Screening donor groups of organic dyes for dye-sensitized solar cells

Abstract

Based on an experimentally synthesized dye D5 (also named d01 in this work), we designed and screened a series of dyes d02–d06 with different electron donors, such as diphenyl ethylene benzene, phenothiazine and perylene and d07–d12, by modifying the donor of d06 using different electron-donating groups. The results indicate that the donor in d12 is a promising electron donor. Therefore, we further designed six novel D–π–A structures of BUCT7–BUCT12 by using the electron donor in d12 as a donor, any two of 3,4-ethylenedioxy thiophene (EDOT), thienothiophene and dithieno[3,2-b:2′,3′-d] thiophene (s-DTT) groups as a π-conjugated bridge, and dicyanovinyl carboxylic acid and dicyanovinyl sulfonic acid as the acceptors. The calculated results indicate that BUCT7–BUCT12 dyes show smaller HOMO–LUMO energy gaps, higher molar extinction coefficients and obvious redshifts compared to the experimentally synthesized d01 dye. In particular, the newly designed BUCT8 dye not only exhibits a redshift of 134 nm and a higher molar extinction coefficient with an increment of 74.1% compared to d01 dye, but also has an extremely broad absorption spectrum covering the entire visible range up to the near-IR region of 1000 nm. In addition, we also found that the dyes with dicyanovinyl sulfonic acid as the electron acceptor are superior to the ones with dicyanovinyl carboxylic acid.

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1. Introduction

A large challenge for our global society is to find ways to replace the slowly but inevitably vanishing traditional fossil fuels caused by the fast consumption due to global development. It is an urgent task to develop renewable and clean energy sources (such as power from wind, water, biomass and solar energy)¹ to avoid negative effects from the current energy systems on climate, environment and health. Actually, solar energy plays a crucial role as a future energy resource, providing our planet with about 10⁴ times more energy than our global daily consumption.² Dye-sensitized solar cells (DSSCs) are photovoltaic devices that use molecules to absorb photons and directly convert them into electric charges without the need for intermolecular transport of electronic excitation. Currently, DSSCs^3–17 are receiving significant attention due to low cost, high efficiency and relatively little maintenance. Therefore, it is very significant to develop highly effective DSSCs for a wide application of renewable energy. However, there are disadvantages which limit the wide use of DSSCs, such as the relatively low efficiencies and the need for a liquid electrolyte.

To date, some investigators have reported a power conversion efficiency (PCE) higher than 11% by using metal–organic sensitizers and liquid electrolytes under AM 1.5.^7,8,18–21 Most recently, Gratzel's research team achieved a PCE exceeding 15%, further promoting the research of solar cells.²² Actually, the PCE of DSSC devices is mainly determined by open circuit photovoltage (V_oc), short circuit current density (J_sc) and fill factor (FF). Improving the three key factors can significantly enhance the PCE of the DSSCs. Generally, the short circuit current density (J_sc) can be increased by improving the light harvesting efficiency (LHE) (LHE can be calculated by the equation of LHE = 1 − 10^−f, where f represents the oscillator strength of adsorbed dye molecules)²³ as well as the molar extinction coefficients of the dyes, and by broadening the absorption spectrum scope in the visible light and near-infrared regions.

In the DSSCs, the dye sensitizers were also considered as one of the most important components to improve the overall device performance. Recently, fully organic dyes with a donor–spacer–acceptor (D–π–A) structure have attracted widespread interest.^9,11,24 In these D–π–A structures, the donor group (D) is an electron-rich unit (such as triphenylamine, indole, dimethyl fluorene, or phenothiazine), linked through a conjugated bridge (π) to the electron-acceptor group (A), which is directly bound to the TiO₂ nanocrystalline surface, usually through a carboxylic or cyanoacrylic group. Important features of these dyes are that they hold the strong absorption of visible light and the charge transfer excited states, ensuring effective charge separation. Currently, designing highly effective D–π–A structures to further improve the PCE of DSSC devices is still a great challenge.²

In order to obtain an excellent dye with a high photocurrent, it should avoid π–π aggregation, which will lead to intermolecular quenching or molecules residing in the system and therefore preventing it from being functionally attached to the TiO₂ surface.^2,25,26 Many organic molecules, such as triphenylamine and phenothiazine, have non-planar structures which suppress the aggregation. Sun et al. introduced triphenylamine as a donor, a thiophene ring as the π bridge and cyanovinyl carboxylic acid as an acceptor to synthesize the D5 dye (also named d01 in this work),⁸ and the resultant DSSC devices exhibit a PCE of 5.1% (the conventional ruthenium dye N719-sensitized DSSC device gives a PCE of about 6.1–6.4% in similar conditions), which has attracted great attention due to its simple structure and outstanding photoelectric performance.^{8,9,12,24,27–36}

