Synergistic co-sensitization of fluorene- and triphenylamine-derived thiazole dyes with N719: A quantum chemical and experimental investigation for enhanced DSSC performance

Hana M. Abumelha; Nuha M. Halawani; Shadiah Albalawi; Jihan Qurban; Khalid Althumayri; Renad Almughathawi; Ali H. Alessa; Nashwa M El-Metwaly

doi:10.25259/AJC_311_2025

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Original Article

:19;

3112025

doi:

10.25259/AJC_311_2025

Synergistic co-sensitization of fluorene- and triphenylamine-derived thiazole dyes with N719: A quantum chemical and experimental investigation for enhanced DSSC performance

Hana M. Abumelha^a,*, Nuha M. Halawani^b, Shadiah Albalawi^c, Jihan Qurban^b, Khalid Althumayri^d, Renad Almughathawi^e, Ali H. Alessa^f, Nashwa M El-Metwaly^b

aDepartment of Chemistry, College of Science, Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia

bDepartment of Chemistry, College of Science, Umm Al-Qura University, Makkah, Saudi Arabia

cDepartment of Physics, Faculty of Science, University of Tabuk, Tabuk, Saudi Arabia

dDepartment of Chemistry, College of science, Taibah University, Al-Madinah Al-Munawarah, Saudi Arabia

eDepartment of Physics, Faculty of Science, Taibah University, Madinah, Saudi Arabia.

fDepartment of Chemistry, Faculty of Science, University of Tabuk, Tabuk, Saudi Arabia

*Corresponding author: E-mail address: hmabumelha@pnu.edu.sa (H. Abumelha)

Received: 2025-03-25, Accepted: 2025-10-28, Epub ahead of print: 2026-02-09, Published: 2026-03-02

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Abstract

To address the limitations of narrow light absorption and charge recombination in dye-sensitized solar cells (DSSCs), two new thiazole-based organic dyes, NMS-1 and NMS-2, incorporating triphenylamine and fluorene donor units, respectively, were designed and synthesized. These molecules were designed to merge the electron-rich character of triphenylamine with the rigid framework of fluorene via a π-conjugated thiazole bridge to achieve synergistic electronic interactions. Co-sensitization of these dyes with the benchmark Ru-based N719 dye significantly enhanced light harvesting and photovoltaic efficiency, yielding power conversion efficiencies (PCEs) of 7.90% (NMS-1+N719) and 7.87% (NMS-2+N719), respectively. Further enhancement to 9.93% was achieved by incorporating chenodeoxycholic acid (CDCA) as a co-adsorbent. This improvement is attributed to suppressed dye aggregation, favorable molecular alignment, and reduced interfacial charge recombination, as confirmed by electrochemical impedance spectroscopy. This study demonstrates how rational donor design and interface engineering can markedly enhance charge transport and overall DSSC performance, establishing thiazole-bridged fluorene and triphenylamine dyes as efficient co-sensitizers for next-generation solar energy devices.

Keywords

Co-sensitization

Dye-sensitized solar cells

Fluorene

N719

Power conversion efficiency

Thiazole dye

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1. Introduction

Dye-sensitized solar cells (DSSCs) are promising photovoltaic systems because they offer low fabrication costs, straightforward processing, and good power conversion efficiency (PCE) under natural lighting conditions [1]. Since the introduction of ruthenium-based sensitizers, such as N3 and N719, DSSCs have achieved efficiencies exceeding 11% under AM 1.5G illumination [2-5]. Further performance improvements have been demonstrated using porphyrin-based dyes in conjunction with cobalt-based redox electrolytes, achieving efficiencies of over 13% [6]. Recently, perovskite solar cells have shown rapid advancements, with certified PCEs surpassing 20% [7]. However, DSSCs remain a viable and attractive option, especially for indoor and low-light applications, owing to their tunable spectral responses and long-term operational stability. By combining dyes with complementary absorption profiles, co-sensitization broadens the light-harvesting capabilities and improves dye coverage on the semiconductor surface, which in turn enhances photocurrent generation and suppresses recombination losses [8]. One of the most successful examples of this approach is the usage of co-adsorbents, such as chenodeoxycholic acid (CDCA), leading to higher efficiencies approaching 10% for nanocrystalline TiO₂-based devices [9]. Although ruthenium-based complexes exhibit outstanding photovoltaic properties, their high cost, intricate synthesis procedures, and dependence on scarce noble metals have encouraged the search for efficient metal-free organic sensitizers [10]. These dyes typically exhibit high molar extinction coefficients, tunable energy levels, and straightforward synthetic modifications, making them ideal candidates for scalable solar technologies [11]. An effective and widely utilized strategy for organic dye design is the assembly of donor–π–acceptor (D–π–A) structures, which link an electron-donor moiety with an electron-acceptor group through a conjugated π-spacer to promote charge transfer. [12]. A variety of donor motifs, including carbazole [13], thiazole oligomers [14], triphenylamine [15], and fluorene, have been explored with promising results [16]. Among these, fluorene and triphenylamine have attracted attention because of their favorable electronic and structural characteristics. Triphenylamine donors exhibit strong electron-donating ability, high thermal stability, and nonplanar geometries that help reduce dye aggregation. Fluorene derivatives are valued for their UV stability, rigidity, and potential to suppress π–π stacking through steric hindrances. Moreover, the incorporation of a thiazole π-bridge has shown promise in enhancing intramolecular charge transfer (ICT) and improving dye-semiconductor interactions owing to its electron-withdrawing nature and planarity [16]. Therefore, this study aimed to design, synthesize, and evaluate two new thiazole-based organic co-sensitizers, NMS-1 and NMS-2, incorporating triphenylamine and fluorene donor units, respectively, for application in (DSSCs). This study seeks to elucidate how the donor structure and interfacial modification with CDCA influence light-harvesting ability, charge recombination, and overall device efficiency when co-sensitized with the benchmark Ru-based dye N719. This study aims to elucidate the structure–property–performance correlations that determine co-sensitization efficiency in DSSCs via the integration of experimental analyses and quantum-chemical simulations.

