Stability Modification of Dye-sensitized Solar Cells by Ruthenium Dyes Embedded on Eggshell Membranes

Dye-sensitized solar cells (DSSCs) have been one of the most promising technologies to convert sunlight into electricity repeatedly based on the mechanism that dyes inject/accept electron into the metal oxides/from redox mediator. Specifically, N719 ([RuL2(NCS)2], L: 4,4′-dicarboxy-2,2′-bipyridine), immobilized on TiO2 through the interaction between its ligands (-COO− and -NCS) and the oxygen on the TiO2 surface, has been used as a conventional DSSC dye with high voltage. Nevertheless, -NCS ligands have been removed from Ru2+ in N719 due to UV irradiation and exchanged with H2O or OH− in electrolyte, resulting in voltage drop. In this work, we developed the first DSSC using the N719-adsorbed Eggshell (ESM)-TiO2 composite to maintain the immobilization of N719 on TiO2 through electrostatic interaction between the protein of ESM and N719. The DSSC using the composite maintained the voltage even after 12 h light irradiation, although the voltage of DSSC without ESM dropped drastically. It means that the ESM contributed to stable photovoltaic performances of DSSCs through the protection of NCS ligands of N719.


Introduction
Renewable energies, such as solar, wind, wave and hydro power, have attracted much attention as the solution to increasing energy and environmental concerns [1].Among them, dye-sensitized solar cells (DSSCs), one of the most promising technologies to convert sunlight into electricity, have been intensely developed due to their lower cost and easier fabrication [2,3].The typical DSSC, which is composed of a working electrode with dye chemically attached to anatase-TiO 2 , a counter electrode, and electrolyte solution including redox mediator I − /I 3 − , operates under the following principles [4]: First, the dyes inject electrons into the conduction band of anatase-TiO 2 upon their excitation by incident light.Second, the injected electrons are transported to the counter electrode through a wire, and then the redox mediator I 3 − is reduced to I − on the counter electrode.Finally, the excited dye then returns to its original state by accepting electrons from the redox mediator I − on the working electrode.As mentioned above, the metal oxide layers, such as anatase-TiO 2 and ZnO, play an important role in improving the performance of DSSCs by immobilizing dyes through the interaction between the ligands of dyes and oxygen on the metal oxides [5][6][7].In the conventional DSSCs, N719 ([RuL 2 (NCS) 2 ]•2TBA (L = 2,2 -bipyridyl-4-4,4 -dicarboxylic acid; TBA = tetra-n-butylammonium) [8,9] have been used as dyes with the ligands (-COO − and -NCS) interacting with oxygen on the surface of TiO 2 and acting as modulators of the voltages of DSSCs.[10][11][12].The -COO − group, the major linker to oxygen on TiO 2 surface, injects electron into the conduction band of TiO 2 from N719 and elevates the lowest unoccupied molecular orbital (LUMO) energy of N719 [13,14].The -NCS group, which is the minor linker to the TiO 2 surface, retrieves electron from a redox mediator and lowers the highest occupied molecular orbital (HOMO) energy [15,16].This HOMO-LUMO gap modulation determines the absorption wavelength between 300 and 800 nm, resulting in the DSSCs exhibiting a high voltage of 0.7 V [4].However, the -NCS ligand has been removed from Ru 2+ in N719 by UV irradiation and exchanged with H 2 O or OH − in electrolyte, resulting in a drop in voltage and photocurrent density [17][18][19][20] (Figure 1).A low-cost and simple solution has also been required for wider application of DSSCs.This suggests that the problem in DSSC technology can be solved by a low-cost system to protect the -NCS ligand of N719.As a solution to the problem, we focused on the structure of eggshell membrane (ESM).ESM, which has been generally discarded as an unwanted byproduct of egg consumption worldwide, is a waste product with enormous energy potential [21].ESM has the hydrophobic structure composition of collagen-like proteins with organic substituents, such as hydroxyl, amine and sulfonic groups [22], resulting in ESM being applied to fuel cells as an insoluble proton-conducting membrane through their substituents [21,23,24].Recently, ESMs have been utilized as low-cost and available biotemplates for the preparation of inorganic nanomaterials [23][24][25][26][27] and adsorbents for the removal of water pollutants [28,29], due to their good adsorption of organic materials and metal ions [30,31] into the hydrophobic cavity.These applications contribute as a simultaneous solution to environmental issues and decrease in waste [30].Furthermore, it is expected that ESM can serve as a functional material to immobilize dyes in DSSCs, because of its adsorption ability in hydrophobic cavity.As demonstrated in a recent report on solar cells containing biomaterials [32] such as mycobacterial protein (conversion efficiency η: 1%) [33], bamboo (η: 5.4%) [34], chitosan (η: 1.8%) [35] and spinach (retention of max current density after irradiation: 77.78%) [36], the use of biomaterials may be critical to the development of a new class of sustainable and greener solar cells.
In this work, for the first time, we report the adsorption of N719 ruthenium dyes on ESMs and the results for the application of ESMs to DSSC (Figure 2).We compare the photovoltaic performance and stability to those of a DSSC without ESMs.Based on the results, we discuss the role of ESMs in preventing the degradation of N719 dyes after long irradiation.

