A Strategy for Enhancing the Performance of Borondipyrromethene

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A Strategy for Enhancing the Performance of Borondipyrromethenes (BODIPY) Dye- Sensitized Solar Cells Ziyi Lu, Mao Liang, Panpan Dai, Kai Miao, Chunyao Zhang, Zhe Sun, and Song Xue J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07356 • Publication Date (Web): 25 Oct 2016 Downloaded from http://pubs.acs.org on October 30, 2016

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The Journal of Physical Chemistry

A Strategy for Enhancing the Performance of Borondipyrromethenes

(BODIPY)

Dye-

Sensitized Solar Cells Ziyi Lu, Mao Liang,* Panpan Dai, Kai Miao, Chunyao Zhang, Zhe Sun, Song Xue* Tianjin Key Laboratory of Organic Solar Cells and Photochemical Conversion, Department of Applied Chemistry, Tianjin University of Technology, Tianjin 300384, P.R.China;

ACS Paragon Plus Environment

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ABSTRACT: Borondipyrromethenes (BODIPY) dyes are one of the most interesting organic dyes because of their unique characteristic in capturing NIR solar radiation. However, the highest reported conversion efficiency of BODIPY dyes is still far behind than other organic dyes. Herein, we demonstrated an efficient strategy for enhancing the photovoltaic performance of BODIPY dyes, i.e., introducing a long linker between the light harvesting antenna and acceptor. Interestingly, the linker length has opposite effects on the BODIPY (M62, M63, M66 and LB1) and triphenylamine (M64 and M65) dyes. This work suggests that the incorporated a long linker in BODIPY dyes shows several favorable characteristics: i) improving the light harvesting capability of BODIPY dyes in longer wavelength without big change of the HOMO levels; ii) efficiently increasing the photocurrent and photovoltage, leading to dramatically improved performance; iii) significantly reducing the charge recombination between electrons in TiO2 and electrolyte acceptor species and oxidized dye molecules at the interface. As a result, M63 with a binary linker display a 4.79 times higher conversion efficiency compared to that of M62 with a single linker. This finding could contribute to the molecular engineering in the design of highly efficient BODIPY dyes.

INTRODUCTION Conversion and storage of renewable energy are central to the sustainable development of mankind owing to the growing global energy demands.1 Dye-sensitized solar cells (DSSCs), thanks to their high efficiency, lower cost and easier production of electronic devices as compared to silicon (Si)–based photovoltaic devices, which is regard as a promising renewable energy supply device.2 Power conversion efficiencies (PCEs) of DSSCs have been substantially increased in the past two decades as the development of photosensitizers. To date, ruthenium(II)


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complexes,3 porphyrin-based dyes4-7 and metal-free organic dyes5-34 have been explored. For a long time, ruthenium(II) complexes and porphyrin dyes are superior to metal-free organic dyes in terms of photovoltaic performance, because the former can harvest the solar light in in the red or near-infrared (NIR) regions. Therefore, design and synthesis of NIR organic dyes is an important research direction. Among the various types of NIR organic dyes, borondipyrromethene35-57 (BODIPY) dyes are appealing because of the remarkable properties (such as high fluorescence quantum yields, molar absorption coefficient, as well as high photophysical stability38) of BODIPY core. Generally, the BODIPY core was employed as a light harvesting antenna.36 Thanks to the versatile BODIPY chemistry, various BODIPY dyes with good light harvesting have been synthesized by substituting at the 3,5-, 2,6-, 4-, and 8-position of the BODIPY core.57 BODIPY dyes with chromophores on the 3,5- and 2,6-positions of the BODIPY core37-39 exhibit absorption bands in longer wavelength region of the visible and near-IR region of the solar spectrum. Song, Mao and co-workers reported series of push–pull dyes by employing the BODIPY as the π-conjugated linker.40-42 Gibson and co-workers prepared BODIPY dyes for NiO-based dye-sensitized solar cells.43 Kubo et al. developed butterfly-shaped BODIPY dyes with a conversion efficiency as high as 6.06%.44 Recently, Qin et al. demonstrated that attaching long alkoxyl groups to the 4position of BODIPY molecules is an efficient way to achieve a better photovoltaic performance.49 However, the lab recorded conversion efficiency of BODIPY dyes is still behind than other organic dyes owing to the severe charge recombination in DSSCs. Therefore, efficient strategy for designing BODIPY dyes need to be established.


