Surface Carbon Shell-Functionalized ZrO2 as Nanofiller in Polymer Gel Electrolyte-Based Dye-Sensitized Solar Cells

We prepare dye-sensitized solar cells (DSSCs) fabricated with a poly (ethylene glycol) based polymer gel electrolytes (PGEs) incorporating surface carbon shell-functionalized ZrO2 nanoparticles (ZrO2-C) as nanofillers (NFs). ZrO2 are polymerized via atom transfer radical polymerization (ATRP) using poly (ethylene glycol) methyl ether methacrylate (POEM) as a scaffold to prepare the ZrO2-C through carbonization. The power conversion efficiency of DSSC with 12 wt% ZrO2-C/PGEs is 5.6%, exceeding that with PGEs (4.4%). The enhanced efficiency is attributed to Lewis acid-base interactions of ZrO2-C and poly (ethylene glycol), catalytic effect of the carbon shells of ZrO2-C, which results in reduced crystallinity, enhanced ion conductivity of electrolytes, decreased counterelectrode/electrolyte interfacial resistance, and improved charge transfer rate. These results demonstrate that ZrO2-C introduction to PGEs effectively improves the performance of DSSCs.

A facile and effective way to improve the conductivity of PGEs is the introduction of nanomaterials to PGEs, such as inorganic metal oxides or carbonaceous materials, as nanofillers (NFs) [12][13][14][15][16]. With metal oxide NFs, the interaction between the Lewis acidic surfaces of metal oxides and Lewis basic polymers in PGEs reduces polymer self-interaction and thus prevents polymer crystallization; this increases the free volume for ion transport and hence enhances ion conductivity. Earth-abundant and cheap conductive carbonaceous materials have some merits compared to other nanomaterials as NFs because of their mechanical and electrochemical properties. Because these conductive carbon compounds create continuous conducting paths in the electrolyte by the overlapping of individual particles, high charge transfer rates arise. Furthermore, the catalytic activity of carbon compounds toward electrochemical reactions decreases the interfacial resistance and thereby improves the energy conversion efficiency [17][18][19].
Low-cost and high-performance DSSCs were reported by Grätzel and O'Regan in 1991, using mesoporous TiO 2 -deposited photoanodes, Pt-coated counter electrodes, and an iodine (I − /I 3 − ) redox couple-based liquid electrolyte [20]. Despite their excellent photovoltaic performance, the poor long-term stability induced by solvent leakage and evaporation has restricted the commercialization of DSSCs. The replacement of liquid electrolytes with PGEs in DSSCs may solve these issues. The functionalization of nanomaterials has received much attention because it easily alters nanomaterial properties. The surface polymerization of metal oxides via atom transfer radical polymerization (ATRP) with hydrophilic polymers can improve NFs particle dispersion in solvents or polymer matrices [21][22][23]. Moreover, these surface-polymerized nanomaterials are good candidates for obtaining carbonaceous-inorganic hybrid materials. Because of the advantageous characteristics of the ZrO 2 , such as their large band gap (~5-5.4 eV) and zero visible-light absorption, good physical and chemical durability, and reliable thermal stability, ZrO 2 are good candidates for NFs [24][25][26].
In this work, we introduce carbon shell-functionalized ZrO 2 (ZrO 2 -C) NFs to a PGEs and investigate the photovoltaic performances of ZrO 2 -C/PGE-based DSSCs. The surface-polymerized ZrO 2 with poly (ethylene glycol) methyl ether methacrylate via ATRP is used as a scaffold to obtain the ZrO 2 -C through carbonization. Detailed properties of ZrO 2 -C are systematically characterized using transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). The DSSCs employing ZrO 2 -C/PGE electrolytes are fabricated and characterized in detail via electrochemical impedance spectroscopy (EIS), intensity modulated voltage spectroscopy (IMVS), incident photon-to-current efficiency (IPCE), and current-voltage curves, respectively.

Surface Modification From ZrO 2 to ZrO 2 -Cl
The surface hydroxyl groups (−OH) of ZrO 2 were converted into chlorine groups (−Cl). First, 3.68 g of DMAP in 20 mL of MC was mixed with 2.8 mL of TEA in a round-bottom flask at 0 • C. Then, 10 g of ZrO 2 in 50 mL of MC was added slowly to the DMAP solution, followed by N 2 gas purging for 30 min. The mixed solution was reacted at room temperature for 24 h by stirring. After the reaction, the resulting solution was washed with methanol and centrifuged three to five times to remove impurities. Finally, the pink-colored ZrO 2 -Cl were obtained and dried in a vacuum oven overnight at room temperature.

