New thieno[3,2-b][1]benzothiophene-based organic sensitizers containing π-extended thiophene spacers for efficient dye-sensitized solar cells

Abstract

Three new thieno[3,2-b][1]benzothiophene (TBT)-based D–π–A organic sensitizers containing the thiophene π-spacer (SGT-121, 123 and 125) have been synthesized for the application of dye-sensitized solar cell (DSSC), where TBT was employed as a new fused π-bridge unit using the advantages of good co-planarity with the linkage between the thiophene unit and the phenyl unit of the triphenylamine group. Specifically, the combination of a dihexyloxyphenyl-substituted biphenylamine donor and the TBT π-bridge plays multifunctional roles, e.g., the enhanced ability of the π-bridge and donor, slow charge recombination and prevention of dye aggregation in the D–π–A sensitizer. The photophysical, electrochemical and photovoltaic properties of the SGT sensitizers were systematically investigated. As a strategy for the improvement of absorption abilities, the various thiophene derivatives, e.g., those with thiophene (T, SGT-121), bithiophene (BT, SGT-123) and thienothio[3,2-b]thiophene (TT, SGT-125) moieties, were incorporated as π-spacers between the TBT π-bridge and the acceptor unit. The introduction of thiophene π-spacers significantly improved the photovoltaic performance (in particular, in terms of the photocurrent J_sc and open-circuit voltage V_oc) compared to SGT-127 without the thiophene unit. The SGT sensitizers were systematically evaluated for DSSCs based on the Co(bpy)₃^2+/3+ (bpy = 2,2′-bipyridine) redox couple. Among the four SGT sensitizers, SGT-123-based DSSC including the BT moiety exhibited the highest power conversion efficiency of 9.69%, J_sc of 16.16 mA cm⁻², V_oc of 830 mV and fill factor of 0.72. These results present the impact of thiophene π-spacers for enhancing the photovoltaic performances of a D–π–A organic sensitizer.

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Introduction

The current shale-gas revolution brings about a new phase of low oil prices, but the global energy experts anticipate being faced with an energy crisis in the near future. As one of the alternate energy technologies, dye-sensitized solar cells (DSSCs) have received continuous attention, owing to their low fabrication cost, transparency, full colour and flexibility with respect to silicon-based solar cells.^1,2 The components of DSSCs, such as sensitizers, redox electrolyte and semiconductor metal oxides, have been investigated in order to achieve a high power conversion efficiency (PCE).^3,4 With the diverse sensitizers as the important component in enhancing the PCE, ruthenium (Ru)-complex-based DSSCs have achieved a PCE over 11%,⁵ whereas the zinc (Zn) porphyrin sensitizer (SM315)-based DSSC has shown a PCE over 13%.⁶ In contrast with the Ru-complex, metal-free organic sensitizers have become attractive sensitizers, owing to their high molar extinction coefficients,⁷ facile engineering of their optical properties,^8–10 low cost and relatively high PCEs over 10%.¹¹ Most of the reported, efficient, metal-free organic sensitizers are commonly composed of the donor–π-bridge–acceptor (D–π–A) configuration, which exhibits efficient intramolecular charge-transfer (ICT) properties for favourable electron injection from the excited dye to the conduction band of TiO₂ (E_CB).^12,13

For new breakthroughs in achieving higher PCEs, the development of the π-bridge unit is fundamental in the molecular engineering of D–π–A sensitizers, which should be introduced as planar-type building blocks of fused heterocycles instead of twisted structures.^4,13,14 The various fused heterocycles such as thienothiophene,¹⁵ dithienothiophene (DTT),¹⁶ dithieno[3,2-b:2′,3′]silole (DTS),¹⁷ 4H-cyclopenta[2,1-b:3,4-b′]dithiophene (CPDT),^8,18,19 dithieno[3,2 b:2′,3′]pyrrole (DTP),²⁰ benzo[1,2-b:4,5-b′]dithiophene (BDT1)²¹ and benzo[2,1-b:3,4-b′]dithiophene (BDT2)²² have been incorporated into the π-bridges with broad and intense spectral absorption in the visible-light region. Among these fused heterocycles, the CPDT π-bridge exhibited a record PCE over 10%,^8,11,23 which has many advantages such as a rigid co-planar structure, good electron-donating ability, enhancement of the photocurrent for efficient charge transfer²⁴ and prevention of dye aggregation by the long alkyl chain.²⁵

By merging insights from the good co-planar structure with the reduction in dihedral angle between the thiophene unit and the phenyl unit of the triphenylamine (TPA) donor,²⁶ we developed thieno[3,2-b][1]benzothiophene (TBT) as the new fused π-bridge for highly efficient metal-free organic sensitizers.²⁷ The TBT derivatives have also been widely employed as core materials for ferroelectric liquid crystals (FLC)²⁸ and organic thin-film transistors (OTFT)²⁹ with good electrical performances, strong fluorescence and high ambient stabilities.³⁰ For optimal D–π–A sensitizers, the dihexyloxyphenyl-substituted biphenylamine donor and TBT π-bridge unit (compound 1 in Scheme 1) were combined as a multifunctional core group, which could play various roles for enhanced ability of the π-bridge and donor, slow charge recombination and prevention of dye aggregation. To enhance the absorption ability of the multifunctional D–π core group, various thiophene derivatives,³¹ which have widely been incorporated into organic sensitizers because of the effective conjugation and co-planarity relative to benzene moieties³² as well as chemical stability and electronic tunability, were also introduced as the π-spacer between the TBT π-bridge and the A unit. Furthermore, to retard charge recombination with the electrolyte and to prevent dye aggregation, the dihexyloxyphenyl-substituted biphenylamine was used as the bulky donor group.^19,33 In this study, we employed thiophene derivatives as π-spacers, such as thiophene (T),³⁴ bithiophene (BT)^35,36 and thienothio[3,2-b]thiophene (TT),^15,37 for SGT-121, SGT-123 and SGT-125 (Scheme 1) in order to evaluate the effect of the π-spacer extension of the sensitizer on the optical properties and device performances.³⁸ SGT-127 has no thiophene π-spacer between the TBT π-bridge and the A unit so that the ability of the electron-rich thiophene unit could be evaluated. By controlling the optical and electrochemical properties of thiophene π-spacers, SGT-123 containing the BT moiety exhibited a significantly enhanced photocurrent and voltage, simultaneously.

Scheme 1 Synthesis of SGT-121, SGT-123, SGT-125 and SGT-127.

Results and discussion

The synthetic routes for the new TBT-based organic sensitizers with the various thiophene derivatives as the π-spacers are illustrated in Scheme 1. The various thiophene derivatives as the π-spacers were introduced between the TBT π-bridge and the cyanoacrylic acid (A) of the D–π–A sensitizers, coded as the T, BT and TT moieties for SGT-121, SGT-123 and SGT-125, respectively. For all SGT sensitizers, the starting material (2) was prepared by bromination using the N-bromosuccinimide (NBS) of 1 followed by the Buchwald–Hartwig C–N coupling reaction³⁹ between 6-bromothieno[3,2-b][1]benzothiophene²⁷ and the biphenylamine donor (Scheme S1†). The aldehyde derivatives (4) were prepared through a Stille coupling reaction⁴⁰ of 2 with the tin derivative (3) of each protected aldehyde. The aldehyde derivative 5 for SGT-127 was prepared through formylation using n-BuLi/N-formylpiperidine with 2 at low temperature. The final sensitizers of SGT-121, SGT-123, SGT-125 and SGT-127 were prepared through Knoevenagel condensation with cyanoacrylic acid.⁴¹ The chemical structures of the final sensitizers were characterized by ¹H NMR, ¹³C NMR and MALDI-TOF mass spectroscopies, and their corresponding characterisation data are summarized in the Experimental section.

