MAPbI 3 Thermal Stability at 150 °C - thermal stability enhancement of

Since those aforementioned low-temperature processed metal oxides in Section 2.5 have ≤150 °C processing temperature, it is important to discuss the thermal stability of MAPbI3 at this temperature. Few works have investigated the thermal stability of MAPbI3 film at 150 °C.

The MAPbI3 degradation at extreme temperatures was not new. Earlier work by Chen et al. (2014) exploited this phenomenon to their solar cell advantages. In their work, they annealed their MAPbI3 films at 150 °C inside the glovebox with a duration of up to 90 min. It was found that annealing at this temperature has a positive effect on the film’s subsequent solar cell efficiency at optimum annealing time. Specifically, at 60 min there is a significant improvement from 0.66% of the unannealed device and 6.64% of the 30 min annealed device, to 12.00% for the 60 min annealed device. Due to the high annealing temperature, the MAI at the grain boundaries of MAPbI3 grains decomposed and escaped, leaving behind PbI2 formed around the GBs. The presence of PbI2 was verified in their XRD result (Figure 2.23(a)). It was this PbI2 formation at interfaces between CTLs and MAPbI3 that passivated the defect sites at GBs and at the charge transport layers|MAPbI3 interfaces (Figure 2.23(b)). This resulted in less charge recombination losses at the GBs, as reflected in longer charges lifetime at 101.3 ns vs 30 min annealed sample’s 91 ns in their photoluminescence analysis. However, further annealing reduced the PCE, dropping the PCE to 10.59% for 90 min and 7.17% for 120 min annealed films, respectively. This was due to the excessive amount of poor conducting PbI2 formation in the film, as reflected in Figure 2.23(a) for 90 min annealed sample. This study demonstrated that MAPbI3 lack of thermal stability at 150 °C can be exploited to achieve controlled self-passivation to reduce recombination losses at GBs’ interfaces.

Figure 2.23: (a) XRD patterns of MAPbI3 annealed at 150°C from 0 to 90 min. (b) Illustration of self-passivation by PbI2 by intentionally thermally degraded MAPbI3 at GBs at 150 °C (Chen et al., 2014).

Eperon et al. (2014) briefly compared the thermal stability of MAPbI3

film against FAPbI3 film at 150 °C under an ambient environment. As shown in Figure 2.24, the FAPbI3 film showed superior thermal stability, demonstrating colour retention over the 60 min annealing time. In contrast, the MAPbI3 film decomposed rapidly to PbI2, showing significant bleaching of colour at 30 min and full conversion to yellow PbI2 at 60 min. This highlighted the instability of using MAI, and suggested future work to substitute MAI with formamidinium (FAI) for a better device operational stability.

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Figure 2.24: Thermal degradation of MAPbI3 vs FAPbI3 films at 150 °C in ambient air over 60 min (Eperon et al., 2014).

H. Yang et al. (2017) compared thermal stability between formamidinium (FA) cations enhanced MA0.7FA0.3Pb(I0.9Br0.1)3 and MAPbI3

films at 150 °C. Their thermal stability test was conducted inside N2 glovebox with oxygen and water concentration less than 10 ppm and 1 ppm, respectively.

As shown in Figure 2.25, the MAPbI3 film showed signs of thermal degradation at 50 min and fully degraded into yellow PbI2 at 80 min. Whereas the FA enhanced MA0.7FA0.3Pb(I0.9Br0.1)3 film retained its initial brown colour throughout the 80 min duration. While colour retention indicates better thermal endurance at 150 °C, their UV-Vis and XRD analyses on the stressed

