Showing papers on "Energy conversion efficiency published in 2008"

Single-cycle nonlinear optics.

Abstract: Nonlinear optics plays a central role in the advancement of optical science and laser-based technologies. We report on the confinement of the nonlinear interaction of light with matter to a single wave cycle and demonstrate its utility for time-resolved and strong-field science. The electric field of 3.3-femtosecond, 0.72-micron laser pulses with a controlled and measured waveform ionizes atoms near the crests of the central wave cycle, with ionization being virtually switched off outside this interval. Isolated sub-100-attosecond pulses of extreme ultraviolet light (photon energy {approx} 80 electron volts), containing {approx} 0.5 nanojoule of energy, emerge from the interaction with a conversion efficiency of {approx} 10{sup -6}. These tools enable the study of the precision control of electron motion with light fields and electron-electron interactions with a resolution approaching the atomic unit of time ({approx} 24 attoseconds).

Electrospun nanofibers in energy and environmental applications

Abstract: Nanotechnology is providing new solutions and opportunities to ensure sustainable energy and environments for the future. Materials of nanofiberous morphology are attractive to solve numerous energy and environmental issues. Nanofibers can be effectively produced by electrospinning, which is a simple and low cost technique. In addition, electrospinning allows the production of nanofibers from various materials e.g. organics and inorganics in different configurations and assemblies. This is highly beneficial for energy devices, where inorganic materials especially metal oxides can be synthesized and electrospun, improving conducting and ceramic properties. Excitonic solar cells fabricated with aligned nanofiberous metal oxide electrodes provide higher solar–electric energy conversion efficiency, whereas fuel cells made with nanofiberous electrodes enable uniform dispersion of catalysts, and thus increase electrocatalytic activity to obtain higher chemical–electric energy conversion efficiency. The nanofibers used in filtration membranes for environmental remediation, minimize the pressure drop and provide better efficiency than conventional fiber mats. The large surface area-to-volume ratio of nanofiber membranes allows greater surface adsorption of contaminants from air and water, and increases the life-time of the filtration media. This review highlights the potential and application of electrospun nanofiberous materials for solving critical energy and environmental issues.

Air-stable inverted flexible polymer solar cells using zinc oxide nanoparticles as an electron selective layer

Abstract: The performance and stability of unencapsulated inverted bulk-heterojunction solar cells with zinc oxide (ZnO) made by different processes as the electron selective contact are compared to conventional bulk-heterojunction solar cells. The low temperature processed inverted devices using ZnO nanoparticles on indium tin oxide plastic substrates showed high power conversion efficiency of ∼3.3%. This inverted device structure possessed much better stability under ambient conditions retaining over 80% of its original conversion efficiency after 40days while the conventional one showed negligible photovoltaic activity after 4days. This is due to the improved stability at the poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)/Ag interface.

Aggregation of ZnO Nanocrystallites for High Conversion Efficiency in Dye‐Sensitized Solar Cells

