Formation of organic–inorganic mixed halide perovskite films by thermal evaporation of PbCl₂ and CH₃NH₃I compounds

Abstract

Organic–inorganic hybrid perovskites were prepared by thermal evaporation from precursor materials PbCl₂ and CH₃NH₃I (MAI). The structures of the perovskite films with various ingredients were characterized by X-ray diffraction (XRD), UV-vis absorption, and scanning electron microscopy (SEM). We found that with the addition of MAI material the evaporated PbCl₂ films were initially transformed to PbI₂, then to standard a stoichiometric perovskite and finally to the MAPbI₃·xMAI phase. The film composition ingredients strongly affect the device performance. An unmatched PbCl₂ to MAI ratio in the evaporated films resulted in reduced conversion efficiency and higher moisture sensitivity. The planar perovskite solar cells with organic charge layers showed negligible photocurrent hysteresis and delivered a power conversion efficiency of 10.5%.

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Introduction

Solar photovoltaic, which converts sunlight directly into electricity, is considered as a clean, sustainable, promising renewable energy conversion technology. Recently organic–inorganic halide perovskites, with features such as excellent optical properties,¹ ambipolar charge transport² and very long electron–hole diffusion length,^3,4 are attracting much attention, and have achieved tremendous progress. In 2009, lead halide perovskites were first reported as photosensitizers in liquid-type sensitized TiO₂ solar cells.⁵ All-solid-state perovskite solar cells were then developed to achieve good stability and high efficiency using spiro-OMeTAD as a hole transporting material.^6,7 To date, a power conversion efficiency of over 16% has been demonstrated by several groups in mesostructured and planar perovskite solar cells.^8–10 It was initially assumed that a mesostructured (or nanostructured) matrix may be necessary for efficient charge transport and collection in perovskite solar cells, but many studies^11,12 confirmed a planar device structure is also very suitable for perovskite solar cells due to their long charge diffusion lengths.

Various processing techniques, including thermal evaporation,^11,13–15 vapor assisted solution process,¹⁶ one-step^6,7,10 and two-step solution process,^9,12,17 have been used for the preparation of high quality perovskite absorbers. For the solution process, a great deal of effort has been put to control the morphology of the resulting film by adjusting the annealing condition,^18,19 using additives^20–22 and mixed solvents,⁸ etc. In contrast, evaporation can offer inherent advantages in obtaining perovskite film with excellent coverage, high material purity and good thickness control. Liu et al.¹¹ first demonstrated vapor-deposited perovskite solar cell with a simple planar device structure. Their findings showed great promise for preparing efficient planar perovskite solar cell by evaporation. Malinkiewicz et al.¹⁵ reported an inverted planar perovskite by evaporation, sandwiched between two organic charge transport layers. This inverted structure allows a relative low processing temperature for fabricating perovskite solar cells. In addition, the evaporation process is free of organic solvent, which may erode the underlying charge layer, thus allowing more choice of organic charge materials.²³

In this paper, we present a detailed study of the evaporated perovskite film under various evaporation conditions. Based the previously reported results^3,24 that the addition of chloride could dramatically increase the photoluminescence lifetime of the photoexcited species and charge diffusion lengths in the absorber, we adopted the PbCl₂ and MAI as the precursor materials. The effect of PbCl₂ to CH₃NH₃I (MAI) ratio on structural, optical and electrical properties of evaporated perovskite films was investigated. Using the planar device structure with low-temperature processed organic charge layers, we obtained an optimal device with an efficiency of 10.5% which didn't show obvious hysteresis.

Experimental section

CH₃NH₃I synthesis

The material CH₃NH₃I (MAI) was synthesized based on the literature.⁷ In short, to prepare MAI, hydroiodic acid (10 mL, 57 wt% in water, Sigma-Aldrich) and methylamine (10 mL, 33 wt% in absolute ethanol, Sigma-Aldrich) were reacted in a round bottomed flask in ice-cold water with stirring for 60 min. Raw CH₃NH₃I was obtained by removing the solvent at 50 °C on a rotary evaporator. The material was then washed in diethyl ether and filtered several times. The precipitate was then dried in a vacuum oven overnight at 50 °C and then stored in nitrogen-filled glovebox.