The absorption and emission spectra of the SGT sensitizers in THF are shown in Fig. 1a, and the photophysical properties are summarized in Table 1. All of the sensitizers show two absorption wavelengths (λ_max) in the ranges of 300–400 and 400–600 nm, respectively. The former corresponds to the π–π* transition and the latter is attributed to intramolecular charge transfer (ICT) from D to A in the D–π–A sensitizer. The λ_max and molar extinction coefficient (ε) of thiophene-based sensitizers are known to increase with the extension of thiophene chain length.^{15,37,38a,38c} Compared to SGT-127, the λ_max and ε values of SGT-121, SGT-123 and SGT-125 were red-shifted by 10, 15.5 and 8 nm, respectively, and were enhanced above 4 × 10³ M⁻¹ cm⁻¹ in the 400–600 nm region, which is consistent with the insertion of electron-rich thiophene units.^38d Although the ε value of planar, fused, TT-based SGT-125 was higher than that of SGT-121, the λ_max of SGT-125 is slightly blue-shifted by 2 nm with respect to SGT-121, despite increasing the π-spacer length from the SGT-121 (2.52 Å) to SGT-125 (4.62 Å). This can be explained by the dihedral angle (4.58°) of SGT-125 between the TBT unit and the TT π-spacer being larger than that of SGT-121 (1.39°), which disturbs charge transfer and results in the hypsochromic shift in absorption wavelength. Interestingly, the longest π-conjugated SGT-123 system exhibited the highest λ_max and ε value as well as the broadest absorption band,^36,38c,38d indicating that the elongation of π-conjugation length may play as a key role in the absorption ability. The emission maximum (λ_em) of each sensitizer appeared at 588 (SGT-127), 621 (SGT-125), 625 (SGT-121) and 646 nm (SGT-123), which follows the same trend as that in the absorption spectra (Fig. 2a and Table 1).

Fig. 1 (a) UV/Vis absorption and emission spectra of the SGT sensitizers measured in THF. (b) Calculated frontier molecular orbitals and experimental energy-level diagram of the SGT sensitizers.

Table 1 Photophysical and electrochemical properties of the SGT sensitizers

Dye	λabs max^a (nm)	ε (M^−1 cm^−1)	λem max^a (nm)	E0–0^b (eV)	Eox^c (V vs. NHE)	E^*ox^d (V vs. NHE)	ΔGinj^e (eV)	θ^f (°)
a Absorption and emission spectra were measured in THF.b E0–0 was determined from the intersection of absorption and emission spectra in THF.c Oxidation potentials of dyes on TiO2 were measured in CH3CN with 0.1 M TBAPF6 and a scan rate of 50 mV s^−1 (calibrated with Fc/Fc^+ as an external reference and converted to NHE by addition of 0.63 V).d Excited-state oxidation potentials were calculated according to Eox − E0–0.e Driving force for electron injection from the E^*ox to the ECB of TiO2 (−0.5 V vs. NHE).f Dihedral angle between TBT unit and thiophene π-spacer.
SGT-121	347	31 227	625	2.30	1.14	−1.18	0.68	1.39
	469	28 007
SGT-123	345.5	25 066	646	2.25	1.10	−1.15	0.65	10.28
	474.5	29 125
SGT-125	345	26 918	621	2.32	1.15	−1.17	0.67	4.58
	467	28 898
SGT-127	344	31 185	588	2.42	1.21	−1.21	0.71	—
	459	23 474

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Fig. 2 TR-PL decay traces of dye-adsorbed mesoporous Al₂O₃ (dotted line) and TiO₂ films (solid line) for SGT-121 (red), SGT-123 (blue), SGT-125 (green) and SGT-127 (black). Excitation wavelength: 393 nm; detection wavelength: λ_{em max} of each sensitizer.

The electrochemical properties of sensitizers were investigated by cyclic voltammetry (CV) in water-free acetonitrile containing 0.1 M TBAPF₆ as the supporting electrolyte at room temperature (Fig. S2†), and the corresponding energy diagram is shown in Fig. 1b with electrochemical data summarized in Table 1. The redox potentials (E_ox) of SGT-121, SGT-123, SGT-125 and SGT-127 correspond to the highest occupied molecular orbital (HOMO) energy levels located at 1.14, 1.1, 1.15 and 1.21 V vs. NHE, respectively, which are more positive than that of the Co²⁺/Co³⁺ redox couple (0.59 V vs. NHE),⁴² and thus offer sufficient driving forces for the regeneration of oxidized dyes. As thiophene π-spacers are inserted between TBT and A, the HOMO energy levels of SGT-121, SGT-123 and SGT-125 are upshifted by about 0.07, 0.11 and 0.06 V, respectively, leading to a decrease in their HOMO–LUMO energy gaps compared with SGT-127. Among them, SGT-123, with the longest π-conjugation length, shows the smallest band gap, which is favourable for the bathochromic shift in the absorption spectra, contributing to improvements in the photocurrent (J_sc). The lowest unoccupied molecular orbital (LUMO) energy levels of sensitizers were calculated using the equation of E_ox − E_0–0, where E_0–0 is the zero–zero transition energy derived from the intersection of the normalized absorption and emission spectra. The LUMO energy levels (E^*_ox) of SGT-121 to SGT-127 are −1.18, −1.15, −1.17 and −1.21 V vs. NHE, which are more negative than the TiO₂ conduction band (E_CB = −0.5 V vs. NHE).⁴³ As shown in Table 1, the E^*_ox − E_CB values, i.e., the driving forces (ΔG_inj) of the sensitizers are sufficient for electron injection from the excited state of the sensitizers into the E_CB of TiO₂ to occur.² The E^*_ox of SGT-127 without a thiophene π-spacer is clearly upshifted with respect to the other sensitizers, which causes the hypsochromic shift in the absorption spectrum and results in a low photocurrent. In contrast, SGT-123 has a more positive E^*_ox value as well as a lower ΔG_inj than the other sensitizers, which can be expected to ensure better electron injection, higher photocurrent and efficiency.

To get an insight into the geometric and electronic properties of the sensitizers, density functional theory (DFT) calculations were performed using the Gaussian 09 program at the B3LYP/6-31G(d,p) level;⁴⁴ the HOMO and LUMO electron distributions are shown in Fig. 1b. The HOMO is distributed along the donor part and the TBT π-bridge, whereas the LUMO is delocalized over the cyanoacrylic acid and thiophene π-spacer. This significant charge separation within the sensitizers can also result in efficient electron injection upon photoexcitation. In addition, as all sensitizers have similar dihedral angles (>40°) between the N atom of the amine donor and the TBT π-bridge, the thiophene π-spacer lengths and their dihedral angles can have important roles in the electronic communication between the TBT π-bridge and the A (Fig. S1†).

