Growth of Compact CH3NH3PbI3 Thin Films Governed by the

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Growth of Compact CH3NH3PbI3 Thin Films Governed by the Crystallization in PbI2 Matrix for Efficient Planar Perovskite Solar Cells Junwei Chen, Zhiyang Wan, Jiandang Liu, Sheng-Quan Fu, Fapei Zhang, Shangfeng Yang, Shanwen Tao, Mingtai Wang, and Chong Chen ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18667 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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ACS Applied Materials & Interfaces

Growth of Compact CH3NH3PbI3 Thin Films Governed by the Crystallization in PbI2 Matrix for Efficient Planar Perovskite Solar Cells Junwei Chena,b, Zhiyang Wana,b, Jiandang Liub, Sheng-Quan Fub, Fapei Zhangc, Shangfeng Yangb, Shanwen Taod, Mingtai Wang,a,* Chong Chene,* a

Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230088, P. R. China b c

High Magnetic Field Laboratory, Chinese Academy of Sciences, Hefei 230031, P. R. China d

e

University of Science and Technology of China, Hefei 230026, China

School of Engineering, University of Warwick, Coventry CV4 7AL, UK

Henan Key Laboratory of Photovoltaic Materials, Henan University, Kaifeng, 475004, PR China

ABSTRACT As a convenient preparation technique, two-step method, which is normally done by spin-coating CH3NH3I onto PbI2 film followed by a thermal annealing, is generally used to prepare solution-processed CH3NH3PbI3 films for planar perovskite solar cells. Here, we prepare the compact CH 3NH3PbI3 thin films by the two-step method at a low temperature ( 80 oC), and investigate the effects of PbI2 crystallization on the structure-property correlation in the CH3NH3PbI3 films. It is found that the importance of the crystallization in PbI2 matrix lies in governing the transition from the (001) plane of trigonal PbI2 to the (002) plane of tetragonal CH3NH3PbI3 in the rapid reaction process for atoms to coordinate into perovskite during spin-coating, which actually determines the morphology and the type of vacancy defects in resulting perovskite; a better crystallized PbI2 film has a much stronger ability to react with CH3NH3I solution and produces larger CH3NH3PbI3 grains with a higher crystallinity; the CH3NH3PbI3/TiO2 planar solar cell derived from a better crystallized PbI2 film exhibits a significantly improved performance and stability as the result of the higher crystallinity inside the perovskite film. Moreover, it is demonstrated that the crystalline PbI2 film matrix subjected to the annealing after a slow heating process prior to contacting CH3NH3I solution is more effective for CH3NH3PbI3 formation than that with a direct annealing history. The results in this paper are expected to provide a guide for preparing the high-quality CH3NH3PbI3 thin films for efficient perovskite solar cells and the CH3NH3PbI3 interfacial films over the layers susceptible to temperature. Keywords: Solar cell, Perovskite, Crystallization, Film preparation, CH 3NH3PbI3 1

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1. INTRODUCTION With the advantages of scalable solution-processibility at low cost and high efficiency, organometal halide (CH3NH3PbX3, X = Cl, Br, I) perovskite solar cells (PSCs) are considered as a potential alternative to conventional crystalline silicon solar cells.1,2 Generally, there are two typical architectures in PSCs: one is mesoscopic architecture, which has a TiO2 mesoporous film as electron accepter and scaffold to support perovskite layer;35 another is planar architecture formed mainly by depositing (CH3NH3PbX3, X = Cl, Br, I) film on a compact TiO2 layer,612 which is of great interest due to its easy fabrication. Among the organometal halide perovskites, methylammoniun lead triiodide (CH3NH3PbI3) is commonly used because of its favorable direct band gap, high charge carrier mobility, long charge transport length, and simple processibility.4,5,1321 There is a strong relationship between the preparation of perovskite film and the device performance in solar cells. To fabricate CH3NH3PbI3-based planar PSCs, the perovskite layer is often prepared on a compact TiO2 film by either one-step or two-step method. In one-step method, the perovskite precursors (i.e., CH3NH3I and PbI2) are coated onto the compact TiO2 film from an organic solution, normally followed by a thermal annealing at 90–150 oC to evaporate solvent.13,16,22,23 However, one-step method suffers from the difficulties in controlling the quality of resulting perovskite films. 2224 Normally, in two-step method, a spin-coated PbI2 film matrix on the compact TiO2 film was loaded CH3NH3I by spin-coating a CH3NH3I solution, and then annealed at 90–150 oC for the conversion of PbI2 into a compact and crystallized CH3NH3PbI3 film.7,18,2527 Prior to contacting CH3NH3I solution, the spin-coated PbI2 films were directly annealed at a certain temperature (e.g., 70 oC)7,25 or not annealed but only dried at room temperature.18,26,27 Relatively, the two-step approach provides a more convenient and efficient way to prepare high-quality CH3NH3PbI3 films for efficient solar cells. There are some reports concerning the CH3NH3PbI3 formation by the two-step method. Padture and coworkers26,27 believed that CH3NH3I first gets infiltrated into the PbI2 film matrix during spin-coating and the followed thermal annealing induces the solid reaction between PbI2 and CH3NH3I to produce CH3NH3PbI3 perovskite, and the multiple iteration of successive spin-coating/annealing process lead to the full conversion of PbI2 into perovskite. Xiao and coworker25 suggested that the CH3NH3PbI3 film is formed by the interdiffusion of PbI2 and CH3NH3I layers 2


