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Two Donor - One Acceptor (2D/1A) Random Terpolymer Comprising of Diketopyrrolopyrrole Quaterthiophene with Various Donor #-Linkers for Organic Photovoltaic Application Balasubramaniyan Sambathkumar, Narayanasastri Somanathan, Shyam Vinod Kumar Panneer, Fatale Shantanu Deepakrao, Subramaniam Sundar Kumar Iyer, Venkatesan Subramanian, Ram Datt, Vinay Gupta, and Suresh Chand J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07479 • Publication Date (Web): 07 Nov 2016 Downloaded from http://pubs.acs.org on November 8, 2016

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The Journal of Physical Chemistry

Two Donor - One Acceptor (2D/1A) Random Terpolymer Comprising of Diketopyrrolopyrrole Quaterthiophene with Various Donor π-Linkers for Organic Photovoltaic Application

B. SambathKumar, †,‡ P. Shyam Vinod Kumar, †,‡ F. Shantanu Deepak rao, § S. Sundar Kumar Iyer, §

V. Subramanian, †,‡ Ram Datt, #,∥ Vinay Gupta, *,#,‡ Suresh Chand #,‡ and N. Somanathan *,†,‡

†

Council of Scientific and Industrial Research (CSIR)-Central Leather Research Institute, Sardar Patel Road, Adyar, Chennai 600020, Tamil Nadu, India

‡

CSIR-Network of Institutes for Solar Energy, New Delhi, India

§

Department of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur 208

016, Uttar Pradesh, India ∥Academy #

of Scientific and Innovative Research, New Delhi 110001, India

Organic and Hybrid Solar Cells, Physics of Energy Harvesting Division, CSIR- National

Physical Laboratory, New Delhi-110012, India

ABSTRACT A series of terpolymer based on diketopyrrolopyrrole (DPP) and quaterthiophene (QT) connecting randomly using various electrons donating π-linker such as fluorene (P4TFDPP), thiophene (P5TDPP), phenyl (P4TPDPP) and vinyl (P4TVDPP) has been designed and synthesized via Suzuki and Stille polymerization. In this work we studied the influence of π-linker, effect on their photo physical properties, energy levels, band gap, packing nature, surface morphology, carrier mobility and photovoltaic behavior were

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analyzed using various technique and correlate with quantum chemical calculation. Bulk heterojunction (BHJ) solar cell were fabricated using these polymer as a donor materials and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor. A high molecular weight with larger absorption coefficient and high hole mobility were achieved for copolymer P4TFDPP compared to other copolymers.

The best photovoltaic’s performance were

achieved for the polymer P4TFDPP with a power conversion efficiency (PCE) of 4.9%, with Voc =0.88 V, Jsc = 9.4 mA/cm2 and FF = 59 % which is the highest PCE, Jsc and FF for fluorene-DPP copolymeric system compared to previously reported in the literature. While P5TDPP, P4TVDPP and P4TPDPP shows a PCE of 4.1%, 2.9% and 3.9% respectively. This work provides, the importance of selecting proper π- linker in random terpolymer provides an efficient approach for enhancing the PCE in polymer solar cells. INTRODUCTION The unique advantage such as light weight, flexibility, solution processability and large area device manufacturing at low-cost using various printing methodology makes bulk heterojunction (BHJ) solar cell as a promising energy resources.1-3 Generally, BHJ has an active layer consists of solid state mixture of conjugated polymer as an electron donor (ptype) and fullerene derivatives as an electron acceptor (n-type) sandwiched between two electrodes with different work function.4,5 The primary strategy for enhancing power conversion efficiency (PCE) of BHJ solar cell is to develop low band gap conjugated copolymers, should exhibit

broad spectrum coverage with high absorption coefficient,

suitable energy level with fullerene derivatives for efficient exciton dissociation and high hole mobility for charge extraction.6,7 In addition molecular weight also play a vital role in regulating conjugation length, absorption coefficient, hole mobility and morphology of the semiconducting polymer.8-10 Recently Wong and Jones et al. demonstrated that PCE increasing significantly from 4.4 to 9.4% as the number-average molecular weight increase 2 ACS Paragon Plus Environment

