π-Spacer effect in dithiafulvenyl-π-phenothiazine dyes for dye-sensitized solar cells

Journal of Power Sources 324 (2016) 484e491

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p-Spacer effect in dithiafulvenyl-p-phenothiazine dyes for dyesensitized solar cells Xiaofeng Zhang a, Faliang Gou a, Dongning Zhao a, Jian Shi a, Hong Gao a, **, Zhenping Zhu b, Huanwang Jing a, b, * a b

State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, China State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan, Shanxi 030001, China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t


Four new dithiafulvenyl-p-phenothiazine dyes have been synthesized for DSSCs.
The inserted p-spacers markedly enhance charge separation in DPP series dyes.
The torsion structures of D-p-D-A dyes suppress their aggregation on TiO2 surface.
DPP-4 sensitized solar cells achieve the best efficiency of 7.66%.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 11 March 2016 Received in revised form 17 May 2016 Accepted 25 May 2016

New dithiafulvenyl-p-phenothiazine dyes have been devised and prepared for dye-sensitized solar cells. Various p-spacers have been successfully introduced into the skeleton of dithiafulvenyl and phenothiazine unit to generate novel D-p-D-A dyes (DPP-1 ~ 4). All dyes have been characterized with NMR, HRMS, UVevis and fluorescence spectra, and taken into cyclic voltammetry measurements. The devices of new dyes have been determined by photoelectrochemical experiments (IV, IPCE and EIS), in which, solar cell of DPP-4 with biphenyl ring p-spacer enhances obviously its photoelectric conversion efficiency to 7.66% reaching 94% of N719-based standard cell and displays good long-term stability with quasi-solid-state electrolyte. Density functional theory (DFT) calculations of new dyes provide further insight into the molecular geometries and the impacts of the torsion angles on their photovoltaic performance. Large dihedral angles in DPP dyes induce good charge separation for efficient unidirectional flow of electron from donor to acceptor. © 2016 Elsevier B.V. All rights reserved.

Keywords: p-Spacer Dithiafulvenyl-p-phenothiazine Torsion angle Dye-sensitized solar cells

1. Introduction

* Corresponding author. State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, Gansu 730000, China. ** Corresponding author. E-mail addresses: [email protected] (H. Gao), [email protected] (H. Jing). http://dx.doi.org/10.1016/j.jpowsour.2016.05.120 0378-7753/© 2016 Elsevier B.V. All rights reserved.

Dye-sensitized solar cells (DSSCs) have attracted more and more interests in both academic and industrial communities since the €tzel and O’Regan in 1991 [1] owing to their easy innovation of Gra fabrication, high efficiency and low costs. PolypyridylRu(II) complex photosensitizers have been proved excellent dyes. In this

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ruthenium dye family, N719 is the most commonly used sensitizer with 11% efficiency [2]. Nevertheless, the resource scarcity and high cost of ruthenium restrain its application in modules and industry [3e5]. As substitutes of the N719 dye for DSSCs, Zn-porphyrin dyes have also showed very good efficiency of photoelectric conversion with various modifications to the porphyrin ring [6e9]. So far, the DSSC devices sensitized by zinc porphyrin dye SM315 in conjunction with a Co(II/III)-based redox have gained superior photovoltaic performance (13.0%) under full sun illumination [10,11]. However, the complicated synthesis and purification for these dyes have limited their development in large scales. Therefore, the metal-free organic dyes possessing donor-p-acceptor (D-p-A) [12] dipolar architecture shall be the best alternatives with high efficiency comparable to those metal-complex dyes in light of several advantages, such as high molar extinction coefficiency, potentially low cost, environmentally friendliness, as well as easier preparation and purification [13e15]. Phenothiazine (PTZ) is a well-known electron-rich heterocyclic compound with a butterfly configuration and introduced in efficient organic dyes to DSSCs [16e24]. Their nonplanar structure impedes molecular aggregations resulting excellent electron separation [23]. Another molecule of tetrathiafulvene (TTF) is recognized as an excellent electron donor in organic semiconductors [25]. The TTF-modified dyes have been reported in literature [26,27]. Recently, dithiafulvenyl (DTF) was also reported as electron donor in a series of D-p-A sensitizers instead of TTF [28e33]. New D-D-A dyes of DTF-PTZ were documented by Jia et al., in which DTF as the donor helps to improve the photoelectric conversion efficiency of DSSC devices [34]. The investigations on classical D-p-A organic dyes demonstrate that p-spacer also plays an important role except for their skeletons and electron donor groups [35e42]. Common p-spacers like phenyl, thiophene and furan ring can increase dyes’ lightharvesting ability, interrupt its co-planarity and retard the back electron transfer resulting in higher efficiency of their DSSC devices [35,40]. Based on the understanding aforementioned, different pspacers were introduced into DTF-PTZ frameworks to build up new DTF-p-PTZ dyes (DPP-1 ~ 4, Fig. 1). The systematically

