Redox Mediator

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Energy, Environmental, and Catalysis Applications

Efficient and Stable Dye-Sensitized Solar Cells Based on a Tetradentate Copper(II/I) Redox Mediator Maowei Hu, Junyu Shen, Ze Yu, Rong-Zhen Liao, Gagik G. Gurzadyan, Xichuan Yang, Anders Hagfeldt, Mei Wang, and Licheng Sun ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10182 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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ACS Applied Materials & Interfaces

Efficient and Stable Dye-Sensitized Solar Cells Based on a Tetradentate Copper(II/I) Redox Mediator Maowei Hu,†, § Junyu Shen,†, § Ze Yu,*,† Rong-Zhen Liao,‡ Gagik G. Gurzadyan,† Xichuan Yang,† Anders Hagfeldt,ǁ Mei Wang,*,† and Licheng Sun†,# †

State Key Laboratory of Fine Chemicals, Institute of Artificial Photosynthesis, DUT-KTH Joint Education and Research Center on Molecular Devices, Institute of Energy Science and Technology, Dalian University of Technology (DUT), Dalian 116024, China. ‡ Key Laboratory of Material Chemistry for Energy Conversion and Storage, Ministry of Education, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, Wuhan 430074, China ǁ Laboratory of Photomolecular Science, École Polytechnique Fédérale de Lausanne, CH-1015 Lausanne, Switzerland. # Department of Chemistry, School of Engineering Sciences in Chemistry, Biotechnology and Health, KTH Royal Institute of Technology, Stockholm 10044, Sweden. §

These authors contributed equally to this work.

ABSTRACT: The identification of an efficient and stable redox mediator is of paramount importance for commercialization of dye-sensitized solar cells (DSCs). Herein, we report a new class of copper complexes containing diamine-dipyridine tetradentate ligands (L1 = N,N′-dibenzyl-N,N′-bis(pyridin-2-ylmethyl)ethylenediamine;

L2

=

N,N′-dibenzyl-N,N′-bis(6-methylpyridin-2-ylmethyl)ethylenediamine) as redox mediators in DSCs. Devices constructed with [Cu(L2)]2+/+ redox couple afford an impressive power conversion efficiency (PCE) of 9.2% measured under simulated one sun irradiation (100 mW cm–2, AM 1.5G), which is among the top efficiencies reported thus far for DSCs with copper complex-based redox mediators. Remarkably, the excellent air, photo, and electrochemical stability of the [Cu(L2)]2+/+ complexes renders an outstanding long-term stability of the whole DSC device, maintaining ~ 90% of the initial efficiency over 500 h under continuous full sun irradiation. This work unfolds a new platform for developing highly efficient and stable redox mediators for large-scale application of DSCs.

KEYWORDS: copper redox mediator, dye-sensitized solar cells, ligand engineering, stability, 1

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energy conversion

1. INTRODUCTION Direct conversion of solar energy to electricity by means of photovoltaic cells offers a prospective solution to tackle the energy and environmental issues.1,

2

Nanocrystalline

dye-sensitized solar cells (DSCs) as one of the most promising next-generation solar cell technologies have been intensively studied over the past two decades, because of some of their advantageous features, such as the use of low-cost materials, ease of fabrication, aesthetic design features (transparency and multicolor options), as well as excellent performance for indoor applications, etc.3, 4 The redox mediator, which takes the responsibilities of effective dye regeneration and charge transport between the two electrodes, is a key component for achieving high overall performance of DSCs.5 The triiodide/iodide has been the most commonly used redox couple since the very beginning of DSC research. However, some shortcomings of this redox mediator, such as a large energy loss for dye regeneration linked to a limited attainable photovoltage, corrosiveness, and competitive light absorption with the dye molecule, impede its potential application.6 Thus, searching for efficient, non-corrosive, and stable redox mediators has been one of the hot topics in the field of DSC research from practical application point of view.7–12 Among all the alternative redox mediators, one-electron outer-sphere transition metal complexes based on Co(III/II),13–18 Fe(III/II),19–22 and Ni(IV/III)23–24 have attracted considerable research interest, because of their easily tunable redox potentials through the modification of ligands to achieve a high photovoltage. So far, the cobalt(III/II) tris(1,10-phenanthroline) ([Co(phen)3]3+/2+) redox system has shown the record power conversion efficiency (PCE) of DSCs exceeding the value of 14%, in conjunction with panchromatic co-photosensitizers.25 Nevertheless, the cobalt complex redox systems are subject to mass-transport limitations derived from the large

