Vanira Trifiletti

Tetraaryl ZnIIPorphyrinates Substituted at β-Pyrrolic Positions as Sensitizers in Dye-Sensitized Solar Cells: A Comparison withmeso-Disubstituted Push-Pull ZnIIPorphyrinates

A facile and fast approach, based on microwave-enhanced Sonogashira coupling, has been employed to obtain in good yields both mono- and, for the first time, disubstituted push-pull Zn(II) porphyrinates bearing a variety of ethynylphenyl moieties at the β-pyrrolic position(s). Furthermore, a comparative experimental, electrochemical, and theoretical investigation has been carried out on these β-mono- or disubstituted Zn(II) porphyrinates and meso-disubstituted push-pull Zn(II) porphyrinates. We have obtained evidence that, although the HOMO-LUMO energy gap of the meso-substituted push-pull dyes is lower, so that charge transfer along the push-pull system therein is easier, the β-mono- or disubstituted push-pull porphyrinic dyes show comparable or better efficiencies when acting as sensitizers in DSSCs. This behavior is apparently not attributable to more intense B and Q bands, but rather to more facile charge injection. This is suggested by the DFT electron distribution in a model of a β-monosubstituted porphyrinic dye interacting with a TiO2 surface and by the positive effect of the β substitution on the incident photon-to-current conversion efficiency (IPCE) spectra, which show a significant intensity over a broad wavelength range (350-650 nm). In contrast, meso-substitution produces IPCE spectra with two less intense and well-separated peaks. The positive effect exerted by a cyanoacrylic acid group attached to the ethynylphenyl substituent has been analyzed by a photophysical and theoretical approach. This provided supporting evidence of a contribution from charge-transfer transitions to both the B and Q bands, thus producing, through conjugation, excited electrons close to the carboxylic anchoring group. Finally, the straightforward and effective synthetic procedures developed, as well as the efficiencies observed by photoelectrochemical measurements, make the described β-monosubstituted Zn(II) porphyrinates extremely promising sensitizers for use in DSSCs.

Thiocyanate-free cyclometalated ruthenium sensitizers for solar cells based on heteroaromatic-substituted 2-arylpyridines

The first examples of thiocyanate-free thiophene-substituted Ru(II) cyclometalated complexes, based on thiophene-derived 2-(2,4-difluorophenyl)pyridine ligands, are presented and investigated as photosensitizers in DSCs. Upon thiophene substitution the complexes presented enhanced optical properties compared to the reference dye with no thiophene substitution. DSCs based on the dithienyl-derived dye showed power conversion efficiencies up to 5.7%, more than twice that containing the complex without the thiophene substitution.

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.

Covalently Functionalized SWCNTs as Tailored p-Type Dopants for Perovskite Solar Cells

The covalent functionalization of (7,6)-enriched single-walled carbon nanotubes (SWCNTs) with oligophenylenevinylene (OPV) moieties terminating with a dimethylamino group is proposed as an efficient way to enhance the affinity of CNTs with spiro-MeOTAD in perovskite-based solar cells. The evidence of SWCNTs functionalization and the degree of OPV substitution on SWCNTs are established from TGA, XPS, TEM, and Raman techniques. Our tailored doping materials afford photovoltaic performances in line with conventional Li-doped spiro-MeOTAD, showing at the same time a significantly improved chemical stability of the perovskite component over time. Furthermore, the comparison of the photovoltaic performances with those obtained with nonfunctionalized SWCNTs suggest that the presence of the organic appends ensures highly reproducible PV performances. These results demonstrate the suitability of this functionalized SWCNT material as a valid doping agent for spiro-MeOTAD, representing a viable alternative to the conventional Li salt.

Dye-sensitized solar cells containing plasma jet deposited hierarchically nanostructured TiO2 thin photoanodes

The investigation of dye-sensitized solar cells (DSCs) using a very thin (0.8 to 4.7 μm) transparent photoanode based on a hierarchically nanostructured TiO2 film prepared via Plasma Assisted Supersonic Jet Deposition (PA-SJD), in combination with the DSC benchmark photosensitizer N719, is presented. The cell photovoltages (nearly 0.8 V) and the amount of adsorbed dye and photocurrent densities, normalized over the film thickness, are higher than those of DSCs prepared with thicker (9 μm) conventional screen-printed TiO2 films. A record efficiency value of 5.7% (standard AM 1.5G solar irradiation) for very thin nanostructured titania photoanodes is obtained with a 2 μm PA-SJD TiO2 film. Electrochemical impedance spectroscopy shows that PA-SJD devices are endowed with higher charge recombination resistance (lower recombination losses) and electron lifetimes compared to conventional nanocrystalline films.

Performance enhancement of a dye-sensitized solar cell by peripheral aromatic and heteroaromatic functionalization in di-branched organic sensitizers

Di-branched dyes based on a triphenylamino (TPA) donor core have been functionalized with different aromatic and heteroaromatic peripheral groups bonded to TPA as auxiliary donors and investigated as sensitizers in DSSC. The different aromatic and heteroaromatic substitution significantly perturbed the optical properties of the dyes compared to the unsubstituted TPA reference dye, in turn leading to enhanced photovoltaic performances in terms of photocurrent, photovoltage, and power conversion efficiency (PCE). The enhancement originated from improved strategic interface interactions between the dye-sensitized titania and the liquid electrolyte, as ascertained by current–voltage, quantum efficiency (IPCE), light harvesting properties (LHE), and impedance spectroscopy (EIS) studies.