To understand the difference of IPCE maximum fluctuation for all SGT sensitizers, the electron injection efficiency (η_inj) was measured for dye-coated Al₂O₃ and TiO₂ films by the time-resolved photoluminescence (TR-PL) (Fig. 2).⁴⁵ The measured PL signal intensity (I_PL) was fitted by three exponentials, I_PL   =   A₁ exp (−t/τ₁)    +   A₂  exp (−t/τ₂)   +     A₃  exp (−t/τ₃) . For all of the sensitizers, the TiO₂ film shows significantly faster PL decay than the Al₂O₃ film, because electron injection occurs at the TiO₂/dye interface but not at the Al₂O₃ interface (Fig. 2). Therefore, the PL decays of the Al₂O₃ films provide the lifetimes of the sensitizers themselves, whereas the PL decays of the TiO₂ films give information about the dynamics of electron injection. As the structural heterogeneity of film samples often gives rise to multi-exponential PL decay, we calculated the amplitude-weighted average lifetimes (τ_av) of the PL decays in order to simplify the comparison between the PL decays of various samples, as summarized in Table 2. We calculated η_inj using the equation of η_inj = 1 − (τ_TiO2)/(τ_Al2O3). The calculated η_inj values of SGT-121, SGT-123, SGT-125 and SGT-127 were 91.2, 98.4, 91.9 and 83.8%, respectively. We can see that η_inj increases in the order of SGT-127 < SGT-121 < SGT-125 < SGT-123, indicating that the E^*_ox value is positively shifted by the insertion of thiophene π-spacers. Considering the significant difference of η_inj values, we can expect that the electron injection efficiency (η_inj) has effect on the IPCE variation and device performance in our DSSCs.

Table 2 Fit parameters of the PL decays shown in Fig. 2 and the calculated electron injection efficiencies

		τ1 (ns)	A1	τ2 (ns)	A2	τ3 (ns)	A3	〈τ〉av^a (ns)	ηinj (%)
a The values of τav were determined using the equation 〈τ〉av = A1τ1 + A2τ2 + A3τ3.
SGT-121	Al2O3	3.82	1.9	1.143	33.8	0.2437	64.3	0.619	91.2
	TiO2	1.246	0.4	0.243	6.2	0.0367	93.4	0.054
SGT-123	Al2O3	3.666	1.2	1.015	32.7	0.2802	66.1	0.561	98.4
	TiO2	1.041	0.0	0.195	0.7	0.0076	99.3	0.009
SGT-125	Al2O3	3.258	1.7	0.903	24.7	0.2060	73.6	0.430	91.9
	TiO2	0.857	0.3	0.176	5.0	0.0246	94.7	0.035
SGT-127	Al2O3	2.684	2.2	0.716	23.8	0.1522	74.0	0.343	83.8
	TiO2	0.931	0.9	0.211	9.5	0.0304	89.6	0.056

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On the basis of in-depth optical and electrochemical evaluations of the sensitizers, their photovoltaic performances were evaluated in Co(bpy)₃^2+/3+ (bpy = 2,2′-bipyridine), exhibiting a higher open-circuit voltage (V_oc) than the traditional I⁻/I₃⁻ redox couple;⁴² the DSSC fabrication details are described in the ESI.† The current–voltage (J–V) and incident photon-to-current conversion efficiency (IPCE) curves are shown in Fig. 3a and b, with the photovoltaic parameters summarized in Table 3. Clearly, DSSCs based on SGT-121, SGT-123 and SGT-125 significantly increased in terms of J_sc to over 14 mA cm⁻² compared to SGT-127 without a π-spacer (8.67 mA cm⁻²), which is attributed to the increased light-harvesting ability (LHE) of the electron-rich thiophene unit.^15,36,37,38a The J_sc values of the DSSCs increased in the order of elongation of the π-spacer length, which is consistent with the absorption and IPCE spectra (Fig. 1a and 3b). As shown in Fig. 3b, the IPCE onsets and maximum IPCE values increase in the same order as J_sc, which is enough to prove the enhanced J_sc of all SGT sensitizers, according to the additional difference in the amount of dye adsorbed on the TiO₂ surface (Table 3). The SGT-123-based DSSC exhibited the highest performance (J_sc, V_oc, fill factor (FF) and PCE = 16.16 mA cm⁻², 830 mV, 0.72 and 9.69%, respectively), whose values are much higher than those of SGT-series sensitizers (Table 3).

Fig. 3 (a) Current–voltage (J–V) characteristics of the DSSCs measured under one sun illuminated AM 1.5G. The TiO₂ film thickness is 5.5 μm for the active layer and 3.3 μm for the scattering layer. (b) The corresponding IPCE spectra.

Table 3 Photovoltaic performances of the DSSCs based on the various sensitizers under one sun illumination (AM 1.5G) (mean of three DSSCs)

Dye^a	Adsorption amount (10^−7 mol cm^−2)	Jsc (mA cm^−2)	Voc (mV)	FF (%)	PCE^b (%)
a Dipping solution: 0.3 mM dye solution (EtOH/THF = 2 : 1) with 20 mM CDCA.b Irradiated light: AM 1.5G (100 mW cm^−2); cell area tested with a metal mask: 0.141 cm^2. Cobalt electrolyte: 0.22 M [Co^II(bpy)3](B(CN)4)2, 0.05 M [Co^III(bpy)3](B(CN)4)3, 0.1 M LiClO4 and 0.8 M TBP in acetonitrile (ACN).
SGT-121	0.99	14.00 ± 0.09	809 ± 4.7	72.71 ± 0.74	8.24 ± 0.06
SGT-123	1.48	16.16 ± 0.57	830 ± 3.2	72.09 ± 0.15	9.69 ± 0.35
SGT-125	1.32	14.74 ± 0.15	773 ± 1.4	71.86 ± 0.58	8.20 ± 0.16
SGT-127	0.91	8.67 ± 0.17	707 ± 5.0	70.45 ± 1.17	4.32 ± 0.05

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Most of the reported photovoltaic performances of DSSCs with thiophene-based sensitizers in the I⁻/I₃⁻ redox electrolyte show an increase in J_sc with the extension of π-conjugation length, whereas their V_oc values decrease with the opposite trend.^38a,38d,46 Unlike this tendency, the V_oc values for our sensitizer-based DSSCs employing the Co(bpy)₃^2+/3+ redox couple increase in the order of SGT-127 (707 mV) < SGT-125 (773 mV) < SGT-121 (809 mV) < SGT-123 (830 mV). In general, the V_oc is dependent upon the difference between the quasi-Fermi level of TiO₂ (E_Fn) and the redox potential of the electrolyte (E_ox).⁴⁷ Considering that the same Co(bpy)₃^2+/3+ redox electrolyte was used for all of the DSSCs, the V_oc may be determined by the E_Fn of TiO₂, which is related to the position of the TiO₂ conduction band (E_CB) and the electron density in TiO₂.⁴⁸ In order to demonstrate these differences in V_oc values in the DSSCs, we conducted several electrochemical measurements using electrochemical impedance spectroscopy (EIS), density of occupied trap states (DOS) of dye-coated TiO₂, intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS); see also Fig. S3† for an extended discussion.

