Amphiphilic Graft Copolymers as Templates for the Generation of Binary Metal Oxide Mesoporous Interfacial Layers for Solid-State Photovoltaic Cells

The binary metal oxide mesoporous interfacial layers (bi-MO meso IF layer) templated by a graft copolymer are synthesized between a fluorine-doped tin oxide (FTO) substrate and nanocrystalline TiO2 (nc-TiO2). Amphiphilic graft copolymers, Poly(epichlorohydrin)-graft-poly(styrene), PECH-g-PS, were used as a structure-directing agent, and the fabricated bi-MO meso IF layer exhibits good interconnectivity and high porosity. Even if the amount of ZnO in bi-MO meso IF layer increased, it was confirmed that the morphology and porosity of the bi-MO meso IF layer were well-maintained. In addtion, the bi-MO meso IF layer coated onto FTO substrates shows higher transmittance compared with a pristine FTO substrate and dense-TiO2/FTO, due to the reduced surface roughness of FTO. The overall conversion efficiency (η) of solid-state photovoltaic cells, dye-sensitized solar cells (DSSCs) fabricated with nc-TiO2 layer/bi-MO meso IF layer TZ1 used as a photoanode, reaches 5.0% at 100 mW cm−2, which is higher than that of DSSCs with an nc-TiO2 layer/dense-TiO2 layer (4.2%), resulting from enhanced light harvesting, good interconnectivity, and reduced interfacial resistance. The cell efficiency of the device did not change after 15 days, indicating that the bi-MO meso IF layer with solid-state electrolyte has improved electrode/electrolyte interface and electrochemical stability. Additionally, commercial scattering layer/nc-TiO2 layer/bi-MO meso IF layer TZ1 photoanode-fabricated solid-state photovoltaic cells (DSSCs) achieved an overall conversion efficiency (η) of 6.4% at 100 mW cm−2.


Introduction
As fossil fuels are depleting, scientists are looking for ways to replace these fuels with renewable energy sources.Solar energy is the most apparent renewable energy due to its abundance, unlimited supply, and lack of carbon dioxide emissions.Solar cells are classified into silicon solar cells, organic solar cells, and dye-sensitized solar cells.Among these, the use of dye-sensitized solar cells (DSSCs) is an attractive option to replace silicon solar cells; these solar cells are environmentally friendly and low-cost compared to silicon solar cells and have relatively high efficiency [1].Liquid electrolytes with a triiodide/iodide redox pair are the most often utilized electrolyte in DSSCs.Although this type of electrolyte has outstanding photovoltaic performance, it has many drawbacks, including solvent evaporation, leakage, chemical degradation, counter electrode corrosion, dye absorption, and inadequate sealing of these cells in long-term applications.Another widely acknowledged idea to increase the operational stability of DSSC devices is to substitute the solid-state electrolyte for the liquid electrolyte.
Alternatively, ZnO has an electron band gap similar to that of TiO 2 but has high electron mobility [28,29].There are many ZnO-based DSSCs with various morphologies, such as ZnO nanorods [14][15][16][17], nanoflowers [18][19][20], nanotubes [21][22][23], and ZnO core-shell structures [24][25][26][27].However, ZnO-based DSSCs exhibit relatively low efficiencies compared to TiO 2 -based DSSCs because a complex that forms between ZnO and carboxylic acid in the dye decreases efficiency.Therefore, ZnO is not used alone as a photoanode but is mixed with other materials to improve the low stability while making use of its high electron mobility.A mesoporous TiO 2 layer serves as the photoanode of DSSCs, and it should act as an appropriate structure to hold sensitizing dye sensitizers in place, allow sunlight to pass through the electrolyte, and transport electrons that produce light to the collector electrode.It is frequently composed of commercial TiO 2 nanoparticles, which are typically 20 nm in size.By lowering the particle size, the surface area and dye absorption capacity are typically increased.However, surface imperfections that serve as electron traps can encourage electron-hole recombination, which will lower a nanoparticulate photoanode's photocurrent density.In the meantime, a mesoporous TiO 2 layer may accurately represent both dye-loading and light-collecting capabilities.In addition to having a high surface area from nanocrystalline aggregates, this structure allows the photoanode to harvest light since hierarchical particle sizes are similar to those in the visible region [30][31][32][33][34][35][36][37].
In this report, binary metal oxide mesoporous interfacial layers (bi-MO meso IF layer) templated by amphiphilic graft copolymer PECH-g-PS are synthesized between an FTO substrate and nc-TiO 2 layer.Atom-transfer radical polymerization (ATRP) was used to create a PECH backbone, and PS (PECH-g-PS) was used as a structure-directing agent to create the bi-MO meso IF layer using a sol-gel method.Improved light harvesting, good interconnectivity, and reduced interfacial resistance were achieved by incorporating the bi-MO meso IF layer between the FTO substrate and nc-TiO 2 layer.Additionally assessed were the photovoltaic characteristics of solid-state photovoltaic cells (DSSCs) based on the bi-MO meso IF layer, nc-TiO 2 layer, and commercial scattering layer.

