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Solution processable methanofullerene-based solar cells are the most widely studied class of organic photovoltaics (OPVs). The evolution of the electronic properties with solvent vapor annealing (SVA) in polyfluorene-copolymer and [6,6]phenyl-C61-butyric acid methyl ester (PCBM) blended OPVs is studied using various scanning probe techniques: light beam induced current spectroscopy (LBIC), conductive atomic force microscopy (c-AFM), and photoconductive AFM (pc-AFM). We demonstrate that SVA improves the power conversion efficiency by 40% while forming mesoscopic PCBM crystallites and a approximately 3 nm copolymer-rich overlayer at the cathode interface. We find that the large crystallites created during annealing do not directly improve the local performance of the device, but instead attribute the performance improvement to the ripened blend morphology and an increase in the hole mobility of the copolymer in comparison to the unannealed blend. The PCBM-rich aggregates act as a sink for excess PCBM, although excess PCBM is initially required to form the appropriate structural features prior to the annealing process.
ACS Nano 2009 Mar 24
PMID:The role of mesoscopic PCBM crystallites in solvent vapor annealed copolymer solar cells. 1922 11

We use photoluminescence (PL) quenching and photoinduced absorption (PIA) spectroscopy to study charge transfer in bulk heterojunction blends of PbSe quantum dots with the semiconducting polymers poly-3-hexylthiophene (P3HT) and poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-para-phenylene vinylene] (MDMO-PPV). PIA spectra from the PbSe blends are compared to spectra from similar blends of the polymers with phenyl-C(61)-butyric acid methyl ester (PCBM) and blends with CdSe quantum dots. We find that the MDMO-PPV PL is quenched, and the PL lifetime is shortened upon addition of PbSe quantum dots, while the PL of the P3HT is unaffected upon blending. However, for PbSe blends with both polymers, the PIA spectra show very little polaronic signal, suggesting that few, if any, long-lived charges are being produced by photoinduced charge transfer.
ACS Nano 2009 Jun 23
PMID:Absence of photoinduced charge transfer in blends of PbSe quantum dots and conjugated polymers. 1944 37

We demonstrate organic photovoltaic cells in which every layer is deposited by solution processing on opaque metal substrates, with efficiencies similar to those obtained in conventional device structures on transparent substrates. The device architecture is enabled by solution-processed, laminated silver nanowire films serving as the top transparent anode. The cells are based on the regioregular poly(3-hexylthiophene) and C(61) butyric acid methyl ester bulk heterojunction and reach an efficiency of 2.5% under 100 mW/cm(2) of AM 1.5G illumination. The metal substrates are adequate barriers to moisture and oxygen, in contrast to transparent plastics that have previously been used, giving rise to the possibility of roll-to-roll solution-processed solar cells that are packaged by lamination to glass substrates, combining the cost advantage of roll-to-roll processing with the barrier properties of glass and metal foil.
ACS Nano 2010 Jan 26
PMID:Fully solution-processed inverted polymer solar cells with laminated nanowire electrodes. 2002 90

Solution processable fullerene and copolymer bulk heterojunctions are widely used as the active layers of solar cells. In this work, scanning time-of-flight secondary ion mass spectrometry (ToF-SIMS) is used to examine the distribution of [6,6]phenyl-C61-butyric acid methyl ester (PCBM) and regio-regular poly(3-hexylthiophene) (rrP3HT) that forms the bulk heterojunction. The planar phase separation of P3HT:PCBM is observed by ToF-SIMS imaging. The depth profile of the fragment distribution that reflects the molecular distribution is achieved by low energy Cs(+) ion sputtering. The depth profile clearly shows a vertical phase separation of P3HT:PCBM before annealing, and hence, the inverted device architecture is beneficial. After annealing, the phase segregation is suppressed, and the device efficiency is dramatically enhanced with a normal device structure. The 3D image is obtained by stacking the 2D ToF-SIMS images acquired at different sputtering times, and 50 nm features are clearly differentiated. The whole imaging process requires less than 2 h, making it both rapid and versatile.
ACS Nano 2010 Feb 23
PMID:Effect of fabrication parameters on three-dimensional nanostructures of bulk heterojunctions imaged by high-resolution scanning ToF-SIMS. 2009 77

The generation and characterization of a robust thienylsilane molecular layer on indium tin oxide substrates was investigated. The molecular layer was found to reduce the oxidation potential required for the electrochemical polymerization of 3,4-ethylenedioxythiophene. The resulting electrochemically prepared poly(3,4-ethylenedioxythiophene):poly(p-styrenesulfonate) (ePEDOT:PSS) films were found to be more uniform in coverage with lower roughness and higher conductivity than analogous films fabricated with bare ITO. A relative improvement in the efficiency of 2,5-diyl-poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) bulk heterojunction solar cells was observed when devices were formed on thienylsilane-modified ITO electrodes, rather than unmodified ITO control electrodes.
ACS Appl Mater Interfaces 2009 Feb
PMID:Thienylsilane-modified indium tin oxide as an anodic interface in polymer/fullerene solar cells. 2035 15

