Galateia E. Zervaki

Efficient sensitization of dye-sensitized solar cells by novel triazine-bridged porphyrin-porphyrin dyads.

Two novel porphyrin-porphyrin dyads, the symmetrical Zn[Porph]-Zn[Porph] (2) and unsymmetrical Zn[Porph]-H2[Porph] (4), where Zn[Porph] and H2[Porph] are the metalated and free-base forms of 5-(4-aminophenyl)-10,15,20-triphenylporphyrin, respectively, in which two porphyrin units are covalently bridged by 1,3,5-triazine, have been synthesized via the stepwise amination of cyanuric chloride. The dyads are also functionalized by a terminal carboxylic acid group of a glycine moiety attached to the triazine group. Photophysical measurements of 2 and 4 showed broaden and strengthened absorptions in their visible spectra, while electrochemistry experiments and density functional theory calculations revealed negligible interaction between the two porphyrin units in their ground states but appropriate frontier orbital energy levels for use in dye-sensitized solar cells (DSSCs). The 2- and 4-based solar cells have been fabricated and found to exhibit power conversion efficiencies (PCEs) of 3.61% and 4.46%, respectively (under an illumination intensity of 100 mW/cm(2) with TiO2 films of 10 μm thickness). The higher PCE value of the 4-based DSSC, as revealed by photovoltaic measurements (J-V curves) and incident photon-to-current conversion efficiency (IPCE) spectra of the two cells, is attributed to its enhanced short-circuit current (J(sc)) under illumination, high open-circuit voltage (V(oc)), and fill factor (FF) values. Electrochemical impedance spectra demonstrated shorter electron-transport time (τd), longer electron lifetime (τe), and high charge recombination resistance for the 4-based cell, as well as larger dye loading onto TiO2.

A mono(carboxy)porphyrin-triazine-(bodipy)2 triad as a donor for bulk heterojunction organic solar cells

In the present article bulk heterojunction (BHJ) solution processed organic solar cells have been prepared using a mono(carboxy)porphyrin-triazine-(bodipy)2 triad (PorCOOH)(BDP)2 as a donor and ([6,6]-phenyl C71 butyric acid methyl ester) (PC71BM) as an acceptor. This donor–acceptor system aims to increase the light capturing process efficiency of the device. The solution processed BHJ organic solar cell with an optimized weight ratio of 1 : 1 (PorCOOH)(BDP)2 : PC71BM processed with THF as a solvent showed an overall power conversion efficiency (PCE) of 3.48% with short circuit current Jsc = 8.04 mA cm−2, open circuit voltage Voc = 0.94 V and fill factor FF = 0.46. The relatively high value of Voc was attributed to the deeper highest occupied molecular orbital energy level of (PorCOOH)(BDP)2. When the active layer of the solar cell was processed using a mixture of 4% v/v of pyridine in THF solvent, it achieved a PCE value of 5.29% with Jsc = 10.48 mA cm−2, Voc = 0.90 V and FF = 0.56. This was ascribed to the enhancement of both the Jsc and the FF values. The higher value of Jsc is explained by the increased absorption profile of the blend, the stronger incident photon to current efficiency (IPCE) response and the higher crystallinity of the active layer, induced by the solvent additive while the enhancement of FF may be due to the better charge transport capability and the charge collection efficiency in the later device.

Dye-sensitized solar cells based on triazine-linked porphyrin dyads containing one or two carboxylic acid anchoring groups

Two porphyrin dyads (5a and 5b) consisting of two meso aryl-substituted zinc-metallated porphyrin units, which are covalently linked through peripheral aryl-amino groups by a 1,3,5-triazine moiety, have been synthesized via stepwise substitution reactions of cyanuric chloride. Both porphyrin dyads 5a and 5b contain a carboxyphenyl meso substituent, while in the former the triazine ring is functionalized by a glycine group, and in the latter by a piperidine group. Photophysical and electrochemical studies of the two dyads, in combination with DFT theoretical calculations, suggest that there is negligible electronic interaction between the porphyrin units in the dyads’ ground states, but the frontier orbital energy levels are suitable for use as sensitizers in dye-sensitized solar cells (DSSCs). Solar cells sensitized by dyads 5a and 5b have been fabricated, and they were found to exhibit power conversion efficiencies (PCEs) of 5.28 and 3.50%, respectively, under illumination (AM1.5, 100 mW cm−2), and TiO2 films of 10 μm thickness. As revealed by the photovoltaic measurements (J–V curves) and the incident photon to current conversion efficiency (IPCE) spectra of the two solar cells, the higher PCE value of the 5a based solar cell is attributed to its enhanced short circuit current (Jsc), higher open circuit voltage (Voc), and fill factor (FF) values. Also, the 5a sensitized solar cell exhibits a larger dye loading value. This is attributed to the presence of two carboxylic acid anchoring groups in its molecular structure (compared to one carboxylic acid and one hindered piperidine-type anchoring group of 5b), which result in a more effective binding capacity onto the TiO2 film. Furthermore, electrochemical impedance spectra (EIS) demonstrated that the 5a based solar cell exhibits longer electron lifetime (τe) and more effective suppression of the recombination reactions of the injected electrons and the electrolyte.

