Thienothiophene-benzoxadiazole based conjugated copolymer for organic photovoltaic application

Materials Today Communications 11 (2017) 132–138

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Thienothiophene-benzoxadiazole based conjugated copolymer for organic photovoltaic application Ranjith Kottokkaran ∗ , Vishnumurthy A, Arul Varman Kesavan, Vinila N V, Praveen C. Ramamurthy Department of Materials Engineering, Indian Institute of Science, Bangalore 560012, India

a r t i c l e

i n f o

Article history: Received 28 February 2017 Accepted 21 March 2017 Keywords: Conducting polymer Copolymer Organic solar cells Bulk heterojunction

a b s t r a c t In present study a novel conjugated polymer of alternating thienothiophene and benzoxadiazole based units (poly(TT-alt-DTOT)) is synthesized and characterized. The polymer shows excellent thermal stability, good solubility and exhibit an optical band gap of ∼1.81 eV. Density functional theory (DFT) calculation of poly(TT-alt-DTOT) indicates that highest occupied molecular orbital (HOMO) is completely delocalized throughout the polymer and the lowest unoccupied molecular orbital (LUMO) is localized mainly along its acceptor part. Furthermore bulk heterojunction solar cells were fabricated using poly(TT-alt-DTOT) as donor and fullerene derivative (PC60 BM and PC70 BM) as the acceptors. Preliminary device study of the fabricated device shows a power conversion efficiency of 2.06% with a short circuit current density of 8.8 mA cm−2 , an open circuit voltage of 0.76 V. This polymer can be further optimized for efficient photovoltaic material. © 2017 Elsevier Ltd. All rights reserved.

1. Introduction Polymer or organic solar cells are of great interest due to the potential of large area flexible devices through easy processing and cost effective production [1–3]. The most successful organic solar cell, bulk heterojunction solar cells (BHJ) consist of interpenetrating networks of donor and acceptor. Mostly conjugated polymers and fullerene derivatives such as [6,6]-phenyl-C61-butyric acid methyl ester (PC60 BM) and [6,6]-phenyl-C71-butyric acid methyl ester (PC70 BM) are used as donors and acceptors respectively [4,5]. For the better device performance tuning of the chemical properties of conjugated polymers are one of the important aspects. Optical and electrochemical band gap tuning of the conjugated polymers have been achieved by using various architectures [6]. Band-gap engineering of the conjugated polymers can be carried out by either raising the HOMO level or lowering the LUMO level. This can be achieved by using suitable donor-acceptor (D-A) architecture [7–10]. Incorporation of various donor and acceptor moieties to the polymeric backbone will perturb the molecular orbitals and effectively alter the band gap [11].

∗ Corresponding author at: Department of Materials Engineering, IISc, Bangalore 560012, India. E-mail address: [email protected] (R. Kottokkaran). http://dx.doi.org/10.1016/j.mtcomm.2017.03.003 2352-4928/© 2017 Elsevier Ltd. All rights reserved.

There are many strategies that have been proposed to control the electronic energy levels of conjugated polymers including D-A approach [12–14]. Here, in addition to the D-A architecture, quinoidal character of the polymeric backbone can be increased by incorporating fused hetero aromatic systems [15,16]. Such a moiety, thieno[2,3-b]thiophene (TT) is an interesting building blocks for conjugated polymers because TT moiety stabilizes the quinoidal structure of the polymeric back bone and hence reduces the band gap of the polymer [17]. Due to the presence of rigid and coplanar fused rings, TT moieties possess highly delocalized ␲-electron system and strong intermolecular ␲–␲ stacking. It is reported that the field effect mobility of conjugated polymers having TT moieties are higher due to the formation of large crystalline domains [18]. By considering all these facts such as higher mobility and lower band gap, TT moiety can act as a good donor unit for the design of efficient conjugated polymer. In order to alter the HOMO-LUMO gap various acceptor moieties such as benzothiadiazole, quinoxaline, benzoxadiazole (BO) have been used for copolymerization [19,20]. Benzothiadiazole is a medium electron-withdrawing moiety, which on copolymerization results coplanar low band gap polymers with deeper HOMO levels [21,22]. Moreover, BO moieties contain highly electronegative oxygen atom, rather than sulfur in benzothiadiazole moiety [23,24]. Due to this, BO moieties can lower both the HOMO and LUMO energy levels. Hence, it is possible to obtain air stable conjugated polymers by incorporating BO moiety as the electron-withdrawing