2


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molecular size of cobalt complexes (six coordination number), which may limit further efficiency improvement.26–28 Moreover, large internal reorganization energy between the d7 (high-spin) and d6 (low-spin) states for cobalt complexes requires additional driving force for dye regeneration.29 In the last few years, Cu(II/I) complexes have gained growing interest as redox mediators and hole-transport materials in DSCs, due to some of their appealing properties, such as smaller size with respect to six-coordinated cobalt complexes, low toxicity, and high earth reserve of the copper metal.26–40 Fukuzumi et al. first introduced blue copper complexes as redox mediators in DSCs, in combination with a ruthenium photosensitizer N719.31 A modest PCE of 1.4% was obtained

under

full

sunlight

illumination,

based

on

bis(2,9-dimethyl-1,10-phenantroline)copper(II/I) ([Cu(dmp)2]2+/+) with a distorted tetragonal geometry. Since then, considerable research efforts have been devoted to designing new ligands for copper(II/I) complexes, mainly consisting of phenantroline and pyridine derivatives.35–41 Nevertheless, the performance of these copper complex-based electrolytes was not satisfactory, typically below 8% (some representative photovoltaic performance of DSCs consisting of copper complex-based electrolytes is summarized in Table S1 in the Supporting Information). Recently,

through

optimization

of

the

device

components,

in

particular

with

high-absorption-coefficient organic sensitizers and poly(3,4-ethylenedioxythiophene) (PEDOT) counter electrode, the PCEs of copper(II/I) complex-based electrolytes have been boosted to 9– 11%.26, 28, 29, 33 The ligand environment (type and denticity) of a metal complex plays a crucial role in determining the electrochemical properties and stability of the complexes.16 Bach et al. have successfully demonstrated that DSC devices containing the cobalt complex with a hexadentate pyridyl ligand exhibited outstanding stability in comparison to its bipyridine counterparts, owing to the chelate effect of the hexadentate ligand.42 It has also been reported that most of the copper

3



complexes with tetradentate ligands exhibited good stability for electrochemical water oxidation during bulk electrolysis.43–45 It is noticeable that the high-performing copper complex redox mediators reported thus far were exclusively dominated by bidentate ligands, i.e. phenantroline and pyridine derivatives.26, 27, 29 In this sense, the long-term stability of copper complex-based redox mediators seemingly constitutes a problematic issue and so far the research in terms of this subject has been rather scarce. Kloo and co-workers investigated the long-term stability of DSCs containing copper complexes based on bidentate pyridyl ligands.26 Such DSCs displayed a high PCE of 9.0% measured under one sun illumination, however, a significant efficiency drop of ~30% was detected within the first 20–40 hours under continuous light irradiation. Herein, we report a new class of copper complexes with diamine-dipyridine tetradentate ligands, [Cu(L1)]2+/+ (L1 = N,N′-dibenzyl-N,N′-bis(pyridin-2-ylmethyl)ethylenediamine) and [Cu(L2)]2+/+ (L2 = N,N′-dibenzyl-N,N′-bis(6-methylpyridin-2-ylmethyl)ethylenediamine), as redox mediators in DSCs. Our motivation is to develop a copper complex-based redox electrolyte that combines high efficiency and simultaneously high long-term stability. Compared to [Cu(L1)]2+/+ complexes, the introduction of methyl substituents on the ortho position of pyridines in complexes [Cu(L2)]2+/+ resulted in a more positive redox potential (300 mV), and thus a high Voc. The smaller internal reorganization energy gave rise to a high dye regeneration yield and photocurrent density, which led to an impressive PCE of 9.2% measured under simulated one sun illumination (100 mW cm–2, AM 1.5G). More strikingly, [Cu(L2)]2+/+ complexes exhibited excellent air, photo and electrochemical stability, and thus high long-term stability of the whole DSC device, maintaining ~ 90% of the initial efficiency over 500 h under continuous full sun irradiation.

2. EXPERIMENTAL SECTION 2.1 Synthesis of ligands and copper complexes 4


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Synthesis

of

N,N′-dibenzyl-N,N′-bis(pyridin-2-ylmethyl)ethylenediamine

(L1).