Abstract: As a relatively new class of photovoltaic devices with a photoelectrochemical system consisting of a dye-sensitized semiconductor film and an electrolyte, dye-sensitized solar cells (DSSCs) have been regarded as a promising alternative to conventional solid-state semiconductor solar cells. They are relatively cost-effective, are easy to manufacture, and can be readily shaped with flexible substrates to satisfy the demands of various applications. Avery important feature of DSSCs is the photoelectrode, which includes mesoporous wide-bandgap oxide semiconductor films with an enormous internal surface area, typically a thousand times larger than that of bulk films. To date, the highest solar-to-electric conversion efficiency of over 11% has been achieved with films that consist of 20-nm TiO2 nanocrystallites sensitized by ruthenium-based dyes. However, further improving the energy conversion efficiency of DSSCs remains a challenge. Competition between the generation and recombination of photoexcited carriers in DSSCs is a main bottleneck for developing higher conversion efficiency. One possible solution is to use one-dimensional nanostructures that are able to provide a direct pathway for the rapid collection of photogenerated electrons and, therefore, reduce the degree of charge recombination. However, such one-dimensional nanostructures seem to have insufficient internal surface area, which limits their energy conversion efficiency at a relatively low level, for example, 1.5% for ZnO nanowires and 4.7% for TiO2 nanotubes. [9] Another way to increase efficiency is to increase the light-harvesting capability of the photoelectrode film by utilizing optical enhancement effects, which can be achieved by means of light scattering by introducing scatterers into the photoelectrode film. Usami, Ferber and Luther, and Rothenberger et al. have demonstrated theoretically that optical absorption by TiO2 nanocrystalline films can be promoted by additionally admixing large TiO2 particles in an optimal volume ratio. This idea was verified experimentally when TiO2 nanocrystalline films were combined with large SiO2, Al2O3, or TiO2 particles. [14–17] By coupling a photonic crystal layer to conventional TiO2 nanocrystalline films as the light scatterer, Nishimura et al. and Halaoui et al. also succeeded in enhancing the light-harvesting capability of solar-cell photoelectrodes. However, the drawback is that the introduction of larger particles into nanocrystalline films will unavoidably lower the internal surface area of the photoelectrode film and, therefore, counteract the enhancement effect of light scattering on the optical absorption, whereas the incorporation of a layer of TiO2 photonic crystal may lead to an undesirable increase in the electron diffusion length and, consequently, increase the recombination rate of photogenerated carriers. Herein we report hierarchically structured ZnO films as the photoelectrodes in DSSCs for the enhancement of energy conversion efficiency. The films are comprised of polydisperse ZnO aggregates consisting of nanosized crystallites. The aggregates are submicrometer-sized and, thus, can function as efficient light scatterers, while the nanocrystallites provide the films with the necessary mesoporous structure and large internal surface area. An overall energy conversion efficiency up to 5.4% has been achieved from the film including polydisperse ZnO aggregates, much higher than 1.5–2.4% for ZnO nanocrystalline films, 0.5–1.5% for ZnO nanowire films, 23] and 2.7–3.5% for uniform ZnO aggregate films. Polydisperse ZnO aggregates were synthesized by the hydrolysis of zinc salt in polyol medium at 160 8C, similar to the method reported by Jezequel et al. Rapid heating at a rate of 10 8Cmin 1 was intentionally used to obtain polydisperse aggregates, that is, with a relatively wide size distribution. The resulting colloidal dispersion was drop-cast onto a fluorine-doped tin oxide (FTO) coated glass substrate to form a film of approximately 9 mm in thickness, and the film was subsequently annealed at 350 8C for 1 h in air to remove residual solvents and any organic compounds as well as to improve the contact between the film and the substrate and the connection between the nanocrystallites and between the aggregates. Figure 1 shows the scanning electron microscopy (SEM) images of ZnO film with polydisperse aggregates and a schematic illustration showing the structure of an aggregate. Figure 1a indicates that the film is well stacked with submicrometer-sized ZnO aggregates. Figure 1b presents the highly disordered structure of the film assembled by polydisperse ZnO aggregates with diameters ranging from several tens to several hundreds of nanometers. Figure 1c is a magnified SEM image of an individual ZnO aggregate, revealing that the ZnO aggregate is nearly spherical in [*] Dr. Q. F. Zhang, Dr. T. P. Chou, B. Russo, Prof. G. Z. Cao Department of Materials Science and Engineering University of Washington Seattle, WA 98195 (USA) Fax: (+1)206-543-3100 E-mail: gzcao@u.washington.edu Homepage: http://depts.washington.edu/solgel/

Application of Highly Ordered TiO2 Nanotube Arrays in Flexible Dye-Sensitized Solar Cells

Abstract: TiO2 nanotube arrays prepared by electrochemical anodization of Ti foils show impressive light to electricity conversion efficiency in the dye-sensitized solar cells (DSCs). The length of the TiO2 nanotube arrays (5−14 µm) was controlled by varying the anodization time from 2 to 20 h. The influence of nanotube lengths on the photovoltaic performance of DSCs was investigated by impedance. A flexible DSC using TiO2 nanotube arrays on a Ti foil as a working electrode and polyethylene naphthalate (ITO/PEN) as counterelectrode in combination with solvent-free ionic liquid electrolyte achieved 3.6% photovoltaic conversion efficiency under simulated AM 1.5 sunlight.

