The Influence of Argon Plasma on Organic Perovskite MAPbI₃ Film Doped with Inorganic Perovskite CsPbI₃ Quantum Dots (QDs)

Abstract

In this study, the inorganic perovskite cesium lead triiodide (CsPbI₃) quantum dots (QDs) produced by hot-injection method were added into the hybrid perovskite methylamine lead triiodide (CH₃NH₃PbI₃; MAPbI₃) to form composite perovskite film. It is not easy for argon (Ar) to react with perovskite. Therefore, argon plasma was used to optimize the properties of the surface. However, methylamine lead triiodide molecular will be degraded by excessive wattage. Therefore, the influence of plasma power acting on composite perovskite film was investigated. The experimental results show that the light absorption capacity can be increased by argon plasma power of 140 watt (W) acting on the surface of films because organic impurities are removed and surface morphology of film is changed.

1. Introduction

To find the main source of clean and sustainable energy on the earth, researchers have long sought to convert solar into electricity through the photovoltaic effect of light-absorbing semiconductors. As the absorber layer material of solar cells, monocrystalline silicon, polycrystalline silicon, and amorphous silicon have attracted much attention due to their high efficiency and stability. However, there are some disadvantages for silicon, such as high-temperature process and vacuum process. Based on the above statements, perovskite methylamine lead triiodide (CH₃NH₃PbI₃; MAPbI₃) has been greatly studied due to its excellent power conversion efficiency, high quantum yield, low-cost manufacturing, and significant absorbance performance [1,2,3]. Needless to say, it has been widely used in solar cells, light-emitting diodes (LED), and photodetectors [4]. Nevertheless, MAPbI₃ is easily degraded into PbI₂ under long-term exposure to moisture, heat, and air, and it will affect the efficiency of the perovskite solar cell [5,6]. Thus, improving the properties of hybrid perovskite MAPbI₃ films has been the main direction of research in recent years. Some literature has reported that doping can improve the quality of the film. For example, Yating Zhang et al., used doped graphene oxide to enhance the photoelectric conversion efficiency of MAPbI₃ [7]; Feng Hao et al., used doped Cl-HOBT to improve the crystallinity and reduce the trap density of MAPbI₃ [8]. The addition of inorganic perovskite cesium lead iodide (CsPbI₃) quantum dots (QDs) to organic MAPbI₃ during the production process is a potential way to improve perovskite film, due to the interaction of organic and inorganic functional groups decreasing the degradation of perovskite films [9]. Furthermore, the surface morphology of the perovskite film is also a key factor that affects the performance of the perovskite solar cell; surface modification techniques have been tested to optimize the surface properties of the perovskite film—including vacuum flash-assisted [10], gas-assisted [11], and plasma-assisted solution processes [12]—because MAPbI₃ is easily degraded into PbI₂ in high-temperature conditions. Therefore, plasma-assisted solution processes that do not require a high-temperature environment are the first choice. There are different gases used in plasma treatment, such as oxygen [9], nitrogen [13] and argon [14]. However, both oxygen and nitrogen will undergo a chemical reaction with perovskite.

Thus, argon plasma treatment (APT) is the preferred method to remove organic contaminants from the surface of the film and to optimize the surface morphology. In addition, there are many studies which show that small surface holes are beneficial to the film [15]. However, excess argon ion bombardment will lead to degraded MAPbI₃ to PbI₂, so the wattage of APT is an important factor. In this article, we describe composite perovskite films based on doping of CsPbI₃ QDs into MAPbI₃. This innovative film, proposed as a potential material, is optimized by APT at different powers, and its formation mechanism and structural properties are presented.

2. Materials and Methods

2.1. Materials

All materials contain cesium carbonate (Cs₂CO₃, 99.9%), lead (II) iodide (PbI₂, 99.9985%), oleic acid (C₁₈H₃₄O₂, analytical reagent 90%), oleyl amine (C₁₈H₃₅NH₂, 90%), 1-octadecene (ODE, technical grade 90%), toluene (anhydrous, 99.8%), hexane (analytical reagent, 97%), methyl acetate (MeOAc, anhydrous 99.5%), methylammonium iodide (CH₃NH₃I, 99%), dimethyl sulfoxide ((CH₃)₂SO, 99%), and gamma-butyrolactone (C₄H₆O₂, 99.9%), as shown in

Table 1. Preparation parameters of CH₃NH₃PbI₃, Cs-oleate precursor, and CsPbI₃ QDs.

