Insulation Ability and Morphological Effect of ZrO₂ Spacer Layer in Carbon-Based Multiporous Layered Electrode Perovskite Solar Cells

Abstract

Fully printable carbon-based multiporous layered electrode perovskite solar cells (MPLE−PSCs) are close to being commercialized due to their excellent stability, their ability to easily be scaled up, and their amenability to mass production via non-vacuum fabrication processes. To improve their efficiency, it is important that detailed studies of the morphologies of mesoporous electrodes be carried out. In this study, we prepared five types of ZrO₂ spacer layers for MPLE−PSCs, and the morphology of ZrO₂ and device performance were evaluated using a scanning electron microscope, nitrogen adsorption/desorption measurements, electrode resistance measurements, UV-visible light reflectance measurements, and current density–voltage measurements. The results reveal that the adequate specific surface area and pore size distribution of mesoporous ZrO₂ provided high insulation ability when used as spacers between electrodes and light absorbance, resulting in a 10.92% photoelectric conversion efficiency with a 23.22 mA cm⁻² short-circuit current density. This information can serve as a guideline for designing morphologies useful for producing high-efficiency devices.

1. Introduction

Perovskite solar cells (PSCs) are composed of halide perovskite materials (ABX₃: A = MA⁺ (methylammonium: CH₃NH₃), FA⁺ (formamidinium: CH(NH₂)₂), Cs⁺, Rb⁺, K⁺; B = Pb²⁺, Sn²⁺, Ge²⁺, Zn²⁺; X = I⁻, Br⁻, Cl⁻), which are used as light absorbers [1,2,3,4,5,6,7,8]. They attracted a lot of attention in the photovoltaic industry and various science communities due to the rapid increase in photoelectric conversion efficiency (PCE) and low solution-based fabrication costs [9,10,11,12,13]. PCE increased from 3.8% in 2009 [1] to 27.0% in 2025 [14]. The performance of these cells is close to that of traditional high-performance commercialized solar cells, such as crystallized Si and cadmium telluride. This high performance is a result of the excellent properties of halide perovskite materials, such as their high charge mobility, long balanced carrier diffusion length, and low exciton binding energy [15,16,17,18,19,20,21,22]. Recently, various research studies have been conducted to improve material and device stability and scale up PSCs for their commercialization [23,24,25,26,27,28,29,30,31,32,33].

Therefore, in this study, we propose the practical commercialization of fully printable carbon-based multiporous layered electrode perovskite solar cells (MPLE−PSCs) in outdoor applications. These solar cells, developed by Prof. Han’s group, are quite cost-effective and highly stable [34]. In such devices, perovskite absorbers are filled in a triple-layer scaffold consisting of mesoporous TiO₂ (the electron transport layer), ZrO₂ (the spacer layer), and carbon (the hole transport layer and back electrode). Because metal electrodes (Au and Ag) have been replaced with carbon electrodes, the cost of the material for these devices is significantly lower than that of conventional thin-film perovskite solar cells. In addition, they have excellent durability because the organic charge transport layers are eliminated [23,24,31,35,36,37]. Solar cells with this structure can be made in open-air ambient conditions using fabrication processes based on the screen printing method. These methods result in low fabrication costs, easy scale-up, and mass production. Currently, the highest PCEs of MPLE−PSCs are reported to be 22.2% (covered 0.1 cm² mask) and 18.2% (57.5 cm² module) [38].

In an MPLE−PSC, perovskite precursor solution must be filled uniformly in the mesoporous TiO₂/ZrO₂/carbon layer to ensure the device’s high performance. This is a very important factor as it affects the material parameters greatly (such as composition, film thickness, and morphology). Many studies reported on perovskite materials, the process of introducing a precursor, pre-modification with additives, and post-treatment [39,40,41,42,43,44,45,46,47,48,49]. Despite this, it is important to study the morphologies and structures of mesoporous electrodes to attain high performance. In our previous studies, we achieved a uniform filling of perovskite light-absorbing crystals by controlling the morphologies of the mesoporous TiO₂ electron transport layer and carbon layer [50,51,52]. In particular, the optimized pore size made it possible to deposit the perovskite crystals throughout the MPLE, which improved light absorption and increased the short−circuit current density (J_SC) [52]. However, detailed studies on the pore sizes of the spacer ZrO₂ layer have not yet been reported.