The

preparation method of L1 was similar to the procedures in the literature as shown in Scheme 1.46 N,N'-dibenzylethylenediamine (2.4 g, 10.0 mmol) and 2-(chloromethyl)pyridine hydrochloride (2.8 g, 22.0 mmol) were dissolved in 150 mL acetonitrile, K2CO3 (3.5 g, 25.0 mmol) was added, and the reaction was refluxed for 12 h. When the reaction was cooled to room temperature, the solid was removed through filtration. The solution was concentrated to 70 mL by vacuum, the white product (3.3 g, 78%) was obtained from recrystallization in acetonitrile at –10 ℃. Anal. Calcd for C28H30N4 (%): C 79.59, H 7.16, N 13.26; found: C 79.62, H 6.76, N 13.45. 1H NMR (CDCl3, 400 MHz): δ 2.68 (s, 4H), 3.57 (s, 4H), 3.70 (s, 4H), 7.11 (m, 2H), 7.25 (m, 10H), 7.45 (d, 2H, J = 8.1 Hz), 7.57 (m, 2H), 8.48 (d , 2H, J = 5.0 Hz).

13

C NMR (CDCl3, 500 MHz): δ

159.22, 147.75, 138.31, 135.21, 127.71, 127.13, 125.82, 121.64, 120.70, 59.57, 57.95 and 50.77. ESI-MS: Calcd for (M+H)+, m/z 423.25; found: 423.24. Scheme 1. Synthetic route to L1

Synthesis of N,N′-dibenzyl-N,N′-bis(6-methylpyridin-2-ylmethyl)ethylenediamin (L2). The preparation method of L2 was similar to L1 (Scheme 2), the yield of L2 was 80%. Anal. Calcd for C30H34N4 (%): C 79.96, H 7.61, N 12.43; found: C 79.99, H 7.62, N 12.41. 1H NMR (CDCl3, 400 MHz): δ 2.42(s, 6H), 2.58 (s, 4H), 3.50 (s, 4H), 3.61 (s, 4H), 6.88 (d, 2H, J = 6.0

5



Hz), 7.17 (m, 12H), 7.38 (t, 2H, J = 6.6 Hz). 13C NMR (CDCl3, 400 MHz): δ 158.64, 156.14, 138.31, 135.42, 127.64, 127.06, 125.72, 120.07, 118.28, 59.68, 57.93, 50.72 and 23.38. ESI-MS: Calcd for (M+H)+, m/z 450.28; found: 450.27.

Scheme 2. Synthetic route to L2

Synthesis of [Cu(L1)](BF4). The mixture of Cu(BF4)·4CH3CN (0.314 g, 1.0 mmol) and L1 (0.422 g, 1.0 mmol) in acetonitrile (40 mL) was stirred at room temperature for 8 h. The solution was concentrated to about 20 mL by evaporation under vacuum, and the yellow crystals of [Cu(L1)](BF4) were obtained by diffusing diethyl ether into the resulting solution. Yield: 0.46 g (80 %); elemental analysis calcd for C28H30N4BF4Cu (%): C 58.70, H 5.28, N 9.78; found: C 58.75, H 5.30, N 9.74; TOF-MS: calcd for (M − BF4)+ (C28H30N4Cu): m/z 485.1766; found: 485.1765. Synthesis of [Cu(L1)](BF4)2. The mixture of Cu(BF4)2·6H2O (0.345 g, 1.0 mmol) and L1 (0.422 g, 1.0 mmol) in acetonitrile (40 mL) was stirred at room temperature for 8 h. The solution was concentrated to about 20 mL by evaporation under vacuum, and the blue crystals of [Cu(L1)](BF4)2 were obtained by diffusing diethyl ether into the resulting solution. Yield: 0.58 g (86%); elemental analysis calcd for C28H30N4B2F8Cu (%): C 50.98, H 4.58, N 8.49; found: C 51.01, H 4.61, N 8.48; TOF-MS: calcd for (M − H2O − 2BF4)2+ (C28H30N4Cu): m/z 242.5883; found: 242.5883. 6


