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1,5-Naphthyridine-based conjugated polymers as co-sensitizers for dye-sensitized solar cells
Solar Energy 194 (2019) 682–687
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1,5-Naphthyridine-based conjugated polymers as co-sensitizers for dyesensitized solar cells Muhammad Manshaa,c,1, Muhammad Younasb,c,1, Muhammad Ashraf Gondalb,c, Nisar Ullaha,
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a
Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Laser Research Group, Department of Physics, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia c Center of Research Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia b
A R T I C LE I N FO
A B S T R A C T
Keywords: 1,5-Naphthyridine-based conjugated polymers N3 dye Co-sensitization Solar cell
1,5-naphthyridine-based conjugated polymers (P1 and P2) were synthesized and extensively characterized by 1H NMR, thermogravimetric analysis (TGA), FT-IR. These polymers were employed as co-sensitizers in DSSCs sensitized with Ru (II) based N3 dye. In comparison to the N3 sensitized device, the P1 and P2 co-sensitized solar cells demonstrated enhanced open circuit voltage, (VOC) of 825 and 788 mV and better fill factor (FF) of 59 and 58%, respectively. The co-sensitization of P1 and P2 with N3 increased the overall efficiencies to 5.88% and 6.21%, respectively, as compared to 5.58% for N3 sensitizer alone. The fabricated device based on N3 cosensitized with P2 displayed higher charge recombination resistances as compared to the devices based on N3 alone or N3 co-sensitized with P1. The conjugated polymers are believed to enhance light harvesting ability and reduce the charged recombination in the co-sensitized solar cells.
1. Introduction Given the limited resource of fossils fuel and climate change considerations, dye-sensitized solar cells (DSSCs) is considered a promising technology for low cost solar to electricity conversion (Zouhri, 2018). As a new generation photovoltaic device, DSSCs is considered superior to the conventional silicon-based photovoltaic devices owing to its flexibility and cost-effective production methods (Lim et al., 2012). Consequently, enormous efforts have been devoted to improving the efficiency of DSSCs (Barbera et al., 2017). DSSC is composed of a photoanode, made of a dye-adsorbed onto a thin film of a wide band gap semiconductor (usually TiO2) deposited on a fluorine doped tin oxide (FTO) coated glass substrate, an electrolyte/hole transporter and a counter electrode. Absorption of a photon by a dye molecule leads to excitation of its ground-state electron, which in turn is injected into the conduction band of semiconductor. The oxidized dye molecule takes electron back from a redox species present in the electrolytic solution to produce ground-state structure. The circuit is completed when oxidized species are reduced back at the counter electrode (Grätzel, 2003). The efficiency of DSSCs is mainly depends upon anode, the photosensitizer, the counter electrode and the electrolyte. Nevertheless, the sensitizer, a key component of DSSCs, plays a key role on the device efficiency and stability. Consequently, metal-free organic dyes and metalorganic
complexes-based sensitizers have been developed for light harvesting (Mansha et al., 2018). Until now ruthenium complexes have emerged as the most efficient sensitizers, allowing 10–11% solar-to-electric power conversion efficiencies (Cao et al., 2009). However, limitations such as heavy-metal toxicity, limited ruthenium resource, difficulty in purifications and stability issues make these complexes unfavorable for large-scale applications. The use of single dye-sensitization strategy suffers from (1) achieving wide optical absorption with high extinction coefficient and (2) high recombination dynamics of charge carriers. These limitations have been overcome by employing the co-sensitization method, which allows to achieve wide spectral absorption with high extinction co-efficient (ε) and also increases electron recombination lifetime from 0.1 ms to 1 s (Kumar et al., 2019). Organic dyes-based sensitizers enjoy advantages of higher molar absorption coefficient, ease of modification and are more environmental friendly. The performance of DSSC has been improved by employing co-sensitization based on combination of metal-free organic dyes and metal-based dyes (Younas et al., 2018, Luo et al., 2014; Naik et al., 2017a; Sharma et al., 2013; Siddiqui et al., 2017; Singh et al., 2013; Su’ait et al., 2015; Wang et al., 2014; Yella et al., 2011). Consequently, fabrication of DSSCs by combination of Ru (II) complex and metal-free organic push-pull configured dyes such as thiophene, triphenylamine, carbazole, indole, phenothiazine as co-
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Corresponding author. E-mail address: [email protected] (N. Ullah). 1 These authors contributed equally. https://doi.org/10.1016/j.solener.2019.11.022 Received 6 June 2019; Received in revised form 3 November 2019; Accepted 6 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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solutions of polymer (15 mg/mL).
sensitizers have been explored in the past (Lee et al., 2018; Naik et al., 2017b). The incorporation of an organic dye with Ru (II) complex in cosensitized device facilitate enhanced harvesting of incident photons which in turn resulted in increased power conversion efficiencies (PCEs) (Clifford et al., 2011). Recently, conjugated polymers have been implicated for metal free DSSCs application (Mozaffari et al., 2014). The advantages of these materials as sensitizer include controllable energy levels (HOMOLUMO) by the deployment of electron-rich and electron-deficient alternating units in the polymer backbone, cost-effective structural modification and large absorption coefficients (Kanimozhi et al., 2010). Moreover, in comparison to small molecular dyes, polymeric dyes can be attached onto the surface of metal oxide surface more stably through several anchoring interactions along a long polymer chain and thus provide more stability. Nevertheless, utilization of conjugated polymers as a sensitizer is not frequently explored (Fang et al., 2011; Naik et al., 2018a). Among the few studies, polythiophene (Mwaura et al., 2006; Senadeera et al., 2005) donor (D)-acceptor (A) co-polymer (Kanimozhi et al., 2010), poly epinephrine and poly dopamine (Mozaffari et al., 2014) and substituted carbazole based dyes as photosensitizers have been reported. It has to be borne in mind that the transfer of photogenerated electrons through a series of interfaces is needed in order for a successful power conversion. However, back-electron transfer reaction, wherein electrons from the conduction band of TiO2 are backinjected to the electrolyte, is considered the main obstacle. To overcome this problem, a wide range of diverse material-based thin energy insulating barrier films have been employed (Shrestha et al., 2019). Lee et al., have successfully used conjugated polymers-based thin film both as co-sensitizer and back-electron injection barrier layer for enhanced electron density at TiO2 (Han et al., 2006). Based on the above consideration, herein we wish to report the synthesis 1,5-naphthyridine-based conjugated polymers and their utilization as as co-sensitizers with a ruthenium-based dye N3 for DSSCs. The structural architect of P1 and P2 contains a central donor moiety, dialkoxyphenylene core, attached to two methoxyphenoxy-6-(3-methoxyphenoxy)-1,5-naphthyridine functions. We envisioned that the incorporation of P1 and P2 would enhance harvesting of incident photons and offer more stability by firm attachment of the polymers to the surface of metal oxide through several anchoring interactions along a long polymer chain. The fabricated devices based on N3 co-sensitized with P1 or P2 have shown greater power conversion efficiencies (PCEs) as compared to devices based on the individual P1 and P2 or N3 alone.