In order to design outstanding dye donors with non-planar structures, we first screened a series of organic dyes with D–π–A structures by considering the d01 dye as the prototype. Then, we recommend novel dye formed by the screened donor, π-conjugated bridge and acceptor groups for highly effective DSSCs with near-infrared light harvesting.³⁶

2. Computational details

Density functional theory (DFT) calculations were performed at the B3LYP level with the 6-31G(d) basis sets for the geometric optimization of these dye molecules without any symmetry constraints. Vibrational frequency calculations were also carried out at the same level to confirm that these dye structures are local minima on the potential energy surfaces. The calculation results show that there is no imaginary frequency, indicating that these structures are stable in theory.

Previous investigations indicate that time-dependent DFT (TDDFT) is a highly efficient and accurate way to calculate the vertical excitation energy, electric properties and optical absorption.^24,25 However, the different exchange-correlation (XC) functionals for charge-transfer excitations often show significant effects. To select a suitable functional, we adopted different XC functionals, including B3LYP, CAM-B3LYP and WB97XD in the TDDFT calculations to evaluate the vertical excitation energies of d01. The calculated vertical excitation energies of the different functionals were 1.959, 2.490 and 2.594 eV, with errors of 0.646, 0.115 and 0.011 eV, respectively, compared to 2.605 eV for the experimental data. Apparently, the vertical excitation energies of the dyes were underestimated severely by the B3LYP functional, and the long range-corrected (LC) functional (WB97XD) was in good agreement with the experimental values. Therefore, we adopted the TD-WB97XD functional with 6-311+g(d,p) basis sets,^37,38 and combined the conductor polarizable continuum model (CPCM)⁴ in acetonitrile solution to predict the optical properties of the newly designed dye molecules. All calculations were performed using a suite of the Gaussian 09 packages.³⁹

3. Results and discussion

3.1 Screening of the electron donor

The experimentally synthesized well-known organic dye D5 is shown in Fig. 1.⁸ Here it is named as d01 for simplification. The dyes d02–d06 (Fig. 1) were obtained by using indoline, phenothiazine, carbazole, tetrahydroquinoline and perylene groups as donors to replace the triphenylamine group of d01. It was found from the optimized geometric structures (Fig. S1†) that the π-conjugated bridges of d01–d06 are planar structures, which are greatly beneficial for the photo-induced electron transfer from the electron donor to the electron acceptor. However, considering the electron donor rather than the π-conjugated bridge, we expect that its structure is non-planar, because it can effectively prevent the intermolecular π–π aggregation and accumulation on the TiO₂ surface, which causes the electron transfer between molecules and affects the PCE of DSSCs.

Fig. 1 Molecular structures of the dyes with different donors.

The triphenylamine dye d01, as a sensitizer of the DSSCs, can expand the light absorption ability of the dye. Its non-planar donor group can also inhibit aggregation of the dye on the TiO₂ surface,⁸ and therefore was widely used as an electron donor for other sensitizers.^{25,27,29,31,32,40} The electron donor in the d02 molecule is similar to d01, in which the three benzene rings of the d02 donor are mutually twisted, and the benzene ring linked π-conjugated bridge in the d02 donor and the π-conjugated group have a small distortion angle (Fig. S1†). The phenothiazine donor in d03 is a well-known heterocyclic compound with electron-rich sulfur and nitrogen heteroatoms, and the phenothiazine ring is non-planar with a butterfly conformation. Thus, phenothiazine is a potential hole-transport semiconductor in organic devices,^34,41–44 showing unique electronic and optical properties with a D–π–A structure. The electron donors in d04 and d06 have planar conformations, which lead to the aggregation of the dye on the TiO₂ surface and therefore affects the PCE of the DSSC. The donor of d05 is non-planar due to the modification by using four methyl groups. The above observations indicate that the molecular conformation of the desired dye can be designed by introducing suitable electron-donating groups. Compared to the optimized donor structures of d01, the d02, d03 and d05 dyes may be suitable candidates for DSSCs.