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Synthesis of [Cu(L2)](BF4). The mixture of Cu(BF4)·4CH3CN (0.314 g, 1.0 mmol) and L2 (0.449 g, 1.0 mmol) in acetonitrile (40 mL) was stirred at room temperature for 8 h. The solution was concentrated to about 20 mL by evaporation under vacuum, and the yellow crystals of [Cu(L2)](BF4) were obtained by diffusing diethyl ether into the resulting solution. Yield: 0.54 g (90%); elemental analysis calcd for C30H34N4BF4Cu (%): C 59.96, H 5.70, N 9.32; found: C 59.97, H 5.72, N 9.31; TOF-MS: calcd for (M − BF4)+ (C30H34N4Cu): m/z 513.2079; found: 513.2080. Synthesis of [Cu(L2)](BF4)2. The mixture of Cu(BF4)2·6H2O (0.345 g, 1.0 mmol) and L2 (0.449 g, 1.0 mmol) in acetonitrile (40 mL) was stirred at room temperature for 8 h. The solution was concentrated to about 20 mL by evaporation under vacuum, and the blue crystals of [Cu(L2)](BF4)2 were obtained by diffusing diethyl ether into the resulting solution. Yield: 0.61 g (89%); elemental analysis calcd for C30H34N4B2F8Cu (%): C 52.39, H 4.98, N 8.15; found: C 52.41, H 4.50, N 8.14; TOF-MS: calcd for (M − 2BF4)2+ (C16H22N4Cu): m/z 256.6040; found: 256.6039. 2.2 Device Fabrication. The DSC devices were fabricated as reported previously.47 First, fluorine-doped tin oxide (FTO) conducting glasses (Pilkington, TEC 15) were sequentially cleaned by detergent, deionized water and ethanol for 30 min, respectively. The cleaned FTO glasses were soaked in 40 mM TiCl4 aqueous solution at 70 ℃ for 30 min, and then sintered at 500 ℃ for 30 min. DSC devices were constructed with a double layer TiO2 film consisting of a ~ 6 µm transparent layer and a ~ 4 µm scatting layer. The transparent layer was fabricated by screen-printing of TiO2 18NR-T paste three times, every time dried at 130 ℃ for 5 min, and the scattering layer was fabricated by screen-printing of TiO2 TPP200 paste one time. After sintering at 500 ℃ for 30 min, the TiO2 films were treated with TiCl4 aqueous solution at 70 ℃ for 30 min again. The sintered photoanode TiO2 films were immersed into a 0.1 mM Y123 dye solution in acetonitrile

7



and tert-butanol mixed solvent (1 : 1, v/v) overnight containing 0.4 mM chenodeoxycholic acid. The unattached dyes were rinsed by ethanol and acetonitrile. The counter electrode poly(3,4-ethylenedioxythiophene) (PEDOT) was prepared by an electrodeposition method as reported previously.48 The dye-sensitized TiO2 photoanode and counter electrode PEDOT were assembled by a 25 µm Surly film. The liquid electrolyte was injected into the interlayer by a predrilled hole, which was then sealed with a Surlyn film and a glass cover. The electrolytes consist of 0.25 M Cu(I), 0.065 M Cu(II), 0.1 M lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) and 0.6 M 4-tert-butylpyridine (TBP) in acetonitrile. The active area of the DSC devices was 0.16 cm2. The photocurrent density-voltage measurements were performed with a black mask (1mm wider than the active area).

3. RESULTS AND DISCUSSION The crystal structures of [Cu(L1)]2+, [Cu(L2)]2+, and [Cu(L2)]+ are shown in Figure 1. The crystallographic data, selected bond lengths and angles are summarized in Tables S2 and S3 (Supporting Information). The copper centers of the complexes are coordinated by the tetradentate N4-ligands. L1 and L2 have a similar composition, where the only difference is the methyl groups attached to the ortho position of the pyridines for L2. Such a small change in the ligand substituents causes quite different molecular structures of [Cu(L1)]2+ and [Cu(L2)]2+. The four nitrogen atoms and copper atom are almost on the same plane for [Cu(L1)]2+ (Figure 1d). The lengths of the Cu–N coordination bonds are ranged from 2.017 to 2.061 Å. By stark contrast, there is a ca. 60 degree deflection occurred in the N1–Cu–N3 angle of [Cu(L2)]2+ as compared to [Cu(L1)]2+ (Figure 1e). For [Cu(L2)]2+, the bond length of Cu–N3 is increased from 2.017 to 2.206 Å, thus [Cu(L2)]2+ has a distorted tetrahedral coordination geometry. The Cu(I) complex [Cu(L2)]+ also displays a tetrahedral configuration as shown in Figure 1f, although there are small changes of the bond lengths and angles relative to the Cu(II) complex. The 8


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dihedral angle (N1–Cu–N3) is not noticeably altered (103.6° for [Cu(L2)]2+, 120.1° for [Cu(L2)]+).

