Manipulating the Morphology of P3HT–PCBM Bulk Heterojunction Blends with Solvent Vapor Annealing

Using grazing incidence X-ray scattering, we observe the effects of solvent vapors upon the morphology of poly­(3-hexylthiophene)–phenyl-C₆₁-butyric acid methyl ester (P3HT–PCBM) bulk heterojunction thin film blends in real time; allowing us to observe morphological rearrangements that occur during this process as a function of solvent. We detail the swelling of the P3HT crystallites upon the introduction of solvent and the resulting changes in the P3HT crystallite morphology. We also demonstrate the ability for tetrahydrofuran vapor to induce crystallinity in PCBM domains. Additionally, we measure the nanoscale phase segregated domain size as a function of solvent vapor annealing and correlate this to the changes observed in the crystallite morphology of each component. Finally, we discuss the implications of the morphological changes induced by solvent vapor annealing on the device properties of BHJ solar cells

Effect of Backbone Regioregularity on the Structure and Orientation of a Donor–Acceptor Semiconducting Copolymer

A regioregular (RR) donor–acceptor conjugated copolymer based on cyclopenta­[2,1-<i>b</i>:3,4-<i>b</i>′]­dithiophene (CDT) and pyridal­[2,1,3]­thiadiazole (PT) structural units was prepared by using polymerization reactions involving reactants specifically designed to avoid random orientation of the asymmetric PT heterocycle along the copolymer backbone. Compared to its regioirregular (RI) counterpart, the RR polymer exhibits a 2 orders of magnitude increase in hole mobility from 0.005 to 0.6 cm² V^–1 s^–1. To probe the reason for this difference in mobility, we examined the crystalline structure and its orientation in thin films of both copolymers as a function of depth via grazing incidence wide-angle X-ray scattering (GIWAXS). In the RI film, the π–π stacking direction of the crystallites is mainly perpendicular to the substrate normal (edge-on orientation) while in the RR film the crystallites adopt a mixed π–π stacking orientation in the center of the film as well as near the interface between the polymer and the dielectric layer. These results demonstrate that control of backbone regularity is another important design criterion to consider in the synthesis and optimization of new conjugated copolymers with asymmetric structural units

Electric Field Induced Selective Disordering in Lamellar Block Copolymers

External electric fields align nanostructured block copolymers by either rotation of grains or nucleation and growth depending on how strongly the chemically distinct block copolymer components are segregated. In close vicinity to the order–disorder transition, theory and simulations suggest a third mechanism: selective disordering. We present a time-resolved small-angle X-ray scattering study that demonstrates how an electric field can indeed selectively disintegrate ill-aligned lamellae in a lyotropic block copolymer solution, while lamellae with interfaces oriented parallel to the applied field prevail. The present study adds an additional mechanism to the experimentally corroborated suite of mechanistic pathways, by which nanostructured block copolymers can align with an electric field. Our results further unveil the benefit of electric field assisted annealing for mitigating orientational disorder and topological defects in block copolymer mesophases, both in close vicinity to the order–disorder transition and well below it

Orientation Control of Block Copolymers Using Surface Active, Phase-Preferential Additives

Orientation control of thin film nanostructures derived from block copolymers (BCPs) are of great interest for various emerging technologies like separation membranes, nanopatterning, and energy storage. While many BCP compositions have been developed for these applications, perpendicular orientation of these BCP domains is still very challenging to achieve. Herein we report on a new, integration-friendly approach in which small amounts of a phase-preferential, surface active polymer (SAP) was used as an additive to a polycarbonate-containing BCP formulation to obtain perpendicularly oriented domains with 19 nm natural periodicity upon thermal annealing. In this work, the vertically oriented BCP domains were used to demonstrate next generation patterning applications for advanced semiconductor nodes. Furthermore, these domains were used to demonstrate pattern transfer into a hardmask layer via commonly used etch techniques and graphoepitaxy-based directed self-assembly using existing lithographic integration schemes. We believe that this novel formulation-based approach can easily be extended to other applications beyond nanopatterning

Scalable and Selective Dispersion of Semiconducting Arc-Discharged Carbon Nanotubes by Dithiafulvalene/Thiophene Copolymers for Thin Film Transistors

