1 Introduction

Once the published work in 1991 by O’regan and Grätzel, who used a trimeric ruthenium structure adsorbed on the nanoporous edge of a semiconductor (Titanium dioxide) to achieve PCE over 7.12%, dye-sensitized solar cells (DSSCs) gave rise to a new, low-cost, and high-power conversion efficiency (PCE) devices around 1991, making a case for the origins of photovoltaic (PV) cells [1]. As a result of this research, scientists have concentrated considerable effort on developing new structures with higher PCE. Small organic compounds [2, 3], porphyrins [4, 5], and ruthenium compounds [6, 7] have all been employed as DSSCs. The metal-to-ligand charge-transfer (MLCT) transition is responsible for the absorption of common ruthenium-based dyes (N-719, N-749, N3, and so on) over a broad wavelength range of 350 to 900 nm [6,7,8]. Metals’ usage in DSSCs on a broad scale is limited due to their elevated manufacturing resources and environmental impacts [9]. To achieve an effective electrical transfer between dye and semiconductor, novel metal-free dyes with maximum molar absorptivity coefficients, a broad range of light absorption, and high reliability to absorb on the surface of TiO2 are needed [10]. On the other hand, metal-free organic dyes have various benefits over the previous class, including simple availability, design diversity, high molar extinction coefficients, adjustable absorption spectrum range from visible to near-infrared (NIR), and low cost of fabrication [11,12,13]. Consequently, large numbers of metal-free organic dyes have been created and explored as prospective DSSC photosensitizers, and system efficiency connections have been discovered [14, 15]. When utilized in DSSCs, several metal-free dyes based on indole, phenoxazine, carbazole, thiophene, and phenothiazine, are good sensitizers with good photovoltaic performance. A thiophene core has been recognized as a viable entity among the above-mentioned heterocyclic compounds because of its adjustable electrochemical and spectroscopic properties [16,17,18,19].

Also, the chemical structure of dyes should have specific functional group, which act as anchoring moieties for binding the dye to the semiconductor surface. Several published photosensitizers possessing cyanoacetic acid, barbituric acid, and rhodamine cores were exposed to be efficient anchoring units [20, 21]. Nonetheless, the majority of those organic compounds have a single anchor, and there have been few investigations into dual-tethered organic dyes for co-sensitizing ruthenium dyes. Dual-anchored dyes have a tighter link to the photoanode and an electrostatic repulsion with efficient prevention of intermolecular accumulation that reduces electron recombination processes as compared to single-anchored dyes [22, 23]. As shown in Fig. 1, novel organic compounds with dual anchoring functionalities are presented. The framework was used to construct three novel bichalcophene-pyrimidine-2,4,6-trione-based sensitizers, F1, F2, and F3, incorporating dual anchoring moieties of barbituric analogs. A comparative study of the photophysical, electrochemical, and photovoltaic characteristics of three bichalcophene derivatives was carried out. Figure 1 illustrates the molecular architecture of F1, F2, and F3. Co-sensitization is a very good approach for improving the light-harvesting potential of DSSCs using two or more organic dyes within the same fabricating device [24, 25]. Several essential characteristics must be met to fabricate a co-sensitization system with high performance: (1) The co-sensitizers should have structural properties that reduce dye incorporation within the semiconductor film; (2) they should have high molar extinction coefficients (ε) that can compensate for the main sensitizer's lack of an absorption spectrum; and (3) a small structure of co-sensitizers is preferred to fill the void left by the main sensitizer [26,27,28]. Furthermore, in order to be commercialized, organic dyes must have a basic structure, be straightforward to synthesis with a few processes, and offer distinct features like configurable molecular architecture, cheap cost, adaptable electrochemical, and absorption characteristics. The simple structure of bichalcophene sensitizers (F1-3), consists of segments that, are widely used in the manufacture of most efficient DSSCs dye-sensitized solar cells and charge transport materials [29], π-conjugation represented in bithiophene and bifuran moieties, and the dual anchoring dyes (1,3-dimethyl are preferred to monoanchoring dyes mainly due to certain favorable inherent properties. As a result, we combined the new dyes with N-719 (Fig. 1), a conventional great-efficiency bipyridyl Ru (II) photosensitizer, to improve the photovoltaic performance of DSSCs with complementing optical absorption [30] and explore their photovoltaic capabilities further. Interestingly, in the co-sensitization process, all photovoltaic parameters, including photovoltage, photocurrent, and fill factor, are enhanced.