2.2. Synthesis of P1 To a solution of mixture of monomer 2 (0.52 g, 0.70 mmol) and dialdehyde 6 (0.30 g, 0.70 mmol) in anhydrous DMF (40 mL) was added sodium tert-butoxide (0.34 g, 3.48 mmol) and the mixture was heated at 100 °C for 24 h under nitrogen atmosphere. After cooling to room temperature, the reaction mixture was poured into 75 mL of methanol followed by centrifuging and decantation allowed to obtain the solid product. The small molecule impurities and oligomers were then removed by re-dissolving the residues in THF and re-precipitating it from methanol, isopropanol, and hexane gave P1 as a dark yellow solid (81%). 1H NMR (500 MHz, CDCl3): δ 8.01 (2H, br.), 7.48 (2H, br.), 7.44 (2H, br.), 7.20 (2H, br.), 7.15 (2H, br.), 7.11 (2H, br.), 7.09 (2H, br.), 7.05 (2H, br.), 4.07 (4H, br.), 3.81 (6H, br.), 1.90–1.85 (4H, br.), 1.53 (4H, m), 1.33 (32H, m), 0.85 (6H, br.). FT-IR (KBr): 3026, 2925, 2848, 1596, 1503, 1356, 1246, 1029 cm−1. GPC analysis: Mn = 3340, Mw = 5720, PDI = 1.70. Anal. Calcd for (C56H72N2O6)n: C, 77.38; H, 8.35; N, 3.22. Found: C, 76.86; H, 8.58; N, 2.90. 2.3. Synthesis of P2 By adopting procedure for the synthesis of P1, P2 was synthesized from the reaction of dialdehyde 6 and monomer 3 as a light-yellow solid (57%). 1H NMR (500 MHz, CDCl3): δ 8.25 (2H, br.), 8.02 (2H, br.), 7.84 (2H, br.), 7.61 (2H, br.), 7.45 (2H, br.), 7.35 (2H, br.), 7.19 (2H, br.), 7.15 (2H, br.), 3.96 (4H, br.), 3.79 (6H, br.H), 2.06 (4H, br.H), 1.92–1.85 (4H, br.), 1.53 (4H, m), 1.32 (20H, m), 1.01 (6H, br.), 0.91 (6H, br.). FTIR IR (KBr): 3077, 2923, 2853, 1593, 1506, 1473, 1348, 1278 cm−1. GPC analysis: Mn = 3618, Mw = 5930, PDI = 1.62. Anal. Calcd for (C48H58N2O6)n: C, 76.16; H, 7.46; N, 3.70. Found: C, 75.84; H, 7.68; N, 3.38. 2.4. Fabrication of PSSC Fluorine doped tin oxide (FTO) conductive glasses with area 1.5 cm2 were physically washed with soap and cotton buds followed by ultrasonically washing sequentially with deionized water, acetone and isopropanol for 10 min each. After drying washed FTO at 100 °C for 30 min, titanium dioxide (TiO2) paste was coated on the cleaned FTO with doctor blade method and calcined first at 450 °C for 30 min. The as prepared TiO2 coated anode were soaked into pure N3 dye solution (0.5 mM in ethanol) and also in pure polymers P1 and P2 solutions (15 mg/ml in THF) for 24 h. The two photoanodes sensitized with pure N3 dye were further separately soaked into P1 and P2 solutions for next 24 h for co-sensitization effect. Counter electrodes were prepared using platinum (Pt) paste with doctor blade method and Pt coated FTO were sintered at 450 °C for 30 min. All the photoanode were removed from respective solutions and rinsed with ethanol to remove unanchored dye/polymer. Both photoanode and counter electrode were joined together with super glue and electrolyte (I−/I3−) was poured with 5 μm micropipette. The active area of the PSSC was 0.25 cm2. To estimate the dyes loading on the photoanode of TiO2, samples TiO2/N3, TiO2/N3/ P1, and TiO2/N3/P2 sensitized with N3, N3 + P1 and N3 + P2, respectively, were placed into a 20 mM KOH solution and stirred at room temperature for 24 h in order to desorb the dyes. The concentrations of the desorbed dyes were determined by using UV–vis spectroscopy (Han et al., 2010). For this purpose, calibration curves were plotted for N3 (0.5 mM), N3 + P1 (0.5 mM and 15 mg/mL) and N3 + P2 (0.5 mM and 15 mg/mL) solution by making a series of dilutions, ranging from 0.5 mM to 0.0078 mM. In each calibration curve the value of coefficient of determination (R2) was maintained > 0.999. The total amount of N3 obtained after desorption of TiO2/N3 solar cell was found to be 0.090 mM per 0.25 cm2 area. Similarly, desorption of TiO2/N3/P1 and TiO2/N3/P2 lead to the total calculated amounts of 0.146 mM and
2. Experimental 2.1. Materials and Instruments 1 H and 13C NMR on 500 MHz spectrometer (JEOL JNM-LA, JEOL USA Inc.) and FTIR spectrophotometer (Perkin Elmer 16F PC, Perkin Elmer Inc. USA) was used for Infrared spectra. The UV–vis absorption spectra on Cary 5000 spectrophotometer (UV–vis-NIR, Agilent Technologies). SDT Q600 (V20.9 Build 20) thermal analyzer was used for thermogravimetric analysis (TGA). Samples were heated to 800 °C at a rate of 10 °C/min rise under oxygen atmosphere, with a purge rate of 50 mL/min. Fluorine doped tin oxide (FTO TCO22-7/LI; 2.2 mm, 7 Ω/ Seq), titanium paste (TiO2 Ti-nanoxide T/SP), Ruthnizer (N3 Ruthenizer 535), electrolyte (I−/I3− Iodolyte AN-50) and platinum paste (Pt Platisol T) were purchased from Solaronix, Switzerland. The current–voltage (J-V) characteristics were measured using 1.5G (100 mW/cm2) IV-5 solar simulator (PV measurements Incorporation Sr. No. 83) with Keithley 2400 as a source meter. Bandgaps determination and estimation of HOMO and LUMO level of P1 and P2 cosensitized with N3 dye was carried out by cyclic voltammetry (CV), using the electrochemical workstation (BioLogic Science Instruments (Model: VMP3 SAS)). Solution of N3 dye (0.5 mM) was deposited on glassy carbon electrode first, which was dried followed by addition of
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Scheme 1. Synthesis of monomers 2, 3, 6 and polymers P1 & P2.
0.174 mM for N3 + P1 and N3 + P2, respectively.
3. Results and discussions 3.1. Synthesis and characterization The naphthyridine based conjugated polymers P1 and P2 were synthesized as outlined in Scheme 1. Horner-Emmons reaction between dialdehyde 6 and 2 or 3 (Mansha et al., 2017b, 2017a; Mehmood et al., 2017) monomers furnished polymers P1 and P2, respectively, in good yield (Scheme 1). The chemical structures of polymers P1 and P2 were characterized on the basis of 1H NMR, 13C NMR and IR spectra (Figs. S1–S4). In 1H NMR of P1 and P2, the characteristic peaks for the phosphonate groups of monomers 2 and 3 at 4.02–4.01 ppm as well as aldehydic peaks of monomer 6 at 10.08 ppm were disappeared. Likewise, the absence of aldehydic peaks (2855, 2725, and 1710 cm−1) in the IR spectrum of P1 and P2 suggested the formation of P1 and P2. The thermogravimetric analysis of P1 and P2 indicated that both polymers exhibited high thermal stability with no appreciable loss of mass up to 275 °C. However, the onset decomposition temperatures (Td) of P1 and P2 were found to be as 325 and 375 °C, respectively. Both polymers displayed abrupt major weight loss above Td, which hinted the decomposition of polymer backbone (Fig. 1). The presence of alkoxy groups on these polymers made them soluble in organic solvents like methylene chloride, chloroform and tetrahydrofuran. The UV–Vis absorption spectra of N3 dye, polymers (P1 & P2) and co-sensitized N3 with polymers (P1 & P2) were recorded on TiO2 films, as shown in Fig. 2. The spectra of P1 and P2 displayed broad band absorptions around 500–250 nm. Likewise, the spectrum of N3 dye revealed the absorption edge starting from 700 nm whereas the first broad band appeared near 525 nm along with multiple bands in the high frequency region. However, in case of N3 dye co-sensitized with polymers (P1 & P2), the first broad band at 525 nm was slightly blue shifted. In addition, the spectra further exhibited multiple bands in the
Fig. 1. TGA graph of P1 and P2.