Synthesis of ZrO 2 -g-POEM via ATRP
The surface-modified ZrO 2 -Cl were polymerized by POEM via ATRP. First, 20 mL of POEM in 10 mL of DMSO was prepared in a 100 mL round flask. Then, 0.0264 g of CuCl and 0.072 mL of HMTETA were added into the POEM solution, followed by 1 g of ZrO 2 -Cl; the mixture was then N 2 gas-purged for 30 min. The mixture was then reacted in an oil bath at 90 • C for 24 h. The resulting solution was washed with methanol and centrifuged three to five times. Finally, the as-obtained ZrO 2 -g-POEM were dried in a vacuum oven at room temperature for 24 h.

Preparation of ZrO 2 -C
The ZrO 2 -g-POEM were used as a template to prepare ZrO 2 -C through carbonization. The 0.1 g of ZrO 2 -g-POEM were placed in a tube furnace and carbonized at 500 • C for 30 min in Ar flow with the heating rate of 3 • C min −1 .

Preparation of ZrO 2 -C/PGEs
ZrO 2 -C/PGEs were prepared by solution-casting method. The ZrO 2 -C, PEG, LiI, MPII and I 2 were dissolved in acetonitrile and cast on the Teflon-coated dish. The solvent was completely removed by vacuum drying. The concentrations of ZrO 2 -C were fixed at 9, 12 or 15 wt% relative to poly (ethylene glycol). The mole ratio of ether oxygen to iodine salt was fixed at 20, and the iodine content was fixed at 10 wt% with respect to the salt. As a control, 12 wt% ZrO 2 -g-POEM/PGEs and 12 wt% ZrO 2 /PGEs were prepared following the same procedures.

Fabrication of ZrO 2 -C/PGE-Based DSSCs
The photoanodes and counter electrodes were prepared as follow [27][28][29]. First, FTO glass substrates were rinsed with chloroform and 2-propanol for 30 min each. As a photoanode, 2 wt% titanium diisopropoxide bis(acetylacetonate) in butanol solution was spin-coated (1500 rpm, 30 s) onto the conductive side of the FTO glass, followed by calcination at 450 • C for 30 min. Commercial TiO 2 paste was deposited onto the as-prepared photoanode via the doctor blade technique and dried at 50 • C and 80 • C for 1 h each. Subsequently, this photoanode was sintered at 450 • C for 30 min. Then, the as-obtained photoanode was immersed in 0.1 M N719 dye solution in ethanol at 50 • C for 3 h under dark condition. To prepare the counter electrode, 1 wt% H 2 PtCl 6 in 2-propanol solution was spin-coated (1500 rpm, 30 s) onto the conductive side of the FTO glass followed by calcination at 450 • C for 30 min. Then, as prepared ZrO 2 -C/PGEs were drop casted on the photoanodes. The ZrO 2 -C/PGE-cast photoanode was superimposed on the counter electrode. Finally, the assembled cell was dried in a vacuum oven for a day at room temperature and sealed with an epoxy resin.

Characterization
The surface morphologies of the ZrO 2 -C were observed by transmission electron microscopy (TEM) (JEM-F200, JEOL, Tokyo, Japan). Thermogravimetric analysis (TGA) was performed by a TG209 F3 (NETZSCH) instrument. Fourier-transform infrared (FT-IR) spectra of the samples were collected using a PerkinElmer instrument from 4000 to 750 cm −1 . Current density-voltage (J-V) measurements were acquired using a potentiostat (Compactstat.h, Ivium Technologies) equipped with a 150-W Xe lamp (Sunlite, ABET Technologies) as a light source. Electrochemical impedance spectroscopy (EIS) was measured at the open-circuit voltage (V oc ) in the frequency range 0.01 Hz-100 kHz with an amplitude of 10 mV. Incident photo-to-electron conversion efficiency (IPCE) spectra were obtained by using a CE system (PEIPCE100S, HS-Technologies). Intensity-modulated voltage spectroscopy (IMVS) was used to check the charge recombination properties. The photovoltaic parameters were calculated by the following equations: where η is the power conversion efficiency (%), J sc is the short-circuit current density, V oc is the open-circuit voltage, P in is the incident light power, and FF is the fill factor. J max and V max are the current density and voltage in the J-V curves, respectively, at the maximum power output.

