Improvement of photovoltaic performance of the inverted planar perovskite solar cells by using CH3NH3PbI3−xBrx films with solvent annealing

Accepted Manuscript Improvement of photovoltaic performance of the inverted planar perovskite solar cells by using CH3NH3PbI3−xBrx films with solvent annealing Shan Wang, Weijia Zhang, Denghao Ma, Zhaoyi Jiang, Zhiqiang Fan, Qiang Ma, Yilian Xi PII:

S0749-6036(17)31297-1

DOI:

10.1016/j.spmi.2017.07.009

Reference:

YSPMI 5119

To appear in:

Superlattices and Microstructures

Received Date: 26 May 2017 Revised Date:

3 July 2017

Accepted Date: 3 July 2017

Please cite this article as: S. Wang, W. Zhang, D. Ma, Z. Jiang, Z. Fan, Q. Ma, Y. Xi, Improvement of photovoltaic performance of the inverted planar perovskite solar cells by using CH3NH3PbI3−xBrx films with solvent annealing, Superlattices and Microstructures (2017), doi: 10.1016/j.spmi.2017.07.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT Improvement of photovoltaic performance of the inverted planar perovskite solar cells by using CH3NH3PbI3-xBrx films with solvent annealing Shan Wang, Weijia Zhang*, Denghao Ma, Zhaoyi Jiang, Zhiqiang Fan, Qiang Ma, Yilian Xi Center of Condensed Matter and Material Physics, School of Physics and Nuclear Energy Engineering, Beihang University, Beijing 100191, People’s Republic of China *Corresponding author.

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E-mail: [email protected]. Telephone number: 011+86+10+82338147

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ABSTRACT：In this paper, the CH3NH3PbI3-xBrx films with various Br-doping contents were successfully prepared by solution processed deposition and followed by annealing process. This method simultaneously modified the morphology and composition of the CH3NH3PbI3 film. The effects of annealing treatment of CH3NH3PbI3-xBrx films under N2 and DMSO conditions on the microstructure of films and photoelectric properties of the solar cells were systematically investigated. The relationship of the component ratio of RBr/I= CH3NH3PbI3-xBrx/CH3NH3PbI3 in the resulting perovskite versus CH3NH3Br concentration also was explored. The results revealed that the CH3NH3PbI3-xBrx films annealed under DMSO exhibited increased grain sizes, enhanced crystallinity, enlarged bandgap and reduced defect density compared with that of the N2 annealing. It also was found that the RBr/I linearly increased in the resulting perovskite with the increased of CH3NH3Br concentration in the methylammonium halide mixture solutions. Furthermore, the photovoltaic performances of devices fabricated using DMSO precursor solvent were worse than that of DMF under N2 annealing atmosphere. When CH3NH3Br concentration was 7.5 mg ml-1, the planar perovskite solar cell based on CH3NH3PbI3-xBrx annealed under DMSO showed the best efficiency of 13.7%. Keywords: solvent annealing, crystallization, Br-doping, photoluminescence, CH3NH3PbI3-xBrx, solar cell 1. Introduction

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Recently, Organic-inorganic hybrid perovskite solar cells have drawn considerable attention because of the unparalleled progress in boosting power conversion efficiency [1-4]. Organometallic halide materials such as CH3NH3PbI3 are the most important components in perovskite solar cells due to their excellent photoelectric performance including high molar absorption coefficient, low exciton binding energy, bipolar charge transport properties and long carrier diffusion length [5-7]. To improve the device performance, it is important to optimize the fabrication method and surface morphology of CH3NH3PbX3 films. CH3NH3PbX3 can be generated by the reaction between PbX2 and CH3NH3X using a variety of preparation methods such as solution spin coating, thermal-evaporation deposition [8-10] and N2 blow drying method [41-43]. Among them, solution spin coating method has the advantages of low cost and simple operation, which is widely adopted. The film crystallinity and morphology quality are paramount in determining the performance of perovskite solar cell. One problem is that solution-process results in poor film quality with low crystallinity using DMF as the precursor solvent due to the quick reaction between PbI2 and CH3NH3I and the rapid crystallization of CH3NH3PbI3 films [11-13]. Efforts have been made to improve this condition. Xiao et al. utilized DMF solvent annealing to tune the crystallization

