Molecular Control of the Band Edge Movement and the

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Molecular Control of the Band Edge Movement and the Recombination Process in Donor-Acceptor Hemicyanine-Sensitized Solar Cells Neeta Karjule, Munavvar Fairoos Mele Kavungathodi, and Jayaraj Nithyanandhan J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b04623 • Publication Date (Web): 11 Sep 2017 Downloaded from http://pubs.acs.org on September 12, 2017

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The Journal of Physical Chemistry

Molecular Control of the Band Edge Movement and the Recombination Process in Donor-Acceptor Hemicyanine-Sensitized Solar Cells Neeta Karjule†,‡, Munavvar Fairoos Mele Kavungathodi † and Jayaraj Nithyanandhan*,†,‡ †

Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, CSIRNetwork of Institutes for Solar Energy, Dr. Homi Bhabha Road, Pune 411008, India. ‡

Academy of Scientific and Innovative Research, New Delhi 110025, India.

ABSTRACT The presence of downward shift in the band edge and the recombination reactions in the hemicyanine-sensitized solar cell reduces the open-circuit potential (VOC) and the short-circuit current density (JSC), which in turn decreases the dye cell performance. Choosing either an electrolyte possessing minimum over-potentials or a systematic dye design which can efficiently suppress the diffusion of charged species towards the TiO2 can improve the overall power conversion efficiency (PCE). Here, a series of donor-acceptor (D-A) hemicyanine dyes were synthesized utilizing a planar heterotriangulene (HT) or triphenylamine (TPA) donor and alkyl functionalized indolium carboxylic acid acceptor unit. By introducing strong HT donor instead

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of TPA, the photophysical, and electrochemical properties of D-A dyes are significantly modulated. The strong donor nature of HT and effective passivation of surface by hydrophobic alkyl chains close to the anchoring group for NC3 dye exhibits an average PCE of 4.34% with a VOC of 0.416 V, JSC of 20.04 mA cm-2, and fill factor (ff) of 52.03% under simulated AM 1.5G illumination (100 mW cm-2) without 3α, 7α-dihydroxy-5β-cholic acid co-adsorbent (CDCA). The intrinsic dipole of the hemicyanine dye and the presence of Li+ ions in iodide/triiodide redox couple without tert-butylpyridine (TBP) additive induces a downward shift in conduction band edge (ECB) of TiO2. By rational molecular design, the extend of shift in ECB is controlled and enhanced the VOC. Electrochemical impedance spectroscopy (EIS) studies revealed the high charge transfer resistance (Rct) and long lifetime (τ) of injected electrons in HT-based dyes than that of TPA derivatives, which provide insight into the passivation of Li+ and I- ions by current D-A dye design possessing alkyl functionalities to increase both the JSC and VOC.

INTRODUCTION Dye-sensitized solar cells (DSSCs) have received considerable attention as low-cost alternatives to conventional inorganic silicon-based solar cells.1,2 The high efficiency photoelectrochemical cells using ruthenium (II)-polypyridyl complex were first investigated by O'Regan and Grätzel in 1991 with a PCE of 7.12% under simulated 1 sun illumination.3 Recently, the development of near-infrared (NIR) light-harvesting metal-free sensitizers showed efficiency more than 10% and emerged as an alternative to metallated dyes. Compared to the metal complexes,4,5,6 organic dyes with D-A configuration have several advantageous features, such as high molar extinction coefficient (ε), low material cost, and diverse structural designs.


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By judiciously choosing electron donating and withdrawing moieties in D-A dyes, its light capturing abilities can be further improved but the energy levels of the dyes have to be properly matched with conduction band edge of the TiO2 and redox energy level of the I-/I3- to facilitate efficient injection and regeneration.7 Hemicyanine dyes, a class of metal-free ionic dyes, are much attractive in terms of its feasible synthesis and strong absorption in the far-red region.8 However, the JSC and VOC of these class of dyes comparatively less than the non-ionic ones because of less light-harvesting ability in the NIR region and downward shift of the TiO2 ECB up on chemisorption, respectively.9 Due to the ionic nature, selection of electrolyte is important for hemicyanine sensitization because of the additives such as protons,10 TBP,11 lithium salts (Li+),12 and ionic liquids13 critically control the VOC, JSC and ff parameters of the device. The adsorption of charged species on TiO2 surface can influence the ECB position. The key challenge in ionic dye cell is to enhance the JSC and VOC by utilizing molecular structure even though the intrinsic dipole moment plays a negative role. To enhance the JSC and VOC of D-A hemicyanine ionic dyes, the molecular structure of donor part and alkyl functionalities play an important role. Connecting a strong donor the photo response of the sensitizers can extent towards NIR region and also increase the injection efficiency, which in turn improves JSC.7 Often used arylamine donors in DSSC, including triphenylamine, indoline, phenothiazine, fluorene and carbazole based dyes have been designed and proved as efficient D-A systems.14,15 But the ε of triarylamine dyes at NIR region was poor compared

