Molecular design of organic sensitizers absorbing over a broadened visible region for dye-sensitized solar cells

Abstract

A group of sensitizers P1–P7, as well as the reference molecule XS54, were theoretically designed and investigated, and the properties of photo-induced charge transfer were characterized by density functional theory (DFT) and time-dependent density functional theory (TD-DFT). Research shows that P5 and P6 possess a broad spectral response, and exhibit strong electron migration ability along the conjugated bridge. The benzothiadiazole (BTD) unit being placed near the accepter unit can effectively extend the spectral range and improve the electron delocalization. Our theoretical design promotes the deeper understanding of dye-sensitized solar cells that absorb over a broad visible region.

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Introduction

Dye-sensitized solar cells (DSSCs) have been intensively studied as the promising candidate for the low-cost alternative energy solution, since the breakthrough first made by O'Regan and Grätzel in 1991.¹ As one of the most crucial components of the DSSC device, the dye sensitizer has always been the issue of concern and explored by scientific research. Generally, every significant improvement of the photovoltaic conversion efficiency of the dye-sensitized solar cell accompanies the development and synthesis of new sensitizers.^2–7 However, it is difficult to design a kind of unique sensitizer with both broad spectrum and strong solar absorptivity simultaneously. Hence, the co-sensitization strategy, which employs two or three dyes with complementary spectral responses, was promoted to achieve panchromatic light harvesting.^8,9

XS54-XS56 are three typical organic donor–π bridge–acceptor (D–π–A) dyes, featuring the new conjugated spacers developed by Wang and coworkers: dithieno[3,2-b:20′,30′-d]pyrrole (DTP) units with different hexyloxyphenyl (HOP) substituents.¹⁰ The special bridge unit 4-HOP-DTP enables XS54 with a strong molar absorption coefficient and distinctly enhanced short-circuit photocurrent density (J_sc), and brings a power conversion efficiency (PCE) of even up to 8.14%. However, the red response of XS54 is not satisfying enough. On the other hand, porphyrin dyes, with the obvious absorption at Soret (400–430 nm) and Q (500–750 nm) band in the visible region, have also been increasingly used as the sensitizers for DSSC.^11–13 If combing the porphyrin unit and the traditional D–π–A molecule together, more efficient dye sensitizer would be expected.

In fact, for the theoretical research of the DSSC, the results considering the polarization effect^14,15 of the semiconductor surface to the sensitizers would be more accurate compared with the experiment, just as previous work of us (RSC Adv., 2013, 3, 12133–12139) did. Therein, the adsorption behaviors of the dye on the TiO₂ clusters and the photo-induced charge-transfer property of the dye/cluster interface were discussed. However, the optimized geometries of the large-sized dye/cluster interfaces usually require expensive computation. This work intends to provide more considerations for the combinatorial design of the organic sensitizers from the perspective of the broadened absorption, because the spectral response region in the UV-Visible spectrum of the sensitizers is one important factor affecting the efficiency of the DSSC. Therefore, we did not consider the influence of the semiconductor in the present work.

The main focus of this work is the theoretically design and selection for the organic sensitizers with stronger absorption and complementary spectra in broad visible region based on XS54 molecule. Two chemical groups (benzothiadiazole (BTD) and porphyrin units) are considered to be inserted in the π-conjugated bridge of XS54 for the molecular design, and the total absorption of the selected designed sensitizers as well as the reference molecule XS54 should cover the whole UV-Visible spectrum as far as possible. Our combinatorial design provides more choices for the experimental and theoretical research of the co-sensitizing or parallel tandem DSSCs.

Methods

The ground-state geometrical optimization and the excitation energy of all the organic molecules are theoretically studied using density functional theory (DFT)^16,17 and time-dependent density functional theory (TD-DFT),^18,19 respectively. Different exchange-correlation functionals with the basis set 6-31g(d,p)^20,21 are tested on the reference molecule XS54, including: three hybrid functionals B3LYP,²² PBE0,²³ and BHandH²⁴ functionals, with 20, 25, and 50% of Hartree–Fock (HF) exchange, respectively, and one range-separated functional CAM-B3LYP.²⁵ PBE0 gives the closer results to the experimental data (see Table 1), and is hence employed for the following calculations to the designed molecules in this work. The optimized structures and the excitation energies of the designed sensitizers P1–P7 are carried out with DFT/PBE0/6-31g(d,p) and TD-DFT/PBE0/6-31g(d,p), respectively. The absorption spectra are simulated by Lorentzian functions with a full width at half maximum (FWHM) of 15 nm. The charge difference densities (CDDs) of the organic sensitizers are visualized with the 3D real space analysis method.^26–31 All the calculations were carried out with Gaussian 09 program packages.³²

Table 1 The computed vertical transition energies of the lowest excited state λ_max (nm) for XS54 in gas phase using TDDFT/6-31g(d,p). The oscillator strengths (f) are also listed

