High-Performance and Stable Gel-State Dye-Sensitized Solar Cells Using Anodic TiO₂ Nanotube Arrays and Polymer-Based Gel Electrolytes

Highly ordered and vertically oriented TiO₂ nanotube (NT) arrays were synthesized with potentiostatic anodization of Ti foil and applied to fabricate gel-state dye-sensitized solar cells (DSSCs). The open structure of the TiO₂ NT facilitates the infiltration of the gel-state electrolyte; their one-dimensional structural feature provides effective charge transport. TiO₂ NTs of length <i>L</i> = 15–35 μm were produced on anodization for periods of <i>t</i> = 5–15 h at a constant voltage of 60 V, and sensitized with N719 for photovoltaic characterization. A commercially available copolymer, poly­(methyl methacrylate-<i>co</i>-ethyl acrylate) (PMMA-EA), served as a gelling agent to prepare a polymer-gel electrolyte (PGE) for DSSC applications. The PGE as prepared exhibited a maximum conductivity of 4.58 mS cm^–1 with PMMA-EA (7 wt %). The phase transition temperature (<i>T</i>_p) of the PGE containing PMMA-EA at varied concentrations was determined on the basis of the viscosities measured at varied temperatures. <i>T</i>_p increased with increasing concentration of PMMA-EA. An NT-DSSC with <i>L</i> = 30 μm assembled using a PGE containing PMMA-EA (7 wt %) exhibited an overall power conversion efficiency (PCE) of 6.9%, which is comparable with that of a corresponding liquid-type device, PCE = 7.1%. Moreover, the gel-state NT-DSSC exhibited excellent thermal and light-soaking enduring stability: the best device retained ∼90% of its initial efficiency after 1000 h under 1 sun of illumination at 50 °C, whereas its liquid-state counterpart decayed appreciably after light soaking for 500 h

Design and Characterization of Heteroleptic Ruthenium Complexes Containing Benzimidazole Ligands for Dye-Sensitized Solar Cells: The Effect of Fluorine Substituents on Photovoltaic Performance

We designed heteroleptic ruthenium complexes (<b>RD12–RD15</b>) containing fluoro-substituted benzimidazole ligands for dye-sensitized solar cells (DSSCs). These dyes were synthesized according to a typical one-pot procedure with the corresponding ancillary ligands produced in two simple steps; they were prepared into DSSC devices according to the same conditions of fabrication. The eventual devices show a systematic trend of increasing <i>V</i>_OC and decreasing <i>J</i>_SC with fluorine atoms of increasing number substituted on the ligand. The charge-extraction results show that upward shifts of the TiO₂ potential occurred when the fluoro-substituted dyes were sensitized on TiO₂ with a systematic trend of shift <b>N719</b> > <b>RD15</b> (with 5 F) > <b>RD12</b> (with 2 F) <b>></b> <b>RD5</b> (no F); the intensity-modulated photovoltage spectra indicate that those fluoro substituents retard charge recombination with the electron lifetimes (τ_R) in the order <b>RD15</b> > <b>RD12</b> > <b>RD5</b> > <b>N719</b>, consistent with the variation of <i>V</i>_OC for the systems

Design and Characterization of Heteroleptic Ruthenium Complexes Containing Benzimidazole Ligands for Dye-Sensitized Solar Cells: The Effect of Fluorine Substituents on Photovoltaic Performance

We designed heteroleptic ruthenium complexes (<b>RD12–RD15</b>) containing fluoro-substituted benzimidazole ligands for dye-sensitized solar cells (DSSCs). These dyes were synthesized according to a typical one-pot procedure with the corresponding ancillary ligands produced in two simple steps; they were prepared into DSSC devices according to the same conditions of fabrication. The eventual devices show a systematic trend of increasing <i>V</i>_OC and decreasing <i>J</i>_SC with fluorine atoms of increasing number substituted on the ligand. The charge-extraction results show that upward shifts of the TiO₂ potential occurred when the fluoro-substituted dyes were sensitized on TiO₂ with a systematic trend of shift <b>N719</b> > <b>RD15</b> (with 5 F) > <b>RD12</b> (with 2 F) <b>></b> <b>RD5</b> (no F); the intensity-modulated photovoltage spectra indicate that those fluoro substituents retard charge recombination with the electron lifetimes (τ_R) in the order <b>RD15</b> > <b>RD12</b> > <b>RD5</b> > <b>N719</b>, consistent with the variation of <i>V</i>_OC for the systems

High-Performance Large-Scale Flexible Dye-Sensitized Solar Cells Based on Anodic TiO₂ Nanotube Arrays

A simple strategy to fabricate flexible dye-sensitized solar cells involves the use of photoanodes based on TiO₂ nanotube (TNT) arrays with rear illumination. The TNT films (tube length ∼35 μm) were produced via anodization, and sensitized with N719 dye for photovoltaic characterization. Pt counter electrodes of two types were used: a conventional FTO/glass substrate for a device of rigid type and an ITO/PEN substrate for a device of flexible type. These DSSC devices were fabricated into either a single-cell structure (active area 3.6 × 0.5 cm²) or a parallel module containing three single cells (total active area 5.4 cm²). The flexible devices exhibit remarkable performance with efficiencies η = 5.40 % (single cell) and 4.77 % (parallel module) of power conversion, which outperformed their rigid counterparts with η = 4.87 % (single cell) and 4.50 % (parallel model) under standard one-sun irradiation. The flexible device had a greater efficiency of conversion of incident photons to current and a broader spectral range than the rigid device; a thinner electrolyte layer for the flexible device than for the rigid device is a key factor to improve the light-harvesting ability for the TNT-DSSC device with rear illumination. Measurements of electrochemical impedance spectra show excellent catalytic activity and superior diffusion characteristics for the flexible device. This technique thus provides a new option to construct flexible photovoltaic devices with large-scale, light-weight, and cost-effective advantages for imminent applications in consumer electronics

