A triazoloquinoxaline and benzodithiophene bearing low band gap copolymer for electrochromic and organic photovoltaic applications

Synthetic Metals 228 (2017) 111–119

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Research paper

A triazoloquinoxaline and benzodithiophene bearing low band gap copolymer for electrochromic and organic photovoltaic applications

MARK

⁎

Serife O. Hacioglua, ,1, Naime A. Unlua, Ece Aktasa, Gonul Hizalana, Esra D. Yildizb, ⁎ Ali Cirpana,c,d,e, Levent Topparea,d,e,f, a

Department of Chemistry, Middle East Technical University, 06800 Ankara, Turkey Department of Physics, Hitit University, 19030 Corum, Turkey c Department of Micro and Nanotechnology, Middle East Technical University, Ankara 06800, Turkey d Department of Polymer Science and Technology, Middle East Technical University, 06800 Ankara, Turkey e The Center for Solar Energy Research and Application (GÜNAM), Middle East Technical University, 06800 Ankara, Turkey f Department of Biotechnology, Middle East Technical University, 06800 Ankara, Turkey b

A R T I C L E I N F O

A B S T R A C T

Keywords: Conjugated polymers Triazoloquinoxaline Benzodithiophene Organic solar cell Copolymer

A new triazoloquinoxaline and benzodithiophene based copolymer was synthesized to investigate its electrochemical, optical and photovoltaic behaviors. According to the polymer design, combination of two acceptor units (benzotriazole and quinoxaline) which contribute imine bonds to the structure and a triazoloquinoxaline unit for enhancing electron accepting ability was pursued. As a result of electrochemical studies, the copolymer PTQBDT has a low lying HOMO energy level as −5.23 eV which increases the chemical stability of the resulting polymer and leads to a higher Voc. In addition, the copolymer has an ambipolar character with two well-defined redox couples in the n-doped state and multichromic behavior. In the context of optical studies, PTQBDT has wide absorption range in the visible region with a tail in the NIR region, which yields a low band gap of 1.20 eV. Organic photovoltaic devices were designed using PTQBDT (the electron donor) and PC71BM (the electron acceptor) for the preliminary studies. The resulting device exhibits a power conversion efficiency of 2.0% with a current density of 8.07 mA cm−2, an open-circuit voltage of 0.45 V, and a fill factor of 55%. The carrier mobility of the PTQBDT was calculated as 3.00 × 10−3 cm2 V−1 s−1 via space-charge-limited current (SCLC) method.

1. Introduction

an active layer became more popular since a variety of achievable colors can be achieved via structural alternations. Synthesis of D-A type conjugated polymers is crucial not only for color tuning but also for processability, fast switching time, high optical contrast and stability of conjugated polymers for electrochromic applications [9]. Synthesis of novel donor-acceptor (DA) conjugated polymers with suitable physicochemical properties is also important to control photovoltaic properties for efficient organic solar cells. Conjugated polymers must have: (1) a broad absorption in visible and near- infrared regions to harvest sun light efficiently to increase JSC, (2) suitable alignment of HOMO and LUMO for efficient charge separation and higher VOC, (3) good charge mobility to overcome recombination process and facilitate the charge transport efficiency for high FF and JSC, (4) processability in the fabrication of OSCs and suitable morphology, and (5) nanoscale phase separation for effective charge separation and extraction [10]. Therefore selection of suitable donor-acceptor combination is impor-

Conjugated polymers are of high interest due to a variety of fields like light emitting diodes, photovoltaic diodes, field effect transistors and electrochromic devices [1–4]. They have diverse advantages such as low cost, light weight, ease of fabrication in large-scale, feasibility of flexible devices and tunability of physiochemical properties [5–7]. Most of the physicochemical properties of such polymers are affected by their band gap. Among all band gap-engineering strategies, use of donoracceptor (D-A) is the most effective strategy to alter the band gap of conjugated polymers. In this approach, such units are alternated along the polymer backbone. It is a fact that the choice of different units for the development of D-A type conjugated polymers affects all physicochemical properties by controlling the degree of aromaticity, the planarity of the backbone, and the electron density of polymers [8]. For electrochromic devices, the use of D-A conjugated polymers as

⁎ Corresponding authors at: Middle East Technical University Department of Chemistry, Department of Biotechnology, Department of Polymer Science and Technology, The Center for Solar Energy Research and Application (GÜNAM, Turkey. E-mail addresses: [email protected] (S.O. Hacioglu), [email protected] (L. Toppare). 1 On leave from Sinop University.

http://dx.doi.org/10.1016/j.synthmet.2017.04.017 Received 27 January 2017; Received in revised form 10 March 2017; Accepted 24 April 2017 Available online 29 April 2017 0379-6779/ © 2017 Elsevier B.V. All rights reserved.

