Investigation of the effect of a nitro group as an anchor group in dye-sensitized solar cells

Abstract

As a result of the increasing in the world population, the development of technology to meet increasing energy needs and to find other energy resources is one of the biggest tests for society. Triphenylamine and its derivatives attract the attention of researchers due to their many applications such as in solar cells, electronics, and medicine. The nitro group, an electron-accepting chromophore, is a good candidate for D–π–A sensitizers for dye-sensitized solar cells. In this study, the power conversion efficiency of an anchor nitro group in dye-sensitized solar cells and the synthesis of new compounds containing a donor triphenylamine unit, functionalized with a nitro group as an anchor group and possessing a π-conjugated structure are investigated. The structures of the compounds are determined by FTIR, UV-Vis spectrometry, NMR, and MS. Considering the photovoltaic performance of the produced dye-sensitized solar cell devices, the anchor group, and its acceptor effect, it is observed that the efficiency increases, in the order, F2 < F1. The highest power conversion efficiency value of 0.45% was obtained with the F1-based dye-sensitized solar cell under amplitude modulation irradiation (100 mW cm⁻²). From the obtained results, it can be seen that an increase in the number of electron-donor groups located close to the anchor group increases the power conversion efficiency.

Introduction

Global warming has become one of the most important factors threatening the world. Fossil fuels, which are mainly used for energy requirements, take first place among the causes of global warming. Intensive consumption of fossil fuels causes depletion of the world’s fossil fuel reserves, environmental pollution, and contributes to the greenhouse effect. Various renewable energy sources are used as alternatives and serve as solutions to these problems. Of all the forms of renewable energy, solar energy is the most popular. Solar cells attract significant attention as one of the most convenient ways to convert solar energy into electricity. ¹ Traditional costly and less efficient silicon-based solar cells are being replaced by dye-sensitized solar cells (DSSCs).

In DSSCs, ruthenium-containing dyes show superior photovoltaic performance compared to other metal complexes. ² However, the large-scale use of ruthenium-based paints in DSSC applications is limited. As ruthenium has high production costs and is toxic, it causes environmental pollution. Therefore, many metal-free organic dyes are being tested as effective dye-sensitizing solar cells. ³ Among metal-free organic dyes, triphenylamine (TPA) dyes are promising candidates as high-throughput DSSCs. ⁴

TPA dyes are widely used as sensitizers in DSSCs due to their strong electron-donating ability. They exhibit high molar extinction coefficients due to intermolecular π–π* transitions. TPA dyes are widely used as sensitizers because they are strong electron donors, and their derivatives occupy an important place in many industrial applications. For example, they are widely used in organic photovoltaic functional materials, ¹ and as block structures for conductive polymers ⁵ and high-spin polyradicals.^6,7 TPA derivatives have also been used in pharmacology and in biosensor construction. Furthermore, as an organic semiconductor, TPA is used as a hole-transfer material for optoelectronic applications. In addition to its very good pore-carrying properties, it also prevents aggregation due to its non-planar structure. ¹

In this study, we describe the synthesis and solar cell applications of organic electronic materials with D–π–A properties. The constructed products possess a nitro group–containing moiety, as an anchor group, high absorption bands in the visible region, and demonstrate attached to the TPA structure of their UV spectrum.

Results and discussion

Characterization

The new ligands F1 and F2 were synthesized in good yields by the condensation reaction of 4-(diphenylamino)benzaldehyde with 4-nitroaniline or 2-chloro-4-nitroaniline (Scheme 1). The FTIR spectra of F1 and F2 show characteristic Schiff base stretching bands at 1619 and 1570 cm⁻¹. These intense bands are assigned to the C=N stretching frequencies of the ligands and are characteristic of the azomethine moiety of most Schiff base compounds. The absorption bands of the substrates at 3480–3350 cm⁻¹ (NH₂) and 1686 cm⁻¹ (C=O) disappear in the infrared spectra of F1 and F2, which indicate that the condensation had occurred.

Scheme 1.

Synthesis of F1 and F2.

