4.2 Thermal Endurance of P-MAPbI 3 Film
4.2.3 SEM Analysis of P-MAPbI 3 Film
Figure 4.3(a) shows the morphology of MAPbI3 control film prepared by annealing at 100 °C for 10 min. It can be observed that the individual grains of MAPbI3 can hardly be identified by the thermionic SEM as they are too small.
Measurement done on those grains that are barely visible showed that the film has average grain size of 213 nm, which is the typical size obtained from antisolvent assisted spin-coating (Huang et al., 2018; Jeon et al., 2018; Lan et al., 2020; H. Li et al., 2019; Zhang et al., 2017). When the film was annealed at 150 °C for 1 h, GBs was significantly noticeable under the SEM (Figure 4.3(b)).
Cracks and voids are also observed around the grains.
The enlargement in grain size and crack formation with increased annealing temperature was also reported by Huang et al. (2018), attributing Ostwald recrystallization for such phenomena. They suggested that the small sized MAPbI3 crystals dissolved at elevated temperature (>100 °C), and subsequently formed larger crystal to reduce surface energy. However, due to their short annealing time of 10 min and maximum temperature capped at 140 °C, the degree of voids and crack formation of their samples were not as severe as this work. Indeed, the reduction of XRD peak intensity of (222) plane (Figure 4.2) from the control sample to the 1 h sample supported this suggestion.
There are also pin-holes spotted in 1 h sample, which may be caused by moisture in the ambient air process during annealing. While large MAPbI3 islands are clearly observed by distinguishing the GBs, there are also small grains inside the islands, suggesting the dissolution and merging of the crystal were still in the process.
Interestingly, by prolonging the annealing time to 2 h, distinctive individual large MAPbI3 grains and small grain clusters were observed, with a transitioning zones highlighted by red arrows between these two extremes (Figure 4.3(c–d)). This further strengthen the suggestion of Ostwald ripening phenomenon was taking place (Huang et al., 2018). In fact, the Ostwald ripening sealed or shrunken some of the pin-holes in 2 h sample, as shown by the red circles in Figure 4.3(c–d).
Figure 4.3: SEM images of (a) control P-MAPbI3 film, (b) 1 h sample, (c) and (d) 2 h sample with red arrows and circles highlighting transitioning zones and sealed pin-holes, respectively.
Figure 4.4 shows the SEM images of 3 h sample. Unlike 2 h sample, the 3 h sample is lack of small MAPbI3 grain clusters but composed of majority larger individual MAPbI3 grains. This indicates that Ostwald ripening was still taking place from 2 h to 3 h of annealing, with majority of the small grains observed from 2 h sample consumed into formation of larger grains. Although
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the film demonstrated strong thermal endurance for the first three hours according to Figure 4.3 and Figure 4.4(a), it is not immune to thermal induced degradation. As shown in Figure 4.4(b), rice-shaped grains started to precipitate out from MAPbI3 grains in a region of the 3 h sample. Considering that PbI2
peak from XRD analysis (Figure 4.2) was observed in the 3 h sample, it is likely that these rice-shaped grains are the PbI2 grains. Shan et al. (2019) reported the precipitation of PbI2 grains at GBs of MAPbI3 grains when the film was annealed unencapsulated at 145 °Cin ambient air with relative humidity of ~ 45% in just 1 h. Conings et al. (2015) also demonstrated that MAPbI3 film heated at 85 °C in N2 or O2 only atmosphere for 24 h showed needle-like PbI2
grains growing out on top of the MAPbI3 film. While their XRD analysis was not able to detect the presence of PbI2 needles, this work was able to detect the presence of PbI2 peak at 12.6 ° (Figure 4.2). The peak detection may be due to annealing at high temperature of 150 °C that promoted the formation of PbI2
grains to the amount above the detection limit of XRD.
Figure 4.4(c) illustrates the SEM image of 4 h sample. The rice-like PbI2
grains density on top of the MAPbI3 grains has increased. This was also accompanied by the formation of new pin-holes. Unlike the 2 h sample that demonstrated pin-holes sealed by Ostwald recrystallization, these new pin-holes were resulted from degradation of MAPbI3 since there was no sign of sealing on the pin-holes. In addition, the large MAPbI3 grain features from 2 h and 3 h samples disappeared, which may also be linked to the degradation of the grains.
The 5 h sample (Figure 4.4(d)) undergone drastic changes, whereby the PbI2 grains previously formed on top of the degrading MAPbI3 grew into larger grains, obscuring the difference between MAPbI3 and PbI2 grains. Voids around the GBs was also observed due to the loss of methylammonium iodide (MAI) by decomposition altering the film morphology.There are also regions with many small grains lacking distinct GBs, showing “fused” grains resulted from the loss of MAI, as shown in Figure 4.4 (e).
Such fusing at GBs was also reported by Li et al. (2017) who annealed their film at 85 °C for 24 h in perovskite solar cell structure. Their fusing phenomenon was initiated by the migration and reaction of MAI to silver counter electrode at elevated temperature. Different from them, this work observed fusing phenomenon without the presence of silver electrode, suggesting that some MAI decomposed during prolonged extreme annealing temperature. The regions where MAI were decomposed from the original MAPbI3 grains were replaced by PbI2 grains that fused the remaining MAPbI3
grains. As a result, PbI2 presence was significant and is reflected in the 5 h XRD analysis (Figure 4.2), accompanied by the observable bleaching of the dark- brown colour of the film (Figure 4.1f). The morphology of P-MAPbI3 films under different annealing durations is summarized in Table 4.1.
Figure 4.4: SEM images of (a) 3 h sample, (b) 3 h sample (higher magnification) with red circle highlighting evolution of PbI2 grains precipitated on top of the large MAPbI3 grain, (c) 4 h sample, (d) 5 h sample, (e) other parts of 5 h sample showing GBs fusing phenomenon.
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Table 4.1: Summary of SEM analysis on P-MAPbI3 films with different annealing durations.
Sample Description
Control Film comprised of many small individual MAPbI3 grains that is similar to those prepared from antisolvent washing.
1 h Grain enlargement began due to Ostwald Recrystallization.
2 h Grains enlarged significantly under continuous Ostwald recrystallization, film morphology comprised of large grains surrounded by small grain clusters, with transitioning zone between these two extremes.
3 h Grains further enlarged under continuous Ostwald recrystallization, the film comprised of mostly large grains, while the small grain cluster from 2 h film consumed for grain enlargement. Thermal degradation begins due to formation of PbI2 needles.
4 h Large grains started to decompose into smaller grains due to thermal degradation, accompanied by formation of pin-holes and more PbI2 needles.
5 h Pin-holes and voids formed around remaining MAPbI3 grains as thermal degradation continues, suggesting volatile MAI escaping the MAPbI3 film under prolonged heating at 150 °C.