properties of methylammonium lead iodide perovskite single crystals

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The technique for growing CH3NH3PbI3 single crystals from saturated solutions in ... CH3NH3PbI3 (methylammonium lead iodide perovskite, MAPbI3).
Journal of Structural Chemistry. Vol. 58, No. 8, pp. 1567-1572, 2017. Original Russian Text © 2017 E. S. Yudanova, T. A. Duda, O. E. Tereshchenko, O. I. Semenova.

PROPERTIES OF METHYLAMMONIUM LEAD IODIDE PEROVSKITE SINGLE CRYSTALS E. S. Yudanova1, T. A. Duda1, O. E. Tereshchenko1,2, and O. I. Semenova1*

UDC 548.55:548.58

The technique for growing CH3NH3PbI3 single crystals from saturated solutions in concentrated hydroiodic acid is improved by introducing a reducing agent (hypophosphorous acid). The structure of perovskite is confirmed by single crystal XRD. By energy dispersive spectroscopy and X-ray photoelectron spectroscopy it is established that the stoichiometry of the grown crystals corresponds to the CH3NH3PbI3 compound. Changes in the photoluminescence intensity during in-air measurements show that the crystals synthesized using the reducing agent are more stable in the external environment with laser exposure than without it. DOI: 10.1134/S0022476617080133 Keywords: synthesis, crystal growth, photoluminescence, photoelectron spectroscopy, solar cell.

INTRODUCTION Recent years have seen the rising interest in a novel semiconductor material organic-inorganic perovskite (OIP) CH3NH3PbI3 (methylammonium lead iodide perovskite, MAPbI3). Owing to a combination of unique physical properties (high charge carrier mobility, optimal band gap, high solar, X-ray, and γ absorption coefficients, effective radiative recombination of charge carriers) this material finds application in the design of thin-film solar cells (SCs) [1], light-emitting devices [2, 3], X-ray and γ radiation sensors [4]. For the first time, OIP was used as a photoactive material for SCs in 2009; the SC performance was ∼3.5%, but the cells rapidly degraded in the electrolyte [5]. In 2013, the group of Professor Michael Grätzel used a new solid hole conductor and thus obtained SCs with the efficiency of about 15% [6], which was called the Breakthrough of the Year in physics by the Science journal. As a result of active researches, the record SC efficiency was 20.2% in 2015 [7]. A drop in the SC performance due to OIP layer degradation in the ambient environment remains the key problem [8]. Currently, the research efforts are focused on the improvement of OIP synthesis techniques to boost the material stability in the atmosphere. It is still topical and important to study the physical properties of semiconductor OIPs not only in the form of polycrystalline films, but also in the form of single crystals, which allows the elimination of the effect of grain boundaries on charge carrier recombination processes. There are several known techniques for growing MAPbI3 crystals: growth from a supersaturated solution of OIP precursors in concentrated hydroiodic (HI) acid without a reducing agent with a decrease in the solution temperature from 65 °C to 40 °C [9]; growth from a saturated γ-butyrolactone solution with an increase in the solution temperature from 80 °C to 110 °C [10]; crystal growth from a γ-butyrolactone solution in dichloromethane antisolvent vapor [11]; growth from

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Rzhanov Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, Russia; *[email protected]. 2Novosibirsk National Research State University, Russia. Translated from Zhurnal Strukturnoi Khimii, Vol. 58, No. 8, pp. 1617-1622, November-December, 2017. Original article submitted April 27, 2017. 0022-4766/17/5808-1567 © 2017 by Pleiades Publishing, Ltd.

