An Elegant Face-down Liquid-space-restricted Deposition of CsPbBr3

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An Elegant Face-down Liquid-space-restricted Deposition of CsPbBr3 Films for Efficient Carbon-based All-Inorganic Planar Perovskite Solar Cells Pengpeng Teng, Xiaopeng Han, Jiawei Li, Ya Xu, Lei Kang, Yangrunqian Wang, Ying Yang, and Tao Yu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00358 • Publication Date (Web): 27 Feb 2018 Downloaded from http://pubs.acs.org on February 28, 2018

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ACS Applied Materials & Interfaces

An

Elegant

Face-down

Liquid-space-restricted

Deposition

of

CsPbBr3 Films for Efficient Carbon-based All-Inorganic Planar Perovskite Solar Cells Pengpeng Teng, 1 Xiaopeng Han, 2,3 Jiawei Li, 2,3 Ya Xu, 2,3 Lei Kang, 2,3 Yangrunqian Wang, 2,3 Ying Yang, 1* and Tao Yu 2,3,4* 1

College of Materials Science and Technology, Nanjing University of Aeronautics and

Astronautics, Nanjing 210016, P. R. China 2

National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P. R.

China 3

Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University, Nanjing 210093, P.

R. China 4

Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing

210093, P.R. China Corresponding author, E-mail: [email protected] (Ying Yang). [email protected] (Tao Yu)

Abstract It is a great challenge to obtain the uniform films of bromide-rich perovskites such as CsPbBr3 in two-step sequential solution process (two-step method), which was mainly due to the decomposition of the precursor films in solution. Herein, we demonstrated a novel and elegant face-down liquid-space-restricted deposition to inhibit the decomposition and fabricate high-quality CsPbBr3 perovskite films. This method is highly reproducible and the surface of the films was smooth and uniform with an average grain size of 860 nm. As a consequence, the planar PSCs without hole-transport-layer based on CsPbBr3 and carbon electrodes exhibit enhanced power conversion efficiency (PCE) along with high open circuit voltage (VOC). The PSCs has achieved a PCE of 5.86% with a VOC of 1.34 V, which to our knowledge is the highest 1

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performing CsPbBr3 PSC in planer structure. Our results suggest an efficient and low-cost route to fabricate the high quality planer all-inorganic PSCs.

Keywords: CsPbBr3; liquid-space-restricted deposition; face-down dipping; planer structure; perovskite solar cells

Introduction Since the first report of the organic-inorganic hybrid halide perovskites in solar cells,1 the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has surged from 3.8% to a certified 22.1% over the past few years.2-5 Despite the rapid increase in PCE, the instability of organic-inorganic hybrid halide PSCs has not been resolved.6-7 Problem is mainly due to the poor stability of hybrid perovskite materials and organic hole-transport materials (HTMs).8-10 In order to avoid this problem, inorganic perovskite materials and inorganic HTMs have attracted extensive attention recently.11-17 CsPbBr3 is a promising inorganic perovskite material which has good stability against oxygen, moisture and heat. Kulbak et al. first demonstrated PSCs with CsPbBr3 by using two-step sequential solution process, since no solvent can dissolve both the CsBr and the PbBr2.18 After using PTAA as HTM in PSCs, the PCE reached up to about 6%.19 Chang et al. replaced organic HTM and metal electrode with carbon electrode in CsPbBr3 mesoporous PSCs, which achieved a PCE of 5% ( VOC = 1.29 V).20 Almost at the same time, Liang et al. have reported all-inorganic PSCs without any organic component, which achieved a PCE of 6.7% ( VOC = 1.24 V) in mesoporous structure.11 All these PSCs based on CsPbBr3 have achieved a relatively increased stability. However, the PSCs based on CsPbX3 in planer structure has rarely been reported by using two-step method. Planer structure has many advantages and was widely used in organic−inorganic hybrid halide PSCs.21-23 Furthermore, Snaith et al. demonstrated 2


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that the uniform CsPbX3 film deposition is challenging by using two-step method.24 In addition, we find that the CsPbBr3 precursor films in CsBr/methanol solution decompose quickly and CsPbBr3 films based on two-step method were poor. A key to optimize the performance of PSCs is to control the crystallization dynamics.14,25,26 Thus, developing an approach to inhibit the decomposition of CsPbBr3 precursor films and enhance the crystallization of CsPbBr3 perovskite films would promote the development of inorganic PSCs. Since studies on the dipping reaction of fabricating inorganic perovskite films in two-step method are rare, further research is urgently required to investigate the dipping process in solution. In this study, we demonstrate a face-down liquid-space-restricted deposition (face-down dipping process) to prepare high-quality CsPbBr3 perovskite film on compact TiO2 layer. Our method is found to inhibit the decomposition of CsPbBr3 precursor films in solution and obtain smooth, uniform and high-quality perovskite films after annealing, which greatly increases the PCE and VOC in planar all-inorganic PSCs.

