Research highlights:
1. The molecular isomers of the reaction product structure MFAI and DMFAI of MACl in FA-based perovskite were determined.
2. Grow MFAPbI 3 single crystal to achieve selective separation of cis/trans-MFAI isomers.
3. Comprehensively analyze the solubility, phase stability and band gap of single crystals grown in different solvents, as well as the effect of MACl on single crystals.
1. Problems with MACl as a FA-based perovskite additive
A promising route to improve PSC performance is the introduction of chlorine-containing organic additives, such as methylammonium chloride (MACl). MACl can effectively inhibit the phase transition of FAPbI 3 , promote grain growth, increase crystallinity, and ultimately improve device efficiency. MACl, as a widely used additive, has found wide applications in high-efficiency PSCs adopting nip and pin structures. It is worth noting that during high-temperature annealing, most MACl additives evaporate and deprotonate. MA 0 can react chemically with lead methylamine ions (FA + ) to form lead methylamine ions (MFA + ) as a by-product. Despite their importance, the chemical interactions between methylamine lead iodide perovskites, MACl additives, and their by-products remain poorly understood.
2. Introduction to results
In view of this, the Sang Il Seok team at the Ulsan National Institute of Science and Technology in South Korea found that the reaction of FA + and MA0 mainly produces a mixture of cis-trans-N-methylmethylamine iodide (MFAI) isomers, in which cis- MFAI dominates. Furthermore, MFAI subsequently reacts with PbI to form fully formed cis-MFAPbIH- phase perovskite. Through the growth of single crystals, the effects of MFAI on the crystal growth, phase stability, and band gap of methylamine-based perovskites were elucidated. This study provides valuable insights into the role of these byproducts in affecting the efficiency and long-term stability of future PSCs.
3. Results and Discussion
Point 1: Determination of the two isomers of MFAI and DMFAI products
In previous work (DOI:10.1002/anie.202212700), the researchers found that 2-methoxyethanol (2-ME) solvent can help the 2H-FAPbI 3 (δ-FAPbI 3 ) phase transform into 3C phase (α-FAPbI 3 ). The black stable 3C-FAPbI 3 crystal obtained from the 2-methoxyethanol solvent can be seen through the X-ray diffraction pattern (Fig. 1a) , which exhibits the peak of the cubic phase. The XRD data in Figure 1a shows that as the MACl content increases, the yellow powder exhibits the most significant peaks at 11.6° and 11.4°. Even when annealed at high temperature (170°C), the yellow crystals do not turn black, which is contrary to the common idea that MACl additives can stabilize the α-FAPbI 3 phase. The 1H NMR data in Figure 1b shows that when a solid mixture of FAPbI 3 containing 100 mol% MACl is directly dissolved in DMSO-d 6 solvent at room temperature , it is comparable to the 1H NMR spectrum of pure FAPbI 3 black powder in DMSO-d 6 In comparison, no new products were significantly formed. However, the 1H NMR spectra in Figure 1c and 1d show that when FAPbI 3 is heated with 35 mol% MACl in 2-ME solvent, four new peaks appear in the yellow powder, which are 2.81, 2.96, 7.95 and 7.89 ppm respectively. When MACl was further increased to 100 mol%, three additional peaks appeared at 2.79, 3.02, and 7.99 ppm. These peaks can be attributed to MFAI and N,N'-dimethylmethylamine iodide (DMFAI). However, the specific peak assignments have not been determined, and the molecular structures of MFAI and DMFAI remain controversial.