Fig. 1
figure 1

Molecular structures of compounds F1-3 and N-719 [31]

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2 Synthesis

2.1 Preparation of 5-arylidine-1,3-dialkylbarbituric acids 3a,b

2.1.1 5-[(5-Bromothiophen-2-yl)methylene]-1,3-dimethylpyrimidine-2,4,6-trione (3a)

To a solution of 1,3-dimethylbarbituric acid (4.68 g, 30 mmol) and 5-bromothiophene-2-carboxaldehyde (5.73 g, 30 mmol) in methanol (50 mL) was added few drops of conc. HCl. The reaction mixture was heated at reflux for 3 h and the precipitate was filtered off, washed with methanol and recrystallized from ethanol/EtOAc to afford compound 3a. Yield 81%, mp 248–250 °C. IR (KBr) ν′ 3095, 3072 (sp2 C–H), 2990, 2949 (sp3 C–H), 1727, 1660 (CO), 1540, 1493 (C=C) cm−1. 1H-NMR (DMSO-d6); δ 3.21 (s, 6H; 2 × N-CH3), 7.53 (d, J = 4.2 Hz, 1H, thiophene H’s), 8.04 (d, J = 4.2 Hz, 1H, thiophene H’s), 8.52 (s, 1H, methine H’s). MS (EI) m/e (rel.int.); 328, 330 (M+, 8; Br isotopes), 249 (30), 56 (100). Anal. Calc. for C11H9BrN2O3S (329.17): C, 40.14; H, 2.76; N, 8.51. Found: C, 39.89; H, 2.85; N, 8.60.

2.1.2 5-[(5-Bromofuran-2-yl)methylene]-1,3-dimethylpyrimidine-2,4,6-trione (3b)

Compound 3b was obtained adopting the same methodology used for preparation of 3a, starting with 5-bromofuran-2-carboxaldehyde instead of 5-bromothiophene-2-carboxaldehyde. Yield 86%, mp 196–198 °C. IR (KBr) ν′ 3178, 3132 (sp2 C–H), 2995, 2952 (sp3 C–H), 1725, 1662 (CO), 1572, 1412 (C=C) cm−1. 1H NMR (CDCl3); δ 3.39, 3.41 (2 s, 6H; 2 × N-CH3), 6.69 (d, J = 3.9 Hz, 1H, furan H’s), 8.34 (s, 1H, methine H’s), 8.60 (d, J = 3.9 Hz, 1H, furan H’s). MS (EI) m/e (rel.int.); 312, 314 (M+, 6; Br isotopes), 233 (100), 176 (53). Anal. Calc. for C11H9BrN2O4 (313.10): C, 42.20; H, 2.90; N, 8.95. Found: C, 42.03; H, 3.05; N, 8.89.

2.2 General procedure for preparation of bichalcophene-5,5′-bis-(methylidenes) bis-1,3-dialkylbarbituric acids F1 and F2

2.2.1 5,5′-[2,2′-Bithiophene-5,5′-diylbis(methylidene)]-bis-(1,3-dimethylpyrimidine-2,4,6-trione) (4a, F1)

