Synthesis, characterization and evaluation of new alternative ruthenium complex for dye sensitized solar cells

Elemental analysis collected in Table 1 confirmed the postulated chemical formula for dyes [RuL2X2], where X = NCS− or NCSe−.Table 1 Elemental analyses data and some physical properties of Ru dyes.The calculated % and measured values of the elemental analysis are in good agreement with a slight differences as shown in Table 1.FTIR spectra of dyes [RuL2X2] are shown in Fig. 3a,b. The main IR vibrational bands with their tentative assignments are given in Table 2. Two bands at 3433 cm−1 and 3078 cm−1 (S–N3) dye at 3407 cm−1, 3066 cm−1 (Se–N3) for stretching υ(OH). Band at 2923 cm−1, 2908 cm−1 for υ(sp2 C–H) for SCN−, SeCN− respectively. Absorption bands at 1708 cm−1 1713 cm−1 for SCN−, SeCN−, respectively assigned υC=O of protonated carboxylic COOH+2 group. Bands at 1369 cm−1, 1363 cm−1 in SCN− and SeCN− respectively (υC=N) of pyridine moiety. Doublet with peaks at 2105 cm−1, 1980 cm−1 for SCN−; 2079 cm−1, 1988 cm−1 for SeCN− characterizing cis-configuration of two-thiocyanate or selenocyanate. NCS− or NCSe− group has two characteristic vibrational modes, υN=C, υC=X, (X = S or Se) diagnose coordination mode of ambidentate SCN− or SeCN− ligands19,20. N-coordination of thiocyanate or selenocyanate group is confirmed by the presence of υC=S, υC=Se vibrational at 780 cm−1, 766 cm−1 respectively. Thiocyanate or selenocyanate groups coordinated to Ru through S or Se atoms. Weak stretching vibration band υC=S & υC=Se appeared at 700 cm−121,22.Figure 3(a) FT-IR spectra of [cis-[RuL2 (NCS)2] and (b) [cis-[RuL2 (NCSe)2] respectively.Table 2 Main IR bands (υ’, cm−1) with their tentative assignments.Molecular structures and chemical compositions of the dyes are confirmed from mass spectra, Fig. 4. Molecular ion signal as function of (m/z) giving molecular weights of dyes at the last spectral line.Figure 4Mass spectra: cis-[RuL2(NCS)2] and cis-[RuL2(NCSe)2] respectively.Dye molecules are ionized into charged molecular ions on bombardment by electron beam that break some sample’s molecules into charged fragments ions separated according to (m/z) ratio of mass spectra of [RuL2 (NCX)2], X = S or Se. Based on fragments ions, Fig. 5 is postulated for fragmentation of the dye molecules. Peaks at 705, 799 for X = S, Se, Main fragments (m/z) with their tentative assignments are given in Table 3.Figure 5Proposed conductive fragmentation pathway for [RuL2(NCS)2].Table 3 Relative intensities of the major molecular ions in Ru complexes.Fragmentation pattern indicated dyes formed as [RuL2 (NCX)2]23. Mass spectra of the dyes are in good agreement with elemental analysis and FTIR spectral data and confirmed the formulation of dyes.The new Se–N3 dye followed the same proposed fragmentation pathway except that S atom replaced by Se atom.UV–Vis. absorption electronic spectra of the dyes were measured in the ethanolic solution of the two complexes RuL2(NCS)2] and RuL2(NCSe)2] and were shown in Fig. 6. Intenses absorption band ligand-centered (LCT) π–π* transitions at 311 nm, 307 nm respectively with transition energy analogous to those of similar of Ru(II) polypyridine and agreed with transition energy reported for N3 dye24,25,26. Both complexes displayed MLCT bands at 350 to 600 nm range27. [RuL2(NCS)2] exhibits two broad bands at 394 nm and 533 nm. The bands for [RuL2(NCSe)2] at 371 nm, and 492 nm are attributed to MLCT absorptions. MC appeared as a shoulder for LC which disappeared in spectra of both complexes because of