Results and Discussion
Scheme 1 shows the reaction scheme by which the ZrO 2 are surface-polymerized by hydrophilic POEM via ATRP. First, surface -OH groups of the ZrO 2 are reacted with CPC at 0 • C for 24 h. The -OH groups of ZrO 2 are substituted by -Cl groups, which are subsequently used as ATRP initiators [30,31]. The as-obtained ZrO 2 -Cl are further polymerized by hydrophilic POEM at 90 • C for 24 h via ATRP. The surface POEM-polymerized ZrO 2 (ZrO 2 -POEM) are used as a template for obtaining ZrO 2 -C through carbonization at 500 • C for 30 min in Ar flow. Scheme 2 depicts the overall reaction scheme for obtaining ZrO 2 -C from the ZrO 2 NPs. Photographs of the ZrO 2 , ZrO 2 -g-POEM, and ZrO 2 -C were shown in Figure S1. The white color of the ZrO 2 was attributed to the large band gap of 5-5.4 eV. Upon surface polymerization with POEM, the color changed to light brown because of the surface POEM chains. After carbonization, black-colored ZrO 2 -C was clearly seen because of the surface graphitic carbon shells.  The magnified TEM image clearly shows no surface residues on the ZrO 2 ( Figure 1d). However, after the surface-initiated ATRP reaction with POEM,~5-nm-thick shells of POEM are visible around the surfaces of the ZrO 2 , as shown in Figure 1b. Figure 1e depicts these shells more clearly, demonstrating that the ZrO 2 are thoroughly surface-polymerized with POEM chains. After thermal carbonization, the thin shells remain as graphitic carbon around the ZrO 2 , as shown in Figure 1c,f. Therefore, this reveals that with the introduction of the POEM polymer shells on the surfaces of the ZrO using the ATRP and following the carbonization process, the ZrO 2 -C was successfully generated. To characterize the organic groups of the various modified ZrO 2 , the FT-IR spectra of ZrO 2 , ZrO 2 -g-POEM, and ZrO 2 -C are shown in Figure 2. In all samples, peaks near 750 cm −1 are observable and attributed to the stretching vibration of the Zr-O group [32]. In the ZrO 2 -g-POEM, a broad 3300 cm −1 peak is apparent from the bending vibrations of H-O-H molecules adsorbed on the surface hydrophilic POEM chains. Furthermore, peaks at 1100 and 1715 cm −1 are attributed to the stretching vibrations of the ether group (C-O-C) and the carbonyl group (C=O) of surface POEM, respectively [33]. The spectrum of the ZrO 2 -C shows none of the above-mentioned peaks, demonstrating that all of the surface POEM chains are carbonized. The weight portions of inorganic ZrO 2 and organic POEM in ZrO 2 -g-POEM were easily calculated using TGA plots, as shown in Figure S2a. TGA was performed in air at 10 • C min −1 . In the temperature range 20-200 • C, the weight fell drastically, attributed to the evaporation of water molecules adsorbed on the surfaces of the ZrO 2 -g-POEM. At temperatures above 250 • C, the surface POEM chains were decomposed into CO 2 and H 2 O. Finally, almost all ZrO 2 -g-POEM were converted to ZrO 2 -C at the temperature of 350 • C, suggesting that the ZrO 2 -POEM included about 8 wt% POEM. To calculate the weight portion of the carbon shells on the ZrO 2 -C, the same analysis was performed in Ar as shown in Figure S2b. By similar calculations as those in Figure S2a, about 6 wt% POEM was decomposed during the carbonization process, meaning that 2 wt% of carbon remained as shells on the surface of the ZrO 2 -C. This implies that ZrO 2 -C were successfully prepared from the carbonization of ZrO 2 -POEM synthesized via ATRP.
