Indium tin oxide (ITO) is the premier choice for transparent conductive electrodes in optoelectro... more Indium tin oxide (ITO) is the premier choice for transparent conductive electrodes in optoelectronic devices despite its inherent brittleness. Here we report the fabrication of a grating-like structure that obviates ITO's mechanical limitations while retaining its resistivity and optical qualities. ITO nanopatterned films exhibited a resistivity <1.3 × 10 Ω cm, which surpassed all previously reported values for flexible ITO, with a normal transmission >90% across the whole visible spectrum range. We demonstrate the nanopatterned ITO retains extraordinary flexibility and durability on heat-sensitive substrates, accommodating cyclic bending to a curvature diameter of at least 3.2 mm for over 50 cycles of compressive and decompressive flexing without significant deterioration of its resistivity or optical properties. Moreover, 2-dimensional extrapolation shows that multiaxial bending is also feasible while maintaining mechanical flexibility, durability, and optical transparency.
The major challenge in solar cell technology dwells in achieving an efficient absorption of photo... more The major challenge in solar cell technology dwells in achieving an efficient absorption of photons with an effective carrier extraction. In all cases, light absorption considerations call for thicker modules while carrier transport would benefit from thinner ones. This dichotomy is the fundamental problem limiting the efficiencies of photovoltaics, especially promising low-cost polymer solar cells. We present experimental and theoretical
Organic photovoltaics with nanopatterned photonic crystal photoactive layers offer an alternative... more Organic photovoltaics with nanopatterned photonic crystal photoactive layers offer an alternative to conventional planar devices that suffer from a competition between optical absorption and electronic processes. Our recent studies have shown that nanopatterned devices exhibit enhanced absorption and exciton creation profiles as compared to planar cells. Improved absorption results in part from the excitation of resonant optical modes where certain
Wiley Interdisciplinary Reviews: Nanomedicine and Nanobiotechnology, 2016
Structural coloration has attracted great interest from scientists and engineers in recent years,... more Structural coloration has attracted great interest from scientists and engineers in recent years, owing to fascination with various brilliant examples displayed in nature as well as to promising applications of bio-inspired functional photonic structures and materials. Much research has been done to reveal and emulate the physical mechanisms that underlie the structural colors found in nature. In this article, we review the fundamental physics of many natural structural colors displayed by living organisms as well as their bio-inspired artificial counterparts, with emphasis on their connections, tunability strategies, and proposed applications, which aim to maximize the technological benefits one could derive from these photonic nanostructures. For further resources related to this article, please visit the WIREs website.
ABSTRACTWe have fabricated ordered arrays of gold nanocrystals on FIB-processed silicon substrate... more ABSTRACTWe have fabricated ordered arrays of gold nanocrystals on FIB-processed silicon substrates using electroless deposition. We have also fabricated ordered arrays of silver nanocrystals on silicon with diameters 40–60 nm separated by 180 nm center-to-center, using pulsed-laser deposition (PLD) to deposit silver on the substrate. The metal nanocrystal arrays are characterized using SEM as well as AFM and energy dispersive x-ray (EDX) analysis. AFM confirms particle sizes measured in SEM, and EDX analysis demonstrates that Ag preferentially clusters at sites that have been damaged by the ion beam. These results suggest that the FIB-PLD combination can be used to create ordered arrays of Ag nanocrystals with diameters of 10 nm or less.
Proceedings of SPIE - The International Society for Optical Engineering, 2011
We show the effects of hole shape, size, and periodicity variation on extraordinary transmission ... more We show the effects of hole shape, size, and periodicity variation on extraordinary transmission through a tuned metamaterial of 4 pairs of alternating layers of Ag-Al2O3 with 20 nm thicknesses under illumination of TM light. The advantage of a metamaterial over a Ag film of similar thickness is the tunability of the surface plasmons coupling k-vector. Because their cross sections appear the same in fourier space, incident TM light sees the same structure for circular, square, and rectangular holes transmission is unaffected by these shape variations. Further, as we increase the hole size, the transmission does not exhibit the expected enhancement as both the 250 nm and 150 nm diameter holes are both too small to have their maxima overlap with the tuned transmission enhancement of the metamaterial. For the metamaterial, we are able to tune it to support surface plasmons from periodicities ranging from 130 nm to 215 nm, which effectively shifts the transmission peaks from the blue end of the spectrum in Ag films across the visible and into the red.
