[1]Benzothieno[3,2‑<i>b</i>]benzothiophene-Based Organic Dyes for Dye‑Sensitized Solar Cells

Three new metal-free organic dyes with the [1]­benzothieno­[3,2-<i>b</i>]­benzothiophene (BTBT) π-bridge, having the structure donor-π-acceptor (D-π<i>-</i>A) and labeled as <b>19</b>, <b>20</b> and <b>21</b>, have been designed and synthesized for application in dye-sensitized solar cells (DSSC). Once the design of the π-acceptor block was fixed, containing the BTBT as the π-bridge and the cyanoacrylic group as the electron acceptor and anchoring unit, we selected three donor units with different electron-donor capacity, in order to assemble new chromophores with high molar extinction coefficients (ε), whose absorption features well reflect the good performance of the final DSSC devices. Starting with the <b>19</b> dye, which shows a molar extinction coefficient ε of over 14,000 M^–1 cm^–1 and takes into account the absorption maximun at the longer wavelength, the substitution of the BFT donor unit with the BFA yields a great enhancement of absorptivity (molar extinction coefficient ε > 42,000 M^–1 cm^–1), until reaching the higher value (ε > 69,000 M^–1 cm^–1) with the BFPhz donor unit. The good general photovoltaic performances obtained with the three dyes highlight the suitable properties of electron-transport of the BTBT as the π-bridge in organic chromophore for DSSC, making this very cheap and easy to synthesize molecule particularly attractive for efficient and low-cost photovoltaic devices

Electrochemical Assessment of the Band-Edge Positioning in Shape-Tailored TiO₂‑Nanorod-Based Photoelectrodes for Dye Solar Cells

Three families of linear shaped TiO₂ anatase nanocrystals with variable aspect ratio (4, 8, 16) and two sets of branched TiO₂ anatase nanocrystals (in the form of open-framework sheaf-like nanorods and compact braid-like nanorod bundles, respectively) were employed to fabricate high-quality mesoporous photoelectrodes and then implemented into dye-sensitized solar cells to elucidate the intrinsic correlation holding between the photovoltaic performances and the structure of the nanocrystal building blocks. To this aim, the chemical capacitance and the charge-transfer resistance of the photoelectrodes were extrapolated from electrochemical impedance spectroscopy measurements and used to draw a quantitative energy diagram of the dye-sensitized solar cells realized, on the basis of which their photovoltaic performances have been discussed. It has thus been revealed that photoanodes made from braid-like branched-nanorod bundles exhibited the most favorable conditions to minimize recombination at the interface with the electrolyte due to their deep distribution of trap states, whereas linear-shaped nanorods with higher aspect-ratios result in more remarkable downshift of the conduction band edge

Sustainability of Organic Dye-Sensitized Solar Cells: The Role of Chemical Synthesis

The synthesis of a novel and efficient π-extended D-A-π-A organic sensitizer (<b>G3</b>, η = 8.64%) for dye-sensitized solar cells has been accomplished by applying the green chemistry pillars, aiming at overriding traditional routes involving organometallic intermediates with innovative synthetic strategies for reducing the waste burden and dye production costs. It has been demonstrated that the obtainment of a complex target sensitizer can be exclusively pursued via direct arylation reactions. Green metrics comparison with those of a traditional synthetic pathway clearly indicates that the new approach has a lower environmental impact in terms of chemical procedures and generated wastes, stressing the importance of the synergy between the molecular design and the synthetic plan in the framework of environmentally friendly routes to back up sustainable development of third-generation photovoltaics. Additionally, the stability of the <b>G3</b>-based photovoltaic devices was also investigated in aging tests on large area devices, evidencing the excellent potentialities of the proposed structure for all practical applications involving inorganic semiconductor/organic dye interfaces

