In order to realise a cost effective solution for solar energy mounting effort has been expended over the last 20 years developing low cost printable solar cells. Historically, the two competing concepts of organic and dye-sensitized solar cells have been battling to reach commercial viability and now both concepts generate over 10% solar power conversion efficiency. However, simultaneously, the cost of crystalline silicon solar cells has continuously dropped, making both low cost and higher efficiency a requirement for the emerging solar technologies. Generating high currents is half the story, however the maximum attainable power conversion efficiency is limited by the fundamental energy losses required to separate excitons and collect free charge carriers in the typically disordered semiconductors. This loss can be quantified as the difference in electrical potential energy between the band gap of the semiconductor and the open-circuit voltage of the solar cell. For organic and dye-sensitized solar cells this loss-in-potential is typically 0.65 to 0.8 eV, but for a single junction solar cell the theoretical minimum, as determined by the Shockley Quasar limit is in the region of 0.25 eV. Here I will present a new hybrid solar cell concept based on a printable mesoporous superstructured self-assembled semiconductor combined with an organic hole conductor. The minimal fundamental losses result in a loss-in-potential as small as 350 meV and the full sun power conversion efficiency is over 10% in a single junction device. I will describe the concept, operating principles, recent progress and a likely path to over 20% solar power conversion efficiency.

Over the past few years, the quantum dot sensitized solar cells (QDSCs) have an explosive growth as they show promise toward next generation photovoltaic devices. We have recently focused on the fabrication of double-layers nanostructures that can be used as electrodes for QDSSC. In this presentation, I will describe our recent results on double layers ZnO photoanode - top layer of ZnO nanotetrapods and bottom layer of ZnO nanoparticles or ZnO nanoarrays. Our approach of double-layered structure primarily addresses the following points: 1) prevent the back electron transfer from conducting layer (FTO) to electrolyte; and 2) utilize ZnO intrinsically excellent electronic properties to easily transport the carries rapidly to the collections elecdrode. Then through secondary growth, the hierarchical nanostructures consisting of branched ZnO nanostructures are amenable to assembly into a highly connected network with excellent electron transport, and exhibit over 5% power conversion efficiency (PEC) with co-sensitizing CdS and CdSe semiconductor nanocrystals.

The assembly of nanoscale building blocks in engineered mesostructures is one of the fundamental goals of nanotechnology. In this communication, we report on a novel fabrication method exploiting self-assembly from the gas-phase: Scattered Ballistic Deposition (SBD). SBD is a general physical phenomenon that arises from the interaction of a supersonic molecular beam with an ambient gas and enables the growth of quasi-1D hierarchical mesostructures (HM). Overall, they resemble a forest composed of individual, high aspect-ratio, tree-like structures, assembled from crystalline nanoparticles. The hierarchical quasi-1D nature of each tree represents an innovative compromise between nanorods/nanotubes (better electron transport) and the conventional isotropic nanoparticle photoanode (high surface area). We demonstrate the successful application of the novel quasi-1D hierarchical TiO2 mesostructures deposited by Pulsed Laser Deposition assisted SBD onto FTO coated glass for two different solar energy conversion and storage technologies: Dye Sensitized Solar Cells and Photoelectrochemical hydrogen generation. The so-fabricated Hierarchical Mesoporous Photoanodes (HMP) exhibit tunable properties controlled by deposition conditions and thermal treatments: high surface area (from 50 to 350 m2g-1); roughness factors up to 100; porosity and pore size distribution (5-20 nm); grain size (10-30 nm); refractive index (1.5-1.9); amorphous or anatase phase. In contrast to typical nanoparticle based mesoporous photoanodes (NMP), they are characterized by vertical channels through the entire thickness that assure execellent diffusional path for liquid electrolytes and easy infiltration for solid hole transporting materials. Optimized photoanodes show enhanced light trapping capabilities with high broadband scattering efficiency. Hence, upon sensitization of the transparent TiO2 HMP with visible light sensitive species, like dyes or Quantum Dots, they show increased optical density and broader absorption features with respect to standard NMP. These characteristics make the novel HMP ideal for solar capture and conversion technologies employing liquid or solid electrolytes. As an example, Solid State Dye Sensitized Solar Cells (SSDSC) fabricated with the novel hierarchical TiO2 photoanode show higher short circuit current, Jsc, in accordance to the higher optical density measured, and higher power conversion efficiency than conventional NMPs. A similar effect is obtained when the HMPs are used for hydrogen generation, upon sensitization with CdS QDs by the successive ionic layer adsorption and reaction (SILAR) technique. The as synthesized CdS/TiO2 electrodes showed higher photocurrent density than pure nanostructure TiO2 electrode and of CdS sensitized NMP.

Highly efficient nanostructured solar cells based on quantum dot sensitizers are presented. Inorganic or organic-inorganic hybrid quantum dots are directly deposited on mesoporous TiO2 film using a spin coating or a successive ionic layer absorption and reaction method, where the deposited sensitizer plays a role in absorbing light, thereby generating electrons and holes, and the TiO2 layer acts as an electron acceptor. Either a hole transporting material or a redox electrolyte can be used as an electron donor. For the case of perovskite-type organic-inorganic hybrid sensitizers, conversion efficiency of 6.5% was achieved at AM 1.5G 1 sun illumination when it was contacted with iodide-based redox electrolyte. Further improvement was achieved using a hole transport material, where efficiency as high as 9% was demonstrated based on ca. 1 mu;m-thick TiO2 layer. For the case of chalcogenide-based quantum dot sensitizers, photocurrent density exceeding 26 mA/cm2 was achieved by controlling metal-sulfur chemical bonding nature. The demonstrated photovoltaic properties are so far the highest performance among the reported quantum-dot-sensitized solar cells.

Thin films are widely used in various applications, including but not limited to simple reflective coatings for mirrors, electrodes for lithium batteries, conducting substrates for electronic circuits, gas sensors and solar cells. As the scope of their applications has widened over the years so has the need to obtain different structural motifs for thin films. A large variety of fabrication techniques are commonly employed to obtain these structures. Pulsed laser deposition (PLD) can be used to obtain fims varying from extremely compact and only a few angstroms thick to micron thick porous structures.In this work we introduce a model for predicting different structures as a function of laser parameters and deposition environments in a pulsed laser deposition system.cThis is followed by a comparison of simulated and experimentally obtained structures. This model is then used to obtain unique hierarchical structures especially suitable for photophysical applications. We investigate the superiority of this unique structure over random nanoparticle networks as photoanodes for dye- sensitized solar cells (DSSC). Different metal oxides (TiO2, Nb2O5, Ta doped TiO2) are grown using this method and their photophysical behaviors are then compared.

Light trapping in photovoltaic devices has recently attracted immense attention because of the intrinsic limitation in film thickness of the photo-induced charge generation layer. To compensate for the limited light absorption within a given thickness related to the charge carrier recombination, particularly in thin film solar cells, there have been many trials on how to efficiently trap light. With relevance to dye-sensitized solar cells (DSCs) which have low-cost of fabrication and high power conversion efficiency, the light trapping strategy is particularly useful to enhance the photo current generation as well as the power conversion efficiency. In the present study, we introduce a novel strategy to trap incident light effectively by three dimensional patterned TiO2 photoanodes in DSCs. Among different geometries of electrodes fabricated by the soft lithographic technique, pyramid-shaped TiO2 photoanodes show the highest absorbance and photocurrent-voltage performance, which originate from the total reflection at the interfaces between TiO2 photoanodes and bulk electrolyte. In order to create even cheaper three dimensional pattern masters, randomized pyramid structures were developed using the texturing of crystalline silicon substrates with anisotropic wet etching. Furthermore, by the combination of randomized pyramids with a scattering layer, we demonstrate the 36% increase in the power conversion efficiency of DSCs over the conventional planar photoanodes.

Fabrication of a structured substrate for dye-sensitized solar cells (DSSCs) is a new approach to improve the performance of DSSCs. There are several approaches to improve the energy conversion efficiency of DSSCs. In the past, studies of new dyes, electrolytes, semiconducting materials and nanostructures for photoanodes have been extensively investigated; however, there have been few studies of the conductive substrate to improve the efficiency by reducing the carrier collection distances. In this research, we focused on the fabrication of patterned conductive oxides, ITO (In doped tin oxide) and FTO (F doped tin oxide), on conductive glass substrates. The conductive oxides are deposited by pulsed laser deposition through a polymer mask to create an array of conical conductive pillars. The optical and electronic properties of these patterned structures were studied. Complete devices were also fabricated depositing nano-TiO2 on the substrates, with and without pattern for comparison. Photoelectrochemical characterizations were employed and the performance of these devices was investigated.

We investigated the photovoltaic performance of DSSCs based on hierarchical twin-scale inverse opal TiO2 electrodes. The colloidal assembly of meso-scale polystyrene particles in macro-scale inverse opal structures produced templates for the meso-scale iverse opal electrodes. We investigated the influence of electrodes thickness and diameter of meso-pores. Electron transport characteristics were investigated by using a electrochemical impedance spectroscopy. The result showed that smaller pores performed higher efficiencies, due to high specific area and the low electron injection resistance. We achieved a maximum photovoltaic conversion efficiency of 6.90% using 12 mu;m thick electrodes with a meso-pore diameter of 35 nm

Three-dimensional highly-branched nanosuperstructures have shown substantially improved efficiency for water splitting owning to their long optical path for efficient light absorption, short 1-D conducting channels for rapid electron-hole separation and charge transportation, as well as high surface areas for fast interfacial charge transfer and electrochemical reactions. However, methods for synthesis of these 3-D nanostructures are usually complicated, time-consuming and costly. In this work, we investigated a rational one-step strategy to synthesize ZnO 3-D superstructures via chemical vapor deposition by designed catalysts. By precisely engineering the morphology of the catalysts, we have obtained 3-D superstructures from 1-D nanowire catalysts and from 2-D network catalysts (porous gold) in a large area, respectively. Compared with 1-D ZnO naonwire arrays grown from commonly used 0-D dot catalysts, dramatic enhancement of photoelectrochemical (PEC) efficiency (150%) was observed from PEC anode made of 3-D nanosuperstructures. This result may inspire a general paradigm for synthesis of 3-D semiconductor nanostructures for various applications.

Numerous three dimensional (3D) nanofabrication methods have been proposed for novel applications in energy devices, optical components, catalyst supports, metamaterials, and etc. However, highly periodic 3D fabrication in large area and volume has limited success. Here I present our recent efforts to expand the limit in size of highly periodic 3D nanostructures through Proximity field nanoPatterning which uses conformal phase masks with outstanding scalability and easiness of the large area patterning. After brief overview of 3D nanofabrication technique and potential application fields, technical advantages of PnP will be discussed. Current strategy to increase the in plane size of conformal phase masks and photosensitive material promises 5 by 5 inch 3D nanostructures with the thickness ~0.1 mm. Examples of using the polymeric 3D nanostructures as it is for optical coating, or as templates for the infiltration of elastomers, oxides, and metals for stretchable conductors and high surface area applications prove the importance of large area 3D nanostructures. We believe nanostructures from PnP can be sacrificial templates answering a major challege of nanotechnology: perfect alignment of nanomaterial in large scale.

Modern day micro- and nano-scale device fabrication relies on lithographic methods to transfer the pattern created on a thin-polymer film onto the substrate of interest. While successfully employed in semiconductor industry for the last four decades, conventional lithographic methods now face critical challenges for further miniaturization of device components at the nanometer scale. In addition the cost associated with conventional lithographic methods and tools has been increasing rapidly. While a few alternative cheaper methods such as micro contact printing (or soft lithography), nanoimprint lithography (NIL), scanning - probe- based techniques , and dip - pen lithography are now being used for some unique applications, these methods suffer from several issues such as polymer thickening and displacement, stamp wear etc. preventing large scale commercial manufacturing. We have developed a novel micro - and nano - patterning method that is compatible with current CMOS fabrication process and is based on Silicon Nanomembrane (Si- NM) technology. This method utilizes a Silicon - on- Insulator (SOI) wafer with micro- and nano - patterns formed on the device layer for creating a master that can be used at least hundreds of times to transfer the pattern to the substrate of interest with high fidelity. Micro and nano-meter size features can easily be obtained since master fabrication employs conventional photolithography (deep - UV or e - beam lithography) processes. After the master is fabricated, the pattern transfer to the substrate is achieved by either Deep Reactive Ion Etching (DRIE) or UV exposure without a mechanical contact. During this pattern transfer process, Si-NM master is brought to close proximity to the substrate, which is coated with a thin layer of polymeric material, usually a photoresist. A highly anisotropic etch is performed to remove the polymer layers which are exposed through the Si-NM. In addition to polymeric materials, by carefully selecting the conditions of etching environment, micro- and nano-patterns inorganic materials can also be transferred. The process is also flexible enough to accommodate flexible substrates as well as flat, rigid substrates. The whole pattern transfer procedure is CMOS process compatible. This micro- and nano-fabrication method might enable large scale, lower-cost manufacturing of electronic and photonic devices, biological and chemical sensors, photovoltaics, fuel cell etc. that require micro- and nanoscale structuring in their designs.

The search for different synthetic pathways towards porous materials is of scientific and technological interest due to the ever increasing importance of such media as catalysts or catalyst supports, adsorbents, and separation materials, amongst others. Finding new effective methods to introduce hierarchical, multi-layered, porosity or defined mesoporosity, into zeolites and related inorganic oxides is one major target in material chemistry, as such structuring would help overcome the diffusion / mass transport limitations of more traditional (microporous) zeolites. With particular regard to the synthesis of a mesoporous zeolite, a significant challenge is the preservation of both promising mesoporous architectures and pore wall crystallinity. In this presentation, promising routes (e.g. dry gel or hydrothermal conversions) to achieving this goal will be discussed with a particular focus on the employment of highly porous nitrogen-containing (carbonaceous) materials as functional, solid, nanostructured templates. Surface nitrogen functionalities of these templates will be tuned (e.g. via quaternarization / nitrogen condensation etc) to enable favourable interaction(s) with the silicic acid (or silicate) species during condensation and crystallization of the inorganic phase. This use of a “solid nanostructured ammonium salt”, coupled with the influence of the amount and chemical structure (e.g. bonding motifs) of nitrogen functionalities on the replication quality into zeolitic and related (monolithic) materials will be discussed. The presented approach provides a tool box for the preparation of a wide range of related hierarchically porous inorganic solids, composites, and supported metal or metal substituted analogues.

Batteries, fuel cells and solar cells, among many other high-current-density devices, could benefit from the precise meso- to macroscopic structure control afforded by the silica sol-gel process. The porous materials made by silica sol-gel chemistry are typically insulators, however, which has restricted their application. Here we present a simple, yet highly versatile silica sol-gel process built around a multifunctional sol-gel precursor that is derived from the following: amino acids, hydroxy acids or peptides; a silicon alkoxide; and a metal acetate. This approach allows a wide range of biological functionalities and metals—including noble metals—to be combined into a library of sol-gel materials with a high degree of control over composition and structure. We demonstrate that the sol-gel process based on these precursors is compatible with block-copolymer self-assembly, colloidal crystal templating and the Stöber process. As a result of the exceptionally high metal content, these materials can be thermally processed to make porous nanocomposites with metallic percolation networks that have an electrical conductivity of over 1,000 S/cm. This improves the electrical conductivity of porous silica sol-gel nanocomposites by three orders of magnitude over existing approaches, opening applications to high-current-density devices.

