Mono- and bi-cyanoacrylic acid substituted phenothiazine based sensitizers for dye sensitized solar cells

Journal Pre-proof Mono- and bi-cyanoacrylic acid substituted phenothiazine based sensitizers for dye sensitized solar cells E. Kavery, S. Muruganantham, S. Prabhu, R. Renganathan, Chin Sim Yee, Md. Maksudur Rahman Khan

PII:

S0030-4026(19)31945-X

DOI:

https://doi.org/10.1016/j.ijleo.2019.164046

Reference:

IJLEO 164046

To appear in:

Optik

Received Date:

30 September 2019

Revised Date:

10 December 2019

Accepted Date:

10 December 2019

Please cite this article as: E. K, S. M, S. P, R. R, Chin SY, Khan MMR, Mono- and bi-cyanoacrylic acid substituted phenothiazine based sensitizers for dye sensitized solar cells, Optik (2019), doi: https://doi.org/10.1016/j.ijleo.2019.164046

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Mono- and bi-cyanoacrylic acid substituted phenothiazine based sensitizers for dye sensitized solar cells

E. Kavery1, S. Muruganantham1, S. Prabhu2, R. Renganathan1*, Chin Sim Yee2 and Md.

2

School of Chemistry, Bharathidasan University, Tiruchirappalli, Tamil Nadu 620 024, India.

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Maksudur Rahman Khan2

Faculty of Chemical and Process Engineering Technology, Universiti Malaysia Pahang,

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Lebuhraya Tun Razak, 26300 Kuantan, Pahang, Malaysia

Corresponding author E-mail: [email protected] Fax: 91-431-2407045/2407020, Tel: 91-

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Highlights

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431-2772326

Phenothiazine with mono- & bi-cyanoacrylic acid dyes reported for DSSC application.

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Dyes having proper energy levels for the DSSC processes.

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Nature of anchoring groups influences the DSSC performance of the dye.

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

Abstract

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Phenothiazine and cyanoacrylic acid moiety based sensitizers were synthesized for dye sensitized solar cell application. Absorption and electrochemical properties of the sensitizers having mono- and bi-substituted cyanoacrylic acids were studied. The mono-cyanoacrylic acid substituted phenothiazine sensitizer has more light harvesting ability due to its high molar extinction coefficient. The photovoltaic performance of mono-cyanoacrylic acid substituted phenothiazine sensitizer was slightly greater compared to the bi-cyanoacrylic acid substituted

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phenothiazine sensitizer which was attributed to the effect of anchoring groups in the sensitizer.

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Keywords: Phenothiazine; Anchoring group; Solar energy materials; Spectroscopy; Dye

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sensitized solar cells 1. Introduction

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During the past few decades, dye sensitized solar cells (DSSCs) have been attracted important consideration due to its advantages such as eco-friendly, low cost, quick device

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fabrication and high energy conversion efficiency[1, 2]. The coordination complexes with heavy metal ions based DSSCs are considered as the most efficient devices which shows up to 12%

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efficiency[3], but cannot be used for large scale application due to the limited resources and high cost. Dyes obtained from the extract of natural plant like Murraya Koenigii, flower of Hibiscus Sabdarifa and aloe vera juice sensitizer based DSSCs are significant because of their non– toxic,

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environmental friendliness and availability[4-6]. But the lower efficiency and high cost of extraction process are the main disadvantages of this natural extract dye sensitized solar cell. The metal-free organic dyes have been considered to be a promising candidate for the potential application due to their cost-effective, high molar extinction coefficient and tunable electrical and structural properties[7, 8]. The design and synthesis of organic dyes based on coumarin, triphenylamine, carbazole, phenothiazine have been developed for the application of DSSCs [9-

2

12]. Phenothiazine, a well-known non-planar heterocyclic compound contains electron-rich heteroatoms such as sulfur and nitrogen with a butterfly conformation. The phenothiazine dyes have long alkyl chain and two anchoring groups are more stable and flexible, thus the enhanced photovoltaic performance can be achieved [10, 13]. In this work, two phenothiazine based (D-A) type sensitizers of PTZ 1 with mono- and PTZ 2 with bi-substituted cyanoacrylic acids were synthesized and used for DSSC application. HOOC

CHO

CN

cyanoacetic acid, ammonium acetate, acetic acid 120oC

S N

N H

N

S N

CHO

Br

NBS, CHCl3 0oC

S

CHO

N

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S

DMF, POCl3, DCE 90oC

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DMF, POCl3, DCE 90oC S

COOH

PTZ 1

2

1- bromobutane, KOH, DMSO 30oC

CN

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N

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S

OHC

4

3

NC

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1

HOOC

S

4-cyanophenylboronic acid, Pd(pph3)4, THF Reflux 2M K2CO3

CN

NC

cyanoacetic acid, ammonium acetate, acetic acid

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N

120oC

PTZ 2

S

CHO

N

5

Scheme 1. Synthesis of sensitizers PTZ 1 and PTZ 2.