Thin film nanostructured photovoltaic cells are increasing in efficiency and decreasing the cost ... more Thin film nanostructured photovoltaic cells are increasing in efficiency and decreasing the cost of solar energy. FDTD modeling of branched nanowire &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;forests&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; are shown to have improved optical absorption in the visible and near-IR spectra over nanowire arrays alone, with a factor of 5 enhancement available at 1000 nm. Alternate BNW tree configurations are presented, achieving a maximum absorption of over 95% at 500 nm.
Vertically aligned bundles of TiO2 nanocrystals were fabricated by pulsed laser deposition (PLD) ... more Vertically aligned bundles of TiO2 nanocrystals were fabricated by pulsed laser deposition (PLD) and tested as a photoanode material in dye sensitized solar cells (DSSC) using scanning electron microscopy (SEM), light absorption spectroscopy (UV-Vis), and incident photon-to-current efficiency (IPCE) experiments. An optimal background pressure of oxygen during deposition was discovered to produce a photoanode structure that simultaneously achieves high surface
The metal-insulator transition present in chromium doped VO2 nanoscale film has been observed by ... more The metal-insulator transition present in chromium doped VO2 nanoscale film has been observed by electron spin resonance (ESR) spectroscopy. VO2 is a highly correlated electron system with numerous practical applications pertaining to this transition, including ultra-fast optical switching and smart windows. We use Cr^3+ transition metal ions with concentration of order one percent as a probe in a 140 nm film to detect this transition. The film exhibited a four-fold decrease in chromium intensity as temperature increased through the transition temperature (˜67^o C). ESR signal intensities were also used to characterize the hysteretic behavior of this particular transition; these results are in agreement with hysteresis observed through optical means. A discussion of how changes in ESR relaxation times affect signal intensities, as monitored through the transition, will be presented.
ABSTRACT Dye-sensitized solar cells (DSSCs) were manufactured with liquid iodine/iodide electroly... more ABSTRACT Dye-sensitized solar cells (DSSCs) were manufactured with liquid iodine/iodide electrolyte solution in acetonitrile and a solid electrolyte based on plasticized poly(ethylene oxide) (PEO). Two types of titania electrodes manufactured from two-ingredient precursors were used to assess the efficacy of the solid electrolyte and the improvement produced by two chemical treatments. Solid electrolyte devices with a liquid electrolyte pretreatment applied to the electrolyte-oxide interface demonstrated improved photovoltaic performance compared to untreated solid electrolyte devices and solid electrolyte devices given an electrode-bulking sol treatment. Devices using electrodes made from hydrothermally synthesized titanate nanotubes resulted in lower performance than those made from the typically-used titania nanopowder, with significant performance loss resulting from the substitution of solid for liquid electrolyte, indicating the critical dependence of performance on electrolyte infiltration into the oxide layer.
Several perfluoropolyether (PFPE)-based elastomers for high-resolution replica molding applicatio... more Several perfluoropolyether (PFPE)-based elastomers for high-resolution replica molding applications are explored. The modulus of the elastomeric materials was increased through synthetic and additive approaches while maintaining relatively low surface tension values (&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;25 mN/m). Using large area (&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;4 in.(2)) master templates, we experimentally show the relationship between mold resolution and material properties such as modulus and surface tension for materials used in this study. A composite mold approach was used to form flexible molds out of stiff, high modulus materials that allow for replication of sub-20 nm post structures. Sub-100 nm line grating master templates, formed using e-beam lithography, were used to determine the experimental stability of the molding materials. It was observed that as the feature spacing decreased, high modulus PFPE tetramethacrylate (TMA) composite molds were able to effectively replicate the nanograting structures without cracking or tear-out defects that typically occur with high modulus elastomers.
Light trapping strategies are frequently suggested to improve organic photovoltaic (OPV) cell eff... more Light trapping strategies are frequently suggested to improve organic photovoltaic (OPV) cell efficiencies. However, one cannot overlook the side-effects to charge carrier collection which are introduced when seeking optical enhancements. A comprehensive electro-photonic model is utilized to study the optical and electrical effects of patterning poly[[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b 0 ]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] and poly(3-hexylthiophene) based solar cells with simple optical grating structures. It is found that for the most part, optical absorption improvements are attenuated by enhanced electrical losses. Optimized device structures that overcome this tradeoff are proposed and their detailed electrooptical characteristics are discussed. When the hole mobility is smaller than the electron mobility, the results suggest that in general, an inverted structure has a better chance to outperform a flat active layer than a conventional architecture in an OPV cell with the ridge-patterned back electrode.
RESUMEN El objetivo del estudio fue conocer las características socioeconómicas de los productore... more RESUMEN El objetivo del estudio fue conocer las características socioeconómicas de los productores de tres municipios con agricultura de riego en Aguascalientes, y su actitud hacia riesgos de inversión y de mercado. La información se obtuvo por medio de ...