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from 19 kg mol

–1

to 112 kg mol

weight 136 kg mol

–1

–1

and also they found that further increasing molecular

results decreasing

PCE due to lower solubility. Thus optimum

molecular weight with good solubility guarantees for high PCE.11-12 In general, such copolymer can be synthesized using push – pull design which consists of electron donating (D) and electron accepting (A) unit placed alternating in polymer backbone.13-14 However due to strong intermolecular charge transfer between D-A, most low band gap copolymer suffer shift in absorption maxima towards visible to nearinfrared leading to weak absorption profile between 400-600nm.15 These limit the number of absorbed photon in visible range results in low current density.16 Therefore a new strategy is needed to develop ideal low band gap copolymer. On the other hand, random terpolymer strategies have been considered as an effective approach for designing ideal conjugated polymers.17 Unlike traditional D-A copolymers, random terpolymer consists of three monomeric units. The judicious selection of additional monomeric unit plays a key role in fine-tuning the absorption, HOMO, LUMO, band gap, packing ability, crystallinity, charge transport and active layer morphology.18-19 Recently Kim et al. demonstrate the change in crystalline nature of polymer with respect to the addition of third comonomer unit.20 In the same way, Jo’s group reported random terpolymer with broad light absorption and they fine tune the HOMO, LUMO energy levels and band gap by systematically varying the composition of two acceptor co-monomeric units. As a result, the corresponding terpolymer has higher efficiency than their parent polymer.21-22 In general, random terpolymer can be synthesized using two different methods: (1) Copolymerization of two electron accepting (2A) and one electron donating (1D) unit, optimize their crystalline nature, light absorption band, HOMO/LUMO position and charge carrier mobility of copolymer.23-27 (2) Copolymerization of two electron donating (2D) and one electron accepting (1A) unit shows a complementary light absorption in both visible and 3 ACS Paragon Plus Environment


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NIR helps in harvesting more number of photons result in maximizing photocurrent of the device.28-30 However, random terpolymer using 2D/1A system with high PCE are scare in the literature. Among various high efficiency promising D-A type conjugated polymers, diketopyrrolo[3,4-c]pyrrole (DPP) are used as an acceptor (A) unit due to its well conjugated rigid structure, high molar extinction coefficient, molecular planarity and electron deficient lactam group. These properties lead to DPP based polymers exhibits low band gap, high charge carrier mobility, compact molecular packing and high crystallinity.31-33 Of various donor (D) unit thiophene derivatives have been extensively used as D unit in D-A type copolymers due to its effective conjugation length, the 2D lamellar arrangement with strong π-π stacking and essential solubility for device processability. Significantly, thiophene derivatives exhibit strong absorption between 400 – 700 nm.34-36 Taking this above advantage, a combination of DPP and thiophene derivatives in a polymer backbone offers an ideal donor material for BHJ application. In this work, we designed and synthesized a series of random terpolymer (2D/1A system) by copolymerizing DPP as electron acceptor (A) and quaterthiophene as electron donor (1D) with various electron donating linker groups (2D) such as fluorene (P4TFDPP), phenyl (P4TPDPP), thiophene (P5TDPP) and vinyl (P4TVDPP) unit. These combinations offer a wide range of optical wavelength, optimized energy level and enhanced in their photovoltaic parameter. Their structure-property relationships of these four random terpolymer were systematically studied using thermal, optical, electrical, density functional theory calculation and photovoltaic behavior. EXPERIMENTAL SECTION Materials


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2-Thiophenecarbonitrile, dimethyl succinate, potassium tert-butoxide, 2-methyl-2-butanol, 5,5′-Dibromo-2,2′-bithiophene,

3-Hexylthiophene-2-boronic

acid

pinacol

ester,

2,5-

Bis(trimethylstannyl)thiophene, 1,4-Benzenediboronic acid bis(pinacol) ester, trans-1,2Bis(tributylstannyl)ethane,

tetrakis(triphenylphosphine)palladium(0),

aliquat,

9,9-

dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester and N-bromosuccinimide (NBS) were purchased from Aldrich chemical company and used without any further purification. Instrumental details 1