The EIS of the DSSCs was measured under a −0.79 V applied bias voltage in the dark, except for the SGT-127-based DSSC (−0.74 V) with the lower V_oc. Nyquist and Bode plots are shown in Fig. 4a and b, and the EIS data estimated with an appropriate equivalent circuit (EC)⁴⁹ are listed in Table 4. In the Nyquist plots (Fig. 4a), the three semicircles located in the high-, middle- and low-frequency regions are assigned to the charge-transfer resistance (R_ct) at the Pt/electrolyte interface, the charge recombination resistance (R_rec) at the TiO₂/dye/electrolyte interface and mass transport of the redox couple in the bulk electrolyte solution, respectively.⁵⁰ With the large radius of the middle semicircle, the R_rec values increased in the order of SGT-125 (27.3 Ω) < SGT-121 (27.97 Ω) < SGT-123 (83.73 Ω) (Table 4), indicating that the SGT-123-based DSSC can retard charge recombination more efficiently than the other sensitizers, resulting in the highest V_oc. Furthermore, the capacitance (C_μ) at the TiO₂/dye/electrolyte interface can also significantly affect the V_oc of DSSCs, owing to the E_CB shift of TiO₂.⁵¹ The difference in the C_μ was slight, but it decreased in the order of SGT-123 > SGT-121 > SGT-125, implying an upshift in the E_CB. The electron lifetimes (τ_r) estimated by τ_r = C_μR_rec were 9.65 (SGT-121), 29.66 (SGT-123), 9.57 (SGT-125) and 10.56 ms (SGT-127), which is in agreement with the observed V_oc values for SGT-123 (830 mV), SGT-121 (809 mV), SGT-125 (773 mV) and SGT-127 (707 mV). This trend could be also confirmed in the mid-frequency region of the Bode phase plot (Fig. 4b). The longer electron lifetime of SGT-123 compared to the other sensitizers is associated with more effective suppression of the back reaction of the injected electron with Co(bpy)₃³⁺ in the electrolyte, leading to a higher V_oc and improved device efficiency. Moreover, the photovoltaic performance can be confirmed by the charge-collection efficiency (η_cc) derived from η_cc = (1 + R_tr/R_rec)⁻¹ or η_cc = (1 + τ_n/τ_r)⁻¹.^42a,48 The η_cc values of the DSSCs calculated from the above EIS parameters are shown in Fig. 4c and Table 4. The comparatively high R_rec value observed for SGT-123 can retard charge recombination, resulting in an increased η_cc of the DSSC. The calculated η_cc values for SGT-121, SGT-123, SGT-125 and SGT-127 were equal to be 95.22, 97.23, 94.36 and 93.7%, respectively. Moreover, the electron transport, recombination time and η_cc values of all DSSCs were also obtained from IMVS and IMPS measurements (Fig. S3†), which were in good agreement with the EIS results.

Fig. 4 (a) Nyquist and (b) Bode plots obtained from the DSSCs based on the various sensitizers in the dark with a forward bias of −0.79 V, except for SGT-127 (−0.74 V). (c) Charge-collection efficiency (η_cc) of the DSSCs according to EIS measurements. (d) The exponential distribution of the DOS deduced by plotting the capacitive charge of electrons in the surface states versus the applied potential for the TiO₂/SGTs/CDCA electrodes. The capacitive charging current at a negative potential is attributed to the filling of surface states below the conduction band edge of TiO₂ for all structures.

Table 4 EIS, IMVS and IMPS parameters for the DSSCs based on various SGT sensitizers

Device	EIS^a						IMVS & IMPS
	Rtr (Ω)	Rrec (Ω)	Cμ (mF)	τn (ms)	τr (ms)	ηcc (%)	τr (ms)	τn (ms)	ηcc (%)
a Values calculated from EIS data measured at a forward bias of −0.79 V (−0.74 V for SGT-127) under dark conditions. Rtr: transport resistance; Rrec: charge-recombination resistance; Cμ: chemical capacitance; τn: transport time; τr: electron lifetime; ηcc: charge-collection efficiency.
SGT-121	1.40	27.97	0.34	0.48	9.65	95.22	34.94	5.95	82.67
SGT-123	2.36	83.73	0.36	0.84	29.66	97.23	38.08	5.40	85.43
SGT-125	1.63	27.3	0.35	0.57	9.57	94.36	22.19	4.46	79.42
SGT-127	2.44	36.38	0.29	0.70	10.56	93.70	50.14	11.14	77.88

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To further investigate the differences in V_oc with changes in the trap state induced by each sensitizer, CV was performed for each SGT sensitizer-absorbed TiO₂ electrode with CDCA in 0.1 M LiClO₄ and acetonitrile (Fig. S4†). Fig. 4d shows the capacitive currents of the different electrodes at the TiO₂/LiClO₄ interface, which exhibited gradual onsets under a forward potential. The distribution of the trap states can be estimated from the density of states (DOS) calculation: DOS = (dQ/dV)(N_A/F),⁵² where Q is the total number of surface trapping sites, N_A is Avogadro's number, F is the Faraday constant and V is the potential applied to the electrode. Compared with the onsets of the capacitive currents, the onsets move upwards in the order of SGT-127 (−0.212 V) < SGT-125 (−0.224 V) < SGT-121 (−0.242 V) < SGT-123 (−0.261 V), indicating that the upshift of E_CB in the above order can contribute to the increase in the V_oc.⁴⁸ Therefore, the highest V_oc of the SGT-123-based DSSC can be explained by the negative shift in the E_Fn level of its electrode. Considering all factors for the V_oc differences, the enhancement of V_oc can be caused either by a negative shift in the E_CB or the retarded recombination of the injected charge in TiO₂.^47a,48,53 In this respect, both the slower charge-recombination rate observed by EIS and IMVS analyses (Fig. 4) and the negative shift in the E_CB according to the DOS results (Fig. 4d) influence the increase in V_oc for the SGT-123-based DSSC. The V_oc of the SGT-125 based DSSC decreased by 36 mV compared to SGT-121, which can be explained by the strong aggregation of introduced TT π-spacer.^31c The V_oc results also suggest that the π-conjugation length of the sensitizers can slightly impact dye adsorption and suppress the recombination by complex formation with the redox electrolyte.⁵⁴ Thus, the longest π-conjugation length of SGT-123 can decrease the recombination possibility between the charge and the Co(bpy)₃³⁺ ion, which results in an enhancement of the V_oc.²³

To obtain insights into the mass-transport limitation of Co(bpy)₃³⁺ ions and the recombination kinetics, photocurrent transients⁵⁵ and open-circuit voltage decay (OCVD) measurements⁵⁶ were recorded for the complete DSSCs. As shown in Fig. 5a, the ratio of the initial peak current to the steady-state current in the photocurrent transients slightly decreased in the order of SGT-123 > SGT-121 > SGT-125 > SGT-127, indicating that the retarded mass transport can also lead to losses in the photocurrents and the DSSC performances. As can be seen in Fig. 5b, after 1 s of illumination, the V_oc of the SGT-127-based DSSC exhibited a sharp decay, owing to fast electron recombination related to the electron lifetime. On the other hand, the V_oc of the DSSC based on SGT-123 showed remarkably slower decay rates relative to the other DSSCs, which means a slower recombination rate and a longer electron lifetime. In other words, there were more electrons surviving the back-reactions in the DSSC based on SGT-123 (inset in Fig. 5b).