To a solution of bromo compound 3a (1.56 g, 5 mmol), and Pd(PPh3)4 (200 mg) in toluene (40 mL) was added hexa-n-butylditin (2.9 g, 5 mmol). The reaction mixture was heated under N2 at 120 °C for 12 h, then the precipitate was filtered and washed with hexanes, recrystallized from DMF to give compound 4a in 79% yield, mp > 300 °C, lit. mp. 438 °C [32]. IR (KBr) ν′ 3080 (sp2 C–H), 2957 (sp3 C–H), 1723, 1657 (CO), 1551, 1461 (C=C) cm−1. 1H-NMR (TFA-d, CF3COOD); δ 3.50, 3.52 (2 s overlapped, 12H, 4 × N-CH3), 7.76 (d, J = 4.0 Hz, 2H, thiophene H’s), 8.01 (d, J = 4.0 Hz, 2H, thiophene H’s), 8.90 (s, 2H, methine H’s), MS (EI) m/e (rel.int.); 498 (M+, 100), 466 (5), 328 (9), 249 (16). Anal. Calc. for C22H18N4O6S2 (498.53): C, 53.00; H, 3.64; N, 11.24. Found: C, 52.74; H, 3.71; N, 11.12.

2.2.2 5,5′-[2,2′-Bifuran-5,5′-diylbis(methylidene)]-bis-(1,3-dimethylpyrimidine-2,4,6-trione) (4b, F2)

Compound 4b was obtained adopting the same procedure used for the preparation of compound 4a using bromo compound 3b instead of 3a. Yield 73%, mp > 300 °C (DMF). IR (KBr) ν′ 3174, 3113 (sp2 C–H), 2956 (sp3 C–H), 1728, 1663 (CO), 1571, 1463 (C=C) cm−1. 1H-NMR (TFA-d, CF3COOD); δ 3.51 (s, 12H, 4 × N-CH3), 7.38 (d, J = 4.0 Hz, 2H, furan H’s), 8.60 (s, 2H, methine H’s), 8.73 (d, J = 4.0 Hz, 2H, furan H’s). MS (EI) m/e (rel.int.); 466 (M+, 12), 270 (100). HRMS calc. for C22H19N4O8 (MH+): 467.1190. Observed: 467.1203. Anal. Calc. for C22H18N4O8 (466.41): C, 56.65; H, 3.89; N, 12.01.Found: C, 56.46; H, 4.03; N, 12.07.

2.2.3 5-((5-(5-((1,3-dimethyl-2,4,6-trioxotetrahydropyrimidin-5(2H)-ylidene)methyl)furan-2 yl)thiophen-2-yl)methylene)-1,3-dimethylpyrimidine-2,4,6(1H,3H,5H)-trione (7, F3)

Firstly; 5-(5-formyl-2-thienyl)-2-furaldehyde 6 was prepared adopting a Suzuki cross coupling conditions [33], reacting 5-bromothiophenealdehyde (5 mmol) with 5-formyl-furanboronic acid (6 mmol) in 1,4-dioxane and Pd(PPh3)4 as a catalyst and anhydrous K2CO3 as a base. The dialdehyde 6 was attained 78% yield as a yellow solid, mp 184–185 °C and it was identical with the reported literature [34]. Secondly, dialdehyde compound 6 (1 mmol) was condensed with 1,3-dimethylbarbituric acid (2 mmol) in 30 mL methanol/acetic acid (2:1) under reflux for 24 h. The resultant precipitate was filtered off while hot. The precipitate was recrystallized from DMF to furnish compound 7 in 69% yield, mp > 300 °C. IR (KBr) ν′ 2925 (sp3 C–H), 1725, 1660 (CO), 1557, 1463 (C=C) cm−1. 1H-NMR (TFA-d, CF3COOD); δ 3.50, 3.52 (two s overlapped, 12H; 4 × N-CH3), 7.32–7.37 (m, 1H), 7.77–7.80 (m, 1H), 7.99–8.02 (m, 1H), 8.61 (s, 1H), 8.74 (s, 1H), 8.92–8.94 (m, 1H). MS (EI) m/e (rel.int.); 482 (M+, 55), 466 (18), 449 (16), 66 (100). Anal. Calc. for C22H18N4O7S (482.46): C, 54.77; H, 3.76; N, 11.61.Found: C, 54.91; H, 3.87; N, 11.40.