Figure 1 Characterization of FAPbI 3 prepared with different amounts of MACl
To this end, the researchers synthesized white crystals of MFAI and DMFAI. As shown in Figure 2a and Figure S7, the 1H NMR data of pure MFAI has two sets of peaks: a significant peak (2.81 ppm) and a weak peak (2.96 ppm) appearing in the upper field region with an integration ratio of approximately 10:1 for the methyl functional group (−CH 3 ) hydrogen. At the same time, a strong peak (7.96 ppm) and a weak peak (7.91 ppm) appeared in the lower field region with an integration ratio of about 10:1, which were divided into hydrogen in the amine carbon (−CH). As shown in Figure 2b, the 13 C NMR spectrum of pure MFAI crystal also has two sets of peaks in the upper field area, 27.85 (main peak) and 32.76ppm corresponding to methyl carbon; in the lower field area, there are two peaks, 155.10 (main peak) and 158.75 ppm, attributable to the amine carbon. These data indicate that in DMSO-d solution , two isomers of MFAI exist in a ratio of approximately 10:1. DMFAI crystals were synthesized with excess methylamine, and their 1H NMR spectrum showed two strong peaks and one weak peak at 2.79 and 3.02 ppm (Fig. 2c), corresponding to methyl hydrogen, respectively. Furthermore, only one strong peak (8.00 ppm) was observed in the downfield region.
Figure 2 NMR spectrum analysis of synthesized white MFAI and DMFAI crystals
The 13 C NMR data of Figure 2d shows two strong peaks at 28.23 and 32.77 ppm, corresponding to the two methyl carbons of DMFAI. In the lower field region, a strong peak (156.00 ppm) and a very weak peak (159.06 ppm) appeared, corresponding to the amine carbon. According to the integral, DMFAI also exists in two isomers, and their ratio is approximately 1:0.035. These results confirm that the MACl additive does react with FAPbI 3 at high temperatures in 2-methoxyethanol solvent , producing products that correspond well to MFAI and DMFAI, and that each product has two isomers.
Point 2: Determination of cis-trans isomers of MFAI and DMFAI
To determine the accurate chemical structures of MFAI and DMFAI and distinguish each isomer, the researchers performed solid-state NMR and solution 2D NMR experiments. In the solid-state 1H NMR spectrum shown in Figure 3a, there are only four peaks for MFAPbI 3 or MFA cations. According to the integral ratio, they are −CH 3 , −CH, −NH 2 and −NH respectively from the upper field to the lower field . It can be seen that the hydrogen on the nitrogen attached to the methyl group (−NH) has a larger chemical shift compared to −NH . Considering the broad peaks of solid-state NMR, a small amount of hydroiodic acid (HI) was introduced into the DMSO-d 6 solution of MFAI to obtain a finer spectrum, and a split spectrum could be observed (Fig. 3b). Based on the solid-state 1H NMR and integration ratio of the MFA cation, −CH (7.96 ppm), −NH 2 (split into two sets of peaks, 8.76/8.78 and 9.1 ppm) and −NH (9.46 ppm) can be determined. It is worth noting that one of the −NH 2 peaks of MFAI has a more obvious split at 8.76 and 8.78 ppm. This is consistent with the reported situation in the literature that FAI containing HI splits into multiple peaks in DMSO-d6 solution : FAI itself has no isomers, but the four hydrogen atoms have different chemical environments due to partial resonant double bonds (cis and trans positions), which can be differentiated using NMR. Hydrogen atoms in the trans position have more pronounced peak splitting and smaller chemical shifts. Considering that FAI adopts the resonance structure of part of the carbon-nitrogen double bond, MFAI is likely to adopt a similar structure and form cis-trans isomers due to the introduction of methyl groups (Figure 3c and 3d).
Figure 2 NMR spectrum analysis of synthesized white MFAI and DMFAI crystals
The 13 C NMR data of Figure 2d shows two strong peaks at 28.23 and 32.77 ppm, corresponding to the two methyl carbons of DMFAI. In the lower field region, a strong peak (156.00 ppm) and a very weak peak (159.06 ppm) appeared, corresponding to the amine carbon. According to the integral, DMFAI also exists in two isomers, and their ratio is approximately 1:0.035. These results confirm that the MACl additive does react with FAPbI 3 at high temperatures in 2-methoxyethanol solvent , producing products that correspond well to MFAI and DMFAI, and that each product has two isomers.