Perovskite film evaporation

The evaporation was carried out in Angstrom Engineering EvoVac deposition system with integrated Innovative Technology glovebox. The PbCl₂ and MAI materials were both evaporated from ceramic crucibles. The evaporation rate was monitored by the quartz crystals. The quartz crystals could be reused after dipping in DMF solution and then washing in ethanol. New quartz crystal was used for each evaporation. The undulating evaporated CH₃NH₃I film makes it difficult to measure the accurate thickness. To avoid the tedious calibration process for the evaporation (or deposition) rate of each material, we set the tooling factor (which is a ratio of the film deposition rate monitored by sensor to that on the substrate) to be 100% during all the co-evaporation process. Initially, the evaporator sources were heated slowly to an level that the evaporation rate was just around the desired range, the process were then carried out in auto mode. A desired evaporation rate can be obtained via heating control system from the feedback of their sensors. After the evaporation rate were stabilized for a prolonged time, the PbCl₂ film and MAI film were then co-evaporated onto the substrates with substrate baffle open. Various MAI evaporation rate (0–8 Å s⁻¹) were employed. The evaporation rate of PbCl₂ material was maintained at constant monitored rate of 2 Å s⁻¹.

Solar cell fabrication

Solar cells with FTO/PEDOT:PSS/Perovskite/PCBM/Ag planar device structure were prepared. Before deposition of other films, the FTO-coated glass substrates were ultrasonically cleaned with detergent, deionized water, acetone and ethanol sequentially, and were then blow-dried in nitrogen. After that, PEDOT:PSS solution (Clevious Al 4083) was spin-coated on FTO-coated substrates at 2000 rpm for 45 s, and subsequently the PEDOT:PSS films were annealed at 150 °C for 20 min. After cooling down, perovskite absorbers were evaporated on PEDOT:PSS films. The evaporated perovskite films were then annealed at 100 °C for 45 min on a hot plate. 30 mg ml⁻¹ PCBM (FEM. Inc.) solution in chlorobenzene (Sigma-Aldrich) was then spin-coated at 1000 rpm for 45 s. Finally, Ag top contact was deposited by evaporation through an aperture mask under a base pressure of 7 × 10⁻⁶ Torr. The active device area is 0.12 cm².

Measurement and characterization

X-ray diffraction (XRD) analysis was performed on a D/max-RB diffractometer (Rigaku) using Cu Kα radiation at a scan rate of 6° min⁻¹. The energy-dispersive X-ray (EDX) compositions and spectra were performed using energy-dispersive spectroscopy (EDS) combined with a field-emission scanning electron microscope (SEM, Hitachi S4500). SEM images were obtained using Hitachi S4500 and Hitachi S5200. Film absorbance spectra were measured by Shanghai UV-vis SP-752 spectrometer. Current–voltage measurements and power conversion efficiencies were obtained using Keithley 2400 at room temperature under AM 1.5G illuminations (1000 W m⁻²) from a solar simulator which was calibrated using a standard silicon solar cell device.

