perylene-diimide and thiophene based D-π-A low bandgap polymer sensitizers for application in dye sensitized solar cells

Journal Pre-proof Diketopyrrolopyrrole/perylene-diimide and thiophene based D-π-A low bandgap polymer sensitizers for application in dye sensitized solar cells Dipanjan Giri, Sagar Kumar Raut, Sanjib K. Patra PII:

S0143-7208(19)32252-1

DOI:

https://doi.org/10.1016/j.dyepig.2019.108032

Reference:

DYPI 108032

To appear in:

Dyes and Pigments

Received Date: 20 September 2019 Revised Date:

6 November 2019

Accepted Date: 7 November 2019

Please cite this article as: Giri D, Raut SK, Patra SK, Diketopyrrolopyrrole/perylene-diimide and thiophene based D-π-A low bandgap polymer sensitizers for application in dye sensitized solar cells, Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2019.108032. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Diketopyrrolopyrrole/Perylene-diimide and Thiophene based D-π-A Low Bandgap Polymer Sensitizers for Application in Dye Sensitized Solar Cells Dipanjan Giri,a Sagar Kumar Rauta and Sanjib K. Patra*a a

Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur-721302, WB, INDIA, E-mail: [email protected]; Tel: +913222283338

TOC

Diketopyrrolopyrrole/Perylene-diimide and Thiophene based D-π-A Low Bandgap Polymer Sensitizers for Application in Dye Sensitized Solar Cells Dipanjan Giri,a Sagar Kumar Rauta and Sanjib K. Patra*a a

Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur-721302, WB, INDIA, E-mail: [email protected]; Tel: +913222283338

Abstract In this study, diketopyrrolopyrrole-alt-thiophene (P1 and P2) and perylene-diimidealt-thiophene (P3 and P4) based donor-π-acceptor (D-π-A) copolymers have been synthesized from the corresponding monomers through Pd-catalyzed Sonogashira polymerization protocol. The well defined and soluble π-conjugated copolymers having alkyl and fluoroalkyl substituents (P1-P4) have been characterized by multinuclear NMR spectra as well as by tetradetector GPC studies showing molecular weight (Mn) in the range of 18-20 kDa with good polydispersity indices of 1.31-1.48. The donor-acceptor based copolymers absorb broadly throughout the visible region. Notably, perylene diimide-thiophene based copolymers (P3 and P4) exhibits an absorption onset at ca. 800 nm corresponding to a bandgap of 1.63 and 1.61 eV (Egopt). DFT computational studies of the model π-conjugated units have also been investigated to understand the molecular geometries and electronic properties of the polymeric unit. The synthesized D-π-A polymers have been utilized as active materials for polymer-sensitized solar cells (PSSCs). The copolymers are effectively adsorbed onto the surface of nanostructured TiO2 photoanode as a result of facile interaction of the anchored -C=O units with the metal oxide surface. The spectral profile of the polymer films on mesoporous oxide surface approximately similar to the solution absorption spectra of the polymer. Interestingly, the polymers featuring perylene diimide unit (P3 and P4) exhibit promising power conversion efficiency (PCE) of 2.71 and 2.96% with a short circuit current (JSC) of 7.54 and 7.85 mA·cm-2 respectively, and IPCE of 42-45% under 1.5 AM illumination.

1

Introduction Considerable research interest has been directed to green and renewable energy resources including solar light to acess economic and promising technology for accomplishing the increasing energy demands. In this direction, various types of photovoltaic cells, categorized as first generation (crystalline silicon-based),1 second generation (thin film of CIGS and CdTe)2 and third generation (organic and hybrid materials)3 have been developed by various research groups worldwide over the past two decades.1-3 Despite the enormous development of high performance photovoltaic cells based on the first and second generation configuration, high manufacturing cost, heavy weight, lack of flexibility restricts their widespread applications. To overcome these limitations, the recent trend is to develop organic small molecule/polymer based solar cells, as the most promising alternatives of metallurgical-grade Si for their multiple advantages.4 Dye sensitized solar cells (DSSCs) have received noteworthy interest as a potential alternative of conventional silicone based inorganic solid state solar cells. The reasons are multifold such as use of relatively low cost, possibility of tuning the electronic parameters of the synthesized dye-sensitized materials by chemical functionalization, very high photon to electricity conversion efficiency, opportunity of large scale assembly on both the hard or flexible substrates. O’Regan and Grätzel have first reported the promising applications of nanosized mesoporous TiO2 film electrodes in DSSCs.5 DSSCs based on organic/inorganic hybrid architectures with ruthenium(II) complexes dye sensitizers such as the Ru(4,4′dicarboxylic acid-2,2′-bipyridine)2(NCS)2, N3, Z-907, N719, and black dye (N749) have already shown a good power conversion efficiencies (PCE), due to their high molar extinction coefficient and wide-range of absorption in the visible light.6 However, ruthenium is a rare metal at high production cost with a low annual yield and difficulty of purification. Use of such metal containing dyes, exhibits a major disadvantages in terms of instability at high temperature which restricts the potential accessibility of cell fabrications and investigations of cell performance at ambient conditions. In view of these considerations, a great effort has been made toward pure organic dyes due to its easy synthetic protocol, wide variety of the structures, low cost, higher molar absorption coefficients, and no resource limitation.7 Among the organic dyes, semiconducting π-conjugated polymers such as polythiophenes,8 polyfluorenes, polytriphenyl9 donor-acceptor based polymers10 have been realized as

promising candidates as dye sensitizers because of their thermal and

environmental stability, solution processability, excellent mobility and conductivity, 2

sufficient exciton generation under illumination, easy deposition on several substrates, broad absorption and high extinction coefficient. In this direction, Liu and co-workers reported πconjugated D-π-A based polymer containing an electron donating triphenylamine (TPA) and an electron accepting cyanoacetic acid with conjugated thiophene units as the linkers and investigated the photovoltaic efficiency. A power conversion efficiency (PCE) of 3.39% was achieved with a JSC of 9.20 mA.cm-2 (VOC = 0.71 V, FF = 0.51).10d G. D. Sharma’s group investigated an alternating phenylenevinylene copolymer with TPA units for the photovoltaic applications. A PCE of 3.78% was obtained with a short circuit current density (JSC) of 7.50 mA.cm-2.9f The presence of triphenyl unit in polymer increases both the π-conjugation and reduces the dye aggregation due to its 3D propeller like geometry. Baek′s group developed D-π-A type copolymer comprising of electron donating (carbazole) and electron accepting (cyanoacetic acid) unit as a photosensitizer and obtained a remarkable PCE of 4% with a JSC of 8.43 mA·cm-2 (VOC = 0.67, FF = 0.73).10b Zhang and coworkers synthesized a series of πconjugated polymers based on phenothiazine unit with 9,9-dioctylfluorene and 9,9dioctylcarbazole as a photosensitizer for dye-sensitized solar cells (DSSCs). The polymers showed a efficiency in the range of 3.0-4.0% with JSC of 8.12 mA.cm-2 and VOC of 0.77 V (FF = 0.71).10a A PCE of 2.43% was achieved with JSC of 6.30 mA·cm-2 (Voc = 0.66 V, FF = 0.58) for the DSSC based on diketopyrrolopyrrole (dpp) based copolymers as photosensitizer developed by Patil and his co-workers.10c Taking into consideration, one of the strategies to access highly efficient panchromatic dyes having high molecular absorption coefficient is the judicial installation of donor-acceptor type push-pull substituent through molecular engineering in the polymeric backbone. Polymers composed of aromatic and hetero-aromatic ring structures have been particularly outstanding from a materials perspective view. In addition, the energy levels (HOMO-LUMO) of the polymers can be judiciously tuned by introducing alternating electron-rich/deficient push-pull units in polymer backbone. Moreover, a high density of anchoring groups of a polymer chain can facilitates the photo-excited electron-hole pair transfers on the semiconducting metal oxide structure via proper harmonizing of their HOMO-LUMO energy levels with the respective electrodes and the electrolytes. The large absorption coefficients and broad absorption spectrum are the essential properties for DSSCs since the high efficiency can only be achieved when the excited state charge transfer from the polymer to the photo-anode is feasible followed by the reduction of photo-oxidized polymer by the electrolyte. These relatively new category of solar cells using polymeric dye 3