2. Materials and Methods

2.1. General procedure for chalcone co-sensitizers NMS-1-2.

A mixture of the aromatic aldehydes, 4-(diphenylamino)benzaldehyde (2.73 g, 0.012 mol) (2), 9,9-dimethyl-9H-fluorene-3-carbaldehyde (2.22 g, 0.01 mol) (3), and 4-(2-aminothiazol-5-yl)benzoic acid (1) (2.20 g, 0.01 mol) dissolved in ethanol (150 mL) and 4 mL acetic acid. The reaction mixture was subjected to reflux for a duration of five h. Upon completion of the reaction, solid sensitizers NMS-1-2 were formed at room temperature. The product sensitizers were subsequently filtered and washed with cold alcohol (Scheme 1).

Synthesis of the thiazole co-sensitizer NMS-1-2.

Molecular structures of dyes NMS-1-2 and the benchmark N719 dye.

4-(2-((4-(Diphenylamino)benzylidene)amino)thiazol-5-yl)benzoic acid (NMS-1). Deep Orange crystals, 82% yield m.p. = 296-298°C. IR (KBr): ν _max 1702 (C=O, COOH), 1673 cm^-1 (C=C). δ ¹H NMR (DMSO-d₆): δ 6.92 (t, J=8.00 Hz, 2H), 7.04 (d, J = 8.00 Hz, 4H), 7.18-7.21 (m, 6H), 7.36 (d, J = 8.00 Hz, 2H), 7.83 (d, J = 8.00 Hz, 2H), 8.11 (s, 1H), 8.25 (d, J = 8.00 Hz, 2H), 8.95 ppm (s, 1H). ¹³CNMR: δ 124.3 (3C), 124.8 (3C), 125.8 (3C), 126.2 (4C), 127.1, 128.6, 129.8 (4C), 130.9, 131.3, 135.0, 136.9, 137.9, 145.4, 147.8, 155.5, 165.6, 168.9 ppm. Analysis calcd. For C₂₉H₂₁N₃O₂S (475.57): C, 73.24; H, 4.45; N, 8.84. Found: C, 73.12; H, 4.78; N, 8.64%.

4-(2-(((9,9-Dimethyl-9H-fluoren-3-yl)methylene)amino)thiazol-5-yl)benzoic acid (NMS-2). Yellow crystals were obtained with a yield of 67%. = 320-322°C. IR (KBr): ν _max 3066, 3015 (CH), 1701 (C=O, COOH), 1687 (C=O), 1596 cm^-1 (C=C) cm^-1. ¹HNMR (DMSO-d₆): δ 2.10 (s, 6H), 7.13 (t, J=8.00 Hz, 1H), 7.21 (t, J = 8.00 Hz, 1H), 7.27 (d, J = 8.00 Hz,1H), 7.45 (d, J = 8.00 Hz, 1H), 7.57-7.61 (m, 2H), 7.68 (d, J = 8.00 Hz, 2H), 7.98 (s, 1H), 8.10 (d, J = 8.00 Hz, 2H), 8.15 (s, 1H), 8.87 ppm (s, 1H). ¹³CNMR: δ 26.6, 49.0, 123.9, 124.3, 124.3 (2C), 124.6, 127.1 (2C), 127.5, 128.0, 128.6, 129.2 (2C), 136.0, 136.7, 136.9, 137.3, 137.9, 151.1 (2C), 154.3 (2C), 157.7, 165.6, 168.9 ppm. Analysis calcd. for C₂₆H₂₀N₂O₂S (424.52): C, 73.56; H, 4.75; N, 6.60; Found: C, 74.02; H, 4.63; N, 6.23%.

3. Results and Discussion

3.1. Synthesis

The thiazole-based co-sensitizers NMS-1 and NMS-2 were successfully synthesized via an acid-catalyzed Schiff base (Figure 1) condensation reaction between 4-(2-aminothiazol-5-yl)benzoic acid (1) and the corresponding aromatic aldehydes: 4-(diphenylamino)benzaldehyde (2) for NMS-1, and 9,9-dimethyl-9H-fluorene-3-carbaldehyde (3) for NMS-2. The reactions were performed under reflux in ethanol with a catalytic quantity of glacial acetic acid for 5 h. The targeted compounds yielded 82% for NMS-1 and 67% for NMS-2, respectively.

