Polymer-based Thermoelectric Devices

Currently, over 50% of all energy generated in the US is lost as waste heat, and thermoelectric generators offer a promising means to recoup some of this energy, if their efficiency is improved. While organic thermoelectric materials lack the efficiency of their inorganic counterparts, they are composed of highly abundant resources and have low temperature processing conditions. Recently, a new class of redox-active polymers, radical polymers, has exhibited high electrical conductivity in an entirely amorphous medium. In addition, these radical polymers have a simple synthetic scheme and can be highly tunable to provide desired electrical properties. In this study, the thermoelectric properties of a nitroxide radical-based polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), is evaluated in a doped state. 4-ethylbenzenesulfonic acid (EBSA) is used to dope PTMA solutions. The Seebeck coefficient and conductivity measurements were collected to calculate the thermoelectric power factor of the material at an average temperature of 40 ˚C. We expect to find that doped PTMA has a peak power factor of ~10-2 μW m-1 K-2. While these power factor values would not exceed a state-of-the-art organic semiconductor, they would show that radical polymers are a viable alternative to pi-conjugated semiconducting polymers. These redox-active polymers are still a new type of semiconducting polymer; therefore, this study could suggest that further research is necessary to determine their full capabilities and the radical solutions they may have to offer

Detecting Trace Explosives with Organic Electronic Devices

Trinitrotoluene (TNT) is a commonly used explosive and poses a significant risk to security arenas across the globe. The use of organic electronics for the detection of explosive residues allows for large scale, solution-processible, and environmentally stable devices with a high selectivity for TNT detection. Currently, fluorescence-based sensors are used in TNT detection, but the synthesis of the fluorescent molecules can be complicated and costly. Hence, we introduce a new design paradigm to overcome this limitation. Specifically, organic field-effect transistors (OFETs) were created using 6,13-bis(triisopropylsilylethynyl) (TIPS) pentacene as the active material to collect a baseline mobility and the on current to off current ratio (ON/OFF). Then, blends of TIPS-pentacene and varying concentrations of TNT were used in OFETs, and the change in the ON/OFF and charge carrier mobility were evaluated. With the introduction of TNT, the ON/OFF increases in value and it was observed that the concentration of the TNT in the film blend has an effect on how much the ON/OFF and hole mobility increases. The measured change in the ON/OFF were used to create a calibration curve that shows the dependence of the TNT concentration. A device that incorporates the TIPS-pentacene FET could eventually be used to sweep an area or surface for the presence of dangerous explosives through a change in an electrical signal in the device and interpretation of the calibration curves

Radical Polymers as Anodic Charge Extraction Layers in Small Molecule Organic Photovoltaic Devices

Organic photovoltaic (OPV) devices based on the copper (II) phthalocyanine(CuPc)/ fullerene(C60) system are an innovative photovoltaic technology optimal for situations requiring low-cost, transparent, and flexible devices. Furthermore, the high degree of reproducibility of this system allows for the ready study of new OPV technologies. Here, we have used this system to elucidate systematic structure-property-performance relationships for a new OPV anode modifier. The addition of interfacial modifier materials between the organic CuPc/C60 layers and the metallic anode drastically can improve efficiency. Radical polymers are a class of polymers with aliphatic backbones and pendent stabilized radical groups. Here, we utilize poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) to examine the feasibility of radical polymers as anode modifiers. OPV devices utilizing a PTMA thin film deposited onto an ITO substrate (anode) with subsequent CuPc and C60 active layers followed by a BCP cathode modifier and an aluminum layer (cathode) were fabricated using thermal evaporation. Device performance was evaluated by measuring current density as a function of voltage during simulated solar radiation. Addition of a thin layer of PTMA between the ITO and CuPc layers increased device power conversion efficiency to approximately 0.95% from a control of 0.57%, likely due to enhancement of the crystal structure of the CuPc layer. The addition of interfacial modifiers significantly increases the overall efficiency, and consequently, viability of CuPc/C60 OPV devices, and this logic should be extendable to a myriad of other polymer based solar cell designs

Design of Triblock Polymers for Water Filtration as Nanoporous Membranes

Clean, usable water is quickly becoming a less abundant natural resource for residential, commercial, and industrial applications. Developing advanced and efficient membranes as filtration components for water-treatment processes will help supply a growing society the clean water it needs. Triblock polymers have recently become of interest for their potential to create membranes that have higher selectivity while also having higher flux values than current commercially available ultrafiltration membranes. The synthesis of a triblock polymer consisting of polyisoprene (PI), polystyrene (PS), and either poly(N,N-dimethylacrylamide) (PDMA) or poly(tert-butyl acrylate) (PtBA) is reported. Each block of the polymer is synthesized via a sequential reverse addition-fragmentation chain transfer (RAFT) polymerization mechanism to achieve controlled, high molecular weights and narrow molecular weight distributions. The triblock polymer is synthesized such that the volume fractions of the PI, PS, and PDMA/PtBA blocks are about 25%, 45%, and 30%, respectively, to achieve optimal mechanical properties and pore functionality within the membrane. Subsequently, the membrane is prepared following the non-solvent induced phase separation (SNIPS) method

Surface Tension, Interfacial Tension and Phase Behavior: Interactions of Surfactant/Polymer Solutions with Crude Oil