higher frequency region with higher intensity as compared to the spectrum of N3 alone. The rise in the absorption was due to the occupancy of empty pores on surface of TiO2 by polymers, which offers higher absorption coefficient and availability of greater amount photosensitizer. The lower energy orbitals provided by the conjugated polymers resulted in the participation of maximum number of incident photons in the high energy region for the electronic transition (Gao et al., 2012). 3.2. Photovoltaic performance To measure the photovoltaic performance of P1 and P2, five different polymer sensitized solar cells (PSSCs) were fabricated. Details on solar cell structures and parameters e.g. Voc (mV), Jsc (mW/cm2), FF (%) and efficiency η (%) are given in Table 1. The current density verses voltage (J-V) curves are provided in Fig. 3. As seen from Table 1 and Fig. 3, pure P1 and P2 exhibited lower efficiencies. However, co-sensitization of P1 and P2 with N3 increased the overall efficiencies to 5.88% and 6.21%, respectively, as compared to 5.58% for N3 alone. In 684
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The incident photon-to-electron conversion efficiency (IPCE), a measure of external quantum efficiency (EQE), is usually determined to assess the performance of solar cells in certain wavelength range. In other words, EQE or IPCE is a strong function of wavelength. The IPCE spectra of TiO2/N3, TiO2/N3/P1, and TiO2/N3/P2 fabricated DSSCs solar cells are given in Fig. S6. It is evident from Fig. S6, that the main pattern of EQE for all the fabricated solar devices are very similar. However, quantitatively the value of EQE was found in the order of TiO2/N3/P2 > TiO2/N3/P1 > TiO2/N3 for the DSSC solar cells. According to Grätzel, IPCE depends greatly on wavelength-based light harvesting efficiency of solar cell (LHE (λ)), the yields of injecting the generated electrons to conduction band energy level (ECB) of the semiconductor and on the collection efficiency of photogenerated electrons (Grätzel, 2005). In addition, the LHE (λ) depends three important factors: (i) The dye adsorption or dye loading on the surface of semiconductor, (ii) light absorbance features of the dye and iii) scattering of light at the surface of photoanode. The IPCE (Fig. S6) and UV–Vis (Fig. 2) spectra for all the fabricated DSSC solar cells explains the dependence of LHE (λ). Similarly, the presence of conjugated polymers enhance the electronic injection into the ECB of semiconductor as the combination of copolymers with N3 led to lower band gap. As for as charge recombination and transport is concerned, the electrochemical analysis shows that resistance to recombination of charges (R2) is higher for TiO2/N3/P2 as compared to other fabricated DSSC solar cells (Fig. 4 and Table 2) (Younas et al., 2019a,b).
Fig. 2. UV–Vis absorption spectra of N3 dye, polymers (P1 & P2) and co-sensitized N3 with polymers (P1 & P2) adsorbed on TiO2 films. Table 1 Photovoltaic properties of fabricated PSSCs. Cell Structure
Jsc (mA/cm2)
Voc (mV)
FF (%)
η (%)
TiO2/Pure N3/Pt TiO2/Pure P1/Pt TiO2/Pure P2/Pt TiO2/N3 + P1/Pt TiO2/N3 + P2/Pt
14.33 1.32 1.06 12.1 13.6
758 613 595 825 788
51 63 56 59 58
5.58 0.51 0.36 5.88 6.21
3.3. Electrochemical impedance spectroscopy (EIS) studies of fabricated DSSCs To study the photovoltaic behavior such as the interfacial charge recombination, EIS analysis of the fabricated devices was conducted. In general, charge recombination dynamics greatly influence the photo voltages of DSSCs. Therefore, EIS spectra of the devices fabricated with N3 alone and co-sensitized with P1 and P2 were carried out at ambient
Fig. 3. J-V curves of DSSCs sensitized with 0.5 mM N3 and co-sensitized with 15 mg/mL of P1 and P2.
addition, both the P1 and P2 based co-sensitized devices exhibited enhanced open circuit voltage, VOC of 825 and 788 mV and better fill factor, FF of 59 and 58%, than N3 alone (Table 1). These results are highly reproducible (Fig. S5 and Table S1). The improvement in the VOC of co-sensitized DSSCs fabricated with polymers is presumably due to the effective adsorption of co-sensitizer along with Ru-II based N3 molecules on the surface of TiO2 which in turn facilitated the suppression of the unwanted back reaction (Naik et al., 2018b). In addition, mixed dyes tend to shift ELUMO close to the CB TiO2 which subsequently enhances electron injection in co-sensitized DSSCs compared to mono-sensitized cell (Kumar et al., 2019).
Fig. 4. (a) EIS Nyquist curves of fabricated DSSCs sensitized with pure N3 (0.5 mM) and co-sensitized with P1 and P2 (15 mg/mL) and (b) equivalent circuit of Nyquist plot. 685
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ferrocenium (FC/FC+). Based on the Bredas Equation, the first oxidation potential and reduction potential are correlated to ionization potential (Ip) and electron affinity (Ea), respectively. Band gap (Eg) of N3 alone and its mixture with P1 and P2 can be determined by subtracting Ea from Ip.
Table 2 Charge recombination resistance (R2) at anode interface (TiO2/N3, TiO2/ N3 + P1 or TiO2/N3 + P2/electrolyte) and HOMO-LUMO energy level calculations from cyclovoltammetry. Sample
DSSCs
R2 (Ohm)
HOMO (eV)
LUMO (eV)
Eg (eV)
1 2 3
N3 N3 & P1 N3 & P2
72 88 108
5.08 4.82 4.88
3.92 3.83 4.09
1.16 0.99 0.79
EHOMO = [(Eox − E1/2) + 4.8] eV EHOMO = [(Ered − E1/2) + 4.8] eV
Eg = EHOMO (Ip) − EHOMO (Ea) conditions at different frequencies (100 mHz–1 MHz) by employing AC voltage. Fig. 4a indicates the electrochemical impedance (Nyquist plot) of the fabricated devices and equivalent circuit model is given in Fig. 4b. Usually EIS is exhibited by three semicircles (R1, R2 and R3). The first semicircle (R1) at higher frequency represents the interface resistance at counter electrode and electrolyte. Likewise, the second semicircle at intermediate frequency (R2) and third semicircle at lower frequency (R3) is related to resistance at photoanode/dye/electrolyte interface and diffusion of the carrier transport existing inside the electrolyte through redox couple (Zw) also called as Warburg diffusion impedance. In addition, the Rs (sheet resistance) shows the point of first arc at the highest frequency. The resistance at interface of counter electrode and electrolyte is given as R1 whereas the resistance to the recombination at photoanode and electrolyte interface expressed by R2. Therefore, the higher value of R2 indicates longer lifetime of electrons (Younas et al., 2019a,b). Moreover, C1 and C2 represent capacitances to the interfaces against R1 and R2. The photovoltaic behavior of fabricated devices, N3 alone, and co-sensitized with P1 and P2, suggested the appearance of R2 resistance at photoanode/dye/electrolyte and the remaining arcs might have been overshadowed by the second arc (Li et al., 2010; Nair et al., 2011). As evident from Fig. 4 and Table 2, R2 value (108 Ω) for the device fabricated with N3 co-sensitized with P2 is higher as compared to the devices N3 alone (72 Ω) or N3 co-sensitized with P1 (88 Ω). In other words, fabricated devices based on N3 alone or N3 co-sensitized with P1 possess lower charge recombination resistances as compared to the device based on N3 co-sensitized with P2. Cyclic voltammetry (CV) has been widely used to estimate the HOMO/LUMO energy levels wherein the onset oxidation and reduction potentials are considered corresponding to HOMO and LUMO energy levels. The voltammograms of N3, P1 and P2 alone, and N3 + cosensitized with P1 and P2 were determined by making their thin films on the glassy carbon electrode in dry acetonitrile, in the presence of an electrolyte tetrabutylammonium perchlorate (TBAPC) at a scan rate of 50 mV/s (Fig. S7). Auxiliary electrode of platinum sheet and reference electrode of Ag/AgCl/3M KCl were used. Before employing the sample, all measurements were calibrated with standard of ferrocene/
The HOMO and LUMO energy level are summarized in the table 2 and their energy band diagram is given in Fig. 5. As illustrated in Fig. 5, the HOMO levels of P1 and P2 were found to be higher than that of HOMO level of N3. The band gap determined by these voltammograms, suggested a significant drop in the bandgaps of N3 co-sensitized with the P2 and P1 as compared to N3 alone. Likewise, the LUMO of N3 co-sensitized with P1 and P2 have higher value than that of N3 alone, which hinted that there was enough possibility for the charge transfer (Lim et al., 2018). 4. Conclusions In conclusion, synthesis of two 1,5-naphthyridine-based conjugated polymers (P1 and P2) was accomplished by employing Horner-Emmons polymerization reaction. After extensive structural characterization, these conjugated polymers were used as co-sanitizers in N3 dye based DSSCs. The detailed investigation revealed that the device fabricated with co-sensitizers enhanced the photovoltaic performance solar cells as compared to N3 alone sensitized solar cell. P2 as co-sensitizer gave a better photovoltaic performance with efficiency enhancement of 6.21% as compared P1 (5.88%). The fabricated device based on N3 co-sensitized with P2 displayed higher charge recombination resistances as compared to the devices based on N3 alone or N3 co-sensitized with P1. This study suggests that the use of 1,5-naphthyridine-based conjugated polymers as co-sensitizers in DSSCs inherits promise for further exploration to develop DSSCs with higher efficiencies. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements The financial support from KFUPM project # DF181029 and research facilities provided by Center of Research Excellence in
Fig. 5. Energy Band diagram of N3, N3 & P1 and N3 & P2. 686
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Renewable Energy (CORE-RE) are gratefully acknowledged.
