Three-Dimensional Monolayer Graphene and TiO₂ Hybrid Architectures for High-Efficiency Electrochemical Photovoltaic Cells

Three-dimensional (3D), tubular-structured monolayer graphene networks were hybridized with TiO₂ nanoparticular layer for futuristic and robust electrode applications. A continuous form of 3D graphene with good carrier mobility provides a direct pathway for electrons to the current collector for a photoanode in dye-sensitized solar cells. This characteristic feature, coupled with its energy level, ensures an enhanced charge collection efficiency. Particular attention was paid to the graphene surface functionalization and the effective loading of TiO₂ nanoparticles to improve the light harvesting and minimize electron recombination for a photoanode. The optimal hybrid structure resulted in a 10% enhanced energy conversion efficiency, compared to the TiO₂-based analogue without graphene. The impedance spectra confirmed that the increase in photovoltaic performance was mainly driven by the efficient charge collection through the 3D, tubular-structured monolayer graphene. This new electrode prototype can serve as a good complement to conventional TiO₂ nanostructures

Organic−Inorganic Hybrid Compounds Containing Polyhedral Oligomeric Silsesquioxane for Conservation of Stone Heritage

Alkoxysilane solutions based on tetraethoxysilane (TEOS) have been widely used for the consolidation of decaying heritage stone surfaces. TEOS-based products polymerize within the porous structure of the decaying stone, significantly increasing the cohesion of the grains of stone components. However, they suffer from practical drawbacks, such as crack formation of the gel during the drying phase due to the developing capillary force and dense gel fractures left inside of the stone. In this study, a TEOS-based stone consolidant containing functional (3-glycidoxypropyl)trimethoxysilane (GPTMS) and polyhedral oligomeric silsesquioxane (POSS) has been prepared in order to reduce gel crack formation during the drying phase. The addition of nanometer-sized POSS and/or GPTMS having a flexible segment reduces the capillary force developed during solvent evaporation. The properties of the TEOS/GPTMS/POSS composite solutions were compared with those of commercial products (Wacker OH and Unil sandsteinfestiger OH 1:1). The gelation time was similar to that of commercial consolidants, and the TEOS/GPTMS/POSS solution was stable over a period of up to 6 months. The addition of POSS and GPTMS provided a crack-free gel, while the gel from the commercial consolidants exhibited cracks after drying. The surface hydrophobicity of the treated decayed granite increased with the addition of POSS and GPTMS, and it was higher than that of the commercial product, implying the possibility of POSS and GPTMS as barriers to the penetration of water. This result implies that the TEOS/GPTMS/POSS solution showed a high suitability for the consolidation of granite heritage

Crystalline Matrix of Mesoporous TiO₂ Framework for Dye-Sensitized Solar Cell Application

In the present study, a well-ordered columnar porous TiO₂ matrix is designed via inverted triblock copolymers self-assembly and introduced as a photoanode for dye-sensitized solar cells (DSSCs). The inverted triblock copolymer, polystyrene-<i>b</i>-poly­(ethylene oxide)-<i>b</i>-polystyrene, with the hydrophobic polystyrene segments at both ends of a hydrophilic poly­(ethylene oxide) chain is synthesized by atom transfer radical polymerization. These reverse-featured triblock copolymers allow facile stacking to 3-dimensional (D) columnar porous matrix from 2-D porous film via hydrophobic–hydrophilic interaction. A 3-D matrix with well-ordered cylindrical pores is favorable to current flow by providing a direct electron pathway. DSSCs with a 3-D matrix of 2 μm thickness show an enhanced photocurrent density of 8.1 mA cm^–2 and higher photoconversion efficiency of 4.23% compared with those of TiO₂ nanoparticle photoelectrode under the illumination of 1 sun (AM 1.5 G 100 mW cm^–2). For the first time, we address that a 3-D metal oxide electrode with columnar pore is demonstrated via reverse-featured triblock copolymer and analyzed with relationships between their structural features and impedance spectroscopy for DSSCs

Exploring Interfacial Events in Gold-Nanocluster-Sensitized Solar Cells: Insights into the Effects of the Cluster Size and Electrolyte on Solar Cell Performance

