Melt Processing of Hybrid Organic–Inorganic Lead Iodide Layered

Hybrid organic–inorganic perovskite (HOIP) materials have attracted considerable interest over the last few decades because of their structural dive...
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Melt Processing of Hybrid Organic−Inorganic Lead Iodide Layered Perovskites Tianyang Li, Wiley A. Dunlap-Shohl, Qiwei Han, and David B. Mitzi* Department of Mechanical Engineering and Materials Science, Duke University, Durham, North Carolina 27708, United States S Supporting Information *

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Previous reports have shown that thermal properties, especially the melting temperatures of Sn-based two-dimensional (2D) layered HOIPs, can be tuned via the alteration of the organic cation structure and that melt processing of high quality films can be achieved.13,14 Considering the similarities in crystal structure, it seems reasonable to question whether this approach can be extended to 2D lead iodide perovskites as well. Here we demonstrate that Pb-based HOIPs have ∼25−45 °C higher melting points compared to analogous Sn-based compounds with the same organic cations, i.e., the melting temperatures vary from ∼200 to 260 °C. The higher temperatures pose an additional challenge for translating the melt-based film deposition from tin(II)- to lead(II)-based HOIPs, without encountering decomposition. Nevertheless, we also show that, with suitable choice of organic cation, highly crystalline Pb−halide-based HOIP films can be melt processed in air and at moderate temperature, thereby opening up the possibility of using this versatile approach for preparing films over the broad range of applications currently envisioned for the HOIP family. 2D layered lead iodide perovskite phases (known as the Ruddlesden−Popper phases) have a general formula of (RNH3)2MAn−1PbnI3n+1. Recently, alkylammonium- and phenethylammonium-based layered perovskites have shown promising results in efficiency/stability for photovoltaic and light emitting devices.22−25 However, given that some alkylammonium-based layered lead iodide perovskites have been reported to undergo considerable decomposition before melting,26 we choose to focus on phenethylammonium (PEA) derivatives as the organic cation. The structure of PEA2PbI4 is shown in Figure S1 as an example. To understand how these organic cations influence the thermal properties, we create several analogous compounds with different phenethylammonium derivatives: phenethylammonium (PEA), beta-methylphenethylammonium (β-Me-PEA), 4-methoxyphenethylammonium (4-MeO-PEA), 2-fluorophenethylammonium (2-F-PEA), 3-fluorophen-ethylammonium (3-F-PEA) and 4-fluorophenethylammonium (4-F-PEA). These six compounds were studied using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (see Table 1 and Figure 1). Compared with the analogous 2D Sn systems,13,14 Pb-based HOIPs with the same organic cation all show higher melting temperature, Tm, by ∼25−45 °C (Table S1). This shift in melting temperature likely arises from the higher bond ionicity

ybrid organic−inorganic perovskite (HOIP) materials have attracted considerable interest over the last few decades because of their structural diversity and novel magnetic, electrical and optical properties.1 Early interest in HOIPs also focused on moving beyond simple organic or inorganic devices, to organic−inorganic hybrid electronics, in which useful attributes of organic and inorganic systems combine within a single molecular-scale composite.2 In recent years, lead−halide-based hybrid perovskites, owing to their appropriate band gap size, strong visible light absorption, effective separation of photogenerated carriers and facile processing techniques, have been successfully integrated as active layers within photovoltaic and light emitting devices, and this has given rise to a strong renewed interest in HOIP research.3−8 One key aspect of the HOIP field that renders these systems particularly attractive for application is the diversity of low-cost and simple approaches for fashioning the hybrids into thin-film form for device integration. So far, the main research focus on processing has been directed toward solution processing (especially spin-coating) of HOIP thin films,9,10 with power conversion efficiencies of associated record photovoltaic devices rapidly increasing to >20% after less than a decade since their introduction.11 Solution processing can be particularly attractive as it provides facile pathways to make thin films at low temperature and does not require complicated vacuum systems. Despite these advantages, solution processing faces several limitations, including the use of toxic solvents (e.g., N,N-dimethylformamide), requirement of additional postannealing steps, difficulties in adapting laboratory fabrication methods to industrial scale, and, in some cases, limited grain sizes within the films. Furthermore, solvent−solute reactions have been implicated in transformation of the organic cation component of the perovskite, possibly reducing device performance.12 Alternatively, melt processing has been applied in processing polymers, inorganic materials as well as hybrid materials that melt before decomposition.13−17 It also provides opportunities for roll-toroll, lamination, capillary filling and extrusion methods.18−21 Further, it has the potential for increasing industrial throughput relative to the more traditional layer-by-layer fabrication methods because other components of the device may be deposited simultaneously on different substrates, then laminated in the final melt-processing step. Compared to spin coating for lead halide perovskite film preparation, melt processing has inherent advantages such as removal of the need for solvents, potential crystal orientation control with the use of a temperature gradient, and one-step self-encapsulation without the need for an external adhesive. © 2017 American Chemical Society