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2.3. Synthesis of monomer and polymer

tant for the synthesis of p-type conjugated polymers. Recently, an electron-rich benzo[1,2-b:4,5-b’]dithiophene (BDT) unit has been utilized as the attractive building block due to its symmetric and coplanar structure. These properties increase intermolecular π-π* interaction [11]. Therefore, in this study we coupled BDT with triazoloquinoxaline for the synthesis of low band gap copolymers (PTQBDT) with suitable energy levels for both electrochromic and organic solar cell applications. Two acceptor units i.e. benzotriazole and quinoxaline, were combined for the synthesis of triazolequinoxaline. This combination not only increases the imine moieties and the coplanarity of the polymer backbone but also decreases optical band gap to harvest more photons [12,13]. This study emphasizes optoelectronic and photovoltaic properties of copolymer i.e. PTQBDT. Optical and electrochemical studies of PTQBDT showed a low optical band gap with a suitable HOMO level. Therefore, BHJ OSCs were fabricated using PTQBDT and PC71BM. The preliminary results showed a power conversion efficiency of 2.0% with a current density of 8.07 mA cm−2, an open-circuit voltage of 0.45 V, and a fill factor of 55%. The hole mobility of the PTQBDT was calculated as 3.00 × 10−3 cm2 V−1 s−1 via SCLC method.

2.3.1. Synthesis of 4,9-bis(5-bromothiophen-2-yl)-2-dodecyl-6,7-diphenyl2H-[1,2,3]triazolo[4,5-g]quinoxaline 2-Dodecyl-6,7-diphenyl-4,9-di(thiophen-2-yl)-2H- [1,2,3]triazolo [4,5 g]quinoxaline (70 mg, 0.107 mmol) was dissolved in 30 mL chloroform. The reaction medium was filled with argon and the temperature was set up at 0 °C using an ice bath. At the same time, 2 equivalents of NBS (38 mg, 0.213 mmol) were dissolved in 15 mL chloroform under an inert atmosphere and added to the reaction medium drop wise. After addition of NBS, the reaction was warmed to room temperature and stirred for 18 h. The reaction was controlled with TLC and after reaction was completed, the product was extracted with saturated sodium thiosulfate solution. The collected organic layer was washed with water and dried with anhydrous magnesium sulfate. The residue was purified by flash column chromatography (silica gel, CHCl3–hexane, 1:1) to yield a purple solid (45 mg, 52%). 1 H NMR (400 MHz, CDCl3) δ 8.50 (d, J = 4.2 Hz, 2H), 7.61-7.58 (m, 4H), 7.40-7.33 (m, 6H), 7.07 (d, J = 4.2 Hz, 2H), 4.62 (t, J = 7.4 Hz, 2H), 1.30 (m, 2H), 1.25-1.15 (m, 18H), 0.80 (m, 3H).13C NMR (101 MHz, CDCl3) δ 151, 140,5, 136.9, 136.1, 130.8, 129.8, 128.3, 128.2, 127, 125.7, 118.5, 61.9, 56.6, 30.8, 30.2, 28.8, 28.6, 28.5, 28.4, 28.3, 27.9, 25.6, 21.7, 13.1.

