Synthesis of POSS-Based Ionic Conductors with Low Glass Transition Temperatures for Efficient Solid-State Dye-Sensitized Solar Cells

Replacing liquid-state electrolytes with solid-state electrolytes has been proven to be an effective way to improve the durability of dye-sensitized solar cells (DSSCs). We report herein the synthesis of amorphous ionic conductors based on polyhedral oligomeric silsesquioxane (POSS) with low glass transition temperatures for solid-state DSSCs. As the ionic conductor is amorphous and in the elastomeric state at the operating temperature of DSSCs, good pore filling in the TiO₂ film and good interfacial contact between the solid-state electrolyte and the TiO₂ film can be guaranteed. When the POSS-based ionic conductor containing an allyl group is doped with only iodine as the solid-state electrolyte without any other additives, power conversion efficiency of 6.29% has been achieved with good long-term stability under one-sun soaking for 1000 h

POSS-Based Electrolyte for Efficient Solid-State Dye-Sensitized Solar Cells at Sub-Zero Temperatures

To expand the application of solid-state dye-sensitized solar cells (ssDSSCs) to low temperatures, it is necessary to develop new solid electrolytes with low glass transition temperature (<i>T</i>_g). The <i>T</i>_g is regulated by varying the length of alkyl chain that is connected with the nitrogen atom in the imidazolium ring linked to the polyhedral oligomeric silsesquioxane (POSS). The <i>T</i>_g as low as −8.8 °C is achieved with the POSS grafted with methyl-substituted imidazolium. The effect of alkyl group on the conductivity, <i>T</i>_g, and photovoltaic performance has also been investigated. The conductivity and power conversion efficiency increase with the alkyl length, while the <i>T</i>_g first increases and then decreases with the alkyl length. Among the synthesized POSS-based ionic conductors, the POSS grafted with the methyl-substituted imidazolium yields the highest power conversion efficiency of 6.98% at RT due to its highest conductivity, and the efficiency (6.52%) is still good at −4 °C, as its <i>T</i>_g (−8.8 °C) is lower than the working temperature (−4 °C). This finding suggests that the POSS-based solid electrolyte is promising for subzero-temperature applications of ssDSSCs

NiS₂/Reduced Graphene Oxide Nanocomposites for Efficient Dye-Sensitized Solar Cells

NiS₂ nanoparticles and nanocomposites of NiS₂ with reduced graphene oxide (NiS₂@RGO) have been successfully prepared via a facile hydrothermal reaction of nickel ions and sulfur source in the absence or presence of graphene oxide. NiS₂@RGO nanocomposites exhibit excellent electrocatalytic performance for reduction of triiodide, owing to the improved conductivity and positive synergetic effect between NiS₂ and RGO. As a consequence, the dye-sensitized solar cell with the NiS₂@RGO counter electrode (CE) produces a power conversion efficiency of 8.55%, which is higher than that (7.02%) for the DSSC with the NiS₂ CE, higher than that (3.14%) for the DSSC with the RGO CE, and also higher than that (8.15%) for the DSSC with the reference Pt CE under the same conditions

Ionic Conductor with High Conductivity as Single-Component Electrolyte for Efficient Solid-State Dye-Sensitized Solar Cells

Imidazolium iodide is an often used component in iodine-based dye-sensitized solar cells (DSSCs), but it cannot operate an efficient DSSC in the absence of iodine due to its low conductivity. For this study, lamellar solid iodide salts of imidazolium or piperidinium with an N-substituted propargyl group have been prepared and applied in solid-state DSSCs. Owing to the high conductivity arising from the lamellar structure, these solid-state ionic conductors can be used as single-component solid electrolytes to operate solid-state DSSCs efficiently without any additives in the electrolyte and post-treatments on the dye-loaded TiO₂ films. With a propargyl group attached to the imidazolium ring, the conductivity is enhanced by about 4 × 10⁴-fold as compared to the alkyl-substituted imidazolium iodide. Solid-state DSSC with the 1-propargyl-3-methylimidazolium iodide as the single-component solid-state electrolyte has achieved a light-to-electricity power conversion efficiency of 6.3% under illumination of simulated AM1.5G solar light (100 mW cm^–2), which also exhibits good long-term stability under continuous 1 sun soaking for 1500 h. This finding paves the way for development of high-conductivity single-component solid electrolytes for use in efficient solid-state DSSCs