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process, finally generating uniform perovskite film and significantly improving the performance of the devices [14]. Inspired by the work, Liu et al. further systematically investigated the crystallization behavior of the perovskite films annealed under different solvent vapor atmospheres. They found that DMSO could significantly improve the crystallization process and quality of the films compared with other solvents. Their study increased power conversion efficiency[15]. In addition to improving the crystallinity and morphology quality of the film, incorporating Br into the iodide-based perovskite has been proved to be an effective way to further refine photovoltaic properties [16-19]. For example, Zhu et al. found that CH3NH3PbI3-xBrx films were much stable than CH3NH3PbI3 films when processing in air [20-21]. Besides Suarez et al. revealed that CH3NH3PbI3-xBrx could increase carriers mobility and decrease recombination loss [22-24]. Moreover, in the seminal work of Edri et al. [25] they found that the VOC of CH3NH3PbBr3 solar cell can reach up to 1.3V, which obviously increased the device's open circuit voltage. However, the reaction towards CH3NH3PbI3-xBrx films makes it difficult to achieve a high quality perovskite film with larger grain sizes and good crystallinity[26]. Furthermore, excessive incorporation of bromine caused poor film quality[27], thus reducing the efficiency of the solar cells. On base of these, we hope to prepare high quality CH3NH3PbI3-xBrx film by solvent annealing process which simultaneously modifies the morphology and composition of the films. The effects of solvent annealing treatment on the structure and the photoelectric properties of the CH3NH3PbI3-xBrx film are complex and have not yet been well investigated, especially on the photo-absorption and photoluminescence. Therefore, this work has great potential for the improvement of device performance and is well worth discussion. This paper proposed a method to simultaneously modify the morphology and composition of the film. The CH3NH3PbI3-xBrx films were first prepared by converting the spin-coated PbI2 films using 0.252M precursor mixture solutions with various CH3NH3Br contents. Then the films were annealed under N2 and DMSO vapor, respectively. We studied the effects of annealing treatment of CH3NH3PbI3-xBrx films under N2 and DMSO conditions on the microstructure of films and photoelectric properties of the solar cells, using scanning electron microscope (SEM), X-ray diffraction (XRD), UV-vis spectrum, and Time-resolved photoluminescence decay in comparison with the film annealed under N2 atmosphere. Moreover, we further studied the photovoltaic performances of perovskite solar cells fabricated using DMSO solvent and annealing under N2 atmosphere. The relationship of the component ratio of RBr/I= CH3NH3PbI3-xBrx/CH3NH3PbI3 in the resulting perovskite and Br contents in the mixed precursor solution also was invested. Finally, the photovoltaic devices were prepared and the photovoltaic performance was evaluated in various process conditions. 2. Experimental Section

2.1. Precursor solution preparation To prepare the 0.252 M methylammonium halide mixture solutions with the CH3NH3Br concentration to be 0 mg m l-1, 2.5 mg m l-1, 5 mg m l-1, 7.5 mg m l-1 and 10 mg ml-1, the CH3NH3Br and CH3NH3I (0.0000 g and 0.0400g, 0.0025 g and 0.0366g, 0.0050 g and 0.0329 g, 0.0075 g and 0.0294 g, 0.0100 g and 0.0259 g) were dissolved in 1ml isopropanol stirring at room temperature. And the PbI2 solution was prepared by dissolving 461mg PbI2 in 1mL DMF precursor solvent.

ACCEPTED MANUSCRIPT 2.2. Solar cell fabrication Solar cells with ITO/PEDOT: PSS/CH3NH3PbI3-xBrx/PC60BM/BCP/Ag planar device structures were prepared. 2.2.1. Preparation of the PEDOT: PSS films

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The ITO glass substrates with sheet resistance of about 10 Ωsq-1 were ultrasonically cleaned with deionized water, acetone, and ethanol for 15min, sequentially and then cleaned by UV–ozone treatment for 20min to remove the surface of organic pollutants, meanwhile increasing the surface hydrophilicity. The PEDOT: PSS solution was spin-coated on the as-cleaned ITO substrates at 3500 rpm for 30 s, and subsequently annealed at 130 ° C for 12 min, as shown in Fig. 1(a). 2.2.2. Perovskite film fabrication

