2.7 PMMA and Caffeine Potential in Enhancing MAPbI3 Thermal
Figure 2.31: (a) Illustration of I− escaping along GBs (Yuan and Huang, 2016). (b–c) Illustration of possible ions (Pb2+, MA+ and I−) and their migration to adjacent vacancy and (d) I− hopping to adjacent vacancy (Eames et al., 2015).
In reality, a perovskite film is comprised of many small grains that correspond to a large number of GBs. These GBs further reduce the activation energy for I− migration to half since there are only half of the chemical bonds present, and more iodine vacancy sites compared to the bulk structure (Yuan and Huang, 2016). This caused high ionic movement at GBs, as demonstrated by Shao et al. (2016). Their C-AFM analysis revealed strong hysteresis behaviour (Figure 2.32(b)) at GBs when the dark current measurement was
(c) (b)
(d)
Iodine vacancy
I− Pb2+
I− hopping
MA+
Pb2+
(a)
probed at that location (blue square region of Figure 2.32(a)), indicating high ionic movement at the GBs. In contrast, only negligible hysteresis (Figure 2.32(c)) was observed when the probing is done at the bulk of the grain (red triangle region of Figure 2.32(a)), indicating bulk ion migration is extremely low. To further verify their observation, they fabricated two PSCs with different grain sizes and measure their time to reach stabilized PCE under illumination.
As shown in Figure 2.32(d), the PSC with large, columnar grains that corresponded to low number of GBs showed instant stabilized 19.0% PCE response upon illumination. On the other hand, the PSC with many small grains suffered from hysteresis, taking as long as 300 s to reach stabilized 16.7%. This indicates that GBs are severe defect sites, enabling the I− to escape from the perovskite layer and cause degradation.
Figure 2.32: (a) C-AFM image of perovskite film with a blue square marks the GB between two grains while the red triangle marks the centre of the grain. (b–c) Their corresponding dark current measurements. (d) PSCs with different grain morphology in the perovskite layer and their corresponding PCE response upon illumination (Shao et al., 2016).
(a)
(b) (c)
Figure 2.32: Continued.
Since the defects at GBs acted as a migration pathway for ionic movement, it can also act as thermal degradation pathway of perovskite film.
At elevated temperature, the I− ions gain more energy for migration and escape via GBs (Chen and Zhou, 2020). This may explain the observed evolution of PbI2 grains around MAPbI3 grains in Figure 2.2 and Figure 2.23 when the MAPbI3 films were thermally stressed at elevated temperatures. Therefore, to enhance thermal stability, especially at 150 °C, these defects have to be passivated first (Lee et al., 2019).
A good example of enhancing thermal stability by defect passivation is shown by Li et al. (2018). In their work, they cross-linked trimethylolpropane triacrylate (TMTA) around the GBs of their MAPbI3 film to provide throughout passivation. This was achieved by cross-linking the monomers (Figure 2.33(a)) located at MAPbI3 GBs at 140 °C for 10 min after deposition of MAPbI3 film (Figure 2.33(b)). The TMTA was able to passivate the GB defects by forming a coordinative bonding with the I− deficient Pb2+ sites at GBs via its carbonyl (C=O) groups, as shown in Figure 2.33(c). The coordinative interaction was
(d)
verified by the shifting of FTIR spectral feature (Figure 2.33(d)) of pristine TMTA’s C=O bond at 1734 cm−1 to weaker wavenumber at 1725 cm−1 after the introduction of MAPbI3 to the TMTA polymer. The ionic conduction activation energy increased from 0.21 eV from the film without TMTA to 0.48 eV with passivation, indicating TMTA restrict ion migrations. This improved the thermal stability of the MAPbI3 film as PSC with TMTA cross-linked retained more than 80% of its initial PCE even after thermal stressing at 85 °C 1000 h (Figure 2.33(e)). In contrast, PSC without TMTA degraded rapidly to 45% in just 300 h. This proved TMTA can passivate the defects at GBs, suppressing the escape of volatile MAI and enhancing the thermal stability of the device.
Figure 2.33: (a) Molecular structure of TMTA monomer and (b) cross- linking into TMTA polymer. (c) Illustration of TMTA passivating GB defects. (d) FTIR analysis showing weakened C=O bond of TMTA after MAPbI3 introduction. (e) Thermal stability and PCE decay of PSCs with or without TMTA passivation (Li et al., 2018).
