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Tuning the Photovoltaic Performance of DSSCs by Appending Various Donor Groups on Trans-Dimesityl Porphyrin Backbone Ravi Kumar, Vediappan Sudhakar, Kamal Prakash, Kothandam Krishnamoorthy, and Muniappan Sankar ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00458 • Publication Date (Web): 04 May 2018 Downloaded from http://pubs.acs.org on May 4, 2018

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ACS Applied Energy Materials

Tuning the Photovoltaic Performance of DSSCs by Appending Various Donor Groups on TransDimesityl Porphyrin Backbone Ravi Kumar,†# Vediappan Sudhakar,‡# Kamal Prakash,† Kothandam Krishnamoorthy,*‡ and Muniappan Sankar* † †

Department of Chemistry, Indian Institute of Technology (IIT) Roorkee, Roorkee-247667, India ‡

Polymers and Advanced Materials (PAM) Laboratory, CSIR-NCL, Pune-411008, India. [email protected], [email protected]

KEYWORDS. Donor groups, push-pull porphyrins, electrochemistry, optical absorption, Photovoltaics, light harvesting efficiency. ABSTRACT: Five cost effective porphyrin based sensitizers with various donor groups (bisthiophene, N,N-dimethylaminophenyl, triphenylamine, carbazole and phenothiazine) and meso-dimesityl as auxillary groups have been synthesized through the shortest possible synthetic route for the practical commercialization of these dyes. Trans-A2BC porphyrin dyes show power conversion efficiencies (η) ranging from 5.3% to 7.11% under 1 sun illumination and highly depend on donor strength of appended moiety. Our molecular design incorporates trans-10,20dimesityl groups which prevents the π-π aggregation between the porphyrin units onto the TiO2 surface. The porphyrin dye (RA-200-Zn) bearing phenothiazine as donor unveils the highest 1 ACS Paragon Plus Environment

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photon-to-current conversion efficiency (PCE) of 7.1% and showed butterfly conformation which can restrict the intermolecular aggregation. Further, this dye shows a broader absorption on TiO2 surface and significantly increased IPCE values in visible region as compared to the other dyes, which assured better light harvesting ability with high short-circuit current density (Jsc) of 14.2 mA cm-2.

Introduction Environmentally favorable solar energy is the potential alternative to the traditional fossil fuel consumption for the development of clean and economical energy source. It is environmental friendly energy resource and also reduces the greenhouse gas emission. Si-based solar cells showed PCE (η) upto 10-20% but their large scale application has been retarded due to its high production cost. It can be replaced by DSSCs1-5 which are cheaper and exhibit high photovoltaic performance. For DSSCs, several Ru(II) based polypyridyl complexes were used as the most effective TiO2 sensitizers6-9 since the seminal paper published by Grätzel and co-workers1 in 1991. However, the limited resources, high cost of noble metal ruthenium, toxicity to environment and complicated syntheses restrain their application in large scales.10,11,12 On the other hand, versatile organic dyes based on triphenylamine,13-16 indoline,17,18 aniline,19,20 carbazole,21-24 cyanine,25,26 fluorene,27,28 coumarin,29-32 and phenothiazine33-37 donors are also applied for DSSCs due to their facile synthesis, easy structural modifications, higher molar absorption coefficients and stability.38-40 Many ferrocenyl linked triphenylamine based D-π-A organic dyes with high efficiency have been reported by Misra et al.41-43 However, porphyrins and phthalocyanines have attracted much attention as they owe excellent stability, high molar extinction coefficients, ease of modification via peripheral substitutions.44-52 The ‘push–pull’ 2 ACS Paragon Plus Environment

