Variations in Power Conversion Efficiency on n-Type Dye-Sensitized Solar Cells with Synthesized TiO₂ Nanoparticle: A Thickness Effect of Active Layer

Abstract

Recently, many researchers have made progress in studies aimed at enhancing the power conversion efficiency (PCE) of n-type dye-sensitized solar cells (DSSCs). This paper presents a systematic investigation focused on improving the PCEs of n-type DSSCs by synthesizing TiO₂ nanoparticle active layers and varying their thickness. The study found that increasing the TiO₂ layer thickness up to 17 µm resulted in a steady 41% increase in PCE, primarily owing to the enhanced photocurrent density in the n-type DSSCs. This improvement is attributed to the enhanced light harvesting effect. As a result, n-type DSSC with 17 µm thick TiO₂ layer demonstrates a relatively high J_sc value of 8.42 mA/cm², achieving an overall PCE of 4.02%. In contrast, the n-type DSSC with a 6 µm thick TiO₂ layer exhibits a much lower J_sc value of 5.55 mA/cm², leading to a reduced PCE. This result represents at least a 52% increase in current density, indicating that the optimal thickness of the TiO₂ active layer is a critical factor influencing the PCE of DSSCs.

1. Introduction

Since the dye-sensitized solar cell (DSSC) was first developed in 1991 with an initial efficiency of 7.12% [1], considerable efforts have been made to improve its power conversion efficiency (PCE) [2,3,4,5,6,7,8]. Despite considerable advancements over the past two decades, the highest efficiencies reported remain at approximately 12% [2,3,7], and achieving efficiencies above 10% is uncommon without highly optimized fabrication conditions. Given the considerable advantages of DSSCs, such as their manufacturing cost being only one-fifth that of conventional silicon solar cells [1,2,3], and their versatility in applications, including the production of richly colored and transparent products, the potential of DSSCs remains substantial. The research results will remain valuable. To enhance the efficiency of DSSCs, various techniques have been proposed, including the deposition of a thin tunneling barrier layer on the substrate [9,10] or the oxide surface [11], co-sensitization using different dyes [12], and post-treatment with a TiCl₄ precursor [13,14]. Additionally, surface texturing has been widely adopted because it confines the incident light in the electrode, thereby increasing the photocurrent density [15].

To date, titanium oxide (TiO₂), a well-known photocatalytic semiconductor, has typically been used as the active layer in the working electrode of DSSCs owing to its relatively large band gap and suitable conduction band energy. It is well established that achieving high PCE requires the energy band structure of TiO₂ to be well-aligned with that of the dye. Specifically, if the conduction band energy of TiO₂ is higher than the LUMO energy of the dye, electron injection from the dye becomes challenging. Given the properties of commonly used ruthenium-based dyes (N3, N719), the selection of compatible oxides is extremely limited. This makes TiO₂ an ideal material because its conduction band energy is approximately 0.2 eV lower than the LUMO of dyes, facilitating efficient electron injection. Additionally, TiO₂ is well-suited for use as a scattering layer owing to its chemical stability and dye absorption capability. As a result, many DSSCs are constructed usingTiO₂ nanoparticle films with a TiO₂ scattering layer on top of the active layer. For instance, Grätzel et al. especially used a mesoporous TiO₂ film layer to achieve a 1000-fold increase in surface area [1,16]. By incorporating a scattering layer, the overall PCE can be further enhanced.

The thickness of the TiO₂ active layer is also a crucial factor influencing the PCE of DSSCs. As the thickness increases, the surface area of the TiO₂ layer expands, leading to greater dye adsorption [17,18,19]. However, this also lengthens the electron diffusion time, which can compete with the electron lifetime, thereby impairing electron transport properties. Additionally, the increased thickness reduces light transmittance, preventing the concentration of photoelectrons from increasing in proportion to the increased dye adsorption [20]. If we can control both surface area and thickness of the TiO₂ active layer by changing crystal structure, particle size, and morphology, a high PCE will be achieved. For this purpose, our research group has synthesized TiO₂ nanoparticles, nanowires and nanorods continuously for the last 15 years [21,22,23], even though we did not obtain the result expected. Despite this situation, in this paper, we aim to present a systematic study of the impact of TiO₂ active layer thickness on the PCE of n-type DSSCs. To achieve this, we synthesized other types of TiO₂ nanoparticles using the hydrothermal method and examined the effect of their thickness on the PCE of the fabricated n-type DSSCs. The performance was then compared to that of DSSC based on commercially available P-25 TiO₂.

