As the energy demand increases, the development of various energy sources has become increasingly important. Although significant advancements have been achieved in silicon solar cells, they are hindered by high manufacturing costs. In contrast, organic–inorganic halide perovskites have garnered attention as solar cell materials owing to their advantages such as tunable bandgap, high optical absorption coefficients, long carrier diffusion lengths, and low production costs [1–6]. Since their introduction in 2009, perovskite solar cells have achieved power conversion efficiencies (PCEs) comparable to those of silicon solar cells owing to continuous research efforts [7]. However, the fabrication of organometallic halide perovskites is significantly affected by external factors such as light, heat, and moisture [8,9]. Particularly, the moisture in ambient air can react with perovskite precursors and degrade the film quality [10]. Although this issue can be mitigated by fabricating devices in well-controlled inertgas gloveboxes, the intricacies associated with this process impede the production of perovskite thin films.

Recently, the fabrication of perovskite thin films under ambient air conditions has been actively researched. Luo et al. [11] fabricated a stable perovskite film under ambient air conditions via solvent engineering. Further, Yoon et al. [12] induced uniform perovskite films through the rapid crystallization of perovskites via surface engineering.

In this study, perovskite films were fabricated under ambient air conditions using additive engineering. To replace the anti-solvent process, vacuum flash-assisted solution processing was used to remove the solvent from the spin-coated films [13]. The additives reduced the influence of moisture in the ambient air, thereby eliminating pinholes on the surface and inducing a uniform film. Under optimal conditions, a PCE of 19.00 % was obtained in ambient air [relative humidity (RH): 50 %]. Moreover, the characteristics of the perovskites were investigated based on device performance and via X-ray diffraction (XRD) and scanning electron microscopy (SEM), and improved film quality was observed as compared with that of the pristine film.

Patterned indium tin oxide (ITO) (145 nm, 11 Ω) glass substrates were cleaned via ultrasonic treatment in acetone and isopropyl alcohol for 15 min and then dried for 1 h in an oven at 135 ∘C. [2-(3,6-Dimethoxy-9H-carbazol-9-yl)ethyl]phosphonic acid (MeO-2PACz) was dissolved in anhydrous ethanol at 3 mmol%; the prepared solution was spin coated at 4,000 rpm for 30 s onto the ultraviolet–ozone-treated ITO substrates and then annealed for 10 min at 100 ∘C in a N₂-filled glovebox. The perovskite precursor solution was prepared by dissolving cesium iodide (0.254 M), formamidinium iodide (1.088 M), formamidinium bromide (0.109 M), lead iodide (1.232 M), and lead bromide (0.217 M) in dimethylformamide: dimethyl sulfoxide in a 9:1 volume ratio. The precursor solution was stirred for 12 h in the N₂-filled glovebox. Oleylamine was then added directly to the perovskite solution (0.1 wt%). The additives were added to the prepared perovskite solution in optimized amounts [methylammonium chloride (MACl) = 0.025 M, pyridine = 0.2 M, and NH₄Cl = 0.075 M], followed by additional overnight stirring in a glovebox. The fully dissolved solution was spin coated onto the MeO-2PACz layer at 3,000 rpm for 10 s in ambient air. To perform vacuum flash-assisted solution processing, the substrate was placed into a home-built chamber, which was evacuated and the pressure was decreased from 1 atm to 10⁻³ Torr over 20 s. The as-pumped perovskite films were thermally annealed at 100 ∘C for 10 min. C60 was thermally evaporated to a thickness of 25 nm. After the C60 layer was deposited, bathocuproine solution (dissolved in 0.5 mg/mL of absolute ethanol) was spin coated onto the C60 layer without further annealing. Ag electrodes were thermally evaporated to a thickness of 100 nm.

The current density–voltage (J–V) characteristics were measured using a Keithley 4200 source meter. The measurement was conducted under air mass 1.5 global (100 mW cm⁻²) light simulated using an Oriel solar simulator (Class AAA solar simulator, Newport Corporation, France). All the electrical tests were conducted in ambient air.

The morphology of the perovskite thin film was examined using field emission SEM (FE-SEM) (SUPRA 40, Carl Zeiss AG).