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stacked together by spin-coating during the followed thermal annealing, and the resulting CH 3NH3PbI3 film thickness can be controlled by the thickness of underneath PbI 2 film. Su and coworker28 found a high annealing temperature above 120 oC will cause the decomposition of CH3NH3PbI3 to release PbI2 in the two-step method. On the other hand, Wu and coworkers29 found that, when immersing in the isopropanol solution of CH3NH3I, the crystallized PbI2 film cast from DMF reacts with CH3NH3I much more rapidly than the amorphous PbI2 film cast from DMSO, but the solvent-retarded PbI2 crystallization favours a more complete conversion of PbI2 into perovskite for more efficient solar cells. While the initial PbI2 film matrix may serve as a template for the formation of perovskite by allowing the insertion of organic cations into layered

structure4,30

PbI2

morphology

is

and

the

PbI2

important to CH3NH3PbI3

formation,19 however, the detailed formation

Figure 1. The structure of solar cells used in this experiment. HTL stands for hole transport layer.

mechanism of CH3NH3PbI3 perovskite in the two-step method is not clear enough yet, and we are lacking the knowledge of what happens during spin-coating process; in particular, how the crystallization in PbI2 film matrix affects the structure and property of resulting CH 3NH3PbI3 film, which is still an open question. Understanding the CH3NH3PbI3 formation is undoubtedly of vital importance for the adjustment of material property31,32 and the optimized fabrication of efficient planar PSCs by the two-step method. In this paper, we prepare the compact CH3NH3PbI3 thin films for efficient CH3NH3PbI3/TiO2 planar solar cells by a two-step method at a low temperature ( 80 oC), in which different thermal histories were used to modify the PbI2 film crystallization. The architecture of the solar cells fabricated in this experiment is shown in Figure 1. Our results clearly indicate that the crystallization in PbI 2 film is very important to the formation of high-quality CH3NH3PbI3 film because it governs the rapid reaction between the PbI2 and interdiffused CH3NH3I during spin-coating process and the final morphology and type of vacancy defects in resulting perovskite, and a better crystallized PbI2 film produces larger CH3NH3PbI3 grains with a higher crystallinity for more efficient solar cells.

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2. EXPERIMENTAL SECTION 2.1. CH3NH3I Synthesis. CH3NH3I was synthesized by reacting methylamine (40% in methanol, Sinopharm) and hydroiodic acid (57 wt% in water, Aldrich) at 1:1.1 mole ratio in a 250 mL round bottom flask at 0 oC under vigorous stirring for 2h.13 The precipitate was recovered by evaporation under reduced pressure, washed with diethyl ether cooled by ice-water bath for three times, recrystallized from diethyl ether, and dried at 60 oC in vacuum oven for 24 h. 2.2. Perovskite film preparation. FTO (SnO2:F) coated glass sheets (14 /, 400 nm FTO in thickness, Nippon Sheet Glass Co.) were patterned into stripes (16  4 mm2) by HCl solution and Zn powder, and washed with acetone, isopropanol, and deionized water, respectively. The patterned FTO sheets were further cleaned with UV-Ozone for 15 min and washed with ethanol before use. TiO 2 precursor solution was prepared by mixing absolute ethanol, tetrabutyltitanate and acetic acid in a volume ratio of 20:5:0.5. A patterned FTO substrate was spin-coated (2500 rpm, 40 s) with the TiO2 precursor solution under ambient condition, and then kept in a humid air (50% relative humidity) overnight in order to get a full hydrolysis of TiO2 precursor and a good polymerization of Ti-O network; a compact TiO2 film (ca., 100 nm thick) was formed on the FTO sheet after a sinter at 550 oC for 30 min in air. The CH3NH3PbI3 films on compact TiO2 films were prepared by a two-step method. Firstly, the PbI2 (99%, Aldrich) solution (462 mg/ml) in N,N-dimethylformamide (DMF, 99.8%, Sigma-Aldrich) was spin-coated (5000 rpm, 30 s) onto the TiO2 film and dried overnight at room temperature in a glovebox (O2 1 ppm, H2O  1 ppm) under N2 atmosphere, resulting in as-cast PbI2 film (as-PbI2). A freshly spin-coated PbI2 film was slowly heated from room temperature to 70 oC at a heating rate of 2 oC/min and maintained at 70 oC for 30 min under N2 atmosphere, resulting in annealed PbI2 film (h-PbI2). Then, CH3NH3I solution (30 mg/ml) in isopropanol (IPA, 99.5%, Sigma-Aldrich) was loaded onto either as-PbI2 or h-PbI2 film and kept there for 20 s under ambient condition, followed by spin-coating (4000 rpm, 20 s) to get an intermediate composite film, which is referred to as CH3NH3I@as-PbI2 or CH3NH3I@h-PbI2 intermediate film. In order to get the CH3NH3PbI3 films on TiO2 surfaces for solar cells, the CH3NH3I@as-PbI2 and CH3NH3I@h-PbI2 intermediate films were annealed directly at 75 oC for 30 min