Hybrid photovoltaics (PV) composed of an organic light absorbing and electron donating species and an inorganic electron acceptor species have been suggested as stable and robust alternatives for all-organic photovoltaics. In such devices the key process of charge generation occurs across the organic-inorganic interface and is therefore predominantly influenced by the organic-inorganic interfacial area, chemical composition and electronic coupling. Accordingly, special efforts have been made to manipulate the organic and inorganic phases into high surface area nanoscale morphologies with organic-inorganic intimate contact, while controlling phase continuity to allow charge transport to the electrodes. We have shown that directed self-assembly of amphiphilic molecules, surfactants or block-copolymers, combined with sol-gel chemistry can be harnessed to co-assemble conjugated polymers and metal oxide precursors into highly ordered hierarchical 3D mesostructured hybrid films amenable for integration into PV cells. In such systems the self-assembly of the structure directing agent not only determines the bulk film morphology but also controls the organic-inorganic interfacial structure on a molecular level, as determined by energy filtered transmission electron microscopy and solid state NMR. The controlled interfacial interactions are further translated to the efficiency of charge generation and PV device performance. We will also show that optically active organic molecules judiciously designed to self-assemble into fibers can be used to template metal oxides, TiOx or ZnO, from sol gel. Under such conditions the organic molecules serve as both the optically active component and the structure directing agent effectively replacing the non-active surfactant. A combination of X-ray scattering, electron and probe microscopies, FTIR and NMR spectroscopies yield insights on the general morphology and distributions of inorganic and organic species within the materials over multiple length scales. Optical measurements and PV device performance allow the correlation between the nano and mesoscopic morphology, structure of the multicomponent interface and the macroscopic behavior of the devices.

K_xCoO₂ is a relatively unexplored complex metal oxide which can be a good candidate for thermoelectric applications due to its low electrical resistivity (~10 mOmega;cm at RT) and reasonable Seebeck coefficient (~30 µV/K at RT). Thermoelectric properties of K_xCoO₂ can be improved by synthesizing it in the form of nanosheets due to reduction in thermal conductivity via phonon confinement and scattering in nano-structures. Here, a scalable nanomanufacturing technique is reported for batch fabrication of 2D complex metal-oxide nanosheets of K_xCoO₂. Hierarchically ordered oxide nanosheet composites can also be useful for other energy applications such as fuel cells and alkali-ion batteries, because they have high chemical and mechanical stability to serve as electrically conductive supports for electro-catalytic reactions in fuel cells and they can provide extremely high surface area as battery electrodes which can lead to increased charge-discharge rates. We report a sol-gel based, high temperature bottom-up synthesis which is a cost-effective route capable of producing tens of thousands of nanosheet layers self-organized into a macro-scale pellet. The synthetic procedure consists of sol-gel coordination of metal ions, auto-combustion, pressurized pellet formation, kinetic demixing, and calcination. The nanosheets are uniform in length and shape with highly anisotropic dimensions of nanometer sheet thickness and millimeter lateral lengths (10-5:1:1), and are readily delaminated into free-standing nanosheets. Selected area electron diffraction (SAED) studies and dark field imaging performed via transmission electron microscopy (TEM) indicated that the material did not decompose during exfoliation and the exfoliated nanosheets are single crystalline. The work was supported in part by the National Science Foundation under Agreement No. DMR-1149036 and the DOE Office of Basic Energy Science under Award Number DE-SC0001086.

The membrane technology to resolve the challenges relevant to water supply and sanitation has been developed by employing various chemical functions or physically controlled structures within membranes. Among various approaches, the introduction of highly ordered nanostructures, such as integrating one-dimensional nanomaterials, creating interconnected nanopores, or using the two-dimensionally nanoporous block copolymer (BCP) thin films has been extensively investigated.. Here, we demonstrate a novel means to generate the hierarchically porous ultrafiltration (UF) membranes with high flux and superior size-separation selectivity via exploiting the strategy of multi-scale self-assembly. To create the hierarchically porous structures, nanopores from the phase-separated BCP thin films are selectively embedded inside the macropores of three-dimensional inverse-opal (3D-IO) structures. The outer skeletal frame of macroporous 3D-IO structures is prepared by UV-curing the poly(urethane acrylate) (PUA) prepolymer using a template of highly ordered opal-structure of polystyrene (PS) microspheres. The constructed 3D-IO structures offer several advantages of large-area fabrication capability as well as reinforced mechanical stability. Then, the cylindrically ordered BCP thin films of polystyrene-block-poly(methyl methacylate) (PS-b-PMMA) are introduced inside the macroporous 3D-IO template, leading to an installment of BCP nanoflanges suspended in necking pores of 3D-IO structures. After selective removal of the PMMA phase, nanopores of BCP nanoflanges (5~30 nm in diameter) are perfectly embedded within the macropores of 3D-IO structures (150~400 nm in diameter), completing a hierarchically nanoporous membrane structure. In addition, a feasibility of this hierarchically porous and free-standing film for UF membrane applications is investigated. As a result, notable membrane performance with high selectivity can be obtained while retaining the outstanding permeability which is greater with an order of magnitude as compared to conventional UF membranes. Therefore, the BCP nanoflange-embedded hierarchically porous membranes have a great promise for the next-generation membrane technology due to their narrow pore size distribution, improved permeability, and highly mechanically strengthened properties.

Zeolites are a class of materials with ordered micropores (smaller than 2 nm), that can be used for gas separation, catalysis, and adsorption. Hierarchical zeolites is a novel class of zeolites with the typical ordered zeolitic microporosity, as well as mesopores (2-50 nm), which by allowing for fast transport of bulky molecules, enable improved performance in petrochemical and biomass processing. Methods for the preparation of hierarchical zeolites involve either multifunctional structure-directing agents (SDA), and/or post-synthesis processing, such as pillaring or desilication/dealumination, which may deteriorate their performance and increase their costs. Starting from a low-cost structure-directing agent, by one-step hydrothermal crystal growth approach, we report the synthesis of a new hierarchical zeolite composed of perpendicularly connected microporous nanosheets using repetitive branching approach. Although this approach has been applied in the synthesis of other nanostructures, such as semiconductor tetrapods, it has not been explored for the formation of pillared zeolites. The zeolite nanosheets have thickness of 2 nm and contain micropores typical of pentasil zeolites. The house-of-cards arrangement of the nanosheets creates a permanent network of 2-7 nm mesopores, which, along with the high external surface area and reduced micropore diffusion length, enable higher reaction rates for bulky molecules compared to those of other mesoporous and conventional MFI zeolites. Reference: Xueyi Zhang, Dongxia Liu, Dandan Xu, Shunsuke Asahina, Katie A. Cychosz, Kumar Varoon Agrawal, Yasser Al Wahedi, Aditya Bhan, Saleh Al Hashimi, Osamu Terasaki, Matthias Thommes, and Michael Tsapatsis, “Synthesis of self-pillared zeolite nanosheets by repetitive branching”, Science, 2012 (in press).

A facile approach has been developed for synthesis of microporous carbon spheres. Microporous carbon particles were obtained by pyrolyzing and carbonizing crosslinked poly(styrene-co-methyl methacrylate) (PS-PMMA) copolymer spheres. Micropores were formed via the selective decomposition of methyl methacrylate (MMA) groups during carbonization. The resultant pores had an average diameter of 1.6 nm. The surface area and pore volume increased as the fraction of MMA was increased to 20 wt%, reaching 1135 m2/g and 40%, respectively. The micropores were randomly dispersed throughout the matrix, which could be explained by comparing the reactivity ratio of styrene and MMA. The electrocatalytic activity of the microporous carbon spheres was estimated by cyclic voltammetry, which revealed 6 times higher than the one of the carbon derived from PS particles.

Rapid charge and discharge without sacrificing energy density is an increasingly sought-out feature of electrical energy storage devices, but rapid charging and discharging causes dramatic capacity reductions (and safety issues) in most rechargeable batteries. Supercapacitors offer high rate performance, but have much lower energy densities than batteries. A storage technology that combines the rate performance of supercapacitors with at minimum the energy density of batteries would revolutionize portable and distributed power. Using a bicontinuous electrode formed via a colloidal crystal templating process, we demonstrate charge and discharge rates of up to 400C and 1,000C for lithium-ion and nickel-metal hydride chemistries, respectively, with minimal capacity loss (400C is a 9 second charge or discharge and 1,000C is a 3.6 second charge or discharge). The final structure also has energy densities comparable to current commercial systems, and, as will be discussed, the potential exists for even higher energy densities. Key to the exceptional kinetics is the bicontinuous nanoarchitecture of the electrode, which consists of an electrolytically active material sandwiched between rapid ion and electron transport pathways. Finally, a full-cell lithium-ion battery constructed from a bicontinuous lithiated MnO2 cathode and a conventional graphite anode was charged to 90% capacity in 2 minutes. The 3D bicontinuous electrode approach presented here is quite general, and as we will show, is applicable to many battery chemistries including ultra-high energy density materials such as silicon, and unique battery architectures, including in-chip integrated microbatteries.

Electrical power sources with high energy and power density that are size comparable to electronics components are important for advancements in microelectronics and MEMS. To date, on-chip electrical power has almost always been provided by capacitors, despite the rather lower energy density and rather large size of capacitors. Here we present a novel approach for microbattery cells with interdigitated 3D porous electrodes capable of both high power density and high energy density. The electrodes have a 3D periodic inverse opal structure that provides tuning of the ion diffusion and electron conduction lengths while maintaining a large volume of active material. The 3D electrodes were fabricated by electrodepositing active electrode material on micropatterned inverse opal nickel scaffolds. A nickel - tin alloy was used for the anode and a lithiated manganese oxide for the cathode active material. The scaffolds were formed by electrodepositing nickel through a colloid assembly formed from 330 - 500 nm diameter colloidal particles, resulting in a Ni inverse of the colloid structure, on a gold patterned glass substrate. The electrodes in the microbattery cell are interdigitated such that each finger is an alternating anode or cathode 34 mu;m wide and 15 mu;m tall with 11 mu;m spacing between the electrodes. The footprint area of the tested cells was ~2 mm2. The microbattery cells demonstrate supercapacitor performance, up to 7.4 mW / cm2mu;m power density and 0.6 mu;Wh / cm2mu;m energy density, at 1000 C discharge rates while maintaining battery performance, up to 23 mu;W / cm2mu;m power density and 15 mu;Wh / cm2mu;m energy density, at 1 C discharge rates. Compared to previous published research on 3D microbatteries, the present work achieves a 2 X increase in energy density and a 2000 X increase in power density. Diffusion simulations show that the power density is limited by solid state lithium ion transport in the cathode. Using the simulation and experimental data, we have developed a set of design rules for high energy and high power density microbatteries.

Inorganic copper and tin compounds have attracted a lot of research attention because of their possible usage in various energy applications such as solar cells, sensors, and Li-ion batteries . In this study, by using copper hydroxystannate (CuSn(OH)6) nanorods as precursors inks, we have designed and developed a solution based synthesis method for tuning and scale-up the final nanostructured (Cu, Sn)-based oxides and alloys. By modifying precursors, concentration, duration time and pH value of growth solution in one batch, large scale CuO and Cu-Sn alloy nanostructures (particles, spindles, and nanorods) have been successfully synthesized with controllable composition. Electron microscopy and spectroscopy have been utilized to investigate structure, morphology and chemical composition of these nanostructured compounds. The electrochemical and photo-catalytic performance has been studied on these nanostructured compounds. The electroless co-reduction mechanism and corrosion assist mechanism have been proposed to illustrate the formation process of the nanostructured Cu-Sn alloy and CuOx, respectively. This low temperature solution based method can be used to synthesize large scale free-standing nanoparticles with tunable composition and architectures for various energy conversion and storage device applications. *puxian.gao@ims.uconn.edu

The development of higher capacity materials and new battery architectures is essential to increasing Li-ion battery energy and power densities. Alloying anodes like SnO have significantly higher capacities than carbon which is used in most commercial Li-ion batteries. However, Sn also undergoes large volume changes during the alloying process, which leads to pulverization and delamination. Decreasing particle size has been shown to reduce the volumetric stresses involved in lithiation and improve electrochemical performance of alloying anodes. Understanding the relationship between microstructure and electrochemical performance is important for designing next generation electrodes. In this presentation, we will demonstrate that atomic layer deposition (ALD) can be used to hierarchically fabricate 3-D SnO anodes with high capacities and excellent rate capabilities. ALD is a self limiting vapor phase process that can be used to coat high aspect ratio 3-D structures with conformal, uniform thin films. By utilizing a vapor phase process, different 3-D structures can serve as current collectors including carbon nanotube paper and nickel nanowires. Thin films of SnO were deposited by ALD on 3-D current collectors and showed significantly improved electrochemical cycling performance compared to conventional SnO anodes prepared by solution processing and mechanical mixing. By employing complex 3-D structures, thin film electrodes will be shown to achieve significant areal capacities. The effect of the thickness of nanometer scale SnO thin films on electrochemical performance was also systematically studied, and it will be demonstrated that there is a critical SnO film thickness in order to maximize its electrochemical performance as a Li-ion battery anode.

While significant effort has been devoted to novel electrode architectures, the simplicity of traditional porous batteries still comprise the great majority of the power source industry. Critical to the development of high energy density batteries is the excessive microstructural tortuosity that arises from suboptimal particle packing and non-ideal particle morphologies. Traditionally, the tortuosity of a porous media has been estimated through the classical Bruggeman relation, tau; = ε^(-0.5) where tau; is the tortuosity and ε is the porosity. However, recent experimental results demonstrate that tortuosity measurements frequently deviates from the classical relation when the particles configuration deviates from ideality. In this paper, we systematically investigate the effects of particle polydispersity, morphological anisotropy and shape randomness on the porous electrode tortuosity and electrochemical response. Different packing configurations, ranging from ordered to random are analyzed. We rationalize the effects of porosity from experimentally obtained three-dimensional X-ray tomography microstructures by using the computer-generated microstructure and to analyze optimal particle configurations. The ultimate goal is to fundamentally understand the impact of different particle configurations, its associated processing, and suggest optimal architectures for improved energy and power densities.

The improvement of battery performance requires the rationale optimization of the composite electrode. The development of “new tools”, i.e. experimental techniques as well as methodologies, is needed to understand the so-called composition-architecture-properties and performance relationships. The broadband dielectric spectroscopy (BDS) technique is applied for the first time to a composite material used as an electrode for lithium battery. The electrical properties (permittivity, resistivity and conductivity) are measured from low frequencies (a few Hz) to microwaves (a few GHz). Model samples prepared from different LiFePO4 varying in particle size, carbon coating content and binder contents are studied. The results demonstrate that the broadband dielectric spectroscopy technique is very sensitive to the different scales of the electrode architecture involved in the electronic transport, from interatomic distances to macroscopic sizes, as well as to the morphology at these scales, coarse or fine distribution of the constituents. When the frequency increases, different kinds of polarizations appear from interatomic distances to macroscopic sizes and give rise to dielectric relaxations in the following order: (a) space-charge polarization (low-frequency range) due to the interface sample/current collector; (b) polarization of carbon coated LiFePO4 clusters (micronic or submicronic scale) due to the existence of resistive junctions between them; and (c) electron hopping between sp2 domains within the carbon coating (nanometric scale). This work opens up new prospects for a more fundamental understanding as well as a more rational optimization of the electronic transport in composite electrodes for lithium batteries, as well as in the field of other battery technologies and conductor-insulator composite materials. Financial funding from UMICORE and the ANR program n° ANR-09-STOCK-E-02-01 is acknowledged.