An inverted organic solar cell employing a sol-gel derived ZnO electron selective layer and thermal evaporated MoO3 hole selective layer

Abstract: We reported an efficient inverted bulk-heterojunction [regioregular of poly(3-hexylthiophene): (6,6)-phenyl C61 butyric acid methyl ester] solar cell with a highly transparent sol-gel derived ZnO film as electron selective layer and MoO3 as hole selective layer. By modifying the precursor concentration of sol from 0.75 to 0.1M, the optical transmittance of ZnO film increases from 75% to 95%. This improvement in transmittance increases the short-circuit density of inverted solar cell from 5.986 to 8.858 mA/cm2 without sacrificing the open-circuit voltage and fill factor of the device. We also demonstrated that the device incorporated with MoO3 has a larger open-circuit voltage and fill factor than the device without MoO3. Power conversion efficiency of 3.09% was achieved under simulated AM 1.5G illumination of 100 mW/cm2.

Organic Solar Cells Using Nanoimprinted Transparent Metal Electrodes

Abstract: Cost effective and highly efficient renewable energy is becoming ever more important in our age of rising energy prices and global climate change. Solar energy is a nonexhaustible and green energy. Organic solar cells (OSC) have the merits of low cost and simplistic fabrication in addition to compatibility with flexible plastic substrates over large areas. They have therefore been considered a promising energy conversion platform for clean and carbon-neutral energy production. In recent years, the power conversion efficiency of OSCs based on conjugated polymers has steadily increased through improved energy harvesting, enhanced exciton separation in improved device structures, and optimization of processing parameters, e.g., solvent evaporation time, and annealing conditions. Most OSCs are built on indium tin oxide (ITO) coated substrates because ITO offers transparency in the visible range of the electromagnetic spectrum as well as good electrical conductivity. However, ITO is not the optimum electrode for solar cell applications as it has been reported that the band structure of ITO hinders efficient photocurrent generation. Moreover, the poor mechanical stability of ITO can cause device failure when an ITO-coated flexible substrate is bent. In addition, the limited supply of indium and the increasing demand from the rapidly expanding display market have increased the cost of ITO drastically, which potentially prevents the realization of low cost and large scale OSC fabrication. Therefore, there is a strong need to find alternative materials that can replace ITO as high transparency electrode. Some examples that have been investigated recently are nanotube networks, and Ag wire grids. In this communication, we report on high transparency metal wire grid electrodes for organic solar cell applications. The high transparency metal electrodes are fabricated by nanoimprint lithography (NIL), and have several advan-

Highly efficient inverted polymer solar cell by low temperature annealing of Cs2CO3 interlayer

Abstract: We demonstrate a highly efficient inverted bulk heterojunction polymer solar cell based on regioregular poly(3-hexylthiophene):[6,6]-phenyl C61 butyric acid methyl ester with a low temperature annealed interfacial buffer layer, cesium carbonate (Cs2CO3). This approach improves the power conversion efficiency of the inverted cell from 2.3% to 4.2%, with short-circuit current of 11.17mA∕cm2, open-circuit voltage of 0.59V, and fill factor of 63% under AM1.5G 100mW∕cm2 irradiation. This result is comparable to the previous regular structure device on the same system. Ultraviolet photoelectron spectroscopy shows that the work function of annealed Cs2CO3 layer decreases from 3.45to3.06eV. Further x-ray photoelectron spectroscopy results reveal that Cs2CO3 can decompose into low work function, doped cesium oxide Cs2O upon annealing, which is accountable for the work-function reduction and device efficiency improvement.