sensitizers are known as polymer-sensitized solar cells or PSSCs.8-10 The efficiencies of DSSCs based on π-conjugated semiconducting polymers remain low because of inappropriate design and long term stability issues. Hence, there is a huge scope of exploring judicially designed polymer based sensitizer to overcome the drawbacks of the existing sensitizers. In this work, we report the synthesis of diketopyrrolopyrrole/perylenediimide and thiophene-based copolymers P1-P4 using 3-substituted 2,5-diethynylthiophene monomers (3octlyoxythiophene and 3-pentadecafluorooctyloxythiophene) and the dibromo-functionalized diketopyrrolopyrrole/perylenediimide monomers via Pd (II)/Cu(I) catalyzed Sonogashira polymerization. The structures, molecular weights, and optical properties of these copolymers were unambiguously characterized by various techniques including multinuclear NMR spectra, UV-vis spectroscopy and tetradetector GPC. The -C=O groups at the backbone of the copolymers are capable to anchor on the surface of mesoporous TiO2 film. The donoracceptor based copolymers (P1-P4) have been explored as dye sensitizer in PSSCs with the iodide/triiodide (I-/I3-) electrolyte system exhibiting promising PCE. Results and Discussion Design and synthesis of polymers Motivated by the current research trend in the field of organic dye sensitized photovoltaics, A-alt-B type copolymers introducing diketopyrrolopyrrole and perylenediimide into polythiophene scaffold have been selected. The polythiophene unit has been preferred due to its interesting electrical and optical properties for the presence of a delocalized electronic structure. Importantly, high charge carrier density and mobility (up to 0.1 cm2V-1s-1)11 are highly advantageous. The perfluoroalkyl groups are preferred as appendage to thiophene unit to provide highly ordered molecular π-π-stacking in the polymer backbone. The strategy can provide a route to enhance the internal dipole strength followed by facile electron transportation in the polymer backbone.12 On the other hand, the diketopyrrolopyrrole or perylenediimide moiety was selected for its broad and intense absorption in UV-vis region with high molecular

absorption

coefficients.

The

-C=O

group

in

the

backbone

of

diketopyrrolopyrrole or perylenediimide unit can be adsorbed on the TiO2 surface resulting the injection of electrons from the excited state of polymers into the conduction band of TiO2.10c

4

The key starting precursor, diethynyl functionalized 3-octyloxythiophene (6) and 3-pentadecafluoro-octyloxythiophene (7) were synthesized involving stepwise Sonogashira coupling followed by desilylation using K2CO3 in DCM/MeOH from 2,5dibromo-3-octyloxythiophene

(2)

and

2,5-dibromo-3-pentadecafluoro-

octyloxythiophene (3) respectively as outlined in scheme 1.

Scheme 1 Step wise synthesis of the monomers 6 and 7 (yield in parenthesis). The characterization and purity of the all the monomers were determined by multinuclear NMR spectral analysis (ESI†). The disappearance of the signal corresponding to TMS at 0.22 ppm, and appearance of the two singlets at 3.49, 3.34 ppm (for 6) and 3.53, 3.36 ppm (for 7) for the ethynyl protons confirm complete conversion of 6-7 from 4-5 respectively.

19

F{1H}NMR also confirms the inclusion of

fluoroalkyl substituents by showing six different signals at -80.7, -119.4, -121.9, 122.7, -123.2 and -126.1 ppm for 7. The compounds 6 and 7 were further confirmed by MALDI-TOF study showing the molecular ion peaks at 288.251 (M)+ and 543.854 (M)+

respectively.

On

the

other

hand,

dioctylpyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione

3,6-bis(5-bromothiophen-2-yl)-2,5(10)13

and

N,N-dioctyl-1,7-

dibromoperylene-3,4,9,10-tetracarboxylic acid diimide (12)14 were synthesized from commercially available 2-thiophenecarbonitrile and perylene-3,4,9,10-tetracarboxylic dianhydride respectively by adaptation of literature procedure.13-14 Alkylation of thiophene-diketopyrrolopyrrole (8) and dibromo-substituted perylenediimide (11) was carried out by treating with octylbromide in DMF and octylamine in propionic acid respectively. The thienyl protons of 10 in 1H NMR spectrum shows doublet at 8.68 and 7.25 ppm, whereas the aromatic protons of 12 exhibits two doublets at 9.46 and 8.68 ppm and a singlet at 8.89 ppm. The -CH2 protons of octyl group adjacent to nitrogen appear as characteristic triplet in the range of 4.02-4.21 ppm. For developing high-performance polymer solar cells, the strategy of donoracceptor (D-π-A) alternating copolymers can offer the unique feature of tuning the energy

levels

and

bandgap.

To

diketopyrrolopyrrole/perylenediimide

obtain based 5

wide

absorption

π-conjugated

thiophene

and

semiconducting

copolymers, Sonogashira polymerization of analytically pure monomers, 6-7 and 10/12, was carried out in degassed anhydrous THF using Pd(PPh3)2Cl2 (2 mol%) as a catalyst as shown in scheme 2. The polymerization reaction was carried out under refluxing condition with vigorous stirring for 48 h under argon atmosphere. The crude polymers were isolated by precipitation in methanol from their concentrated CHCl3 solution. It was further purified through Soxhlet extractor using MeOH, followed by hexanes to remove the oligomers and catalyst residue. Finally, the well-defined polymers are obtained from subsequent extraction in distilled chloroform. The synthesized polymers with long alkyl/fluoroalkyl groups are readily soluble in common organic solvents such as THF, toluene and chlorinated solvents. The πconjugated polymers having wide absorption band, were unambiguously characterized by multinuclear NMR, FTIR and tetradetector Gel Permeation Chromatography (GPC). In 1H NMR spectrum of P1, the thienyl protons of the diketopyrrolopyrrole unit exhibits two multiplets in the range of 8.51-8.42 and 8.34-8.32 ppm, whereas the thienyl proton of thiophene resonates as broad singlet centered at 7.35 ppm. The (Th)CH2-O- (Th = thienyl) and -N-CH2(octyl) protons resonate in the range of 4.594.46 ppm, whereas the -O-CH2(octyl) protons are appeared as broad signal centred at 3.65 ppm. The aromatic carbons of P1 resonate at 151.2-120.2 ppm, whereas the alkyl carbons resonate in the range of 60.2-14.2 ppm. Similar characteristic observation was noted in 1H and 13C{1H}NMR spectra for the polymer P2. 19F{1H} NMR alsoconfirms the inclusion of perfluoroalkyl group in polymer P2 by showing six different signals at -80.7, -119.2, -121.9, -122.7, -123.2, -126.2 ppm. For the polymers, P3 and P4, the thienyl and aromatic protons of the perylenediimide resonates as multiplet in the range of 7.88-7.32 ppm. The (Th)CH2-O- and -N-CH2(octyl) protons resonate in the range of 4.83-4.43 ppm as broad signal. The -O-CH2(octyl) and -O-CH2-CF2(fluoroalkyl) in P3 and P4 are appeared as broad multiplet centred at 3.58 and 4.05 ppm respectively. The other octyl protons resonate as multiplet in the region of 2.17-0.72 ppm. The aromatic carbon resonates in the range of 152.2-123.1 ppm, whereas the alkyl protons resonate in the range of 66.2-14.2 ppm.