Scheme 3 illustrates the possible mechanism of ZrO 2 -C/PGEs in counterelectrode and electrolyte. The carbon shells of the ZrO 2 -C in the PGEs catalyze the reduction reaction of I 3 − to I − . Near the counter electrode, the carbon shells of the ZrO 2 -C and the counter electrode are in contact, forming a continuous electrically conductive medium, rather than an ion-conducting network accompanying the redox reactions, as shown in Scheme 3a. Ideally, zero distance is achieved between the photoanode and counterelectrode, decreasing the interfacial resistances [17,34]. Furthermore, the partial Lewis acid-base interactions with the PGEs reduce the crystallinity of the polymer and facilitate ion diffusion through the amorphous region of the electrolyte, thus enhancing ion conductivity, as shown in Scheme 3b. The bifunctional properties of ZrO 2 -C contribute to enhancing the overall conversion efficiency of DSSCs. To investigate the coordinative interactions in ZrO 2 -C/PGEs, the FT-IR spectra were measured and presented in Figure 3. Before measurement, all electrolytes were dried in vacuum oven overnight at room temperature to evaporate solvent completely. Depending on the concentrations of the ZrO 2 , ZrO 2 -g-POEM, and ZrO 2 -C in the PGEs, significant deviations in peaks between 1050 and 1100 cm −1 were observed. These arise from the interaction between Lewis basic ether groups (C-O-C) in PEGs and the Lewis acidic surfaces of the ZrO 2 . The ether peak at 1078 cm −1 in PGEs is shifted to 1083, 1086, and 1084 cm −1 after the incorporation of 9, 12, and 15 wt% ZrO 2 -C, respectively. The positive shifting of the ether peak implies increasing interactions and amorphous regions. From this perspective, the optimum concentration of ZrO 2 -C is 12 wt%. A higher concentration of ZrO 2 -C induces particle aggregation, which decreases the active sites. Interestingly, the C-O-C peak of 12 wt% ZrO 2 -g-POEM/PGEs is located at 1082 cm −1 , which is lower than that of the 12 wt% ZrO 2 /PGEs (1085 cm −1 ) and ZrO 2 -C/PGEs. These FT-IR results can be explained by the fact the Lewis basic surface of POEM chains in the ZrO 2 -g-POEM can deactivate the Lewis acidity. To confirm the photovoltaic performances of the DSSCs utilizing ZrO 2 -g-POEM/PGEs, J-V curves under 1 sum illumination (100 mW cm −2 ) are characterized in Figure 4a. Moreover, the photovoltaic parameters of J sc , V oc , FF, and η are summarized in Table 1. The J sc is theoretically calculated by the following equation: where q is the elementary charge, η lh is the light-harvesting efficiency, η inj is the electron injection efficiency, η col is the electron collection efficiency, and I 0 is the light flux. Moreover, the V oc is estimated by the following equation: where V FB is the potential of the Fermi level of the photoanode and V red is the standard reduction potential of a redox couple. For the 9, 12, and 15 wt% ZrO 2 -C/PEGs-based DSSCs, the η values reach 5.2, 5.6, and 4.9%, respectively, while that of the PGE-based DSSC is 4.4%, demonstrating that the introduction of ZrO 2 -C to the PGEs effectively improves the photovoltaic performance of DSSCs. The significantly enhanced J sc and V oc also contribute to the high photovoltaic performances, although FF remains unchanged. Two main factors cause these performance enhancements: (1) the Lewis acid-base interaction between poly (ethylene glycol) and ZrO 2 -C in the PGEs decreased the crystallinity and increased the amorphous region where redox couples could diffuse easily, yielding high ion conductivity; (2) the catalytic effect of the ZrO 2 -C enabled electron transfer to the counter electrode, thus increasing the charge-transfer rate and decreasing the interfacial resistance.  DSSCs with 12 wt% ZrO 2 -g-POEM and ZrO 2 /PGEs show η values of 4.7% and 4.8%, respectively. Surface polymerization with POEM chains deactivates the surface properties of the ZrO 2 NPs as NFs, yielding an inferior J sc compared to that of the 12 wt% ZrO 2 -C/PEGs and ZrO 2 /PGEs. Furthermore, the accumulation of Lewis acids such as Li + salts in PGEs on the photoanode can shift the conduction band edge of TiO 2 downward [35][36][37]. On the basis of the above studies, it can reasonably be concluded that Li + salts preferentially adsorbed on the surfaces of the ZrO 2 -C instead of on the TiO 2 photoanode is the most dominant factor in determining the enhanced V oc (0.67 V) in 12 wt% ZrO 2 -g-POEM/PGE-based DSSC. The FF is related to not only