The structures of NMS-1 and NMS-2 were elucidated using Fourier transform infrared (FT-IR), nuclear magnetic resonance (¹HNMR & ¹³CNMR) spectroscopy, and elemental analysis (Figure S1-9). The FTIR spectrum of NMS-1 exhibited strong absorption bands at 1702 cm^-1, matching the C=O stretching vibration of the carboxylic acid group. The ¹H NMR spectrum of NMS-1 displayed a singlet at δ 8.95 ppm corresponding to the imine proton (–CH=N–), confirming the formation of the Schiff base. The ¹³C NMR spectrum of NMS-1 showed resonances characteristic of aromatic and heterocyclic carbons in the range of δ 124.3-155.5 ppm. The carbonyl carbon of the COOH group appeared at δ 168.9 ppm. For NMS-2, ¹H NMR spectrum showed a singlet at δ 8.87 ppm for the imine proton and a distinct singlet at δ 2.10 ppm for the two methyl groups (6H total) on the fluorene unit, confirming successful condensation and substitution. In the ¹³C NMR spectrum, the methyl carbons resonated at δ 26.6 ppm, whereas the quaternary carbon attached to both methyl groups appeared at δ 49.0 ppm.

Figures S1-9

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3.2. Photophysical characterization of the compounds

The photophysical characteristics of the thiazole-based co-sensitizers NMS-1 and NMS-2 revealed the typical dual-band absorption profile commonly observed in organic dyes, which were determined in DMF solution and have been shown in Figure 2 and Table 1.

UV-Vis absorption spectra of thiazole sensitizers NMS-1-2.

Table 1. Photophysical characterization of the co-sensitizer NMS-1-2.

Sensitizer	λ_max (nm)	Ɛ (10⁴ M^-1cm^-1)	λ_onset/nm	Experimental E_0-0 (eV)
NMS-1	355, 501	41.92, 5.14	636	1.94
NMS-2	301, 476	1.58, 4.65	580	2.13

Figure 2 reveals two characteristic absorption bands: the one appearing in the ultraviolet region (300-400 nm) is due to π–π* transitions, and the dominant visible-region band (400-600 nm) originates from intramolecular charge-transfer (ICT) excitation [17], which promotes an efficient photoinduced charge separation. NMS-1 and NMS-2 exhibited λ_max at 501 nm and 476 nm, respectively, with molar extinction coefficients (ε) of 5.14 and 4.65 × 10⁴ M⁻¹ cm⁻¹, respectively. These ε values significantly exceed that of the reference Ru-based dye N719 (∼ 1.48 × 104 M-1). cm^-1 (Figure S1). The integration of a thiazole spacer and a conjugated donor system, triphenylamine in NMS-1 and fluorene in NMS-2, contributed to the broad absorption spectra and enhanced ICT. Owing to the planar configuration and high electron-donor strength of the triphenylamine unit, NMS-1 exhibited extended conjugation, which caused a significant red-shift in the absorption spectrum relative to that of NMS-2. [18], The optical band gap (E_0-0) was 1.94 eV for NMS-1 and 2.13 eV for NMS-2. The smaller band gap of NMS-1 indicates enhanced π-electron delocalization and more efficient excitation, which are critical for improved solar photon utilization.

When the NMS dyes were adsorbed onto TiO₂, their absorption spectra became broader and more structured than in solution, reflecting effective binding and strong electronic coupling with the semiconductor surface [19], useful for improving the photocurrent short-circuit current density (J_SC) in the designed DSSCs. Notably, a hypsochromic (blue) shift in the absorption maxima was noticed for both dyes after adsorption, attributing to H-type aggregation or carboxylic acid deprotonation during binding to the TiO₂ surface [19]. Such aggregation affects the molecular planarity and dye packing, and although it may limit conjugation to some extent, it often reflects effective dye loading on the semiconductor. This improved co-sensitization of NMS-1-2 with N719 coverage results from the complementary absorption of the metal-free dyes in the mid-visible range and N719’s strong absorption in both the visible and near-UV regions [20]. During co-adsorption, the development of J-type aggregates induces bathochromic displacement of the absorption band, which extends the spectral coverage and leads to more effective light collection [20]. Among the co-sensitized systems, the combination of NMS-1 with N719 showed superior spectral extension compared to NMS-2 + N719, likely due to the stronger ICT character and broader visible absorption of the triphenylamine-based NMS-1 (Figure 3). This synergy between the dyes enhances the photon capture efficiency, reduces competitive adsorption, and promotes charge separation within the device. Further improvement was observed when CDCA was introduced as a co-adsorbent in the ternary system (NMS-1-2 + N719 + CDCA). The presence of CDCA led to absorption spectrum broadening and reduced undesirable dye aggregation by occupying vacant TiO₂ sites and positioning the dye molecules appropriately [21]. The optimized molecular arrangement supports the generation of beneficial J-aggregates, as reflected by bathochromic displacement and intensified absorption bands.

UV-visible absorption spectra of thiazole NMS-1-2 and NMS-1-2+N719 dyes on TiO2.

3.3. Electrochemical characterization

To study the electrochemical characteristics of the synthesized dyes, cyclic voltammetry (CV) measurements were performed on the thiazole-based sensitizers NMS-1 and NMS-2 to assess their redox activity and energy-level alignment, critical for DSSC performance [22]. These electrochemical experiments provided values for both the ground-state oxidation potential (GSOP) and the excited-state oxidation potential (ESOP), from which the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels were derived, respectively. Understanding the relative positions of these energy levels is essential for evaluating the feasibility of electron injection from the excited dye into the TiO₂ conduction band and subsequent regeneration by the electrolyte (Eq. 1, 2) The relevant data, summarized in Table 2, are illustrated in the energy-level diagram (Figure 4), providing a comprehensive view of the electronic alignment of the dyes with the DSSC components.

