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

Abstract

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.

1. Introduction

Polymer gel electrolytes (PGEs), which are quasi-solid-state electrolytes, have promising properties such as appropriate ion conductivity, guaranteed stability, mechanical strength, and good electrode affinity [1,2,3,4]. These beneficial characteristics allow widespread PGEs application in electrochemical systems such as lithium-ion batteries, supercapacitors, and dye-sensitized solar cells (DSSCs) [5,6,7,8,9]. Despite the attractive electrochemical properties of PGEs, crystalline polymer matrices can limit ion diffusion in PGEs. Hindered charge transport causes decrease in ion conductivity and energy conversion efficiency [10,11].

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 TiO₂-deposited photoanodes, Pt-coated counter electrodes, and an iodine (I⁻/I₃⁻) 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 ZrO₂, such as their large band gap (~5–5.4 eV) and zero visible-light absorption, good physical and chemical durability, and reliable thermal stability, ZrO₂ are good candidates for NFs [24,25,26].

In this work, we introduce carbon shell-functionalized ZrO₂ (ZrO₂-C) NFs to a PGEs and investigate the photovoltaic performances of ZrO₂-C/PGE-based DSSCs. The surface-polymerized ZrO₂ with poly (ethylene glycol) methyl ether methacrylate via ATRP is used as a scaffold to obtain the ZrO₂-C through carbonization. Detailed properties of ZrO₂-C are systematically characterized using transmission electron microscopy (TEM), Fourier-transform infrared (FTIR) spectroscopy, and thermogravimetric analysis (TGA). The DSSCs employing ZrO₂-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.

2. Experimental Section

2.1. Materials

ZrO₂ powder, 4-(dimethylamino) pyridine (DMAP, > 99%), 2-chloropropionyl chloride (CPC, > 97%), triethylamine (TEA, > 99.5%), methylene dichloride (MC, > 99.8%), copper (I) chloride (CuCl), 1,1,4,7,10,10-hexamethyl triethylenetetramine (HMTETA), dimethyl sulfoxide (DMSO), poly(ethylene glycol) methyl ether methacrylate (POEM, M_n = 500 g/mol), poly(ethylene glycol) (PEG, M_n = 100,000 g/mol), LiI, I₂, 1-methyl-3-propylimidazolium iodide (MPII), H₂PtCl₆, and titanium diisopropoxide bis (acetylacetonate) were purchased from Sigma-Aldrich. Commercial TiO₂ paste (Ti-Nanoxide, D20) and ruthenium dye (535-bisTBA, N719) were purchased from Solaronix, Switzerland. Ethanol, acetonitrile, chloroform, and 2-propanol were purchased from J.T. Baker. Deionized water (> 18 MΩm) was obtained with a water purification system made by Millipore Corporation. Fluorine-doped tin oxide (FTO, TEC-7) conducting substrates were purchased from Pilkington, France. All chemicals were used without further purification.

2.2. Surface Modification From ZrO₂ to ZrO₂-Cl

The surface hydroxyl groups (−OH) of ZrO₂ 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 ZrO₂ in 50 mL of MC was added slowly to the DMAP solution, followed by N₂ 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 ZrO₂-Cl were obtained and dried in a vacuum oven overnight at room temperature.

2.3. Synthesis of ZrO₂-g-POEM via ATRP

The surface-modified ZrO₂-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 ZrO₂-Cl; the mixture was then N₂ 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 ZrO₂-g-POEM were dried in a vacuum oven at room temperature for 24 h.

2.4. Preparation of ZrO₂-C

The ZrO₂-g-POEM were used as a template to prepare ZrO₂-C through carbonization. The 0.1 g of ZrO₂-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.

2.5. Preparation of ZrO₂-C/PGEs

ZrO₂-C/PGEs were prepared by solution-casting method. The ZrO₂-C, PEG, LiI, MPII and I₂ were dissolved in acetonitrile and cast on the Teflon-coated dish. The solvent was completely removed by vacuum drying. The concentrations of ZrO₂-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% ZrO₂-g-POEM/PGEs and 12 wt% ZrO₂/PGEs were prepared following the same procedures.

2.6. Fabrication of ZrO₂-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 TiO₂ 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% H₂PtCl₆ 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 ZrO₂-C/PGEs were drop casted on the photoanodes. The ZrO₂-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.