The

19

F{1H} NMR of P4 confirms the

inclusion of -C7F15 substituents by showing the signals at -80.7, -119.2, -121.9, -122.7, -123.1, -124.9, -126.1 ppm. The formation of the polymers was further confirmed by tetradetector Gel Permeation Chromatography (in THF with polystyrene standards) as shown in Figure 1a. The number-average molecular weights (Mn) of the well-defined

6

π-conjugated polymers (P1-P4) are found to be in the range of 18-22 kDa with the very good polydispersity indices of 1.31-1.48 respectively (Table 1). R O O

C 8 H17 N S

R

n N C8 H17

P1; R = -C 7 H15 (61%) P2; R = -C 7 F15 (55%)

O

O Pd(PPh3)Cl2 (2 mol%) CuI (2 mol%), Et3N, S

R O

THF, 60 oC, 36 h S

6 : R = -C7 H15 7 : R = -C7 F15 12

O

O

N C8 H 17

C8 H 17 N C 8H 17 N

S

Br

Br O

O

C8H17 N

S Br

O N C 8H17 10

N C 8H 17

O

O

O n

O

S

S

10

P3; R = -C 7H 15 (56%) P4; R = -C 7F15 (50%)

O Br 12

Scheme 2 Synthesis of A-alt-B type D-π-A copolymers by Sonogashira polymerization.

Figure 1. (a) Tetradetector GPC trace (RI response) of the synthesized polymers, (P1-P4), using THF as eluent. (b) TGA thermograms of the polymers P1-P4 recorded at a rate of 10 °C/min under N2. Thermal stability of the polymers is essential for device application. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) experiments were performed to determine the thermal properties of the polymers, P1P4. TGA was carried out in inert (nitrogen) atmosphere at heating rate of 10 °C/min from ambient to 700 °C to find out the thermal stability of the polymers. The onset point of decomposition (Td) was determined from the temperature at first weight loss 7

from (≥5% weight loss) TGA experiment. The decomposition temperatures of the polymers (P1-P4) at were obtained as 291, 300, 305 and 315 °C respectively as shown in Figure 1b. Thus, all the new polymers exhibit good thermal stability, suitable for applications in DSSCs and other optoelectronic devices. Noticeable glass transition temperature was not observed for the polymers during DSC analysis. Table 1Characterization data for the synthesized polymers P1-P4. Tdc (oC)b Polymer Mn(kDa) Mw (kDa)a PDIa 20.1 26.3 1.31 291 °C P1 22.5 31.1 1.38 300 °C P2 18.7 26.3 1.41 305 °C P3 20.5 30.3 1.48 315 °C P4 a Obtained from tetradetector GPC (gel permeation chromatography), b Calculated from TGA studies, cDecomposition temperature

Photophysical studies The optical properties of the synthesized donor-acceptor based semiconducting πconjugated polymers (D-π-A) were investigated by UV-vis absorption studies in chloroform and as well as in solid state (as thin film) to understand the electronic properties in the ground state (Figure 2). The polymersP1 and P2 are dark purple in color with high extinction coefficients, whereas the polymers P3 and P4 are dark blue. Intense absorption bands were observed from 350 to 800 nm for the polymers (P1-P4). For all the polymers the absorption maxima corresponding to π-π* transitions appear in the region of 371-397 nm (ɛ= 1.24-2.17 × 104 M-1cm-1) (Table 2).15 The major absorption bands with λmax of 527 nm and 566-568 nm were observed for the polymers P1 and P2 having diketopyrrolopyrrole as acceptor unit. For the polymers P3 and P4, a broad absorption maxima at 542 and 586 nm was observed, with shoulder at 678 and 685 nm respectively. These low energy absorption bands are featured as intramolecular charge transfer (ICT) between thiophene donor and perylenediimide acceptor moieties.10c,13b-c It may be noted that the absorption maxima of P2 and P4 (having fluoroalkyl groups on thienyl unit) are slightly red-shifted by 6-9 nm than that of P1 and P3 (having alkyl groups on thienyl core). Generally, incorporation of fluoralkyl groups on polythiophene induces remarkable blue shift in absorption maxima as reported by Collard and coworkers.16 However, this effect is not observed in P2 and P4 as the fluoroalkyl groups are not directly attached, but connected to thienyl core through (Th)–CH2-O-CH2R (R = C7F15).The solid state absorption spectra for P1-P4 were recorded as thin films (spin coated on quartz 8

substrates). The solid state absorption spectra were similar to that of solution spectra. Interestingly, for polymers P3 and P4, the absorption maxima were red-shifted compared to its absorption maxima recorded in CHCl3. Hence, the absorption studies suggest that the band gap can be finely tuned by varying the electron withdrawing property in the donor-acceptor polymer back bone. Most importantly, the newly synthesized donor-acceptor based semiconducting polymers P1 and P2 show wide absorption band up to ca. 700 nm, whereas for the polymers P3 and P4 having diketopyrrolopyrrole unit exhibit broad absorption up to ca.800 nm. Being low Eg (optical) value, the synthesized D-π-A type polymer dyes could be a potential candidate in DSSCs as active polymeric dye photosensitizer materials. All the polymers are highly emissive in reddish to purple spectral region with λem in the range of 550-800 nm with a Stoke's shift of 1500-1800 cm−1with good quantum yield (Ф). The fluorescence life time of all the donor-acceptor based π-conjugated polymers are in the range of 4.1-5.5 ns (Figure S28b). Table 2. Photophysical data of the polymers P1-P4. Polymer

Absorbance (Solution)a λmax (ε x 10-4 in M-1cm-1)

Absorbance (Film)b

Emission (Solution)

Stoke’s shift

λmax(nm)

λem(nm)

Φ (%)c

τav (ns)d

(cm-1)

P1

354 (2.71), 371 (2.17), 527 (2.50), 566 (3.29)

356, 372, 525, 566

580, 630

25

5.29

1826

P2

355 (1.32), 372 (1.24), 527 (2.25), 568 (2.80)

362, 378, 534, 572

581, 629

29

5.49

1707

P3

385 (1.79), 469 (1.24), 542 (1.25, br), 678 (sh)

405, 479 (sh), 598, 687

571, 618, 671

23

4.11

3547

P4

397 (2.21), 472 (1.54), 586(1.56, br), 685 (sh)

393, 483 (sh), 606, 695

540, 583, 646

24

4.28

1584

a

The photophysical properties of the synthesized π-conjugated copolymers were measured in CHCl3 in 2×10-5 M concentration. bMeasured as polymer film on quartz plate. cMeasured by relative method with quinine sulphate (in 0.1 M sulphuric acid) as the reference (Φ = 0.54). dThe florescence lifetime decay experiment was conducted in CHCl3 solution (2×10-5 M concentration).

9

Figure 2. Solid state UV-Vis spectra (spin coated on quartz plate from CHCl3 solution of polymers) of the polymers P1-P4 at 28 °C. The

cyclic

voltammetric

experiments

of

these

thiophene

and

dikketopyrrolopyrole/perylenediimide based π-conjugated copolymers were conducted to investigate the electrochemical properties of the polymers and to evaluate the HOMO and LUMO energy levels. The cyclic voltammograms of thin film, drop-casted onto glassy carbon disc electrode from the DCM solutions of the polymers were shown in Figure 3. The cyclic voltammetric measurement was carried out in dry CH3CN using nBu4NPF6 (0.1 M) as supporting electrolytes, glassy carbon disc working electrode, Pt wire counter electrode and Ag/AgCl reference electrode under argon atmosphere. The electrochemical data are summarized in Table 3. The onset reduction potential of P1-P4 were located at -0.83, -0.75, 0.63 and -0.67 V respectively. The LUMO energy levels of P1-P4 were calculated as -3.46, 3.54, -3.66 and -3.62 eV respectively from the onset reduction potentials considering the known reference level for ferrocene (4.80 eV below the vacuum level) following the equation, ELUMO (eV) = -[Eredonset – Ered,Fc] - 4.80 (Fc = ferrocene).17 Whereas, the HOMO energy levels were estimated from onset oxidation potentials of the polymers. For P1 and P2 the HOMO energy levels were calculated as -5.22 and -5.20 eV respectively (considering the known reference level for ferrocene, -4.80 eV below the vacuum level) according to the equation; EHOMO (eV) = -[Eoxonset - Eox,Fc] - 4.80.17 On the other hand, for P3 and P4, the HOMOs were calculated from the equation EHOMO = ELUMO + Egopt,as there was no prominent oxidation wave in that potential window.18 The optical band gap was estimated by considering the equation of the 1240/λcut-off (λcut-off is the edge of the solid state absorption spectra), and the Egopt was found to be 1.96, 1.95, 1.63and 1.61 eV (λcut-off= 632, 635, 760 and 770 nm respectively) for P1-P4.