In Situ Growth of Co_0.85Se and Ni_0.85Se on Conductive Substrates as High-Performance Counter Electrodes for Dye-Sensitized Solar Cells

We present herein a facile one-step low-temperature hydrothermal approach for in situ growth of metal selenides (Co_0.85Se and Ni_0.85Se) on conductive glass substrates. The as-prepared metal selenides on conductive substrates can be used directly as transparent counter electrodes for dye-sensitized solar cells (DSSCs) without any post-treatments. It is found that graphene-like Co_0.85Se exhibits higher electrocatalytic activity than Pt for the reduction of triiodide. As a consequence, the DSSC with Co_0.85Se generates higher short-circuit photocurrent and power conversion efficiency (9.40%) than that with Pt

Reduced Graphene Oxide–TaON Composite As a High-Performance Counter Electrode for Co(bpy)₃^3+/2+-Mediated Dye-Sensitized Solar Cells

We report herein the investigation of TaON nanoparticles incorporating a reduced graphene oxide (RGO) nanocomposite as a counter electrode for application in Co­(bpy)₃^3+/2+ (bpy = 2,2′-bipyridine)-mediated dye-sensitized solar cells (DSSCs). The RGO–TaON nanocomposite has been prepared by mixing graphene oxide (GO) and presynthesized TaON nanoparticles in ethanol/water followed by the facile hydrazine hydrate reduction of GO to RGO. Compared with RGO or TaON alone, the RGO–TaON nanocomposite shows a much higher electrocatalytic activity for the reduction of Co­(bpy)₃³⁺ species owing to synergistic effects, resulting in significantly improved solar-cell performance when it is applied as the counter electrode in DSSCs. An efficiency of 7.65% for the DSSC with the RGO–TaON counter electrode is obtained, competing with the efficiency produced by the Pt counter electrode; additionally, the former exhibits a much better electrochemical stability than the latter in a Co­(bpy)₃^3+/2+ acetonitrile solution

Molecular Engineering of Quinoxaline-Based Organic Sensitizers for Highly Efficient and Stable Dye-Sensitized Solar Cells

A series of quinoxaline based metal-free organic sensitizers has been designed and synthesized for dye-sensitized solar cells (DSSCs). The absorption, electrochemical, and photovoltaic properties for all sensitizers have been systematically investigated. It is found that the incorporation of quinoxaline unit instead of thienopyrazine unit results in a negative shift of the lowest unoccupied molecular orbital levels for <b>FNE44</b>, <b>FNE45</b>, <b>FNE46</b>, and <b>FNE47</b>, in comparison to <b>FNE32</b>, which induces a remarkable enhancement of the electron injection driving force from the excited organic sensitizers to the TiO₂ semiconductor. Moreover, when the alkyl substituents are removed from the spacer part in <b>FNE44</b> to the donor part in <b>FNE45</b> and <b>FNE46</b>, a more conjugated system and a bathochromically shifted maximum absorption band can be realized, which consequently results in an increased light harvesting efficiency and photogenerated current. In addition, the length of the alkyl substituents on the donor part has a certain influence on the DSSC performance. Combining the three contributions, <b>FNE46</b>-based DSSC with liquid electrolyte displays the highest power conversion efficiency (η) of 8.27%. Most importantly, a η of 7.14% has been achieved for <b>FNE46</b> based quasi-solid-state DSSC and remained at 100% of the initial value after continuous light soaking for 1000 h, which indicates that <b>FNE46</b> is appropriate for promising commercial application. Our findings will facilitate the understanding of the crucial importance of molecular engineering and pave a new path to design novel metal-free organic dyes for highly efficient and stable DSSCs

Characterization of Perovskite Obtained from Two-Step Deposition on Mesoporous Titania

