Cubic structure of the mixed halide perovskite CH₃NH₃PbI_3−xCl_x via thermal annealing

Abstract

A methodology has been developed to obtain a cubic structure of the mixed halide perovskite CH₃NH₃PbI_3−xCl_x that involves thermal annealing of a vacuum-deposited perovskite layer. In this process, a tetragonal–cubic transition of the mixed halide perovskite CH₃NH₃PbI_3−xCl_x has been observed using a variety of characterization techniques, e.g., X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical and Hall effect measurements. According to the XRD, the tetragonal structure of CH₃NH₃PbI_3−xCl_x changed into the cubic phase when the temperature increased to above the transition temperature, at which point all iodine atoms move towards the central axis while PbI₆ rotates around the C–N bond. After annealing, the stable cubic structure of the mixed halide perovskite CH₃NH₃PbI_3−xCl_x has a smaller band gap and a better optical absorption than the original tetragonal structure. Meanwhile, the microstructure has shown an increase of grain size after annealing, which could be given the fastest mobility of 13.5 cm² V⁻¹ S⁻¹, yielded from the Hall effect measurements. The potential benefits of the stable cubic structure and large grain size in the mixed halide perovskite CH₃NH₃PbI_3−xCl_x are to improve the device performance of solar cells from the viewpoint of bandgap engineering and crystalline quality.

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1. Introduction

Recently, the highest record of power conversion efficiency (PCE) > 20% has been achieved in emerging solar cells by the use of absorbers of the lead halide perovskite CH₃NH₃PbX₃ (X = Cl, Br, I and mixed).¹ The reason for such high performance is due to the excellent properties of polycrystalline perovskites, e.g., direct-optical absorption² and balanced bipolar long carrier diffusion length (∼1 μm),³ which is several orders longer than that for other emerging semiconductors in photovoltaics.⁴ Hence, in order to further enhance the device efficiency, larger grain sizes are one of the critical factors to ensure excellent optical absorption and low non-radiative recombination within the thicker perovskite absorbers.^5,6 In addition, the unique hysteretic current–voltage behaviours of perovskite solar cells are also closely related to the hybrid organic–inorganic crystal structure and their surface defect states.^7,8

In principle, the symmetry of the atomic structure within the crystal can have an influence on the band gap and optical absorption range of perovskite absorbers.^9–12 CH₃NH₃PbI₃ has three stable crystal structures, i.e., cubic, tetragonal and orthorhombic,^13–15 of which the former provides the highest symmetry structure and stability at high temperatures.^10,16 It was found that along with the crystal structure transforming from a lower symmetry structure (orthorhombic) to a higher symmetry structure (tetragonal) in CH₃NH₃PbI₃, its band gap decreases and hence enables greater absorption for a much longer wavelength.^9,12,15 Thus, it is expected that with further transformation into a cubic structure that has an increased symmetry, the optical band gap of CH₃NH₃PbI₃ would further decrease, thereby matching the solar spectrum better and lead towards an enhanced optical absorption. However, for most CH₃NH₃PbI₃ devices, only the use of a tetragonal crystal structure obtained by either solution processing^17–19 or vacuum deposition²⁰ has been reported. None of them have considered the approach of a thermal annealing-induced phase-transition to prepare the cubic structure of CH₃NH₃PbI_3−xCl_x perovskite for the improvement of perovskite solar cells.

In general, doping a small amount of bromine (Br) or chlorine (Cl) within the CH₃NH₃PbI₃ thin films effectively enables the phase transition from a tetragonal to a cubic structure.^21,22 In addition, it is confirmed that the incorporation of Cl within CH₃NH₃PbI₃ increases the carrier mobility²³ and diffusion length³ as well as the carrier transport at the interface.²⁴

