Tamar Segal-Peretz

Facile infiltration of semiconducting polymer into mesoporous electrodes for hybrid solar cells

Hybrid composites of semiconducting polymers and metal oxides are promising combinations for solar cells. However, forming a well-controlled nanostructure with bicontinuous interpenetrating networks throughout the photoactive film is difficult to achieve. Pre-structured “mesoporous” metal oxide electrodes can act as a well-defined template for latter polymer infiltration. However, the long range infiltration of polymer chains into contorted porous channels has appeared to elude the scientific community, limiting the advancement of this technology. Here we present a structural and electronic characterisation of poly(3-hexylthiophene) (P3HT) infiltrated into mesoporous dye-sensitized TiO2. Through a combination of techniques we achieve uniform pore filling of P3HT up to depths of over 4 μm, but the volumetric fraction of the pores filled with polymer is less than 24%. Despite this low pore-filling, exceptionally efficient charge collection is demonstrated, illustrating that pore filling is not the critical issue for mesoporous hybrid solar cells.

Molecular pathways for defect annihilation in directed self-assembly

Significance A molecular model is used to calculate the free energy of formation of ordered and disordered copolymer morphologies. We rely on advanced methodologies to identify the minimum free energy pathways that connect such states of the material. Our predictions for defect formation and annealing are compared with experimental observations. Our results provide a detailed molecular view of isolated block copolymer defects, which measure approximately 5 nm and represent isolated events in large areas. They are true “needles in the hay stack” that can only be studied by concerted molecular simulations and dedicated access to production-level fabrication tools. We show that defect annealing is an activated process, where defects are eliminated by operating near the order−disorder transition. Over the last few years, the directed self-assembly of block copolymers by surface patterns has transitioned from academic curiosity to viable contender for commercial fabrication of next-generation nanocircuits by lithography. Recently, it has become apparent that kinetics, and not only thermodynamics, plays a key role for the ability of a polymeric material to self-assemble into a perfect, defect-free ordered state. Perfection, in this context, implies not more than one defect, with characteristic dimensions on the order of 5 nm, over a sample area as large as 100 cm2. In this work, we identify the key pathways and the corresponding free energy barriers for eliminating defects, and we demonstrate that an extraordinarily large thermodynamic driving force is not necessarily sufficient for their removal. By adopting a concerted computational and experimental approach, we explain the molecular origins of these barriers and how they depend on material characteristics, and we propose strategies designed to overcome them. The validity of our conclusions for industrially relevant patterning processes is established by relying on instruments and assembly lines that are only available at state-of-the-art fabrication facilities, and, through this confluence of fundamental and applied research, we are able to discern the evolution of morphology at the smallest relevant length scales—a handful of nanometers—and present a view of defect annihilation in directed self-assembly at an unprecedented level of detail.

Quantitative three-dimensional characterization of block copolymer directed self-assembly on combined chemical and topographical prepatterned templates

Characterization of the three-dimensional (3D) structure in directed self-assembly (DSA) of block copolymers is crucial for understanding the complex relationships between the guiding template and the resulting polymer structure so DSA could be successfully implemented for advanced lithography applications. Here, we combined scanning transmission electron microscopy (STEM) tomography and coarse-grain simulations to probe the 3D structure of P2VP-b-PS-b-P2VP assembled on prepatterned templates using solvent vapor annealing. The templates consisted of nonpreferential background and raised guiding stripes that had PS-preferential top surfaces and P2VP-preferential sidewalls. The full 3D characterization allowed us to quantify the shape of the polymer domains and the interface between domains as a function of depth in the film and template geometry and offered important insights that were not accessible with 2D metrology. Sidewall guiding was advantageous in promoting the alignment and lowering the roughness of the P2VP domains over the sidewalls, but incommensurate confinement from the increased topography could cause roughness and intermittent dislocations in domains over the background region at the bottom of the film. The 3D characterization of bridge structures between domains over the background and breaks within domains on guiding lines sheds light on possible origins of common DSA defects. The positional fluctuations of the PS/P2VP interface between domains showed a depth-dependent behavior, with high levels of fluctuations near both the free surface of the film and the substrate and lower fluctuation levels in the middle of the film. This research demonstrates how 3D characterization offers a better understanding of DSA processes, leading to better design and fabrication of directing templates.

