Developing Composite Insulating Cross-Arms for 400 kV Lattice Towers

\u3cp\u3ePolymorphism of organic semiconducting materials exerts critical effects on their physical properties such as optical absorption, emission and electrical conductivity, and provides an excellent platform for investigating structure–property relations. It is, however, challenging to efficiently tune the polymorphism of conjugated polymers in aggregated, semi-crystalline phases due to their conformational freedom and anisotropic nature. Here, two distinctly different semi-crystalline polymorphs (β\u3csub\u3e1\u3c/sub\u3e and β\u3csub\u3e2\u3c/sub\u3e) of a low-bandgap diketopyrrolopyrrole polymer are formed through controlling the solvent quality, as evidenced by spectroscopic, structural, thermal and charge transport studies. Compared to β\u3csub\u3e1\u3c/sub\u3e, the β\u3csub\u3e2\u3c/sub\u3e polymorph exhibits a lower optical band gap, an enhanced photoluminescence, a reduced π-stacking distance, a higher hole mobility in field-effect transistors and improved photocurrent generation in polymer solar cells. The β\u3csub\u3e1\u3c/sub\u3e and β\u3csub\u3e2\u3c/sub\u3e polymorphs provide insights into the control of polymer self-organization for plastic electronics and hold potential for developing programmable ink formulations for next-generation electronic devices.\u3c/p\u3

On the homocoupling of trialkylstannyl monomers in the synthesis of diketopyrrolopyrrole polymers and its effect on the performance of polymer-fullerene photovoltaic cells

Homocoupling of monomers in a palladium-catalyzed copolymerization of donor-acceptor polymers affects the perfect alternating structure and may deteriorate the performance of such materials in solar cells. Here we investigate the effect of homocoupling bis(trialkylstannyl)-thiophene and -bithiophene monomers in two low band gap poly(diketopyrrolopyrrole-alt-oligothiophene) polymers by deliberately introducing extended oligothiophene defects in a controlled fashion. We find that extension of the oligothiophene by one or two thiophenes and creating defects up to at least 10% does not significantly affect the opto-electronic properties of the polymers or their photovoltaic performance as electron donor in solar cells in combination with [6,6]-phenyl C71 butytic acid methyl ester as acceptor. By using model reactions, we further demonstrate that for the optimized synthetic protocol and palladium-catalyst system the naturally occurring defect concentration in the polymers is expected to be less than 0.5%

The effect of side-chain substitution and hot processing on diketopyrrolopyrrole-based polymers for organic solar cells

\u3cp\u3eThe effects of cold and hot processing on the performance of polymer-fullerene solar cells are investigated for diketopyrrolopyrrole (DPP) based polymers that were specifically designed and synthesized to exhibit a strong temperature-dependent aggregation in solution. The polymers, consisting of alternating DPP and oligothiophene units, are substituted with linear and second position branched alkyl side chains. For the polymer-fullerene blends that can be processed at room temperature, hot processing does not enhance the power conversion efficiencies compared to cold processing because the increased solubility at elevated temperatures results in the formation of wider polymer fibres that reduce charge generation. Instead, hot processing seems to be advantageous when cold processing is not possible due to a limited solubility at room temperature. The resulting morphologies are consistent with a nucleation-growth mechanism for polymer fibres during drying of the films.\u3c/p\u3

Relation between the electronic properties of regioregular donor-acceptor terpolymers and their binary copolymers

By analyzing the optical band gap and energy levels of seven different regioregular terpolymers in which two different electron-rich donor moieties are alternating with a common electron-deficient acceptor unit along the backbone, we establish a direct correlation with the properties of the corresponding binary copolymers in which one donor and one acceptor are combined. For this study, we use diketopyrrolopyrrole as the common acceptor and different π-conjugated aromatic oligomers as donors. We find that the optical band gap and frontier orbital energies of the terpolymers are the arithmetic average of those of the parent copolymers with remarkable accuracy. The same relationship is also found for the open-circuit voltage of the bulk heterojunction solar cells made with the ter- and copolymers in combination with [6,6]-phenyl-C71-butyric acid methyl ester. Comparison of these findings with data in the literature suggests that this is a universal rule that can be used as a tool when designing new π-conjugated polymers. The experimental results are supported by a semiempirical quantum chemical model that accurately describes the energy levels of the terpolymers after parametrization on the energy levels of the copolymers and also provides a theoretical explanation for the observed arithmetic relations

The influence of siloxane side-chains on the photovoltaic performance of a conjugated polymer

\u3cp\u3eThe effect of gradually replacing the branched alkyl side chains of a diketopyrrolopyrrole (DPP) conjugated polymer by linear side chains containing branched siloxane end groups on the photovoltaic performance of blends of these polymers with a common fullerene acceptor is investigated. With an increasing proportion of siloxane side chains, the molecular weight and solubility of the polymers decreases. While the siloxane containing polymers exhibit a higher hole mobility in field-effect transistors, their performance in solar cells is less than the polymer with only alkyl sides chains. Using grazing-incidence wide-angle X-ray scattering, transmission electron microscopy, and fluorescence spectroscopy we identify two main reasons for the reduced performance of siloxane containing polymers in solar cells. The first one is a somewhat coarser phase-separated morphology with slightly wider polymer fibers. This is unexpected as often the fiber width is inversely correlated with polymer solubility. The second one is stronger non-radiative decay of the pristine polymers containing siloxane side chains.\u3c/p\u3