Fig. 5 (a) Photocurrent transient dynamics of the DSSCs with various sensitizers. (b) Open-circuit voltage decay (OCVD) curves for the same DSSCs. The inset shows the electron lifetimes of the same DSSCs.

Conclusions

A series of sensitizers based on the π-spacer units of thiophene derivatives, e.g., thiophene (T), bithiophene (BT) and thienothio[3,2-b]thiophene (TT) moieties, containing dihexyloxyphenyl-substituted biphenylamine as an electron donor, thieno[3,2-b][1]benzothiophene (TBT) as the new fused π-bridge and cyanoacrylic acid as acceptor/anchor, has been specifically developed as SGT-121, SGT-123 and SGT-125 for high-efficiency DSSCs. We evaluate the effect of extended thiophene π-spacers in D–π–A organic sensitizers on the absorption, energy levels and photovoltaic performances of the sensitizers compared with SGT-127 without thiophene, which acts as the reference sensitizer. It is found that the electronic distributions of all SGT sensitizers exhibit good charge separation between the HOMO and LUMO energy levels. On the other hand, the extension of the π-spacer length close to the acceptor group clearly red-shifts the absorption band, increases the molar extinction coefficients and positively shifts the LUMO levels compared to SGT-127, which leads to enhanced DSSC performances. The photovoltaic performances of SGT sensitizers are investigated in the Co(bpy)₃^2+/3+ (bpy = 2,2′-bipyridine) redox couple, exhibiting a higher V_oc than in the I⁻/I₃⁻ electrolyte. As the π-conjugation length of the sensitizer is increased, the J_sc and IPCE onset values are enhanced in the order of the π-conjugation length. From the results of EIS, DOS and IMVS analyses, the SGT-123-based DSSC exhibits the highest V_oc, indicating a slower charge-recombination rate and a negative shift of the E_CB of TiO₂. Consequently, the DSSC based on SGT-123 containing BT shows the best performance with a PCE of 9.69%, J_sc of 16.16 mA cm⁻², V_oc of 830 mV and FF of 0.72, which are much higher than those of the SGT-127 reference sensitizer (4.32%, 8.67 mA cm⁻², 707 mV and 70.45%). These structure–performance relationships provide valuable information for the modification of thiophene-based π-spacers in organic sensitizers.

Experimental section

Materials and synthesis

All reactions were carried out under a nitrogen atmosphere. Solvents were distilled from appropriate reagents. All reagents were purchased from Sigma-Aldrich, TCI and Alfa Aesar. 6-Bromothieno[3,2-b][1]benzothiophene,²⁷ bis(2′,4′-dihexyloxybiphenyl-4-yl)-amine,¹⁹ [5-(1,3-dioxolan-2-yl)thiophen-2-yl]tributylstannane (3a),⁵⁷ [5′-(1,3-dioxolan-2-yl)-2,2′-bithiophen-5-yl]tributylstannane (3b)⁵⁸ and [5-(1,3-dioxolan-2-yl)thieno[3,2-b]thiophen-2-yl]tributylstannane (3c)⁵⁹ were synthesized according to previous reports, but with slight modification. The synthetic routes for SGT-121, SGT-123, SGT-125 and SGT-127 are showed in Scheme 1 and the details can be depicted as follows.

N,N-Bis(2′,4′-dihexyloxybiphenyl-4-yl)-thieno[3,2-b][1]benzothiophen-6-amine (1)²⁷. Bis(2′,4′-dihexyloxybiphenyl-4-yl)-amine (0.402 g, 0.56 mmol), 6-bromothieno[3,2-b][1]benzothiophene (0.15 g, 0.56 mmol), Pd₂(dba)₃ (0.02 g, 0.02 mmol), P(t-Bu)₃ (0.01 mL, 0.02 mmol), and sodium-tert-butoxide (0.57 g, 5.91 mmol) in dry toluene (15 mL) was heat to reflux for 1 h. After cooling to room temperature, saturated ammonium chloride solution was added to the reaction solution. The solvent was removed by rotary evaporation, and the crude product was purified by column chromatography on silica gel with CH₂Cl₂/n-hexane (v/v, 1 : 3) as eluent to afford 1 as a yellow sticky solid (0.4 g, 80%). ¹H NMR (300 MHz, CDCl₃) δ 7.73 (d, J = 8.4 Hz, 1H), 7.62 (s, 1H), 7.41–47 (m, 5H), 7.27–7.30 (m, 4H), 7.15–7.17 (d, 4H), 6.52–6.55 (m, 4H), 3.94–4.01 (q, 8H), 1.84 (m, 8H), 1.28–1.50 (m, 24H), 0.91 (m, 12H). ¹³C NMR (300 MHz, CDCl₃): δ 159.6, 157.05, 146.0, 145.2, 144.1, 137.2, 134.8, 133.0, 130.9, 130.3, 128.0, 127.0, 123.5, 122.9, 122.5, 121.3, 120.4, 119.1, 105.3, 100.4, 68.4, 31.7, 29.4, 25.9, 22.7, 14.1. MS (MALDI-TOF) m/z calculated for C₅₈H₇₁NO₄S₂: 910.3186; found, 909.2919.

N,N-Bis(2′,4′-dihexyloxybiphenyl-4-yl)-2-bromothieno[3,2-b][1]benzothiophen-6-amine (2)²⁷. The compound 1 (0.68 g, 0.75 mmol) was dissolved in 100 mL of chloroform. N-Bromosuccinimide (0.13 g, 0.75 mmol) was added in one portion. A mixture was stirred for 1 h and the mixture was poured into water. Aqueous was extracted with dichloromethane and the combined organic layer was dried over anhydrous MgSO₄. The solvent was removed by rotary evaporation, and the crude product was purified by column chromatography on silica gel with CH₂Cl₂/n-hexane (v/v, 1 : 2) as eluent to afford 2 as a yellow sticky solid (0.6 g, 81%). ¹H NMR (300 MHz, CDCl₃) δ 7.65 (m, 2H), 7.49–52 (d, 6H), 7.27–7.33 (m, 4H), 7.19–21 (d, J = 8.4 Hz, 4H), 6.56 (m, 4H), 4.01 (q, 8H), 1.91 (m, 8H), 1.33–1.49 (m, 24H), 0.91 (m, 12H). ¹³C NMR (300 MHz, CDCl₃): δ 159.6, 156.9, 1445.8, 145.7, 145.4, 142.9, 135.6, 134.9, 134.0, 133.1, 130.9, 130.3, 127.4, 123.7, 123.1, 122.3, 121.1, 118.4, 112.8, 105.2, 100.3, 68.3, 31.7, 29.4, 25.8, 22.7, 14.1.

[5-(1,3-Dioxolan-2-yl)thiophen-2-yl]tributylstannane (3a). Compound 3a was synthesized according to previously reported methods.^{57 1}H NMR (300 MHz, CDCl₃): δ 7.27–7.28 (d, J = 3.3 Hz, 1H), 7.04–7.05 (d, J = 3.3 Hz, 1H), 6.15 (s, 1H), 4.13–4.16 (m, 2H), 4.01–4.04 (m, 2H), 1.50–1.58 (m, 6H), 1.29–1.36 (m, 6H), 1.06–1.11 (m, 6H), 0.86–0.92 (m, 9H).