2. Experimental 2.1. Materials

2.3.2. Synthesis of PTQBDT 4,9-Bis(5-bromothiophen-2-yl)-2-dodecyl-6,7-diphenyl-2H-[1,2,3] triazolo[4,5-g]quinoxaline (76 mg, 0.0942 mmol) and 2,6-bis(trimethylstannyl)-4,8-bis(2-ethylhexyloxy)benzo[1,2-b:4,5-b’]dithiophene (76.4 mg, 0.0989 mmol) were dissolved in anhydrous toluene (4 mL) under inert atmosphere. After the mixture was purged with argon to remove O2, tris(dibenzylideneacetone)dipalladium(0)(4.5 mg, 0.005 mmol) as the catalyst and tri(o-tolyl)phosphine (12 mg, 0.04 mmol) as the ligand were added to reaction mixture and the mixture was refluxed for 2 days under argon. After polymerization reaction was completed, the end cappers 2- bromothiophene and tributyl(thiophen-2-yl)stannane were added to the reaction medium. Then the solvent was removed under reduced pressure and the crude product was washed with methanol, acetone, and hexane using a Soxhlet extractor to remove oligomers. Finally the polymer was precipitated into methanol to afford a green solid (57 mg, 40%). GPC: Mn: 23000 gmol−1, Mw: 31000 gmol−1, PDI: 1.35 .1H NMR (400 MHz, CDCl3) δ 8.70 (triazoloquinoxaline), 7.75 (BDT), 7.45 (thiophene), 5.23 (OeCH2), 3.40 (NeC), 2.10 (CH), 0.90-1.60 (CH2, CH3)

1

H-benzotriazole, potassium tert-butoxide, 1-bromododecane, bromine, hydrobromic acid, sulfuric acid, nitric acid, thiophene, tributyltin chloride, bis(triphenylphosphine)- palladium(II) dichloride, ammonium chloride N-bromosuccinimide (NBS), acetic acid, ethanol magnesium sulfate, dichloromethane, hexane, chloroform,2,6-bis(trimethylstannyl)-4,8-bis(2-ethylhexyloxy)benzo[1,2- b:4,5-b]dithiophene, toluene, NBS, p-toluenesulfonic acid, bis(dibenzylideneacetone)palladium(0) and tri(o-tolyl)phosphine were purchased from Sigma Aldrich Chemical Co. Ltd and used without further purification. Tetrahydrofuran (THF) and toluene were dried over Na/benzophenone ketyl and distilled before use. Air sensitive reactions were conducted under argon atmosphere. 2-Dodecyl-2H-benzo[d][1,2,3]triazole, 4,7dibromo-2-dodecyl-2H-benzo[d][1,2,3]triazole, 4,7-dibromo-2-dodecyl-5,6-dinitro-2H-benzo[d][1,2,3]triazole, 2-dodecyl-5,6-dinitro-4,7di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole, 2-dodecyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole-5,6-diamine and 2-dodecyl-6,7diphenyl-4,9-di(thiophen-2-yl)-2H-[1,2,3]triazolo[4,5-g]quinoxaline were synthesized according to previously described methods [14].

2.2. Measurements 2.4. Organic solar cell (OSC) fabrication and characterization 1

H and 13C NMR spectra were recorded on a Bruker Spectrospin Avance DPX-400 Spectrometer using trimethyl silane (TMS) as the internal reference and the chemical shifts were reported in ppm. For all electrochemical studies, the polymer was dissolved in CHCl3 (5 mg mL−1) and spray processed on an ITO coated glass substrate. Cyclic voltammetry studies were carried out using a Gamry Reference 600 potentiostat/galvanostat in a three-electrode cell bearing an ITOcoated glass slide (the working electrode), a Pt wire (the counter electrode), and a Ag wire calibrated against Fc/Fc+. 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in anhydrous acetonitrile (ACN) was used for electrochemical studies. Corresponding HOMO and LUMO energy levels were calculated from the onset of oxidation and reduction potentials. For optical and spectroelectrochemical studies, Varian Cary 5000 UV–vis-NIR spectrophotometer was used and UV–visNIR spectra were recorded under ambient conditions. Average molecular weight of the polymer was determined by gel permeation chromatography (GPC) on a Polymer Laboratories GPC 220 using polystyrene as the standard and THF as the solvent.