485

investigations about the optical, photochemical, and photovoltaic performance of their DSSC subassemblies were carefully carried out. The effects of fine structural adjustment with different pspacers were analyzed by DFT calculations.

2. Experimental section 2.1. General Solvents were treated by standard methods. All reagents were utilized as received without further purification. FTO-based glass devices were ordered from Nippon Sheet Glass Co. Ltd. (15 Ohm cm2). The NMR spectra were recorded on a Varian Mercury 300 MHz and a JEOL ECS 400 MHz spectrometer with tetramethylsilane (TMS) as an internal standard. The HRMS spectra were obtained by using an Orbitrap spectrometer. The UVevis absorption spectra were acquired by utilizing an UV-3600 spectrophotometer in tetrahydrofuran at room temperature. The fluorescence spectra were recorded on an F-7000 fluorescence spectrophotometer.

2.2. Syntheses and characterization of dyes The synthetic routes of the dyes are shown in Scheme 1. Compound 1, 3, 4, 5, 6, 7, 8, 9-3 and dye T2-1 were synthesized according to literature [22,23,28,43e46]. Their 1H NMR spectra were consistent with that in references. The aldehyde 5 reacted with cyanoacetic acid via Knoevenagel condensation leading to dye DP1. Obtained aromatic aldehydes 9-n reacted with 4,5bis(hexylthio)-1,3-dithiol-2-thione (compound 1) by HornerWittig condensation to generate D-p units 10-n that can be transferred to key intermediates 11-n by Suzuki coupling reactions with core phenothiazine 8. Finally, desired dyes DPP-n were synthesized via Knoevenagel reactions of 11-n and cyanoacetic acid. The synthetic procedures and NMR data of new dyes and intermediates are detailed in supplementary information.

Fig. 1. Structural features of new organic dyes.

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Scheme 1. Synthetic routs of dyes. (a) NaH, n-BuBr, THF; (b) POCl3, DMF, C2H4Cl2; (c) 1, P(OEt)3, toluene; (d) cyanoacetic acid, piperidine, CHCl3; (e) POCl3, DMF, C2H4Cl2; (f) NBS, THF; (g) bis(pinacolato)diboron, Pd(PPh3)2Cl2, KOAc, toluene; (h) P(OEt)3, toluene; (i) 10-n, Pd(PPh3)4, K2CO3, DME/H2O.

2.3. Preparation of photovoltaic devices The TiO2 paste containing polyethylene glycol (PEG 20000, 50% weight) was coated onto a commercial FTO glass by a vacuum spincoater. After drying in the air, a square TiO2 film (0.45 cm  0.45 cm) would be reserved on FTO plate by scraping the needlessly surrounding TiO2. The object was then sintered at  450e500 C for 1 h in Muffle furnace resulting a layer of 15 mm thickness. The obtained TiO2 electrode was immerged straightforward into a 0.5 mM dye solution in THF for 24 h, swashed with CH3CN, and dried with argon stream. The cell was assembled by

agglutinating this sensitized TiO2 photoanode and a platinumcoated FTO glass as counter electrode. The acetonitrile electrolyte including 0.5 M TBP, 0.6 M DMPII, and 0.05 M I2 was perfused into the cell under vacuum, which was sealed by hot glue. Quasi-solidstate gel electrolyte was prepared by mixing 5 wt % poly(vinylidenefluoride-co-hexafluoropropylene) in a redox solution containing 0.05 M LiI, 0.1 M I2, 0.6 M DMPImI and TBP in 3methoxypropionitrile under heating until all solids were dissolved. After introducing the hot gel solution into the internal space of the cell under vacuum, a uniform polymer electrolyte was formed between the working and counter electrodes. The hole was

