Sampreetha
Thampy‡
a,
Boya
Zhang‡
a,
Jong-Goo
Park
b,
Ki-Ha
Hong
*b and
Julia W. P.
Hsu
*a
aDepartment of Materials Science and Engineering, University of Texas at Dallas, Richardson, TX 75080, USA. E-mail: jwhsu@utdallas.edu
bDepartment of Materials Science and Engineering, Hanbat National University, Yuseong-Gu, Daejeon, 34158, Republic of Korea. E-mail: kiha.hong@hanbat.ac.kr
First published on 16th October 2020
The thermal stability and decomposition pathway of formamidinium iodide (FAI, HC(NH2)2I) in contact with NiO and TiO2 are investigated by combined experimental studies and density functional theory (DFT) calculations. Based on the decomposition temperature, we find that the stability decreases as FAI ∼ FAI + TiO2 > FAI + NiO. Moreover, FAPbI3 in contact with NiO and TiO2 shows similar thermal stability behaviour to FAI. The bulk decomposition of FAI occurs via the formation of sym-triazine, and can also produce HCN, and NH4I at ∼280 °C, which further decomposes to NH3 and HI above 300 °C. When FAI comes into contact with NiO, the interfacial reaction triggers decomposition at a much lower temperature (∼200 °C), resulting in the formation of NiI2 as the solid product while releasing NH3 and H2O into the gas phase; sym-triazine and HCN are observed near the FAI bulk decomposition temperature. In contrast, when FAI comes into contact with TiO2, the decomposition temperature is similar to bulk FAI; however, HCN is released at a lower temperature (∼260 °C) compared to sym-triazine. The difference in the degradation behavior of FAI with NiO and TiO2 is elucidated using DFT calculations. Our results show that the interfacial reaction between the organic component of perovskite material and NiO occurs similarly for MA and FA, which thereby can induce device instability.
While previous work focused on the thermal stability and degradation mechanism in formamidinium iodide (FAI, HC(NH2)2I) and FAPbI3 by themselves, here we investigate the thermal stability and decomposition pathway of FAI in contact with NiO, a commonly used hole transport layer material,27,35 or TiO2, a commonly used electron transport layer material.36 In our previous work, we showed that the inorganic component of the perovskite materials—PbI2—does not undergo any change in this temperature range (<400 °C), both by itself or in contact with metal oxides.22 The instability of halide perovskites primarily arises from the decomposition of the organic component. Hence, studying FAI degradation will provide an understanding of FAPbI3 stability. The thermal stability and degradation reactions are studied using thermogravimetric analysis complemented with differential scanning calorimetry (TGA-DSC), as well as the temperature-programmed desorption technique combined with mass spectrometry (MS) and Fourier transform infrared spectroscopy (TPD-MS-FTIR) for simultaneous detection and unequivocal identification of gas-phase decomposition products. The solid decomposition products are examined by X-ray diffraction (XRD). Density functional theory (DFT) modelling is employed to explain the experimental results. This combination allows us to construct an accurate delineation of the decomposition pathways.
Eads = Et(molecule adsorbed surface) − E(surface) − E(molecule) | (1) |
ΔE = E(HCN* + NiO) + E(HI* + NiO) + E(NH3* + NiO) − Et((HC(NH2)2I)* + NiO) − 2E(NiO) | (2) |
To identify the volatile decomposition products associated with thermal events observed in TGA-DSC, we performed TPD-MS-FTIR experiments with neat FAI, FAI + NiO, and FAI + TiO2 powders. Such simultaneous detections of evolved gases by FTIR and MS help to accurately identify molecular species as ionization probability and fragmentation into smaller ions in MS complicate the analysis while many organic moieties have overlapping vibrational frequencies in FTIR. The top panel in Fig. 2 shows FTIR temperature profiles representing infrared absorption intensity versus temperature for evolved gases. Comparing the observed IR spectra of the gas species released from the decomposition to NIST database,45 we assign the evolved gases at 967 cm−1, 3600–3800 cm−1, 739 cm−1, and 1551 cm−1 to ammonia (NH3, black), water (H2O, blue), hydrogen cyanide (HCN, red), and sym-triazine ((HCN)3, green), respectively. Note that the higher wavenumber region is used for H2O to avoid the overlap with NH3 signals in the 1500–1600 cm−1 region. The full FTIR line spectra at different temperatures for the three samples are shown in Fig. S4 (ESI†). In MS, a molecule can have several fragments with different mass to charge ratio (m/z) values. By comparing the intensity ratios at different m/z of all detected ions to NIST database,46 we identify the released gases to be NH3, H2O, HCN, and sym-triazine (Fig. S5, ESI†). Using the m/z of the parent ions, NH3+ (m/z = 17), H2O+ (m/z = 18), HCN+ (m/z = 27), and sym-triazine ((HCN)3+, m/z = 81), the MS temperature profiles are shown in the bottom panel of Fig. 2. For neat FAI (Fig. 2a and d), the FTIR and MS results show the decomposition temperature (Td), at which gases begin to evolve, to be ∼280 °C. While Td of the neat FAI observed here is closer to Perez et al.'s work (260 °C),30 Ma et al. reported it to be at 245 °C.18 The Td differences in these three works could arise from the different ramping rates used in heating the samples and the geometry of the experimental apparatus.17 In contrast, when FAI is in contact with NiO (Fig. 2b and e), gases begin to evolve at ∼200 °C, 100 °C lower than Td of neat FAI. When FAI is in contact with TiO2 (Fig. 2c and f), HCN starts to appear at ∼260 °C.