Metrology of DSA process using TEM tomography

Directed self-assembly (DSA) of block copolymers (BCPs) is a rising technique for sub-20 nm patterning. To fully harness DSA capabilities for patterning, a detailed understanding of the three dimensional (3D) structure of BCPs is needed. By combining sequential infiltration synthesis (SIS) and scanning transmission electron microscopy (STEM) tomography, we have characterized the 3D structure of self-assembled and DSA BCPs films with high precision and resolution. SIS is an emerging technique for enhancing pattern transfer in BCPs through the selective growth of inorganic material in polar BCP domains. Here, Al2O3 SIS was used to enhance the imaging contrast and enable tomographic characterization of BCPs with high fidelity. Moreover, by utilizing SIS for both 3D characterization and hard mask fabrication, we were able to characterize the BCP morphology as well as the alumina nanostructures that would be used for pattern transfer.

Mesostructured silica containing conjugated polymers formed within the channels of anodic alumina membranes from tetrahydrofuran-based solution.

The synthesis of mesostructured silica from a tetrahydrofuran (THF)-based sol gel was carried out in the channels of an anodic alumina membrane (AAM) using the evaporation-induced self-assembly (EISA) method. Two different nonionic surfactants were used as structure-directing agents, the triblock copolymer Pluronic P123 and the oligomer surfactant Brij56. The effect of the relative humidity and surfactant concentration on the type of mesophase and orientation of the in-channel mesostructures was studied using transmission electron microscopy (TEM) and grazing incidence small angel X-ray scattering (GISAXS). The in-channel structures obtained in this study were primarily of the 2D hexagonal phase with a circular orientation in which the hexagonally packed cylinders form a spiral-like shape from the channel wall inward. In addition, a columnar orientation of the hexagonal phase, in which the axes of the hexagonally packed cylinders are oriented parallel to the channel axes, was also observed. Finally, the use of the THF-based synthesis allowed the in situ incorporation of the highly hydrophobic yellow-emitting conjugated polymer poly[9,9-dioctylfluorene-co-benzothiadiazole] into the in-channel mesostructure upon its formation. The conjugated polymer was well distributed within the mesostructure and maintained its optical properties.

New Insights into Sequential Infiltration Synthesis

Sequential infiltration synthesis (SIS) is a process derived from ALD in which a polymer is infused with inorganic material using sequential, self-limiting exposures to gaseous precursors. SIS can be used in lithography to harden polymer resists rendering them more robust towards subsequent etching, and this permits deeper and higher-resolution patterning of substrates such as silicon. Herein we describe recent investigations of a model system: Al2O3 SIS using trimethyl aluminum (TMA) and H2O within the diblock copolymer, poly(styrene-block-methyl methacrylate) (PS-b-PMMA). Combining in-situ Fourier transform infrared absorption spectroscopy, quartz-crystal microbalance, and synchrotron grazing incidence small angle X-ray scattering with high resolution scanning transmission electron microscope tomography, we elucidate important details of the SIS process: 1) TMA adsorption in PMMA occurs through a weakly-bound intermediate; 2) the SIS kinetics are diffusion-limited, with desorption 10× slower than adsorption; 3) dynamic structural changes occur during the individual precursor exposures. These findings have important implications for applications such as SIS lithography.

Staining Block Copolymers using Sequential Infiltration Synthesis for High Contrast Imaging and STEM tomography

T. Segal-Peretz, J. Winterstein, M. Biswas, J.A. Liddle, Jeffrey W. Elam, N. J. Zaluzec, P.F. Nealey 1 Institute for Molecular Engineering, University of Chicago, Chicago, USA 2 Materials Science Division, Argonne National Laboratory, Argonne, USA 3 Center for Nanoscale Science and Technology, National Institute of Standards and Technology, Gaithersburg, USA 4 Energy Systems Division, Argonne National Laboratory, Argonne, USA 5 Electron Microscopy Center, NST Division , Argonne National Laboratory, Argonne, USA

