Morphology and Oxygen Sensor Response of Luminescent Ir-Labeled Poly(dimethylsiloxane)/Polystyrene Polymer Blend Films

Polymer films consisting of a linear poly(dimethylsiloxane) end-functionalized with a luminescent Ir(III) complex (Ir−PDMS), blended with polystyrene (PS), function as optical oxygen sensors. The sensor response arises by quenching of the luminescence from the Ir(III) chromophore by oxygen that permeates into the polymer film. The morphology and luminescence oxygen sensor properties of blend films consisting of Ir−PDMS and PS have been characterized by fluorescence microscopy, atomic force microscopy, and scanning electron microscopy. The investigations demonstrate that microscale phase segregation occurs in the films. In blends that contain a relatively small amount of Ir−PDMS in PS (ca. 10 wt %), the Ir−PDMS exists as circular domains, with diameters ranging from 2 to 5 μm, surrounded by the majority PS phase. For larger weight fractions of Ir−PDMS in the blends, the film morphology becomes bicontinuous. A novel epifluorescence microscopy method is applied that allows the construction of Stern−Volmer quenching images that quantify the oxygen sensor response of the blend films with micrometer spatial resolution. These images provide a map of the oxygen permeability of the polymer blend films with a spatial resolution of ca. 1 μm. The results of this investigation show that the micrometer-sized Ir−PMDS domains display a 2−3-fold higher oxygen sensor response compared to the surrounding PS matrix. This result is consistent with the fact that PDMS is considerably more gas permeable compared to PS. The relationship of the microscale morphology of the blends to their performance as macroscale optical oxygen sensors is discussed

Tuning Polymer Light-Emitting Device Emission Colors in Ternary Blends Composed of Conjugated and Nonconjugated Polymers

We report here the utilization of a ternary polymer blend system consisting of two conjugated polymers, the orange emitting poly(2-methoxy-5-(2'-ethyl-hexyloxy)-p-phenylenevinylene) (MEH-PPV) and blue emitting poly(9,9-dioctylfluorene) (PFO), with the inert matrix polymer poly(methyl methacrylate) (PMMA) to show voltage-controlled color tuning in micro- and nano-phase-separated domains. Morphological characterization using atomic force microscopy showed 100-600 nm diameter sized conjugated polymer domains depressed into the surface when processed from toluene. Fluorescence microscopy and transmission electron microscopy were utilized to show that the conjugated polymers, present as the minority phase relative to PMMA, tend to remain together in domains phase separated from the matrix. Photophysical and electroluminescence studies showed efficient Forster energy transfer from PFO to MEH-PPV when equal concentrations of conjugated polymers were utilized with emission occurring at 570 nm, leading to a bright yellow light-emitting device. Ternary blends containing an excess of PFO relative to MEH-PPV showed voltage tunable (8-12 V) yellow to green electroluminescence. The ternary blend devices showed higher efficiencies than the binary blend devices consisting of PFO/PMMA. These results demonstrate that the phase-separated morphology having an excess of the high band gap polymer is essential for obtaining voltage-controlled variable color emission in polymer light-emitting devices

Variable band gap conjugated polymers for optoelectronic and redox applications

We report here on the utilization of variable band gap conjugated polymers for optoelectronic redox applications comprising organic photovoltaics, color tunable light emitting diodes, and electrochromics. For the evaluation of morphology in photovoltaicdevices, atomic force microscopy, and optical microscopy provided direct visualization of the blend film structure. The evolution of the morphology in two and three component blends incorporating poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenlenevinylene] (MEH-PPV), poly(methylmethacrylate) (PMMA), and [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) was investigated. It was found that while insulating PMMA can be used to modulate the phase separation in these blends, a bicontinuous network of donor and acceptor was required to achieve the best device results. Similarily, a MEH-PPVcopolymer with a decreased conjugation length has been used for investigating inter- and intramolecular photoinduced charge transfer in the presence of PMMA and PCBM.We fabricated MEH-PPV/PCBM solar cells that have power conversion efficiencies up to 1.5% with a range of 0.7-1.5%, dependent on the nature of the MEH-PPV used. This further indicates that in addition to blend morphology, polymer structure is critical for optimizing device performance. To this end, the concept of an ideal donor for photovoltaic devices based on poly[2,5-di(3,7-dialkoxy)-cyanoterephthalylidene] is described and two donor-acceptor polymers based on cyanovinylene (CNV) and dioxythiophene are discussed as representative examples of soluble narrow band gap polymers synthesized in our group. For light emitting applications, utilization of two blue emitting conjugated polymers poly (9,9-dioctylfluorene) (PFO) and poly[(9,9-dihexylfluorenyl-2,7-diyl)-co-(9,ethyl-3,6-carbazole)] (PFH-PEtCz)is presented for a color tunable polymer light emitting diode that emits orange, green, and blue light with a voltage range of 7-10 V as a function of the total conjugated polymer content in PMMA and is attributed to the phase separation between the conjugated polymers. Finally, the narrow band gap conjugated polymer, poly[bis(3,4-propylenedioxythiophene-dihexyl)]-cyanovinylene has been characterized for its electrochromic properties, illustrating the multifunctional nature of variable band gap conjugated polymers

Variable band gap conjugated polymers for optoelectronic and redox applications

We report here on the utilization of variable band gap conjugated polymers for optoelectronic redox applications comprising organic photovoltaics, color tunable light emitting diodes, and electrochromics. For the evaluation of morphology in photovoltaic devices, atomic force microscopy, and optical microscopy provided direct visualization of the blend film structure. The evolution of the morphology in two and three component blends incorporating poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenlenevinylene] (MEH-PPV), poly(methylmethacrylate) (PMMA), and [6, 6]-phenyl C-61-butyric acid methyl ester (PCBM) was investigated. It was found that while insulating PMMA can be used to modulate the phase separation in these blends, a bicontinuous network of donor and acceptor was required to achieve the best device results. Similarity, a MEH-PPV copolymer with a decreased conjugation length has been used for investigating inter- and intramolecular photoinduced charge transfer in the presence of PMMA and PCBM. We fabricated MEH-PPV/PCBM solar cells that have power conversion efficiencies up to 1.5% with a range of 0.7-1.5%, dependent on the nature of the MEH-PPV used. This further indicates that in addition to blend morphology, polymer structure is critical for optimizing device performance. To this end, the concept of an ideal donor for photovoltaic devices based on poly[2,5-di(3,7-dialkoxy)-cyanoterephthalylidene] is described and two donor-acceptor polymers based on cyanovinylene (CNV) and dioxythiophene are discussed as representative examples of soluble narrow band gap polymers synthesized in our group. For light emitting applications, utilization of two blue emitting conjugated polymers poly (9,9-dioctylfluorene) (PFO) and poly[(9,9-dihexylfluorenyl-2,7-diyl)-co-(9,ethyl-3,6-carbazole)] (PFH-PEtCz) is presented for a color tunable polymer light emitting diode that emits orange, green, and blue light with a voltage range of 7-10 V as a function of the total conjugated polymer content in PMMA and is attributed to the phase separation between the conjugated polymers. Finally, the narrow band gap conjugated polymer, poly [bis(3,4-propylenedioxythiophene-dihexyl)]-cyanovinylene has been characterized for its electrochromic properties, illustrating the multifunctional nature of variable band gap conjugated polymers