The Influence of Oxygen Plasma on Methylammonium Lead Iodide (MAPbI3) Film Doped with Lead Cesium Triiodide (CsPbI3)

In recent years, the study of organic–inorganic halide perovskite as an optoelectronics material has been a significant line of research, and the power conversion efficiency of solar cells based on these materials has reached 25.5%. However, defects on the surface of the film are still a problem to be solved, and oxygen plasma is one of the ways to passivate surface defects. In order to avoid destroying the methylammonium lead iodide (MAPbI3), the influence of plasma powers on film was investigated and the cesium triiodide (CsPbI3) quantum dots (QDs) were doped into the film. In addition, it was found that oxygen plasma can enhance the mobility and carrier concentration of the MAPbI3 film.


Introduction
Hybrid halide perovskite (HHP) has become a promising candidate for third-generation solar cell absorption layers. Since Kojima et al. [1] first reported it in 2009, this material has shown great potential due to its excellent photovoltaic response. In the past few years, the power conversion efficiency (PCE) of solar cells based on this material has increased from 3.8 percent in 2009 [1] to 25.5 percent [2,3]. The high efficiency of lead-based perovskite solar cells can be attributed to band gaps close to the Shkreli-Quessel limit, high light absorption coefficients, excellent electric mobility and large electric diffusion lengths [4,5]. The performance of HHP solar cells is expected to be greatly improved due to their excellent optical properties. However, polycrystalline perovskite materials show a shorter photoluminescence life, indicating the existence of electric defects [6]. These defects originate from the crystal boundary inside the perovskite, which may capture free-charged carriers. In this article, references are presented that mention that trace amounts of oxygen are beneficial to methylammonium lead iodide (MAPbI 3 ) films [7,8]. Moreover, defects can be decreased in number by introducing oxygen. Additionally, oxygen plasma treatment was used to remove excess surface ligand and precursor, and the absorption and electrical performance was also improved. However, excess oxygen ions cause oxidation and ion bombardment [9], leading to a decrease in the stability and structural destruction of the film. Thus, cesium triiodide (CsPbI 3 ) quantum dots (QDs) were doped into the MAPbI 3 film to achieve anion exchange in order to make the overall structure more stable [10], and the absorption was enhanced due to the unique optical properties of the QDs. In addition, the power of the oxygen plasma was tuned to investigate its influence on the MAPbI 3 . For the investigation, X-ray photoelectron spectroscopy (XPS) was used to analyze the surface composition of the MAPbI 3 -doped CsPbI 3 QDs. Furthermore, it was found that the oxygen plasma was able to improve the carrier concentration and mobility of the MAPbI 3 . This innovative film as a potential material was optimized by oxygen plasma treatment at different powers. In this article, this work provides in-depth details to illustrate the investigation of the formation mechanism and surface properties. Figure 1 shows the transmission electron microscope (TEM) image of CsPbI 3 QDs, and the d-space of CsPbI 3 QDs is 0.30 nm. Figure 2a plots the absorbance spectra of a MAPbI 3 film without and with CsPbI 3 QDs and further optimization by means of oxygen plasma treatment at 0 to 80 W. The results show the phenomenon that the absorption of the MAPbI 3 film was enhanced from 350 to 750 nm after doping CsPbI 3 QDs, and this is a great improvement in absorption compared to the MAPbI 3 film without CsPbI 3 QDs. The reason for this is the excellent optical properties and the wide energy gap of the CsPbI 3 QDs [11,12]. Multiple studies demonstrated that the light-harvesting ability of the perovskite film could be enhanced by doping with QDs [13,14]. In addition, the surface of the perovskite films was treated with the oxygen plasma at 20 to 80 W. It can be observed that the strongest absorbance of the films was at 20 W, because the excess ligands and impurities were removed from the surface. However, the absorbance gradually decreased in the power range of 40 to 80 W due to the degradation of MAPbI 3 caused by high-power oxygen ion bombardment [15]. Figure 2b shows the photoluminescence (PL) results of the pure MAPbI 3 film and MAPbI 3 films doped with CsPbI 3 QDs with and without oxygen plasma treatment at 20 W. The PL intensity of the MAPbI 3 film doped with CsPbI 3 QDs was drastically enhanced compared to that of pure MAPbI 3 film due to the Cs ion exchange process [16]. The PL intensity of MAPbI 3 film with QDs treated with oxygen plasma was also enhanced compared to the MAPbI 3 film not treated with QDs. This is attributed to the removal of the ligands and impurities. In addition, the reason for the enhancement might be the passivation of surface defects with plasma treatment, and this was caused by the replacement of the defects [17]. It is observed that the luminescence peak of the composite perovskite film exhibits a shift towards higher wavelengths from 770.4 to 776.7 nm, caused by the doping with CsPbI 3 QDs. In addition, the sample showed a shift to lower wavelengths from 776.7 to 773.2 nm after treatment with oxygen plasma at 20 W, which may be due to the removal of excess ligands and precursors or the decrease in the number of defects on the surface [18,19]. The decrease in the number of defects resulted in the increase in the band gap, leading to the shift in the PL peak.

