Ángela Sastre-Santos

Energetic factors governing injection, regeneration and recombination in dye solar cells with phthalocyanine sensitizers

Phthalocyanines (Pcs) are promising candidates for photon collectors for the near-infrared region of the solar spectrum in dye solar cells (DSC). Using two new Pc sensitizers, differing only in their Zn or metal-free center, we discuss the behaviour of the Pc dyes as sensitizers in DSC, and their influence on the solar cell performance, in comparison with standard N719 dye, at identical electrolyte conditions. Based on the separate identification of recombination resistance and the energy levels of titania by impedance spectroscopy, we determine the recombination rates and we also map the energies of the relevant components of the solar cells. An integration of the absorbance of the three sensitizers with the solar spectral distribution shows that electron injection should be similar for all the dyes (although 20% less in the case of the Pc with Zn center). However, since the LUMO level of these Pcs is low on the energy scale, it is necessary to use an electrolyte that keeps the conduction band of titania down to obtain some injection from the Pcs. Hence, the photovoltage is limited because the conduction band of titania is at a much lower position than in normal DSCs. We find that the conduction band of titania with the metal-free Pc is located on a similar energy level as for N719, and the recombination rate is not significantly different from the N719 cell. The main reason for the lower performance of this Pc is the lower injection than the ruthenium complex. In the case of the ZnPc, the conduction band is at higher energy than for N719. This allows that the Voc obtained in these two samples becomes nearly the same despite the lower electron injection in the Zn phthalocyanines.

Perylenediimides as non-fullerene acceptors in bulk-heterojunction solar cells (BHJSCs)

Perylenediimides are ideal candidates for the substitution of fullerene derivatives as electron acceptors in bulk heterojunction organic solar cells due to their extremely intense light absorbance, high electron mobility and excellent photochemical stability. In this review article, we briefly highlight the recent progress in highly efficient perylenediimide dyes characterized by a reduction of the cofacial stacking without an adverse impact in their charge-transport properties, which has led to PCEs higher than 8.4%.

Control of Photoinduced Electron Transfer in Zinc Phthalocyanine−Perylenediimide Dyad and Triad by the Magnesium Ion

Photoexcitation of a zinc phthalocyanine-perylenediimide (ZnPc-PDI) dyad and a bis(zinc phthalocyanine)-perylenediimide [(ZnPc) 2-PDI] triad results in formation of the triplet excited state of the PDI moiety without the fluorescence emission, whereas addition of Mg (2+) ions to the dyad and triad results in formation of long-lived charge-separated (CS) states (ZnPc (*+)-PDI (*-)/Mg (2+) and (ZnPc) 2 (*+)-PDI (*-)/Mg (2+)) in which PDI (*-) forms a complex with Mg (2+). Formation of the CS states in the presence of Mg (2+) was confirmed by appearance of the absorption bands due to ZnPc (*+) and PDI (*-)/Mg (2+) complex in the time-resolved transient absorption spectra of the dyad and triad. The one-electron reduction potential ( E red) of the PDI moiety in the presence of a metal ion is shifted to a positive direction due to the binding of Mg (2+) to PDI (*-), whereas the one-electron oxidation potential of the ZnPc moiety remains the same. The binding of Mg (2+) to PDI (*-) was confirmed by the ESR spectrum, which is different from that of PDI (*-) without Mg (2+). The energy of the CS state (ZnPc (*+)-PDI (*-)/Mg (2+)) is determined to be 0.79 eV, which becomes lower that of the triplet excited state (ZnPc- (3)PDI*: 1.07 eV). This is the reason why the long-lived CS states were attained in the presence of Mg (2+) instead of the triplet excited state of the PDI moiety.

Advances in phthalocyanine-sensitized solar cells (PcSSCs)

Phthalocyanines are among the most promising of efficient molecules for commercial application in dye sensitized solar cells due to their extremely intense red absorbance and excellent photochemical stability. In this highlight article, we briefly review the recent progress in phthalocyanine dyes making a tour around the different strategies employed to increase their photovoltaic performances taking into consideration the crucial factors governing the dye solar cell processes, and conclude with future perspectives.