Here, the cubic structure and the larger grain size have been successfully obtained for the vacuum-deposited CH₃NH₃PbI_3−xCl_x layer through a proper thermal annealing. During annealing, the iodine atoms move towards the central axis while the PbI₆ metal halide framework rotates around the C–N bond (i.e., the crystal axis) in the tetragonal structure, and finally reaches the stable cubic structure at the phase transition temperature. As the temperature increases further, the PbI₂ phase separates from the bulk CH₃NH₃PbI_3−xCl_x perovskite, owing to the process of bond breaking of organic cations (CH₃NH₃⁺) away from the PbI₆ metal halide framework. Moreover, based on observations, an enhanced optical absorption with a red-shifted absorption wavelength has been achieved by transforming the crystal structure from tetragonal to cubic after annealing at 90 °C. Transport properties from Hall effect measurements suggested that the microstructure and grain sizes both have an impact on the carrier mobility, whereas the highest mobility (13.5 cm² V⁻¹ S⁻¹) was achieved after annealing at 100 °C due to the reduced defect density.

2. Experimental

2.1 CH₃NH₃PbI_3−xCl_x fabrication

CH₃NH₃I (99.999% purity) and PbCl₂ (99.999% purity) were both received from commercial outlets. Before vacuum deposition, the substrates were ultrasonically cleaned in sequence with a detergent and methanol for 5 min, and subsequently these substrates were further treated with UV-ozone for 15 min. Then the substrates were fixed on a sample holder and transferred into a vacuum chamber, and two crucibles were heated as the base pressure (Mini-spectros, Kurt. J. Lesker) reached to <5.6 × 10⁻⁶ mbar. CH₃NH₃I (99.999% purity) and PbCl₂ (99.999% purity) were heated in alumina crucibles, and 500 mg and 200 mg were employed, respectively, each time. During the deposition process the molar ratio of CH₃NH₃I to PbCl₂ was fixed at 3 : 1 to ensure the formation of stable CH₃NH₃PbI_3−xCl_x, and the thickness was monitored using a calibrated quartz crystal microbalance and further calibrated via cross-sectional measurements. After finishing the deposition, all thin films were thermally annealed in a nitrogen-filled glove-box. In our work, the as-grown thin films were annealed at 70 °C, 80 °C, 90 °C, 100 °C, 110 °C and 120 °C for 1 hour. After thermal annealing, most of the thin films change to reddish-brown colours from shallow oranges, while the film annealed at 120 °C did not show the same observation. Finally, as the annealed thin films cooled down to room temperature, some of the samples were transferred to a vacuum chamber again and metallic electrode deposition (Ag) was completed though a shallow mask to satisfy the Hall effect measurement, while the rest of the samples were characterized using various measuring methods. The thickness of the CH₃NH₃PbI_3−xCl_x film is 530 nm.

2.2 Characterization

The films were characterized using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), UV-visible light absorption and Hall effect measurements. X-ray diffraction characterization was performed (XRD, Rigaku TTR III) using Cu Kα (λ = 1.5406 Å) radiation. The surface morphologies of the CH₃NH₃PbI_3−xCl_x films were characterized using scanning electron microscopy (SEM, FEI Quanta 200), and their band structure was studied using photoluminescence (PL) spectroscopy (iHR320, Horiba JY). Moreover, UV-visible light absorption of CH₃NH₃PbI_3−xCl_x was measured using AnalytikJena AG, and the Hall mobilities were obtained using a Nanometrics HL5500 Hall system (notice that a 3.8 nA DC mean current and a 200 mV applied voltage were used when performing the Hall effect measurements).

3. Results and discussion

Pure CH₃NH₃PbI₃ has a tetragonal structure at room temperature,^15,16 yet can undergo a tetragonal–cubic transition at around 54.4 °C.¹⁴ Typically, CH₃NH₃PbI₃ obtained from solution processing after thermal annealing (>80 °C) shows a tetragonal structure.⁵ After doping with elements that have smaller atomic radii, such as bromine (Br) and chlorine (Cl), a halide perovskite with a cubic structure could be achieved without affecting the optical absorption and electrical properties.^11,21,22,25 Details of the crystal structure are compared in Fig. 1(a) and (b) according to the reported atomic coordinates in the unit cells,¹⁵ in which the organic cations (CH₃NH₃⁺) and PbI₆ are located in different sites, and the orientation of CH₃NH₃⁺ is distinct within the two phases. Theoretically calculated XRD results are plotted in Fig. S(1 and 2).† It is noted that the calculated results are consistent with those experimentally observed for both the cubic and tetragonal crystal structures.^21,26,27 The band gap of the cubic structure of CH₃NH₃PbI₃ is around 1.55 eV,¹⁵ while the band gap of CH₃NH₃PbI_3−xCl_x with a cubic structure is 1.57 eV.¹²

