Organic double D–π–A sensitizers based on 2,2′-(2,2 diphenylethene-1,1-diyl)dithiophene: π-conjugation fragment effect on the photovoltaic properties

Abstract

The development of new dye sensitizers to further reveal the influence of changes in structural components on the photovoltaic performance is of great significance to dye-sensitized solar cells (DSSCs). The propeller-shaped 2,2′-(2,2-diphenylethene-1,1-diyl)dithiophene (DPDT) unit was introduced to construct sensitizers for the first time. Three DPDT-bridged double D–π–A organic sensitizers (A6, A8, A9) were prepared by altering the π-conjugation fragment. The photophysical, electrochemical and photovoltaic properties of the sensitizers were systematically investigated to assess the role of the terminal fragment of dyes in DSSCs. The results show that, compared to the mono-anchoring congener AZ6, the di-anchoring sensitizer A6 displayed a comparable power conversion efficiency (PCE = 8.21%) and a higher short-circuit current density (J_SC). Replacement of the terminal thiophene with a phenyl ring (A9) can effectively increase the photovoltage by 70 mV with an efficiency of 8.14%, which is 1.5 times higher than that of A8 (5.36%) with 2-cyanoacrylic acid at the meta-position of the phenyl ring. These results indicate that the terminal fragments of sensitizers have a significant effect on the photovoltaic performance.

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Introduction

The field of dye-sensitized solar cells (DSSCs) has grown at an alarming speed and significant achievements have been made in metal complex-sensitized^1,2 and pure organic dye-sensitized solar cells in the last three decades.^3–5 As a crucial assembly unit, sensitizers fundamentally affect the performance of DSSCs and have grown rapidly in recent decades.^6–8 Numerous ruthenium complex dyes and zinc porphyrin sensitizers with high efficiency such as CYC-B11⁹ (11.5%), SM315¹⁰ (13.0%), XW51¹¹ (11.1%), and so on have been reported. Among various dyes,^12,13 metal-free photosensitizers have also aroused great research interest due to the flexible molecular design, high molar extinction coefficients, and cost-effectiveness. In addition to the donor–π–acceptor (D–π–A) featured sensitizers,¹⁴ new types of dyes have been developed to seek ideal sensitizers with the characteristics of being aggregation-resistant and having strong absorption. Recently, some studies have found that di-anchoring dyes^15,16 have a stronger affinity to photoanodes, are more stable than the corresponding mono-anchored dyes, and can provide more electron injection pathways.

However, compared with mono-anchoring dyes, di-anchoring dyes^17–19 usually suffer a decreased open-circuit voltage (V_OC) due to the change of the conduction band edge (E_CB) of TiO₂ and dark current. Therefore, more new fragment compositions were introduced into the di-anchoring dyes to overcome this shortcoming.^20–22 Recently, the light-emitting molecule tetraarylethylene²³ has attracted our interest, which can effectively promote the orderly aggregation of dyes without introducing aggregation-induced emission (AIE) characteristics. Since the Su group²⁴ first introduced the tetraphenylethylene (TPE) core into X-shaped dyes, more dyes have been introduced into this fragment at different sites. For example, the Lin group²⁵ synthesized a series of di-anchoring TPE-tethered YL dyes, providing a stronger ability to suppress charge recombination compared to the congener dyes. Recently, Zheng and co-workers found that different triarylethylene (TAE)²⁶ units, when used as a π-spacer fragment of dyes, have an impact on photovoltaic properties. The DPTP unit has similar AIE properties, which may be more beneficial to the light absorption of dyes due to the electron-rich characteristics of thiophene. Therefore, it is still potentially valuable to introduce the DPTP unit into dye molecules.