The spacer layer is an essential factor within a solar cell; its purpose is to separate the anode from the cathode. In MPLE−PSCs, the spacer layer prevents short−circuiting between electrons in the TiO₂ and holes in the carbon electrode [53,54,55,56]. Moreover, the light transmitted from the “mesoporous−TiO₂ + perovskite” layer can be absorbed by the perovskite crystals in the spacer layer, thus improving light absorption [54]. In principle, the materials used for spacer layers should have a wide band gap and be able to withstand high-temperature processes applied to remove paste bonding agents (polymer binder thickeners) [51]. In previous studies, some materials, such as ZrO₂, Al₂O₃, and SiO₂, and their composites have been reported [34,57,58,59,60,61,62]; in recent studies, ZrO₂ has been commonly used as the spacer layer in high-efficiency MPLE−PSCs [38,43,44,45,46]. These ZrO₂ layers are often 1 to 3 µm thick, and the crystal should be larger than 20 nm to avoid cracks in the film [54,56]. Thus, while the spacer layer has been optimized in terms of film thickness and particle size, the ideal standard has not yet been established for parameters related to the mesoporous structure. In particular, pore size and specific surface area significantly influence the introduction and packing ratios of perovskite light-absorbing crystals.

In this study, in order to verify the influence of the morphology (especially pore size) of mesoporous ZrO₂ on the corresponding device, we applied five different ZrO₂ spacer layers to MPLE−PSCs. These ZrO₂ spacer layers were dispersed in paste and deposited via screen printing. MPLE−PSCs composed of <glass/fluorine-doped tin oxide (FTO)/compact TiO₂/mesoporous TiO₂ + perovskite/mesoporous ZrO₂ + perovskite/carbon + perovskite> are shown in Figure 1a. The thickness of these layers was controlled using a screen printing method, and the values were 0.3 to 0.5 μm for mesoporous TiO₂, 1.4 to 1.6 μm for mesoporous ZrO₂, and 10 to 15 μm for mesoporous carbon, as shown in the scanning electron microscope (SEM) images. MPLE−PSC devices were fabricated in ambient open air via spray pyrolysis deposition and screen printing. 5−AVA−MAPbI₃ was used as the light-absorbing material (Figure 1b); this perovskite material has been used since the initial development of MPLE−PSCs, and it provides excellent reproducibility and durability [34,35,36]. However, recent studies have shown that the performance of devices containing perovskite materials, including MAI and 5−AVAI, varies significantly depending on the measurement conditions [63,64]. Therefore, it is important to note that the photoelectric conversion performance shown in this study cannot be replicated in real operating conditions. The morphology of ZrO₂ was evaluated using scanning electron microscopy (SEM) and nitrogen adsorption/desorption measurements. The insulation ability of the mesoporous ZrO₂ layer between the cathode and anode was evaluated in terms of its electrical resistance before the deposition of the perovskite precursor solution. The light absorbance of MPLE−PSCs was investigated using UV-visible reflection spectra. To investigate the performance of MPLE−PSCs, we performed current density–voltage (J−V) measurements and incident photon-to-current efficiency (IPCE) measurements. Through these measurements, it was confirmed that the specific surface area, pore volume, and size distribution of ZrO₂ affected the J_SC and open circuit voltage (V_OC) of the devices due to the change in light absorption and insulating ability.

2. Materials and Methods

2.1. Materials

Lead iodide (PbI₂, 99.99%), methylammonium iodide (MAI, 99.0%), 5−ammonium valeric acid iodide (5−AVAI, 97.0%), acetic acid, and ethyl cellulose (45–55 mPa·s) were purchased from Tokyo Chemical Industry Co., Ltd., Tokyo, Japan. γ−butyrolactone (GBL, 99.5%), ethanol (99.5%), and α−terpineol (95.0%) were purchased from Kanto Chemical Co., Inc., Tokyo, Japan. Titanium diisopropoxide bis (acetylacetonate) (TAA, 75 wt.% in isopropanol) was purchased from Sigma-Aldrich, St. Louis, MO, USA. ZrO₂ particles (EP, UEP, SRP−2, and SPZ) were obtained from Daiichi Kigenso Kagaku Kogyo Co., Ltd., Osaka, Japan. ZrO₂ particles (NP−ZRO2−4−500) were purchased from American Elements, Los Angeles, CA, USA. TiO₂ paste and carbon paste were prepared in the laboratory using the same process reported in our previous paper [52,65].