3 Results and discussion

3.1 Chemistry

Preparation of the new bithiophene-pyrimidine-2,4,6-trione derivative 4a (F1) began with Knoevenagel condensation reaction by treatment of 5-bromothiophene-2-carboxaldehyde with 1,3-dimethylbarbituric acid to afford the bromo compound 3a, followed by a stille-type homocoupling reaction using hexabutylditin mediated homocoupling conditions [35] for 3a in the presence of catalytic Pd(PPh3)4 using refluxing toluene as a solvent to afford the desired compound F1. Adopting the same methodology used for the preparation of 3a, starting with 5-bromofuran-2-carboxaldehyde instead of 5-bromothiophene-2-carboxaldehyde to afford the bromo compound 3b, which also underwent a stille homocoupling reaction to afford the anticipated bifuran-pyrimidine-2,4,6-trione derivative 4b (F2) as outlined in Scheme 1. The newly synthesized bichalcophene-pyrimidine-2,4,6-trione derivatives were established via their corresponding spectral analysis. In all cases, the IR spectra of the derivative products reported the existence of carbonyl groups. 1H-NMR spectrum of bromothiophene methylene-1,3-dimethylbarbituric acid 3a displayed a singlet signal characteristic of the 1,3-dimethylbarbituric acid moiety at δ 3.21 (6H), two doublet signals of the thiophene moiety at δ 7.53 and 8.04 (each signal integrated for one proton), in addition to a singlet signal of methine proton at δ 8.52 (1H), and its mass spectrum exhibited a molecular ion peak at m/z 328, 330 of bromine isotopes and a fragment peak at m/z 249 formed from the parent molecular ion through losing bromine. Moreover, the 1H-NMR spectrum of the bithiophene-pyrimidine-2,4,6-trione derivative F1 showed two singlet overlapped signals integrated for 12 protons characteristic for two 1,3-dimethylbarbituric acid moieties at δ 3.50 (6H) and 3.52 (6H) and a singlet signal at δ 8.90 (2H) of the methine H’s; in addition to two doublet signals of the symmetrical bithiophene moiety at δ 7.76 and 8.01 (each signal integrated for two protons); and its mass spectrum showed a molecular ion peak at m/z 498 as a base peak. Further, the 1H-NMR spectrum of bromofuran methylene-1,3-dimethylbarbituric acid 3b displayed two singlet signals characteristic for the 1,3-dimethylbarbituric acid moiety at δ 3.39 (3H) and 3.41 (3H), two doublet signals of the furan moiety at δ 6.69 and 8.60 (each signal integrated for one proton), in addition to a singlet signal of methine proton at δ 8.34 (1H); and its mass spectrum showed a molecular ion peak at m/z 312, 314 of bromine isotopes and a fragment peak at m/z 233 as a base peak produced from the parent molecular ion through losing bromine. Moreover, the 1H-NMR spectrum of the bifuran-pyrimidine-2,4,6-trione derivative F2 showed singlet signal characteristics for two 1,3-dimethylbarbituric acid moieties at δ 3.51 (12H) and a singlet signal at δ 8.60 (2H) of the methine protons and two doublet signals of the symmetrical bifuran moiety at δ 7.38 and 8.73 (each signal integrated for two protons); and its mass spectrum showed a molecular ion peak at m/z 466 and a fragment peak at m/z 270 as a base peak.