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O H3 C

+

K2CO3, DMF K

S S

-

S

+

K K CO , DMF 2 3

O

0o C

H3 C

+ S

133

O

O

S

S

O

CH3

H3C

O

O O

CH3

CH3

CH3 KOH/H2O

H 3C

HCl S

S

S

S

Br

180 oC

CH3 S

OH

Cu2O, Quinoline O

NBS, Chloroform H 3C

O

S

S

HO

o Br 0 C, 12 h

CH 3

H 3C

CH 3

H3 C

SnBu 3

Pd(0) 12 h S

S

S

S o

BuLi, -78 C

Bu 3 Sn

S

S

S

S

SnBu 3

Tributyl stannyl chloride H3 C

CH 3

H3 C

CH 3

Scheme 1. Synthesis of Monomer 1.

unit in the polymeric back-bone. However, the solubility of BO polymers is less in common organic solvents. In order to enhance the solubility the dodecyloxy chains in the BO ring can be used. Most of the reported BO polymers possess lower molecular weight due to the steric hindrance of the BO molecule during the polymerization. Thiophene spacers can reduce the steric hindrance and results high molecular weight conjugated polymers. In this work a novel conjugated polymer was designed to enhance charge carrier mobility, higher crystallinity, desirable HOMO-LUMO levels and enhanced solubility and there by photovoltaic performance. An alternating copolymer of TT based donor (3,4-dimethyl-2,5-di(thiophen-2-yl)thieno[2,3-b]thiophene) and BO based acceptor, DTOT (5,6-didodecyloxy-4,7-di(thiophene-2yl) −2,1,3-benzoxadiazole) was synthesized (poly(TT-alt-DTOT)). Bulk hetero-junction solar cells were fabricated using poly(TTalt-DTOT) as donor and PC60 BM or PC70 BM as acceptor and photovoltaic parameters are evaluated. 2. Experimental 2.1. Materials Ethyl bromoacetate, Acetylacetone, Copper(II) oxide (Cu2 O), N-Bromosuccinimide, 2-tributylstannyl thiophene, Tetrakis (triphenylphosphine) palladium (Pd(PPh3 )4 ), Tributylstannyl chloride, 1-Bromododecane, Sodium azide, tetrabutylammonium bromide, triphenylphosphine and tetrabutylammonium hexafluorophosphate (TBAPF6 ) were purchased from Sigma Aldrich and used without further purification. n-BuLi solution (1.6 M in hexane) was purchased from Across chemicals and used as such. Solvents dichloromethane (DCM), chloroform and dimethylformamide (DMF) were distilled before use. 2.2. Characterization 1 H NMR of compounds was carried out by Bruker ultra shield 400 MHz NMR. Thermo LCQ Deca XP MAX was used to record the mass spectrum of the compound. FTIR spectra of polymers were obtained on a Thermo-Nicolet 6700 spectrometer, using KBr pellets. Gel permeation chromatography (GPC) was used to obtain the molecular weight of the polymers and was determined by Waters make GPC instrument with reference to polystyrene standards