Fig. 1 Crystal structure of the mixed halide perovskite with a cubic phase (a) and (b) a tetragonal phase, where black spheres represent lead, purple spheres are iodine and the others represent the organic cations (CH₃NH₃⁺).

Thermal treatment provides a route to control both the crystalline structure and grain size. Fig. 2(a) and (b) illustrate the XRD evolution of the CH₃NH₃PbI_3−xCl_x perovskite on the as-grown thin films which are treated by post-annealing in a nitrogen-filled glove box at 80 °C, 90 °C, 100 °C and 110 °C for 1 hour. The as-grown CH₃NH₃PbI_3−xCl_x perovskite shows the tetragonal-structured characteristic peaks of (002), (110), (004) and (220).^14,21,28 Additionally, in the mixed halide perovskite films, it is found that the dominant component of the thin films is CH₃NH₃PbI₃ not CH₃NH₃PbI_3−xCl_x, which is consistent with the literature^20,29–31 as only a small fraction of Cl atoms was observed from the EDS measurements, see the ESI (Table S1†). After the thin films were annealed at 80 °C for 1 hour, the (110) and (220) peaks decreased while the (002) and (004) peaks became dominant.

Fig. 2 XRD (a and b) patterns of the perovskite layer before and after thermal annealing at 80 °C, 90 °C, 100 °C, and 110 °C, the stars represent the peaks of the substrates.

As the annealing temperature increased to 90 °C, peaks corresponding to the tetragonal structure completely disappeared, instead, the (100) and (210) peaks of the cubic structure for CH₃NH₃PbI_3−xCl_x emerged, which indicates a phase transition from the tetragonal to the cubic structure (i.e., space group Pm3̄m, lattice constant is 6.276 Å). Moreover, a strong (001) peak of PbI₂ also appeared. As the annealing temperature increased much higher, to 100 °C and 110 °C, the peak intensity of PbI₂ (001) become much higher, indicating that more of the CH₃NH₃PbI_3−xCl_x perovskite thin film dissociated into the CH₃NH₃I gas and PbI₂ crystals due to the chemical bonds between the organic cations and the PbI₆ cages being broken at a high temperature. The color of the CH₃NH₃PbI_3−xCl_x films (see in Fig. S3†) changed from a reddish-dark-brown to a yellow colour as the annealing temperature increased from 90 °C to 110 °C, further confirming that the optical absorption of the films is reduced. Some pioneering work predicted that PbI₂ existing at grain boundaries can function as a passivator³² and facilitates carrier transport.^29,33 The bi-phasic materials of the perovskite phase and PbI₂ within the films have been observed from low magnification SEM images in Fig. S4(b and c)† as the annealing temperature was over 90 °C.

For the lead halide perovskite film, there are three major dynamic mechanisms for the phase transition. (i) Organic cations (CH₃NH₃⁺) move towards the C–N bonds or with respect to the crystal axis with the temperature and rotation often enhanced during the structural phase transition from a lower symmetry to a higher symmetry;¹⁰ (ii) the orientation of highly ordered CH₃NH₃⁺ along the different directions changes³⁴ and dynamic disorder appears in the cubic phase;¹⁶ (iii) it is often accompanied by the rotation of PbI₆ cages around the crystal axis.¹⁴