Synthesis of Amphiphilic Graft Copolymers, PECH-g-PS
The amphiphilic graft copolymers PECH-g-PS were used as structure-directing agents for the bi-MO meso IF layer, which was synthesized using the ATRP method, as described in previous research [38].Briefly, 2 g of PECH was dissolved in 25 mL of toluene while stirring at 70 • C for 3 h.After cooling the solution to room temperature, 12 g of styrene, 0.16 g of CuCl, and 0.48 mL of hexamethyltriethylenetetramine (HMTETA) were added to the solution.The resulting green mixture was stirred until it became a homogeneous solution, then it was purged with nitrogen for 30 min.The mixture was placed in a 100 • C oil bath for 8 h.After polymerization, the resultant polymer solution was diluted with tetrahydrofuran (THF).After passing the solution through a column with activated Al 2 O 3 to remove the catalyst, the solution was precipitated into methanol.The amphiphilic graft copolymers PECH-g-PS were obtained and dried in a vacuum oven overnight at room temperature.

Preparation of Binary Metal Oxide Mesoporous Interfacial Layer (bi-MO Meso IF Layer)
A series of samples was prepared by varying the amount of ZnO from 0 to 6 mg.First, 0.04 g of amphiphilic graft copolymers PECH-g-PS was dissolved in 1.5 mL of THF with vigorous stirring for 2 h.Separately, 150 µL of HCl and 150 µL of H 2 O were added to a 300 µL solution of TTIP sequentially, followed by vigorous stirring for 15 min.The TTIP/HCl/H 2 O solution was then added to the amphiphilic graft copolymers' PECH-g-PS solution.Next, different amounts of ZnO (0, 2, 4, and 6 mg) were added to the solution and aged for 3 h with vigorous stirring at room temperature.Based on the amount of ZnO (0, 2, 4, and 6 mg), the bi-MO meso IF layers were named bi-MO meso IF layer TZ0, bi-MO meso IF layer TZ1, bi-MO meso IF layer TZ2, and bi-MO meso IF layer TZ3, respectively.

Preparation of Photoanode
Before depositing the nanocrystalline TiO 2 layer (nc-TiO 2 layer), cleaned FTO was coated with a bi-MO meso IF layer solution using a spin coater, followed by calcination at 450 • C for 30 min.Then, the nc-TiO 2 layer was deposited onto the bi-MO meso IF layer using a doctor-blade method and dried at 50 • C followed by 80 • C for 1 h each.Finally, this photoanode was calcined at 450 • C for 30 min.

Fabrication of DSSCs
DSSCs with an active area of 0.16 cm −1 were fabricated according to our previous method [39][40][41].First, the FTO glass was sonicated with isopropanol and chloroform and dried in air.For the dense-TiO 2 layer, titanium (IV) bis(ethyl acetoacetato) diisopropoxide in 1-butanol was spin coated onto FTO glass, followed by calcination at 450 • C for 30 min.Then, the nc-TiO 2 layer was deposited onto the dense-TiO 2 layer-coated FTO glass using a doctor-blade method and dried at 50 • C and 80 • C for 1 h each, and calcined at 450 • C for 30 min.This photoanode was immersed in a dye solution of 13 mg N719 in 50 mL of ethanol for 3 h at 50 • C. Subsequently, the resulting photoanode was washed several times with ethanol and dried in air.Solid-state electrolytes, containing LiI, I 2 , and PEG 10K in acetonitrile solvent, was cast onto the photoanode.A Pt counter electrode was prepared by spin coating 1 wt% H 2 PtCl 6 in an isopropanol solution onto FTO glass and calcined at 450 • C for 30 min.Subsequently, the prepared photoanode and Pt counter electrode were superimposed and sealed with epoxy resin.