Results and discussion

The CH₃NH₃PbI_3−xCl_x perovskite films were prepared by co-evaporation method with various MAI evaporation rate but at a fixed PbCl₂ evaporation rate. The details of the co-evaporation are described in experimental section. The deposition condition and EDX composition data of the prepared perovskite thin films are shown in Table 1, where the evaporation rate ratio of CH₃NH₃I to PbCl₂ is referred to as y_rt. All EDX analysis was performed at electron accelerating voltage of 10 kV. It can be seen that when the evaporation rate of CH₃NH₃I was increased from 0 to 8 Å s⁻¹, the Cl content in the films dropped greatly from 64.45% down to 0.18%, while the Pb content only from 35.55% to 19.86%. Therefore, it is easy to conclude that the loss of Cl occurred in the evaporating process. In the solution-processed perovskite film employing PbCl₂ and MAI with a molar ratio of 1 : 3, many studies have showed that Cl tended to get lost during the annealing, which accords well with our results. Cl was believed to sublime in the form of CH₃NH₃Cl¹⁸, thus the loss of Cl increased with the incorporation of MAI. Although there is a negligible amount of Cl content inside fully annealed perovskite films, it is still considered the addition of chloride play a key role in improving film growth and charge transport.^20,21,24,25 Particular attention was paid to the I to Pb ratio in evaporated film. When the process condition y_rt equals 2, the I to Pb ratio is about 2, which can be attributed to the PbI₂ formation. For y_rt = 3, the I to Pb ratio is about 3, which is close to that of the standard stoichiometric MAPbI_3−xCl_x perovskite (Pb/I = 1/3).

Table 1 Composition variation (measured by EDX) of the evaporated films at different deposition conditions^a

Sample	Evaporation rate ratio yrt	Evaporation rate (Å s^−1)		EDX composition
		CH3NH3I	PbCl2	Pb (at%)	I (at%)	Cl (at%)
a The evaporation rate ratio of CH3NH3I to PbCl2 is referred to as yrt, and yrt = 0 means only PbCl2 film was deposited.
S1	0	0	2	35.55	0	64.45
S2	2	4	2	32.18	62.10	5.72
S3	3	6	2	25.96	73.43	0.60
S4	4	8	2	19.86	79.96	0.18

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In order to identify the phase constitution, X-ray diffraction (XRD) analysis was conducted to study the structural evolution of evaporated perovskite films at different deposition conditions. Fig. 1a shows the XRD patterns of four samples listed in Table 1. Much difference can be observed between the samples deposited with different amounts of MAI. When only PbCl₂ (y_rt = 0) is deposited, the sample exhibits peaks at 19.9°, 23.0°, 25.1° and 32.4°, which is in agreement with PbCl₂ XRD data according to JCPDS card 26-1150. For the sample with deposition condition y_rt = 2, the peak positions of the X-ray reflection are comparable to the powder-diffraction standard data (JCPDS 07-0235) for PbI₂, indicating the formation of PbI₂ from the reaction of PbCl₂ and MAI. It can be also seen that the PbCl₂ peaks disappear entirely although there was a little Cl (5.72 at%) contained in the film, which can be explained by low crystallinity of possible remaining PbCl₂ or limited determination precision of XRD used. In this case, no peaks related to perovskite MAPbI₃ phase are found from the XRD pattern, indicating the amount of evaporated MAI is not sufficient. As the amount of MAI was further increased (y_rt = 3), two strong peaks appear at 14.2° and 28.5°, which correspond to the perovskite CH₃NH₃PbI₃ (110) and (220) crystalline planes, respectively. Compared with the mesostructured perovskite layers in other literatures,¹⁷ the evaporated planar perovskite film shows an extremely preferred orientation along (110) plane, perhaps due to lack of the restrictions from oxide scaffolds in the formation of the perovskite layer. For the samples with deposition condition y_rt = 4, some extra peaks appeared, which means that the amount of evaporated MAI is too-much. In contrast to the X-ray diffractogram of powdered MAI in Fig. S3,† we found those extra peaks don't originate from the MAI material and may be attributed to the possible compound MAPbI₃·xMAI.

Fig. 1 (a) XRD spectra, (b) UV-vis absorbance and (c–g) photographs of the evaporated films at different deposition conditions.

