Synthesis of novel isophorone-based dyes for dye-sensitized solar cells

Abstract

Four organic dyes containing isophorone as the π-bridge unit were synthesised and their photophysical and electrochemical properties were characterised. They were then used to fabricate dye-sensitised solar cells. Arylamine derivatives and cyanoacrylic acid functioned as electron donors (D) and electron acceptors (A), respectively, to form a D–π–A system without the need for a Pd catalyst. YC-1, with a long chain hexyloxy group and a strong phenothiazine donor moiety, improved both the open-circuit voltage (V_oc) and short-circuit current (J_sc) and reduced charge recombination. The optimum device had a J_sc of 14.86 mA cm⁻², a V_oc of 0.67 V, a fill factor of 0.62 and a photon-to-current conversion efficiency of 60% at 385–605 nm, corresponding to an overall conversion efficiency of 6.18%. The photophysical properties of the dye-sensitised solar cells were analysed using a time-dependent density functional theory model with the B3LYP functional. The electronic nature of the devices was elucidated using electrochemical impedance spectroscopy and controlled intensity modulated photo spectroscopy.

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1. Introduction

The global problems of the limited supply of fossil fuels and the need to reduce CO₂ emissions have prompted the investigation and development of renewable and environmentally friendly sources of energy. Solar energy is widely acknowledged as a potential alternative energy resource. Compared with silicon-based photovoltaic devices, dye-sensitized solar cells (DSSCs) have received considerable attention because they are easy to fabricate, have a wide absorption range because their structures can be tuned, are colourful and transparent, and can be manufactured at low cost.¹ O'Regan and Grätzel² reported that DSSCs based on ruthenium-based complexes, such as N3, N719, black dye and Z907, yielded a high power conversion efficiency of >11%. Diau et al.³ used a porphyrin dye and achieved a maximum performance of 13%. However, another paper on a metal–complex sensitizer reported a complicated synthetic route, difficult purification and a low yield.^2d

The photosensitizer is the most crucial factor determining the performance of DSSCs and numerous metal-free organic dyes have been investigated and shown to have a high performance, similar to that of N719.⁴ Metal-free organic dyes are environmentally friendly, low cost, have a high structural flexibility and can be easily prepared. Most organic dyes have linear-shaped molecules consisting of a strong dipole, an electron donor (D), a π-bridge, an electron acceptor (A) and a D–π–A system. To achieve an optimum D–π–A system for DSSCs, it is critical to improve the short-circuit current (J_sc) and open-current voltage (V_oc).

In general, the π-bridge unit in the sensitizer has mainly been based on the use of thiophene,⁵ furan,⁶ pyrrole,⁷ benzene,⁸ diketopyrrolopyrrole⁹ or benzothiadiazole¹⁰ as linkers. However, the C–C bond that connects the bridges unit between the donor or acceptor moiety of sensitizers is usually formed by Pd-catalysed cascade reactions, such as Sonogashira, Heck, Suzuki–Miyaura or Stille coupling reactions. Although a Pd-based cross-coupling reaction is a powerful method for linking two moieties through a simple, time-efficient and versatile synthesis route, Pd is expensive and not environmentally friendly.

Data on the use of the simple and low-cost isophorone structure as the π-bridge for DSSCs are rare. Tian and co-workers^11,12 reported high efficiencies for NKX-2753 and D-3, which incorporated coumarin and indoline derivatives, respectively, as the donor groups. In this study, we modified and designed four YC series dyes using isophorone as the π-bridge between the arylaimine electron donor and the cyanoacrylic acid acceptor without using a Pd catalyst. We have previously reported the use of phenothiazine, which has electron-rich nitrogen and sulphur heteroatoms and a molecular structure with a slight bend in the central ring.¹³ A long chain increases the solubility of a structure and reduces the dark current by covering the TiO₂ surface.^4h,14 We therefore investigated whether the introduction of an arylamine (YC-4) or phenothiazine (YC-1) with long alkoxy chains into DSSCs enhanced their light-harvesting function and V_oc; YC-2 and YC-3 served as references. The synthetic procedures are described in Scheme 1 and all the structures were confirmed based on spectroscopic data.

Scheme 1 Synthetic route for the organic dyes. Reagents and conditions: (i) ethyl-2-cyanoacetate, NH₄OAc, benzene, acetic anhydride, reflux; (ii) n-BuLi, DMF, THF, −78 °C; (iii) compound 2, piperidine, CH₃CN, 90 °C; and (iv) LiOH, EtOH/H₂O/THF, 50 °C.