Fig. 2 TPD-MS-FTIR results: (a–c) FTIR temperature profiles (absorbance versus temperature) and (d–f) MS signals of evolved gases for FAI (left), FAI + NiO (middle), and FAI + TiO2 (right). Signals at the characteristic vibrational frequencies of NH3 (967 cm−1), H2O (3600–3800 cm−1), HCN (739 cm−1), and sym-triazine (1551 cm−1) gases are used in FTIR profiles and represented with black, blue, red, and green lines, respectively, in (a–c). MS signals of m/z = 17 (black), m/z = 18 (blue), m/z = 27 (red), and m/z = 81 (green) represent NH3+, H2O+, HCN+, and sym-triazine parent ions, respectively. All assignments are based on the NIST database.45,46 |
Correlating the TPD results with TGA-DSC results, we can attribute the first endothermic peak in DSC (Fig. 1a, blue dashed line) to bulk decomposition of neat FAI, releasing sym-triazine and HCN gases simultaneously at Td ∼ 280 °C as detected by both FTIR and MS (Fig. 2a and d). In addition, FTIR (Fig. 2a) also shows NH3 evolution at a higher temperature ∼340 °C, indicating that bulk FAI decomposition does not produce NH3 directly; this temperature corresponds to the second endothermic peak in DSC. From these results, we can infer that the bulk decomposition of neat FAI occurs via a two-step process. Previous work on neat FAI thermal decomposition also reported sym-triazine, HCN, and NH3 as the gaseous products, but no detection of HI or I2.18,30 We also did not observe HI in FTIR or MS (Fig. 2a and d) as HI is known to adhere to cold surfaces in the apparatus.10 The fact that NH3 is not detected by MS (Fig. 2d) can be attributed to its low concentration, i.e. below the MS detection limit; FTIR is sensitive to the N–H symmetric deformation mode, but the intensities of these peaks are extremely low in this case (Fig. S4a, ESI,† black dotted rectangle in the zoomed-in view).
In the case of FAI + NiO, we observe two distinct degradation processes, with gaseous products of NH3, H2O, sym-triazine, and HCN (Fig. 2b and e). At 200 °C, NH3 and H2O are released simultaneously, corresponding to the first endothermic peak in DSC (Fig. 1b, blue dashed line), while sym-triazine and HCN only begin to evolve at ∼270 °C, which aligns well with the second endothermic peak in DSC. Because the high-temperature evolved gases are the same as neat FAI and also occur at a similar temperature, we attribute this process to bulk decomposition of FAI. The low-temperature event, during which NH3 and H2O are released, is a new degradation pathway that is not previously known. To accentuate the interfacial effects, we increased the molar ratio of FAI to NiO to 1:4. With excess NiO, both FTIR (Fig. S6a, ESI†) and MS (Fig. S6c, ESI†) results are dominated by the NH3 and H2O evolution at 200 °C, confirming that the low-temperature process arises from the interaction of FAI with NiO. At the same time, the sym-triazine signal is low in FTIR and not observed in MS, indicating that a very small amount is produced whereas HCN is detected using both techniques. It is noteworthy that the Td ∼200 °C and the released NH3 and H2O gas products are the same as observed for the decomposition of MAI in contact with NiO.22
For FAI + TiO2 samples, although there is no significant change in Td from neat FAI, HCN is released first at a lower temperature of ∼260 °C while sym-triazine is released at ∼280 °C (Fig. 2c and f). Similar to neat FAI, FTIR shows evidence of NH3 released at ∼340 °C (Fig. 2c and Fig. S4c, ESI†), in agreement with the DSC result (Fig. 1c) where we observed a high temperature endothermic peak at ≥300 °C. With the increased molar ratio of FAI:TiO2 to 1:4, Td remains the same as 1:1 FAI:TiO2, but much more HCN is produced compared to sym-triazine (Fig. S6b and d, ESI†).