Fig. 2 shows the energy levels for d01–d06 in acetonitrile solution. The results indicate that the LUMO energies of d01–d06 are higher than the conduction band edge (CBE) of the TiO₂ electrode (about −4.0 eV),²⁸ shown as a red dashed line in Fig. 2. That is to say, these excitation state molecules could inject electrons into the TiO₂ electrode successfully. In addition, the HOMO energies of d01–d06 are lower than the potential (about −4.6 eV) of the I⁻/I₃⁻ redox electrolyte.²⁸ Therefore, these dye molecules that lose electrons could obtain electrons quickly from the electrolyte. Compared to the energy gap of d01, the d05 and d06 dyes may be suitable candidates for DSSCs. However, the HOMO energy of d05 is so close to the potential of the redox electrolyte that it only has a low reduction driving force, if we increase the π-conjugated bridge length.

Fig. 2 Energy levels for the d01–d12 dyes.

The UV-visible absorption spectra of the d01–d06 dyes in acetonitrile solution are shown in Fig. 3. The maximum absorption peaks of d02–d06 are 457, 452, 455, 498 and 506 nm, respectively, in which d02–d04 exhibit blueshifts of 21, 26 and 23 nm, respectively, while d05–d06 exhibit redshifts of 20 and 28 nm, respectively, compared to that of d01 (478 nm). The maximum molar extinction coefficients of d02–d06 also increase greatly (see Table 1). The optical absorption spectra redshift and the maximum molar extinction coefficient indicate that the d06 dye may be a good candidate as a photosensitizer for the DSSC device, which is consistent with the band gap results above. However, the planar conformation of the electron donor in the d06 molecule limits its application as an excellent photosensitizer for DSSCs. In order to construct a non-planar donor structure in d06, we designed a series of dyes, named d07–d12, by using different electron-donating groups such as amino, methoxy, butyl, phenyl, aminobenzene and methylbenzene groups, as shown in Fig. 1. The optimized geometries in Fig. S1† show that the donors in the d07–d09 molecules still have a planar structure, but the electron-donating groups in d10–d12 create a non-planar structure with twist angles of 31.3°, 29.7°and 31.5°, respectively, with the donor plane. So, d10 and d12 may be good candidates for DSSCs. Synthetically considering their energy levels and absorption spectra, we find that the properties of d12 are slightly better than d10. So, we selected the electron donor in d12 as a donor for designing further novel dyes.

Fig. 3 UV-visible absorption spectra of the d01–d12 dyes. Inset: enlarged absorption spectra of the d01–d12 dyes in the range of 400–540 nm.

Table 1 The TDDFT calculated maximum absorption wavelengths λ (nm), corresponding vertical excitation energies E_ex (eV), oscillator strengths f and the maximum molar extinction coefficients ε (10⁴ M⁻¹ cm⁻¹)

Dye	λ	Eex	f	ε	Dye	λ	Eex	f	ε
d01	478	2.5890	1.5910	6.444	d10	505	2.4530	1.9242	7.811
d02	457	2.7107	2.0130	8.112	d11	509	2.4343	1.9260	7.820
d03	452	2.7419	1.8777	7.605	d12	506	2.4474	1.9316	7.841
d04	455	2.7250	1.9082	7.728	BUCT7	588	2.1089	2.7494	11.229
d05	498	2.4874	1.9284	7.810	BUCT8	612	2.0259	2.7544	11.216
d06	506	2.4500	1.8382	7.458	BUCT9	574	2.3767	2.1601	9.751
d07	510	2.4308	1.7940	7.294	BUCT10	578	2.145	2.2509	9.658
d08	512	2.4213	1.7947	7.305	BUCT11	557	2.2264	2.7933	12.017
d09	508	2.4429	1.8968	7.690	BUCT12	562	2.2053	2.7824	11.878

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3.2 Design of novel D–π–A dyes

On the basis of the above screening for the donor structure and the screening for the π-conjugated bridges in our previous research,³⁶ we designed a further six novel dyes BUCT7–BUCT12 by employing the electron donor in d12 as a donor, any two of 3,4-ethylenedioxy thiophene (EDOT), thienothiophene and dithieno[3,2-b:2′,3′-d] thiophene (s-DTT) groups as the π-conjugated bridge, and dicyanovinyl carboxylic acid and dicyanovinyl sulfonic acid as the acceptors. The structures of BUCT7–BUCT12 are presented in Fig. 4. Interestingly, the optimized geometric structures (Fig. S1†) of the π-conjugated bridges of BUCT7–BUCT12 are planar, which is greatly beneficial for the photo-induced electron transfer from the electron donor to the electron acceptor, while their donors are non-planar, which inhibits aggregation of the dyes on the TiO₂ surface.