2. Experimental

Characterisation and reagents

All reactions and manipulations were conducted under a nitrogen atmosphere and all solvents were freshly distilled according to standard procedures. ¹H- and ¹³C-NMR spectra were recorded on Bruker spectrometers (AV 300/AV 400/AVIII HD 400/AV 500 MHz) using CDCl₃ as the solvent. Chemical shifts (δ) were reported downfield from the peak with respect to tetramethylsilane. The absorption spectra were recorded on a Shimadzu UV-1800 spectrophotometer and the emission spectra and photoluminescence quantum yield were obtained using a Hitachi F-4500 spectrofluorometer. The redox potentials were measured by cyclic voltammetry on a CHI 620 analyser. All measurements were conducted in tetrahydrofuran solutions containing 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte under ambient conditions after purging for 10 min with N₂. A conventional three-electrode configuration was used; this consisted of a glassy carbon working electrode, a platinum counter electrode and an Ag/Ag⁺ reference electrode calibrated using ferrocene/ferrocenium as an internal reference. Mass spectra were recorded using a JEOL JMS-700 double-focusing mass spectrometer.

Ethyl-2-cyanoacetate, ammonium acetate, benzene, n-butyllithium (1.6 M in hexane), N,N-dimethylformamide, piperidine, lithium hydroxide (LiOH), acetic anhydride and acetonitrile (MeCN) were separately purchased from ACROS, Alfa, Merck, Lancaster, TCI, Sigma-Aldrich and Showa. Chromatographic separations were performed using pre-coated silica gel plates (Kieselgel si 60, 40–63 µm; Merck).

Fabrication and characterisation of DSSCs

Fluorine-doped tin oxide (FTO) conducting glass (TCO22-7; sheet resistance, 7 Ω square⁻¹) and Ti-nanoxide T/SP and Ti-nanoxide R/SP titanium oxide pastes were purchased from Solaronix S.A. (Switzerland). A thin film of TiO₂ (10–12 µm transparent + 5 µm scattering) was coated on a 0.25 cm² FTO glass substrate, which was then immersed in a THF or CH₂Cl₂ solution containing 3 × 10⁻⁴ M dye sensitisers for 12 h, then rinsed with anhydrous acetonitrile and dried. Another piece of FTO glass with a 100 nm thick layer of sputtered Pt was used as a counter electrode. The active area was controlled at dimensions of 0.25 cm² by adhering a 60 µm thick piece of polyester tape to the Pt electrode. The photocathode was placed on top of the counter electrode and tightly clipped to form a cell; an electrolyte was then injected into the seam between the two electrodes. An acetonitrile solution containing LiI (0.5 M), I₂ (0.05 M) and 4-tert-butylpyridine (TBP; 0.5 M) was used as electrolyte 1 (E1). A solution of 3-dimethylimidazolium iodide (1.0 M), LiI (0.05 M), I₂ (0.03 M), guanidinium thiocyanate (0.1 M) and TBP (0.5 M) in MeCN : valeronitrile (85 : 15, v/v) was used as electrolyte 2 (E2). Devices composed of the commercial dye N719 under the same conditions (3 × 10⁻⁴ M; Solaronix) were used as references. The cell parameters were determined under incident light with an intensity of 100 mW cm⁻² (measured using a thermopile probe; Oriel 71964) generated by a 300 W solar simulator (Oriel Sol3A Class AAA Solar Simulator 9043A, Newport) and passed through an AM 1.5 filter (Oriel 74110). The light intensity was further calibrated using an Oriel reference solar cell (Oriel 91150) and adjusted to 1.0 Sun. The monochromatic quantum efficiency was recorded using a monochromator (Oriel 74100) under short-circuit conditions. The electrochemical impedance spectra of the DSSCs were recorded using an impedance/charge extraction method (CEM)/intensity-modulated photovoltage spectroscopy (IVMS) analyser (Zahner Ennium).