	B3LYP	PBE0	BHandH	CAM-B3LYP	Exp^10
λmax	568	530	425	419	520
f	0.9361	1.0990	1.9058	1.8725	—

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Results and discussion

Fig. 1 gives the chemical structure of the reference molecule XS54, in which the yellow-shadowed special bridge group (4-HOP-DTP) was demonstrated to be the crucial factor for the high efficiency of the XS54-based DSSC due to the stronger molar absorption coefficients brought by it.¹⁰ From Fig. S1 of the ESI,† the absorption spectrum trend of XS54 calculated at the TD-DFT/PBE0/6-31g(g,p) theory level agrees better with the experimental one¹⁰ compared with the other functionals. Table 2 lists the calculated details of the main excited states of XS54 in UV-Visible spectral region from 350 nm to 750 nm, including the excitation energy, absorption strength, and the Kohn–Sham transition components for each excited state. The calculated first excited state S1 contributes the strongest absorption peak centered at 520 nm in the experimental observation, corresponding to the electric transition from the highest occupied molecular orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO). And S3 involves in the electric transition from HOMO-1 to LUMO. CDDs in Table 3 present the charge separation of XS54 after the light excitation. For S3, the direction of the charge transfer is generally from triphenylamine donor to cyanoacrylic acid acceptor with the only tiny disturbance by the charge transfer from 4-HOP-DTP to the backbone of the organic molecule. As for the strongest S1 excited state, it is an intra-molecular charge transfer (ICT) state (see Table S1†) with the electron transferring from donor to accepter unit upon the excitation, which just confirms the good performance of the solar cell based on XS54.

Fig. 1 The chemical structure of the reference sensitizer XS54. The blue and yellow shadows represent the donor and bridge unit, respectively.

Table 2 The calculated excitation energy during 350 nm and 750 nm, absorption strengths, the Kohn–Sham transition components with the corresponding percentage contribution to each excited state for XS54 at TDDFT/PBE0/6-31d(d,p) theory level in vacuum. Only the excited states with relatively stronger oscillator strength are listed

Excited state	Excitation energy (eV, nm)	f	Kohn–Sham transition components
S1	2.34, (530)	1.0990	H–>L 0.70179 (99%)
S3	3.02, (411)	0.9158	H-1–>L 0.63901 (82%)

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Table 3 The charge difference density (CDD) for XS54

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Although, XS54 provides an encouraging efficiency, its spectral response is not wide enough. On this issue, in order to gain more broadened absorption, a series of organic sensitizers P1–P7 are designed based on XS54 and theoretically investigated. Fig. 2 gives the chemical structures and the optimized geometries of the designed sensitizers. Two substituents (porphyrin and BTD) are adopted to replace or insert on both sides of the conjugated spacer 4-HOP-DTP, since the porphyrin dyes possess obvious absorption in near-infrared region, and BTD unit is another widely used effective electron-withdrawing moiety for sensitizers.⁷ In some of the designed sensitizers (P1, P2, P3, P6 and P7), carbon–carbon triple bond is used to link up the porphyrin unit and the acceptor unit to keep a better planarity at the conjugated system, ensuring the fluency of the electron transfer to the direction of the acceptor unit.

Fig. 2 The chemical structures and optimized geometries of the designed dyes.

According to the energy levels of all the sensitizers in Fig. S2,† the LUMO levels of all the designed dyes are above the conduction band edge of the TiO₂ electrode, which enables the sensitizers enough driving force for the electron injection. And the HOMO levels are all below the energy level of I⁻/I₃⁻ electrolyte, as is beneficial for the reduction of the excited dyes. However, P7 possesses the lowest LUMO level and highest HOMO level among the designed molecules, predicting a weaker ability of electron injection and reduction of the excited sensitizer, and it is not a wise choice as an efficient sensitizer. Fig. 3 gives the absorption spectra of each designed sensitizer. Table 4 gives the details of the excited states of the designed sensitizers contributing to the absorption bands in the red and near infrared region. In Fig. 3, the absorption strengths of P1 and P3 in the red region are relatively weak compared with the others', although they both show a total pure charge-transfer characteristic of the excited states corresponding to the absorption in the near-infrared region (see Table 5). This shows that porphyrin linked directly with the acceptor unit extends indeed the spectral response, but weakens the absorption strength in the near-infrared region, which is not desired by an ideal sensitizer for DSSC. Thus, we put 4-HOP-DTP next to the cyanoacrylic acid group as an auxiliary acceptor, such as P2 and P6. And the absorption in red region of the two dyes is remarkably increased as expected, which is probably due to the stronger electron-withdrawing ability of the 4-HOP-DTP unit. P2 and P6 possess the similar absorption spectrum and similar charge difference densities for the dominant excited states in red region. The hole and electron localize mainly on the donor part and accepter unit, respectively, and the distinct hole–electron coupling is found along the conjugating bridge, indicating a long electron delocalization range of the dye molecule. The only difference is that, except for S1 state, P6 possesses one more excited state S2 than P2 in the red region (600–700 nm), which is probably caused by the longer electron delocalization length caused by the inserted BTD unit near the donor part (see Table 5). P6 is more promising as a candidate for efficient sensitizers than P2.

