Jason B. Baxter

Nanowire-based dye-sensitized solar cells

We describe the design and performance of a ZnO nanowire-based dye-sensitized solar cell. ZnO nanowires with a branched structure were employed as the wide-band-gap semiconductor to construct dye-sensitized solar cells which exhibit energy conversion efficiencies of 0.5% with internal quantum efficiencies of 70%. The nanowires provide a direct conduction path for electrons between the point of photogeneration and the conducting substrate and may offer improved electron transport compared to films of sintered nanoparticles. The devices have light harvesting efficiencies under 10%, indicating that current densities and efficiencies can be improved by an order of magnitude by increasing the nanowire surface area.

Synthesis and characterization of ZnO nanowires and their integration into dye-sensitized solar cells

ZnO nanowires, grown on transparent conducting oxide substrates from aqueous solutions of methenamine and Zn(NO3)2, were integrated as the wide band gap semiconductor into dye-sensitized solar cells. ZnO nanowires and their growth mechanisms were studied using electron microscopy, x-ray diffraction and photoluminescence measurements. The solution growth method forms dense arrays of long nanowires oriented normal to the substrate surface because nanowires growing at off-normal angles are prevented from growing further when they run into neighbouring wires. Dye-sensitized solar cells with ZnO nanowires were assembled and characterized using optical and electrical measurements. Short circuit current densities of 1.3?mA?cm?2, and overall power conversion efficiencies of 0.3% were achieved with 8??m long nanowires. Photocurrent and efficiency increase with increasing nanowire length and improved light harvesting. Low surface area and a shunt that appears under light illumination limit the solar cell performance. Internal quantum efficiencies were similar for nanowires of all lengths, indicating that electron transport is not limited by the nanowire dimensions for aspect ratios less than 70.

Photovoltaic manufacturing: Present status, future prospects, and research needs

In May 2010 the United States National Science Foundation sponsored a two-day workshop to review the state-of-the-art and research challenges in photovoltaic (PV) manufacturing. This article summarizes the major conclusions and outcomes from this workshop, which was focused on identifying the science that needs to be done to help accelerate PV manufacturing. A significant portion of the article focuses on assessing the current status of and future opportunities in the major PV manufacturing technologies. These are solar cells based on crystalline silicon (c-Si), thin films of cadmium telluride (CdTe), thin films of copper indium gallium diselenide, and thin films of hydrogenated amorphous and nanocrystalline silicon. Current trends indicate that the cost per watt of c-Si and CdTe solar cells are being reduced to levels beyond the constraints commonly associated with these technologies. With a focus on TW/yr production capacity, the issue of material availability is discussed along with the emerging technologies of dye-sensitized solar cells and organic photovoltaics that are potentially less constrained by elemental abundance. Lastly, recommendations are made for research investment, with an emphasis on those areas that are expected to have cross-cutting impact.

Nanoscale design to enable the revolution in renewable energy

The creation of a sustainable energy generation, storage, and distribution infrastructure represents a global grand challenge that requires massive transnational investments in the research and development of energy technologies that will provide the amount of energy needed on a sufficient scale and timeframe with minimal impact on the environment and have limited economic and societal disruption during implementation. In this opinion paper, we focus on an important set of solar, thermal, and electrochemical energy conversion, storage, and conservation technologies specifically related to recent and prospective advances in nanoscale science and technology that offer high potential in addressing the energy challenge. We approach this task from a two-fold perspective: analyzing the fundamental physicochemical principles and engineering aspects of these energy technologies and identifying unique opportunities enabled by nanoscale design of materials, processes, and systems in order to improve performance and reduce costs. Our principal goal is to establish a roadmap for research and development activities in nanoscale science and technology that would significantly advance and accelerate the implementation of renewable energy technologies. In all cases we make specific recommendations for research needs in the near-term (2–5 years), mid-term (5–10 years) and long-term (>10 years), as well as projecting a timeline for maturation of each technological solution. We also identify a number of priority themes in basic energy science that cut across the entire spectrum of energy conversion, storage, and conservation technologies. We anticipate that the conclusions and recommendations herein will be of use not only to the technical community, but also to policy makers and the broader public, occasionally with an admitted emphasis on the US perspective.

Growth mechanism and characterization of zinc oxide hexagonal columns

We report on the growth mechanism, structure, and luminescence properties of ZnO hexagonal columns grown from Zn vapor and air plasma. Single-crystal ZnO columns grow in the [0001] direction through repeated nucleation and growth of epitaxial hexagonal pyramids on the c-planes. Homoepitaxial nucleation of three-dimensional ZnO pyramids is most likely due to the Ehrlich–Schwoebel effect. This mechanism produces columns that are a few hundred nanometers in diameter and up to 2 μm in length. Convergent beam electron diffraction shows that the columns grow with Zn polarity in the [0001] direction. Cathodoluminescence and photoluminescence measurements show near-bandedge emission (3.29 eV) with no emission associated with oxygen vacancies at 2.5 eV.