(a) (b)
(c)
Figure 2.33: Continued.
Similar to TMTA, PMMA also contains C=O groups along the polymer chain and has demonstrated its ability to passivate the iodine vacancy defects.
In the work from Bi et al. (2016), PMMA was used as a templating agent to control the nucleation and crystal growth of perovskite film. During the spin- coating of perovskite precursor solution, PMMA dissolved in antisolvent (chlorobenzene and toluene) was dripped onto the wet precursor to induce fast crystallization. The PMMA regulated the nucleation and coordinated the crystal growth, which led to the formation of high-quality perovskite grains. This yielded a high efficiency, hysteresis-free PSC with 21.6% PCE. The lack of hysteresis indicated PMMA had suppressed the GB defects at HTL|perovskite interface. Their FTIR analysis revealed weakened PMMA carbonyl (C=O) bond from 1735 cm−1 to 1723 cm−1 upon introduction of PbI2 (Figure 2.34(a)). This demonstrated ability of PMMA to donate electron from its C=O groups to the iodine deficient Pb2+ defects at GBs, passivating the defect and eliminating the hysteresis. Furthermore, their XPS analysis (Figure 2.34(b)) also revealed PMMA eliminated the metallic Pb signal, which may correlate to a reduced number of iodine vacancies.
(d) (e)
Figure 2.34: (a) FTIR analysis showing weakened C=O bond when PMMA formed coordinative bonding with PbI2. (b) XPS analysis revealing PMMA as templating agent (red line) eliminated Pb signal that was otherwise present in the control sample (black line) (Bi et al., 2016).
Similarly, Peng et al. (2018) also used PMMA interlayers to passivate the interface defects at ETL|perovskite and HTL|perovskite (Figure 2.35(a)).
Their FTIR analysis (Figure 2.35(b)) also revealed the carbonyl group (C=O) bond of PMMA weakened from 1732 cm−1 to 1724 cm−1 upon introduction to PbI2. This showed that PMMA is able to act as a Lewis-base site, donating its electrons from the carbonyl groups (C=O) to form coordinated bonding with Pb2+ defects at the GBs located at both interfaces. As a result, this suppressed ion migration and eliminated hysteresis behaviour, improving the PCE from 18.5% for the device without passivation to 20.8% for PMMA passivated device.
The PMMA also demonstrated stronger coordination with FAI, as its FTIR spectral feature further weakened to 1720 cm−1. This suggests PMMA may also prevent FAI from escaping via GBs or interfaces and can be applied to MAPbI3
to enhance the thermal stability at 150 °C by simultaneously passivating the Pb2+ defects and supressing escape of MAI at GBs.
(a) (b)
Pb
Figure 2.35: (a) Structure of PSC with PMMA as interlayer at ETL and HTL. (b) FTIR analysis showing weakened C=O bond when PMMA formed coordinative bonding with PbI2 or FAI (Peng et al., 2018).
Chen et al. (2020) used PMMA as a matrix to form CsPbI3|PMMA composite film that functions as a light-emitting diode (LED). Their FTIR analysis also showed a strong interaction between PMMA with the Pb2+ of CsPbI3 nanocrystals. Different from previous works that investigated shifting of PMMA C=O wavenumber at ~1732 cm−1, they focused on the emergence of a new peak at 1640 cm−1 located between symmetric stretch mode of PMMA COO− at 1604 cm−1 and C=O at 1729 cm−1. This new peak only emerged after adding CsPbI3 nanocrystal to PMMA. At the same time, the COO− at 1640 cm−1
(a)
(b)
decreased as the amount of CsPbI3 nanocrystal is presence in the composite film.
Based on this observation, they also suggested C=O to interact strongly with Pb2+.
Figure 2.36: (a) FTIR analysis of PMMA film (black line at the bottom) to 4, 6, 8 and 10 wt.% CsPbI3|PMMA composite (dark green line at the top) showing the emergence of a new peak at 1640 cm−1 wavenumber. (b) Suggested PMMA coordinate with Pb2+ of CsPbI3 via C=O group (Chen et al., 2020).
Since PMMA can suppress the ionic migration and hysteresis behaviour of perovskite material, in theory, it should also enhance the thermal stability at elevated temperature as the MAI cannot escape easily via the defect at GBs.