H and

13

C NMR spectra of compounds were recorded on a Bruker 400 MHz NMR

spectrometer. UV-Visible absorbance study was carried out on a Varian Carey 50 Bio UVVisible spectrophotometer. Elemental analyses for polymers were carried out on a Euro Vector S.P.A, Euro EA 3000 CHNS Elemental analyzer. Cyclic voltammetry measurements were carried out using a CHI 600D electrochemical workstation with a platinum disc electrode as the working electrode, Ag/AgCl electrode as the reference electrode and a platinum wire electrode as the counter electrode. Measurements in cyclic voltammetry were performed by coating a thin layer of polymer on a platinum disc electrode. Thermal stability was analyzed using a Mettler TOLEDO TGA/SDTA 851e at a heating rate of 10 °C min-1. Xray diffraction (XRD) data for polymer were obtained using a Bruker AXS D8 Advance Xray diffractometer Cu Ka wavelength was used for XRD experiments. Atomic force microscopy (AFM) was recorded for active layer d on Nova 1.0.26 RC1 atomic force microscope in semi contact mode with NT-MDT solver software. Transmission electron microscope (TEM) analysis were measured for polymer:PC71BM blend films using Philips CM 12 transmission electron microscope operated at 120 kV accelerating voltage.



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Computational Study Theoretical studies based on Density Functional Theory (DFT) calculations were carried out to understand the effect of varying different functional donor chromophores by considering dimers of each polymer as models.37 The geometries of the dimers were optimized using DFT B3LYP/6-31G* method38-39 in gas phase without any symmetry constraints. The energy profiles of these structures were then subjected to frequency calculations at the same level of theory to confirm their nature in the potential energy surface. Time Dependent Density Functional Theory (TD-DFT) was used with the PBE0/6-31+G* level40 to study the optical characteristics of the model dimers in chloroform solvent environment as implemented in Polarizable Continuum Model (PCM) 41.

Solar cell and mobility measurement The ITO-coated glass substrates were cleaned by detergent. Then it was sonicated in deionized water, acetone and isopropyl alcohol for 15 minutes and dried in a nitrogen stream. A thin layer of MoOx (10 nm) as hole transport layer was thermal evaporated in a glove box at a chamber pressure of ∼ 10−7 torr. The active layer was prepared by dissolving polymer and PCBM with the ratio of 1:2 (15 mg/ml) in chloroform/ortho-dichlorobenzene (CF/O-DCB; 94:6) co-solvent mixture. The photoactive solution was spin coated on ITO/ MoOx layer at 1000 rpm for 60s under N2 environment. The active layer thickness was approximately 130 nm. On the top of the active layer 100 nm of Al was thermally deposited at a pressure of approximately (2 × 10−6 Torr). The effective device area of the cell was 0.25 cm2. The current density – voltage characterization were studied using Keithley 2400 Source- Measure Unit. The photocurrent was measured using Xe lamp source under 1 sun AM 1.5 solar illumination.


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The Hole only device were constructed using device configuration of ITO /PEDOT:PSS (30 nm) /Active layer (50 nm) /Au (100 nm). The hole mobility was calculated from dark current density-voltage (J-V) curve by fitting the space charge limited current (SCLC) model using Mott–Gurney equation 9 = μ

8

where J is the current density, d is the film thickness of the active layer, µ is the hole mobility, ɛr is the relative dielectric constant (assumed as 2.75 for conjugated polymers) of the transport medium, ɛo is the permittivity of free space (8.854 × 10−12 Fm−1), d is the thickness of the polymer and V is the effective voltage. RESULTS AND DISCUSSION Synthesis Figure 1 illustrates the chemical structure of four terpolymer P4TFDPP, P4TPDPP, P5TDPP and P4TVDPP and Scheme 1 outlined the synthetic route for monomers and terpolymer. The DPP (M1) and quaterthiophene (M2) monomer were prepared based on literature method. 4243

The polymer P4TVDPP and P5TDPP were synthesized using Stille coupling whereas

polymer P4TFDPP and P4TPDPP were prepared by Suzuki coupling. For all the polymers the linkage group is equal to the addition of two monomers M1 and M2. The detailed synthesis of monomer and terpolymer are given in supporting information (SI). P4TFDPP, P4TPDPP and P5TDPP exhibit good solubility in common organic solvent such as tetrahydrofuran, chloroform, chlorobenzene and o-dichlorobenzene, whereas P4TVDPP shows moderate solubility in room temperature. Gel permeation chromatography (GPC) was carried out to find the weight-average molecular weight (Mw), number-average molecular weight (Mn), and polydispersity index (PDI) in tetrahydrofuran solution relative to polystyrene standard and the results are tabulated in Table 1. Among all the random



