Recent Progress on Reduced Graphene Oxide-Based Counter Electrodes for Cost-Effective Dye-Sensitized Solar Cells

Improved Performance of Dye-Sensitized Solar Cells with TiO2 Nanoparticles/Zn-Doped TiO2 Hollow Fiber Photoanodes

In this study, dye-sensitized solar cells (DSSCs) were fabricated using double-layer photoanodes consisting of TiO2 nanoparticles (NPs) and Zn-doped TiO2 hollow fibers (HFs). The TiO2 HFs were prepared by co-axial electrospinning and used as the light-scattering layer in the DSSC. The thickness variations of the TiO2 NP and Zn-doped TiO2 HF photoanode layers affect the performance of the DSSC, especially the short-circuit photocurrent density. The thickness of the TiO2 NP layer significantly affected the absorbance of photons and N719 dye molecules in the double-layer photoanode, while that of the Zn-doped TiO2 HF layer affected the scattering of light, as indicated by the low light transmittance in the photoanode. Conventional DSSCs consist of single-layer photoanodes, and exhibit relatively low efficiency, i.e., 1.293% and 0.89% for TiO2 NP and Zn-doped TiO2 HF, respectively. However, herein, the highest efficiency of the DSSC (3.122%) was achieved with a 15 μm NP-5 μm HF photoanode, for which the short-circuit photocurrent density, open-circuit photovoltage, and fill factor were 15.81 mA/cm2, 0.566 V, and 34.91%, respectively.

Design of SnO₂ Aggregate/Nanosheet Composite Structures Based on Function-Matching Strategy for Enhanced Dye-Sensitized Solar Cell Performance

Hierarchical SnO₂ nanocrystallites aggregates (NAs) were prepared with a simple room temperature–based aqueous solution method followed by simple freeze-drying treatment. The as-prepared SnO₂ NAs were subsequently combined with SnO₂ nanosheet–based structures from the viewpoint of a function-matching strategy, and under an optimized condition, a power conversion efficiency (PCE) of 5.59% was obtained for the resultant hybrid photoanode, a remarkable 60% enhancement compared to that of dye-sensitized solar cells (DSCs) fabricated with bare SnO₂ NAs architecture. The significantly enhanced efficiency can be attributed to the combination of the desirable electron transport property obtained by the intentionally introduced SnO₂ nanosheets (NSs) and the effectively retained inherent characteristics of SnO₂ NAs, i.e., large surface area and strong light-scattering effect. This work provides a promising approach for the rapid development of highly efficient SnO₂ photoanode film-based DSCs with the properties of simplicity of operation and control over the photoanode composition.

Large energy storage efficiency of the dielectric layer of graphene nanocapacitors

Electric capacitors are commonly used in electronic circuits for the short-term storage of small amounts of energy. It is desirable however to use capacitors to store much larger energy amounts to replace rechargeable batteries. Unfortunately existing capacitors cannot store sufficient energy to be able to replace common electrochemical energy storage systems. Here we examine the energy storage capabilities of graphene nanocapacitors, which are tri-layer devices involving an Al film, Al₂O₃ dielectric layer, and a single layer of carbon atoms, i.e., graphene. This is a purely electronic capacitor and therefore it can function in a wide temperature interval. The capacitor shows a high dielectric breakdown electric field strength, of the order of 1000 kV mm^-1 (i.e., 1 GV m^-1), which is much larger than the table value of the Al₂O₃ dielectric strength. The corresponding energy density is 10-100 times larger than the energy density of a common electrolytic capacitor. Moreover, we discover that the amount of charge stored in the dielectric layer can be equal or can even exceed the amount of charge stored on the capacitor plates. The dielectric discharge current follows a power-law time dependence. We suggest a model to explain this behavior.

Highly-stable and -flexible graphene/(CF3SO2)2NH/graphene transparent conductive electrodes for organic solar cells

We first employ highly-stable and -flexible (CF₃SO₂)₂NH-doped graphene (TFSA/GR) and GR-encapsulated TFSA/GR (GR/TFSA/GR) transparent conductive electrodes (TCEs) prepared on polyethylene terephthalate substrates for flexible organic solar cells (OSCs). Compared to conventional indium tin oxide (ITO) TCEs, the TFSA-doped-GR TCEs show higher optical transmittance and larger sheet resistance. The TFSA/GR and GR/TFSA/GR TCEs show work functions of 4.89 ± 0.16 and 4.97 ± 0.18 eV, respectively, which are not only larger than those of the ITO TCEs but also indicate p-type doping of GR, and are therefore more suitable for anode TCEs of OSCs. In addition, typical GR/TFSA/GR-TCE OSCs are much more mechanically flexible than the ITO-TCE ones with their photovoltaic parameters being similar, as proved by bending tests as functions of cycle and curvature.

High quality reduced graphene oxide flakes by fast kinetically controlled and clean indirect UV-induced radical reduction

This work highlights a surprisingly simple and kinetically controlled highly efficient indirect method for the production of high quality reduced graphene oxide (rGO) flakes via UV irradiation of aqueous dispersions of graphene oxide (GO), in which the GO is not excited directly. While the direct photoexcitation of aqueous GO (when GO is the only light-absorbing component) takes several hours of reaction time at ambient temperature (4 h) leading only to a partial GO reduction, the addition of small amounts of isopropanol and acetone (2% and 1%) leads to a dramatically shortened reaction time by more than two orders of magnitude (2 min) and a very efficient and soft reduction of graphene oxide. This method avoids the formation of non-volatile species and in turn contamination of the produced rGO and it is based on the highly efficient generation of reducing carbon centered isopropanol radicals via the reaction of triplet acetone with isopropanol. While the direct photolysis of GO dispersions easily leads to degradation of the carbon lattice of GO and thus to a relatively low electric conductivity of the films of flakes, our indirect photoreduction of GO instead largely avoids the formation of defects, keeping the carbon lattice intact. Mechanisms of the direct and indirect photoreduction of GO have been elucidated and compared. Raman spectroscopy, XPS and conductivity measurements prove the efficiency of the indirect photoreduction in comparison with the state-of-the-art reduction method for GO (hydriodic acid/trifluoroacetic acid). The rapid reduction times and water solvent containing only small amounts of isopropanol and acetone may allow easy process up-scaling for technical applications and low-energy consumption.

Shedding light on the soft and efficient free radical induced reduction of graphene oxide: hidden mechanisms and energetics

Reduction of graphene oxide in aqueous dispersions by strongly reducing free radicals has been found to be a very powerful approach – functional groups are removed softly but efficiently, and non-volatile impurities as well as defects are avoided.