2.2.3. Fabrication of devices

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The CH3NH3PbI3-xBrx films were prepared by solution-processed deposition and followed by annealing process. Firstly, the PbI2 films were successfully prepared (Fig.1b) then the pre-configured methylammonium halide mixture solutions was spin-coated on the PbI2 films, at 3000rpm for 35s. (Fig.1c) Subsequently as shown in Fig. 1(e1) and (e2), the spin-coated CH3NH3PbI3-xBrx films were transferred onto a hot plate for annealing under N2 and DMSO atmospheres at 100 °C for 60 min, respectively. The corresponding samples with the CH3NH3Br concentration to be 0 mg m l-1, 2.5 mg m l-1, 5 mg m l-1, 7.5 mg m l-1 and 10 mg m l-1, were labeled as S0, S2.5, S5, S7.5, and S10, respectively.

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2.3. Characterization

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PCBM solution in chlorobenzene (20 mg ml-1) was then spin-coated on the prepared perovskite film at 1500 rpm for 40 s and annealed in a glove box at 100 ° C for 30 min. (Fig.1f) Finally, 7 nm BCP barrier layer and 100 nm Ag electrode were deposited by thermal evaporation. (Fig.1h) The above experiment process of preparing perovskite solar cells was schematically shown in Fig. 1.

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The surface morphology of pervoskite films and the cross-sectional SEM image of device were obtained using the FEI NOVA NANOSEM 450.The XRD diffractions of films were characterized by Bruker D8 X-ray diffractometer at room temperature. The absorption spectra of the pervoskite films were measured using UV-vis spectrometer Hitachi U-3300 with an integrated sphere. The steady-state and time-resolved photoluminescence spectra of the films were performed with an Edinburgh Instruments FLS 980 fluorescence spectrometer. The J-V curves were measured by Keithley 2400 at room temperature under AM 1.5G illuminations (100mWcm-2) from a solar simulator, and calibrated using a standard silicon solar cell device. 3. Results and discussion 3.1. Morphology of the CH3NH3PbI3-xBrx films Fig. 2 showed the surface morphology of the CH3NH3PbI3-xBrx films prepared under N2 and DMSO annealing conditions. As exhibited in Fig. 2(a1)-(e1), the morphology of the perovskite film annealed under N2 changed significantly with the increase of CH3NH3Br concentration in the

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methylammonium halide mixture solutions, especially the grain sizes became larger. However, when the CH3NH3Br concentration increased to 10 mg ml-1, crack appearance and pinhole formation were clear as shown in Fig. 1(e1). This indicated that the incorporation of CH3NH3Br in the precursor solution could induce the Oswald ripening [28-29], which leaded to the increase of the grain sizes of the films. But the high-concentration CH3NH3Br treatment only induced Br/I halide exchange reaction without obvious differences in grain sizes, and decreased the film morphology quality, which was consistent with Yang's research report [27]. To summarize, an appropriate CH3NH3Br doping concentration was required. From the Fig. 2(a2)-(e2), it can be seen that the perovskite film annealed under DMSO showed a relatively flat surface with fine crystallites. And with the increase of CH3NH3Br concentration in the methylammonium halide mixture solutions, the grain sizes first increased and then decreased, which was the same as that in N2 annealing process. Compared with films annealed under N2 atmosphere, it can be observed that the grain sizes of the film increased and the crystallinity improved obviously under DMSO annealing vapor. These could be explained that during the annealing treatment, the DMSO vapor could corrode the perovskite film surface and cause a strong recrystallization process thus resulted in large grains and high quality perovskite crystals formation [15, 30]. This method of densifying the film was similar to liquid sintering [31]. 3.2. Crystal phase of CH3NH3PbI3-xBrx films