(E,Z)-2-Cyano-2-(3-((E)-2-(10-(4-(hexyloxy)phenyl)-10H-phenothiazin-3-yl)vinyl)-5,5-dimethylcyclohex-2-en-1-ylidene)acetic acid (YC-1). Compound E-1 (5 g, 18.3 mmol) and 2 M LiOH were placed in a one-necked flask with ethanol, water and THF and heated to 50 °C for 3 h. The reaction was neutralised by adding 1 M HCl at room temperature and the product was extracted using methylene choride. The organic layer was dried over anhydrous MgSO₄ and evaporated under vacuum. The product was then purified using silica gel column chromatography with methylene chloride as the eluent. A red solid (YC-1) was obtained at a yield of 83%. δ_H (500 MHz, CDCl₃) 7.27 (s, 1H), 7.26 (s, 1H), 7.10–7.13 (m, 3H), 6.90–6.97 (m, 3H), 6.81–6.83 (m, 4H), 4.04 (t, 2H, J = 6.15 Hz), 2.96 (s, 1H), 2.65 (s, 1H), 2.39 (s, 1H), 2.37 (s, 1H), 1.82–1.87 (m, 2H), 1.52–1.54 (m, 2H), 1.35–1.38 (m, 4H), 1.03–1.06 (m, 6H), 0.93 (t, 3H, J = 6.3 Hz). δ_C (125 MHz, CDCl₃) 168.8, 167.1, 166.5, 159.0, 154.0, 153.6, 145.4, 143.6, 134.5, 132.4, 131.8, 130.2, 130.1, 128.3, 127.6, 126.9, 126.8, 126.5, 125.9, 125.2, 124.9, 123.8, 122.7, 120.0, 118.9, 117.1, 116.5, 116.3, 115.9, 115.6, 97.6, 96.8, 68.3, 44.9, 41.1, 39.2, 38.8, 31.9, 31.6, 31.5, 29.2, 28.3, 28.1, 25.7, 22.6, 14.0. MS (FAB, 70 eV): m/z (relative intensity) 590 (M⁺, 100); HRMS calculated for C₃₇H₃₈N₂O₃S, 590.2603; found, 590.2597.

(E,Z)-2-Cyano-2-(3-((E)-4-(diphenylamino)styryl)-5,5-dimethylcyclohex-2-en-1-ylidene)acetic acid (YC-2). Compound YC-2 was synthesised using the same procedure as for YC-1. A red solid (YC-2) was obtained at a yield of 87%. δ_H (400 MHz, CDCl₃) 7.90 (s, 0.5H), 7.39 (d, 1H, J = 8.6 Hz), 7.38 (d, 1H, J = 8.6 Hz), 7.29–7.33 (m, 4H), 7.08–7.16 (m, 6.5H), 7.00–7.05 (m, 2H), 6.90–6.97 (m, 2H), 3.00 (s, 1H), 2.69 (s, 1H), 2.46 (s, 1H), 2.44 (s, 1H), 1.10 (s, 3H), 1.07 (s, 3H). δ_C (100 MHz, CDCl₃) 169.0, 168.0, 167.4, 167.3, 154.3, 154.0, 149.2, 149.1, 146.9, 135.7, 135.6, 129.4, 129.3, 129.2, 128.5, 128.3, 127.6, 125.7, 125.3, 125.2, 123.9, 123.8, 123.7, 122.2, 122.1, 117.1, 116.4, 97.5, 96.7, 45.0, 41.2, 39.3, 38.9, 31.9, 31.5, 28.3, 28.1. MS (FAB, 70 eV): m/z (relative intensity) 460 (M⁺, 100); HRMS calculated for C₃₁H₂₈N₂O₂, 460.2151; found, 460.2149.

(E,Z)-2-Cyano-2-(5,5-dimethyl-3-((E)-4-(naphthalen-1-yl-(phenyl)amino)styryl)cyclohex-2-en-1-ylidene)acetic acid (YC-3). YC-3 was synthesised using the same procedure as for YC-1. A red solid (YC-3) was obtained at a yield of 85%. δ_H (400 MHz, CDCl₃) 7.91 (t, 2H, J = 8.2 Hz), 7.87 (s, 0.5H), 7.83 (d, 1H, J = 8.2 Hz), 7.47–7.53 (m, 2H), 7.32–7.41 (m, 4H), 7.25–7.29 (m, 2H), 7.14–7.17 (m, 2H), 7.02–7.07 (m, 2H), 6.87–6.94 (m, 4.5H), 2.99 (s, 1H), 2.68 (s, 1H), 2.43 (s, 1H), 2.42 (s, 1H), 1.08 (s, 3H), 1.06 (s, 3H). δ_C (100 MHz, CDCl₃) 154.4, 154.0, 149.8, 149.7, 147.2, 142.6, 135.9, 135.8, 135.3, 131.1, 130.0, 129.3, 129.2, 128.6, 128.5, 128.3, 127.9, 127.4, 127.2, 127.1, 126.6, 126.3, 125.5, 123.9, 123.5, 123.4, 123.3, 123.2, 120.2, 120.1, 117.1, 116.4, 97.3, 96.5, 45.0, 41.2, 39.3, 38.9, 31.9, 31.5, 28.3, 28.1. MS (FAB, 70 eV): m/z (relative intensity) 510 (M⁺, 100); HRMS calculated for C₃₅H₃₀N₂O₂, 510.2302; found, 510.2291.