4


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under N2 atmosphere. For simplicity, the CH3NH3PbI3 films derived from as-PbI2 and h-PbI2 layers are referred to as as-CH3NH3PbI3 and h-CH3NH3PbI3 films, respectively. 2.3. Solar cell fabrication. 1 ml of 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenyl-amine)-9,9'-spirobifluorene (Spiro-MeOTAD) (Xi’an Polymer Light Technology Corp.) solution (80 mg/ml) in chlorobenzene (anhydrous, 99.8%, Sigma-Aldrich) was mixed with 17.5 µL of lithium bis(trifluoromethylsulphonyl)imide (Li-TFSI, 99.95%, Sigma-Aldrich) solution (520 mg/ml) in acetonitrile (99.9%, Fisher) and 29 µL of 4-tert-butylpyridine (96%, Sigma-Aldrich). The mixture was spin-coated (4000 rpm, 30 s) onto as-CH3NH3PbI3 or h-CH3NH3PbI3 layer, and thermally annealed at 65 oC for 15 min in N2 atmosphere to get a Spiro-MeOTAD film as hole transport layer (HTL, ca. 100 nm in thickness). Finally, an Au electrode (100 nm thick) was thermally evaporated onto the Spiro-MeOTAD layer through a shadow mask to form an overlapped area of 1  4 mm2 between FTO and Au electrode, which defines the active area of each device. The devices were sealed in the glovebox under N2 atmosphere. 2.4. Instruments and characterization. X-ray diffraction (XRD) data were collected on a Philips X’Pert Pro MPD diffractometer with monochromated Cu K radiation ( = 1.54056 Å). Scanning electron microscopy (SEM) measurements were performed on a field-emission scanning electron microscope (FESEM, FEI-Sirion 200). UVvis absorption spectra were recorded on a Shimadzu UV-2550 spectrophotometer. Room temperature photoluminescence (PL) spectra were obtained by a Hitachi F-7000 spectrofluorophotometer. Transient absorption spectroscopy (TAS) measurements were carried out on a laser flash photolysis spectrometer (LKS 80, UK). Ultraviolet photoelectron spectroscopy (UPS) of the samples was carried out on a Thermao ESCALAB 250 photoelectron spectrometer (Thermo Electron Corporation) under an ultrahigh vacuum chamber (9.51010 mbar) with a He ultraviolet photon discharge lamp (21.22 eV) and a resolution of 0.1 eV. During the UPS measurements, a bias voltage of 8.0 V with respect to the ground was applied to the samples for the measurement of the binding energy of secondary electron cut-off. The slow positron annihilation spectrum (SPAS) measurements (Home-made in University of Science and Technology of China) were carried out to determine the defect states in perovskite layers, by using a 20 mCi 22Na source with a polycrystalline W foil moderator and the gamma-ray energy was measured with a 5



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high-purity Germanium (HPGe) detector. During the SPAS measurements, the energies of incidence positron beam varied from 0 up to 20 keV, and the vacuum in sample chamber was maintained at approximately 10−8 Pa. The mean depth of positron corresponding to incident energy in SPAS was calculated33,34 by the relationship of Z = (A/) × E1.6, where Z is the mean depth (nm),  is the density of the material (g/cm3), E is the incident energy (keV) and A = 40 g cm2 keV1.6. We took  = 4.16 g/cm3 for tetragonal CH3NH3PbI3 with I4/mcm space group.35 The film samples for XRD, SEM, UVvis absorption, UPS and SPAS measurements were fabricated on the compact TiO2 films, while those for PL and TAS measurements were prepared on clean quartz substrates. The

hole

mobility

in

as-CH3NH3PbI3

space-charge-limited current (SCLC) method,

and

h-CH3NH3PbI3

films

was

measured

using

by making the hole-only devices with a structure of

Glass/Cr/Au/PEDOT:PSS/CH3NH3PbI3/MoO3/Au. PEDOT:PSS was first spin-coated (2000 rpm, 60 s) over Cr/Au (3 nm:30 nm) substrate, then the perovskite films were prepared on the PEDOT:PSS layer by two-step method similar to that for solar cell preparation. The MoO 3/Au (6 nm:60 nm) top electrode was evaporated onto the perovskite layer through a shadow mask. The current-voltage (JV) characteristics of solar cells were measured under AM1.5 illumination with an intensity of 100 mW/cm 2 from a 94023A Oriel Sol3A solar simulator (Newport Standford, Inc.), and the light intensity from a 450 W xenon lamp was calibrated with a standard crystalline silicon solar cell. The JV curves were collected with an Oriel® I-V test station (PVIV-1A, Keithley 2400 Source Meter, Labview 2009 SP1 GUI Software, Newport). Intensity modulated photovoltage spectroscopy (IMVS) was measured on a controlled intensity modulated photo spectroscopy (CIMPS, Zahner Co., Germany) in ambient conditions with a background intensity of 15.85 mW/cm2 and within the frequency range of 1 Hz–50 kHz. During the JV and IMVS measurements, the illumination was limited to the overlapped area (1×4 mm2) between FTO and Au by a photomask attached to each device. Incident photon-to-current conversion efficiency (IPCE) spectra were measured with a QE/IPCE Measurement Kit (Newport, USA) that was automatically controlled by Oriel ® Tracq Basic V6.5 software with a light from a 300 W xenon lamp focusing through a monochromator (74125 Oriel Cornerstone 260 1/4 m) onto the solar cells. The light intensity and photocurrent generated were

6


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measured with a 2936-R dual channel power/current meter and an 818-UVcalibrated silicon-UV photodetector.

3. RESULTS AND DISCUSSION 3.1. Formation of CH3NH3PbI3 layer. Figure

2

illustrates

the

preparation

of

CH3NH3PbI3 films by two-step method. To get the differently crystallized PbI2 film matrixes, the spin-coated PbI2 film was not annealed to give sample as-PbI2 film, while it was first slowly heated to 70

o

C and then annealed at this

temperature to provide sample h-PbI2. Based on those PbI2 films, the intermediate films (i.e.,

Figure 2. Schematic illustration for sample preparation on compact TiO2 films. To prepare as-PbI2 film, the spin-coated PbI2 film is dried at room temperature; the spin-coated PbI2 film is first slowly heated at a rate of 2 oC/min to 70 oC and then annealed at this temperature for 30 min to get h-PbI2 film; the perovskite films are obtained by annealing the intermediate films directly at 75 oC for 30 min.