new 3-hexyl-2,5-diphenylthiophene: phenylene vinylenes copolymers as colorimetric sensor for iodide anion. J. Appl. Polym. Sci. 134. Mehmood, U., Mansha, M., Ullah, N., 2017. 3-Hexyl-2,5-diphenylthiophene:phenylene vinylene-based conjugated polymer for solar cells application. Dyes Pigments 144, 218–222. Mozaffari, S., Nateghi, M.R., Zarandi, M.B., 2014. Effects of multi anchoring groups of catecholamine polymer dyes on the electrical characteristics of metal free dye-sensitized solar cells: a comparison study. Sol. Energy 106, 63–71. Mwaura, J.K., Zhao, X., Jiang, H., Schanze, K.S., Reynolds, J.R., 2006. Spectral broadening in nanocrystalline TiO2 solar cells based on poly(p-phenylene ethynylene) and polythiophene sensitizers. Chem. Mater. 18, 6109–6111. Naik, P., Abdellah, I.M., Abdel-Shakour, M., Su, R., Keremane, K.S., El-Shafei, A., Vasudeva Adhikari, A., 2018a. Improvement in performance of N3 sensitized DSSCs with structurally simple aniline based organic co-sensitizers. Sol. Energy 174, 999–1007. Naik, P., Abdellah, I.M., Abdel-Shakour, M., Acharaya, M., Pilicode, N., El-Shafei, A., Adhikari, A.V., 2018b. An efficient aniline-based co-sensitizer for high performance N3-sensitized solar cells. Chem. Select 3, 12297–12302. Naik, P., Elmorsy, M.R., Su, R., Babu, D.D., El-Shafei, A., Adhikari, A.V., 2017a. New carbazole based metal-free organic dyes with D-Π-A-Π-A architecture for DSSCs: synthesis, theoretical and cell performance studies. Sol. Energy 153, 600–610. Naik, P., Su, R., Elmorsy, M.R., El-Shafei, A., Adhikari, A.V., 2017b. New di-anchoring A–π-D–π-A configured organic chromophores for DSSC application: sensitization and co-sensitization studies. Photochem. Photobiol. Sci. 17, 302–314. Nair, A.S., Jose, R., Shengyuan, Y., Ramakrishna, S., 2011. A simple recipe for an efficient TiO2 nanofiber-based dye-sensitized solar cell. J. Colloid Interface Sci. 353, 39–45. Senadeera, G.K.R., Kitamura, T., Wada, Y., Yanagida, S., 2005. Photosensitization of nanocrystalline TiO2 films by a polymer with two carboxylic groups, poly (3-thiophenemalonic acid). Sol. Energy Mater. Sol. Cells 88, 315–322. Sharma, G.D., Singh, S.P., Kurchania, R., Ball, R.J., 2013. Cosensitization of dye sensitized solar cells with a thiocyanate free Ru dye and a metal free dye containing thienylfluorene conjugation. RSC Adv. 3, 6036. Shrestha, N.K., Yoon, S.J., Bathula, C., Opoku, H., Noh, Y.-Y., 2019. Hole-induced polymerized interfacial film of polythiophene as co-sensitizer and back-electron injection barrier layer in dye-sensitized TiO2 nanotube array. J. Alloys Compd. 781, 589–594. Siddiqui, M.N., Mansha, M., Mehmood, U., Ullah, N., Al-Betar, A.F., Al-Saadi, A.A., 2017. Synthesis and characterization of functionalized polythiophene for polymer-sensitized solar cell. Dyes Pigments 141, 406–412. Singh, S.P., Chandrasekharam, M., Gupta, K.S.V., Islam, A., Han, L., Sharma, G.D., 2013. Co-sensitization of amphiphilic ruthenium (II) sensitizer with a metal free organic dye: improved photovoltaic performance of dye sensitized solar cells. Org. Electron. 14, 1237–1241. Su’ait, M.S., Rahman, M.Y.A., Ahmad, A., 2015. Review on polymer electrolyte in dyesensitized solar cells (DSSCs). Sol. Energy 115, 452–470. Wang, Y., Chen, B., Wu, W., Li, X., Zhu, W., Tian, H., Xie, Y., 2014. Efficient solar cells sensitized by porphyrins with an extended conjugation framework and a carbazole donor: from molecular design to cosensitization. Angew. Chem. Int. Ed. 53, 10779–10783. Yella, A., Lee, H.-W., Tsao, H.N., Yi, C., Chandiran, A.K., Nazeeruddin, M.K., Diau, E.W.G., Yeh, C.-Y., Zakeeruddin, S.M., Grätzel, M., 2011. Porphyrin-sensitized solar cells with cobalt (II/III)-based redox electrolyte exceed 12 percent efficiency. Science 334, 629–634. Younas, M., Gondal, M.A., Dastageer, M.A., Baig, U., 2019a. Fabrication of cost effective and efficient dye sensitized solar cells with WO3-TiO2 nanocomposites as photoanode and MWCNT as Pt-free counter electrode. Ceram. Int. 45, 936–947. Younas, M., Gondal, M.A., Mehmood, U., Harrabi, K., Yamani, Z.H., Al-Sulaiman, F.A., 2018. Performance enhancement of dye-sensitized solar cells via cosensitization of ruthenizer Z907 and organic sensitizer SQ2. Int. J. Energy Res. 42, 3957–3965. Younas, M., Gondal, M.A., Dastageer, M.A., Harrabi, K., 2019b. Efficient and cost-effective dye-sensitized solar cells using MWCNT-TiO2 nanocomposite as photoanode and MWCNT as Pt-free counter electrode. Sol. Energy 188 (2019), 1178–1188. Zouhri, K., 2018. The effect of iodide and tri-iodide on the dye sensitized solar cell. Renew. Energy 126, 210–225.