Received: June 8, 2017 Revised: July 11, 2017 Published: July 12, 2017 6200

DOI: 10.1021/acs.chemmater.7b02363 Chem. Mater. 2017, 29, 6200−6204

Communication

Chemistry of Materials

samples) to the melting transition. Similar structural transitions have been observed/characterized within the analogous Snbased systems.13,14 A clear goal for enabling melt processing must be to lower the melting temperature safely below the decomposition point of the HOIP. Despite the fact that the variation in decomposition temperatures is relatively small for all six 2D HOIPs, the temperatures where the structural transition and melting transition occur can depend quite significantly on the choice of the organic cation. For example, for F-substituted PEA compounds, both the structural transition and melting temperature increase when the position of the F atom moves from the 2- to 4-position on the phenyl ring. Although most of the compounds start to decompose before melting, β-MePEA2PbI4 melts at a much lower temperature, with minimal peak overlap between melting and decomposition in the DSC scan. The variation in structural transition and melting temperatures presumably arises from the change in strength of the hydrogen bonding interaction between the organic and inorganic component, as well as other subtle structural changes within the inorganic layer. The identity and position of the substituent on the phenyl ring (or on the alkyl chain) impacts the interaction between the cation and anionic sublattice through steric and electronic effects. These effects, which for example may lead to different tilting of the PbI6 octahedra within the crystal structure, impact the band gap energy and position of the associated exciton peak and may also help to determine the thermal properties.28 Interestingly, the correlation between F-substitution site and melting temperature, as well as the low melting temperature of the β-Me-PEA analog reflect the results from the analogous Sn-based systems,13,14 showing that melt-processing materials design rules can translate from one metal-based system to another. A more detailed study of the correlation between the crystal structure and phase transition temperature is currently underway. Choosing the right organic cation enables the formation of a stable melt at a relatively low temperature (∼200 °C), in order to process films of the 2D Pb perovskites. This can be feasibly achieved by heating powdered samples of the HOIP on a substrate (1.2-mm-thick glass) covered with a thin (8-μmthick) Kapton sheet in an ambient environment. A small amount of extra corresponding organic iodide salt is added and mixed to compensate for any possible loss from decomposition. The substrate is placed on a hot plate that maintains the temperature 5 °C above the HOIP melting point. When the powder melts, the yellow melt liquid spreads between the substrate and the Kapton sheet by the capillary effect. The film is then quickly pressed from the top by a separate preheated plate and then removed from the hot plate to cool down. Using β-Me-PEA2PbI4 or 4-MeO-PEA2PbI4 as the active melt processing materials, thin films with excellent crystallinity can be achieved with negligible detected decomposition or secondary phases. X-ray diffraction (XRD) patterns of asprepared films only show series of narrow (00l) peaks, due to the 2D nature and preferred orientation of these compounds. An additional very small peak at 2θ ≈ 12.7° is from PbI2, and is only visible using a square root or logarithmic intensity scale, as shown in Figure 2a,b. The peak positions and relative peak intensities of the melt-processed and spin-coated films show no obvious differences. A closer examination of the XRD patterns show that the d-spacing values extracted from the first five (00l) peaks are almost identical, whereas the full width at halfmaximum (fwhm) widths of the peaks are smaller for the melt-

Table 1. Transition Temperatures and Enthalpies for Structural and Melting Transitions in Pb−Halide HOIPs β-Me-PEA2PbI4 2-F-PEA2PbI4 4-MeO-PEA2PbI4 PEA2PbI4 3-F-PEA2PbI4 4-F-PEA2PbI4 a

Ts (°C)

ΔH (kJ/mol)

Tm (°C)

ΔH (kJ/mol)

170.7 188.3 181.2 214.2 199.2 211.6 244.3

17.5 0.99 9.8 26.0 20.3 27.1 43.5

207.0

9.5

245.8 247.1 252.9 261.4 258.9

13.0a 12.9 12.9a 25.3a 17.7a

Estimated from overlapping peaks.

Figure 1. Derivative of the weight loss (black lines) and heat flow (red lines) as a function of temperature for six related 2D lead iodide HOIPs.

in the inorganic backbone.27 Note that a previous study has shown that Ge-based layered perovskites can have an even lower melting point than Sn-based, consistent with this trend.26 All six compounds from the current study, as shown in the TGA weight loss curves (Figure S2), clearly undergo three stages of decomposition. No weight loss is observed below 200 °C. Between 200 and 280 °C, about 40%−45% of the initial weight is lost, most likely due to the organic component partial dissociation. Above 280 °C and below 340 °C, the rest of the organic component is lost and PbI2, which comprises ∼47% of the original weight, remains in the solid form. Above 340 °C, the slow weight loss can be attributed to sublimation of PbI2. From the DSC scans of these compounds, one or more endothermic transitions can be observed, followed by a very broad exothermic peak starting at ∼220 °C, which corresponds to HOIP decomposition. The first endothermic transition (first two transitions for β-Me-PEA2PbI4) labeled as Ts are assigned as structural transitions, while the later endothermic transition (Tm) corresponds (determined from visual examination of the 6201