Experimental Section Materials and reagents All chemicals were purchased commercially and used without further purification. PbBr2 (99.999%), CsBr (99.999%), DMF (99.8%), methyl alcohol (99.9%), isopropanol (99.9%), and conductive carbon paste were purchased from Shanghai MaterWin New Materials Co., Ltd. (China).

Preparation of compact TiO2/FTO glass substrates FTO glass substrates were cleaned by sonication in diluted detergent, deionized water, acetone, and absolute alcohol for 30 min successively. Then the substrates were dried by nitrogen stream. Finally, FTO glass substrates were cleaned by UV-ozone treatment for 30 min. The compact TiO2 layer was deposited on FTO glass by spin-coating a TiO2 sol at 4000 rpm for 30 s and annealing in air at 450 °C for 60 min. 3



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Preparation of CsPbBr3 films and planar all-inorganic PSCs

Figure 1. Schematic process for the preparation of CsPbBr3 films obtained by conventional dipping process and face-down dipping process.

All the films and PSCs were fabricated in ambient air. Figure 1 shows the experimental process for preparing CsPbBr3 films by conventional dipping process (face-up

dipping

process)

and

face-down

dipping

process.

Firstly,

a

dimethylformamide (DMF) solution of 1.0 M PbBr2 was kept at 75 °C and was 4


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spin-coated onto the compact TiO2 (c-TiO2) layer at 2000 rpm for 30 s, then the substrate was dried on a hot plate at 80 °C for 30 min. Secondly, PbBr2 films were face-up and face-down dipped in a methanol solution of 15 mg/mL CsBr at 50 °C for different reaction time and rinsed with isopropanol, dried in air. Then the films were annealed at 250 °C for 5 min on a hot plate to obtain the CsPbBr3 perovskite films. Additionally, in order to obtain the PSCs, a commercial carbon paste was painted on the CsPbBr3 layer by silkscreen printing, and heated at 120 °C for 10 min.

Characterizations The XRD patterns were performed on a Rigaku Ultima III X-ray instrument with the speed of 5º/min, using Cu-Kα radiation (λ=0.154178nm). Surface and cross-section morphology images of the samples were collected by Carl Zeiss SIGMA. The J-V measurements of the PSCs were carried out in the air with the relative humility 60% under AM1.5 illuminations (100 mW cm-2) cast by an Oriel 92251A-1000 sunlight simulator. The light intensity was calibrated by the standard reference of a Newport silicon solar cell. The J-V curves of PSCs were recorded by the scans (a voltage step of 10 mV and a delay time of 50 ms) under AM 1.5 G simulated solar illuminations. The PL spectra were obtained on a fluorescence spectrophotometer with the excitation wavelength of 515 nm at the room temperature. UV-visible absorption spectra were conducted using a Shimadzu UV-2550 spectrometer with an integrating sphere. The IPCE spectra of the PSCs with an area of 0.09 cm2 was measured by a monochromatic light in the range 300-700nm.

5



Results and discussion

(a)

(b)

Figure 2. (a) XRD patterns and (b) SEM images of CsPbBr3 perovskite films face-up dipped at 50 °C for 2 min, 3 min, 4 min and 6 min, respectively.

During the face-up dipping process, we found that the CsPbBr3 precursor films in solution were easy to decompose, and the yellow films gradually turned to colorless in a short time (as is shown in Figure S1). To investigate CsPbBr3 perovskite films in detail, the PbBr2 films were dipped for 2 min, 3 min, 4 min and 6 min. X-ray diffraction (XRD) patterns and scanning electron microscopy (SEM) images are shown in Figure 2. In Figure 2a, the diffraction peaks at 15.5°, 21.9° , 31.0° and 34.9° correspond to the planes of (100), (110), (200) and (210) of CsPbBr3. The diffraction peak of PbBr2 at 12.0° appears in 2 min and 3 min, indicating an incomplete conversion of PbBr2.20 When the dipping reaction time is more than 3 min, the presence of the diffraction peaks of CsBr at 29.7° may indicate the precipitation of CsBr. Finally, the diffraction peaks correspond to the CsPbBr3 perovskite film almost disappeared and presents an obvious CsBr diffraction peaks. The SEM image of PbBr2 film is shown in Figure S2. Figure 2b shows the SEM images of CsPbBr3 perovskite films dipped for 2 min, 3min, 4 min and 6 min, respectively. A rough film with many cubic crystals was obtained when the PbBr2 film was dipped for 2 min. For 3 min, many pinholes and some new crystals appeared on the film. According to the XRD patterns, these new crystals 6


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could be CsBr crystals. For more than 4 min, the film started to disappear and much more CsBr crystals appeared on it. This result implies that the films obtained by face-up dipping process were poor, which is mainly due to the decomposition of CsPbBr3 precursor films.

