Pyrrolidinium containing perovskites with thermal stability and water resistance for photovoltaics

Abstract

The commonly employed methyl-ammonium containing perovskites are unstable at high temperature or under moisture attack, thus limiting their commercial applications. To overcome this barrier, a new perovskite possessing both thermal stability and water resistance has been constructed, involving pyrrolidinium rings (CH₂)₄NH₂PbI₃ (PyPbI₃), through a simple solution-processing method. It presents not only prolonged moisture resistance and a favorable bandgap of 1.80 eV, as determined by P-XRD and UV absorption, but also excellent thermal stability, as verified via the in situ XRD technique. The new structure may be applicable to perovskite solar cells with improved thermal stability and water resistance.

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1. Introduction

With the rapid improvement of the power conversion efficiency (PCE) from 3.8% to 24.2% ¹ in less than one decade, perovskite solar cells (PSCs) have been widely considered as a promising candidate for next generation photovoltaics due to their suitable bandgap,² excellent absorption properties³ and simple solution-processing synthesis methods.^4,5 However, despite their impressive performance, a major bottleneck of commercialization remains. PSCs are not only unstable in humid environments,^6,7 but also show poor thermal stability at the working temperature⁸ (85 °C). The most commonly employed PSC material CH₃NH₃PbI₃ (MAPbI₃) was shown to be intrinsically thermally-unstable.⁸ In order to fight against the moisture instability alone, various strategies have been developed, including device encapsulation,⁹ and surface passivation,¹⁰ but little improvement has been achieved. As a result, replacing MA cations seems inevitable.^8,11–13

Although replacement with various cations has been tested, such as formamidine (FA),^14,15 hydrazine (HA)¹⁶ and many others,¹⁷ still none resolves the stability issue. This may be attributed to the hydrophilic nature of these cations.^10,18 Therefore, a hydrophobic structure has been selected and introduced into the perovskite structure to enhance the water resistance in some recent attempts.^19–24 In this regard, three-membered ring based aziridinium lead iodide has been simulated,²⁵ which showed a theoretical bandgap as low as 1.49 eV and good water resistance. Yet, due to the extreme toxicity of aziridine,²⁶ it is impractical to apply to real devices. Although such toxicity can be eliminated by applying a four-membered azetidinium ring-based perovskite AzPbI₃, it raises the bandgap to 2.15 eV, which is significantly higher than that of both MAPbI₃ and FAPbI₃, resulting in a low PCE of less than 1%.^27,28 Moreover, the structure of AzPbI₃ is extremely distorted,²⁷ because of the severe strain in the four-membered azetidine rings, which aggravates the thermal instability.

Based on these developments, a novel five-membered pyrrolidinium ring-based perovskite PyPbI₃ was synthesized in our previous attempt,²⁹ to release the strain on the Az cations. Such a structure was proved to be highly ordered and moisture stable,²⁹ offering a possible solution to the long-standing moisture instability problem. However, the thermal stability of PyPbI₃ remains uncertain, which is imperative, crucial, and needs to be investigated.

It is, therefore, the purpose of this study to resolve the stability problems in MA-based perovskites by investigating the thermal stability of PyPbI₃. Based on the results of in situ X-Ray diffraction (XRD), UV absorption and photoluminescence, we were able to uncover the thermal stability of PyPbI₃, which remains intact up to 135 °C, as well as to confirm the smaller bandgap of PyPbI₃ (1.80 eV) than that of AzPbI₃ (∼2.15 eV). In addition, PyPbI₃ was found to possess enduring moisture resistance after exposure to ambient air for 4 months. These results not only make PyPbI₃ a promising candidate for photovoltaics, but also present a possible solution to the long-standing stability problem of perovskite solar cells.

2. Experimental

2.1 Film formation

1 mmol of pyrrolidinium hydroiodide (98%, TCI) and 1 mmol of lead iodide (99.999%, Sigma) were dissolved in 1 ml DMF (99.99%, Sigma), respectively. The drop casting method and spin coating method were adopted to fabricate the perovskite thin films. After annealing at 120 °C for 40 minutes, polycrystalline films were obtained from both methods. The structure was confirmed via P-XRD (Fig. S1, ESI†) and H-NMR spectra (Fig. S2, ESI†).

2.2 In situ XRD measurements

X-Ray diffraction (XRD) patterns of the polycrystalline films were collected using a Bruker-AXS D8 DISCOVER X-ray diffractometer with CuKα1 radiation (λ = 1.79026 Å) in the range of 8–78° (2θ) with a step size of 0.002° and a time setting of 0.1 s per step. For in situ XRD experiments, the films were placed in a sample chamber armed with a temperature-control stage. The thermal XRD patterns were collected every 30 minutes.