Point 2: Determination of cis-trans isomers of MFAI and DMFAI
To determine the accurate chemical structures of MFAI and DMFAI and distinguish each isomer, the researchers performed solid-state NMR and solution 2D NMR experiments. In the solid-state 1H NMR spectrum shown in Figure 3a, there are only four peaks for MFAPbI 3 or MFA cations. According to the integral ratio, they are −CH 3 , −CH, −NH 2 and −NH respectively from the upper field to the lower field . It can be seen that the hydrogen on the nitrogen attached to the methyl group (−NH) has a larger chemical shift compared to −NH . Considering the broad peaks of solid-state NMR, a small amount of hydroiodic acid (HI) was introduced into the DMSO-d 6 solution of MFAI to obtain a finer spectrum, and a split spectrum could be observed (Fig. 3b). Based on the solid-state 1H NMR and integration ratio of the MFA cation, −CH (7.96 ppm), −NH 2 (split into two sets of peaks, 8.76/8.78 and 9.1 ppm) and −NH (9.46 ppm) can be determined. It is worth noting that one of the −NH 2 peaks of MFAI has a more obvious split at 8.76 and 8.78 ppm. This is consistent with the reported situation in the literature that FAI containing HI splits into multiple peaks in DMSO-d6 solution : FAI itself has no isomers, but the four hydrogen atoms have different chemical environments due to partial resonant double bonds (cis and trans positions), which can be differentiated using NMR. Hydrogen atoms in the trans position have more pronounced peak splitting and smaller chemical shifts. Considering that FAI adopts the resonance structure of part of the carbon-nitrogen double bond, MFAI is likely to adopt a similar structure and form cis-trans isomers due to the introduction of methyl groups (Figure 3c and 3d).
Figure 3 Characterization and identification of MFAI isomers by NMR spectroscopy
To further determine which isomer is dominant, 2D-NOESY experiments were performed on MFAI versus HI in DMSO-d solution . As shown in Figure 3e, the two blue dots marked within the circle represent the hydrogens between 2.81 (−CH 3 ) of the major isomer and 8.78/8.76 (trans, −NH 2 ) and 9.46 ppm (−NH) interaction. This means that the trans hydrogens on −CH and −NH of the main isomer are relatively very close in space, and this structure can only exist in the cis isomer of MFAI, as shown in Figure 3c Two red hydrogen atoms. The trans hydrogens on −CH and −NH of trans-MFAI are relatively very far apart in space, as shown by the two red hydrogen atoms in Figure 3d, indicating that cis-MFAI (2.81 ppm, 7.96 ppm) is dominant The reaction product contains a small amount of trans-MFAI isomer (2.96 ppm, 7.91 ppm). According to the 1H NMR spectrum, it can be determined accordingly that the strong peak at 27.85 ppm on the 13 C NMR spectrum of MFAI is the methyl carbon (−CH 3 ) of cis-MFAI , while the methyl carbon of trans-MFAI moves downfield to 32.76 ppm. . The strong peak at 155.10 ppm belongs to the amine carbon (−CH) of cis-MFAI, while the weak peak at 157.85 ppm belongs to trans-MFAI.
Point 3: Mechanism of reaction and conversion of cis-trans isomers of MFAI and DMFAI
NMR peaks for all the above substances, including FAI for better comparison. According to the 13C chemical shifts of −CH of different amine carbon compounds , such a trend was found: cis-MFAI (155.10 ppm) < (cis, trans)DMFAI (156.0 ppm) < FAI (157.12 ppm) < trans-MFAI (157.85 ppm) < (trans, trans)DMFAI (159.06 ppm). This trend indicates that the −CH in the cis-amine carbon is more shielded and therefore has a smaller chemical shift, which means that the carbon in the cis-amine carbon has more electrons. Therefore, the researchers infer that −CH 3 in cis-MFAI will give more electrons to the −CH cation in the amine cation, thereby increasing the stability of cis-MFAI. The schematic diagram of the reaction and transformation mechanism is shown in Figure 4a. First, at high temperatures, the methylammonium chloride additive is deprotonated to produce neutral methylamine, which attacks the positively charged carbon of the double bond in the FA cation and removes one NH molecule to produce MFAI; if there is excess chlorine Methylammonium or methylamine, which will further react to form DMFAI. Cis-MFAI can form temporary cis-3-MFAI through the single and double bonds formed by resonance. Then, the single bond of −CH–NH–CH in cis 3-MFAI can rotate more freely and become trans 3-MFAI, which will return to the trans MFAI isomer through a partial double bond resonance mode. Therefore, cis-trans MFAI isomers will reach dynamic equilibrium in solution through configuration inversion. The same mechanism is valid for DMFAI. The relationship between cis-trans MFAI isomers and 3-MFAI/1-MFAI isomers is shown in Figure 4b. The 3-MFAI/1-MFAI structure also exists as cis-trans isomers, which are named cis-1-MFAI, cis-3-MFAI, trans-1-MFAI and trans-3-MFAI. .