CH3NH3I@as-PbI2 and CH3NH3I@h-PbI2) were obtained by spin-coating CH3NH3I solution. Finally, the intermediate films were annealed at 75 oC to get the perovskite films (i.e., as-CH3NH3PbI3 and h-CH3NH3PbI3). Note, we also prepared the reference PbI 2 film (ref-PbI2) by directly annealing the spin-coated PbI2 film at 70 oC for comparison; our results showed that the ref-PbI2 film had a lower crystallinity than h-PbI2 one, and the reference solar cell made from ref-PbI2 film exhibited a much lower performance than the device from h-PbI2 film (refer to Figure S6 and Table S1 discussed in Section 3.2). Obviously, the crystalline PbI2 film matrix subjected to the annealing after a slow heating process before contacting CH3NH3I solution is more effective for CH3NH3PbI3 formation than that with a direct annealing history. 3.1.1. XRD data. The diffraction peaks of FTO substrates in all the samples are detectable, informing that the X-ray can penetrate through the whole film to reach FTO layer. However, the TiO 2 layer on FTO substrate is not evidently observed in XRD measurements, for which the reason is attributed to the too weak diffraction peaks of thin TiO2 film (i.e., 100 nm thick, refer to Figure 5). Note, when we used a

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thicker TiO2 film, the diffraction peaks of TiO2 film were weakly observable, as shown in Figure S1 in Supporting Information (SI). Both as-PbI2 and h-PbI2 films exhibit the diffraction peaks at 12.70°, 25.46° and 38.63°, which are respectively indexed to the (001), (002) and (003) crystal planes of trigonal PbI 2 with

space group (JCPDS 80-1000),

suggesting a preferential orientation with (001) planes parallel to substrate plane.36 Obviously, the XRD diffraction peak of h-PbI2 film is much stronger than that of as-PbI2 one, informing a much higher crystallinity in h-PbI2 film.27,29,37 Both as-CH3NH3PbI3 and h-CH3NH3PbI3 films display the strong diffraction peaks of tetragonal CH3NH3PbI3 with I4/mcm space group at 14.15°,

Figure 3. XRD patterns of (a) as-PbI2, (b) h-PbI2, (c) CH3NH3I@as-PbI2, (d) CH3NH3I@h-PbI2, (e) as-CH3NH3PbI3, (f) h-CH3NH3PbI3 films and (g) CH3NH3I powder. The black dots identify the XRD patterns of FTO. Insets 1 and 2 show the XRD peaks at around 14o and 20o, respectively.

28.43° and 31.90° for (002)/(110), (004)/(220) and (310) crystal planes, respectively.16,20,26,27,38,39 The diffraction peaks of PbI2 at 12.70°and free CH3NH3I40 at 19.68°and 29.70o (Inset 2 to Figure 3) are absent in as-CH3NH3PbI3 and h-CH3NH3PbI3 films, indicating the formation of pure CH3NH3PbI3. Therefore, both as-PbI2 and h-PbI2 films have reacted completely with CH3NH3I into crystalline CH3NH3PbI3 after annealing at 75 oC (Figure 2). Since the XRD peaks of h-CH3NH3PbI3 film is much stronger than that of as-CH3NH3PbI3 one and both films have a comparable thickness (ca. 200 nm, refer to Figure 5), it is reasonable that the crystallinity in h-CH3NH3PbI3 film is higher than in as-CH3NH3PbI3 one. In order to get the details of CH3NH3PbI3 formation, we measured the XRD patterns of CH3NH3I@as-PbI2 and CH3NH3I@h-PbI2 intermediate films. Actually, the intermediate films provide the information on perovskite formation at room temperature (RT) during spin-coating process. Both intermediate films display the typical diffraction peaks of CH3NH3PbI3, indicating the following reaction during spin-coating process.27 8


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PbI2 solid + CH3NH3I solution

CH3NH3PbI3 solid

(1)

Careful inspection shows that the (002)/(110) crystal planes of CH3NH3PbI3 in the intermediate films have a bit larger d-spacing as compared to the pure CH3NH3PbI3 films (Inset 1 to Figure 3), indicating that the (001) crystal planes of trigonal PbI2 with

space group is changed into the (002)/(110) crystal

planes of tetragonal CH3NH3PbI3 with I4/mcm space group during spin-coating process, leading to the observed d-spacing expansion of perovskite. Interestingly, in the intermediate films, the (001) peak of PbI 2 at 12.70o is still observed, but the diffraction peaks of free CH 3NH3I at 19.68°and 29.70°are not evident. This infers that, when contacting with each other during spin-coating process, the PbI2 in either as-PbI2 or h-PbI2 film is not fully converted into CH3NH3PbI3 perovskite, the CH3NH3I molecules left in the intermediate films are in a coordinated state (or not in a free state), and the excess of free CH 3NH3I molecules have be removed away from the film matrix by spin-coating. Moreover, the (001) diffraction peak of PbI2 remaining in CH3NH3I@as-PbI2 film is much stronger than in CH3NH3I@h-PbI2 one, inferring that the well crystallized h-PbI2 has a much stronger ability to react with CH3NH3I into perovskite and more coordinated states during spin-coating process, in agreement with previous observations. 27,29 Obviously, the thermal annealing of the intermediate films at 75 oC (Figure 2) promotes the further solid-state reaction between PbI2 and CH3NH3I residues and the improvement of perovskite crystallization,27 in which a better crystallized PbI2 film generates perovskite layer with a higher crystallinity. 3.1.2. UV-vis absorption. The chemical reaction

during

preparing

CH3NH3PbI3

perovskite films can also be indicated by the changes in the film colours (Inset to Figure 4). Both as-PbI2 and h-PbI2 films are yellow in colour. As either as-PbI2 or h-PbI2 film contacts CH3NH3I solution during spin-coating, a rapid change happens in colour (i.e., Reaction 1) and produces

gray

CH3NH3I@as-PbI2

or

Figure 4. UV-vis absorption spectra of (a) as-PbI2, (b) h-PbI2, (c) CH3NH3I@as-PbI2, (d) CH3NH3I@h-PbI2, (e) as-CH3NH3PbI3 and (f) h-CH3NH3PbI3 films. The inset images are the photographs showing the colour changes in those films.