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copolymer P4TFDPP shows slightly higher number-averaged molecular weight compared to analogous copolymer probably due to dihexyl fluorene linker unit helps in increasing solubility and degree of polymerization. While the polymer with vinyl as linker shows relatively low molecular weight due to limited solubility. Geometry To investigate the structure property relationship of terpolymer ground-state geometry optimizations was performed with two repeating unit (n=2) using DFT B3LYP/6-31G* method. The optimized geometry of dimers is shown in Figure 2. Notably P4TFDPP and P4TPDPP show highly twisted conformations and their dihedral angles vary between 19° to 25° when compared to P5TDPP and P4TVDPP dimers for which the angles range from 1° to 10° between monomer thiophene and linker unit (fluorene, phenyl, thiophene and vinyl). The large dihedral angle for P4TFDPP and P4TPDPP probably due to its bulky nature of fluorene and benzene units in the polymer backbone. Optical Properties The optical properties of the terpolymer were analyzed using ultraviolet-visible (UV-vis) absorption spectroscopy in dilute chloroform solution and spin coated films on a quartz substrate. Their normalized absorption spectra and absorption coefficient in thin film are shown in Figure 4 and their corresponding optical parameters are listed in Table 1. All the terpolymer exhibit two absorption band, the first absorption band at short wavelength region attributed to localized π–π* transitions (LT) of the polymer backbone. The second absorption band at higher wavelength region corresponds to intramolecular charge transfer (ICT) from donor to acceptor. Similar absorption trend is noticed in both solution and thin film absorption spectra. However compared to solution, LT and ICT band in thin film are strongly red shifted due to strong intermolecular interaction and more aggregated in the solid state. As 8 ACS Paragon Plus Environment

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noted in absorption spectra, P4TFDPP possess a narrow absorption band with absorption maxima at 642 and 659 nm in solution and solid state, owing to high aromaticity, weak electron donating and large dihedral angle with adjacent monomer which affect the planarity and conjugation of polymer backbone.44 In comparison when phenyl unit was used ~100 nm red shift with absorption maxima 755 nm is observed in thin film with respect to fluorene unit probably due to its increase in conjugation of phenyl unit compared to fluorene. Replacing phenyl by thiophene leads to a red shift in a longer wavelength region with absorption maxima at 834nm in thin film explains the increasing electron donating nature of thiophene. When vinyl unit was used, broad and longer wavelength was observed in both solution and thin film with absorption 895 nm in the thin film possibly due to high degree of coplanarity in polymer backbone leads to strong π–π interchain interactions.45 It is worth noting that among all the polymer P4TFDPP exhibit maximum absorption coefficient (5.7 X 104 cm-1, 659 nm) which is higher than that of P4TPDPP (5.3 X 104 cm-1, 754 nm), P5TDPP(4.6 X 104 cm-1, 752 nm) and P4TVDPP(4.1

X

104 cm-1, 804 nm) probably due to high molecular weight which

strongly effect the intermolecular packing and conjugation length. The optical band gaps of polymer were estimated using onset of absorption in thin film. The optical band gaps of P4TFDPP, P4TPDPP, P5TDPP and P4TVDPP are 1.65, 1.42, 1.21 and 1.18 eV respectively. This illustrated that addition of the third comonomer on terpolymer system able to fine-tune the absorption ability. To gain more insight the spectroscopic behaviour of random terpolymer TDDFT study was performed. Table 2 summarized calculated excited-state vertical transition energies, oscillator strengths, and transition electronic configurations. All the modelled oligomers exhibit S0 → S1 transition principally a HOMO →LUMO transition (54%-88%), even if other close lying valence molecular orbitals (HOMO−1, LUMO+2 and LUMO+3) also involves in the excitation. The simulated absorption spectra of the four polymers using 9 ACS Paragon Plus Environment


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TD-DFT calculations on model dimers are shown in Figure 5. The results show a broad optical profile for these systems covering the entire region from 400 to 900 nm. Similar to the experimental results, all the polymers show two major absorption transitions, one at the higher energy region and the other at the lower energy band. The λmax of the four systems ranges from about ~643 to 843 nm which are in the order of P4TFDPP < P4TPDPP < P5TDPP < P4TVDPP. Electrochemical properties In order to understand the redox nature and estimate the HOMO/LUMO energy levels and electrochemical band gap of terpolymer, cyclic voltammetry (CV) was employed. Figure 6 displays the cyclic voltammogram of terpolymer films on the platinum electrode as working electrode in 0.1 mol/L tetrabutylammonium hexafluorophosphate (Bu4NPF6) acetonitrile solution. For calibration ferrocene/ferrocenium ion (Fc/Fc+) redox couple was used as an external standard with a potential of 0.40 V against (Ag/Ag+) potential assuming redox potential of Fc/Fc+ has an absolute energy level of -4.80 eV to vacuum. The HOMO, LUMO energy level and electrochemical band gap are estimated from the first onset oxidation potential (Eox) and the reduction potential (Ered) using the following equation.46 HOMO = −e(Eox + 4.4) (eV)