2.2. Preparation of ZrO₂ Paste for Mesoporous ZrO₂ Layer

A total of 4.5 g of ZrO₂ particles, 8.85 g of α–terpineol, 4.05 g of ethyl cellulose dissolved in α–terpineol (10 wt.%), Al₂O₃ beads (10 mm), and two sizes of ZrO₂ beads (5 mm and 2 mm) were placed in an agate pot and crushed and stirred in a planetary ball mill (PULVERISETTE 7, Fritsch Japan Co., Ltd., Yokohama, Japan) for 1 h (700 rpm); then, the paste was milled at a low speed (300 rpm) for 5 min to remove air bubbles.

2.3. Preparation of Perovskite Precursor Solution

The (5−AVA)_0.05(CH₃NH₃)_0.95PbI₃ precursor solution (1.2 mol L⁻¹ in GBL) was prepared in a glove box filled with nitrogen gas. The precursor solution was prepared by mixing 0.60 mmol of PbI₂, 0.57 mmol of MAI, and 0.03 mmol of 5−AVAI in 0.5 mL of GBL. The solution was stirred at 800 rpm for 17.5 h at 70 °C.

2.4. Fabrication of MPLE−PSCs

The perovskite solar cells were fabricated under atmospheric open-air conditions on fluorine-doped tin oxide (FTO)-coated glass substrates (TEC−15, Nippon Sheet Glass Co., Ltd., Tokyo, Japan). We patterned the substrates via laser etching to separate the FTO conductive layers. Then, the patterned substrates were ultrasonically cleaned with detergent solution and ethanol for 15 min each. A compact TiO₂ hole-blocking layer was deposited on the patterned FTO glass heated at 500 °C using the spray pyrolysis method. To prepare the precursor solution, 0.66 mL of TAA was diluted with 22.5 mL of ethanol. Then, the mesoporous TiO₂ electron transport layer was screen-printed, left to rest at room temperature for 10 min, and after that, dried on a hot plate at 50 °C and 140 °C for 5 min each. The TiO₂ layer was annealed at 500 °C for 55 min (25 min rising and 30 min holding) in an electric furnace. The mesoporous ZrO₂ spacer layers were similarly screen-printed on the mesoporous TiO₂ layer, left to rest at room temperature for 10 min, and after that, they were dried on a hot plate at 50 °C and 140 °C for 5 min each. The ZrO₂ layer was annealed on a hot plate at 200 °C for 20 min [66]. Then, the carbon back electrode was screen-printed on the mesoporous ZrO₂ spacer, left to rest at room temperature for 10 min, and then it was dried on a hot plate at 125 °C for 10 min. The carbon layer-coated substrate was annealed at 400 °C for 1 h on a hot plate. The substrates were then cooled to room temperature and diced into individual cells, and electrical contacts were attached to both electrodes via ultrasonic soldering. The surrounding area of the multiporous-layered electrodes was masked with heat-resistant polyimide tape so that the perovskite solution would fill the multiporous-layered electrodes. Finally, per cell, 4.0 µL of perovskite precursor solution was drop-casted into the porous electrode layer in the masked area by polyimide tape (the cell and area sizes were 196 and 120 mm²). The solution-dropped devices were left to rest at 25 °C in a Petri dish for 30 min. Then, with the Petri dish covered, they were heated on a hot plate at 50 °C for 90 min. Afterwards, the Petri dish was removed and the devices were dried at 50 °C for 10 min to complete the crystallization of the perovskite material. All perovskite infiltration and crystallization processes were conducted under atmospheric open-air conditions, with room temperature ranging from 15 °C to 25 °C and relative humidity between 20% and 70%.