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In order to examine the crystal structural variation, we conducted XRD patterns of the as-synthesized perovskite films annealed under N2 and DMSO vapor. As shown in Fig. 3(a), for the CH3NH3PbI3 film exposured to N2 vapor, several main peaks centered at 14.08 °, 19.75 °, 28.48 ° and 31.85 °could be indexed to the ( 110), (112), (220), and (310) planes, respectively [32]. In addition, the relative intensity of different diffraction peaks of CH3NH3PbI3-xBrx had negligible change compared with CH3NH3PbI3, indicating Br incorporation caused no preferred grain orientation. The difference could be explained that I and Br had the similar atom sizes while the sizes of Cl was smaller than I [33]. Fig. 3(b) showed the corresponding (200) plane magnified XRD patterns in the region of 2θ=28.0-29.0º. With the increased CH3NH3Br concentration, the diffraction peak gradually shifted to the larger angle indicating that the lattice constant decreased, which confirmed the formation of mixed I/Br lead halide perovskites [34]. The DMSO annealing-induced the XRD patterns change were in accord with that of the samples annealed under N2, but the intensity of the diffraction peaks increased obviously, as shown in Fig. 3 (c)-(d).This exhibited that the DMSO vapor could lead the recrystallization of the grains and cause the rearrangement of the grain for a more favored stacking arrangement [15]. In order to further investigate the effect of N2 and DMSO annealing treatments on different Br doping perovskite films, the XRD patterns of the samples in the 2θ = 28-29º region were fitted by a bi-Gaussian function. The XRD fitting Chart of the perovskite film with a CH3NH3Br concentration of 5 mg m l-1 annealed under N2 was shown in Fig. 3(e). The blue curve characterized the CH3NH3PbI3 component and the brown curve represented CH3NH3PbI3-xBrx component. The ratio of two components RBr/I= CH3NH3PbI3-xBrx/CH3NH3PbI3 in perovskite films could be obtained from the ratio of the Gaussian curve integral area. It can be observed from Fig. 3 (f) that the ratio of the two components in perovskite films linearly increased with the growth of CH3NH3Br concentration in the methylammonium halide mixture solutions, which further confirmed the incorporation of bromine. Besides the ratio change of the two components in the resulting perovskite films under

ACCEPTED MANUSCRIPT DMSO annealing condition was consistent with that of the N2, but the value was significantly higher than that of N2 condition. This indicated that the DMSO annealing was beneficial to drive full reaction of the two stacking precursor layers into CH3NH3PbI3-xBrx perovskite film. 3.3. UV–vis absorption spectra of CH3NH3PbI3-xBrx films

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To explore the variation in optical properties of different Br-doped film samples after annealing treatment under N2 and DMSO, UV–vis absorption spectra was conducted, and the corresponding results were shown in Fig. 4(a). With the increase of CH3NH3Br concentration in the methylammonium halide mixture solutions, the absorption edge of the CH3NH3Pb3-xBrx samples annealed under N2 gradually moved toward the short wavelength showing blue shift phenomenon. This indicated the formation of CH3NH3Pb3-xBrx films. In addition, the absorption intensity of CH3NH3Pb3-xBrx films was higher than that of the CH3NH3PbI3. It was mainly due to the larger absorption coefficient of CH3NH3PbI3-xBrx samples and the increase of grain sizes of capping layer[35]. When the CH3NH3Br concentration was 7.5 mg ml-1, the absorption intensity reached a maximum which should be attributed to better film morphology. It was found that films annealed under DMSO condition showed obviously only difference between that annealed under N2, as shown in Fig. 4(a). In this case, the absorption intensity of the films annealed under DMSO vapor became stronger, which had a great effect on the performance of the solar cells. This exhibited that the annealing treatment under DMSO could lead to larger grains growth and improved crystallization process, resulting in strong light harvesting capability. The corresponding optical band gaps (Eg) values of the film samples were calculated according to the Tauc equation [21] and were shown in Fig. 4 (b). For the N2 annealing condition, the Eg value of the CH3NH3PbI3 film was 1.55eV, which was consistent with the previous report result [21], while the other film samples S2.5, S5.0, S7.5, and S10 were 1.59eV, 1.60eV, 1.61eV and 1.63eV, respectively. Furthermore, it can be perceived that with the increase of CH3NH3Br concentration in the methylammonium halide mixture solutions, the band gap values of the film samples were gradually increased. This was because the orbital hybridization of Br (4p) with I (5p) and Pb (6s) as well as the structural transformation of the CH3NH3PbI3-xBrx compound [21, 35]. Meanwhile, for the DMSO annealed samples, Eg presented a nearly linear relationship with the increased of CH3NH3Br contents in the methylammonium halide mixture solutions. Moreover, the Eg the film samples annealed under DMSO was higher than that of the N2 annealing atmosphere, which implied the sufficient reaction of two stacking precursor layers under DMSO vapor, facilitating the incorporation of bromine. 3.4. Steady-state photoluminescence spectra of CH3NH3PbI3-xBrx films Fig. 5(a), (b).showed the steady-state photoluminescence spectra of CH3NH3PbI3-xBrx thin films with various Br-doping contents synthesized in N2 and DMSO annealing atmosphere. For the perovskite films after exposured to the N2 vapor, it can be seen that with the increase of CH3NH3Br concentration in the methylammonium halide mixture solutions, the emission peak was obviously blue-shifted, which was in agreement with their band gap variations. Besides the peak intensity increased compared with that of CH3NH3PbI3 films indicating that Br-droping was beneficial to suppress the nonradiative recombination of the excitons [21]. The peak intensity ran up to the maximum at 7.5 mg ml-1 CH3NH3Br followed by a decrease with higher CH3NH3Br concentration duing to the deterioration of film morphology which resulted in increased interface