(1)

LUMO = −e(Ered + 4.4) (eV)

(2)

Egec = LUMO-HOMO (eV)

(3)

The electrochemical parameters of terpolymer are summarized in Table 2. The HOMO energy level of terpolymer P4TFDPP, P4TPDPP, P5TDPP and P4TVDPP are calculated to be -5.46, -5.25, 5.11 and -5.04 eV, respectively. Especially fluorene substituted terpolymer exhibit deep HOMO level -5.46 eV than the electron donating thiophene (-5.11 eV) and vinylene (-5.04 eV) counterpart respectively. These deep lying HOMO energy level helps in 10 ACS Paragon Plus Environment

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enhancing Voc, since it is directly proportional to HOMO of conjugated donor and LUMO of fullerene derivatives.47-49 Thus higher Voc is expected from P4TFDPP. Meanwhile HOMO value of P4TPDPP has -5.25 eV appear in-between fluorene and thiophene counterpart respectively. This clearly explains that HOMO of the polymer is mainly controlled by electron donor unit.50 The LUMO level of the terpolymer is calculated from the reduction onset value and it is found to be -3.75 eV, -3.79 eV, -3.85 eV and -3.82 eV. The LUMO value for all the terpolymer was positioned within an appropriate range and was sufficiently larger (>0.3 eV) than that of LUMO of [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM, ca. 4.3 eV) . This sufficient LUMO-LUMO offset assures efficient dissociation of excitons and charge transfer from terpolymer to fullerene acceptor.51 Finally, the electrochemical band gap (Egec) were calculated based on HOMO and LUMO energy difference and it is shown in Table 3. The Egec calculated from CV measurement is higher than Egop from absorption spectroscopy probably due to interface barriers for charge injection between polymer and electrode surface.50 These demonstrate that a simple donor structural modification helps in fine tuning HOMO, LUMO and energy level of the polymer. The HOMO, LUMO energy levels of the terpolymer and PC71BM are shown in Figure 7. The optimized structures of the model dimers along with the FMO charge density contours are shown in Figure 8. P4TFDPP and P4TPDPP show highly stabilized HOMO levels which can be attributed to the presence highly aromatic fluorene and benzene units in these polymers. However, there is not much stabilization in the LUMO levels of these systems resulting in wide energy gaps of 2.11 and 2.07 eV respectively. In the case of P5TDPP and P4TVDPP, the HOMO energies are destabilized while the LUMO energies are stabilized which result in lower energy gaps of 1.84 and 1.74 eV respectively in these systems. The charge density maps of these donors elicit that the HOMOs of the polymers were delocalized over the whole molecule while the LUMOs are localized on the DPP unit. Exception where the LUMOs are extending over the



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Whole backbone in P4VTDPP and HOMO delocalizing on the DPP and thiophene units in the case of P4TFDPP. The above result reveals that all the terpolymer exhibit clear charge separation from HOMO to LUMO.

XRD Analysis In order to evaluate the packing nature of random terpolymer, XRD measurement was carried out in thin film spin coated on glass substrate. Their diffraction patterns are shown in Figure 9. All the four terpolymer exhibit weak diffraction peak at wide angle at 2θ =23.12°, 24.31°, 23.42° and 24.43° for P4TFDPP, P5TDPP, P4TPDPP and P4TVDPP which corresponds to the d spacing of 3.84 Å, 3.66 Å, 3.79 Å and 3.64 Å attributed to π-π stacking between polymer backbones. These results indicate that P4TVDPP exhibit strong intermolecular π-π stacking compared to P5TDPP, P4TPDPP and P4TFDPP. This effective intermolecular interaction is probably due to more planar vinyl unit compared to thiophene, phenyl and bulky fluorene unit. From Figure 9 similar XRD pattern first and second order diffraction peaks are noticed for polymer P5TDPP, P4TPDPP and P4TVDPP at small angle region which corresponds to the lamellar stacking. The first order diffraction peak at 4.98°, 4.95° and 4.88° for P5TDPP, P4TPDPP and P4TVDPP which correspond to the d spacing of 17.7 Å, 17.84 Å and 18.08 Å while the second order diffraction peak at 9.95°, 9.64° and 9.72° correspond to d spacing of 8.85Å, 9.16Å and 9.08Å respectively. Whereas P4TFDPP doesn't exhibit a distinct diffraction peak at small angle region probably due to flexible dihexyl alkyl chain in bulky fluorene group disrupts the lamellar order but not the π-π stacking.