Figure 4 Mechanistic insights into the isomerization of MFAI and DMFAI
In order to further prove that MFAI exists in the form of two cis-trans MFAI isomers, and to study whether MFAI will react with PbI to form new compounds, the researchers grew MFAPbI 3 single crystals. The crystal structure in Figure 5a shows that the MFA cation is disordered multiple times, but all adopt the cis configuration. Figures 5b and 5c show the crystal structure with an ordered cis-MFA cation, with different orientation views. The two carbon-nitrogen bond lengths are almost equal (1.314/1.316 Å), corresponding to a partial carbon-nitrogen double bond. When solving the crystal structure of MFAPbI3 , it was found that due to its larger size and more stretched configuration, it is difficult for trans-MFA cations to enter the pores surrounded by lead-iodine octahedrons (Fig. 3d). In the solid-state NMR data of MFAPbI3 single crystal powder, only one set of peaks for MFA cations was observed (Figure 3a and Figure 5d), further indicating that all cations in MFAPbI3 may adopt a cis structure.
Figure 5 Crystal structure of yellow cis-MFAPbI 3 single crystal
The similar XRD pattern in Figure 6a further shows that MFAPbI 3 and 2H-FAPbI 3 have similar crystal structures, which also indicates that the obtained yellow MFAPbI 3 single crystal is a pure phase and further proves that all cations adopt cis-MFA structure. However, the main peak of cis-MFAPbI 3 is shifted to a lower angle of 11.4°. This is because the size of MFA cations is larger than that of FA cations, causing the lattice to expand. The XRD peak at 11.4° is consistent with the results in Figure 1a, further proving that yellow MFAPbI crystals are produced during the heating process of MACl and FAPbI in 2 -ME solvent . The XRD pattern in Figure 6b shows that the strong peak of the (010) crystal plane shifts to 11.6°, which is located between 2H-FAPbI 3 (11.79°) and 2H-MFAPbI3 (11.40°). This is consistent with the XRD angle of FAPbI 3 -35% mol MACl in Figure 1a , indicating that approximately 35% of the MFA cations can form a solid solution with the FA cations. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) measurements were performed. The results show that 2H-MFAPbI remains stable from room temperature to 250 °C and does not transform into the 3C-MFAPbI phase or decompose (Figure 6c). The above data indicate that the introduction of MFA will induce the formation of 2H phase perovskite and prevent its transformation to the 3C phase. The absorption spectrum in Figure 6d shows that the band gap of 2H-MFAPbI is 2.4 eV, which is larger than the 2.1 eV of 2H - FAPbI .
Figure 6 Crystal structure of yellow cis-MFAPbI 3 single crystal
Point 4: The role of MFAI within FAPbI 3
A single crystal example is used to discuss whether the reaction product of MFAI between MACl and FAPbI 3 leads to an increase in the band gap and how MFAI affects the nucleation and crystal growth of the 3C-FAPbI 3 phase. It was found that MFAPbI 3 was not completely soluble (less than 0.05 M) in the 2-ME solvent at room temperature (RT) , while in 2-ME, FAPbI 3 showed a high solubility of 2.22 M (Figure 7a). Although the solubility of MFAPbI in GBL increased to 0.67 M at RT , it was still much lower than the solubility of FAPbI in GBL ( 1.92 M at RT). In addition, the solubility of MFAPbI in GBL significantly decreased to 0.42 M when heated at 60 °C , indicating that MFAPbI in GBL also follows the inverse temperature solubility law like FAPbI . It was inferred that due to its low solubility, MFAPbI 3 is more likely to nucleate and crystallize in GBL or 2-ME solvents. Therefore, FA-based perovskite single crystals with cubic phase cannot be directly grown using 2-ME solvent in the presence of MFAI.