We report a simple and scalable method to enrich large quantities of semiconducting arc-discharged single-walled carbon nanotubes (SWNTs) with diameters of 1.1–1.8 nm using dithiafulvalene/thiophene copolymers. Stable solutions of highly individualized and highly enriched semiconducting SWNTs were obtained after a simple sonication and centrifuge process. Molecular dynamics (MD) simulations of polymer backbone interactions with and without side chains indicated that the presence of long alkyl side chains gave rise to the selectivity toward semiconducting tubes, indicating the importance of the roles of the side chains to both solubilize and confer selectivity to the polymers. We found that, by increasing the ratio of thiophene to dithiafulvalene units in the polymer backbone (from pDTFF-1T to pDTFF-3T), we can slightly improve the selectivity toward semiconducting SWNTs. This is likely due to the more flexible backbone of pDTFF-3T that allows the favorable wrapping of SWNTs with certain chirality as characterized by small-angle X-ray scattering. However, the dispersion yield was reduced from pDTFF-1T to pDTFF-3T. MD simulations showed that the reduction is due to the smaller polymer/SWNT contact area, which reduces the dispersion ability of pDTFF-3T. These experimental and modeling results provide a better understanding for future rational design of polymers for sorting SWNTs. Finally, high on/off ratio solution-processed thin film transistors were fabricated from the sorted SWNTs to confirm the selective dispersion of semiconducting arc-discharge SWNTs

Enhanced Solid-State Order and Field-Effect Hole Mobility through Control of Nanoscale Polymer Aggregation

Efficient charge carrier transport in organic field-effect transistors (OFETs) often requires thin films that display long-range order and close π–π packing that is oriented in-plane with the substrate. Although some polymers have achieved high field-effect mobility with such solid-state properties, there are currently few general strategies for controlling the orientation of π-stacking within polymer films. In order to probe structural effects on polymer-packing alignment, furan-containing diketopyrrolopyrrole (DPP) polymers with similar optoelectronic properties were synthesized with either linear hexadecyl or branched 2-butyloctyl side chains. Differences in polymer solubility were observed and attributed to variation in side-chain shape and polymer backbone curvature. Averaged field-effect hole mobilities of the polymers range from 0.19 to 1.82 cm²/V·s, where PDPP3F-C16 is the least soluble polymer and provides the highest maximum mobility of 2.25 cm²/V·s. Analysis of the films by AFM and GIXD reveal that less soluble polymers with linear side chains exhibit larger crystalline domains, pack considerably more closely, and align with a greater preference for in-plane π–π packing. Characterization of the polymer solutions prior to spin-coating shows a correlation between early onset nanoscale aggregation and the formation of films with highly oriented in-plane π-stacking. This effect is further observed when nonsolvent is added to PDPP3F-BO solutions to induce aggregation, which results in films with increased nanostructural order, in-plane π–π orientation, and field-effect hole mobilities. Since nearly all π-conjugated materials may be coaxed to aggregate, this strategy for enhancing solid-state properties and OFET performance has applicability to a wide variety of organic electronic materials

Scalable and Selective Dispersion of Semiconducting Arc-Discharged Carbon Nanotubes by Dithiafulvalene/Thiophene Copolymers for Thin Film Transistors

We report a simple and scalable method to enrich large quantities of semiconducting arc-discharged single-walled carbon nanotubes (SWNTs) with diameters of 1.1–1.8 nm using dithiafulvalene/thiophene copolymers. Stable solutions of highly individualized and highly enriched semiconducting SWNTs were obtained after a simple sonication and centrifuge process. Molecular dynamics (MD) simulations of polymer backbone interactions with and without side chains indicated that the presence of long alkyl side chains gave rise to the selectivity toward semiconducting tubes, indicating the importance of the roles of the side chains to both solubilize and confer selectivity to the polymers. We found that, by increasing the ratio of thiophene to dithiafulvalene units in the polymer backbone (from pDTFF-1T to pDTFF-3T), we can slightly improve the selectivity toward semiconducting SWNTs. This is likely due to the more flexible backbone of pDTFF-3T that allows the favorable wrapping of SWNTs with certain chirality as characterized by small-angle X-ray scattering. However, the dispersion yield was reduced from pDTFF-1T to pDTFF-3T. MD simulations showed that the reduction is due to the smaller polymer/SWNT contact area, which reduces the dispersion ability of pDTFF-3T. These experimental and modeling results provide a better understanding for future rational design of polymers for sorting SWNTs. Finally, high on/off ratio solution-processed thin film transistors were fabricated from the sorted SWNTs to confirm the selective dispersion of semiconducting arc-discharge SWNTs

Scalable and Selective Dispersion of Semiconducting Arc-Discharged Carbon Nanotubes by Dithiafulvalene/Thiophene Copolymers for Thin Film Transistors