to

hemicyanine,16,17

merocyanine,18,19

porphyrins,20

phthalocyanines21

and

squaraines,22,23,24 and which have been attracted interest for the design of organic dyes possessing tunable absorption in the far-red to NIR region with high ε value. Squaraines and porphyrins are highly studied NIR dyes. Noticeably, the effort towards high performance hemicyanine is not


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much explored even though it shows good light capturing ability towards NIR region. Among the

few

reports,

hemicyanine

dyes

comprising

triphenylamine,17,25,26

fluoranthene,27

phenothiazine9 and tetrahydroquinoline28 as donor units showed a strong absorption in the range of 500–650 nm in solution with ε of ~104-105 M-1cm-1. However, the PCE (2-5.5%) under simulated AM 1.5G illumination (100 mW cm-2) was moderate compared to other far-red active dyes because of formation of aggregates upon adsorption on the TiO2 surface and adsorption of charges (dipole pointing towards TiO2) on TiO2 surface that alters the conduction band position. The D-A structure of hemicyanine dyes exhibit broad absorption in the far-red region in solution due to the presence of highly electron withdrawing indolium unit as an electron acceptor which facilitate the efficient intermolecular charge transfer (ICT). In addition to ICT, the selfaggregation in solution or on surfaces due to the strong intermolecular van der Waals forces8 can form H- or J- type aggregates which leads to blue- and red- shifted absorption, respectively.9 In the previous reports, bridged triphenylamine (heterotriangulene)29 has been recognized as an efficient donor units for far-red harvesting squaraine sensitizers.30 Planar and rigid moiety provides an excellent degree of conjugation between donor and acceptor that gives faster electron injection, assemble via π-π stacking while anchoring on the TiO2 surface,31 and localizes the positive charge resulting after injection. It exhibited red-shifted absorption spectrum leading to a significant enhancement in photocurrent density and device efficiencies.32 Though the aggregation phenomenon is detrimental to the hemicyanine device efficiency, controlling the aggregation of ionic dyes on TiO2 surface helps to broaden the absorption spectrum which can contribute for photocurrent to achieve high PCE. In DSSCs, several strategies were proposed for diminishing the dye aggregation and charge recombination by focusing on the electron donor, πspacer, and acceptor unit to achieve high performance. The addition of optically transparent co-


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adsorbent like CDCA also employed to reduce the aggregation and improve the JSC and VOC of DSSC devices fabricated with organic dyes.33 But the higher concentration of CDCA competes with dyes in chemisorption and decreases the JSC after certain adsorbent/dye ratio. The alkyl chains, the in-plane or out-of-plane with respect to conjugated backbone, improve the JSC and VOC due to the increased intermolecular distance and effective screening of TiO2 surface from Li+ and I- ions present in electrolyte which retards the dark current.30 Intrigued from the position of out-of-plane alkyl functionalities reported for organic dyes, D-A hemicyanine dye with both in-plane and out-of-plane groups near to the carboxylic acid anchoring unit can avoid the aggregation and hence increase the JSC. Addressing the second obstacle in hemicyanine DSSC, the intrinsic dipole moment of ionic dye (because of the positive charge near to the acceptor moiety) points towards the TiO2 and diffusion of Li+ ions present the electrolyte towards the surface shifts the ECB downwards and which increases the electron injection but reduces the VOC.34 The commonly employed additive TBP to provide upward shift in ECB drastically reduces the JSC and increases the VOC of hemicyanine DSSC.35 Trade-off between JSC and VOC of hemicyanine dye cells were tried to overcome by using polysulfide electrolytes where the small cations are judiciously replaced with tetrabutylammonium cation.28 Through judicious molecular design of D-A configurations possessing bulky alkyl groups near to the anchoring group can be utilized to passivate the TiO2 surface from the small ions present in the iodide/triiodide electrolyte. There are reports in the literature for enhancement of voltage and current after substituting a weak donor with a strong one.15