Attempts to harness the enormous capacity of silicon in lithium ion batteries have pointed to the use of composite structures that allow nanoscopic silicon to expand and contract during electrochemical cycling in an isolated space, thereby protecting the silicon from fusion, fragmentation, and decoupling from a conductive pathway. Designed internal composite architectures offer many advantages over conventional materials, but also present new opportunities for device failure. We are studying the cyclic stability of high capacity model Si composite anodes with an emphasis on developing a mechanistic understanding of the structural characteristics that provide robust functionality. Our goals are to understand how the silicon is confined in these model systems, how the structures contribute to cycling stability, and how to best analyze these structures with in situ and ex situ methods. Two types of composite anodes were fabricated as films on quartz wafers. The first type were layered structures of graphene oxide (GO) sheets and silicon nanoparticles. The second type were vertically aligned carbon nanotube arrays (VACNTs) coated with silicon both with and without a final hard oxide capping layer. For in situ analysis, we used a multi-beam optical stress sensor (MOSS) system that allows measurement of wafer curvature and calculation of internal mechanical stresses experienced during electrochemical cycling. Ex situ analysis included electron microscopy of focused ion beam cross sections, nanoindentation, and x-ray diffraction. The results of our analysis will aid the design of future high capacity anodes with high cyclic stability.

High power energy storage devices, such as supercapacitors and Li-ion batteries, are critical for the development of zero-emission electrical vehicles, large scale smart grid, and energy efficient cargo ships and locomotives. The energy storage characteristics of supercapacitors and Li-ion batteries are mostly determined by the specific capacities of their electrodes, while their power characteristics are influenced by the maximum rate of the ion transport. Conductive polymers and metal oxides exhibit fast and electrochemically-reversible Faradaic redox reactions and offer high ion storage capabilities [1-3]. However, these materials may suffer from low surface area and low electrical conductivity. Here we report on the formation of metal-oxide/carbon and polymer/carbon nanocomposites for use in supercapacitor and Li-ion battery electrodes. These hierarchically structured composites circumvent the limitations of pure polymers and metal oxides and offer a unique combination of high energy and ultra-high power characteristics, unattainable in the individual material components. The reported electrodes were synthesized via chemical vapor deposition (CVD), atomic layer deposition (ALD) and wet chemistry routes. Their microstructure and chemistry was characterized using scanning and transmission electron microscopy (SEM and TEM), energy dispersive spectroscopy (EDS), Fourier Transform Infrared Spectroscopy (FTIR), Raman spectroscopy, X-ray diffraction (XRD). Their pore size distribution and specific surface area was characterized using gas sorption techniques. The produced electrodes were tested in both 2016 coin cells and pouch cell configurations in various electrolytes, including aqueous, organic and ionic liquids. Acknowledgement: This work was partially supported by US AFOSR and AMRDEC, US Army RDECOM Keywords: chemical vapor deposition, PANI, supercapacitors, Li-ion batteries References: 1. S. Boukhalfa, K. Evanoff, and G. Yushin, Energy & Environmental Science, 2012, 5, 6872-6879. 2. J. Benson, S. Boukhalfa, A. Magasinski, A. Kvit, and G. Yushin, Acs Nano, 2012, 6, 118-125. 3. I. Kovalenko, D. Bucknall, and G. Yushin, Advanced Functional Materials, 2010, 20, 3979-3986.

Electrochemical energy storage (EES) has always disregarded Moore&’s Law [1]. Improving EES performance requires redesigning the reaction interphases within which occur the fundamental processes that store energy in batteries and electrochemical capacitors. Our route to a new EES performance curve is to apply an architectural perspective in which nanometric feature sizes are integrated via interpenetrating transport pathways such that electrical and molecular communication is wired throughout an ultraporous form that can be readily scaled to macroscale sizes [2-4]. We use aerogel-like carbon nanofoam papers as our architectural test-bed because they provide a low cost and scalable nanocomposite that offers an optimal balance of critical architectural features: (1) open, 3D interconnected macropores sized at 20-to-250 nm co-continuous with (2) ~20-nm pore walls of a size that reduces dead weight and volume while retaining mechanical strength and flexibility without compromising electronic conductivity. Charge-storage or catalytic functionality can then be imparted to internal carbon walls simply by transporting reactants within the 3D “plumbing” of the macroporous foam. Self-limiting modification strategies allow us to incorporate conformal, nanoscopic functional “paints” of metal(Mn, Ti, Ru, Fe)oxides or polymer (redox-active or electron insulating) throughout the macroscopic thickness (70 mu;m to 0.3 millimeter) of carbon nanofoam paper. For instance, painting the carbon walls with 10-nm-thick MnOx increases the mass-, geometric-, and volume-normalized capacitance (2- to 10-fold) relative to the native carbon nanofoam without significantly altering its high-rate character. The oxide-modified paper is now a multifunctional electrode structure that can be used in an aqueous asymmetric electrochemical capacitor or as an air cathode in a Zn/air cell to electrocatalyze oxygen reduction and provide pulse power. Our redesign of electrode structures using modified carbon nanofoam papers has catalyzed breakthroughs in our work within a broad range of multifunctional energy storage and conversion, including asymmetric electrochemical capacitors, air cathodes for metal-air batteries, 3-D batteries, and semifuel cells. [1] D.R. Rolison, L.F. Nazar, MRS Bull. 2011, 36(7), 486-493. [2] J.W. Long, B. Dunn, D.R. Rolison, H.S. White, Chem. Rev.2004, 104, 4463-4492. [3] D.R. Rolison, J.W. Long, Acc. Chem. Res.2007, 40, 854-862. [4] D.R. Rolison, J.W. Long, J.C. Lytle, A.E. Fischer, C.P. Rhodes, T.M. McEvoy, M.E. Bourg, A.M. Lubers, Chem. Soc. Rev.2009, 38, 226-252. [5] J.C. Lytle, J.M. Wallace, M.B. Sassin, A.J. Barrow, J.W. Long, J.L. Dysart, C.H. Renninger, M.P. Saunders, N.L. Brandell, D.R. Rolison, Energy Environ. Sci.2011, 4, 226-252.

Batteries and electrochemical capacitors (ECs) represent the most widely used types of electrochemical energy storage devices. ECs are frequently overlooked as an energy storage technology despite the fact that these devices provide greater power, much faster response times, and longer cycle life than batteries. A key limitation to this technology, which is based on high surface area carbon electrodes and electrical double-layer capacitance, is its low energy density. In this paper we review our recent work on mesoporous transition metal oxide architectures that store charge through surface or near surface redox reactions, a phenomenon termed pseudocapacitance. The faradaic nature of pseudocapacitance leads to significant increases in energy density and thus represents an exciting future direction for ECs. Our results with mesoporous TiO2, MoO3 and Nb2O5 films demonstrate that this porous architecture is very beneficial for capacitive energy storage. Enhanced device performance is realized through integration of one or more of the following design rules into electrode architecture: assembling small nanosized building blocks to increase surface area, maintaining an interconnected open mesoporosity to facilitate solvent diffusion, flexibility in the structure to facilitate volume expansion during ion transport, well defined nanodimensional domains, possibly with oriented crystalline layers to facilitate ion intercalation into the lattice, and creation of effective electron transport pathways. Cyclic voltammetry of the mesoporous films at various sweep rates confirms that charge storage is primarily from pseudocapacitive processes with only a minor contribution from double-layer capacitance. The pseudocapacitive behavior exhibited by the mesoporous transition metal oxide films represents a very promising direction for designing electrochemical capacitors that can achieve increased energy density while still maintaining high power density.

We synthesized flexible supercapacitor electrode materials by carbonizing a common livestock biowaste in the form of chicken eggshell membranes. The carbonized eggshell membrane (CESM) is a three-dimensional macroporous carbon film that resembles construction paper. At higher magnification the CESM is actually composed of interwoven connected microporous carbon fibers containing around 10 wt% oxygen and 8 wt% nitrogen. Despite relatively low surface area of 221 m2 g-1, exceptional specific capacitances of 297 F g-1 and 284 F g-1 are achieved in basic and acidic electrolytes in 3-electrode system, respectively. This yields an unusually high volumetric energy density in the range of 600 F cm-3. Furthermore the electrodes demonstrate excellent cycling stability: only 3% capacitance fading is observed after 10,000 cycles at a current density of 4 A g-1. These very attractive electrochemical properties are discussed in the context of the unique structure and chemistry of the material.

Nanowires have attracted great attentions due to their very large surface/volume ratio to contact with electrolyte, continuous conducting pathways for electrons through the electrodes and facile strain relaxation during battery operation. However, they suffer poor cycling properties as nanowires aggregate with each other as a result of the high surface energy. Recent attention has been focused on the synthesis and application of complex-structure nanomaterials, which can have superior electrochemical performance than single-structured materials. Hierarchical structures with high surface/body ratios, large surface areas, better permeability and more surface active sites can significantly increase energy density, power density and cycle performance, solve self-aggregation of nanowire electrode material and have potential in elecnot;trochemical applications. In this abstract, we report three ways to reduce the aggregation of nanowire-based electrode materials for electrochemical energy storage, which are hierarchical heterostructured nanowire assembly, ultralong hierarchical nanowires assembly and in situ chemical oxidative reaction. To increase energy density and cycle performance, our group has synthesized the three-dimensional hierarchical MnMoO4/CoMoO4 heterostructured nanowires by a facile micro-emulsion & refluxing method under mild conditions. We fabricated asymmetric supercapacitors based on hierarchical heterostructured nanowires. Compare with pure one-dimensional nanowires, hierarchical heterostructured nanowires increase specific capacitance and energy density up to an order of magnitude, and good reversibility with a cycling efficiency of 98% after 1,000 cycles. To increase power density and solve self-aggregation of nanowire electrode material, our group has designed and synthesized ultralong hierarchical vanadium oxide nanowires using the low-cost starting materials by electrospinning combined with annealing. This novel nanostructure exhibits a high performance for lithium ion batteries, providing a high discharge capacity of 390 mAh/g and improved cycling stability, which results from reduced self-aggregation of the nanomaterials. It is expected that our study may extend effective and helpful methods in directions that will solve the challenge of property degradation in energy storage and open new applications.

Hierarchical Co3O4 nanoarchitectures, such as single crystalline plum-like spheres and octahedra on and around MWCNTs, 1D porous nanowires, 2D porous nanosheets, 3D twin-spheres with urchin-like structure and 3D porous nanowalls, have been prepared successfully. The growth mechanisms of the nanoarchitectures have been investigated extensively to understand their formations. It was found that the hierarchical nanostructures can be effectively tuned through adjusting the reaction parameters. The electrochemical properties of the materials will be also presented. The surpercapacitors fabricated with the nanoarchitectures show excellent performances in specific capacitances and stability. The novel hierarchical nanostructures with pores and 3D networks may offer pathways for electron and ions transportation and are thus promising materials for energy storage.

Functional nanocomposites that contain biomaterials and non-biomaterials are one of the main subjects of recent studies because of their wide range of potential applications. Here, we demonstrate for the first time that a porous DNA hydrogel can be an excellent template for the combination of conducting polyelectrolytes to produce high-performance supercapacitor electrodes. DNA hydrogel-synthetic polymer hybrid supercapacitors have been constructed from the electrostatic deposition of conductive polyelectrolyte multilayers onto a DNA hydrogel followed by coating with Mn3O4 nanoparticles as a pseudocapacitor. The performances of the supercapacitors with respect to specific capacitance, cycling stability, power density, and energy density have been systematically studied for various numbers of polyelectrolyte multilayers. The specific capacitance of these DNA hydrogel-based supercapacitors reached 105.4 F/g with a power density of 8230 kW/kg, which is higher than the specific capacitance and power density of commercially available supercapacitors. These conceptually novel hybrid electrodes have the potential to be a platform technology for the creation of biocompatible and implantable energy storage devices for in vivo applications.

We have successfully fabricated an asymmetric supercapacitor with high energy and power densities by using graphene hydrogel (GH) with 3D interconnected pores as the negative electrode and vertically-aligned MnO2 nanoplates on nickel foam (MnO2-NF) as the positive electrode in a neutral aqueous Na2SO4 electrolyte. Our results have shown that assembling graphene nanosheets into GH with a 3D network structure during the chemical reduction of graphene oxides (GO) can effectively prevent the aggregation of graphene nanosheets. Consequently, GH as electrode materials provides large active surface areas and facilitated electrolyte ion transportation and demonstrated an enhanced energy storage capacity and a good rate capability. For the positive electrode, a hierarchical composite of MnO2-NF was prepared by electrodeposition of MnO2 nanoplates on highly-conductive porous nickel foam through cathodic electrodeposition and directly used as electrodes for supercapacitors without further processing. Importantly, the porous structure of nickel foam allows a higher mass loading of active materials per area, facilitates the electrolyte ion transportation and reaction with the active materials, and eventually leads to improved utilization efficiency of active materials. Compared with anodic deposition in which a positive potential is applied to oxidize starting Mn2+ into Mn4+ and deposit as MnO2, cathodic deposition of MnO2 can avoid the oxidation and dissolution of low-cost metallic current collectors. Collectively, taking advantages of GH with 3D interconnected pores, vertically-aligned MnO2 nanoplates with sufficient free spaces, and robust contact between MnO2 and nickel foam, we have found that the fabricated asymmetric supercapacitor of GH//MnO2-NF can be cycled reversibly in a wide potential window of 0-2.0 V and exhibits a much higher energy density of 23.2 Wh kg-1 than that of the symmetric supercapacitors of GH//GH (5.5 Wh kg-1) and MnO2-NF//MnO2-NF (6.7 Wh kg-1) at current density of 1 A g-1. Furthermore, the asymmetric supercapacitor displays remarkable cycling stability with capacitance retention of 83.4% after 5000 continuous charge/discharge cycles.

Novel hierarchically nanoarchitecture of CoO@TiO2 was successfully constructed with hydrothermal growth of CoO nanowires and followed atomic layer deposition (ALD) of TiO2 on the surface of it. When the structure of CoO@TiO2 were tested as supercapacitor electrode, improved electrochemical properties were achieved, largely due to the conformal structure design providing enlarged specific surface area and fast ion transport. As a result, the hierarchical structure shows ~2-4 times of capacitance compared to the nanowire alone, which reveals the beauty of the rational designed core-shell structure. The novel structures with ALD provide an exciting direction to rational design abundant promising nanostructures with different functional composites and for many other applications, such as lithium ion battery, catalyst and sensor.