Indeed, McKenna et al. (2017) have demonstrated excellent capability of PMMA in enhancing the thermal stability of perovskite film. In their work, they investigated the thermal stability of MAPbI3-xClx perovskite films encapsulated in different polymers. These polymers were ethyl cellulose (EC), poly(4- methyl-1- pentene) (PMP), polycarbonate (PC) and PMMA. The PMMA encapsulation demonstrated to enhance the thermal stability of the film at various elevated temperatures. When the encapsulated film was thermally stressed at 60 °C under ambient air for 432 h, PMMA encapsulation was able to
(a) (b)
retain the perovskite phase without degradation to PbI2, while other polymer encapsulations demonstrated degradation to PbI2, as shown in their XRD analysis in Figure 2.37(a). UV-Vis analysis (Figure 2.37(b)) also revealed PMMA encapsulated film to retain its band edge absorbance at 750 nm even after thermal stressing at 100 °C for 200 h. However, instead of suggesting Lewis base passivation by PMMA as a factor in enhancing thermal stability, they attributed the thermal stability enhancement to low water vapour transmission rate offered by PMMA. No defect passivation by the PMMA was mentioned.
Figure 2.37: MAPbI3-xClx films on glass slides, encapsulated with different polymers and their corresponding XRD patterns after thermal stressing at 60 °C for 432 h in ambient air. (b) MAPbI3-xClx films with different polymer encapsulations and their corresponding absorbance at 750 nm over 200 h thermal stressing at 100 °C (McKenna et al., 2017).
(a)
Figure 2.37: Continued.
Caffeine is also a small molecule that contains C=O groups capable of passivating the defects. Wang et al. (2019a) addressed the problem of iodine migration by adding caffeine to their MAPbI3 film. In their work, caffeine as additive was added to the MAPbI3 precursor solution. Upon film formation, the caffeine occupied the GBs and thoroughly passivated them. Same as previously discussed works, its passivation effect was verified by confirming the FTIR spectral feature shifting of C=O bonds from 1652 cm−1 to 1657 cm−1 upon introduction to MAPbI3. Its passivation effect was so strong that it prevented the migration of iodine from MAPbI3 layer to react with Ag counter electrode.
This was reflected by the EDX mapping (Figure 2.38(a)) conducted on two different PSCs after stressed at 85 °C for 1300 h. The PSC without caffeine showed Ag cluster and iodine signal presence in both MAPbI3 and Ag counter electrode layers, indicating a typical migration-induced degradation. However, the device with caffeine showed a very clear separation of iodine and Ag layers, demonstrating the strong passivation effect of caffeine despite prolonged stressing at elevated temperature. This enabled the caffeine added PSC to retain
(b)
over 80 % of the initial PCE even after 1300 h of stressing in N2 environment (Figure 2.38(b)). XRD analysis (Figure 2.38(c)) also revealed caffeine resisted MAPbI3 phase from degrading into PbI2. This shows caffeine may enhance thermal stability of MAPbI3 at 150 °C.
Figure 2.38: (a) Cross-sectional EDX mapping of PSCs with and without caffeine additive. (b) PCE degradation and (c) XRD patterns of caffeine added MAPbI3 vs. reference MAPbI3 film after 1300 h of 85 °C thermal stress test (Wang et al., 2019a).
w/o caffeine
with caffeine
(b) (a)
Figure 2.38: Continued.
The molecular structures for both PMMA and caffeine are shown in Figure 2.39. Through their C=O groups, the PMMA and caffeine have demonstrated their capability to passivate defects and enhancing thermal stability. However, there is still no investigation on whether they are capable of enhancing the thermal stability of MAPbI3 film at 150 °C. Hence, this work aims to investigate this unexplored field. They are chosen because they do not degrade and is non-volatile at 150 °C, hence they are expected to enhance the thermal stability. Successful thermal stability enhancement of the MAPbI3 film would allow low-temperature metal oxide sols to be directly annealed on it, realizing fabrication of all metal oxide CTL PSC with enhanced operational stability and lowered fabrication cost, shorten the steps to commercialization.
(c)
Figure 2.39: Molecular structures of (a) PMMA and (b) caffeine (Peng et al., 2018; Wang et al., 2019a).