Appendix A. Supplementary material Supplementary data to this article can be found online at https:// doi.org/10.1016/j.solener.2019.11.022. References Barbera, E., Sforza, E., Guidobaldi, A., Di Carlo, A., Bertucco, A., 2017. Integration of dyesensitized solar cells (DSC) on photobioreactors for improved photoconversion efficiency in microalgal cultivation. Renew. Energy 109, 13–21. Cao, Y., Bai, Y., Yu, Q., Cheng, Y., Liu, S., Shi, D., Gao, F., Wang, P., 2009. Dye-sensitized solar cells with a high absorptivity ruthenium sensitizer featuring a 2-(Hexylthio) thiophene conjugated bipyridine. J. Phys. Chem. C 113, 6290–6297. Clifford, J.N., Martínez-Ferrero, E., Viterisi, A., Palomares, E., 2011. Sensitizer molecular structure-device efficiency relationship in dye sensitized solar cells. Chem. Soc. Rev. 40, 1635–1646. Fang, Z., Eshbaugh, A.A., Schanze, K.S., 2011. Low-bandgap donor−acceptor conjugated polymer sensitizers for dye-sensitized solar cells. J. Am. Chem. Soc. 133, 3063–3069. Gao, Y., Zou, X., Sun, Z., Huang, Z., Zhou, H., 2012. Enhancement of electron transfer efficiency in solar cells based on PbS QD/N719 dye cosensitizers. J. Nanomater. 2012, 415370. Grätzel, M., 2003. Dye-sensitized solar cells. J. Photochem. Photobiol. C Photochem. Rev. 4, 145–153. Grätzel, M., 2005. Solar energy conversion by dye-sensitized photovoltaic cells. Inorg. Chem. 44 (20), 6841–6851. Han, Z., Liu, L., Forsberg, E., 2006. Ultra-compact directional couplers and Mach-Zehnder interferometers employing surface plasmon polaritons. Opt. Commun. 259, 690–695. Han, J., Fan, F., Xu, C., Lin, S., We, M., Duan, X., Wang, Z.L., 2010. ZnO nanotube-based dye-sensitized solar cell and its application in self-powered devices. Nanotechnology 21, 405203. Kanimozhi, C., Balraju, P., Sharma, G.D., Patil, S., 2010. Diketopyrrolopyrrole-based donor−acceptor copolymers as organic sensitizers for dye sensitized solar cells. J. Phys. Chem. C 114, 3287–3291. Kumar, K.A., Subalakshmi, K., Karl Chinnu, M., Senthilselvan, J., 2019. Polarization effect of dye-sensitizers on the current density and photovoltaic efficiency of co-sensitized DSSCs using metal-free and metal-based organic dyes. J. Mater. Sci. Mater. Electron. 30, 230–240. Lee, H., Kim, J., Kim, D.Y., Seo, Y.Y., 2018. Co-sensitization of metal free organic dyes in flexible dye sensitized solar cells. Org. Electron. 52, 103–109. Li, S., Lin, Y., Tan, W., Zhang, J., Zhou, X., Chen, J., Chen, Z., 2010. Preparation and performance of dye-sensitized solar cells based on ZnO-modified TiO2 electrodes. Int. J. Miner. Metall. Mater. 17, 92–97. Lim, I., Bui, H.T., Shin, C.Y., Shrestha, N.K., Bathula, C., Lee, T., Noh, Y.-Y., Han, S.-H., 2018. Study of PEDOT and analogous polymer film as back-electron injection barrier and electrical charge storing materials. Mater. Lett. 211, 1–4. Lim, I., Yoon, S.J., Lee, W., Nah, Y.-C., Shrestha, N.K., Ahn, H., Han, S.-H., 2012. Interfacially treated dye-sensitized solar cell with in situ photopolymerized iodine doped polythiophene. ACS Appl. Mater. Interfaces 4, 838–841. Luo, J., Xu, M., Li, R., Huang, K.-W., Jiang, C., Qi, Q., Zeng, W., Zhang, J., Chi, C., Wang, P., Wu, J., 2014. N -Annulated Perylene as an efficient electron donor for porphyrinbased dyes: enhanced light-harvesting ability and high-efficiency Co(II/III)-based dye-sensitized solar cells. J. Am. Chem. Soc. 136, 265–272. Mansha, M., Khan, I., Ullah, N., Qurashi, A., 2017a. Synthesis, characterization and visible-light-driven photoelectrochemical hydrogen evolution reaction of carbazolecontaining conjugated polymers. Int. J. Hydrogen Energy 42, 10952–10961. Mansha, M., Mehmood, U., Ullah, N., 2018. Poly(phenylene cyanovinylenes) carbazole based conjugated polymer as a photosensitizer for dye-sensitized solar cells. Mater. Lett. 231, 56–59. Mansha, M., Sohail, M., Ullah, N., 2017b. Synthesis, characterization, and properties of
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1,5-Naphthyridine-based conjugated polymers as co-sensitizers for dyesensitized solar cells Muhammad Manshaa,c,1, Muhammad Younasb,c,1, Muhammad Ashraf Gondalb,c, Nisar Ullaha,
T ⁎
a
Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Laser Research Group, Department of Physics, King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia c Center of Research Excellence in Nanotechnology (CENT), King Fahd University of Petroleum and Minerals (KFUPM), Dhahran 31261, Saudi Arabia b
A R T I C LE I N FO
A B S T R A C T
Keywords: 1,5-Naphthyridine-based conjugated polymers N3 dye Co-sensitization Solar cell
1,5-naphthyridine-based conjugated polymers (P1 and P2) were synthesized and extensively characterized by 1H NMR, thermogravimetric analysis (TGA), FT-IR. These polymers were employed as co-sensitizers in DSSCs sensitized with Ru (II) based N3 dye. In comparison to the N3 sensitized device, the P1 and P2 co-sensitized solar cells demonstrated enhanced open circuit voltage, (VOC) of 825 and 788 mV and better fill factor (FF) of 59 and 58%, respectively. The co-sensitization of P1 and P2 with N3 increased the overall efficiencies to 5.88% and 6.21%, respectively, as compared to 5.58% for N3 sensitizer alone. The fabricated device based on N3 cosensitized with P2 displayed higher charge recombination resistances as compared to the devices based on N3 alone or N3 co-sensitized with P1. The conjugated polymers are believed to enhance light harvesting ability and reduce the charged recombination in the co-sensitized solar cells.