(1)

G S O P = - [E_{o n s e t}^{o x d} (e V) + 4.7)] e V

(2)

ESOP = (GSOP - E_{0 - 0}) eV

Table 2. Electrochemical data for thiazole sensitizers NMS-1 and NMS-2.

Sensitizer	Experimental data						Theoretical calculation
Sensitizer	E_OX (HOMO)	E_ox^* (LUMO)	E_0-0 (eV)	∆G_inject (eV)	∆G_reg (eV)	∆G_rec (eV)	E_OX (HOMO)	E_ox^* (LUMO)	E_0-0 (eV)
NMS-1	-5.58	-3.64	1.94	-0.56	0.38	1.38	-5.56	-3.64	1.92
NMS-2	-5.45	-3.32	2.13	-0.88	0.25	1.25	-5.43	-3.35	2.10

Experimental data for GSOP and ESOP of thiazole co-sensitizer NMS-1-2.

The measured LUMO energies of NMS-1 (-3.64 eV) and NMS-2 (-3.32 eV) exceed the TiO₂ conduction-band potential (-4.20 eV), verifying that both sensitizers possess the necessary energetic alignment for efficient electron injection into TiO₂ [22]. The HOMO energy levels, derived from the oxidation onset potentials, were -5.58 eV for NMS-1 and -5.45 eV for NMS-2, both of which were sufficiently below the redox potential of the I⁻/I₃⁻ electrolyte (-5.20 eV). This ensures thermodynamic favorability for dye regeneration, with sufficient driving force to facilitate rapid electron donation from the electrolyte to the oxidized dye [22].

The greater HOMO depth of fluorene in NMS-2 suggests improved oxidative stability, whereas the shallower HOMO of NMS-1 contributes to a more favorable electron injection (ΔG_inj) Eq. 3-5.

(3)

Δ G_{i n j} (e V) = E_{O X}^{d y e^{*}} - E_{C B}

(4)

Δ G_{r e g} (e V) = E_{O X}^{d y e} - E_{r e d o x}

(5)

Δ G_{r e c .} (e V) = E_{O X}^{d y e} - E_{C B}

NMS-2 showed a more negative (ΔG_inj) of -0.88 eV, compared to -0.56 eV for NMS-1, highlighting its superior ability to transfer electrons to the conduction band. This stronger driving force in NMS-2 is consistent with the presence of a highly electron-rich fluorene donor group, which facilitates better orbital overlap and faster charge transfer kinetics. The corresponding free energies of dye regeneration (ΔG_reg) were 0.38 eV for NMS-1 and 0.25 eV for NMS-2, indicating that both dyes can be efficiently reduced by the redox couple, although NMS-2 offers a more favorable profile. Additionally, the recombination driving force (ΔG_rec), which represents the energy barrier preventing back electron transfer from TiO₂ to the oxidized dye, was calculated to be 1.38 eV for NMS-1 and 1.25 eV for NMS-2. These high ΔG_rec values confirm the strong suppression of charge recombination in both systems, which is an essential factor for maintaining a high open-circuit voltage (V_OC) and overall device efficiency [23]. The electrochemical and optical parameters derived for NMS-1 and NMS-2 closely aligned with the DFT-calculated frontier molecular orbitals and bandgap values, strongly supporting the validity of the computational models employed. This consistency highlights the robustness of integrating theoretical predictions with experimental validation in the rational design of efficient DSSC sensitizers.

3.4. Molecular modeling

The molecular optimization of NMS-1 and NMS-2 was achieved through density functional theory (DFT) employing the B3LYP/6-31G(d) basis set, which facilitated a comprehensive evaluation of their geometry and electronic configuration. As shown in Figure 5, NMS-1, the triphenylamine donor adopts a planar conformation that supports strong electron delocalization along the π system. In contrast, the fluorene unit in NMS-2 introduced torsion that slightly disrupted conjugation. These structural differences influence the charge distribution within the molecules. For NMS-1, the HOMO is localized on the triphenylamine and thiazole moieties, while the LUMO extends across the π-bridge to the anchoring group (carboxylic acid group). This separation facilitates charge transfer after photoexcitation. NMS-2 shows a similar pattern, though with less pronounced orbital separation. The HOMO is located across the fluorene and thiazole moieties, while LUMO continues across the π-bridge to the anchoring group (carboxylic acid group). The calculated energy levels align with experimental measurements, confirming that both dyes have sufficient potential to inject electrons into TiO₂ and be regenerated by the electrolyte. The electronic structure of NMS-1 and NMS-2, combined with their more favorable geometry, suggests stronger light absorption and more efficient charge separation features that are expected to support higher solar cell performance.

FMO for thiazole sensitizers NMS-1-2. NMS-1 (4-(2-((4-(Diphenylamino)benzylidene)amino)thiazol-5-yl)benzoic acid); NMS-2, 4-(2-(((9,9-Dimethyl-9H-fluoren-3-yl)methylene)amino)thiazol-5-yl)benzoic acid.

3.5. Molecular electrostatic potential for thiazole sensitizers NMS-1-2.

Molecular electrostatic potential (MEP) provides insight into the charge distribution and reactivity of the NMS-1 and NMS-2 co-sensitizers [24]. As shown in Figure 6, both molecules exhibit regions of low electrostatic potential (red) localized around the carboxylic acid anchoring groups. In NMS-1, the triphenylamine donor appears in a region of higher potential (blue), indicating it may act as an electrophilic site. Similarly, in NMS-2, the fluorene unit displays localized positive potential, suggesting a comparable role.