Water-induced degradation of polymer photovoltaics based on the active materials poly(3-hexylthiophene) (P3HT) or poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEHPPV) was studied. The solar cell devices comprised a bulk heterojunction formed by the active material and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) in a standard device geometry. The use of H2(18)O in conjunction with time-of-flight secondary ion mass spectrometry enabled mapping of the parts of the device that were induced by water. A comparison was made between the two active materials and between devices that were kept in the dark and devices that had been subjected to illumination under simulated sunlight. Devices that were exposed to ambient humidity were compared to devices exposed to saturated humidity. Finally, a comparison was made between results obtained using H2(18)O and earlier work involving 18O2. Water was found to have behavior similar to but not identical with molecular oxygen.
ACS Appl Mater Interfaces 2009 Jan
PMID:Water-induced degradation of polymer solar cells studied by H2(18)O labeling. 2035 61

We report on two pairs of platinum acetylide based polymers and model oligomers utilizing a 2,1,3-benzothiadiazole (BTD) acceptor moiety flanked on either side by either 2,5-thienyl donor units (Pt2BTD-Th and p-PtBTD-Th) or (3,4-ethylenedioxy)-2,5-thienyl donors (Pt2BTD-EDOT and p-PtBTD-EDOT). Both oligomer/polymer pairs absorb strongly throughout the visible region; however, because the (ethylenedioxy)thiophene moiety is a stronger donor than thiophene, the latter oligomer/polymer pair has a correspondingly lower band gap and, therefore, harvests light more efficiently at longer wavelengths. p-PtBTD-Th exhibits a relatively narrow molecular weight distribution with a number-average molecular weight (Mn) of 22 kDa, while p-PtBTD-EDOT exhibits a comparable Mn of 33 kDa but has a high polydispersity index likely due to aggregation. We provide a complete report of the photophysical and electrochemical characterization of the two oligomer/polymer pairs. The photophysical studies reveal that the materials undergo relatively efficient intersystem crossing. In a discussion of the energetics of photoinduced electron transfer from the platinum polymers to [6,6]-phenyl C61 butyric acid methyl ester (PCBM), it is noted that while the singlet state is quenched efficiently, the triplet state is not quenched, indicating that charge generation in the photovoltaic materials must ensue from the singlet manifold. Finally, organic photovoltaic devices based on blends of p-PtBDT-Th or p-PtBDT-EDOT with PCBM were characterized under monochromatic and simulated solar (AM1.5) illumination. Optimized devices exhibit an open-circuit voltage (Voc) of approximately 0.5 V, a short-circuit current density (Isc) of approximately 7.2 mA cm(-2), and a fill factor of approximately 35%, which yields overall power conversion efficiencies of 1.1-1.4%.
ACS Appl Mater Interfaces 2009 Jan
PMID:Low-band-gap platinum acetylide polymers as active materials for organic solar cells. 2035 67

Two novel soluble compounds T and A that contain a central dihexyloxy-p-phenylenevinylene unit, intermediate moieties of thiophene or anthracene, respectively, and terminal cyano-vinylene nitrophenyls were synthesized and characterized. They showed moderate thermal stability and relatively low glass transition temperatures. These compounds displayed similar optical properties. Their absorption was broad and extended up to about 750 nm with the longer-wavelength maximum around 640 nm and an optical band gap of approximately 1.70 eV. From the current-voltage characteristics of the devices using both compounds T and A, it was concluded that both compounds behave as p-type organic semiconductors with hole mobility on the order of 10(-5) cm(2)/(V s). The power conversion efficiency (PCE) of the devices based on these compounds was 0.019% and 0.013% for compounds A and T, respectively. When compounds A and T were blended with [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), the PCE dramatically increased up to 1.66% and 1.36% for devices with A:PCBM and T:PCBM, respectively. The efficiencies of the devices were further enhanced upon thermal annealing up to 2.49% and 2.33% for devices based on A:PCBM and T:PCBM, respectively.
ACS Appl Mater Interfaces 2009 Aug
PMID:Novel p-phenylenevinylene compounds containing thiophene or anthracene moieties and cyano-vinylene bonds for photovoltaic applications. 2035 87

Ambipolar organic thin-film transistors with a bilayer structure of poly(3-hexylthiophene) and [6,6]phenyl C(61) butyric acid methyl ester were fabricated using a simple solution-based, contact-film-transfer method. The transistors exhibited balanced electron and hole mobilities of 2.1 x 10(-2) and 1.1 x 10(-2) cm(2) V(-1) s(-1), respectively. Complementary inverters based on two identical ambipolar transistors showed good performance with a gain of 14.
ACS Appl Mater Interfaces 2009 Sep
PMID:Bilayer ambipolar organic thin-film transistors and inverters prepared by the contact-film-transfer method. 2035 7

An inverted-type organic bulk-heterojunction solar cell inserting zinc oxide (ZnO) as an electron collection electrode, fluorine-doped tin oxide (FTO)/ZnO/[6,6]-phenyl-C(61)-butyric acid methyl ester:regioregular poly(3-hexylthiophene) (PCBM:P3HT)/poly(3,4-ethylenedioxylenethiophene):poly(4-styrenesulfonic acid) (PEDOT:PSS)/Au, was fabricated in air and characterized by an alternating current impedance spectroscopy (IS). In the IS measurement, we observed reproducibly the electric resistance and capacitance components originating from ZnO and organic active layers, and we found that the depletion layer functioning to take out the photocurrent to the external circuit was formed in both the ZnO and PCBM:P3HT layers at the ZnO/PCBM:P3HT interface. In this letter, we propose that this IS measurement is effective for evaluating the electric properties of several layers with capacitance components in organic thin-film solar cells.
ACS Appl Mater Interfaces 2009 Oct
PMID:Characterization of inverted-type organic solar cells with a ZnO layer as the electron collection electrode by ac impedance spectroscopy. 2035 41


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