10

Figure 3. Cyclic voltammogram of P2 and P4 polymer film in acetonitrile, using TBAPF6 as supporting electrolyte, GC working electrode. Potentials are relative to the Ag/AgCl reference electrode. Scan rate at 100 mV/s.

Table 3. HOMO/LUMO and Egvalues of P1-P4 Polymer

Epa,Va

Epc,Va

EHOMO, eV

ELUMO, eVd

Egopt, eVf (from CV)g

P1

0.93

-0.83

-5.22b

-3.46 (-3.26)e

1.96 (1.76)

P2

0.91

-0.75

-5.20b

-3.54 (-3.25)e

1.95 (1.66)

P3

-

-0.63

-5.29c

-3.66

1.63

P4

-

-0.67

-5.23c

-3.62

1.61

a

Onset of oxidation and reduction waves. bEHOMO (eV) = -[Eoxonset - Eox, Fc] - 4.80. cHOMOs of P3 and P4 were calculated from the equation EHOMO = ELUMO + Egopt, as there was no prominent oxidation wave in that potential window. dLUMO was determined from onset of reduction wave using the equation, ELUMO (eV) = -[Eredonset – Ered,Fc] - 4.80. eLUMO calculated from ELUMO = EHOMO + Egopt is in parenthesis. fEgopt from solid state absorption spectra. gEg = LUMO-HOMO, calculated from CV in parenthesis.

Theoretical calculations To further examine structure property relationship especially the relation between molecular geometries and electronic properties of P1-P4, density functional theory (DFT) calculation, using the Gaussian 09 program based on Becke’s three-parameter set with LeeYang-Parr correlation functional (B3LYP) and the 6-31G(d,p) basis set, were performed on the model diketopyrrolopyrrole-thiophene and perylenediimide-thiopheneunits (Figure S37).19 To simplify the computational calculation, the long alkyl and fluoroalkyl groups are 11

replaced by -CH3 and -CF3 groups respectively. Figure 4 shows the energy-minimized optimized structures and corresponding highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the corresponding model monomers in the ground state. All the model monomers have an almost planer geometrical conformation with very small torsion angles, which will facilitate the electron/hole transport along the molecule backbone by extended π-delocalization. For the all the model monomers of P1-P4, the electron density in HOMOs is basically delocalized along the entire donor and acceptor units. For the model monomers of P3 and P4, the electron density in LUMOs is more localized in the acceptor units only.20

Figure 4. Optimized molecular geometries (B3LYP/6-31G(d,p)) and HOMO/LUMO of the analog D-π-A model units obtained from DFT calculations. Photovoltaic studies Before studying the photovoltaic application, the energy band diagrams of the polymers was evaluated to understand the feasibility of the polymers for acting as sensitizer in polymer sensitized solar cells (PSSCs). The low Eg values and favourable HOMO/LUMO 12

energy levels make these polymers as potential candidate as polymeric dye-sensitized materials for solar cells as shown in Figure 5. At first, the dye sensitized device was fabricated

as

FTO/TiO2-blocking

layer/TiO2-nanoparticle/dye/electrolyte/Pt/FTO,

as

illustrated in Figure S30. TiO2 (CB: -4.05 eV) was employed as the electron transport layer (ETL) due to its high electron mobility (0.017 cm2 V-1s-1).21 Since the valance band edge of TiO2 (VB: -7.32 eV) is much lower than those of the highest occupied molecular orbitals (HOMOs) of all the copolymers (P1-P4), the TiO2mesoporous layer serves as hole blocking layer (HBL) also. On the other hand, the Pt layer was used as counter electrode because of its small electron affinity. The dye-covered TiO2 electrode and Pt-counter electrode were assembled into a sandwich type cell fashioned, followed by injecting the electrolyte through the hole (drilled on counter electrode). Finally, the hole was sealed using a cover glass (0.1 mm thickness).22 Figure S27 shows the normalized absorption spectra of polymers (P1-P4) adsorbed on TiO2 film on FTO. Slight broadening and red-shift by 7-12 nm in λmax of absorption profile on TiO2 film, compared to the absorption spectra as film on quartz plate, was observed.10c

Figure 5. The energy diagram (HOMO and LUMO energy level were obtained from electrochemical and optical studies). The photovoltaic performances of DSSCs, fabricated using P1-P4 as a dye sensitizer, was investigated under the illumination of AM 1.5G solar light from a 3A solar simulator equipped with a 300 W xenon lamp. J–V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a Keithley model 2450 digital source meter with an area of 0.125 cm2. The photocurrent density-voltage (J-V) curve of polymer-sensitized solar cells (P1-P4) under illumination intensity of 100 mW/cm2 is shown in Figure S31 and the photovoltaic parameters are summarized in Table S1. All the devices were fabricated under identical conditions. P1 displayed a short-circuit current (Jsc) of 4.75 13

mA·cm-2, an open-circuit voltage (Voc) of 0.56 V and a fill factor (FF) of 0.48, thus leading to an overall efficiency (η) of 1.27%, whereas P2 having fluoroalky side chain exhibited a slightly higher value of short-circuit current (Jsc) of 5.73 mA·cm-2 with efficiency (η) of 1.67 %. On the other hand, polymer P3 and P4 showed a short-circuit current (Jsc) of 4.15 and 5.41 mA·cm-2 with an efficiency of (η) of 1.21 and 1.71% respectively. Similar values of open-circuit voltage (Voc: 0.58 V and 0.61 V for P3 and P4) and a fill factor (FF: 0.51 and 0.52 for P3 and P4) were achieved (Table S1). The increase of the efficiency of the perfluoroalkyl containing polymers (P2 and P4) can be attributed due to the slight improvement of both the Voc and FF. For the fluoroalkyl containing polymers (P2 and P4), crystalline peaks were obtained in powder X-ray diffraction (PXRD) (Figure S25). The diffraction pattern of thermally annealed P2 films (annealed at 100 °C for 6h in vacuum) exhibited intense (100) reflection at 2θ = 20.67° (d100 = 4.29 Å), which are presumably due to the π-π stacking of the polymers chain.23 Like P2, copolymer P4 showed intense peak at 2θ = 19.69° (d100 = 4.50 Å). Furthermore, the introduction of fluoroalkyl group in appropriate position of the polymer can enhance the internal dipole moment within the polymer backbone.12 This strategy helps to provide ordered microstructure at the donor-acceptor interface, which would decrease charge recombination and facilitating additional driving forces to improve exciton dissociation at the TiO2/dye/electrolyte interface.12 These collective results lead the improvement of short-circuit currents (Jsc) and fill factors (FF), thus increasing the photovoltaic performance of the PSSCs for the perfluoroalkyl containing polymers (P2 and P4).24 Now to further improve the efficiency of the device, the dye loading time was extended to 8 h from 5 h to increase the concentration of dye polymers on TiO2 surface. Interestingly, for all the sensitizers (P1-P4), a higher efficiency was observed along with higher Voc and FF (Table S2). For polymer P1 and P2 the PCE (%) of 2.06 (Voc = 0.57 V, FF = 0.58) and 2.32 (Voc = 0.58 V, FF = 0.58) were obtained whereas for the polymer P3 (Voc= 0.56 V, FF = 0.57) and P4 (Voc = 0.58 V, FF = 0.59) relatively higher value of PCE (%) was witnessed (2.30 and 2.51 respectively). For further increasing of the dye adsorption time no significant increase of performance of the devices was observed. Hence, keeping the dye loading time constant at 8 h, the TiO2 electrode was modified using double blocking layer (TiCl4 necking treatment) as FTO/TiO2-blocking layer/TiO2nanoparticle/TiO2-blocking layer/dye/electrolyte/Pt/FTO for better electron transportation rate and with aim of further optimizing the photovoltaic efficiency.25 The schematic device 14

