Chin-Li
Wang
a,
Min
Zhang
b,
Yu-Hsin
Hsiao
a,
Chuan-Kai
Tseng
a,
Chia-Lin
Liu
a,
Mingfei
Xu
b,
Peng
Wang
*b and
Ching-Yao
Lin
*a
aDepartment of Applied Chemistry, National Chi Nan University, Puli, Nantou Hsien 54561, Taiwan. E-mail: cyl@ncnu.edu.tw; Fax: +886-49-2917956; Tel: +886-49-2910960 ext. 4152
bState Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, 130022, China. E-mail: peng.wang@ciac.ac.cn
First published on 9th November 2015
Three porphyrin dyes with different electron-donating groups were prepared to compare their photovoltaic performance in dye-sensitized solar cells. LWP12 is based on LD31 which has a complicated, piece-together electron-donating group, whereas LWP13 and LWP14 have merged substituents with electron-donating and absorption-broadening abilities. The results in this work suggest that consolidated anthryl donors may be used to design highly efficient photosensitizers.
Broader contextWith a great variety of colorful photosensitizers to choose from, a dye-sensitized solar cell (DSSC) has been considered as one of the possible means for solar energy conversion. In order to improve the photovoltaic performance of DSSCs, dyes should have broad light-harvesting ability to absorb a wide spectrum of solar energy and a push–pull structure to facilitate charge transfer processes efficiently. To accomplish both, however, chemical structures of many efficient porphyrin dyes have become very complicated. In this work, we aim to simplify the structures of porphyrin dyes by using consolidated anthryl donors to provide both absorption-broadening as well as electron-donating effects. Simplicity is not simple. Only one of the porphyrin dyes under investigation achieves overall efficiency greater than 10%. |
In order to achieve higher photovoltaic performance, dyes should have broad light-harvesting ability and a push–pull (or donor/acceptor) structure. To accomplish both, however, many of the efficient porphyrin dyes have complicated chemical structures. For example, porphyrin SM315 uses a bulky bis(2′,4′-bis(hexyloxy)-[1′-biphenyl]-4-yl)amine donor,9 WW-6 employs an N-annulated perylene substituent,13 LD31 attaches a dioctylaminophenyl-ethynyl-anthryl group,27 and LDD1 and YDD6 consist of porphyrin dimers.28,29 In this work, we aim to explore the possibility of using substituents with simpler chemical structures to provide both electron-donating and absorption-broadening abilities.
Chart 1 depicts chemical structures of the porphyrins under investigation (denoted as LWP12, LWP13, and LWP14). The more complicated LWP12 is based on LD31 with an additional benzothiadiazole group. In contrast, LWP13 and LWP14 utilize consolidated donors. A dioctylamino group is directly attached to anthracene for LWP13 whereas a diarylamino group is put onto the anthryl group for LWP14. All three donor groups are linked to a meso-position of the porphyrin core via an ethynyl bridge. For the anchoring groups, a benzothiadiazole is placed between the benzoic acid and the ethynyl bridge for a stronger push–pull effect.9,10 For diarylamino-anthracene, it is worth mentioning that a similar donor bearing two methoxy groups was used in a metal-free co-sensitizer recently.18 However, it has been shown that attaching longer alkyl chains at the phenyl groups of diarylamine would be a better choice than methoxy groups (i.e. YD2 vs. YD3).15
Although the chemical structures of LWP13 and LWP14 are simpler than that of LWP12, they should still offer both electron-donating as well as absorption-broadening effects owing to the dialkylamino/diarylamino and the anthryl groups, respectively. By comparing these three porphyrins, we hope to achieve high PCE of the DSSC with a simpler dye.
Dye | Absorption/nm (log![]() |
Emission/nm | E 1/2/V vs. SCE | S0/S+/eV | S*/S+/eV | |
---|---|---|---|---|---|---|
Ox(1) | Red(1) | |||||
a Potential determined by differential pulse voltammetry due to overlapped oxidative waves. b Taken from ref. 25. c Taken from ref. 27. | ||||||
LWP12 | 490(5.06), 518(4.98), 709(5.05) | 736 | +0.79a | −1.14 | −5.53 | −3.81 |
LWP13 | 484(5.17), 694(4.91) | 720 | +0.64a | −1.19 | −5.38 | −3.63 |
LWP14 | 477(5.14), 696(4.98) | 722 | +0.83a | −1.17 | −5.57 | −3.82 |
LD14b | 459(5.40), 667(4.82) | 682 | +0.74 | −1.32 | −5.48 | −3.65 |
LD31c | 449(5.10), 521(4.92), 691(4.94) | 713 | +0.86 | −1.15 | −5.60 | −3.84 |
Fig. 2 compares the fluorescence spectra of the LWP12–14 porphyrins. The emission maxima are listed in Table 1. The fluorescence bands of LWP12–14 strongly resemble the mirror images of the corresponding Q bands, implying that the spacing between vibrational energy levels is similar for the ground and excited states, and the same transitions are favourable for both absorption and emission. Although the emission maximum wavelengths are similar, the trend is the same as that of the Q bands: LWP12 > LWP14 > LWP13. The Stokes shifts of LWP12, LWP13, and LWP14 are calculated to be 482, 522, and 483 cm−1, respectively. In comparison with a known dye (ESI†), fluorescence quantum yields are estimated to be 3.3%, 3.4% and 3.9% for LWP12, LWP13, and LWP14, respectively. These values are comparable to a very common porphyrin, 5,10,15,20-tetraphenylporphinato zinc(II) or ZnTPP (3.3%). Similar quantum yields and Stokes shifts among the LWP porphyrins may be related to the similarity in their chemical structures (from benzoic acid to anthracene).