Up to now, di-anchoring dyes^27–30 have not been extensively studied due to the difficulties in the design and synthesis of di-anchoring dyes. It is easy to find that little attention was paid to the modulation of the π-spacer fragment, which dramatically affected the optical properties and the interfacial charge recombination. Thiophene and phenyl rings are the most commonly used π-spacer components. Dyes containing thiophene as a π-spacer component always display a higher J_SC value but a lower open-circuit voltage,^31,32 while dyes with phenyl as a π-spacer component usually exhibit opposite results.^33–35 By inserting an additional terminal phenyl ring, the reported dyes GY50³⁶ and 2³⁷ can greatly suppress the charge recombination and significantly improve the V_OC and J_SC. Simultaneously, the position of the acceptor in sensitizers has an influence on the binding mode and photovoltaic properties.³⁸ For instance, Galoppini et al.³⁹ reported that the anchor group of sensitizers in the meta position (m-ZnTCPP) favored a planar binding mode to the metal oxide surfaces. D’Souza and Gao^40,41 revealed that dyes with a para- or meta-position acceptor showed better DSSC performance than dyes with an ortho-position acceptor. However, related investigations are still scant, especially in metal-free di-anchoring dyes.

Based on the above background, we introduced the propeller-shaped DPDT unit into double D–π–A sensitizers (A6, A8, A9) for the first time and connected it with different terminal fragments of dyes (Fig. 1). The cyclic thiourea functionalized triphenylamine was selected as the donor unit due to its excellent optical characteristics and the ability to inhibit charge recombination.⁴² Bithiophene or 2-phenylthiophene was selected as the π-spacer with cyanoacrylic acid as the acceptor at the meta- or para-position of the phenyl ring. The dyes are applied in DSSCs to understand which is the dominant factor affecting the V_OC of double D–π–A sensitizers: the conduction band edge (E_CB) of TiO₂ or the ability to suppress the dark current? In addition, it is important to clarify that the different geometric configurations caused by the positions of the para- or meta-anchoring group of benzene may affect the loading mode of dyes on TiO₂ film. The reported D–π–A sensitizer AZ6 was synthesized as a reference. The photophysical, electrochemical and photovoltaic properties of the dyes were studied systematically. In particular, the synthetic method provides more space and feasibility for further modulation of the tetraarylethylene fragment.

Fig. 1 Chemical structures of sensitizers A6, A8, and A9.

Results and discussion

Material synthesis and characterization

The synthetic routes of A6, A8, and A9 are shown in Scheme 1. The raw material C₆S₂TPAB(OH)₂ was synthesized according to our previously reported method.⁴² Intermediates 3a–3c and 4a–4c were synthesized via Suzuki coupling. The target dye molecules A6, A8, and A9 were obtained from 4a–4cvia the Knoevenagel condensation reaction. All of the new compounds were characterized by ¹H NMR, ¹³C NMR, IR spectroscopy and HRMS or MALDI-TOF-MS (see the ESI†).

Scheme 1 Synthetic routes of dyes A6, A8, and A9.

Photophysical properties

The UV-vis absorption spectra of the dyes measured in dichloromethane and on TiO₂ films are shown in Fig. 2. The detailed parameters are summarized in Table 1. All the dyes showed two major absorption bands. The absorption band at 300–400 nm is attributed to the π–π* transition of the conjugated system and the absorption band at 400–670 nm is ascribed to the intramolecular charge transfer (ICT) from the functionalized triphenylamine donor to the cyanoacetic acid acceptor. Compared with the single D–π–A dye AZ6, the double D–π–A dye A6 exhibited broader absorption characteristics with a higher molar extinction coefficient (ε) in the whole absorption region due to conjugation extension. It is worth noting that the π–π* electron transition band of the three D–π–A dyes showed a higher ε than their ICT bands, which indicated that a stronger π–π* interaction occurred in the dyes due to the bulky donor configuration. This phenomenon can also be found in other reported dyes.^34,43,44 Dye A8 showed a narrower and less intense ICT band than A9, which was caused by the poor coplanarity of the π-spacer and the meta-position acceptor molecular configuration may weaken the charge transfer. When the phenyl ring was replaced with a thiophene group, the absorption properties of sensitizer A6 were further enhanced. The ε values at λ_max of the absorption spectra for A6, A8, and A9 were 56 210 M⁻¹ cm⁻¹, 34 556 M⁻¹ cm⁻¹, and 41 648 M⁻¹ cm⁻¹, respectively, suggesting that the absorption characteristics can be effectively fine-tuned by adjustment of the π-spacer segments and the acceptor position.