The ITO electrodes were cleaned with detergent (Hellmanex, 2%), deionized water and isopropyl alcohol for 15 min by sonication. After ultrasonic cleaning oxygen plasma cleaning was carried out. PEDOT:PSS was filtered through 0.45 μm pore sized filter and spin coated on the ITO substrate. Then ITO coated substrates were placed on hot plate at 135 °C to remove water for 15 min. Polymer:PCBM blends were prepared and stirred overnight and filtered with 0.2 μm PTFE. Filtered solution was spin-coated in a nitrogen filled glove box. Finally, lithium fluoride (0.6 nm) and aluminum (100 nm) were evaporated onto the active layer with an average rate of 0.1 Å/s and 1 Å/s in a glove box (H2O < 0.1 ppm and O2 < 0.1 ppm) under high vacuum (1 × 10−6mbar), respectively. The active area of the cells was determined as 0.06 cm2. The current density-voltage (J-V) characteristics were measured with a Keithley 2400 source measurement unit under AM 1.5 solar simulator. The incident photon to current efficiencies (IPCE) of organic solar cells were recorded using a Newport quantum efficiency measurement. 112

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Scheme 1. Synthetic pathway and chemical structure of PTQBDT.

compound 6. 2-Dodecyl-6,7-diphenyl-4,9-di(thiophen-2-yl)-2H-[1,2,3] triazolo[4,5-g]quinoxaline (7) was synthesized via condensation reaction of 6 with benzyl using PTSA as the catalyst. Then, in order to get the dibrominated monomer (8), bromination reaction of 7 was performed in the presence of NBS and chloroform. Finally 4,9-bis(5bromothiophen-2-yl)-2-dodecyl-6,7-diphenyl-2H-[1,2,3]triazolo[4,5-g] quinoxaline (8) and (4,8-bis(heptan-3-yloxy)benzo[1,2-b:4,5-b']dithiophene-2,6-diyl)bis(trimethylstannane) were polymerized by Stille coupling in the presence of Pd2(dba)3 and tri(o-tolyl)phosphine to get copolymer PTQBDT. After synthesis, purification of copolymer was performed by Soxhlet extraction with methanol, hexane, and acetone to remove oligomeric units. Gel permeation chromatography (GPC) was used to determine the molecular weight of the polymer using THF as the solvent and polystyrene as the reference. Number average molecular weight of PTQBDT was 23000 gmol−1, with Mw of 31000 gmol−1

3. Results and discussion 3.1. Synthesis and characterization The synthetic routes were illustrated in Scheme 1. 2-Dodecyl-6,7diphenyl-4,9-di(thiophen-2-yl)-2H-[1,2,3]triazolo[4,5-g]quinoxaline (7) was synthesized according to previously described methods [14]. 4,7-Dibromo-2-dodecyl-2H-benzo[d][1,2,3]triazole (3) was synthesized by alkylation of 1H-benzo[d][1,2,3]triazole (1) and then by bromination of alkylated product (2) with HBr and bromine. Then, nitration of 3 using sulfuric acid and fuming nitric acid resulted in 4,7-dibromo-2dodecyl-5,6-dinitro-2H-benzo[d][1,2,3]triazole(4). The Stille coupling reaction of 4 and tributyl(thiophen-2-yl)stannane gave 2-dodecyl-5,6dinitro-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole (5) and the reduction of compound 5 with iron dust and acetic acid yielded 113

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Fig. 1. Single scan cyclic voltammogram of PTQBDT in 0.1 M TBAPF6/ACN solution at 100 mV/s. Table 1 Summary of electrochemical and spectroelectrochemical properties of PTQBDT. Ep-doping (V)

Ep-dedoping (V)

En-doping (V)

En-dedoping (V)

HOMO (eV)

LUMO (ev)

Egec (eV)

λmax

λmax

thinfilm

solution

(nm)

Egop (eV)

(nm) PTQBDT

0.70/ 1.05

0.45/ 0.92

−1.15 /−1.60

−0.88/ −1.34

−5.23

−3.83

1.40

452/725

465/789

1.20

Fig. 2. Scan rate dependency of PTQBDT at 50, 100, 150, 200, 250 mV/s in 0.1 M TBAPF6/ACN solution on ITO electrode.