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Hole mobility measurement To investigate the charge carrier ability of the copolymer hole-only device were measured for both pristine polymer and Polymer:PC71BM blend using single charge-carrier device configuration ITO /PEDOT: PSS (30 nm) /Polymer or Polymer: PC71BM (50 nm) /Au (100 nm). The hole mobility of the pristine polymer and Polymer: PC71BM film were estimated from the current density – voltage (J-V) curves and their log–log (J–V) plot is shown in Figure 10 and their values are listed in Table 4. In general intermixing fullerene moiety with polymer results, either decrease or increase the hole mobility due to PC71BM intercalation effect which may disrupt or enhance the polymer crystallization.52,53 In both pristine and blend film the hole mobility value decrease in the order of P4TFDPP> P5TDPP> P4TPDPP> P4TVDPP. However compared to pristine film the hole mobility value of blend film shows approximately two order of magnitude lower value, point out inhibition of hole mobility by fullerene. Overall P4TFDPP exhibit high mobility compared to other polymers, even though it exhibit large dihedral angle and poor lamellar order. Their high mobility value is probably due to high molecular weight, good polymer solubility, and favorable morphology which are consistent with AFM, TEM and photovoltaic behavior. Device testing and morphological behavior. The photovoltaic properties of terpolymer were analyzed by fabricating conventional BHJ PSCs with the device structure of ITO/MoOx (10 nm)/ POLYMER:PC71BM (130 nm)/Al (100 nm). The active layer was prepared by dissolving polymer and PC71BM (1:2 wt%) chloroform and 6 vol% O-Dichlorobenzene co-solvent with a total concentration of 15 mg/mL. Their current density – voltage (J-V) curve are shown in Figure 11 and their corresponding photovoltaic’s parameters are summarized in Table 4. Furthermore a nonlinear trend was noticed in PCE of terpolymer in the order of P4TFDPP (4.9%) > P5TDPP (4.1%) >



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P4TPDPP (3.9%) > P4TVDPP (2.9%). Interestingly P4TFDPP with fluorene linked terpolymer exhibit higher PCE 4.9% with a Voc of 0.88 V, Jsc of 9.4 mA/cm2 and a FF of 59 %. The higher Voc of the device is probably due to the presence of fluorene unit in the polymer chain which helps to obtain lower lying HOMO energy level. Since Voc of the device is directly correlate with HOMO of donor and LUMO of an acceptor. These results are well consistent with electrochemistry data. To the best of our knowledge, these are the highest PCE, Jsc and FF values compared to previously reported Fluorene – Diketopyrrolopyrrole copolymeric derivatives. 44, 54-57 In contrast PCE decreased by replacing linkage monomer units from fluorene to thiophene, phenyl and vinyl group. The decrease in PCE of P5TDPP (4.1%) and P4TPDPP (3.9%) probably due to its lower hole mobility compared to P4TFDPP. To verify the high device performance of P4TFDPP/PC71BM compared to other Polymer/PC71BM blend, external quantum efficiency (EQE) spectra is measured under optimized device condition and their spectral response are shown in Figure 11. Even though P4TFDPP exhibit a narrow spectral range from 300 – 750 nm it shows high photo response more than 50% from 400 to 700 nm. Moreover P4TPDPP, P5TDPP and P4TVDPP show a broad spectral coverage from 300 to 1000 nm. Particularly P4TPDPP shows high EQE value 46 % at 680 nm and 43 % at 750 nm clear up the high Jsc of P4TPDPP based device. In addition P4TVDPP and P5TDPP shows a photoresponce at higher wavelength for example P4TVDPP shows EQE value of 30 and 33% corresponding to 845 and 775 nm whereas P5TDPP shows 31% at 740 nm. This measured EQE trend is in good agreement with the optical absorption and photocurrents in J–V curve. To pinpoint the impact of surface morphology of the active layer on the device

performance, the morphology of polymer:PC71BM blend films were analyzed by using atomic force microscopy (AFM) based on their device configuration. The AFM height images of the blend layer are shown in Figure 12. The root-mean-square (rms) roughness 14 ACS Paragon Plus Environment