Figure 1 shows a schematic of the method used to obtain perovskite active-layer thin films. An optimized amount of the additive was added to the perovskite precursor solution (refer to Section 2). To investigate the effect of moisture on the perovskite, we compared the performance and morphology of the perovskite fabricated in the N₂ glovebox and under ambient air conditions with three different levels of humidity. The glove box was filled with ambient air and the humidity was adjusted using a humidifier. Figures 2(a) and 2(b) show the J–V curves of the perovskite solar cells. The detailed photovoltaic parameters are listed in Table I. Figures 2(c) and 2(d) show the SEM images of the perovskite film surface fabricated in the N₂ glove box with ambient air, and the inset image shows a cross-section of the perovskite film. As shown in Table I, the efficiency of the devices fabricated in the N₂ glove box was 18.55 %. However, the efficiency of the devices fabricated under ambient air conditions decreases with increasing humidity. The performances at RH levels of 30, 50, and 70 % are 17.41, 15.20, and 15.09 %, respectively. This indicates that a higher moisture content in ambient air reduces the performance of perovskite solar cells. Furthermore, a comparison of the SEM images in Figs. 2(c) and 2(d) indicate that the perovskite film fabricated in the N₂ glove box exhibits a uniform morphology without pinholes, whereas the perovskite film fabricated under RH 70 % conditions shows many pinholes. Evidently, these defects contributed to a reduction in performance. To mitigate the influence of moisture and obtain high-quality perovskite films, we used various additives (MACl, pyridine, and NH₄Cl).

Figure 1. Schematic of the synthesis of perovskite film.

Figure 2. J–V curves of perovskite solar cells fabricated in (a) N₂ glove box and (b) ambient air with three different levels of humidity. SEM image of perovskite film fabricated in (c) N₂ glove box and (d) ambient air.

Table I

Photovoltaic parameters of perovskite solar cells..

	Jsc (mA/cm2)	Voc (V)	FF (%)	PCE (%)
N2 condition	20.812 (20.783)	1.130 (1.100)	78.88 (76.30)	18.55 (17.45)
Air_RH30 %	21.326 (21.040)	1.060 (1.053)	77.03 (74.27)	17.41 (16.56)
Air_RH50 %	19.793 (19.648)	1.040 (1.018)	73.84 (69.20)	15.20 (13.85)
Air_RH70 %	20.529 (17.329)	1.010 (0.780)	72.80 (59.71)	15.09 (11.05)

To investigate the effects of various additives on the device performance of perovskite solar cells, we fabricated perovskite solar cells under 50 % RH conditions using optimized amounts of additives. The optimal molar concentration for additives were 0.025, 0.200, and 0.075 M for MACl, pyridine, and NH₄Cl, respectively. Figure 3 and Table II show the J–V curves and photovoltaic parameters, respectively. As shown in Table II, by applying MACl, pyridine, and NH₄Cl, we achieved increased efficiencies of 17.67, 17.73, and 19.00 %, respectively, as compared with the efficiency of the pristine film (15.20 %). In particular, the device with NH₄Cl addition showed the best performance with short-circuit current (J_SC), open-circuit voltage (V_OC), and fill factor (FF) of 21.469 mA cm⁻², 1.130 V, and 78.33 %, respectively. The improved photovoltaic parameters resulting from the additives can be attributed to various reasons, and an additional analysis was conducted to investigate this accurately.

Figure 3. J–V curves of perovskite solar cell fabricated in ambient air without and with various additives (MACl, pyridine, and NH₄Cl).

Table II

Photovoltaic parameters of perovskite solar cell fabricated under ambient air condition without and with various additives (MACl, pyridine and NH₄Cl)..

	Jsc (mA/cm2)	Voc (V)	FF (%)	PCE (%)
Air_RH50 %	19.793 (19.648)	1.040 (1.018)	73.84 (69.20)	15.20 (13.85)
MACl	20.609 (20.513)	1.080 (1.025)	79.40 (66.93)	17.67 (14.35)
Pyridine	20.571 (19.933)	1.120 (1.070)	76.94 (66.18)	17.73 (14.20)
NH4Cl	21.469 (20.874)	1.130 (1.102)	78.33 (77.03)	19.00 (17.72)