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N

N

N

N

N

N O

I

O

I

O

I

OH

OH

HO NC1

NC2

N

NC3

N

N

N

N

I

O

N O

I

OH

HO NC4

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NC5

O

I

OH NC6

Figure 1. Molecular structures of HT-based NC1-3 and TPA-based NC4-6 based hemicyanine dyes. In this report, we have designed and synthesized six hemicyanine dyes in which either HTbased (NC1-3) or TPA-based (NC4-6) used as electron donors and 1-alkyl-5-carboxy-3,3dialkyl-indol-1-ium moiety (indolium unit) as an electron acceptor contains anchoring group. The molecular structures of NC1-3 and NC4-6 dyes are shown in Figure 1. Planarization of donor and out-of-plane alky groups plays an important role in improving the VOC, JSC and


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concomitant improvement in the PCE of hemicyanine dyes without co-adsorbent CDCA.36,37 Our results indicate that the rational molecular design of NC1-3 modulated the energy level, spectral response, and interfacial charge transfer property that helps to achieve high JSC and VOC compared to NC4-6.

RESULTS AND DISCUSSION

Synthesis of NC1-6 dyes Synthesis of hemicyanine dyes were fairly straight forward, and the adopted synthetic route of NC1-6

are

depicted

in

Scheme

1.

4,4,8,8,12,12-Hexamethyl-8,12-dihydro-4H-

benzo[9,1]quinolizino[3,4,5,6,7-defg]acridine-2-carbaldehyde(1),38

4-(diphenylamino)benzal-

dehyde (2)39 and 1-alkyl-5-carboxy-3,3-dialkyl-indol-1-ium units (3a-3c)30 were synthesized according to reported procedures. A condensation reaction between 3a-3c and monoformlyated amine intermediate (1 and 2) yielded the NC1-6 dyes. The structures of all dyes are characterized by 1H- and 13C-NMR spectroscopy and mass spectrometry (HRMS-ESI). The spectral data were consistent with the proposed structures. All dyes have good solubility in MeOH, CH3CN, CHCl3, DMSO, and t-BuOH.


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Scheme 1. Synthesis of HT-based NC1-3 and TPA-based NC4-6 based hemicyanine dyes.

General Procedure 4,4,8,8,12,12-Hexamethyl-8,12-dihydro-4H-benzo[9,1]quinolizino[3,4,5,6,7-defg]acridine-2carbaldehyde (100 mg, 0.25 mmol) or 4-(diphenylamino)benzaldehyde (100 mg, 0.37 mmol) and compound 3a-3c (1.2 eq.) were added into CH3CN (10 mL) with piperidine as the catalyst and refluxed for 16 h. Then solvent was removed under reduced pressure and the residue was purified by column chromatography on silica with methanol/dichloromethane as eluent to afford NC1-3 as a blue solid and NC4-6 as a violet solid.

(E)-5-carboxy-2-(2-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[9,1]quinolizino [3,4,5,6,7-defg]acridin-2-yl)vinyl)-1,3,3-trimethyl-3H-indol-1-ium (NC1): Yield: 70 mg, 46%. 1H NMR (200 MHz, MeOH-d4) δ: 8.65 - 8.36 (m, 1 H), 8.31 - 8.09 (m, 4 H), 7.74 (d, J =


8

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8.3 Hz, 1 H), 7.62 - 7.43 (m, 5 H), 7.33 - 7.15 (m, 2 H), 4.16 (s, 3 H), 1.89 (s, 6 H), 1.70 (s, 12 H), 1.65 (s, 6 H).

13

C NMR (100 MHz, MeOH-d4) δ: 184.1, 156.6, 145.7, 144.4, 139.7, 132.8,

132.4, 132.2, 132.1, 131.5, 130.8, 128.4, 126.3, 126.2, 124.9, 124.8, 114.8, 53.3, 37.0, 36.7, 34.7, 34.4, 32.9, 27. HRMS (ESI): m/z calcd for C41H41N2O2 [M]+: 593.3163; found: 593.3163.