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2. Experimental

The sensitizers were synthesised by following the methods reported in the literature [10,

14] as shown in Scheme 1. N-alkylation of phenothiazine with n-butyl group results in compound 1, which upon Vilsmeier-Haack formylation gave the carbaldehyde compounds 2 & 3. The resulted compound 3 was brominated to yield the compound 4 which subsequently treated with 4-cyano phenylboronic acid to yield the compound 5. Knoevenagel condensation between 3

cyanoacetic acid and compound 2 and 5 in the presence of ammonium acetate formed the desired phenothiazine based sensitizers PTZ 1 and PTZ 2 respectively. The structures of synthesized materials were characterized by 1H NMR and 13C NMR spectra recorded on a Bruker Avance (400 MHz) NMR spectrometer. The high-resolution mass (HRMS) spectra were collected using Thermo Exactive Orbitrap mass spectrometer. The FTIR spectra were recorded using Agilent resolutions pro spectrum in the range of 4000-400 cm-1.

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Absorption spectra of the compounds were obtained using Jasco V630 Spectrophotometer. The

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electrochemical properties were investigated by cyclic voltammetry (CV) by using 0.1M n-

TBAPF6 as supporting electrolyte using Biologic SP-50 potentiostat. The optimized geometries

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and energy levels of the sensitizers PTZ 1 and PTZ 2 were done with DFT technique. The

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current-voltage characteristics were measured using a solar simulator (AM-1.5 Filter, 150 W Xenon Lamp, 100 mW/cm2, Newport instrument).

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3. Results and Discussion

The absorption spectra of the sensitizers in tetrahydrofuran (THF) solvent are shown in Fig.

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1(a). Both the sensitizers exhibit two absorption peaks, the first peak observed from 300 to 320 nm attributed to the localized π-π* transition. The second broad peak observed from 420 to 520 nm assigned to the delocalized intramolecular charge transfer process. The absorption peaks of PTZ 1 was red-shifted than PTZ 2 which ascribed the higher visible light absorption of the

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sensitizer [8]. The molar extinction coefficient of PTZ 1 was very high compared to PTZ 2 (Table 1), thus the PTZ 1 can harvest more solar light than PTZ 2 which is beneficial for higher DSSC efficiency[15].

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a

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b

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Fig. 1. (a) UV-Visible and (b) cyclic voltammetry (CV) spectra of the sensitizers.

The CV measurements (Fig. 1(b)) show that the ground state oxidation potentials

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corresponding to the HOMO levels of PTZ 1 and PTZ 2 are 1.13 and 1.06 eV (vs. NHE) respectively. The HOMO levels (EHOMO) of the sensitizers were more positive than the

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iodine/iodide (I-/I3-) redox potential value (0.4 V), indicating the electron regeneration of the

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sensitizers from I- ions is favoured process [16]. The band gap energy (E0-0) of PTZ 1 and PTZ 2 were calculated from their absorption threshold and found to be 2.70 and 2.84 eV respectively.

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The excited state potentials corresponding to the LUMO levels of PTZ1 and PTZ 2 were calculated by using the formula: EHOMO - E0–0 and found to be -1.57 and -1.78 eV respectively. The LUMO levels of the sensitizers were more negative than the conduction band (CB) edge (Ecb) of the TiO2 (-0.5 eV vs.NHE), which implies that electron injection from the excited

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sensitizers molecule into the CB of TiO2 is feasible.

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a

(a)

(c)

(b)

(g) b

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(f)

(e)

(d)

Fig. 2. (a & b) Optimized structure, (c & d) (HOMO) and (e & f) (LUMO) of PTZ 1 and PTZ 2

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respectively and (g) I-V curves of the sensitizers.