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Figure 1. Chemical structures of the M62-67 and LB1. In this work, we focus on the effect of the distance between the BODIPY antenna and acceptor. We prepared two BODIPY dyes. i.e. M62 and M63 (Figure 1) with a dithieno[3,2-b:2′,3′d]pyrrole (DTP) and DTP-DTP linker at the 8-position of BODIPY core, respectively. Specially, the BODIPY unit was replaced by the triphenylamine in the M62 and M63 dyes to give M64 and M65 (Figure 1), respectively. To further evaluate the effectiveness of modification on 8-position of BODIPY, M66 and LB1 (Figure 1) with different length of linker were also investigated. For comparison, M67 without the light harvesting antenna or triphenylamine donor has also been prepared. Impressively, it is found that M63 featuring the DTP-DTP unit exhibited 4.79 times higher efficiency than that of M62 with single DTP unit, sharply contrasting an enhancement of efficiency in the M64 cells as compared to those of M65. Joint electrical, photophysical, and photovoltaic studies indicate that keeping the Boron atom (with a negative charge) away from the TiO2 surface could be a promising strategy for achieving high performance BODIPY dyesensitized solar cells. Indeed, this strategy was further confirmed by another BODIPY dye M66, which incorporates a benzene-DTP linker, exhibiting much higher efficiency as compared to that of M62. Our word provides a new guideline for designing BODIPY dyes.


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EXPERIMENTAL SECTION Materials Pd(PPh3)4, n-BuLi, PINBOP 2,4-Dimethylpyrrole, DDQ, TEA, BF3·Et2O, ethyl cyanocetate and cyanocetic acid were purchased from Energy Chemical (China). All other solvents and chemicals were analytical grade and used without further purification. Synthesis of dyes The synthetic routes to the M62-67 depicted in Scheme S1 and S2 in the SI. The sensitizers and important intermediates were well-characterized by 1H and 13C NMR and HRMS. Synthesis of M62 To the round bottom flask, compound 4 (210 mg, 0.288 mmol), piperidine (0.08 mL, 0.864 mmol), cyanoacetic acid (36.76 mg, 0.432 mmol) and acetonitrile (8 mL) were added. The reaction mixture was refluxed for 10 hours. Additional cyanoacetic acid (24.5 mg, 0.288 mmol) and piperdine (0.054 mL, 0.579 mmol) were added. The mixture was refluxed for 7 hours. The crude product was purified by column chromatography on silica gel (CH2Cl2-MeOH, v/v = 10: 1) to give a desired orange solid (20% yield). Mp: 88-91 oC. IR (KBr): 3543, 3467, 3410, 3238, 2956, 2917, 2852, 2358, 2344, 1636, 1622, 1543, 1509, 1469, 1399, 1258, 1097, 1018 cm-1. 1H NMR (400 MHz, CDCl3): δ 8.32 (s, 1H), 7.69 (s, 1H), 7.3 (d, J = 9.11 Hz, 1H), 6.86 (s, 1H), 6.66 (s, 1H), 6.6 (d, J = 9.11 Hz, 1H), 6.04 (s, 2H), 4.03 (t, J = 6.38 Hz, 2H), 3.96 (t, J = 6.38 Hz, 2H), 2.58 (s, 6H), 1.86-1.82 (m, 2H), 1.76 (s, 6H), 1.55-1.52 (m, 4H), 1.39-1.38 (m, 4H), 1.12 (s, 6H), 0.97-0.93 (m, 3H), 0.8-0.78 (m, 3H). 13C NMR (100 MHz, CDCl3): δ 182.97, 160.06, 154.6, 149.87, 145.13, 139.84, 127.84, 127.58, 123.99, 121.61, 120.27, 115.59, 112.58, 105.36, 101.38,