(E)-5-carboxy-2-(2-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4H-benzo[9,1] quinolizino[3,4,5,6,7-defg]acridin-2-yl)vinyl)-1-hexyl-3,3-dimethyl-3H-indol-1-ium

(NC2):

Yield: 90 mg, 53%. 1H NMR (200 MHz, MeOH-d4) δ: 8.57 (m, 1 H), 8.32 - 8.15 (m, 4 H), 7.76 (d, J = 8.5 Hz, 1 H), 7.60 - 7.47 (m, 5 H), 7.32 - 7.21 (m, 2 H), 4.70 (t, J = 7.3 Hz, 2 H), 1.90 (s, 6 H), 1.70 (s, 12 H), 1.67 (s, 6 H), 1.57 - 1.28 (m, 8 H), 0.95 - 0.85 (m, 3 H);

13

C NMR (100

MHz , MeOH-d4) δ: 183.7, 156.8, 144.7, 132.8, 132.5, 132.3, 132.1, 131.4, 128.4, 126.3, 126.2, 124.9, 114.9, 53.3, 47.5, 37.0, 36.7, 34.7, 32.8, 32.5, 29.6, 27.5, 27.2, 23.7, 14.4; HRMS (ESI): m/z calcd for C46H51N2O2 [M]+: 663.3945; found: 663.3940.

(E)-5-carboxy-3-decyl-2-(2-(4,4,8,8,12,12-hexamethyl-8,12-dihydro-4Hbenzo[9,1] quinolizino[3,4,5,6,7-defg]acridin-2-yl)vinyl)-1-hexyl-3-octyl-3H-indol-1-ium (NC3): Yield: 140 mg, 62%.1H NMR (200 MHz, MeOH-d4) δ: 8.68 - 8.52 (m, 1 H), 8.34 - 8.10 (m, 4 H), 7.75 (d, J = 8.3 Hz, 1 H), 7.62 - 7.40 (m, 5 H), 7.33 - 7.17 (m, 2 H), 4.73 (t, J = 6.9 Hz, 2 H), 2.63 (t, 2 H), 2.44 (t, 2 H), 2.02 - 1.90 (m, 2 H), 1.70 (s, 12 H), 1.64 (s, 6 H), 1.56 - 1.14 (m, 8 H), 1.02 (m, 25 H), 0.91-0.85 (m, 4 H), 0.67 (t, 6 H).

13

C NMR (100 MHz, MeOH-d4) δ: 182.1, 155.6,

146.1, 141.5, 140.3, 137.9, 132.9, 132.5, 132.4, 132.3, 131.4, 130.7, 128.4, 126.4, 126.3, 124.9, 114.5, 63.1, 47.5, 42.3, 42.2, 37.1, 36.7, 34.8, 33.0, 32.8, 32.7, 30.7, 30.5, 30.3, 30.2, 30.1, 30.0,


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29.9, 29.8, 27.9, 24.7, 24.6, 23.7, 14.5 HRMS (ESI): m/z calcd for C62H83N2O2 [M]+: 887.6449; found: 887.6448. (E)-5-carboxy-2-(4-(diphenylamino)styryl)-1,3,3-trimethyl-3H-indol-1-ium(NC4): Yield: 90 mg, 52%. 1H NMR (500 MHz, MeOH-d4) δ: 8.46 - 8.34 (d, J = 15.0 Hz, 1 H), 8.32 - 8.17 (m, 2 H), 7.97 (d, J = 8.2 Hz, 2 H), 7.78 (d, J = 8.2 Hz, 1 H), 7.48 - 7.38 (m, 5 H), 7.32 - 7.19 (m, 6 H), 6.96 (d, J = 7.6 Hz, 2 H), 4.08 (s, 3 H), 1.85 (s, 6 H);

13

C NMR (100 MHz, MeOH-d4) δ:

184.4, 168.6, 157.3, 155.7, 146.9, 146.7, 144.5, 134.9, 132.4, 132.1, 131.3, 128.2, 127.7, 127.7, 125.0, 119.8, 115.2, 109.0, 53.1, 34.5, 27.1; HRMS (ESI): m/z calcd for C33H29N2O2 [M]+: 473.2224; found: 473.2229.