2.5. Characterization

SEM images of the cross-sectional structure of MPLE−PSCs and mesoporous ZrO₂ spacer layers were obtained using a scanning electron microscope (JSM–6510, JEOL, Akishima, Japan) at an accelerating voltage of 10 kV. The specific surface area and pore size distribution were calculated from nitrogen adsorption/desorption isotherm profiles (BELSORP MINI X, MicrotracBEL Corp., Osaka, Japan) using the Brunauer–Emmett–Teller (BET) and the Barrett–Joyner–Halenda (BJH) methods, respectively. UV-visible light reflectance measurements were taken using a UV-visible spectrophotometer (V−650, JASCO, Hachioji, Japan). Current density–voltage (J−V) curves were measured with a DC voltage current source (B2901A, Agilent, CA, USA) under a solar simulator (AM1.5G, 100 mW cm⁻²) equipped with a 500 W xenon lamp (YSS−100A, Yamashita Electric, Tokyo, Japan). The irradiation intensity of the AM1.5G solar simulator was calibrated using a reference Si photodiode (Bunko Keiki, Tokyo, Japan). The photosensitive area was limited to 0.09 cm² by masking with 3 mm × 3 mm aperture. The measurement voltage ranged from −0.05 to 1.05 V; forward and reverse scanning were performed; the step was 0.01 V; and the scanning delay time was 10 msec. Measurements were taken three times in the forward direction and three times in the reverse direction, with an interval of 2 min between each measurement. The device was continuously illuminated for photoactivation between measurements [67]. The optical aging under AM1.5G was performed under open-circuit conditions. To take the incident photon-to-current efficiency (IPCE) measurement, a 150 W xenon lamp (Peccell Technologies, Kawasaki, Japan) equipped with a monochromator was used as a light source to obtain the spectrum. Before the measurement, calibration was performed with a Si photodiode.

3. Results and Discussion

First, to confirm the morphology of ZrO₂, five different ZrO₂ powders (product−names SRP−2, EP, UEP, SPZ, and NP−ZRO2−4−500 (denoted “4−500”)) were used to prepare the mesoporous ZrO₂ spacer layers shown in the SEM images (Figure 2). All of the ZrO₂ layers were not removed from the substrate during the processes of printing and annealing, and no cracks were observed. In the SRP−2 and EP layers (Figure 2a,b), large aggregates of about 1 μm were observed. Other ZrO₂ (UEP, SPZ, and 4−500) layers (Figure 2c–e) showed no aggregates, and dispersed ZrO₂ particles were identified. The particle sizes were 0.1 µm to 0.2 µm for SPZ and less than 0.1 µm for UEP and 4−500.

To reveal the pore characteristics of the ZrO₂ layers, nitrogen adsorption/desorption measurements were performed to obtain the specific surface area and pore size distribution of ZrO₂. The specific surface area of ZrO₂ (A_s), total pore volume (V_p), and medium pore diameter (d_p) are shown in

Table 1. Parameters of the morphology of ZrO₂ from nitrogen adsorption/desorption measurements.

ZrO2	Specific Surface Area: As [m2/g]	Total Pore Volume: Vp [cm3/g]	Medium Pore Diameter: dp [nm]
SRP−2	32.0	0.338	48.1
EP	18.8	0.239	40.7
UEP	18.0	0.316	28.0
SPZ	5.80	0.194	98.2
4−500	17.4	0.196	29.4

. A_s was calculated based on the BET plots shown in Figure 3a. Pore size distributions were obtained using the BJH method (Figure 3b). The 4−500 and UEP powders of ZrO₂ have pores ranging from 30 to 40 nm. UEP has a higher number of pores than 4−500. SPZ of ZrO₂ has the largest number of large pores at around 100 nm. In addition, in the EP and SRP−2 of ZrO₂, pores with a wide range of sizes are observed. SRP−2 has pores less than 10 nm, but it is difficult for perovskite precursor solutions to penetrate these small pores [52]. From the above morphology analysis results (Figure 2 and Figure 3), it can be stated that larger ZrO₂ particles have larger pore sizes, and the distribution of pore size is influenced by the homogeneity of the particles—in this case, the presence of aggregates is apparent in the SRP−2 and EP layers.

To evaluate the insulation ability of the above mesoporous ZrO₂ layers, multiporous layered electrodes composed of <FTO/compact TiO₂/mesoporous TiO₂/mesoporous ZrO₂/mesoporous carbon> were fabricated. The insulation resistance of the electrodes was measured before introducing the perovskite precursor solution, as shown in Figure 4a. The high resistance across the etch line on the FTO glass substrate denotes the prevention of contact between the carbon layer and FTO (or TiO₂). The insulation resistance values for each ZrO₂ powder are shown in the boxplots in Figure 4b. The measurement apparatus had a maximum resistance limit of 20 MΩ. Hence, when the resistances between the electrodes showed “over range”, we marked the data as “20 MΩ”. In total, 10 cells were used for one type of ZrO₂ (50 cells in total). Devices with SPZ had lower insulation resistance compared to the other ZrO₂ materials. SRP−2 showed high insulation performance (more than 1 MΩ). Furthermore, we investigated the relationship between ZrO₂ morphology and insulation performance. As shown in Figure 4c–e, the morphology parameters obtained from Table 1 are plotted on the x-axis, and the insulation resistance of 10 devices for each ZrO₂ and their average values are plotted on the y-axis. The insulation ability increased with the increase in A_s and V_p and with the decrease in the medium d_p. These results indicate that the morphology of ZrO₂ affects the insulation ability of the device when applied as a spacer layer.