Characterization
Using an ATR facility, the FT-IR spectra of materials were obtained using an Excalibur Series FTIR (DIGLAB Co.) instrument, covering the frequency range of 4000 to 400 cm −1 .XRD was used to identify the phase of specimens at 40 kV and 300 mA, using Rigaku CuKa radiation (λ = 1.5406Å).In the 2 theta range of 5 to 60 • , data were gathered using a step interval of 0.1 • and a measuring period of 2 s per point.Surface and cross-sectional images were characterized by field emission scanning electron microscopy (FE-SEM, SU 8000, Hitachi).An aberration-corrected SEM, outfitted with an energy-dispersive X-ray spectrometer (EDS), operating at an acceleration voltage of 200 kV, was used to perform the detailed nanoarchitecture of the bi-MO meso IF layer.Transmittance spectra were obtained using UV-vis spectroscopy (MEGA 500, Scinco Co.) in the wavelength range of 300-800 nm.In order to measure photocurrent and voltage, a Keithley model 2400 source measuring unit was used.The light source was an ABE Technologies class A solar simulator (model 11,000) with a 1000 W xenon lamp (Oriel, 91193).A certified reference Si solar cell (Fraunhofer Institute for Solar Energy System, Mono-Si + KG filter, Certificate No. C-ISE269) was used to adjust the light intensity, to produce a sunlight intensity of one (100 mW/cm 2 ).To stop more light from entering through the lateral space, a black mask with an opening was placed over the DSSCs during the photocurrent-voltage measurements.The computation of the photoelectrochemical performances was conducted using Equations ( 1) and (2): which is composed of the following: Pin is the incident light power; FF is the fill factor; η is the overall energy conversion efficiency; J max (mA/cm 2 ) and V max (V) are the current density and voltage in the J-V curve, respectively, at the point of maximum power output; and Jsc is the short-circuit current density (mAcm 2 ).The EIS data were measured using a compactstat electrochemistry analyzer (IVIUM Technologies) with a frequency range of 0.01 Hz to 0.1 MHz and potential modulation of 0.2 V. Incident photo to current conversion efficiency (IPCE) spectra were acquired with a 300 W xenon lamp and a monochromator, equipped with order sorting filters (K3100) at a spectral resolution of 5 nm.The IPCE value was calculated using the following formula: where h and c stand for the terms Planck's constant and the speed of light in a vacuum, respectively.I is the photocurrent density (mA/cm 2 ), and λ and p are the wavelength (nm) and intensity (mA/cm 2 ) of the incident monochromatic light, respectively.Under AM 1.5 (100 mW/cm 2 ) light illumination, the EIS data were measured using a compactstat electrochemistry analyzer (IVIUM Technologies) with a frequency range of 0.01 Hz to 0.1 MHz and potential modulation of 0.2 V.

Measurement of Dye Adsorption Value
The dye-sensitized photoanode was dipped into 5 mL of a 0.01 M NaOH solution in ethanol-H 2 O (1:1).The mixture was stirred until the complete desorption of the dye.This mixture solution was measured by UV-Vis spectroscopy at 515 nm in absorption mode.Absorbance at 515 nm was used to calculate the amount of adsorbed N719 dye molecules, according to the Beer-Lambert law, A = εlc (4 where A is the absorbance of the UV-visible spectra at 515 nm, ε = 14,100/M cm is the molar extinction coefficient of the dye at 515 nm, l is the path length of the light beam, and c is the dye concentration [42].