Synthesis of CH3NH3PbI3	Cs-Oleate Precursor	Synthesis of CsPbI3 QDs
CH3NH3I 198.75 mg	Cs2CO3 0.1 g	PbI2 0.173 g
GBL(C4H6O2) 0.5 mL	Oleic acid 0.5 mL	1-octadecene 10 mL
PbI2 576.25 mg	1-octadecene 10 mL	Oleic acid 1 mL
DMSO(C2H6OS) 0.5 mL		Oleyamine 1 mL
		Cs-oleate precursor 1.6 mL

. All chemicals in this work were used without further treatment.

2.2. Solution Preparation and Synthesis for Cs-Oleate Precursor, CsPbI₃ QDs, and CH₃NH₃PbI₃

All the experiments were performed in a glove box filled with nitrogen, H₂O < 1 ppm and O₂ < 1 ppm. The schematic diagram of the experiment is shown in Figure 1.

2.3. Synthesis Process of CH₃NH₃PbI₃

CH₃NH₃I and PbI₂ were mixed into the 50 mL sample bottle, then DMSO and GBL were added into the mixture powder in nitrogen atmosphere and stirred at 300 rpm for 24 h.

2.4. Synthesis Process of Cs-Oleate Precursor

Cs₂CO₃, OA, and ODE were poured into a 50 mL sample bottle and stirred for 1 h at 120 °C. Then, the Cs-oleate precursor was stored at 100 °C to avoid precipitation.

2.5. Synthesis Process of CsPbI₃ QDs

Both 1-octadecene and PbI₂ were added into a 50 mL sample bottle and were dried at 120 °C for 1 h. Then, oleic acid and oleyamine were poured. The Cs-oleate precursor was swiftly injected into the solution when the solution was heated to 185 °C. After 5 s, the reaction solution was cooled by ice bath.

2.6. Purification Process of CsPbI₃ QDs

MeOAc (the volume ratio of crude solution/Meoac is 1:3) was poured in CsPbI₃ crude solution to separate CsPbI₃ QDs before centrifuge at 8000 rpm for 5 min. The supernatant solution was removed, and the precipitate was dissolved in 3 mL of hexane. Then, MeOAc (the volume ratio of crude solution/Meoac is 1:1) was added to precipitate CsPbI₃ QDs again and centrifuge at 8000 rpm for 2 min. Finally, the QDs were dispersed in 3 mL of hexane and centrifuged at 4000 rpm for 5 min to remove excess PbI₂ and precursors. CsPbI₃ QDs can be separated after centrifugation to remove the transparent liquid.

2.7. Fabrication of Thin Films

MAPbI₃ (50 μL) and CsPbI₃ QDs (1 mg) were mixed and spin-coated on a glass substrate in a glove box. The composite perovskite film was annealed at 80 °C for 15 min, then the films were treated by APT for different powers for 2 s, as shown in

Table 2. Types of film and power range of argon plasma.

Film	Power
MAPbI3	0 W
MAPbI3 with QDs	0 W
MAPbI3 with QDs	100 W
MAPbI3 with QDs	120 W
MAPbI3 with QDs	140 W
MAPbI3 with QDs	160 W
MAPbI3 with QDs	180 W
MAPbI3 with QDs	200 W

.

2.8. Characteristic Measurements

The absorption spectrum of the thin film is measured by ultraviolet/visible (UV/vis) absorption spectroscopy (HITACHI, U-3900, Tokyo, Japan). The crystalline of the thin film is measured by grazing incidence X-ray diffraction (GIXRD; Bruker, D8 Discover, Billerica, MA, USA). The surface roughness of films was measured by atomic force microscopy (AFM; Park Systems, XE-70, Suwon, South Korea). The surface morphology of the thin film is measured by scanning electron microscope (SEM; JEOL, 6330, Tokyo, Japan).

3. Results and Discussion

Figure 2a shows that the absorbance spectrum of the perovskite film with and without CsPbI₃ QDs in the wavelength range from 350 to 850 nm.

It can be observed that the absorbance area of the composite perovskite film is larger than that of the pure MAPbI3 film. The reason is that CsPbI₃ QDs enhance the absorbance of MAPbI₃ at short wavelengths. The α-CsPbI₃ QDs is a wide energy gap material, so α-CsPbI₃ QDs have stronger absorbance at short wavelengths [16,17].

The surface of composite perovskite films is further optimized via APT at different powers, from 100 to 200 W, as shown in Figure 2b. Absorbance of composite perovskite film is enhanced after the composite perovskite film is treated by APT at 100 W to 140 W. The reason was that APT causes an increase for the roughness of composite perovskite films and a decrease for the band-gap of composite perovskite films.

However, the absorbance will not gradually increase as the wattage increases. When the plasma power is over 160 W, the surface of MAPbI₃ film was degraded to PbI₂, resulting in a small amount of macula appearing on the surface of the composite perovskite film [9].

