Enrique D. Gomez

Conjugated Block Copolymer Photovoltaics with near 3% Efficiency through Microphase Separation

Organic electronic materials have the potential to impact almost every aspect of modern life including how we access information, light our homes, and power personal electronics. Nevertheless, weak intermolecular interactions and disorder at junctions of different organic materials limit the performance and stability of organic interfaces and hence the applicability of organic semiconductors to electronic devices. Here, we demonstrate control of donor-acceptor heterojunctions through microphase-separated conjugated block copolymers. When utilized as the active layer of photovoltaic cells, block copolymer-based devices demonstrate efficient photoconversion well beyond devices composed of homopolymer blends. The 3% block copolymer device efficiencies are achieved without the use of a fullerene acceptor. X-ray scattering results reveal that the remarkable performance of block copolymer solar cells is due to self-assembly into mesoscale lamellar morphologies with primarily face-on crystallite orientations. Conjugated block copolymers thus provide a pathway to enhance performance in excitonic solar cells through control of donor-acceptor interfaces.

Effect of Ion Distribution on Conductivity of Block Copolymer Electrolytes

Energy-filtered transmission electron microscopy (EFTEM) was used to determine the distribution of lithium ions in solid polymer electrolytes for lithium batteries. The electrolytes of interest are mixtures of bis(trifluoromethane)sulfonimide lithium salt and symmetric poly(styrene-block-ethylene oxide) copolymers (SEO). In contrast to current solid and liquid electrolytes, the conductivity of SEO/salt mixtures increases with increasing molecular weight of the copolymers. EFTEM results show that the salt is increasingly localized in the middle of the poly(ethylene oxide) (PEO) lamellae as the molecular weight of the copolymers is increased. Calculations of the inhomogeneous local stress field in block copolymer microdomains, modeled using self-consistent field theory, provide a quantitative explanation for this observation. These stresses, which increase with increasing molecular weight, interfere with the ability of PEO chains to coordinate with lithium cations near the walls of the PEO channels where ion mobility is expected to be low.

Transient photovoltaic behavior of air-stable, inverted organic solar cells with solution-processed electron transport layer

The short-circuit current density of inverted organic solar cells comprising a solution-processed titania electron transport layer increases with continuous illumination in air and saturates after 10 min. On extended exposure (>2 days), the open-circuit voltage of the devices increases also. The improvement in device characteristics over short time scales is attributed to the filling of shallow electron traps in titania. With an increase in photoconductivity of titania, the short-circuit current increases accordingly. The increase in open-circuit voltage on extended exposure to air is attributed to an increase in the electrostatic field across the diodes when polythiophene is doped by oxygen.

Directly patternable, highly conducting polymers for broad applications in organic electronics

Postdeposition solvent annealing of water-dispersible conducting polymers induces dramatic structural rearrangement and improves electrical conductivities by more than two orders of magnitude. We attain electrical conductivities in excess of 50 S/cm when polyaniline films are exposed to dichloroacetic acid. Subjecting commercially available poly(ethylene dioxythiophene) to the same treatment yields a conductivity as high as 250 S/cm. This process has enabled the wide incorporation of conducting polymers in organic electronics; conducting polymers that are not typically processable can now be deposited from solution and their conductivities subsequently enhanced to practical levels via a simple and straightforward solvent annealing process. The treated conducting polymers are thus promising alternatives for metals as source and drain electrodes in organic thin-film transistors as well as for transparent metal oxide conductors as anodes in organic solar cells and light-emitting diodes.

Solvent-dependent electrical characteristics and stability of organic thin-film transistors with drop cast bis(triisopropylsilylethynyl) pentacene

The solvent from which the active layer is drop cast dramatically influences the electrical characteristics and electrical stability of thin-film transistors comprising bis(triisopropylsilylethynyl) pentacene. Casting from high boiling solvents allows slower crystallization; devices cast from toluene and chlorobenzene thus exhibit mobilities >0.1 cm2/V s and on/off ratios of ∼106. More importantly, the solvent choice influences the device stability. Devices from toluene exhibit stable characteristics, whereas devices from chlorobenzene show hystereses on cycling, with dramatic threshold voltage shifts toward positive voltages. The instability in chlorobenzene devices is attributed to the migration of water and solvent impurities to the charge transport interface on repetitive testing.