Results and Discussion
The preparation of binary metal oxide mesoporous interfacial layers (bi-MO meso IF layer) templated by amphiphilic graft copolymers, Poly(epichlorohydrin)-graft-poly(styrene) and PECH-g-PS, is illustrated in Scheme 1.The templating amphiphilic graft copolymer, PECH-g-PS, which is composed of a hydrophilic PECH backbone chain and hydrophobic PS side chain, was used as a structure-directing agent in the sol-gel reaction.The hydrophilic PECH backbone undergoes a Ti precursor and ZnO NP selective reaction to form a selfassembled micelle.The PECH-g-PS/Ti precursor and ZnO NP hybrid phase splits into nanometer-sized domains upon calcination at 450 • C. The Ti precursor and ZnO NP sits in the hydrophilic PECH domains to form a bi-MO meso IF layer around the hydrophobic PS areas that produce pores.The chemical method for the ATRP synthesis of the amphiphilic graft copolymers PECH-g-PS employing direct initiation of the PECH chlorine atoms is shown in Figure S1.The ATRP-based "grafting from" approach is a popular and effective polymerization technique for creating functional sol-gel templates [30].The FT-IR spectra of the amphiphilic graft copolymers, PECH-g-PS and pristine PECH, are shown in Figure S2.The ether stretching band on the hydrophilic backbone chain of the pristine PECH is responsible for the strong absorption bands that were seen at about 1102 cm −1 .Three absorption bands at 1455, 1499, and 1600 cm −1 emerged during PS graft polymerization; these bands were ascribed to C=C stretching vibrations of PS.The hydrophobic PS side chain was successfully grafted using ATRP from the chlorine atoms on the hydrophilic PECH backbone, as evidenced by the FT-IR spectroscopic data.
splits into nanometer-sized domains upon calcination at 450 °C.The Ti precursor and ZnO NP sits in the hydrophilic PECH domains to form a bi-MO meso IF layer around the hydrophobic PS areas that produce pores.The chemical method for the ATRP synthesis of the amphiphilic graft copolymers PECH-g-PS employing direct initiation of the PECH chlorine atoms is shown in Figure S1.The ATRP-based "grafting from" approach is a popular and effective polymerization technique for creating functional sol-gel templates.[30] The FT-IR spectra of the amphiphilic graft copolymers, PECH-g-PS and pristine PECH, are shown in Figure S2.The ether stretching band on the hydrophilic backbone chain of the pristine PECH is responsible for the strong absorption bands that were seen at about 1102 cm −1 .Three absorption bands at 1455, 1499, and 1600 cm −1 emerged during PS graft polymerization; these bands were ascribed to C=C stretching vibrations of PS.The hydrophobic PS side chain was successfully grafted using ATRP from the chlorine atoms on the hydrophilic PECH backbone, as evidenced by the FT-IR spectroscopic data.Figure 1 displays the XRD pa ern of the bi-MO meso IF layer on FTO that was created using the amphiphilic graft copolymers' PECH-g-PS template.A number of strong peaks, centered at 2 theta = 25.5, 37.9, and 54.3, were seen in the bi-MO meso IF layer TZ0, TZ1, TZ2, and TZ3; these correspond to the (101), (004), and (105) reflections of the anatase form of TiO2, respectively.[43] The (110), (101), (200), and (211) crystal planes of the rutile SnO2 phase of the FTO substrate are represented by a number of strong peaks at 2 theta values of 26.8, 33.9, 38.1, and 51.6, respectively.[44] Furthermore, small diffraction peaks identified as (100), (101), and (110) reflections of the hexagonal wur ite structure of ZnO were seen in the XRD pa ern for the bi-MO meso IF layers TZ1, TZ2, and TZ3 on the FTO glass substrate.[45] Overall, Figure 1 shows that, according to the instrumental analysis for material characterization, the bi-MO meso IF layers were successfully prepared on the FTO substrate.
Figure 2 displays the surface and cross-sectional FE-SEM images of the double-layer structure associated with the nc-TiO2 layer and bi-MO meso IF layer TZ1 on the FTO substrate.A uniformly distributed mesoporous structure with high porosity can be seen in Figure 2a, which indicates that a higher surface area was achieved in the bi-MO meso IF layer TZ1, leading to a higher dye adsorption value in solid-state photovoltaic cells.In Figure 2a,b, the surface pore size and cross-sectional thickness of the bi-MO meso IF layer TZ1 were found to be approximately 30~50 nm and 500 nm, respectively.FE-SEM images of a bi-MO meso IF layer TZ1 with a robust a achment structure to the FTO substrate and nc-TiO2 layer is displayed in Figure 2c,d Figure 1 displays the XRD pattern of the bi-MO meso IF layer on FTO that was created using the amphiphilic graft copolymers' PECH-g-PS template.A number of strong peaks, centered at 2 theta = 25.5, 37.9, and 54.3, were seen in the bi-MO meso IF layer TZ0, TZ1, TZ2, and TZ3; these correspond to the (101), (004), and (105) reflections of the anatase form of TiO 2 , respectively [43].The (110), (101), (200), and (211) crystal planes of the rutile SnO 2 phase of the FTO substrate are represented by a number of strong peaks at 2 theta values of 26.8, 33.9, 38.1, and 51.6, respectively [44].Furthermore, small diffraction peaks identified as (100), (101), and (110) reflections of the hexagonal wurtzite structure of ZnO were seen in the XRD pattern for the bi-MO meso IF layers TZ1, TZ2, and TZ3 on the FTO glass substrate [45].Overall, Figure 1 shows that, according to the instrumental analysis for material characterization, the bi-MO meso IF layers were successfully prepared on the FTO substrate.
