Antisolvent diffusion-induced growth, equilibrium behaviours in aqueous solution and optical properties of CH₃NH₃PbI₃ single crystals for photovoltaic applications

Abstract

Crystallization and decomposition of organolead trihalide perovskites (OTPs) are very sensitive to the presence of water in precursor or in ambient conditions. Thus, understanding equilibrium behaviours (crystallization and decomposition) of OTPs in aqueous solution is very critical for OTP solar cells fabricated with water-based precursor solutions. Here, equilibrium behaviours in an aqueous solution of CH₃NH₃PbI₃ (MAPbI₃) single crystals (MSCs) were studied. Diethyl ether, as an antisolvent, effectively diffused and induced MSC growth by screening different solvents (diethyl ether, tetrahydrofuran, dichloromethane, and chloroform). The structure transforms from the initial PbI₂ to intermediate (H_xPbI_2+x·xH₂O) and finally MSCs were observed by X-ray diffraction. Decomposition of MSCs in aqueous solution was significantly enhanced by potassium iodide coordination and inhibited by CH₃NH₃I (MAI) addition. We ascribed this inhibition behaviour to suppressing MAI migration from the MSC crystal structure. Finally, the optical properties of MSC were studied.

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Introduction

Organolead trihalide perovskites (OTPs) possess a tunable band gap,^1,2 high charge carrier mobility,³ and long electron–hole diffusion.⁴ With the continuous efforts from researchers worldwide, the power conversion efficiency of photovoltaic (PV) cells based on OTPs have been updated to 20.1%.⁵ Moreover, one of the most attractive features for OTPs is that high-performance devices can be made with low-cost solution processing. However, solution-processed OTP thin films often encounter non-uniformity and incomplete coverage.⁶ Consequently, understanding and controlling the crystallization process in solution processing is meaningful and has become one of the priorities of OTP-based scientists and manufacturers. The surface coverage of CH₃NH₃PbI₃ (MAPbI₃) is reduced by single-step spin-coating using a mixed solution of PbI₂ and CH₃NH₃I (MAI) in γ-butyrolactone (GBL) or N,N-dimethylformamide (DMF) solvent.⁷ The use of CH₃NH₃Cl (MACl) precursor can slow down the crystallization process of MAPbI₃, thereby improving film morphology and extending the carrier's lifespan.^8–10 The precursor solubility or crystallization is controlled by 1,8-diiodooctane,¹¹ HI,¹ dimethyl sulfoxide (DMSO) solvent additive,¹² or solvent¹³ for highly uniform and dense perovskite layers. Meanwhile, exposure of MAPbI₃ precursor films to chlorobenzene,¹⁴ dichlorobenzene, DMSO,¹⁵ toluene,¹² diethyl ether,¹⁶ and DMF¹⁷ can be an alternative approach to facilitate the crystallization or recrystallization of perovskites. Recently, Wang et al. developed a new precursor, HPbI₃, which enables crystallization of highly uniform formamidium lead iodide (FAPbI₃) films through a one-step spin-coating process.¹⁸

Besides the solution chemical of OTP polycrystal film, solution-grown MAPbI₃ single crystal (MSC) is beneficial to accurately understand their intrinsic physical, chemical, and optoelectronic properties. OTP single crystal is grown by cooling-induced crystallization, which may cause different crystal phases arising from the temperature-dependent phase transitions in OTPs.^3,4,19 Recently, mild heating of CH₃NH₃PbBr₃ DMF solution at about 50 °C will form red single crystals after several hours without solvent evaporation.²⁰ Kadro et al. presented a facile method for the growth of freestanding crystals of MAPbI₃ from GBL solution using inverse solubility at high temperatures.²¹ Almost at the same time, Saidaminov et al. also reported the growth of shape-controlled OTP single crystal by inverse temperature crystallization.²² Shi et al. reported an antisolvent vapor-assisted [dichloromethane (DCM)] crystallization approach to create MAPbX₃ single crystals.²¹ Although the idea of antisolvent vapor diffusion induced crystallization methods in organic solvent DMF for OTP single crystal have been reported, crystallization and decomposition of OTP single crystal in aqueous solution are still unclear.

In this work, our study is focus on antisolvent diffusion-induced growth, and equilibrium behaviours in aqueous solution of OTP single crystal. MSCs were synthesized in MAI and H_xPbI_2+x·xH₂O precursor by antisolvent diffusion-induced (ASDI) method at room temperature. The structure transform from initial PbI₂ in HI aqueous solution to finally MSCs was investigated by X-ray diffraction (XRD). On the other hand, decomposition of MSCs in aqueous solution was significantly enhanced by potassium iodide addition, and inhibited by CH₃NH₃I addition. We ascribed this inhibition behaviours to suppressing MAI migration from the MSC crystal structure. Finally, the optical properties of MSC were studied.