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68.88, 68.5, 31.59, 31.27, 29.24, 28.84, 25.75, 25.49, 22.62, 22.44, 14.04, 13.88. HRMS (ESI) calcd for C43H48N4O4S2BF2 (M+H+): 797.3185, found: 797.3178. Synthesis of M63 The synthesis procedure is similar to that of compound M62. The crude product was purified by column chromatography on silica gel (CH2Cl2-MeOH, v/v = 100: 1) to give a desired purple solid (33% yield). Mp: 99-101 oC. IR (KBr): 3552, 3475, 3415, 3238, 2928, 2361, 2070, 2022, 1638, 1618, 1524, 1398, 1133, 1125, 1086 cm-1. 1H NMR (400 MHz, CDCl3): δ 8.17 (s, 1H), 7.42 (s, 1H), 7.34-7.3 (m, 2H), 7.03 (s, 1H), 6.94 (s, 1H), 6.77 (s, 1H), 6.67-6.66 (m, 2H), 6.626.57 (m, 2H), 6.01 (s, 2H), 4.05-4.01 (m, 4H), 3.98-3.92 (m, 4H), 2.54 (s, 6H), 1.86-1.81 (m, 4H), 1.74 (s, 6H), 1.62-1.51 (m, 16H), 1.11 (s, 12H), 0.95-0.91 (m, 6H), 0.76-0.7 (m, 6H). 13C NMR (100 MHz, CDCl3): δ176.82, 167.57, 160.14, 159.97, 156.04, 154.54, 154.48, 149.28, 140.54, 133.53, 132.72, 131.38, 127.61, 127.47, 121.42, 120.59, 119.96, 119.49, 119.39, 116.66, 115.06, 105.54, 105.48, 101.63, 101.33, 70.57, 70.362, 69.16, 68.87, 68.47, 65.99, 34.08, 31.87, 31.54, 31.3, 31.28, 29.73, 29.64, 29.6, 29.47, 29.44, 29.3, 29.26, 29.21, 29.19, 29.13, 28.88, 27.17, 26.47, 25.72, 25.71, 25.54, 25.37, 24.91, 22.63, 22.57, 22.37, 22.35, 15.17, 14.6, 14.06, 13.99, 13.84. HRMS (ESI) calcd for (M+H+): 1250.4985, found: 1250.4965. Synthesis of M66 The synthesis procedure is similar to that of compound M62. The residue was purified by column chromatography on silica gel (CH2Cl2-MeOH, v/v = 10: 1) to give a desired dark orange solid (25% yield). Mp: 90-92 oC. IR (KBr): 3549, 3473, 3408, 3238, 2956, 2928, 2857, 2364, 2336, 1636, 1616, 1543, 1509, 1402, 1306, 1184, 1159 cm-1. 1H NMR (400 MHz, CDCl3): δ 8.32 (s, 1H), 7.65 (s, 2H), 7.51 (s, 1H), 7.23 (s, 3H), 7.1-7.08 (m, 1H), 6.64-6.62 (m, 1H), 6.54 (s, 1H), 6.01 (s, 2H), 3.99 (s, 2H), 3.91 (s, 2H), 2.58 (s, 6H), 1.85 (s, 2H), 1.48 (s, 4H), 1.36-1.31 (m,


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10H), 1.04 (s, 6H), 0.86 (s, 3H), 0.68 (s, 3H).

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C NMR (100 MHz, CDCl3): δ 174.38, 160.07,

155.74, 154.37, 142.82, 140.83, 135.69, 134.41, 131.31, 128.76, 127.45, 121.33, 119.9, 115.25, 109.23, 105.39, 101.37, 68.82, 68.45, 51.42, 45.59, 37.1, 34.13, 31.92, 31.62, 31.34, 29.59, 29.48, 29.44, 29.35, 29.28, 29.15, 28.94, 25.78, 25.55, 24.97, 24.81, 22.68, 22.62, 22.39, 14.57, 14.1, 14.05, 13.9, 8.59, 1.01. HRMS (ESI) calcd for C49H52N4O4S2BF2 (M+H+): 873.3499, found: 873.3470. Optical and electrochemical measurements as well as fabrication and characterization of DSSCs are shown in the SI. RESULTS AND DISCUSSION Molecular Design In this work, a series of Meso (8)-BODIPY dyes were design and synthesized. Thanks to its structure, BODIPY can be engineered to three types of BODIPY dyes for DSSCs.57 There are two reasons for our choice. Firstly, in a study on BODIPY dyes, Akkaya et al. proposed that the meso-position (8-position) is better suited for efficient electron injection compared to the 2position of the BODIPY core.37 Secondly, donors can easily be introduced at the 3,5- positions for M62, M63 and M66, which is beneficial to light harvesting. This work we just focus on the influence of the linker on the performance of the Meso (8)-BODIPY dyes, without considering an optimum structure of them. Photophysical Properties. The UV-visible absorption and emission properties of these dyes are summarized in Table 1. As presented in Figure 2a, the reference dye LB1 exhibits a major absorption peak (λmax) located


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at 503 nm, corresponding to the lowest energy transition (S0 →S1) of the BODIPY core. In contrast, the absorption of M62 shows two peaks located at 427 and 519 nm, respectively. Simultaneously, the absorption range is also extended. Moreover, M63 displays a new absorption band in the range from 650-800 nm, as the linker between the antenna and acceptor increased. The M62 and M63 display a red-shifted λmax in comparison with M66, indicating DTP unit is better than benzene unit in terms of light harvesting. Overall, absorption properties of these BODIPY dyes can be tuned by modification the substitution on the 8-position of BODIPY core. Moreover, BODIPY core indeed does behave as a light harvesting antenna to enhance the spectra response. M67 without the antenna chromophore displays a blue-shifted absorption peak (59 nm) and attenuated molar absorption coefficient.