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then sealed with a Surlyn film covered with a thin glass slide under heat. 2.4. Measurements of photoelectrochemistry and electrochemistry The JV curves of photoelectric devices were illustrated by a Keithley 2601A Source Meter under irradiation of San-Ei XES-301S solar simulator. A black mask with aperture area of 0.16 cm2 was used during measurement to avoid stray light. The incident light intensity was calibrated to 100 mW/cm2 with a standard Si solar cell (IEC 60904-3). The incident photon-to-current conversion efficiency (IPCE) of photoelectric devices was measured by a Crown facility. IPCE (l) ¼ (sample current  reference IPCE (l))/reference current. The cyclic voltammetry data were collected by a CHI 660E instrument using a three-electrode system: Ag/AgCl, glassy carbon, and Pt electrode. The electrochemical impedance spectroscopy (EIS) was obtained by using a Zahner impedance analyzer (IM6ex). The frequency of the AC signal ranged between 0.1 Hz and 1 MHz and its amplitude was 0.01 V.

Fig. 3. Normalized PL spectra of organic dyes in THF.

3. Results and discussion 3.1. Photophysical and electrochemical properties The UVevis spectra of dyes in THF solutions and on TiO2 films were illustrated in Fig. 2. Their emission spectra were depicted in Fig. 3. The UVevis values, molar absorption coefficients (ε), and fluorescence data were demonstrated in Table 1. Compared with T2-1, DP-1 reveals a new peak at 365 nm in its UVevis spectrum attributing to the DTF chromophores. When different phenyl rings are inserted between DTF and PTZ moieties as p-spacers, new

Fig. 4. JV characteristics of DSSCs based on organic dyes.

absorption bands of DTF evidently appear around 380 nm in the DTF-p-PTZ dyes of DPP-1 ~ 3 with much larger ε values than DP-1. When the phenyl ring is replaced by a biphenyl ring in the dye of DPP-4, the largest molar absorption coefficients have been achieved implying the strongest light-harvesting ability. In comparison with their absorption spectra in THF solution, the absorption bands of DPP-sensitized TiO2 films evidently shift toward visible band, which can benefit their light harvesting abilities. The electrochemical properties of dyes were studied by utilizing a cyclic voltammetry method (Fig. S1) and listed in Table 1. The first oxidation potentials (Eox) are associated with the HOMO levels [27]. All Eox values of these sensitizers are more positive than the redox potential of I/I 3 (0.4 V vs NHE) to ensure thermodynamically downhill regeneration of the oxidized dyes. The LUMO levels of these dyes (calculated by EoxE0-0, in Table 1) are similar and more negative than the Ecb (conduction-band-edge energy level) of the TiO2 electrode (0.5 V vs NHE) [47], indicating that they have sufficient driving forces for the electron injection process from the excited state of dyes into the conduction band of TiO2. 3.2. Photovoltaic performance of DSSCs

Fig. 2. (a) UVevis absorption spectra of organic dyes in THF. (b) UVevis absorption spectra of organic dyes on TiO2 surface.

The photovoltaic performance of DSSCs based on these new dyes were investigated and presented in Fig. 4. Firstly, when the DTF unit as an electron donor group is connected directly to phenothiazine, resulted dye of DP-1 presents better efficiency

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Table 1 Absorption, emission, and electrochemical properties of T2-1, DP-1 and DPP-1 ~ 4.a Dye

T2-1 DP-1 DPP-1 DPP-2 DPP-3 DPP-4 a b c d e

Absorption

Emission

Oxidation potential data

l/nm (ε/M1 cm1)a

lmaxb/nm

lmax/nm

E0e0c/V

Eoxd/V (vs NHE)

ESþ/S*e/V (vs NHE)

312(23,400), 317(32,169), 314(32,182), 307(36,513), 309(35,467), 314(42,497),

465 485 481 443 431 415

620 642 634 608 615 611

2.32 2.28 2.26 2.31 2.29 2.30

1.07 0.71 0.68 0.76 1.06 0.70

1.25 1.57 1.58 1.55 1.23 1.40

437(14,145) 467(14,892) 378(28,000), 450(17,927) 383(30,753) 387(28,135) 382(40,639)

Absorption, emission data were collected using a solution of dye in THF (5  104 M) at room temperature. Absorption spectra of dyes adsorbed onto TiO2. E0e0 was calculated using the intersection of normalized emission and absorption spectra. The oxidation potentials were measured in a THF solution of 0.1 M TBAPF6 (tetrabutylammonium hexafluorophosphate) under a scan rate of 50 mV s1 (vs NHE). ESþ/S* ¼ Eox  E00.