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CH3NH3I@h-PbI2 intermediate films. After annealing of the intermediate films at 75 oC, dark-brown as-CH3NH3PbI3 and h-CH3NH3PbI3 films are obtained. Both as-PbI2 and h-PbI2 films exhibit the absorption band edge at ca. 530 nm, which characterizes the band gap of crystallized PbI2.29,36 The absorption peaks that resemble the iodoplumbate complexes41 at 374 nm for PbI3 and 414 nm for PbI42 are also evident in the PbI2 films. The as-CH3NH3PbI3 and h-CH3NH3PbI3 films display the typical absorption characteristics of CH 3NH3PbI3, with an absorption band at 770 nm characterizing its band gap (Eg) (i.e., Eg = 1.61 eV) and an evident shoulder peak at about 500 nm, in agreement with previous reports.3,7,4143 No absorption peaks for iodoplumbate complexes are evident in the resulting CH3NH3PbI3 films, indicating the transformation of iodoplumbate complexes into perovskite. However, h-CH3NH3PbI3 film has a higher absorption than as-CH3NH3PbI3 one due to the higher crystallinity in former case.37,43 Except for a lower absorbance, both CH3NH3I@as-PbI2 and CH3NH3I@h-PbI2 intermediate films display the absorption features similar to perovskite films, suggesting that perovskite has predominated the intermediate films, in agreement with the XRD data (Figure 3). The absorbance of CH3NH3I@as-PbI2 is smaller than CH3NH3I@h-PbI2, also indicating that a better crystallized perovskite is resulted from h-PbI2 during spin-coating process.37,43 3.1.3. SEM observation. Both as-PbI2 and h-PbI2 films (ca. 100 nm thick) are porous and consists of crystalline PbI2 flakes that are stacking with a flat-on orientation over TiO2 layer (ca. 100 nm thick) and irregular in shape and lateral size, but h-PbI2 is denser than as-PbI2 (Figure 5a,b), similar to the previous reports. 27,29 Clearly, annealing as-PbI2 film at 70 oC after a slow heating history does not remarkably change the film morphology, but increases the flake

Figure 5. Bird-view (above) and sectional (below) SEM images of (a) as-PbI2, (b) h-PbI2, (c) CH3NH3I@as-PbI2, (d) CH3NH3I@h-PbI2, (e) as-CH3NH3PbI3 and (f) h-CH3NH3PbI3 layers on compact TiO2 films. The scale bar is for all the images.

thickness (ca. 1535 nm in as-PbI2, 4050 nm in h-PbI2). As for the resulting perovskite films (Figure 5e,f), 10


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both as-CH3NH3PbI3 and h-CH3NH3PbI3 films are compact without evident voids/pinholes and have a thickness of ca. 200 nm. The as-CH3NH3PbI3 film is mainly formed by stacking spherical crystal grains with a size of 50100 nm, while the h-CH3NH3PbI3 film dominantly consists of elongated crystal grains with a size of 80200 nm in width and 200600 nm in length and most of the grains have a thickness/height extending the full perovskite layer. SEM observations on the intermediate films (Figure 5c,d) reveal two interesting phenomena as follows: one is that the morphologies of CH3NH3I@as-PbI2 and CH3NH3I@h-PbI2 intermediate films (both are ca. 180 nm in thickness) are very similar to as-CH3NH3PbI3 and h-CH3NH3PbI3 films, respectively; another is that no interface between CH3NH3I and PbI2 layers is distinguishable in both intermediate films, indicating a long-range interdiffusion of CH3NH3I solution into either as-PbI2 or h-PbI2 solid film for reaction.25 The SEM observations visualize that, during spin-coating process, CH3NH3I can rapidly interdiffuse into the PbI2 film matrix and promptly react with PbI 2 therein, the better crystallized h-PbI2 film favours the formation of lager CH3NH3PbI3 crystals, and the morphology in the resulting perovskite film mainly depends on the reaction between CH3NH3I solution and PbI2 solid film during spin-coating process. 3.1.4. SPAS data. Previous results44,45 have revealed that there are often lots of vacancy defects in CH3NH3PbI3 films, which result from the loss and/or mismatch of anion (I) and cation (Pb2, CH3NH3) due to ionic transport for example. The intermediate and perovskite films were studied with SPAS measurements (Figure 6). The parameter S in SPAS, defined as the ratio of the area under a fixed number of central channels to the total area under annihilation line shape in Doppler broadening technique, is very

Figure 6. Energy-S (a) and S-W (b) plots of samples measured by SPAS. The straight line in (b) serves as a guide for eyes.

sensitive to vacancy defect density.33,34,46 The perovskite films (i.e. as-CH3NH3PbI3 and h-CH3NH3PbI3) 11



have a much smaller S than the intermediate films (i.e., CH3NH3I@as-PbI2 and CH3NH3I@h-PbI2) in the depth range of 10200 nm corresponding to film body, indicating that the thermal annealing of intermediate films at 75 oC produces the perovskite with a significantly reduced density of vacancy defects.47 Moreover, h-CH3NH3PbI3 and CH3NH3I@h-PbI2 films have a smaller S than as-CH3NH3PbI3 and CH3NH3I@as-PbI2 ones, respectively, suggesting that the better crystallized h-PbI2 film produces a lower density of vacancy defects in the intermediate and perovskite films. The results from the Energy-S plots are in a good agreement with XRD data (Figure 3). The defect type can be further characterized by the S–W plot in SPAS measurements, where W characterize the fraction of annihilations with the high-momentum core electrons and is sensitive to the chemical composition around annihilation site, and the change in the slope of S–W linear relationship provides a direct evaluation of the variation in vacancy defect type. 33,48 Our results show that the S–W plots of the intermediate and perovskite films have the same slope, which strongly indicates that the types of vacancy defects are the same in those films.47 Therefore, the thermal annealing of intermediate films at 75 oC (Figure 2) only reduces the density of vacancy defects but does not change the defect types. In order to further support the conclusions from SPAS measurements, we additionally carried out the PL, TAS and UPS measurements of as-CH3NH3PbI3 and h-CH3NH3PbI3 films (Figures S2S4 in SI). First, the quartz substrate (non-quenching) was used to replace TiO2 film to prepare as-CH3NH3PbI3 and h-CH3NH3PbI3 films, and the intrinsic optical properties regarding the defects in as-CH3NH3PbI3 and h-CH3NH3PbI3 films were revealed by PL and TAS spectra of those films on quartz substrates. The higher band emission of h-CH3NH3PbI3 film than that of as-CH3NH3PbI3 one indicates that the better crystallized h-PbI2 film produces the perovskite layer with a higher crystallinity and less defects in that the reduced density of defects will lead to a less non-radiative energy loss by the charge recombination via defects (Figure S2). The time constant () extracted from TAS decay (i.e., 19.15 ns for as-CH3NH3PbI3 and 36.88 ns for h-CH3NH3PbI3 films) actually reflects the bulk charge recombination inside those films (Figure S3). Clearly, the h-CH3NH3PbI3 film has a higher crystallinity and a less density of vacancy defects to capture holes for a reduced charge recombination. Furthermore, the presence of different vacancy defects at varied energy levels eventually leads to the changes in the valence band (VB) level of semiconductors.49 Our UPS 12


