Thomas P. Russell

P3HT/PCBM Bulk Heterojunction Organic Photovoltaics: Correlating Efficiency and Morphology

Controlling thin film morphology is key in optimizing the efficiency of polymer-based photovoltaic (PV) devices. We show that morphology and interfacial behavior of the multicomponent active layers confined between electrodes are strongly influenced by the preparation conditions. Here, we provide detailed descriptions of the morphologies and interfacial behavior in thin film mixtures of regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM), a typical active layer in a polymer-based PV device, in contact with an anode layer of PEDOT-PSS and either unconfined or confined by an Al cathode during thermal treatment. Small angle neutron scattering and electron microscopy show that a nanoscopic, bicontinuous morphology develops within seconds of annealing at 150 °C and coarsens slightly with further annealing. P3HT and PCBM are shown to be highly miscible, to exhibit a rapid, unusual interdiffusion, and to display a preferential segregation of one component to the electrode interfaces. The ultimate morphology is related to device efficiency.

P3HT Nanopillars for Organic Photovoltaic Devices Nanoimprinted by AAO Templates

Free-standing nanorod arrays of poly(3-hexylthiophene) (P3HT) were fabricated on indium tin oxide/glass substrates using anodic aluminum oxide (AAO) templates. The AAO templates were treated with a low molecular weight polydimethylsiloxane mold-release agent to reduce their surface energy of the template and interactions with the P3HT. Using a thermal nanoimprinting process, the templates were easily removed, generating nanorods on the surfaces of P3HT thin films. These unique structures were investigated for application in organic photovoltaic devices.

Unidirectionally aligned line patterns driven by entropic effects on faceted surfaces

A simple, versatile approach to the directed self-assembly of block copolymers into a macroscopic array of unidirectionally aligned cylindrical microdomains on reconstructed faceted single crystal surfaces or on flexible, inexpensive polymeric replicas was discovered. High fidelity transfer of the line pattern generated from the microdomains to a master mold is also shown. A single-grained line patterns over arbitrarily large surface areas without the use of top-down techniques is demonstrated, which has an order parameter typically in excess of 0.97 and a slope error of 1.1 deg. This degree of perfection, produced in a short time period, has yet to be achieved by any other methods. The exceptional alignment arises from entropic penalties of chain packing in the facets coupled with the bending modulus of the cylindrical microdomains. This is shown, theoretically, to be the lowest energy state. The atomic crystalline ordering of the substrate is transferred, over multiple length scales, to the block copolymer microdomains, opening avenues to large-scale roll-to-roll type and nanoimprint processing of perfectly patterned surfaces and as templates and scaffolds for magnetic storage media, polarizing devices, and nanowire arrays.

High aspect ratio sub-15 nm silicon trenches from block copolymer templates.

High-aspect-ratio sub-15-nm silicon trenches are fabricated directly from plasma etching of a block copolymer mask. A novel method that combines a block copolymer reconstruction process and reactive ion etching is used to make the polymer mask. Silicon trenches are characterized by various methods and used as a master for subsequent imprinting of different materials. Silicon nanoholes are generated from a block copolymer with cylindrical microdomains oriented normal to the surface.

ABC triblock copolymer vesicles with mesh-like morphology.

Polymer vesicles made from poly(isoprene-b-styrene-b-2-vinyl pyridine) (PI-b-PS-b-P2VP) triblock copolymer confined within the nanopores of an anodic aluminum oxide (AAO) membrane are studied. It was found that these vesicles have well-defined, nanoscopic size, and complex microphase-separated hydrophobic membranes, comprised of the PS and PI blocks, while the coronas are formed by the P2VP block. Vesicle formation was tracked using both transmission and scanning electron microscopy. A mesh-like morphology formed in the membrane at a well-defined composition of the three blocks that can be tuned by changing the copolymer composition. The nanoscale confinement, copolymer composition, and subtle molecular interactions contribute to the generation of these vesicles with such unusual morphologies.