(E,Z)-2-(3-((E)-4-(Bis(4-(hexyloxy)phenyl)amino)styryl)-5,5-dimethylcyclohex-2-en-1-ylidene)-2-cyanoacetic acid (YC-4). Compound YC-4 was synthesised using the same procedure as for YC-1. A red solid (YC-4) was obtained at a yield of 87%. δ_H (400 MHz, CDCl₃) 7.89 (s, 0.5H), 7.28–7.33 (m, 2H), 7.01–7.10 (m, 4H), 6.85–6.94 (m, 8.5H), 3.94–3.98 (m, 4H), 3.00 (s, 1H), 2.67 (s, 1H), 2.43 (s, 1H), 2.41 (s, 1H), 1.76–1.82 (m, 4H), 1.47–1.50 (m, 4H), 1.36–1.38 (m, 8H), 1.08 (s, 3H), 1.06 (s, 3H), 0.94 (t, 6H, J = 6.52 Hz). δ_C (100 MHz, CDCl₃) 168.7, 168.0, 166.9, 156.2, 156.1, 154.5, 154.1, 150.1, 150.0, 139.7, 139.6, 136.0, 135.9, 128.6, 127.4, 127.3, 127.2, 126.6, 125.3, 123.3, 119.1, 119.0, 117.5, 116.7, 115.4, 97.5, 96.6, 68.2, 45.0, 41.2, 39.3, 38.9, 31.9, 31.6, 31.5, 29.3, 28.3, 28.1, 25.7, 22.6, 14.0. MS (FAB, 70 eV): m/z (relative intensity) 660 (M⁺, 100); HRMS calculated for C₄₃H₅₂N₂O₄, 660.3922; found, 660.3916.

Quantum chemistry computations

The structures of the dyes were optimized using the B3LYP/6-31G* hybrid functional. For the excited states, time-dependent density functional theory with the B3LYP functional was used. All analyses were performed using Q-Chem 3.0 software. The frontier orbital plots of the highest (HOMO) and lowest (LUMO) occupied molecular orbitals were drawn using GaussView 04.

3. Results and discussion

Synthesis

The structures of the dyes are shown in Fig. 1 and their synthetic sequences are outlined in Scheme 1. The YC series contained an isophorone moiety as the bridge unit in the sensitizer. Its synthesis started with isophorone yielding an ester unit B (E/Z 1/1) through Knoevenagel condensation and the isomer ratio was verified using proton NMR (Fig. S1†).¹² A bromo group was replaced with an aldehyde on the donor moiety of D-1–4 by n-butyl lithium and DMF. The donor part was then connected to the π-bridge and acceptor part by using the Knoevenagel condensation reaction to yield E-1–4. Four final products were obtained at high yield and were easily purified through the base hydrolysis of an ester to construct the cyanoacrylic acid acceptor unit. The structures of all new compounds were characterised spectroscopically.

Fig. 1 Structures of the YC series organic dyes.

Absorption spectra

Fig. 2 shows the ultraviolet-visible absorption spectra of all dyes in CH₂Cl₂ solution and Fig. 3 shows the ultraviolet-visible absorption spectra of all dyes on TiO₂; the photochemical and electrochemical parameters are listed in Table 1. All the sensitizers exhibited broad, high-intensity absorption peaks in the range 344–504 nm. The short wavelength region at 344–370 nm was attributed to localized π–π* transitions, whereas the long wavelength region in the range 476–504 nm was attributed to intramolecular charge-transfer transitions (ICTs). Compared with the other three compounds, the intensity of the π–π* transition of YC-1 was stronger with the phenothiazine donor moiety, indicating that the phenothiazine donor increased π-overlapping relative to the fan structure of triphenylamine. YC-1 and YC-4 showed broader and higher intensity ICT transitions because the alkoxy chain improved not only the donating ability of the donor moiety, but also the coplanar conformation of the bridge moiety between the donor and acceptor. A slightly different optimized structure was evidenced by theoretical computations using time-dependent density functional theory with the B3LYP functional in combination with the standard 6-31G* basis set (see Fig. 5). The phenomenon was consistent with the dipole moments revealed by the computation results (Table S1†). The absorption photocurrent (J_sc) and the intensity of absorption showed the same trends in solution.¹⁵

Fig. 2 Absorption spectra of dyes in CH₂Cl₂ solution.

