4.3 Thermal Endurance of Caffeine Boosted MAPbI 3 Film
4.3.3 SEM Analysis of C-MAPbI 3 Film
Figure 4.8 shows the top and cross-sectional views of the C-MAPbI3
film. It can be seen from Figure 4.8(a) that the control film (heated at 100 °C for 10 min) composed of many small individual grains, which is similar to the P-MAPbI control sample (Figure 4.3(a)). Cross-sectional SEM analysis was
10 15 20 25 30 35 40 45 50
Normalized Intensity
2θ (°)
(314)
(224)
(220)
Control 1 h 2 h 3 h 4 h 5 h
(110) (222)
PbI2 (001)
MAPbI3
(112) (211) (202) (312)
conducted to gain better insight into the aspect ratio of the grains. The aspect ratio is defined by the size of individual grains to the thickness of the film. In agreement to the top SEM view, the cross-sectional view (Figure 4.8(b)) shows that the ~ 380 nm thick C-MAPbI3 film is composed of multi-layered small aspect ratio (<1) grains. As such, there are GBs spanning across the horizontal and vertical directions of the film. This small aspect ratio of grains was also observed on (FAPbI3)1−x(MAPbBr3)x film by M. Kim et al. (2017) and MAPbI3
film by Huang et al. (2018), both of the films were 100 °C annealed for 10 min.
Figure 4.8: (a) Top and (b) cross sectional SEM images of C-MAPbI3
control sample.
Figure 4.9 shows the morphology of the 1 h C-MAPbI3 film. From the top view images, it can be seen that the grain sizes increased drastically from nanometres for control sample (Figure 4.8) to micrometres for 1 h sample (Figure 4.9), which is similar to the large individual grains from 2 h P-MAPbI3
(Figure 4.3(c–d)). However, unlike the 1 h and 2 h P-MAPbI3 samples that showed clear evidence of dissolution into small crystal as part of Ostwald recrystallization process, the current 1 h C-MAPbI3 sample did not contain any small grains clusters, only large grains of MAPbI3. According to Huang et al.
(2018), the Ostwald recrystallization process should initiated with dissolution
(a) (b)
of small grains and later formation of large grains to reduce the surface energy.
Based on the aforementioned C-MAPbI3 XRD result (Figure 4.7) that showed clear retention of other MAPbI3 peaks, especially the (222) plane at 31.8 o, the lack of small grain clusters suggested the caffeine addition may skipped the dissolution process and straight towards Ostwald recrystallization induced grain growth. The increase in grain size was significant that most of their GBs, as shown in Figure 4.9(b), were perpendicular to the substrate. Such observation is similar to Huang et al. (2018), in which 130 °C annealed sample for 10 min yielded GBs that were mostly of perpendicular to the substrate.
Figure 4.9: (a) Top and (b) cross-sectional SEM images of 1 h C-MAPbI3
film.
For 2 h C-MAPbI3 sample (Figure 4.10(a)), the grain size increased substantially when compared to 1 h counterpart (Figure 4.9) as the recrystallization process continued. As a result, the aspect ratio of grains was larger than 1, and resulted grains with vertical GBs (Figure 4.10(b)). In fact, Figure 4.10(c) confirms the large aspect ratio (>1) of the grains sticking to the PMMA layer. M. Kim et al. (2017) also demonstrated vertical GBs in their (FAPbI3)1−x(MAPbBr3)x films annealed at 300 °C and 400 °C for just 8 and 4 s, respectively. At 150 °C, it would require longer annealing time of 2 h to achieve
(b) (a)
the same effect as their 300 °C and 400 °C annealed samples. No PbI2 needles observed for both the 1 h and 2 h samples, which is supported by the lack of PbI2 peak in XRD analysis (Figure 4.7).
Figure 4.10: SEM images of 2 h C-MAPbI3 film: (a) Top, (b) cross-section and (c) lower magnification cross-section showing large aspect ratio grains stuck to the PMMA layer peeled upward.
Figure 4.11 shows the morphology of 3 h C-MAPbI3 sample.