Gold nanoclusters (Au NCs) with molecule-like behavior have emerged as a new light harvester in various energy conversion systems. Despite several important strides made recently, efforts toward the utilization of NCs as a light harvester have been primarily restricted to proving their potency and feasibility. In solar cell applications, ground-breaking research with a power conversion efficiency (PCE) of more than 2% has recently been reported. Because of the lack of complete characterization of metal cluster-sensitized solar cells (MCSSCs), however, comprehensive understanding of the interfacial events and limiting factors which dictate their performance remains elusive. In this regard, we provide deep insight into MCSSCs for the first time by performing in-depth electrochemical impedance spectroscopy (EIS) analysis combined with physical characterization and density functional theory (DFT) calculations of Au NCs. In particular, we focused on the effect of the size of the Au NCs and electrolytes on the performance of MCSSCs and reveal that they are significantly influential on important solar cell characteristics such as the light absorption capability, charge injection kinetics, interfacial charge recombination, and charge transport. Besides offering comprehensive insights, this work represents an important stepping stone toward the development of MCSSCs by accomplishing a new PCE record of 3.8%

Nanocomposite Coatings on Biomedical Grade Stainless Steel for Improved Corrosion Resistance and Biocompatibility

The 316 L stainless steel is one of the most commonly available commercial implant materials with a few limitations in its ease of biocompatibility and long-standing performance. Hence, porous TiO₂/ZrO₂ nanocomposite coated over 316 L stainless steels was studied for their enhanced performance in terms of its biocompatibility and corrosion resistance, following a sol–gel process via dip-coating technique. The surface composition and porosity texture was studied to be uniform on the substrate. Biocompatibility studies on the TiO₂/ZrO₂ nanocomposite coatings were investigated by placing the coated substrate in a simulated body fluid (SBF). The immersion procedure resulted in the complete coverage of the TiO₂/ZrO₂ nanocomposite (coated on the surface of 316 L stainless steel) with the growth of a one-dimensional (1D) rod-like carbonate-containing apatite. The TiO₂/ZrO₂ nanocomposite coated specimens showed a higher corrosion resistance in the SBF solution with an enhanced biocompatibility, surpassing the performance of the pure oxide coatings. The cell viability of TiO₂/ZrO₂ nanocomposite coated implant surface was examined under human dermal fibroblasts culture, and it was observed that the composite coating enhances the proliferation through effective cellular attachment compared to pristine 316 L SS surface

Effective Passivation of Nanostructured TiO₂ Interfaces with PEG-Based Oligomeric Coadsorbents To Improve the Performance of Dye-Sensitized Solar Cells

A novel poly­(ethylene glycol) (PEG) based oligomeric coadsorbent was employed to passivate TiO₂ photoanodes resulting in the large increase in both open-circuit voltage (<i>V</i>_oc) and short-circuit current density (<i>J</i>_sc) primarily because of the reduced electron recombination by the effective coverage of vacant sites as well as the negative band-edge shift of TiO₂. The effective suppression of electron recombination was evidenced by electrochemical impedance spectroscopy (EIS) and by stepped light-induced transient measurements of photocurrent and voltage (SLIM-PCV). The work function measurements also showed that the existence of coadsorbents on TiO₂ interfaces is capable of shifting the band-edge of TiO₂ photoanodes upwardly resulting in the increase in photovoltage. In addition, the coadsorbent was proven to be effective even in the presence of common additives such as LiI, 4-<i>tert</i>-butylpyridine, and guanidinium thiocyanate. The effect of Li⁺ cation trapping by ethylene oxide units of the coadsorbent was particularly notable to significantly increase <i>V</i>_oc at a small expense of <i>J</i>_sc. Consequently, the introduction of novel PEG-based oligomeric coadsorbents for TiO₂ photoanodes is quite effective in the improvement of photovoltaic performance because of the simultaneous increase in both <i>V</i>_oc and <i>J</i>_sc