The properties of perovskite films are sensitive to the fabrication method, which plays a crucial role in the performance of perovskite solar cell. In this work, we fabricate organo-lead iodide perovskite on mesoporous TiO₂ films through two different two-step deposition methods, respectively, for the purpose of studying the crystal growth of perovskite film and its effect on light harvesting efficiency, defect density, charge extraction rate, and energy levels. The crystal growth exerts a significant influence on the morphology and hence the film properties, which are found to correlate with the performance of solar cells. It is found that vapor deposition of methylammonium iodide in the PbI₂ lattice gives a more complete coverage on mesoporous TiO₂ with a flatter surface and Fermi level closer to the middle of the band-gap, resulting in higher light absorption in the visible spectral region, lower defect density, and faster charge extraction, as compared to the sequential solution deposition. For this reason, the vapor-processed perovskite film achieves higher short-circuit photocurrent and power conversion efficiency than the solution-processed film

A Near-Infrared Dithieno[2,3‑<i>a</i>:3′,2′‑<i>c</i>]phenazine-Based Organic Co-Sensitizer for Highly Efficient and Stable Quasi-Solid-State Dye-Sensitized Solar Cells

A novel near-infrared (NIR) organic sensitizer FNE53 with a strong electron-withdrawing unit, dithieno­[2,3-<i>a</i>:3′,2′-<i>c</i>]­phenazine, has been designed and synthesized for quasi-solid-state dye-sensitized solar cells (DSSCs). By simply fusing the two thiophene rings on quinoxaline unit in sensitizer FNE48, the intramolecular charge transfer (ICT) band bathochromically shifts from 542 nm for FNE48 to 629 nm for FNE53 in toluene solution. The absorption spectrum of sensitizer FNE53 covers the whole visible region and extends to the NIR region, which exhibits complementary absorption profile to another organic dye FNE46 based on quinoxaline. When FNE46 and FNE53 are used as cosensitizers for metal-free cocktail-type quasi-solid-state DSSCs, sensitizer FNE53 not only extends the photoresponse range but also suppresses the intermolecular interactions among the dye molecules. Therefore, the cocktail-type quasi-solid-state DSSC displays much higher IPCE value compared with that for the DSSC sensitizer based on FNE53 and a broader IPCE response in comparison to that for the DSSC sensitizers based on FNE46, respectively. After the molar ratio between the two cocktail dyes is optimized, the highest energy conversion efficiency of 8.04% is achieved in a metal-free quasi-solid-state DSSC cosensitized with FNE46 and FNE53, which exhibits good long-term stability after continuous light soaking for 1000 h

Facile and Selective Synthesis of Oligothiophene-Based Sensitizer Isomers: An Approach toward Efficient Dye-Sensitized Solar Cells

Two sets of isomeric organic dyes with <i>n</i>-hexyl (<b>DH</b> and <b>AH</b>) or 2-ethylhexyl (<b>DEH</b> and <b>AEH</b>) groups substituted at the spacer part have been designed and straightforwardly synthesized via a facile and selective synthetic route. The structure difference between the isomers stands at the position of the incorporated alkyl chains which are introduced into the terthiophene spacer close to the donor (<b>D</b>) or anchor (<b>A</b>) side. The relationship between the isomeric structures and the optoelectronic properties are systematically investigated. It is found that, in the <b>D</b> series dyes, the alkyl group is much closer to the aromatic donor moiety, which brings about strong steric hindrance and therefore causes a remarkable twist in the molecular skeleton. In contrast, a more planar chemical structure and more effective π-conjugation are realized in the <b>A</b> series dye isomers. Consequently, the <b>A</b> series isomeric dyes demonstrate bathochromically shifted absorption bands, resulting in the improved light-harvesting capability and enhanced photo-generated current. However, the <b>D</b> series isomeric dyes with more twisted molecular skeleton have suppressed the intermolecular interactions and retarded the charge recombination more efficiently, which induces higher open-circuit photovoltage. Combining the two effects on the performance of the fabricated dye-sensitized solar cells (DSSC), the influence from the short-circuit photocurrent plays a more significant role on the power conversion efficiency (η). As a result, isomer <b>AEH</b>-based DSSC with quasi-solid-state electrolyte displays the highest η of 7.10% which remained at 98% of the initial value after continuous light soaking for 1000 h. Promisingly, a η of 8.66% has been achieved for <b>AEH</b>-based DSSC with liquid electrolyte containing Co­(II)/(III) redox couple. This work presents the crucial issue of molecular engineering and paves a way to design organic sensitizers for highly efficient and stable DSSCs