Polymer films such as biaxial oriented polypropylene are widely used as the active dielectric in current capacitor designs due to their low dielectric loss and high reliability. However, increasing the energy density of capacitors requires dielectric materials with improved permittivity or dielectric strength. Current approaches in the nanocomposite area are limited by random morphologies that exhibit drastic reductions in breakdown strength at higher filler loadings. We report on a promising approach that utilizes an extreme density of internal interfaces (500-1000 m^2/g) and ultrafine morphology, which effectively traps locally injected carriers, thereby inhibiting breakdown events. We demonstrate that the dielectric breakdown field (EBD) can be increased up to intermediate inorganic volume fractions by creating uniform one-dimensional nanocomposites (nanolaminates) rather than blends of spherical inorganic nanoparticles and polymers. Free standing nanolaminates of highly aligned and dispersed montmorillonite in polyvinyl butyral exhibited enhancements in EBD up to 30 vol % inorganic (70 wt % organically modified montmorillonite). Enhancements are observed up to five times the inorganic fraction known for random nanoparticle dispersions, and are two to four times greater compared to currently used volume fractions of nanoparticles. A trade-off between increased path tortuosity and polymer-deficient structural defects is the origin of the observed breakdown characteristics. This implies that an idealized PNC morphology to retard the breakdown cascade perpendicular to the electrodes will occur at intermediate volume fractions and resemble a discotic nematic phase where highly aligned, high-aspect ratio nanometer thick plates are uniformly surrounded by nanoscopic regions of polymer.

Electron transfer between charge storage materials and their underlying, supporting electrodes is an important aspect of pseudocapacitor optimization. An interfacial bottleneck is especially important in heterogeneous, nanostructured pseudocapacitors where extremely small supporting electrodes can lead to high interfacial current densities. Our research investigates the role of interface chemistry and geometry in optimizing carbon-MnO2 heterostructures, with the goal of understanding how electron transfer rates at nanoscale electrode interfaces can be improved through surface chemistry and geometry. To this end, we have fabricated nanoscale supporting electrodes out of graphitic carbon nanotubes and Pt films, with surface areas ranging from 0.002 to 20 um2. Conformal coatings of MnO2 are electrochemically deposited on these supports using a pulsed deposition scheme that allows good control over the coating morphology and thickness. The technique achieves virtually identical MnO2 coatings on supporting electrodes that range from the microscale into the truly nanoscale regime. Cyclic voltammetry in LiClO4 electrolytes allows quantitative analysis of the heterostructures&’ capacitance and resistivity. While the former is a simple function of the MnO2 volume, the latter is a complex and non-ohmic function that depends on the nanoelectrode geometry, chemistry, and interfacial current density. For the same specific capacitance, interfacial resistivity can vary over four orders of magnitude. This result is a severe tradeoff that may limit the applicability of nanoscale conductive supports like carbon nanotubes in high power pseudocapacitors.

Ordered hierarchical nanostructured carbons (OHNCs) have unique porous structure, namely, mesopores (2 nm

Morphology in bulk heterojunction solar cells plays an important role in determining power conversion efficiency. How to control the solid state structure is an art of practice. We studied this issue by developing donor polymers that bear pendant groups functioning as good solvent for fullerene derivative. A series of new polymers with different aryl chloride-functionalized side chains were synthesized to investigate potential solvation effects in polymer solar cells. Grazing incidence wide angle x-ray scattering (GIWAXS) data revealed that the lamellar spacing distance is shortened as the stacking force of attached aryl chlorides is enhanced. The decrease in lamellar spacing is more pronounced when polymer was blended with fullerene derivatives, indicating a clear solvation effect. Solar cell performance of devices fabricated using different processing solvents provides insights into this solvation effect, including the unanticipated observation that the use of high-boiling-point additives does not always yield a better result for this class of polymers.

Solar cells based on the polymer:fullerene bulk heterojunction (BHJ) represent one of the most promising technologies for next-generation solar energy conversion due to their low-cost and scalability. In the last fifteen years, research efforts have led to organic photovoltaic (OPV) devices with power conversion efficiencies (PCEs) >8%, but these values are still insufficient for the devices to become widely marketable. To further improve solar cell performance, a thorough understanding of the complex structure-property relationships in OPV devices is required. In this work, we have determined that the superior performance of PTB7:fullerene bulk heterojunction (BHJ) solar cells, one of the best-performance OPV systems, is attributed to hierarchical nanomorphologies with optimum crystallinity and nanoscale intermixing of copolymers with fullerenes, which together promote exciton dissociation, and consequently, contribute to photocurrent. We have also discovered that the formation mechanism of hierarchical nanomorphologies is a competitive yet synergistic mechanism of crystallization and liquid-liquid phase separation (either nucleation and growth or spinodal decomposition). Fine-tuning morphologies from molecular ordering to nanophases enables us to understand the fundamental physics underlying the superior performance of PTB copolymers and, thereby, realize improvement in the performance of PTB:fullerene BHJ solar cells with power conversion efficiencies (PCEs) up to approximately 9%, very close to a commercially viable PCE of 10%.

The morphology of bulk heterojunction organic solar cells plays a critical role in photovoltaic performance. Morphological and structural traits such as domain size and purity, extent of molecular miscibility, and polymer crystallinity often correlate with device performance. In this work, we present a system that is comparatively free from the morphological constraints of a precise bulk heterojunction morphology. When blended with C61-based fullerene, the fluorinated polymer, poly[4,8-(3-butylnonyl)benzo[1,2-b:4,5-b&’]dithiophene-alt-2-(2-butyloctyl)-5,6-difluoro-2H-benzo[d][1,2,3]triazole] (BnDT-FTAZ), yields a fill factor above 65% even with factor of two changes in domain size and relative domain purity. Furthermore, the high fill factors are achieved using active layers of 250 nm, which exceeds those typically used to prevent free carrier recombination losses nearly absent in these devices. A possible explanation for these observations is the low (<5%) molecular miscibility of fullerene in polymer, which indicates an inherit thermodynamic incompatibility of the donor and acceptor, thereby, by a mechanism yet to be precisely determined, suppressing recombination. More generally however, these results indicate that BnDT-FTAZ is an excellent candidate for roll-to-roll processing because precise control of the morphology and active layer thickness is not required.

The molecular to mesoscale morphology of active layers in organic photovoltaic devices is one of the most important aspects determining device performance but, unfortunately, has proven to be the most difficult to measure, control and optimize. This is largely due to the lack of high resolution tools that can quantitatively distinguish between the similar materials used. Soft x-rays - those resonant with molecular orbitals in the molecules - have been demonstrated to possess a quantitative, high contrast between the constituent materials, and in a scattering experiment can yield global statistics on domain size and composition from the micron size scale down to the nanometer level [1,2]. Recently, we have used polarized x-rays to reveal noncrystalline molecular orientational ordering at domain interfaces [3]. Such ordering, along with molecular-scale mixing driven by a fundamental miscibility of the materials, is likely very important to the processes of charge separation and recombination in devices. The formation of domains or bicontinuous structures at the mesoscale clearly are important as well, controlling the longer range charge transport. All of these aspects can be quantitatively measured via resonant x-ray scattering. With the future use of coherent x-rays, reciprocal space data could be inverted to real space maps or even tomograms of the device morphology. The continued development of resonant scattering continues to improve the precise measurement and correlation of morphology to device processes, allowing for the identification of processing routes to control and optimize the hierarchical structure within these devices. [1] S. Swaraj et al., Nano Letters 10, 2863 (2010). [2] H. Yan et al., ACS Nano 6, 677 (2012). [3] B. A. Collins et al., Nature Materials 11, 536 (2012).

2,1,3-benzothiadiazole (BT) is one of the most wildly used functional themes for high performance semiconducting polymers utilized in organic solar cells. Recent research found a modified theme, i.e. naphtha[1,2-c:5,6-c]bis[1,2,5]thiadiazole (NT) based devices, to show higher performance when both themes were used with PBDT [1]. Since the morphologies of active layer are considered as the one of the main reason leading to different device characteristics, the morphologies of those PBDT-NT and PBDT-BT polymers based blends were extensively examined. By utilizing 2D x-ray diffraction, resonant soft x-ray scattering, polarized x-ray scattering (P-SoXS) [2] and x-ray absorption microscopy, the crystallinity, domain size/purity, molecular orientational ordering and miscibility of BT and NT based blend films were probed, respectively, and correlated to the device performance. Surprising, although the overall chemical structure of the BT and NT polymers are not drastically different, a totally different morphology is observed in the two blend films. The results showed that BT based devices exhibited low crystallinity and small, impure domains, which is consistent with the high miscibility of fullerene with PBDT-BT measured separately. In contrast, NT based devices were highly crystalline, and had large (>80 nm size) and pure domains. The purity is again consistent with the low miscibility with fullerene measured independently. Efficiency of ~6% has been achieved despite a domain size much larger than the nominal exciton diffusion length. Furthermore, P-SoXS revealed molecular orientational ordering of the NT polymer relative to the PCBM dispersions. Such ordering has previously not been considered as an important parameter in solar cells, as there was no tool prior to P-SoXS to measure it. It might, however, be just as or more important than crystallinity to explain the high performance despite the large domains. Reference [1], M. Wang, X. Hu, P. Liu, W. Li, X. Gong, F. Huang, Y. Cao, Journal of the American Chemical Society 2011, 9638-9641. [2], B.A. Collins, J.E. Cochran, H. Yan, E. Gann, C. Hub, R. Fink, C. Wang, T. Schuettfort, C.R. McNeill, M.L. Chabinyc, H. Ade, Nature Materials 2012, 11, 1-8.

Control over the nanoscale morphology and molecular packing of electron donor and acceptor materials is a key challenge in the design of active layers for organic photovoltaics. For simple blends of the two materials, solvent or thermal annealing steps have been shown to improve molecular scale organization, and therefore device efficiency. However, extended annealing can yield coarsening of domains and thus an optimum in performance is typically seen for intermediate annealing treatments that strike a balance between crystallization and phase separation. Block-copolymers or co-oligomers containing both donors and acceptors can provide superior control over nanoscale morphology, but also place fundamental limits on how crystals of each material can form. Templated crystallization of one material, seeded by nanocrystals of the other, can provide opportunities for tailoring crystalline donor/acceptor nanostructures. In particular, we describe a new approach for the formation of “shish-kebabs” of P3HT and perylene tetracarboxy diimide (PDI) that relies on coupled crystal modification during solvent casting. The presence of P3HT in solution inhibits nucleation of PDI crystals, but once formed, a PDI nanowire “shish” serves as a nucleation site for crystalline P3HT “kebabs”. This coupled process of crystal growth not only enhances formation of P3HT crystals, but also passivates the edges of growing PDI fibers, allowing the lateral dimensions of the acceptor domains to be tuned from ~ 200 nm down to 25 nm simply through changes in mixing ratio. The mechanism is general to several organic solvents and perylene derivatives, as well as P3HT with different molecular weights, thus providing a simple and robust route to form highly crystalline nanophase separated organic donor/acceptor assemblies.

Blockcopolymers feature unique properties for organizing in ordered structures on length scales of several tenths of nanometers due to microphase separation. This special attribute enables the formation of ideal donor and acceptor domains for photovoltaic devices in the size of the exciton diffusion length. Therefore we designed different amphiphilic blockcopolymers, carrying a hole conductor segment and a block which coordinates inorganic semiconductor nanoparticles as electron acceptors. Utilizing controlled radical polymerization and “click” chemistry techniques, defined amphiphilic blockcopolymers were synthesized. Amorphous tetraphenylbenzidine groups were chosen as hole conductor while the hydrophilic block comprises of anionic polystyrene sulfonate. A defined precursor polymer was first synthesized by reversible addition fragmentation chain transfer polymerization (RAFT). Secondly the hole conductor moiety was introduced by “click” chemistry to attain the semiconducting amphiphilic blockcopolymer poly(N,N&’-bis(4-methoxyphenyl)-N-phenyl-N&’-4-triazolylphenyl-(1,1‘-biphenyl)-4,4‘-diamine)-block-poly(triethylammonium styrene sulfonate (PTPD-b-PTeaSS). Following this procedure we synthesized several polymers with varying composition and molecular weight. All the polymers were characterized by SEC, NMR and DSC. These blockcopolymers create narrow distributed micelles in aqueous solution exhibiting domain sizes suitable for photovoltaic applications. The strong anionic sulfonate groups maintain the good solubility even at low pH values, which is important for electrostatic stabilization of inorganic nanoparticles. Thus bulky organic ligands, which reduce the connectivity between the particles and in consequence the conductivity, are avoided. Furthermore the sulfonate groups offer high loading capacities for various cationic nanoparticles while maintaining the solubility. First results indicate preferential loading of cationic CdSe nanorods in the micellar shell of the self-assembling blockcopolymers. In addition to their characteristics in solution we examined the thin film morphologies of these blockcopolymers. The as-prepared films indicate microphase separation, but without any order. However, during annealing processes vertical cylindrical domains are created independent of the blockcopolymer composition. This enables the individual optimization of the domain size, while maintaining good transport pathways for electrons and holes to the respective electrodes. The advantages of possible high loading capacity and ideal alignment combined with the processability from aqueous dispersions promises a novel alternative for preparation of solar cells with controlled domain sizes in the desired length scale and orientation. The design, synthesis and characterization of these novel amphiphilic semiconductor blockcopolymers suitable for solar cell applications will be presented. This procedure can easily be extended to a variety of similar polymers.

Organic solar cells are easy and inexpensive to manufacture, however, a major limitation to the full exploitation of their potential is the limited light absorption. This limitation results from the need to use thin films due to limited carrier mobility, and because of band absorption rather than an absorption threshold. Light management thus has the potential for a breakthrough in enhancing energy conversion efficiency in organic photovoltaics. We analyze the use of a periodic structure for light trapping at a specific band of frequencies by optical resonance of the light captured inside the structure, enhancing the absorption at this specific band. This approach is aimed at expanding the absorbed portion of the solar spectrum. The photovoltaic device is integrated within the photonic crystal laterally rather than vertically, preventing issues of charge transport induced by adding the light manipulating layer below/ above the photovoltaic one. We present theoretical modeling of the enhancement in light absorption based on numeric simulation using the Finite Difference Time Domain (FDTD) method. We compare the spatial distribution of absorption within the active layer in the presence of the photonic structure to that without it. The active layer is made of a poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blend, embedded within a periodic structure made of Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) or indium-doped SnO2 (ITO). The model predicts periodic dimensions in which the absorption is significantly enhanced at 660 nm, where the blend&’s absorption is weak, without harming the absorption over other parts of the solar spectrum. The spatial distribution of the absorption shows a significant impact of the presence of interfaces between the photonic structure and active blend, perpendicular to the device plane. The addition of a flash layer beneath the structure or a light-reflector layer on top of it significantly modifies both the absorption enhancement and the absorption spatial distribution.

Performance degradation is one of the most important metrics for the evaluation of solar cells. We have shown that when electrical performance of Al/Ca/P3HT:PC61BM/PEDOT:PSS/ITO devices degrades average molecular ordering of the active layer remains constant illustrating that there is not a simple correlation between active layer morphology and device performance during device aging. This observation is based on GIWAXS measurements, which show no change in film crystallinity, while significant degradation of device performance is observed. In order to control the access of water and oxygen to the solar cell surface, P3HT:PC61BM-based solar cells were encapsulated using PET, Kapton and silica glass. Electrical measurements at the device level point to water diffusion as the primary determining factor of the degradation rate of the solar cell&’s electrical properties. The fact that water diffusion determines degradation rate of OPV solar cells is good news for high-throughput manufacturability of OPV solar cells because humidity decrease on the production floor is feasible, while elimination of oxygen would be more challenging. Experiments that test whether water diffusion controls degradation rate in all polymer-based photovoltaics are currently being performed. MPN is grateful to the Director&’s Fellowship Program for financial support. Use of the Center for Nanoscale Materials and the Advanced Photon Source was supported by the U. S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under contract No. DE-AC02-06CH11357.