2.3 Characterization

UV-vis spectra were acquired from thin film samples at room temperature using a Cary 5000 spectrophotometer. The Tauc plot was used to estimate the bandgap of the material. The photoluminescence test is conducted using an Ar-ion laser (excitation wavelength = 514.5 nm) at room temperature from Melles Griot (35LAP431208) at a power of 130 mW with a repetition rate of 50 Hz. The PL signals were collected using a liquid N₂-cooled silicon CCD. Scanning electron microscopy was carried out on a JEOL JSM-7000F instrument operating at a 0.1–5.0 kV landing voltage.

A Nicolet 6700 FT-IR spectrometer was used to acquire the FTIR spectra in the wavenumber range of 500–4500 cm⁻¹ with a resolution of 1 cm⁻¹ in DRIFT mode, dispersing the powdered samples in dry KBr powder.

TGA measurements on the PyPbI₃ samples were performed using a Netzsch 409 PC Luxx station, with a temperature range from ambient to a maximum of 1600 Celsius.

3. Results and discussion

To investigate the thermal stability of PyPbI₃, in situ XRD with a high temperature stage was employed. The initial temperature was set at 40 °C, and gradually increased to 135 °C. The temperature was held for 30 min for each stage. As shown in Fig. 1, no obvious change was observed in the XRD patterns for all stages, which indicated the thermal stability of PyPbI₃. In addition, TGA measurements were also taken to evaluate the thermal stability of PyPbI₃, as illustrated in Fig. S3 (ESI†).

Fig. 1 In situ thermal XRD of PyPbI₃.

Moreover, the XRD test also confirmed the long-term moisture resistance of PyPbI₃ on a thin film sample which has been exposed to a humid environment for 4 months. Such a test is still of huge significance for the enduring stability even though the moisture stability of PyPbI₃ has been previously tested via dripping water for 30 min.²⁹ No decomposition was found in the XRD patterns, as indicated in Fig. 2, which presented excellent moisture resistance of PyPbI₃. These results prove that PyPbI₃ is stable not only at high temperature way above working conditions, but also in moist environments for an extensive period of time.

Fig. 2 XRD patterns of PyPbI₃ before (black) and after (red) exposure to ambient air for 4 months.

To further ensure the suitability of PyPbI₃ in photovoltaic application, the bandgap needs to be determined, which is given by UV absorption and steady-state photoluminescence (PL) tests. The onset of the absorption spectra is located at 660 nm, which reveals a bandgap of the material of 1.80 eV as derived from the Tauc plot, with an absorption coefficient of more than 10⁴, as shown in Fig. 3. Such a result is consistent with the photoluminescence result (Fig. 4), which indicated the bandgap value of PyPbI₃ (∼1.80 eV) is, indeed, significantly lower than that of AzPbI₃ (∼2.15 eV), making it more promising for photovoltaic applications.

Fig. 3 UV absorption and the Tauc plot (inset) of the PyPbI₃ thin film.

Fig. 4 Steady state photoluminescence (PL) result of PyPbI₃.

Other than the bandgap, another important factor that influences the device performance is the film morphology, which was detected by field emission SEM (JEOL JSM-7000F), as shown in Fig. 5. Clearly, the PyPbI₃ thin film consists of many large grains, with an average size of 2–3 μm, which is comparable to that of other studies.^4,27,30

Fig. 5 SEM image of the PyPbI₃ thin film.

The defect density and the conductivity of the crystal were found to be 2.3 × 10¹⁶ cm⁻³ and 0.0177 (Ohm cm)⁻¹, respectively, following the method of the literature.^31,32 The value is comparable to that of MA-based perovskites.³³

3.2 Discussion

As demonstrated, PyPbI₃ has presented exceptional thermal stability and long-term moisture resistance, which is in sharp contrast to the literature, where the usual MA and FA based perovskites show neither moisture resistance nor thermal/phase stability. Obviously, for practical solar cell applications, the PyPbI₃ structure is much more favourable due to its outstanding thermal/moisture stability.

This superiority of the bandgap and stability is attributed to the unique 1D structure of PyPbI₃, as illustrated in its single XRD patterns (Fig. S4, ESI†), which show a hexagonal structure, with the PbI₂ inorganic chains arranged in a face sharing manner and organic pyrrolidinium cations embedded between them. One huge benefit of this face sharing arrangement is that it results in a larger volume in the inorganic cage than 3D perovskites, as compared in Table 1 and Fig. 6, where the intramolecular force is weakened and imposes little constraint on the rotation of the Py cation. As a result, the organic cation is free to move in the cage and this leads to 12-fold dipole orientations even at room temperature, as verified by our single crystal XRD²⁹ and FTIR (Fig. S5, ESI†) results. Therefore, the room temperature phase of PyPbI₃ remains at the working temperature.