forbidden transition because it is d–d transition and may be obscured under LC band. Spectra of both complexes exhibit an intense band at 208 nm and 246 nm due to LMCT. Table 4 shows two absorptions in both dyes are red-shifted compared with that of [Ru(bpy)3]2+28 due to: distortion by existence of two 2,2′-bipyridine-4,4′-dicarboxylic acid molecules, two X = NCS−1 or NCSe−1 instead of three bipyridine molecules in each complex, and replacement of one strong bipyridine ligand by two weaker ligand molecules NCS−1 or NCSe−1 decreased charge transfer energy. Red shift of MLCT in N3–S relative N3–Se is due to stronger distortion in Ru-NCS− complex than Ru-NCSe−. Electronic transition in [RuL2(NCSe)2] dye is more probable than that of [RuL2(NCS)2] dye due to the wide absorption bands at 371 nm, 492 nm.Figure 6UV–Vis. spectra: (a) [cis-[RuL2(NCS)2], (b) [cis-[RuL2(NCSe)2].Table 4 Electronic absorption spectral data for Ru complexes.Figure 7a,b shows the photoluminescence properties of the Ru(II) polypyridine complexes dissolved in absolute ethanol, where (a) is the N3-dye and (b) is the N3–Se dye. The measurements were carried out with Spectro fluorometric characteristics: Emission spectra at an excitation wavelength of 380 nm.Figure 7Photoluminescence spectra: (a) N3-dye, (b) N3–Se dye.Emission occurs from lowest-lying triplet 3MLCT excited state29,30,31. Emission bands at λmax. 700, 701 nm for [RuL2(NCS)2] and [RuL2(NCSe)2] respectively. Broad absorption band in UV–Vis. electronic absorption spectra showed that complexes have inter molecular charge transfer characteristics. Conjugation makes tuning between LUMO and HOMO energy levels in excited state and increases resonating structures of pyridine rings25.Comparing between the dyes, the wider PL band for N3–Se dye at 700 nm indicated that it has more efficient PL activity than N3–S dye.Redox and electrochemical behavior of the dyes were clarified via cyclic voltammetry (Fig. 8a,b). Dyes are reversibly reduced on cathodic polarization and are oxidized on anodic polarization. Reversible redox behavior of dyes confirmed optical activity by donating lone pair electrons to TiO2. Single redox wave in cyclic voltammogram indicated high purity of the dyes32. Reversible cyclic voltamograms indicated that both dyes can be regenerated after oxidation. Table 5 shows polarization parameters for both N3 and N3–Se dyes. Dyes exhibited a signal reversible electrochemical wave over examined potential range with oxidation potential, E1/2 0.241 V for N3-dye and E1/2 0.686 V for Se-dye.Figure 8Cyclic voltamograms: (a) N3–S dye, (b) N3–Se Dye; (c,d) Nyquist plots of N3 dye, N3–Se dye respectively, (e) Equivalent circuit model.Table 5 Polarization parameters for N3–S and N3–Se dyes.Under experimental conditions used in recording cyclic voltammograms, value of parameter \(\Delta {E}_{P}\) represented ΔELUMO–HOMO in eV33. Se-dye has lower ΔELUMO–HOMO than N3dye. The much lower ∆IP for Se-dye indicates that nearly reversible equilibrium behaviors of this dye rather than N3 dye i.e. Se-dye is easily regenerated more than N3-dye. Adsorption strength of N3-dye and Se-dye on TiO2 electrode can be represented in Nyquist impedance diagrams (Fig. 8c,d). Impedance parameters (Table 6) were obtained via nonlinear fitting of Nyquist plots to the theoretical equivalent semicircle model shown in Fig. 8e 34.Table 6 Impedance parameters for N3–S and N3–Se dyes.Resistance of solution and adsorbed film of dye molecules is represented by element Rs, Resistor, Rct is connected parallel to capacitor, Qdl