Ultrastrong Plasmon–Exciton Coupling by Dynamic Molecular Aggregation

Plasmon–exciton polaritons arise from the coherent coupling of the localized plasmon of metal nanoparticles and the exciton of nearby resonant nanoemitters. The behavior of such systems is strictly defined by the initial choice of the metallic and excitonic materials, with only weak control possibilities, essentially limited to polarization-related effects or photoswitchable molecules. Here we propose a new strategy to control the plasmon–exciton splitting, based on the number of excitonic dipoles involved in the interaction. By integrating plasmonic arrays in a microfluidic device and injecting a dilute near-infrared cyanine dye solution, we are able to probe in real time the emergence and evolution of the strong plasmon–exciton coupling regime. When dye molecules selectively aggregate on silver as a result of chemical affinity, we observe a continuous increase of the Rabi splitting up to an exciton energy fraction as high as 35%, compatible with an ultrastrong coupling regime

NiO/MAPbI_3‑xCl_<i>x</i>/PCBM: A Model Case for an Improved Understanding of Inverted Mesoscopic Solar Cells

A spectroscopic investigation focusing on the charge generation and transport in inverted p-type perovskite-based mesoscopic (Ms) solar cells is provided in this report. Nanocrystalline nickel oxide and PCBM are employed respectively as hole transporting scaffold and hole blocking layer to sandwich a perovskite light harvester. An efficient hole transfer process from perovskite to nickel oxide is assessed, through time-resolved photoluminescence and photoinduced absorption analyses, for both the employed absorbing species, namely MAPbI_3‑<i>x</i>Cl_<i>x</i> and MAPbI₃. A striking relevant difference between p-type and n-type perovskite-based solar cells emerges from the study

Ultrastrong Plasmon–Exciton Coupling by Dynamic Molecular Aggregation

Plasmon–exciton polaritons arise from the coherent coupling of the localized plasmon of metal nanoparticles and the exciton of nearby resonant nanoemitters. The behavior of such systems is strictly defined by the initial choice of the metallic and excitonic materials, with only weak control possibilities, essentially limited to polarization-related effects or photoswitchable molecules. Here we propose a new strategy to control the plasmon–exciton splitting, based on the number of excitonic dipoles involved in the interaction. By integrating plasmonic arrays in a microfluidic device and injecting a dilute near-infrared cyanine dye solution, we are able to probe in real time the emergence and evolution of the strong plasmon–exciton coupling regime. When dye molecules selectively aggregate on silver as a result of chemical affinity, we observe a continuous increase of the Rabi splitting up to an exciton energy fraction as high as 35%, compatible with an ultrastrong coupling regime

Ultrathin TiO₂(B) Nanorods with Superior Lithium-Ion Storage Performance

The peculiar architecture of a novel class of anisotropic TiO₂(B) nanocrystals, which were synthesized by an surfactant-assisted nonaqueous sol–gel route, was profitably exploited to fabricate highly efficient mesoporous electrodes for Li storage. These electrodes are composed of a continuous spongy network of interconnected nanoscale units with a rod-shaped profile that terminates into one or two bulgelike or branch-shaped apexes spanning areas of about 5 × 10 nm². This architecture transcribes into a superior cycling performance (a charge capacitance of 222 mAh g^–1 was achieved by a carbon-free TiO₂(B)-nanorods-based electrode vs 110 mAh g^–1 exhibited by a comparable TiO₂-anatase electrode) and good chemical stability (more than 90% of the initial capacity remains after 100 charging/discharging cycles). Their outstanding lithiation/delithiation capabilities were also exploited to fabricate electrochromic devices that revealed an excellent coloration efficiency (130 cm² C^–1 at 800 nm) upon the application of 1.5 V as well as an extremely fast electrochromic switching (coloration time ∼5 s)

Nanoscale Study of the Tarnishing Process in Electron Beam Lithography-Fabricated Silver Nanoparticles for Plasmonic Applications