Light emitting diodes (LEDs) are attracting a lot of interest as candidates for next-generation lighting sources due to their low energy consumption and long lifetimes. However, in order to realize the future LED-based solid-state lighting, the quantum efficiency of LEDs is needed to be improved further. The external quantum efficiency of LEDs is mainly determined by the internal quantum efficiency (IQE) and the light extraction efficiency (LEE). Recently, the IQE of a GaN LED was drastically increased due to the rapid development of growth techniques using metalorganic chemical vapor deposition (MOCVD) processes. In case of the LEE, although the use of a patterned sapphire substrate (PSS) can yield an LEE enhancement up to 80%, further enhancements in the LEE are strongly required for the continued development of high-power low-consumption LEDs. Herein, we describe the fabrication of corrugated inorganic oxide surface via direct single step conformal nanoimprinting to achieve enhanced light extraction in light emitting diodes (LEDs). Nanoscale zincoxide (ZnO) and indium tin oxide (ITO) corrugated layer were created on a nonplanar GaN LED surface including metal electrode using ultraviolet (UV) assisted conformal nanoimprinting and subsequent inductively coupled plasma reactive ion etching (ICP-RIE) treatment. The total output powers of the surface corrugated LEDs increased by 45.6% for the patterned sapphire substrate LED and 41.9% for the flat c-plane substrate LED without any degradation of the electrical characteristics. The role of the nanoscale corrugations on the light extraction efficiency enhancement was examined using 3-dimensional finite-difference time-domain (FDTD) analysis. It was found that light scattering by surface corrugation plays important role to redirect the trapped light into radiative modes. This straightforward inorganic oxide imprint method with inherent flexibility provides an efficient way to generate nanoscale surface textures for the production of high power LEDs and optoelectronic devices.

In this research, we suggest a facile method to synthesize nanoporous TiO₂ thin film. Silver/TiO₂ co-sputtering led to the nanocomposite film which consists of silver nanoclusters and surrounding TiO₂ matrix, and subsequently, Ag nanoclusters in nanocomposite were selectively etched by just immersing in nitric acid. Nanoporous-TiO₂ photoelectrodes fabricated by this simple and straightforward process were applied to dye-sensitized solar cell (DSSC), and showed the power-conversion efficiency of 3.4% under 1 sun condition at the thickness of only 1.8 mu;m. [1] C. Nahm, H. Choi, J. Kim, D.-R. Jung, C. Kim, J. Moon, B. Lee, and B. Park, Appl. Phys. Lett.99, 253107 (2011). [2] S.-C. Yang, D.-J. Yang, J. Kim, J.-M. Hong, H.-G. Kim, I.-D. Kim, and H. Lee, Adv. Mater.20, 1059 (2008). Corresponding Author: Byungwoo Park: byungwoo@snu.ac.kr

Nano titania films used in many advanced energy technology fields typically are prepared by screen printing. An alternative method that can be used to build mesoporous anatase films as they are applied for example in dye-sensitized solar cell (DSSC) photoanode fabrication, is electrophoretic deposition (EPD). Multiple layer deposition is often required to create barriers against interfacial charge recombination. It is the scope of this work to explore the potential of EPD as directed self assembly method to prepare nano titania/metal oxide composite films via its coupling with simultaneous electrolytic deposition of hydrous metal oxides. To this end, 3 different salts, Mg(NO3)2, Zn(NO3)2, and AlCl3, were employed as charging agent as well as binder in a P25 nanotitania 5 vol% isopropanol aqueous suspension. These nano hydrous oxides were found to distribute uniformly within the TiO2 film at approximately 2-3wt% content significantly increasing film adhesion. In order to provide insight in the role of these co-deposited metal oxides as potential charge recombination blocking barriers, the photovoltaic properties of mixed nano composite films were studied in detail by employing electrochemical techniques like EIS and OCVD. Thus, the TiO2-Al2O3 electrode was found to exhibit the highest charge recombination resistance at the TiO2/electrolyte interface (Rct) and as consequence slightly higher Voc among the three composite films. However, its conversion efficiency (4.14%) was the lowest because it suffered from very high electron transport resistance (Rt) in the TiO2 network. By comparison the TiO2-MgO film resulted in 5.40 % conversion efficiency and the TiO2-ZnO film in 5.85% efficiency-both exhibiting significantly lower Rt resistance. The obtained results point to the need for simultaneous optimization of the nanocomposite TiO2/metal oxide film structure that delivers high interfacial charge recombination resistance while maintain low the overall electron transport resistance of the film.

Due to potentials as a low cost energy conversion device from solar to electric energy, Dye sensitized solar cells (DSCs) have been extremely developed and reached recently power conversion efficiency of exceeding 12%. In counter electrode of dye sensitized solar cell, oxidized ions in electrolyte are reduced with electrons. The charge transfer resistance in interface between electrolyte and counter electrode is one of the important factors determining fill factor of DSCs. Commonly used platinum counter electrode has high catalytic activity resulting in high power conversion efficiency but it is scarce and expensive so low cost materials with high catalytic activity is needed to replace platinum. Not only intrinsic catalytic activity of materials but functional nanostructure of electrode provides chances to improve photovoltaic performances of DSCs. Mesoporous structure can maximize an intrinsic catalytic activity of counter electrode materials and improve interface between electrolyte and counter electrode due to its internal surface area, open pores and interconnected framework. We applied mesoporous carbon materials prepared by using hard template or soft template methods and reported comparable efficiency with conventional platinum counter electrode in iodine electrolyte (I3-/I-). When we also used in Quantum dot sensitized solar cell, mesoporous carbon counter electrode showed higher fill factor and better stability than platinum. We also synthesized ordered mesoporous TiN-carbon composites and used in disulfide/thiolate electrolyte which is promising one of iodine free electrolytes because conventional platinum showed poor catalytic activity with large charge transfer resistance in this electrolyte. The ordered mesoporous TiN-carbon composite showed two fold improvement of efficiency (6.71%) with lower charge transfer resistance than Platinum (3.32%). It is attributed to synergetic effect obtained from mesoporous structure and high catalytic activity of TiN. In addition to carbon materials, it has also been proved that the application of mesoporous structure to inorganic materials such as partially reduced tungsten oxide (WO3-x) improves catalytic activity, thereby lowering charge transfer resistance and increasing power conversion efficiency.

Hierarchically structured TiO2 (HS-TiO2) was prepared on a flexible ITO-PEN (polyethylene naphthalate) substrate via electrospray deposition using a commercially available TiO2 nanocrystalline powder in order to fabricate flexible DSSCs under low-temperature (<150°C) conditions. The cell efficiency increased when using flexible ITO-PEN substrates post-treated by either a mechanical compression treatment or a chemical sintering treatment using titanium tetrabutoxide (TTB). The mechanical compression treatment reduced the surface area and porosity of the HS-TiO2; however, this treatment improved the inter-particle connectivity and physical adhesion between the HS-TiO2 and ITO-PEN substrate, which increased the photocurrent density of the as-pressed HS-TiO2 cells. The electron diffusion coefficients of the as-pressed HS-TiO2 improved upon compression treatment whereas the recombination lifetimes remained unchanged. An additional chemical sintering post-treatment involving TTB was tested for its effects on DSSC efficiency. The freshly coated TiO2 submitted to TTB hydrolysis in water at 100°C yielded an anatase phase. TTB treatment of the HS-TiO2 cell after compression treatment yielded faster electron diffusion, providing an efficiency of 5.57% under 100 mW cm-2, AM 1.5 global illumination.

It has been considered that the optimization of semiconducting electrode nanostructures is one of the primary factors to improve electron transport and enhance the overall conversion efficiency of dye sensitized solar cells (DSSCs). Additionally, it has been also reported that the introduction of carbon materials such as carbon nanotubes and graphene into photoanodes induced an increase of DSSC performance due to high electrical conductivity of carbons. In our recent study, graphitic thin films embedded with TiO₂ nanoparticles were included in the photoanodes of conventional DSSC configuration, and the cell efficiency was improved up to ~ 60%. The aim of this study is to integrate various carbon moieties as essential elements of DSSCs and systematically investigate their effects on the cell efficiency. First, we demonstrate block copolymer (BCP) mediated synthetic protocols for hybrid carbon/TiO₂ beads. Commercially available triblock copolymers were utilized as a building block to generate well-defined hybrid nanostructures as well as a carbon source. For the hybrid carbon/TiO₂ bead fabrication, the surface of SiO₂ beads was covered with a mixture of TiO₂ sol-gel precursor and BCPs followed by calcination under inert atmosphere. Next, we introduced a series of different types of carbon precursors such as carbohydrates, pyrrole and BCPs to distribute carbons evenly throughout photoanode layer and find the best candidate as a carbon source for constructing hybrid photoanode in DSSCs in terms of conductivity, convenience for use, ease of integration into photoanode. Finally, we present a low-cost fabrication route to prepare hybrid Pt/carbon counter electrodes which are decorated with 5-30 nm Pt nanodots based on a combined BCP inverse micelle lithography and direct carbonization process. Simple variations such as the concentration of Pt precursor, the immersing time and the order of UV stabilization step allowed for the fine-tuning of the size of Pt NPs. This work represents facile strategies to produce carbon-based hybrid components in DSSCs with a better conductivity and electron transport, which can lead to an enhanced DSSC performance.

Dye sensitized solar cell (DSSC), now a matured field of research which attracted extensive interest for past two decades, has been considered as a cost-effective alternative for conventional thin film based photovoltaic device. While overall conversion efficiency of conventional TiO2 paste based mesoporous layer on rigid glass substrate have reached over 12%, high temperature annealing process was unavoidable for removal of organic binder and ensure necking between nanoparticles. Low temperature process providing well crystallized nanoporous structure is required in order to incorporate flexible polymer substrate into DSSC device for practical advantages such as low cost, light weight, dimensional flexibility and shock resistivity. Zinc oxide(ZnO) has long been expected to be candidate material for DSSCs for having similar valence band edge, conduction band edge position of TiO2. However, ZnO is expected to have advantages such as higher electron mobility and morphology controllability largely owing to anisotropic crystal structure of hexagonal wurzite. Unique crystallographic characteristics of ZnO combined with highly non-equilibrium state of pulsed laser induced ablated species enabled direct room temperature deposition of highly crystalline ZnO nanostructure free from any chemically induced additives or organic/inorganic impurities. In this presentation, we demonstrate direct deposition of porous hierarchical ZnO structure on flexible PEN/ITO as well as rigid glass/ITO substrate. Film morphology was carefully modified by varying ambient gas pressure, where internal surface area was finely tuned as a function of atmosphere gas pressure. Deposited film was sensitized with conventional N719 dye for DSSC application. Optimized structure on rigid substrate showed short circuit current density of 13.1mA/cm2, open circuit voltage of 0.55V and overall efficiency of 3.89% under 1.5AM light illumination with no further heat treatment. Device performance of fabricated DSSCs was discussed in terms of morphology of the ZnO nanostructure.

Hydrogen generation through water splitting using sunlight is promising technology as solar energy storage. Hematite, which has a bandgap of ~2eV, is an attractive material as a photoanode to harvest substantial portion of solar energy. However, the short diffusion length of minority carriers (~10nm assisted by band bending at the electrolyte junction) limits effective light absorption, and as a result, hydrogen generation is limited near the surface. Here, we investigate an increase in photocurrent generation by Mie resonance, an EM resonance within a wavelength-scaled dielectric nanostructure, combined with waveguiding effects. Array of hematite nanowires on the thin layer of ITO is proposed as a model of optical system and optimized by determining specific thickness of the ITO layer, size and pitch of the nanowire array. Light intensity inside the hematite nanowire increases when the feature size of the nanowire is comparable with the wavelength of incident light. In particular, when the diameter of the wire is ~200nm, high order mode of Mie resonance is driven, resulting in enhanced light intensity near the surface and thereby improved photocurrent generation. Furthermore, with optimal pitch of nanowire, scattered light from the nanowire propagates in the ITO layer and couples again with the array of the nanowires, leading to additional Mie resonance. Full field EM simulation indicates that, with properly chosen pitches and size of the model, substantial enhancement of light absorption, about 2 times higher than the planar counterparts, can be achieved near the surface of the nanowire. Mie resonance coupled with waveguiding effect is also experimentally demonstrated, showing the strong peak of absorption enhancement at resonant spectrum region.

Solar thermal fuels make use of molecules that undergo reversible photo-isomerization to store solar energy and convert it into thermal energy. Because solar thermal fuels produce no emissions and can store and convert energy within one material, they are an attractive option for a renewable alternative energy source. The trans to cis azobenzene photo-isomerization has drawn attention as a candidate material for solar thermal fuels. However, both isomers are photoactive in similar regions of the solar spectrum, and the metastable cis isomer exhibits a higher absorption coefficient, leading to a photo-stationary (storage) state with a significant amount of the lower energy trans isomer and an energy storage capacity of only twenty percent of the maximum value. We explore a novel solution to this problem, designing close-packed semi-crystalline azobenzene/template nanostructures to sterically inhibit the cis-to-trans photo-isomerization pathway without preventing the trans-to-cis photo-isomerization. Using various levels of theory from DFT to GW/Bethe-Salpeter equation approach and TDDFT, we calculate the absorption properties of trans- and cis- photoisomers and determine storage efficiency of solar thermal fuels. Our work demonstrates the possibility of using geometric and chemical modifications to simultaneously tune the ground and excited state behavior in these molecules, and could lead to large efficiency improvements in solar thermal fuels.

Sunlight can be harvested and transduced into useful energy using semiconductor-liquid junction solar cells, which generate power through the transfer of electronic energy to molecules dissolved in fluid solution. If two such interfacial electron-transfer events are efficiently coupled, chemical bonds can be formed, an important step to powering our planet with fuels derived from renewable energy. Toward this, free-standing periodic arrays of crystalline silicon microwires partially embedded in a Nafion proton-exchange membrane were used to photogenerate H₂, and I₃^-, from aqueous HI solutions via sunlight-driven artificial photosynthesis. Ordered arrays of Si microwires were grown on planar Si(111) substrates by a chemical-vapor-deposition process, employing Cu growth catalysts. Nanoparticle electrocatalysts were deposited on the Si microwires to catalyze light-driven hydrogen evolution and iodide oxidation at p- and n-type Si, respectively. Open-circuit photovoltages measured for each type of wire array were 400 - 500 mV, with n⁺p-Si in HCl/H₂ and n-Si in acidic I₃^-/I^-. Corrosion of Si was attenuated through methylation of Si atop sites via a two-step chlorination-alkylation surface chemistry procedure. This resulted in stable iodide oxidation under continuous simulated sunlight illumination for weeks. Si microwire arrays partially embedded in Nafion proton-exchange membrane were mechanically removed from the Si substrate to yield free-standing devices. Pt electrocatalysts were deposited on the microwire backsides by electron-beam evaporation to complete the functional fuel-forming devices. Solar-to-hydrogen efficiencies and Faradaic yields for product formation were quantified. These systems are sustainable because the HI fuel precursor is inorganic, thus not capable of liberating CO₂, and HI can be regenerated in a flow battery / fuel cell as H₂ + I₂.