2. Results and Discussion

The solar energy power (P) can be calculated using the following equation:

P = IV

(1)

The energy conversion efficiency (η) of DSSCs is defined as the ratio of the maximum electrical power output (P_max) to the solar energy power input (P). P_max is calculated by multiplying the maximum current (J_mp) by the maximum voltage (V_mp). The solar energy power input is determined as the product of the irradiance of the incident light, measured in W/m², according to the following equation:

η = P_max/P = J_mp × V_mp/P

(2)

The ideal power density of DSSCs is a product of the maximum short-circuit current J_sc and maximum open-circuit voltage V_oc. The fill factor (ff) is a measure of the quality of the solar cell, and it can be calculated by comparing J_sc and V_oc to J_mp and V_mp by the following equation:

ff = (J_mp × V_mp)/(J_sc × V_oc)

(3)

Therefore, we can obtain the power conversion efficiency (η) of DSSCs by the following equation.

η = P_max/P = (J_sc × V_oc × ff)/P

(4)

where P (100 mW/cm²) is the solar cell testing standard under terrestrial conditions with air mass AM1.5.

According to Equation (4), achieving high ƞ in DSSCs requires maximizing both J_sc and V_oc, as well as ff. Thus, researchers have focused on integrating metal oxide nanostructures into DSSCs to increase dye adsorption and thereby enhance the photocurrent (J_sc).The fundamental operating principle of DSSCs mirrors that of photocatalytic reactions, where the generation of hole–electron pairs is crucial for both high PCE and efficient catalysis. Among various metal oxide materials, TiO₂ nanostructures are particularly favored for DSSC and photocatalysis applications owing to their stability and low production cost.

Figure 1 shows a schematic diagram of the doctor-blade method used for coating TiO₂ active layers. The thickness of the active layer can be precisely controlled by adjusting both the number of 3M tape layers and the number of doctor-blade coating cycles. Using these techniques, the TiO₂ active layers were accurately fabricated in a thickness range of 6–24 µm. Detailed procedures for the fabrication and characterization of TiO₂ based DSSCs have been previously published [21,22].

Figure 2a presents a typical XRD pattern of TiO₂ nanoparticles synthesized and annealed at 700 °C. The prominent diffraction peak at 25° corresponds to the (101) rutile plane, indicating that the primary crystal growth direction is along the {101} direction. A smaller diffraction peak at 38° is attributed to the (101) plane of anatase phase. Additionally, several other peaks corresponding to either (200) and (211) rutile or (211) anatase crystallites are also observed. By analyzing the peak areas (and considering their FWHM values), we calculated the crystalline phase ratio of rutile to anatase, which is very similar to that of P-25, with values of 78% rutile and 22% anatase. Park et al. reported that anatase crystals generally have smaller particle sizes compared to rutile crystals, which results in a larger surface area for anatase. Consequently, more dye molecules can be adsorbed on the anatase surface, leading to improved light absorption and increased photocurrent. Additionally, the interactions between particles in anatase crystals are stronger than those in rutile crystals, which enhance electron mobility and boost photocurrent [24]. Conversely, rutile crystals are more stable and more effective at scattering light compared to anatase crystals, and finding the optimal combination of these phases remains a crucial research objective. Figure 2b shows an atomic force microscopy (AFM) image of the same film depicted in Figure 2a, with a scan size of 10 × 10 µm². The image reveals craters approximately 0.5 µm in size on the surface of the fluorine-doped tin oxide (FTO) glass substrate, along with small nanosized clusters [see the enlarged image in Figure 2b]. The average surface roughness was measured at 0.274 nm, indicating a relatively larger surface area compared to a film coated with commercially available P-25 powder. This rough surface increases surface haze owing to a decrease in particle size. Therefore, this rough surface can produce more electrons in photovoltaic cells, resulting in the increase of photocurrent (J_sc) from our synthesized TiO₂ nanoparticles.