Further insight into the degradation process can be gained by examining the solid decomposed products using XRD. The XRD patterns taken before heating and after heating at each temperature are shown in Fig. S7 (ESI†). The XRD patterns of neat FAI, FAI + NiO, and FAI + TiO2 measured after heating to 250 °C are shown in Fig. 3. For neat FAI (top panel), XRD shows strong NH4I peaks (maroon squares) and weak sym-triazine signals (green inverted triangles), indicating that neat FAI decomposition results in the formation of NH4I as the solid product. The XRD patterns of FAI + NiO (Fig. 3, middle panel) show predominantly NiI2 peaks (violet diamonds) along with weak NH4I signals (maroon squares). The formation of NiI2 coincides with the observed weight loss of FA only in the TGA of FAI + NiO (Fig. 1b), further substantiating the reaction between FAI and NiO, i.e. the interfacial reaction. NiI2 has been identified previously as the reaction product of halide perovskite and NiO.12,21,22 On the other hand, in the XRD pattern of FAI + TiO2 (Fig. 3, bottom panel), no peaks of TiI4, only TiO2 reflections (blue dumbbells), are observed at 250 °C. Thus, the absence of TiI4 is consistent with the weight loss in TGA of FAI + TiO2 (Fig. 1c), which corresponds to the total weight of FAI in the sample and supports the fact that no reaction occurs between TiO2 and FAI.
(3a) |
(3b) |
To explain our experimental observations and to elucidate the degradation pathways in neat FAI and when FAI contacts NiO or TiO2, we employed DFT calculations. The energy changes for decomposition reactions in the gas phase and on oxide surfaces are calculated with adsorption energy data sets and are represented in Fig. 4 and Table S1 (ESI†). Here our calculation results shed light on the conflicting reports of whether sym-triazine or HCN is formed first. Fig. 4 shows that, for a given sample, neat FAI, FAI + NiO, or FAI + TiO2, the reactions that produce sym-triazine (green dashed lines) have lower energies than equivalent reactions that produce HCN (red dashed lines). Thus, sym-triazine is a thermodynamically favored product over HCN, i.e.reaction (3a) dominates over (3b). The middle panel in Fig. 4 shows that contact with NiO (pink pentagons) and TiO2 (blue dumbbells) lowers the energies of both reaction (3a) and (3b) compared to neat FAI (black circles). The reaction energies decrease as FAI > FAI + TiO2 > FAI + NiO, consistent with the decrease in Td observed in TPD-FTIR-MS (Fig. 2), i.e. 280 °C for FAI > 260 °C for FAI + TiO2 > 200 °C for FAI + NiO. Note that our calculations present the thermodynamic energy changes for selected reactions, but not the activation energy barrier heights. Thus, the negative energy of reaction (3a) on NiO indicates that the reaction is thermodynamically favoured but does not mean that it will occur spontaneously.
While Perez et al. and Akbulatov et al. proposed NH4I in FAI or FAPbI3 decomposition,30,32 our experimental results are the first to unambiguously identify its presence in the decomposition products. The XRD results of partially decomposed FAI after 250 °C heat treatment (Fig. 3, top panel) show the existence of sym-triazine (green inverted triangles) and NH4I (maroon squares). Our DFT calculations (Table S1, ESI†) show that when FAI decomposes, the formation of NH4I from NH3 and HI is energetically favorable (−0.45 eV). In this case (black circle), the lowest energy reaction produces sym-triazine + NH4I (0.24 eV, green dashed line, right column), while the reaction that produces sym-triazine + NH3 + HI, i.e.reaction (3a), has a higher energy of 0.69 eV (green dashed line, middle column). Moreover, the sym-triazine + NH4I reaction requires less energy than the equivalent reaction that produces HCN + NH4I (1.2 eV, red dashed line, right column). We therefore propose that FAI first transforms to sym-triazine and NH4I. sym-triazine and HCN readily desorb at ∼280 °C in the TPD experiment (Fig. 2a and d), while NH4I is still a solid at this temperature. As the temperature further rises, NH4I undergoes complete decomposition to NH3 + HI by 340 °C with NH3 being detected (Fig. 2a). The TGA-DSC of neat NH4I shown in Fig. S8 (ESI†) provides further evidence that NH4I decomposes ≥300 °C. Comparing the DSC curves (red) of NH4I with FAI (Fig. 1a), the endothermic peak at ∼330 °C matches well with the second endothermic peak in FAI, supporting our hypothesis that FAI decomposition occurs in two steps with the release of NH3 (Fig. 2a) as the result of NH4I decomposition. Since the reaction to form NH4I is highly exothermic, the reformation of solid NH4I on the colder parts of the experimental apparatus can occur, which has been suggested previously.32 In neat FAI and FAI + TiO2 TPD experiments, we observed white deposits lining the exhaust capillary of the cell. Comparing the XRD pattern of this white deposit (Fig. S9, ESI,† orange) to neat NH4I (Fig. S9, ESI,† maroon), it is identified as NH4I. Furthermore, NH4I TPD results show no NH3 or HI (m/z = 128) gases (Fig. S10a, ESI†) and a large amount of white deposits in the exhaust capillary of the sample cell (Fig. S10b, ESI†). Thus, based on these results, we determine that the neat FAI undergoes decomposition via sym-triazine + NH4I first and NH4I further decomposes to NH3 + HI at higher temperature, with the possibility of NH4I reformation on colder surfaces.