Advanced oil recovery techniques, beyond primary and secondary recovery, are required in order to produce additional oil in existing reservoir rock. Here, we evaluated a combination of polymer and surfactant aqueous solutions, in order to generate a working fluid capable of achieving high-performance enhanced oil recovery (EOR). In this recovery process, surfactant is added to the water flooding mixture in order to lower the interfacial tension between the oil and the water. If the interfacial tension can be decreased by ~1,000-fold, then the aqueous solution can mobilize and displace the oil. Moreover, a polymer is added to the aqueous solution in order to increase the viscosity of the working fluid. Aqueous solutions with a viscosity higher than the oil viscosity can produce a stable flow of oil. However, the exact combination and concentration needed for these two key components to be effective is dependent on each oil reservoir and requires several experiments and specific tuning in order to yield an effective design. In order to determine the optimal combination, the effects of the average molecular weight of the polymers, the surfactant chemistry, and their combinations in salt solutions (at varying salt concentrations) were investigated. Specifically, the surface tension of aqueous solutions against air and the interfacial tension against oil and the phase behavior of the polymer-surfactant systems were evaluated with a model hydrocarbon, dodecane, and with crude oil. By varying the molecular properties of the surfactant and the polymer, we found a technically promising surfactant-polymer combination for potential EOR application

Development and Evaluation of Low-Cost CO2 Sensors for Buildings

There is a significant opportunity to improve building energy efficiency and indoor environmental quality by accurately monitoring CO2 levels. However, current CO2 sensors tend to be expensive or require regular recalibration. This work presents research related to the initial development and evaluation of two novel CO2 sensors based on chemiresistive and resonant mass sensing techniques. Prototype sensors were assessed in a bench-top test chamber at temperatures, humidity levels, and CO2 concentrations, typical of indoor environments. Under these conditions, prototype sensors required only 60 mW of power, or less. Further, each sensor was developed to have a footprint of less than 25 mm2 and a cost of less than $50. Given the relative low cost, small size, and potential for low power consumption, these sensors may serve as an attractive alternative to the commercial CO2 sensors that are currently available

Polythiophene-containing block copolymers for organic photovoltaic applications.

University of Minnesota Ph.D. dissertation. August 2009. Major: Chemical Engineering. Advisors: C. Daniel Frisbie and Marc A. Hillmyer. 1 computer file (PDF); xiii, 275, appendix: pages 257-275.Poly(3-alkylthiophene)s (P3ATs) have become the most common electron-donating material in organic photovoltaics (OPVs), and recent advances in the fabrication of polythiophene-fullerene bulk heterojunction solar cells have allowed for devices with power conversion efficiencies of up to ~6% to be realized. This efficiency has only been possible through enhancements in the active layer microstructure. This key factor allowed for better separation of the bound electron-hole pair (exciton), generated by absorption of light. Understanding how exciton dissociation and the active layer morphology affect device performance will facilitate cell optimization, ultimately leading to higher device efficiencies. Consequently, we developed two new classes of polythiophene-based block copolymers to better understand these phenomena. First, we synthesized well-defined diblock and triblock copolymers with the structures: poly(3-alkylthiophene)-b-polylactide (P3AT-PLA) and polylactide-b-poly(3-alkylthiophene)-b-polylactide (PLA-P3AT-PLA). We have observed that kinetic factors dominate phase separation for a semicrystalline polythiophene block. However, if the polythiophene moiety was amorphous the polymers self-assembled into thermodynamically stable, ordered microstructures with domain spacings on the scale of interest for charge separation in OPV cells (ca 30 nm). Polylactide was chosen as the second moiety in the block copolymers because it could be selectively etched from the polythiophene matrix with a gentle alkaline bath. This procedure led to the formation of nanoporous templates that could generate ordered bulk heterojunctions. In the second approach, P3AT chain ends were terminated with fullerene to create an internal electron acceptor-donor-acceptor, methylfulleropyrrolidine-poly(3-alkylthiophene)-methylfulleropyrrolidine (C60-P3AT-C60). Microphase separation occurred between the polymer chain and fullerene end groups, which suggested the creation of two distinct semicrystalline regimes. A compositionally similar blend of P3HT and C60 showed a similar microstructure. This comparable domain formation, coupled with the possibility of enhanced charge transfer, makes C60-P3AT-C60 a promising candidate as a material in bulk heterojunction organic photovoltaic devices.Boudouris, Bryan W.. (2009). Polythiophene-containing block copolymers for organic photovoltaic applications.. Retrieved from the University Digital Conservancy, https://hdl.handle.net/11299/54044

Systematic Control of the Nanostructure of Semiconducting-Ferroelectric Polymer Composites in Thin Film Memory Devices

In polymer-based ferroelectric diodes, films are composed of a semiconducting polymer and a ferroelectric polymer blend sandwiched between two metal electrodes. In these thin films, the ferroelectric phase serves as the memory retention medium while the semiconducting phase serves as the pathway to read-out the memory in a nondestructive manner. As such, having distinct phases for the semiconducting and ferroelectric phases have proven critical to device performance. In order to evaluate this crucial structure–property relationship, we have fabricated ordered ferroelectric devices (OFeDs) through common lithographic techniques to establish systematically the impact of nanoscale structure on the macroscopic performance. In particular, we demonstrate that there is an optimal domain size (∼400 nm) for the interpenetrating networks, and we show that the ordered device, with semiconducting domains that span the entire length of the active layer film, provides a significant increase in the ON/OFF ratio relative to the blended film fabricated using standard solution blending and spin-coating techniques. This improved performance occurs due to a combination of the ordered nanostructure and the nature of the ferroelectric-semiconductor interface. As this is the first demonstration of macroscopic OFeDs, this work helps to elucidate the underlying physics of the device operation and establishes a new archetype in the design of polymer-based, nonvolatile memory devices