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Fig. 2 Fluorescence spectra of LWP12–14 (2.0 × 10−6 M of each porphyrin in THF, excited wavelengths per nm: LWP12, 490; LWP13, 484; LWP14, 477). |
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Fig. 3 Cyclic voltammograms (bold lines) of LWP12, LWP13 and LWP14 in THF/TBAP. For the oxidation, differential pulse voltammograms were also measured (thin lines) to resolve the overlapped waves. |
In general, the first porphyrin-ring reduction potentials of LWP12–14 are similar to that of LD31, but all positively shifted from that of LD14. This can be attributed to LWPs having more conjugated double bonds than LD14. On the other hand, the oxidation waves exhibit overlapped waves and stronger currents. Differential pulse voltammetry (DPV) was therefore employed to resolve the overlapped signals. As shown in Table 1 and Fig. 3, the first oxidation potentials of LWP12, 13, and 14 were found to be +0.79, +0.64, and +0.83 V vs. SCE, respectively. These potentials were used to estimate the S0/S+ energy levels (vide infra). The oxidation potentials of the porphyrins show a trend of LWP14 > LWP12 > LWP13. LWP13 being easier to oxidize than LWP12 implies that the consolidated donor of LWP13 is more effective than the complicated donor of LWP12. Also bearing a consolidated donor group, however, LWP14 is more difficult to oxidize than LWP12. This might be related to the di-arylamine group. Due to its bulkiness and the sp3-nitrogen atom, it would be difficult for the di-arylamine to be co-planar with the anthracene. As a result, π-conjugation might be hampered. This suggestion is consistent with the DFT-optimized structure of LWP14 (Fig. 4).
Interestingly, the reduction potentials of the LWP porphyrins show a different trend of LWP13 > LWP14 > LWP12. This phenomenon may be attributed to the different patterns of the HOMOs and the LUMOs.30
Fig. 4 shows the frontier orbital patterns of LWP12–14 calculated by density-functional theory (DFT)33 at the B3LYP/LanL2DZ level. The MOs of LWP12–14 are consistent with the Gouterman's four-orbital model with deviation.30 For example, the HOMO-1 and LUMO+1 patterns resemble those of the a2u orbital and one of the eg orbitals in Gouterman's model. Because the donor and acceptor groups are attached to the porphyrin core structure via ethynyl bridges, delocalization of the MO patterns from the porphyrin centre to the substituents can be expected. In contrast, the patterns of the HOMOs and LUMOs concentrate at the electron-donating and electron-withdrawing groups, respectively, with a small contribution from the porphyrin core. Since the acceptor and donor groups dominate the LUMOs and HOMOs, respectively, it is not surprising to observe different trends in the reduction and oxidation potentials. In addition, the concentration of the HOMO and LUMO patterns at the donor and acceptor sides, respectively, suggests a stronger push–pull tendency. This is a welcome merit for n-type DSSCs. This suggestion is consistent with the calculated dipole moment of LWP12 (with benzothiadiazole, 11.90 Debye) being greater than that of LD31 (without benzothiadiazole, 10.85 Debye).
Fig. 5 illustrates the energy-level diagram of LWP12–14, upon comparing the ground-to-oxidized states (S0/S+), the first singlet excited-to-oxidized states (S*/S+) of each dye, the conduction bands (CB) of TiO2, and the redox energy of the electrolyte. The first oxidation potentials were used to estimate the S0/S+ levels. The zero–zero excitation energies (E0–0) obtained from the intersection of the corresponding normalized absorption and emission spectra were used to estimate the energy gaps between the S*/S+ and the S0/S+ levels.1 As suggested in the figure, the S*/S+ levels of LWP12–14 are considerably higher than the conduction bands of TiO2 and the S0/S+ levels are noticeably lower than the redox energy of the electrolyte. Therefore, LWP12–14 should all be capable of injecting electrons into the CB of TiO2 upon excitation and the resulting cations should be efficiently regenerated by the electrolyte.
To unveil the obvious difference in the maxima of EQEs, the yields of electron injection (ϕei) from the excited states of dye molecules into the conduction band of TiO2 were estimated by resorting to the time correlated single photon counting (TCSPC) technique34 and employing the corresponding dye-grafted alumina films as control. As depicted in Fig. S10 (ESI†), due to the absence of favorable energy offsets for dye-grafted alumina films, the photoluminescence (PL) decays (blue lines) arise from the radiative and radiationless deactivations of excited-state dye molecules. Upon switching to the titania samples, considerable PL quenching is monitored owing to the occurrence of expeditious electron injection. By integrating the areas of PL traces of dye-coated titania and alumina films, the ϕei value for LWP12, LWP13, and LWP14 can be derived, being 81%, 79%, and 84%, respectively. Thereby we can conclude that the ϕei value is not the key factor controlling EQE summits.