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defects. Meanwhile, it was worth noting that the DMSO annealing-induced PL spectra change was similar to that obtained from N2-treated films in Fig. 4 (b). However, the luminous intensity was obviously enhanced which can be ascribed to the fully grown film with better crystallinity and reduced trap density. In order to further probe the effect of annealing treatment of CH3NH3PbI3-xBrx films under N2 and DMSO conditions on the microstructure of films and photoelectric properties of the solar cells, the PL spectra of samples were fitted by a bi-Gaussian function. As shown in Fig. 5 (c), the PL spectrum of the DMSO-treated film with a CH3NH3Br concentration of 2.5 mg ml-1 was fitted by a bi-Gaussian function. Particularly, the PL spectra contained two evident emission peaks. One with a blue broad full-width at half maximum (FWHM) of 41.0 nm was characterized as CH3NH3PbI3-xBrx component, whereas the other red PL spectrum showing a FWHM of 70.2 nm represented the CH3NH3PbI3 component. The ratio of two components RBr/I= CH3NH3PbI3-xBrx/CH3NH3PbI3 in perovskite films could be estimated from the ratio of the Gaussian curve integral area. As shown in Fig. 5 (d), with the increase of CH3NH3Br concentration in the methylammonium halide mixture solutions, the R Br/I under N2 annealing condition linearly increased, which was similarly noticed under DMSO annealing. However, in comparison to the N2 annealing, the R Br/I obtained under DMSO annealing were relatively high which was in accordance with the XRD results in Fig. 3, indicating that DMSO vapor was beneficial to the incorporation of bromine and promoted the formation of the mixed–halide CH3NH3PbI3-xBrx. Based on these above mentioned results, it could be concluded that the relatively high-concentration CH3NH3Br incorporation only formed CH3NH3PbI3-xBrx perovskite films without significantly improving perovskite performance, suggesting that a proper CH3NH3Br concentration was required [27].

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3.5. Time-resolved PL decay curves of CH3NH3PbI3-xBrx films The time-resolved PL decay were measured to further investigate the carrier recombination in the prepared films, as shown in Fig. 6.(a), (b).All curves were fitted with double-exponential function of time t [36]:

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I ( t ) = A 0 + A1 exp( − t / τ 1 ) + A 2 exp( − t / τ 2 ) （1）

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Where A1, A2 were the fitting parameters and τ1, τ2 represented exciton or carrier lifetime. It can be obtained from previous reports [37-39] that the PL decay was governed by a fast and a slow recombination process. The fast decay component, τ1, which might come from bimolecular recombination and the slow decay component, τ2 could be due to recombination of carriers in the radiation channel. The fitted τ1 and τ2 were summarized in Table1. For the samples annealed under N2, τ1 did not change obviously and τ2 increased first and then decreased with the increased of CH3NH3Br concentration in the methylammonium halide mixture solutions, as shown in Table1 (a). In the CH3NH3PbI3 film, τ2 was smaller, suggesting a severe recombination occurred. With the increased of CH3NH3Br concentration in the methylammonium halide mixture solutions, τ2 increased gradually, indicating increased carrier lifetime of the film which represented the reduced recombination in the film. It was associated with the improvement of film quality. When CH3NH3Br concentration was 7.5mg m l-1, τ2 reached the maximum leading to the longest carrier lifetime. By further increasing CH3NH3Br concentration to10 mg ml-1, τ2 dropped, which showed that excessive incorporation of CH3NH3Br degraded the perovskite film,