Fig. 2 UV-vis absorption spectra of the dyes (a) in CH₂Cl₂ solution and (b) on the TiO₂ film.

Table 1 Photophysical and electrochemical properties of the dyes

Dye	λ max (nm)	ε (M^−1 cm^−1)	E ox (eV)	E red (eV)	E HOMO (eV)	E LUMO (eV)	E g (eV)
a Absorption spectra of sensitizers in CH2Cl2 (10^−5 M). b E HOMO = −e (Eox + 4.71) (eV) and ELUMO = −e (Ered + 4.71) (eV); Eg = e(Eox − Ered) (eV).
A6	432	56 210	0.21	−0.99	−4.92	−3.72	1.20
	484	47 420
A8	413	34 556	0.25	−1.18	−4.96	−3.53	1.43
A9	447	41 648	0.27	−1.14	−4.98	−3.57	1.41
AZ6	501	36 939	0.31	−1.06	−5.02	−3.65	1.37

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In the case of the dyes absorbed on the TiO₂ surface, the absorption bands were broader and red-shifted (ca. 50 nm) than those in solution, which is favorable to improving the light-harvesting ability. In order to further verify the binding mode of the sensitizers on the TiO₂ film, the FT-IR analysis of the dyes anchored on the TiO₂ film was carried out.⁴⁵ As shown in Fig. S2 (ESI†), asymmetric stretching (υ_as, around 1598 cm⁻¹) and symmetric stretching (υ_s, around 1408 cm⁻¹) bands appeared, whereas the –COOH peak (around 1724 cm⁻¹) of the pure dyes disappeared. This result indicates that all three double D–π–A dyes adsorbed on the TiO₂ film in the bidentate adsorption mode, which may improve the affinity of the dye to the TiO₂ film. The desorption experiment²⁵ of the dyes in an alkaline solution (Fig. S3, ESI†) showed that the di-anchoring dyes were more difficult to extract than the mono-anchoring dyes. The result is consistent with its binding mode.

Electrochemical properties

Cyclic voltammetry (CV) was performed to evaluate the feasibility of the electron injection and dye regeneration processes.⁴⁶ As shown in Fig. 3, all dyes had sufficiently high LUMO levels, which indicates that electrons can be effectively injected from the excited dyes into the conduction band of TiO₂ (−4.0 eV vs. vacuum). Meanwhile, the HOMO levels of the dyes were more negative than the redox potential of the iodide/triiodide electrolyte (−4.80 eV vs. vacuum), ensuring that these oxidized dyes could be easily regenerated. For A8 and A9, similar energy levels demonstrate that the acceptor position has a slight influence on the energy levels. Replacing the phenyl with a thiophene ring, the LUMO level of dye A6 significantly shifted down, thus leading to a narrow energy gap, which may be favorable for generating a higher photocurrent. The HOMO–LUMO energy gap values decreased in the order: A8 (1.43 eV) > A9 (1.41 eV) > AZ6 (1.37 eV) > A6 (1.20 eV), which is consistent with the absorption characteristics.

Fig. 3 CV-derived HOMO and LUMO energy levels of the dyes.