Moreover, a tris(4,4′-dimethyl-2,2′-bipyridine)cobalt (Co-Me2bpy) electrolyte with a more negative redox potential was used to inspect the influence of hole injection efficiency (ϕhi) on the EQE summit. As shown in Fig. S11 (ESI†), the employment of the Co-Me2bpy electrolyte just slightly enhances the EQE summits of cells with LWP12 and LWP13, suggesting that the ϕhi value is also not the controlling factor of the EQE summits. Thereby, we speculate that the low EQE summits of LWP12 and LWP13 cells should be ascribed to the inferior charge collection yields, which can be originated from the swift interfacial charge recombination as observed in the following electrical analysis.
The current density–voltage (J–V) characteristics (Fig. 7) were further examined under the irradiance of 100 mW cm−2, simulated AM1.5 sunlight. The detailed device parameters are compiled in Table 2. The cell with LWP14 exhibits a considerably larger short-circuit photocurrent density (JSC) of 17.22 mA cm−2 than that of 12.07 and 10.06 mA cm−2 for LWP12 and LWP13, respectively, which is in good agreement with the preceding EQE measurements. Moreover, with respect to the low open-circuit photovoltages (VOC) of LWP12 (0.731 V) and LWP13 (0.706 V), an obviously higher VOC of 805 mV was achieved with the LWP14 dye, contributing to a higher η of 10.3%. The J–V curves under various metal-mesh attenuated lights were also measured and plotted VOC against JSC (Fig. 8). It is easy to perceive that LWP14 dye features an evidently improved VOC at a given JSC with respect to that of LWP12 and LWP13.
Dye |
J
EQESC![]() |
J SC (mA cm−2) | V OC (mV) | FF (%) | η (%) |
---|---|---|---|---|---|
a J EQESC is derived via wavelength integration of the product of the standard AM1.5 emission spectrum (ASTM G173-03) and the EQEs measured at the short-circuit. The validity of measured photovoltaic parameters is evaluated by comparing the calculated JEQESC with the experimentally measured JSC. | |||||
LWP12 | 12.16 ± 0.09 | 12.07 ± 0.08 | 731 ± 3 | 73.8 ± 0.5 | 6.5 ± 0.08 |
LWP13 | 10.44 ± 0.07 | 10.06 ± 0.06 | 706 ± 5 | 78.0 ± 0.4 | 5.5 ± 0.10 |
LWP14 | 16.79 ± 0.06 | 17.22 ± 0.06 | 805 ± 4 | 74.1 ± 0.5 | 10.3 ± 0.07 |
![]() | ||
Fig. 8 Plots of open-circuit photovoltage (VOC) vs. short-circuit photocurrent density (JSC). The solid lines are a guide to the eye. |
Note that for a fixed electrolyte, the enhancement of VOC stems from the up-shift of the electron quasi-Fermi-level (EF,n) of TiO2, which is positively correlated with the energy level of the conduction-band edge (Ec) of TiO2 and/or the electron density of TiO2. Therefore, to dissect the origins of the dye structure correlated VOC difference, the charge extraction (CE)35 and transient photovoltage decay (TPD)36 measurements were further carried out. As shown in Fig. 9a, more charges stored in the mesoporous titania film (QCE) can be observed for the LWP14 cell with respect to the LWP12 and LWP13 cells at a certain VOC, indicating a relatively lower Ec position of titania for LWP14. However, the cell with LWP14 dye exhibits over two orders of magnitude longer half lifetime (tTPD1/2) for photo-injected electrons at a given QCE compared with the LWP12 and LWP13 cells as presented in Fig. 9b, which outreaches the adverse effect of a lower Ec, explaining its superior photovoltage at a given Jsc (Fig. 8). The loading amount (cm) of dye molecules on TiO2 was measured by recording the small but reliable light-absorption change of a dyeing solution at a certain volume, being 0.48 × 10−8 mol cm−2 μm−1 for LWP12, 0.99 × 10−8 mol cm−2 μm−1 for LWP13, and 0.92 × 10−8 mol cm−2 μm−1 for LWP14. The slower charge recombination for LWP14 can be ascribed to a higher cm and an improved steric hindrance of the bis(4-octylphenyl)amino group to prevent the cobalt(III) ions to be in close proximity to titania. Moreover, it is valuable to note that the remarkably fast charge recombination could also result in inferior charge collection yields, providing a clue for the lower EQE maxima of LWP12 and LWP13, which have been proved by impedance spectroscopy (IS) measurements.37 As shown in Fig. S12 (ESI†), the LWP14 cell features an obviously longer electron diffusion length at a given potential bias with respect to the LWP12 and LWP13 cells.
Footnote |
† Electronic supplementary information (ESI) available: Dye synthesis and characterization, PL transient experiments of various semi-conductor films, and photovoltaic measurements using different Co-electrolytes. See DOI: 10.1039/c5ee02505b |
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