Engineering the Kinetics of Directed Self-Assembly of Block Copolymers toward Fast and Defect-Free Assembly

Directed self-assembly (DSA) of block copolymers (BCPs) can achieve perfectly aligned structures at thermodynamic equilibrium, but the self-assembling morphology can become kinetically trapped in defective states. Understanding and optimizing the kinetic pathway toward domain alignment is crucial for enhancing process throughput and lowering defectivity to levels required for semiconductor manufacturing, but there is a dearth of experimental, three-dimensional studies of the kinetic pathways in DSA. Here, we combined arrested annealing and TEM tomography to probe the kinetics and structural evolution in the chemoepitaxy DSA of PS- b-PMMA with density multiplication. During the initial stages of annealing, BCP domains developed independently at first, with aligned structures at the template interface and randomly oriented domains at the top surface. As the grains coarsened, the assembly became cooperative throughout the film thickness, and a metastable stitch morphology was formed, representing a kinetic barrier. The stitch morphology had a three-dimensional structure consisting of both perpendicular and parallel lamellae. On the basis of the mechanistic information, we studied the effect of key design parameters on the kinetics and evolution of structures in DSA. Three types of structural evolutions were observed at different film thicknesses: (1) immediate alignment and fast assembly when thickness < L0 ( L0 = BCP natural periodicity); (2) formation of stitch morphology for 1.25-1.45 L0; (3) fingerprint formation when thickness >1.64 L0. We found that the DSA kinetics can be significantly improved by avoiding the formation of the metastable stitch morphology. Increasing template topography also enhanced the kinetics by increasing the PMMA guiding surface area. A combination of 0.75 L0 BCP thickness and 0.50 L0 template topography achieved perfect alignment over 100 times faster than the baseline process. This research demonstrates that an improved understanding of the evolution of structures during DSA can significantly improve the DSA process.

Directed self-oriented self-assembly of block copolymers using chemically modified surfaces

A review of the factors affecting directed self-assembly (DSA) of block copolymers on chemical nanopatterns is presented. The review starts with the formation of chemical patterns suitable for DSA, including patterns and processes that are compatible with industrial fabrication. The critical role of thermodynamics in DSA is then presented, including the impact of chemical pattern geometry and chemistry on the final assembly. The foundation in thermodynamics is also used to explain defect generation and the results observed in DSA with density multiplication. The effect of kinetics in the DSA process is reviewed, focusing on reducing process time to levels that are acceptable for manufacturing process, and the role that kinetics can play in creating trapped defect states. Efforts to achieve sub-10-nm patterning with DSA are shown, including solvent annealing, the use of top-coats, and the synthesis of block copolymers with blocks having similar surface energies. Applications that could benefit from DSA are reviewed, focusing on bit patterned media and fin field-effect transistors. Finally, the integration of DSA on a 300-mm wafer line is presented, including a defect source analysis of DSA on the wafer line.

Controlling morphology and charge transfer in ZnO/polythiophene photovoltaic films

The organic–inorganic interfacial chemical composition and interaction have a critical influence on the performance of corresponding hybrid photovoltaic devices. Such interfaces have been shown to be controlled by using surfactants to promote contact between the organic electron-donating conjugated polymer species and the inorganic electron-accepting metal oxides. However, the location of the surfactant at the organic–inorganic interface often hinders donor–acceptor charge transfer and limits the device performance. In this study we have replaced the conventional surfactant with an optically and electronically active amphiphilic polythiophene that both compatibilizes the conjugated polymer and metal oxide, and plays an active role in the charge generation process. Specifically, a polythiophene with hydrophilic urethane side-groups allows the formation of a fine continuous ZnO network in P3HT with an organic–inorganic interfacial chemical interaction and a high surface area. A combination of FTIR, absorption and photoluminescence life-time spectroscopies yields insights into the composition and nano-scale interaction of the components, while high-resolution electron microscopy reveals the morphology of the films. The control over morphology and electronic coupling at the hybrid interface is manifested in photovoltaic devices with improved performances.