In Fig. 1b–g we present UV-vis absorption spectra and photographs of the samples with different deposition rate ratios. The PbCl₂ film shows white color and an extremely low absorbance. When the amount of MAI is insufficient, the sample exhibits a yellow color due to the presence of PbI₂. When amount of MAI is moderate or excess, the samples after annealing exhibit a dark brown color due to the formation of perovskite phase, and the corresponding absorbance spectra also display an onset of drop at 780 nm, which is in agreement with previously reported results.⁷ Additional features related to the stability must be also mentioned. The perovskite film with moderate MAI demonstrated good stability, as evidenced by the absorbance spectra change over time (Fig. S4†). In contrast, the perovskite film with excess MAI exhibits rapid degradation in absorbance and the film color varies from dark brown to white (Fig. 1g) after exposing to the air (relative humidity more than 60%) for only 20–30 min. It is noted that the degradation is probably different from the hydrolyzation of perovskite to yellow PbI₂. However, the transition is reversible. The brown dark perovskite phase will recover again upon heating. Similar results shown in Fig. S2† were also found for the samples with surface composition gradient. We believe that the excess MAI in the perovskite film affects the morphology and crystalline phase, thus reducing the environmental stability.

To further investigate the morphology of the films, we performed SEM analysis. Fig. 2 shows the SEM micrographs of the evaporated films deposited with different MAI to PbCl₂ ratios. It can be seen that all the films have dense microstructures and show a 100% coverage which is hard to realize in solution-processed films. When the film was deposited with either PbCl₂ (y_rt = 0) or insufficient MAI (y_rt = 2), the grain size of the films is within a small range. For the film with y_rt = 3, considerable grain growth with smooth surface can be observed. Smooth films could lower the dark current and reduce the density of interface states,²⁶ which is essential for fabricating a high-quality planar solar cell. When y_rt was increased to 4, loose porous surface morphology was formed. The significant difference can be contributed to the presence of excess MAI due to its high surface activity and hygroscopic nature.

Fig. 2 SEM images of the evaporated films with different deposition ratios.

The above structural and optical results predict that the perovskite film with y_rt = 3 should yield good photovoltaic performance. To find the optimal deposition condition, detailed optimizations of evaporation rate ratios around y_rt = 3 were carried out. Three perovskite solar cells with FTO/PEDOT:PSS/Perovskite/PCBM/Ag planar structures were prepared. Fig. 3 shows a cross-sectional SEM image of the complete device. The thicknesses of PEDOT:PSS, Perovskite and PCBM layers are about 30 nm, 400 nm and 100 nm, respectively. The average device parameters with the standard deviations are summarized in Table 2. It is observed that three sets of cells demonstrated a low statistical spread in open circuit voltage (V_oc), which can be explained by good coverage of the evaporated perovskite films. The main differences in the solar cells performance result from fill factor (FF) and short circuit current density (J_sc). For the solar cells with y_rt = 3, the average short circuit current density (J_sc) is only 9.08 mA cm⁻², which may be attributed to the non-exactly-matching of MAI to PbCl₂ ratio. When the deposition ratio y_rt reaches 2.9, the mean power conversion efficiency (PCE) of 7.61% was achieved with increased J_sc and FF. Reducing y_rt to 2.8 results in decreased PCE. Therefore, the optimal deposition ratio y_rt determined in this work is 2.9.

Fig. 3 Cross-sectional SEM image of a complete FTO/PEDOT:PSS/CH₃NH₃PbI_3−xCl_x/PCBM/Ag perovskite device (inset shows a photograph of the perovskite device).