(MEP) for thiazole sensitizer NMS-1-2 co-sensitizers. NMS-1 (4-(2-((4-(Diphenylamino)benzylidene)amino)thiazol-5-yl)benzoic acid); NMS-2, 4-(2-(((9,9-Dimethyl-9H-fluoren-3-yl)methylene)amino)thiazol-5-yl)benzoic acid.

3.6. Photovoltaic device characterizations

The photovoltaic performance of DSSCs employing thiazole-based co-sensitizers NMS-1 and NMS-2 was evaluated; the current–voltage (J-V) curves have been shown in Figure 7, and the subsequent device parameters have been summarized in Table 3. Co-sensitized cells demonstrated significantly enhanced photocurrent and efficiency. One co-sensitized (NMS-1+N719) device reached a (Jsc) of 18.84 mA.cm^-2, open-circuit voltage (Voc) of 691 mV, and PCE of 7.90%, while the (NMS-2+N719) cell achieved 18.06 mA cm^-2 (Jsc), 663 mV (Voc), and 7.87% efficiency. These improvements of co-sensitizers (NMS-1-2+N719) than sensitization by N719 which are primarily attributed to the broader spectral coverage and more effective dye loading enabled by the co-sensitization approach [25,26]. The dye loading measurements confirmed more efficient surface coverage, 1.77 × 10^-5 mol∙cm^-2 for NMS-1+N719 and 1.65 × 10^-5 mol∙cm⁻² for NMS-2+N719, compared to 1.53 × 10^-5 mol∙cm^-2 for N719 alone.

J-V curves of thiazole sensitizers (NMS-1-2 +N719) and (NMS-1-2 +N719 + CDCA).

Table 3. Photovoltaic characterization for sensitizers thiazole NMS-1-2.

Sensitizers	V_OC (mV)	J_SC (mA/cm²)	FF (%)	η_cell (%)	Dye loading/10^-5 mol.cm^-2
Sensitizers	V_OC (mV)	J_SC (mA/cm²)	FF (%)	η_cell (%)	N719	NMS-1-2	CDCA	Total
(N719)	675	17.60	61.44	7.30	-	-	-	1.53
(NMS-1+N719)	691	18.84	60.68	7.90	0.95	0.82	-	1.77
(NMS-2+N719)	663	18.06	65.72	7.87	0.92	0.73	-	1.65
(NMS-1+N719+CDCA)	707	19.18	65.63	8.90	0.78	0.59	0.83	2.20
(NMS-2+N719+CDCA)	723	20.18	68.05	9.93	0.73	0.78	0.84	2.35

To further optimize dye alignment and suppress recombination, CDCA was introduced as a co-adsorbent in the ternary system [27]. The (NMS-1+N719+CDCA) device showed improved performance, reaching a Jsc of 19.18 mA cm^-2, Voc of 707 mV, and a PCE of 8.90%. Remarkably, the (NMS-2+N719+CDCA) system exhibited the highest overall performance, with a Jsc of 20.18 mA.cm^-2, Voc of 723 mV, FF of 68.05%, and a PCE of 9.93%. These results mark a substantial improvement compared to N719 alone, representing a relative PCE increase of approximately 36%. The superior performance of the NMS-2-based ternary system (NMS-2+N719+CDCA) may be attributed to the structural compatibility between the fluorene donor unit and the packing behavior promoted by CDCA, as shown in Figure 7 [27]. CDCA further contributes by enhancing dye spacing, promoting vertical orientation, and minimizing recombination, as reflected in the elevated Voc values. Dye loading analysis supports photovoltaic performance, with total dye coverage reaching 2.35×10^-5 mol cm^-2 for NMS-2+N719+CDCA and (2.20 × 10^-5mol.cm⁻²) for NMS-1+N719+CDCA. Such findings demonstrate that NMS-1 and NMS-2 are efficient co-sensitizers when paired with Ru-based complexes, especially when interface engineering strategies are applied to optimize dye organization on the semiconductor surface.

Figure 8 presents the incident photon-to-current efficiency (IPCE) spectra of the DSSCs assembled with the individual thiazole dyes (NMS-1 and NMS-2) and their corresponding co-sensitized systems with N719, both in the absence and presence of CDCA. Upon co-sensitization with the Ru-based dye N719, both NMS-1 and NMS-2 systems demonstrated a pronounced increase in spectral response and a broader light-harvesting range compared with the single-dye device. The maximum IPCE values reached 80% for the NMS-1 + N719 cell and 77% for the NMS-2 + N719 configuration, whereas the reference N719-only device achieved 73%. This improvement originates from the absorption behavior of the two organic sensitizers and their electron-injection capability when coupled with N719 [28]. In addition, the NMS-1 + N719 co-sensitized device exhibited a slight red shift in its spectral response compared with NMS-2 + N719, a trend that matches the red-shifted UV–Vis absorption observed for NMS-1 in solution and on TiO₂ films. Further enhancement of the photoresponse was observed after incorporating CDCA as a co-adsorbent. The maximum IPCE increased to 81% for the NMS-1 + N719 + CDCA device and 83% for the NMS-2 + N719 + CDCA system. Although the fluorene-containing NMS-2 + N719 + CDCA configuration exhibited the highest response, the NMS-1-based cell also displayed a substantial improvement relative to its binary counterpart. The outperforming behavior of the NMS-2-based ternary system could be linked to the structural compatibility between the rigid, planar fluorene donor and CDCA, which enables better dye organization and packing on the TiO₂ surface. This arrangement promotes efficient charge separation and suppresses electron recombination [29]. Moreover, CDCA acts as a molecular spacer that mitigates dye aggregation and optimizes surface coverage, allowing for more uniform adsorption of both dyes. This cooperative interaction between CDCA and the fluorene moiety enhances the charge-transfer dynamics and promotes the collection efficiency within the co-sensitized system. Consequently, the combined effects of extended spectral coverage, improved molecular alignment, and reduced recombination account for the notable rise in the IPCE and overall photovoltaic performance observed in the NMS-2 + N719 + CDCA device.

