New diketopyrrolopyrrole-based organic dyes for highly efficient dye-sensitized solar cells

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New diketopyrrolopyrrole-based organic dyes for highly efficient dye-sensitized solar cells

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Fang Zhang a,b, Ke-Jian Jiang a,⇑, Jin-Hua Huang a, Shao-Gang Li a, Ming-Gong Chen b,⇑, Lian-Ming Yang a, Yan-Lin Song a,⇑ a

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b

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Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China School of Chemical Engineering of Anhui University of Science and Technology, Huainan 232001, PR China

a r t i c l e

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i n f o

Article history: Received 8 March 2014 Received in revised form 8 April 2014 Accepted 8 April 2014 Available online xxxx

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Keywords: Diketopyrrolopyrrole Dyes/pigments Sensitizers Solar cell

a b s t r a c t Three diketopyrrolopyrrole (DPP) dyes (ICD-3, ICD-4 and ICD-5) with a D-p-A conjugation were designed and synthesized, where a symmetric phenyl-DPP-phenyl unit was used to connect a substituted diphenylamine and a thienyl acrylic acid, and two n-hexyl or 2-ethyl-hexyl chains were introduced on the periphery of the DPP macrocycle. The dyes were characterized by photophysical, electrochemical, and density functional theory calculations. Among the three dyes, the ICD-5-based DSC afforded the best photovoltaic performance: a short circuit photocurrent density (Jsc) of 16.34 mA/cm2, an open circuit voltage (Voc) of 753 mV, and a fill factor (FF) of 0.74, corresponding to an overall conversion efficiency (g) of 9.10% using I /I3 redox couple-based liquid electrolyte under AM 1.5 conditions. The experimental results demonstrate that the DPP-based sensitizer is a promising option for DSCs, and rational molecular engineering is crucial for constructing highly efficient charge transfer sensitizers. Ó 2014 Published by Elsevier B.V.

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1. Introduction

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Dye-sensitized solar cells (DSCs) are one of the most promising alternatives to the conventional inorganic semiconductor photovoltaic devices due to their potential low material/fabrication cost and relatively high conversion efficiencies [1]. As one of the key components, the sensitizer is exerting a significant influence on the power conversion and stability in DSCs. With ruthenium complexes [2a–d] and zinc porphyrin dyes [2e–f], high power conversion efficiencies of up to 8–12% have been achieved. On the other hand, metal-free organic dyes, usually with electron donor-p conjugation bridge-electron acceptor (D-p-A) configuration, have been intentionally investigated due to

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⇑ Corresponding authors. Tel.: +86 62619942 E-mail addresses: [email protected] (K.-J. Jiang), [email protected]. cn (M.-G. Chen), [email protected] (Y.-L. Song).

their molecular tailoring flexibility and raw material abundance, exhibiting comparable conversion efficiencies [3]. In the D-p-A organic dyes, triarylamine and cyanoacrylic acid were widely used as donor and acceptor, respectively, and various p-conjugated bridges were employed to bridge the donor and acceptor units to construct a large number of D-p-A dyes for DSCs. The conjugation bridge is of paramount importance in tuning the molecular energy gap, and the electronic and steric structures, strongly affecting device performances [4]. Diketopyrrolopyrrole (DPP) chromophore has a uniquely planar conjugated bicyclic structure with electron-withdrawing property, and its derivatives are extensively used as high-performance pigments due to its exceptional photochemical, mechanical and thermal stability [5]. Following their successful investigations in a series of optical electronic devices [6], it was first tested as sensitizers for DSCs in 2010 [7a]. So far, a series of

http://dx.doi.org/10.1016/j.orgel.2014.04.009 1566-1199/Ó 2014 Published by Elsevier B.V.