configuration for double blocking layer based dye sensitized devices is depicted in Figure 6. The surface morphology of the TiO2 film was analyzed through AFM and FESEM studies. The surface roughness of the mesoporous TiO2 electrode was investigated with AFM studies. The root-mean-square (RMS) surface roughness of single blocking layer was found to be 13.4 nm, whereas the doubly modified mesoporous TiO2 layer displayed a RMS roughness of 10.5 nm with homogeneous distribution of the TiO2 nanoparticle (20-30 nm size). The thickness of the TiO2 nanoparticle layer was measured as 400-450 nm. Figure 7b showed more homogeneous spherical type nanoparticle network (using double blocking layer in the device) than the Figure 7a with single TiCl4 blocking layer modification (vide supra).22 This also suggests the efficient electron injection on the TiO2 layer.25 The FESEM images also reveal the similar morphology of the TiO2 layer. The photocurrent density-voltage (J-V) curve of polymer-sensitized solar cells (for P1-P4) under illumination intensity of 100 mW/cm2 is shown in Figure 8, and photovoltaic parameter is tabulated in Table 4. P1 shows a PCE of 2.28% (Jsc = 6.65 mA·cm-2) with an open-circuit voltage (Voc) of 0.57 V and a fill factor (FF) of 0.60, on the other hand for the P2 (fluorinated functionalized polymer) showed a higher short-circuit current (Jsc) of 7.31 mA·cm-2, an open-circuit voltage (Voc) of 0.57 V and a fill factor (FF) of 0.58 with the overall efficiency of 2.42%. For the perylenediimide based π-conjugated polymer P3, a Jsc value of7.54 mA·cm-2

Figure 6. Typical device configuration of polymer sensitized solar cells (PSSCs) with TiCl4double blocking layer treatment.

15

Figure 7. Atomic force microscopy images (AFM) of TiO2 nanoparticle (a) with single blocking layer, (b) with double blocking layer on FTO surface. (c) FESEM of the TiO2mesoporous layer. (d) TiO2mesoporous thickness (cross section of the device with FTO). with PCE of 2.71% (Voc= 0.58 V, FF = 0.61) was obtained, whereas polymer P4 shows a best efficiency of 2.96% with short-circuit current (Jsc) of 7.85 mA·cm-2 (Voc= 0.58 V, FF = 0.63). Hence, the double blocking layer based dye sensitized devices displayed much higher efficiency than the single blocking layer based devices (Table 4).26 Therefore, we conclude that implementation of double layer treatment on TiO2 mesoporous layer improved the PCE by enhancing JSC, as a result of decrease in the charge trapping on the doubly modified TiO2 moseporous electrode. Thus, less roughness in the surface reduces the possibility of charge trapping and enhances the charge transport.27 The higher power conversion efficiency of the solar cells with P3 and P4 as polymer dye sensitizer than that of P1 and P2 could be ascribed due to higher electronic conjugation and the superior light harvesting ability in the solar spectrum region and low band gap of P3 and P4 (Table 4). Table 4 Photovoltaic data of the PSSCs.a Polymer FF (%)

VOC (V)

JSC (mA/cm2)

PCE (%)b

P1

60

0.57

6.65

2.28 (2.31)

P2

58

0.57

7.31

2.42 (2.55)

P3

61

0.59

7.54

2.71 (2.80)

P4

63

0.60

7.85

2.96 (3.03)

a

Double blocking layer (TiCl4 necking treatment) with 8h dye loading time. bAverage of 20 devices. Best PCE in parenthesis.

Figure 8. (a) Current-voltage characteristics of the PSSCs based on P1-P4 polymer dye

16

sensitizers. (b) Variation of photocurrent with illumination intensity for PSSCs based on P4 as sensitizer (with double layer TiCl4 modification). The variation of short circuit photocurrent density (of the devices based on P4) with the incident illumination intensity (Pin) was also measured with different dye loading time to understand the contribution of photogenerated electrons towards photocurrent. A directly proportional relationship was obtained between JSC and Pin with a highest Jsc value of 6.45 and 7.85 mA·cm-2 for dye adsorption time of 5 h and 8 h respectively at 100 mWcm-2of Pin. Similar result was observed for other copolymer based devices by varying the dye adsorption time. In the photocurrent vs PIN plot (Figure 8b), the slope (0.092) is 23% steeper in case of 8 h dye loading time compared to that (0.070) of 5 h dye loading for the device based on P4. This indicates higher collection of photogenerated electrons at the mesoporous TiO2 surface of the device for longer dye loading period. It clearly signifies that Jsc of the PSSCs is mainly influenced by the amount of dye adsorbed on the mesoporous TiO2 surface. Higher the dye loading on the TiO2 surface, higher the photocurrent density, which enhances the electron transfer efficiency in the TiO2 film as well as to the external circuit. To confirm the reproducibility of PSSCs fabrication, the performance statistics of twenty devices is depicted as a histogram in Figure 9. The devices were fabricated within 24 hours of each other by following the standard procedure. Conversion efficiencies up to 3.0% were obtained for P4, with an average being 2.6 ± 0.2% for all the devices. This statistical graph shows excellent reproducibility with good stability at ambient condition (up to 60h, Figure S34) of the method described here for the fabrication of polymer based dye sensitized solar cells.

17

Figure 9. Histogram depicting reproducibility of PSSCs (for P4) power conversion efficiencies. Batch of 20 PSSCs devices were produced with a 24 hour time period (Similar observation was noted for the all other polymers as shown in Fig S33). Finally, the incident photon to current conversion efficiency (IPCE) spectrum obtained for P1-P4 as sensitizers are compared and depicted in Figure 10. The IPCE spectra of the perylenediimide-alt-thiophene copolymers (P3-P4) based device showed a red-shifted of ca. 28 nm with a broadened response as compared to that of diketopyrrolopyrrole-altthiophene (P1-P2) based devices. The maximum IPCE value of ~42-45% at 605 nm was obtained for the P3-P4 based solar cell with a broad band in the region of 520-750 nm. On the other hand, polymers P1-P2 exhibit a relatively lower IPCE value of~37% at 577 nm with a region of 530-730 nm, which is 18% lower than that of P3 and P4. This result indicates that the synthesized polymers could be acts as potential candidates as dye sensitizers for DSSCs.

Figure 10.The incident photon to current conversion efficiency (IPCE) spectrum of P1-P4.

Conclusion In conclusion, diketopyrrolopyrrole and perylenediimide based D-π-A alternating copolymers P1-P4 were synthesized via Pd-catalyzed Sonogashira coupling polymerization, and successfully characterized by various spectroscopic tools. The wide absorption band in the visible region with high absorption coefficient and favourable HOMO/LUMO energy levels allowed us to explore as polymeric dye sensitizer to fabricate the PSSCs. The PSSCs exhibited an overall power conversion efficiency about 2.28, 2.49, 2.71 and 2.96% for P1, P2, P3 and P4 respectively. The higher power conversion efficiency of perylenediimide based copolymers (P3 and P4) based PSSCs can be attributed form the red-shifted and

18

broadened absorption spectrum relative to diketopyrrolopyrrole based polymer sensitizers (P1 and P2), as well as increased amount of photogenerated electron injection into the conduction band of TiO2. For the perfluoroalkyl containing polymers (P2 and P4), the higher value of PCE than that of P1 and P3 can be attributed due to the enhanced π-stacking interactions and thus increasing charge carrier mobility. The doubly TiCl4-modified TiO2photoanode showed better performance due to the reduced back recombination and more charge carriers in the external circuit. High reproducibility of the PSSCs devices with along with good stability up to 60 h suggest that the well-organized donor-acceptor based πconjugated copolymer dyes, developed in this work are highly promising. This work certainly providesa platformfor further designing the efficient photoactive polymeric sensitizers. Materials and Instrumentation All the air and moisture sensitive reactions and manipulations were carried out under an atmosphere of pre-purified N2 or Ar by using standard Schlenk techniques. The glasswares were oven-dried (at 180 °C) and cooled under vacuum. Tetrahydrofuran and diethyl ether were dried over Na/benzophenone. All chemicals were purchased from Aldrich unless otherwise noted. Silica gel (60–120 and 100–200 mesh) used for column chromatography, was purchased from Merck. Pd(PPh3)2Cl2 was synthesized following the literature method.28 1