Experimental section

Synthesis of CH₃NH₃I

CH₃NH₃I were synthesized from 10 mL hydroiodic acid (57% wt%, in water, J&K Chemical), by reacting 27 mL methylamine (40% in methanol, Sigma) in a 250 mL round bottomed flask at 0 °C for 2 h with stirring. The precipitates were recovered by evaporating the solutions at 50 °C. The products were dissolved in ethanol, recrystallized using diethyl ether, and finally dried at 60 °C in a vacuum oven for 24 h. The phase purity of the sample was confirmed by NMR as shown in Fig. S2† [1H NMR (d6-DMSO) δ 7.51 (br s, 3H, CH₃C̲H̲₃NH₃⁺I⁻), 2.38 (s, 3H, C̲H̲₃NH₃NH₃⁺I⁻)].

Antisolvent diffusion-induced growth (ASDI) of CH₃NH₃PbI₃ single crystals

In a typical procedure, A 50 mL round bottom flask was charged with HI (1.82 mL, HI 55% wt%, 0.023 mmol, in water, energy). The liquid was degassed by passing a stream of nitrogen through it for 1 min and keeping it under a nitrogen atmosphere throughout the experiment. PbI₂ (115 mg, 0.25 mmol) was dissolved in HI solution upon heating the flask to 120 °C using an oil bath, forming a bright yellow solution. To the hot yellow solution was added solid CH₃NH₃I (40 mg, 0.25 mmol) dissolving immediately. The stirring was discontinued and the solution was left to cool back to room temperature. The solution was transferred to cuvette. Antisolvent inflow into cuvette very slowly along cuvette wall. The cuvette was left to grow single crystal at room temperature without disturbance. The single crystal were washed by diethyl ether and dried in vacuum as raw materials for characterization.

Single crystal XRD characterization

Data was collected on a Bruker SMART 1000 CCD diffractometer with the use of Mo-Kα radiation (λ = 0.71073 Å). The structures were solved by direct method and expanded with the use of difference Fourier techniques with the SHELXL-97 program.

MAPbI₃ single crystal film characterization

Powders XRD measurements were obtained using PANalyticalX'Pert diffractometer (Cu Kα radiation at λ = 1.54 Å) sampling at 2° min⁻¹, 40 ekV and 100 mA. UV-Vis absorption spectrum and diffuse reflectance spectrum was obtained using UV/Vis/NIR spectrometer (Perkinelmer, lambda, 750S). Steady-state photoluminescence was carried out on LS55 Luminescence Spectrometer (Perkin Elmer instrument).

Results and discussions

To determine the antisolvent that can induce the growth of MAPbI₃ single crystal, we selected four solvents (diethyl ether, tetrahydrofuran, DCM, and chloroform). As described in the Experimental section, four kinds of solvents inflowed into the perovskite precursors. The final volume ratio of inflowed solvent and perovskite precursors was about 10 : 1. As shown in Fig. S1,† the volume of perovskite precursors slightly increased when diethyl ether and tetrahydrofuran were inflowed, which indicated that diethyl ether and tetrahydrofuran diffused into the perovskite precursors. However, the volume of the perovskite precursors with methylene chloride or chloroform did not increase. MAPbI₃ single crystal was observed after 2 days with diethyl ether as antisolvent, whereas single crystals in other solvents were not observed (Fig. 1a and b). The basic idea of ASDI crystallization is the elegant combination of two miscible solvents. However, for dichloromethane and chloroform, neither of them are miscible with the water-based precursor solution, as shown in Fig. 1a and b. They hardly diffused into the water-based precursor solution and hardly increased the saturation of the perovskite precursors. Thus, dichloromethane and chloroform can not be regarded as antisolvent for ASDI crystallization in this study. For diethyl ether, it is miscible with the water-based precursor solution. The saturation of the perovskite precursors increased by diethyl ether diffusion, and the precursors showed gradual nuclear formation and growth to single crystals. Thus, diethyl ether is effective antisolvent to diffuse and induce MSC growth. Different concentration of perovskite precursors must affect the number of crystal nuclear and structure in the process of ASDI growth. When the concentration of perovskite precursors is 0.09595 mol L⁻¹, no single crystal was observed even after seven days. This concentration was too low to reach the sursaturation in the process of antisolvent diffusion. When the concentration increased to 0.1097 mol L⁻¹ and 0.1371 mol L⁻¹, black single crystals were observed after 2 days. From SEM (Fig. 1c and d), we can see that single crystal processed high quality with crack-free, smooth surfaces, well-shaped borders. Single crystal X-ray diffraction (XRD) data (ESI†) showed that the as-grown crystals were attributed to MAPbI₃, which crystallizes in the tetragonal I4/mcm space group (a = b = 8.8719 Å, c = 12.6770 Å) in agreement with the previous results.²² Detailed information of the single crystal is summarized in ESI.† The phase purity of the single crystals was confirmed by powder XRD from a large batch of crystals (Fig. 1e and f). We found that the fit between the experimental patterns and calculated curves was good.