Scheme 1
scheme 1

Synthesis scheme for the preparation of symmetrical bichalcophene dyes F1 and F2

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Scheme 2 outlines the preparation of the new unsymmetrical bichalcophene dye 7 (F3) starting with Suzuki coupling by treatment of bromothiophene aldehyde 2a with (5-formylfuran-2-yl)boronic acid (5) to furnish dialdehyde compound 6, which is subsequently condensed with 2 equivalents of 1,3-dimethylbarbituric acid to afford the target compound F3. The newly synthesized bichalcophene-pyrimidine-2,4,6-trione derivative F3 was assured based on the spectral data. Thus, the IR spectra of bichalcophene-pyrimidine-2,4,6-trione derivative F3 indicated the presence of carbonyl groups. 1H-NMR spectrum of compound 7 displayed two singlet overlapped signals characteristic of two 1,3-dimethylpyrimidine-2,4,6-trione moieties at δ 3.50 (6H) and 3.52 (6H), and four protons of thiophene and furan moieties, in addition to two singlet signals of bis methylidenes at δ 8.61 (1H) and 8.74 (1H). Additionally, its mass spectrum showed a molecular ion peak at m/z 482 (M+).

Scheme 2
scheme 2

Synthesis scheme for the preparation of unsymmetrical bichalcophene dye F3

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3.2 Optical measurements

The UV/visible absorption spectra of dyes F1-3 measured in DMF solution (2 × 10–5 M) are presented in Fig. 2, and their related parameters are summarized in Table 1.

Fig. 2
figure 2

Absorption spectra of bichalcophene derivatives F1-3 in DMF

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Table 1 Photophysical parameters of bichalcophene derivatives F1-3
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Compounds F1-3 exhibited wide absorption in the 350–550 nm region, which corresponds to the conjugated system’s π–π* transition and intramolecular charge transfer (ICT). Compound F1 showed a bathochromic shift with maximum molar extinction coefficient (ε), which is explained by the availability of bithiophene in its architecture, which causes an increase in conjugation, and hence the absorption occurred at a longer wavelength [36]. E0-0 (energy gap) was calculated from the beginning of the lowest possible-energy peak [37], and the values are F1 (2.32 eV), F2 (2.57 eV), and F3 (2.33 eV), respectively, compared to the E0-0 of N-719 (1.92 eV). In the same spectral region, the molar extinction values obtained for F1-3 are higher than those for N-719 dye. Because of its complementary light-harvesting capacity, F1-3 dye may be a good co-sensitizing option with N-719 in DSSC. F1-3 have maximum absorption wavelengths of 509 nm (ε = 5.72 × 104 M−1 cm−1), 512 nm (ε = 2.11 × 104 M−1 cm−1), and 514 nm (ε = 4.17 × 104 M−1 cm−1), respectively. N-719, on the other hand, had a maximum absorption wavelength of 512 nm (ε = 1.42 × 104 M−1 cm−1).

Figure 3 illustrates the normalized UV/visible absorption peaks of compounds F1-3 on TiO2 thin film. The absorbance spectra of the solid film were wider, suggesting that the adsorption of the substance on the TiO2 surface was in high quantities. The interaction between the Ti4+ ions of the semiconductor and the carbonyl group in the pyrimidine ring of the investigated compounds causes the spectra to broaden, resulting in a drop in the dye particle’s π* energy level, which is advantageous for enhancing the photovoltaic performance of the DSSCs, particularly the photocurrent [38]. The adsorption of substances on the surface of the TiO2 appeared in the sequence F1 > F3 > F2.

Fig. 3
figure 3

Normalized Absorption of sensitized electrodes

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Cyclic voltammetry (CV) studies are used to measure the redox potentials of structures F1-3 in addition to the thermodynamic possibility of charge transfer [39]. To adjust the reduction potential, ferrocene (0.4 V vs. normal hydrogen electrode (NHE)) is used as an external reference. The oxidation onset of F1-3, which is comparable to the ground state oxidation potential (GSOP), was evaluated using CV graphs (Fig. 4). The excited state oxidation potential was calculated from the obtained GSOP and E0-0 as shown in Eq. 1 (a value of 4.7 was added to obtain values in eV)

$${\text{ESOP}} = [{ }\left( {\left( {{\text{GSOP }}\left( {\text{V}} \right) + 4.7} \right) - - E_{0 - 0} } \right]{\text{eV}}$$
(1)
Fig. 4
figure 4