Synthesis and Photoinduced Electron Transfer of Phthalocyanine―Perylenebisimide Pentameric Arrays

Zinc phthalocyanine-perylenebisimide pentameric arrays, ZnPc(PDI)(4) 1 and 2, have been synthesized. ZnPc(PDI)(4) 1 has no substituents in the PDI bay positions, while ZnPc(PDI)(4) 2 presents four phenoxy groups in the bay positions of each perylene. In both cases, the PDI moieties are directly connected to the ZnPc. As a consequence of aggregation, photoexcitation of 1 affords the intermolecular exciplex rather than the charge-separated state. In contrast to 1, photoexcitation of 2, which contains sterically demanding substituents in the PDI moieties, affords the charge-separated (CS) state, which was clearly detected by its transient absorption spectrum in femtosecond laser flash photolysis measurements. The CS lifetime was determined to be 26 ps. The addition of Mg(ClO(4))(2) to a benzonitrile solution of 2 and the photoexcitation affords the long-lived CS state with the lifetime of 480 micros, whereas no such long-lived CS state was formed in the case of 1 under such conditions. The remarkable elongation of the CS lifetime results from the strong binding of Mg(2+) to the PDI(*-) moiety in the CS state.

Red-light-emitting electrochemical cell using a polypyridyl iridium(III) polymer.

A deep-red phosphorescent ionic iridium(III) complex is prepared and incorporated into a polymer. Both the complex (1) and the polymer (2) were used as the single active material in solid-state light-emitting electrochemical cells (LECs). The devices built up using 1 and 2 emit in the deep-red region of the visible spectrum with CIE coordinates x = 0.710; y = 0.283 and x = 0.691; y = 0.289, respectively, making them one of the deepest-red emitting LECs reported. It is the first example of a polymeric LEC incorporating an ionic iridium complex, which exhibits increased stabilities compared with the device based on the small molecular weight complex.

Submillisecond-lived photoinduced charge separation in a fully conjugated phthalocyanine–perylenebenzimidazole dyad

A fully electronically conjugated phthalocyanine–perylenemonoimidebenzimidazole system, ZnPc–PMIBI 2, where the conjugation goes through the imide position of the perylene has been synthesized. The preparation was made possible by the condensation of a new unsymmetrically substituted diaminophthalocyanine, ZnPc(NH2)2, with a perylene monoanhydride monoimide. Both the experimental and the computational (DFT) results indicate that ZnPc–PMIBI exhibits significant intramolecular electronic interactions. The lifetime of the charge-separated (CS) state was extended to 0.26 ms, corresponding to the longest value ever reported for a covalent phthalocyanine–peryleneimide system in solution, and is attributed to the synergy of an extremely low CS energy, lower than the triplet energy of each chromophore, together with the coupling between both units, allowing fast charge separation.

Interfacial photo-induced charge transfer reactions in perylene imide dye sensitised solar cells

The interfacial electron transfer reactions of a perylene imide based dye sensitised solar cell (DSSC) with an efficiency of 3.15% under standard conditions have been characterised. The observed interfacial charge transfer dynamics under different sensitisation conditions have been correlated with the device efficiency.

Efficient electroluminescence from a perylenediimide fluorophore obtained from a simple solution processed OLED

Simple solution processed organic light emitting diodes are used to screen the performance of two types of highly efficient, narrow band red emitting fluorescent perylenediimides (PDIs). PDIs substituted at the diimide positions seem to form aggregates in the thin film architecture as evidenced by the shifted electroluminescent spectrum. When substituted on the bay position and when used both as the emitting and the electron transporting specie, bright electroluminescence with a narrow width around 610 nm reaching 500 cd m−2 at moderate voltages was observed, demonstrating the usefulness of these fluorophores for OLED applications.

Supramolecular interactions in dye-sensitised solar cells

A new Li+-coordinating perylene monoimide dye has been synthesised. The dye was used as a sensitiser in dye-sensitised solar cells (DSCs), and the effect of Li+ coordination on the device performance was analyzed using various spectroscopic techniques.