Fig. 3 Normalized absorption spectra of YC series organic dyes on TiO₂.

Table 1 Photochemical and electrochemical parameters of the dyes

Dye	λ max ^a (nm)/ε (M^−1 cm^−1)	λ max (TiO2)	E ox ^b (V)	E 0–0 (eV)	E red ^c (V)	J sc (mA cm^−2) E1/E2^e	V oc (V) E1/E2	FF E1/E2	η ^d (%) E1/E2	Loading amount (10^−7 mol cm^−2)
a Absorption and emission are in CH2Cl2. b Oxidation potential in THF (10^−3 M) containing 0.1 M (n-C4H9)4NPF6 at a scan rate of 50 mV s^−1 (vs. NHE). c E red calculated by Eox − E0–0. d Performance of DSSCs measured in a 0.25 cm^2 working area on an FTO (7 Ω square^−1) substrate. e Electrolyte 1 (E1): LiI (0.5 M), I2 (0.05 M) and TBP (0.5 M) in MeCN. Electrolyte 2 (E2): 3-dimethylimidazolium iodide (1.0 M), LiI (0.05 M), I2 (0.03 M), guanidinium thiocyanate (0.1 M) and TBP (0.5 M) in MeCN : valeronitrile (85 : 15, v/v). f See ref. 20.
YC-1	491 (28 500)	436	0.61	2.03	−1.42	14.86/11.91	0.67/0.69	0.62/0.63	6.18/5.17	13.4
YC-2	478 (29 300)	416	0.83	2.17	−1.34	12.18/10.15	0.64/0.71	0.63/0.64	4.92/4.61	6.3
YC-3	476 (36 300)	405	0.84	2.18	−1.34	12.43/10.78	0.65/0.67	0.63/0.61	5.08/4.43	8.3
YC-4	504 (40 300)	434	0.60	2.01	−1.41	13.44/12.42	0.66/0.69	0.60/0.61	5.37/5.20	11.8
N719	—	—	1.10^f	2.60^f	−1.50^f	16.22/16.32	0.73/0.75	0.59/0.60	7.02/7.34	—

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Fig. 4 HOMO–LUMO energy levels of the YC series sensitizer.

Fig. 5 Optimized structure of YC series organic dyes estimated by time-dependent DFT/B3LYP (6-31G* basis set).

The absorption spectra of all the dyes absorbed on the TiO₂ surface are shown in Fig. 3. Compared with the absorption spectra of YC-2 and YC-3, those of YC-1 and YC-4 are broader, representing advantageous spectral properties for light harvesting. All the dyes exhibited a blue shift of approximately 55–71 nm in the film ICT absorption band with respect to those in solution and the spectral differences between the dyes decreased. The blue shift, a common phenomenon for most organic dyes, appears to be a result of either the deprotonation of the carboxylic acid or strong interactions between the dye and the semiconductor surface when it was anchored onto the TiO₂ surface.

Electrochemical properties

The oxidation potentials (E_ox), corresponding to the HOMO level of the dyes, were measured using cyclic voltammetry. It is clear that the long hexyloxy chain in the arylamine donor moiety effectively influences the electron delocalization and oxidation potential. The lowest ionization potentials in CH₂Cl₂ solutions decreased in the order of YC-2 ≈ YC-3 > YC-4 ≈ YC-1 (Fig. 4 and S19†). The LUMO levels of the dyes were estimated according to the E_ox values and the zero–zero band gaps at the start of the absorption spectra (Table 1). YC-1 and YC-4 had a narrower HOMO–LUMO energy gap, consistent with the wider absorption ranges in Fig. 2. This observation was confirmed by theoretical calculations. For appropriate functioning of the DSSCs, all the sensitizers were maintained at a more positive potential than the redox potential of the iodine/iodide electrolyte (0.4 V vs. NHE) to ensure a sufficient driving force to regenerate the oxidized dyes from the electrolytes. The LUMO levels of all sensitizers were higher than the conduction bands (−0.5 V vs. NHE) of TiO₂. The energy levels of all dyes were in accordance with the requirements for efficient electron flow.

