Microwave Induced Crystallization of the Hybrid Perovskite

Tom Kollek , Daniel Wurmbrand , Susanne T. Birkhold , Eugen Zimmermann , Julian Kalb , Lukas Schmidt-Mende , and Sebastian Polarz. ACS Applied Materia...
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Microwave Induced Crystallization of the Hybrid Perovskite CH3NH3PbI3 from a Supramolecular Single-Source Precursor Tom Kollek, Christian Fischer, Inigo Göttker-Schnetmann, and Sebastian Polarz Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b01263 • Publication Date (Web): 05 Jun 2016 Downloaded from http://pubs.acs.org on June 7, 2016

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Microwave Induced Crystallization of the Hybrid Perovskite CH3NH3PbI3 from a Supramolecular Single-Source Precursor Tom Kollek, Christian Fischer, Inigo Göttker-Schnetmann, Sebastian Polarz* (*) Department of Chemistry, University of Konstanz, D-78457 Konstanz, Germany Supporting Information available ABSTRACT: Hybrid perovskites like CH3NH3PbI3 (MAPI) have been the recent spotlight of materials science for their outstanding properties as a new class of semiconductors. Their solution-based synthesis offers versatile processing, but complex chemical interactions of the organic cation, the coordinating solvent and inorganic component are influencing the crystal formation, and most likely the optoelectronic properties. Consequently, solid intermediate states can arise to limit the controlled crystallization of defined MAPI morphologies. In particular, the design of desired nanostructures bears major difficulties and hampers of defined single source precursors owing structure related properties. Here, we introduce a new systematic precursor system based on various oligo-glycols with single-source characteristics, addressing MAPI pre-organized on the molecular level. Elevated temperature trigger the formation of MAPI starting from a stable ionic liquid precursor. This enables the unique access for a microwave- assisted synthesis of MAPI scaffolds that reveal a local photoluminescence enhancement behavior at the outer surface.

Perovskites (ABX3) have become one of the most versatile

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classes of solids in diverse and highly functional materials. Whereas oxidic perovskites are well-known since decades, the family of hybrid perovskite materials (HYPE) like 3 CH3NH3PbI3 (MAPI) have provoked enormous scientific attention as it was used as absorber material in photovoltaic 4 5 devices with liquid electrolyte in 2009 and in 2012 , in an allsolid device architecture. Since then, especially for their unique semiconducting properties and their advanced per6 formance for innovative photovoltaic devices. Further prom7 8 ising areas of application have arisen as LEDs , lasers , pho9 10 todetectors and in spintronics. As a result, the search for reliable synthesis routes for HYPE materials including novel nanostructures like defined nanoparticles and thin films has 11 been proven to be a difficult task, in particular for MAPI. Allocation of unique HYPE nanostructures and insights how such features influence functional properties stands at the beginning, and yet synthesis protocols comprising defined molecular precursors have not developed. We introduce a new precursor system with single-source characteristics, addressing MAPI pre-organized on the molecular level. The formation of the perovskite is induced by temperature tuning the molecular architecture of the precursor to be a stable liquid. This enables unique access to a microwave assisted synthesis of MAPI scaffolds that reveal a local photoluminescence enhancement behavior at the outer surface. There exists a notable repertoire of synthetic protocols to HYPEs ranging from two-step procedures to direct crystalli12-14 zation. As nucleation and growth are difficult to control for such deposition-precipitation from saturated solutions by evaporating of the solvent, the results are dependent on various experimental parameters which reduces the repro15 ducibility. Conditions like oxygen rich atmosphere, humidi16-17 ty or processing temperature, lead to a mixture of crystal sizes, surface chemistry and morphologies making it hard to