Figure 7 Comprehensive analysis of solubility, phase stability and band gap of single crystals grown in different solvents
During actual device preparation, most of the MACl additives will evaporate during the annealing process, leaving less than 10% MA and 5% MFA cations in the final polycrystalline film. Therefore, the researchers used GBL as the solvent and used the inversion solubility method to grow and compare the band gaps of the following four perovskite single crystals: (A) α-FAPbI 3 , (B) (FAPbI 3 ) 0.95 (MFAPbI 3 ) 0.05 , (C)(FAPbI 3 ) 0.9 (MAPbI 3 ) 0.1 , and (D)FAPbI 3 -10% mol MACl. Due to its low solubility, 5% mol MFAPbI 3 is more prone to phase separation at high temperatures. By optimizing the concentration to avoid phase separation of MFAPbI 3 , a single crystal of (FAPbI 3 ) 0.95 (MFAPbI 3 ) 0.05 (composition B) was grown. . Figure 7c shows that at 1% MFA residue, the smallest band gap of 1.412 eV can be obtained. The band gap slightly increases by 0.003 eV compared to 1.409 eV of 3C-FAPbI 3 (composition A). Next, in the single crystal grown on annealed (FAPbI 3 ) 0.9 (MAPbI 3 ) 0.1 (composition C), the XRD pattern in Figure 7b also shows the cubic phase. Figure 7c shows that a minimum band gap of 1.422 eV can be obtained. Compared with 1.409 eV of 3C-FAPbI3 , the band gap increases by about 0.013 eV, indicating that chloride ions mainly cause the band gap increase of 3C-FAPbI3 single crystal caused by MACl additive. Figure 7d shows the average result of multiple measurements: approximately 2% chlorine increases the band gap by approximately 0.01 eV. In comparison, 1% MFAI is associated with an increase in the band gap of approximately 0.003 eV, while approximately 7% MA cations barely affect the band gap. Therefore, it is concluded that the residual chloride ions mainly lead to the increase in the band gap of FAPbI 3 single crystal when MACl additive is introduced.
4. Summary
Overall, this study reveals the reaction of methylamine-based perovskites with commonly used MACl additives, thereby systematically investigating the molecular structures of their reaction products, especially the MFAI and DMFAI isomers. Among these isomers, cis-MFAI and (cis, trans)-DMFAI dominate. In addition, the successful growth of MFAPbI 3 single crystals achieved the selective separation of cis/trans-MFAI isomers. Interestingly, MFAPbI exhibits a face-sharing lead-iodine octahedral structure similar to 2H- FAPbI . Furthermore, forming a solid solution alloy containing MFA cations suppresses the phase transition of 2H-FAPbI to 3C-FAPbI when using 2-ME solvent . Studies of four perovskite single crystals grown in GBL solvent showed that MFAPbI3 has extremely little solubility and is prone to phase separation. It is worth noting that in this case, it is difficult for larger-sized MFA cations to enter the 3C-FAPbI 3 single crystal. By comparing the components and band gaps, it was determined that chloride ion, as the main factor in the MACl additive, plays a key role in increasing the band gap of 3C-FAPbI 3 single crystal. This comprehensive study provides fundamental chemical insights and valuable scientific guidance for preparing efficient and stable perovskite solar cells.
5. References
Chen. L et al. Deciphering Reaction Products in Formamidine-Based Perovskites with Methylammonium Chloride Additive . JACS
DOI:10.1021/jacs.3c12755(2023)
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