A notable difference in neat FAI vs. FAI + oxides is the decomposition via NH4I vs. NH3 + HI. Since solid NH4I is not stable on either NiO (0.21 eV) or TiO2 (0.67 eV) surfaces (Table S1, ESI†), it dissociatively adsorbs as NH3* and HI* on the oxide surfaces instead of as NH4I*. This difference in the adsorption characteristics on oxide surfaces explains the presence of NH4I (maroon squares) in the XRD of neat FAI (Fig. 3, top panel), while a lower amount is found in that of FAI + NiO (Fig. 3, middle panel) and none in that of FAI + TiO2 (Fig. 3, bottom panel). Hence, the decomposition of FAI on NiO or TiO2 follows decomposition reaction (3a). We next analyse the effects of oxide surfaces on FAI decomposition pathways based on the adsorption energies of molecules summarized in Table 1. As shown in Fig. 4, FAI decomposition into sym-triazine, NH3, and HI (reaction (3a)) on NiO has a significantly lower reaction energy (−0.09 eV, green dashed lines, middle column) compared to neat FAI decomposing into sym-triazine and NH4I (0.24 eV, green dashed lines, right column). The low energy of reaction (3a) for FAI + NiO is in good agreement with the experimental results, where we also observed NH3 (not NH4I) and H2O at 200 °C, and sym-triazine and HCN at 270 °C (Fig. 2b and e). The low desorption temperature of NH3 (200 °C) on the NiO surface is attributed to its low binding energy (−0.77 eV, Table 1) and hence it can be readily desorbed from the surface. On the other hand, the HI gas that must be formed as a decomposition product and adsorbed on the surface as HI* reacts further with NiO producing H2O and NiI2. H2O desorbs from the surface as water vapor and is detected in both FTIR and MS (Fig. 2b and e, blue), and NiI2 remains as a solid decomposed product as observed in XRD (Fig. 3, middle panel, violet diamonds). Thus, the detection of NiI2, NH3, and H2O at low temperature (∼200 °C) points to the prevalence of the interfacial reaction of FAI with NiO. To further substantiate these findings, we performed TGA-DSC and TPD-FTIR-MS experiments on NH4I + NiO (1:1 molar ratio). We also observed the decomposition of this sample starting at 220 °C and a clear endothermic peak at 250 °C (Fig. S11, ESI†), while both FTIR (Fig. S12a, ESI†) and MS (Fig. S12b, ESI†) in the TPD experiment detect evolution of NH3 (black) and H2O (blue) gases starting at 220 °C. Thus, the similar behaviours of FAI + NiO and NH4I + NiO unambiguously show that FAI reacting with NiO at the interface results in FAI decomposing prematurely. In addition, this result also validates the simulation that NH4I is unstable on the NiO surface. As pointed out earlier, FAI and MAI when in contact with NiO show similar thermal stability, with both undergoing interfacial decomposition at ∼200 °C. These results suggest that the intrinsic stability is dictated by the oxide, rather than the perovskite.