Preparation 2.2.1. EMS
The inner ESM was obtained from a gently broken commercial chicken egg, washed with distilled water and dried at 25 • C in the air for 24 h.The ESM was ground into powder in an agate mortar.

N719-Adsorbed ESM-TiO 2 Composite
ESM powder (400 mg) was soaked in a solution of water/ethanol (50:50 vol%), including 1 mg of N719.Then, the residue, N719-adsorbed ESM, was obtained and dried at 25 • C. To achieve a strong interaction between the TiO 2 and N719-adsorbed ESM, we performed an in situ synthesis of anatase-TiO 2 by using the sol-gel method [37][38][39].N719-adsorbed ESM-TiO 2 composite was obtained by adding N719-adsorbed ESM to the aqueous titanium oxide precursor solution.The composite was pasted onto the FTO substrate several times and heated at 60 • C for a few hours.This substrate was used as the working electrode of the DSSC (the loading area: 1 cm × 1 cm).

Characterization 2.3.1. Scanning Electron Microscopy (SEM) with Energy Dispersion X-ray Spectroscopy (EDX)
SEM was performed on JSM-6610 (JEOL, Tokyo, Japan) to observe the morphology of the samples.The samples (ESM and N719-adsorbed ESM-TiO 2 composite) on carbon tapes were sputtered with gold in vacuo three times for 2 min.SEM observation was conducted under high vacuum at a voltage of 15 kV and a working distance of 10 mm in secondary electron mode.
EDX was performed to reveal the distribution of the components, N719 and TiO 2 , and absence of eggshell on ESM by calculation of the fluorescence X-ray yield derived from Ru, Ti and Ca.The observation was conducted under the same conditions as those of SEM observations in the EDX with a silicon drift detector (JED-2300, JEOL).

X-ray Diffraction (XRD)
The XRD measurement was performed to confirm the form of TiO 2 in N719-adsorbed ESM.The XRD pattern of N719-adsorbed ESM was recorded in the 2θ angles of 5-60 • on a Rigaku MiniFlex600 (Rigaku, Tokyo, Japan) diffractometer using a Cu-target tube at a voltage of 40 kV, a current of 15 mA and a step scan of 0.02 • .The intensities were averaged by calculating four times.

Fabrication of DSSCs
N719-adsorbed ESM-TiO 2 composite working electrode and the Pt-sputtered counter FTO electrode were placed face-to-face and assembled into C1.The electrolyte solution was composed of 0.5 M 4-t-butyl phenol, 0.05 M I 2 and 0.1 M LiI in acetonitrile.For control experiments, we prepared the DSSC-designated C2 by using the N719-TiO 2 composite, made by the same in situ sol-gel method without ESM, working electrode (the loading area: 2 cm × 4 cm).