(a)

(b)

(c)

Figure 3. (a) XRD patterns and (b) SEM images of CsPbBr3 perovskite films face-down dipped at 50 °C for 10 min, 20 min, 30 min, 40 min, 50 min, and 60 min, respectively. (c) Plot of average grain size versus dipping time.

For face-down dipping process, the CsPbBr3 precursor films in solution were very stable, and color of films did not change obviously within one hour (as is shown in Figure S3). Then, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were carried out to analyze CsPbBr3 perovskite films obtained by face-down dipping process. Figure 3a shows the XRD patterns of CsPbBr3 films face-down dipped at 50 °C for 10 min, 20 min, 30 min, 40 min, 50 min and 60 min, respectively. Figure 3b shows the corresponding SEM images. As shown in Figure 3a, the film dipped for 10 7



min shows an obvious diffraction peak of PbBr2, indicating the incomplete conversion of PbBr2. Increasing dipping time to more than 20 min, the diffraction peak of PbBr2 nearly disappeared. Pure phase CsPbBr3 perovskite films could be attained through appropriate face-down dipping time. As seen from Figure 3b, the microstructure of grains differs from each other. For dipping time less than 20 min, the PbBr2 film converted to CsPbBr3 film with many small grains on it. Extend the dipping time to more than 20 min, grains became bigger, films became smooth and uniform for longer reaction time. However, too long reaction time such as 50 min, the surface of CsPbBr3 films became rougher, and many pinholes appeared. And, as revealed in Figure 3c, the average grain sizes were increased from 430 nm to 860 nm when the films were dipped from 10 min to 40 min. For increasing dipping time to 60 min, the average grain sizes were decreased to 510 nm.

(a)

(b)

Figure 4. (a) Room-temperature steady-state PL spectra and (b) UV-Vis absorption spectra of the CsPbBr3 films prepared by face-up dipping and face-down dipping process on glass substrates.

To investigate the optical properties of the films by both dipping methods, we carried out steady-state photoluminescence (PL) measurements and ultraviolet–visible spectroscopy (UV-Vis). As shown in Figure 4a, the emission peak of both films is at ~ 527 nm. In comparison, the PL intensity of the face-down film is stronger than that of the face-up sample. It is indicates that there are fewer defect sites in the film prepared by face-down dipping process.25 Figure 4b shows the UV-Vis absorption spectra. It is 8


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obvious that two different dipping methods have the similar absorption onset at ~ 540 nm, which corresponds to the bandgap of ~ 2.3 eV.11 However, the absorption intensity of the film prepared by face-down dipping process increased. This enhancement of the light absorption is probably caused by the high crystallinity of the film.27,28 Transient PL decay measurement was carried out to investigate both films (as is shown in Figure S4), this result further proved the decreased defect density and a larger charge carrier diffusion length in the film obtained by face-down dipping process.29

Figure 5. Schematic illustration of the dipping reaction by face-up dipping and face-down dipping process.

Considering the XRD, SEM, PL and UV-vis results, we inferred that the films obtained by face-down dipping process are obviously better than these obtained by face-up dipping process. Here we explain the enhancement of CsPbBr3 perovskite films. As schematically illustrated in Figure 5, during the face-up dipping and face-down dipping process, the PbBr2 film in CsBr/methanol solution was converted to CsPbBr3 precursor film. For face-up dipping reaction, the conversion of PbBr2 film is quicker than face-down dipping process. However, the CsPbBr3 precursor film decomposed quickly before the complete conversion of PbBr2, which is challenging to obtain a high-performance CsPbBr3 films. After annealing, the CsPbBr3 perovskite films obtained by this process are rough and grain sizes are small. X-ray diffraction (XRD) was carried out to further analyze precursor film which decomposed 9



completely in solution (as is shown in Figure S5). It shows that CsPbBr3 precursor film decomposed to PbBr2 and CsBr in solution. Face-down dipping process could inhibit the decomposition of CsPbBr3 precursor films by liquid-space restriction, which provides a kinetic control of crystal growth. Furthermore, the face-down dipping process provides enough time for PbBr2 film to convert and for CsPbBr3 precursor films to crystallize. Thus, this process can fabricate high-performance CsPbBr3 perovskite films. To further understand the advantage of face-down liquid-space-restricted deposition, the CsPbBr3 films above were used to fabricate PSCs. The structure of the PSC is FTO/c-TiO2/CsPbBr3/Carbon. Cross-sectional scanning electron microscopy (SEM) image is shown in Figure 6a. The J-V curves and basic photovoltaic parameters (including short circuit current density (JSC), VOC, fill factor (FF) and PCE) of PSCs based on face-down dipping process are presented in Figure S6a and Table S1. The cell with CsPbBr3 film face-down dipped for 10 min obtained a PCE of 3.59% with a