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a saturated OIP solution in a concentrated HI medium with a reducing agent (hypophosphorous acid) in a temperature gradient provided by heating the bottom of the solution-containing vessel on an oil bath to 75° and cooling the solution surface by atmospheric air [12]. The studies [9, 12] report the formation of intermediate hydrate structures (CH3NH3)4PbI6⋅2H2O at temperatures below 40 °C. The crystals prepared in [10, 11] were grown using the γ-butyrolactone solvent banned in the Russian Federation. It should be noted that the electrophysical characteristics of the crystals synthesized in γ-butyrolactone solutions are clearly inferior to the results from [9, 12]. We have developed a new method of crystal growth based on a combination of the techniques from [9, 12], which enabled us to grow OIP single crystals with a decrease in the solution temperatures to room values without the formation of hydrate structures, with the crystals synthesized being air-stable.

EXPERIMENTAL CH3NH3PbI3 was synthesized in three stages. In the first stage, methylammonium iodide was obtained by reaction (1); to this end, 25% aqueous solution of methylamine (0.28 М) was mixed with an aqueous solution of HI (57%) taken in excess (0.3 М). H3PO2 (2 wt.%) was preliminarily added to the HI solution for its stabilization CH3NH2 + HI → CH3NH3I.

(1)

The synthesis was performed at 0 °C with constant stirring of the solution for 2 h. The prepared solution was dried in the atmosphere at room temperature for several days. The precipitate was washed with high purity chloroform. Lead acetate trihydrate Pb(CH3COOH)2⋅3H2O in an amount of 28 g was dissolved in 100 ml of an aqueous HI solution (57%) to form a PbI2 precipitate by the reaction Pb(CH3COO)2⋅3H2O + 2HI → PbI2↓ + 2CH3COOH + 3H2O.

(2)

Then, 12 g of methylammonium iodide synthesized by reaction (1) were added to the obtained solution. The third stage occurred by the reaction CH3NH3I + PbI2 → CH3NH3PbI3.

(3)

The resulting solution was evaporated at a temperature of 65 °C with constant stirring for several hours and then gradually cooled to room temperature. It should be noted that no intermediate hydrate structures (CH3NH3)4PbI6⋅2H2O formed when the solution temperature was decreased below 40 °C, in contrast to that observed in [9, 12]. The powder synthesized was filtered and washed with diethyl ether. Bulky MAPbI3 crystals were grown from a saturated solution of an OIP powder at decreasing temperature. HI was used as a solvent. The data on the temperature dependence of the OIP solubility in stabilized (2% of the H3PO2 reducing agent) and non-stabilized HI solutions are important for the chosen growth technique. The data on the OIP solubility in nonstabilized HI in a temperature range 60-67 °C are reported in [9]. The OIP solubility in stabilized HI has not been previously studied, hence, we obtained the solubilities in an extended temperature range 45-65 °C. The saturated solution was prepared by dissolving 38 g of an OIP powder in 100 ml of stabilized 57% HI solution at 65 °C for 24 h. During the growth the solution temperature was controllably decreased from 65 °C to 21 °C with a step of 0.1°C in 40 min. The growth process lasted for 12 days. Large crystals prepared were washed with diethyl ether and annealed in an atmosphere at 60 °C to improve the quality of the crystal surface. For comparison, we have also grown OIP crystals from a non-stabilized HI solution. Saturation was reached by dissolving a substance powder in a HI solution at 65 °C. The solution temperature was precisely decreased from 65 °C to 45 °C with a step of 0.1°C in 60 min, because at a temperature below 40 °C the formation of intermediate hydrate structures in the form of semitransparent needle-shaped yellow crystals was observed. An analogous growth process was described in [9, 12]. The elemental composition of the prepared OIP crystals was determined by scanning electron microscopy (SEM) on a ZEISS CrossBeam 1540XB with an Oxford X-Max energy dispersive detector. The experiments for the study of the crystal 1568