potentials were evolved at 0.70 V/1.05 V with 0.45 V/0.92 V dedoping potentials on the anodic scan. The reduction peaks were recorded at −1.15 V/-1.60 V with corresponding n-type dedoping potentials as −0.88 V/-1.34 V. As seen in Fig. 1, PTQBDT has well defined two reversible redox couples in the n doped state which is a rare property for these type of polymers. This behavior can be attributed to the combination of two strong acceptor units (benzotriazole and quinoxaline) in the structure. Combination of these units increases the imine bonds in the polymer backbone and enhances the electron accepting ability of triazoloquinoxaline unit. HOMO and LUMO energy levels of PTQBDT were calculated from the onsets of the oxidation and the reduction peaks as −5.23 eV and −3.83 eV, respectively. The electronic band gap was calculated as

and 1.35 PDI. 3.2. Electrochemical properties The electrochemical properties of the polymer were investigated via cyclic voltammetry (CV) in an electrolyte solution of 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) in acetonitrile (ACN) in the potential range between − 1.90 V and 1.20 V. PTQBDT was dissolved in CHCl3 (5 mg mL−1) and spray processed on ITO electrodes for electrochemical and optical studies. The redox behaviors of the polymer were investigated by single scan CV as depicted in Fig. 1. PTQBDT has two reversible redox couples both in the p type and the n type doped states. As reported in Table 1, the polymer oxidation 114

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Fig. 5. Percent transmittance changes as a function of time at 1210 nm and 1560 nm for PTQBDT.

Fig. 3. Electronic absorption spectra in 0.1 M TBAPF6/ACN solution between 0.0 V (black) and 1.3 V (turquoise) for PTQBDT.

Fig. 6. The absorption spectra of PTQBDT in chloroform solution and in thin film and absorption spectra of PTQBDT blend film with and without PC71BM.

Single scan CVs of PTQBDT were recorded at five different scan rates to investigate the relationship between the peak current and the scan rate. As seen in Fig. 2(a,b), a linear relationship between the current density and scan rate proves the formation of well adhered polymer film and non-diffusion controlled redox processes.

3.3. Spectroelectrochemistry For spectroelectrochemical studies, PTQBDT was dissolved in CHCl3 and spray coated onto ITO electrodes. Optical studies for the polymer were carried out by recording the changes in the absorption spectra under a variety of applied voltage by UV–vis-NIR spectrophotometer. The potentials were swept from 0.0 V to 1.1 V in 0.1 M ACN/TBAPF6 solution. Optical characterizations are crucial for both observation of absorption range of the polymer and corresponding λmax and Eg values. The λmax could be defined as the wavelength at which a polymer reveals a π-π* transition. As seen in Fig. 3, PTQBDT has two distinctive absorption maxima at around 452 nm and 725 nm which are essential for a neutral state green color. The corresponding optical band gap (Eg) value of the polymer was calculated from the onset of the lowest energy π-π* transition and reported as 1.20 eV. Optical results show that, PTQBDT is low band gap polymer with a strong absorption in the visible region. In addition, this wide absorption tails into NIR region as reported in Fig. 3. During oxidation, while the neutral state absorptions were depleting, new absorption bands aroused which could be explained by the formation of free charge carriers such as polarons (radical cations) and

Fig. 4. (a) Electronic absorption spectra of PTQBDT in the neutral and oxidized states. (b) Colors of the polymer at different states.

1.40 eV from the difference between the HOMO and LUMO energy levels. HOMO/LUMO energy levels which were crucial for organic photovoltaic applications were calculated and reported relative to the vacuum. Ambipolar character of PTQBDT makes it applicable in different research fields such as; batteries, supercapacitors, and lightemitting diodes [15]. All electrochemical results were given in Table 1. As seen in Table 1 PTQBDT has relatively a low lying HOMO energy level as −5.23 eV. These desired behaviors of the polymer should increase its chemical stability in ambient conditions and in addition, the deep HOMO levels of polymers will lead higher Voc during the organic solar cell applications since the Voc depends on the difference between the LUMO level of acceptor group and the HOMO level of the donor [16].

115

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Fig. 7. (a) Energy band diagram for ITO/PEDOT:PSS/PTQBDT/PC71BM/LiF/Al. (b) Schematic representation of device configuration.

lowest optical band gap as 1.20 eV while that of PTQF is [17] 1.40 eV, PBDT-BTzQx-C12 [16] is 1.38 eV and PFDTBTz-Q-2OC1 [13] is 1.63 eV, respectively.