Figure 3 presents (a) scanning electron microscopy (SEM) and (b) electron dispersion spectroscopy (EDS) data for the TiO₂ nanoparticles synthesized after calcining at 450 °C and annealing at 700 °C. As shown in Figure 3a, the SEM image reveals spherical nanoparticles with some aggregate morphologies and an average particle size of 25 nm (see SEM inset). To determine the composition ratio of the synthesized TiO₂ nanoparticles, we measured the EDS spectrum. Figure 3b shows the EDS spectrum along with elemental mapping data (inset), which shows peaks corresponding only to Ti and O, with a weight percent ratio of Ti to O at 49.88:50.12. This indicates that a slightly excess of oxygen likely attributed to water adsorption or further oxidation in the air exhibits. Considering the differences in atomic sensitivity in EDS, the atomic percent indicates 75% O and 25% Ti, which falls within acceptable error limits. Using theseTiO₂ nanoparticles, we fabricated the active layers of n-type DSSCs with varying film thicknesses by adjusting the number of 3M tape layers and doctor-blade coating cycles, ranging from 1 to 5 layers.

Figure 4 shows the SEM surface morphologies (a–c) and cross-sectional images (d–g) of TiO₂ nanoparticle film layers with varying numbers of doctor-blade coatings such as 1 layer (d), 2 layer (e), 3 layer (f), and 5 layer (g), respectively. By adjusting both the numbers of 3M tape layers and doctor-blade coating cycle, we controlled the thickness of the TiO₂ nanoparticle film layers. The thickness of the TiO₂ active layers varied as follows: 6 µm (1 layer), 12 µm (2 layers), 17 µm (3 layers), and 24 µm (5 layers) with an increasing number of doctor-blade coating cycles. As the thickness increased, the surface roughness also increased, indicating an increase in surface area (Figure 4a–c). When we fabricated films thicker than 17 µm (3 layers), both optical transmittance and PCE of the n-type DSSCs decreased, largely owing to minimal changes in dye absorption [see Figure 5b as well]. Therefore, in this study, we excluded TiO₂ nanoparticle films thicker than 17 µm (3 layers). The cross-sectional images (d–g) clearly illustrate the variations in TiO₂ nanoparticle film thickness with different numbers of doctor-blade coating cycles. Based on this study, it is evident that, while light efficiency increases with thickness up to a certain point, it subsequently decreases when the thickness exceeds this optimal level. This decrease is likely attributable to increased electron diffusion time and reduced light transmittance, as described earlier. As the layer thickness increases, however, the distance that electrons generated by the dye must travel to reach the electrode becomes too great, which can lead to recombination and reduced electron injection efficiency into the electrode.

Figure 5a shows UV-Visible absorption spectra measured after N719 dye adsorption on the TiO₂ active layers, as shown in Figure 4. The baseline UV-Visible absorption spectrum was recorded from a bare FTO glass substrate without dye adsorption. The intensity of the dye adsorption peaks (as well as peak area), particularly around 500 nm, which correspond to the π–π* transition in the N719 dye molecule, increases with the thickness of the TiO₂ active layers. This suggests that a thicker TiO₂ layer, with its larger surface area, allows for greater dye adsorption. Using Beer’s law, we calculated the relative amounts of adsorbed dye on the TiO₂ active layers based on the data presented in Figure 5a. Figure 5b shows the variation in thickness (left, black color) and dye adsorption amounts (right, red color) with the number of doctor-blade coating cycles. As the number of coating cycles increases, both thickness and dye adsorption amounts increase from 1.2 × 10⁻⁷ mol/cm³ (6 µm, 1 layer) to 2.9 × 10⁻⁷ mol/cm³ (24 µm, 5 layers). However, the rate of increase begins to level off after three coating cycles. This indicates that the amount of dye molecules increases with the thickness of the active layers up to a critical thickness. Additionally, we observed that the PCE of DSSCs with active layer thicknesses exceeding 17 µm (3 layer) did not improve and even declined at 24 µm (see

Table 1. Photovoltaic characteristics of TiO₂ nanoparticle-based DSSCs with different thicknesses.