We report a simple and scalable method to enrich large quantities of semiconducting arc-discharged single-walled carbon nanotubes (SWNTs) with diameters of 1.1–1.8 nm using dithiafulvalene/thiophene copolymers. Stable solutions of highly individualized and highly enriched semiconducting SWNTs were obtained after a simple sonication and centrifuge process. Molecular dynamics (MD) simulations of polymer backbone interactions with and without side chains indicated that the presence of long alkyl side chains gave rise to the selectivity toward semiconducting tubes, indicating the importance of the roles of the side chains to both solubilize and confer selectivity to the polymers. We found that, by increasing the ratio of thiophene to dithiafulvalene units in the polymer backbone (from pDTFF-1T to pDTFF-3T), we can slightly improve the selectivity toward semiconducting SWNTs. This is likely due to the more flexible backbone of pDTFF-3T that allows the favorable wrapping of SWNTs with certain chirality as characterized by small-angle X-ray scattering. However, the dispersion yield was reduced from pDTFF-1T to pDTFF-3T. MD simulations showed that the reduction is due to the smaller polymer/SWNT contact area, which reduces the dispersion ability of pDTFF-3T. These experimental and modeling results provide a better understanding for future rational design of polymers for sorting SWNTs. Finally, high on/off ratio solution-processed thin film transistors were fabricated from the sorted SWNTs to confirm the selective dispersion of semiconducting arc-discharge SWNTs

The Role of Solvent Additive Processing in High Performance Small Molecule Solar Cells

The use of small volumes of a high boiling point liquid as a “solvent additive” is a deposition processing method that has been implemented in most high/record performing polymer:fullerene-based bulk heterojunction (BHJ) solar cell devices. Recently, solvent additive processing has been employed in a solution processable small molecule (SPSM) BHJ system, viz., 5,5′-bis­{(4-(7-hexylthiophen-2-yl)­thiophen-2-yl)-[1,2,5]­thiadiazolo­[3,4-<i>c</i>]­pyridine}­3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene:[6,6]-phenyl C₇₁ butyric acid methyl ester (<i>p</i>-DTS­(PTTh₂)₂:PC₇₁BM), when a small amount, 0.25 v/v %, diiodooctane (DIO) was added to the casting solution, several key device metrics increased, leading to a high power conversion efficiency (PCE) of 6.7%. X-ray diffraction experiments show that the amount of additive added to the casting solution to make <i>p</i>-DTS­(PTTh₂)₂:PC₇₁BM thin films has several effects on the structure at multiple length scales: for example, the number and orientation of <i>p</i>-DTS­(PTTh₂)₂ crystallites, different π–π stacking distances, and the nanoscale domain size. Additionally, we utilize energy filtered transmission electron microscopy (EFTEM), a technique that significantly enhances the contrast between <i>p</i>-DTS­(PTTh₂)₂ and PC₇₁BM in real space, to further verify the effect of increasing domain size as the additive concentration is increased. Tomographic reconstruction of the TEM micrographs provides a 3D representation of the BHJ structure. These studies show how domain size and tortuosity in all dimensions change due to solvent additive processing, and the overall finding is that the nanostructures of <i>p</i>-DTS­(PTTh₂)₂ have enhanced connectivity when 0.25 v/v % DIO was used. Finally, we show evidence of solvent additive retention in <i>p</i>-DTS­(PTTh₂)₂:PC₇₁BM films when 1 v/v % DIO is used (but absent for 0.25%). This finding, in conjunction with the appearance of two populations of π–π stacking distances when 1 v/v % DIO is used, leads to the identification of one of the specific points of interaction between DIO and <i>p</i>-DTS­(PTTh₂)₂

Scalable and Selective Dispersion of Semiconducting Arc-Discharged Carbon Nanotubes by Dithiafulvalene/Thiophene Copolymers for Thin Film Transistors

We report a simple and scalable method to enrich large quantities of semiconducting arc-discharged single-walled carbon nanotubes (SWNTs) with diameters of 1.1–1.8 nm using dithiafulvalene/thiophene copolymers. Stable solutions of highly individualized and highly enriched semiconducting SWNTs were obtained after a simple sonication and centrifuge process. Molecular dynamics (MD) simulations of polymer backbone interactions with and without side chains indicated that the presence of long alkyl side chains gave rise to the selectivity toward semiconducting tubes, indicating the importance of the roles of the side chains to both solubilize and confer selectivity to the polymers. We found that, by increasing the ratio of thiophene to dithiafulvalene units in the polymer backbone (from pDTFF-1T to pDTFF-3T), we can slightly improve the selectivity toward semiconducting SWNTs. This is likely due to the more flexible backbone of pDTFF-3T that allows the favorable wrapping of SWNTs with certain chirality as characterized by small-angle X-ray scattering. However, the dispersion yield was reduced from pDTFF-1T to pDTFF-3T. MD simulations showed that the reduction is due to the smaller polymer/SWNT contact area, which reduces the dispersion ability of pDTFF-3T. These experimental and modeling results provide a better understanding for future rational design of polymers for sorting SWNTs. Finally, high on/off ratio solution-processed thin film transistors were fabricated from the sorted SWNTs to confirm the selective dispersion of semiconducting arc-discharge SWNTs