New solution processed bulk-heterojunction organic solar cells based on a triazine-bridged porphyrin dyad as electron donor

An unsymmetrical porphyrin dyad (ZnP)-[triazine-Npip]-(ZnPCOOH) or P-tNp-P′ consisting of two zinc-metallated porphyrin units covalently linked through their peripheral aryl-amino groups to a central triazine group, to which an N-piperidine group is also attached, has been used in combination with PC71BM ([6,6]-phenyl C71 butyric acid methyl ester) as electron donor and acceptor, respectively, for the active layer of solution-processed bulk hetero-junction (BHJ) organic solar cells. The photophysical properties of P-tNp-P′ and PC71BM blend films, as well as cyclic voltammetry measurements of the porphyrin dyad, suggest that P-tNp-P′ can effectively harvest photons and transfer electrons to PC71BM. The BHJ organic solar cell based on the P-tNp-P′ : PC71BM active layer blend in 1 : 1 weight ratio processed from THF resulted in a power conversion efficiency (PCE) of 2.91%. A significant improvement of the overall photovoltaic efficiency was achieved up to 4.16% when the active layer blend was processed from a 3% v/v mixture of pyridine in THF. This was ascribed to an enhancement of the short circuit current Jsc of the solar cell, which is related to the different surface morphology of the P-tNp-P′ : PC71BM active layer blend upon addition of pyridine. The pyridine-modified active layer exhibits a higher degree of crystallinity, as confirmed by the X-ray diffraction pattern and AFM images of the corresponding film, which results in an increase of exciton dissociation efficiency and more balanced charge transport.

Donor-π-acceptor, triazine-linked porphyrin dyads as sensitizers for dye-sensitized solar cells

Two porphyrin dyads with the donor-π-acceptor molecular architecture, namely (ZnP)-[triazine-gly]-(H2PCOOH) and (ZnP)-[triazine-Npip]-(H2PCOOH), which consist of a zinc-metalated porphyrin unit and a free-base porphyrin unit covalently linked at their peripheries to a central triazine group, substituted either by a glycine in the former or a N-piperidine group in the latter, have been synthesized via consecutive amination substitution reactions of cyanuric chloride. The UV-vis absorption spectra and cyclic-voltammetry measurements of the two dyads, as well as theoretical calculations based on Density Functional Theory, suggest that they have suitable frontier orbital energy levels for use as sensitizers in dye-sensitized solar cells. Dye-sensitized solar cells based on (ZnP)-[triazine-gly]-(H2PCOOH) and (ZnP)-[triazine-Npip]-(H2PCOOH) have been fabricated, and they were found to exhibit power conversion efficiency values of 5.44 and 4.15%, respectively. Photovoltaic measurements (J–V curves) and incident photon to current conversion efficiency spectra of the two solar cells suggest that the higher power conversion efficiency value of the former solar cell is a result of its enhanced short circuit current, open circuit voltage, and fill factor values, as well as higher dye loading. This is ascribed to the existence of two carboxylic acid anchoring groups in (ZnP)-[triazine-gly]-(H2PCOOH), compared to one carboxylic acid group in (ZnP)-[triazine-Npip]-(H2PCOOH), which leads to a more effective binding onto the TiO2 photoanode. Electrochemical impedance spectra show evidence that the (ZnP)-[triazine-gly]-(H2PCOOH) based solar cell exhibits a longer electron lifetime and more effective suppression of charge recombination reactions between the injected electrons and electrolyte.