Room temperature magnetic materials from nanostructured diblock copolymers

Nanostructured magnetic materials are important for many advanced applications. Consequently, new methods for their fabrication are critical. However, coupling self-assembly to the generation of magnetic materials in a simple, straight-forward manner has remained elusive. Although several approaches have been considered, most have multiple processing steps, thus diminishing their use of self-assembly to influence magnetic properties. Here we develop novel block copolymers that are preprogrammed with the necessary chemical information to microphase separate and deliver room temperature ferromagnetic properties following a simple heat treatment. The importance of the nanostructured confinement is demonstrated by comparison with the parent homopolymer, which provides only paramagnetic materials, even though it is chemically identical and has a higher loading of the magnetic precursor. In addition to the room temperature ferromagnetic properties originating from the block copolymer, the in situ generation densely functionalizes the surface of the magnetic elements, rendering them oxidatively stable.

Highly ordered nanoporous template from triblock copolymer.

Silica nanoporous templates from poly(1,4-isoprene)-block-polystyrene-block-poly(2-vinyl pyridine) (IS2VP) were prepared. The films of IS2VP spin-coated from toluene showed a dimple-type structure with short-range lateral order. When the films were exposed to a mixed solvent vapor of toluene/hexane, a highly ordered and oriented core-shell structure, consisting of an outer shell of PI, a middle shell of PS, and a core of P2VP, was obtained. The PI was degraded by UV-ozone treatment and removed. A film of polydimethylsiloxane (PDMS) was spin coated onto the remaining film with dimple-type structures and, upon heating, was drawn into the interstitial regions by capillary action. Exposure to oxygen plasma converted the PDMS into silica and degraded all other remaining polymers. This led to highly ordered and oriented nanoporous silica that could be used as an etching mask for transfer of the pattern or templates for metal loading.

Phase transition behavior in thin films of block copolymers by use of immiscible solvent vapors

The phase transitions in thin films of a block copolymer, BCP, that forms micelles when exposed to immiscible solvent vapors, were investigated at two different temperatures. Pre-swelling with a non-volatile solvent was used to prevent dewetting prior to the addition of a volatile solvent used to affect the annealing. The temperature and, therefore, the vapor pressure of the solvent were varied, which gave rise to remarkably different kinetic pathways of lateral ordering of the BCP. At 22 °C, a rapid annihilation of defects occurred with exchange of BCP chains between domains. At 17 °C, a slow coalescence of two adjacent domains led to enhanced lateral ordering of the swollen BCP microdomains and also led to domains twice as large as those observed at 22 °C. These studies suggest that the large interfacial energy arising from the immiscibility of the components, coupled with an incommensurability of the swollen film thickness with the period of the BCP morphology, gave rise to the observed differences and subsequent lateral ordering of the BCP microdomains.

Circular Nanopatterns over Large Areas from the Self-Assembly of Block Copolymers Guided by Shallow Trenches

We report the fabrication of ultradense circular nanolines of block copolymer (BCP) microdomains over macroscopic areas. These lines were generated by the directed self-assembly (DSA) of BCPs on the topographically patterned substrates, where the trenches with circular shape are patterned on a flat substrate. The width of the trench and the distance between trenches are varied for commensurability issues, and difference BCPs are used to demonstrate the generality of this strategy. When a commensurability condition is satisfied, BCPs on the topographically patterned substrates undergo a DSA with solvent annealing, resulting in a flat film with an areal density amplification of the circular patterns over large areas. The methodology described here may provide an easy approach to high densities of circularly shaped nanopatterns for data storage device manufacturing.

Controlled Orientation of Block Copolymers on Defect‐Free Faceted Surfaces

The effect of a faceted surface geometry on controlling the direction of well-defined line patterns of block copolymer (BCP) microdomains over macroscopic areas is reported. Facets with asymmetric base angles can control the direction of BCP microdomains oriented either parallel or perpendicular to the facets depending on BCP film thickness.