We have investigated the photovoltaic properties of inverted solar cells comprising a bulk hetero... more We have investigated the photovoltaic properties of inverted solar cells comprising a bulk heterojunction film of poly(3-hexylthiophene) and phenyl-C(61)-butyric acid methyl ester, sandwiched between an indium-tin-oxide/Al-doped zinc oxide (ZnO-Al) front, and tungsten oxide/aluminum back electrodes. The inverted solar cells convert photons to electrons at an external quantum efficiency (EQE) exceeding 70%. This is a 10-15% increase over EQEs of conventional solar cells. The increase in EQE is not fully explained by the difference in the optical transparency of electrodes, interference effects due to an optical spacer effect of the metal-oxide electrode buffer layers, or variation in charge generation profile. We propose that a large additional splitting of excited states at the ZnO-Al/polymer interface leads to the considerably large photocurrent yield in inverted cells. Our finding provides new insights into the benefits of n-type metal-oxide interlayers in bulk heterojunction solar cells, namely the splitting of excited states and conduction of free electrons simultaneously.
An array of periodic surface features were patterned on mesoporous niobium oxide films by a soft-... more An array of periodic surface features were patterned on mesoporous niobium oxide films by a soft-lithographic technique with the goal of constructing a photonic crystal (PC) structure on the back side of the oxide. The oxide films, fabricated by mixing sol-gel derived niobium oxide nanoparticles and hydroxypropyl cellulose, were employed as photoelectrodes in dye-sensitized solar cells (DSSCs), and their performance evaluated against their flat counterparts. The surface patterns were imprinted using a photocurable perfluoropolyether (PFPE) soft-replica of a silicon master with a two-dimensional array of cylindrical posts (200 nm (D) × 200 nm (H)) in hexagonal geometry. The PC on the niobium oxide surface caused large changes in optical measurements, particularly in the blue wavelengths. To evaluate the optical effect on solar energy conversion, the incident photon-to-current conversion efficiency (IPCE) was measured in the patterned devices and the control group. The IPCE of patterned niobium oxide anodes exhibited a relative enhancement in photocurrent generation over the wavelength range corresponding to the higher absorption in optical measurements.
Indium tin oxide (ITO) is the premier choice for transparent conductive electrodes in optoelectro... more Indium tin oxide (ITO) is the premier choice for transparent conductive electrodes in optoelectronic devices despite its inherent brittleness. Here we report the fabrication of a grating-like structure that obviates ITO's mechanical limitations while retaining its resistivity and optical qualities. ITO nanopatterned films exhibited a resistivity <1.3 × 10 Ω cm, which surpassed all previously reported values for flexible ITO, with a normal transmission >90% across the whole visible spectrum range. We demonstrate the nanopatterned ITO retains extraordinary flexibility and durability on heat-sensitive substrates, accommodating cyclic bending to a curvature diameter of at least 3.2 mm for over 50 cycles of compressive and decompressive flexing without significant deterioration of its resistivity or optical properties. Moreover, 2-dimensional extrapolation shows that multiaxial bending is also feasible while maintaining mechanical flexibility, durability, and optical transparency.
The major challenge in solar cell technology dwells in achieving an efficient absorption of photo... more The major challenge in solar cell technology dwells in achieving an efficient absorption of photons with an effective carrier extraction. In all cases, light absorption considerations call for thicker modules while carrier transport would benefit from thinner ones. This dichotomy is the fundamental problem limiting the efficiencies of photovoltaics, especially promising low-cost polymer solar cells. We present experimental and theoretical
Organic photovoltaics with nanopatterned photonic crystal photoactive layers offer an alternative... more Organic photovoltaics with nanopatterned photonic crystal photoactive layers offer an alternative to conventional planar devices that suffer from a competition between optical absorption and electronic processes. Our recent studies have shown that nanopatterned devices exhibit enhanced absorption and exciton creation profiles as compared to planar cells. Improved absorption results in part from the excitation of resonant optical modes where certain
Wiley Interdisciplinary Reviews: Nanomedicine and Nanobiotechnology, 2016
Structural coloration has attracted great interest from scientists and engineers in recent years,... more Structural coloration has attracted great interest from scientists and engineers in recent years, owing to fascination with various brilliant examples displayed in nature as well as to promising applications of bio-inspired functional photonic structures and materials. Much research has been done to reveal and emulate the physical mechanisms that underlie the structural colors found in nature. In this article, we review the fundamental physics of many natural structural colors displayed by living organisms as well as their bio-inspired artificial counterparts, with emphasis on their connections, tunability strategies, and proposed applications, which aim to maximize the technological benefits one could derive from these photonic nanostructures. For further resources related to this article, please visit the WIREs website.