DFT calculations

The molecular geometry conformations and electronic properties of dyes A6–A9 were simulated by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). The frontier molecular orbitals of the dyes are shown in Fig. 4 and the corresponding energy levels are listed in Table S2 (ESI†). Compared with A9, A8 presents a more twisted molecular configuration and a shorter distance between the two acceptors (13.9 Å for A8 and 20.0 Å for A9), which has an advantage in inhibiting charge recombination. In contrast, A6 shows better coplanarity from the bithiophene to the acceptor, which is favorable for its effective intramolecular charge transfer process. The HOMO levels of the three dyes are mainly concentrated on the donor moieties. For di-anchoring dyes, the charges are mainly located on both sides of their two branches, implying double electron injection channels. According to TD-DFT theoretical calculations, the ICT in A8 and A9 is largely from the HOMO to LUMO+2 (Table S3, ESI†), while for A6 is largely from the HOMO to the LUMO.

Fig. 4 Frontier molecular orbitals of dyes A6, A8, A9, and AZ6.

Photovoltaic performance of DSSCs

The DSSC performances of the dyes were evaluated under simulated AM 1.5G solar light (100 mW cm⁻²) and the relevant parameters are tabulated in Table 2. The photocurrent density–voltage (J–V) curves and incident photon to current conversion efficiency (IPCE) spectra are plotted in Fig. 5. The J_SC values increased in the sequence: A8 (10.26 mA cm⁻²) < A9 (16.60 mA cm⁻²) < AZ6 (16.84 mA cm⁻²) < A6 (18.63 mA cm⁻²). Obviously, the A8-based DSSC displayed an inferior photocurrent, although A8 has a higher loading amount (0.74 × 10⁻⁷ mol cm⁻²) than A6. This result indicates that the loading amount has a small influence on its photocurrent as observed in other reported dyes.^47,48 The narrow UV-vis absorption band of A8 in solution and on the TiO₂ film is consistent with the integration and intensity of the IPCE spectrum. All the factors contribute to a low J_SC value of A8. It is worth noting that the V_OC values showed an opposite tendency: A8 (731.8 mV) > A9 (717 mV) > AZ6 (683.7 mV) > A6 (647.0 mV). Also compared with the mono-anchoring dye AZ6, the A6-based DSSC showed a lower V_OC, which may be caused by the following two aspects. Firstly, compared with the mono-anchoring dye AZ6 (0.84 × 10⁻⁷ mol cm⁻²), the di-anchoring dye A6 released more protons (2 × 0.51 × 10⁻⁷ mol cm⁻²) to the TiO₂ surface and thus decreased the E_CB of the TiO₂. Furthermore, the dye A6 with more active sulfur sites is easier to form dye–iodine complexes,^49,50 causing serious charge recombination. However, replacing the thiophene with a phenyl ring in the π-spacer, higher V_OC values were observed in both A8- and A9-based DSSCs. Compared with dyes AZ6 and A6, dyes A8 and A9 have a better loading capacity, indicating that more protons were released and then the V_OC was decreased. The higher V_OC values indicate that the benzene ring in the π-spacer effectively inhibits charge recombination and then reduces dark current. The DSSC based on A8 showed a slightly higher V_OC value than that based on A9, which may be due to the larger steric hindrance. This reasonable speculation is also supported by the dark current test results (Fig. 5). Overall, the DSSC based on A6 obtained a power conversion efficiency (PCE = 8.21%) with the highest photocurrent. When the acceptor position was altered from the meta- to the para-position, the efficiency increased by 52% from A8 (PCE = 5.36%) to A9 (PCE = 8.14%), which confirmed that the suitable acceptor position is crucial for the device performance. The A6-based DSSC presented a broad spectral response and extended the spectral region to 750 nm. The integration area of the IPCE spectra of the three dyes is in good accordance with the light-harvesting characteristics and J_SC values.