and THF as the eluent. UV–vis absorption spectra of the polymer were measured on a Perkin Elmer Lambda 25 UV–vis spectrometer. CH660D CH instruments with a Faraday cage set up was used for the electrochemical studies of the synthesized polymers. Thermogravimetric analysis (TGA) measurements were conducted with NETZSCH STA 409 analyzer under argon at a scan rate of 10 ◦ C/min in an alumina crucible. Photovoltaic (PV) characteristics were carried out by Oriel sol 3ATM solar simulator and Keithley 2420-C 3A source meter. 3. Results and discussion 3.1. Synthesis Alternating copolymer poly(TT-alt-DTOT) was synthesized by the stille polymerization of the monomers. Synthetic steps of the intermediate compounds and monomers are as shown in Schemes 1 and 2 and are described in the supporting information. Schematic of poly (TT-alt-DTOT) is as shown in Scheme 3. The synthesized polymer has number-average molecular weight (Mn) of 16460 with polydisperse index (PDI) of 1.35. 3.2. Density functional theoretical (DFT) calculations The structural and electronic properties of the polymer at the ground state energy were studied by computational calculations using Gaussian 03 software. The quantum chemical calculation was carried out using BP86 functional and the 6-311++ g(d,p) as the basis set. After optimizing the geometrical structure of the polymer, time dependant density functional theory (TDFT) calculations were carried out to predict the absorption properties of the polymer. All side chains in the polymer were replaced by methyl group in order to simplify the calculation. Optimized geometrical structure of the polymer is as shown in Fig. 1. The HOMO of the polymer is completely delocalized throughout the polymer and the LUMO is localized mainly along the acceptor part of the polymer (Fig. 2). The HOMO and LUMO energy level obtained from this calculation are −4.83 eV and −3.49 eV respectively. The calculation was carried out for only one repeating unit. This could be the reason for observed small difference between the values of electrochemical and theoretical calculations.

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R OH DMF, K2CO3

HO

O

O

R R Fumming HNO3

O

O

R

O

O

R

NaN3, TBAB

AcOH

C12H25Br

R

PPh3, Toluene NO 2

O 2N

N

N O

DCM, AcOH R

R O

R

O

R

O

O

R

Br

S

S N

Br

S S

Br

N

Br

SnBu 3

S

N

N

N

N O

O

O

R

O

Pd(pph3)4,Toluene

NBS CHCl3,AcOH

O

Br2

Scheme 2. Synthesis of Monomer 2.

R O Bu 3Sn

S

S

S

S

S

Br

S

CH3

N

Br

N O

Toluene

Pd(PPh3)4

H 3C

CH3 R

R

O H

R

SnBu 3

+ H3C

O

S

S

S

S

O

S

S

H n

N

N O

Scheme 3. Synthesis of poly (TT-alt-DTOT).

3.3. Optical characterization

Fig. 1. Optimized geometry of the polymer.

UV–vis characterization of poly(TT-alt-DTOT) was carried out in both solution and film state. A very dilute solution of the polymer in o-dichlorobenzene was used for the measurements. Thin film of the polymer was fabricated by spin coating the solution on a glass plate. UV–vis absorption spectra of poly(TT-alt-DTOT) are as shown in Fig. 3. Two characteristics absorption peaks are

Fig. 2. Optimized molecular orbital surfaces of the (a) HOMO and (b) LUMO energy level of the molecule.

R. Kottokkaran et al. / Materials Today Communications 11 (2017) 132–138

135

1000000

690 nm

Intensity (a.u)

800000

600000

400000

200000

0 600

650

700

750

800

850

Wavelength (nm)

observed in Fig. 3, which is the characteristics property of donoracceptor type polymers. Absorption peak due to ␲-␲* transition of the conjugated backbone of the polymeric chain is observed at about 386 nm for both solution and film. Absorption peak due to intra molecular charge transfer (ICT) between the acceptor and donor moieties in the copolymer is observed at about 515 nm for the solution state which red shifted to ∼534 nm for the film. This shows that polymeric chains are well aligned in the film state as compared to solution. Optical band gap of the polymer is determined by considering the absorption onset as 683 nm and is determined as 1.81 eV. Time dependent DFT (TDFT) calculation was also carried out to simulate the absorption spectrum of the molecule. Simulated absorption spectrum of the polymer is also shown in Fig. 3. As explained in the experimental results, theoretical calculation also shows two distinct absorption bands. Shorter wavelength absorption observed at about 337 nm and higher wavelength absorption peak is observed at about 473 nm. The theoretical calculation was carried out for a single monomeric unit and the alkyl chains are not considered during the calculation. This could be the reason for the difference in the experimental and theoretical absorption peaks whereas the trend of absorption matches well with experimental results. Fluorescence emission study of poly(TT-alt-DTOT) was carried out for a dilute solution of the copolymer in chloroform. Fluorescence spectrum of the copolymer excited using its absorption maxima is as shown in Fig. 4. Polymer is found to be highly emissive and emission maxima is found to be 690 nm with a stoke shift of 175 nm. Time Resolved Emission: Time resolved emission experiments were carried out in the dilute chloroform solutions. Polymer was excited with its absorption maxima and its time resolved emission was measured. Time resolved Fluorescence emission spectrum of the polymer is as shown in Fig. 5. The lifetime of the excited species was found to be 1.5 × 10−11 s. 3.4. Electrochemical studies Electrochemical studies of poly(TT-alt-DTOT) were carried out by cyclic voltammetry studies using cetonitrile was used as the solvent and TBAPF6 (0.1 M) as the supporting electrolyte. Platinum (Pt) wire was used as the counter electrode and non-aqueous Ag/AgCl electrode was used as the reference electrode. The polymer in chloroform solution was coated on the Pt working electrode and carried out the measurements with a scan rate of 100 mV/sec. The redox

