plane and its optimization for device performance. Therefore, this review is devoted to strategies for improving μ_out-of-plane in neat CP films and the implications for more complex systems, such as D:A blends which are relevant to OPV devices. The specific strategies discussed for improving μ_out-of-plane include solvent/field processing methods, chemical modification, thickness confinement, chemical additives, and different post-annealing strategies, including annealing with supercritical fluids. This review leverages the most recent fundamental understanding of mechanisms of charge transport and connections to morphology, identifying robust design strategies for targeted improvements of μ_out-of-plane.

Conducting microhelices from self-assembly of protein fibrils

Herein we utilize insulin to prepare amyloid based chiral helices with either right or left handed helicity. We demonstrate that the helices can be utilized as structural templates for the conducting polymer alkoxysulfonate poly(ethylenedioxythiophene) (PEDOT-S). The chirality of the helical assembly is transferred to PEDOT-S as demonstrated by polarized optical microscopy (POM) and Circular Dichroism (CD). Analysis of the helices by conductive atomic force microscopy (c-AFM) shows significant conductivity. In addition, the morphology of the template structure is stabilized by PEDOT-S. These conductive helical structures represent promising candidates in our quest for THz resonators.

Anticorrelation between Local Photoluminescence and Photocurrent Suggests Variability in Contact to Active Layer in Perovskite Solar Cells

We use high-resolution, spatially resolved, laser beam induced current, confocal photoluminescence, and photoconductive atomic force microscopy (pcAFM) measurements to correlate local solar cell performance with spatially heterogeneous local material properties in methylammonium lead triiodide (CH₃NH₃PbI₃) perovskite solar cells. We find that, for this material and device architecture, the photocurrent heterogeneity measured via pcAFM on devices missing a top selective contact with traditional Au-coated tips is significantly larger than the photocurrent heterogeneity observed in full devices with both electron- and hole-selective extraction layers, indicating that extraction barriers at the Au/perovskite interface are ameliorated by deposition of the organic charge extraction layer. Nevertheless, in completed, efficient device structures (PCE ≈ 16%) with state-of-the-art nickel oxide and [6,6]-phenyl-C61-butyric acid (PCBM) methyl ester contacts, we observe that the local photoluminescence (PL) is weakly anticorrelated with local photocurrent at both short-circuit and open-circuit conditions. We determine that the contact materials are fairly homogeneous; thus the heterogeneity stems from the perovskite itself. We suggest a cause for the anticorrelation as being related to local carrier extraction heterogeneity. However, we find that the contacts are still the dominating source of losses in these devices, which minimizes the impact of the material heterogeneity on device performance at present. These results suggest that further steps to prevent recombination losses at the interfaces are needed to help perovskite-based cells approach theoretical efficiency limits; only at this point will material heterogeneity become crucial.

Organic Optoelectronic Materials: Mechanisms and Applications

Organic (opto)electronic materials have received considerable attention due to their applications in thin-film-transistors, light-emitting diodes, solar cells, sensors, photorefractive devices, and many others. The technological promises include low cost of these materials and the possibility of their room-temperature deposition from solution on large-area and/or flexible substrates. The article reviews the current understanding of the physical mechanisms that determine the (opto)electronic properties of high-performance organic materials. The focus of the review is on photoinduced processes and on electronic properties important for optoelectronic applications relying on charge carrier photogeneration. Additionally, it highlights the capabilities of various experimental techniques for characterization of these materials, summarizes top-of-the-line device performance, and outlines recent trends in the further development of the field. The properties of materials based both on small molecules and on conjugated polymers are considered, and their applications in organic solar cells, photodetectors, and photorefractive devices are discussed.

Insights into the nanoscale lateral and vertical phase separation in organic bulk heterojunctions via scanning probe microscopy

Solution processed polymer (donor) and fullerene (acceptor) bulk heterojunctions are widely used as the photo active layer in organic solar cells. Intimate mixing of these two materials is essential for efficient charge separation and transport. Identifying relative positions of acceptor and donor rich regions in the bulk heterojunction with nanometer scale precision is crucial in understanding intricate details of operation. In this work, a combination of Ar(+)2000 gas cluster ion beam and scanning probe microscopy is used to examine the lateral and vertical phase separation within regio-regular poly(3-hexylthiophene)(P3HT):phenyl-C60-butyric acid methyl ester (PCBM) bulk heterojunction. While the Ar(+)2000 gas cluster ion beam is used as a sputter tool to expose the underneath layers, scanning probe microscopy techniques are used to obtain two-dimensional (2D) electrical maps (with sub-2 nm lateral resolution). The electrical mapping is decoded to chemical composition, essentially producing lateral and vertical maps of phase separation. Thermal stress causes large PCBM-rich hillocks to form, and consequently affecting the balance of P3HT:PCBM heterojunctions, hence a negative impact on the efficiency of the solar cell. We further developed a method to analyze the efficiency of exciton dissociation based on the current maps and a loss of 20% in efficiency is observed for thermally degraded samples compared to fresh un-annealed samples.

Electron Transport Nanostructures of Conjugated Polymer Films Visualized by Conductive Atomic Force Microscopy

We have successfully measured electron transport nanostructures of conjugated polymer thin films by conductive atomic force microscopy, using an air-stable electron-injecting electrode coated with ethoxylated polyethylenimine. Electron- and hole-transport networks in donor/acceptor polymer blends can be selectively observed by using an appropriately coated electrode. This approach enables us to visualize phase-separated nanostructures of donor/acceptor polymer blends for thin-film electronic devices based on their semiconducting properties.

Conductance measurements of individual polypyrrole nanobelts

We present here a study on the electrical conduction properties of individual polypyrrole nanobelts by using conductive atomic force microscopy and discuss a general effect while probing soft materials. A length-dependent analysis demonstrates that the tip could induce local defects into the polymer structure and, thus diminishes the electrical conduction.

Photon assisted conducting atomic force microscopy study of nanostructured additives in P3HT:PCBM

P3HT:PCBM demonstrates an enhanced current with additives (Ag NPs and Gr ). Continuous conducting paths attribute the Gr additives with an order of higher magnitude than the Ag NPs. The results are consistent and were studied using photon assisted conducting microscopy.