ABSTRACTWe have fabricated ordered arrays of gold nanocrystals on FIB-processed silicon substrate... more ABSTRACTWe have fabricated ordered arrays of gold nanocrystals on FIB-processed silicon substrates using electroless deposition. We have also fabricated ordered arrays of silver nanocrystals on silicon with diameters 40–60 nm separated by 180 nm center-to-center, using pulsed-laser deposition (PLD) to deposit silver on the substrate. The metal nanocrystal arrays are characterized using SEM as well as AFM and energy dispersive x-ray (EDX) analysis. AFM confirms particle sizes measured in SEM, and EDX analysis demonstrates that Ag preferentially clusters at sites that have been damaged by the ion beam. These results suggest that the FIB-PLD combination can be used to create ordered arrays of Ag nanocrystals with diameters of 10 nm or less.
Proceedings of SPIE - The International Society for Optical Engineering, 2011
We show the effects of hole shape, size, and periodicity variation on extraordinary transmission ... more We show the effects of hole shape, size, and periodicity variation on extraordinary transmission through a tuned metamaterial of 4 pairs of alternating layers of Ag-Al2O3 with 20 nm thicknesses under illumination of TM light. The advantage of a metamaterial over a Ag film of similar thickness is the tunability of the surface plasmons coupling k-vector. Because their cross sections appear the same in fourier space, incident TM light sees the same structure for circular, square, and rectangular holes transmission is unaffected by these shape variations. Further, as we increase the hole size, the transmission does not exhibit the expected enhancement as both the 250 nm and 150 nm diameter holes are both too small to have their maxima overlap with the tuned transmission enhancement of the metamaterial. For the metamaterial, we are able to tune it to support surface plasmons from periodicities ranging from 130 nm to 215 nm, which effectively shifts the transmission peaks from the blue end of the spectrum in Ag films across the visible and into the red.
Thin film nanostructured photovoltaic cells are increasing in efficiency and decreasing the cost ... more Thin film nanostructured photovoltaic cells are increasing in efficiency and decreasing the cost of solar energy. FDTD modeling of branched nanowire &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;forests&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39; are shown to have improved optical absorption in the visible and near-IR spectra over nanowire arrays alone, with a factor of 5 enhancement available at 1000 nm. Alternate BNW tree configurations are presented, achieving a maximum absorption of over 95% at 500 nm.
Vertically aligned bundles of TiO2 nanocrystals were fabricated by pulsed laser deposition (PLD) ... more Vertically aligned bundles of TiO2 nanocrystals were fabricated by pulsed laser deposition (PLD) and tested as a photoanode material in dye sensitized solar cells (DSSC) using scanning electron microscopy (SEM), light absorption spectroscopy (UV-Vis), and incident photon-to-current efficiency (IPCE) experiments. An optimal background pressure of oxygen during deposition was discovered to produce a photoanode structure that simultaneously achieves high surface
The metal-insulator transition present in chromium doped VO2 nanoscale film has been observed by ... more The metal-insulator transition present in chromium doped VO2 nanoscale film has been observed by electron spin resonance (ESR) spectroscopy. VO2 is a highly correlated electron system with numerous practical applications pertaining to this transition, including ultra-fast optical switching and smart windows. We use Cr^3+ transition metal ions with concentration of order one percent as a probe in a 140 nm film to detect this transition. The film exhibited a four-fold decrease in chromium intensity as temperature increased through the transition temperature (˜67^o C). ESR signal intensities were also used to characterize the hysteretic behavior of this particular transition; these results are in agreement with hysteresis observed through optical means. A discussion of how changes in ESR relaxation times affect signal intensities, as monitored through the transition, will be presented.
ABSTRACT Dye-sensitized solar cells (DSSCs) were manufactured with liquid iodine/iodide electroly... more ABSTRACT Dye-sensitized solar cells (DSSCs) were manufactured with liquid iodine/iodide electrolyte solution in acetonitrile and a solid electrolyte based on plasticized poly(ethylene oxide) (PEO). Two types of titania electrodes manufactured from two-ingredient precursors were used to assess the efficacy of the solid electrolyte and the improvement produced by two chemical treatments. Solid electrolyte devices with a liquid electrolyte pretreatment applied to the electrolyte-oxide interface demonstrated improved photovoltaic performance compared to untreated solid electrolyte devices and solid electrolyte devices given an electrode-bulking sol treatment. Devices using electrodes made from hydrothermally synthesized titanate nanotubes resulted in lower performance than those made from the typically-used titania nanopowder, with significant performance loss resulting from the substitution of solid for liquid electrolyte, indicating the critical dependence of performance on electrolyte infiltration into the oxide layer.