(E)-5-carboxy-2-(4-(diphenylamino)styryl)-1-hexyl-3,3-dimethyl-3H-indol-1-ium(NC5): Yield: 85 mg, 43%. 1H NMR (200 MHz, MeOH-d4) δ: 8.46 - 8.38 (m, 1 H), 8.31 - 8.18 (m, 2 H), 7.94 (d, J = 8.5 Hz, 2 H), 7.75 (d, J = 8.5 Hz, 1 H), 7.52 - 7.39 (m, 5 H), 7.34 - 7.24 (m, 6 H), 6.97 (d, J = 9.0 Hz, 2 H), 4.57 (t, J = 7.3 Hz, 2 H), 1.85 (s, 6 H), 1.40 - 1.27 (m, 8 H), 0.92 - 0.85 (m, 3 H).

13

C NMR (100 MHz, MeOH-d4) δ: 183.7, 157.2, 155.8, 146.7, 145.4, 144.6, 134.8,

132.4, 131.3, 128.3, 127.8, 127.6, 125.1, 119.8, 115.0, 53.2, 47.4, 32.6, 30.9, 29.5, 27.5, 27.2, 23.7, 14.4. HRMS (ESI): m/z calcd for C37H39N2O2 [M]+ : 543.3006; found: 543.3014.

(E)-5-carboxy-3-decyl-2-(4-(diphenylamino)styryl)-1-hexyl-3-octyl-3H-indol-1-ium (NC6): Yield: 180 mg, 64%. 1H NMR (200 MHz, MeOH-d4) δ = 8.52 - 8.38 (m, 1 H), 8.29 8.15 (m, 2 H), 7.96 (d, J = 8.2 Hz, 2 H), 7.74 (d, J = 8.2 Hz, 1 H), 7.51 - 7.39 (m, 5 H), 7.36 7.24 (m, 6 H), 6.98 (d, J = 9.0 Hz, 2 H), 4.61 (t, J = 6.8 Hz, 2 H), 2.58 - 2.30 (m, 4 H), 1.96 1.82(m, 2 H), 1.39 - 1.18 (m, 11 H), 1.11 (br. s., 21 H), 0.91 - 0.81 (m, 9 H), 0.60 - 0.48 (m, 2


10

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H).

13

C NMR (100 MHz, MeOH-d4) δ: 182.1, 156.0, 146.7, 146.6, 141.5, 134.9, 132.4, 131.3,

128.3, 127.8, 127.6, 125.0, 119.9, 114.6, 63.0, 47.5, 42.4, 33.1, 33.0, 32.7, 30.7, 30.5, 30.3, 30.2, 30.1, 29.9, 29.9, 27.8, 24.7, 24.7, 23.8, 23.8, 23.7, 14.6, 14.6, 14.4. HRMS (ESI): m/z calcd for C53H71N2O2 [M]+: 767.5510; found: 767.5515.

Photophysical Properties NC1-3 dyes have strong light absorption in far-red region in solutions. The strong planar HT donor and electron withdrawing cationic indolium carboxylic acid acceptor units provide the broad and strong absorption in the ultraviolet and visible region. Figure 2 shows the absorption spectra of the dyes in MeOH solution and on a transparent mesoporous TiO2 film. Table 1 summarizes the optical properties for NC dyes. NC1-3 dyes exhibited absorptions bands in the range of 440 to 730 nm are assigned to the ICT between donor and acceptor and apart from this a weak π-π* transitions occurs between 350 to 450 nm. On the other hand, NC4-6 exhibits strong absorption band between 400-680 nm and less intense absorption between 300-380 nm. As shown in Figure 2a, NC1-3 showed red shift in the maximum absorption wavelength (λmax), which arises from HT core, it has rigid coplanar structure that gives the better conjugation compared to propeller-donor TPA. And, within the each series, when introducing the long or branched alkyl groups the absorption peak is red shifted. It indicates that the out-of-plane alkyl segments on indolium sp3-C atom and linear chains at the N- atom of indolium units increases the ICT character and red shift the transition due to the formation of dye-solvent complex which stabilizes the ICT state.40Among NC1-3 dyes, the red-shifted absorption with respect to NC1 was in the order of NC2 (7 nm) NC2> NC3 and NC4 ≈ NC5>NC6, due to progressive increase of alkyl group at N- and sp3-C atoms of indolium unit which is near to the TiO2 surface that reduces the dye anchoring to the Ti (V) sites (Figure S25). The branched and