Surface passivation of high‐efficiency silicon solar cells by atomic‐layer‐deposited Al2O3

Abstract: Atomic-layer-deposited aluminium oxide (Al2O3) is applied as rear-surface-passivating dielectric layer to passivated emitter and rear cell (PERC)-type crystalline silicon (c-Si) solar cells. The excellent passivation of low-resistivity p-type silicon by the negative-charge-dielectric Al2O3 is confirmed on the device level by an independently confirmed energy conversion efficiency of 20·6%. The best results are obtained for a stack consisting of a 30 nm Al2O3 film covered by a 200 nm plasma-enhanced-chemical-vapour-deposited silicon oxide (SiOx) layer, resulting in a rear surface recombination velocity (SRV) of 70 cm/s. Comparable results are obtained for a 130 nm single-layer of Al2O3, resulting in a rear SRV of 90 cm/s. Copyright © 2008 John Wiley & Sons, Ltd.

Vibration energy harvesting by magnetostrictive material

Abstract: A new class of vibration energy harvester based on magnetostrictive material (MsM), Metglas 2605SC, is designed, developed and tested. It contains two submodules: an MsM harvesting device and an energy harvesting circuit. Compared to piezoelectric materials, the Metglas 2605SC offers advantages including higher energy conversion efficiency, longer life cycles, lack of depolarization and higher flexibility to survive in strong ambient vibrations. To enhance the energy conversion efficiency and alleviate the need of a bias magnetic field, Metglas ribbons are transversely annealed by a strong magnetic field along their width direction. To analyze the MsM harvesting device a generalized electromechanical circuit model is derived from Hamilton’s principle in conjunction with the normal mode superposition method based on Euler‐Bernoulli beam theory. The MsM harvesting device is equivalent to an electromechanical gyrator in series with an inductor. In addition, the proposed model can be readily extended to a more practical case of a cantilever beam element with a tip mass. The energy harvesting circuit, which interfaces with a wireless sensor and accumulates the harvested energy into an ultracapacitor, is designed on a printed circuit board (PCB) with plane dimension 25 mm × 35 mm. It mainly consists of a voltage quadrupler, a 3 F ultracapacitor and a smart regulator. The output DC voltage from the PCB can be adjusted within 2.0‐5.5 V. In experiments, the maximum output power and power density on the resistor can reach 200 μW and 900 μ Wc m −3 , respectively, at a low frequency of 58 Hz. For a working prototype under a vibration with resonance frequency of 1.1 kHz and peak acceleration of 8.06 m s −2 (0.82 g), the average power and power density during charging the ultracapacitor can achieve 576 μ Wa nd 606 μ Wc m −3 , respectively, which compete favorably with piezoelectric vibration energy harvesters. (Some figures in this article are in colour only in the electronic version)

A luminescent solar concentrator with 7.1% power conversion efficiency

Abstract: The Luminescent Solar Concentrator (LSC) consists of a transparent polymer plate, containing luminescent particles. Solar cells are connected to one or more edges of the polymer plate. Incident light is absorbed by the luminescent particles and re-emitted. Part of the light emitted by the luminescent particles is guided towards the solar cells by total internal reflection. Since the edge area is smaller than the receiving one, this allows for concentration of sunlight without the need for solar tracking. External Quantum Efficiency (EQE) and current–voltage (I –V) measurements were performed on LSC devices with multicrystalline silicon (mc-Si) or GaAs cells attached to the sides. The best result was obtained for an LSC with four GaAs cells. The power conversion efficiency of this device, as measured at European Solar Test Installation laboratories, was 7.1% (geometrical concentration of a factor 2.5). With one GaAs cell attached to one edge only, the power efficiency was still as high as 4.6% (geometrical concentration of a factor 10). To our knowledge these efficiencies are among the highest reported for the LSC. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