VOC of 1.35 eV, a JSC of 4.65 mA/cm2 and a FF of 56.86% due to the incomplete conversion of PbBr2. The PCE, JSC and FF improved gradually when the dipping time increase from 20 min to 40 min and the VOC remained stable at about 1.35 eV, which is higher than previous reports.19,30 It suggests that smooth and uniform perovskite film is in favor of carriers transport. Furthermore, increase the dipping time to more than 40 min, the performance of PSCs decreased dramatically due to the higher number of defects, such as pinholes.31-32 We investigated the J-V curves under reverse and forward scans and the results are shown in Figure S6b, which demonstrated a hysteresis for CsPbBr3 planner PSCs. We believe that the hysteresis is due to the polarization of ionic charges in CsPbBr3 layer.33

10


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(a)

(b)

(c)

(d)

Figure

6.

(a)

Cross-sectional

SEM

image

of

a

completed

FTO/c-TiO2/CsPbBr3/Carbon planer all- inorganic PSC. (b) J-V curves and (c) IPCE spectra for the best PSCs fabricated by face-down and face-up dipping process. (d) The PCE histogram of the fabricated PSCs with the CsPbBr3 films prepared by face-down dipping process.

After optimizing the both methods, as shown in Figure 6b, the best PSC based on face-down dipping process yields a PCE of 5.86% with a VOC of 1.34 V, JSC of 6.46 mA/cm2, and a FF of 68.04%. This result is much higher than the best PCE of 3.6% in planer structure.11 However, the cell prepared by face-up dipping process only yields a PCE of 2.36% with a VOC of 1.28 V, JSC of 3.48 mA/cm2, and a FF of 45.46%. To verify the excellent photoelectric conversion characteristics of the cells dipped for 40 min, we measured the incident photon conversion efficiency (IPCE) values of PSCs fabricated by face-down and face-up dipping process. As is revealed in Figure 6c, the IPCE of the cell with face-down dipping process is significantly greater than face-up dipping process. Moreover, as shown in Figure 6d, the average PCE of the fabricated 11



PSCs with the CsPbBr3 films face-down dipped for 40 min is 5.39%, which suggests that this face-down liquid-space-restricted deposition is manageable and reproducible. As given in Figure S7a, the steady-state current density stabilized within seconds to about 4.74 mA/cm2 at 0.984 V for the PSC with the uniform CsPbBr3 film, yielding a stabilized PCE of around 4.66%. All the above results indicate that our strategy can obtain high-quality CsPbBr3 films with large grains. In addition, the PCEs stability of CsPbBr3 PSCs was further investigated and shown in Figure S7b. The devices were kept in an ambient environment with a relative humidity (RH) of 50−60% and a temperature of 25 °C. Our result indicated that the CsPbBr3 PSCs have a relatively good stability over a period of 240 h.

Conclusions In summary, a face-down liquid-space-restricted deposition was presented to fabricate high-quality CsPbBr3 films as the light absorber layer in carbon-based all-inorganic planner PSCs. Compared to the films obtained by conventional dipping method, the CsPbBr3 perovskite film is smooth and uniform with larger grains, which could significantly increase the performance of devices. The planer PSCs based on the high-quality CsPbBr3 perovskite film show a PCE of 5.86% with high VOC of 1.34 V, which is much higher than the device employed by the face-up film. Consequently, we hope that our work will provide an elegant and low-cost route to obtain smooth, uniform and efficient CsPbX3 or Bromide-rich films.

Associated Content Supporting Information The samples of CsPbBr3 precursor films, SEM image of PbBr2 film, normalized transient PL spectra of the CsPbBr3 films on glass substrates, XRD pattern of CsPbBr3 precursor films decomposed completely, J-V curves of the PSCs, photovoltaic parameters of the PSCs based on face-down dipping process, Steady-state current density and PCE output, and PCE stability test of the CsPbBr3 PSCs. 12


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Acknowledgements This work was financially supported by the 111 project (No. B12021), the National Natural Science Foundation of China (61377051) and the Priority Academic Program Development of Jiangsu Higher Education Institutions. We thank Jianyong Feng, Weidong Zhu, Huiting Huang and Qingxiao Meng for informative discussions and experimental and technical assistances.

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An Elegant Face-down Liquid-space-restricted Deposition of CsPbBr3

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