Silver is the ideal material for plasmonics because of its low loss at optical frequencies, though it is often replaced by a lossier metal, gold. This is because of silver’s tendency to tarnish, an effect which is enhanced at the nanoscale due to the large surface-to-volume ratio. Despite chemical tarnishing of Ag nanoparticles (NPs) has been extensively studied for decades, it has not been well understood whether resulted by sulfidation or oxidation processes. This intriguing quest is herein rationalized by studying the atmospheric corrosion of electron beam lithography-fabricated Ag NPs, through nanoscale investigation performed by high-resolution transmission electron microscopy (HRTEM) combined with electron energy loss (EEL) and energy dispersive X-ray (EDX) spectroscopies. We demonstrate that tarnishing of Ag NPs upon exposure to indoor air of an environment located inside a rural site, not particularly influenced by naturally and human-made sulfur sources, is caused by chemisorbed sulfur-based contaminants rather than via an oxidation process. Furthermore, we show that the sulfidation occurs through the formation of crystalline Ag₂S bumps onto Ag surface in place of a homogeneous growth of a silver sulfide film. From a single 2D Z-contrast scanning transmission electron microscopy image, a method for 3D reconstruction of silver nanoparticles with extremely high spatial resolution has been derived thus establishing the preferential nucleation of Ag₂S bumps in proximity of lattice defects located on the NP surface. Finally, we also provide a straightforward and low-cost solution to achieve stable Ag NPs by passivating them with a self-assembled monolayer of hexanethiols. The sulfidation mechanism inhibition allows to prevent the increased material damping and scattering losses

Addressing the Function of Easily Synthesized Hole Transporters in Direct and Inverted Perovskite Solar Cells

Two simple small molecules are designed and successfully implemented here as hole-transporting material (HTM) in perovskite-based solar cells (PSCs). With the aim of elucidating the interconnection between molecular structure, properties, and their role in the working devices, these HTMs are implemented in both thin planar direct (n–i–p) and inverse (p–i–n) geometries. It is observed how the HTM layer morphology influences the photovoltaic performance. Moreover, from analysis of the different devices, fundamental information is retrieved on the factors influencing small molecule hole extracting/transporting functionality in PSCs. Specifically, two main roles are identified: When HTMs are introduced as growing substrate (p–i–n), there is a positive impact on the device performance via influence of perovskite formation; meanwhile, their efficacy in transporting the holes governs the performance of direct configurations (n–i–p). These findings can be extended to a wide family of small molecule HTMs, providing general rules for refining the design of novel and more efficient ones

Influence of Porphyrinic Structure on Electron Transfer Processes at the Electrolyte/Dye/TiO₂ Interface in PSSCs: a Comparison between meso Push–Pull and β‑Pyrrolic Architectures

Time-resolved photophysical and photoelectrochemical investigations have been carried out to compare the electron transfer dynamics of a 2-β-substituted tetraarylporphyrinic dye (ZnB) and a 5,15-meso-disubstituted diarylporphyrinic one (ZnM) at the electrolyte/dye/TiO₂ interface in PSSCs. Although the meso push–pull structural arrangement has shown, up to now, to have the best performing architecture for solar cell applications, we have obtained superior energy conversion efficiencies for ZnB (6.1%) rather than for ZnM (3.9%), by using the I^–/I₃^–-based electrolyte. To gain deeper insights about these unexpected results, we have investigated whether the intrinsic structural features of the two different porphyrinic dyes can play a key role on electron transfer processes occurring at the dye-sensitized TiO₂ interface. We have found that charge injection yields into TiO₂ are quite similar for both dyes and that the regeneration efficiencies by I^–, are also comparable and in the range of 75–85%. Moreover, besides injection quantum yields above 80%, identical dye loading, for both ZnB and ZnM, has been evidenced by spectrophotometric measurements on transparent thin TiO₂ layers after the same adsorption period. Conversely, major differences have emerged by DC and AC (electrochemical impedance spectroscopy) photoelectrochemical investigations, pointing out a slower charge recombination rate when ZnB is adsorbed on TiO₂. This may result from its more sterically hindered macrocyclic core which, besides guaranteeing a decrease of π-staking aggregation of the dye, promotes a superior shielding of the TiO₂ surface against charge recombination involving oxidized species of the electrolyte