[5′-(1,3-Dioxolan-2-yl)-2,2′-bithiophen-5-yl]tributylstannane (3b). Compound 3b was synthesized according to previously reported methods.^{58 1}H NMR (300 MHz, CDCl₃): δ 7.26–7.27 (d, J = 3.3 Hz, 1H), 6.99–7.05 (m, 3H), 6.07 (s, 1H), 4.12–4.14 (m, 2H), 3.99–4.04 (m, 2H), 1.52–1.65 (m, 6H), 1.28–1.38 (m, 6H), 1.08–1.13 (m, 6H), 0.87–0.93 (m, 9H).

[5-(1,3-Dioxolan-2-yl)thieno-3,2-b-thiophen-2-yl]tributylstannane (3c). Compound 3c was synthesized according to previously published reports.^{59 1}H NMR (300 MHz, CDCl₃): δ 7.34–7.36 (d, 1H), 7.23–7.24 (d, 1H), 6.16 (s, 1H), 4.12–4.16 (m, 2H), 4.00–4.05 (m, 2H), 1.54–1.63 (m, 6H), 1.28–1.37 (m, 6H), 1.10–1.15 (m, 6H), 0.87–0.93 (m, 9H).

5-(6-[Bis(2′,4′-dihexyloxybiphenyl-4-yl)amino]thieno[3,2-b][1]benzothiophen-2-yl)thiophene-2-carbaldehyde (4a). A mixture of compound 2 (0.1 g, 0.10 mmol), 3a (0.05 g, 0.11 mmol) and Pd(PPh₃)₂Cl₂ (0.01 g, 0.01 mmol) in dry toluene (20 mL) was heated at 110 °C for 12 h under an inert N₂ atmosphere. After complete consumption of the starting materials, dilute hydrochloric acid was added and the mixture was stirred at room temperature for 10 min and extracted with dichloromethane. The organic phase was washed in brine, dried over MgSO₄ and the solvent was evaporated under vacuum. The crude product was purified by column chromatography on silica gel with CH₂Cl₂/n-hexane (v/v, 1 : 1) to afford 4a as a yellow sticky solid (0.05 g, 48%). ¹H NMR (300 MHz, CDCl₃): δ 9.87 (s, 1H), 7.69 (d, J = 5.2 Hz, 1H), 7.67 (s, 1H), 7.58 (d, J = 1.8 Hz, 1H), 7.53 (d, 1H), 7.51 (s, 1H), 7.45–7.48 (d, J = 8.7 Hz, 4H), 7.30–7.82 (m, 3H), 7.15–7.18 (d, J = 8.7 Hz, 4H), 6.53 (m, 4H), 3.94 (q, 8H), 1.73–1.82 (m, 8H), 1.29–1.50 (m, 24H), 0.86 (m, 12H).

5-(6-[Bis(2′,4′-dihexyloxybiphenyl-4-yl)amino]thieno[3,2-b][1]benzothiophen-2-yl)-2,2′-bithiophene-5-carbaldehyde (4b). The synthetic procedure for 4b was similar to that for 4a, except that compound 3b (0.06 g, 0.12 mmol) was used instead of 3a. After complete consumption of the starting materials, dilute hydrochloric acid was added and the mixture was stirred at room temperature for 10 min and extracted with dichloromethane. The organic phase was washed in brine, dried over MgSO₄ and the solvent was evaporated under vacuum. The crude product was purified by column chromatography on silica gel with CH₂Cl₂/n-hexane (v/v, 2 : 1) to afford 4b (0.09 g, 74%). ¹H NMR (300 MHz, CDCl₃): δ 9.83 (s, 1H), 7.63 (d, 1H), 7.62 (s, 1H), 7.58 (d, J = 1.8 Hz, 1H), 7.45–7.47 (d, J = 6.9 Hz, 4H), 7.32 (s, 1H), 7.23–7.25 (m, 4H), 7.19 (d, 2H), 7.14 (m, 4H), 6.54 (m, 4H), 3.95 (q, 8H), 1.72–1.81 (m, 8H), 1.27–1.50 (m, 24H), 0.88 (m, 12H).

5-(6-[Bis(2′,4′-dihexyloxybiphenyl-4-yl)amino]thieno[3,2-b][1]benzothiophen-2-yl)thieno[3,2-b]thiophene-2-carbaldehyde (4c). The synthetic procedure for 4c was similar to that for 4a, except that compound 3c (0.07 g, 0.13 mmol) was used instead of 3a. After complete consumption of the starting materials, dilute hydrochloric acid was added and the mixture was stirred at room temperature for 10 min and extracted with dichloromethane. The organic phase was washed in brine, dried over MgSO₄ and the solvent was evaporated under vacuum. The crude product was purified by column chromatography on silica gel with CH₂Cl₂/n-hexane (v/v, 2 : 1) to afford 4c (0.07 g, 59%). ¹H NMR (300 MHz, CDCl₃): δ 9.92 (s, 1H), 7.66 (d, 1H), 7.59 (d, 1H), 7.43–7.48 (m, J = 8.7 Hz, 6H), 7.27–7.34 (m, 4H), 7.19 (d, J = 8.7 Hz, 4H), 6.55 (m, 4H), 3.97 (q, 8H), 1.71–1.80 (m, 8H), 1.28–1.48 (m, 24H), 0.89 (m, 12H).

5-[6-Bis(2′,4′-dihexyloxybiphenyl-4-yl)amino]thieno[3,2-b][1]benzothiophene-2-carbaldehyde (5). n-BuLi (0.06 mL, 2.5 M in n-hexane) was added drop-wise to a solution of compound 2 (0.106 g, 0.11 mmol) in anhydrous THF (50 mL) at −78 °C under a N₂ atmosphere. The reaction was kept at −78 °C for 1 h. Then, N-formylpiperidine (0.02 mL, 0.21 mmol) was added, the mixture was allowed to warm to room temperature and stirred overnight. The solution was extracted with ether and the combined organic layers were dried over anhydrous Na₂SO₄ before being evaporated to dryness. The residue was purified by chromatography on silica gel, eluting with CH₂Cl₂/hexane(v/v, 1/2) to afford 5 as a yellow sticky solid (50 mg, 49%). ¹H NMR (300 MHz, CDCl₃) δ 9.95 (s, 1H), 7.91 (s, 1H), 7.792 (d, J = 8.4 Hz, 1H), 7.44–7.55 (m, 6H), 7.17 (d, J = 8.4 Hz, 6H), 6.55 (m, 4H), 3.95 (q, 8H), 1.71–1.82 (m, 8H), 1.29–1.49 (m, 24H), 0.88 (m, 12H).