composition were also carried out on a SPECS (Germany) photoelectron spectrometer with a PHOIBOS-150-MCD-9 hemispherical analyzer and a FOCUS-500 X-ray monochromator (AlKα radiation, hν = 1486.74 eV, 200 W). The binding energy scale (Ebind) was pre-calibrated by positions of the core level peaks from the Au4f7/2 (84.00 eV) and Cu2p3/2 (932.67 eV) standards. The binding energy and the full width at half maximum were determined with an accuracy of 0.05 eV. The structure of the crystals was examined on a Bruker X8 APEX diffractometer (MoK radiation, graphite monochromator). The calculations were performed using the SHELX-97 software [13]. The photoluminescence (PL) spectra were measured at a temperature of 20 °C for the crystals grown from stabilized and non-stabilized HI solutions. First, the PL spectra of the crystal surface exposed to atmosphere for a long time were measured, then the crystal surface was cleaved and the PL intensity was measured again, thus providing information about its change with time. For the PL spectra GaN diode laser radiation (wavelength 405 nm) with a power density of 10 W/cm2 was used. The spectra were analyzed on a spectrometer equipped with a CCD chamber.

RESULTS AND DISCUSSION We have developed a new technique for growing OIP crystals, which allowed us to prepare 5×5 mm single crystals with pronounced faceting (Fig. 1). We managed to prevent the undesirable formation of the intermediate hydrate structures of MAPbI3 crystals by introducing a reducing agent (hypophosphorous acid H3PO2) into the initial solution used for crystal growth. To explain the observed growth effect we used the data from [14] where the kinetics and mechanism of chemical transformations in the H3PO2-containing solutions were considered. In that work, it was shown that the (H2PO2)– anion formed in the solution shifted the equilibrium of the interaction reaction toward the formation of HI (reaction 4) and, by interacting with iodine, formed a dissociated anion of phosphoric acid (reaction 5) H2PO −2 + I2 + H2O → H2PO 3− + 2HI,

(4)

H2PO 3− + I2 + H2O → H2PO −4 + 2HI.

(5)

In the non-stabilized solution the I2 formation process by the reaction 4HI + O2 → 2H2O + I2 is irreversible due to iodine release, and the HI concentration in the solution decreases. Note that the use of hypophosphorous acid as the reducing agent in [12] had no positive effect, because, in our opinion, the growth was conducted in an open volume at a constant solution temperature of 75 °C, which led to the evaporation of the solution stabilizer and, consequently, enhanced the HI decomposition effect. This caused the growth of undesirable hydrate forms at temperatures below 40 °C. The addition of hypophosphorous acid also affects the OIP solubility in HI. Previously, in [9], the OIP solubility in the non-stabilized HI solution was studied at temperatures of 60 °C, 64 °C, 65 °C, and 67 °C where the amount of the

Fig. 1. Image of one of OIP crystals synthesized.

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dissolved substance was estimated from the weight of the precursor introduced into the solution, which was not entirely correct. We obtained the data on the OIP solubility in the non-stabilized HI solution at temperatures of 60 °C and 65 °C, which coincided with those previously reported in [9] (Fig. 2a). The OIP solubility in the HI solution with H3PO2 was studied at temperatures of 45 °C, 50 °C, 55 °C, 60 °C, and 65 °C. It is found that the OIP solubility in stabilized HI is almost twice higher than that in non-stabilized HI (Fig. 2b). This can be associated with an increase in the solvent concentration due to the prevented decomposition into molecular iodine and water, as well as with the formation of other chemical forms of lead in the solution. A non-linear character of the solubility curve in HI stabilized H3PO2 (Fig. 2b) indicates the existence of a chemical interaction between ions in the stabilized solution and this interaction is most likely to be related to the formation of more intricate iodide lead complexes. The introduction of hypophosphorous acid also changes the chemical equilibriums on the crystal-solution interface, with possible selective adsorption of stabilizer ions H2PO 3− and H2PO −4 . The adsorption phenomenon changing the surface state needs a further, more thorough study, which is planned for the nearest future. According to the data of energy dispersive spectroscopy (EDS) (Fig. 3) and X-ray photoelectron spectroscopy (XPS) the grown crystals were stoichiometric with the composition CH3NH3PbI3. Fig. 4 depicts the survey XPS spectrum of the CH3NH3PbI3 crystal surface. It should be noted that, despite a prolonged exposure to the air, a relatively small amount of oxygen (nearly a monolayer) was found on the sample surface. Since the spectra do not have chemically shifted components of the lines from the elements typical of oxides, we can argue that oxygen is mainly contained in adsorbed water molecules (or phosphorous anions) on the OIP surface. By X-ray diffraction it is established that the prepared crystals have the space group I4/mcm, the lattice constants a = 8.8776(7) Å, b = 8.8776(7) Å, c = 12.6702(8) Å, V = 998.56(17) Å3. The obtained crystallographic characteristics confirm the formation of CH3NH3PbI3 OIP, which agrees well with the CCDC data reported in [15]. Monocrystallinity of