Results
To investigate the oxygen plasma's effect on the surface of the MAPbI 3 film at different powers, XPS was conducted, as shown in Figure 3. Figure 3a shows the Pb 4f 7/2 and 4f 5/2 core level represented by two peaks centered on 137.9 and 142.8 eV, respectively. In addition, it is found that the core levels of the MAPbI 3 film shift towards higher binding energies for 20 W curves. This result could be due to the increase in the number of oxygen ions on the surface due to the oxygen plasma treatment. Figure 3b shows the peaks of the I 3d 5/2 and 3d 3/2 core level. It can be observed that two peaks appear at 625 and 635 eV after oxygen plasma treatment, and the intensity of the two peaks was enhanced with the increase in the plasma powers. The results reveal the bonding of the iodine in the MAPbI 3 film and the oxygen ions, which causes the generation of IO 3 − . The IO 3 − improves iodine interstitial defects and results in the deep trap state being closer to the valence band [20]. Thus, the charge carrier losses can be reduced, and this means that the electric property of the MAPbI 3 film could be enhanced by the formation of IO 3 − . However, the content of iodine in the film gradually decreased with increase in IO 3 − , as shown in Figure 3b, and this phenomenon is explained by the degradation of MAPbI 3 due to the high plasma power. The XPS binding energy spectra for O 1s core levels are shown in Figure 3c. It can be seen that the spectra present two peaks. The O II peak corresponds to the chemisorbed oxygen atoms on the surface [21], and the O I peak corresponds to the bond of lead and oxygen.
The O 1s XPS spectrum shows that the intensity of the O II peak increased with the increase in plasma power, and it can be speculated that defects are replaced with oxygen. In order to perform further analysis, the core level spectra of O 1s were peak-fitted for each plasma power, as shown in Figure 3d-h. It can be observed that only O II peak appeared for the 0 and 20 W curves, and the O I peak appeared for over 40 W curves. It is conjectured that the excessive oxygen plasma power is caused the degradation of the film and the oxygen ion species bound to the lead in MAPbI 3 , as reported in other studies [22,23]. In Figure 3g,h, the intensity of the O II peak is shown to drastically increase, indicating that the MAPbI 3 was destroyed because of high power plasma. The above result is consistent with the result shown in Figure 2a.   Figure 3d-h. It can be observed that only OII peak appeared for the 0 and 20 W curves, and the OI peak appeared for over 40 W curves. It is conjectured that the excessive oxygen plasma power is caused the degradation of the film and the oxygen ion species bound to the lead in MAPbI3, as reported in other studies [22,23]. In Figure 3g-h, the intensity of the OII peak is shown to drastically increase, indicating that the MAPbI3 was destroyed because of high power plasma. The above result is consistent with the result shown in Figure 2a. In Table 1, the mobility of the MAPbI3 doped with CsPbI3 QDs was higher than that of the pure MAPbI3 film. The stability could be improved by doping with CsPbI3 QDs, and the number of defects on the film can be decreased, leading to the higher mobility [10]. For the analysis of the conduction mechanism, the mobility and the carrier concentration were measured through Hall effect measurement. Figure 4 shows the mobility and carrier concentration of different films treated with various powers of oxygen plasma from 0 to 80 W. It can be observed that the mobility was enhanced for 20 In Table 1, the mobility of the MAPbI 3 doped with CsPbI 3 QDs was higher than that of the pure MAPbI 3 film. The stability could be improved by doping with CsPbI 3 QDs, and the number of defects on the film can be decreased, leading to the higher mobility [10]. For the analysis of the conduction mechanism, the mobility and the carrier concentration were measured through Hall effect measurement. Figure 4 shows the mobility and carrier concentration of different films treated with various powers of oxygen plasma from 0 to 80 W. It can be observed that the mobility was enhanced for 20 W curves. As mentioned previously in Figure 3c, the defect was replaced by oxygen, and this leads to the improvement of mobility. Furthermore, it is shown that oxygen is a method to suppress nonradioactive recombination and to improve the photovoltaic performance [24]. The plasma treatment also caused the decrease in the grain boundary of the MAPbI 3 film, which can be seen in a previous study [10]. With the decrease in the grain boundaries, the scattering of charge carriers reduced and the mobility of the film increased [25]. However, an excessive supply of oxygen plasma with power over 20 W caused the formation of bonds between carbon, lead and oxygen. This effect leads to the degradation of MAPbI 3 [10,15], and therefore the mobility dropped from 6.06 × 10 3 to 1.08 × 10 4 cm 2 /V s . Compared to the untreated film, the carrier concentration of the treated MAPbI 3 film was also enhanced. The result was attributed to the reduction in charge carrier losses, resulting from the formation of IO 3 − , as mentioned earlier in Figure 3b. In addition, interstitial iodine defects can decrease the carrier lifetimes of MAPbI 3 [26], and the appearance of IO 3 − reveals the passivation of interstitial iodine defects [27]. The reason for the decreased carrier concentration at higher plasma powers is the same as that for the decreased the mobility. Based on the above results, the oxygen plasma power of 20 W is the better condition.  In Figure 5, the contact angle of the MAPbI 3 films doped with CsPbI 3 QDs further illustrates the effect of the oxygen plasma power and demonstrates the influence of oxygen ion bombardment. When the plasma power increased from 0 to 20 W, the contact angle of the film dropped from 91.59 degrees to 71.30 degrees. This might be due to increase in the surface energy, and the higher surface energy was caused by oxygen atoms on the surface [28]. Moreover, oxygen plasma can significantly improve surface hydrophilicity, and the increase in the hydrophilicity could be due to the carbonyl group [29,30], which might be caused by the chemisorbed oxygen atoms on the surface. However, the contact angle decreased to 81.89, 82.1 and 81.55 degrees at 40, 60 and 80 W, respectively. This means that the MAPbI 3 is destroyed and oxygen ions started to bond with lead with the increase in plasma power. In addition, the oxygen ions also bond to the carbon in MAPbI 3 , leading to the formation of CO x , and the CO x is not the carbonyl group on the surface, but the gas [31,32]. Although the higher plasma powers caused the degradation of the film and the formation of CO x , the carbonyl group still appeared on the surface. Thus, the contact angle would not change obviously at powers of 60 and 80 W.