Fig. 4. Fluorescence spectra of dilute solution of the co-polymer in chloroform solvent. 50000

40000

Intensity (a.u.)

Fig. 3. UV–vis spectra of the synthesized polymer solution and thin film.

30000

20000

10000

0

400

600

800

1000

1200

1400

Wavelength (nm) Fig. 5. Time resolved fluorescence spectrum of the polymer.

potential of ferrocene/ferrocenium was measured as 0.11 V to the Ag/AgCl electrode. The redox potential of ferrocene/ferrocenium was assumed to be −4.80 eV to vacuum [25]. HOMO energy levels of the polymer is calculated by using the empirical formula [26]. HOMO = − (4.80 + Eox, onset) eV LUMO = (HOMO + Eg, opt) eV Cyclic voltammogram of the polymer is as shown in Fig. 6. The HOMO level was calculated as −5.28 eV from the onset of oxidation potential. However, the polymer did not show any sharp change in potential during the reduction process. Hence, by considering the optical band gap, LUMO of the polymer was determined as −3.47 eV, which agrees with the theoretical calculation. This HOMO-LUMO energy level shows that by the incorporation of BO moiety, deeper HOMO levels can be achieved and hence obtained an air stable polymer. 3.5. Thermal stability studies Thermal stability studies of the polymer were carried out by thermo gravimetrical analysis under nitrogen atmosphere. Thermogram of the copolymer is as shown in Fig. 7. The slight weight

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Fig. 6. Cyclic voltammogram of the polymer.

Weight reduction (%)

100

90

80

Fig. 8. Capacitance-voltage and inset shows Mott-Schottky characteristic of the (a) poly(TT-alt-DTOT): PC60BM and (b) poly(TT-alt-DTOT):PC70BM bulk hetero junction solar cell.

70

100

200

300

400

500

Temperature (°C) Fig. 7. Thermo gravimetric analysis of the copolymer.

loss around 100 ◦ C is associated with the materials may be due to moisture loss. The decomposition temperature of the copolymer is observed to be >300 ◦ C. 5% weight loss of the polymer is found to be at ∼290 ◦ C which indicates excellent thermal stability of the copolymer. This excellent thermal stability of the polymer could be due to the rigid polymeric backbone. No thermal transition was observed during differential scanning calorimetric measurement, which also could be due to the rigid backbone of the polymer chain.

3.6. Device fabrication The bulk hetero-junction photovoltaic devices were fabricated using polymer with two different types of acceptor molecule such as PC60 BM and PC70 BM. The blend solutions of poly(TT-alt-DTOT) and fullerene derivatives were prepared in the optimal weight ratio 1:1 with 20 mg mL−1 concentration in dichlorobenzene. The hole transport layer PEDOT:PSS was spin casted over cleaned ITO and annealed at 110 ◦ C for 10 min in the ambient condition. The as deposited film then transferred into the nitrogen filled glove box for the active layer deposition and metal contact fabrication. Active layer was spin coated with 1000 rpm for 40 s followed by annealing at 140 ◦ C for 15 min. To complete the device structure, ∼100 nm aluminum (Al) metal cathode contact was deposited by thermal evaporation technique with the evaporation rate of 1 Å per second under vacuum (1 × 10−6 mbar). The thickness of the active layer was ∼100 nm.