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compare material properties. Furthermore, the intrinsic chemistry of the best performing HYPEs like MAPI is delicate to the stoichiometric ratio of the precursor solution leading to lower dimensional cluster formation, and ion migration in 19-20 the final perovskite phase. Thus, different crystallization pathways of MAPI occur which are (i) crystallization from supersaturated solution, (ii) dissolution crystallization or (iii) 21 in-situ transformation. Several of the mentioned problems needed to be considered earlier for other important classes of materials, for example metal chalcogenides. One learned the generation and employment of defined precursors bears major benefits and 22 opens new ground for materials synthesis. In particular, the chemical and physical properties of the precursor that can be tuned by means of molecular architecture and material formation is triggered by stimuli like reactants or temperature. The special gain in a liquid materials precursor are unique 23 processing steps, for example the possibility of infiltration into templates. Further, if a distinct compound as a precursor contains all elements needed and its transformation leads direct to this material the term 'single-source' could be ap24 plied. Besides the common precursor, like salt solutions of PbI2 and MAI based on coordinating solvents (e.g. DMF, DMSO), 25 that include several low dimensional intermediate states , chemistry of defined precursors for HYPEs is poorly developed and only few papers have already addressed this topic. In 2015 Zhao et al. published an interesting report about HPbI3 as a precursor compound to replace PbI2 in HYPE 26 synthesis. Yang et al. used a complex, solid precursor comprising dimethylsulfoxide intercalated in a PbI2/ forma27 midinium phase. Our group introduced a CH3NH3PbI3TEG2 compound (TEG (triethylene glycol), which transforms into MAPI via adding an anti-solvent and a crystal-to-crystal 28 transformation. A defined liquid, single-source precursor

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system for the synthesis of HYPE nanostructures does currently not exist and represents a major challenge, we pursuit here. A systematic series of precursor-compounds owing length defined oligo-glycol derivatives (oGs), as glycol, glycolmonomethylether and glycoldimethylether were evaluated. + MAPbI3oG (MA (methyl ammonium, CH3NH3 ; oG (oligoglycol derivative = diethylene glycol DEG (⇒1), triethylene glycol TEG (⇒2), triethylene glycolmonomethylether TEGOME (⇒3) and triglyme (⇒4)) was prepared, resulting in solid, crystalline compounds. Experimental details are given in the supporting information section. The structure of the precursor phases was determined from single-crystal Xray diffraction analysis and is summarized in Fig. S-1a-e with compound (3) as representative shown in Fig. 1a.

Figure 1. (a) Photographic image of a solid-precursor (3) and an excerpt of its structure determined from single-crystal Xray diffraction. (b) Photographic image of the liquid precursor (5) and proposed structure of the liquid. (c) Unit cell of MAPI. oG depicted in grey 'stick mode'; MA depicted in red 2+ 'stick mode'; Pb ≅ black spheres; I ≅ blue spheres; [PbI6] depicted as green polyhedra. A joint feature in all precursor structures (1-4) are onedimensional ([PbI3] )n chains formed by face-shared lead iodide octahedral . The oGs and the methyl ammonium form a complex cation that surrounds the ([PbI3] )n chains. The nature of the oG has a significant influence on the structure, the symmetry and the physical properties of the crystalline

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precursor phases (see Fig. S-1). For compounds (4), (3), (1) one oG unit coordinates to one MA (see also Fig. 1a). As a result the charge of MA is only partially shielded, and this causes a strong interaction with the ([PbI3] )n and leads to high melting points determined from differential calorimetry (DSC) measurements given in Fig. S-2 (Tmp(4) = 109 °C, Tmp(3) = 95 °C, Tmp(1) = 85 °C). The oG-based precursor show the structural motif of CsNiBr3 with an increased cation size + comprising of MA and the glycol respectively (tolerance 29 factor>1). The structural resemblance (e.g. face-centered symmetry) between (3) as a representative and the target perovskite MAPI is shown in Fig. 1a, c. When TEG is used as an alternative oG, an important deviation arises (see Fig. S-1c), as now two oG entities coordinate to one MA and effectively shield the charges. The melting point decreases significantly to Tmp(2) = 67 °C. Assuming predominantly electrostatic interactions, the data indicate (agreeing with Coulomb law) a qualitative correlation between Tmp and the cation to anion distance. The average distance d(N-Pb) can be extracted from single crystal data showing an increase of 150% from 4.9 Å for (1) to 7.5 Å for (2). Therefore, further decrease of the melting point is expected, if the distance between MA and ([PbI3] )n can be enlarged. This can be realized for instance by using a steric more demanding oG compared to TEG. For tripropylene glycol (TPG), compound (5), crystallization does not occur anymore, as proven by DSC (see also Fig. S-3a). This novel precursor is liquid at room temperature (see Fig. 1b). UV-VIS data (given in Fig. S-3b) of the liquid precursor phase (5) reveals an absorption maximum at λmax(5) = 397 nm. This is between the characteristic wavelength for zero dimensional 4PbI6 (λmax = 372 nm) clusters and one dimensional ([PbI3] )n 30 chains (λmax= 460 nm). Thus, λmax(5) fits to isolated, but rather short ([PbI3] )n chains dispersed in TPG coordinating to MA (Fig. 1b). ATR FT-IR spectra given in Fig. S-3c proves the presence of MA and the strong similarity to the used solvent tripropylene glycol as further mark of a liquid behavior. An important feature of all precursor presented here is the ability for the direct formation of MAPI induced by temperature. Comparable to a “masked” perovskite phase with CsNiBr3 structure, that transforms to MAPI via an irreversible phase transition without evaporating the glycol. Below a critical temperature, the precursor state is stable. Although all precursors were explored regarding the T-induced formation of MAPI (for compound (3) see data given in Fig. S4), our focus here lies on the liquid system (5). The transformation is studied in detail using in situ UV/VIS spectroscopy and powder X-ray diffraction (PXRD) given in Fig. 2. The formation of MAPI (Egap = 1.52 eV; λgap = 815 nm) is traced by optical spectroscopy, because the precursor phase (see above) does not absorb in this region (Fig. 2a). At a temperature of T = 75 °C a first signal characteristic for MAPI can be observed after 35 min, followed by complete conversion of the precursor until 100 min. The kinetics of this crystallization process, including the nucleation and growth is adjusted with temperature (Fig. 2b). The emerging MAPI phase is confirmed unambiguously by PXRD measurements shown in Fig. 2c. With prolonged heat treatment, the amorphous halo at 2θ= 22°, caused by the short-range order (Fig. 1b), disap-