IPCE of thiazole sensitizers (NMS-1-2 +N719) and (NMS-1-2 +N719 + CDCA).

3.7. Electrochemical impedance spectroscopy

Electrochemical impedance spectroscopy (EIS) was performed to probe the charge transport and recombination dynamics in the fabricated DSSCs, as shown in Figure 9 [30]. In the Nyquist plots, the semicircle observed at intermediate to low frequencies relates to the charge recombination resistance (R_rec) at the TiO₂/dye/electrolyte interface. A higher semicircle radius reflects higher recombination resistance, indicating slower electron recombination and longer electron lifetimes, both of which are favorable for device performance. The Nyquist plots reveal a clear trend in R_rec among the devices, with semicircle radii increasing in the following order: NMS-2+N719 < N719 < NMS-1+N719 < NMS-2+N719+CDCA < NMS-2+N719+CDCA. This trend strongly correlates with the (Voc) values of the respective devices, suggesting that improved recombination resistance contributes to the observed enhancement in Voc [30]. The addition of CDCA further improved recombination resistance in both dye systems. For both NMS-1 and NMS-2, co-adsorption with CDCA resulted in noticeably larger semicircle radii compared to the corresponding binary systems without CDCA. This enhancement is attributed to CDCA’s role in mitigating dye aggregation, improving dye packing, and passivating surface trap states, all of which help suppress recombination pathways at the TiO₂ interface.

Nyquist plots for co-sensitizers (NMS-1-2+N719) and (NMS-1-2+N719+CDCA).

4. Conclusions

DSSCs remain a competitive photovoltaic technology, with state-of-the-art efficiencies for Ru-based and porphyrin systems reaching over 13% under optimized conditions. However, the continued dependence on noble-metal complexes drives the search for cost-effective, stable, and environmentally benign metal-free organic sensitizers. In this context, two new D–π–A-structured thiazole dyes, NMS-1 and NMS-2, were synthesized, employing triphenylamine and fluorene as donor units, respectively, and investigated as co-sensitizers with N719. The co-sensitized devices exhibited notable performance, achieving power-conversion efficiencies of 7.90% for NMS-1 + N719 and 7.87% for NMS-2 + N719. Upon the incorporation of CDCA as a co-adsorbent, further enhancement was realized, with efficiencies rising to 8.90% for NMS-1 + N719 + CDCA and 9.93% for NMS-2 + N719 + CDCA. The enhanced behavior of the NMS-2 system could be due to the favorable interfacial interactions between the rigid fluorene donor, CDCA, and N719, which facilitate orderly dye organization, higher loading, and suppressed charge recombination. These outcomes underscore how donor design and interfacial engineering can substantially advance the performance of organic co-sensitizers. Overall, this study contributes to the ongoing state-of-the-art development of high-efficiency, metal-free DSSCs by offering a rational molecular framework that couples optical tunability with improved charge-transfer dynamics.

Acknowledgment

Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2025R22), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia.

CRediT authorship contribution statement

Hana M. Abumelha, Nuha M. Halawani: Data curation, formal analysis, methodology, and software; Shadiah Albalawi, Jihan Qurban: Investigation and writing – review & editing; Khalid Althumayri, Renad Almughathawi: formal analysis, investigation, writing-original draft. Ali H. Alessa, Nashwa M. El-Metwaly: Supervision and administration of research group.

Declaration of competing interest

There are no conflicts of interest.

Data availability

The data included in article/supplementary material/references in the article.

Declaration of generative AI and AI-assisted technologies in the writing process

The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.

Supplementary data

Supplementary material to this article can be found online at https://dx.doi.org/10.25259/AJC_311_2025.

References

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1. Introduction

2. Materials and Methods

2.1. General procedure for chalcone co-sensitizers NMS-1-2.

3. Results and Discussion

3.1. Synthesis
3.2. Photophysical characterization of the compounds
3.3. Electrochemical characterization
3.4. Molecular modeling
3.5. Molecular electrostatic potential for thiazole sensitizers NMS-1-2.
3.6. Photovoltaic device characterizations
3.7. Electrochemical impedance spectroscopy

4. Conclusions

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[1] O’Regan B., Grätzel M.. A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO₂ films. Nature. 1991;353:737-740. https://doi.org/10.1038/353737a0
[Google Scholar]