High efficiency n-type Si solar cells on Al2O3-passivated boron emitters

Abstract: In order to utilize the full potential of solar cells fabricated on n-type silicon, it is necessary to achieve an excellent passivation on B-doped emitters. Experimental studies on test structures and theoretical considerations have shown that a negatively charged dielectric layer would be ideally suited for this purpose. Thus, in this work the negative-charge dielectric Al2O3 was applied as surface passivation layer on high-efficiency n-type silicon solar cells. With this front surface passivation layer, a confirmed conversion efficiency of 23.2% was achieved. For the open-circuit voltage Voc of 703.6mV, the upper limit for the emitter saturation current density J0e, including the metalized area, has been evaluated to be 29fA∕cm2. This clearly shows that an excellent passivation of highly doped p-type c-Si can be obtained at the device level by applying Al2O3.

Bamboo-type TiO2 nanotubes: improved conversion efficiency in dye-sensitized solar cells.

Abstract: Bamboo-type TiO2 nanotube layers were produced by alternating voltage anodization of Ti. In comparison to smooth TiO2 morphologies the stratified nanotube structure shows significantly increased light harvesting and conversion efficiencies when used in dye sensitized solar cells.

Vertically Aligned p-Type Single-Crystalline GaN Nanorod Arrays on n-Type Si for Heterojunction Photovoltaic Cells

Abstract: Vertically aligned Mg-doped GaN nanorods have been epitaxially grown on n-type Si substrate to form a heterostructure for fabricating p-n heterojunction photovoltaic cells. The p-type GaN nanorod/n-Si heterojunction cell shows a well-defined rectifying behavior with a rectification ratio larger than 10(4) in dark. The cell has a high short-circuit photocurrent density of 7.6 mAlcm2 and energy conversion efficiency of 2.73% under AM 1.5G illumination at 100 mW/cm2. Moreover, the nanorod array may be used as an antireflection coating for solar cell applications to effectively reduce light loss due to reflection. This study provides an experimental demonstration for integrating one-dimensional nanostructure arrays with the substrate to directly fabricate heterojunction photovoltaic cells.

Polydisperse Aggregates of ZnO Nanocrystallites: A Method for Energy‐Conversion‐Efficiency Enhancement in Dye‐Sensitized Solar Cells

Abstract: ZnO films consisting of either polydisperse or monodisperse aggregates of nanocrystallites were fabricated and studied as dye-sensitized solar-cell electrodes. The results revealed that the overall energy-conversion efficiency of the cells could be significantly affected by either the average size or the size distribution of the ZnO aggregates. The highest overall energy-conversion efficiency of ∼4.4% was achieved with the film formed by polydisperse ZnO aggregates with a broad size distribution from 120 to 360 nm in diameter. Light scattering by the submicrometer-sized ZnO aggregates was employed to explain the improved solar-cell performance through extending the distance travelled by light so as to increase the light-harvesting efficiency of photoelectrode film. The broad distribution of aggregate size provides the ZnO films with both better packing and an enhanced ability to scatter the incident light, and thus promotes the solar-cell performance.

A 97.8% Efficient GaN HEMT Boost Converter With 300-W Output Power at 1 MHz

Abstract: A 175-to-350 V hard-switched boost converter was constructed using a high-voltage GaN high-electron-mobility transistor grown on SiC substrate. The high speed and low on-resistance of the wide-band-gap device enabled extremely fast switching transients and low losses, resulting in a high conversion efficiency of 97.8% with 300-W output power at 1 MHz. The maximum efficiency was 98.0% at 214-W output power, well exceeding the state of the art of Si-based converters at similar frequencies.

Efficient squaraine-based solution processable bulk-heterojunction solar cells.

Abstract: We report the fabrication of organic bulk-heterojunction solar cells based, for the first time, on squaraine/PCBM blends. The most efficient device, solution-processed in air, exhibits Jsc = 5.70 mA/cm2, Voc = 0.62 V, fill-factor = 0.35, and power conversion efficiency = 1.24%, one of the highest to date for a small molecule solution-processed bulk-heterojunction cell.

