3.1 Synthesis and Characterization of Polymeric Metal Complexes.
It can be seen from Scheme 1, -Pyridin-3-yl-1H-pyrrole-2-carbaldehyde (1) was attained by Ma Dawei carbon nitrogen coupling reaction of 3-Bromo-pyridine and pyrrole-3-carboxaldehyde, 3-(2-Vinyl-pyrrol-1-yl)-pyridine (2) was prepared from 1-Pyridin-3-yl-1H-pyrrole-2-carbaldehyde by addition reaction of aldehyde group, 5-Vinyl-quinolin-8-ol (3) was synthesized by Reimer-Tiemann reaction of 8-hydroxyqzlinoline. The synthesis of 2-cyano-3-(8-hydroxy-quinolin-5-yl)acrylic acid (4) was gained by 8-hydroxyqzlinoline and cyanoacetic acid. The NMR spectra of compound 1, 2, 3 and 4, are shown in Fig. S1, Fig. S2, Fig. S3 and Fig. S4, respectively. The four target polymeric metal complexes were obtained by Yamamoto coupling reaction of metal complexes PY-Cd, PY-Zn, PY-Cu and PY-Ni with BDTT.
Table 1 summarizes the results of elemental analysis, FT-IR spectroscopy and gel permeation chromatography (GPC) for the four polymer metal complexes.The data of GPC show that the number average molecular weights (Mn) of BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY-Cu and BDTT-PY-Ni are 11.79, 10.11, 12.62 and 10.05 kg mol−1, with PDI values ranging from 1.84 to 2.11 and N values of 8, 9, 10 and 8 for the four dyes, respectively.These results, together with the elemental analysis, reflect that the four target polymers have been successfully synthesized.
The FT-IR spectra of metal complexes (PY-Cd, PY-Zn, PY-Cu and PY-Ni) and the four polymeric metal complexes (BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY-Cu and BDTT-PY-Ni) are shown in Fig. S5 and Fig. S6.
According to Fig. S5, the peak around 3420 cm−1 is the hydroxyl peak, which is the stretching vibration peak of the hydroxyl group; The absorption peak around 2210 cm−1 is the C≡N absorption peak on the auxiliary ligand. The absorption peak for C=C is around 1610 cm−1 and the absorption peak for C=N is around 1565 cm−1. The absorption peaks for the four metal complexes C-O-M occur at around 1110 cm−1 and N-M at around 505 cm−1. Notably, the absorption peak of PY-Cu at 1686 cm−1 is generated by the stretching vibration of C=O. However, the other complexes do not have significant absorption peaks in the vicinity, which is due to the strong absorption peaks in the vicinity covering the stretching vibration peaks of C=O.
As seen in Fig. S6, the C-O-M absorption peaks of BDTT-PY-M appear at 1103 cm−1-1112 cm−1 and N-M absorption peaks appear at 493 cm−1-515 cm−1. Compared with the corresponding complexes, the absorption peaks of the above functional groups are having a certain redshifted, which attributed to expanding the conjugate system by introducing the BDTT. At about 2920 cm−1 and 2850 cm−1, the C-H stretching vibration peak of the alkyl chain appeared, which further indicated that the donor BDTT had been successfully introduced into the polymer. Combined with the results of gel permeation chromatography and elemental analysis, the four polymeric metal complexes have been synthesized successfully.
