We describe the layer-by-layer (LBL) fabrication of multilayer films and photovoltaic cells using poly(phenylene ethynylene)-based anionic conjugated polyelectrolytes as electron donors and water-soluble cationic fullerene C60 derivatives as acceptors. LBL film deposition was found to be linearly related to the number of bilayers as monitored by UV-vis absorption. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) of the multilayer films revealed an aggregated but relatively uniform morphology devoid of any long-range phase separation. The maximum incident monochromatic photon to current conversion efficiency (IPCE) of the photovoltaic cells was 5.5%, the highest efficiency reported to date for cells fabricated by using the LBL fabrication technique, and since the thin film cells do not provide complete absorption of the incident light, the current generation per photon absorbed may be as much as 10%. The cells exhibited open circuit voltages of 200-250 mV with highest measured short circuit currents up to 0.5 mA/cm2 and fill factors around 30%. The power conversion efficiencies measured at AM 1.5 solar conditions (100 mW/cm2) varied between 0.01 and 0.04%, and similar to the IPCE results, the efficiency is a function of the thickness of the PV active layer.

This work demonstrates a simple process to form insoluble poly(3,4-propylenedioxythiophene) (Poly[ProDOT]) films by defunctionalization of a soluble form, which is useful in the fabrication of multilayer polymer devices. Three new ester disubstituted Poly[ProDOTs], soluble in common organic solvents, including chloroform, methylene chloride, toluene, tetrahydrofuran, and ethyl acetate, have been synthesized by oxidative polymerization using iron(III) chloride. These Poly[ProDOT-diesters] have the expected repeat unit stuctures along with GPC determined number-average molecular weights ranging from 9000 to 12 000 g mol-1. Dilute polymer solutions in toluene exhibited red fluorescence with quantum efficiencies from 0.24 to 0.32. Homogeneous thin films were formed by spray casting polymer solutions onto ITO coated glass slides and compared to films prepared by electropolymerization. These Poly[ProDOT-diesters] are electroactive, switching from a dark blue-purple to a transmissive sky blue when potentials are applied between −0.9 and +0.3 V vs Fc/Fc.+ A thin film saponification method was developed and rendered the spray-cast films insoluble by submersion into 0.1 M KOH in hot methanol for 1 h to remove the solubilizing ester group. In the case of a bis(heptanoate) Poly[ProDOT-diester] (1), spectroelectrochemistry showed minimal change in its electronic spectra after methanolysis and the resulting alcohol-substituted polymer (Poly[ProDOT-diol] (4)) could be repeatably switched between neutral and oxidized states in subsecond times. In contrast, when a more highly functionalized Poly[ProDOT-tetraester] (3) was used, a small change in the electronic spectrum which is associated with a distinct color change from burgundy to blue was observed upon methanolysis to Poly[ProDOT-tetraol] (6). The insoluble alcohol-substituted polymer films were found to be efficient hole transport layers in polymer light-emitting diodes.

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 Förster 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.

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.