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symmetric and asymmetric DPP dyes have been reported, and the device performances have been greatly improved [7]. In 2012, Han’s group introduced a strong electron donor indoline in a symmetric DPP dye, giving efficiency of 7.4% [7d]. Following the report, an asymmetric DPP sensitizer with intense absorption in the red/near-IR region was prepared with an efficiency of 7.7%, and a higher efficiency of 8.6% was achieved by co-sensitization with another spectrally complementary organic sensitizer [7f]. Very recently, the same group furthermore modified the structure of asymmetric DPP sensitizers, giving a high efficiency of 10.1% employing Co2+/Co3+ redox couple [8]. In our previous report, we found that compact DPP-based dyes ICD-1 presented broad absorption spectral with efficient charge transfer in the molecule, leading to a power conversion efficiency of 8.61% in DSC with an I /I3 redox couple-based electrolyte [9]. Therefore, DPP-based sensitizers is a desirable building block for constructing highly efficient sensitizers in DSCs. In this article, we further modified the structure of the compact DPP-based dye molecules, and three new DPP dyes, ICD-3, ICD-4, ICD-5, were designed, as shown in Scheme 1. In these dyes, two different substituted triphenylamine donors and different alkyl chains on the periphery of the DPP macrocycle were employed. It was found that ICD-5 dye with 4-(hexyloxy)-N-(4-(hexyloxy)phenyl)-Nphenylaniline donor and ethyl-hexyl chains showed the

highest power conversion efficiency of 9.10% in DSCs with an I /I3 based electrolyte under standard AM 1.5 conditions.

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2. Experimental section

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2.1. Measurement and characterization

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NMR spectra were recorded on a BRUKER AVANCE 400 MHz instruments. The residual solvent protons (1H) or the solvent carbons (13C) were used as internal standards. 1H NMR data are presented as follows: chemical shift in ppm (d) downfield from tetramethylsilane (multiplicity, coupling constant (Hz), integration). The following abbreviations are used in reporting NMR data: s, singlet; br. s, broad singlet; d, doublet; t, triplet; q, quartet; dd, doublet of doublets; m, multiplet. UV–vis absorption spectra were recorded on a HP 8453 spectrophotometer. Mass spectra were taken on a Bruker Daltonics Inc. APEXII FT-ICR spectrometer. The photocurrent–voltage (I–V) characteristics were recorded at room temperature using a computer-controlled Keithley 2400 source meter under air mass (AM) 1.5 simulated illumination (100 mW cm 2, Oriel, 67,005). The action spectra of monochromatic incident photo-to-current conversion efficiency (IPCE) for solar cells were performed using a commercial setup (PV-25 DYE, JASCO). A 300 W Xenon lamp was employed as light

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Scheme 1. Chemical structures of the DPP-based dyes: ICD-1–ICD-5.

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source for generation of a monochromatic beam. Calibrations were performed with a standard silicon photodiode. IPCE is defined by IPCE(k) = hcJsc/euk, where h is Planck’s constant, c is the speed of light in a vacuum, e is the electronic charge, k is the wavelength in meters (m), Jsc is the short-circuit photocurrent density (A m 2), and u is the incident radiation flux (W m 2).

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2.2. Materials

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All reagents were obtained from Alfa Aesar Chemical Co., Aladdin Chemical Co., and J&K Chemical Co. and they were used as received unless otherwise specified. All manipulations involving air-sensitive reagents were performed in the atmosphere of dry argon. The solvents (tetrahydrofuran (THF) and toluene) were purified by routine procedures and were distilled under dry argon before being used. 1,4-diketo-3,6-bis(4-bromophenyl)pyrrolo[3,4-c]-pyrrole (DPP) were prepared according to the reference procedures [9]. 3,6-bis(4-bromophenyl)-2, 5-dihexylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2a) and 5-(4-(4-(4-bromophenyl)-2,5-dihexyl-3,6-dioxo-,3,5,6-tetrahydropyrrolo[3,4-c]pyrrol-1-yl)phenyl)thiophene-2carbaldehyde (3a) were synthesized in the method reported previously [9]. 3,6-Bis(4-bromophenyl)-2, 5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4(2H, 5H)-dione (2b) was prepared according to Ref. [10].