Sample Name	PCE (%)	Voc (V)	Jsc (mA/cm2)	ff (%)
Base Cell (P-25) (0 µm)	3.77	0.74	7.29	69.6
TEST-6 1 layer (6 µm)	2.86	0.73	5.55	70.7
TEST-6 2 layer (12 µm)	3.76	0.71	7.67	68.9
TEST-6 3 layer (17 µm)	4.02	0.70	8.42	68.2
TEST-6 5 layer (24 µm)	3.91	0.73	8.25	66.7

). This suggests that, as the TiO₂ active layer becomes thicker, the distance electrons generated by the dye must travel to reach the electrode becomes too long. Consequently, recombination may occur, preventing efficient electron injection into the electrode. Therefore, achieving high surface roughness (i.e., a large surface area) combined with an optimal TiO₂ active layer thickness can absorb more dye molecules, enhancing the light-trapping or scattering effect, particularly in the long-wavelength region. This leads to an increase in the PCE of the photovoltaic device. This result indicates that TiO₂ nanoparticle-based DSSCs are highly effective in enhancing light-trapping or light-scattering properties in photovoltaic cells by adjusting thickness and surface area. However, it is important to note that, while nanoparticle films demonstrate considerably higher PCE compared to other nanostructures, they also face a clear disadvantage in charge transport owing to the excessive number of adsorbed dyes.

The photocurrent (J_sc) and photovoltage (V_oc) of the solar cell devices, with an active area of 0.25 cm², were measured using simulated sunlight at AM-1.5 produced by solar simulator. Figure 6 shows the J–V curves for TiO₂ nanoparticle film-based DSSCs with varying active layer thicknesses. The photovoltaic characteristics of these n-type DSSC devices are summarized in Table 1. As illustrated in Figure 6 and detailed in Table 1, the TiO₂ nanoparticle film-based DSSCs with active layer thicknesses of less than 12 µm (2 layers) exhibit considerably lower PCE values compared to the commercially available P-25 based DSSC, which has a PCE of 3.77%. This decrease is likely attributable to the relatively low crystalline quality (poor polycrystallinity) of the synthesized TiO₂ nanoparticles. Despite the lower PCE values compared to P-25 based DSSCs, the experiment shows that J_sc gradually increases with the thickness of TiO₂ nanoparticle active layers, from 1 to 3 layers (see the y-axis changes in Figure 6). This increases in J_sc results in an increase in PCE from 2.86% to 4.02%, which surpasses the P-25 based DSSC. In the case of 5 layer, however, the PCE was dropped to 3.91% due mostly to the decrease of J_sc and ff rather than V_oc. This indicates that if a thickness of active layer has over optimum value, the distance electrons generated by the dye should long travel to reach the electrode owing to relatively prolonged recombination lifetime (This will be confirmed later). From the Table 1, we concluded that the considerable increase in PCE by 41% is primarily attributed to a large improvement in photocurrent density (at least 52%) in the n-type DSSCs, although the values of V_oc and ff remained relatively constant. Ohno et al. reported the analysis results of composition ratio between rutile and anatase crystalline phase, as well as average particle sizes with commercially available P-25 (Degussa, Frankfurt, Germany) TiO₂ powder of 70–80% anatase and 20–30% rutile, with average particle sizes ranging from 20 to 30 nm, respectively [25]. Since our synthesized TiO₂ nanoparticles have a similar composition ratio (78% anatase and 22% rutile, with little high amount of anatase due possibly to relatively low annealing temperature [26]) and average particle size (25 nm) with P-25, they can be comparable to each other even though we obtained a relatively low PCE value.

To understand this result in detail, we performed an analysis using electrochemical impedance spectroscopy (EIS). Figure 7a shows the Nyquist plots of the EIS data for the same DSSCs shown in Figure 6. The EIS data reveal four components: Z₁ (high-frequency region, 100 kHz–500 Hz), Z₂ (middle-frequency region, 500 Hz–1 Hz), Z₃ (low-frequency region, 1 Hz–100 mHz), and the Ohmic series resistance of the TCO (R_h). In Figure 7a, three distinct semicircles are observed in the measured frequency range of 100 mHz–100 kHz. R_h in the high-frequency region is associated with the resistance of the electrolyte and the FTO, while the impedances (or resistances) Z₁, Z₂, and Z₃ correspond to different aspects of the charge transfer and diffusion processes. Z₁ is the charge transfer interface resistance between the Pt counter electrode and the electrolyte (R₁), Z₂ is the charge transfer resistance at the TiO₂/electrolyte interface (R₂), and Z₃ is the Warburg diffusion resistance in the electrolyte (R₃) [27]. As we can see in the Figure 7b, however, in the analysis of a traditional equivalent circuit model for DSSCs, a constant photo-generated current source, a series parasitic resistance (i.e., series resistance, R_s) and a parallel parasitic resistance (i.e., parallel resistance, R_p) are generally included. For an understanding of the electronic behaviors of the solar cell, therefore, two types of resistance values, series and parallel resistance, were identified to measure the change in charge-carrier mobility with the energy level of the transport layer due to changes inside the solar cell. Because internal impedance is inversely proportional to solar cell performance, DSSCs with thicker TiO₂ active layers exhibit relatively lower impedance compared to those with thinner TiO₂ layers [see the maximum values of each second semicircle in Figure 7a]. Thus, our data in Figure 7a confirm a decreasing trend in all impedances with increasing thickness of the TiO₂ active layer. Among the resistances R₁, R₂, and R₃, the R₂ value (32.13 Ω) for the DSSC with a thick TiO₂ active layer of 17 µm (3 layers) showed lower impedance compared to those of DSSCs with thinnerTiO₂ layers (see also