1. Introduction Given the limited resource of fossils fuel and climate change considerations, dye-sensitized solar cells (DSSCs) is considered a promising technology for low cost solar to electricity conversion (Zouhri, 2018). As a new generation photovoltaic device, DSSCs is considered superior to the conventional silicon-based photovoltaic devices owing to its flexibility and cost-effective production methods (Lim et al., 2012). Consequently, enormous efforts have been devoted to improving the efficiency of DSSCs (Barbera et al., 2017). DSSC is composed of a photoanode, made of a dye-adsorbed onto a thin film of a wide band gap semiconductor (usually TiO2) deposited on a fluorine doped tin oxide (FTO) coated glass substrate, an electrolyte/hole transporter and a counter electrode. Absorption of a photon by a dye molecule leads to excitation of its ground-state electron, which in turn is injected into the conduction band of semiconductor. The oxidized dye molecule takes electron back from a redox species present in the electrolytic solution to produce ground-state structure. The circuit is completed when oxidized species are reduced back at the counter electrode (Grätzel, 2003). The efficiency of DSSCs is mainly depends upon anode, the photosensitizer, the counter electrode and the electrolyte. Nevertheless, the sensitizer, a key component of DSSCs, plays a key role on the device efficiency and stability. Consequently, metal-free organic dyes and metalorganic
complexes-based sensitizers have been developed for light harvesting (Mansha et al., 2018). Until now ruthenium complexes have emerged as the most efficient sensitizers, allowing 10–11% solar-to-electric power conversion efficiencies (Cao et al., 2009). However, limitations such as heavy-metal toxicity, limited ruthenium resource, difficulty in purifications and stability issues make these complexes unfavorable for large-scale applications. The use of single dye-sensitization strategy suffers from (1) achieving wide optical absorption with high extinction coefficient and (2) high recombination dynamics of charge carriers. These limitations have been overcome by employing the co-sensitization method, which allows to achieve wide spectral absorption with high extinction co-efficient (ε) and also increases electron recombination lifetime from 0.1 ms to 1 s (Kumar et al., 2019). Organic dyes-based sensitizers enjoy advantages of higher molar absorption coefficient, ease of modification and are more environmental friendly. The performance of DSSC has been improved by employing co-sensitization based on combination of metal-free organic dyes and metal-based dyes (Younas et al., 2018, Luo et al., 2014; Naik et al., 2017a; Sharma et al., 2013; Siddiqui et al., 2017; Singh et al., 2013; Su’ait et al., 2015; Wang et al., 2014; Yella et al., 2011). Consequently, fabrication of DSSCs by combination of Ru (II) complex and metal-free organic push-pull configured dyes such as thiophene, triphenylamine, carbazole, indole, phenothiazine as co-
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Corresponding author. E-mail address: [email protected] (N. Ullah). 1 These authors contributed equally. https://doi.org/10.1016/j.solener.2019.11.022 Received 6 June 2019; Received in revised form 3 November 2019; Accepted 6 November 2019 0038-092X/ © 2019 International Solar Energy Society. Published by Elsevier Ltd. All rights reserved.
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solutions of polymer (15 mg/mL).
sensitizers have been explored in the past (Lee et al., 2018; Naik et al., 2017b). The incorporation of an organic dye with Ru (II) complex in cosensitized device facilitate enhanced harvesting of incident photons which in turn resulted in increased power conversion efficiencies (PCEs) (Clifford et al., 2011). Recently, conjugated polymers have been implicated for metal free DSSCs application (Mozaffari et al., 2014). The advantages of these materials as sensitizer include controllable energy levels (HOMOLUMO) by the deployment of electron-rich and electron-deficient alternating units in the polymer backbone, cost-effective structural modification and large absorption coefficients (Kanimozhi et al., 2010). Moreover, in comparison to small molecular dyes, polymeric dyes can be attached onto the surface of metal oxide surface more stably through several anchoring interactions along a long polymer chain and thus provide more stability. Nevertheless, utilization of conjugated polymers as a sensitizer is not frequently explored (Fang et al., 2011; Naik et al., 2018a). Among the few studies, polythiophene (Mwaura et al., 2006; Senadeera et al., 2005) donor (D)-acceptor (A) co-polymer (Kanimozhi et al., 2010), poly epinephrine and poly dopamine (Mozaffari et al., 2014) and substituted carbazole based dyes as photosensitizers have been reported. It has to be borne in mind that the transfer of photogenerated electrons through a series of interfaces is needed in order for a successful power conversion. However, back-electron transfer reaction, wherein electrons from the conduction band of TiO2 are backinjected to the electrolyte, is considered the main obstacle. To overcome this problem, a wide range of diverse material-based thin energy insulating barrier films have been employed (Shrestha et al., 2019). Lee et al., have successfully used conjugated polymers-based thin film both as co-sensitizer and back-electron injection barrier layer for enhanced electron density at TiO2 (Han et al., 2006). Based on the above consideration, herein we wish to report the synthesis 1,5-naphthyridine-based conjugated polymers and their utilization as as co-sensitizers with a ruthenium-based dye N3 for DSSCs. The structural architect of P1 and P2 contains a central donor moiety, dialkoxyphenylene core, attached to two methoxyphenoxy-6-(3-methoxyphenoxy)-1,5-naphthyridine functions. We envisioned that the incorporation of P1 and P2 would enhance harvesting of incident photons and offer more stability by firm attachment of the polymers to the surface of metal oxide through several anchoring interactions along a long polymer chain. The fabricated devices based on N3 co-sensitized with P1 or P2 have shown greater power conversion efficiencies (PCEs) as compared to devices based on the individual P1 and P2 or N3 alone.
2.2. Synthesis of P1 To a solution of mixture of monomer 2 (0.52 g, 0.70 mmol) and dialdehyde 6 (0.30 g, 0.70 mmol) in anhydrous DMF (40 mL) was added sodium tert-butoxide (0.34 g, 3.48 mmol) and the mixture was heated at 100 °C for 24 h under nitrogen atmosphere. After cooling to room temperature, the reaction mixture was poured into 75 mL of methanol followed by centrifuging and decantation allowed to obtain the solid product. The small molecule impurities and oligomers were then removed by re-dissolving the residues in THF and re-precipitating it from methanol, isopropanol, and hexane gave P1 as a dark yellow solid (81%). 1H NMR (500 MHz, CDCl3): δ 8.01 (2H, br.), 7.48 (2H, br.), 7.44 (2H, br.), 7.20 (2H, br.), 7.15 (2H, br.), 7.11 (2H, br.), 7.09 (2H, br.), 7.05 (2H, br.), 4.07 (4H, br.), 3.81 (6H, br.), 1.90–1.85 (4H, br.), 1.53 (4H, m), 1.33 (32H, m), 0.85 (6H, br.). FT-IR (KBr): 3026, 2925, 2848, 1596, 1503, 1356, 1246, 1029 cm−1. GPC analysis: Mn = 3340, Mw = 5720, PDI = 1.70. Anal. Calcd for (C56H72N2O6)n: C, 77.38; H, 8.35; N, 3.22. Found: C, 76.86; H, 8.58; N, 2.90. 2.3. Synthesis of P2 By adopting procedure for the synthesis of P1, P2 was synthesized from the reaction of dialdehyde 6 and monomer 3 as a light-yellow solid (57%). 1H NMR (500 MHz, CDCl3): δ 8.25 (2H, br.), 8.02 (2H, br.), 7.84 (2H, br.), 7.61 (2H, br.), 7.45 (2H, br.), 7.35 (2H, br.), 7.19 (2H, br.), 7.15 (2H, br.), 3.96 (4H, br.), 3.79 (6H, br.H), 2.06 (4H, br.H), 1.92–1.85 (4H, br.), 1.53 (4H, m), 1.32 (20H, m), 1.01 (6H, br.), 0.91 (6H, br.). FTIR IR (KBr): 3077, 2923, 2853, 1593, 1506, 1473, 1348, 1278 cm−1. GPC analysis: Mn = 3618, Mw = 5930, PDI = 1.62. Anal. Calcd for (C48H58N2O6)n: C, 76.16; H, 7.46; N, 3.70. Found: C, 75.84; H, 7.68; N, 3.38. 