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2.3. Synthesis

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2.3.1. Synthesis of 3b An oven-dried 100 mL three-necked flask was charged with compound 2 (1.81 g, 2.7 mmol), Pd(PPh3)4 (0.21 g, 0.18 mmol), and sodium carbonate (4.9 g, 46 mmol). The flask was evacuated and backfilled with nitrogen, with the operation being repeated twice. 40 mL THF and 10 mL H2O were added and the blend was heated at 45 °C for 0.5 h, then a solution of 5-formyl-2thiophenylboronic acid (0.47 g, 3.0 mmol) in 10 mL of THF was added via syringe at this time. The temperature was increased to 80 °C and maintained for 16 h. After cooling to room temperature, dichloromethane were added and the organic phase was filtered through a silica-gel pad. The filtrate was washed with water (3  20 mL) and brine (3  20 mL). The combined organic phases were dried on MgSO4, filtered and evaporated under reduced pressure and the residue purified by silica-gel column chromatography to give an orange-red fraction compound 5 (0.7 g, yield = 37%). 1H NMR(CDCl3, 400 MHz) d(ppm): 0.70–0.79 (m, 6H), 0.81–0.86 (m, 6H), 1.05–1.24 (m, 16H), 1.37 (m, 2H), 3.70 (m, 4H), 7.42 (d, J = 4 Hz, 1H), 7.49 (d, J = 8.4 Hz, 2H) 7.55 (d, J = 8.8 Hz, 2H), 7.65 (d, J = 8 Hz, 2H), 7.70 (d, J = 3.6 Hz, 1H),7.76 (d, J = 8.4 Hz, 2H), 9.87 (s, 1H).

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2.3.2. Synthesis of 4a An oven-dried 100 mL three-necked flask was charged with compound 3 (0.7 g, 1.0 mmol), Pd(OAc)2 (22 mg, 0.1 mmol), PtBu3 (20 mg, 0.1 mmol), bis(4-(tert-butyl)phenyl)amine (0.37 g, 1.30 mmol), tBuONa (0.48 g, 5.0 mmol). The flask was evacuated and backfilled with nitrogen, with the operation being repeated twice. Then 20 mL toluene

3

were added and the mixture was heated at 90 °C for 16 h. After cooling to room temperature, dichloromethane were added and the organic phase was filtered through a silica-gel pad. The filtrate was with water (320 mL) and brine (320 mL). The combined organic phases was dried on MgSO4, filtered and evaporated under reduced pressure and the residue purified by silica-gel column chromatography to give a deep-red solid(0.61 g, 62%). 1H NMR(CDCl3, 400 MHz) d(ppm): 0.69–0.82 (m, 12H), 0.95–1.24 (m, 8H), 1.35 (s, 18H), 1.45–1.51 (m, 8H), 1.75–1.1.78 (m, 2H), 3.79 (m, 4H), 6.85 (d, J = 8.8 Hz, 4H), 6.90 (d, J = 8.8 Hz, 2H), 7.11 (d, J = 8.8 Hz, 4H),7.48 (d, J = 4 Hz, 1H), 7.71(d, J = 9.2 Hz, 2H), 7.76 (d, J = 4 Hz, 1H), 7.79 (d, J = 8.4 Hz,2H),7.85 (d, J = 8.4 Hz, 2H), 9.92 (s, 1H).

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2.3.3. Synthesis of 4b The reaction and purification was performed in the same manner as for 4a. 1H NMR(CDCl3, 400 MHz) d(ppm): 0.69–0.82 (m, 6H), 0.89–0.93 (m, 6H), 1.10–1.24 (m, 16H), 1.33–1.36 (m, 10H), 1.45–1.51 (m, 2H), 1.62– 1.64 (m, 4H), 3.78 (m, 4H), 3.80–3.91 (m, 4H), 6.84 (d, J = 8.8 Hz, 4H), 6.89 (d, J = 8.8 Hz, 2H), 7.13 (d, J = 8.8 Hz, 4H), 7.46 (d, J = 4 Hz, 1H), 7.73 (d, J = 9.2 Hz, 2H), 7.75 (d, J = 4 Hz, 1H), 7.78 (d, J = 8.4 Hz, 2H), 7.84 (d, J = 8.4 Hz, 2H), 9.92 (s, 1H).

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2.3.4. Synthesis of 4c The reaction and purification was performed in the same manner as for 4a. 1H NMR(CDCl3, 400 MHz) d(ppm): 0.70–0.84 (m, 12H), 0.89–0.93 (m, 6H), 1.10– 1.24 (m, 16H), 1.33–1.36 (m, 12H), 1.45–1.51 (m, 2H), 1.75–1.82 (m, 4H), 3.79 (m, 4H), 3.95 (t, J = 6.4 Hz, 4H), 6.85 (d, J = 8.8 Hz, 4H), 6.90 (d, J = 8.8 Hz, 2H), 7.11 (d, J = 8.8 Hz, 4H), 7.48 (d, J = 4 Hz, 1H), 7.71 (d, J = 9.2 Hz, 2H), 7.76 (d, J = 4 Hz, 1H), 7.79 (d, J = 8.4 Hz, 2H), 7.85 (d, J = 8.4 Hz, 2H), 9.92 (s, 1H).