Figure 2 displays the surface and cross-sectional FE-SEM images of the double-layer structure associated with the nc-TiO 2 layer and bi-MO meso IF layer TZ1 on the FTO substrate.A uniformly distributed mesoporous structure with high porosity can be seen in Figure 2a, which indicates that a higher surface area was achieved in the bi-MO meso IF layer TZ1, leading to a higher dye adsorption value in solid-state photovoltaic cells.In Figure 2a,b, the surface pore size and cross-sectional thickness of the bi-MO meso IF layer TZ1 were found to be approximately 30~50 nm and 500 nm, respectively.FE-SEM images of a bi-MO meso IF layer TZ1 with a robust attachment structure to the FTO substrate and nc-TiO2 layer is displayed in Figure 2c,d, which results in improved electron transfer and suppressed electron recombination in solid-state photovoltaic cells.Ti and Zn ions are uniformly deposited on the bi-MO meso IF layer TZ1, according to the results of energy dispersive X-ray spectroscopy (EDS) mapping pictures displayed in Figure S3.Also, it was shown that the shape, porosity, and metal ion distribution of the bi-MO meso IF layer were preserved, even when the concentration of ZnO in the layer varied, as shown in Figures S4-S6.
dispersive X-ray spectroscopy (EDS) mapping pictures displayed in Figure S3.Also, it was shown that the shape, porosity, and metal ion distribution of the bi-MO meso IF layer were preserved, even when the concentration of ZnO in the layer varied, as shown in Fig- ures S4-S6.As seen in Figure 3, the transmi ance spectra of the bi-MO meso IF layer on FTO were acquired in the wavelength range of 300-800 nm, in which the bi-MO meso IF layer exhibited higher transmi ance compared with the pristine FTO and dense TiO2 layer on FTO.This is most likely caused by the bi-MO meso IF layer that formed on the FTO substrate, which improved the uniformity of the film surface and ultimately increased the light transmi ance of the device.When light passes through a medium with a different As seen in Figure 3, the transmittance spectra of the bi-MO meso IF layer on FTO were acquired in the wavelength range of 300-800 nm, in which the bi-MO meso IF layer exhibited higher transmittance compared with the pristine FTO and dense TiO 2 layer on FTO.This is most likely caused by the bi-MO meso IF layer that formed on the FTO substrate, which improved the uniformity of the film surface and ultimately increased the light transmittance of the device.When light passes through a medium with a different refractive index vertically, the reflectance R can be expressed as the following equation: where n 1 and n 2 represent the refractive index of each medium.The constructive and destructive interferences are expressed as the following equations: where n and d represent the refractive index and the thickness of medium, respectively.λ represents the wavelength of the light and m is a constant (m = 1, 2, 3. ..).According to Equation ( 5), reflection occurred at the air/bi-MO meso IF layer and bi-MO meso IF layer/FTO glass interfaces and was proportional to the difference in refractive index.
According to Equations ( 6) and ( 7), constructive and destructive interferences occurred, depending only on the thickness and refractive index of the bi-MO meso IF layer, as seen in Figure 3, because the thickness and refractive index of the FTO glass are unchanged.The refractive index of the bi-MO meso IF layer could be influenced by the relative amount of pristine TiO 2 (n = 2.6) and ZnO (n = 2.0) [46,47].The lower refractive index of the bi-MO meso IF layer was observed as the amount of ZnO was increased, indicating that the porosity of the bi-MO meso IF layer influenced the refractive index.
Nanomaterials 2024, 14, x FOR PEER REVIEW 8 of 13 meso IF layer was observed as the amount of ZnO was increased, indicating that the porosity of the bi-MO meso IF layer influenced the refractive index.The current density-voltage (J-V) curves of solid-state photovoltaic cells (DSSCs) with the bi-MO meso IF layer, templated by amphiphilic graft copolymers PECH-g-PS, were performed at 100 mW/cm 2 , as seen in Figure 4a, and cell performance and related parameters, including short-circuit current density (JSC), open-circuit voltage (VOC), fill factor (FF), overall conversion efficiency (η), and dye adsorption amount, are summarized in Table 1.The overall conversion efficiency of the bi-MO meso IF layer used as the photoanode was higher than that of the nc-TiO2 layer/dense-TiO2 layer, indicating that the nc-TiO2 layer/bi-MO meso IF layer TZ1 achieved the highest value (5.0%).A higher JSC was observed in solid-state photovoltaic cells (DSSCs) with the bi-MO meso IF layer TZ1 than those with the dense-TiO2 layer.JSC is a function of dye adsorption value and light har- The current density-voltage (J-V) curves of solid-state photovoltaic cells (DSSCs) with the bi-MO meso IF layer, templated by amphiphilic graft copolymers PECH-g-PS, were performed at 100 mW/cm 2 , as seen in Figure 4a, and cell performance and related parameters, including short-circuit current density (J SC ), open-circuit voltage (V OC ), fill factor (FF), overall conversion efficiency (η), and dye adsorption amount, are summarized in Table 1.The overall conversion efficiency of the bi-MO meso IF layer used as the photoanode was higher than that of the nc-TiO 2 