22

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long alkyl groups on the indolium units play an important role for preventing surface aggregation and reduce the charge recombination even without any co-adsorbent. Therefore, the addition of a large amount of CDCA is not beneficial for NC3 or NC6 dye to enhance the photovoltaic performance.36 All the dyes with HT donor showed high JSC with and without CDCA than TPA donor. The low VOC of NC dyes with I−/ I3− redox couple are consistent with that of earlier reports on hemicyanine dyes.27,17 The poor PCE of NC4-6 series is largely because of lower JSC and which is dropped by 48% going from NC1 to NC4 and maximum JSC loss for NC6 around 69%. Notably, HT donor connected to indolium units provides more conjugated and planar molecular structure that significantly showed high JSC with and without CDCA than TPA donor. Table 3. Photovoltaic parameters for NC1-3 and NC4-6 with and without co-adsorbent

Dye Loading NC Dyes

2

JSC (mA/cm )

VOC (V)

ff (%)


η (%)a

(×10-7 mol cm-2)b

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Page 24 of 41

NC1

16.65±0.95

0.385±0.006 58.62±0.61

3.76±0.24

3.43

NC1/CDCA (1:10)

21.67±0.74

0.414±0.005 48.35±0.61

4.33±0.15

1.72

NC2

15.86±0.83

0.399±0.008 59.63±0.57

3.78±0.23

2.63

NC2/CDCA (1:5)

20.63±1.24

0.412±0.009 45.93±1.87

3.90±0.24

1.36

NC3

20.04±0.94

0.416±0.002 52.03±0.37

4.34±0.23

1.89

NC3/CDCA (1:5)

14.24±0.69

0.404±0.008 57.77±0.53

3.32±0.26

0.77

NC4

6.46±0.51

0.347±0.003 61.43±1.33

1.37±0.12

3.89

NC4/CDCA (1:20)

11.94±0.39

0.372±0.007 48.41±0.69

2.14±0.08

1.75

NC5

4.19±0.68

0.343±0.006 58.77±0.63

0.85±0.14

4.59

NC5/CDCA (1:5)

8.38±0.46

0.358±0.005 49.13±0.57

1.47±0.11

2.08

NC6

4.99±0.39

0.329±0.005 59.34±1.61

0.98±0.12

1.62

NC6/CDCA (1:5)

6.83±0.41

0.320±0.009 54.93±0.67

1.20±0.11

0.61

a

Electrolyte: 0.5 M LiI and 0.05 M I2 in CH3CN. Dye cell area was 0.18 cm2 and measurements carried out under 1 sun intensity (100 mW/cm2), bThe desorption experiment was carried out in 2 M ethanolic HCl at room temperature.

NC1-3 dye cells were exhibited impressive I-V and IPCE characteristics compared to the TPA counterparts. Comparing the IPCE spectra of HT and TPA based dyes, the device sensitized with NC1-3 displayed broader plateau region (400−700 nm) with the excellent light-harvesting in longer wavelength region extended to 780 nm than that of NC4-6 (480−710 nm) (Figure 6b). In particular, NC3 exhibited maximum plateaus around 79% at 580 nm without any co-adsorbents, proving that long and branched alkyl side chains on the acceptor is an effective way for the suppression of dye aggregation, and significantly contribute to the high performance of NC3 dye. NC1 sensitized cell with CDCA showed the IPCE maximum plateaus around 90% at 560 nm with co-adsorbents. However, with the same conditions NC4 exhibited IPCE maximum plateaus around 50% at 500 nm. Apparently, NC1-3 has higher light harvesting capability than


24

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TPA based dyes, indicating that the introduction of HT donor provides effective conjugation leading to a relative high photocurrent density and VOC.