represents capacitance of electric double layer (edl) at electrode/solution interface. System heterogeneity is represented by parameter n. Electron transfer resistance from dye to TiO2 across surface of the working electrode is represented by charge transfer resistance (Rct). The rate of electron transfer equals reciprocal 1/Rct.For lifetime measurements for N3–S and N3–Se Dyes, Fig. 9a shows raw FLIM data for N3–S dye, with PL intensity decay curve fitted by sum of two exponential functions with more than 98% of counts have 0.85 ns half lifetime. The right panel of Fig. 9a shows phasor plot Intensity decays for corresponding FLIM image was shown in left panel of Fig. 9a. In phasor plot, each pixel in FLIM image is represented in 2D diagram with two coordinates S and G (given by Eqs. 1, 2) based on phase shift (φ) between transmitted wave and resulting PL wave and demodulation factor (m) in laser source35:$${\text{S}} = {\text{m}}\;{\text{sin}}\left( \varphi \right) = \omega \tau /\left( {{1} + \omega^{{2}} \tau^{{2}} } \right)$$
(1)
and$${\text{G}} = {\text{m}}\;{\text{cos}}\left( \varphi \right) = {1}/\left( {{1} + \omega^{{2}} \tau^{{2}} } \right)$$
(2)
where τ is lifetime, ω = 2πf is laser modulation angular frequency (20 MHz). From Eqs. (1, 2),$$\tau = \omega \left( {{\text{S}}/{\text{G}}} \right)$$
(3)
Figure 9Raw FLIM data: (a) N3–S, (b) N3–Se dye. Middle panel shows PL intensity decay curve of each dye with fitting curve. Curve at the bottom panels is the fitting residual. Left panel in a, b is phasor plot representations from fluorescence FLIM data.Phasor plot for N3–S dye showed point clusters located at edge of phasor plot semicircle, indicating that N3–S dye decay is single exponential decay with an average τ 0.8 ns as calculated from last equations.For N3–Se dye (Fig. 9b), decay curve was fitted by sum of two exponential functions with 75% counts have 2.5 ns, 25% counts have 30 ns lifetime. Phasor plot shows relatively broad point clusters at left side of the circle, indicating that the decay is multi-components exponential decay.From light current–Voltage. Characteristics for the Fabricated DSSC clarified as follows:Efficiency (η) of DSCC is calculated using Eq. (3);$$\eta = {\text{J}}_{{{\text{sc}}}} {\text{V}}_{{{\text{oc}}}} {\text{FF}}/{\text{I}}_{0}$$
(4)
where I0 is the incident irradiation power, Jsc is the short circuit current density (current density corresponding to V = 0), Voc is open circuit voltage (voltage corresponding to zero current density, Jsc = 0), and FF is fill factor.Figure 10a–c showed cell I–V curves for DSSCs and the cell parameters when using commercial N3, prepared N3–S and N3–Se dyes, respectively.Figure 10Current–voltage characteristics of solar cell using: (a) commercial N3 dye, (b) Prepared N3 dye, and (c) N3–Se dye.Table 7 collected the cell parameters: For commercial N3 dye purchased from solaronix, the measured photovoltaic parameters are: Jsc = 17.813 mA/cm2, Voc = 0.678 V, FF = 0.607 and η = 7.3%. For the prepared N3–S dye, the measured photovoltaic parameters areJsc 11.2 mA/cm2, Voc = 0.650 V, FF = 0.681 and η = 5%. For N3–Se dye, the measured photovoltaic parameters are Jsc = 6.67 mA/cm2, Voc = 0.6004 V, FF = 0.77 and η = 7.30%. Results indicated a comparable high cell performance for the commercial and prepared N3 dyes. For N3–Se dye which is prepared for the first time, it indicates a good cell performance with an efficiency of about 3.09%.Table 7 Output cell parameters for sensitizers dyes: commercial N3, prepared N3–S and N3–Se respectively.