Both compounds exhibit good solubility in common polar organic solvents such as ethanol, methanol, and chloroform. The UV-Vis spectra of the ligands (Figure 1) showed two types of transitions, the first occurring in the 270–320 nm range, which is due to transitions containing molecular orbitals that can be assigned to the π–π* transition. The second type of transition occurs in the 320–450 nm range, which can be assigned to the n–π* transition, in which the bands become wider.

Figure 1.

Absorption spectra of ligands F1 and F2 in methanol.

In the mass spectra, the molecular ion peaks of F1 and F2 appear at m/z 392.00 [M + H]⁺ and m/z 428.11 [M + H]⁺, thereby data confirming the proposed structures.

Compounds F1 and F2 containing a nitro group as an anchor group attached to the TPA structure were found to be pure by ¹H NMR, with the aromatic protons being observed in their expected regions. In the ¹H NMR spectrum of F1, peaks were observed in the range of 7.61–6.95 ppm for the 14 protons of the TPA unit. The signal at 8.44 ppm is due to the N=CH proton and those at 8.14–6.81 ppm are due to the four protons of the nitrobenzene ring. In the ¹H NMR spectrum of F2, peaks are observed in the range of 7.28–6.68 ppm for the 14 protons of the TPA ring system. The signal at 8.15 ppm is that of the N=CH group, while the resonances at 7.93–7.61 ppm are for the three protons of the nitrobenzene ring.

For DSSC fabrication, transparent TiO₂ paste was deposited on the conductive side of an FTO (fluorine-doped tin oxide) substrate using Doctor Blade’s method. The photoanode was prepared by immersing the coated FTO substrate in a solution containing 0.3 mM F1 and F2 sensitizers in EtOH. The DSSC parameters short-circuit current (J_sc), open-circuit potential (V_oc), and the fill factor (FF) were extracted from the current density–voltage (J-V) curve to find the cell efficiency, as illustrated in Figure 2. Moreover, in our DSSC device, the effective area of the devices was 0.51 cm², which is larger than those of previously reported devices.

Figure 2.

Current density–voltage curves of illuminated DSSCs using dyes F1 and F2.

The current–voltage (I-V) curves recorded under light and in the dark clearly show a correlation between the structure of the sensitizer and the power conversion efficiency (PCE) of the device. It was observed that a higher PCE value was obtained with the J_sc, V_oc, and FF values obtained from the F1-based DSSC device produced. This higher PCE value was observed because the NO₂ anchor group in the structure of sensitizer F1 contributes with wider absorption and better light-collecting capacity in the visible region after adsorbing to the TiO₂ surface when compared to the other sensitizer derivative F2. It is seen that the values of the open current voltages obtained from compounds F1 and F2 are higher than those of other nitro anchor–containing compounds in the literature. ⁸ A higher J_sc value was obtained from the device prepared with the F1 sensitizer. The values obtained for the devices can be attributed to the wider absorption in the visible region and the higher molar extinction coefficient, as well as the differences in the molecular structures. The experimental result for the F1 and F2 devices and N719 is summarized in Table 1. N719 dye was used as a standard sensitizer for comparison with the synthesized dyes, and its PCE was measured as 1.032%.

Table 1.

Photovoltaic parameters for the DSSCs sensitized from F1, F2, and N719.

Sample	Jsc (mA cm−2)	Voc (V)	FF	η (%)
N719	6.70	0.66	0.38	1.70
F1	2.10	0.61	0.37	0.45
F2	1.70	0.57	0.20	0.19

The effect of the electron-withdrawing chlorine atom close to the anchor NO₂ group has a negative impact on achieving a better PCE value. It can be seen from the obtained PCE values that the electron density on the anchor NO₂ in sensitizer F2 is reduced by the electron-withdrawing Cl atom, so even if the open I-V value of the device is good, electron transfer to the semiconductor decreases.