Table 2 Photovoltaic parameters of the DSSCs based on A6, A8, A9 and AZ6^a

Dye	J SC (mA cm^−2)	V OC (mV)	FF	PCE (%)	Dye loading (10^−7 mol cm^−2)
a Performances of DSSCs were measured with a 0.25 cm^2 working area under AM 1.5G solar light irradiation (100 mW cm^−2), the TiO2 layer thickness is 13 μm and the photovoltaic data are the averaged values of six parallel cells.
A6	18.63 (18.11 ± 0.96)	647.0 (648.2 ± 7.5)	0.68 (0.69 ± 0.01)	8.21 (8.08 ± 0.32)	0.51
A8	10.26 (10.01 ± 0.26)	731.8 (733.0 ± 6.7)	0.71 (0.71 ± 0.01)	5.36 (5.23 ± 0.15)	0.74
A9	16.60 (16.36 ± 0.47)	717.0 (712.0 ± 9.4)	0.68 (0.69 ± 0.01)	8.14 (8.05 ± 0.11)	0.91
AZ6	16.84 (16.84 ± 0.50)	683.7 (689.0 ± 20.2)	0.72 (0.71 ± 0.02)	8.31 (8.24 ± 0.27)	0.84

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Fig. 5 Device characterization of DSSCs based on organic dyes AZ6, A6, A8 and A9 with the iodide liquid electrolyte: (a) J–V plots (measured under one sun or dark conditions); (b) IPCE spectra; and (c) Nyquist and (d) Bode plots (measured in the dark).

In addition to the PCE of the DSSCs, the stability of the dye-based DSSCs was also evaluated to further reveal the differences between mono-anchoring dyes and di-anchoring dyes. The cells were stored at room temperature and approximately 25% relative humidity. Fig. S6 (ESI†) shows the variation of the photovoltaic parameters of DSSCs recorded under continuous light irradiation (AM 1.5G, 100 mW cm⁻²) over a period of 3000 h. The A6-based solar cell showed the best long-term stability, which can still retain 98% of the initial value after 3000 h. After 3000 hours of aging, the A8-based solar cell and A9-based solar cell retained 90% and 82% of the initial efficiencies, respectively. As a comparison, the AZ6-based solar cell could retain 78% of its initial value after 3000 h of one sun soaking. This result confirms that the bidentate binding mode of di-anchoring dyes is beneficial for enhancing the stability of DSSCs.

Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) analysis was performed to investigate the photovoltaic properties and charge transfer processes at the electrode/electrolyte interface of DSSCs under dark conditions. As shown in Fig. 5, the charge-transfer resistance at the dye-adsorbed TiO₂/electrolyte interface was assessed through the diameter of the semicircle. The diameter of the semicircle increased in the order: A6 < AZ6 < A9 < A8. The electron recombination lifetime (τ_e) was obtained from the Bode phase plot (Fig. 5d) using the equation τ_e = 1/2πf_max, which increased in the order: A6 (6.5 ms) < AZ6 (8.0 ms) < A9 (21.3 ms) < A8 (29.9 ms). The larger semicircle and longer lifetime imply less interfacial charge recombination. The results coincided well with the V_OC values and dark current test results, which further confirmed that the phenyl ring effectively retarded the charge recombination process.

Conclusions

We synthesized three DPDT-based double D–π–A organic sensitizers by altering the terminal fragment. The di-anchoring sensitizers showed robust affinity to the TiO₂ surface and better long-term stability than the corresponding mono-anchoring dye AZ6. Owing to the extension of conjugation, the A6-based DSSC achieved a high photocurrent. By altering the terminal fragment of the dyes, the photocurrent and voltage can be improved. As a result, the A6-based DSSC achieved an efficiency of 8.21% with a superior J_SC of 18.63 mA cm⁻². When replacing the terminal thiophene with a phenyl ring, the A9-based DSSC yielded a comparable efficiency of 8.14% with a higher V_OC value of 717 mV. Further study on new double D–π–A sensitizers through a molecular engineering strategy is in progress.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support from the National Science Foundation Committee of China (21673134 and 21543012), Program for Science & Technology Innovation Team of Shaanxi Province (2018TD-030), the International Science and Technology Cooperation Project of Shaanxi Province (2021KW-20) and the Fundamental Research Funds for the Central Universities (GK202101005).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1ma00728a

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