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The optimized structures of these sensitizers are presented in Fig. 2 (a and b) which has a butterfly-shaped structure. The electron distribution in the frontier molecular orbitals (HOMO

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and LUMO) of PTZ 1 and PTZ 2 is shown in Figure 2 (c to f) and their corresponding energy gaps are estimated to be 2.70 and 2.86 eV respectively which found to be consistent with

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experimental values. The HOMO of the sensitizers is mainly populated on phenothiazine part which is the electron-richest part of the molecule. Upon light illumination, electrons are excited

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from HOMO of the sensitizers to their corresponding LUMO through an intramolecular charge transfer, and then located on the acceptor (ester group) of phenothiazine chain. The electron located in the ester group of the sensitizer is then transferred to the TiO2 electrode [17]. The photocurrent density-voltage (J-V) plots of the DSSCs fabricated using the prepared

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sensitizers are shown in Fig. 2(g) and the corresponding short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF) and efficiency (η) values are summarized in Table 1. The devices were fabricated under ambient atmosphere and not under any inert atmospheric condition which revealed the stability of the materials in water or air. Moreover, the efficiency of the fabricated DSSCs was measured after fifteen days by refilling the electrolyte and showed no significant

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changes, which confirmed more stability of the fabricated devices. The DSSC fabricated using PTZ 1 show better photovoltaic performance than PTZ 2 which may be due to the higher binding nature of the sensitizer with TiO2 compared to PTZ 2[17]. In PTZ 1, there are two carboxyl moieties (bi anchoring group) in cyanoacrylic acid groups which pull the electron density from phenothiazine core and inject it into TiO2 through ester linkage which results in higher binding of the sensitizer with TiO2. However, in PTZ 2 there is one carboxyl moiety

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(mono anchoring group) in the cyanoacrylic acid group which pulls the electron cloud towards

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the ester group and inject it to TiO2 but, the cyano moiety present in phenyl ring resist the flow of electron from the phenothiazine core to the carboxyl group [18]. This may result in lower

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binding nature of PTZ 2 with the TiO2. Tailoring the architecture of phenothiazine based

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molecules as sensitizer for DSSC results in the improved efficiency. Yong et al. demonstrated that changing the length of alkyl chain at the N atom of phenothiazine based D-D-π-A type

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sensitizers has improved the efficiency up to 8.18% [19]. Wang et al. modified the terminal donor groups of D-D-π-A type phenothiazine based dyes with simple and bulky groups and

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achieved higher efficiency of 7.13% with a simple donor group [20]. Gao et al. modified the π linker, oligothiophene in the phenothiazine based dyes and showed a high efficiency of 7.48% [21]. Buene et al. reported the effect of π-spacers such as benzene, furan and thiophene in phenothiazine based sensitizers and showed a higher efficiency of 4.34% for thiophene and furan

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modified sensitizers [22]. The effect of number of anchoring groups in the phenothiazine based dyes to achieve a better efficiency of 7.06% was also demonstrated by Eiamprasert et al. [13]. Slodek et al. recently modified the phenothiazine based dyes with 3,5-bis(trifluoromethyl)phenyl and p-methoxyphenyl groups and achieved a higher efficiency of 6.21% [8]. Albeit the lower efficiency of PTZ 1 and PTZ 2 in comparison to the above literature reports, the present work

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has established the first demonstration of employing the prepared sensitizers for DSSC application. The structural modification of the sensitizers with different donor groups, anchoring moieties and substituent at the N atom to achieve high efficiency should be explored and it is underway. Table 1. Absorption, electrochemical and photovoltaic performances of the sensitizers.

(M-1 cm-1)

314 &

8.45 &

459

4.26

305 &

6.08 &

436

2.24

1.13

-1.57

1.06

-1.78

Voc

cm-2)

(mV)

3.0

558

2.6

FF

539

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(nm)

LUMO Jsc (mA

(%)

0.56

1.10

0.55

0.91

4. Conclusion

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PTZ 2

HOMO

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PTZ 1

ε × 104

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Sensitizer λmax

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Phenothiazine based sensitizers consisting of mono- and bi-cyanoacrylic acid (PTZ 1 and PTZ 2 respectively) as anchoring groups were synthesized and characterized using NMR, HRMS and FTIR techniques. The absorption spectra of the sensitizers revealed that these are suitable for DSSC application. The HOMO and LUMO energy levels of the sensitizers obtained

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from CV and computational studies suggest the feasibility of the electron transfer process in the DSSCs. The DSSC efficiency of PTZ 1 and PTZ 2 was 1.10 % and 0.91 % respectively. The slightly lower efficiency of PTZ 2 may be due to the presence of 4-cyanophenyl group, which hinders the flow of electron from the sensitizer to the TiO2. Declaration of interests Nil 8

Acknowledgement The authors thank DST NM (Ref. No. SR/NM/NS-26/2013/19.02.2014), DST (Ref. No.

1/7/2015 & Ref No.UGC-EF-7855, 2016-2017) for financial support.

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