Several perfluoropolyether (PFPE)-based elastomers for high-resolution replica molding applicatio... more Several perfluoropolyether (PFPE)-based elastomers for high-resolution replica molding applications are explored. The modulus of the elastomeric materials was increased through synthetic and additive approaches while maintaining relatively low surface tension values (&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt;25 mN/m). Using large area (&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt;4 in.(2)) master templates, we experimentally show the relationship between mold resolution and material properties such as modulus and surface tension for materials used in this study. A composite mold approach was used to form flexible molds out of stiff, high modulus materials that allow for replication of sub-20 nm post structures. Sub-100 nm line grating master templates, formed using e-beam lithography, were used to determine the experimental stability of the molding materials. It was observed that as the feature spacing decreased, high modulus PFPE tetramethacrylate (TMA) composite molds were able to effectively replicate the nanograting structures without cracking or tear-out defects that typically occur with high modulus elastomers.
Light trapping strategies are frequently suggested to improve organic photovoltaic (OPV) cell eff... more Light trapping strategies are frequently suggested to improve organic photovoltaic (OPV) cell efficiencies. However, one cannot overlook the side-effects to charge carrier collection which are introduced when seeking optical enhancements. A comprehensive electro-photonic model is utilized to study the optical and electrical effects of patterning poly[[4,8-bis[(2-ethylhexyl)oxy] benzo[1,2-b:4,5-b 0 ]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] and poly(3-hexylthiophene) based solar cells with simple optical grating structures. It is found that for the most part, optical absorption improvements are attenuated by enhanced electrical losses. Optimized device structures that overcome this tradeoff are proposed and their detailed electrooptical characteristics are discussed. When the hole mobility is smaller than the electron mobility, the results suggest that in general, an inverted structure has a better chance to outperform a flat active layer than a conventional architecture in an OPV cell with the ridge-patterned back electrode.
RESUMEN El objetivo del estudio fue conocer las características socioeconómicas de los productore... more RESUMEN El objetivo del estudio fue conocer las características socioeconómicas de los productores de tres municipios con agricultura de riego en Aguascalientes, y su actitud hacia riesgos de inversión y de mercado. La información se obtuvo por medio de ...
We have investigated the photovoltaic properties of inverted solar cells comprising a bulk hetero... more We have investigated the photovoltaic properties of inverted solar cells comprising a bulk heterojunction film of poly(3-hexylthiophene) and phenyl-C(61)-butyric acid methyl ester, sandwiched between an indium-tin-oxide/Al-doped zinc oxide (ZnO-Al) front, and tungsten oxide/aluminum back electrodes. The inverted solar cells convert photons to electrons at an external quantum efficiency (EQE) exceeding 70%. This is a 10-15% increase over EQEs of conventional solar cells. The increase in EQE is not fully explained by the difference in the optical transparency of electrodes, interference effects due to an optical spacer effect of the metal-oxide electrode buffer layers, or variation in charge generation profile. We propose that a large additional splitting of excited states at the ZnO-Al/polymer interface leads to the considerably large photocurrent yield in inverted cells. Our finding provides new insights into the benefits of n-type metal-oxide interlayers in bulk heterojunction solar cells, namely the splitting of excited states and conduction of free electrons simultaneously.
An array of periodic surface features were patterned on mesoporous niobium oxide films by a soft-... more An array of periodic surface features were patterned on mesoporous niobium oxide films by a soft-lithographic technique with the goal of constructing a photonic crystal (PC) structure on the back side of the oxide. The oxide films, fabricated by mixing sol-gel derived niobium oxide nanoparticles and hydroxypropyl cellulose, were employed as photoelectrodes in dye-sensitized solar cells (DSSCs), and their performance evaluated against their flat counterparts. The surface patterns were imprinted using a photocurable perfluoropolyether (PFPE) soft-replica of a silicon master with a two-dimensional array of cylindrical posts (200 nm (D) × 200 nm (H)) in hexagonal geometry. The PC on the niobium oxide surface caused large changes in optical measurements, particularly in the blue wavelengths. To evaluate the optical effect on solar energy conversion, the incident photon-to-current conversion efficiency (IPCE) was measured in the patterned devices and the control group. The IPCE of patterned niobium oxide anodes exhibited a relative enhancement in photocurrent generation over the wavelength range corresponding to the higher absorption in optical measurements.
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Papers by Rene Lopez