Computational analysis

The effects of YC-1–4 on the device performance were further explored through theoretical models. Complete geometrical optimizations were performed using the B3LYP/6-31G* hybrid functional implanted in Q-Chem 3.0.¹⁶ The optimized molecular geometry of YC-1–4 is shown in Fig. 5, which suggests that the conformation of the isophorone bridge unit is nearly coplanar. However, the orientation of the dimethyl group on the slightly bulky isophorone ring prevents intramolecular aggregation. The electron distributions of frontier orbitals are shown in Fig. S17† and, as depicted, the electron densities in the HOMOs were mainly distributed around the amine donor moieties and those in the LUMOs were mainly distributed around the cyanoacrylic acid of the acceptor moieties. A delicate balance between charge separation and recombination must be maintained by modifying the molecular structures. A disturbance in the π-conjugation, either by a distortion of the molecular geometry or an interruption of the intramolecular aggregation by a long chain, may reduce the charge migration rate. As long as the charge-separated state has a longer lifetime, a higher device quantum efficiency can be achieved.¹⁷ A difference in the Mulliken charge distribution between the ground state (S₀) and excited state (S₁) is plotted in Fig. 6 based on the estimate generated using the time-dependent DFT/B3LYP model (see Table S2†). YC-1 and YC-4 showed the optimum charge separation in the ICT state (bar chart, Fig. 6). Both compounds had a slightly higher V_oc value than YC-2 and YC-3 in DSSCs using E1.

Fig. 6 Difference of Mulliken charges between ground state (S₀) and excited state (S₁) estimated by the time-dependent DFT/B3LYP model.

Photoexcitation pumps an electron from the HOMO to the LUMO and therefore shifts a considerable number of electrons from the donor to the acceptor. Consequently, the HOMO–LUMO energy gap in YC-1 and YC-4 was lower than that in YC-2 and YC-3 and the ICT band in the former structures exhibited a bathochromic shift with respect to the latter. This also explains the dipole moment (YC-1, 12.8343 D; YC-4, 11.9272 D) and zero–zero energy of the YC series sensitizers.

The probability of transition by excitation was estimated using the time-dependent DFT/B3LYP model and the calculated oscillator strength (f) is given in Table S1.† The f values for the lowest energy transitions of the ICT transitions (the S₁ state) are all relatively high, ranging from 0.61 to 1.13. The results are consistent with the absorptivity measured in solution.

Photovoltaic performance of DSSCs

DSSC devices composed of the synthesised dyes were fabricated on the surface of TiO₂ according to a standard procedure.

Two types of electrolytes were used to investigate the performance of the devices: E1 consisted of LiI (0.5 M), I₂ (0.05 M) and TBP (0.5 M) in MeCN, whereas E2 was composed of 3-dimethylimidazolium iodide (1.0 M) and guanidinium thiocyanate (0.1 M) in addition to LiI (0.05 M), I₂ (0.03 M) and TBP (0.5 M) in a mixed solvent of MeCN and valeronitrile (85 : 15, v/v). Experiments involving co-absorption with deoxycholic acid (DCA) were conducted. The incident photocurrent conversion efficiencies (IPCEs) and photocurrent–voltage (J–V) curves of all the dyes were measured under AM 1.5 solar light (100 mW cm⁻²); the short-circuit current (J_sc), open-circuit voltage (V_oc), fill factor (FF) and solar-to-electrical photocurrent density (η) are summarised in Table 1, where N719 serves as the reference dye.

The J–V plots and IPCEs of all devices with E1 are shown in Fig. 7. An apparent improvement in the V_oc values was observed using E2; the values were approximately 0.02–0.05 V higher than those obtained using E1 (Table 1). The different interactions in different solvents might cause this variation in the chemical and physical properties between the sensitizer and the TiO₂ surface.¹⁸ Two types of solvents were used: THF and CH₂Cl₂. The devices fabricated using CH₂Cl₂ gave the optimum results (Table S3†). Therefore the parameters of all the YC series dyes listed in Tables 1 and 2 are those of devices using the CH₂Cl₂ solvent system.