XRD measurements were performed to study the changes in the perovskite crystallinity induced by the additives. Figure 4 shows the XRD patterns of perovskite crystals with and without additives. The peak intensity of the perovskite (100) crystals significantly increased when MACl and pyridine were added, as compared with the values under pristine conditions. However, when NH₄Cl was added, the peak intensity of the (100) crystals remains at a level similar to that under pristine conditions. Moreover, we calculated the crystal size to assess the crystalline properties of perovskite, and the results are listed in Table III. Crystal sizes of the perovskite were estimated using Scherrer’s equation: D = 0.9 λ / full width at half maximum (FWHM) cos θ. Here, D, θ, and λ represent the crystal size, Bragg angle, and wavelength of X-rays, respectively [14]. The average crystal size increased to 74.18 and 77.57 nm when MACl and pyridine were added, respectively, as compared with 70.57-nm crystal size under pristine conditions. However, when NH₄Cl was added, the crystal size decreased to 61.59 nm. The XRD analysis indicates that the addition of MACl and pyridine enhanced the crystallinity and increased the crystal size. This may induce improved charge transport within the perovskite, thereby resulting in the observed increase in the perovskite solar cell efficiency with MACl and pyridine [15]. However, no significant enhancement was indicated in the XRD analysis results of the perovskite with NH₄Cl addition, which exhibited the highest efficiency. For further examination in this regard, we performed SEM analysis.

Figure 4. XRD data of perovskite film fabricated in ambient air without and with various additives (MACl, Pyridine, and NH₄Cl): (a) entire patterns and (b) enlargement of (100) plane.

Table III

XRD parameters and average crystal size of perovskite solar cells..

	2 θ (°)	FWHM (°)	Interplanar distance (nm)	Average crystal Size (nm)
Pristine_50 %	14.22	0.119	0.314	70.57
MACl	14.21	0.113	0.314	74.18
Pyridine	14.19	0.108	0.314	77.57
NH4Cl	14.11	0.136	0.316	61.59

To confirm the morphological properties of the perovskite induced by the additives, we obtained FE-SEM images of the perovskite film surface. Figures 5(a)–(c) show SEM images of the perovskite film with additives, with the inset showing a cross-section of the perovskite film. Figure 5(d) shows the histogram of quantitative grain size obtained from surface SEM images. As shown in Figs. 5(a)–(c), the application of various additives resulted in a uniform film without pinholes, as compared to that of the pristine film [Fig. 2(d)]. Pinholes within the perovskite film can act as defects, impeding charge transport and leading to recombination. Therefore, the formation of uniform films without pinholes may contribute to the reproducibility of the perovskite and improvement in the device performance. As evident from Fig. 5(d), the average grain sizes are 88.2, 86.3, 123.2, and 158.1 nm for pristine, MACl-, pyridine-, and NH₄Cl-added perovskites, respectively. The grain size of the perovskite treated with NH₄Cl was approximately twice that of the pristine perovskite. This significant increase in grain size can reduce the density of grain boundaries, leading to decreased charge recombination and, consequently, enhanced device performance [16]. Moreover, the reduced surface area for moisture penetration owing to fewer grain boundaries prevents moisture from reacting with the perovskite precursors [17]. This is advantageous for fabricating perovskites in humid ambient air environments.

Figure 5. (a–c) Surface and cross-sectional SEM images of perovskite films fabricated in ambient air with various additives (MACl, pyridine, and NH₄Cl). (d) Histogram of quantitative grain size from surface SEM image.

Finally, the application of additives reduced the number of pinholes in the perovskite films under ambient air conditions, leading to a high PCE. Particularly, the addition of NH₄Cl doubled the grain size, resulting in a decrease in grain boundaries. This prevents charge recombination, thereby increasing efficiency and reducing moisture penetration from ambient air. This is the reason for the observed best PCE in the perovskite devices treated with NH₄Cl additive.

Perovskite films were fabricated via additive engineering under ambient air conditions. This enhanced the perovskite morphology, enabling the fabrication of high-performance perovskite solar cells under humid conditions. Particularly, the perovskite film with NH₄Cl additive exhibited a significant reduction in pinholes as compared to that of the pristine film and nearly doubled the grain size. These results can help enhance charge transport within the device and reduce recombination, thereby increasing the device efficiency. Furthermore, the reduced grain boundaries prevent moisture in the air from reacting with the perovskite precursors, thereby enabling stable perovskite formation in ambient air. Under optimized conditions, we achieved an efficiency of 19.00 % in ambient air with 50 % RH. This study affords an approach to achieving high-quality perovskite films under ambient air conditions.

This work was supported by a 2-Year Research Grant of Pusan National University.

The authors declare no conflicts of interest.