Fig. 1 Crystal growth, SEM and diffraction. (a) Four solvent (diethyl ether, tetrahydrofuran, dichloromethane, chloroform) was inflowed into perovskite precursors. (b) Four solvent was inflowed into perovskite precursors after 2 days. SEM of MAPbI₃ single crystal formed in 0.1097 mol L⁻¹ (c) and 0.1371 mol L⁻¹ (d) PbI₂. Experimental and calculated powder XRD profiles of as-prepared MAPbI₃ single crystal at different arrange: (e) 10–18°, (f) 10–70°.

Fig. 2 XRD patterns of PbI₂, PbI₂ in H₂O, PbI₂ in HI aqueous solution and MSC at different arrange: (a) 10–20°; (b) 20–70°. (c) Schematic illustration of the configurations of layered PbI₂ structure, PbI₂·xH₂O, H_xPbI_2+x·xH₂O and MAPbI₃ single crystal.

In order to study the transformation process in the single crystal growth in aqueous solution, we carried out XRD to study structure transformation. Compared to solubility of PbI₂ in H₂O, GBL or DMF, PbI₂ powder was easily dissolved in HI aqueous solution at room temperature, forming a bright yellow solution. This result indicated that coordination between PbI₂ and HI was formed, resulting in good solubility of PbI₂ in HI aqueous solution. In order to determine the structure of the coordination compound, the XRD characterization was carried out. For comparison, pure PbI₂ and PbI₂ dispersed in H₂O were also measured. As shown in Fig. 2a, the diffraction peak of pure PbI₂ at 12.8° was attributed to (001) with a lattice spacing of 6.98 Å, indicating that a layered crystal structure of PbI₂, agreeing with literature.²³ Compared to pure PbI₂, the diffraction peaks at low angle of the powder obtained from PbI₂ dispersed in H₂O moved from 12.8° to 12.6°, while other diffraction peaks from 16° to 70° was not obviously moved. This result indicated that the H₂O molecule inserted into the layered PbI₂ structure increased the lamellar distance following reaction (1).

PbI₂(s, yellow) + xH₂O(aq) ⇌ PbI₂·xH₂O(s, yellow) (1)

PbI₂(s, yellow) + xH₂O(aq) + xHI(aq) ⇌ H_xPbI_2+x·xH₂O(aq) (2)

The XRD pattern of the coordination compound for PbI₂ in HI aqueous solution was different from that of pure PbI₂, which indicated that edge-sharing PbI₆ octahedral was changed by HI coordination. The moles of PbI₂ and HI in solution were 0.25 mol and 0.023 mol, so the formed coordination compound was not stoichiometry HPbI₃ but H_xPbI_x+2. Powder XRD of PbI₂ dissolved in aqueous HI also showed a diffraction peak at 12.6°, indicating that the structure was also layered with H₂O. Thus, the composition of PbI₂ dissolved in aqueous HI might be H_xPbI_2+x·xH₂O, as shown in reaction (2). Based on XRD analysis of MSC, the diffraction peaks of PbI₂ moved from 12.8° to 14.1°, illustrating that the layered structure disappeared and converted into a three-dimensional MAPbI₃ single-crystal structure [reaction (3)].

Understanding the decomposition of MSC in aqueous solution is helpful to solve the stability of MSC in the water. The decomposition of MSCs in different aqueous solutions, as shown in Fig. 3, were analyzed. MSC in aqueous solution turned into yellow crystal rather than dissolving after 30 min. However, the yellow crystal recovered to black after 12 h (inset in Fig. 3). This colour recovery phenomenon was also observed when polycrystal MAPbI₃ film were exposed to flow of 98 ± 2% relative humidity gas for a period of 24 h, and subsequently flushed with dry nitrogen gas (0% relative humidity).²³ Thus, the initial colour change from black to yellow of the MSCs decomposition process in aqueous solutions might be not an acid–base reaction of the methylammonium cation, but rather hydration of MSCs (as illustrated in eqn (4)).

MAPbI₃(s, black) + H₂O(aq) ⇌ (CH₃NH₃)₄PbI₆·2H₂O(s, yellow) (4)

MAPbI₃(s, black) + xHI(aq) ⇌ MAI(aq) + H_xPbI_2+x(aq) (5)

MAPbI₃(s, black) + xKI(aq) ⇌ MAI(aq) + K_xPbI_2+x(aq) (6)

Fig. 3 Photograph of the stability of MAPbI₃ single crystal in different solution at different time. (1) H₂O, (2) saturated KI aqueous solution, (3) saturated MAI aqueous solution, (4) 55% wt% HI aqueous solution, (5) saturated KI in 55% wt% HI aqueous solution, (6) saturated MAI in 55% wt% HI aqueous solution. Schematic illustration of hydatation from CH₃NH₃PbI₃ to (CH₃NH₃)₄PbI₆·H₂O.