To understand the performance of photoelectrochemical systems, it is necessary to develop a high fidelity, predictive simulation tool for device performance on the mesoscale. On this length scale (100nm to 1cm), light absorption, carrier transport, catalysis, electrolyte and product transport through a potentially multiphase solution and ion-conducting membrane are coupled together. We report here on modeling of light absorption and carrier transport characteristics of a monolithic, Si-microwire based, Z-scheme photoelectrochemical device. This design capitalizes on high aspect ratio structures that orthogonalize light absorption and carrier collection to improve EQE in indirect bandgap materials. It also increases catalytic surface area and decreases material usage in comparison to planar models. Experiment and modeling have revealed that Si is an attractive photocathode for a Z-scheme design, but challenges exist for photoanode selection and design. Designs that incorporate photoanodes including TiO₂, WO₃, BiVO₄, and ZnSnN₂ have been explored to optimize efficiency with respect to optoelectronic device performance. A thorough analysis of light absorption in integrated Si/WO₃ tandem designs reveals the power of modeling. Light absorption and carrier transport are investigated by simulation using full-field electromagnetic and device simulations to optimize design. As an example, we investigated Si wires (d=2µm, L=100µm, pitch=7µm), conformally-coated with 50µm of a 100nm-thick contact layer and 1µm of WO₃. Exact dimensions vary. The initial design for a Si/WO₃ tandem device contacted with aluminum resulted in a WO₃-limited, short circuit current of 1.04mA/cm², under AM1.5 illumination and unity IQE. Spatially-resolved light absorption profiles revealed that the aluminum strongly absorbs in a surface plasmon mode, motivating use of a transparent contact (ITO). However, a transparent contact shortens photon path lengths through WO₃, thereby reducing short circuit current to 0.7mA/cm². With the use of light management techniques, iterative design optimization afforded a greater than 200% increase in short circuit current, achieving 2.55mA/cm². These findings demonstrate that material selection and geometric design is crucial to optimization of light absorption. Experimentally, these tandem devices have been successfully fabricated, consisting of n-type Si microwires with p⁺ emitter layers, which are conformally-coated with sputtered ITO and electrodeposited WO₃. Performance approaches electrochemical conditions for unassisted water splitting, achieving V_OC=1.2V and J_SC=0.5 mA/cm² in 1M H₂SO₄, under 1 sun illumination. Ongoing work focuses on achieving unassisted solar water splitting, matching experimental and simulation results, and using simulation to guide experimental device design. The speed and low cost of simulation in comparison to experiment enables the rapid exploration of design space towards accelerated device optimization.

One of the critical disadvantages of solar energy conversion system is in that the photoreceptors cannot absorb the entire photo-band of sunlight, because the light-conversion efficiency in the UV-A region is significantly lower than in the visible region. A reasonable solution for this problem can be realized by applying a spectral conversion film (SCF) from UV-A to visible wavelengths. In this paper, we demonstrate a new strategy for fabricating effective SCF. Our approach can be defined by the facts that molecularly-ordered nano-fiblirs (MOFs), containing fluorophores such as pyrene and anthracene having lambda;max in the UV-A region, are encapsulated in a polymer thin film such as PMMA and EVA, and therefore the composite polymer film is colorless and transparent. Furthermore, the fluorophores can orient in the MOFs and consequently yield their excimers leading to remarkably high Storks shift. Here, we introduce an excellent example as our SCF: the pyrene-functionalized L-glutamide (g-Pyr) [1] as a MOFs-forming and energy transfer donor, and fluorescent dyes (e.g. diphenylantrathene, perylene, and coumarin) as energy transfer acceptors were mixed with poly(ethylene-co-vinylacetate) (EVA) in a toluene/ethylacetate (1 : 1) mixture. The thin film was prepared by casting on a substrate and dried for 1 h at 25 °C. The obtained g-Pyr-doped EVA film showed high absorption in the UV-A region (315 - 400 nm) due to a pyrenyl group of g-Pyr but almost no absorption at 400 - 900 nm, and showed strong emission in the visible region (400 - 550 nm) based on the excimeric emission and energy transfers. When the SCF was put on the top of a compound semiconductor solar cell and the power-conversion efficiency was measured by using the solar simulator with AM1.5, significantly effective enhancement of the power conversion was observed. [1] T. Sagawa, S. Fukugawa, T. Yamada and H. Ihara, Langmuir, 2002, 18, 7223-7228.

Soft-lithography has some advantages such as a convenient process and cheap cost, comparing with conventional photo-lithography. In these reasons, organic-inorganic composites (OICs) can be adopted for making patterns as a mask in the surface texturing of silicon solar cell. In this study, we make OICs which consists of silica domains with noble metal clusters and entire grounds of polymers such as poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) or polystyrene -block-poly(2-vinylpyridine) (PS-b-P2VP). Silica domains in OICs are selectively etched in the well-known etching solution (HF and HNO3) and polymer domains acted as a barrier against etching. This etching solution starts to isotropically etch from the silica domain in OIC mask. Isotropic etching makes hemisphere-shaped holes on the surface of silicon wafer. The hemisphere-shaped holes were not enough to decrease reflectance of silicon wafer effectively. So we add noble metal clusters in the silica domains in order to make deep nano-sized holes by metal-assisted chemical etching. This polymer/silica/metal hybrid resin makes hierarchical pattern on the surface of silicon wafer showing both nano and micro sized pores. Silica domains in OICs make hemisphere shaped holes having micro sized holes and metal clusters make nano-sized pores over the silica domains. The diameter and depth of nanopores are about ~10nm and 100~200nm, respectively. It is possible that the size of etched hole is controllable with the size control of silica domain. The hierarchical pattern of polymer/silica/metal hybrid resin is expected to decrease effectively reflectance of silicon wafer and contribute to the improved efficiency in silicon-solar cell.

Solar energy harvesting in a sustainable and efficient way is more important today than ever before. One promising approach is to produce energy-rich solar fuels, e.g. hydrogen and methanol, from abundant resources, e.g. CO2 and H2O through an artificial photosynthetic approach. Regardless of the details in various methodologies, oxygen evolution from water holds the key to an efficient artificial photosynthetic system, because water is the only potential electron and proton source on earth that is capable to support terawatt scale energy products. Also, in order to achieve efficient solar fuel production, a higher TOF (100 s-1nm-2) is required for keeping up with the photon flux at high solar intensity. It is known that Pt is capable to reduce proton to hydrogen at a rate of 290 s-1 per surface atom. Therefore, oxygen-evolving from water is the rate-limiting step and searching for efficient water oxidation catalysts is vital for building up an efficient solar fuel system. Recent progress indicates that cobalt and manganese spinels could be highly efficient water oxidation catalysts. Although the origin of high activity is still unclear, it has been hypothesized that the metal sitting in the octahedral sites dominates the overall performance of spinel catalyst because of its cubane structure. Here, we will show our recent studies in mesoporous binary spinel systems, which suggest the metal sitting at the tetragonal site has huge impact on the water oxidation activity of spinel catalysts. A wide range of AB2O4 mixed oxide spinels with highly ordered mesoporous structures have been synthesized for the first time and their structures have been carefully characterized. The photocatalytic oxygen evolution activities of as-prepared samples were investigated using Ru(bpy)32+/Na2S2O8 method. The preliminary results indicate the element at tetragonal sites has significant impact on the overall photocatalytic activities. References: 1. Jiao, F. &Frei, H. Nanostructured cobalt and manganese oxide clusters as efficient water oxidation catalysts. Energy & Environmental Science 3, 1018-1027 (2010). 2. Boppana, V. B. R. & Jiao, F. Nanostructured MnO2: an efficient and robust water oxidation catalyst. Chemical Communications 47, 8973-8975 (2011).

The fabrication of hierarchically structured porous materials with the building blocks of perovskite niobate nanosheets H2(Ca,Na)NbnO3n+1(n=2,3,4,5) is demonstrated based on colloidal self assembly. Aurivillius phases Bi2(Ca,Na)NbnO3n+3(n=2,3,4,5) were chemically exfoliated into two dimensional perovskite nanosheets by using an efficient organic-free one-step method. Dispersion of the nanosheets is very sensitive to pH and can be adjusted from full dispersion to full agglomeration. Under controlled conditions, the nanosheets form either highly disordered 3-D structures or layered structures by self assembly. The strong hydrogen bonds between the protonized surfaces of the nanosheets are found to be responsible for the self assembly of the colloidal suspension. The microstructures are analyzed by in-situ high temperature X-ray diffraction. The photocatalytic activities of the nanosheets are characterized with respect to decoloration of methylene blue and water splitting. With the large surface area and high surface hydroxyl group density, the well dispersed nanosheets showed enhanced activity and demonstrated the potential for technological application.

The greenhouse effect has become a threat to the living environment of mankind. CO2 is the primary greenhouse gas which is released mainly by the burning of fossil fuels and the expanded human activity. At the same time, CO2 is also a nontoxic, cheap, and highly functional carbon source. The conversion of CO2 at room temperature and atmospheric pressure using solar light is a highly challenging and promising approach to close the CO2 cycle, and to develop photosynthesis mimetic approaches. This option requires technologies both to capture CO2 from the atmosphere and to convert it into fuel. To satisfy the two requisites, layered double hydroxides (LDHs) were chosen with the expectation of (1) sorption capacity for CO2 in the layered space and (2) tunable semiconductor properties (photocatalytically active) as a result of the choice of metal cations. In this work, ordered layered double hydroxides (LDHs) consisting of zinc and/or copper hydroxides were synthesized and combined with aluminum or titanium by the co-precipitation method at constant pH. These LDH compounds were then applied as photocatalysts to convert gaseous CO2 and H2O to carbon fuels under full spectra. Zn-Ti LDH was the most active for CO2 photoreduction and the major product was CH4 and CO, which was online characterized by Gas Chromatography. Powder X-ray diffraction (PXRD), scanning electron microscope (SEM), highresolution transmission electron microscopy (HRTEM) and Brunauer-Emmett-Teller (BET) revealed that the prepared LDHs samples possess high crystallinity, hierarchical structure as well as large specific surface. The mechanism of photocatalytic reduction of CO2 with H2O over the catalyst samples was investigated by DRIFTs. The reaction kinetic was discussed. It is suggested that the rate limiting reaction is the water splitting reaction, similar to the natural photosynthetic systems. When gas phase hydrogen was allowed in the system, the hydrogenation conversion rate was increased significantly. This work demonstrates the potential application of LDHs in the field of photocatalysis

The development of photocatalysts for hydrogen evolution and CO2 reduction under visible-light is important in respect of the fossil fuel shortage and global warming problem. We have developed N-doped Ta2O5 (N-Ta2O5) powder with p-type conduction [1] and achieved the visible-light-induced selective reduction of CO2 to HCOOH for the first time by N-Ta2O5 combining with a ruthenium complex [2, 3]. In order to improve photocatalytic activity of N-Ta2O5, it is desired to control its morphology and porous form. The higher surface area can enhance the interaction of N-Ta2O5 with the reactants, while the submicron-sized spherical morphology is expected to enhance optical absorption due to the light scattering [4]. In this paper, we report the synthesis of submicron-sized mesoporous N-Ta2O5 spheres (N-CMTS) and their photocatalytic activities. First, crystallized mesoporous tantalum pentoxide spheres (CMTS) with particle diameters of ca. 100-500 nm and composed of Ta2O5 nanocrystals were synthesized by a combination of the sol-gel process and heat-treatment with the aid of carbon reinforcement. The specific surface area of the CMTS was up to 105 m2 g-1 and the pore diameter was controllable in the range of 5.6-17 nm by changing the crystallization temperature. Visible light-sensitive p-type N-doped Ta2O5 (N-CMTS) containing 5 at% N was successfully obtained by treatment of CMTS with ammonia. The yellow-colored N-CMTS retained the mesoporosity and morphology of CMTS after the nitridation process. A hybrid photocatalyst comprised of an N-CMTS semiconductor linked with a ruthenium-complex electrocatalyst exhibited higher activity for the photoconversion of CO2 to formic acid under visible-light irradiation (ge;410 nm) than hybrid photocatalyst with fine N-doped Ta2O5 (N-Ta2O5) particles due to their larger surface area and morphology. The synthesis method presented herein can be widely applied to highly-efficient photocatalysis for solar energy conversion and solar fuel generation. [References] [1] T. Morikawa, et. al., Appl. Phys. Lett., 96 (2010) 142111. [2] S. Sato, et. al., Angew. Chem. Int. Ed., 49 (2010) 5101. [3] T. M. Suzuki, et. al., Chem. Commun., 47 (2011) 8673. [4] Q. Zhang, et. Al., Adv. Funct. Mater., 18 (2008) 1654.

Solar water splitting for hydrogen generation is known to have the potential for future carbon-free, renewable and storable source of energy. Therefore, significant research effort has been made in this area over the last four decades [1]. Recently, metal nitrides (i.e., GaN, InGaN) have attracted considerable attention, as they possess all the requirements (i.e., band gap, band alignment and corrosion resistant) for water splitting [2]. On the other hand, we have recently shown that one-dimensional (1D) GaN nanowires offer significantly improved photocatalytic activity over commonly used GaN nanoparticles for overall water splitting [3]. Moreover, the surface band bending and therefore carrier transport properties of nanowires can be engineered by controlled dopant (Si and Mg) incorporation, which can further enhance the photocatalytic activity. In this context, we have studied the effect of Si and Mg ion dopants on the water splitting activity of GaN nanowire arrays. GaN nanowires are grown on Si (111) substrate using radio frequency plasma assisted molecular beam epitaxy (MBE). The as-grown nanowires are doped with tetravalent (Si4+) and divalent (Mg2+) ions to make it n and p type, respectively. The doping density is controlled by tuning the effusion cell temperatures of Si and Mg. Room temperature photoluminescence (PL) measurement shows the peak position near ~365 nm for both Si and Mg doped samples, which corresponds to the bandgap of GaN. The photocatalytic activity of Si and Mg doped GaN nanowires are tested by individually performing H2 and O2 half reactions in the presence of respective sacrificial reagents. The H2 half reaction demonstrates significantly enhanced photocatalytic activity for Mg doped GaN nanowires, compared to Si doped and non-intentionally doped nanowires. The enhanced activity of Mg doped nanowires is further correlated with X-ray photoelectron spectroscopy (XPS) valence spectrum, which indicates the presence of downward band bending at the wire surface, compared to the upward band bending commonly observed for Si-doped GaN nanowires. To enhance the photocatalytic activity for H2 evolution, Rh and Cr2O3 nanoparticles are deposited on the GaN nanowire surfaces using photodeposition process from liquid precursors. Successful deposition of metallic Rh core and amorphous Cr2O3 shell are confirmed by XPS and high resolution transmission electron microscopy on the nanowire surface. The overall water splitting activity of Mg doped GaN are significantly higher than Si doped and undoped GaN nanowires. A detailed correlation between the enhanced photocatalytic activity and surface band engineering, due to controlled dopant incorporation is being investigated and will be reported. [1] Kudo, A.; Miseki, Y., Chem. Soc. Rev. 38, 253 (2009). [2] Fuji, K., Kusakabe, K., Ohkawa, K., Jpn. J. Appl. Phys., 44, 7433 (2005). [3] Wang, D. et al. Nano Lett. 11, 2353 (2011).