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Figure 2. (a) Absorption spectra of the M62-67 and LB1 dyes; Normalized absorption and emission spectra of the dyes with (b) and without (c) BODIPY core. The solvent is dichloromethane. Table 1. Optical and electrochemical data of the M62-67 and LB1.

Stokes shift / nm

E0–0/eV

HOMO/ V vs NHE

LUMO/ V vs NHE

0.031

19

2.35

1.17

-1.18

533

0.028

16

2.36

1.09

-1.26

52.6

598

0.052

105

2.25

0.94

-1.31

498

65.8

637

0.020

101

2.07

0.67

-1.39

502

497

73.1

536

0.012

34

2.42

1.15

-1.27

M67

460

411

34.3

517

0.0041

57

2.55

1.27

-1.28

LB1

503

506

110.0

519

0.59

16

2.43

0.81

-1.62

ε/103M– Emax/ Quantum 1 cm–1 nmc yield

Dye

λmax/ nma

λmax/ nmb

M62

519

516

69.4

538

M63

517

510

52.4

M64

493

463

M65

536

M66

The absorption data a and emission c of dyes in solution. The absorption data of sensitized electrodes.b

When the BODIPY antenna was replaced by triphenylamine, absorption of dyes changed. M65 exhibits distinct absorption properties (λmax = 536 nm) with a 42 nm red-shifted λmax relative to that of M64 (λmax = 493 nm) with increased conjugated length (Figure 2b). Clearly, the expansion of π-conjugated linker is beneficial to the interactions between donor and acceptor and thus decrease the energy gap of the triarylamine organic dyes.16 However, this effect is not pronounced for BODIPY dyes. As a matter of fact, M63 shows a blue-shifted λmax (by 2 nm) in comparison with that for M62, which is contrary to the fact observed in TPA dyes. In addition, the HOMO/LUMO gap of M63 (E0-0 = 2.36 eV) is slightly higher than that of M62 (E0-0 = 2.35 eV), sharply contrasting a 0.18 eV attenuation for the M65 (E0-0 = 2.07 eV) with respect to that of M64 (E0-0 = 2.25 eV). In other words, prolonging the linker on the 8-position of BODIPY core


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does not lead to a decrease in energy gap. An interesting feature that is different from that of traditional organic dyes. This means that we can widen the spectral response of dyes but not sacrifice the driving force for electron injection or dye regeneration. In contrast, substitution at the 3,5-positions results in a significant decrease in energy gap.37,49 Thus, we can introduce substituents at different position of the BODIPY core to finely tune the energy levels of BODIPY dyes.

Figure 3. (a) Absorption sensitized electrodes (3 µm TiO2 films); (b) absorption of M62, M63 and M66 dyes in DCM with and without TEA. The absorption spectra of sensitized electrodes are shown in Figure 3a. It can be found that the absorption bands of sensitized electrodes are more wide than those in solution. A phenomenon related to the strong interaction of the anchoring group with the titanium dioxide.58 Compared with the absorption spectrum in DCM, the ICT band for M64 and M65 are blue-shifted by 30 and 38 nm, respectively. To identify the origin of the observed blue shift for sensitized electrodes, absorption of dyes in DCM with and without triethylamine (TEA) were recorded. Similar blue shifts have been recorded (see Figure S1), indicating that the blue shift in absorption peak for M64 and M65 sensitized electrodes can be attributed the deprotonation effect.59 Such a deprotonation would diminish the electron-accepting ability of the acceptor moiety, because


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carboxylate-TiO2 unit is a weaker electron acceptor compared to the carboxylic acid. This in turn may substantially decrease the donor-acceptor interaction in the dye and lead to a blue shift in the absorption spectrum. Meanwhile, some protons enter the solution and some protons combine with TiO2. Nevertheless, this sharp hypsochromic shift of absorption peaks has not been observed in these BODIPY dyes. For instances, the λmax of M62 and M63 sensitized films are 516 and 510 nm, showing a small blue shift of 3 and 7 nm, respectively. When TEA was added in CH2Cl2 solutions, the λmax of M62, M63 and M66 was almost unchanged (see Figure 3b), indicating that BODIPY antenna is not a strong donor, since cationic moiety can be regard as a class of strong electron acceptor. This effect was also observed in some organic dyes with the DA-π-A structure.15 Electrochemical Properties. Differential pulse voltammetry (DPV) of sensitized electrodes was measured using a ZAHNER (Germany) electrochemical analyzer (Figure 4a). The derived energy levels are tabulated in Table 1. As presented in Figure 4b, a significantly negative shift of the HOMO (0.27 V) and a small negative shift of the LUMO (0.08 V) can be observed for M65 vs M64, resulting in a reduce of the energy gap between the LUMO and HOMO. In contrast, prolonging the linker on the 8-position of BODIPY core has small influence on the HOMO energy levels for BODIPY dyes. We found that (see Figure 4b) the HOMO levels of M63 (1.09 V) and M66 (1.15 V) are closer to that of M62 (1.17 V). Note that, as shown in Figure 4b, the HOMO levels of M62 and M63 are much lower than those of M64 and M65, respectively. It appears that by the elongation of linker on the 8-position of BODIPY core, the light harvesting capability can be improved while retaining the HOMO levels. In addition, we notice that the HOMO level of M67 is even lower (see Figure 4b), being 1.27 V, in comparison with that of M62 (1.17 V). Therefore, the