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structures and/or π-conjugation are the common structural designs for efficient porphyrin dyes that produce panchromatic behavior in visible and near infrared regions. These type of architecture further promotes charge transfer from electron-donating moiety to porphyrin acceptor.53 After obtaining a breakthrough efficiency of 12.3% using Zn(II) porphyrin dye YD2o-C8,53 Gratzel et al. further improved the cell performance to 12.7% (GY50) and 13% (SM315) by attaching an electron-accepting benzothiadiazole group between porphyrin and acceptor moiety and by appending a bulky diphenylamine donor respectively under one sun illumination.54,55,56 Nevertheless, the mass production of these bridged conjugated frameworks involves multistep synthesis along with expensive palladium catalyzed coupling reactions under harsh reaction conditions which results in low yields of these sensitizers. In this regard, Hung et al reported many porphyrin sensitizers with three p-carboxyphenyl acceptor groups as mesosubstituents without the use of expensive metal (Pd)-catalyzed coupling reactions.57,58 Recently, we also reported a series of five porphyrinic dyes with efficient synthetic methodology for DSSCs application.59 He et al reported porphyrin dyes with a simple meso-acrylic acid and two trans-mesityl groups substituted zinc porphyrin as a sensitizer with power conversion efficiency of 5.1%.60 But this architecture still involves Pd(0)-catalyzed Heck coupling of meso-bromo substituted porphyrin to append the acrylic acid acceptor group on periphery leading to many synthetic steps with overall low yields and thus production of high cost DSSCs. However, with these simple architectures, achieving power conversion efficiency more than 5% is still a challenge. In this regard, our continuous efforts to develop simple, efficient, stable, and cost-effective sensitizers involving fewer synthetic steps results in the synthesis of five trans-mesityl A2BC 3 ACS Paragon Plus Environment


porphyrin Zn(II) complexes (RA-196-Zn to RA-200-Zn) by appending various donor groups on trans-dimesityl porphyrin skeleton (Chart1) with a power conversion efficiency ranging from 5.3% to 7.1%. Bulky trans-meso-dimesityl porphyrin skeleton was chosen as alkyl and alkoxy chains are well known to prevent dye aggregation.61-66 It also provides the oxidative stability to porphyrin dyes as well as prevent self-quenching resulting from porphyrin aggregation adsorbed onto the TiO2 leading to high cell performance. Further, the distance between the two dye molecules is widened since it is well known that the two mesityl groups lie parallel to TiO2 surface60 and make effective shield toward recombination process with the electrolyte. It enhances the electron injection efficiency (ϕinj).

Chart 1. Molecular Structures of targeted trans- A2BC Dyes. A benzene ring between porphyrin and the carboxylic acid group functions as a spacer. Power conversion efficiencies (η) of 5.3 - 7.1% were obtained with our molecular design which depend upon the donor strength of the appended moieties. Among all, RA-200-Zn has demonstrated the

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highest η value of 7.11% which is ascribed as strong electron donor capability of phenothiazine moiety.67-69

Results and discussion Synthesis and Characterization. All employed dyes were synthesized according to MacDonald [2+2] condensation using modified literature methods.59 5-mesityldipyrromethane, aldehyde containing electron donating group (R-CHO) and methyl 4-formylbenzoate (C) were condensed to get trans-A2BC-ester porphyrins (1a-1e) in good yields. In general, small quantity of TFA was used to prepare many symmetric porphyrins. Initially, we have used small quantities of TFA for the preparation of precursor porphyrins. For example, we used 0.113 mmol TFA for 1.89 mmol of dipyrromethane and 0.95 mmol of different aldehydes. However, this ratio didn’t produce good yield of porphyins and most of the aldehyde precursors remain unreacted. Hence, we increased the quantity of TFA to 1.13 mmol and obtained very good yields of target porphyrins. The synthetic methodologies were given in the supporting information (SI). We have also provided the large scale of synthesis of porphyin 1a showing the scalable synthetic approach of our porphyrins. In our synthetic method, we used less amount of TFA as compared to the methods reported by Lindsey for the preparation of sterically hindered porphyrins.59,70,71 1a-1e were further transformed into corresponding carboxyporphyrins (2a-2e) via base hydrolysis in quantitative yields (Scheme 1).



Scheme 1. Synthetic routes for targeted porphyrinic dyes.