H (600 MHz and 400 MHz), 13C{1H} (150 MHz and 100 MHz) NMR spectra were

obtained from Bruker Lambda spectrometer using CDCl3 unless otherwise mentioned. Spectra were internally referenced to residual solvent peaks (δ = 7.26 ppm for proton and δ = 77.2 for carbon (middle peak) in CDCl3. All coupling constants (J) are given in Hz. The HRMS was recorded in ESI+ mode (70 eV) in Waters mass spectrometer (Model: XevoG2QTOF). The absorption and fluorescence spectra were collected using a Shimadzu (Model UV-2450) spectrophotometer and a Hitachi (Model F-7000) spectrofluorimeter, respectively. FT-IR spectroscopy was recorded in Spectrum-BX (Perkin Elmer). Solid state PL spectra were recorded in Flurolog Horiba (Model FL-1016, Spectracq). MALDI-TOF study was performed by using Bruker MALDI-TOF-UltrafleXtreme instrument. Morphology and EDX analysis of polymer thin films were carried out by JEOL JSM5800 (Japan) Scanning Electron Microscope with Oxford EDS detector. Thermogravimetric analysis (TGA) was carried out using Perkin Elmer Pyris Diamond TG/DTA instrument. Gel Permeation Chromatography (GPC) using a Viscotek VE 2001 Triple-Detector Gel Permeation Chromatograph equipped with automatic sampler, pump, injector, inline degasser, column oven (30 °C), styrene/divinylbenzene columns with pore sizes of 500 Å and 100,000 Å, VE 3580 19

refractometer, four-capillary differential viscometer and 90° angle laser and low angle laser (7°) light scattering detector (VE 3210 & VE270). HPLC grade THF was used as the chromatography eluent, at a flow rate of 1.0 mL/min. Samples were dissolved in the eluent (1 mg/mL) and filtered with a Ministart SRP 15 filter (polytetrafluoroethylenemembrane of 0.2 µm pore size) before analysis. Calibration of all three detectors (refractive index, laser light scattering and viscometry) was performed using polystyrene standards (Viscotek). This equipment allows the absolute measurement of homopolymer molecular weights and PDIs. Cyclic voltammetric studies were performed on a BASi Epsilon electrochemical workstation in acetonitrile with 0.1 M tetra-n-butylammoniumhexafluorophosphate (TBAPF6) as the supporting electrolyte at room temperature. The working electrode was a BASiPt disc electrode, the reference electrode was Ag/AgCl and the auxiliary electrode was a Pt wire. The ferrocene/ferrocenium couple occurs at E1/2 = +0.51 (70) V versus Ag/AgCl under the same experimental conditions. Photovoltaic measurements employed an AM 1.5 solar simulator equipped with a 300 W xenon lamp (Enli Technology Co., Ltd., class-3A solar simulator, Model F5-3A) and Keithley model 2450 digital source. The action spectra of monochromatic incident photon-to-current conversion efficiency (IPCE) for solar cell were performed by using a commercial setup (QE-R3011, IPCE Measurement System, Enli Technology Co., Ltd., Taiwan). AFM measurement was carried out with Nanosurf FlexAFM-5 with C3000 controller. Experimental section The experimental details, solid state photophysical studies, sample preparation for FESEM studies etc. have been thoroughly discussed in SI. The preparation of TiO2 screen printing paste, TiO2 nanoparticles, fabrications of TiO2 electrode, Pt-counter electrode, electrolyte solution have been elegantly discussed in SI. Fabrication of Polymer-Sensitized Solar Cells Photovoltaic measurements are employed with AM 1.5 solar simulator equipped with a 300 W xenon lamp (Enlitech class-3A solar simulator, Model F5-3A). The power of the simulated light was calibrated to 100 MW/cm2 by using a reference Si photodiode equipped with an IR-cut off filter (KG-5) in order to reduce the mismatch between the simulated light and AM 1.5 (in the region of 350–750 nm) to less than 2%. I–V curves were obtained by applying an external bias to the cell and measuring the generated photocurrent with a Keithley model 2450 digital source meter. The voltage step and delay time of photocurrent were 0.02 V and 1 ms, respectively. 20

DSSCs assemblage:The dye-covered TiO2 electrode and Pt-counter electrode were assembled into a sandwich type cell fashioned. A drop of the electrolyte was put on the hole in the back of the counter electrode. It was introduced into the cell via vacuum backfilling. The cell was placed in a small vacuum chamber to remove inside air. Exposing it again to ambient pressure causes the electrolyte to be driven into the cell. Finally, the hole was sealed using a cover glass (0.1 mm thickness). Light reflection losses were eliminated using a selfadhesive black plastic tape. Syntheses.Synthesis of the compounds 1, 2, 4, 6, and 8-12 have been adopted from previously reported literature procedure13-14 and its characterization is included in SI. 2,5-dibromo-3-((2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-pentadecafluorooctyloxy)methyl)thiophene (3): Sodium hydride (225 mg, 9.40 mmol) and anhydrous 20 mL THF were placed in a 100 mL

Schlenk

flask

under

an

inert

atmosphere.

2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-

pentadecafluorooctnol (3.02 g, 7.52 mmol) was added and the mixture was stirred for 15 minutes. 2,5-Dibromo-3-bromomethylthiophene (2.10 g, 6.27 mmol) was added and the reaction was stirred for 24 h at room temperature. The solution was poured into water and extracted with diethyl ether. The organic layer was dried over anhydrous MgSO4 and filtered. The solvent was concentrated. The crude compound was purified by silica gel column chromatography (hexanes) to give colorless oil. Yield: 3.32 g, 81 %. 1

H NMR (400 MHz, CDCl3): δ 6.97 (s, 1H, thienyl), 4.74 (s, 2H, -CH2-O), 3.95 (t, J = 12 Hz,

2H, -CH2-CF2-,).

13

C{1H} NMR (100 MHz, CDCl3): δ 137.5 (C, thiophene), 130.5 (CH,

thiophene), 127.2-121.2 and 115.9-108.1 (multiplets, CF2, fluoroalkyl), 118.7, 118.5 (C, thiophene), 68.1 (-CH2-O), 67.2-66.6 (multiplet, -O-CH2-).

19

F{1H} NMR (377.3 MHz,

CDCl3): δ -80.75, -119.36, -122.01, -122.73, -123.24, -126.10. MALDI-TOF (m/z): C13H5F15Br2SO, calculated value 651.418 (M)+, found 651.447(M)+. Synthesis

of

5:

To

a

solution

of

2,5-dibromo-3-((2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-

pentadecafluorooctyloxy)methyl)thiophene (504 mg, 0.76 mmol), PdCl2(PPh3)2 (5 mol%), CuI (5 mol%), and 20 mL THF were added. Then 5 mL triethylaminewas added to it at room temperature while stirring. The resulting solution was stirred for 5 min before trimethylsilylacetylene (158 g, 1.61 mmol) was added. The reaction mixture was stirred for 12 h at room temperature and poured into cold water. The aqueous layer was extracted with DCM. The organic layer was dried over anhydrous MgSO4, and the crude product was concentrated in vacuo. The product 5 was purified using silica gel column chromatography

21

(hexane:ethylacetate as eluent) to get dark brown sticky semi-solid product. Yield: 392 mg. 75%. 1

H NMR (400 MHz, CDCl3): δ 7.12 (s, 1H, thienyl), 4.64 (s, 2H, -CH2-O ), 3.92 (t, 2H, O-

CH2-), 0.31 (TMS).

19

122.6, -123.2, -126.1.