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results (Figure S4) showed that as-CH3NH3PbI3 and h-CH3NH3PbI3 films have almost the same VB level (5.51 eV), further supporting the same type of vacancy defects in those films. 3.1.5. Perovskite growth. Clearly, the crucial CH3NH3PbI3 perovskite growth predominantly takes place during spin-coating CH3NH3I solution on PbI2 film, while the followed thermal annealing mainly induces the further reaction between PbI2 and CH3NH3I solid residues and the perovskite crystallization to a high crystallinity without evidently changing the morphology and the type of vacancy defects formed therein during the spin-coating process. The perovskite formation in the two-step method is schematically illustrated in Figure 7. First, the spin-coated PbI2 crystallize into the flakes with a trigonal

space group by

stacking (001) planes along the c-axis direction through van der Waals force and having dangling bonds on the (001) edges (I in Figure 7). Second, once contacting CH3NH3I solution during spin-coating process, the CH3NH3I molecules rapidly intercalate into the interlayer space between stacked (001) planes of PbI2 crystals and turn into a partially or fully coordinated state,50,51 where some of the intercalated CH3NH3I molecules coordinate with the dangling bonds of PbI2 and rearrange into the

Figure 7. Schematic illustration for the changes in crystalline structures during CH3NH3PbI3 formation. (I) is PbI2, (II) and (III) are perovskite framework in intermediate film, and (IV) is CH3NH3PbI3 perovskite. The dash lines in (I) mean the defects due to the loss and/or mismatch of I and Pb2 ions. The defects in (II, III) and (IV) are attributed to the loss/mismatch of CH3NH3, I and Pb2 ions, and a more full coordination in (IV) than in (II, III) depicts less defects in perovskite than in perovskite framework. (III) is (II) after rotating for a clear view of (110) crystal planes along [001] direction.

perovskite with tetragonal I4/mcm space group that has many defects, resulting in the intermediate perovskite framework (II, III in Figure 7) similar to the case of CH3NH3PbI3 perovskite colloid formation in one-step method.51 The CH3NH3PbI3 are normally undistiguishable by XRD,16,20,26,27,38,39

(002) and (110) palnes of

and whether the (001) plane of PbI2 is

preferentially changed into (002) or (110) plane of CH3NH3PbI3 has not been clarified yet up to now. Our 13



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XRD data clearly show that the d-spacing expansion of the (002)/(110) planes of CH3NH3PbI3 occurs during the formation of the perovskite (Figure 3). We think that the (001) plane of PbI2 is very likely transformed into the similarly layered (002) plane of CH3NH3PbI3 (II in Figure 7) rather than its (110) plane, because the (110) planes of CH3NH3PbI3 are not in a layer structure and the d-spacing expansion is not easily to take place in them as a result of intercalating molecules (III in Figure 7). Finally, the followed thermal annealing of the perovskite framework at 75 oC induces the further reaction between PbI2 and CH3NH3I solid residues and the rearrangements of CH3NH3, I and Pb2 ions therein into a much better crystallized perovskite, where the atoms tend to the states of full coordination (Figure S5 in SI). Compared to the perovskite framework, the resulting perovskite has a significantly reduced density of vacancy defects, a higher crystallinity, and a smaller d-spacing distance of (002) planes after the elimination of intercalated solvent and partially coordinated CH3NH3I molecules (Figure 3). A better crystallized PbI 2 film (i.e., h-PbI2) has a much stronger ability to react with CH3NH3I solution to produce both better crystallized perovskite framework and more coordinated states, consequently leading to a better crystallized CH3NH3PbI3 perovskite film with fewer defects and larger crystal grains (Figures 36). Since the morphology and the type of vacancy defects in the resulting CH3NH3PbI3 perovskite are almost same to those in the intermediate perovskite framework, the important effect of PbI 2 crystallization on the perovskite growth reasonably lies in governing transition from trigonal PbI 2 to tetragonal CH3NH3PbI3 in the rapid reaction process for atoms to coordinate into the perovskite framework during spin-coating CH3NH3I solution on PbI2 film.

Table 1. Device performance of the solar cells.a Perovskite

Voc (V)

Jsc (mA cm-2)

FF (%)

 (%)

Rs (k)

Rsh (k)

e (ms)

as-CH3NH3PbI3

0.85±0.02

6.23±0.07

45.51±3.66

2.45±0.15

1.20±0.05

17.92±2.39

0.44±0.05

h-CH3NH3PbI3

1.09±0.01

13.56±0.20

62.44±0.97

9.20±0.13

0.27±0.01

40.18±5.65

0.70±0.09

a

Each of the photovoltaic performance data with standard deviations represents the average of three devices that are measured under AM1.5 illumination (100 mW/cm2) in ambient conditions; Rs and Rsh are the series resistance and shunt resistance correlated with the slope characteristics at Voc and Jsc, respectively.