3.2 Photophysical Properties of Polymeric Metal Complexes.
According to the UV-Vis spectrum of PY-M and BDTT-PY-M (Fig. 1), the maximum absorption wavelengths of PY-Cd, PY-Zn, PY-Cu and PY-Ni occur at 373 nm, 362 nm, 348 nm and 342 nm respectively, this may be due to the stronger electronic interactions between the larger radius coordination metals and the same
ligands. The corresponding polymeric metal complexes (BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY-Cu and BDTT-PY-Ni) have two absorption peaks. The polymer metal complexes have a certain red shift in the 350-400 nm range compared to the corresponding complexes. This is may be due to BDTT of strong electronic capability and good planarity was introduced. Notably, the polymeric metal complexes have an extra absorption peak with a maximum absorption wavelength of 506 nm, 488 nm, 478 nm and 464 nm respectively, which is due to the charge transfer between the
Table 1
Molecular weights and thermal properties of four polymeric metal complexes BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY-Cu and BDTT-PY-Ni
polymer | Mna[×103g mol−1] | Mwa[×103g mol−1] | PDI | N | Tgb[℃] | Tdc[℃] |
BDTT-PY-Cd | 11.79 | 22.75 | 1.93 | 8 | 163 | 314 |
BDTT-PY-Zn | 10.11 | 20.62 | 2.04 | 9 | 154 | 296 |
BDTT-PY-Cu | 12.62 | 26.62 | 2.11 | 10 | 149 | 287 |
BDTT-PY-Ni | 10.05 | 18.49 | 1.84 | 8 | 142 | 268 |
a Determined by gel permeation chromatography with polystyrene as standard. |
b Determined by DSC with a heating rate of 20 ℃/min under nitrogen. |
c The temperature at 5% weight loss under nitrogen. |
donor BDTT and the complex in the molecule. By comparing the absorption spectra between different polymeric metal complexes, it was found that the maximum absorption wavelength of the polymeric with large coordination metal radius has a certain degree of red shift relative to the polymeric with small coordination metal radius. This may be due to the stronger the feedback π bond formed by the coordination metal with a large radius and the better plane configuration.
According to Table 1, the absorption coefficients of the four polymeric metal complexes were all above 18000 L·mol−1cm−1 for the maximum absorption wavelength. Among them, BDTT-PY-Cd has the largest absorption of 23,357 L-mol-1cm-1, which reflects the high absorption coefficient of the polymeric metal complexes. In summary, the molecular design of polymeric metal complexes can broaden the spectral absorption range of this class of dyes by introducing ligand metals with strong coordination ability.
3.3 Electrochemical Properties of Polymeric Metal Complexes.
The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the dye sensitiser can estimate the cyclic voltammogram (CV) for the onset of oxidation (Eox) and reduction potential (Ered). A suitable energy level can be effective in improving the efficiency of the dye. The HOMO and LUMO energy levels and the electrochemical band gap (Eg) of the polymeric metal complexes can be calculated according to reference [37].
The cyclic voltammetry curves of the four polymers and ferrocene, Fig. 2 shows the HOMO and LUMO energy levels of BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY-Cu and BDTT-PY-Ni and the data is collected in Table 2. The Eox of four polymers are 0.996, 1.011, 1.034 and 1.135 V, respectively; and their HOMO are respectively -5.336, -5.351, -5.374 and -5.475 eV, which was significantly lower than the redox potential (-4.83 eV) [38] of I−/I3−, indicating that the polymeric metal complexes could be effectively regenerated. The Ered of four polymers are -1.049, -1.055, -1.071 and -1.026 V respectively; and their LUMO energy levels are -3.291, -3.285, -3.269 and -3.314 eV respectively, which are higher than the titanium dioxide conduction band energy levels (-4.26 eV) by about 0.97eV, indicating that there is sufficient driving force to make electron injection into the semiconductor. The electrochemical band gaps (Eg) of BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY-Cu and BDTT-PY-Ni were 2.045, 2.066, 2.105 and 2.161 eV respectively, have smaller energy level gaps and can achieve light absorption in a longer wavelength range. The energy level gap of BDTT-PY-Cd is the smallest, which can explain why the absorption range of UV-Vis absorption spectrum is the largest.
Table 2
Optical and electrochemical property of four polymeric metal complexes BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY-Cu and BDTT-PY-Ni.