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pears and only the signals characteristic for CH3NH3PbI3

become visible.

Figure 2. In situ investigations to the formation of MAPI from the liquid precursor phase (5). (a) UV/VIS data; bottom window ≅ spectrum of the unreacted precursor; top window ≅ spectrum after complete conversion; middle window ≅ temporal evolution map of UV/VIS data, absorption values brightness encoded. (b) Temperature dependency conducted from UV/VIS kinetic data (λ= 700 nm) while heating up from room temperature to T1= 125 °C, T2= 100 °C and T3= 75 °C. (c) PXRD kinetic data of MAPI crystallization on a Si substrate at T3= 75 °C with t1= 0 min, t2= 10 min, t3= 30 min, t4= 45 min and t5= 60 min. SEM micrographs of MAPI thin film section grown on silicon (d), truncated cubic morphology grown in solution (e), scalebars ≅ 5 µm respectively. The crystallization of MAPI crystallization can be either initiated from homogeneous solution or heterogeneously in the presence of a substrate like silicon; the latter tracked by PXRD (Fig. 2c). In the early crystallization stage, the signals with c-component are much weaker than expected which indicates a preferential growth in (a,b)-direction, parallel to the substrate. Thus, a thin coating of crystalline MAPI with lateral extension of 50-100 µm (Fig. 2d) can be obtained. The generation of extended, monodomain films is an important aspect for all applications of HYPEs in optoelectronic devic31 es. With prolonged crystallization a significant change in orientation is observed, as the (112) signal at 2θ= 19.9° and related planes are increasing (see Fig. 2c, S-5a). The change in crystallization direction perpendicular to the substrate becomes clear, when the structural resemblance of the glycol precursor and the MAPI phase were considered (S-5b). Thus the liquid precursor can direct the crystal growth like a crys32 tallization capper. The unique properties of the new, liquid precursor phase allows further possibilities to produce refined surface patterns. As proof of concept, we show that lines of MAPI nanoparticles can be written on silicon substrates by spatially resolved excitation with a laser beam (see Fig. S-6). Homogeneous nucleation and bulk growth in solution is observed by conventional light microscopy (see Fig. S-7a and video) leading to various crystal shapes (hexagonal, cubic, truncated cubes; Fig. 2e and Fig. S-7b). There are only few large crystals indicating the precursor phase (5) enables a slow nucleation rate with a limited number of critical nuclei. With prolonged time of crystallization, truncated cubic shapes grow to cubic bulk crystals, which agrees with a 17 Volmer-Weber growth mechanism. It can be expected that higher nucleation rates can be reached, if the increase of temperature is accelerated. Therefore, microwave radiation applied to the liquid precursor (5) is suitable to form MAPI. Microwave synthesis can be used to generate a wide range of inorganic materials as recently 33 reviewed by Niederberger et al., but it has yet not been used for the preparation of HYPE materials by now. Our results

are shown in Fig. 3 and further information is given in Fig. S8. Considering the reaction parameters a major advantage in the use of microwaves is a related La-Mer crystallization 34 behavior including a short nucleation period and a prolonged growth period (Fig. S-8a).