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BODIPY antenna has some donating capability. On the other hand, the LUMO of the M62-67 are higher than -1.18 V, providing high driving forces for electron injection (ECB of TiO2 is -0.5 V versus NHE).

Figure 4. (a) DPV of sensitized electrodes. (b) HOMO, LUMO and energy gap of studied dyes based on DPV measurements. Theoretical Approach To gain insight into the geometrical and electronic properties of the resulted sensitizers, density functional theory (DFT) and time-dependent DFT (TDDFT) calculations were conducted using B3LYP/6-31G (d) level. Figure 5 shows the HOMO, HOMO-1, LUMO and LUMO+1 of BODIPY dyes. Major electron excitations and corresponding electron transition are listed in Table 2. It can be found that the main absorption band for LB1 is based with one excitation (HOMO→LUMO) calculated at 423 nm. By contrast, there are several electron excitations (HOMO-1→LUMO, HOMO-1→LUMO+1, HOMO→LUMO+1) for M62, which corresponds


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well with the experimental result that M62 have multiple absorption peaks in a broad region. Similar trend can be found for M63 and M66. The absorption band at around 700 nm for M63 in dichloromethane can be assign to the electron excitation HOMO→LUMO+1. In view of the fact, the replacement of traditional benzoic acid by longer linker and stronger acceptor is beneficial to BODIPY dyes.

Figure 5. HOMO, HOMO-1, LUMO and LUMO+1 of BODIPY dyes that are involved in vertical electronic transitions (isodensity value = 0.02). Frontier molecular orbitals for the M64 and M65 are shown in Figure S2. The superiority of M62, M63 and M66 as compared to LB1 can also be observed from the frontier orbitals. The HOMO of LB1 is located on the BODIPY antenna, and the LUMO is delocalized also on the same moiety. It is interesting to find that there is no electron distribution on the benzoic acid, indicating that benzoic acid could be considered as an anchor but not an


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acceptor. Obviously, the replacement of traditional benzoic acid by longer linker and stronger acceptor strongly affects the electron distribution on dyes. For example, the LUMO of M62, M63 and M66 more delocalized than that in LB1. Moreover, an apparent electron distribution across the DTP linker and cyanoacrylic group can be observed in LUMO of them, demonstrating that efficient energy transfer from the all parts of molecular to the cyanoacrylic acid moiety occurs under light irradiation. In other words, cyanoacrylic acid in M62, M63 and M66 acts as anchor and acceptor. Additionally, it should be note that the low-energy absorption for M62 (HOMO-1 → LUMO transition) includes two process electron transfers, i.e. from DTP to BODIPY and DTP to cyanoacrylic group. Similar cases can be found for M63 and M66. This difference in electron transfer processes may be another reason for attenuation of deprotonation effect for BODIPY dyes. Table 2. Calculated TDDFT Excitation Energies (eV, nm), Oscillator Strengths (f), and Molecular Orbital Transition assignment. Dye M62

nm (eV) 510.80 (2.4273)

Oscillator strength (f) 0.3404

MOs compositions (C) 209→211（HOMO-1→LUMO）72.9% 209→212（HOMO-1→LUMO+1）18.7%

M63

435.17 (2.8491)

0.5097

210→212（HOMO→LUMO+1）70.3%%

429.42 (2.8872)

1.0006

209→212（HOMO-1→LUMO+1）72.7%

654.88 (1.8932)

0.1501

331→332（HOMO→LUMO）24.5% 331→333（HOMO→LUMO+1）72.2%

592.34 (2.0931)

1.8278

331→332（HOMO→LUMO）74.6% 331→333（HOMO→LUMO+1）24.5%

437.22 (2.8357)

0.5033

330→333（HOMO-1→LUMO+1）85.2%

419.91 (2.9527)

