Scaling from single-point sap velocity measurements to stand transpiration in a multispecies deciduous forest: Uncertainty sources, stand structure effect, and future scenarios

9 páginas.-- 5 figuras.-- 2 tablas.-- 58 referencias[EN] A major challenge in studies estimating stand water use in mixed-species forests is how to effectively scale data from individual trees to the stand. This is the case for forest ecosystems in the northeastern USA where differences in water use among species and across different size classes have not been extensively studied, despite their relevance for a wide range of ecosystem services. Our objectives were to assess the importance of different sources of variability on transpiration upscaling and explore the potential impacts of future shifts in species composition on the forest water budget. We measured sap velocity in five tree species (Fagus grandifolia Ehrh., Acer rubrum L., Acer saccharum Marsh., Betula alleghaniensis Britton, and Betula papyrifera Marsh.) in a mature stand and a young stand in New Hampshire, USA. Our results showed that the greatest potential source of error was radial variability and that tree size was more important than species in determining sap velocity. Total sapwood area was demonstrated to exert a strong controlling influence on transpiration, varying depending on tree size and species. We conclude that the effect of potential species shifts on transpiration will depend on the sap velocity, determined not only by radial variation and tree size, but also by the sapwood area distribution in the stand.[FR] Les études dont le but est d'estimer l'utilisation de l'eau a` l'échelle du peuplement dans les forêts mélangées font face a` un défi majeur : comment passer efficacement de l'échelle des arbres individuels a` l'échelle du peuplement. C'est le cas pour les écosystèmes forestiers dans le nord-est des États-Unis où les différences dans l'utilisation de l'eau entre les espèces et parmi les différentes catégories de taille n'ont pas fait l'objet d'études approfondies malgré leur pertinence pour une vaste gamme de services de l'écosystème. Nos objectifs consistaient a` évaluer l'importance des différentes sources de variation sur l'extrapolation de la transpiration et a` explorer les impacts potentiels des changements futurs dans la composition en espèces sur le bilan hydrique de la forêt. Nous avons mesuré la vitesse de la sève chez cinq espèces d'arbre (Fagus grandifolia Ehrh., Acer rubrum L., Acer saccharum Marsh., Betula alleghaniensis Britton et Betula papyrifera Marsh.) dans un peuplement mature et dans un jeune peuplement au New Hampshire (É.-U.). Nos résultats ont montré que la plus grande source potentielle d'erreur était la variation radiale et que la vitesse de la sève était davantage déterminée par la taille des arbres que par l'espèce. La surface totale de bois d'aubier avait un effet très déterminant sur la transpiration qui variait selon la taille et l'espèce d'arbre. Nous concluons que l'effet des changements potentiels dans la composition en espèces sur la transpiration dépendra de la vitesse de la sève qui est principalement déterminée par la variation radiale et la taille des arbres mais aussi de la distribution de la surface de bois d'aubier dans le peuplement.This work was funded by the University of New Hampshire and the New Hampshire Agricultural Experiment Station. The Bartlett Experimental Forest is operated by the USDA Forest Service Northern Research Station. S. Mcgraw, P. Pellissier, C. Breton, S. Alvarado-Barrientos, R. Snyder, and Z. Aldag assisted in the field and in the lab. The 2011 stand inventory was led by S. Goswami. Tree heights were measured and compiled by C. Blodgett, T. Fahey, and L. Liu. A. Richardson shared meteorology and solar radiation data from the Bartlett Amerflux tower. The stands used in this experiment are maintained and monitored by the MELNHE project under the direction of R. Yanai and M. Fisk, with funding from NSF grants DEB 0235650 and DEB 0949324Peer reviewe

Titanium Carbide and Titanium Nitride-Based Nanocomposites as Efficient Catalysts for the Co²⁺/Co³⁺ Redox Couple in Dye-Sensitized Solar Cells

Two different kinds of nanocomposites were developed by electrochemical deposition of poly­(3,4-ethylenedioxythiophene) (PEDOT) into porous hard template films of TiC or TiN nanoparticles, in order to evaluate their use as alternative catalysts in dye-sensitized solar cells (DSSC) utilizing a Co²⁺/Co³⁺ polypyridyl redox mediator. Cyclic voltammograms indicate that both types of nanocomposite show comparable catalytic activity to platinum-coated electrodes. However, electrochemical impedance spectroscopy (EIS) reveals that electron transfer resistances are significantly reduced with the porous nanocomposite electrodes (<1 Ω), to about an order of magnitude lower than those observed for the Pt coated electrode. As a result, DSSCs with the composite counter electrodes achieved equivalent or higher photovoltaic conversion efficiencies compared to cells with pristine PEDOT or Pt coated electrodes. In particular, the highest efficiency (8.26%) was achieved with a DSSC using a TiN-PEDOT counter electrode