In our case, the phase transition of the structure is majorly attributed to the temperature-induced rotation of slightly distorted PbI₆ octahedra along the crystal-axis of (001).¹⁶ Fig. 3(a)–(c) show the atomic arrangements on the (100), (002) and (110) lattice planes of the tetragonal and cubic structures. It is found that atoms on the (002) and (110) lattice planes of the tetragonal structure are iodines, while the (100) plane is composed of Pb atoms and organic cations (CH₃NH₃⁺) for the cubic structure. But as the as-grown thin film of the tetragonal-structured CH₃NH₃PbI_3−xCl_x perovskite was heated, all the iodides on the (110) lattice planes started to move towards the crystal-axis, and led to the symmetric structure of the PbI₆ cages and a reduction in the chemical bond lengths between the atoms (see in Fig. 3(a)), resulting in the dominant diffraction peaks changing from (110) to (002) as the annealing temperature increased to 80 °C. As the temperature further increased from 80 °C to 90 °C, all iodine atoms aligned with the central axis owing to the PbI₆ cages rotating around the crystal-axis, which resulted in the formation of perfect PbI₆ octahedra in such crystals, i.e., a cubic structure.

Fig. 3 Atomic arrangements (a–c) on the (110) and (002) lattice planes of the tetragonal structure and the (100) lattice plane of the cubic structure.

Fig. 4 shows the surface morphology of the CH₃NH₃PbI_3−xCl_x perovskite thin film characterized after annealing at the different temperatures. For the as-grown thin films, the surface shows a crack-like morphology with a few isolated CH₃NH₃PbI_3−xCl_x islands, and the grain size is around less than 500 nm. After the annealing temperature increased to 70 °C and 80 °C, some isolated islands started to connect into a continuous film. After the annealing temperature reached 80 °C, the isolated islands merged together to form uniform thin films.

Fig. 4 SEM images of the vacuum-deposited MAPbI_3−xCl_x perovskite layer before (a) and after thermal annealing (b–f) at 70 °C, 80 °C, 90 °C, 100 °C and 110 °C.

As the annealing temperature reached 90 °C, not only did the structure change from tetragonal to cubic, but also the grain size became larger. As the temperature reached 100 °C, the largest grain size of around 5 microns was obtained but a void was also seen in Fig. 4(e). This void could have originated from the dissociation of CH₃NH₃PbI_3−xCl_x, as discussed with respect to the X-ray diffraction. As the temperature increased to 110 °C, smaller grains emerged again which could be due to the large amount of PbI₂ crystals dissociated from the bulk sample. Furthermore, the coexistence of PbI₂ and CH₃NH₃PbI_3−xCl_x within the thin films after annealing at temperatures of over 90 °C has also been confirmed with high contrast SEM images at 100 °C and 110 °C (Fig. S4(c and d)†), as bright crystals of PbI₂ were observed.

The band gaps of the CH₃NH₃PbI_3−xCl_x films were determined using the photoluminescence (PL) spectrum as shown in Fig. 5(a). Compared to the as-grown film and the film after annealing at 80 °C, the red-shift of the PL peak is due to the increased grain size.^9,35 As the annealing temperature changed from 80 °C to 90 °C, which corresponds to the phase transition from tetragonal to cubic, the PL peak position red-shifted from 768 nm to 778 nm, the red-shift is around 10 nm. The change in the PL peak position is due to variation in the band-edge emission, not the different grain-induced defect states^36,37 and the onset of the light absorption demonstrated an enhanced absorption at the long wavelengths in Fig. 5(b) for the cubic CH₃NH₃PbI_3−xCl_x. This confirmed that the higher symmetry cubic structure has a smaller band gap compared to the structure of CH₃NH₃PbI_3−xCl_x. Similar observations have been reported for orthorhombic–tetragonal transitions.^9,13

Fig. 5 Photoluminescence (PL) spectra (a) and light absorption spectra (b) of the vacuum-deposited CH₃NH₃PbI_3−xCl_x perovskite layer (530 nm) on the quartz substrates before and after thermal annealing at 80 °C, 90 °C, 100 °C and 110 °C, with an excitation wavelength of 467 nm, where the dashed line indicates the PL peaks.