0.3197

327→332（HOMO-4→LUMO）61.6%


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331→334（HOMO→LUMO+2）27.6% M66

486.82 (2.5468)

0.0001

229→231（HOMO-1→LUMO） 99.3%

481.48 (2.5751)

1.6145

230→231（HOMO→LUMO） 93.2% 230→232（HOMO→LUMO+1）4.1%

LB1

422.52 (2.9344)

0.5692

229→232（HOMO-1→LUMO+1）95.2%

423.42 (2.9282)

0.5820

96→97（HOMO→LUMO）96%

Photovoltaic Performance The photocurrent-voltage (J-V) characteristic curves (measured under the 100 mW cm-2 simulated AM1.5 sunlight) for iodine cells based on BODIPY dyes and TPA dyes are shown in Figure 6a and Figure 6b, respectively. Photovoltaic data including short–circuit photocurrent density (JSC), open–circuit photovoltage (VOC) and fill factor (FF) values are gathered in Table 3. The M63 sensitized device yields JSC, VOC and FF values of 8.9 mA cm–2, 553 mV and 0.70, respectively, resulting in a power conversion efficiency (PCE) of 3.45%. In contrast, reduce the length of the linker can dramatically lower JSC of M62 sensitized devices, being 2.3 mA cm–2. Besides JSC, M62 exhibited a VOC of 455 mV, which is almost 100 mV lower than M63. As a result, M63 with a binary linker display a 4.79 times higher conversion efficiency compared to that of M62 (PCE = 0.72%) with a single linker. We speculated that the distance between the light harvesting antenna and acceptor dominate the performance of BODIPY dyes. If it was the case, a group inserted between the BODIPY core and DTP unit would result in a dye with somewhat improved the cell performance. Therefore, we designed and synthesized M66 with a benzene-DTP linker to verify our supposition. We observed a similarity of the photovoltaic data for M66 (JSC = 8.3 mA cm–2, VOC = 540 mV and FF = 0.71) as compared to M63. Obviously,


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PCE of M66 (3.18%) is significant higher than that of M62. In contrast, LB1 exhibits a low PCE of 0.29%. These results were in good agreement with our speculation.

Figure 6. J–V characteristic curves for M62, M63, M66 and LB1 dyes (a) and M64, M65 and M67 dyes (b). In contrast to the BODIPY dyes sensitized cells, TPA dyes sensitized cells exhibit an opposite trend. Figure 6b shows the J−V curves of M64, M65 and M67, and detailed parameters are gathered in Table 3. M64 displays an impressive overall conversion efficiency of 7.67%, while that for M65 was only 4.19%. This result indicates that DTP-DTP linker adversely effected performance of triarylamine dyes. This negative effect of DTP-DTP linker also found for C253.25 The π-conjugation extension from M64 to M65 has caused a JSC attenuation of 5.7 mA cm-2, sharply contrasting a 6.6 mA cm-2 enhancement in BODIPY dyes. The JSC differences were clearly observed in IPCE spectra for cells based on BODIPY dyes (Figure 7a) and TPA dyes (Figure 7b). The experiment results suggested that structure–property relationship for BODIPY dyes is different from that of arylamine organic dyes. Under the same conditions, M67-based cell gave a PCE of 2.94%. Apparently, M67 suffers from its narrow IPCE response (Figure 7b) due to lack of arylamine donor, leading to lower photocurrent and thus efficiency as compared to those of M64. Note that, despite its narrow IPCE response, the PCE of M67 is higher than that of


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M62 containing a light harvesting antenna. This indicates that the BODIPY antenna has some negative effects on photovoltaic performance of cells. In our opinion, BODIPY core is a twoedged sword. It can act as an antenna to absorption sun light, but it also induce a negative effect related to recombination of charge. In spite of that, we can reduce this negative effect by modifying the structure of the BODIPY dyes. On the other hand, we can see LB1 exhibits a peak of IPCE around 500 nm, in good agree with its absorption spectra. Therefore, this peak is an evidence for photocurrent following excitation of BODIPY.

Figure 7. IPCE spectra for M62, M63, M66 and LB1 dyes (a) and M64, M65 and M67 dyes (b). Table 3. Photovoltaic parameters for M62-67 and LB1.a PCE / %

Dye load / 10-8 mol cm-2

0.69±0.01

0.72±0.2

10.4

553±8

0.70±0.01

3.45±0.2

9.1

16.3±0.3

692±6

0.68±0.01

7.67±0.2

11.6

M65

10.6±0.3

582±8

0.68±0.01

4.19±0.2

9.3

M66

8.3±0.3

540±10

0.71±0.01

3.18±0.2

9.7

Dye

JSC/ mA cm–2

M62

2.3±0.4

455±8

M63

8.9±0.3

M64

VOC/mV

FF


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a

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M67

7.5±0.3

560±8

0.70±0.01

2.94±0.2

15.6

LB1

1.1±0.3

390±9

0.67±0.01

0.29±0.1

5.8

Irradiating light: AM 1.5G (100 mW cm–2).