Quasi-Solid-State Dye-Sensitized Solar Cells on Plastic Substrates

Interest has recently grown around flexible dye-sensitized solar cells (DSCs) for their potential low cost roll-to-roll production process and their wide range of applications. However, flexible DSCs do not perform as well as their glass substrate equivalents, and standard devices using liquid electrolytes are challenged with long-term stability issues. Consequently, constructing stable flexible solar cells presents a major challenge. This article focuses on flexible quasi-solid-state DSCs (QS-DSCs), constructed with poly­(vinylidenefluoride-co-hexafluoropropylene)-based gel electrolytes and submicrometer mesoporous TiO₂ beads on plastic substrates. The influence of the gel electrolyte composition was investigated and optimized by varying the polymer content and introducing inorganic fillers. The diffusion behavior of the gel electrolytes was studied by means of voltammetric measurements. Electrochemical impedance spectroscopy gave an understanding of the role of polymer and inorganic fillers with regard to the charge recombination process. Transient photocurrent measurements and scanning electron microscopy coupled with energy X-ray dispersive spectrometry revealed that infiltration of the electrolyte through the photoanode was advantageous for films made with TiO₂ beads over TiO₂ P25 particles. A record power conversion efficiency of 6.4% for flexible QS-DSCs was obtained with an optimized gel electrolyte, constituting a promising step toward the fabrication of stable flexible DSCs

In-Depth Understanding of the Morphology–Performance Relationship in Polymer Solar Cells

It is well-established that thermal annealing optimizes the morphology and improves the efficiency of P3HT-based organic solar cells, but the effects of different cooling rates after annealing are not well understood. In this paper, we use a model system based on poly­(3-hexylthiophene) (P3HT) and phenyl-C₆₁-butyric acid methyl ester (PCBM) to examine the relationship between morphology and device performance for annealing before (preannealing) and after (postannealing) the application of the electrode, with different cooling rates and in different device architectures. In the conventional structure, postannealing is confirmed to significantly enhance efficiency. The device prepared with a slow cooling rate (3.6%) shows a higher average power conversion efficiency than that prepared with a fast cooling rate (3.3%). The microstructural changes underlying this 10% increase in device performance and further effects of cooling rate, pre- and postannealing, and device architecture are comprehensively examined with a combination of synchrotron-based techniques, including grazing incidence wide-angle X-ray scattering, near-edge X-ray absorption fine structure spectroscopy, and X-ray photoelectron spectroscopy. The best device in the conventional architecture (postannealed with slow cooling rate) shows a more face-on orientation and narrower orientational distribution of P3HT crystallites. In addition, postannealing leads to PCBM diffusion toward the blend/top electrode interface. The enrichment of PCBM at the blend/top electrode interface plays a positive role in aiding electron collection at the electrode in the conventional structure, but it has a negative effect on the performance of the inverted structure, where hole collection at the top electrode instead is required. For this reason, in an inverted structure, preannealed films with slow cooling exhibit the best photovoltaic performance

4-<i>tert</i>-Butylpyridine Free Hole Transport Materials for Efficient Perovskite Solar Cells: A New Strategy to Enhance the Environmental and Thermal Stability

Organic semiconductors as hole transport materials (HTMs) often require additives, such as LiTFSI and <i>tert</i>-butylpyridine (TBP), in order to enhance their hole conductivities. However, the combination of lithium salts and TBP leads to significant HTM morphological deformation and poor device stability. Here we have successfully applied tetrabutylammonium (TBA) salts to replace both LiTFSI and TBP. A high power conversion efficiency of 18.4% has been achieved for the devices with TBATFSI, which is higher than the control devices with LiTFSI and TBP (18.1%). We also found that the anions in the TBA salts play important roles in the hole conductivity and uniformity of the HTM layer, as well as the hysteresis of the devices. More importantly, the devices with TBATFSI and TBAPF₆ demonstrated significantly enhanced environmental and thermal stability. This new strategy of using TBA salts is promising for developing stable organic HTM thin films for solar cell applications

Surface State Recombination and Passivation in Nanocrystalline TiO₂ Dye-Sensitized Solar Cells

The relative role of surface state recombination in dye-sensitized solar cells is not fully understood, yet reductions in the recombination rate are frequently attributed to the passivation of surface states. We have investigated reports of trap state passivation using an Al₂O₃-coated TiO₂ photoanode achieved through atomic layer deposition (ALD). Electrochemical characterization, performed through impedance measurements and intensity modulated photovoltage spectroscopy (IMVS), data showed that the Al₂O₃ deposition successfully blocked electron recombination and that the chemical capacitance of the film was unchanged after the ALD treatment. A theoretical model outlining the recombination kinetics was applied to the experimental data to obtain charge transfer rates from conduction band states, exponentially distributed traps, and monoenergetic traps. The determined electron transfer rates showed that the deposited Al₂O₃ coating did not selectively passivate trap states at the nanoparticle surface but reduced recombination rates equally from both conduction band states and surface states. These results imply that the reduction in the recombination rates reported in core–shell structured photoanodes cannot be attributed to a modification of surface traps, but rather to the weakened electronic coupling between electrons in the film and the electrolyte species

Solvent-Mediated Intragranular-Coarsening of CH₃NH₃PbI₃ Thin Films toward High-Performance Perovskite Photovoltaics