3.6.1. Mott-Schottky characteristics All the voltage dependent capacitance measurement was carried out at 20 mV and with 20 kHz frequency. Carrier concentration and built-in voltage (Vbi) was computed using Mott-Schottky relation. Fig. 8(a) and (b) shows the capacitance-voltage characteristics of the poly(TT-alt-DTOT):PC60 BM and poly(TT-alt-DTOT):PC70 BM active layer thickness respectively and the inset show their corresponding Mott-Schottky characteristics curve. The computed Vbi for poly(TT-alt-DTOT):PC60 BM and poly(TT-alt-DTOT):PC70 BM blend device under dark condition was found to be 740 ± 10 mV and 770 ± 10 mV respectively. Similar way the Vbi was calculated for the device under illuminated condition and calculated value is 720 ± 10 mV and 740 ± 10 mV. By assuming dielectric constant value (␧) as 3 for poly(TT-alt-DTOT):PC60 BM active layer, acceptor carrier concentration was calculated and it was found to be (2.1 ± 0.08) × 1016 cm−3 in dark condition and (2.9 ± 0.07) × 1016 cm−3 under illumination. Using the same ␧ value for poly(TT-alt-DTOT):PC70 BM BHJ, acceptor concentration was calculated and the obtained value is (6.1 ± 0.06) × 1016 cm−3 and (6.5 ± 0.09) × 1016 cm−3 respectively device under dark and light condition. 3.6.2. Capacitance-frequency (C-f) and density of state (DOS) characteristics Fig. 9(a) and (b) shows frequency dependent capacitance and inset shows DOS spectra. Frequency dependent capacitance spectra of the device were measured. Depletion capacitance at lower frequency (4 kHz) was calculated and it was found to be (142.30 ± 0.4) × 10−10 Fm−2 and (131.98 ± 0.4) × 10−10 Fm−2 respectively under dark and light condition for PC60 BM acceptor. Computed capacitance at 4 kHz was (165.24 ± 0.4) × 10−10 Fm−2 and (159.26 ± 0.4) × 10−10 Fm−2 respectively for PC70 BM acceptor device under dark and illuminated condition. Reduction in capacitance at lower frequency regime is observed in both acceptor type

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137

35

EQE (%)

28 21 14

PC70BM PC60BM

7 0 300

375

450

525

600

675

Wavelength (nm) Fig. 10. External quantum efficiency spectra of poly(TT-alt-DTOT):PC70 BM and poly(TT-alt-DTOT):PC60 BM blend device.

6.0 light with PCBM(70)

-2

Current density (Am )

4.5 light with PCBM(60)

3.0

1.5

-0.9 Fig. 9. Frequency dependent capacitance spectra of (a) poly(TT-alt-DTOT):PC60 BM and (b) poly(TT-alt-DTOT):PC70 BM active layer and inset shows their corresponding density of states plot.

-0.6

-0.3

0.0 0.0

0.3

0.6 0.9 Voltage (V)

Fig. 11. Semi log current verses voltage of characteristics of the fabricated device.

BHJ. Reduction in capacitance at lower frequency regime of BHJ under illuminated condition is associated with the de-trapping of shallow trapped charge carriers under light condition. Density of states was derived from the C(f) characteristics and is shown in the inset of C-f characteristics. Since, the depletion layer thickness in organic devices is approximately equal to the thickness of the active layer, thickness of the active layer was considered as the depletion width in the calculation. Obtained DOS is analyzed by assuming Gaussian shape defect distribution. From the Gaussian distribution of defect states disorder parameter (␴) was calculated and was found to be (71.79 ± 0.08) eV and (74.91 ± 0.07) eV respectively for PC60 BM acceptor device under dark and light condition. Similar way computed ␴ for PC70 BM acceptor device is (77.79 ± 0.06) eV and (79.59 ± 0.07) eV respectively in dark and illuminated condition. Determined position of maximum defect distribution (Xc) for poly(TT-alt-DTOT): PC60 BM active layer BHJ (422.7 ± 0.2) eV and (424.1 ± 0.2) eV respectively. Similar way computed Xc in dark and light condition for poly(TT-alt-DTOT):PC70BM active layer it is (429.0 ± 0.3) eV and (431.5 ± 0.3) eV in dark and light respectively. 3.6.3. Quantum efficiency (QE) characteristics Fig. 10 shows the external quantum efficiency spectra of poly(TT-alt-DTOT) with two different acceptor materials such as PC60 BM and PC70 BM. The obtained area under the EQE curve represents photocurrent generated region which is related to charge generation in poly(TT-alt-DTOT) and charge transfer to the acceptor material (PC60 BM and PC70 BM). In this case, the photon energy values higher than 1.81 eV is responsible for the photocurrent generation in the donor material. Calculated maximum external quantum efficiency (EQE) was (34 ± 1.3) % and (31 ± 1.2)