IMVS and EIS Characterization In the case of triphenylamine dyes, shorten the linker successfully realizes a significant increase in VOC by 110 mV (the VOC of M64 and M65 are 692 and 582 mV, respectively). Surprisingly, same modification resulted in a decrease in VOC for BODIPY dyes. The VOC of M62 (VOC = 455 mV) sensitized cells are considerably lower than those of M63 (VOC = 553 mV) sensitized cells. The opposite trend of photovoltaic performance for BODIPY dyes and TPA dyes is worthy of investigation. It is widely recognized in DSSCs that severe interfacial recombination in DSSCs may result in the energy losses. So, controlled intensity modulated photovoltage spectroscopy (IMVS) as well as electrochemical impedance spectroscopy (EIS) measurements were carried out. Figure 8a shows the extracted charge density (dn) at open circuit potential as a function of VOC for the studied DSSCs. All curves parallel to each other with small changes (< 10 mV). Evidently, large VOC difference between these dyes does not result from the small CB shift. Thus, the root cause of the observed VOC alternations for these dyes is closely correlated with the charge recombination rate at the interface of titania and electrolyte.


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Figure 8. Charge density at open circuit potential as a function of open-circuit photovoltage. (a) and electron lifetime as a function of EF (b) for M62-67 and LB1. Figure 8b displays the electron lifetime (τ) as a function of quasi-Fermi-level (EF) for M62-67 and LB1. At a given value of EF the electron lifetimes (measured by IMVS) for TPA dyes are larger than those of BODIPY dyes, which is in good agreement with the sequence of VOC values. Note that, at a fixed EF, the electron lifetime of M63 is increased by 6.37-fold as compared to that of M62, providing sound evidence for the aforementioned a 98 mV VOC enhancement from M62 to M63. In contrast, M65 exhibits a remarkably enhanced electron recombination between electrons in TiO2 and electrolyte acceptor species and oxidized dye molecules with an extension of the linker conjugation. The electron lifetime of M64 is 17.3-fold higher compared to that of M65, thus remarkably increasing the VOC by 110 mV. In addition, we notice that the lifetime of M67 based DSSC given by IMVS are higher than that of M62, but close to that of M63. Obviously, the BODIPY antenna in M62 has a negative influence on the rate of charge recombination between titania electrons and redox electrolytes. Fortunately, the charge recombination rate in BODIPY dyes sensitized DSSCs could be significantly alleviated by increasing the linker length. Typical EIS Nyquist plots (Figure 9a) and Bode phase (Figure 9b) plots for the studied DSSCs measured in the dark under a forward bias of -0.5 V. The larger semicircle at middle-frequency regions of Nyquist plots are assigned to the interfacial charge transfer resistances (RCT) at the TiO2/dye/electrolyte interface. Generally, RCT is depends on the charge recombination rate between the titania electrons and redox electrolytes. For example, at a faster charge recombination, it is expected that the RCT is smaller. The RCT are in the order LB1 < M62 < M66 < M63, with the extension of the linker. In contrast, RCT decreased in the order M64 > M65. Apparently, the elongation of linker on the 8-position of BODIPY core is beneficial to control


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the charge recombination rate between the titania electrons and redox electrolytes. However, an accelerated recombination of electrons in TiO2 with redox electrolytes occurred as the dyes became longer in TPA dyes. Note that, this RCT order for all dyes is consistent with the sequence of JSC values, i.e. LB1 < M62 < M66 < M63 < M64 < M65. This means large RCT also improved charge collection efficiency at the photoanode.60 From the Bode phase plot, the calculated electron lifetimes were extracted using τ = 1/ωmin.61 The obtained electron lifetimes increased in the order LB1 < M62 < M66 < M63, as the distance between the BODIPY antenna and acceptor increased. And the electron lifetime of M64 is longer than that of M65 and M67. These results are similar to the trend observed in IMVS measurements.