the deep penetration of the electrolyte into the photoanode but also dark current at the photoanode, the efficiency of the recovery of the redox mediator at the counterelectrode [38,39]. Therefore, the relatively similar values of FF indicate that the incorporation of the ZrO 2 -g-POEM and ZrO 2 -C to PGEs does not relate to achieving deep penetration of the electrolyte into the photoanode. The electrochemical properties of the DSSCs are further characterized using EIS data, as shown in Figure 4b. The EIS Nyquist plots are measured from high (1 MHz) to low (0.01 MHz) frequency at the V oc under one sun illumination (100 mW cm −2 ). The equivalent circuit model ( Figure  S3) includes the elements R S , R-CE, R-TiO 2 , and W S , representing the ohmic resistance of the FTO substrate (R S ), interfacial charge-transfer resistance between the counter electrode and electrolyte (R-CE), interfacial charge-transfer resistance between the photoanode and electrolyte (R-TiO 2 ), and the redox couple mass-transfer resistance (W S , Warburg diffusion resistance) [40,41]. The electro-kinetic properties of the DSSCs are summarized in Table 1. The DSSCs with 12 wt% ZrO 2 -g-POEM/PGEs, ZrO 2 /PGEs, and PGEs have similar R-CE values of 14.5, 13.6, and 14.2 Ω, respectively. Moreover, the DSSC with 12 wt% ZrO 2 -C/PGEs has the lowest R-CE, R-TiO 2 , and W S . This result indicates the enhanced electro-kinetic performance of the 12 wt% ZrO 2 -C/PGE-based DSSCs. While the 15 wt% ZrO 2 -C/PGE-based DSSCs has the highest R-CE of 16.4 Ω due to the aggregation induced by high ZrO 2 -C concentration. The mechanism underlying the photovoltaic performance improvement in the 12 wt% ZrO 2 -C/PGE-based DSSCs was investigated by measuring the electron transport and charge recombination properties through and IMVS measurements. Figure 4c shows that the τ r values of the 12 wt% ZrO 2 -C/PGE-based DSSCs are always greater than those of the PEGs over the light intensity ranges. This significant improvement of electron transport and reduced charge recombination in the 12 wt% ZrO 2 -C/PGE-based DSSCs. We also confirm that the high recollection efficiency at the counterelectrode as well as slower recombination at the photoanode site of the 12 wt% ZrO 2 -C/PGE-based DSSCs which again agrees with the previous discussed EIS analysis. IPCE curves were obtained to characterize the electrochemical behavior of the DSSCs based on ZrO 2 -C/PGEs, as shown in Figure 4d. The IPCE spectra indicate that the DSSCs with the ZrO 2 -C/PGEs showed a considerably higher IPCE value than the DSSCs without the PGEs over the entire spectrum. The maximum IPCE value of the 12 wt% ZrO 2 -C/PGE-based DSSCs were 80% at λ = 520 nm, which increased up to 60% with the introduction of the PGEs. The enhanced IPCE value of the 12 wt% ZrO 2 -C/PGE-based DSSCs are attributed to the improved electron collection and injection values.

Conclusions
We demonstrated that the introduction of ZrO 2 -C in PGEs enhanced the power conversion efficiency of DSSCs. Hydrophilic POEM containing ligand oxygens were polymerized from the surface of ZrO 2 after the introduction of the chlorine atoms via surface-initiated ATRP method. The ZrO 2 -g-POEM is used as a template to obtain the ZrO 2 -C through carbonization. The power conversion efficiency of the 12 wt% ZrO 2 -C/PGE-based DSSCs reached 5.6% at 100 mW cm −2 ; this is higher than those of PEGs (4.4%), ZrO 2 /PGEs (4.8%), and 12 wt% ZrO 2 -g-POEM/PGEs (4.7%). The higher cell performance of 12 wt% ZrO 2 -C/PGE-based DSSCs is due to reduced crystallinity, enhanced ion conductivity of electrolytes, decreased counterelectrode/electrolyte interfacial resistance and improved charge-transfer rate. This work highlights the potential of applying surface carbon shell-functionalized nanomaterials for the development of efficient photovoltaic devices with improved efficiency and offers future opportunities for the development of energy conversion and storage devices.