Performance analysis of near-field thermophotovoltaic devices considering absorption distribution

Abstract: This paper elucidates the energy transfer and conversion processes in near-field thermophotovoltaic (TPV) systems, considering local radiation absorption and photocurrent generation in the TPV cell. Radiation heat transfer in a multilayered structure is modeled using the fluctuation–dissipation theorem, and the electric current generation is evaluated based on the photogeneration and recombination of electron–hole pairs in different regions of the TPV cell. The effects of near-field radiation on the photon penetration depth, photocurrent generation, and quantum efficiency are examined in the spectral region of interest. The detailed analysis performed in the present work demonstrates that, while the near-field operation can enhance the power throughput, the conversion efficiency is not much improved and may even be reduced. Subsequently, a modified design of near-field TPV systems is proposed to improve the efficiency.

Schottky-quantum dot photovoltaics for efficient infrared power conversion

Abstract: Planar Schottky photovoltaic devices were prepared from solution-processed PbS nanocrystal quantum dot films with aluminum and indium tin oxide contacts. These devices exhibited up to 4.2% infrared power conversion efficiency, which is a threefold improvement over previous results. Solar power conversion efficiency reached 1.8%. The simple, optimized architecture allows for direct implementation in multijunction photovoltaic device configurations.

Pyroelectric energy conversion: Optimization principles

Abstract: In the framework of microgenerators, we present in this paper the key points for energy harvesting from temperature using ferroelectric materials. Thermoelectric devices profit from temperature spatial gradients, whereas ferroelectric materials require temporal fluctuation of temperature, thus leading to different applications targets. Ferroelectric materials may harvest perfectly the available thermal energy whatever the materials properties (limited by Carnot conversion efficiency) whereas thermoelectric material's efficiency is limited by materials properties (ZT figure of merit). However, it is shown that the necessary electric fields for Carnot cycles are far beyond the breakdown limit of bulk ferroelectric materials. Thin films may be an excellent solution for rising up to ultra-high electric fields and outstanding efficiency. Different thermodynamic cycles are presented in the paper: principles, advantages, and drawbacks. Using the Carnot cycle, the harvested energy would be independent of materials properties. However, using more realistic cycles, the energy conversion effectiveness remains dependent on the materials properties as discussed in the paper. A particular coupling factor is defined to quantify and check the effectiveness of pyroelectric energy harvesting. It is defined similarly to an electromechanical coupling factor as k2 = p2thetas0/(epsivthetas 33 CE), where p, thetas0, epsivthetas 33, Ce are pyroelectric coefficient, maximum working temperature, dielectric permittivity, and specific heat, respectively. The importance of the electrothermal coupling factor is shown and discussed as an energy harvesting figure of merit. It gives the effectiveness of all techniques of energy harvesting (except the Carnot cycle). It is finally shown that we could reach very high efficiency using lang111rang0.75Pb(Mg1/3Nb2/3)-0.25PbTiO3 single crystals and synchronized switch harvesting on inductor (almost 50% of Carnot efficiency). Finally, practical implementation key points of pyroelectric energy harvesting are presented showing that the different thermodynamic cycles are feasible and potentially effective, even compared to thermoelectric devices.

Efficient Polythiophene/Polyfluorene Copolymer Bulk Heterojunction Photovoltaic Devices: Device Physics and Annealing Effects†