Figure 9. EIS Nyquist (a) and Bode (b) plots for the M62-67 and LB1. In a study on the root cause of open-circuit voltage, Mori and Hagfeldt proposed that the smaller polyene dyes show a surface-protecting effect preventing recombination upon increased dye loading, whereas the larger dyes enhance the recombination.62 In our work, IMVS and EIS measurements revealed that M64 and M65 follows this trend because of same reasons. We believe that M63 also suffers from poor surface blocking and dye-iodine interaction just like M65. Given that there are no other influence factors, M62 should be superior to M63 in terms of


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photovoltaic performance. However, M63 displays an opposite rule, indicating that M63 suffer from other factors. Therefore, we suggest that the determining factor for the photovoltaic performance of BODIPY dyes is the BODIPY core, because the most apparent structural difference between BODIPY dyes and TPA dyes is that the former ones contain a negative charged center in the terminal. To examine the difference among these dyes, the electrostatic potential of M62, M63, M64, M66 and LB1 have been calculated, and the results are plotted in Figure 10. A negative charge can be observed at the Boron atom in the BODIPY core in M62, M63, M66 and LB1, whereas M64 has no negative charges at the terminate part of molecular. Previous studies have proposed that the indirect electrostatic attraction of I3- by negatively charged atoms in the dyes increases the local concentration of I3- in the vicinity of TiO2, facilitating charge recombination.63 IMVS and EIS clearly revealed that this is indeed the case. Herein, Li+ in electrolytes would feel attractive force from the Boron atom at the BODIPY core, and then, I3- would feel attractive force from Li+, and thereby the more pronounced charge recombination. Importantly, our work demonstrates that the length of the linker between the light harvesting antenna and acceptor strongly affect the photovoltaic performance of BODIPY dyes. The calculated distance between the Boron atom and the acceptor are 8.8, 14.6, 18.9 and 22.5 Å (determined by the ChemBio3D ultra 11.0 program) for LB1, M62, M66 and M63, respectively. With the increasing of the distance, the photovolatic performance increased in the order LB1 (PCE = 0.29%) < M62 (PCE = 0.72%) < M66 (PCE = 3.18%) < M63 (PCE = 3.45%). We supposed that BODIPY core has strong interaction with interaction with cations in the electrolyte, which outweighs the interaction between the electrolyte acceptor species and sulphur atom in DTP unit. In other words, keeping the Boron atom (with a negative charge) away from the TiO2


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surface is the key to achieve a better performance of BODIPY dyes. Based on this study, we proposed an empirical law, i.e. the negative effect of BODIPY core would be smaller for the BODIPY based DSSCs if the distance between the Boron atom and the acceptor is more than 18.9 Å. Nevertheless, this distance may be shorter once substituent groups introduced at the terminal of BODIPY dyes.

Figure 10. Electrostatic potential plots of M62, M63, M64, M66 and LB1. CONCLUSIONS For the purpose of exploring new strategy for rational design of BODIPY dyes and understand the unique characteristic of BODIPY dyes, we prepared a series of new BODIPY dyes (M62, M63, M66 and LB1) with different linker between the BODIPY antenna and acceptor.


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Accompanied with the increasing length of linker at the 8-position of BODIPY core, the light harvesting capability in longer wavelength is improved without big change of the HOMO levels. Surprisingly, M63 with a longer linker achieves much better photovoltaic performances than that of M62 with a short linker, in sharp contrast the trend for triphenylamine dyes (M64 and M65). Joint IMVS and EIS measurements revealed that the BODIPY core has strong interaction with interaction with cations in the electrolyte. A factor that outweighs the interaction between the sulphur atom and electrolyte acceptor species and surface blocking of the dye layer. Therefore, keeping the Boron atom (with a negative charge) away from the TiO2 surface is the key for achieving high performance BODIPY dyes sensitized solar cells. Although the efficiency of M63 is not good enough, we believe that this strategy is useful for design BODIPY dyes or other organic dyes containing negative charges. Further structural modification of BODIPY dyes is underway in our lab. ASSOCIATED CONTENT Supporting Information. Optical and electrochemical measurements as well as fabrication and characterization of DSSCs. The detailed synthetic procedures and structure characterizations of the new compounds and long-term stability of the DSSCs based on M62-66 (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author * E-mail: [email protected] * E-mail: [email protected] Phone: +86 60214259


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT We gratefully acknowledge the financial support from the National Science Foundation of China (No. 21373007, 21376179), and the Tianjin Natural Science Foundation (13JCZDJC32400, 14JCYBJC21400). REFERENCES 1. Li, C.; Han, X.; Cheng, F.; Hu, Y.; Chen, C.; Chen, J., Phase and Composition Controllable Synthesis

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a Novel

Triazine Bridged


Triad:

Synthesis,

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Dye-Sensitized Solar Cells: Combined Effects of Molecular Structure of Dyes and Electrolytes. J. Am. Chem. Soc.2008, 130, 17874-17881.

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A Strategy for Enhancing the Performance of Borondipyrromethene

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