Figure 3. Data for microwave-assisted synthesis (Pmw= 700 W) of MAPI from the liquid precursor phase (5). (a) PXRD and (b) confocal fluorescence microscopy micrograph of hollow cubic particles with corresponding photoluminescence intensity profile, scalebar ≅ 5 µm. SEM photographs of resulting MAPI morphologies (c) scalebar ≅ 20 µm, (d) scalebar ≅ 5 µm. Using microwaves as thermal initiator the crystallization is taking place within minutes. PXRD data (Fig. 3a) clearly prove the formation of crystalline MAPbI3. As expected, the number of formed particles increased (Fig. 3c) compared to the conventional-thermal treatment described before. Obviously, the crystalline perovskite particles display a unique morphology. Crystals started to dissolve preferentially starting from the center of the (100) surfaces of the initial, cubic particles (Fig. 3c; Fig. S-8b). The edges and corners of the

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particles remain stable. Therefore, the microwave synthesis appears as a promising route towards the generation of hollow HYPE nanostructures. The dissolution does not occur, when the power of the applied microwaves is lower (Pmw = 25 W); see data given in Fig. S-8b, resulting in compact particles. The applied microwave power is crucial to the MAPI crystallization, and dissolution process as shown in S-8c. With low microwave power (Pmw < 50 W) crystallization is favored even for extended heating protocols (see S-8d). It is noteworthy, that some of the particles deviate from the cubic shape. For a crystalline system with (pseudo)-cubic symmetry (like perovskites) cubic shapes (reflecting (100) surfaces and symmetry equivalents) and octahedral shapes (reflecting (111) surfaces) are expected. A tetrahedral shape like shown in Fig. 3d is unexpected, because it represent a varia35 tion of crystal 'Tracht'. Confocal fluorescence microscopy is a powerful tool to examine surface characteristics of hybrid perovskite films and 36 the local distribution of defects in optoelectronic devices. The obtained particles are an ideal model to identify morphology-related photoluminescence properties that is regarded as a crucial parameter in device optimizations. The cubic scaffold particles (Fig. 3b) reveal a significant enhancement in photoluminescence intensity at the outer [001] crystal edges and corners compared to the etched (001) surface and their respected inner edges and corners. As surface defects generally result in trap-assisted, non-radiative recombination, we conclude the surface chemistry of inner and outer edges is different and leads to a local variation of photoluminescence intensity. Such a local photoluminescence enhancement is only observed for the hollow particles but not for the compact (see Fig. S-8e), and could be of further interest in light-emitting and lasing applications. We showed that the development of a liquid single source precursor together with its thermo-sensitive property opens a great potential in the synthesis of unique MAPI materials. For this manipulation on the molecular scale, a deeper understanding of solvent interactions in liquid and solid precursors needed to be considered. We therefore provide a suitable model system, to study the influence of oligo-glycols (by varying, chain length, coordinating and side groups) on the precursor level and the final MAPI phase. It can be imagined, that the reactivity as well as the direction of crystallization can be manipulated which could open up new pathways for different morphologies of the MAPI phase. With the liquid precursor different structures can be coated or be infiltrated, which then are thermally converted into a HYPE coating afterwards. Aerosol processes relying on liquid precursors will be possible as well. The preparation of the liquid precursor is simple, enables easy up-scaling and is based on non-hazardous solvents which could be an important aspect for the application of hybrid perovskites in industrial context. Last but not least, a proof of concept for the spatially resolved MAPI formation using a focused laser has been presented, and is worth pursuing.

ASSOCIATED CONTENT Supporting Information Detailed experimental procedures, single crystal data and analysis (DSC, UV-Vis, IR, SEM, microscopy and confocal

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fluorescence microscopy images) is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

Author Contributions T. Kollek performed the precursor synthesis, materials preparation and characterization. C. Fischer contributed to the experimental work with kinetic studies on the temperature induced material preparation. I. Göttker Schnetmann performed SC-XRD measurements and solved the structures. S. Polarz designed the research and wrote the paper. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT We thank the Baden-Württemberg foundation for financial support (project “SUPERSOL”; program “Biomimetic Materials Synthesis”). We thank P. Grauss and J. Boneberg for performing the initial experiments using laser indentation and M. Winterhalder for confocal fluorescence microscopy measurements.

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