Chemical Bath Deposition of ZnO Nanowires at Near-Neutral pH Conditions without Hexamethylenetetramine (HMTA): Understanding the Role of HMTA in ZnO Nanowire Growth

Chemical bath deposition (CBD) is an inexpensive and reproducible method for depositing ZnO nanowire arrays over large areas. The aqueous Zn(NO(3))(2)-hexamethylenetetramine (HMTA) chemistry is one of the most common CBD chemistries for ZnO nanowire synthesis, but some details of the reaction mechanism are still not well-understood. Here, we report the use of in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy to study HMTA adsorption from aqueous solutions onto ZnO nanoparticle films and show that HMTA does not adsorb on ZnO. This result refutes earlier claims that the anisotropic morphology arises from HMTA adsorbing onto and capping the ZnO {10 1 0} faces. We conclude that the role of HMTA in the CBD of ZnO nanowires is only to control the saturation index of ZnO. Furthermore, we demonstrate the first deposition of ZnO nanowire arrays at 90 °C and near-neutral pH conditions without HMTA. Nanowires were grown using the pH buffer 2-(N-morpholino)ethanesulfonic acid (MES) and continuous titratation with KOH to maintain the same pH conditions where growth with HMTA occurs. This semi-batch synthetic method opens many new opportunities to tailor the ZnO morphology and properties by independently controlling temperature and pH.

Commercialization of dye sensitized solar cells: Present status and future research needs to improve efficiency, stability, and manufacturing

Dye sensitized solar cells (DSSCs) have received a tremendous amount of attention since the first report of a 7% efficient cell in 1991. Confirmed record efficiencies are now 11.2% for small cells and 9.9% for submodules, and low-cost production methods are enabling manufacturing of DSSC products for a variety of markets. This review describes the present status of DSSC devices and manufacturing as well as research challenges that must be addressed to continue the rapid commercialization of DSSC technology. These challenges fall into the categories of improving efficiency, stability, and manufacturability. Efficiency improvements will hinge on the development of new combinations of dyes, redox couples, and photoanodes. Best-case lifetimes are determined by the kinetics of various molecular-level processes, and realization of these lifetimes will require improved encapsulation of cells and modules. Low-cost and sustainable manufacturing of DSSC modules depends on use of high-throughput roll-to-roll process...

Enhanced charge transfer kinetics of CdSe quantum dot-sensitized solar cell by inorganic ligand exchange treatments.

Enhancement of the charge transfer rate in CdSe quantum dot (QD) sensitized solar cells is one of the most important criteria determining cell efficiency. We report a novel strategy for enhancing charge transfer by exchanging the native, long organic chain to an atomic ligand, S(2-), with a simple solid exchange process. S(2-)-ligand exchange is easily executed by dipping the CdSe QDs sensitized photoanode into a formamide solution of K2S. The results show that this exchange process leads to an enhancement of the electronic coupling between CdSe QD and TiO2 by removing the insulating organic barrier to charge transfer, while maintaining its quantum confined band structure. This treatment significantly increases the charge transfer rate at the interfacial region between CdSe QDs and TiO2 as well as between the CdSe QDs and Red/Ox coupling electrolyte, as verified by time-resolved photoluminescence and electrochemical impedance spectroscopy measurements. Finally, the S(2-)-treated photoanode exhibits a much higher photovoltaic performance than the conventional MPA or TGA-capped CdSe QDs sensitized solar cell. The findings reported herein propose an innovative route toward harvesting energy from solar light by enhancing the carrier charge transfer rate.

Infrared detection of hydrogen-generated free carriers in polycrystalline ZnO thin films

The changes in the free-carrier concentration in polycrystalline ZnO films during exposure to H2 and O2 plasmas were studied using in situ attenuated total reflection Fourier transform infrared spectroscopy. The carrier concentration and mobility were extracted from the free-carrier absorption in the infrared using a model for the dielectric function. The electron density in polycrystalline zinc oxide films may be significantly increased by >1019cm−3 by brief exposures to hydrogen plasma at room temperature and decreased by exposure to O2 plasmas. Room-temperature oxygen plasma removes a fraction of the H at donor sites but both elevated temperatures (∼225°C) and O2 plasma were required to remove the rest. We demonstrate that combinations of O2 and H2 plasma treatments can be used to manipulate the carrier density in ZnO films. However, we also show the existence of significant drifts (∼15%) in the carrier concentrations over very long time scales (hours). Possible sites for H incorporation in polycrystal...

Ultrafast Carrier Dynamics in Nanostructures for Solar Fuels

Sunlight can be used to drive chemical reactions to produce fuels that store energy in chemical bonds. These fuels, such as hydrogen from splitting water, have much larger energy density than do electrical storage devices. The efficient conversion of clean, sustainable solar energy using photoelectrochemical and photocatalytic systems requires precise control over the thermodynamics, kinetics, and structural aspects of materials and molecules. Generation, thermalization, trapping, interfacial transfer, and recombination of photoexcited charge carriers often occur on femtosecond to picosecond timescales. These short timescales limit the transport of photoexcited carriers to nanometer-scale distances, but nanostructures with high surface-to-volume ratios can enable both significant light absorption and high quantum efficiency. This review highlights the importance of understanding ultrafast carrier dynamics for the generation of solar fuels, including case studies on colloidal nanostructures, nanostructured photoelectrodes, and photoelectrodes sensitized with molecular chromophores and catalysts.