NiO(001) (eV) | TiO2(001) (eV) | |
---|---|---|
NH3* | −0.77 | −1.43 |
HI* | −1.28 | −2.15 |
sym-triazine* | −0.87 | −1.42 |
HCN* | −0.39 | −0.83 |
NH4I* | −1.39 | −2.46 |
CH(NH2)2I* | −1.56 | −3.78 |
NH–CH–NH2* | −0.74 | −1.62 |
While interfacial decomposition is evident for FAI + NiO with lower Td, the released gases at 200 °C are only NH3 and H2O. The reason why sym-triazine is not detected at 200 °C along with NH3 is due to its higher adsorption energy than NH3* (−0.87 eV vs. −0.77 eV, Table 1). Thus, we observe sym-triazine from both interfacial and bulk decomposition starting at 270 °C as a result of its strong adsorption energy on the NiO surface. While HCN has a lower adsorption energy (Table 1), the reason that HCN is not observed at low temperature is because on the NiO surface, the reaction energy for FAI decomposing to produce sym-triazine is lower than HCN (−0.09 vs. 0.75 eV, Table S1, ESI†). Therefore sym-triazine, not HCN, is the decomposition product at 200 °C. At higher temperatures, configurational entropy favors HCN over sym-triazine. The free energies of HCN and sym-triazine can be estimated by considering entropy contribution using the values from JANAF table: −TΔS @227 °C = −1.15 eV.50 The entropy of sym-triazine is assumed to be 1/3 of the entropy of HCN. The free energy difference then becomes 0.07 eV at 227 °C and −0.11 eV at 327 °C. This implies that HCN can be generated from sym-triazine between 227 °C and 327 °C, without revoking the attack of hydrogen radicals from HI previously proposed.18 However, the formation energy for HCN from sym-triazine is higher by 0.84 eV on NiO (Table S1, ESI†) compared to 0.58 eV for FAI + TiO2. Therefore, HCN evolves at 270 °C along with sym-triazine on the NiO surface. The lower reaction energy along with lower Td supports the dominance of the interfacial reaction when FAI is in contact with NiO. Based on these results, the interfacial decomposition reaction of FAI + NiO can be written as:
(4) |
In contrast to NiO, the energy for FAI decomposition on the TiO2 surface according to reaction (3a) is 0.42 eV (green dashed lines, middle column) which is slightly higher than neat FAI decomposing into sym-triazine and NH4I (0.24 eV, green dashed lines, right column), so FAI + TiO2 mostly follow the bulk FAI decomposition pathway. Experimentally, we do not observe a significantly different Td (Fig. 1c, 2c and f). Similar to neat FAI, sym-triazine, HCN, and NH3 are the decomposed gaseous products. The decomposition of FAI + TiO2 should also produce HI. However, there is no reaction between TiO2 and HI* as the formation enthalpy of TiI4 is less negative compared to NiI2 (−0.9 eV vs. −2.4 eV),51,52 and is consistent with no TiI4 in the XRD (Fig. 3, bottom panel). Moreover, sym-triazine and NH3 are released at higher temperatures of 280 °C and 340 °C, respectively, while HCN is detected at a lower temperature of 260 °C. This is because the adsorption energies of sym-triazine* (−1.42 eV), NH3* (−1.43 eV), and HI* (−2.15 eV) are quite strong on TiO2 compared to the NiO surface, which explains why we cannot detect them under 280 °C. On the other hand, the HCN* binding energy (−0.83 eV) is significantly lower than the other three molecules and HCN formation energy from sym-triazine is also lower (−0.58 eV), and readily desorbs from the TiO2 surface.
It is noteworthy that the HCN formation is also affected by the oxide surface. We see that a higher amount of HCN is formed on both NiO and TiO2 surfaces compared to neat FAI. This is because the formation energy of HCN from sym-triazine on NiO (−0.84 eV) and TiO2 (−0.58 eV) surfaces is lower compared to neat FAI (−0.94 eV). In addition, the adsorption energies of HCN on both NiO and TiO2 surfaces are lower than sym-triazine (Table 1). Thus, the lower formation energy compared to FAI coupled with lower adsorption energy of HCN on oxide surfaces explains the larger amount of HCN detection on the FAI + TiO2 (Fig. 2f) and FAI + NiO samples (Fig. 2e), in particular when FAI is mixed with excess oxides (1:4 molar ratios, Fig. S6, ESI†).
Finally, the decomposition of FAI to gas-phase FA and HI (Fig. 4, orange solid lines and Table S1, ESI†) for all three cases is unfavourable, consistent with TPD results where we did not observe FA (m/z = 44) in any samples.
Footnotes |
† Electronic supplementary information (ESI) available: FAPbI3 synthesis and XRD, adsorption configurations, TGA-DSC, FTIR line spectra, TPD-MS bar charts, TPD-MS-FTIR of 1:4 FAI + NiO and FAI + TiO2, XRD patterns, reaction energy table, TGA-DSC of NH4I, XRD of FAI after TPD, TPD-MS of NH4I, TGA-DSC and TPD-MS-FTIR of 1:1 NH4I + NiO are provided. See DOI: 10.1039/d0ma00624f |
‡ These two authors contributed equally. |
This journal is © The Royal Society of Chemistry 2020 |