Fig. 2. Comparison of the OIP solubility in HI (2) with the data from [9] (a); the temperature dependence of the OIP solubility in HI with H3PO2 (1) and in HI (2) (b).

Fig. 3. Energy dispersive spectrum of the MAPbI3 crystals. 1570

Fig. 4. Survey XPS spectrum of the CH3NH3PbI3 single crystal surface. large crystals was not proved, although the state of the developed surface of 5 mm crystals visually appeared to be rather uniform. The chemical stability of the CH3NH3PbI3 crystals was examined by recording the PL spectra of the air-exposed crystal against time. Fig. 5a depicts the evolution of the PL spectra for the crystal grown without the reducing agent: the diagram clearly demonstrates the chemical activity of the crystal in the atmosphere, which changes the PL intensity under laser exposure. A significant decrease in the crystal PL intensity with an increase in its air exposure duration is probably related to the interaction of the crystal surface with the atmosphere (water vapor, oxygen), which, consequently, alters the surface layer nature and thickness. This effect was not observed for the crystals grown from the HI solution with the addition of H3PO2 (Fig. 5b). Here, a time change in the PL intensity on the freshly cleaved surface had a more complicated character: during the first five minutes the PL intensity decreased by 25% and remained practically unchanged with further exposure, although it showed a tendency to increase. This demonstrated the chemical stability of the crystal to atmospheric and laser exposure. This phenomenon can be related to adsorption or even the incorporation of H3PO2 oxidation products in some crystal faces during its growth. Therefore, we have showed that the use of H3PO2 leads to a very important property of OIP, namely, its stability under the light radiation effect, which is critically important in the work of SCs. From the position of the PL peak it can be inferred that for both types of crystals the band gap is 1.6 eV, which agrees with the literature data [1]. Both properties are

Fig. 5. Time change in the PL spectra for the crystals grown from the non-stabilized solution (1 is the surface after storage for a month, 2 is the exposure for 30 s, 3 is the fresh cleavage) (a), the air exposure dependence of the PL spectra for the crystals grown from the stabilized solution (b).

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crucial given the promising application of this compound as SC. Looking forward, we intend to perform a more extensive study of the properties of the crystals grown with the reducing agent.

CONCLUSIONS We have developed the technique for growing MAPbI3 crystals from an aqueous HI solution (57%) stabilized with hypophosphorous acid H3PO2 (2%), whose real stoichiometry and the structure type were determined by XPS, EDS, and X-ray diffraction. The technique enables the growth of OIP crystals in a temperature range from 65 °C to 21 °C without the formation of the hydrate by-product (CH3NH3)4PbI6⋅2H2O. It was established that the addition of the reducing agent (hypophosphorous acid) doubled the OIP solubility in the aqueous HI solution. By recording the PL spectra it was shown that the band gap of the crystals was 1.6 eV, which was the best for materials used in photovoltaics. The crystals grown with the reducing agent are stable when exposed to air, which makes it possible to study the physical properties of this semiconductor material without using the protective coatings. The authors are grateful to research engineer D. A. Piryazev (the Crystal Chemistry Laboratory, Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences) for performing the single crystal XRD experiment and to leading engineer A. A. Guzev (Institute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences) for developing the equipment for crystal growth.

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