Electrochemical Impedance Analysis EIS measurement was carried out under dark conditions to elucidate the interfacial charge recombination process in DSSCs.60 Basically, VOC and JSC are affected by charge transfer processes occur at the TiO2/dye/electrolyte interface. The VOC is calculated from the potential difference between the quasi- Fermi level of TiO2 (EFn) and the chemical potential of redox species (EF,redox) in the electrolyte (Equation 4). However, the redox potential of electrolyte is kept constant, as the same (I−/ I3−) redox couple used in the experiment. A variation in VOC value is explained by the position of TiO2 conduction band (ECB) and the electron density in the semiconductor (nc), it is closely related to the surface charge and charge recombination, respectively as expressed in Equation 5.61,62

where kB is the Boltzmann constant, T is the absolute temperature (293 K), nc is the free electron density, and Nc is the density of accessible states in the conduction band. For better understanding the interface kinetics, we have analyzed NC1, NC3, NC4 and NC6 as shown in Figure 7. The capacitance (Cµ) extracted from the Nyquist plot with different applied potential using RS+R1/C1+R2/C2 circuit model (Figure S26). Nyquist plot of NC3 with different applied potential showed in Figure S27 and we have carried out similar experiment for other dyes. We have plotted capacitive response of the cell under a series of applied voltage to


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Page 26 of 41

scrutinize effect on the position of TiO2 conduction band. As shown in Figure 7a, for best cells, the Cµ was decreased in the sequence of NC1 ≥ NC3 > NC4 > NC6 at a given applied potential, indicating a sequential upward shift of ECB (Table 4) which leads to increase in VOC in the order of NC1 ≥ NC3 > NC4 > NC6. The increased VOC of NC3 dye without any coadsorbent is because of the upward shift in the ECB owing to effective passivation TiO2 surface from ions (Li+ and I-) present in the electrolyte.34 In fact, after adsorption on to the TiO2 surfaces, the different dyes have some difference in dipole moment. Predominantly, the dipole moments points from dyes to TiO2 (vertical) upon dye adsorption which influences a conduction band shift and affects the VOC.63 However, within the NC1-3 dyes, the improved VOC of NC3 without CDCA can be observed from the diminished recombination due to sp3-C and N-alkyl chain.


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Figure 7. Plots of EIS analysis of NC1, NC3, NC4 and NC6 dye cells. (a), (b) and (c) Cµ, Rct and τ as a function of voltage, respectively, (d) Mott–Schottky plots at the semiconductor/electrolyte interface. The Rct and τ of NC dyes were described as shown in Figure 7b and Figure 7c. The Rct was observed in the order of NC1 (25.63 Ω), > NC3 (18.91 Ω)> NC4 (11.75 Ω) > NC6 (9.82 Ω) at applied bias of -0.36 V. For NC1sensitized cells Rct was higher than NC4. Therefore, NC1 or NC3 can suppress charge recombination more efficiently than NC4 or NC6, resulting in the highest VOC. Table 4. EIS parameters for the DSSCs based on NC1, NC2, NC3 and NC4.


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NC Dyes

Rct (ohm)

Cm (mF/cm2)

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τ (ms)

NC1/CDCA (1:10)

25.63

0.70

17.86

NC3/CDCA (1:0)

18.91

0.60

11.32

NC4/CDCA (1:20)

11.75

0.29

3.47

NC6/CDCA (1:5)

9.82

0.14

1.37

EIS data measured at a applied bias of −0.36 V under dark conditions, Rct: charge transport resistance; Cµ: chemical capacitance; τ: electron lifetime.

The longer electron lifetime indicates slower charge recombination rate at the interface result into a higher VOC.64 The calculated electron lifetime of NC1 (17.86 ms) and NC3 (11.32 ms) is longer than NC4 (3.47 ms) and NC6(1.37 ms) at applied bias of -0.36 V, and which is consistent with observed improvement in VOC (Figure 7c). NC3 dyes can suppress charge recombination much more efficiently than other dyes even without any co-adsorbents and which is mainly because of the alkyl chain in the N- and sp3-C atoms of indolium unit gives out-of-plane branching of hydrophobic chains that reduces the intermolecular interactions to some extent when adsorbed on TiO2 and also provides effective surface blocking for the charge recombination between electrons injected on the TiO2 film and I3-. However the high charge transfer resistance and longer electron lifetime in NC1-3 determines the remarkable improvement in VOC values as compared to NC1-3 dyes. Improving the VOC of hemicyanine dyes by the addition of TBP is nearly impossible because of the low laying LUMO and effect of TBP on ECB of semiconductor.9,65 TBP is one of the most important additives in the electrolytes that are adsorbed onto the TiO2/electrolyte interface, thus