Cyclic voltammograms of bichalcophene derivatives F1-3

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Table 2 displays the electrochemical data obtained for structures F1-3. As per studies, the mechanism of successful electron injection and dye regeneration, is dependent on the relative positions of the GSOP, ESOP, conduction band (CB) of the TiO2, and the redox potential of the electrolyte [40]. Table 2 shows that the calculated GSOP levels of dyes F1 (− 5.82 eV), F2 (− 6.29 eV), and F3 (− 5.99 eV) are more positive than \(I_{3}^{ - } / I^{ - }\) redox couples (− 5.2 eV), reflecting a sufficient thermodynamic driving force for the iodine cells.

Table 2 Electrochemical parameters for bichalcophene derivatives F1-3
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Furthermore, the ESOP levels of F1-3 are much more negative than the CB of the TiO2 as shown by − 3.50 eV, − 3.72 eV, and − 3.66 eV, respectively, indicating that electrons are efficiently injected from the excited dye to the CB of the TiO2 [41]. All the dyes in the fabricated devices satisfied the basic requirements for dye regeneration with effective electron transfer to the conduction band of the semiconductor (TiO2) [42].

The energy level diagram of bichalcophene derivatives F1-3 is shown in Fig. 5. The distance between both the LUMO value of dyes F1-3 and the CB band of TiO2 was then used to calculate the negative free energies for electron injection. Their negative free energy levels were determined to be in the following order: F1 (0.7 eV) > F3 (0.54 eV) > F2 (0.48 eV). Based on the data, the highest value of F1 indicates that its electrons are transferred with high efficiency to the CB of TiO2.

Fig. 5
figure 5

Calculated energy levels for bichalcophene derivatives F1-3

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3.3 Theoretical studies

With the Gaussian 09 at the B3LYP/6-31G* stage, the optimal geometry and electron densities of the HOMOs and LUMOs of compounds F1-3 were examined [43]. (Figs. 6, 7).

Fig. 6
figure 6

Optimized structures for bichalcophene derivatives F1-3

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Fig. 7
figure 7

Electron distribution on molecular orbital for bichalcophene derivatives F1-3

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Figure 7 shows the different behavior of the electronic distributions for the HOMO and LUMO for dyes F1-3. According to the results of a density functional theory investigation, the HOMO charge density of symmetrical bithiophene dye F1 is predominantly dispersed across the bithiophene moiety, whereas the LUMO of F1 fills up on the barbituric ring. The HOMO is present on the bifuran ring in symmetrical bifuran dye F2, but the LUMO is not fully extended to the barbituric ring. However, in the case of unsymmetrical bichalcophene dye F3, the HOMO dispersed over the thiophene-furan moiety, whereas the LUMO was found to be centered just on the barbituric ring, which was joined to the thiophene ring. In comparison to the other dyes F2 and F3, these results show that F1 has well-overlapped HOMO and LUMO molecular orbitals all over the molecules, including the two barbituric rings. Furthermore, light-illumination generated electrons in F1 may move from dye to TiO2 semiconductor through the HOMO–LUMO transition with high activation [44].

In this work, the molecular electrostatic potential (MESP) model was used to study specific molecular characteristics of organic dyes. Using their optimal configurations on the matching molecular surface, the electrostatic potential (ESP) maps of F1-3 dyes are shown in Fig. 8. The ESP maps that are created here show the charge densities in the molecules in three dimensions. In terms of color, the areas of electrostatic potential usually rise in the correct sequence: blue > green > yellow > orange > red. However, regions of low potential (red color) and high potential (orange color) are distinguished by an excess of electrons and a shortage of electrons, respectively [45]. As can be seen in Fig. 8, F1 has a lower potential than the other dyes, suggesting the availability of electrons in the red portion of the structure.