Table 2. DSSC parameters of internal resistance (R₂), peak frequency (f_p,max), electron lifetime (τ_e), and recombination lifetime (τ_r) extracted from EIS measurements [Figure 7a] and voltage decay curve (Figure 8).

Sample Name	R2 (Ω)	fp,max (Hz)	τe (ms)	τr (s)
TEST-6 1 layer (6 µm)	39.35	19.08	5.24	1.38
TEST-6 2 layer (12 µm)	35.60	17.51	5.71	1.29
TEST-6 3 layer (17 µm)	32.13	16.57	6.04	0.92
TEST-6 5 layer (24 µm)	30.54	18.09	5.53	1.32

). However, the EIS data of thickness shows an abnormal trend for the relatively high second semicircle and low impedance compared with other data. This suggests a longer electron diffusion time, indicating that the DSSC with the thickerTiO₂ layer below critical value can has a higher electron density and reduced electron recombination, which improves charge transport. Consequently, the DSSC with the 17 µm thick TiO₂ active layer demonstrates the highest photocurrent (8.42 mA/cm²) and the best cell performance (η = 4.02%) in this study. However, the PCE begins to decline when the thickness exceeds 17 µm (3 layers) owing to extended electron diffusion times and reduced light transmittance in thicker layers of lower-quality TiO₂ nanoparticles. This indicates the need for further experiments to enhance light harvesting through improved light-trapping or light-scattering techniques. Exploring alternative active layers, such as 1-dimensional TiO₂ nanostructures, may offer better performance compared to nanoparticles.

In order to understand in detail the main reason for the thickness effect on the J_sc and PCE, calculations of electron lifetime (τ_e) and recombination lifetime (τ_r) were carried out as following procedures. The electron diffusion length (L_n) is determined by the diffusion coefficient (D_e) and the electron lifetime (τ_e) [16].

L_n = D_eτ_e

(5)

According to Equation (5), we can realize that the L_n becomes longer as the τ_e increases. The electron lifetime (τ_e) is calculated by Equation (6) using the peak frequency (f_p,max) of the second semicircle from the Nyquist diagram shown in Figure 7a [28].

τ_e = 1/f_p,max

(6)

The peak frequency (f_p,max) of the second semicircle and the electron lifetime (τ_e) for the different TiO₂ thicknesses are shown in Table 2, together with the values of R₂ and recombination lifetime (τ_r). The peak frequency (f_p,max) decreases and the electron lifetime (τ_e) increases as the thickness of TiO₂ increases. Based on our obtained EIS data shown in Figure 7a, relative impedance values are obtained from the DSSCs with different thickness of TiO₂ active layers [29]. We could also obtain a value of electron diffusion time (τ_e) theoretically by using Equation (6) and its results are summarized in Table 2. The obtained τ_e values are 5.24 × 10⁻³ (6 µm), 5.71 × 10⁻³ (12 µm), 6.04 × 10⁻³ (17 µm), and 5.53 × 10⁻³ s (24 µm), respectively. The increase in the τ_e makes the electron diffuse and transfer more easily due to the increase in the L_n, since the τ_r is closely related to both electron lifetime (τ_e) and electron diffusion length (L_n). Recombination lifetimes (τ_r) were therefore determined from the photovoltage (V_oc) decay curve (see Figure 8) and realized a tendency to oppose between τ_e and τ_r (see Table 2). As shown in Figure 7a (see also Table 1) and Figure 8, the highest efficiency cell showed the lowest τ_r owing to lowest recombination. Therefore, in this work, the J_sc and PCE are increased as the TiO₂ thickness increases up to 17 μm because the internal resistance (R₂) related to the electron transport in the TiO₂/dye/electrolyte interface is decreased and the electron lifetime (τ_e) is increased, reflecting a very short electron recombination time (τ_r). The performance of the DSSC with a thickness above 17 μm, however, is decreased because the recombination of the electron is strengthened. Finally, it is demonstrated that the optimal TiO₂ thickness is 17 μm for the best performance of the DSSC.

