Engineering the electrochromism and ion conduction of layer-by-layer assembled films

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Chemical Engineering, 2003.Includes bibliographical references.This work applies the processing technique of layer-by-layer (LBL) assembly to the creation and development of new electrochemically active materials. Elements of the thin-film electrochromic cell were chosen as a particular focus for LBL fabrication. Layer-by-layer assembly is the ideal processing tool to tailor the electrochemical systems within electrochromic cells because modulating processing conditions can greatly impact the nanoscale composition and morphology of the resultant films. For the first time, this control was used to: 1) intelligently design electrochromic LBL assembled composite films that facilitated ion motion for faster switching and exhibited enhanced or shifted coloration, 2) combine multiple electrochromic materials into novel LBL assembled composites with even higher contrast, faster switching, and multiple colored states, and finally 3) develop and optimize several LBL assembled polymer electrolyte films that display high ionic conductivity and sound mechanical integrity. Electrochromic cell elements were chosen not only for their undeveloped commercial potential, but also because they incorporate multifunctional material systems with alternative applications. Studies of LBL fabrication and the operation of electrochromic cells provide insight into intermolecular interactions, internal and external film interfaces, thin film electrochemistry, and charged species mobility in polymer solids. First investigated was the capability of LBL assembly to alter the properties of electrochromic films by varying molecular blending.(cont.) The electrochromophores for this investigation were appropriated from all corners of the materials spectrum, including discrete electrochromic polymers, conjugated polymers, soft colloidal suspensions, and inorganic particle dispersions. In each system, the influence of assembly conditions and film composition was elucidated; in particular systems the hydrophobicity, acidity, and morphology of the films were found to impact the electrochemistry and optical character of the films, providing a means to modulate these properties by directing LBL assembly design choices. Because of the high uniformity and thickness control allowed by LBL assembly, the contrast and switching performance of all LBL assembled electrochromic films were in general superior to those of films containing the same electrochromophores fabricated by other methods. One particularly promising system involved novel LBL assembled films containing the same electrochromophores fabricated by other methods. One particularly promising system involved novel LBL assembled films containing electrochromic metal hexacyanoferrate nanocrystals of the Prussian blue family. These films displayed fast and deep coloration; synthetic nanocrystal variation extended absorbance over a broad spectral range so that these inorganic/polymer composite films could potentially be considered as elements in a full-color switchable CMYK display. The power of the LBL assembly technique was leveraged further with the successful fabrication of "dual electrochrome" electrodes ...by Dean M. DeLongchamp.Ph.D

A Simple and Robust Approach to Reducing Contact Resistance in Organic Transistors

Efficient injection of charge carriers from the contacts into the semiconductor layer is crucial for achieving high-performance organic devices. The potential drop necessary to accomplish this process yields a resistance associated with the contacts, namely the contact resistance. A large contact resistance can limit the operation of devices and even lead to inaccuracies in the extraction of the device parameters. Here, we demonstrate a simple and efficient strategy for reducing the contact resistance in organic thin-film transistors by more than an order of magnitude by creating high work function domains at the surface of the injecting electrodes to promote channels of enhanced injection. We find that the method is effective for both organic small molecule and polymer semiconductors, where we achieved a contact resistance as low as 200 Ωcm and device charge carrier mobilities as high as 20 cm2V−1s−1, independent of the applied gate voltage

Self-Assembly of ABC Bottlebrush Triblock Terpolymers with Evidence for Looped Backbone Conformations

Bottlebrush block copolymers offer rich opportunities for the design of complex hierarchical materials. As consequences of the densely grafted molecular architecture, bottlebrush polymers can adopt highly extended backbone conformations and exhibit unique physical properties. A recent report has described the unusual phase behavior of ABC bottlebrush triblock terpolymers bearing grafted poly(dl-lactide) (PLA), polystyrene (PS), and poly(ethylene oxide) (PEO) blocks (LSO). In this work, a combination of resonant soft X-ray reflectivity (RSoXR), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and self-consistent field theory (SCFT) was used to provide insight into the phase behavior of LSO and underlying backbone chain conformations. Consistent with SCFT calculations, RSoXR measurements confirm a unique mesoscopic ACBC domain connectivity and decreasing lamellar periods (d0) with increasing backbone length of the PEO block. RSoXR and NEXAFS demonstrate an additional unusual feature of brush LSO thin films: when the overall film thickness is ∼3.25d0, the film–air interface is majority PS (>80%). Because PS is the midblock, the triblocks must adopt looping configurations at the surface, despite the preference for the backbone to be extended. This result is supported by backbone concentrations calculated through SCFT, which suggest that looping midblocks are present throughout the film. Collectively, this work provides evidence for the flexibility of the bottlebrush backbone and the consequences of low-χ block copolymer design. We propose that PEO blocks localize at the PS/PLA domain interfaces to screen the highest χ contacts in the system, driving the formation of loops. These insights introduce a potential route to overcome the intrinsic penalties to interfacial curvature imposed by the bottlebrush architecture, enabling the design of unique self-assembled materials

Measuring Order in Regioregular Poly(3-hexylthiophene) with Solid-State ¹³C CPMAS NMR

We report on measurements of order in semicrystalline, high molar mass poly­(3-hexylthiophene) (P3HT) by solid-state ¹³C cross-polarization magic angle spinning (CPMAS) nuclear magnetic resonance (NMR) measurements. The relative degree of crystallinity was estimated for two films with different drying conditions via X-ray diffraction (XRD) and differential scanning calorimetry (DSC). Order determined by ¹³C NMR does not necessarily correlate with crystallinity, indicating that local order can occur in noncrystalline regions. Slow main chain dynamics influence the ¹³C NMR peak widths at lower temperatures (<0 °C), with side chain motions influencing the main chain motions. At higher temperatures (>0 °C), where narrower thiophene resonances are observed, these main chain conformation rearrangements occur on fast time scales (≪3 ms). This room-temperature dynamic disorder suggests that P3HT may be classified as a conformationally disordered (CONDIS) crystal