14


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3.2. Solar cells. Both as-CH3NH3PbI3 and h-CH3NH3PbI3

perovskite

films

have

a

comparable thickness of 200 nm (Figure 5), and the planar PSCs (Figure 1) are fabricated from these films. Figure 8a compares the J–V curves of the solar cells, and the averaged overall photovoltaic performance is presented in Table 1. The as-CH3NH3PbI3 devices show a quite low short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF) and power conversion efficiency

().

The

performance

of

as-CH3NH3PbI3 devices is lower than the

Figure 8. (a) Typical J–V curves, (b) IPCE spectra and (c) IMVS spectra of the solar cells based on as-CH3NH3PbI3 (○) and h-CH3NH3PbI3 (□). (d) shows the band structures for Voc origin, where Ef,e and Ef,h are the quasi-Fermi levels of photogenerated electrons in TiO2 and photogenerated holes in perovskite, respectively. The solid symbols on plot (c) identify fmin points.

similarly structured planar PSCs based on the CH3NH3PbI3 made from non-annealed PbI2 film by two-step method,18 for which the reason may be attributed to the much thicker perovskite layer (350 nm) and yttrium-doped TiO2 compact film used in the latter case, rather than the PbI2 residue that is normally regarded as an important factor to deteriorate the performance of devices with a perovskite layer prepared by two-step method10,27,39,52 because no PbI2 residue is detected in the perovskite films (Figure 3). However, the as-CH3NH3PbI3 devices exhibit a much higher Jsc, FF and  but a comparable Voc, as compared to the similarly structured planar PSCs with Ag counter electrode, in which CH 3NH3PbI3 layer was prepared by dipping non-annealed PbI2 film in CH3NH3I solution.27 We also prepared the reference solar cells with the CH3NH3PbI3 layer (200 nm thick) derived from the PbI2 film directly annealed at 70 oC (i.e., ref-PbI2 film), and got the devices with an efficiency of 7.60% (Figure S6 and Table S1 in SI), higher than the similarly structured planar PSCs ( = 6.0%) with CH3NH3PbI3 (410 nm thick) derived from the PbI2 film directly annealed at 70 oC7, but much lower than h-CH3NH3PbI3 devices (Table 1). The results of the reference devices strongly suggest that the CH3NH3PbI3 film resulting from the PbI2 film matrix subjected to annealing after a slow heating process (Figure 2) has a higher photovoltaic performance than that from directly annealed PbI2 film matrix. In contrast, h-CH3NH3PbI3 devices show a significantly higher 15



performance, of which the Jsc, Voc and FF and  are much higher than those in the similar solar cells with CH3NH3PbI3 (410 nm thick) prepared by two-step method from directly annealed PbI2 films.7 In particular, the Voc of h-CH3NH3PbI3 devices is comparable to the planar devices with CH3NH3PbI3 films prepared by the two-step method using a higher temperature (120 oC)24 or one-step method.8,16 Noticeably, those reported solar cells generally have a higher Jsc, and  as well, than h-CH3NH3PbI3 devices, which can be accounted for by the much thicker CH3NH3PbI3 films (i.e., 300500 nm) used in the previous experiments.8,16,27 Noticeably, we also studied the JV property of the solar cells under reverse scan and the device stability. Our results showed that h-CH3NH3PbI3 device had a smaller hysteresis in JV property than the as-CH3NH3PbI3 one (Figure S7a), indicating that the h-PbI2 matrix leads to a lower hysteresis because of the larger crystal grains with a higher crystallinity in the resulting h-CH3NH3PbI3 film, in agreement with previous reports;53 moreover, after a storage in a glovebox under N2 atmosphere for 30 days, the as-CH3NH3PbI3 and h-CH3NH3PbI3 devices retained 84% and 96% of their initial efficiencies, respectively (Figure S7b), inferring that the device derived from the h-PbI2 matrix has a higher stability. Our JV data clearly show that a better crystallization in PbI 2 film before deposition of CH3NH3I is very importance for efficient solar cells. The IPCE of h-CH3NH3PbI3 devices is much higher than that of as-CH3NH3PbI3 ones in the whole absorption range (Figure 8b), suggesting that the h-CH3NH3PbI3 devices have a higher ability to convert photons into photocurrent. IMVS, a powerful dynamic photoelectrical method for studying the charge recombination properties in solar cells, measures the photovoltage (Uph) response to a small sinusoidal perturbation superimposed on the steady background light under open-circuit conditions.54 The frequency (fmin) of the lowest imaginary component of IMVS response provides an evaluation of electron lifetime (e) in the devices according to the relation e = (2fmin)–1. The IMVS spectra of our solar cells are in a distorted semicircle shape (Figure 8c), similar to the observations in inorganic heterojunction solar cells,55 polymer-based solar cells56 and the PSCs reported by others.5760 The h-CH3NH3PbI3 device has a higher photovoltage (Uph), in agreement with the higher Voc. The e values for as-CH3NH3PbI3 and h-CH3NH3PbI3 devices are on a millisecond scale (Table 1). Since our (Figure S3) and others12 data showed that the average charge recombination lifetime in CH3NH3PbI3 is several to tens of