2.7. Characterization

The surface morphologies of the ZrO₂-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:

FF = V_max·J_max/V_oc·J_sc

(1)

η = V_max·J_max/Pin·100 = V_oc·J_sc·FF/P_in·100

(2)

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.

3. Results and Discussion

Scheme 1 shows the reaction scheme by which the ZrO₂ are surface-polymerized by hydrophilic POEM via ATRP. First, surface -OH groups of the ZrO₂ are reacted with CPC at 0 °C for 24 h. The -OH groups of ZrO₂ are substituted by -Cl groups, which are subsequently used as ATRP initiators [30,31]. The as-obtained ZrO₂-Cl are further polymerized by hydrophilic POEM at 90 °C for 24 h via ATRP. The surface POEM-polymerized ZrO₂ (ZrO₂-POEM) are used as a template for obtaining ZrO₂-C through carbonization at 500 °C for 30 min in Ar flow.

Scheme 2 depicts the overall reaction scheme for obtaining ZrO₂-C from the ZrO₂ NPs. Photographs of the ZrO₂, ZrO₂-g-POEM, and ZrO₂-C were shown in Figure S1. The white color of the ZrO₂ 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 ZrO₂-C was clearly seen because of the surface graphitic carbon shells.

Figure 1 shows the TEM images of ZrO₂, ZrO₂-g-POEM, and ZrO₂-C taken after dispersion in ethanol. The ZrO₂ have the average diameter of 40 nm and polygonal shapes with smooth surfaces. The magnified TEM image clearly shows no surface residues on the ZrO₂ (Figure 1d). However, after the surface-initiated ATRP reaction with POEM, ~5-nm-thick shells of POEM are visible around the surfaces of the ZrO₂, as shown in Figure 1b. Figure 1e depicts these shells more clearly, demonstrating that the ZrO₂ are thoroughly surface-polymerized with POEM chains. After thermal carbonization, the thin shells remain as graphitic carbon around the ZrO₂, 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 ZrO₂-C was successfully generated.

To characterize the organic groups of the various modified ZrO₂, the FT-IR spectra of ZrO₂, ZrO₂-g-POEM, and ZrO₂-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 ZrO₂-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 ZrO₂-C shows none of the above-mentioned peaks, demonstrating that all of the surface POEM chains are carbonized.

The weight portions of inorganic ZrO₂ and organic POEM in ZrO₂-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 ZrO₂-g-POEM. At temperatures above 250 °C, the surface POEM chains were decomposed into CO₂ and H₂O. Finally, almost all ZrO₂-g-POEM were converted to ZrO₂-C at the temperature of 350 °C, suggesting that the ZrO₂-POEM included about 8 wt% POEM. To calculate the weight portion of the carbon shells on the ZrO₂-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 ZrO₂-C. This implies that ZrO₂-C were successfully prepared from the carbonization of ZrO₂-POEM synthesized via ATRP.

Scheme 3 illustrates the possible mechanism of ZrO₂-C/PGEs in counterelectrode and electrolyte. The carbon shells of the ZrO₂-C in the PGEs catalyze the reduction reaction of I₃⁻ to I⁻. Near the counter electrode, the carbon shells of the ZrO₂-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 ZrO₂-C contribute to enhancing the overall conversion efficiency of DSSCs.

To investigate the coordinative interactions in ZrO₂-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 ZrO₂, ZrO₂-g-POEM, and ZrO₂-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 ZrO₂. 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% ZrO₂-C, respectively. The positive shifting of the ether peak implies increasing interactions and amorphous regions. From this perspective, the optimum concentration of ZrO₂-C is 12 wt%. A higher concentration of ZrO₂-C induces particle aggregation, which decreases the active sites. Interestingly, the C-O-C peak of 12 wt% ZrO₂-g-POEM/PGEs is located at 1082 cm⁻¹, which is lower than that of the 12 wt% ZrO₂/PGEs (1085 cm⁻¹) and ZrO₂-C/PGEs. These FT-IR results can be explained by the fact the Lewis basic surface of POEM chains in the ZrO₂-g-POEM can deactivate the Lewis acidity.