Table 2 Photovoltaic performance of DSSCs based on various dyes. Dye

Jsc/mA cm2

Voc/V

FF (%)

ƞ (%)

T2-1 DP-1 DPP-1 DPP-2 DPP-3 DPP-4 N719

8.60 9.33 10.02 11.86 11.51 13.84 14.15

0.73 0.67 0.68 0.78 0.72 0.78 0.80

70 73 73 73 73 71 72

4.39 4.56 4.97 6.75 6.05 7.66 8.15

(4.56%) than T2-1 (4.39%) within their devices that is consistent with literature report [33]. When substituted phenyl rings are inserted between DTF and PTZ moieties as p-spacers, obtained DTF-p-PTZ dyes of DPP-1 ~ 3 exhibit higher photoelectric conversion efficiency in their DSC cells (Table 2). What accounts for these phenomena? They should attribute to more efficient charge separation since the p-spacers can interrupt the conjugation of DTF and PTZ. Comparing the efficiency of the DSSCs sensitized by DPP-2 with that of DPP-1, the marked enhancements of both Jsc and Voc could be attributed to the electron withdrawing effect of fluorine atom, which facilitated the intramolecular electron transfer from DTF to Anchor [42,48]. The performance of DPP-3 sensitized solar cells is better than that of DPP-1, but is worse than that of DPP-2 due to an electron donor group of hexyloxy. Based on the understandings about p-spacer effect, an elongated p-spacer of a biphenyl ring is inserted between DTF and PTZ moieties to give DPP-4. The DSSC of it exhibits the best photovoltaic performance (ƞ ¼ 7.66%, Voc ¼ 0.78 V, Jsc ¼ 13.84 mA cm2), which puts down to more complete charge separation caused by the disconjugated biphenyl ring in DPP-4. Therefore, p-spacer of biphenyl ring adjusted D-p-D-A dye leads to an evident enhancement in conversion efficiency of their DSC devices from 4.56% (DP-1) to 7.66% (DPP-4).

Fig. 5. IPCE of DSSCs based on organic dyes.

between their p-spacers and PTZ are found in DPP-1 ~ 3 (34.9 , 36.2 and 34.8 ). The torsion structure should augment intramolecular charge separation efficiency and suppress intermolecular aggregation on the surface of TiO2 leading to the improvements of their DSSC performance comparing with DP-1 in DSSC. Moreover, in DPP-4, two large torsions (35.2 and 36.3 ) between DTF and PTZ induce more complete electron separation and stronger light-harvesting (350e450 nm, Fig. 2) than DPP-1 ~ 3. Thus, DPP-4 sensitized solar cells achieved the highest photoelectric conversion efficiency 7.66%. The energy level of TiO2 conduction band is exactly located between LUMO and HOMO of new dyes (Fig. S2), which satisfies the electron injection from LUMO to the conduction band of TiO2. 3.4. Electrochemical impedance spectroscopy

3.3. DFT calculations and energy levels To comprehend the different results of dyes, the energies and geometries of their molecular orbitals were calculated and optimized by the standardized DFT method of B3LYP/6-31G(d, p) [34e37]. The diagrams of HOMO-1, HOMO, LUMO and LUMOþ1 are drawn in Fig. 6. It is clear to see that the electron density in the HOMO of DPP dyes is distributed along the DFT, p-spacer and half of PTZ, and the LUMO shows localized electron distributions though another half of PTZ and cyanoacrylic acid. These electron distributions will therefore ensure efficient electron injection from the dye to the conduction band of TiO2. The dihedral angle of PTZ and DTF in DP-1 without p-spacer is only 13.9 . Large torsion angles

