Ze-Min Ju

Picolinic acid as an efficient tridentate anchoring group adsorbing at Lewis acid sites and Brønsted acid sites of the TiO2 surface in dye-sensitized solar cells

We developed a novel efficient tridentate anchoring group which can anchor dyes onto the TiO2 surface via synchronously choosing Lewis acid sites and Bronsted acid sites of TiO2. For the purpose of comparing the traditional carboxylate anchoring group to picolinic acid, two new D–π–A porphyrin dyes (JA1 and JA2) differing only in anchoring groups have been synthesized and applied in dye-sensitized solar cells. Picolinic acid as an anchoring group in the dye JA2 not only extended the scope of the spectral response, but also improved the charge transport properties and enhanced the electron injection efficiency. The PCE of the JA1 based-device (carboxylate as the anchoring group) was 5.76%. The PCE of the JA2 based-device was 7.20%, which increased by 25% compared with JA1. The dye TTR2 was used as a cosensitizer; it would not just make up for the poor absorption of porphyrin dyes in the 470–550 nm range, but also would suppress the main dye aggregation and reduce the charge recombination rate. We found that the picolinic acid anchor was more suitable for the cosensitization system than the carboxylate anchor, for there was almost no competitive adsorption between JA2 and TTR2. The JA2 + TTR2 based-device showed the highest PCE of 8.98% under AM 1.5 G irradiation.

Promising alkoxy-wrapped porphyrins with novel push-pull moieties for dye-sensitized solar cells†‡

Dye-sensitized solar cells (DSSCs) have been considered as very promising third generation solar cells. Porphyrins are promising candidates as highly efficient sensitizers for DSSCs because of their superior light-harvesting ability in the visible region and their mimicking of photosynthesis. This paper focuses on the structure modification of porphyrin dyes for efficient DSSCs, which was based on a rational design using density functional theory (DFT) before the experiment. We synthesized and fully characterized four porphyrin dyes, named ZLD13, ZLD14, ZLD15 and ZLD16. On one hand, we used 5-ethynylthiophene-2-carboxylic acid to replace 4-ethynylbenzoic acid as the electron-withdrawing anchoring group for the first time. Property studies indicate that the aggregation of porphyrin molecules can be sufficiently suppressed via this modification. On the other hand, 4,4′-di(2-thienyl)triphenylamine moiety, which has been proved to be a electron donor group for triarylamine dyes in our previous reports, was introduced to porphyrin dyes, and energy conversion efficiencies (η) were improved by 76% (ZLD15 vs. ZLD13). After the two modifications, the energy conversion efficiency (η) of ZLD16 is comparable with an N719-based reference cell under the same conditions. Enhancement of photovoltaic performances from ZLD13 to ZLD16 is partly due to the decreased dark current and charge recombination rate.

Improvement of photovoltaic performance of DSSCs by modifying panchromatic zinc porphyrin dyes with heterocyclic units

A series of novel panchromatic D–D–π–A porphyrin dyes have been synthesized and applied to dye-sensitized solar cells. Three porphyrin dyes named JP1, JP2 and JP3, and their photophysical and electrochemical properties and photovoltaic performance were investigated and compared with reference dye YD2-O-C8. 2-Hexylthiophene chromophores were introduced to the donor groups, which extended the π-conjugation system effectively, then broadened the range of spectral response and improved the charge separation between the donor and acceptor moieties in the excited state. Moreover, this paper used thiophene-2-carboxylic acid instead of the traditional benzoic acid as an anchor group, which can make the molecules arrange to tilted orientation when adsorbed on the TiO2 surface, and this may effectively suppress the dye aggregation and prevent charge recombination. These dyes were clearly red-shifted when compared with dye YD2-O-C8. Especially for dye JP3, its maximum absorption peak was red shifted 20 nm with respect to dye YD2-O-C8 from 645 to 665 nm, and the molar extinction coefficient (6.2 × 104 M−1 cm−1) of JP3 is double that of YD2-O-C8 (3.1 × 104 M−1 cm−1) at the Q band. Dye JP3 extended the spectral response to 750 nm. The density functional theory (DFT) calculations indicated that the electronic density of the HOMO was increased by the additional thiophene units in these dyes when compared with YD2-O-C8, and this will improve the conjugation and electron donating ability. The power conversion efficiencies of JP1, JP2 and JP3 are 5.09%, 5.62% and 6.40% respectively under AM 1.5G irradiation, which are 74.5%, 82.3% and 93.7% of the YD2-O-C8 based-device (6.83%) under the same conditions.

Improvement of dye-sensitized solar cells performance through introducing different heterocyclic groups to triarylamine dyes

This paper focuses on the structure modification of triphenylamine dyes for efficient dye-sensitized solar cells (DSSCs). Three D–D–π–A dyes (TTR1–3), with triphenylamine moiety and its derivatives as the electron donor, thiophene ring as the π-bridge, and 2-(1,1-dicyanomethylene)rhodanine (DCRD) as the electron acceptor, were synthesized and fully characterized. Nanocrystalline TiO2-based DSSCs were fabricated using these dyes to investigate the effect of different donor groups introduced into triphenylamine on their photovoltaic performances. The overall power conversion efficiency (PCE) of DSSCs based on TTR1–3 with chenodeoxycholic acid (CDCA) coadsorbant are 5.20%, 5.71% and 6.30%, respectively, compared to 6.62% achieved with N719. Introduced heterocyclic group with alkyl lain into triphenylamine decreased dye absorbed amount but significantly improved the value of the open circuit voltage (Voc) and the short-circuit photocurrent (Jsc), which result from the fact that they can effectively suppress the charge recombination and prevent aggregation between adjacent molecules on TiO2. We also researched the effect of sensitization for single dyes on their photovoltaic performances. The PCEs of DSSCs soaked for 32 h increase slightly compared to those of DSSCs soaked for 16 h, which result from the adsorption quantity on the TiO2 surface. We found that, with soaking twice in 32 h, the Jsc and Voc were both obviously improved compared with soaking once in 32 h. These results provide a new approach for enhancing the photovoltaic performances of DSSCs based on single dye.

Syntheses, characterization, and properties of five coordination compounds based on the ligand tetrakis(4-pyridyloxymethylene)methane

A nonplanar tetrahedral pyridine ligand has been synthesized and applied to assemble five metal–organic frameworks (MOFs) with novel structural features under solvothermal conditions, namely, {[Cu2(TPOM)(adi)2](H2O)4}n (1), {[Zn2(TPOM)(glu)2](H2O)8}n (2), {[Cd2(TPOM)(1,4-chdc)2(H2O)4](H2O)4}n (3), {[Ni(TPOM)(suc)(H2O)2](H2O)2}n (4), and {[Zn2(TPOM)(1,4-chdc)(NO3)2](H2O)2}n (5) (TPOM = tetrakis(4-pyridyloxymethylene)methane, adi = adipic acid, glu = glutaric acid, chdc = 1,4-cyclohexanedicarboxylic acid, suc = succinic acid). These compounds were characterized by elemental analyses, IR spectroscopy and X-ray single-crystal diffraction. Compounds 1 and 3 reveal 3-fold interpenetrating 3D frameworks with sqc969 and new topologies, while compound 2 possesses a 2-fold interpenetrating 3D framework with qtz topology. Compound 4 exhibits a non-interpenetrating 3D structure with the extension of the cage structure, in which there are only two pyridine nitrogen atoms in TPOM involved in the coordination. It is different from compounds 1–3, which may take distinct coordination modes under different conditions. In compound 5, the coordination mode of TPOM is also different from those of compounds 1–3; it is a 2D structure with a 2-fold interpenetrating framework.