DOI: 10.1021/acs.chemmater.7b02363 Chem. Mater. 2017, 29, 6200−6204

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Figure 3. Absorption (a, b) and photoluminescence (c, d) spectra of spin-coated and melt-processed films of β-Me-PEA2PbI4 (a, c) and 4MeO-PEA2PbI4 (b, d).

processed and spin-coated films of β-Me-PEA2PbI4 exhibit broad asymmetrical photoluminescence peaks at 2.39 and 2.37 eV, respectively (Figure 3c). The long tail on the lower energy side of the spectrum may suggest a high concentration of defect states between the conduction and valence band.30,31 On the other hand, the 4-MeO-PEA2PbI4 films yield much narrower peaks centered at 2.37 eV (Figure 3d), with the melt-processed peak slightly broader than the spin-coated one. We speculate that the slight variation of absorption and photoluminescence peak broadening for spin coating and melt processing could arise from an increase in shallow defects close to the band edge for the latter, originating from organic cation or halogen deficiency.32,33 In conclusion, as for Sn-based systems, the thermal properties of 2D lead iodide HOIPs can be tailored using the organic cation structure, providing successful candidates that melt below the decomposition temperature. The melting temperatures of Pb-based perovskites are 20−45 °C higher compared to the Sn-based analogues with the same phenethylammonium-based organic cations and melt-processing materials design rules appear to translate from one metalbased system to another (e.g., ordering of melting temperature with substitution position on the phenyl ring of the cation13,14). Furthermore, melt processed films of these 2D lead-based HOIPs, prepared in the ambient air and with high phase purity and crystallinity, have been demonstrated for the first time. This demonstration can potentially open up exciting new avenues for device fabrication, such as roll-to-roll techniques and direct lamination or encapsulation of active HOIP films between two substrates. Notably, one of the limitations of melt processing is that the material candidates must melt before decomposition, whereas most HOIP materials tend to have poor thermal stability at high temperatures. The key lies in finding the right materials that can meet such criteria and at the same time show suitable properties for optoelectronic applications. Further efforts to understand the structural/ compositional underpinning of melting temperature variation, demonstration of melt-processed lower band gap and higher dimensional n > 1 perovskite films, and development of

Figure 2. X-ray diffraction patterns (square root scale) of spin-coated and melt-processed films of (a) β-Me-PEA2PbI4 and (b) 4-MeOPEA2PbI4; SEM images of (c, d) spin-coated and (e, f) melt-processed films of β-Me-PEA2PbI4 (c, e) and 4-MeO-PEA2PbI4 (d, f). Scale bar: 50 μm. Inset in panel a: melt-processed β-Me-PEA2PbI4 film on glass substrate with Kapton sheet cover.

processed films, indicating better crystallinity compared with the spin coated analogs (Table S2). Examination of the as prepared films using scanning electron microscopy (SEM) reveals the high quality of the melt-processed films (Figure 2e,f). Both spin-coated and melt-processed films show good coverage free of pinholes, with very large lateral grain size of at least 10 μm, consistent with the narrow peaks in the XRD data. Based on cross section SEM, the thickness of the meltprocessed films is estimated to be around 500 to 800 nm, depending on the amount of sampling loading and applied pressure during melting. Figure 3a shows the absorption spectra of the melt-processed and spin-coated β-Me-PEA2PbI4 films. For the spin-coated film, a strong broad exciton absorption peak appears at 2.45 eV (estimated from the peak maximum), with a weaker second feature at 2.80 eV. The band gap energies of these layered lead iodide perovskites are typically around 2.5 eV; however, because of the broadening of the exciton peak at room temperature, this transition obscures the direct observation of the band edge.29 Thus, in this case we refrain from extracting the exact band gap energy or examining the absorption profile very close to the band edge. For the analogous melt-processed film, the same exciton peak still can be seen, but with a slight (0.03 eV) shift to lower energy. For 4-MeO-PEA2PbI4 films (Figure 3b), the exciton peak appears at 2.41 eV, which also partially overlaps with the weak band-edge absorption on the right. The melt-processed film shows a similar exciton peak at 2.39 eV, which appears to be slightly broader. Both melt6202

DOI: 10.1021/acs.chemmater.7b02363 Chem. Mater. 2017, 29, 6200−6204

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Chemistry of Materials mechanisms to control film crystallographic orientation, thickness and quality via precise temperature/pressure control, are all important areas of future study and should provide a means of enabling device application of this approach.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02363. Synthesis and melt processing procedures, characterization details, TGA weight loss profiles, XRD patterns of the six compounds, and detailed XRD comparison of spin-coated and melt-processed films (PDF)

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AUTHOR INFORMATION

Corresponding Author

*D. B. Mitzi. E-mail: [email protected]. ORCID

David B. Mitzi: 0000-0001-5189-4612 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This material is based upon work financially supported by the ONR through award number N00014-17-1-2207. The work was performed in part at the Duke University Shared Materials Instrumentation Facility (SMIF), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI). All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of the ONR or NSF.

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REFERENCES

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