To confirm the light absorbance at the photosensitive area of these devices, UV−vis reflection measurements were performed using the same process as previously reported [52]. The perovskite precursor solution was introduced into the TiO₂/ZrO₂/carbon scaffold and crystallized via annealing to obtain solar cells. Usually, perovskite crystallization can occur on the whole area in mesoporous electrodes, and the photosensitive area shows a dark black color (Figure 5a, left side). In this case, the value of absorbance will be high, and then a large value for the short-circuit current density (J_SC) can be expected. On the contrary, when crystallization does not occur homogeneously, the photosensitive area shows a white or gray color derived from TiO₂ and ZrO₂ (Figure 5a, right side), and it gains low absorbance. Therefore, it is important to study reflection absorbance to evaluate the crystallization behavior of the perovskite precursor on mesoporous ZrO₂ morphologies. Figure 5b shows the reflection absorption spectra of the device using different ZrO₂ sources. The results show that the device using UEP achieved the highest absorbance in the visible light range (350 nm to 750 nm).

To evaluate the photoenergy conversion performance of the fabricated MPLE−PSCs, we obtained the performance parameters, J−V curves, IPCE spectrum, and boxplots prepared based on each parameter, as shown in

Table 2. Best-performing parameters and hysteresis index (HI) of the devices fabricated with different mesoporous ZrO₂ layers.

ZrO2		JSC [mA cm−2]	VOC [V]	FF [%]	PCE [%]	HI [%]
SRP−2	Reverse	22.80	0.823	58.0	10.88	13.2
	Forward	23.79	0.783	50.7	9.44
EP	Reverse	22.38	0.831	53.9	10.03	17.0
	Forward	23.17	0.780	46.1	8.32
UEP	Reverse	23.22	0.840	56.0	10.92	22.8
	Forward	24.10	0.783	44.7	8.43
SPZ	Reverse	16.08	0.741	45.3	5.40	17.2
	Forward	16.54	0.700	38.6	4.47
4−500	Reverse	22.43	0.831	56.5	10.54	17.7
	Forward	23.14	0.779	48.1	8.67

and

Table 3. Parameters of the devices fabricated with different mesoporous ZrO₂ layers (n = 10 cells).

ZrO2	JSC [mA cm−2]	VOC [V]	FF [%]	PCE [%]
SRP−2	16.20 ± 3.85	0.778 ± 0.052	55.6 ± 5.9	7.08 ± 2.22
EP	18.48 ± 3.33	0.777 ± 0.062	51.5 ± 7.5	7.83 ± 1.63
UEP	22.00 ± 1.11	0.806 ± 0.027	52.6 ± 4.1	9.35 ± 1.08
SPZ	11.88 ± 2.66	0.618 ± 0.085	48.3 ± 10.4	3.58 ± 1.26
4−500	19.33 ± 3.10	0.756 ± 0.074	51.2 ± 4.5	7.67 ± 2.27

and Figure 6a–d. The hysteresis index (HI) in Table 2 was calculated using Equation (1) as follows:

$$\mathrm{H}\mathrm{I}={\displaystyle \frac{\mathrm{P}\mathrm{C}\mathrm{E}\left(\mathrm{R}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{e}\right)-\mathrm{P}\mathrm{C}\mathrm{E}\left(\mathrm{F}\mathrm{o}\mathrm{r}\mathrm{w}\mathrm{a}\mathrm{r}\mathrm{d}\right)}{\mathrm{P}\mathrm{C}\mathrm{E}\left(\mathrm{R}\mathrm{e}\mathrm{v}\mathrm{e}\mathrm{r}\mathrm{s}\mathrm{e}\right)}}$$

(1)