Abstract: Here the influence of annealing on the operational efficiency of all-polymer solar cells based on blends of the polymers poly(3-hexylthiophene) (P3HT) and poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthiophen-5-yl)-2,1,3-benzothiadiazole]-2′,2″-diyl) (F8TBT) is investigated. Annealing of completed devices is found to result in an increase in power conversion efficiency from 0.14 to 1.20%, while annealing of films prior to top electrode deposition increases device efficiency to only 0.19% due to a lowering of the open-circuit voltage and short-circuit current. By studying the dependence of photocurrent on intensity and effective applied bias, annealing is found to increase charge generation efficiency through an increase in the efficiency of the separation of bound electron-hole pairs following charge transfer. However, unlike many other all-polymer blends, this increase in charge separation efficiency is not only due to an increase in the degree of phase separation that assists in the spatial separation of electron-hole pairs, but also due to an order of magnitude increase in the hole mobility of the P3HT phase. The increase in hole mobility with annealing is attributed to the ordering of P3HT chains evidenced by the red-shifting of P3HT optical absorption in the blend. We also use X-ray photoelectron spectroscopy (XPS) to study the influence of annealing protocol on film interface composition. Surprisingly both top and bottom electrode/blend interfaces are enriched with P3HT, with the blend/top electrode interface consisting of more than 95% P3HT for as-spun films and films annealed without a top electrode. Films annealed following top electrode deposition, however, show an increase in F8TBT composition to ∼15%. The implications of interfacial composition and the origin of open-circuit voltage in these devices are also discussed.

Metal and dielectric nanoparticle scattering for improved optical absorption in photovoltaic devices

Abstract: The influence of electromagnetic scattering by Au and silica nanoparticles placed atop silicon photovoltaic devices on absorption and photocurrent generation has been investigated. The nanoparticles produce substantial increases in power transmission into the semiconductor and consequently photocurrent response from ∼500to>1000nm. Increases in power conversion efficiency under simulated solar irradiation of up to 8.8% are observed experimentally, and numerical simulations provide quantitatively accurate predictions of these observed enhancements. Additional simulations indicate that these concepts can be applied to a broad range of photovoltaic device structures, including those based on low-index materials for which conventional antireflection coatings are problematic.

Molecular Design of Coumarin Dyes for Stable and Efficient Organic Dye-Sensitized Solar Cells

Abstract: A series of coumarin dyes (NKX-2593, NKX-2807, and NKX-2883) with one or two -CN groups as electron acceptors were synthesized and applied as dye sensitizers for dye-sensitized solar cells. Compared with the dye containing one -CN group, linking one more -CN group to the π-conjugation bridge positively shifts the lowest unoccupied molecular orbital and thus red-shifts the maximum absorption band, harvesting more photons in the long-wavelength region for photoelectric conversion. Among the three dyes studied, NKX-2883 showed the best photovoltaic performance, yielding 7.6% power conversion efficiency using a volatile electrolyte and demonstrating good photostability under visible light soaking with 6% of power conversion efficiency for 1000 h using a nonvolatile electrolyte.

Simultaneous Use of Small‐ and Wide‐Angle X‐ray Techniques to Analyze Nanometerscale Phase Separation in Polymer Heterojunction Solar Cells

Abstract: Using grazing-incidence small-angle X-ray scattering and wide-angle X-ray diffraction techniques to analyze the nanoscale phase separation of P3HT and PCBM after annealing, the effects of the sizes of the PCBM clusters and P3HT crystallites on the power conversion efficiency of bulk heterojunction solar cells is studied. This approach allowed us to investigate the effects of the sizes of the PCBM clusters and P3HT crystallites on the power conversion efficiencies of bulk heterojunction solar cells. It appears that improved power conversion efficiency requires the value of Rg of the PCBM clusters to be greater than 20nm and the value of D100 of the P3HT crystallites to be greater than 16nm for an active layer thickness of ca. 100nm.

Control of the nanoscale crystallinity and phase separation in polymer solar cells

Abstract: Grazing-incidence x-ray diffraction and atomic force microscopy were performed on bulk heterojunction regioregular poly(3-hexylthiophene) (RR-P3HT) [6,6]-phenyl-C71-butyric acid methyl esters spin-cast films with different film processing conditions to correlate the crystalline nanostructure of P3HT with the corresponding solar cell performance. The increase in long wavelength absorption for solvent annealed films is related to highly conjugated crystal structure of RR-P3HT phase-separated in the active layer. Upon thermal annealing, the solvent annealed 50-nm-thick device shows high solar cell performance with fill factor up to 73% and power conversion efficiency of 3.80%.