Due to the well-defined molecular structures and extremely narrow size distribution, virus capsids have been widely exploited as three-dimensional scaffolds for hierarchical assembly of inorganic materials and functional molecules. Researchers have built a variety of functions into capsids to enable site-specific binding of inorganic materials, drug delivery, light harvesting, and use as high-surface area electrodes. To take advantage of the power of viral scaffolds and direct their organization into more complex hierarchical structures to achieve higher functionalities, various techniques have been investigated for patterning viruses over large length scales. However, very few are able to control the inter-virus spacing and to position individual viruses with nanoscale precision. On the other hand, the use of DNA origami tiles as templates has been recently reported for directed assembly of many nanoscale objects, such as gold and silver nanoparticles, RNA molecules, and carbon nanotubes with exquisite precision. In this work, we explored the use of DNA origami tiles as inorganic-biological interfacial templates for tunable photo-energy conversion by selective immobilization of photoactive bacteriophage MS2 capsids and gold nanoparticles, serving as optical emitters and antennas, respectively. The virus capsids were modified on their interior surfaces to include choromophores as light harvesting centers and on their exterior surfaces with single strand DNA as probe strands. These optically active virus capsids were then precisely assembled on DNA origami tiles bearing the complementary probe strands for linkage and gold nanoparticles for photo-energy conversion enhancement. Since the precise control over the spatial proximity between photo-emitting viruses and nanoparticle antennas is crucial to achieve photoemission enhancement, we took advantage of DNA origami as programmable templates to define an emitter-antenna separation of between ~0 and 70 nm. We also developed a correlative approach to studying the distance dependence of energy conversion efficiency that utilized atomic force microscopy to measure the nanometer scale spacing between the capsid and the nanoparticle and single-particle fluorescence microscopy to determine the energy conversion efficiency of each single hierarchical structure unit. Our results show a systematic dependence of energy conversion efficiency on separation, which are consistent with physical models of distance-dependent quenching at short length scales and enhancement at longer lengths scales. Efforts are currently underway to implement these origami tiles into higher order structures to obtain improved light harvesting yield and photo energy conversion efficiencies. A feasible solution is to organize the origami tiles at micrometer-scale using complimentary anchor strands patterned by scanned probe lithography or micro-contact printing.

Photosynthesis is one of the most famous energy transfer process used by photosynthetic organisms to transform carbon dioxide and water into sugar and oxygen by capturing sun energy. And this process occurs in the chloroplasts in many photosynthetic organisms such as leaves and green algae. Here, we increase the density of photosynthetic organisms by concentrating and immobilizing them in a proper matrix. To enhance the mass transfer in high density, we fabricated micro-sized hydrogel particles containing photosynthetic organisms using photopolymerization process. We also measured oxygen production in the photosynthetic microparticles to figure out whether this unit can run the photosynthetic process or not. This work may have a significant impact on the design of artificial photosynthetic reactor. In this experiment, we chose green algae to be concentrated in hydrogel because they have higher concentration of chloroplasts than general plant leaves. To increase the photosynthetic efficiency or apply additional engineering factors to photosynthetic processes, the immobilization of photosynthetic organisms in proper matrix might be a good solution. To achieve this, we should immobilize the photosynthetic organisms in appropriate matrix and make them alive at least more than several days supporting mass transport of their reactants and products. The mixture of polyethylene glycol acrylate (PEGDA: Mw. 700), which is hydrogel prepolymer, and photoreactive initiator (5% in PEGDA) was used to provide hydrogel matrix for immobilizing photosynthetic organelles. This material is photopolymerized by photochemical reaction. Algae, whose scientific name is Chlamydomonas, was added to this polymer as 2:1 to immobilize the algae. With this mixture, we fabricated micro-sized particles containing Chlamydomas using maskless photolithography technique capable of manufacturing micro-sized polymer particle with any sizes and any shapes. We designed few hundred micrometer particles as a unit of artificial photosynthetic reactor because a group of small sized particles obtains higher mass transport efficiency than thin wide films by increasing the influx and outflux due to its large surface/volume ratio. Using these photosynthetic microstructures, we observed their oxygen production changing the concentration of photosynthetic organisms and other conditions, to optimize their functions. In conclusion, we immobilized concentrated green algae in polymeric microparticles through photopolymerization. High surface/volume ratio of the downsized photosynthetic microparticles helps to increases the mass transport efficiency of algae&’s reactants and products, enhancing viability of embedded algae. The scheme shown here may have a significant impact on next artificial photosynthetic reactor. Acknowledgement: This work was supported by the National Research Foundation of Korea(NRF) grant funded by the Korea government(MEST) (2011-0030269).

Fiber membranes have been shown to be very useful for heterogeneous catalysis and also as supports for catalytic metal nanoparticles and nanowires. In this research, a few techniques have been presented to functionalize TiO₂ nanofibers with Ag nanostructures with an aim to increase the specific surface area and effective photoactive sites for more efficient photocatalysis. Pristine TiO₂ nanofibers are prepared by electrospinning a precursor solution of Ti(IV) isopropoxide at 15 kV followed by pyrolysis at 510 ^oC. Hierarchical nanostructures of Ag nanoparticles are grown on TiO₂ nanofibers by reduction of AgNO₃ with different concentrations on the surface of the TiO₂ nanofibers during calcination at 450 ^oC. The functionalization of TiO₂ nanofibers with Ag nanostructures enhances the trapping of photoexcited electrons allowing for more electrons available to participate in redox reactions by minimizing electron-hole recombination. Hierarchical growth of TiO₂ nanorods on TiO₂/Ag nanofibers was observed by dip-coating pristine TiO₂ nanofibers in AgNO₃-ethanol solution followed by heat treatment at 450 ^oC for 1 h. Structural characterization of the prepared nanofiber catalysts was done using high-resolution TEM operated at 200 kV. High-resolution lattice-fringe measurements showed the presence of mixed-phase anatase and rutile TiO₂ nanocrystals along with elemental Ag nanoparticles. High-angle annular dark-field (HAADF) imaging was performed to study the distribution of Ag nanoparticles on the surface of the TiO₂/Ag nanofibers. HAADF provides for Z-contrast imaging making it easy to differentiate heavier atomic-number (Z) elements from lighter ones. Surface roughness and porosity were observed by performing tilt-series imaging followed by reconstruction to build a 3D electron tomograph of a single nanofiber. BET specific surface-area analysis was used to measure the total specific surface area and active surface area for different fiber samples. Photocatalytic activities of various pure TiO₂ and TiO₂/Ag nanofibers were investigated by studying the decomposition of methyl red and rhodamine-B dyes, under UV-light irradiation in a photoreactor. It was found that the TiO₂/Ag nanofibers had more than 10 times the activity compared to the pure TiO₂ nanofibers. Further, higher activities can be achieved by increasing the total specific surface area of such nanofiber catalysts. Some other ways to enhance the photoactivity of such TiO₂-based nanofiber-catalysts include optimum loading of Ag nanoparticles and reduction of AgNO₃ on TiO₂ nanofiber surface, with a suitable catalyst, to produce Ag nanowires on TiO₂/Ag nanofibers. These TiO₂/Ag nanofiber catalysts are excellent candidate materials for photoelectrochemical splitting of H₂O for hydrogen generation.

One of today&’s technological challenges is to effectively convert the sun energy to fuels such as hydrogen.1,2 The conventional metal oxide-based approach, however, suffers from very low efficiency under direct solar irradiation, due to the large bandgap.2 On the other hand, the bandgap of metal nitrides, i.e. InGaN can be varied across nearly the entire solar spectrum.3 In this regard, we have developed a multilayer n-InGaN nanowire system for photoelectrochemical hydrogen generation, which showed good photocurrent with high incident photon to current efficiency (IPCE) (up to ~ 2.45%) under visible light illumination. To make the system more practical, we utilized salt-based solution and further compared the results with acidic solution. In this experiment, silicon doped InGaN nanowire arrays were grown by a Veeco Gen II MBE on n-type, Si (111) substrate. Scanning electron microscopy image of n-InGaN shows that nanowires are vertically aligned to the substrate with a high degree of size uniformity. Their average lengths and diameters are about 400 nm and 50 nm, respectively, with estimated areal density of 1.5 × 1010 cm-2. The InGaN nanowire sample was characterized by photoluminescence spectroscopy, which showed a strong emission peak at ~ 540 nm at room temperature. Three-electrode configuration was used to carry out photoelectrochemical experiments, including an Ag/AgCl as a reference electrode, Pt wire as a counter electrode, and n-InGaN as a working electrode. All PEC measurements were conducted in one molar solutions of either potassium or hydrogen bromide, i.e. KBr or HBr, as electrolyte. A 375 nm cut-off filter was used to remove the ultraviolet light and prevents the GaN layer being excited. The photocurrent versus bias potential curves show the flat band potential depends on the electrolyte and is ~ -0.30 V and -0.45 V in HBr and KBr, respectively. Photocurrents are ~ 0.4 mA/cm2 and 0.6 mA/cm2 at zero bias in KBr and HBr, respectively, indicating that both electrolytes are suitable for hydrogen generation. A significant enhancement (up to a factor of 5) of the photocurrent density was observed in HBr in comparison to KBr. The stability is also confirmed by experiments performed over an extended period of time. Hydrogen generation experiment was carried out under zero bias. The results showed about 1000 µmole hydrogen was produced after 8 hours. The IPCE was measured in both electrolytes at various applied potentials. Under zero bias, the efficiency of InGaN was 0.17% and 0.53% at 410 nm for KBr and HBr, respectively. The IPCE increases by applying higher biases to the photoelectrode. For example at 0.5 V bias, the system showed an efficiency of ~ 2.45% in HBr and 0.77% in KBr at 410 nm, respectively. 1. Bolton, J. R. Solar Energy 1996, 57, (1), 37-50. 2. Gratzel, M. Nature 2001, 414, (6861), 338-344. 3. Aryal, K.; Pantha, B. N.; Li, J.; Lin, J. Y.; Jiang, H. X. Applied Physics Letters 2010, 96, (5), 052110-3.

Recently, the necessity for renewable energy is increasing according to the high oil price, environmental pollution, and global warming. The solar harvesting devices that can converting solar energy to electric energy- is the eco-friendly technology that does not generates pollution, and it can get the electric power wherever sun shines[1-3]. Especially, organic solar cell, one of different type of solar cell is manufactured on circuit board with thin semiconductor film coats, it's producing is simple and low cost, and mass production is possible.In this work, an optical combination of polymeric donor and acceptor materials using genetic algorithm was achieved for organic harvesting devices. We have investigated the organic semiconductor combination factors affecting organic solar cell performance. The poly(3-hexylthiophene) and [6,6]-phenyl C₆₁butyric acid methylester blends composed bulk-heterojunction structure was selected as the organic semiconductor in order to verify the proposed model. Through the analysis the genetic algorithm for organic functional materials, it was concluded that the suggested model can find optimal combination for high efficiency energy harvesting devices. Acknowledgement This research was supported by Basic Science Research Program through the National Research Foundation of Korea(NRF) funded by the Ministry of Education, Science and Technology(2011-0013873) References 1. C. W. Tang, C. W, Appl. Phys. Lett, 48, 183 (1986) 2. C. Waldauf, P. Schilinsky, J. Hauch, C. J. Brabec, Science Direct, ELSEVIER (2003) 3. G. Li, R. Zhu, Y. Yang, Nature Photonics, Vol. 6, No. 1 (2012)

Zinc oxide (ZnO), a II-VI semiconductor with a wide bandgap of 3.37 eV with a large exciton binding energy of 60 meV, has attracted extensive attention as a promising materials for applications in photovoltaic cells. ZnO has polar terminated (001) face and nonpolar (100) face. The nonpolar surfaces are more stable than polar face, leading to faster growth along the polar surface and therefore, one dimensional(1-D) ZnO nanostructure can be easily synthesized. 1-D ZnO nanostructure provide direct conduction pathway for the rapid collection of photogenerated electrons, which diminish the charge recombination during interparticle hops. In this report, we synthesis a hierarchical ZnO nanowire structure which has larger surface area than 1-D ZnO using hydrothermal growth and RF-sputtering deposition. The morphology and absorbance of hierarchical ZnO has been charicterized by SEM and UV-visable spectroscopy respectively.

Our living environment has an abundance of energies in the forms of light, thermal, mechanical (such as vibration, sonic wave, wind and hydraulic), magnetic, chemical and biological. Harvesting these types of energies is of critical importance for long-term energy needs and sustainable development of the world. Over the years, rationally designed materials and technologies have been developed for converting solar and mechanical energies into electricity. Photovoltaic relies on approaches such as inorganic pn junctions, organic thin films, and organic-inorganic heterojunctions. Mechanical energy generators have been designed based on principles of electromagnetic induction and piezoelectric effect. Innovative approaches have to be developed for conjunctional harvesting of multiple types of energies using an integrated structure/material so that the energy resources can be effectively and complimentarily utilized whenever and wherever one or all of them are available. We report a hybrid cell that is designed for simultaneously harvesting solar and mechanical [1,2], and chemical and mechanical [3] energies using nanotechnology. The two energy harvesting approaches can work simultaneously or individually, and they can be integrated in parallel and serial for raising the output current and voltage, respectively. Our study demonstrates an innovative approach for developing integrated technologies for effectively scavenging available energies in our environment around the clock. [1] C. Xu, X.D. Wang and Z.L. Wang, “Nanowire structured hybrid cell for concurrently scavenging solar and mechanical energies”, JACS, 131 (2009) 5866-5872. [2] M.B. Lee, R.S. Yang, C. Li and Z.L. Wang “Nanowire-quantum dot hybridized cell for harvesting sound and solar energies”, J. Phys. Chem. Letts., 1 (2010) 2929-2935. [3] B.J. Hansen, Y. Liu, R.S. Yang and Z.L. Wang “Hybrid Nanogenerator for Concurrently Harvesting Biomechanical and Biochemical Energy”, ACS Nano, 4 (2010) 3647-3652.

Solar thermal fuels store energy from sunlight by the rearrangement of molecular bonds upon the photochemical formation of a metastable isomer, which is later released as heat upon reversion to the stable isomer. Such materials are a potential solution for capturing, converting, transporting, and delivering solar energy sustainably and cleanly, but previous solar thermal fuel systems were impractical due either to low energy densities or poor photochemical properties. We have recently found that the energy density and storage lifetime of photochromic molecules with favorable photochemical properties can be engineered by templating the molecules at the nanoscale, for example, by chemically appending functionalized azobenzenes to single-walled carbon nanotubes (SWCNTs). In such a material, the azobenzenes store a portion of the light energy absorbed by the stable trans isomer by photoisomerizing to the metastable cis isomer. This stored energy is later released as heat when returning to the trans isomer. High packing densities (e.g., 1 azobenzene appended per 8 C atoms in the SWCNT) lead to predicted increases of 20% in the energetic the barrier dictating the energy storage lifetime and of 260% in the amount of thermal energy stored in the case of functionalized azobenzenes bound to single-walled carbon nanotubes. This talk discusses the strategies for synthesizing solar thermal fuels using our templating approach and the experimental validation of the predicted increases in defining energy differences.