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value of blend films P4TFDPP, P5TDPP, P4TPDPP and P4TVDPP are 1.21, 1.71, 1.56 and 2.16 nm respectively. Smooth surface roughness with phase separated domains in blend film can observed for P4TFDPP, P5TDPP and P4TPDPP.58 Whereas in the case of P4TVDPP blend are homogenous with ambiguous phase separation probably due to its limited solubility and de-mixing with fullerene. In order to deeply examine the morphology of the active layer TEM analysis was performed and it is shown in Figure 13. P4TFDPP, P5TDPP and P4TPDPP show well-distinct bright and dark areas. The bright area corresponds to polymer rich domain and the darker one corresponds to phase separated PC71BM domain due to high electron scattering density.59 In particular P4TFDPP blend shows finer phase separation with interpenetrated morphology and even film thickness compared to P4TPDPP and P5TDPP blend shows height variation. We can conclude that this characteristic morphology helps in enhancing efficiency which is in good agreement with their photovoltaic performance.44 Whereas P4TVDPP blend film shows ill-defined morphology in which PC71BM are segregated as isolated nano island in the width of around 5 to 30nm clearly point out the poor miscibility between P4TVDPP and PC71BM.60 This result clearly illustrate the low PCE and mobility of P4TVDPP: PC71BM blend compared to others. CONCLUSIONS In summary, we synthesized a series of random terpolymer based on diketopyrrolopyrrole and quaterthiophene with various electrons donating linking segment such as fluorene, thiophene, and phenyl and vinyl comonomer unit. A comparative analysis is performed based on how these linking units on the polymer backbone affect their molecular weight, optical, energy level, packing nature, mobility and photovoltaic performance. We found that fluorene-containing terpolymer exhibit deep lying HOMO energy level compared to other polymers which are beneficial for getting higher Voc. Impressive PCE 4.9% with



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Voc =0.88 V, Jsc = 9.4 mA/cm2 and FF = 59 % is achieved probably due to its high molecular weight, favorable phase segregation, high mobility and better miscibility with fullerene provide good structural compatibility for device performance. Our works demonstrate that the importance of selecting the suitable π –linkers is the key factor for enhancing device performance for random terpolymer systems.

ASSOCIATED CONTENT Supporting information It contains detailed synthetic procedure for monomers and copolymers, 1H,

13

C NMR,

MALDI mass spectra. AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (+91-044-24437189) *E-mail: [email protected]

(+91-9958144324)

ACKNOWLEDGEMENTS The authors thank Council of Scientific and Industrial Research (CSIR), India, for financial support through TAPSUN- NWP 54 project. Authors would also like to thank Dr A. Ajayaghosh, J. D. Sudha, and R. Ramakrishnan Photo sciences and Photonics group, NIIST, Trivandrum, India for AFM measurement.

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Table 1. Optical, Thermal properties and Molecular weight of terpolymer

λmax (nm) Polymer

a

Solution

b

λonset

Thin film

(nm)

c

Egopt

d

Td (oC)

e

Mn(g/mol)

f

Mw(g/mol)

g

PDI

(eV)

(thin film) P4TFDPP

437,601,642

455,607,659

752

1.65

422

26,402

41437

1.57

P4TPDPP

458,637,674

513,690,755

873

1.42

418

12,506

28,857

2.3

P5TDPP

478,605,738

561,752,834

1020

1.21

367

22,488

37371

1.66

P4TVDPP

514,792,872

569,804,895

1050

1.18

386

4299

9771

2.27

a

Solution absorption in CHCl3. bAbsorption for thin films on quartz. cOptical band gap estimated from the onset of the thin film. d5% weight loss temperature measured by TGA under N2. eNumber-average molecular weight. fWeight-average molecular weight. g Mw/Mn.