Table 2 Photovoltaic Properties of PTQBDT and Summary of Carrier Mobilities with Different Blend Ratio of Active Layer. The values in parenthesis refer to average values. Polymer:PC71BM

Jsc (mA/cm2)

Voc (V)

FF (%)

PCE (%)

Thickness (nm)

Carrier Mobilities (cm2/Vs)

1:2

7.15

0.35

0.28

121

1.02 × 10−4

1:3

5.22

0.37

0.45

119

9.84 × 10−4

1:4

8.07

0.45

0.55

0.70 (0.69) 0.86 (0.75) 2.0 (1.80)

110

3.00 × 10−3

3.4. Electrochromic switching studies Kinetic studies were done to explore the optical contrast and switching times at certain wavelengths. A square-wave potential step method and spectrophotometer were coupled in these studies to obtain the electrochromic switching abilities of the polymer. For this purpose, the polymer was spray coated on ITO and the electrochromic switching studies were recorded via switching the potentials between the doped and dedoped states. Switching time is the time required for one full switch of the electrochromic material between the two extreme states. During the kinetic studies, the wavelengths were determined from the spectroelectrochemical studies of the PTQBDT. Due to the wide absorption range of the polymer in the visible region, both polaronic and bipolaronic regions were observed at longer wavelengths. As shown in Fig. 5, the PTQBDT film revealed 9% optical contrast with a 1.2 s switching time at 1210 nm and 12% optical contrast with a switching time of 0.6 s at 1560 nm

bipolarons (dications) at 1210 nm and 1560 nm, respectively. Two absorption peaks in the visible spectrum yielded a neutral state green polymer PTQBDT which is an important and rare property for conjugated polymers. In the oxidized state, the corresponding polymer exhibited transmissive gray color since the polaronic and bipolaronic absorptions were not in the visible region, both neutral/oxidized state absorptions and corresponding colors were depicted in Fig. 4. Results of optoelectronic studies of conjugated polymers are crucial to apply these materials in OPVs. The polymers with low band gap and high absorption coefficient harvest more light makes a possible candidate for optoelectronic applications. As illustrated in Fig. 4a PTQBDT has wide range absorption in the visible region with a tail in the NIR region which yields relatively low band gap polymer (Egop = 1.20 eV). During the design of triazoloquinoxaline unit as the acceptor group, it is aimed to combine two popular and efficient acceptor units (benzotriazole and quinoxaline) in one structure. The use of two acceptors yielded to a stronger acceptor ability than either molecule alone due to the number of imine bonds in the polymer backbone. [14] In literature, different benzotriazole derivatives were used as active layers for OPVs. In terms of optical properties PTQBDT has one of the

3.5. Photovoltaic properties The PTQBDT showed two marked absorptions, the first at the shorter wavelength refer to the π–π* transition of the main chain and the second band at the longer wavelength was due to the intermolecular charge transfer (ICT) interaction between the benzodithiophene (BDT) donor unit and the triazoloquinoxaline acceptor unit [18] (Fig. 6). Hai et al. synthesized and characterized BHJ cells for the similar polymers based on benzodithiophene and triazoloquinoxaline and they obtained a lower short circuit current since the polymer has excessive alkyl chains that may prevent the main backbone from π-π stacking hence, decreasing the JSC [19]. The nanoscale phase separation between 116

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Fig. 8. J/V curves measured under AM 1.5G irradiation of 100 mW/cm2 (a) and IPCE spectra of polymer:PC71BM prepared from chloroform and dichlorobenzene mixture (b).

Fig. 9. J-V curve of ITO/PEDOT:PSS/PTQBDT:PC71BM/LiF/Al device for PTQBDT:PCBM for (1:4) active layer ratio.

polymer is lower than the PC71BM amount in the best performance device, its contribution to absorption of light is lower. Thus, lower spectral response was obtained at 800 nm for PTQBDT/PC71BM (1:4) film.

polymer and fullerene domains was not observed distinctly in the active layers [20]. The absorption spectra of PTQBDT in chloroform solution and in thin film and absorption spectra of PTQBDT blend film with and without PC71BM are reported in Fig. 6. As seen, since the amount of 117

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units was synthesized via Stille coupling reaction. The polymer showed good electrochemical, spectroelectrochemical behaviors with promising organic photovoltaic properties. The copolymer PTQBDT has an ambipolar character and shows two reversible redox couple during ntype doping which can refer to the enhanced electron accepting ability of triazoloquinoxaline group. Low lying HOMO level and multichromic character with neutral state green color make it applicable in electrochromic devices. In terms of optical behaviors, the resulting polymer has low band gap (1.20 eV) which could be explained with the wide absorption range in the visible region tailing to the NIR region. Preliminary organic solar cell applications were performed and PTQBDT/PC71 BM (1:4) showed the best PCE of 2.0% with a Jsc of 8.07 mA cm−2, a Voc of 0.45 V, and a FF of 55%. Finally, SCLC method was used to calculate the carrier mobility of the PTQBDT as 3.00 × 10−3 cm2 V−1 s−1.

