All the reagents and solvents were acquired from Sigma-Aldrich, including 3-formylchromone, Rhodamine-6G, hydrazine hydrate (85%), silver nitrate, sodium borohydride, acetic acid, ethanol, acetone, and iodide/triiodide electrolyte. Water-based colloidal graphite used in making the counter electrode was purchased from Electron Microscopy Sciences (EMS). The titanium dioxide (TiO₂) film was prepared with Titanium (IV) oxide, anatase (<25 nm) powder purchased from Sigma-Aldrich. The conducting fluorine-doped tin oxide (FTO) glass (50 mm × 50 mm × 2.2 mm) was obtained from Sigma-Aldrich.

The performance of the solar cell was evaluated using a 150 W fully reflective solar simulator with a standard illumination of air-mass 1.5 global (AM 1.5 G filter) having an irradiance of 100 mW/cm² (Sciencetech Inc, London, Ontario, Canada). An Interface1010E potentiostat/galvanostat/ZRA used for current, voltage, and impedance measurements were purchased from GAMRY Instruments (Warminster, PA, USA). The particle size and surface morphology of the synthesized-silver nanoparticles and silver-nanoparticle-infused titanium dioxide were examined with a JEOL transmission electron microscope with an accelerating voltage of 120 KV and a LaB6 electron gun (MA, USA). All absorption and fluorescence spectra were recorded using an Agilent Cary 60 UV/Vis absorption spectrometer and a Cary Eclipse fluorescence spectrophotometer, respectively.

Silver nanoparticles (AgNPs) were prepared following a previously reported method [12] using sodium borohydride (NaBH₄) to reduce silver nitrate (AgNO₃). Briefly, silver nanoparticles (1 mM) and sodium borohydride (2 mM) solutions were prepared in ice-chilled pure water. The two solutions were mixed slowly with constant stirring, and a yellowish color solution appeared. The mixture was cooled for 20 minutes in ice bath, where the ice bath is used to slow down the reaction and give better control over final particle size/shape. Silver ions were reduced and clustered to form monodispersed nanoparticles as a transparent solution in the aqueous medium. The silver nanoparticle was characterized by UV-V spectroscopy (Agilent Cary 60 UV/Vis’s) and High Resolution-Transmission Electron Microscopy (120 KV JEOL).

The titanium dioxide paste was prepared by mixing TiO₂ powder (1 g), ethylene glycol (1 mL), and glacial acetic acid (3 mL). Silver nanoparticle-based titanium dioxide paste was prepared by mixing TiO₂ powder (1 g), ethylene glycol (1 mL), and silver nanoparticle (3 mL) [13] [14] [15] . The anode was prepared by subsequently administering TiO₂ paste via the doctor-blade method, using a glass rod and adhesive tape to form a 2 × 2 cm rectangle, on the conductive surface of the FTO glass [16] . The FTO glass was air dried for 10 min and then annealed at 450˚C for 30 min. The annealed titanium dioxide was immersed in RdS1 solution overnight. To prepare the cathode, colloidal graphite was applied to the conductive surface of the FTO glass and then dried at 80˚C for 10 min [17] . The components of the DSSC were assembled by fitting the TiO₂-coated FTO glass on top of the colloidal graphite-coated FTO glass, followed by the introduction of redox iodide/triiodide electrolyte solution. The electrolyte was dropped between the photoanode and counter electrode and allowed to spread down by capillary action. The systematic complete assembly of a plasmonic DSSC and energy diagram is shown in Figure 1 (Scheme 1, Scheme 2).

Rhodamine hydrazide intermediate was synthesized according to Yang’s method [18] . A mixture of rhodamine hydrazide intermediate (100 mg, 0.21 mmol), Chrome-3-carboxaldehyde (22 mg, 0.04 mmol) and 2 mL of ethanol was placed in a microwave vessel (Scheme 1). The resulting mixture was stirred and placed in a reactor. The reaction vessel was then run under pressure and irradiation at a specific temperature and time. After cooling, the reaction mixture was filtered and washed with cold ethanol. After drying, the solid product was isolated, and obtained, yield 80%. ¹H-NMR (d₆-DMSO), δ (ppm): 8.5 (s, 1H), 8.4 (s, 1H), 8.0 (m, 1H), 7.9 (m, 1H), 7.71 (m, 1H), 7.60 (m, 3H), 7.50 (m, 1H), 7.00 (d, 1H), 6.27 (s, 2H), 6.10 (s, 2H), 5.01 (s, 2H, -NH), 3.14 (t, 4H, NCH₂CH₃), 1.87 (s, 6H, -CH₃), 1.21 (t, 6H, NCH₂CH₃). ¹³C-NMR (DMSO), δ (ppm): 165.23, 152.07, 151.33,

147.35, 132.31, 129.48, 128.00, 127.01, 123.43, 122.13, 117.79, 104.99, 95.85, 64.96, 55.99, 37.45, 18.53, 17.06, 14.20. HRMS (MALDI): m/z Calcd for S1 (M + 1): 585.2496; Found: 585.2504.