Figure 1. Molecular structures of (a) [Cu(L1)]2+, (b) [Cu(L2)]2+, and (c) [Cu(L2)]+ as ball-and-stick drawings. Uncoordinated counterions, solvent molecule, and hydrogen atoms are omitted for clarity. The first coordination environment of (d) [Cu(L1)]2+, (e) [Cu(L2)]2+, and (f) [Cu(L2)]+.

For copper complexes, internal reorganization energy (λ) is crucial for the electron-transfer behavior upon the twisting of the bond angles.27 In order to elucidate the effect of the introduction of methyl groups in L2 on the internal reorganization energy, we further carried out density functional theory (DFT) calculations to estimate the λ values of the copper complexes associated with the oxidation of the copper(I) species. The computational details are described in Figure S1 and Table S4 in the Supporting Information. The λ of [Cu(L2)]2+/+ is predicted to be 0.33 eV in acetonitrile, which is substantially lower than that of [Cu(L1)]2+/+ (0.81 eV). Lower reorganization energy observed for [Cu(L2)]2+/+ complexes is anticipated to result in faster dye regeneration, which will be discussed in the following section. The UV-vis absorption spectra of the copper complexes studied are shown in Figure 2. All the 9



peaks ranging from 258 to 310 nm are attributed to π→π* absorptions arising from the pyridine and benzene units of the ligands. The very weak broad absorptions observed at 641 nm for [Cu(L1)]2+ and 630 nm for [Cu(L2)]2+ are caused by Cu(II) d-d transitions. The metal-to-ligand charge transfer (MLCT) transitions of Cu(I) species are observed at 355 nm for [Cu(L1)]+ (ε = 1530 M–1 cm–1) and 342 nm for [Cu(L2)]+ (ε = 2917 M–1 cm–1), respectively. It is worth pointing out here that the absorption behaviors of the diamine-dipyridine tetradentate ligands used in this work are quite different from previously reported bidentate phenanthroline and pyridine ligands. The Cu(I) complexes containing these reported ligands exhibit maximum absorptions in the range of 400–500 nm, which are competitive with dye absorptions in the visible light region (the maximum absorption values of some representative copper(I) complexes are summarized in Table S1, Supporting Information). Since there is no macrocyclic aromatic conjugation system in the complexes studied in this work, the absorptions of of [Cu(L1)]2+/+ and [Cu(L2)]2+/+ are mainly located in the UV-light region. Therefore, they display almost negligible visible-light absorption competition with the sensitizer Y123 (Figure S2, Supporting Information), the absorption of which lies in the range of 400–600 nm (λmax = 513 nm, ε = ̴ 50000 M–1 cm–1).

Figure 2. UV−vis absorption spectra of (a) L1, [Cu(L1)]+, [Cu(L1)]2+ and dye Y123; (b) L2, [Cu(L2)]+, [Cu(L2)]2+ and dye Y123 in acetonitrile solutions. Concentrations: 0.1 mM for the

10


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ligands and the copper complexes, and 0.02 mM for dye Y123.

The electrochemical properties of [Cu(L1)]2+/+ and [Cu(L2)]2+/+ were studied by cyclic voltammetry measurements using a three-electrode system. As shown in Figure 3a, [Cu(L1)]+ and [Cu(L2)]+ display reversible redox peaks at E1/2 = 0.31 V and 0.61 V versus normal hydrogen electrode (NHE), respectively, which are ascribed to the Cu(II/I) couples rather than the redox peaks of ligands (Figure S3, Supporting Information). Notably, by the introduction of two methyl groups to the ligand, the redox potential of [Cu(L2)]+ is positively shifted by 300 mV relative to that of [Cu(L1)]+, strongly correlated with the steric-hindrance effect of L2. Compared to the crystal structure of [Cu(L1)]2+ (Table S3, Supporting Information), the bond length of Cu–N3 for [Cu(L2)]2+ is obviously increased due to the influence of methyl substituents. The coordination bond is thus weakened, so the Cu(II) core in [Cu(L2)]2+ could be readily reduced to Cu(I) owing to the low electron density of the central metal. This redox potential shift caused by steric-hindrance effect is also supported by DFT calculations, which give a shift of the redox potential by 350 mV (Figure S1, Supporting Information). These results highlight that a small change in the ligand could make a considerable influence on the redox potential of copper complexes. The redox potential difference observed for these two redox couples also implies that higher photovoltage is expected to be achieved for [Cu(L2)]2+/+, which will be discussed in the following part. In addition, the HOMO (the highest occupied molecular orbital) energy level of dye Y123 is determined to be 1.06 V vs. NHE (Figure 3a), indicating that these copper complexes are well suited as redox mediators in DSCs with sufficient driving forces for dye regeneration (Figure 3b).