To confirm the photovoltaic performances of the DSSCs utilizing ZrO₂-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. Photovoltaic and electrochemical parameters of DSSCs fabricated with ZrO₂-C/PGEs that were obtained under one sun illumination (AM 1.5, 100 mW cm⁻²). ^a.

Electrolyte	Voc (V)	Jsc (mA/cm2)	FF	η (%)	RS	R-CE	R-TiO2	WS
9 wt% ZrO2-C/PGEs	0.67	12.8	0.61	5.2	13.1	12.8	30.4	1.4
12 wt% ZrO2-C/PGEs	0.66	14.0	0.61	5.6	14.1	12.6	26	1.2
15 wt% ZrO2-C/PGEs	0.64	12.8	0.60	4.9	12.2	16.4	32.6	3.0
12 wt% ZrO2-g-POEM/PGEs	0.67	11.5	0.61	4.7	14.0	14.5	30.6	1.7
12 wt% ZrO2/PGEs	0.66	11.9	0.61	4.8	15.8	13.6	31	2
PGEs	0.64	11.4	0.60	4.4	11.4	14.2	34.5	2.4

^a A typical cell had an active area of ca. 0.40 cm⁻² and was masked using an aperture of the same area during the J-V measurements. In addition, thickness of the photoanode was approximately 10 µm.

. The J_sc is theoretically calculated by the following equation:

J_sc = qη_lhη_injη_colI₀

(3)

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 I₀ is the light flux. Moreover, the V_oc is estimated by the following equation:

V_oc = |V_FB − V_red|

(4)

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% ZrO₂-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 ZrO₂-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 ZrO₂-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 ZrO₂-C enabled electron transfer to the counter electrode, thus increasing the charge-transfer rate and decreasing the interfacial resistance.

DSSCs with 12 wt% ZrO₂-g-POEM and ZrO₂/PGEs show η values of 4.7% and 4.8%, respectively. Surface polymerization with POEM chains deactivates the surface properties of the ZrO₂ NPs as NFs, yielding an inferior J_sc compared to that of the 12 wt% ZrO₂-C/PEGs and ZrO₂/PGEs. Furthermore, the accumulation of Lewis acids such as Li⁺ salts in PGEs on the photoanode can shift the conduction band edge of TiO₂ 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 ZrO₂-C instead of on the TiO₂ photoanode is the most dominant factor in determining the enhanced V_oc (0.67 V) in 12 wt% ZrO₂-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 ZrO₂-g-POEM and ZrO₂-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-TiO₂, 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-TiO₂), 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% ZrO₂-g-POEM/PGEs, ZrO₂/PGEs, and PGEs have similar R-CE values of 14.5, 13.6, and 14.2 Ω, respectively. Moreover, the DSSC with 12 wt% ZrO₂-C/PGEs has the lowest R-CE, R-TiO₂, and W_S. This result indicates the enhanced electro-kinetic performance of the 12 wt% ZrO₂-C/PGE-based DSSCs. While the 15 wt% ZrO₂-C/PGE-based DSSCs has the highest R-CE of 16.4 Ω due to the aggregation induced by high ZrO₂-C concentration. The mechanism underlying the photovoltaic performance improvement in the 12 wt% ZrO₂-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% ZrO₂-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% ZrO₂-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% ZrO₂-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 ZrO₂-C/PGEs, as shown in Figure 4d. The IPCE spectra indicate that the DSSCs with the ZrO₂-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% ZrO₂-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% ZrO₂-C/PGE-based DSSCs are attributed to the improved electron collection and injection values.

4. Conclusions

We demonstrated that the introduction of ZrO₂-C in PGEs enhanced the power conversion efficiency of DSSCs. Hydrophilic POEM containing ligand oxygens were polymerized from the surface of ZrO₂ after the introduction of the chlorine atoms via surface-initiated ATRP method. The ZrO₂-g-POEM is used as a template to obtain the ZrO₂-C through carbonization. The power conversion efficiency of the 12 wt% ZrO₂-C/PGE-based DSSCs reached 5.6% at 100 mW cm⁻²; this is higher than those of PEGs (4.4%), ZrO₂/PGEs (4.8%), and 12 wt% ZrO₂-g-POEM/PGEs (4.7%). The higher cell performance of 12 wt% ZrO₂-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.