The Na–O2 battery offers an interesting alternative to the Li–O2 battery, which is still the source of a number of unsolved scientific questions. In spite of both being alkali metal–O2 batteries, they display significant differences. For instance, Li–O2 batteries form Li2O2 as the discharge product at the cathode, whereas Na–O2 batteries usually form NaO2. A very important question that affects the performance of the Na–O2 cell concerns the key parameters governing the growth mechanism of the large NaO2 cubes formed upon reduction, which are a requirement of viable capacities and high performance. By comparing glyme-ethers of various chain lengths, we show that the choice of solvent has a tremendous effect on the battery performance. In contrast to the Li–O2 system, high solubilities of the NaO2 discharge product do not necessarily lead to increased capacities. Herein we report the profound effect of the Na+ ion solvent shell structure on the NaO2 growth mechanism. Strong solvent–solute interactions in long-chain ethers shift the formation of NaO2 toward a surface process resulting in submicrometric crystallites and very low capacities (ca. 0.2 mAh/cm2 (geom)). In contrast, short chains, which facilitate desolvation and solution-precipitation, promote the formation of large cubic crystals (ca. 10 um), enabling high capacities (ca. 7.5 mAh/cm2 (geom)). This work provides a new way to look at the key role that solvents play in the metal–air system.
Further development of quantum dot-sensitized solar cells (QDSCs) will require long-term stability in addition to the continuous increase of photovoltaic (PV) conversion efficiency achieved in the last years. We report a robust S(2-)/S(n)(2-) electrolyte that has been specifically designed for compatibility with CdSe quantum dots in sensitized solar cells. The new pyrrolidinium ionic liquid reaches 1.86% efficiency and a short-circuit current close to 14 mA·cm(-2) under air-mass 1.5 global illumination and improves the device lifetime with good photoanode stability over 240 h. PV characterization showed that the solar cell limitations relate to poor catalysis of regeneration at the counter electrode and high recombination. Further improvement of these factors in the robust electrolyte configuration may thus have a significant impact for advancing the state-of-the-art in QDSCs.
The photovoltaic properties of nanostructured ZnO films sensitized with the indoline derivative dye D149 were studied. The performance of dye-sensitized solar cells built from ZnO building blocks with different morphology, (a) randomly oriented nanoparticle network and (b) nanowire arrays, was compared. The nanoparticle networks were prepared by the standard doctor blade technique from commercial ZnO powders and the nanowire arrays were electrodeposited in aqueous media. Two different lengths for the nanowire arrays (2.5 and 5 mm) were considered. The characterization included electron microscopy, adsorption measurements, optical spectroscopy, current-voltage characteristics, open-circuit voltage versus light intensity, incident-photon-to-current efficiency, open circuit voltage decay and impedance spectroscopy under illumination. In spite of the smaller dye loadings of the nanowires with respect to the nanoparticles, the former showed a remarkably effective integrated optical absorption (in the range from 370 to 700 nm, 57% versus 69% for the latter). However the photocurrents for nanowires were lower than expected from this good absorption, which suggests that recombination rather than solar light harvesting can be the limiting factor in these nanowire-based solar cells. The impedance analysis and the open-circuit voltage decays showed smaller recombination resistances and shorter lifetimes for the nanowire-based solar cells. However, the interpretation of the recombination resistances, capacitances and lifetimes in the case of the nanowires is likely affected by space-charge effects and back-reaction through the substrate. An understanding of the effects discovered in this study is very valuable for the development of strategies to enhance the energy conversion efficiency for the ZnO nanowire array based solar cells.
1-Dimensional nanostructured ZnO electrodes have been demonstrated to be potentially interesting for their application in solar cells. Herein, we present a novel procedure to control the ZnO nanowire optoelectronic properties by means of surface modification. The nanowire surface is functionalized with ZnO nanoparticles in order to provide an improved contact to the photoactive P3HT:PCBM film that enhances the overall power conversion efficiency of the resulting solar cell. Charge extraction and transient photovoltage measurements have been used to successfully demonstrate that the surface modified nanostructured electrode contributes in enhancing the exciton dissociating ratio and in enlarging the charge lifetime as a consequence of a reduced charge recombination. Under AM1.5G illumination, all these factors contribute to a considerably large increase in photocurrent yielding unusually high conversion efficiencies over 4% and external quantum efficiencies of 87% at 550 nm for commercially available P3HT:PCBM based solar cells. The same approach might be equally used for polymeric materials under development to overcome the record reported efficiencies.
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