Polymer | λa,max (nm) | εmax (L mol−1cm−1) | Ered (V) | Eox (V) | HOMO (eV) | LUMO (eV) | Eg (eV) |
BDTT-PY-Cd | 506 | 23357 | -1.049 | 0.996 | -5.336 | -3.291 | 2.045 |
BDTT-PY-Zn | 488 | 22072 | -1.055 | 1.011 | -5.351 | -3.285 | 2.066 |
BDTT-PY-Cu | 478 | 20069 | -1.071 | 1.034 | -5.374 | -3.269 | 2.105 |
BDTT-PY-Ni | 464 | 18541 | -1.026 | 1.135 | -5.475 | -3.314 | 2.161 |
3.4 Thermal Propertiesof Polymeric Metal Complexes.
The actual working conditions of dye sensitisers are generally at high temperatures, so good thermal stability of the sensitiser is required for practical use. The thermal properties of the four polymeric metal complexes were measured using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) at a
heating rate of 20°C/min under a nitrogen atmosphere. The four polymeric metal complexes (BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY -Cu and BDTT-PY-Ni) show outstanding thermal stability, as shown in Fig. 3, with data collected in Table 1, and their onset decomposition temperatures (Td) of 314, 296, 287 and 268°C, respectively. In contrast, their glass transition temperatures (Tg) were 163, 154, 149 and 142°C, respectively. In addition, since no fixed melting point was detected, it was speculated that the four polymers were amorphous structures. These data show that these four polymers are good enough for DSSC applications and that they have excellent thermal stability.
3.5 Photovoltaic Properties of Polymeric Metal Complexes.
The photovoltaic performance tests in DSSCs are mainly used to evaluate the potential of dye sensitisers for application into DSSCs. Photovoltaic devices in DSSCs consisting of polymers were fabricated according to standard procedures and their performance parameters were obtained by testing at standard light intensities (AM 1.5G, 100 mW/cm2). Fig. 4 shows the photocurrent-photovoltage (J-V) curves for the polymeric metal complexes (BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY-Cu and BDTT-PY-Ni). The test data related to open circuit voltage (Voc), short circuit current density (Jsc), fill factor (ff) and incident photon-to-current conversion efficiency (IPCE) are shown in Table 3 and Fig. 5.
According to Fig. 4, the short-circuit current densities (Jsc) of the four polymeric metal complexes were BDTT-PY-Cd (17.45 mA/cm2), BDTT-PY-Zn (14.75 mA/cm2), BDTT-PY-Cu (13.94 mA/cm2) and BDTT-PY-Ni (12.00 mA/cm2), respectively. This is consistent with the UV-vis absorption test results, indicating that the better the
absorption spectrum, the higher the photocurrent generated. The open-circuit voltage (Voc) of the four polymeric metal complexes were 0.78, 0.76, 0.74 and 0.75 V, respectively, with corresponding fill factors (ff) of 70.38, 70.43, 71.70, and 66.91%, respectively. The above experimental data indicated that Jsc is the most important factor affecting the efficiency of the polymeric. The BDTT-PY-Cd has the highest
PCE with 9.73 %, which is to be attribute to a larger radius, better planarity, and stronger intramolecular electron transfer ability of the polymeric. As shown in Fig. 5, the IPCE curve of the four polymeric metal complexes is over 65 %, among which BDTT-PY-Zn and BDTT-PY-Cd exceed 70 %. The IPCE curve is basically consistent with the UV-visible absorption curve, which further reflects the photoelectric performance of the polymeric metal complex.
Table 3
Photovoltaic parameters of devices based on four polymeric metal complexes BDTT-PY-Cd, BDTT-PY-Zn, BDTT-PY-Cu and BDTT-PY-Ni in DSSCs at full sunlight (AM 1.5 G, 100 mW cm−2)
Polymer | Solvent | Jsc(mA cm−2) | Voc(V) | ff(%) | η (%) |
BDTT-PY-Cd | DMF | 17.45 | 0.78 | 70.38 | 9.73 |
BDTT-PY-Zn | DMF | 14.75 | 0.76 | 70.43 | 8.02 |
BDTT-PY-Cu | DMF | 13.94 | 0.74 | 71.70 | 6.82 |
BDTT-PY-Ni | DMF | 12.00 | 0.75 | 66.91 | 6.12 |