11



Figure 3. (a) Cyclic voltammograms of [Cu(L1)]+, [Cu(L2)]+ (both in 1 mM) and Y123 (absorbed on the TiO2 film) as well as the blank CV of a GC electrode in acetonitrile solution contain 0.1 M nBu4NPF6 at a scan rate of 50 mV s–1. (b) Schematic energy diagram of DSC device based on the Y123 sensitized TiO2 film with [Cu(L1)]2+/+ and [Cu(L2)]2+/+ redox couples. Potentials are reported relative to NHE.

Two new electrolytes based on [Cu(L1)]2+/+ and [Cu(L2)]2+/+ redox mediators were further tested in DSCs, in conjunction with a high-absorption-coefficient organic sensitizer Y123 and a PEDOT counter electrode. The details of solar cells fabrication are described in the Experimental Section. The photocurrent density-voltage (J−V) characteristics of the best-performing solar cells for each redox mediator measured at 100 mW cm–2 illumination (AM 1.5G) are presented in Figure 4a, and the corresponding parameters are summarized in Table 1. The devices employing [Cu(L2)]2+/+ electrolyte exhibit an impressive PCE of 9.2%, with an open-circuit voltage (Voc) of 0.87 V, a short-circuit current density (Jsc) of 15.6 mA cm–2, and a parameters of the [Cu(L1)]2+/+ redox system are lower than the corresponding values of [Cu(L2)]2+/+, especially for the Voc and Jsc, which result in a PCE of merely 5.0%. Generally, Voc is mainly determined by the potential gap between the quasi-Fermi energy level of the TiO2 and the redox potential of the redox mediator in the electrolyte.27, 49 Thus, the 180 mV gain in Voc for 12


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the [Cu(L2)]2+/+ electrolyte should be primarily attributed to its more positive redox potential. The incident photon-to-current conversion efficiency (IPCE) spectra of DSCs based on these two redox electrolytes are shown in Figure 4b. The device containing [Cu(L2)]2+/+ complexes exhibits higher IPCE values than the [Cu(L1)]2+/+ counterpart over the entire region measured, yielding a broad IPCE plateau of over 80% in the range of 400 to 580 nm. The trend observed from the IPCE measurements agrees well with the Jsc obtained from J–V measurements. IPCE is the product of the light-harvesting efficiency (LHE), electron injection yield, dye regeneration yield and charge collection efficiency.4, 50 Since the same dye and working electrode are used for both of these two electrolytes, similar LHE and electron injection yield can be envisioned. Therefore, the discrepancy observed in IPCE measurements could be a consequence of less efficient dye regeneration and/or lower charge collection efficiency for [Cu(L1)]2+/+ based electrolyte. As discussed previously, we studied the influence of the introduction of methyl groups in the ligand on the internal reorganization energy of copper complexes. The internal reorganization energies are crucial in the electron-transfer properties of the redox shuttles.27 The larger internal reorganization energy of [Cu(L1)]2+/+ complexes determined by the DFT calculations implies a slow electron-transfer process for this redox couple, resulting in a low dye regeneration yield and thus a low photocurrent density. To gain insight into dye regeneration kinetics of these two redox systems, nanosecond transient absorption spectroscopy (TAS) was further performed and the results are depicted in Figure 4c. In the presence of an inert electrolyte (0.1 M Li-TFSI and 0.6 M TBP in acetonitrile), the decay of the absorbance signal, arising from the recombination between the injected electrons in the conduction band of TiO2 and the oxidized dye molecule, shows a half-time (t1/2) of 16.5 µs. With copper complexes, the decay of the signal is significantly accelerated, indicating that the redox mediators regenerate the oxidized dye molecules. The regeneration half-time for the [Cu(L2)]2+/+ complexes is 0.45 µs, which is much shorter than that of the

13



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[Cu(L1)]2+/+ redox mediator (1.22 µs) under the same conditions. The dye regeneration yield (φreg) can be calculated as follows.