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Table 2. Calculated Excited-State Transition Energies, Electronic Configurations, and Oscillator Strengths of the Main Transitions Determined by TDDFT at the B3LYP/631G(d,p) Level of Theory for the Model Systems P4TFDPP, P4TPDPP, P5TDPP and P4TVDPP Excitation

Oscillator

Energy

Strength

Model

Transitions

Main Configuration

%Contribution

S0→S1

HOMO→LUMO

54%

HOMO-1→LUMO

22%

molecule

P4TFDPP

P4TPDPP

P5TDPP

P4TVDPP

nm

eV

644

1.93

3.657

509

2.44

1.501

S0→S8

HOMO→LUMO+2

65%

479

2.59

1.875

S0→S10

HOMO-1→LUMO+3

55%

669

1.85

3.623

S0→S1

HOMO→LUMO

77%

477

2.60

1.709

S0→S10

HOMO-1→LUMO+2

34%

HOMO→LUMO+3

31%

HOMO-1→LUMO+3

18%

771

1.61

4.900

S0→S1

HOMO→LUMO

82%

517

2.40

1.162

S0→S9

HOMO-1→LUMO+2

49%

HOMO→LUMO+3

34%

843

1.47

4.838

S0→S1

HOMO→LUMO

88%

514

2.41

0.839

S0→S10

HOMO-1→LUMO+2

53


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Table 3. Electrochemical properties of terpolymer

Polymer

Eoxonset Eredonset

a

P4TFDPP

1.06

-0.65

-5.46

-3.75

1.71

P4TPDPP

0.85

-0.61

-5.25

-3.79

1.46

P5TDPP

0.71

-0.55

-5.11

-3.85

1.26

P4TVDPP 0.64

-0.58

-5.04

-3.82

1.22

a

HOMO

a

LUMO

b

Egec(eV)

Calculated according to the formula HOMO/LUMO =-e(Eox/red + 4.40) (eV).

b

Egec =( LUMO-HOMO)

Table 4. Photovoltaic properties of Polymer/PC71BM under optimized condition and hole mobility under dark condition.

Polymer

Voc (V)

FF

Jsc

PCE

(mA/cm2)

µh (cm-2 V-1 S-1)

µh (cm-2 V-1 S-1)

(Pristine)

(Blend)

P4TFDPP

0.88

9.4

59

4.9%

7.89×10−5

3.54×10−5

P4TPDPP

0.72

10.05

50

3.8%

1.44×10−5

7.11×10−6

P5TDPP

0.77

8.57

61

4.1%

4.31×10−5

1.83×10−5

P4TVDPP

0.69

9.43

44

2.9%

3.91×10−6

1.13×10−6



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Figure Captions: Figure 1: Chemical structures of random terpolymer P4TFDPP, P4TPDPP, P5TDPP and P4TPDPP (x:y=1:1).

Figure 2: Optimized geometry of P4TFDPP, P4TPDPP, P5TDPP and P4TVDPP modeled oligomers by DFT/B3LYP/6-31G*.

Figure 3: TGA thermograms of terpolymer P4TFDPP, P5TDPP, P4TVDPP and P4TPDPP recorded at a heating rate of 10 °C min-1 under N2 atmosphere. Figure 4: Absorption spectra of Terpolymers in (a) dilute chloroform solution (b) thin film Figure 5: Simulated absorption spectra of the model dimers at TD-DFT PBE0/6-31+G* level of theory. Figure 6: Cyclic voltammograms of terpolymer in thin film. Figure 7: Energy level diagram of terpolymer and PCBM. Figure 8: Optimized geometries of the dimers and their respective FMOs at B3LYP/6-31G* level of theory. Figure 9: XRD spectra of terpolymer in thin film. Figure 10: Double logarithmic plot of the current density (J) versus applied voltage (V) for pristine Polymer and Polymer: PC71BM blends. Figure 11: (a) J-V curves and (b) EQE spectra of Polymer:PC71BM blend under optimized condition. Figure 12: AFM image of P4TFDPP (a), P4TPDPP (b), P5TDPP (c) and P4TVDPP (d) blended with PC71BM under optimized condition. Figure 13: TEM image of P4TFDPP (a), P4TPDPP (b), P5TDPP (c) and P4TVDPP (d) blended with PC71BM under optimized condition.


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Scheme 1

Synthetic route for monomer and random polymers P4TFDPP, P5TDPP, P4TVDPP and P4TPDPP.



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Figure 1


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Figure 2

Figure 3



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Figure 4


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Figure 5

Figure 6



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Figure 7


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Figure 8



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Figure 9


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Figure 10



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Figure 11


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Figure 12

Figure 13



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TOC graphic


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