Results and Discussion
Figure 3a,b shows an SEM image of the surface of ESM powder.It was revealed that the ESM powder was a spherical aggregate composed of thin fiber.This is because collagen, interwoven and coalescing fiber in ESM was cut by grind in the mortar.Additionally, no Ca ion was detected in the ESM (Figures 3c,d and S1a-d, Tables 1 and S1-S4).According to EDX mapping (Figure 3e,f), Ru and Ti were homogeneously detected on the surface of N719-adsorbed ESM-TiO 2 composite, indicating that N719 and TiO 2 were distributed on the surface of the ESM fibers homogeneously.Moreover, Figure 4a shows an XRD pattern of the N719-adsorbed ESM-TiO 2 composite.The peaks were assigned to (101), (004), ( 200), ( 105) and (211) of anatase TiO 2 (quasi-stable phase) [38], although a part of TiO 2 forms a rutile-form (most stable phase).In addition, a strong absorption occurred at 325 and 525 nm in the UV-Vis spectroscopic observation of N719-absorbed ESM-TiO 2 composite (Figure 4b).These absorption peaks were assigned as superposition of TiO 2 and the phenyl groups of their amino acids in ESM (300 nm), N719 (380, 525 nm) and TiO 2 (325 nm).Therefore, the N719-adsorbed ESM-TiO 2 composite was composed of the homogeneous EMS, N719 and TiO 2 , and it was evident that TiO 2 was prepared through the in situ method without decay of N719.
The photovoltaic performances of DSSCs were evaluated using the illustrated structure in Figure 5a, and the J-V curves of the C1 and C2 were obtained before and after 12 h irradiation, respectively (Figure 5b,c).Although C1 exhibited lower photocurrent densities and voltages than C2, before irradiation, the photovoltaic performances of C1 were higher than those of C2 after 12 h irradiation.The activation, the gradual resistance-decrease process, might occur in C1 during irradiation [40], enhancing the photovoltaic performances after 12 h irradiation in C1.In addition, Tables 2 and 3 show the photovoltaic performances, short-circuit photocurrent densities J SC , open-circuit voltages V OC , fill factors (FF) and power conversion efficiencies (PCE) of C1 and C2 before and after irradiation, respectively.Before irradiation, C1 and C2 showed a J SC of 0.441 and 0.682 mA cm −2 , a V OC of 0.212 and 0.356 V, a FF of 0.337 and 0.347 and a PCE of 0.005 and 0.10%, respectively.On the other hand, after irradiation, C1 and C2 showed a J SC of 0.643 and 0.071 mA cm −2 , a V OC of 0.232 and 0.063 V, a FF of 0.347 and 0.223 and a PCE of 0.008 and 0.001%, respectively.It was revealed that ESM-composition improved the photovoltaic stability, although the photocurrent densities and voltages were lower.It is assumed that the enhanced stability of C1 was due to the protection of the -NSC ligand of N719 dyes in ESMs' hydrophobic cavity.As shown in Figure 6, conventional N719, bound to the surface of TiO 2 through the interaction between carboxylate and oxygen, has lost -NCS ligands by irradiation, resulting in a voltage drop in C2.In addition, photocurrent density was also lowered because of loss of the electron acceptor from the redox mediator I − in electrolyte through the redox reaction of I − /I 3 − .On the other hand, in this work, -NCS ligand remained because it was surrounded by the ESMs' cavity [41,42].Additionally, UV light with a wavelength of 300 nm was absorbed in the phenyl groups of ESMs' amino acids, such as Phe and Tyr [43] (Figure 4b), maintaining the molecular structure of N719 and the photovoltaic performances in C1.As shown in Figure 3b, N719-ESM could accept electron from the redox mediator I − even after irradiation, which likely enhanced the stability of photovoltaic performances of DSSCs.

Conclusions
We developed a prototypical DSSC using N719-adsorbed ESM-TiO 2 composites as a working electrode, which is the first example of a DSSC containing ESM.We showed that the new DSSC exhibited more stable performance than a conventional DSSC without ESM.This stability was caused by protection of the dye by ESMs' structure with the hydrophobic cavity.The present findings thus suggest a novel and effective application for the widely available ESMs, which in the past have mainly been used for the adsorption of chemicals such as organic dyes.However, the photovoltaic performances were significantly lower than those in previously reported DSSCs (Table S5) [2,3,[33][34][35], which was likely caused by contamination of rutile TiO 2 , insulating ESM and a small amount of N719 in the electrodes.To improve the values, the DSSC should be optimized by using pure anatase TiO 2 , electronic conductive porous materials and a large amount of N719.

Figure 1 .
Figure 1.The schematic illustration of the interaction between N719 and TiO2 through carboxylate and the surface of TiO2, and the conventional DSSC degradation mechanism by long irradiation.

Figure 1 .
Figure 1.The schematic illustration of the interaction between N719 and TiO 2 through carboxylate and the surface of TiO 2 , and the conventional DSSC degradation mechanism by long irradiation.

Figure 5 .
Figure 5. (a) Schematic view of C1.J-V curves for the as-prepared C1 (b) and C2 (c) before and after 12 h of light irradiation.

Figure 5 .
Figure 5. (a) Schematic view of C1.J-V curves for the as-prepared C1 (b) and C2 (c) before and after 12 h of light irradiation.

Figure 6 .
Figure 6.Schematic illustration of the N719-protection mechanism of ESM.

Figure 6 .
Figure 6.Schematic illustration of the N719-protection mechanism of ESM.

Table 1 .
The elemental analysis at the 001 point.

Table 2 .
The performances (mean values) of C1 and C2 before irradiation.

Table 3 .
The performances (mean values) of C1 and C2 after 12 h irradiation.