To discuss the reasons for the increasing of the absorbance area, tauc plots were obtained from absorption data, as shown in Figure 3.

The band gap of composite perovskite materials was determined by the tauc’s relation as below.

$$\alpha hv=C{\left(hv-Eg\right)}^n$$

(1)

where α is the absorption coefficient, Eg is the band gap, hν is the incident photon energy, C is a constant, and n is the index, which can have different values (2 and 1/2) corresponding to direct allowed transitions and indirect allowed transitions [18,19,20,21].

The band gap of pure MAPbI₃ and composite perovskite films are 1.57 eV and 1.56 eV. When composite perovskite films were treated by APT, the band gap is further reduced to 1.52 eV.

The argon plasma removes the oxygen contamination from the surface of the composite perovskite film and changes the values of the valence band and the conduction band.

When composite perovskite films are treated by APT, the valence band will enlarge and the conduction band will shrink, and the film has a lower band gap which is more conducive to capturing sunlight [12,18]. Furthermore, the report also proved that the lower band gap has better photoelectric properties [19,20].

Roughness of composite perovskite film is increased with increasing argon plasma, as shown in Figure 4. The bombardment of the APT caused fine holes on the surface of the composite perovskite film, and the absorbance of the film could be enhanced by these holes. The reason is that the incident beam is mostly scattered, and the absorption is performed again at the holes to enhance the intensity of harvesting [15].

To investigate the argon plasma effect on the crystal phase of the composite film, the X-ray diffraction (XRD) was measured. Figure 5 shows that the XRD pattern of the composite perovskite films were treated by argon plasma at different powers.

Inorganic CsPbI₃ QDs can further reduce the degradation of MAPbI₃ [21]. The reason is interactions with organic and inorganic functional groups stabilize the perovskite structure [9].

However, the peaks of PbI₂ can be observed for 180 W and 200 W curves, indicating that the power of 180 W and 200 W is too high, leading to the degradation of MAPbI₃, and PbI₂ will affect the efficiency of the perovskite film. Furthermore, there is no peak of metal lead at 31.5° and 36.3°, which means that the plasma treatment of 100 W to 200 W only affects the surface of the perovskite film and does not affect the entire perovskite film [14].

Figure 6 shows the XRD peak fitting patterns of composite perovskite treated by argon plasma at different powers. The peaks are represented in the (100) orientation of CsPbI₃ at 13.97°, the (110) orientation of Pb₃O₄ at 14.05°, and the (110) orientation of MAPbI₃ at 14.16°, respectively [22,23]. The presence of the oxygen pollutant Pb₃O₄ can be observed in Figure 6a. The reason is that oxygen can interact with one or two photoexcited electrons and form superoxide and peroxide. These highly oxidative species react with the inorganic lattice and destroy the Pb-I octahedra by forming Pb-O bonds. The Pb₃O₄ content is gradually reduced when using APT on composite perovskite, as shown in Figure 6b,c. The Pb₃O₄ content is completely removed when APT wattage is increased to 140 W, as shown in Figure 6d. This proves that argon plasma is an effective method to remove oxygen contaminants, preventing formation of superoxide and peroxide, and improving film stability and charge carrier life [24,25].

Figure 7 shows scanning electron microscope (SEM) images of films treated with APT at different powers to further analyze their crystalline growth. In Figure 7a, the initial crystal islands and obvious grain boundaries can be observed. As shown in Figure 7b,c, when the power increased to 100 W and 120 W, the bombardment of the argon plasma caused fine holes on the surface of the film, and these holes can increase the light absorption area of the film as previously mentioned in Figure 4. The cause is that the incident beam is scattered roughly, thus increasing the absorption of the sample [14]. In Figure 7d,e, branch-shaped nicks began to appear around the crystal island, and the nicks of the branch-shaped became more obvious as the plasma power increased. As shown in Figure 7f,g, when the wattage continues to increase, a small amount of flaky debris appears around the crystal island. These fragments can be speculated to be PbI₂ [26], but the MAPbI₃ islands can still maintain their form. This means that the plasma treatment of 100 W to 200 W will only affect the surface of the perovskite film, which is consistent with the XRD measurement results.

4. Conclusions

In this research, the absorbance of composite perovskite film is increased by adding CsPbI₃ QDs and APT. In the XRD observation, it was found that there are PbI₂ peaks at 180 W and 200 W, indicating that the high plasma wattage causes degradation of MAPbI₃. It is observed from SEM that high wattage will damage the surface, so the better parameter is 140 W. From the above analysis, the improved composite film is more stable. Following surface treatment by argon plasma, the light absorption is further improved, which is a good development for perovskite solar cells.