layer/dense-TiO 2 layer, indicating that the nc-TiO 2 layer/bi-MO meso IF layer TZ1 achieved the highest value (5.0%).A higher J SC was observed in solid-state photovoltaic cells (DSSCs) with the bi-MO meso IF layer TZ1 than those with the dense-TiO 2 layer.J SC is a function of dye adsorption value and light harvesting; therefore, the bi-MO meso IF layer TZ1, with high porosity, exhibited high dye adsorption (70 nmol/cm 2 ), and higher light harvesting, which resulted in a higher J SC , as confirmed in the IPCE results (Figure 4c).Note that the dye adsorption value of the nc-TiO 2 layer/dense-TiO 2 layer is 55 nmol/cm 2 (Table 1, Figure S7).Also, the higher electron mobility of the ZnO bi-MO meso IF layer TZ1 enhanced the J SC and overall conversion efficiency compared to the solid-state photovoltaic cells (DSSCs) with the bi-MO meso IF layer TZ0.Interfacial and internal resistance of the solid-state photovoltaic cells (DSSCs) with the bi-MO meso IF layer were investigated using electrochemical impedance spectroscopy (EIS) analysis under 100 mW/cm 2 , as shown in Figure 4b; the resulting equivalent circuit is shown in Figure S8.The electrochemical parameters of the solid-state photovoltaic cells (DSSCs) with bi-MO meso IF layers are listed in Table 2.A frequency range from 0.1 Hz to 0.1 MHz with an AC amplitude of 0.01 V at the V OC was used.The parameters related to resistance consisted of the ohmic series resistance of FTO glass (R s ), charge-transfer resistance at the counter electrode-electrolyte interface (R 1 ), chargetransfer resistance at the photoanode-electrolyte interface (R 2 ), and the Warburg diffusion resistance of the redox I − /I 3 − couple in electrolyte (W s ).Among these parameters, the R 2 value is important in order to investigate how effectively the bi-MO meso IF layer in solid-state photovoltaic cells (DSSCs) worked electrochemically.Solid-state photovoltaic cells (DSSCs) with the nc-TiO 2 layer/bi-MO meso IF layer TZ1 (9.6 Ω) as the photoanode had a smaller R 2 value than those of the other solid-state photovoltaic cells (DSSCs), indicating a smaller charge-transfer resistance between the photoanode and electrolyte, and higher FF and conversion efficiency.Recombination resistance (R rec ) at the photoanodeelectrolyte interface was also measured using EIS analysis under dark conditions, as shown in Figure S9.Measurement was performed at −550 mV, and the frequency range and AC amplitude conditions were the same as the EIS in bright conditions.Solid-state photovoltaic cells (DSSCs) with the nc-TiO 2 layer/bi-MO meso IF layer exhibited smaller R rec values, demonstrating a higher electron recombination rate.This demonstrated that the electron recombination was suppressed by high electron mobility with good connectivity between the FTO, nc-TiO 2 layer, and bi-MO meso IF layer, which helped enhance the FF value.As shown in Figure 4b, the cell performance of solid-state photovoltaic cells (DSSCs) with the nc-TiO 2 layer/bi-MO meso IF layer remained unchanged over 15 days, suggesting that the bi-MO meso IF layer with solid-state electrolytes has enhanced electrochemical stability at the electrode/electrolyte interface.Also, commercial scattering layer/nc-TiO 2 layer/bi-MO meso IF layer TZ1 photoanode-prepared solid-state photovoltaic cells (DSSCs) achieved an overall conversion efficiency (η) of 6.4% at 100 mW cm −2 , as shown in Figure S10.As indicated in Table S1, it should be noted that the efficiency of solid-state photovoltaic cells (DSSCs) with the nc-TiO 2 layer/bi-MO meso IF layer is among the highest values recorded for solid-state photovoltaic cells with a mesoporous layer to date [48][49][50][51][52][53][54].with the nc-TiO2 layer/bi-MO meso IF layer TZ1 (9.6 Ω) as the photoanode had a smaller R2 value than those of the other solid-state photovoltaic cells (DSSCs), indicating a smaller charge-transfer resistance between the photoanode and electrolyte, and higher FF and conversion efficiency.Recombination resistance (Rrec) at the photoanode-electrolyte interface was also measured using EIS analysis under dark conditions, as shown in Figure S9.
Measurement was performed at -550 mV, and the frequency range and AC amplitude conditions were the same as the EIS in bright conditions.Solid-state photovoltaic cells (DSSCs) with the nc-TiO2 layer/bi-MO meso IF layer exhibited smaller Rrec values, demonstrating a higher electron recombination rate.This demonstrated that the electron recombination was suppressed by high electron mobility with good connectivity between the FTO, nc-TiO2 layer, and bi-MO meso IF layer, which helped enhance the FF value.As shown in Figure 4b, the cell performance of solid-state photovoltaic cells (DSSCs) with the nc-TiO2 layer/bi-MO meso IF layer remained unchanged over 15 days, suggesting that the bi-MO meso IF layer with solid-state electrolytes has enhanced electrochemical stability at the electrode/electrolyte interface.Also, commercial sca ering layer/nc-TiO2 layer/bi-MO meso IF layer TZ1 photoanode-prepared solid-state photovoltaic cells (DSSCs) achieved an overall conversion efficiency (η) of 6.4% at 100 mW cm −2 , as shown in Figure S10.As indicated in Table S1, it should be noted that the efficiency of solid-state photovoltaic cells (DSSCs) with the nc-TiO2 layer/bi-MO meso IF layer is among the highest values recorded for solid-state photovoltaic cells with a mesoporous layer to date [48][49][50][51][52][53][54].