F{1H} NMR (377.3 MHz, CDCl3): δ (ppm) -80.7, -119.4, -121.8, 13

C{1H} NMR (100 MHz, CDCl3): δ (ppm) 141.6 (C, thiophene),

132.9 (CH, thiophene), 124.5, 123.1 (C, thiophene), 120.6-110.1 (multiplets, C, fluoroalkyl), 104.1, 100.8, 96.7, 95.3 (C, ethynyl), 68.4 (-CH2-O), 67.0-66.7 (multiplet, -O-CH2-), 0.9 (C, SiMe3). MALDI-TOF (m/z): C23H23F15OSSi2, calculated value 688.640 (M)+, found 688.636 (M)+. Synthesis of 7: Compound 5 (745 mg, 1.08 mmol)and potassium carbonate (330 mg, 2.38 mmol) were dissolved in a solution of 15 mL DCM and 10 mL methanol. The solution was allowed to stir at room temperature for 2 h. The reaction mixture was poured into water and extracted with DCM. The organic extract was washed with brine. The combined organic layers were dried over anhydrous MgSO4. The solvent was removed by rotary evaporation. It was purified using silica gel column chromatography (hexane as eluent) to afford 7 as brown sticky semi-solid like product. Yield: 552 mg, 94 %. 1

H NMR (400 MHz, CDCl3): δ 7.18 (thienyl), 4.67 (s, 2H, -CH2-O), 3.94 (t, 2H, O-CH2-),

3.52 (s, 1H, ethynyl), 3.36 (s, 1H, ethynyl).

19

80.7, -119.4, -121.9, -122.7, -123.2, -126.1.

F{1H} NMR (377.3 MHz, CDCl3): δ (ppm) -

13

C{1H} NMR (100 MHz, CDCl3): δ (ppm)

142.0 (C, thiophene), 132.9 (CH, thiophene), 127.6 (C, thiophene), 123.4 (CH, thiophene), 121.5-115.2 (multiplets, C, fluoroalkyl), 85.4, 82.3 (CH, ethynyl), 75.7, 74.3 (C, ethynyl), 67.7 (-CH2-O), 67.1-66.5 (multiplet, -O-CH2-). MALDI-TOF (m/z): C17H7F15OS, calculated value 543.997 (M)+, found 543.854 (M)+. Synthesis of P1 and P2: 6 (135 mg, 0.21 mmol) /7 (42 mg, 0.11 mmol) and 10 (115 mg, 0.11 mmol/ 155 mg, 0.20 mmol) were dissolved in 10 mL of distilled degassed THF in a 100 mL Schlenk flask under argon atmosphere. PdCl2(PPh3)2 (2 mol%) and CuI (2 mol%) was then added to the reaction flask. The reaction flask was degassed three times by freeze-pump-thaw technique. Then 5 mL triethylamine was added to it at room temperature while stirring. The resulting orange solution was stirred and heated to 65 ºC for 72 h. The color of the reaction turned to dark orange during the course of the reaction. After cooling to room temperature, the solvent was concentrated to minimum volume and polymer was precipitated to a stirring methanol solution. After the complete precipitation the methanol was removed and the dark brown color polymer was washed another two times with methanol. Then it was dried under 22

vacuum. Next the polymer was purified through Soxhlet extraction using distilled hexanes, methanol and at last collected in dry and distilled chloroform. The chloroform part was evaporated to get the brown colored polymers. The polymers (P1/P2) were finally purified by fractional precipitation in cold distilled hexanes from a concentrated dichloromethane solution to achieve well-defined P1/P2. Yield: 102 mg (61%) and 132 mg (55%) respectively. P1: 1H NMR (400 MHz, CDCl3): δ 8.58-8.42 (m, 2H, thienyl), 8.34-8.32 (m, 2H, thienyl), 7.40-7.32 (s, br, 1H, thienyl), 4.59-4.46 (m, 6H, -N-CH2- and Th-CH2-O-), 3.65 (s, br, 2H, O-CH2-), 2.34-2.06 (m, 6H, octyl), 1.54-1.13 (m, 30H, octyl), 0.85-0.78 (m, 10H, octyl). 13

C{1H} NMR (100 MHz, CDCl3): δ 152.0, 151.9, 151.1, 147.3, 143.4, 140.7, 139.5, 135.7,

128.1, 126.4, 124.9, 124.5, 123.7, 121.6, 120.1 (aromatic rings), 114.3, 113.4 (C, pyrrole), 88.5, 87.2, 77.4, 71.0 (C, ethynyl), 67.0, (-CH2-O) 55.5, 34.0, 32.1, 31.6, 30.4, 29.5, 26.7, 24.1, 22.8 (CH2, octyl), 14.3 (CH3, octyl). UV (CHCl3): λmax(ε): 303 (2.65 × 104 M−1cm−1), 351 (2.12 × 104 M−1cm−1), 527 (3.81 × 104 M−1cm−1), 567 (3.88 × 104 M−1cm−1); PL(CHCl3): λem(λex): 580, 630 (400) nm; Tetradetector GPC data: Mn = 20.1 kDa, Mw = 26.3 Da, PDI = 1.31; Td (°C) = 291. P2: 1H NMR (400 MHz, CDCl3): δ 8.52-8.34 (m, 2H, thienyl), 7.61-7.33 (m, 2H, thienyl), 4.66-4.46 (m, 6H, -N-CH2- and Th-CH2-O-), 3.59 (s, br, 2H, -O-CH2-CF2-), 2.34-1.97 (m, 4H, octyl), 1.27-1.13 (m, 20H, octyl), 0.85-0.78 (m, 6H, octyl).

19

F{1H} NMR (377.3 MHz,

CDCl3): δ -80.7, -119.2, -121.9, -122.7, -123.2, -126.2. 13C{1H} NMR (100 MHz, CDCl3): δ 158.2, 157.2, 141.1, 141.0, 140.7, 140.2, 139.5, 132.6, 127.2, 123.5, 121.3, 120.7 (aromatic rings), 119.1-111.4 (multiplets, C, fluoroalkyl), 110.3, 109.3, 107.8, 105.3 (C, ethynyl), 68.7, 68.5 (-NCH2, octyl), 67.0 (-CH2-O), 66.7-66.4 (multiplet, -O-CH2-), 43.7, 32.1, 31.9, 29.8, 29.5, 29.2, 29.1, 27.5, 22.9 (CH2, octyl), 14.3 (CH3, octyl). UV (CHCl3): λmax(ε): 303 nm (2.24 × 104 M−1cm−1), 352 nm (2.01 × 104 M−1cm−1), 527 nm (3.51 × 104 M−1cm−1), 568 nm (3.56 × 104 M−1cm−1); PL(CHCl3): λem(λex): 581, 629 (400) nm; Tetradetector GPC data: Mn = 22.5 kDa, Mw = 31.3 Da, PDI = 1.38; Td (°C) = 300. Synthesis of P3 and P4: 6 (75 mg, 0.21 mmol)/7 (104 mg, 0.12 mmol) and 12 (200 mg, 0.13 mmol/ 131 mg, 0.11 mmol) were dissolved in 10 mL of distilled THF in a 100 mL Schlenk flask under argon atmosphere. Then PdCl2(PPh3)2 (2 mol%) and CuI (2 mol%) was added to the reaction flask. The reaction flask was degassed three times by freeze-pump-thaw technique. Then triethylamine (5 mL) was added to it at room temperature while stirring. The

23

resulting orange solution was stirred and heated to 65 °C for 72 h. The colour of the reaction turned to dark orange during the course of the reaction. After cooling to room temperature, the solvent was concentrated to minimum volume and polymer was precipitated to a stirring methanol solution. After the complete precipitation the methanol was removed and the dark brown color polymer was washed another two times with methanol. Then it was dried under vacuum. Next the polymer was purified through Soxhlet extraction using distilled hexanes, methanol and at last collected in dry and distilled chloroform. The chloroform part was evaporated to get the brown colored polymers. The polymers (P3/P4) were finally purified by fractional precipitation in cold distilled hexanes from a concentrated dichloromethane solution to achieve well-defined P3/P4. Yield: 98 mg (56%) and 112 mg (50%)respectively. P3: 1H NMR (400 MHz, CDCl3): δ 7.88-7.32 (m, 7H, aromatic), 4.68-4.43 (m, 6H, -N-CH2and Th-CH2-O-), 3.58 (s, br, 2H, -O-CH2-), 2.06-2.01 (m, 6H, octyl), 1.29-1.09 (m, 30H, octyl), 0.91-0.79 (m, 9H, octyl).