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2.3.5. Synthesis of ICD-3 An oven-dried 100 mL one-necked flask was charged with compound 4a (3.5  10 4 mol), cyanoacrylic acid (7  10 3 mol), 10 mL of dry THF and 0.4 mL piperidine. The solution was heated to reflux for 15 h and the color turned to deep red. After cooling to room temperature, dichloromethane were added and the organic phase was washed with water (320 mL) and brine (320 mL). The combined organic phase was dried on MgSO4, filtered and evaporated under reduced pressure and the residue purified by silica-gel column chromatography to give a purple solid (86%). 1H NMR(DMSO, 400 MHz) d(ppm): 0.67–0.80 (m, 12H), 0.91–1.22 (m, 8H), 1.35 (s, 18H), 1.41–1.50 (m, 8H), 1.72–1.1.75 (m, 2H), 3.77 (m, 4H), 6.83 (d, J = 8.8 Hz, 4H), 6.88 (d, J = 8.8 Hz, 2H), 7.09 (d, J = 8.8 Hz, 4H), 7.46 (d, J = 4 Hz, 1H), 7.68 (d, J = 9.2 Hz, 2H), 7.73 (d, J = 4 Hz, 1H), 7.76 (d, J = 8.4 Hz, 2H), 7.84 (d, J = 8.4 Hz, 2H). HRMS (ESI): m/z calcd: 968.5, m/z found (M + 1): 968.8.

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2.3.6. Synthesis of ICD-4 The reaction and purification was performed in the same manner as for ICD-3. 1H NMR(DMSO, 400 MHz) d(ppm): 0.66–0.80 (m, 6H), 0.87–0.92 (m, 6H), 1.07–1.21

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(m, 16H), 1.31–1.33 (m, 10H), 1.43–1.49 (m, 2H), 1.60–1.63 (m, 4H), 3.77 (m, 4H), 3.78–3.89 (m, 4H), 6.82 (d, J = 8.8 Hz, 4H), 6.87 (d, J = 8.8 Hz, 2H), 7.10 (d, J = 8.8 Hz, 4H),7.43 (d, J = 4 Hz, 1H), 7.70 (d, J = 9.2 Hz, 2H), 7.74 (d, J = 4 Hz, 1H), 7.76 (d, J = 8.4 Hz, 2H),7.82 (d, J = 8.4 Hz, 2H). MS (MALDI-TOF): m/z calcd: 1000.5, m/z found: 1000.8 (M+).

2.5. Characterization

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The photocurrent–voltage (I–V) characteristics were recorded at room temperature using a computer-controlled Keithley 2400 source meter under air mass (AM) 1.5 simulated illumination (100 mW cm 2, Oriel, 67,005). The action spectra of monochromatic incident photo-tocurrent conversion efficiency (IPCE) for solar cells were performed using a commercial setup (PV-25 DYE, JASCO). A 300 W Xenon lamp was employed as light source for generation of a monochromatic beam. Calibrations were performed with a standard silicon photodiode. IPCE is defined by IPCE(k) = hcJsc/euk, where h is Planck’s constant, c is the speed of light in a vacuum, e is the electronic charge, k is the wavelength in meters (m), Jsc is the shortcircuit photocurrent density (A m 2), and u is the incident radiation flux (W m 2).

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3. Results and discussion

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2.3.7. Synthesis of ICD-5 The reaction and purification was performed in the same manner as for ICD-3. 1H NMR(DMSO, 400 MHz) d(ppm): 0.62–0.79 (m, 12H), 0.86–0.89 (m, 6H), 1.01– 1.06 (m, 16H), 1.30–1.41 (m, 14H), 1.67–1.72 (m, 4H), 3.74 (s, 4H), 3.96 (m, 4H), 6.73 (d, J = 8.8 Hz, 2H), 6.97 (d, J = 8.8 Hz, 4H), 7.16 (d, J = 8.8 Hz, 4H), 7.76–7.78 (m, 4H), 7.90 (s, 4H), 8.11 (s, 1H); 13C NMR(600 MHz, CDCl3) d(ppm): 162.4, 156.1, 150.1, 142.2, 139.4, 130.5, 129.7, 125.6, 118.2, 117.6, 115.5, 109.2, 68.4, 44.5, 38.3, 31.7, 30.7, 30.4, 29.4, 28.6, 25.9, 23.9, 23.1, 22.9, 22.7, 14.2, 14.0, 10.6; MS (MALDI-TOF): m/z calcd: 1056.5, m/z found: 1056.5 (M+).