3-[5-(6-[Bis(2′,4′-dihexyloxybiphenyl-4-yl)amino]thieno[3,2-b][1]benzothiophen-2-yl)thiophene-2-yl]-2-cyanoacrylic acid (SGT-121). Compound 4a (0.05 g, 0.05 mmol), dissolved in CHCl₃ (6 mL) and acetonitrile (6 mL), was condensed with 2-cyanoacetic acid (0.01 g, 0.17 mmol) in the presence of piperidine (0.01 g, 0.17 mmol). The mixture was refluxed for 12 h. After cooling the solution, the organic layer was removed in vacuo. The red solid of SGT-121 was obtained by silica gel chromatography [CH₂Cl₂/MeOH (v/v, 10 : 1)]. Yield: 75% (40 mg). M.p.: 171–173 °C. ¹H NMR (300 MHz, CDCl₃) δ 8.31 (s, 1H), 7.74 (d, J = 8.7 Hz, 1H), 7.69 (d, 1H), 7.60 (s, 1H), 7.57 (d, 1H), 7.52 (s, 1H), 7.46–7.49 (d, J = 8.7 Hz, 4H), 7.28–7.34 (m, 3H), 7.16–7.19 (d, J = 8.7 Hz, 4H), 6.53 (m, 4H), 3.94 (q, 8H), 1.73–1.80 (m, 8H), 1.25–1.48 (m, 24H), 0.86 (m, 12H). ¹³C NMR (300 MHz, DMSO-d₆) δ 162.1, 159.2, 157.7, 156.2, 145.1, 145.1, 143.6, 141.0, 139.9, 137.9, 136.4, 136.1, 133.5, 130.5, 130.2, 127.4, 124.8, 123.2, 122.3, 121.8, 119.6, 119.4, 111.5, 110.7, 109.5, 105.9, 100.2, 67.8, 67.5, 31.0, 30.8, 29.0, 28.7, 28.5, 25.2, 22.1, 22.0, 13.9, 13.8. UV-vis (THF, nm): λ_max (ε) 337 (38 356), 470.5 (28 080). PL (THF, nm): λ_max 626. MS (MALDI-TOF) m/z calculated for C₆₆H₇₄N₂O₆S₃: 1086.4986; found, 1086.3811.

3-[5-(6-[Bis(2′,4′-dihexyloxybiphenyl-4-yl)amino]thieno[3,2-b][1]benzothiophen-2-yl)-2,2′-bithiophene-5-yl]-2-cyanoacrylic acid (SGT-123). Compound 4b (0.09 g, 0.08 mmol), dissolved in CHCl₃ (9 mL) and acetonitrile (9 mL), was condensed with 2-cyanoacetic acid (0.02 g, 0.29 mmol) in the presence of piperidine (0.02 g, 0.28 mmol). The mixture was refluxed for 12 h. After cooling the solution, the organic layer was removed in vacuo. The red solid of SGT-123 was obtained by silica gel chromatography [CH₂Cl₂/MeOH (v/v, 10 : 1)]. Yield: 52% (50 mg). M.p.: 169–171 °C. ¹H NMR (300 MHz, CDCl₃): δ 8.21 (s, 1H), 7.59–7.62 (m, 3H), 7.54 (s, 1H), 7.44–7.46 (m, 5H), 7.24–7.34 (d, 4H), 7.13–7.15 (m, 5H), 6.54 (m, J = 6.9 Hz, 4H), 3.95 (q, 8H), 1.72–1.82 (m, 8H), 1.25–1.47 (m, 24H), 0.86 (m, 12H). ¹³C NMR (900 MHz, DMSO-d₆): δ 159.2, 156.4, 145.1, 143.4, 141.1, 137.8, 137.3, 136.4, 135.6, 132.9, 130.4, 130.1, 127.1, 126.9, 124.6, 123.1, 122.1, 121.7, 105.8, 100.1, 91.8, 70.2, 68.8, 67.8, 67.4, 66.1, 31.0, 30.5, 28.6, 28.5, 25.2, 22.0, 13.8. UV/Vis (THF, nm): λ_max (ε) 345.5 (25 066), 474.5 (29 125). PL (THF, nm): λ_max 646. MS (MALDI-TOF) m/z calculated for C₇₀H₇₆N₂O₆S₄: 1168.4586; found, 1168.1948.

3-[5-(6-[Bis(2′,4′-dihexyloxybiphenyl-4-yl)amino]thieno[3,2-b][1]benzothiophen-2-yl) thieno[3,2-b]thiophene-2-yl]-2-cyanoacrylic acid (SGT-125). Compound 4c (0.07 g, 0.037 mmol), dissolved in CHCl₃ (9 mL) and acetonitrile (9 mL), was condensed with 2-cyanoacetic acid (0.05 g, 0.65 mmol) in the presence of piperidine (0.02 g, 0.32 mmol). The mixture was refluxed for 12 h. After cooling the solution, the organic layer was removed in vacuo. The red solid of SGT-125 was obtained by silica gel chromatography [CH₂Cl₂/MeOH (v/v, 10 : 1)]. Yield: 85% (63 mg). M.p.: 170–172 °C. ¹H NMR (300 MHz, CDCl₃): δ 8.33 (s, 1H), 7.95 (s, 1H), 7.68 (d, J = 8.7 Hz, 1H), 7.56 (d, 1H), 7.42–7.51 (m, 4H), 7.28–7.33 (m, 4H), 7.15–7.18 (d, 5H), 6.55 (m, J = 6.0 Hz, 4H), 3.99 (q, 8H), 1.73–1.82 (m, 8H), 1.25–1.48 (m, 24H), 0.86 (m, 12H). ¹³C NMR (900 MHz, DMSO-d₆): δ 159.2, 156.5, 145.1, 145.0, 143.5, 142.9, 142.0, 141.9, 141.2, 139.6, 137.9, 137.3, 133.1, 130.4, 130.1, 126.7, 123.2, 121.7, 118.3, 117.0, 105.9, 100.1, 91.4, 68.8, 67.8, 31.0, 28.6, 28.5, 25.2, 22.0, 13.9. UV-vis (THF, nm): λ_max (ε) 345 (26 918), 467 (28 898). PL (THF, nm): λ_max 621. MS (MALDI-TOF) m/z calculated for C₆₈H₇₄N₂O₆S: 1142.4430; found, 1142.1804.

3-(6-[Bis(2′,4′-dihexyloxybiphenyl-4-yl)amino]thieno[3,2-b][1]benzothiophene-2-yl)-2-cyanoacrylic acid (SGT-127). Compound 5 (0.05 g, 0.05 mmol), dissolved in CHCl₃ (6 mL) and acetonitrile (6 mL), was condensed with 2-cyanoacetic acid (0.046 g, 0.54 mmol) in the presence of piperidine (0.023 g, 0.27 mmol). The mixture was refluxed for 12 h. After cooling the solution, the organic layer was removed in vacuo. The orange solid of SGT-127 was obtained by silica gel chromatography [CH₂Cl₂/MeOH (v/v, 10 : 1)]. Yield: 64% (42 mg). M.p.: 161–163 °C. ¹H NMR (300 MHz, CDCl₃): δ 8.36 (s, 1H), 7.93 (s, 1H), 7.77 (d, J = 6.9 Hz, 1H), 7.44–7.52 (m, 6H), 7.17–7.29 (m, 6H), 6.54 (m, 4H), 3.97 (q, 8H), 1.73–1.86 (m, 8H), 1.25–1.50 (m, 24H), 0.87 (m, 12H). ¹³C NMR (900 MHz, DMSO-d₆): δ 159.2, 156.50, 156.0, 154.8, 144.9, 141.9, 137.3, 132.8, 130.2, 127.4, 123.7, 123.5, 123.0, 121.7, 105.9, 101.5, 100.1, 99.9, 68.8, 67.8, 67.4, 30.9, 28.6, 25.2, 22.0, 13.9. UV/Vis (THF, nm): λ_max (ε) 344 (31 185), 459 (23 474). PL (THF, nm): λ_max 588. MS (MALDI-TOF) m/z calculated for C₆₂H₇₂N₂O₆S₂: 1004.4832; found, 1004.2767.