Free base carboxyporphyrins (2a-2e) were metallated using Zn(OAc)2•2H2O in CHCl3/MeOH mixture to yield the final porphyrinic dyes in 90-95% yield. This Pd-free facile synthetic method is working in multigram scale with almost same yield. NMR, UVvisible, fluorescence and mass spectrometry were employed to characterize all the intermediates and final products. Density Functional theory (DFT) calculations were employed to have an insight into electron density distribution in frontier molecular orbitals (FMOs) and geometry optimization. Further, cyclic voltammetry, impedance spectroscopy and photocurrent-voltage measurements for the synthesized dyes were also investigated in detail. The detailed synthesis is given in the experimental section in supporting information (SI). NMR and MALDI-TOF-MS spectra of trans-A2BC porphyrins are shown in Figures S1-S20 and S21-S35, respectively in the SI.


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Electronic Spectral Properties UV-visible spectra of trans-A2BC porphyrin dyes were determined in dichloromethane at RT. Figure 1a represents the UV-Vis spectra of RA-197-Zn along with RA-198-Zn dyes and the absorption spectral data are summarized in Table 1. As can be seen from the Figure 1a, these sensitizers exhibited Soret and Q bands in 400-630 nm region which are attributed to the π-π* electronic transitions.

(a)

(b)

Figure1. (a) UV-visible spectra of dyes RA-197-Zn along with RA-198-Zn. (b) Emission spectra of trans-A2BC porphyrin dyes in dichloromethane at RT.

Fluorescence spectroscopic studies were also carried out to have an insight into role of various donor moieties. The Steady state emission spectra of synthesized dyes in CH2Cl2 at 298 K are represented in Figure 1b. Table 1 lists the emission data of synthesized dyes. Fluorescence quenching and fluorescence enhancement respectively in Soret and Q band region were observed for RA-198-Zn, RA-199-Zn and RA-200-Zn which also gives a preliminary idea of enhanced charge transfer from donor moiety to porphyrin core relative to RA-196-Zn and RA-197-Zn. The quantum yield decreases in the order RA-200-Zn > RA-199-Zn > RA-198-Zn > RA-197Zn > RA-196-Zn which refleccts the probability of charge transfer decreases along the series. 7 ACS Paragon Plus Environment


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Table 1. Electronic spectral Data of trans-A2BC porphyrin dyes in CH2Cl2 at 298K. Values in parentheses represents log ɛ. Porphyrin

B band, nm

Q band(s), nm

λem, nm

Φf

Lifetime (τ), ns

RA-196-Zn

423(5.55)

552(4.31), 595(3.82)

611, 653

0.062

1.84

RA-197-Zn

421(5.39)

551(4.29), 592(3.74)

612, 652

0.061

1.62

RA-198-Zn

421(5.46)

551(4.32), 592(3.76)

607, 651

0.060

1.78

RA-199-Zn

423(5.61)

550(4.39), 592(3.74)

604, 650

0.057

1.86

RA-200-Zn

422(5.55)

550(4.35), 591(3.73)

605, 650

0.056

1.62

Figure 2. Normalized UV-Vis absorptions of all dyes with CDCA (0.4mM) on the TiO2 films. We also recorded the fluorescence lifetime for these dyes and listed the data in Table 1. RA-200Zn dye exhibited shorter singlet lifetime and lower quantum yield indicating the enhanced charge-transfer in this push-pull porphyrin. When adsorbed on TiO2 films, the UV-vis spectra (Figure 2) of trans-A2BC porphyrin dyes with CDCA are significantly bathochromic shifted and broader relative to corresponding absorptions in solution. It is well known that the extended absorption region after anchoring on TiO2 surface is due to the interaction between the dye and 8 ACS Paragon Plus Environment

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TiO2, or the intermolecular interaction, or both of them. These observations reflects J-type aggregation of the porphyrins adsorbed on TiO2.72 The increment in Q band intensity after adsorption onto the TiO2 films indicates a strong electronic coupling of dyes with TiO2 film. This strong absorption also indicates improved light harvesting ability of photons and alteration of porphyrin geometry73 which is favorable for DSSCs application.