Fig. 8
figure 8

MESP maps of bichalcophene structures F1-3

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3.4 Photovoltaic performance

Photovoltaic characterizations of co-sensitized devices were performed to establish the structure–property relationship and determine the best electron donating and anchoring groups for bichalcophene co-sensitizers systems on co-adsorption properties and establish a link between the co-adsorber structure and its corresponding solar performance. Photocurrent action spectra for bichalcophene co-sensitizers are depicted in Fig. 9. Impressively, as described in Table 3, the photovoltaic performance of the fabricated devices with 2-bichalcophene co-sensitizers F1-3 was as follows: a photocurrent (Jsc) of 23.28, 19.96 and 21.31 (mA/cm2); photovoltage (Voc) of 0.733, 0.609 and 0.637 V; a fill factor (FF) of 58.40, 57.70 and 58.10%; and a PCE (η) of 9.97, 6.95 and 7.88%, respectively. The DSSC with N-719 showed a Jsc of 20.71 mA. cm−2, a Voc of 0.586 eV, FF of 57.40%, and a PCE of 7.03%. (I–V) characteristics of the cell based on bichalcophene co-sensitizers F1-3 with N-719 are displayed in Fig. 9. Over co-sensitization, all the values of Voc were improved, which suggests that the charge recombination caused by N-719 aggregation is reduced by bichalcophene co-sensitizers F1-3, resulting in better performance of solar devices co-sensitized with F1-3. Besides the enhancement of the values of VOC, the values of (FF) were also enhanced, which indicates that the smaller size of the bichalcophene co-sensitizers F1-3 provides more sufficient covering of the TiO2 surface [46, 47]. The efficiency of the N-719 sensitized solar cell was lower than the co-sensitizers F1-3 that indicates a sharper charge recombination produced from the closer \(\pi\)–\(\pi\) stacking or the dye aggregation [48]. The electron injection efficiency of the main complex dye N-719 was reduced as a result of this aggregate. Following the addition of co-sensitizers to N-719, the efficiency was improved. Thus, co-sensitizers are characterized by their small sizes, enabling for good, decreasing \(\pi\)–\(\pi\) stacking due to the presence thiophene/furan moiety. As a result, charge recombination is inhibited, and device overall performance was improved [49]. In other circumstances, co-sensitizers should provide two roles: they should inhibit organic compounds from forming π–π stacking and they should also have a light harvesting ability. The PCE of the DSSCs fabricated with the bichalcophene co-sensitizers F1-3 gradually increased in the following order: N-719/F2 (6.95%) < N-719/F3 (7.88%) < N-719/F1 (9.97%). The highest efficiency was achieved with co-sensitizer F1, which is mainly related to the chemical structure of the bithiophene donor moiety/π-spacer, which will improve the solubility of the co-sensitizer suppressing the dark current. As a result, the highest value of Voc of F1 reflected a decreased electron recombination with a triiodide redox pair. On other hand, F1 showed the highest value of JSC and showed the highest harvesting ability due to the highest molar extinction co-efficient and the most bathochromic shift in the absorption spectrum toward longer wavelengths.

Fig. 9
figure 9

I–V characteristics of solar devices based on bichalcophenes F1-3 and N-719

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Table 3 Photovoltaic parameters of bichalcophene derivatives F1-3
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For further confirmation of the abovementioned results, F1-3 were fabricated, and their photovoltaic performances were measured individually without the standard dye N-719. Table 3 lists the photovoltaic characteristics associated with the I–V characteristic curves of F1-3 seen in Fig. 10.