% respectively for poly(TT-alt-DTOT):PC60 BM and poly(TT-altDTOT):PC70 BM blend respectively. Photocurrent density of the fabricated device was calculated by integrating the EQE curve over minimum to maximum absorbed wavelength region. Measured value of photo-current density is (7.2.1 ± 0.3) mA cm−2 and (7.8 ± 0.2) mA cm−2 respectively for poly(TT-alt-DTOT):PC60 BM and poly(TT-alt-DTOT):PC70 BM active layer respectively. On comparing the EQE curve, device with PC60 BM and PC70 BM acceptor layer, it is clear that the photocurrent generation is relatively higher in PC70 BM. From the experiment it is clear that, the poly(TT-altDTOT): PC70 BM produce higher photo-current as compared with poly(TT-alt-DTOT): PC60 BM. 3.6.4. Photovoltaic characteristics For all devices, the I–V data were collected with the illumination intensity of one sun condition (1000 W/m2 ) at 1.5 AM. Fig. 12 shows the current-voltage characteristic of the fabricated device at dark and illuminated condition. The fabricated devices show power conversion efficiencies of 1.51% and 2.06% with PC60 BM and PC70 BM respectively. The device with PC70 BM as an acceptor shows 31% increment in the current density, which could be due to the efficient charge transfer between donor molecule and PC70 BM acceptor due to the well aligned energy levels. The increase in Voc from 0.72 V to 0.76 V (5%) also shows that there is a reduction in the recombination at the donor acceptor interface. The semi log(current density)-voltage behaviour with illumination is symmetrical to Y-axis (Fig. 11). Hence exciton separates into free charge carriers predominantly at the D-A interface rather than the semiconductor-metal interface. This result indicates that the

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Appendix A. Supplementary data

120 2

J(A/m )

80

dark PCBM[60] light PCBM[60] dark PCBM[70] light PCBM[70]

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.mtcomm.2017. 03.003.

40 0 0.0

-0.3

-40

References 0.3

0.6

0.9

Voltage (V)

[1] [2] [3] [4]

-80 [5]

-120 [6] [7] Fig. 12. J-V characteristics of the dark and illumination characteristics device with PC60 BM and PC70 BM.

synthesized conjugated polymer is a promising candidate for bulk hetero junction solar cells and further optimization in the device engineering will enhances the performance of the solar cell. 4. Conclusions A novel thienothiophene and benzoxadiazole based conjugated polymer, poly(TT-alt-DTOT) was successfully synthesized using palladium catalyzed Stille coupling polymerization. The synthesized polymer absorbs light up to 700 nm having HOMO-LUMO energy levels as −5.28 and −3.47 eV respectively, indicating that this is a promising candidate for organic photovoltaic application. This polymer was used as the donor material for the fabrication of BHJ solar cells with PC60 BM and PC70 BM as the acceptor materials. Preliminary device results show power conversion efficiencies of 1.51% and 2.06% with PC60 BM and PC70 BM respectively. The enhancement in the device performance with PC70 BM could be due to the effective charge transfers takes place between the donor and PC70 BM interface. Higher Voc ∼0.76 V is achieved and this could be due to the incorporation of thienothiophene moieties in the polymer backbone. Careful optimization of the processing condition and device architecture will further improve the performance of the organic solar cells. Acknowledgments This work is funded by DST NoSB/SR/S3/ME/51/2012 and technical support from Advanced Facility for Microscopy and Microanalysis (AFMM), Indian Institute of Science (IISc).

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