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2.4. DSC fabrication and characterization

3.1. Optical and electrochemical properties

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2.4.1. DSC fabrication The nanocrystalline TiO2 pastes (particle size, 20 nm) were prepared using a previously reported procedure [11]. Fluorine doped thin oxide (FTO, 4 mm thickness, 10 X/sq, Nippon Sheet Glass, Japan) conducting electrodes were washed with soap and water, followed by sonication for 10 min in acetone and isopropanol, respectively. Following a drying period, the electrodes were then submerged in a 40 mM aqueous solution of TiCl4 for 30 min at 75 °C, and then washed by water and ethanol. On the electrodes, an 11 lm thick nanocrystalline TiO2 layer and 6 lm thick TiO2 light scattering layer (particle size, 400 nm, PST-400C) were prepared by screen-printing method. The TiO2 electrodes were heated at 500 °C for 30 min, followed by treating with a 40 mM aqueous solution of TiCl4 for 30 min at 75 °C and subsequent sintering at 500 °C for 30 min [12]. The TiO2 film thickness was measured by a profiler, Sloan, Dektak3. The electrodes were immersed in a dye bath containing 0.2 mM dye (ICD-3, ICD-4 or ICD-5) and 2 mM 3a,7a-dihydroxy-5b-cholic acid (chenodeoxycholic acid) in 4-tert-butanol/acetonitrile mixture/tetrahydrofuran (1:1:0.2, v/v) and kept for 24 h at room temperature. The dyed electrodes were then rinsed with the mixed solvent to remove excess dye. A platinum-coated counter electrode was prepared according to the report [12], and two holes were drilled on its opposite sides. The two electrodes were sealed together with a 25 lm thick thermoplastic Surlyn frame. An electrolyte solution was then introduced through one of the two holes in the counter electrode, and the holes were sealed with the thermoplastic Surlyn. The electrolyte contains 0.68 M dimethyl imidazolium iodide, 0.05 M iodine, 0.10 M LiI, 0.05 M guanidinium thiocyanate, and 0.40 M tert-butylpyridine in the mixture of acetonitrile and valeronitrile (85:15, v/v). All the devices were prepared with a photoactive area of about 0.3 cm2, and a metal mask of 0.165 cm2 was covered on the device for photovoltaic property measurements.

The synthetic routes of ICD-3, ICD-4 and ICD-5 are described in Scheme 2. Their UV–vis absorption spectra examined in dichloromethane solutions were displayed in Fig. 1, and the relevant parameters were collected in Table 1. The three dyes showed broad absorption spectra with two absorption bands covering a wide range in the visible region, corresponding to the p–p transition of the molecules and the intramolecular charge transfer. The maximum absorption wavelength (kmax) of ICD-3 with tertbutyl-substituted triphenylamine donor peaks at 535 nm, which is red-shifted to 542 nm when replacing by hexyloxy-based triphenylamine for ICD-5. Substituting the ethyl-hexyl units on the periphery of the DPP macrocycle with hexyl units leads to further red shift at 556 nm for ICD-4. The emission peaks are located at 642 nm, 670 nm and 684 nm for ICD-3 to ICD-5, respectively. Among the three dyes, ICD-5 shows the highest extinction coefficient (e = 64,100 L mol 1 cm 1). The cyclic voltammetry measurements of the three dyes were performed in a 0.1 M dichloromethane solution of tetrabutylammonium hexafluorophosphate with ferrocene as internal standard at 0.63 V vs. NHE. The first oxidation potentials (E+/0) of ICD-3, ICD-4 and ICD-5 were observed to be 0.91, 0.85 and 0.88 V vs. NHE, respectively, which are assigned to the oxidation of triphenylamine units. All the three potential values are substantially more positive than the iodide/tri-iodide couple redox (0.4 V vs. NHE), indicating that the ground-state sensitizer regeneration is energetically favorable in DSC [10]. The excited-state redox potentials were 1.01 V, 1.05 V and 1.02 V for ICD-3, ICD-4 and ICD-5, respectively, determined by subtracting the optical transition energies E0–0 from E+/0, where the E0–0 were estimated from the intersection of their absorption and emission spectra with values of 1.92 eV for ICD-3 and 1.90 eV for ICD-4 and ICD-5. The values of the excitedstate redox potentials are negative enough to allow their excited state electron transfer into the TiO2 conduction band ( 0.5 V vs. NHE) [11].