Photovoltaic properties of PTQBDT was investigated with the device structure of ITO/PEDOT:PSS/PTQBDT:PC71BM/LiF/Al (Fig. 7). Active layers were prepared from dichlorobenzene (DCB) and chloroform (CHCl3) mixture. This solvent mixture showed the best film-forming property. Effect of polymer PC71BM ratio was investigated (Table 2). The PTQBDT/PC71BM weight ratio of 1:4 showed the best device performance. With the preliminary optimization studies the highest power efficiency of 2.0% was achieved with an open circuit voltage (VOC) of 0.45 V, a short circuit current (Jsc) of 8.07 mA cm−2 and a fill factor (FF) of 55%. IPCE measurements were done in the range of 300–1000 nm using monochromic light. 35% IPCE was obtained with the best performance device as shown in Fig. 8. Further studies will be carried out such as heat treatments and addition of additive to enhance the device performance. The IPCE spectra (Fig. 8) show that PTQBDT based solar cells harvest light efficiently between 350 and 900 nm, as expected from the UV–vis spectra of PTQBDT. However, their absorption below 800 nm is weak, resulting in low IPCE values in this spectral region. In literature Tang and coworkers synthesized different BDT and triazoloquinoxaline bearing copolymers namely; PBDTT-BTzQx-EH-C1, PBDT-BTzQx-EH-C1, PBDT-BTzQx-EH-C12, and PBDT-BTzQx-C12 [19]. The optical band gaps for the polymers were reported as 1.48, 1.44, 1.45 and 1.38 eV, respectively. In terms of optical band gap PTQBDT has lower band gap (1.20 eV) when compared with similar copolymers. In addition, OPV applications were also performed for corresponding polymers and reported [19]. PBDT-BTzQx-EH-C1 revealed highest PCE as 1.40% with 0.62 V Voc and 4.12 mA cm−2 Jsc which were reported as 2.0% with a current density of 8.07 mA cm−2, an open-circuit voltage of 0.45 V for PTQBDT in this study. Hence, PTQBDT has a lower optical band gap and a higher PCE compared to similar copolymers published in literature.

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3.6. The carrier mobility calculated using classic SCLC model The carrier mobility is important in the efficiency improvement of devices such as OLEDs, OFETs and OSCs. It is very much to do with by energetic disorder. This arises from the interaction of each charge with randomly located dipoles in the active layer [21,22]. Therefore, the carrier mobility of organic materials can be measured using different techniques such as time-of-flight measurement (TOF), impedance spectroscopy, dark injection transient and space charge limited current (SCLC) [23]. The TOF technique however, requires quite thick films and also can not predict the practical mobility for a thickness around 50 nm in OSCs or OLEDs [21]. Therefore, in this work, the SCLC method is used for ITO/PEDOT:PSS/PTQBDT:PC71BM/LiF/Al OSCs. Fig. 9 shows log J-log V characteristics of ITO/PEDOT:PSS/ PTQBDT:PC71BM/LiF/Al under dark. The log J-log V plots show two distinct regions at low and high bias, such as ohmic region (JαV) and the SCLC region (JαV2), respectively. When all traps are filled and converted from neutral to charge state, the device enters the trap free SCLC, which represents the Mott-Gurney and Child’s law [24–28]. The SCLC current density is written as:

JSCL =

9 V2 εo θεr μ 3 8 L

(1)

Here, L is thickness of active layer, V is voltage, μ is the carrier mobility, εr (3 for polymer) is the relative permittivity and εo (8.85 × 10−12 F/ m) [29]. permittivity of free space. The carrier mobilities of PTQBDT:PC71BM was calculated by Eq. (1) as shown in Table 2. These results may be useful for new conjugated polymer designs and improvement in the performance of OSCs [18,25–27]. 4. Conclusions The copolymer bearing triazoloquinoxaline and benzodithiophene 118

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