As the annealing temperature increased to 100 °C, the PL peak position exhibits almost no change at 778 nm, but its intensity enhanced and the width of the PL peak reduced, which means that thin films with annealing temperatures at 100 °C have a much lower density of defect states. For the optical absorption in Fig. 5(b) and Fig. S(5 and 6) in the ESI,† a second absorption peak, which corresponds to PbI₂, also emerged after annealing at 100 °C. As the temperature further increased to 110 °C, the PL peak blue shifts to 774 nm. This phenomenon is largely due to the smaller grain size³⁷ and the emerging phase of PbI₂.⁴ Such a change also results in colour variation of the samples, photographed in Fig. S3.† Hence, the corresponding PL and absorption of the films are consistent with the change of grain size and the phase transition after annealing at the different temperatures.

The optical absorption of the CH₃NH₃PbI_3−xCl_x film benefits from the higher symmetry cubic structure and the large grain size after annealing. The corresponding change in the carrier mobility as the annealing temperature increased was further characterized using Hall effect measurements. A Hall effect measurements setup is plotted in Fig. 6(a) and during the measurements an applied magnetic field was rotated to eliminate the effect of the irregular metallic Ag electrode on the measured results.

Fig. 6 Hall effect measurement setup (a), and (b) the mobility and resistivity both plotted against the thermal annealing temperature.

Fig. 6(b) shows CH₃NH₃PbI_3−xCl_x as a p-type semiconductor and the mobility/resistivity as a function of the annealing temperature. The mobility reaches a maximum value of 13.5 cm² V⁻¹ S⁻¹ after annealing at 100 °C and this value is even larger than the mobility from other solution processes.²³ The reason is that the CH₃NH₃PbI_3−xCl_x films have different grain sizes and structures as well as compositions as the thin films were annealed at different temperatures. Obviously, whether the thin films undergo annealing at 70–80 °C or not, the surface morphologies of the thin films have changed considerably but the changes are negligible for the grain size and crystal structure. Therefore, the observed enhancement of mobility (or decrease in resistivity) is primarily contributed by the microstructure. When the temperature rose to 90–100 °C and even 110 °C, all the perovskite in the thin films transformed into a cubic structure, along with a gradual increase in the amount of PbI₂. The largest mobility was achieved when the thin films were annealed at 100 °C, indicating that the large grain size within the CH₃NH₃PbI_3−xCl_x film strongly contributes to the mobility, while an appropriate amount of PbI₂ and a low density of defect states are pre-requisites for high mobility. In short, for the cubic-structured CH₃NH₃PbI_3−xCl_x perovskite, controllable mobility can be achieved by tuning the microstructure and grain size within the thin films after annealing at an appropriate temperature.

4. Conclusions

In summary, the cubic phase for CH₃NH₃PbI_3−xCl_x perovskite has been achieved after thermal annealing at 90 °C, and it is found that the dominant diffraction peaks are different from the as-grown thin films annealed at 80 °C. The fundamental reason for such changes is due to the rotation of the octahedral cages of PbI₆ around the (001) c-axes with increasing temperature which leads to iodine atoms on the (110) plane moving towards the central axis. The results confirmed that the phase transition was accompanied by changes in the microstructure and grain size post-annealing, which in turn govern its optical absorption and non-radiative recombination as well as the defect states. Finally, the highest mobility of 13.5 cm² V⁻¹ S⁻¹ and a longer absorption wavelength of 778 nm have been achieved for the cubic phase after annealing at 100 °C. As such, the cubic-structured CH₃NH₃PbI_3−xCl_x thin films with large crystalline structures and a small amount of PbI₂ at room temperature ensured a low density of defect states and non-radiative recombination. As a result, this paves the way to yielding excellent optoelectronic properties for vacuum-deposited CH₃NH₃PbI_3−xCl_x mixed halide perovskites by adjusting post-annealing conditions.

Acknowledgements

We acknowledge the funding support for this work from the National Natural Science Foundation of China (No. 61166006, 61366002, 61166007 and U1137602) and the Introduction Program of High end scientific and technological talents in Yunnan Province (No. 2013HA019). Also, we wish to acknowledge funding from the Applied Basic Research Programs of Science and the Technology Commission Foundation of Yunnan Province (No. 2010ZC164).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra16516d

‡ Present address: Institute of Modern Optics, School of Physics, Peking University, Beijing 100871, China.

§ Authors contributed equally to this work.

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