The deposition of dense and uniform perovskite films with large grains is crucial for fabricating high-performance perovskite solar cells (PSCs). High-quality CH₃NH₃PbI₃ films were produced by a self-induced intragranular-coarsening approach. The perovskite precursor solution contained a Lewis base, <i>N</i>,<i>N</i>-dimethyl sulfoxide (DMSO), and was deposited using a gas-assisted, one-step, spin-coating method that was followed by a solvent vapor-assisted annealing treatment using a mix of DMSO and chlorobenzene (CBZ). Combining solvent-engineering with gas-assisted deposition helps to form intermediate crystalline entities upon evaporation of the parent solvent but retards the otherwise fast reaction between the precursor ingredients. Subsequent cosolvent annealing induces further grain-coarsening via a facilitated dissolution–precipitation process. This technique produced flat CH₃NH₃PbI₃ films featuring large grain microstructures, with well-coarsened subgrains and a reduction of intragranular defects that minimized carrier recombination. The optimized CH₃NH₃PbI₃ films exhibited enhanced crystallinity, excellent carrier transport and injection, as well as suppressed charge recombination. Benefiting from these advantages, PSCs based on the optimized perovskite films delivered a power conversion efficiency of 17.99% and a stabilized power output above 17.30%. This study presents an effective strategy for the fabrication of high-quality, hybrid perovskite films with potential applications in optoelectronic devices

Benefit of Grain Boundaries in Organic–Inorganic Halide Planar Perovskite Solar Cells

The past 2 years have seen the uniquely rapid emergence of a new class of solar cell based on mixed organic–inorganic halide perovskite. Grain boundaries are present in polycrystalline thin film solar cell, and they play an important role that could be benign or detrimental to solar-cell performance. Here we present efficient charge separation and collection at the grain boundaries measured by KPFM and c-AFM in CH₃NH₃PbI₃ film in a CH₃NH₃PbI₃/TiO₂/FTO/glass heterojunction structure. We observe the presence of a potential barrier along the grain boundaries under dark conditions and higher photovoltage along the grain boundaries compare to grain interior under the illumination. Also, c-AFM measurement presents higher short-circuit current collection near grain boundaries, confirming the beneficial roles grain boundaries play in collecting carriers efficiently

Boosting the Photocurrent Density of p‑Type Solar Cells Based on Organometal Halide Perovskite-Sensitized Mesoporous NiO Photocathodes

The p–n tandem design of a sensitized solar cell is a novel concept holding the potential to overcome the efficiency limitation of conventional single-junction sensitized solar cells. Significant improvement of the photocurrent density (<i>J</i>_sc) of the p-type half-cell is a prerequisite for the realization of a highly efficient p–n tandem cell in the future. This study has demonstrated effective photocathodes based on novel organometal halide perovskite-sensitized mesoporous NiO in liquid-electrolyte-based p-type solar cells. An acceptably high <i>J</i>_sc up to 9.47 mA cm^–2 and efficiency up to 0.71% have been achieved on the basis of the CH₃NH₃PbI₃/NiO solar cell at 100 mW cm^–2 light intensity, which are significantly higher than those of any previously reported liquid-electrolyte-based p-type solar cells based on sensitizers of organic dyes or inorganic quantum dots. The dense blocking layer made by spray pyrolysis of nickel acetylacetonate holds the key to determining the current flow direction of the solar cells. High hole injection efficiency at the perovskite/NiO interface and high hole collection efficiency through the mesoporous NiO network have been proved by time-resolved photoluminescence and transient photocurrent/photovoltage decay measurements. The limitation of these p-type solar cells primarily rests with the adverse light absorption by the NiO mesoporous film; the secondary limitation arises from the highly viscous ethyl acetate-based electrolyte, which is helpful for the solar cell stability but hinders fluent diffusion into the pore channels, giving rise to a nonlinear dependence of <i>J</i>_sc on the light intensity

Four-Terminal Tandem Solar Cells Using CH₃NH₃PbBr₃ by Spectrum Splitting

In this work, the use of a high bandgap perovskite solar cell in a spectrum splitting system is demonstrated. A remarkable energy conversion efficiency of 23.4% is achieved when a CH₃NH₃PbBr₃ solar cell is coupled with a 22.7% efficient silicon passivated emitter rear locally diffused solar cell. Relative enhancements of >10% are demonstrated by CH₃NH₃PbBr₃/CH₃NH₃PbI₃ and CH₃NH₃PbBr₃/multicrystalline-screen-printed-Si spectral splitting systems with tandem efficiencies of 13.4% and 18.8%, respectively. The former is the first demonstration of an all perovskite split spectrum system. The CH₃NH₃PbBr₃ cell on a mesoporous structure was fabricated by the vapor-assisted method while the planar CH₃NH₃PbI₃ cell was fabricated by the gas-assisted method. This work demonstrates the advantage of the higher voltage output from the high bandgap CH₃NH₃PbBr₃ cell and its suitability in a tandem system