Fig. 4 Dye structures of the novel designed BUCT7–BUCT12 dyes.

The energy levels of the BUCT7–BUCT12 dyes in acetonitrile solution are shown in Fig. 5. The results indicate that the LUMO energies of BUCT7–BUCT12 are higher than the conduction band edge (CBE) of the TiO₂ electrode (about −4.0 eV),²⁸ shown as a red dashed line in Fig. 5. That is to say, these excitation state molecules could inject electrons into the TiO₂ electrode successfully. In addition, the HOMO energies of BUCT7–BUCT12 are lower than the potential (about −4.6 eV) of the I⁻/I₃⁻ redox electrolyte. Therefore, these dye molecules that lose electrons could obtain electrons quickly from the electrolyte.²⁸ The calculated results also indicate that the BUCT7–BUCT12 dyes show smaller HOMO–LUMO energy gaps compared to the experimentally synthesized d01 dye.

Fig. 5 Energy levels for the BUCT7–BUCT12 dyes.

Fig. 6 shows the absorption spectra of BUCT7–BUCT12 in acetonitrile solution. The maximum molar extinction coefficients of the BUCT7–BUCT12 dyes are 11.229 × 10⁴, 11.216 × 10⁴, 9.751 × 10⁴, 9.658 × 10⁴, 12.017 × 10⁴ and 11.878 × 10⁴ M⁻¹ cm⁻¹, respectively, exhibiting an increment of 49.9–86.5% compared to d01. The maximum absorption peaks of BUCT7–BUCT12 are 588, 612, 574, 578, 557, and 562 nm, respectively, which show large redshifts ranging from 79 to 134 nm, compared to d01. BUCT7–BUCT12 dyes not only show smaller HOMO–LUMO energy gaps, but also higher molar extinction coefficients and obvious redshifts compared to the d01 dye. In particular, the newly designed BUCT8 dye exhibits not only a redshift of 134 nm and a higher molar extinction coefficient with an increment of 74.1% compared to d01 dye, but also an extremely broad absorption spectrum covering the entire visible range up to the near-IR region of 1000 nm, and could be considered as a panchromatic dye. So, BUCT8 may be a promising candidate for highly efficient DSSC devices.

Fig. 6 UV-visible absorption spectra of for the BUCT7–BUCT12 dyes.

In addition, we found that although BUCT8 has the same structure as BUCT7, except for the electron acceptor, BUCT8 shows a smaller energy gap and a redshift of 24 nm compared to BUCT7. Similar results were observed in the comparisons of BUCT9 and BUCT10, BUCT11 and BUCT12. Therefore, we believe that the dyes with dicyanovinyl sulfonic acid as the electron acceptor are superior to the ones with dicyanovinyl carboxylic acid.

4. Conclusions

We have screened a series of dyes, d01–d12, and found that the donor in d12 is a promising functional electron donor. Then, we designed six novel D–π–A structures, BUCT7–BUCT12, by using the electron donor in d12 as a donor, any two of EDOT, thienothiophene and s-DTT groups as the π-conjugated bridge, and dicyanovinyl carboxylic acid and dicyanovinyl sulfonic acid as the acceptors. Compared to the experimentally synthesized d01 dye, the newly designed BUCT7–BUCT12 dyes show smaller HOMO–LUMO energy gaps, higher molar extinction coefficients and obvious redshifts. In particular, the BUCT8 dye exhibits not only a redshift of 134 nm and a higher molar extinction coefficient with an increment of 74.1% compared to the d01 dye, but also an extremely broad absorption spectrum covering the entire visible range up to the near-IR region of 1000 nm. In addition, we also found that the dyes with dicyanovinyl sulfonic acid as the electron acceptor are superior to the ones with dicyanovinyl carboxylic acid. It is expected that this work will provide guidance for the synthesis of panchromatic dyes and that the use of the BUCT8 dye for highly efficient DSSCs can be confirmed by experiments.

Acknowledgements

This work is supported by the National 863 Program (2013AA031901), NSF of China (no. 91334203, 21274011, 21121064), Outstanding Talent Funding from BUCT and Science Foundation (KYJJ2012-06-026) from CUP.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra17261b

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