Cyclic Voltammetric (CV) Studies CV studies were carried out to know the influence of electron donor (R) groups on porphyrins redox potentials. The studies were carried out in CH2Cl2 having 0.1 M TBAPF6 with 0.1 V/s scan rate at RT. The corresponding voltammograms are represented in Figure 3 and the electrochemical data is summarized in Table 2. The E1/2 values of trans-A2BC porphyrin dyes are shifted cathodically as compared to ZnTPP46 depicting the electron donor effect of appended groups and thus making them a suitable candidate for solar energy conversion.

Table 2. Redox Potentials (vs Ag/AgCl) of Zn Porphyrin Dyes in CH2Cl2 Containing 0.1 M TBAPF6. Porphyrin

Oxidation (V)

Reduction (V)

∆E(V)

I

II

III

I

II

RA-196-Zn

0.860

1.102

1.507

-1.372

-

2.232

RA-197-Zn

0.790

1.008

-

-1.490

-

2.280

RA-198-Zn

0.808

1.005

1.260

-1.476

-

2.284

RA-199-Zn

0.788

1.113

1.416

-1.480

-

2.268

RA-200-Zn

0.790

0,910

1.215

-1.440

-

2.230



Figure 3. CVs of synthesized Zn(II) dyes (~ 1 mM) in dichloromethane.

The estimated oxidation potentials (Highest Occupied Molecular Orbital (HOMO) levels) of synthesized dyes versus NHE are cathodically shifted than that of I-/I3- redox electrolyte (∼0.4 Volts) which indicates the facile regeneration of dyes whereas Lowest Unoccupied Molecular Orbital (LUMO) levels are lying more negative as compared to conduction band (CB) of TiO2. Notably, the third oxidation can be ascribed for porphyrin dyes due to the one electron oxidation of donor moiety. It makes electron injection more efficient from the excited dye species to the semiconductor TiO2 conduction band.74 From the HOMO-LUMO values, the energy-level diagram were constructed as shown in Figure 4 and also compared with electrolyte and conduction band of TiO2 which clearly depicted facile electron transfer process in DSSCs (down-hill process).


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Figure 4. HOMO-LUMO gap of dyes against nanocrystalline TiO2 and iodide/triiodide electrolyte.

Photovoltaic Properties The device fabrication has been described in the supporting information. J-V characteristics of trans-A2BC porphyrin dyes (RA-196-Zn to RA-200-Zn) are depicted in Figure S36a in the SI and their photovoltaic data under one sun illumination (power 100 mW cm-2) with an active area of 0.16 cm2, are given in Table 3. N719 dye is also studied for the comparison under similar condition. The PCE of trans-A2BC porphyrin dyes are found from 4.41% to 6.28%, follows the order as RA-196-Zn (2′-bisthiophene, η = 4.41%) < RA-197-Zn (4′-N,N-dimethylamine, η = 4.87 %) < RA-198-Zn (4′-triphenylamine, η = 5.39%) < RA-199-Zn (9-butyl-3-carbazole, η = 5.52) < RA-200-Zn (10-butyl-3-phenothiazine, η = 6.28%). The IPCE spectra of all Zn(II) poprhyrin dyes was shown in Figre S36b where IPCE values varied from 54-72%. Interestingly, the dye RA-200-Zn with phenothiazine as donor has shown the highest PCE of 6.28%, with a short-circuit current dendity (Jsc) of 21.83 mA/cm2, a open circuit voltage (Voc) of 700 mV and a fill factor (FF) of 0.70. RA-200-Zn also exhibited the highest IPCE value of 72% as compared to others which indicate their higher light harvesting ability due to the strong donor strength of phenothiazene moiety. It is known in the literature that the nonplanar butterfly conformation of 11 ACS Paragon Plus Environment


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phenothiazine75-77 donor impedes the molecular aggregation which is responsible for its high Jsc values (RA-200-Zn). These results indicate that phenothiazine moiety is a strong donor group on porphyrin skeleton leads to facile electron transfer (vide supra) which make it efficient porphyrin dye for photovoltaic studies. As porphyins are having planar structure with extensive πconjugation, so their π-π stacking nature causes the aggregation of porphyrins on the TiO2 which decreases the Jsc of dyes and thus the efficiency of the dyes for DSSC. To minimize the process of porphyrin aggregation, the photovoltaic performance of porphyrin dyes can be enhanced due to the increment in Jsc value. Generally, chenodeoxycholic acid (CDCA) is used to suppress the dye aggregation due to non-planar conformation and bulky nature. We applied 0.4 mM CDCA as co-absorbent for our dyes in order to prevent aggregation. Figure 5a showed the J-V curve of the DSSCs based on the porphyrin dyes along with CDCA and their data is listed in Table 3.