Chemical characterization

The ¹H NMR spectroscopy study was conducted on a Varian Mercury 300 spectrometer using tetramethylsilane (TMS; d = 0 ppm) as the internal standard. Chemical shifts for the ¹H NMR spectra were recorded on a Varian Mercury 300 spectrometer using tetramethylsilane (TMS; d = 0 ppm) as the internal standard. The ¹³C NMR spectroscopy study was conducted on a Bruker Biospin Gmbh AVAVCE II 900 spectrometer using tetramethylsilane (TMS; d = 0 ppm) as the internal standard. Chemical shifts for the ¹³C NMR spectra were recorded on a Bruker Biospin Gmbh AVAVCE II 900 spectrometer using tetramethylsilane (TMS; d = 0 ppm) as the internal standard. MALDI-TOF mass spectra were recorded by a Voyager-DETM STR biospectrometry workstation.

Instrumentations

UV/Vis absorption spectra were obtained in THF on a Shimadzu UV-2401PC spectrophotometer. Photoluminescence spectra were analysed with a Fluorolog FL-3-22 fluorimeter from Horiba-Jobin-Yvon Ltd., which was equipped with a 450 W Xe lamp and two analysing monochromators. Visible emission spectra were recorded with a Hamamatsu R928 photomultiplier. Cyclic voltammetry was performed on a Versa STAT3 (AMETEK) instrument. A three-electrode system was used and consisted of a reference electrode (Ag/AgCl), a working electrode (dye-coated TiO₂ films) and a counter electrode (Pt wire). The redox potentials of the dyes on TiO₂ were measured in CH₃CN with 0.1 M TBAPF₆ at a scan rate of 50 mV s⁻¹. Electrochemical impedance spectra of the DSSCs were measured with an impedance analyser (VersaSTAT3, AMETEK) connected to a potentiostat under dark conditions at room temperature. The spectra were scanned in a frequency range of 0.1–10⁵ Hz with an amplitude of 10 mV at room temperature. Intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS) were performed on an IviumStat (Ivium Technologies, Netherlands) supplied modulated light. IMVS and IMPS were measured under the illumination of an LED light source (λ = 635 nm) with different light intensities. The frequency range was 0.5–500 Hz.

Theoretical calculations

All the theoretical calculations were performed with the Gaussian 09 program package on a computer workstation. The ground-state geometries were fully optimised without symmetry constraints with Becke's three-parameter hybrid functional and Lee, Yang, and Parr's correlational functional (B3LYP) using the 6-31G(d,p) basis set on all atoms. The electron-density-difference diagram and theoretical absorption spectra were calculated by TD-DFT at the same levels of theory in vacuum.

Time-resolved photoluminescence

Steady-state emission spectra of the dye sensitizers were measured with a FluoroLog-322 (Horiba) spectrometer equipped with a 450 W Xe arc lamp. Time-resolved photoluminescence (TR-PL) was measured using a time-correlated single-photon counting spectrometer (FluoTime 200, PicoQuant) equipped with a picosecond diode laser (LDH-P-C-390, PicoQuant) and a hybrid photomultiplier detector (PMA Hybrid 50, PicoQuant). The film samples consisting of dyes adsorbed on a layer of TiO₂ (3.5 μm thickness) or Al₂O₃ (4.0 μm thickness) were excited by laser pulse of approximately 100 ps at a centre wavelength of 393 nm, and the emission was measured at the λ_{em max} of each dye. The time resolution of the TR-PL measurement was approximately 190 ps, whereas it was effectively reduced to <50 ps by deconvolution of the instrument response function.

Transient photovoltage and photocurrent decay measurements

In transient photovoltage measurements, the DSSCs were irradiated by a diode LED (635 nm) and the decay of the open-circuit voltage, caused by a stepwise decrease in a small fraction of the laser intensity, was measured. The resulting photovoltage decay transients were collected and the τ values are determined by fitting the data to the equation exp(−t/τ).

DSSC fabrication

A transparent nanocrystalline layer on the FTO glass plate was prepared by repeated screen printing with the TiO₂ paste (Dyesol, 18NR-T) and then dried at 120 °C. The TiO₂ electrodes were gradually heated under flowing air at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500 °C for 15 min. A paste for the scattering layer containing 400 nm anatase particles (CCIC, PST-400C) was deposited through screen printing and then dried for 1 h at 25 °C. The TiO₂ electrodes were gradually heated under flowing air at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min and at 500 °C for 15 min. The resulting layer was composed of a 5.5 μm-thick transparent layer and a 3.5 mm-thick scattering layer. The thickness of the transparent layer was measured using an Alpha-step 250 surface profilometer (Tencor Instruments, San Jose, CA). The TiO₂ electrodes were again treated with TiCl₄ at 70 °C for 30 min, and then sintered at 500 °C for 30 min. The TiO₂ electrodes were immersed in the dye solution (0.3 mM in THF/EtOH = 1 : 2 containing 20 mM CDCA) and kept at room temperature for 2 h. The counter electrodes were prepared by coating an FTO plate with a drop of H₂PtCl₆ solution (2 mg of Pt in 1 mL of ethanol) and it heating at 400 °C for 15 min. The dye adsorbed on the TiO₂ electrode and the Pt counter electrode were assembled into a sealed sandwich-type cell by heating at 80 °C with a hot-melt film (25 μm thick Surlyn) as a spacer between the electrodes. A drop of the electrolyte solution was placed on a drilled hole in the counter electrode of the assembled cell and was driven into the cell through vacuum backfilling. Finally, the hole was sealed by additional Surlyn and a cover glass (0.1 mm thick).

Photoelectrochemical measurements of DSSCs

Photoelectrochemical data were measured using a 1000 W xenon light source (Oriel, 91193) that was focused to give 1000 W m⁻² at the surface of the test cell, which is equivalent to one sun at AM 1.5G. The light intensity was adjusted with a Si solar cell that was double-checked with an NREL-calibrated Si solar cell (PV Measurement Inc.). The applied potentials and measured cell currents were measured using a Keithley model 2400 digital source meter. Under these conditions, the current–voltage characteristics of the cell were determined by externally biasing the cell and measuring the generated photocurrents. This process was fully automated using Wavemetrics software. A similar data acquisition system was used to control the incident photon-to-current conversion efficiency (IPCE) measurement. Under full computer control, light from a 300 W Xe lamp was focused through a high throughput monochromator onto the photovoltaic cell under test. The monochromator was incremented through the visible spectrum to generate the IPCE (λ) curve as expressed in this equation (IPCE (λ) = 1240(J_sc/l_φ)), where λ is the wavelength, J_sc is the current at short circuit (mA cm⁻²), φ is the incident radiative (W m⁻²). The IPCE curve can be derived from the measured absorption spectrum of the DSSC for comparison.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (No. 2014R1A2A1A10051630 and No. 2014R1A1A1002511).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra15100g

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