Table 1 Comparison of the lattice parameters in various references

Sample	MAPbI3 ^34,35	FAPbI3 ^36	PyPbI3 ^29
Space group	Pm3m	Pm3m	P63/mmc
Crystal system	Cubic	Cubic	Hexagonal
Lattice parameter	a = 6.276(4) Å	a = 6.3620(8) Å	a = 9.3117(5) Å
			c = 8.1080(4) Å
	V = 247.1(0.4) Å^3	V = 257.51(5) Å^3	V = 608.84(7) Å^3

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Fig. 6 Packing image of the infinite [PbI₃⁻]_n chains highlighting the interchain I–I distances. Lead black and iodine purple.

The superb moisture resistance of PyPbI₃ can be attributed to its unique ring structure, which is composed of pyrrolidinium cations. The hydrophobic nature of these cations stops the invasion of water molecules,^13,19 and thus improves the moisture resistance of the perovskites.

Although the PyPbI₃ only device is still under development, promising results were already obtained from PyPbI₃ containing FAPbI₃ devices with a PCE of 19.2%, in addition to much enhanced stability.

4. Conclusions

To summarize, ring based pyrrolidinium lead iodide was synthesized and investigated. The newly fabricated PyPbI₃ exhibits not only outstanding humidity resistance after exposure to air for 4 months, but also excellent thermal stability up to 135 °C, which is significantly higher than the phase transformation temperature of commonly employed MAPbI₃. In addition, PyPbI₃ also possesses a suitable bandgap of 1.80 eV, which enables a theoretical PCE above 26%, as estimated by the Shockley–Queisser limit. Given that no phase transformation was found in PyPbI₃ at the perovskite working temperature, it will be of great potential to replace the existing perovskite structures for solar cell application, not only with both the required stability and desired PCE, but also with stable device performance.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Natural Sciences and Engineering Research Council of Canada. The authors acknowledge SEM support from Canadian Center of Electron Microscopy (CCEM) and XRD support from MAX Diffraction Facility of McMaster University.