(a)

(b)

Figure 5. (a) J-V characteristics of the DSSCs using various trans-A2BC porphyrin dyes. (b) Incident Photon-to-Current Conversion Efficiency (IPCE) action spectra of trans-A2BC dyes.

It is observed that the use of 0.4 mM CDCA significantly enhanced the PCE efficiency of these dyes from 5.30% to 7.11%. The enhancement of photovoltaic performance of dyes after using CDCA depicted the effective suppression of dye aggregation of porphyrins. 12 ACS Paragon Plus Environment

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RA-196-Zn and RA-197-Zn exhibited 20% increment in their PCE efficiency (η) value increased from 4% to 5% whereas RA-198-Zn, RA-199-Zn and RA-200-Zn showed 15%, 18% and 11% increment in PCE, respectively.78 The bulky nature of phenothiazine group already suppressed the dye aggregation in RA-200-Zn, hence only 11% increment was observed in PCE as compared to other porphyins. As opposite, other porphyrins showed more increments in PCE efficiency after using CDCA. Now, the trend in PCE as follows: RA-196-Zn (η = 5.30%) < RA-197-Zn (η = 5.86 %) < RA-198-Zn (η = 6.20%) < RA-199-Zn (η = 6.54) < RA-200-Zn (η = 7.11%). The efficiency of RA-200-Zn was about 75% of the N719 dye based cell which was also fabricated under similar conditions. Table 3. Photovoltaic measurements of trans-A2BC porphyrin dyes with and without CDCA. Porphyrin

IPCE% (at 420 nm)

Voc (V)

Jsc (mA/cm2 )

FF (%)

η (%)

RA-196-Zn

55

0.631±0.022

10.0±0.21

70±2

4.41±0.13

RA-197-Zn

56

0.667±0.015

10.3±0.23

71±2

4.87±0.15

RA-198-Zn

54

0.682±0.020

11.3±0.19

70±2

5.39±0.16

RA-199-Zn

61

0.678±0.240

11.81±0.17

69±1

5.52±0.22

RA-200-Zn

72

0.700±0.019

12.83±0.16

70±1

6.28±0.19

RA-196-Zn + CDCA

63

0.635±0.020

12.1±0.25

69±1

5.30±0.11

RA-197-Zn + CDCA

61

0.670±0.010

12.6±0.20

70±2

5.86±0.14

RA-198-Zn + CDCA

68

0.690±0.021

13.1±0.12

70±1

6.20±0.12

RA-199-Zn + CDCA

72

0.688±0.230

13.7±0.18

70±1

6.54±0.21

RA-200-Zn + CDCA

81

0.705±0.015

14.2±0.19

71±1

7.11±0.18

N719 (ref)

86

0.755

18.1

69

9.43

The average values are taken from 10 cells for each dye .TiO2thickness 8+5 µm and 0.235 cm2, the concentration of porphyrin dye was 0.2 mM in THF/EtOH (v/v: 1/4) mixture and 0.4 mM CDCA was used for preventing dye aggregation, I-/I3 as a redox mediator under AM 1.5 illumination (100 mW/cm2). Dye dipping time 20 h. Please note PV measurements were done without black mask.