=

≈−

/, /,

(1)

The calculated φreg for [Cu(L2)]2+/+ based electrolyte is 97.3%, while the corresponding value for [Cu(L1)]2+/+ complexes is only 92.6%. On the other hand, inefficient dye regeneration could cause a considerably higher equilibrium concentration of photo-oxidized dye molecules, and therefore a faster recombination rate between electrons in the TiO2 and oxidized dye molecules.13 This will result in shorter electron lifetimes, and thus a lower charge collection efficiency. To verify this hypothesis, electrochemical impedance spectroscopy (EIS) measurements were performed for DSC devices containing these two copper complex-based redox mediators recorded under full sun illumination. The Nyquist plots obtained from the EIS results are shown in Figure 4d, and the equivalent circuit model is shown in the inset of Figure 4d.51 In the equivalent circuit model, the region of the high-frequency where the phase is zero on the real axis corresponds to the series resistance (Rs). The left semicircle in the high frequency range (103–105 Hz) corresponds to the charge-transfer resistance at the PEDOT counter electrode/electrolyte interface (RCE) and the Cp corresponds to the capacitance. The right semicircle in the low frequency range (0.1–1 Hz) is related to the Nernst diffusion-limited impedance (ZN) of the redox couples in the electrolyte. The semicircle in the mid-frequency range (1–103 Hz) represents the charge-transfer resistance at the TiO2/dye/electrolyte interface (Rct) and the Cµ corresponds to the capacitance. The Rct of the [Cu(L2)]2+/+ system is estimated to be 48.2 Ω, which is much higher than the corresponding value of the [Cu(L1)]2+/+ system (12.7 Ω). This result implies that a faster electron recombination rate occurs at the TiO2/dye/electrolyte interface for the [Cu(L1)]2+/+ system, resulting in a lower charge collection efficiency. Above all, we can conclude that the introduction of methyl substituents in the [Cu(L2)]2+/+ complexes significantly lowers the internal reorganization energy, which results in 14


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more efficient dye regeneration yield and charge collection efficiency. This in turn leads to a higher Jsc, and ultimately a much improved overall efficiency. Table 1. Photovoltaic parameters of the best DSC devices based on the [Cu(L1)]2+/+ and [Cu(L2)]2+/+ redox couples measured under 100 mW m–2 irradiation (AM 1.5G). Redox Couple

Voc (V)

Jsc (mA cm–2)

FF

PCE (%)

[Cu(L2)]2+/+

0.87

15.6

0.68

9.2

[Cu(L1)]2+/+

0.69

11.5

0.64

5.0

Figure 4. (a) J–V curves measured under illumination (100mW cm–2, AM 1.5G) and in the dark conditions. (b) IPCE spectrum of DSC devices sensitized with Y123 dye. (c) Nanosecond laser transient absorption spectroscopy measurements of Y123 on mesoporous TiO2 with an inert electrolyte, [Cu(L1)]2+/+ and [Cu(L2)]2+/+ based electrolytes. The excitation wavelength was 532 15



nm and probe wavelength was 720 nm. (d) Nyquist plots for DSC devices based on [Cu(L1)]2+/+ and [Cu(L2)]2+/+ electrolytes measured under illumination at a bias voltage of 1.0 V.