Interfacial Degradation of Planar Lead Halide Perovskite Solar Cells

The stability of perovskite solar cells is one of the major challenges for this technology to reach commercialization, with water believed to be the major degradation source. In this work, a range of devices containing different cathode metal contacts in the configuration ITO/PEDOT:PSS/MAPbI₃/PCBM/Metal are fully electrically characterized before and after degradation caused by steady illumination during 4 h that induces a dramatic reduction in power conversion efficiency from values of 12 to 1.8%. We show that a decrease in performance and generation of the S-shape is associated with chemical degradation of the metal contact. Alternatively, use of Cr₂O₃/Cr as the contact enhances the stability, but modification of the energetic profile during steady illumination takes place, significantly reducing the performance. Several techniques including capacitance–voltage, X-ray diffraction, and optical absorption results suggest that the properties of the bulk perovskite layer are little affected in the device degradation process. Capacitance–voltage and impedance spectroscopy results show that the electrical properties of the cathode contact are being modified by generation of a dipole at the cathode that causes a large shift of the flat-band potential that modifies the interfacial energy barrier and impedes efficient extraction of electrons. Ionic movement in the perovskite layer changes the energy profile close to the contacts, modifying the energy level stabilization at the cathode. These results provide insights into the degradation mechanisms of perovskite solar cells and highlight the importance to further study the use of protecting layers to avoid the chemical reactivity of the perovskite with the external contacts

Synergistic Catalytic Effect of a Composite (CoS/PEDOT:PSS) Counter Electrode on Triiodide Reduction in Dye-Sensitized Solar Cells

Inorganic/organic nanocomposite counter electrodes comprised of sheetlike CoS nanoparticles dispersed in polystyrenesulfonate-doped poly(3,4-ethylenedioxythiophene (CoS/PEDOT:PSS) offer a synergistic effect on catalytic performance toward the reduction of triiodide for dye-sensitized solar cells (DSSCs), yielding 5.4% power conversion efficiency, which is comparable to that of the conventional platinum counter electrode (6.1%). The electrochemical impedance spectroscopy (EIS) and cyclic voltammetry measurements revealed that the composite counter electrodes exhibited better catalytic activity, fostering rate of triiodide reduction, than that of pristine PEDOT: PSS electrode. The simple preparation of composite (CoS/PEDOT:PSS) electrode at low temperature with improved electrocatalytic properties are feasible to apply in flexible substrates, which is at most urgency for developing novel counter electrodes for lightweight flexible solar cells

Photoinduced Giant Dielectric Constant in Lead Halide Perovskite Solar Cells

Organic–inorganic lead trihalide perovskites have emerged as an outstanding photovoltaic material that demonstrated a high 17.9% conversion efficiency of sunlight to electricity in a short time. We have found a giant dielectric constant (GDC) phenomenon in these materials consisting on a low frequency dielectric constant in the dark of the order of ε₀ = 1000. We also found an unprecedented behavior in which ε₀ further increases under illumination or by charge injection at applied bias. We observe that ε₀ increases nearly linearly with the illumination intensity up to an additional factor 1000 under 1 sun. Measurement of a variety of samples of different morphologies, compositions, and different types of contacts shows that the GDC is an intrinsic property of MAPbX₃ (MA = CH₃NH₃⁺). We hypothesize that the large dielectric response is induced by structural fluctuations. Photoinduced carriers modify the local unit cell equilibrium and change the polarizability, assisted by the freedom of rotation of MA. The study opens a way for the understanding of a key aspect of the photovoltaic operation of high efficiency perovskite solar cells

Densely Packed Siloxane Barrier for Blocking Electron Recombination in Dye-Sensitized Solar Cells

A challenge in developing photovoltaic devices is to minimize the loss of electrons, which can seriously deteriorate energy conversion efficiency. In particular, minimizing this negative process in dye-sensitized solar cells (DSCs) is imperative. Herein, we use three different kinds of siloxanes, which are adsorbable to titania surfaces and polymerizable in forming a surface passivation layer, to reduce the electron loss. The siloxanes used are tetraethyl orthosilicate (TEOS or compound A), 1-(3-(1<i>H</i>-imidazol-1-yl)­propyl)-3-(3-triethoxysilyl) propyl) urea (compound B), and <i>N</i>-(3-triethoxysilylpropyl)-<i>N</i>′[3-(3-methyl-1<i>H</i>-imidazol-3-ium) propyl] urea iodide (compound C). Titania surface passivation by either compound B or C was comparatively more effective in increasing the electron lifetime than TEOS. In the case of small-sized TEOS combined with either large-sized compound B or C, a thinner and denser passivation layer was presumably developed, thus increasing electron lifetime further. Intriguingly, device AB shows the longest electron lifetime, whereas device AC has the highest energy conversion efficiency among these experimental conditions. These results suggest that, in this special case, the electron lifetime may not be a dominant parameter in determining the energy conversion efficiency