The UV-vis spectra of trans-A2BC dyes with CDCA and without CDCA on TiO2 films were presented in figure 2 and S37 in the SI to demonstrate the non-aggregation of dyes in presence of CDCA. Thus, these studies confirmed that the less extend of dyeaggregation occurs in these porphyrins which was suppressed by using CDCA as coabsorbent. We have already reported five trans-A2BC porphyrin dyes that exhibited the PCE efficiency in the range of 2.11-4.23%. RA-195-Zn exhibited the highest PCE efficiency of 4.23%.59 This work encouraged us to synthesize more poprhyrin dyes having strong electron donating groups on the peripheral position which can be more effective for DSSC cells. Figure S38 in the SI represent the linear curve between donor strength of R moiety and PCE which shows the gradual increment in η values along the series (RA191-Zn to RA-200-Zn) from phenyl to phenothiazine donor. It clearly indicates the electron-donating ability of the appended donor moieties on trans-dimesityl porphyrin skeleton enhances the ability of photon absorption of the dyes, tunes oxidation potentials and exhibits higher power conversion efficiency. To better understand the photocurrent action, the IPCE spectra were collected on a Newport IPCE system at incident wavelength and represented in Figure 5b. The IPCE spectra of these dyes are very familiar with their UV-vis. spectra of dyes adsorbed on TiO2 films (Figure 2). All the fabricated solar cells exhibit photon absorption in the 390-470 (Soret region) and 500-670 nm (Q band) regions which are demonstrated in Table 3. RA-200-Zn dye showed efficient IPCE value (82%) in Soret and (62%) Q bands regions that are in accordance with a Jsc value of 14.2 mA/cm2 obtained from the J-V measurement (Table 3) as compared to other dyes. The dyes are


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arranged in the order of their IPCE value: RA-196-Zn < RA-197-Zn < RA-198-Zn < RA-199Zn < RA-200-Zn which also in good agreement with Jsc and PCE values.

EIS Studies Electrochemical impedance spectroscopy (EIS) was performed to understand the photovoltaic behavior of trans-A2BC dyes fabricated DSSCs.79 The Nyquist plots for the fabricated solar cells under the bias voltage of -0.6 V in dark are shown in Figure 6.

Figure 6. Nyquist plots for synthesized trans-A2BC porphyrin sensitizers.

We observed three semicircles in each of the Nyquist plot. The first semicircle in high frequency region represents the charge transfer resistance at the counter electrode-electrolyte interface, the middle semicircle is ascribed to interfacial charge-transfer resistances (Rct) at TiO2-porphyrin dye-electrolyte interface whereas third semicircle in low frequency area represents the impedance which is correlated with ion diffusion resistance within electrolyte.80,81 Rct represents the charge recombination (RC) rate between the electrolyte (I3-) and injected electrons and a high value of Rct depicts that there is slow electron transfer (ET) rate between I‒/ I3‒ electrolyte and 15 ACS Paragon Plus Environment


the TiO2 surface, which further results in an increment in the Voc values.82 The Rct values follows the order of RA-196-Zn (9 Ω) < RA-197-Zn (11 Ω) < RA-198-Zn (12 Ω) < RA-199-Zn (14 Ω) < RA-200-Zn (16 Ω). This is in good agreement Voc order of RA-196-Zn (635 mV) < RA-197Zn (665 mV) < RA-198-Zn (690 mV) ~ RA-199-Zn (688 mV) < RA-200-Zn (705 mV). From the Rct, we calculated electron lifetime (τ) using the equation as shown below.

τ

= Rct × Cchem where Cchem represents the Chemical capacitance.82 The fitted relationship

between the electron lifetime and capacitance as a function of Voc is shown in Figure 7a. At a given voltage, the largest chemical capacitance is shown by RA-200-Zn (Figure 7b) which is directly correlated to the charge recombination rate.83 Thus, the electron lifetime dependence (τ) against bias voltage (V) is represented in Figure 7b. In general, the higher electron lifetime (τ) indicate that recombination rate will be slower with the electrolyte and thus better photovoltaic performance expected.84 For the five dyes, the predicted electron lifetimes are arranged as following order: RA-196-Zn (14 ms) < RA-197-Zn (24 ms) < RA-198-Zn (31 ms) < RA-199Zn (52 ms) < RA-200-Zn (81 ms) which is in accordance with the Voc order of the trans-A2BC porphyrin dyes. The enhanced parameters for RA-200-Zn viz. high recombination resistance (16 Ω), longer electron lifetime (81 ms) and high capacitance (4.2×10-3 F/cm2) is an indicative of slow charge recombination (RC) between injected electron and I3- in the electrolyte and TiO2 electrode. It is resulted in higher Jsc (14.2 mA/cm2), Voc (0.705 V), FF (71 %) and highest PCE value of 7.11% for RA-200-Zn. The strong electron donating ability and bulky nature of donor retard the RC process and enhance the electron lifetime of molecule. From bisthiophene to phenothiazine donor, both electron donating nature and bulkiness of the groups increased. Hence, electron lifetime from RA-196-Zn to RA-200-Zn increased and large electron lifetime is 16 ACS Paragon Plus Environment