The MSCs in HI aqueous solution was dissolved after 25 min, as shown in reaction (5). Compared to solubility of MSC in HI aqueous solution, the MSCs in saturated KI aqueous solution and in saturated KI of HI aqueous solution were rapidly dissolved after 2 min and 10 min, respectively. Thus, KI promoted the dissolution of single crystal by coordination reaction (6). Interestingly, MSC could still be observed in saturated MAI aqueous solution or saturated MAI in HI aqueous solution after 65 min. These results indicated that excess MAI in solution could enhance the stability of MSC. The dominant reason might be that the massive MAI suppressed hydration of MSCs induced by hydrogen bond between MAI and H₂O molecule. Thus, in preparing OTPs solar cell process, massive MAI around OTP layer may enhance the stability of photovoltaic devices.

The thermal stability of the single crystal was measured by thermogravimetric analysis (Fig. 4a). No apparent weight change was observed from 50 °C to 260 °C. Although there is no obvious change in its quality, we can not determine whether the crystal phase of the perovskite crystal is changed. The weight of the single crystal began to reduce after 260 °C, suggesting gradual degradation of the single crystal structure. Optical band gap (E_g) could be extrapolated method from diffuse reflectance spectrum (DRS) (Fig. 4b) and the transformed Kubelka–Munk spectrum for the MSC film (Fig. 4c). According to the Kubelka–Munk equation, the optical absorption coefficient F(R) can be expressed as F(R) = (1 − R)²/2R, where R is the percentage of reflected light. The relationship of the incident photon energy (hν) and E_g can be expressed as the transformed Kubelka–Munk function, [F(R)hν]^p = A(hν − E_g), where E_g is the band gap energy, A is the absorption constant, h is the Planck’s constant, ν is the frequency of the light (s⁻¹) and p is the power index that is related to the optical absorption process.^24–26 Theoretically p equals to 1/2 or 2 for an indirect or a direct allowed transition, respectively. According to literature, MAPbI₃ can be regard as direct allowed transition. Thus, E_g for the MSC film is determined to approximately 1.5 eV from the intercept of the liner part of [F(R)hν]² plot (Fig. 4c) on the X axis. The MSC had a red-shift absorption band at 850 nm (Fig. 4c), whereas regular polycrystal films had an absorption cut-off at 800 nm. This observation was consistent with the DRS results. The PL peak of the MSCs (776 nm) had a shorter wavelength than the absorption cut-off (Fig. 4d). This overlap of the PL spectra with the absorption spectrum of MSC allowed photon recycling in thick MSC by reabsorbing the emission.

Fig. 4 Thermogravimetric analysis and optical properties of MAPbI₃ single crystals: thermogravimetric analysis (a), diffuse reflectance spectrum (b), transformed Kubelka–Munk spectrum (c) and steady-state absorbance and photoluminescence (d).

Conclusions

In summary, we have developed facile growth of MAPbI₃ single crystals by antisolvent diffusion-induced (ASDI) method at room temperature. Diethyl ether, as an antisolvent, can effectively diffuse and induce MSC growth. Compared to solubility of PbI₂ in H₂O, PbI₂ powder was easily dissolved in HI aqueous solution by coordination between PbI₂ and HI. The structure transform from initial layered PbI₂ to intermediate (H_xPbI_2+x·xH₂O) and finally three-dimensional MAPbI₃ single crystal were observed. On the other hand, decomposition of MAPbI₃ single crystal in aqueous solution was significantly enhanced by potassium iodide coordination, and inhibited by CH₃NH₃I addition. We ascribed this inhibition behaviours to suppressing MAI migration from the MSC crystal structure. These findings should be of interest to OTP-based scientists and manufacturers.

Acknowledgements

This work was financially supported by the National Basic Research Program of China (Grant No. 2011CBA00701), National High Technology Research and Development Program of China (Grant No. 2011AA050510), National Natural Science Foundation of China (Grant No. 21171084), the Returned Overseas Researcher Foundation from the Ministry of Education, the Program for Scientific Research Innovation Team in Colleges and Universities of Shandong Province antae-Shan Scholar Research Fund.

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Footnote

† Electronic supplementary information (ESI) available: Single crystal X-ray diffraction (XRD) data of CH₃NH₃PbI₃. Photograph of the crystallization process by antisolvent-diffusion induced method. NMR of CH₃NH₃I. See DOI: 10.1039/c5ra17579h

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