References

1. NREL Best Research-Cell Efficiency chart, https://www.nrel.gov/pv/assets/pdfs/best-research-cell-efficiencies.20190802.pdf.
2. A. Kojima, K. Teshima, Y. Shirai and T. Miyasaka, Organometal halide perovskites as visible-light sensitizers for photovoltaic cells, J. Am. Chem. Soc., 2009, 131(17), 6050–6051.
3. Y. Wang, Y. Zhang, P. Zhang and W. Zhang, High intrinsic carrier mobility and photon absorption in the perovskite CH₃NH₃PbI₃, Phys. Chem. Chem. Phys., 2015, 17(17), 11516–11520.
4. W. Nie, et al., High-efficiency solution-processed perovskite solar cells with millimeter-scale grains, Science, 2015, 347(6221), 522–525.
5. N. Ahn, et al., Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated via Lewis Base Adduct of Lead(II) Iodide, J. Am. Chem. Soc., 2015, 137(27), 8696–8699.
6. Y. Rong, et al., Challenges for commercializing perovskite solar cells, Science, 2018, 361, eaat8235.
7. J. Yang, B. D. Siempelkamp, D. Liu and T. L. Kelly, Investigation of CH₃NH₃PbI₃ degradation rates and mechanisms in controlled humidity environments using in situ techniques, ACS Nano, 2015, 9(2), 1955.
8. B. Conings, et al., Intrinsic Thermal Instability of Methylammonium Lead Trihalide Perovskite, Adv. Energy Mater., 2015, 5(15), 1500477.
9. Y. Han, et al., Degradation observations of encapsulated planar CH₃NH₃PbI₃ perovskite solar cells at high temperatures and humidity, J. Mater. Chem. A, 2015, 3, 8139–8147.
10. X. Zheng, et al., Defect passivation in hybrid perovskite solar cells using quaternary ammonium halide anions and cations, Nat. Energy, 2017, 2, 17102.
11. R. K. Gunasekaran, et al., Revealing the Self-Degradation Mechanisms in Methylammonium Lead Iodide Perovskites in Dark and Vacuum, ChemPhysChem, 2018, 19, 1507–1513.
12. Y. Y. Zhang, et al., Intrinsic Instability of the Hybrid Halide Perovskite Semiconductor CH₃NH₃PbI₃, Chin. Phys. Lett., 2018, 6(3), 57–62.
13. S.-H. Turren-Cruz, A. Hagfeldt and M. Saliba, Methylammonium-free, high-performance, and stable perovskite solar cells on a planar architecture, Science, 2018, 362, 449–453.
14. T. M. Koh, et al., Formamidinium-containing metal-halide: An alternative material for near-IR absorption perovskite solar cells, J. Phys. Chem. C, 2014, 118, 16458–16462.
15. G. E. Eperon, et al., Formamidinium lead trihalide: A broadly tunable perovskite for efficient planar heterojunction solar cells, Energy Environ. Sci., 2014, 7, 982–988.
16. A. F. Akbulatov, et al., Hydrazinium-loaded perovskite solar cells with enhanced performance and stability, J. Mater. Chem. A, 2016, 4, 18378–18382.
17. R. T. Wang, et al., A KMnF₃ perovskite structure with improved stability, low bandgap and high transport properties, Ceram. Int., 2019, 45, 64–68.
18. T. Niu, et al., High performance ambient-air-stable FAPbI₃ perovskite solar cells with molecule-passivated Ruddlesden-Popper/3D heterostructured film, Energy Environ. Sci., 2018, 11, 3358–3366.
19. L. N. Quan, et al., Ligand-Stabilized Reduced-Dimensionality Perovskites, J. Am. Chem. Soc., 2016, 138, 2649–2655.
20. G. Grancini, et al., One-Year stable perovskite solar cells by 2D/3D interface engineering, Nat. Commun., 2017, 8, 15684.
21. D. Liu, et al., Improved stability of perovskite solar cells with enhanced moisture-resistant hole transport layers, Electrochim. Acta, 2019, 296, 508–516.
22. T. Zhang, et al., Low-temperature processed inorganic perovskites for flexible detectors with a broadband photoresponse, Nanoscale, 2019, 11, 2871–2877.
23. F. Wang, et al., Steering the crystallization of perovskites for high-performance solar cells in ambient air, J. Mater. Chem. A, 2019, 7, 12166–12175.
24. L. Ji, et al., Band alignment of Pb-Sn mixed triple cation perovskites for inverted solar cells with negligible hysteresis, J. Mater. Chem. A, 2019, 7, 9154–9162.
25. C. Zheng and O. Rubel, Aziridinium Lead Iodide: A Stable, Low-Band-Gap Hybrid Halide Perovskite for Photovoltaics, J. Phys. Chem. Lett., 2018, 9(4), 874–880.
26. F. M. D. Ismail, D. O. Levitsky and V. M. Dembitsky, Aziridine alkaloids as potential therapeutic agents, Eur. J. Med. Chem., 2009, 44(9), 3373–3387.
27. S. R. Pering, et al., Azetidinium lead iodide for perovskite solar cells, J. Mater. Chem. A, 2017, 5, 20658.
28. R. Panetta, et al., Azetidinium lead iodide: Synthesis, structural and physico-chemical characterization, J. Mater. Chem. A, 2018, 6, 10135.
29. A. F. Xu, et al., Pyrrolidinium lead iodide from crystallography: A new perovskite with low bandgap and good water resistance, Chem. Commun., 2019, 55, 3251–3253.
30. W. S. Yang, et al., High-performance photovoltaic perovskite layers fabricated through intramolecular exchange, Science, 2015, 348(6240), 1234–1237.
31. D. Shi, et al., Low trap-state density and long carrier diffusion in organolead trihalide perovskite single crystals, Science, 2015, 347(6221), 519–522.
32. Z. Liu, et al., Chemical Reduction of Intrinsic Defects in Thicker Heterojunction Planar Perovskite Solar Cells, Adv. Mater., 2017, 29(23), 1606774.
33. N. Liu, et al., Extremely low trap-state energy level perovskite solar cells passivated using NH₂-POSS with improved efficiency and stability, J. Mater. Chem. A, 2018, 6, 6806–6814.
34. M. T. Weller, O. J. Weber, P. F. Henry, A. M. Di Pumpo and T. C. Hansen, Complete structure and cation orientation in the perovskite photovoltaic methylammonium lead iodide between 100 and 352 K, Chem. Commun., 2015, 51(20), 4180–4183.
35. T. Baikie, et al., Synthesis and crystal chemistry of the hybrid perovskite (CH₃NH₃)PbI₃ for solid-state sensitised solar cell applications, J. Mater. Chem. A, 2013, 1(18), 5628.
36. M. T. Weller, O. J. Weber, J. M. Frost and A. Walsh, Cubic Perovskite Structure of Black Formamidinium Lead Iodide, α-[HC(NH₂)₂]PbI₃, at 298 K, J. Phys. Chem. Lett., 2015, 6(16), 3209–3212.

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Footnote

† Electronic supplementary information (ESI) available: P-XRD of the PyPbI₃ product, H-NMR spectra, TGA curve and FTIR spectra. See DOI: 10.1039/c9tc02800e

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