MA0.7FA0.3Pb(I0.9Br0.1)3 revealed that the FA enhanced film was not entirely immune to thermal degradation at this temperature. At the first 20 min, both of the films showed a similar UV-Vis absorbance to their initial condition as seen in Figure 2.25(b). At 60 min and above, serious bleaching (decreased absorbance) of the MAPbI3 films in the wavelengths from 750 nm to 450 nm was observed. As for FA enhanced film, there was still a significant MAPbI3

characteristic band edge at 750 nm (Figure 2.25(c)). At 80 min, the MAPbI3

band edge at 750 nm disappeared. In contrast, the FA enhanced MA0.7FA0.3Pb(I0.9Br0.1)3 film still retained a small amount of perovskite characteristic band after 80 min. A similar trend was reported in their XRD analysis (Figure 2.25 (d)). The perovskite phase at 14.1 ° disappeared completely after 80 min of thermal stressing, and only left PbI2 peak as thermal decomposition by-product (12.6 °). In contrast, the FA enhanced perovskite film still retained small amount of perovskite phase even after 80 min (Figure 2.25 (e)). This illustrated the intrinsic thermal instability of MAPbI3 at this temperature.

Figure 2.25: Physical appearance of (a) MA0.7FA0.3Pb(I0.9Br0.1)3 and MAPbI3 films on glass slides when heated at 150 °C throughout 80 min.

UV-Vis spectra of (b) MAPbI3 and (c) MA0.7FA0.3Pb(I0.9Br0.1)3 films. XRD patterns of (d) MAPbI3 and (e) MA0.7FA0.3Pb(I0.9Br0.1)3 films (H. Yang et al., 2017).

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Song et al. (2015) reported MAI experiencing weight loss at high temperatures. At 150 °C, the rate of weight loss was 0.002% per minute according to their thermal gravimetric analysis (Figure 2.26). Hence, the decomposition of MAPbI3 to PbI2 due to MAI escaping the film was unavoidable at 150 °C. However, they also observed the annealing time required to achieve MAPbI3 pure phase increased with the concentration of MAI. When the concentration of MAI is higher than PbI2 during precursor preparation, the resulting film was comprised of three-dimensional MAPbI3 grains surrounded by excess MA cations forming low-dimensional perovskites. These low- dimensional perovskites may be removed by annealing at a temperature above 150 °C to achieve a pure MAPbI3 phase. Hence, if the low-temperature metal oxides discussed in Section 2.5 are to be annealed on top of the MAPbI3 layer, one may consider to slightly increase the MAI concentration relative to PbI2 to offset the MAI loss during the metal oxide annealing process.

Figure 2.26: Weight loss of MAI at high temperature (150 °C) (Song et al., 2015).

Yu et al. (2017) also reported rapid decomposition of MAPbI3 at 150 °C.

Through in situ diffuse-reflectance infrared Fourier transform infrared (FTIR) spectroscopy characterization, they were able to observe the reduction of N-H bending modes intensity (~1469 cm⁻¹) throughout the thermal stressing duration (Figure 2.27). This reduction of N-H bending modes allowed them to calculate the thermal degradation rate in the N2 environment at various temperatures.

Below 100 °C, the degradation rate was insignificant and was ≤0.8 ± 0.2% h⁻¹. However, the degradation rate increased significantly to 6±0.5% h⁻¹ when the temperature was increased to 120 °C. At 150 °C, it became more severe with a degradation rate reaching 50% h⁻¹. They also reported the activation energy for decomposition to be ~120 kJ/mol in an N2 atmosphere. It can be further reduced to just ~50 kJ/mol under simultaneous O2 atmosphere exposure and illumination due to potential deprotonation reaction between reduced O2 and MA⁺ cation.

This highlights the rapid degradation nature of MAPbI3 film at 150 °C, especially under ambient air atmosphere due to the reduced activation energy.

Figure 2.27: Reduction in N-H peak over thermal stressing duration at various temperatures (Yu et al., 2017).

Using dimethyl sulfoxide (DMSO) as solvent vapor, Eze and Mori (2016) solvent annealed their MAPbI3 film at 150 °C. Since DMSO is solvent for MAPbI3 precursor, the DMSO vapor in the annealing chamber can react with the small grains of as prepared MAPbI3film (Figure 2.28(a)), depinning them and reforming larger MAPbI3 grains with reduced GBs (Figure 2.28(b)). This improved the performance of the PSC. The reduced number of GBs ensure charges encounter less GB, hence less energy loss at overcoming GBs. This significantly increased the efficiency of the PSC, yielding a champion PCE of 16.77%. In contrast, the as-prepared film only yielded an inferior 10.90% PCE due to efficiency loss at GBs. While 10 min of solvent annealing using DMSO at 150 °C proven to be beneficial, prolonging the annealing time decreased the PCE. When the annealing duration was increased to 20 min, the MAPbI3 film started to experience thermal decomposition and can be observed by the formation of pin-holes on the film (Figure 2.28(c)). As a result, the PCE dropped to 14.57%. More pin-holes formed at 30 min annealing duration (Figure 2.28(d)), which further reduced the PCE to just 9.59%. Hence, when annealing the MAPbI3 film at this temperature, precaution must be taken by not prolonging the solvent annealing duration.