The long-term stability of the electrolyte, in particular the redox mediator itself, is of great importance for the durability of the whole DSC device. With this regard, we first examined the intrinsic stability of the copper complexes. Multiple CV scan measurements as exhibited in Figure S4 (Supporting Information) show that an upshift of approximately 10 mV can be clearly found for [Cu(L1)]+ after 500 cycles. By stark contrast, there is no obvious difference detected for [Cu(L2)]+, indicating an excellent electrochemical stability of the latter complex. The air stability of four copper complexes [Cu(L1)]+, [Cu(L1)]2+, [Cu(L2)]+, and [Cu(L2)]2+ were also evaluated by monitoring the UV-vis absorption changes before and after one month stored in the ambient atmosphere (Figure S5, Supporting Information). It clearly shows that almost no changes were observed for the absorptions for complexes [Cu(L2)]+, [Cu(L2)]2+, and [Cu(L1)]2+ after one month storage in the air. However, for complex [Cu(L1)]+, the absorption at 355 nm totally disappeared. The results indicate that redox couple [Cu(L2)]2+/+ is highly stable in the air. We further investigated the photostability of the copper complexes. As shown in Figure S6 (Supporting Information), the UV-vis absorptions of [Cu(L2)]2+/+ have no significant difference after 100 h continuous light illumination (100 mW cm–2, AM 1.5G), indicating its high photostability. Above all, we can conclude that redox mediator [Cu(L2)]2+/+ shows excellent air, photo, and electrochemical stability. Accordingly, the long-term stability of the whole DSC devices without encapsulations was further studied based on [Cu(L2)]2+/+ electrolyte by using a xenon lamp with a UV cut-off filter under continuous full sun illumination at temperature around 20 °C and humidity about 50%. A mixture of acetonitrile and 3-methoxy-propionitrile (MPN) (4:1, volume ratio) was used to reduce the volatility of the electrolyte. Figure 5 shows the evolution of the photovoltaic parameters and normalized efficiency of a representative DSC

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device containing [Cu(L2)]2+/+ electrolyte over a period of 500 h. Within the first 60 h of continuous illumination, a ̴ 6% performance increase was observed, which was substantially stable compared to previously reported results (distinct efficiency drop of ̴ 30%) involving Cu2+/+-based redox mediator bearing bidentate pyridyl based ligands.26 The device exhibited high stability over the entire testing period, remaining ~ 90% of its original efficiency (representative photovoltaic parameters at different stages during the stability test are listed in Table S5 in the Supporting Information).

Figure 5. Evolution of photovoltaic parameters and normalized PCE of a representative DSC based on [Cu(L2)]2+/+ measured over 500 h under continuous full sun irradiation with a UV cut-off filter. The electrolyte consists of 0.25 M [Cu(L2)]+, 0.065 M [Cu(L2)]2+, 0.1 M Li-TFSI and 0.6 M TBP in acetonitrile/MPN (4:1, v/v).

4. CONCLUSIONS In conclusion, we have reported the application of a new class of copper complexes with diamine-dipyridine tetradentate ligands as redox mediators in DSCs. Tuning the substituents of the tetradentate ligands through the introduction of methyl groups positively shifts the redox potential by 300 mV, due to the steric-hindrance effect of L2. The more positive redox potential 17



of [Cu(L2)]2+/+ leads to a 180 mV gain in Voc. Moreover, the presence of methyl substituents in L2 also makes the coordination geometry of [Cu(L2)]2+/+ maintained during the redox process, which minimizes the internal reorganization energy. This results in a high dye regeneration yield and photocurrent density, and consequently a much improved PCE of 9.2%. More strikingly, [Cu(L2)]2+/+ complexes exhibited excellent air, photo and electrochemical stability, and thus high long-term stability of the whole DSC device, maintaining ~ 90% of the initial efficiency over 500 h under continuous full sun irradiation. The present work underlines that rational design of the coordination sphere of the copper complexes is an effective strategy to fine-tune the redox potential of the electrolyte and electron-transfer kinetics for dye regeneration, which is essential for future design of new copper-based redox mediators in DSCs. It is also worthy of emphasizing that the redox potential of [Cu(L2)]2+/+ (0.61 V vs. NHE) is rather similar to that of the state-of-the-art redox couple [Co(phen)3]3+/2+ (0.62 V vs. NHE).25 This offers the opportunity to couple this copper-based redox mediator with the widely existing sensitizers developed for cobalt-based electrolytes, in particular with broad-spectrum sensitizers (single panchromatic dyes or co-sensitization), to further improve the photocurrent and thus the overall performance. Work on these aspects is underway.

ASSOCIATED CONTENT Supporting Information Additional experimental details including materials, instruments, crystallographic structure determinations, electrochemistry studies, DFT calculations, molecular structure of dye Y123, cyclic voltammograms and UV-vis absorption spectra. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

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Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors acknowledge the financial support by the National Natural Science Foundation of China (21606039, 21673028, 51661135021, 91233201), the Fundamental Research Funds for the Central Universities (DUT17JC39), the Swedish Foundation for Strategic Research (SSF), the Swedish Energy Agency, as well as the Knut and Alice Wallenberg Foundation.

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TOC

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Redox Mediator

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