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shifts the semiconductor's conduction band edge upward and prevents charge recombination with triiodides.12,66 It also helps to improve the VOC by a substantial charge rearrangement upon adsorption onto the TiO2 surface. Also the electrostatic potential (dipole) generated by the dye can shift the TiO2 conduction band.67,68 For this reason, an apparent Fermi level of semiconductor/electrolyte interface was measured by electrochemical analysis. Mott-Schottky analysis of photo-anode was carried out in presence of iodolyte redox couple (0.5 M LiI and 0.05 M I2 in CH3CN) with and without TBP (0.5 M) using three electrode assembly (Figure 7d). After the addition of TBP, around 220 mV up-shift (more negative) of TiO2 flat band potential was observed, and this study support the drastic decrease in PCE of NC dyes using TBP based electrolyte (Figure S24). The efficiency of the cells sensitized by NC was suppressed by a greater when TBP was added (Table S4)9.

CONCLUSION In summary, we have systematically investigated the electronic levels, corresponding photophysical properties, and conversion efficiencies of D−A type hemicyanine dyes which are designed by adopting two different donors and varying the alkyl functionalities on acceptor unit. For controlling the dye aggregation on TiO2 surface and to passivate the surface to avoid charge recombination in-plane and out-of-plane alky groups are incorporated in D-A dyes. The planar and more conjugated structure of HT is beneficial to the harvest longer wavelength photons, which reflects the higher IPCE response in NC1-3 than NC4-6. In particular, the newly designed dyes NC1-3 (HT-based) are more promising candidates for DSSCs which provides significant improvement in JSC and VOC values. The NC1-3 and NC4-6 display distinct IPCE profiles, NC1 dye shows plateau around 80% from 520 to 720 nm and it is only 40% from 480 to 620 nm for


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NC4. Density functional theory (DFT) calculations showed that HT unit has a dihedral angle of 0.05° between the phenyl groups and the plane of the N-bonded C atoms, which is smaller as compared to that for TPA (57.56°) indicates that the HT has excellent electron delocalization than that for TPA. The electrochemical impedance measurement was carried out to investigate the interfacial charge transfer processes for NC1-6 dyes. The NC3 sensitized cells showed photovoltaic efficiency of 4.34 ± 0.23% without co-adsorbents (JSC = 20.04 ± 0.94 mA cm-2, VOC = 0.416 ± 0.002 V, and ff = 52.03 ± 0.37%). The molecular engineering of D-A dye using strong donor and out-of-plane alkyl functionalities brings an upward shift in ECB of TiO2 even in presence of high concentration of Li+ and which is clearly explained by large charge transfer resistance, and a longer injected electron lifetime for HT-NC sensitized cells. This D-A molecular structure is an example of improving VOC and JSC of hemicyanine dyes without altering the electrolyte. Trade-off between ECB and VOC without TBP additive controlled to some extend using new dye design and further increase in efficiency may be attainable with sulfurbased electrolyte.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Materials and instruments, Device fabrication and characterization, UV-vis absorption and emission spectra of dyes, UV-vis absorption of dyes in different solvents, 1H NMR,

13

C NMR

and HRMS spectra, isodensity surface plots for the MOs, UV-vis absorption spectra of desorbed NC dyes in 2 M HCl in EtOH, Simulated absorption spectrum by TD-DFT calculation using


30

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B3LYP/6-31G (d, p), Supplementary photovoltaic performance and schematic diagram of Equivalent circuit model (Rs+R1/C1+R2/C2).

AUTHOR INFORMATION Corresponding Author *Tel: +91-020-25903050. E-mail: [email protected]. Author Contributions N. Karjule and M.F.M.K. contributed equally to this work.

ACKNOWLEDGMENT J. N. thanks Dr. Kothandam Krishnamoorthy, Polymer Science Engineering Division, CSIRNational Chemical Laboratory, Pune, India for his support, help with device fabrication and characterization. This work is financially supported by CSIR-Network Project NWP0054 (CSIRTAPSUN). N. K. thanks UGC for SRF fellowship.

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