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Scheme 2. Synthetic routes of the DPP dyes: ICD-3, ICD-4 and ICD-5.

ICD-3 ICD-4

Absorbance / a.u.

1.2

ICD-5

0.8

0.4

0.0 300

400

500

600

700

Wavelength / nm Fig. 1. UV–vis absorption spectra of ICD-3, ICD-4 and ICD-5 in dichloromethane. 353 354 355

In order to further gain insight into the geometrical configuration and electron distribution of the frontier orbitals of the two dyes, density functional theory (DFT) calcula-

tions were made on a B3LYP/6-31G level as shown in Fig. S2. As shown in Fig. S2, the frontier HOMOs of both the dyes are homogeneously delocalized over the whole triphenylamine and DPP units, while their LUMOs are on the cyanoacrylic acid group and part of the DPP macrocycle. Thus electron communication may be favorable between the donor and the acceptor in all the three dyes, allowing an efficient electron transfer from dye to TiO2 electrode under light irradiation. Fig. 2 shows action spectra in the form of monochromatic incident photon-to-current conversion efficiencies (IPCEs). All the three dye-based devices exhibited high IPCEs of over 70% in the spectral range of 400–650 nm. Among them, ICD-3 and ICD-5-based devices presents higher IPCEs of over 80% than that for ICD-4-based device in the spectral range of 500–650 nm. The relatively low IPCE for ICD-4 may be rationalized by the charge recombination at the dyed TiO2 interface due to the dye aggregation, where two hexyl units were employed in ICD-4 in contrast to large ethyl-hexyl units for ICD-3 and ICD-5 [7d]. Compared to ICD-3, the spectra of ICD-4 and ICD-5

Table 1 Photoelectrochemical properties of ICD-3, ICD-4 and ICD-5 and their solar cell performances. Dye

kmaxa (nm)

e (L mol

ICD-3

535 393 556 393 542 393

53,200 61,700 48,300 42,000 53,500 64,100

ICD-4 ICD-5

1

cm

1

)

kemb (nm)

E0–0c (eV)

E+/0d (V)

E+/⁄e (V)

642

1.92

0.91

1.01

670

1.90

0.85

684

1.90

0.88

Voc (mV)

FF

gf (%)

16.30

770

0.71

8.89

1.05

16.76

680

0.66

7.53

1.02

16.34

753

0.74

9.10

Jsc (mA cm

2

)

a

Absorption in CH2Cl2 solutions (1  10 5 M) at rt. Emission in CH2Cl2 solutions (1  10 5 M) at rt. c E0–0 values were estimated from the intersection of the absorption and emission spectra. d The oxidation potentials of the dyes were measured in CH2Cl2 solutions with tetrabutyl-ammoniumhexafluorophosphate (TBAPF6, 0.1 M) as electrolyte, Pt wires as working and counter electrode, Ag/Ag+ as reference electrode; calibrated with ferrocene/ferrocenium (Fc/Fc+) as an internal reference and converted to NHE by addition of 630 mV. e The estimation was determined by subtracting E0–0 from Es+/0. f The data were recorded under AM 1.5 G simulated solar light at a light intensity of 100 mW cm 2, and represents the average of three devices, where TiO2 films with 11 lm thick nanocrystalline layer and 6 lm thick scattering layer were used with an electrolyte containing 0.68 M dimethyl imidiazolium iodide, 0.05 M iodine, 0.10 M LiI, 0.05 M guani-dinium thiocyanate, and 0.40 M tert-butylpyridine in a mixture of acetonitrile and valeronitrile (85/15, v/v), and each data was averaged by three parallel samples. b

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1.0

35

ICD-3

ICD-3 30

ICD-4

0.8

ICD-5

ICD-5

25

0.6

IPCE

ICD-4

20 15

0.4

10 0.2 5 0.0 400

0 500

600

700

800

0

30

378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399

90

120

150

Fig. 2. IPCE action spectra of ICD-3, ICD-4 and ICD-5.