Size-Controlled Anatase Titania Single Crystals with Octahedron-like Morphology for Dye-Sensitized Solar Cells

A simple hydrothermal method with titanium tetraisopropoxide (TTIP) as a precursor and triethanolamine (TEOA) as a chelating agent enabled growth in the presence of a base (diethylamine, DEA) of anatase titania nanocrystals (HD1–HD5) of controlled size. DEA played a key role to expedite this growth, for which a biphasic crystal growth mechanism is proposed. The produced single crystals of titania show octahedron-like morphology with sizes in a broad range of 30–400 nm; a typical, extra large, octahedral single crystal (HD5) of length 410 nm and width 260 nm was obtained after repeating a sequential hydrothermal treatment using HD3 and then HD4 as a seed crystal. The nanocrystals of size ∼30 nm (HD1) and ∼300 nm (HD5) served as active layer and scattering layer, respectively, to fabricate N719-sensitized solar cells. These HD devices showed greater <i>V</i>_OC than devices of conventional nanoparticle (NP) type; the overall device performance of HD attained an efficiency of 10.2% power conversion at a total film thickness of 28 μm, which is superior to that of a NP-based reference device (η = 9.6%) optimized at a total film thickness of 18–20 μm. According to results obtained from transient photoelectric and charge extraction measurements, this superior performance of HD devices relative to their NP counterparts is due to the more rapid electron transport and greater TiO₂ potential

Cobalt Oxide (CoO_<i>x</i>) as an Efficient Hole-Extracting Layer for High-Performance Inverted Planar Perovskite Solar Cells

CoO_<i>x</i> is a promising hole-extracting layer (HEL) for inverted planar perovskite solar cells with device configuration ITO/CoO_<i>x</i>/CH₃NH₃PbI₃/PCBM/Ag. The devices fabricated according to a simple solution procedure showed the best photovoltaic performance attaining power conversion efficiency (PCE) of 14.5% under AM 1.5 G 1 sun irradiation, which is significantly superior to those of materials fabricated with a traditional HEL such as PEDOT:PSS (12.2%), NiO_<i>x</i> (10.2%), and CuO_<i>x</i> (9.4%) under the same experimental conditions. We characterized the chemical compositions with XPS, crystal structures with XRD, and film morphology with SEM/AFM techniques. Photoluminescence (PL) spectra and the corresponding PL decays for perovskite deposited on varied HEL films were recorded to obtain the hole-extracting characteristics, for which the hole-extracting times show the order CoO_<i>x</i> (2.8 ns) < PEDOT:PSS (17.5 ns) < NiO_<i>x</i> (22.8 ns) < CuO_<i>x</i> (208.5 ns), consistent with the trend of their photovoltaic performances. The reproducibility and enduring stability of those devices were examined to show the outstanding long-term stability of the devices made of metal oxide HEL, for which the CoO_<i>x</i> device retained PCE ≈ 12% for over 1000 h

Hybrid Titania Photoanodes with a Nanostructured Multi-Layer Configuration for Highly Efficient Dye-Sensitized Solar Cells

To construct a hybrid titania photoanode containing nanoparticles and nanorods of varied size in a multilayer (ML) configuration for dye-sensitized solar cells, the essence of our ML design is a bilayer system with additional layers of nanorods of well-controlled size inserted between the transparent and the scattering layers to enhance the light-harvesting capability for photosensitizers with small absorptivity, such as Z907. We measured charge-extraction and intensity-modulated photoelectric spectra to show the advantages of one-dimensional nanorods with an improved electron-transport property and an upward shift of the potential band edge; a favorable ML configuration was constructed to have a cascade potential feature for feasible electron transport from long nanorods, to normal nanorods, to small nanoparticles. On the basis of the ML system reported herein, we demonstrate how the performance of a Z907 device is improved to attain η ∼10%, which is a milestone for its future commercialization

Role of Tin Chloride in Tin-Rich Mixed-Halide Perovskites Applied as Mesoscopic Solar Cells with a Carbon Counter Electrode

We report the synthesis and characterization of alloyed Sn–Pb methylammonium mixed-halide perovskites (CH₃NH₃Sn_<i>y</i>Pb_1–<i>y</i>I_3–<i>x</i>Cl_<i>x</i>) to extend light harvesting toward the near-infrared region for carbon-based mesoscopic solar cells free of organic hole-transport layers. The proportions of Sn in perovskites are well-controlled by mixing tin chloride (SnCl₂) and lead iodide (PbI₂) in varied stoichiometric ratios (<i>y</i> = 0–1). SnCl₂ plays a key role in modifying the lattice structure of the perovskite, showing anomalous optical and optoelectronic properties; upon increasing the concentration of SnCl₂, the variation of the band gap and band energy differed from those of the SnI₂ precursor. The CH₃NH₃Sn_<i>y</i>Pb_1–<i>y</i>I_3–<i>x</i>Cl_<i>x</i> devices showed enhanced photovoltaic performance upon increasing the proportion of SnCl₂ until <i>y</i> = 0.75, consistent with the corresponding potential energy levels. The photovoltaic performance was further improved upon adding 30 mol % tin fluoride (SnF₂) with device configuration FTO/TiO₂/Al₂O₃/NiO/C, producing the best power conversion efficiency, 5.13%, with great reproducibility and intrinsic stability