The employment of transparent conductors in new energy conversion devices, e.g. hybrid-organic photovoltaics (HOPV), requires the development of materials with dedicated functional properties, such as large effective surface area, enhanced light trapping, controlled electrical transport and optical transparency. A promising approach to the tuning and integration of such properties is represented by the synthesis of hierarchically structured materials with morphological and structural features engineered at different length scales, and in this respect Pulsed Laser Deposition (PLD) is a powerful and versatile technique to grow oxides characterized by a wide range of morphologies at the nm and sub-mu;m scale. We here report on the synthesis of hierarchically nanostructured Al-doped ZnO (AZO) films by PLD at room temperature, with properties of interest for application in HOPV. Due to clustering phenomena within the ablation plume, in the presence of O₂ as a background gas, a threshold pressure was identified (100 Pa O₂) above which deposition occurs in the form of nanosized aggregates, constituting hierarchically assembled structures in a mesoporous forest-like fashion. The films exhibit high optical transparency in the visible range (mean value >90%) and remarkably high haze (i.e. scattered-to-transmitted photons ratio, >85%) but very high resistivity (>1 MOmega; cm) due to the reduced degree of connectivity. Low resistivity values (~ 10^-4 Omega; cm) were easily obtained on compact films grown at lower O₂ pressures, where high electron carrier density is provided by stoichiometry reduction (i.e. O vacancies) and the compact morphology results in carrier mobilities higher than 15 cm²/Vs. In order to combine the effective light management due to hierarchical structures with high electrical conductivity, an independent control of morphology and stoichiometry is required. Two possible strategies are here presented. One is based on PLD in mixed Ar:O₂ atmospheres, to decouple morphology (inert gas partial pressure) and stoichiometry (reactive gas partial pressure). At an optimal pressure ratio of 2% O₂:98% Ar, a resistivity of 70 Omega; cm, with mean transparency of 90% and haze of 40%, were obtained. A second strategy consists in AZO films with a graded morphology and structure to achieve spatial separation of functionality. Films constituted by a nano-forest structure (to control light scattering) evolving to a compact transparent conducting layer (to improve electrical conductivity) were produced by PLD by decreasing the deposition pressure during the film growth. An optimal combination of deposition parameters yielded a resistivity of about 3 mOmega; cm, with 80% transparency and 40% haze. We believe that the possibility to combine conductivity, transparency and light scattering into one film can be exploited to improve efficiency in HOPV devices: we underline that all depositions were performed at room temperature to test the compatibility with organic substrates.

The achievement of large-area arrays of interpenetrating polymer and inorganic nanomaterials remains a challenge in the fabrication of ordered bulk heterojunction (OBHJ) hybrid photovoltaics. Bottom-up synthesis and subsequent directed assembly present an economical route that not only allows control across the nanoscale lengths of light absorption and exciton diffusion, but also allows for the persistence of this nanoscale ordering across macroscopic length scales. Here, we demonstrate the achievement of an OBHJ photovoltaic active layer based on the magnetic alignment of Co-doped ZnO nanowire-polythiophene hybrid nanostructures. First, Co-doped ZnO nanowires are grown under a mild-condition solvothermal reaction. Nanowires are then directly grafted with conjugated polythiophenes though carboxylic acid moieties to form ZnO-polymer core-shell nanocomposites. The composite materials are dispersed in polar solvent loaded with polythiophene homopolymer to provide a continuous polymer film upon casting from solution. Room-temperature paramagnetic properties of the Co-doped nanowires provide a handle for bulk out-of-plane alignment of the core-shell structures under a 6T magnetic field. Due to the specific magnetocrystalline anisotropy of the wurtzite ZnO, rotational annealing must be enlisted to break degeneracy of the ‘easy&’ magnetic orientations. The desired vertically-oriented configuration is locked by evaporating the concentrated dispersion during alignment in the field. Finally, PV devices are constructed and tested under 1.5AMG solar simulation to show the utility of using directed assembly by magnetic field to achieve OBHJ hybrid solar cells.

We report on experimental absorption and photocurrent enhancement in ultrathin film solar cells with nanostructured plasmonic back contacts. In this work, we design plasmonic nanostructures to couple incident sunlight into localized resonant modes and propagating waveguide modes of an ultrathin (100nm-thick) amorphous silicon p-i-n solar cell (a-Si:H) for enhanced solar-to-electricity conversion. Three dimensional (3D) electromagnetic simulations (FDTD) allow us to understand and develop optimized designs for high performance devices. In this work, we focused on the efficiency enhancement of solar cells integrated with aperiodic plasmonic and photonic structures with isotropic scattering properties, specifically Vogel spirals. Spiral array enhancements will be compared against random arrays and optimized periodic arrays. Vogel spiral arrays lack translational and orientational invariance but possess circularly symmetric, diffuse diffraction spectra resulting in strong and polarization-insensitive light scattering at the Si absorption gap region. Arrays with different morphologies and nanoparticles sizes were systematically investigated in a large range of geometrical parameters. Array geometries are defined by reactive ion etched (RIE) nano-pillar arrays, etched in a Si substrate. Subsequent conformal depositions are carried out to form the plasmonic back contact, absorbing p-i-n layers, and top transparent and conductive contact layer (ITO). The I-V curves and photocurrent spectra of the solar cells were measured with and without the nanostructured back contacts. Our results demonstrate highly reproducible absorption enhancement from lithographically defined deterministic aperiodic arrays with optimized broadband scattering suitable for the engineering of thin-film Si solar cells and photodetector devices.

We have designed and synthesized III-V on Si1-xGex wire array device structures that offer the potential for >30% photovoltaic efficiencies via III-V growth that allows rapid strain relaxation and defect mitigation through the unique geometry. Optoelectronic simulations of GaAsxP1-x on Si1-xGex solar cells with Ga1-xInxP window layers allowed for careful, current matched device design. Subsequently, ordered, <111> oriented, Si1-xGex wire arrays were grown on Si substrates through a high temperature, atmospheric VLS growth process over the full range of compositions (x=0 to x=1) and oxide masked GaP outer layers were deposited with metal-organic chemical-vapor-deposition. Device fabrication is in progress and will be presented along with materials characterization. Using a modeling approach that combines full field optics simulations and a carrier transport device physics model,(1) we have explored a range of architectures for a series-connected monolithic multijunction GaAs0.9P0.1 /Si0.1Ge0.9 tandem wire cell coated with Ga0.56In0.44P window layers. Realistic optical generation profiles are exported from the electromagnetic simulations into the device physics model, and light IV curves simulated for a variety of geometries, doping profiles, and material lifetimes. The wire array pitch and III-V thickness were altered until current matching was achieved between the GaAs0.9P0.1 and Si0.1Ge0.9 cells at >20 mA/cm2. Realistic tunnel junction doping profiles were then considered, and a Gaussian doped Si0.1Ge0.9 emitter dropped only 4 mV for a 4x1019 cm-3 peak doping, on par with what we have achieved experimentally.(2) Recent experiments have shown that the wire geometry allows for rapid strain relaxation,(3) and simulations suggest that a highly doped, defective III-V region (tau; = 1 ps) around the heterojunction will not adversely affect device performance; rather it will act as a minority carrier mirror. Si1-xGex wires were grown on Si substrates from Cu or Au catalysts in a high temperature, chlorosilane and chlorogermane process. Ge wires were grown at 785oC with GeCl4 in H2.(4) XRD confirms that the Ge is lattice matched and highly crystalline. The unique wire morphology suggests that the strain is rapidly relieved at the wire base and TEM is in progress to further elucidate the mechanism. Si, Si0.7Ge0.3, and Si0.5Ge0.5 wires were grown at 1000oC with GeCl4 and SiCl4.(5) GaAs solar cells will be grown on the Ge wire arrays and the photovoltaic results presented. 1. M. D. Kelzenberg, M. C. Putnam, D. B. Turner-Evans, N. S. Lewis, H. A. Atwater, in Photovoltaic Specialists Conference (PVSC), 2009 34th IEEE. (2009), pp. 001948-001953. 2. M. D. Kelzenberg et al., Energy & Environmental Science 4, 866 (2011). 3. C. V. Falub et al., Science 335, 1330 (March 16, 2012, 2012). 4. E. I. Givargizov, J. Cryst. Growth 20, 217 (1973). 5. B. M. Kayes et al., Applied Physics Letters 91, 103110 (2007).

Silicon nanowire radial p-n junction photovoltaic design is a promising candidate for next generation high efficiency solar cells with unique anti-reflective, light trapping and carrier collection properties. In this study, we present a simple and low-cost etching procedure to fabricate hierarchical multi-diameter single crystalline silicon nanowire arrays for radial junction photovoltaic cells. This method starts with fabrication of well-ordered silicon nanowires by patterned metal assisted chemical etching of single crystalline silicon wafer and followed by radial isotropic/anisotropic etching of nanowires. Successive repetition of this two-step etching procedure form multi-diameter silicon nanowires, e.g. 200 nm at the top, 500 nm at the middle and 800 nm at the bottom. Multi-diameter silicon nanowires showed lower optical reflection compared to single diameter silicon nanowires due to gradual increase in effective reflective index. Optical properties of multi-diameter nanowires were modeled by using finite-difference-time-domain method and their diameter, length, and periodicity were tuned to achieve optimum optical absorption of solar spectrum.

The large absorption coefficient of iron pyrite (FeS2) nanocrystals coupled with their low-cost and vast-abundance shows great promise as a potential photovoltaic absorber. The use of Fe in photovoltaics was proposed nearly 25 years ago in the FeS2 pyrite form, but the low photovoltage, sulfur deficiencies and poor structure uniformity has limited their PV applications. As such, FeS2 as a promising material for commercially viable PV cells has not yet reached fruition. Recent studies have revealed that solution processed colloidal nanocrystals have an overwhelming influence on the cost efficiency of PV cells, compared to their bulk counterparts. We demonstrate that self-assembled bulk heterojunction (BHJ) nanostructures consisting of FeS2 nanocubes (NCs) and CdS quantum dots (QDs), lead to the well-defined percolation network, which significantly improved open-circuit voltage and power conversion efficiency of 1.1% under AM 1.5 solar illumination. The formation of bicontinuous donor/acceptor phases and a well-defined interface in the BHJ photoactive film through self-assembly and sequential ligand exchange treatment can largely enhance charge separation and transport efficiency, which are essential for all inorganic BHJ solar cells in comparison to the FeS2/CdS bi-layer heterostructures. The localized surface plasmon resonances (LSPRs) arising from p-type colloidal FeS2 NCs exhibit plasmonic photoelectron conversion. Our approach can be applied to a wide range of colloidal nanocrystals exhibiting the LSPRs effect and is compatible with solution processing, thereby offering a general tactic to enhancing the efficiency of all inorganic BHJ solar cells and LSPRs-based NIR photodetectors.

A crucial threshold in the usage of the benign class of ZnX systems (X = O, S, Se, or Te) in photocatalysis and photovoltaics is our ability to engineer (i.e., reduce) their band gap to desired values. In the present contribution, we show using first-principles density functional theory (DFT) and hybrid DFT calculations that a nanolayer of ZnX when coherently placed on a substrate of ZnY (Y = O, S, Se, Te; with X and Y being mutually exclusive), would significantly enhance the range of absorption or emission energies. This is an example of heterostructures providing a natural way of manipulating materials and their properties through epitaxial strain and in turn, providing a powerful opportunity to control the bandgap as well as the relative position of band-edges. Our present contribution on ZnX/ZnY nano-hetero-architectures builds on past work in which the possibility of bandgap reduction in bulk single-crystal ZnX systems through imposition of uniaxial strains was unequivocally shown using conventional DFT, hybrid DFT and many-body perturbation theory [1, 2]. Here, we extend that work to study the effect of built-in strains due to constraints of coherency at interfaces between dissimilar systems. We considered (0001), (10-10), and (11-20) interfacial planes for heterostrucutre formation, with thickness of each layer ranging from 2-3 nm. Such architectures lead to both in-plane biaxial strains and quantum confinement. Coherency of the overlayer with the substrate is modeled by appropriately constraining the in-plane lattice parameters of the heterostructure. The highest band gap change is observed when any of the ZnX systems is under a biaxial tensile strain in the (10-10) plane. The largest variation in the band edges is always confined to the unstrained material. It was also found that strained material displays a diminished quantum-confinement tendency compared to the unstrained material. In general this finding opens the possibility to tune the bandgap through proper choice of materials making up the nano-heterostructure. Among the various combinations of systems considered, a ZnSe nanolayer on a ZnTe substrate is particularly promising for solar cell application. ZnSe shows 50% reduction in the band gap from its equilibrium value of 2.7 eV, and the continuous variation of the band gap in the ZnTe side of the heterostructure is an added advantage. [1] Yadav et. al, Phys. Rev. B, 81, 144120 (2010); [2] Yadav et. al., Appl. Phys. Lett.,100, 241903 (2012).

Hybrid organic-inorganic solid-state solar cells combine the unique properties of inorganic semiconductors with the film forming properties of the conjugated polymers. One of the main challenges in these systems is to manipulate the organic and inorganic phases into nanoscale and mesoscale morphologies which allow charge collection and transport while controlling the interfacial interaction of the hybrid interface. Here we present a simple, one-pot synthesis, combining sol-gel chemistry and block copolymer self-organization to co-assemble ruthenium dyes, conjugated polymer and titania precursor into hierarchical 3D mesostructure hybrid films, with highly ordered morphology and controlled chemical composition. Block-copolymer structure directing agent are used to direct TiOx-containing dye and conjugated polymer films with sub-20 nm periodicity, while maintaining a continuous percolation path for the free carriers across 200 nm. The periodicity of the structure imposes the necessary proximity between the electron donating materials (conjugated polymer and ruthenium dyes) and the electron accepting material (titania). A combination of small-angle X-ray scattering, transmission electron microscopy, FTIR spectroscopy, and solid-state NMR spectroscopy yield insights on the compositions, structures, and distributions of inorganic and organic species within the materials over multiple length scales. In particular, both energy filtered TEM imaging and solid-state NMR spectroscopy show that judicious selection of the ruthenium dye and the conjugated polymer enable the control over the distribution of these donor species within the different blocks of the structure directing agent. Moreover, the macroscopic behavior of photovoltaic devices based on the hybrid layer is correlated to the chemical composition of the multicomponent interface.

Inorganic-Organic hybrid photovoltaic (h-PV) devices based on metal oxide-conjugated polymer blends have the potential to be developed as entirely solution processable, scalable devices on rigid and flexible substrates. To overcome performance limitations attempts have focused on the preparation of controlled nanostrucutres [1,2] in order to maximize interfacial area. Like others, our approach is based on a two-stage growth of the active layer: first, the growth an oxide nanorod template followed by polymer infiltration - resulting in a dense interconnected structure. While processing of controlled oxide nanorod morphologies may increase device performance by increasing interfaces or aiding infiltration it is also well known that control of the polymer morphology is crucial for efficient charge transport and the preparation of optimised devices [3]. In this study we present recent results examining the morphology of poly3-(hexylthiophene) (P3HT) films inside ZnO nanorod arrays. Small angle X-ray scattering (SAXS) and X-ray diffraction (XRD) studies have been carried out on P3HT inside nanorod arrays prior to, during and after thermal annealing in order to elucidate the bulk and interfacial polymer morphology. Of particular interest is the role on polymer molecular weight (Mw) and the dimensions between adjacent nanorods on polymer movement - specifically the kinetics of interchain reorganization. We present device characterization of hybrid nanostructures prepared at specific points of the annealing cycles in a bid to allow us to study the correlation between polymer morphology and device performance. Our results show the importance of controlling polymer microstructure on device performance and highlight the importance of correlating processing, structure and device performance. 1. Downing, J. M. et al J. Photon. Energy. 2011 (1) 011117. 2. Downing, J. M., Ryan, M. P., McLachlan M. A., Hydrothermal Growth of ZnO Nanorods: The Role of KCl in Controlling Rod Morphology (submitted) 3. Conings, B. et al J. Phys. Chem. C 2011 115 (33), 16695-16700.