Fig. 4. Electrochemical impedance spectra measured in the dark at a forward bias of 0.7 V for the DSCs employing ICD-3, ICD-4 and ICD-5.

are red shifted about 30 nm and 15 nm, respectively, in good agreement with their absorption spectra. Fig. 3 shows photocurrent density vs. voltage (I–V) curves of ICD-3, ICD-4 and ICD-5, measured under AM 1.5 G simulated solar light at a light intensity of 100 mW cm 2, and the corresponding data were listed in Table 1. The ICD-3-based device gave a short circuit photocurrent density (Jsc) of 16.30 mA/cm2, an open circuit voltage (Voc) of 770 mV, and a fill factor (FF) of 0.71, corresponding to an overall conversion efficiency (g), derived from the equation g = Jsc
Voc
FF/light intensity, of 8.89%. The Voc value (770 mV) is much higher than that (708 mV) for the similar dye ICD-1 with hexyl units on the periphery of the DPP macrocycle in our previous report, indicating two large ethyl-hexyl units in ICD-3 can more effectively retard charge recombination at the interface of the TiO2/electrolyte [13]. The ICD-4 based devices exhibited 16.76 mA cm 2, 680 mV, 0.66% and 7.53%, while the values of ICD-5 are Jsc of 16.34 mA cm 2, Voc of 753 mV, FF of 0.74 and g of 9.10%, respectively. The large difference in the efficiency comes mainly from the differences in the Vocs, and can be explained by the interface charge recombination as mentioned.

For further elucidating the difference in the Voc for the devices sensitized by ICD-3, ICD-4 and ICD-5, electrochemical impedance spectroscopy (EIS) was performed to investigate the FTO/TiO2/dye interfaces as shown in Fig. 4. The Nyquist plots of DSCs were recorded in the dark with a forward bias of 0.7 V, as shown in Fig. 4. Two semicircles from left to right in the Nyquist plot were assigned to the resistances of charge transfer (RPt) on the Pt counter electrode and charge recombination (Rr) at the interface of the TiO2/electrolyte. It is clear that the semicircle for Rr is the smallest for ICD-4, which may indicate large charge recombination at the interface back charge transfer at the TiO2/ the dye interface, which is consistent with the lowest Voc for the ICD-4-based device.

400

4. Conclusions

414

In summary, three novel D-p-A dyes ICD-3, ICD-4 and ICD-5 were designed and synthesized, where phenylDPP-phenyl unit was used a linker to bridge a substituted diphenylamine and a phenyl cyanoacetic acid, and two n-hexyl or 2-ethyl-hexyl chains were introduced on the periphery of the DPP macrocycle. These dyes were employed as sensitizers in DSCs, and gave a high power conversion efficiency of 9.10% using I /I3 redox couplebased electrolyte. Our results indicate that molecular engineering is crucial for constructing highly efficient sensitizers in DSCs, and the results would provide valuable, basic guidelines for rational designs of D-p-A molecules for high-performance DSCs and other optoelectronic devices.

415

Acknowledgements

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The authors greatly appreciate the financial support Q3 from the National Natural Science Foundation of China Q4 (Grant Nos. 21174149, 21102150, 51173190, 21073203, 21076002 and 21121001), the National 863 Program (No. 2011AA050521), the 973 Program (2009CB930404, 2011CB932303 and 2011CB808400), and the Scientific Equipment Program, ACS (YZ201106).

429

ICD-3

Current Density / mA cm-2

377

60

Z' / Ω

Wavelength / nm

ICD-4

15

ICD-5

10

5

0 0

200

400

600

800

Voltage / mV Fig. 3. Photocurrent density vs. voltage (I–V) curves of ICD-3, ICD-4 and ICD-5.

Please cite this article in press as: F. Zhang et al., New diketopyrrolopyrrole-based organic dyes for highly efficient dye-sensitized solar cells, Org. Electron. (2014), http://dx.doi.org/10.1016/j.orgel.2014.04.009

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Appendix A. Supplementary material

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Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.orgel.2014.04.009.

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