Cong et al. ⁹ synthesized a functionalized dye (JY1) with an electron-withdrawing nitro group, in addition to CN and COOH anchor groups, and determined the PCE of the dye as η = 0.81. In another study, Zhang and Cole ⁸ synthesized a new dye to examine the effect of a single NO₂ anchor group on the PCE of the dye, with the value being η = 0.04 in the prepared device. In this study, a very high PCE value was reached compared to the literature using the NO₂ group as an anchor in compounds F1 and F2 used as sensitizers. Badawy et al. ¹⁰ synthesized four innovative phenylacetonitrile and 2-cyanoacetamide derivatives of dyes, and studied how the presence of a thiophene spacer and different auxiliary acceptors affected the photophysical and electrochemical performance of the DSSC devices. They also measured the photovoltaic properties and overall photoconversion efficiencies of the DSSCs for sensitizers SFA5 and SFA6 (Figure 3), achieving the highest value available in the literature of 5.69% for a compound possessing an NO₂ anchor group. ¹⁰ Although the V_oc values of the devices prepared with the F1 and F2 sensitizers are quite close to those prepared with the SFA5 and SFA6 sensitizers, the low PCE value is seen as being due to the effect of the groups directing the electron movement in the sensitizer structure.

Figure 3.

Structures of compounds SFA5 and SFA6. ¹⁰

Conclusion

In this study, sensitizers F1 and F2 containing nitro groups as anchors were synthesized. These sensitizers were used in the manufacture of DSSC devices and the I-V values were measured. The effective photovoltaic performance of the sensitizing F1 DSSC device with a nitro anchor group strongly binds to the TiO₂ surface. Our best result in this study was achieved with the F1-based DSSC device, which exhibited a photovoltaic efficiency of up to 0.45%. It has been observed that the presence of an electron-withdrawing group close to the NO₂ anchor group in the sensitizer structure reduces the PCE of the produced DSSC device, as was the case with F2. These results reveal that direct electron transfer from the donor group in the sensitizer structure to the anchor group leads to an increase in the performance of the sensitized solar cells.

Experimental

Materials

All chemicals were reagent grade from Merck and Fluka. Solvents were distilled from appropriate drying agents prior to use. Commercially available reagents were used without further purification unless otherwise stated. Flash column chromatography was carried out using silica gel 60 (0.04–0.063 mm) from Merck. 4-(Diphenylamino)benzaldehyde was prepared according to the Vilsmeier–Haack reaction, which is widely used to obtain aryl aldehydes. ¹¹

Equipment

FTIR spectra were recorded on a Spectrum One Perkin Elmer FTIR spectrophotometer using attenuated total reflectance (ATR). The mass spectra were acquired on an Agilent 6530 + Agilent high-performance liquid chromatography (HPLC) liquid chromatography–mass spectrometry (LC-MS) Quadrupole time-of-flight (QTOF) instrument, and gas chromatography–mass spectrometry (GC-MS) mass spectra were recorded at the Yıldız Technical University Science and Technology Application and Research Center (BITUAM), Turkey. ¹H NMR spectra were recorded in CDCl₃ solutions on a Varian 500 MHz spectrometer.

Synthesis

Synthesis of 4-{[(4-Nitrophenyl)imino]methyl}-N,N-diphenylaniline (F1)

An ethanol solution of 4-nitroaniline (0.1 g, 0.73 mmol) was added dropwise to a solution of triphenylamine aldehyde (0.2 g, 0.73 mmol) in ethanol (10 mL). The resulting mixture was refluxed for 18 h in the presence of piperidine (0.045 mL, 0.45 mmol) as the catalyst. The solvent was evaporated, and the residue was purified by column chromatography (n-hexane/methanol/chloroform gradient). The obtained product was dried in a vacuum oven to give an orange solid (0.182 g, 63%) is obtained. m.p. 223 °C. UV-Vis (MeOH): λmax/nm (logε/dm³mol⁻¹ cm⁻¹): 698 (5.15), 297 (4.89). FTIR max cm⁻¹: 3057 (Ar-CH), 1570 (C=N), 1507 (−NO₂, asymmetric stretch), 1330 (−NO₂, symmetric stretch), 885 (−NO₂ scissors); ¹H NMR (500 MHz, CDCl₃): d = 8.44 (br s, 1 H), 8.13–8.18 (m, 2 H), 8.10 (d, J = 7.8 Hz, 1 H), 7.61 (d, J = 7.8 Hz, 1 H), 7.39 (t, J = 7.9 Hz, 3 H), 7.30 and 7.28 (t, J = 7.9 Hz, each 1 H), 7.18–7.21 (m, 4 H), 7.15 (d, J = 7.8 Hz, 1 H), 7.07–7.13 (m, 2 H), 6.94 (d, J = 7.8 Hz, 1 H) and 6.81 (br d, J = 7.8 Hz, 1 H); GC-MS: m/z calcd 393.44; found: 392.00 [M–H] ⁺.