2.4. Fabrication of PSSC Fluorine doped tin oxide (FTO) conductive glasses with area 1.5 cm2 were physically washed with soap and cotton buds followed by ultrasonically washing sequentially with deionized water, acetone and isopropanol for 10 min each. After drying washed FTO at 100 °C for 30 min, titanium dioxide (TiO2) paste was coated on the cleaned FTO with doctor blade method and calcined first at 450 °C for 30 min. The as prepared TiO2 coated anode were soaked into pure N3 dye solution (0.5 mM in ethanol) and also in pure polymers P1 and P2 solutions (15 mg/ml in THF) for 24 h. The two photoanodes sensitized with pure N3 dye were further separately soaked into P1 and P2 solutions for next 24 h for co-sensitization effect. Counter electrodes were prepared using platinum (Pt) paste with doctor blade method and Pt coated FTO were sintered at 450 °C for 30 min. All the photoanode were removed from respective solutions and rinsed with ethanol to remove unanchored dye/polymer. Both photoanode and counter electrode were joined together with super glue and electrolyte (I−/I3−) was poured with 5 μm micropipette. The active area of the PSSC was 0.25 cm2. To estimate the dyes loading on the photoanode of TiO2, samples TiO2/N3, TiO2/N3/ P1, and TiO2/N3/P2 sensitized with N3, N3 + P1 and N3 + P2, respectively, were placed into a 20 mM KOH solution and stirred at room temperature for 24 h in order to desorb the dyes. The concentrations of the desorbed dyes were determined by using UV–vis spectroscopy (Han et al., 2010). For this purpose, calibration curves were plotted for N3 (0.5 mM), N3 + P1 (0.5 mM and 15 mg/mL) and N3 + P2 (0.5 mM and 15 mg/mL) solution by making a series of dilutions, ranging from 0.5 mM to 0.0078 mM. In each calibration curve the value of coefficient of determination (R2) was maintained > 0.999. The total amount of N3 obtained after desorption of TiO2/N3 solar cell was found to be 0.090 mM per 0.25 cm2 area. Similarly, desorption of TiO2/N3/P1 and TiO2/N3/P2 lead to the total calculated amounts of 0.146 mM and
2. Experimental 2.1. Materials and Instruments 1 H and 13C NMR on 500 MHz spectrometer (JEOL JNM-LA, JEOL USA Inc.) and FTIR spectrophotometer (Perkin Elmer 16F PC, Perkin Elmer Inc. USA) was used for Infrared spectra. The UV–vis absorption spectra on Cary 5000 spectrophotometer (UV–vis-NIR, Agilent Technologies). SDT Q600 (V20.9 Build 20) thermal analyzer was used for thermogravimetric analysis (TGA). Samples were heated to 800 °C at a rate of 10 °C/min rise under oxygen atmosphere, with a purge rate of 50 mL/min. Fluorine doped tin oxide (FTO TCO22-7/LI; 2.2 mm, 7 Ω/ Seq), titanium paste (TiO2 Ti-nanoxide T/SP), Ruthnizer (N3 Ruthenizer 535), electrolyte (I−/I3− Iodolyte AN-50) and platinum paste (Pt Platisol T) were purchased from Solaronix, Switzerland. The current–voltage (J-V) characteristics were measured using 1.5G (100 mW/cm2) IV-5 solar simulator (PV measurements Incorporation Sr. No. 83) with Keithley 2400 as a source meter. Bandgaps determination and estimation of HOMO and LUMO level of P1 and P2 cosensitized with N3 dye was carried out by cyclic voltammetry (CV), using the electrochemical workstation (BioLogic Science Instruments (Model: VMP3 SAS)). Solution of N3 dye (0.5 mM) was deposited on glassy carbon electrode first, which was dried followed by addition of
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Scheme 1. Synthesis of monomers 2, 3, 6 and polymers P1 & P2.
0.174 mM for N3 + P1 and N3 + P2, respectively.
3. Results and discussions 3.1. Synthesis and characterization The naphthyridine based conjugated polymers P1 and P2 were synthesized as outlined in Scheme 1. Horner-Emmons reaction between dialdehyde 6 and 2 or 3 (Mansha et al., 2017b, 2017a; Mehmood et al., 2017) monomers furnished polymers P1 and P2, respectively, in good yield (Scheme 1). The chemical structures of polymers P1 and P2 were characterized on the basis of 1H NMR, 13C NMR and IR spectra (Figs. S1–S4). In 1H NMR of P1 and P2, the characteristic peaks for the phosphonate groups of monomers 2 and 3 at 4.02–4.01 ppm as well as aldehydic peaks of monomer 6 at 10.08 ppm were disappeared. Likewise, the absence of aldehydic peaks (2855, 2725, and 1710 cm−1) in the IR spectrum of P1 and P2 suggested the formation of P1 and P2. The thermogravimetric analysis of P1 and P2 indicated that both polymers exhibited high thermal stability with no appreciable loss of mass up to 275 °C. However, the onset decomposition temperatures (Td) of P1 and P2 were found to be as 325 and 375 °C, respectively. Both polymers displayed abrupt major weight loss above Td, which hinted the decomposition of polymer backbone (Fig. 1). The presence of alkoxy groups on these polymers made them soluble in organic solvents like methylene chloride, chloroform and tetrahydrofuran. The UV–Vis absorption spectra of N3 dye, polymers (P1 & P2) and co-sensitized N3 with polymers (P1 & P2) were recorded on TiO2 films, as shown in Fig. 2. The spectra of P1 and P2 displayed broad band absorptions around 500–250 nm. Likewise, the spectrum of N3 dye revealed the absorption edge starting from 700 nm whereas the first broad band appeared near 525 nm along with multiple bands in the high frequency region. However, in case of N3 dye co-sensitized with polymers (P1 & P2), the first broad band at 525 nm was slightly blue shifted. In addition, the spectra further exhibited multiple bands in the
Fig. 1. TGA graph of P1 and P2.
higher frequency region with higher intensity as compared to the spectrum of N3 alone. The rise in the absorption was due to the occupancy of empty pores on surface of TiO2 by polymers, which offers higher absorption coefficient and availability of greater amount photosensitizer. The lower energy orbitals provided by the conjugated polymers resulted in the participation of maximum number of incident photons in the high energy region for the electronic transition (Gao et al., 2012). 3.2. Photovoltaic performance To measure the photovoltaic performance of P1 and P2, five different polymer sensitized solar cells (PSSCs) were fabricated. Details on solar cell structures and parameters e.g. Voc (mV), Jsc (mW/cm2), FF (%) and efficiency η (%) are given in Table 1. The current density verses voltage (J-V) curves are provided in Fig. 3. As seen from Table 1 and Fig. 3, pure P1 and P2 exhibited lower efficiencies. However, co-sensitization of P1 and P2 with N3 increased the overall efficiencies to 5.88% and 6.21%, respectively, as compared to 5.58% for N3 alone. In 684
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The incident photon-to-electron conversion efficiency (IPCE), a measure of external quantum efficiency (EQE), is usually determined to assess the performance of solar cells in certain wavelength range. In other words, EQE or IPCE is a strong function of wavelength. The IPCE spectra of TiO2/N3, TiO2/N3/P1, and TiO2/N3/P2 fabricated DSSCs solar cells are given in Fig. S6. It is evident from Fig. S6, that the main pattern of EQE for all the fabricated solar devices are very similar. However, quantitatively the value of EQE was found in the order of TiO2/N3/P2 > TiO2/N3/P1 > TiO2/N3 for the DSSC solar cells. According to Grätzel, IPCE depends greatly on wavelength-based light harvesting efficiency of solar cell (LHE (λ)), the yields of injecting the generated electrons to conduction band energy level (ECB) of the semiconductor and on the collection efficiency of photogenerated electrons (Grätzel, 2005). In addition, the LHE (λ) depends three important factors: (i) The dye adsorption or dye loading on the surface of semiconductor, (ii) light absorbance features of the dye and iii) scattering of light at the surface of photoanode. The IPCE (Fig. S6) and UV–Vis (Fig. 2) spectra for all the fabricated DSSC solar cells explains the dependence of LHE (λ). Similarly, the presence of conjugated polymers enhance the electronic injection into the ECB of semiconductor as the combination of copolymers with N3 led to lower band gap. As for as charge recombination and transport is concerned, the electrochemical analysis shows that resistance to recombination of charges (R2) is higher for TiO2/N3/P2 as compared to other fabricated DSSC solar cells (Fig. 4 and Table 2) (Younas et al., 2019a,b).