[2] Wang P., Zakeeruddin S.M., Moser J.E., Nazeeruddin M.K., Sekiguchi T., Grätzel M.. A stable quasi-solid-state dye-sensitized solar cell with an amphiphilic ruthenium sensitizer and polymer gel electrolyte. Nature Materials. 2003;2:402-407. https://doi.org/10.1038/nmat904
[Google Scholar]

[3] Nazeeruddin M.K., De Angelis F., Fantacci S., Selloni A., Viscardi G., Liska P., Ito S., Takeru B., Grätzel M.. Combined experimental and DFT-TDDFT computational study of photoelectrochemical cell ruthenium sensitizers. Journal of the American Chemical Society. 2005;127:16835-16847. https://doi.org/10.1021/ja052467l
[Google Scholar]

[4] Wang Z.S., Yamaguchi T., Sugihara H., Arakawa H.. Significant efficiency improvement of the black dye-sensitized solar cell through protonation of TiO₂ films. Langmuir: The ACS Journal of Surfaces and Colloids. 2005;21:4272-4276. https://doi.org/10.1021/la050134w
[Google Scholar]

[5] Grätzel M.. Photoelectrochemical cells. Nature. 2001;414:338-344. https://doi.org/10.1038/35104607
[Google Scholar]

[6] Mathew S., Yella A., Gao P., Robin H.B., Curchod B.F.E., Negar A.A., Tavernelli I., Rothlisberger U., Nazeeruddin M.K., Grätzel M.. Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers. Nature Chemistry. 2014;6:242. https://doi.org/10.1038/nchem.1861
[Google Scholar]

[7] Best research-cell efficiencies. Available from: http://www.nrel.gov/ncpv/images/efficiency_chart.jpg [Last accessed Feb 2015].

[8] Islam A., Swetha T., Karim M.R., Akhtaruzzaman M., Han L., Singh S.P.. Tuning of spectral response by co‐sensitization in black‐dye based dye‐sensitized solar cell. Phys Status Solidi A. 2015;212:No. 3, 651-656.‏ https://doi.org/10.1002/pssa.201431654
[Google Scholar]

[9] Sasitharan K., Mora Abarca A.J., Cucinotta F., Pineda L.W., Soto Tellini V.H., Freitag M.. Bile acid derivatives as novel co-adsorbents for enhanced performance of blue dye-sensitized solar cells. Communications Chemistry. 2025;8:75. https://doi.org/10.1038/s42004-025-01433-1
[Google Scholar]

[10] Wang X., Wei N., Cheng Y., Zhang A., Bian Z., Lu H., Zhu X., Liu Y., Wei Y., Bo Z.. Boosting organic solar cell efficiency via tailored end-group modifications of novel non-fused ring electron acceptors. Materials Horizons. 2024;11:6019-6027. https://doi.org/10.1039/d4mh01113a
[Google Scholar]

[11] Gupta K.S.V., Suresh T., Singh S.P., Islam A., Han L., Chandrasekharam M.. Carbazole based A-p-D-p-A dyes with double electron acceptor for dye- sensitized solar cell. Organic Electronics. 2014;15:266. https://doi.org/10.1016/j.orgel.2013.11.020
[Google Scholar]

[12] Zhang J., Zhong A., Huang G., Yang M., Li D., Teng M., Han D.. Enhanced efficiency with CDCA co-adsorption for dye-sensitized solar cells based on metallosalophen complexes. Solar Energy. 2020;209:316-324. https://doi.org/10.1016/j.solener.2020.08.096
[Google Scholar]

[13] Wang Z.S., Koumura N., Cui Y., Takahashi M., Sekiguchi H., Mori A., Kubo T., Furube A., Hara K.. Hexylthiophene-functionalized carbazole dyes for efficient molecular photovoltaics: Tuning of solar-cell performance by structural modification. Chemistry of Materials. 2008;20:3993-4003. https://doi.org/10.1021/cm8003276
[Google Scholar]

[14] Radwan A.S., Elmorsy M.R., Abdel-Latif E., Makhlouf M.M., Badawy S.A.. Innovating dye-sensitized solar cells: Thiazole-based Co-sensitizers for enhanced photovoltaic performance with theoretical insights. Optical Materials. 2023;140:113914. https://doi.org/10.1016/j.optmat.2023.113914
[Google Scholar]

[15] Tarsang R., Promarak V., Sudyoadsuk T., Namuangruk S., Jungsuttiwong S.. Tuning the electron donating ability in the triphenylamine-based D-π-A architecture for highly efficient dye-sensitized solar cells. Journal of Photochemistry and Photobiology A: Chemistry. 2014;273:8-16. https://doi.org/10.1016/j.jphotochem.2013.09.002
[Google Scholar]

[16] Gupta K.S.V., Singh S.P., Islam A., Han L., M. C.. Simple fluorene based triarylamine metal-free organic sensitizers. Electrochimica Acta. 2015;174:581-587. https://doi.org/10.1016/j.electacta.2015.05.158
[Google Scholar]

[17] Alessa A.H., Alqarni S.A., Qurban J., Alghasham H.A., Ashour G.R., Bayazeed A., El-Metwaly N.M.. Synergistic Co-sensitization: Unlocking the potential of carbohydrazide chromophores and metal complexes for high-performance dye-sensitized solar cells. Journal of Molecular Liquids. 2024;408:125354. https://doi.org/10.1016/j.molliq.2024.125354
[Google Scholar]