Synthesis of 4-{[(2-chloro-4-nitrophenyl)imino]methyl}-N,N-diphenylaniline (F2)

An ethanol solution of 2-chloro-4-nitroaniline (0.126 g, 0.73 mmol) was added dropwise to a solution of triphenylamine aldehyde (0.2 g, 0.73 mmol) in ethanol (10.0 mL). The resulting mixture was refluxed for 18 h in the presence of piperidine (0.045 mL, 0.45 mmol) as the catalyst. The solvent was evaporated, and the residue was purified by gradient column chromatography (n-hexane/methanol/chloroform). The obtained product was dried in a vacuum oven to give a yellow solid (0.148 g, 47%). m.p. 137 °C. UV-Vis (MeOH): λmax/nm (logε/dm³mol⁻¹ cm⁻¹): 353 (5.38). FTIR max cm⁻¹: 3057 (Ar-CH), 1582 (C=N), 1522 (–NO₂, asymmetric stretch), 1321 (–NO₂, symmetric stretch), 893 (–NO₂ scissors), 817, 744, 692 (C-Cl); ¹H NMR (500 MHz, CDCl₃): d = 8.15 (d, J = 1.5 Hz, 1 H), 7.93 (dd, J = 7.8, 1.5 Hz, 1 H), 7.61 (d, J = 7.8 Hz, 2 H), 7.28 (t, J = 7.6 Hz, 4 H), 7.19 (s, 1 H), 7.07–7.12 (m, 6 H), 6.93 (d, J = 7.8 Hz, 2 H), and 6.68 (d, J = 7.8 Hz, 1 H); LC-MS (QTOF): m/z calcd 427.88; found: 428.11 [M]⁺.

Fabrication and characterization of the DSSCs

For DSSC fabrication, two transparent TiO₂ pastes were deposited using Doctor Blade’s method on the conductive side of an FTO substrate, as reported earlier. ¹² In order to prepare the dye-sensitized (F1 and F2) TiO₂ films (photoanodes), the coated FTO substrates were immersed into a 0.3-mM dye solution in EtOH. The Pt counter electrodes were prepared on the FTO substrates by casting from a platinum paste solution. Both the photoanode and counter electrode were sealed with Surlyn film. The redox electrolyte was injected between the electrodes through a drilled hole in the counter electrode. The J-V characteristics were investigated using a potentiostat/galvanostat under AM 1.5 global one sun illumination (100 mW cm⁻²) from a solar simulator.

The electrical performance of the solar cell is determined by the short-circuit current (I_sc), the open-circuit voltage (V_oc), the current at the maximum power point (I_mp), the voltage at the maximum power point (V_mp), the maximum power (P_max), the fill factor (FF), and the photovoltaic cell efficiency (η). The solar cell efficiency and fill factor are calculated using equations (1) and (2), respectively

$$\eta =\frac{P_{\max }}{P_{in}}=\frac{I_{sc}.V_{oc}.FF}{P_{in}}$$

(1)

where the FF is most commonly determined from measurement of the I-V curve, and is given by

$$FF=\frac{I_{mp}.V_{mp}}{I_{mp}.V_{oc}}$$

(2)

Supplemental Material

sj-pdf-1-chl-10.1177_17475198231168948 – Supplemental material for Investigation of the effect of a nitro group as an anchor group in dye-sensitized solar cells

Supplemental material, sj-pdf-1-chl-10.1177_17475198231168948 for Investigation of the effect of a nitro group as an anchor group in dye-sensitized solar cells by Fatma Aytan K1l1çarslan and İbrahim Erden in Journal of Chemical Research