16


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nanoseconds, the e measured by IMVS is reasonably related to the interfacial charge recombination at CH3NH3PbI3/TiO2 interface, rather than the bulk charge recombination inside CH3NH3PbI3 film. In order to get insight into the observed performances in solar cells, the hole transport property in CH3NH3PbI3 films was studied by space-charge-limited current (SCLC) method using hole-only devices (Figure S8 in SI). The hole mobilities (μh) in h-CH3NH3PbI3 and as-CH3NH3PbI3 films were measured to be 0.057 and 0.029 cm2V1s1, respectively. These μh values are much higher than the reported SCLC results for CH3NH3PbI3 polycrystalline films,21,61 but about two orders of magnitude lower than those for CH3NH3PbI3 single crystals.62 Compared to as-CH3NH3PbI3 film, h-CH3NH3PbI3 film has a significantly increased hole mobility by ca. 100%, which is reasonably attributed to the higher crystallinity in the later film. The better crystallized h-CH3NH3PbI3 film has less defect states to trap charge carriers for recombination and a better electronic contact between crystalline grains, consequently resulting in the improved transporting pathways for both holes and electrons inside perovskite layer, which can be further supported by the remarkably reduced series resistance (Rs) and increased shunt resistance (Rsh) of their solar cells (Table 1). The observed Jsc, IPCE and e values in as-CH3NH3PbI3 and as-CH3NH3PbI3 devices are thought to predominantly correlate with the charge transport property inside CH 3NH3PbI3 film, which actually depends on the CH3NH3PbI3 crystallinity governed by PbI2 crystallization. With an improved charge carrier transport property in CH3NH3PbI3 layer, the spatial e-h separation inside CH3NH3PbI3 layer becomes more efficient for a reduced bulk charge recombination therein (Figure S3); meanwhile, the holes in perovskite film escape more easily away from perovskite/TiO2 interface, and the spatial e-h separation between the electrons in TiO2 and the holes left in perovskite happens more effectively for a reduced interfacial charge recombination at the perovskite/TiO2 interface (i.e., a larger e) (Figure 8c). As the results of the significantly reduced bulk and interfacial charge recombinations, the solar cells based on h-CH3NH3PbI3 eventually exhibit a much higher photocurrent generation ability to provide higher Jsc and IPCE values. Note, the increased FF in h-CH3NH3PbI3 devices can be accounted for by the reduced Rs and increased Rsh resistances, according to the equivalent circuit model.6365

17



On the other hand, our previous results54,57 have confirmed that the Voc of perovskite solar cells is originally determined by the energy offset between the conduction band (CB) level of n-type nanostructured accepter/transporter (i.e., TiO2 in this experiment) and the VB level of perovskite, and significantly correlates with the splitting of the quasi-Fermi levels of the photogenerated electrons in the accepter/transporter and the photogenerated holes in perovskite. An increased Voc by about 330 mV is observed when changing as-CH3NH3PbI3 into h-CH3NH3PbI3 (Table 1). Since the TiO2 films in both devices are prepared by the same procedure, the TiO2 films reasonably have comparable CB level and quasi-Fermi levels of electrons. Moreover, the VB levels (5.51 eV) of as-CH3NH3PbI3 and h-CH3NH3PbI3 are almost identical (Figure S4). Therefore, the Voc difference between the two devices should not originate from the CB-VB energy offset between TiO2 and perovskite, and also not be related to the quasi-Fermi levels of electrons in TiO2. In fact, trapping holes by vacancy defects will cause the quasi-Fermi levels of holes to shift away from the VB level of CH3NH3PbI3 by the energy offset of eV (Figure 8d). Since the density of vacancy defects is lower in h-CH3NH3PbI3 film than in as-CH3NH3PbI3 one (Figures 3, 6, S2 and S3), the holes in h-CH3NH3PbI3 film inevitably have a lower possibility to be trapped by intraband defects, leading to the quasi-Fermi levels of holes closer to VB level and consequently a large splitting of the quasi-Fermi levels of photogenerated electrons and holes for a larger Voc in solar cells.

4. CONCLUSIONS Compact CH3NH3PbI3 thin films with a thickness of 200 nm for efficient solar cells are prepared by a two-step method via spin-coating CH3NH3I solution over PbI2 film matrixes and using a low temperature ( 80 oC). The low temperature used will facilitate the application of flexible substrates to potentially scaled solar cell fabrication and the formation of CH3NH3PbI3 interfacial films over the substantial layers that are susceptible to temperature. The growth of CH3NH3PbI3 thin films in the two-step method features: (1) the perovskite growth mainly takes place during spin-coating process, where a rapid reaction between PbI2 solid film and CH3NH3I solution occurs upon their contacting to turn CH3NH3I molecules into a coordinated state and the excess free CH3NH3I molecules are removed away by rotating, resulting in an 18


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intermediate film dominated by perovskite framework with lots of defects; (2) the followed thermal annealing of the intermediate film induces the further reaction between PbI2 and CH3NH3I solid residues and the perovskite crystallization to a higher crystallinity, without evidently changing the morphology and the type of vacancy defects therein. The crystallization in PbI2 matrix governs the transition from the (001) plane of trigonal PbI2 to the (002) plane of tetragonal CH3NH3PbI3 in the rapid reaction process for atoms to coordinate into perovskite framework during the spin-coating process. A better crystallized PbI2 film has a higher ability to react with CH3NH3I solution and produces larger perovskite crystal grains with a higher crystallinity. The crystalline PbI2 film matrix subjected to a slow heating before annealing is more effective for CH3NH3PbI3 formation than that with a direct annealing history. The CH3NH3PbI3/TiO2 planar solar cells derived from better crystallized PbI2 films have a significantly improved photocurrent generation ability, as the result of the enhanced charge mobility due to a higher crystallinity in the perovskite film. The Voc in the solar cells correlates tightly with the splitting of quasi-Fermi levels of photogenerated electrons in TiO2 and holes in CH3NH3PbI3, and the vacancy defect density in perovskite film is a key factor limiting Voc.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional XRD and SEM data of TiO2 film; PL, TAS, UPS, and SCLC data of perovskite films; crystal structures for fully coordinated CH3NH3PbI3 crystal; characterization of ref-PbI2 film and its solar cells.

AUTHOR INFORMATION Corresponding Author *E-mails: [email protected] (Wang), [email protected] (Chen). Notes The authors declare no competing financial interest. 19



ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (grant nos. 91333121, 11474286, 51572254, U1404616, U1532156, 11774348, 61704048), the National Key Research and Development Program of China (grant no. 2017YFA0402800), the Science and Technology Project of Auhui Province (grant no. 1604a0902148), and the Major/Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (grant no. 2016FXZY003).

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Growth of Compact CH3NH3PbI3 Thin Films Governed by the

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