Fig. 7 J–V plots and IPCE of DSSCs devices of YC-1–4 with the E1 electrolyte. The plots were measured under a light intensity of 1.0 Sun.

Table 2 Photovoltaic parameters of DSSCs fabricated using YC-1–4 with and without deoxycholic acid

Dye	DCA^a (mM)	J sc (mA cm^−2)	V oc (V)	FF	η ^b (%)
a Concentration of dye 3 × 10^−4 M in CH2Cl2. b Performance of DSSCs measured in a 0.25 cm^2 working area on an FTO (7 Ω square^−1) substrate under AM 1.5 irradiation with E1.
YC-1	0	14.86	0.67	0.62	6.18
	10	12.91	0.67	0.63	5.40
YC-2	0	12.18	0.64	0.63	4.92
	10	12.83	0.66	0.64	5.36
YC-3	0	12.43	0.65	0.63	5.08
	10	12.56	0.66	0.63	5.28
YC-4	0	13.44	0.66	0.60	5.37
	10	13.17	0.65	0.60	5.17

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An apparent improvement in the V_oc values was observed using E2; specifically, the values were approximately 0.02–0.07 V higher those obtained using E1. The lower concentration of Li ions in E2 raised the Fermi energy level of the conduction band of TiO₂, enhancing the V_oc values and subsequently increasing the gap between the conduction band of TiO₂ and E2.¹⁹ Although the V_oc values of all devices with E2 were considerably higher than those with E1, the J_sc values were considerably lower, particularly that of YC-2 (approximately 17%) (see Table 1).

The long hexyloxy chains of YC-1 and YC-4 may prevent contact between I₃⁻ and the surface of TiO₂; these compounds exhibited higher V_oc values than those of the other two sensitisers, which had no long chain.^4c,4j,21 Therefore YC-1 and YC-4 reduce the dark current by lowering the rate of charge recombination by more than YC-2 and YC-3.

The alignment of dyes on the surface of TiO₂ is a crucial factor in the further optimisation of the performance of DSSCs. The sensitiser must be carefully placed vertically on TiO₂ to prevent intermolecular aggregation under dark current conditions. It was examined whether adding a DCA co-absorbent prevented the dye from lying on the TiO₂ surface. For some dyes that are unable to form high-quality films, adding DCA has been reported to effectively improve the film morphology and consequently reduce the charge recombination rate.²² However, adding DCA may not improve the performance of devices that have a high-quality film morphology. Tian and co-workers^12,23 reported the same structure using the YC-2 dye; however, they did not report the effects of adding a DCA co-absorbent on S1 or D-2. In this study, we showed that adding DCA (10 mM) to YC-2 (up to 5.36%) and YC-3 (5.28%) improved the quantum efficiency by 4–9% (Table 2 and Fig. 8). In the presence of DCA, both the V_oc and J_sc values increased considerably, indicating an improvement in film quality and dye loading (Table S4†). This indicates that the addition of a DCA co-absorbent helps the dye to align vertically on the TiO₂ surface rather than lying horizontally. YC-1 and YC-4 seem to form a high-quality monolayer. On adding DCA, both the V_oc and FF values remained nearly the same; however, the J_sc values decreased from 14.86 to 12.91 mA cm⁻² for YC-1 and from 13.44 to 13.17 mA cm⁻² for YC-4. The quantum efficiency showed an overall decrease of 4–13%. The reduction in the J_sc value can be explained by a lower amount of loading because the TiO₂ surface was partly occupied by DCA. The long hexyloxy substituents in YC-1 and YC-4 not only cover the TiO₂ surface, but also form a high-quality film. This effect is supported by the higher resistance in the electrochemical impedance spectra. The relative photovoltaic efficiency of YC-1 with E1 was higher than that with the other electrolytes; this difference was attributed to the higher efficiency of electron injection to the conduction band of TiO₂. YC-1 exhibited an IPCE of 60% in the region 385–605 nm, a J_sc value of 14.86 mA cm⁻², a V_oc value of 0.67 V and an FF of 0.62, corresponding to an overall conversion efficiency of 6.18%.

Fig. 8 J–V curves of DSSCs fabricated with and without a DCA co-absorbent at a light intensity of 1.0 Sun.