Fig. 3 The spectra of all the sensitizers, including the reference (XS54) and designed (P1–P7) ones.

Table 4 The excitation energies of the designed molecules

		Excitation energy (eV, nm)	f	CI
P1	S1	1.82, (680)	0.5497	H–>L	0.69703
P2	S1	1.87, (664)	1.5698	H–>L	0.67296
P3	S1	1.60, (777)	0.5807	H–>L	0.69562
	S2	2.02, (615)	0.4288	H-1–>L	0.58439
P4	S1	1.92, (647)	1.0086	H–>L	0.69102
P5	S1	1.81, (684)	0.9700	H–>L	0.70428
P6	S1	1.91, (648)	1.5152	H–>L	0.60241
	S2	2.05, (605)	0.3739	H-1–>L	0.55472
P7	S1	1.40, (889)	0.5575	H–>L	0.69885
	S2	1.80, (688)	1.0331	H-1–>L	0.67498

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Table 5 The charge difference density (CDDs) of the designed molecules

P1		P2
P4		P5
P3		P6
P7		P7

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As for P4 and P5, BTD unit is added on each side of the 4-HOP-DTP unit. According to the absorption spectra in Fig. 3, several peaks of P4 spread over the whole spectrum region, which seems to make P4 to be a good candidate for efficient sensitizer. However, from the CDDs of P4 in Table 5, a small amount of hole density locates on the 4-HOP-DTP unit after the light-induced excitation and blocks the electron distribution along the conjugated bridge and acceptor. Accordingly, 4-HOP-DTP group is probably more suitable to be used as an assistant electron donor and electron-transport unit near the donor part, and BTD unit should be placed near the acceptor group for the electron receiving and transferring from the donor group to the direction of the acceptor unit. P5 presents a light absorption with comparable strength and breadth as P4. Concerned with the CDDs for P5 in Table 5, hole and electron densities almost separate totally, indicating a pure charge-transfer excited state and the stronger electron migration ability, compared with P4 and XS54, which might be induced by the small overlap and the relatively distinct spatial localization of the involved orbitals for the concerned excited state.³³ And this feature of P5 is beneficial for the electron transfer from the donor to the acceptor group, and then to the conduction band of the TiO₂ film, favorable for the improvement of the efficiency of the DSSC. Now that a group of sensitizers based on XS54 have been designed and the optical properties of them are discussed, we judge that P5 and P6 could be selected out as the promising dyes to co-sensitize the solar cell together with the reference molecule XS54. Fig. 4 gives the total combinatorial spectrum of the three dyes (P5, P6 and XS54). It is seen that, the red region of the combinatorial spectrum are mainly contributed by the S1 state of P5 and P6. Both of them are ICT state corresponding to the HOMO–>LUMO transition, and the electron transfers from donor part to the acceptor unit (see Table S2†). Besides, the valleys between the three main peaks (1, 2 and 3) in the whole combinatorial spectrum are effectively filled by P6 (S2, S3, S9, S10). The combinatorial designed dyes achieve more broadened absorption, on which the efficiency of the DSSC based might be probably exceeding that sensitized by only XS54 molecule.

Fig. 4 The total combinatorial absorption spectrum of the three dyes, including XS54, P5 and P6.

Conclusion

In conclusion, a series of sensitizers P1–P7 had been theoretically designed based on the reference molecule XS54, and the optical absorption properties of them are investigated by quantum chemical method. The calculation shows that P5 and P6 exhibit wide spectral response and stronger absorption strength than XS54. And they both exhibit good electron delocalization upon the photo-induced excitation, especially for P5 with the pure charge-transfer excited state contributing to the strong absorption band in the red region. It is demonstrated that combining the porphyrin unit near the donor part as a conjugated spacer for the traditional D–π–A dye would effectively extend the spectral response region and absorption strength of the sensitizer. BTD unit being putted near the acceptor unit may receive and transfer the electron densities from donor part more effectively. The total combinatorial spectrum of the three dyes shows more broadened spectral response, which is beneficial for the high efficient DSSC. Our theoretical designing promotes the deeper understanding for dye-sensitized solar cell absorbing in broadened visible region, and provides more considerations on the molecular engineering and the co-sensitized DSSC.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (Grant nos 11374045, 11404055 and 11374353), the Fundamental Research Funds for the Central Universities (Grant no. DUT13ZD207), and Program for New Century Excellent Talents in University (Grant no. NCET-12-0077).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra10896e

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