The best-performing device achieved 10.92% PCE, 23.22 mA/cm⁻² J_SC, 0.840 V V_OC, and a 56.0% fill factor (FF) on the reverse scan using UEP. Furthermore, the HI was above 10% for all of the best-performing devices. Since a large HI was obtained regardless of the morphology of ZrO₂, we assume that this is due to the low quality of the perovskite light-absorbing crystals of 5−AVA−MAPbI₃ rather than the result of a problem with ZrO₂ [64]. The IPCE values of the best-performing device using UEP exhibited the highest response across the entire visible light region. The other best-performing devices showed similar trends, as reflected in the J_SC presented in Table 2 and Figure 6a. These results confirm that the J−V measurements were conducted properly. Comparing the results of these five types of ZrO₂ powders, devices using UEP achieved a high performance with good reproducibility. In particular, J_SC increased with the increase in the absorbance value of the device in the visible light region, and reproducibility improved. In contrast, lower absorbance values result in lower J_SC.

The relationships between the electrodes’ insulation resistance and solar cell performance are shown in Figure 6e. The results show linear relationships of PCE, V_OC, FF, and shunt resistance (R_sh) in terms of electrode insulation resistance. In particular, a strong relationship between V_OC and electrode insulation resistance was identified, and this trend is consistent with the findings of a previous study [54]. The series resistance (R_s) with low values (below 10 Ω) in the devices was also commonly observed when the electrode insulation resistance was high. This may be because proper interfacial contact is achieved due to the good separation of the carrier transport layer (TiO₂ and carbon) using the ZrO₂ spacer. The correlation of J_SC and electrode insulation resistance cannot be confirmed because J_SC is a parameter attributed to light absorption. As expected from these results, the high PCE, resulting in the increase in V_OC and FF, with high electrode insulation resistance, is due to the increase in R_sh associated with the suppression of non-radiative recombination in the device.

Next, the relationships between ZrO₂ morphology parameters (A_s, V_p, and medium d_p) and device performances (V_OC, R_sh, and J_SC) are shown in Figure 7. The values of V_OC and R_sh increased with the increase in A_s and V_p and with the decrease in the medium d_p (Figure 7a,b). This trend is consistent with the trend for the electrodes’ insulation resistance outlined in Figure 4c–e. These results indicate that the good separation of the charge transport layers using the ZrO₂ spacer layer can reduce V_OC loss by suppressing the risk of electron and hole recombination. Moreover, it is also interesting to note that J_SC shows the different variation in trends from R_sh to V_OC (Figure 7c). This indicates that controlling the morphology of ZrO₂ can work not only to improve the insulating capacity, but also to assist with the formation of light-absorbing perovskite crystal in solar cells. SRP−2, which has large A_s and V_p, has smaller J_SC and absorbance in the devices. This is likely due to the small (<10 nm) pores contained in SRP−2, which prevent the permeation of perovskite precursor solution and the lower filling rate of light-absorbing crystals. These results can be linked to the findings reported in a previous study [55,56]. Specifically, a small pore size in the ZrO₂ layer can hinder the complete filling of the perovskite light-absorbing crystals throughout the mesoporous electrode. In such cases, the device’s light absorbance may become limited. To make further improvements to achieve high-efficiency devices, it is necessary to carefully control the morphology and thickness of the ZrO₂ and other mesoporous layers. In addition, optimizing the infiltration conditions and the crystallization process of the perovskite material is essential.

4. Conclusions

We investigated the effect of the morphology of the mesoporous ZrO₂ layer on insulation ability and device performance by making a comparison with five different types of ZrO₂. This study outlines how to achieve good separation between TiO₂ and carbon and devices with high absorbance and high efficiency via optimizing the morphological properties of the ZrO₂ spacer layer. In conclusion, as shown in Figure 8, mesoporous ZrO₂ layers should be designed based on the following characteristics to obtain high-efficiency devices:

• Large A_s and V_p and small medium d_p are needed to obtain high insulation ability (Figure 4c–e).
• The pore size distribution should be uniform, from 10 nm to 40 nm, to obtain uniform filling of the perovskite precursor solution and high absorbance (Figure 3b and Figure 5b).

Future morphological research on spacer materials should place particular emphasis on controlling the crystallization behavior of the perovskite light absorber, aiming to achieve further enhancements in light harvesting efficiency and overall device performance. In addition, evaluating the morphological uniformity of the mesoporous layer when the device is scaled up may represent a critical factor in the commercial potential of MPLE−PSCs. These results will be helpful for designing the material morphology of MPLE−PSCs using ZrO₂ spacers and other mesoporous carrier transport layers, or other devices, using porous materials.