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observed for RA-200-Zn porphyrin dye. Therefore, it is observed that the better photovoltaic performance of RA-200-Zn is achieved from its increased light harvesting property, better absorption on TiO2 and effective retardation of charge recombination.

(a)

(b)

Figure 7. Plots of (a) Cµ and (b) τ versus bias voltage of Dye-Sensitized Solar Cells (DDSCs) based on RA-196-Zn to RA-200-Zn.

DFT Studies To study the electronic distribution of frontier molecular orbitals of dyes, we have optimized the geometries using the Gaussian 9.0 program at the B3LYP/LanL2DZ level. In all the cases, porphyrin core exhibited the planar conformation while all the meso-substituents lie more or less perpendicular to the plane of porphyrin skeleton. All these porphyrins showed characteristic tertaphentylporphyrin(TPP) bearing acceptor groups type molecular orbitals a1u, a2u and eg, i.e HOMO is a2u, HOMO-1 is a1u; LUMO and LUMO+1 are eg orbitals, respectively. The FMOs distributions of the synthesized porphyrins are presented in Figure S39 in the SI. The electronic distributions in HOMOs for all the porphyrin dyes are mainly situated over the electron donor 17 ACS Paragon Plus Environment


groups and the porphyrin dye, while the LUMOs are preferably localized over the acceptor phenylcarboxylic group and the porphyrin core. As a result of this distribution, the electrons can be easily transported from HOMOs to LUMOs which result in efficient ejection of the electrons from the donors to the acceptor and finally the electron injection to the TiO2 surface, indicating that the synthesized five trans-A2BC porphyrin dyes can be applied for DSSCs.

Conclusions Five new Zn(II) porphyrin dyes have been synthesized with trans-10,20-dimesityl groups on porphyrin skeleton without the use of expensive Pd-catalyzed coupling reactions. Different donors (bisthiophene, N,N-dimethylaminophenyl, triphenylamine, carbazole and phenothiazine) and carboxyphenyl as acceptor were appended to these ‘push-pull’ porphyrin dyes. These dyes were fully characterized by UV-Vis absorption, fluorescence, 1H NMR and mass spectrometry. Electrochemical studies showed that reduction potentials are cathodically shifted as compared to ZnTPP suggesting the electron donating ability of appended R (donor) groups. The power conversion efficiencies (η) ranging from 5.3%-7.11% have been achieved under 1 sun illumination which depends upon the donor strength of appended moiety. Of all the synthesized dyes, phenothiazine substituted porphyrin (RA-200-Zn) demonstrated 7.11% power conversion efficiency which can be attributed to the electron donating power of the phenothiazine. Eletcronic Supporting Information NMR and MALDI-TOF mass spectra and optimized geometries of targeted trans-A2BC porphyrin dyes are given in the electronic supporting information. Acknowledgements


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MS sincerely thanks SREB (EMR/2016/004016) and BRNS (2012/37C/61/BRNS) for financial assessment. RK and KP thank MHRD, Govt. of India for SRF. Author Contributions #

R.K. and V.S.: These authors contributed equally to this work.

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Graphical Abstract

Five cost effective porphyrin based sensitizers with various donor groups and meso-mesityl as auxillary groups have been synthesized through the shortest possible synthetic route for the practical commercialization of these dyes. The maximum power conversion efficiencies (η) of DSSCs based on these dyes are in the range of 5.3% to 7.11% under 1 sun illumination and highly depend on donor strength of appended moiety. The dye RA-200-Zn with phenothiazine as donor exhibited highest power conversion efficiency of 7.1% under 1 sun illumination.

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