Table 2 Effect of the amount of CH₃NH₃I on photovoltaic performance of perovskite solar cells. The mean values and standard deviations were attained from 8 cells

Evaporation rate ratio yrt	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)
3.0	0.87 ± 0.03	9.08 ± 1.86	0.50 ± 0.05	3.87 ± 0.61
2.9	0.87 ± 0.07	14.68 ± 2.52	0.59 ± 0.02	7.61 ± 1.60
2.8	0.78 ± 0.08	15.89 ± 2.68	0.52 ± 0.06	6.48 ± 1.18

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Recently, there has been increasing discussion as to the photocurrent hysteresis in perovskite devices.^27,28 To understand the phenomenon of the fabricated perovskite solar cells, the PCE differences were investigated under the forward and reverse scans. 20 devices at the process condition of y_rt = 2.9 were prepared. Their performances were characterized under forward (−0.2 V → 1.2 V) and reverse (1.2 V → −0.2 V) scans. The results are shown in Table 3 where detailed photovoltaic parameters are listed in Table S1.† Fig. 4 shows the current–voltage (J–V) curves for the best performing device measured under forward and reverse scans. The champion device (Fig. 4a) exhibited a J_sc of 17.3 mA cm⁻², FF of 0.63, V_oc of 0.97 V, and PCE of 10.50% under forward scans, and a J_sc of 17.0 mA cm⁻², FF of 0.63, V_oc of 0.97 V, and PCE of 10.32% under reverse scans. Fig. 4b shows the external quantum efficiency (EQE) curve for one optimized device. The EQE curve starts increasing rapidly at 800 nm, which is related to the optical absorption of the perovskite absorber, reaches a maximum value of 71% at 560 nm, and then drops slowly at about 400 nm. From Fig. 4a, the champion device doesn't show obvious hysteresis of photocurrent. However, the statistics result (see Fig. 5) shows the devices under reverse scans have a 0.66% PCE higher than that under forward scans. The higher efficiency under reverse scans mainly results from a slightly higher V_oc by 0.03 V and J_sc by 1.08 mA cm⁻². Compared to TiO₂-based perovskite planar solar cell in other literatures,⁸ the difference is rather small, which could be explained by the difference of perovskite preparation process or charge separation interfaces. There is still great room to further increase the conversion efficiency. The best device in this study shows a V_oc close to 1 V, and its performance is mainly limited by the relatively low FF and J_sc, which may indicate that the charge transport and collection for the solar cells are not still very efficient. Further understanding and optimization of the device structure, apart from the process control, may help improve the device performance.

Table 3 Photovoltaic performance of perovskite solar cells with the process condition y_rt = 2.9 under forward and reverse scan directions. The mean values and standard deviations were attained from 20 cells

Evaporation rate ratio yrt	Scan direction	Voc (V)	Jsc (mA cm^−2)	FF (%)	PCE (%)
2.9	Forward	0.85 ± 0.11	13.63 ± 3.29	0.58 ± 0.10	6.91 ± 2.47
	Reverse	0.88 ± 0.09	14.71 ± 2.67	0.57 ± 0.04	7.57 ± 2.06
	Best	0.97	17.30	0.63	10.50

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Fig. 4 (a) J–V curves for the best performing perovskite solar cell measured under forward and reverse scans. (b) External quantum efficiency of a perovskite device with deposition ratio y_rt = 2.9.

Fig. 5 Device performance statistics for 20 cells measured under (a) forward and (b) reverse scans.

Conclusion

In summary, we present a detailed study of the evaporated perovskite film under various evaporation conditions. The effects of PbCl₂ to CH₃NH₃I ratio on structural, optical and electrical properties of evaporated perovskite films were investigated. We found the deposition condition plays an important role in device performance and optimized the evaporation process. The solar cells utilizing the optimized evaporation process and low temperature processed organic charge layer yielded a power conversion efficiency of 10.5% with negligible hysteresis.

Acknowledgements

This work is supported by the National Nature Science Foundation of China (51202227), the Science and Technology Development Foundation of China academy of Engineering Physics (2014A0302015 and 2014B0302054), the National High Technology Research and Development Program of China (2012AA050704), Sichuan International Cooperation Research Project (no. 2014HH0068) and the Fundamental Research Funds for the Central Universities of China (no. 2672012ZYGX2012J065).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra17316c

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