[18] Althagafi I., El-Metwaly N.. Enhancement of dye-sensitized solar cell efficiency through co-sensitization of thiophene-based organic compounds and metal-based N-719. Arabian Journal of Chemistry. 2021;14:103080. https://doi.org/10.1016/j.arabjc.2021.103080
[Google Scholar]

[19] Karuppasamy A., Stalindurai K., Peng J.D., Ho K.C., Ramalingan C.. Organic dyes festooned with fluorene and fused thiazine for efficient dye-sensitized solar cells. Electrochimica Acta. 2018;268:347-357. https://doi.org/10.1016/j.electacta.2018.02.134
[Google Scholar]

[20] Abumelha H.M., Alessa A.H., Alharbi H., Qurban J., Katuah H.A., Saad F.A., El-Metwaly N.M.. Boosting DSSC efficiency via acid-base co-sensitization with N-1 dye. Journal of Photochemistry and Photobiology A: Chemistry. 2024;453:115664. https://doi.org/10.1016/j.jphotochem.2024.115664
[Google Scholar]

[21] Alenazi N.A.. Breaking the 11% efficiency barrier in dye-sensitized solar cells: Synergistic Co-Sensitization with CZ-1, TP-2, and black dye. Arabian Journal for Science and Engineering. 2025 https://doi.org/10.1007/s13369-025-10053-9
[Google Scholar]

[22] Badawy S.A., Abdel-Latif E., Elmorsy M.R.. Tandem dye-sensitized solar cells achieve 12.89% efficiency using novel organic sensitizers. Scientific Reports. 2024;14:26072. https://doi.org/10.1038/s41598-024-75959-0
[Google Scholar]

[23] Gupta K.S.V., Singh S.P., Islam A., Han L., M. C.. Simple fluorene based triarylamine metal-free organic sensitizers. Electrochimica Acta. 2015;174:581-587. https://doi.org/10.1016/j.electacta.2015.05.158
[Google Scholar]

[24] Makangara J.J., Sahini M.G., Surendra babu N.. Theoretical and conceptual framework to design D-π-A type organic dyes for application dye-sensitized solar cells. Journal of the Indian Chemical Society. 2024;101:101118. https://doi.org/10.1016/j.jics.2023.101118
[Google Scholar]

[25] Badawy S.A., Abdel-Latif E., Mohamed W.H., Elmorsy M.R.. Unleashing synergistic co-sensitization of BOA dyes and Ru(ii) complexes for dye-sensitized solar cells: Achieving remarkable efficiency exceeding 10% through comprehensive characterization, advanced modeling, and performance analysis. RSC Advances. 2024;14:25549-25560. https://doi.org/10.1039/d4ra04001e
[Google Scholar]

[26] Jin L., Shi S., Zhao C., Yu X., Lu J., Wang Q., Wei Y.. Y-shaped organic dyes with D2–π–A configuration as efficient co-sensitizers for ruthenium-based dye sensitized solar cells. Journal of Power Sources. 2021;481:228952. https://doi.org/10.1016/j.jpowsour.2020.228952
[Google Scholar]

[27] Salvatori P., Marotta G., Cinti A., Anselmi C., Mosconi E., De Angelis F.. Supramolecular interactions of chenodeoxycholic acid increase the efficiency of dye-sensitized solar cells based on a cobalt electrolyte. The Journal of Physical Chemistry C. 2013;117:3874-3887. https://doi.org/10.1021/jp4003577
[Google Scholar]

[28] Koyyada G., Kumar Chitumalla R., Thogiti S., Kim J.H., Jang J., Chandrasekharam M., Jung J.H.. A new series of EDOT based Co-Sensitizers for enhanced efficiency of cocktail DSSC: A comparative study of two different anchoring groups. Molecules (Basel, Switzerland). 2019;24:3554. https://doi.org/10.3390/molecules24193554
[Google Scholar]

[29] Li J., Wu W.J., Yang J.B., Tang J., Long Y.T., Hua J.L.. Effect of chenodeoxycholic acid (CDCA) additive on phenothiazine dyes sensitized photovoltaic performance. Science China Chemistry. 2011;54:699-706. https://doi.org/10.1007/s11426-011-4227-9
[Google Scholar]

[30] Hossan A.. Cocktail co-sensitization of two novel metal-free dyes and N-719: A promising strategy for enhanced performance of dye-sensitized solar cells. Journal of Photochemistry and Photobiology A: Chemistry. 2023;445:115072. https://doi.org/10.1016/j.jphotochem.2023.115072
[Google Scholar]

Synergistic co-sensitization of fluorene- and triphenylamine-derived thiazole dyes with N719: A quantum chemical and experimental investigation for enhanced DSSC performance

Abstract

Keywords

Co-sensitization

Dye-sensitized solar cells

Fluorene

N719

Power conversion efficiency

Thiazole dye

1. Introduction

2. Materials and Methods

2.1. General procedure for chalcone co-sensitizers NMS-1-2.

3. Results and Discussion

3.1. Synthesis

3.2. Photophysical characterization of the compounds

3.3. Electrochemical characterization

3.4. Molecular modeling

3.5. Molecular electrostatic potential for thiazole sensitizers NMS-1-2.

3.6. Photovoltaic device characterizations

3.7. Electrochemical impedance spectroscopy

4. Conclusions

Acknowledgment

CRediT authorship contribution statement

Declaration of competing interest

Data availability

Declaration of generative AI and AI-assisted technologies in the writing process

Supplementary data

References

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