Scheme 1 .
Scheme 1. Schematic of the bi-MO meso IF layer fabricated based on amphiphilic graft copolymers in this study for solid-state photovoltaic cells.

1 .
Figure1displays the XRD pa ern of the bi-MO meso IF layer on FTO that was created using the amphiphilic graft copolymers' PECH-g-PS template.A number of strong peaks, centered at 2 theta = 25.5, 37.9, and 54.3, were seen in the bi-MO meso IF layer TZ0, TZ1, TZ2, and TZ3; these correspond to the (101), (004), and (105) reflections of the anatase form of TiO2, respectively.[43]The (110), (101), (200), and (211) crystal planes of the rutile SnO2 phase of the FTO substrate are represented by a number of strong peaks at 2 theta values of 26.8, 33.9, 38.1, and 51.6, respectively.[44]Furthermore, small diffraction peaks identified as (100), (101), and (110) reflections of the hexagonal wur ite structure of ZnO were seen in the XRD pa ern for the bi-MO meso IF layers TZ1, TZ2, and TZ3 on the FTO glass substrate.[45]Overall, Figure1shows that, according to the instrumental analysis for material characterization, the bi-MO meso IF layers were successfully prepared on the FTO substrate.Figure2displays the surface and cross-sectional FE-SEM images of the double-layer structure associated with the nc-TiO2 layer and bi-MO meso IF layer TZ1 on the FTO substrate.A uniformly distributed mesoporous structure with high porosity can be seen in Figure2a, which indicates that a higher surface area was achieved in the bi-MO meso IF layer TZ1, leading to a higher dye adsorption value in solid-state photovoltaic cells.In Figure2a,b, the surface pore size and cross-sectional thickness of the bi-MO meso IF layer TZ1 were found to be approximately 30~50 nm and 500 nm, respectively.FE-SEM images of a bi-MO meso IF layer TZ1 with a robust a achment structure to the FTO substrate and nc-TiO2 layer is displayed in Figure2c,d, which results in improved electron transfer and suppressed electron recombination in solid-state photovoltaic cells.Ti and Zn ions are uniformly deposited on the bi-MO meso IF layer TZ1, according to the results of energy