13

C{1H} NMR (100 MHz, CDCl3): δ 152.5, 151.7 (-C),

151.0, 143.8, 143.2, 140.8, 140.5, 133.4, 128.9, 128.2, 127.3, 127.0, 126.2, 125.7, 124.8, 123.5, 120.5, 118.7, 115.8 (aromatic rings), 85.1, 83.4, 77.6, 76.3 (C, ethynyl) 68.5 (-NCH2, octyl), 67.2 (-CH2-O), 55.5, 40.5, 31.9, 30.1, 29.3, 23.9, 22.7 (CH2, octyl), 14.2 (CH3, octyl). UV (CHCl3): λmax(ε): 385 nm (2.89 × 104 M-1cm-1), 542 nm (2.21 × 104 M-1cm-1); PL(CHCl3): λem(λex): 571, 618, 671 (400) nm; Tetradetector GPC data: Mn = 18.7 kDa, Mw = 26.3 Da, PDI = 1.41; Td (°C) = 305. P4: 1H NMR (400 MHz, CDCl3): δ 7.84-7.35 (m, 7H, aromatic), 4.83-4.68 (m, 6H, -N-CH2and Th-CH2-O-), 4.05 (m, 2H, -O-CH2-CF2-), 2.17-1.99 (m, 4H, octyl), 1.26-1.09 (m, 20H, octyl), 0.82-0.72 (m, 6H, octyl). 19F{1H} NMR (377.3 MHz, CDCl3): δ -80.7, -119.2, -121.9, -122.7, -123.1, -124.9, -126.1.

13

C{1H} NMR (100 MHz, CDCl3): δ 145.2, 143.7, 141.3,

141.1, 140.7, 140.3, 139.5, 132.9, 132.7, 132.0, 128.9, 124.7 (aromatic rings), 123.5-114.3 (multiplets, C, fluoroalkyl), 111.8, 110.2, 109.6, 109.3 (C, ethynyl), 68.7 (-NCH2, octyl), 67.8 (-CH2-O), 66.7-66.4 (multiplet, -O-CH2-), 55.4, 53.6, 43.4, 34.0, 32.1, 30.8, 29.9, 29.2, 29.1, 22.8 (-CH2, octyl), 14.2 (-CH3, octyl). UV (CHCl3): λmax(ε): 397 nm (3.26 × 104 M-1cm-1), 586 nm (3.56 × 104 M-1cm-1); PL(CHCl3): λem(λex): 540, 583, 646 (400) nm; Tetradetector GPC data: Mn = 20.5 kDa, Mw = 30.3 Da, PDI = 1.48; Td (°C) = 315.

24

Acknowledgements Authors acknowledge DST, Govt. of India (DST/TM/SERI/FR/193) for the financial support and fellowship of SKR. SKP thanks IIT Kharagpur for funding the purchase of tetradetector GPC through Competitive Research Infrastructure Seed Grant (SGDRI, IIT/SRIC/CHY/NPA/2014-15/81) grant. DG acknowledges IIT KGP for doctoral fellowship. Dr.Surajit Ghosh and Prof.JayantaChakraborty (Department of Chemical Engineering, IIT Kharagpur) are specially acknowledged for helpful discussion in device fabrication and IPCE measurement. References 1. (a) S. Ikhmayies, Advances in silicon Solar cells. Spinger, Switzerland, 2018. (b) J. Liu, Y. Yao, S. Xiao, Review of Status Developments of High-Efficiency Crystalline Silicon Solar Cells. J. Phys D appl. Phys.51 (2018) 123001–123013. (c) C. Battaglia, A. Cuevas, S. De Wolf, High-Efficiency Crystalline Silicon Solar Cells: Status and Perspectives. Energy Environ. Sci.9 (2016) 1552–1576. (d) M. Konagai, Present Status and Future Prospects of Silicon Thin-Film Solar Cells. Jpn. J. Appl. Phys.50 (2011) 030001_1-030001_12. (e) M. A. Green, The Path to 25% Silicon Solar Cell Efficiency: History of Silicon Cell Evolution. Prog. Photovoltaics Res. Appl.17 (2009) 183–189. (f) J. Zhao, A. Wang, M. A. Green, 24.5% Efficiency Silicon PERT Cells on MCZ Substrates and 24.7% Efficiency PERL Cells on FZ Substrates. Prog. Photovoltaics Res. Appl.7 (1999) 471–474. (g) A. Goetzberger, J.Knobloch, V.Voss, Crystalline Silicon Solar Cells, Wiley, 1998. 2. (a) T. Feurer, P.Reinhard, E.Avancini, B.Bissig, J.Löckinger, P. Fuchs, R. Carron, T. P. Weiss, J.Perrenoud, S.Stutterheim, S.Buecheler, A. N. Tiwari, Progress in Thin Film CIGS Photovoltaics–Research and Development, Manufacturing, and Applications. Prog. Photovoltaics Res. Appl. 25 (2017) 645-667. (b) T. D. Lee, A. U. Ebong, A Review of Thin Film Solar Cell Technologies and Challenges. Renew. Sustain. Energy Rev.70 (2017) 1286–1297. (c) J. Ramanujam, U. P. Singh, Copper Indium Gallium Selenide Based Solar Cells - a Review. Energy Environ. Sci.10 (2017) 1306–1319. (d) N. Shahzad, Fabrication of CdTe Based single crystal & polycrystalline Solar Cells. Lambert Academic Publishing, 2015. (e) N. Amin, M. A. Islam, High Efficiency Ultra Thin Cadmium Telluride (CdTe) Solar Cells: Alternative Approaches with Novel Window Layers. Lambert Academic Publishing, 2013. (f) S. E. Habas, H. A. S. Platt, M. F.A. M. Hest, D. S.Ginley, Low-Cost Inorganic Solar Cells : From Ink To Printed Device. Chem. Rev.110 (2010) 6571–6594.

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37

TOC

Diketopyrrolopyrrole-alt-thiophene and perylene-diimide-alt-thiophene based D-π-A low bandgap copolymers have been synthesized, and successfully employed as effective sensitizers in dye sensitized solar cells (DSSC) with optimized power conversion efficiency (η) upto3.03% for the best device.

38

Diketopyrrolopyrrole/Perylene-diimide and Thiophene based D-π-A Low Bandgap Polymer Sensitizers for Application in Dye Sensitized Solar Cells Dipanjan Giri,a Sagar Kumar Rauta and Sanjib K. Patra*a

a

Department of Chemistry, Indian Institute of Technology Kharagpur, Kharagpur-721302, WB, INDIA, E-mail: [email protected]; Tel: +913222283338

Highlights

•

Well defined diketopyrrolopyrrole/perylene-diimide and thiophene based copolymers.

•

Low bandgap D-π-A polymers with broad absorption profile in visible region.

•

PCE up to 3.03% for the polymer based DSSC solar cells.

Indian Institute of Technology Kharagpur Kharagpur-721302, INDIA Dr. Sanjib K. Patra Associate Professor Department of Chemistry

Tel:

+91-3222-283338 (O) +91-3222-283339 (R) Email:[email protected] Homepage:https://www.skplabiitkgp.com/ Date: 6th November, 2019

To The Editor Dyes and Pigments Manuscript ID: DYPI_2019_2153 Dear Prof. Sylvain Achelle,

The authors declare no conflict of interest. All the authors have approved submission of the revised manuscript. We look forward to your positive response on final acceptance of our revised manuscript.

Sincerely,

Dr. Sanjib K. Patra

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