As is well known that electrochemical impedance spectroscopy (EIS) is a useful method of illustrating the important interfacial charge transfer and carrier transportation process in DSSCs [49,50]. Nyquist and Bode plots measured in the dark under forward bias (0.75 V) are depicted in Fig. 7. The larger semicircle at intermediate frequencies in Nyquist plots represents the interfacial charge transfer resistance (Rct) at the TiO2/electrolyte interface [51,52]. DPP-4 based devices present largest Rct that impeded the transportation of photoelectrons from the CB of TiO2 to I 3 species. In the Bode phase plot, the lower frequency peak is indicative of the charge-transfer process of injected electrons in TiO2. In terms of the relation te ¼ 1/(2pf) (f is the peak frequency of lower-frequency

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Fig. 6. Molecular orbitals of organic dyes calculated using DFT methods.

range in Bode plots), the order of electron time (te) would be in reverse with that of the peak frequency of lower-frequency range (f) in their devices. So DPP-1 ~ 3 dyes display longer electron lifetime than that of DP-1 in their cells. The DSSC device of DPP-4 with the longest electron lifetime demonstrates the best photovotaic performance.

3.5. Long-term stability measurements Long-term stability is considered as an important parameter for outdoor application of DSSC in the future. Quasi-solid-state DSSCs have shown greatly improved long-term stability because of no leakage of the electrolyte [53,54]. Therefore, our quasi-solid-state DSSCs have been constructed for the long-term stability test. The photovoltaic parameters of quasi-solid-state DSSCs of DPP-1 ~ 4 are shown in Table 3 and Fig. S3. It can be seen that the photovoltaic performance of the quasi-solid-state DSSCs based on DPP-1 ~ 4 is in accordance with the trend for the DSSCs with liquid electrolyte. The device of DPP-4 exhibits the best performance of 6.59%. Thus, the stability of quasi-solid-state DSSCs based on DPP-4 was evaluated during 500 h under visible-light. Their photovoltaic performance parameters were depicted in Fig. 8 indicating 99% remains of the initial efficiency value.

4. Conclusions In summary, different p-spacers have been successfully introduced into DTF-PTZ dyes giving new structural dyes of DPP-1 ~ 4. Our experimental results demonstrated that when inserting substituted phenyl rings between DTF and PTZ as p-spacers, DPP1 ~ 3 sensitized solar cells display higher photoelectric conversion efficiency in accordance with their excellent absorption propensity in the UVevis absorption spectra. This phenomenon is attributed to an interruption of the conjugation between DTF and PTZ by pspacers. Moreover, the DSSC device of DPP-4 possessing a p-spacer of biphenyl ring presents the best photovoltaic performance (ƞ ¼ 7.66%, Voc ¼ 0.78 V, Jsc ¼ 13.84 mA cm2). Furthermore, quasisolid-state DSSCs based on DPP-4 display an ƞ of 6.59%, and thier ƞ values remain 99% of the initial efficiency value after continuous light soaking for 500 h. DFT calculation results indicate that the torsion structure of DPP-4 causes more complete charge separation and impedes intermolecular aggregation of DPP-4 on TiO2 surfaces. Two torsion angles of biphenyl ring might prevent the back reaction and contribute to the best performance of its DSC devices. Overall, this work might blaze a new path to the structural modification of sensitizers for developing high-performance dye-sensitized solar cells.

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Acknowledgment The authors acknowledge the financial support of this work from the National Natural Science Foundation of China (NSFC 21173106), the Foundation of State Key Laboratory of Coal Conversion (Grant No. J16-17-913) and the Fundamental Research Funds for the Central Universities (No. lzujbky-2014-246). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.05.120. References

Fig. 7. EIS spectra of DSSCs based on organic dyes measured at 0.75 V forward bias in the dark: (a) Nyquist diagram and (b) Bode phase plots. Table 3 Photovoltaic performance of quasi-solid-state DSSCs based on DPP-1 ~ 4. Dye

Jsc/mA cm2

Voc/V

FF (%)

ƞ (%)

DPP-1 DPP-2 DPP-3 DPP-4

9.70 11.39 10.96 13.45

0.64 0.72 0.67 0.72

68 69 69 68

4.22 5.66 5.07 6.59

Fig. 8. Evolutions of photovoltaic performance parameters for DPP-4 based quasisolid-state DSSCs during one sun soaking.

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