Fig. 2. UV–Vis absorption spectra of N3 dye, polymers (P1 & P2) and co-sensitized N3 with polymers (P1 & P2) adsorbed on TiO2 films. Table 1 Photovoltaic properties of fabricated PSSCs. Cell Structure
Jsc (mA/cm2)
Voc (mV)
FF (%)
η (%)
TiO2/Pure N3/Pt TiO2/Pure P1/Pt TiO2/Pure P2/Pt TiO2/N3 + P1/Pt TiO2/N3 + P2/Pt
14.33 1.32 1.06 12.1 13.6
758 613 595 825 788
51 63 56 59 58
5.58 0.51 0.36 5.88 6.21
3.3. Electrochemical impedance spectroscopy (EIS) studies of fabricated DSSCs To study the photovoltaic behavior such as the interfacial charge recombination, EIS analysis of the fabricated devices was conducted. In general, charge recombination dynamics greatly influence the photo voltages of DSSCs. Therefore, EIS spectra of the devices fabricated with N3 alone and co-sensitized with P1 and P2 were carried out at ambient
Fig. 3. J-V curves of DSSCs sensitized with 0.5 mM N3 and co-sensitized with 15 mg/mL of P1 and P2.
addition, both the P1 and P2 based co-sensitized devices exhibited enhanced open circuit voltage, VOC of 825 and 788 mV and better fill factor, FF of 59 and 58%, than N3 alone (Table 1). These results are highly reproducible (Fig. S5 and Table S1). The improvement in the VOC of co-sensitized DSSCs fabricated with polymers is presumably due to the effective adsorption of co-sensitizer along with Ru-II based N3 molecules on the surface of TiO2 which in turn facilitated the suppression of the unwanted back reaction (Naik et al., 2018b). In addition, mixed dyes tend to shift ELUMO close to the CB TiO2 which subsequently enhances electron injection in co-sensitized DSSCs compared to mono-sensitized cell (Kumar et al., 2019).
Fig. 4. (a) EIS Nyquist curves of fabricated DSSCs sensitized with pure N3 (0.5 mM) and co-sensitized with P1 and P2 (15 mg/mL) and (b) equivalent circuit of Nyquist plot. 685
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ferrocenium (FC/FC+). Based on the Bredas Equation, the first oxidation potential and reduction potential are correlated to ionization potential (Ip) and electron affinity (Ea), respectively. Band gap (Eg) of N3 alone and its mixture with P1 and P2 can be determined by subtracting Ea from Ip.
Table 2 Charge recombination resistance (R2) at anode interface (TiO2/N3, TiO2/ N3 + P1 or TiO2/N3 + P2/electrolyte) and HOMO-LUMO energy level calculations from cyclovoltammetry. Sample
DSSCs
R2 (Ohm)
HOMO (eV)
LUMO (eV)
Eg (eV)
1 2 3
N3 N3 & P1 N3 & P2
72 88 108
5.08 4.82 4.88
3.92 3.83 4.09
1.16 0.99 0.79
EHOMO = [(Eox − E1/2) + 4.8] eV EHOMO = [(Ered − E1/2) + 4.8] eV
Eg = EHOMO (Ip) − EHOMO (Ea) conditions at different frequencies (100 mHz–1 MHz) by employing AC voltage. Fig. 4a indicates the electrochemical impedance (Nyquist plot) of the fabricated devices and equivalent circuit model is given in Fig. 4b. Usually EIS is exhibited by three semicircles (R1, R2 and R3). The first semicircle (R1) at higher frequency represents the interface resistance at counter electrode and electrolyte. Likewise, the second semicircle at intermediate frequency (R2) and third semicircle at lower frequency (R3) is related to resistance at photoanode/dye/electrolyte interface and diffusion of the carrier transport existing inside the electrolyte through redox couple (Zw) also called as Warburg diffusion impedance. In addition, the Rs (sheet resistance) shows the point of first arc at the highest frequency. The resistance at interface of counter electrode and electrolyte is given as R1 whereas the resistance to the recombination at photoanode and electrolyte interface expressed by R2. Therefore, the higher value of R2 indicates longer lifetime of electrons (Younas et al., 2019a,b). Moreover, C1 and C2 represent capacitances to the interfaces against R1 and R2. The photovoltaic behavior of fabricated devices, N3 alone, and co-sensitized with P1 and P2, suggested the appearance of R2 resistance at photoanode/dye/electrolyte and the remaining arcs might have been overshadowed by the second arc (Li et al., 2010; Nair et al., 2011). As evident from Fig. 4 and Table 2, R2 value (108 Ω) for the device fabricated with N3 co-sensitized with P2 is higher as compared to the devices N3 alone (72 Ω) or N3 co-sensitized with P1 (88 Ω). In other words, fabricated devices based on N3 alone or N3 co-sensitized with P1 possess lower charge recombination resistances as compared to the device based on N3 co-sensitized with P2. Cyclic voltammetry (CV) has been widely used to estimate the HOMO/LUMO energy levels wherein the onset oxidation and reduction potentials are considered corresponding to HOMO and LUMO energy levels. The voltammograms of N3, P1 and P2 alone, and N3 + cosensitized with P1 and P2 were determined by making their thin films on the glassy carbon electrode in dry acetonitrile, in the presence of an electrolyte tetrabutylammonium perchlorate (TBAPC) at a scan rate of 50 mV/s (Fig. S7). Auxiliary electrode of platinum sheet and reference electrode of Ag/AgCl/3M KCl were used. Before employing the sample, all measurements were calibrated with standard of ferrocene/
The HOMO and LUMO energy level are summarized in the table 2 and their energy band diagram is given in Fig. 5. As illustrated in Fig. 5, the HOMO levels of P1 and P2 were found to be higher than that of HOMO level of N3. The band gap determined by these voltammograms, suggested a significant drop in the bandgaps of N3 co-sensitized with the P2 and P1 as compared to N3 alone. Likewise, the LUMO of N3 co-sensitized with P1 and P2 have higher value than that of N3 alone, which hinted that there was enough possibility for the charge transfer (Lim et al., 2018). 4. Conclusions In conclusion, synthesis of two 1,5-naphthyridine-based conjugated polymers (P1 and P2) was accomplished by employing Horner-Emmons polymerization reaction. After extensive structural characterization, these conjugated polymers were used as co-sanitizers in N3 dye based DSSCs. The detailed investigation revealed that the device fabricated with co-sensitizers enhanced the photovoltaic performance solar cells as compared to N3 alone sensitized solar cell. P2 as co-sensitizer gave a better photovoltaic performance with efficiency enhancement of 6.21% as compared P1 (5.88%). The fabricated device based on N3 co-sensitized with P2 displayed higher charge recombination resistances as compared to the devices based on N3 alone or N3 co-sensitized with P1. This study suggests that the use of 1,5-naphthyridine-based conjugated polymers as co-sensitizers in DSSCs inherits promise for further exploration to develop DSSCs with higher efficiencies. Declaration of Competing Interest The authors declared that there is no conflict of interest. Acknowledgements The financial support from KFUPM project # DF181029 and research facilities provided by Center of Research Excellence in
Fig. 5. Energy Band diagram of N3, N3 & P1 and N3 & P2. 686
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Renewable Energy (CORE-RE) are gratefully acknowledged.
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