Fig. 10
figure 10

I–V characteristics of solar devices based on bichalcophenes F1-3

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The fabricated devices F1-3 displayed a maximum PCE (η) value for sensitizer F1 of 4.41% (JSC = 10.66 mA/cm2, VOC = 0.65 V, and FF = 64.30%) when compared to devices with F2 (JSC = 6.30 mA/cm2, VOC = 0.50 V, and FF = 61.5%), and F3 (JSC = 6.99 mA/cm2, VOC = 0.55 V and FF = 62.4%). The highest PCE value for F1 as a sensitizer due to the highest values of VOC and JSC, which may be attributed to its effective anchoring property of 1,3-dimethyl barbituric acid and electron injection onto the TiO2 surface. Poor performances of other dyes like F2-3 can occur because of the lower molar extinction co-efficient of F2-3 than F1, which had resulted in poor electron injection to the TiO2 surface, further leading to lower JSC values.

Furthermore, our results show that the photovoltaic performance of bichalcophene dyes F1-3 follows the same patterns when co-sensitized with N-719. As a result, better sensitizers transferred to better co-sensitizers, confirming the higher performance.

(EIS) Electrochemical impedance spectroscopy is considered a helpful tool for analyzing the electron charge transfer and chemical capacitance at the TiO2/dye/electrolyte and Pt/electrolyte interfaces in DSSCs fabricated with sensitizers [50, 51]. Nyquist plots for co-sensitized cells based on bichalcophene co-sensitizers F1-3 with the main dye N-719 are displayed in Fig. 11. It had two distinct semicircles. The small semicircle at lower frequency represents the cathode charge transfer resistance, which is directly related to FF, and the large semicircle at middle frequency represents the charge recombination resistance (Rct) from TiO2 to the electrolyte, which is directly related to Voc. In Nyquist plots, the radius of the large semicircle of the cells co-sensitized by F1-3 was all larger than that sensitized by metal complex N-719 only, F1 + N-719 > F3 + N-719 > F2 + N-719 > N-719. This clearly shows that the co-sensitization by bichalcophene co-sensitizers is beneficial in reducing dark current. As a result, Nyquist plots show that co-sensitization helps to suppress charge recombination at the TiO2/dye/electrolyte interface. Co-sensitizers F1-3 displayed larger Rct in relation to N-719 upon co-sensitization devices, due to the small structure of bichalcophene co-sensitizers characterized by various acceptors and anchoring moieties represented in (1,3-dimethylbarbituric acid) that are favorable for depressing undesired charge recombination, which is related to the ability of bichalcophene co-sensitizers to fill the dip from the main dye N-719 and is clearly beneficial in reducing electron recombination and beneficial in reducing the dark current. The foregoing corresponds to the Voc values in Table 3. As a result, Nyquist plots show that co-sensitization helps to suppress charge recombination at the TiO2/dye/electrolyte interface.

Fig. 11
figure 11

Nyquist plots of bichalcophene derivatives F1-3 and N-719 based devices

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4 Conclusion

Three organic dyes, denoted by the letters F1, F2, and F3, were created as sensitizers and co-sensitizers for dye-sensitized solar cells. These bichalcophene compounds were described and evaluated, and their optical and electrochemical parameters were examined. Furthermore, theoretical studies based on the DFT approach were used to investigate their photovoltaic properties. With a PCE of 4.41%, the bithiophene derivative F1 had the highest DSSC efficiency in the photovoltaic characterization when sensitized as a single device. Furthermore, F1, F2, and F3 were co-sensitized with a conventional Ru (II) dye, N-719, to promote light-harvesting across a wider spectral area and therefore enhance efficiency. The photovoltaic cell co-sensitized with F1 surpassed the device using N-719 alone, with an enhanced efficiency of 9.97%, a JSC of 23.28 mA cm−2, VOC of 0.733 V, and a FF of 58.4% (an efficiency of 7.03% for N-719) owing to the maximum molar extinction coefficient complementing the light-harvest ability of N719 dye. Further, the devices were subjected to electrochemical impedance spectroscopy to gain an insight into the interfacial charge transfer and recombination processes while in use. Finally, density functional theory (DFT) has been studied for bichalcophene dyes F1-3 using Guassian09 software and the outcomes comfortably agree with experimental results.