3. Materials and Methods

To prepareTiO₂ nanoparticles suitable for use as an active layer in DSSCs, we used a hydrothermal method, synthesizing the TiO₂ nanoparticles using a titanium isopropoxide solution in a temperature range 50–200 °C. The nanoparticles were then calcined and annealed at 450 °C and 700 °C, respectively. Before calcining and annealing, we measured the average particle sizes, which were initially in the range of 15–20 nm. After calcining and annealing, the particle sizes increased to 25–30 nm. The annealing process aimed to achieve a crystalline phase ratio similar to that of P-25 (Degussa) standard TiO₂ material, which has a crystalline phase ratio of, e.g., 70–80% anatase and 20–30% rutile.

To prepare the TiO₂ paste, we combined 0.5 g of standard P-25 (Degussa) TiO₂ powder with the annealed synthesized TiO₂ nanoparticles. This mixture was ground with 1 mL of water containing acetylacetone (Aldrich, Saint Louis, MO, USA) and 0.25 g of hydroxypropyl cellulose (Aldrich) to prevent particle re-aggregation. The resulting paste was then spread uniformly and evenly applied to the surface of FTO-coated glass substrates using the doctor-blade technique to fabricate DSSC devices. After application, all doctor-bladed film samples were annealed in air at 450 °C for 60 min to remove the solvent. To fabricate the DSSC devices, the annealed, doctor-bladed films were sensitized by immersing them in an ethanol solution containing 0.5 mM N719 dye (Solaronix Inc., Aubonne, Switzerland) for 24 h. After sensitization, the films were sandwiched between a platinum-coated FTO counter electrode and the sensitized film. The electrodes were separated by a 20 µm thick polypropylene spacer, and the internal cell space was filled with a liquid electrolyte (Solaronix Inc., Iodolyte AN-50). The active area of each DSSC was 0.25 cm². To observe the typical characteristics of fabricated n-type DSSC devices, photocurrent density–voltage (I–V) curves (SUN 2000) were measured using a xenon lamp under AM 1.5 filter at 100 mW/cm² illumination by a source meter (Model 2400, Keithley Instrument, Inc., Solon, OH, USA) under open circuit conditions. The analysis of electrochemical impedance spectroscopy (EIS) was then performed with the electrochemical analyzer (Biologic science instrument, Seyssinet-Pariset, France, SP-150) in the range of the frequency from 100 mHz to 100 kHz under AC voltage with a perturbation amplitude of 10 mV applied in the EIS measurement. In order to the light absorption of dye molecules with different TiO₂ thicknesses, UV/VIS spectra are measured from the wavelength of 400 nm to the wavelength of 800 nm by means of the UV/VIS spectrophotometer (OPTIZEN 3220 UV, Mecasys, Daejeon, Republic of Korea).

4. Conclusions

We successfully synthesized TiO₂ nanoparticles with a composition ratio and crystallinity similar to those of commercially available P-25 TiO₂ powder. Subsequently, we fabricated TiO₂ nanoparticle-based n-type DSSCs with varying active layer thicknesses and compared their photovoltaic properties to those of P-25 TiO₂-based DSSCs. The results clearly demonstrate the impact of TiO₂ layer thickness on n-type DSSC performance. As the thickness of the TiO₂ layer increased up to 17 µm, the PCE gradually improved from 2.86% to 4.02%, primarily owing to the increased photocurrent density in the n-type DSSCs. Notably, the DSSC with a 17 µm thick TiO₂ active layer achieved a maximum J_sc of 8.42 mA/cm², which is considerably higher compared to the 5.55 mA/cm² obtained with the 6 µm thick TiO₂ active layer, resulting in a current density increase of at least 52%. However, the relatively poor crystallinity and smaller particle size of the synthesized TiO₂ active layers adversely affected both V_oc and ff. Therefore, further research is essential to address these issues.