Figure 1 .
Figure 1.XRD pa erns of bi-MO meso IF layer on FTO where S, T, and Z denote SnO2, TiO2, and ZnO, respectively.

Figure 2 .
Figure 2. FE-SEM surface images of (a) bi-MO meso IF layer TZ1, FE-SEM cross-sectional images of (b) bi-MO meso IF layer TZ1 on FTO, (c) magnified bi-MO meso IF layer TZ1 on FTO, and (d) nc-TiO 2 layer/bi-MO meso IF layer TZ1 on FTO.

Figure 3 .
Figure 3. Transmi ance spectra of bi-MO meso IF layer on FTO; dense-TiO2 layer on FTO and pristine FTO.

Figure 3 .
Figure 3. Transmittance spectra of bi-MO meso IF layer on FTO; dense-TiO 2 layer on FTO and pristine FTO.

Figure 4 .
Figure 4. (a) J-V curves, (b) EIS Nyquist plots under 1 sun condition, (c) IPCE curves of bi-MO meso IF layer-based solid-state photovoltaic cells, and (d) normalized cell efficiency of bi-MO meso IF layer-based solid-state photovoltaic cells (40-50% relative humidity).Error bars represent the standard deviation of at least five cells.

Figure 4 .
Figure 4. (a) J-V curves, (b) EIS Nyquist plots under 1 sun condition, (c) IPCE curves of bi-MO meso IF layer-based solid-state photovoltaic cells, and (d) normalized cell efficiency of bi-MO meso IF layer-based solid-state photovoltaic cells (40-50% relative humidity).Error bars represent the standard deviation of at least five cells.

Table 1 .
Photovoltaic properties of bi-MO meso IF layer-based solid-state photovoltaic cells at 100 mW/cm 2 (AM 1.5).a, b, c, d, e, f Dye adsorption amount determined from UV-visible absorption spectra for bi-MO meso IF layer-based solid-state photovoltaic cells g .

Table 1 .
Cont.Solid-state electrolyte consists of PEG, LiI, MPII, and I 2 in acetonitrile.bThephotoactive area for solid-state photovoltaic cells was 0.16 cm 2 .c The thickness of the photoanode was approximately 7 µm.d Determined from J-V curves.e Determined from IPCE curves.f Error bars represent the standard deviation of at least five cells.g Dye-adsorbed photoanode active areas were fixed at 0.16 cm 2 . a