Interestingly, although XRD pattern (Figure 4.7) showed no PbI2 peak at 12.6 o, SEM analysis shown in Figure 4.11 revealed precipitation of PbI2 grains on the MAPbI3 grains and at GBs, which is similar to the 3 h P-MAPbI3 sample (Figure
(b) (a)
(c)
4.4). The reason that XRD was unable to detect PbI2 peak may be due to the caffeine passivation effect in reducing the amount of PbI2 precipitation to be below the detection limit of the equipment. Additionally, this suggests that the PbI2 precipitations are independent to the presence of caffeine additive and are time dependent when it is annealed at 150 °C, strictly under encapsulation by PMMA. Despite that, the grain aspect ratio maintained at more than 1, and is clearly supported by the vertical GBs shown in cross-sectional SEM images (Figure 4.11(c–d)).
Figure 4.11: SEM images of 3 h C-MAPbI3 film. (a) Higher (30 k×) and (b) lower (15 k×) magnification top view. (c) Cross-sectional view and (d) grains stuck to PMMA layer that was rolled upward during the cross- section fracturing process.
The increased in severity of decomposition was confirmed by continuously heating the film to 4 h, whereby the large grains decomposed into smaller grains with more PbI2 grains precipitated, as shown in Figure 4.12(a).
Additionally, the cross-sectional morphology of the 4 h sample (Figure 4.12(b))
(a) (b)
(d) (c)
further supports the top view finding by revealing two new characteristics, which are: (i) the loss of vertical GBs, which was replaced by disorientated GBs and (ii) the formation of poor conducting, bright PbI2 spots on some of the grains, pointed by the red arrows. These bright spots were also observed by Meng et al.
(2020) in their (FAPbI3)1–xMAPb(Br3–yCly)x films that were thermally stressed at 150, 200 and 205 °C for 70 min, to which they referred to as “white phase”
PbI2 rich region. These PbI2 needles were indeed supported by the presence of PbI2 XRD peak (Figure 4.7).
Figure 4.12: SEM images of 4 h C-MAPbI3 film. (a) Top view, (b) cross- sectional view, with red arrows representing bright PbI2 spots on the MAPbI3 grains and red circle representing disorientated GBs.
Subsequently, the 5 h sample undergone further morphological changes.
The large MAPbI3 grains from 1–3 h samples no longer present and was replaced by more smaller grains resulted from the thermal induced decomposition, as shown in Figure 4.13(a). A more severe changes in morphology can be found in cross-sectional SEM image from Figure 4.13(b), whereby the vertical GBs of MAPbI3 grains were completely destroyed.
Interestingly, the XRD pattern of 5 h sample (Figure 4.7) showed only little PbI2
peak present in relative to the (110), (220) and (222) dominant MAPbI3 crystals.
This may be due to changes in morphology is more sensitive than phase changes,
(a) (b)
Disorientated GBs
whereby these disorientated grains can individually still be MAPbI3. The morphology of films under different annealing durations is summarized in Table 4.3.
Figure 4.13: (a) Top and (b) cross-sectional SEM images of 5 h C-MAPbI3
film.
Table 4.3: Summary of SEM analysis on C-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. Cross-sectional SEM view reveals film comprised of many small grains resulted in horizontal and vertical GBs in the cross-section.
1 h Grain enlargement due to Ostwald recrystallization. Unlike P- MAPbI3 film, there are no small grain cluster, suggesting caffeine skipped dissolution process and straight to grain enlargement. Film comprised of columnar grains (vertical GBs).
2 h Further grain enlargement under continuous Ostwald recrystallization. Film still comprised of columnar grains (vertical GBs).
(a) (b)
Table 4.3: Continued.
3 h Further grain enlargement occurred. However, sign of thermal degradation began, as indicated by the formation of PbI2 needles.
Film still comprised of columnar grains (vertical GBs).
4 h Degradation continued, with larger grains started to decompose into smaller grains. Columnar grains from 3 h film started to distort as some GBs no longer remained vertical.
5 h More smaller grains formed due to continuous thermal degradation. Cross-sectional SEM image reveals columnar grains disappeared, with significant distortion to the morphology of the grains.