Homocoupling defects in diketopyrrolopyrrole-based copolymers and their effect on photovoltaic performance

We study the occurrence and effect of intrachain homocoupling defects in alternating push-pull semiconducting PDPPTPT polymers based on dithienyl-diketopyrrolopyrrole (TDPPT) and phenylene (P) synthesized via a palladium-catalyzed cross-coupling polymerization. Homocoupled TDPPT-TDPPT segments are readily identified by the presence of a low-energy shoulder in the UV/vis/NIR absorption spectrum. Remarkably, the signatures of these defects are found in many diketopyrrolopyrrole (DPP)-based copolymers reported in the literature. The defects cause a reduction of the band gap, a higher highest occupied molecular orbital (HOMO) level, a lower lowest unoccupied molecular orbital (LUMO) level, and a localization of these molecular orbitals. By synthesizing copolymers with a predefined defect concentration, we demonstrate that their presence reduces the short-circuit current and open-circuit voltage of solar cells based on blends of PDPPTPT with [70]PCBM. In virtually defect-free PDPPTPT, the power conversion efficiency is as high as 7.5%, compared to 4.5-5.6% for polymers containing 20% to 5% defects. © 2014 American Chemical Society

Near-infrared tandem organic photodiodes for future application in artificial retinal implants

Photovoltaic retinal prostheses show great potential to restore sight in patients suffering from degenerative eye diseases by electrical stimulation of the surviving neurons in the retinal network. Herein, organic photodiodes (OPDs) sensitive to near‐infrared (NIR) light are evaluated as photovoltaic pixels for future application in retinal prostheses. Single‐junction and tandem OPDs are compared. In the latter, two nominally identical single‐junction cells are processed on top of each other, effectively doubling the open‐circuit voltage (V OC). Both single‐junction and tandem OPD micropixels can deliver the required charge to stimulate neurons under pulsed NIR light at physiologically safe intensities when connected to stimulating microelectrodes in a physiological saline solution. However, only tandem OPD pixels can cover the entire charge per pulse neural stimulation window due to their higher V OC (≈1.4 V). This demonstrates the viability of high‐resolution retinal prostheses based on flexible OPD arrays

Fluorination as an effective tool to increase the open-circuit voltage and charge carrier mobility of organic solar cells based on poly(cyclopenta[2,1-b:3,4-b′]dithiophene-alt-quinoxaline) copolymers

The effect of fluorination on the optoelectronic properties and the polymer : fullerene solar cell characteristics of PCPDTQx-type (poly{4-(2′-ethylhexyl)-4-octyl-4H-cyclopenta[2,1-b:3,4-b′]dithiophene-alt-2,3-bis[5′-(2′′-ethylhexyl)thiophen-2′-yl]quinoxaline}) low bandgap copolymers is reported. The introduction of fluorine atoms on the quinoxaline constituents is an effective way to lower the HOMO and LUMO energy levels of the alternating copolymers, resulting in an enhanced open-circuit voltage for the devices based on the fluorinated polymers (∼0.1 V per F added). Furthermore, fluorination also improves the charge carrier mobility in the bulk heterojunction blends. Despite the formation of unfavorable photoactive layer morphologies, the best solar cell performance is obtained for the copolymer prepared from the difluorinated quinoxaline monomer, affording a power conversion efficiency of 5.26% under AM 1.5G irradiation, with an open-circuit voltage of 0.83 V, a short-circuit current density of 11.58 mA cm−2 and a fill factor of 55%.\u3cbr/\u3

The impact of device polarity on the performance of Polymer-Fullerene solar cells

Diketopyrrolopyrrole (DPP)‐conjugated polymers are a versatile class of semiconductors for application in organic solar cells because of their tunable optoelectronic properties. A record power conversion efficiency (PCE) of 9.4% was recently achieved for DPP polymers, but further improvements are required to reach true efficiency limits. Using five DPP polymers with different chemical structures and molecular weights, the device performance of polymer:fullerene solar cells is systematically optimized by considering device polarity, morphology, and light absorption. The polymer solubility is found to have a significant effect on the optimal device polarity. Soluble polymers show a 10–25% increase in PCE in inverted device configurations, while the device performance is independent of device polarity for less soluble DPP derivatives. The difference seems related to the polymer to fullerene weight ratio at the ZnO interface in inverted devices, which is higher for more soluble DPP polymers. Optimization of the nature of the cosolvent to narrow the fibril width of polymers in the blends toward the exciton diffusion length enhances charge generation. Additionally, the use of a retroreflective foil increases absorption of light. Combined, the effects afford a PCE of 9.6%, among the highest for DPP‐based polymer solar cells