Fabrication of Composite Perovskite Films
CH 3 NH 3 I (50 µL) and CsPbI 3 (1 mg) were mixed and then spin-coated onto a glass substrate in two steps, at 1000 rpm for 10 s and 5000 rpm for 20 s. Toluene was dropped on the spinning film for 15 s during the second step. Hereafter, the sample was annealed at 90 • C for 15 min to obtain the composite perovskite films. These composite perovskite films were further enhanced by oxygen plasma treatment at different powers from 0 to 80 W. The plasma treatment was carried out by radio frequency (RF) excitation with a power source of 13.56 MHz (Plasma Etch PC-150 plasma etching/cleaning system).

Conclusions
In this article, we report the effect of doping with CsPbI 3 QDs and oxygen plasma treatment on MAPbI 3 film. In addition, the absorbance and PL intensity was enhanced by doping with CsPbI 3 QDs and oxygen plasma treatment. According to the Hall effect measurement, the carrier concentration and mobility was improved at 20 W. However, high powers from 40 to 80 W not only caused surface damage of the films but also destroyed the electric properties. Additionally, the influence of oxygen plasma on the surface composition was investigated by means of XPS. Moreover, the electric property was discussed by Hall effect measurement, and it is found that the mobility and carrier concertation of MAPbI 3 was obviously enhanced through oxygen plasma treatment. According to XPS, the reason for the improvement in the mobility was also explained. In this study, we prove that oxygen plasma is a potential treatment for perovskite films.

Institutional Review Board Statement:
This study did not involve humans or animals.
Informed Consent Statement: This study did not involve humans.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.