Electrochemical impedance spectroscopy and lifetime using CEM/IMVS

Electrochemical impedance spectroscopy was performed to further elucidate the photovoltaic properties of the dyes (Fig. 9). The Nyquist and Bode plots were measured under forward bias and dark conditions. In the Nyquist plots, a semicircle was observed for each dye; this semicircle was associated with the transport process at the TiO₂/electrolyte/dye interface; a larger semicircle corresponds to a larger charge recombination resistance at a low frequency, indicating a lower charge recombination rate and smaller dark current.²⁴ The radius of the larger semicircles increased in the order YC-2 < YC-3 < YC-4 < YC-1 and this trend was consistent with the V_oc values in E1. The electron lifetime can be directly estimated by fitting the plots to the equation τ = 1/(2πf) in a Bode plot (Fig. 9b), where a shift to a lower frequency corresponds to a longer electron lifetime. The larger V_oc values of YC-1 (27.5 ms) and YC-4 (22.4 ms) correspond to a longer electron lifetime and the results showed that the long hexyloxy chain of the arylamine donor could prevent direct contact between iodine and the TiO₂ surface and reduce the charge recombination rate.

Fig. 9 Impedance spectra of YC series dyes using the E1 electrolyte in CH₂Cl₂ without DCA. (a) Nyquist plots (minus the imaginary part of the impedance −Z″ vs. the real part of the impedance Z′ when sweeping the frequency). (b) Bode phase plots at −0.70 V bias in the dark.

We measured the electron lifetime (τ_r) by IMVS (Fig. 10a). The electron lifetime decreased in the order YC-1 (31.41 ms) > YC-4 (24.62 ms) > YC-3 (18.66 ms) > YC-2 (16.77 ms) under the same light intensity; a similar trend was seen in the data by fitting Bode plots. The V_oc value is defined as the energy difference between the Fermi level of TiO₂ and the redox couple of the electrolyte.¹⁸ Considering that the redox couple in the fabricated cells was identical (E1 electrolyte), the difference in V_oc for YC-1–4 should be attributable to the electron density in TiO₂. We also performed CEM measurements to investigate the influence of the V_oc value on the shift in the Femi energy level of the TiO₂ conduction band. The conduction band edge of TiO₂ exhibited a higher up-shift for YC-1 (Fig. 10b) and this trend was consistent with the Nyquist plots. The high V_oc value and electron lifetime were associated with the balance between electron injection and charge recombination.

Fig. 10 IMVS and CEM results for the YC series dyes using E1 in CH₂Cl₂ without DCA. (a) Electron lifetime as a function of V_oc measured by IMVS. (b) Electron density as a function of V_oc by CEM.

4. Conclusions

We have shown that high-performance DSSCs can be achieved using isophorone as the bridge moiety in YC-1–4 dyes and that the dyes can be synthesised without a Pd catalyst. The planar shape of isophorone, with two slightly bulky methyl substituents, resulted in the formation of films on the TiO₂ surface. Both YC-1 and YC-4 exhibited remarkable solar-to-energy conversion efficiencies as a result of their favourable light-harvesting capacity and high absorptivity. The YC-1 and YC-4 sensitisers formed high-quality films on TiO₂ without the use of a DCA co-absorbent. A typical device fabricated using YC-1 had an IPCE of 60% in the region 385–605 nm, a J_sc of 14.86 mA cm⁻², a V_oc of 0.67 V and an FF of 0.62, corresponding to an overall conversion efficiency of 6.18%. YC-1 also showed higher V_oc values and electron lifetimes using the impedance/CEM/IMVS method. Although YC-1 showed the highest performance of 6.18%, this was still lower than NKX-2753 and D-3. However, we used low-cost C–C bond coupling without the requirement for a Pd catalyst to obtain the isophorone derivatives. This facilitated the preparation of useful dyes for solar cells and our investigation of the donor side with long alkyl chain led to simplified device fabrication without the addition of DCA.

Acknowledgements

Financial support from the Ministry of Science and Technology of Taiwan (MOST 103-2113-M-029-007-MY2) and Academia Sinica are gratefully acknowledged. Special thanks to Professor C.-P. Hsu at the Institute of Chemistry Sinica of Taiwan for her assistance with the quantum chemistry computations.

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Footnote

† Electronic supplementary information (ESI) available: Experimental data of compound B–D, the ¹H- and ¹³C-NMR spectra, theoretical calculation, UV/vis spectra, CV spectra, EIS spectra, and the devices incooperating DCA. See DOI: 10.1039/c5ra17898c

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