The optical property of synthesized silver nanoparticles was characterized by UV-Vis spectroscopy and high resolution-transmission electron microscopy (HR-TEM). The absorption spectra of the synthesized silver nanoparticles are shown in Figure 3(a). The absorption band at about 400 nm is apparently due to the surface plasmon resonance (SPR) band of the Ag nanoparticles, and so, it confirms the presence of the silver nanoparticles (AgNPs). The SPR results from the interaction of free electrons and electromagnetic radiation [19] reported that rhodamine 6G dyes can form dimers on the AgNPs surface which is related to localized surface plasmon resonance. This phenomenon enhances the absorption coefficients of the dye and optical absorption, which results in an increase in the efficiency of the solar cell [20] [21] . The AgNPsas well as the AgNPs-infused TiO₂ were characterized by transmission electron microscopy (TEM). The TEM allows for the visualization of the individual nanoparticles. The TEM images of the AgNPs with TiO₂ showed a close interaction of the AgNPs with TiO₂. The sizes of the AgNPs and titanium dioxide were similar, which accounts for the excellent interaction between the nanoparticles. The energy disperses X-ray spectrometry of the sample confirmed the presence of copper (Cu), carbon (C), and silver (Ag). The high percentage of copper in the spectrum is due to the copper TEM grid used for mounting the AgNPs sample. The carbon signal should come from the carbon-supporting film on the copper grid. The images of the analysis are displayed in Figure 2.

The optical property of rhodamine derivative RdS1 was investigated using UV/Vis absorption and fluorescence spectroscopy. The dye solution showed no absorption above 400 nm, as shown in Figure 3(b), which is typical for the most prominent ring-closed form of rhodamine derivatives [22] . The UV/Vis spectrum of RdS1 was recorded in buffer at 25˚C and showed an absorption maximum at λ = 305 nm, which is attributed to the intramolecular π-π* charge transfer transition. Upon addition of copper(II) ion, the absorption peak at 300 nm decreased, and a new absorption band appeared at 525 nm, which can be attributed to the delocalized xanthene moiety of rhodamine and coordination of copper ion, as shown in Figure 3(c). As demonstrated in Scheme 2, a monomeric system forms a 1:1 complex. The dye RdS1 exhibited similar fluorescence spectroscopic properties upon binding with Cu²⁺. As shown in Figure 3(d), the fluorescence emission for RdS1 appeared at 560 nm and as well as significant fluorescence intensity enhancement with 5 equivalents of Cu²⁺ ions.

The current-voltage (I-V) characteristics were measured to study the photoelectric performance of both the bare and plasmonic silver nanoparticles (AgNPs) incorporated dye-sensitized solar cells (DSSCs). The photovoltaic performances of the cells were determined via the measurements of maximum voltage (V_max), maximum current (I_max), open-circuit voltage (Voc), short-circuit current (Isc), fill factor (FF), and the power conversion efficiency (η) of the cell. By measuring the current and voltage of the constructed device, it was possible to determine the solar-to-electric power efficiency of the dye-sensitized solar cells [23] . The dye-sensitized solar cell device efficiency was calculated from Figure 4 using the equation, where P_max and P_in denote the maximum output power and intensity of the incident light, respectively.

η ( % ) = P max P i n = ( V o c ∗ I s c ∗ F F ) / P i n

The solar-to-electric power conversion efficiency of the rhodamine dye-fabricated device was compared to the efficiencies of rhodamine-fabricated devices with silver nanoparticles incorporated DSSCs. Table 1 and Figure 4 show the current-voltage curves of the free rhodamine dye (RdS1), rhodamine-copper(II) complex (RdS1-Cu²⁺), rhodamine with AgNPs (RdS1 + AgNPs), rhodamine-copper(II) complex with AgNPs (RdS1-Cu²⁺ + AgNPs). An increase in electric power efficiency was observed after the rhodamine dye RdS1 was made to interact with AgNPs. The efficiency of the device with RdS1 alone was 0.06% but increased to 0.17% after the addition of AgNPs. However, the solar-to-electric power efficiency of the device decreased to 0.08% with the introduction of AgNPs to the dye-copper(II) complex. Thus, the introduction of silver nanoparticles enhanced the light absorption abilities of the dye which consequently led to an increase in the efficiencies of dye-sensitized solar cells. The enhanced photovoltaic performance could be attributed to the plasmonic effect of the silver nanoparticles that result in a swift transfer of an electron from the AgNPs to the TiO₂ [24] .

The interfacial charge mechanisms of the DSSCs were addressed using electrochemical impedance spectroscopy (EIS). Impedance measurements were carried out on the fabricated DSSCs at frequencies between 1 Hz and 10⁶ Hz under 100 mW/cm² illumination. Figure 5(a) shows the EIS-Nyquist plots of fabricated DSSCs. The semicircle in the high-frequency region corresponds to charge

transport resistance (R_ct1) at the I⁻/ I 3 − graphite interface, and semicircle in the middle-frequency region is attributed to charge transport resistance (R_ct2) at the TiO₂/RdS1 I⁻/ I 3 − interface. An equivalent circuit model of the EIS studies is shown in Figure 5(b). Faster electron transfer rates are associated with smaller resistances, which then result in improved efficiency of the DSSC. Conversely, larger resistances hinder the flow of electrons, thus reducing the performance of the DSSC [25] . From the Bode plots of Figure 6, the electron lifetimes (τ) of the DSSCs created were assessed. The electron lifetimes are inversely proportional to the peak frequency, as shown in the formula. The formula τ = 1/(2πf), where f is the peak frequency associated with the charge transfer and recombination kinetics at the sensitizer adsorbed photoanode/electrolyte interface, the lifetimes (τ) of RdS1, RdS1-Cu²⁺, RdS1 + AgNPs, and RdS1-Cu²⁺ + AgNPs were calculated to be 3.19, 4.00, 5.00, and 2.51 ms respectively. The fabricated DSSC with RdS1

+ AgNPs having a higher electron lifetime compared to the DSSC devices agrees with the I-V measurements of the open-circuit voltage (V_oc) of the solar cells, which leads to the higher efficiency of the dye-sensitized solar cell. The peak frequency and lifetime of each fabricated DSSCs is summarized in Table 2.