Figure 2.28: (a) As prepared MAPbI3 film. (b) 10, (c) 20, and (d) 30 min DMSO solvent annealed MAPbI3 films. The red circles highlight the pin- holes formation as the MAPbI3 film thermally degrade (Eze and Mori, 2016).

Surprisingly, few works showed MAPbI3 can be quite stable at this temperature. Boyd et al. (2018) fabricated inverted MAPbI3 PSC with the structure of ITO|NiOx|MAPbI3|PCBM|SnOx|ITO|Ag (Figure 2.29). The PCBM was deposited either by thermal evaporation or by spin-coating, the SnOx was deposited using ADL, and ITO was deposited using sputtering. They observed when the PCBM was deposited using spin-coating, and when the ITO was deposited on top and the edge of the MAPbI3 layer, the thermal stability at 150 °C drastically improved. The spin-coated PCBM thoroughly filled the valleys of MAPbI3 GBs as opposed to only conformal coating via thermal evaporation. This thoroughly passivated the GBs at the valleys, blocking Ag diffusion into the MAPbI3 layer via this site. While sealing ITO on the top prevented the diffusion of Ag into the perovskite layer and iodine from the

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perovskite layer to the Ag counter electrode, the edge sealing by sputtered ITO was also important as it prevented the escape of volatile MAI via the edge, If left unsealed, the MAI can still escape from the edge and attack the top side of the Ag counter electrode, causing mutual degradation at both the MAPbI3 layer and the Ag counter electrode. These two factors yielded remarkably stable MAPbI3 PSC, as it does not degrade into yellow PbI2 even after 4 h of thermal stressing, as shown in Figure 2.29(d). In contrast, other configurations with either evaporated PCBM or top ITO sealing only or both (Figure 2.29(a–c)) degraded rapidly into yellow PbI2 under the same duration.

MAPbI3 thermal stability enhancement at 150 °C was also reported by Chaudhary et al. (2020). They compared thermal stability of pristine MAPbI3

film and 1-naphthylmethylammoinium iodide (NMAI) added MAPbI3 film (NMA)2(MA)n−1PbnI3n +1 (n represents the amount of MAI). Similar to findings from Eperon et al. (2014) and H. Yang et al. (2017), their pristine MAPbI3 film (n=∞) degraded rapidly to almost fully yellow PbI2 in 90 min (Figure 2.30(a)), supported by their UV-Vis analysis that revealed flattening of MAPbI3 and evolution of PbI2 characteristic band edges located at 750 nm and 500 nm (Figure 2.30(b)), respectively. Upon addition of NMAI to the film (n=40 or 60), the (NMA)2(MA)n−1PbnI3n +1 films retained their dark appearance even after 90 min, with UV-Vis analysis (Figure 2.30(c–d)) confirmed retention of MAPbI3

band edge at 750 nm. This thermal stability enhancement was attributed to bulky NMAI passivating the GB defects, promoting the formation of a more stable 2D structure at GBs and suppressed escape of MAI from there.

Figure 2.30: (a) Physical appearance of (NMA)2(MA)n−1PbnI3n +1 films (n=∞

represents MAPbI3 film) stressed at 150 °C and (b–d) their corresponding UV-Vis absorption spectra throughout the 90 min duration (Chaudhary et al., 2020).

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2.7 PMMA and Caffeine Potential in Enhancing MAPbI3 Thermal

Dalam dokumen thermal stability enhancement of - UTAR Institutional Repository (Halaman 63-75)