Immunogloblin E-mediated immediate allergic reactions to dipyrone: value of basophil activation test in the identification of patients

Background Pyrazolones are a major cause of immediate IgE‐mediated reactions to drugs in many countries.

Azadipyrromethene-based Zn(II) complexes as nonplanar conjugated electron acceptors for organic photovoltaics.

The effectiveness of new a electron acceptor for organic solar cells is demonstrated. The acceptor is a homoleptic zinc(II) complex of 2,6-diphenylethynyl-1,3,7,9-tetraphenylazadipyrromethene. The high power-conversion efficiency obtained is attributed to the acceptors 3D structure, which prevents crystallization and promotes a favourable nanoscale morphology, its high Voc , and its ability to contribute to light harvesting at 600-800 nm.

Engineering the organic semiconductor-electrode interface in polymer solar cells

Engineering the interfaces between organic semiconductors and electrodes minimizes interfacial resistances and enhances the performance of polymer solar cells. Organic semiconductors have intrinsically low free carrier densities, which can lead to large injection barriers if the work functions of the electrodes are not properly matched to the energy levels of the photoactive layers in electronic devices. One approach to engineer this crucial interface is through the judicious selection of electrode materials. Selecting the electrodes so their work functions match the energy levels of the organic semiconductors within the photoactive layer, however, can often compromise the environmental stability of polymer solar cells. One must thus strive to achieve a balance during device fabrication between the bulk properties of the electrode, such as electrical conductivity, and its interfacial properties, such as the energy alignment between the organic semiconductor and the electrode. Another approach to enhance charge extraction at the organic semiconductor-electrode interface is to adsorb molecular layers (MLs) on the electrode prior to the deposition of the photoactive layer. If the adsorbed molecules are preferentially oriented and they possess a net dipole moment, MLs can be utilized to modify the work function of the electrode so to minimize resistive losses during charge extraction. In this approach, one needs to take into account changes in the morphology of the photoactive layer – which undoubtedly also alters device performance – that result due to differences in the surface energy of the ML-modified electrode. As an alternative to completely replacing the electrode, interfacial modification via ML adsorption offers optimization of the charge extraction efficacy at the organic semiconductor-electrode interface independent of the bulk conductivity of the electrode.

Immunologic evaluation of drug allergy.

Hypersensitivity drug reactions (HDR) consist of an individual abnormal response with the involvement of the immunological system. In addition to specific immunological mechanisms where specific antibodies or sensitised T cells participate, release of inflammatory mediators by non-specific immunological recognition may also occur. Within this category are one of the most common groups of drugs, the non-steroidal anti-inflammatory drugs. In addition to chemical drugs new emerging ones with an increasing protagonism are biological agents like humanised antibodies and others. For IgE dependent reactions both in vivo and in vitro tests can be used for the immunological evaluation. Sensitivity of these is not optimal and very often a drug provocation test must be considered for knowing the mechanism involved and/or establishing the diagnosis. For non-immediate reactions also both in vivo and in vitro tests can be used. Sensitivity for in vivo tests is generally low and in vitro tests may be needed for the immunological evaluation. Immunohistochemical studies of the affected tissue enable a more precise classification of non-immediate reactions. The monitorization of the acute response of the reactions has given clues for understanding these reactions and has promising results for the future of the immunological evaluation of HDR.

Direct measurements of exciton diffusion length limitations on organic solar cell performance

Through a combination of X-ray scattering and energy-filtered electron microscopy, we have quantitatively examined the relationship between the mesostructure of the photoactive layer and device performance in PBTTT/PC(71)BM solar cells. We can predict device performance from X-ray structural data through a simple morphological model which includes the exciton diffusion length.