ACCEPTED MANUSCRIPT resulting in increased defects in the film. In addition, for film samples annealed under DMSO, the change trend of τ1 and τ2 were the same as that of N2 condition, but only the values of τ1 and τ2 increased as shown in Table 1(b). This implied that DMSO annealing was beneficial to reduce the defect density and suppresses non-radiative recombination [15], which increased the lifetime of the carriers and improved photovoltaic performance of the solar cell devices.

τ2/ns 14.03 21.04 45.00 45.69 20.91

Sample S0 S2.5 S5 S7.5 S10

τ1/ns 11.09 10.83 10.85 10.52 9.65

τ2/ns 29.52 31.48 43.14 52.34 40.82

annealing solvent N2 N2 N2 N2 N2

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τ1/ns 4.00 4.05 4.14 6.68 6.75

annealing solvent DMSO DMSO DMSO DMSO DMSO

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Sample S0 S2.5 S5 S7.5 S10

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Table1.The time-resolved PL decay parameters of perovskite films with different Br doping content prepared under N2 (a) and DMSO (b) annealing conditions

3.6. Photovoltaic performance of CH3NH3PbI3-xBrx planar solar cells

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Photovoltaic devices were fabricated to investigate the effect of annealing treatment on devices performance. Solar cell with ITO / PEDOT: PSS / CH3NH3PbI3-xBrx / PC60BM / BCP / Ag device structure was prepared where PEDOT: PSS, PC61BM, and Ag film were served as the hole transport, electron transport, and metal contact, respectively. The cross-sectional SEM image of one typical device based on CH3NH3PbI3-xBrx with the 7.5 mg ml-1 CH3NH3Br treatment annealed under DMSO was showed in Fig. 7(a). The J-V characteristic curves of the perovskite solar cells were measured under AM 1.5G (100mWcm-2) light illumination, as shown in Fig. 7 (b), (c) and (d). The detailed photovoltaic parameters of the devices including open-circuit Voltage (VOC), short-circuit current density (Jsc), fill factor (FF) and PCE were summarized in Table 2, and a comparison was presented in Fig. 7(e). For the N2 annealing condition, the sample S0 fabricated using DMF solvent exhibited open circuit voltage (VOC) of 0.89 V, short circuit current (Jsc) of 15.4 mAcm-2, fill factor (FF) of 62.9 %, corresponding to PCE of 8.6%. By comparison, for other samples S2.5，S5.0，S7.5，S10, the VOC were 0.91V, 0.92V, 0.95V, 0.98V, the Jsc were 17.1 mAcm-2, 17.3 mAcm-2，18.1 mAcm-2，16.8 mAcm-2, the FF were 63.7%, 64.0%, 64.2%, 63.7% and PCE were 9.9% ,10.2% ,11%, 10.5%, respectively, as shown in Table 2. Results showed that the open-circuit voltage (VOC) of CH3NH3PbI3-xBrx was higher than that CH3NH3PbI3.This was because that with more Br substituting I, optical band gap increased, resulting in an increase in open circuit voltage [21, 40]. The short-circuit photocurrent density (Jsc) of CH3NH3PbI3-xBrx was also higher than that CH3NH3PbI3 which could be derived from the increased absorption and reduced carrier recombination [21]. In addition, the devices performance of S7.5 samples was better than that of others which could be attributed to the better perovskite film morphology with fine crystallites. Clearly, the perovskite solar cells based on CH3NH3PbI3-xBrx film showed notably improved performance. And it could be further optimized

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by incorporating a proper CH3NH3Br concentration. Compared with N2 annealed condition, the efficiency enhancement of devices fabricated using DMF solvent and annealing under DMSO atmosphere followed the same trend. However, the devices based on the film prepared with DMSO annealing showed higher efficiency than that of N2 conditions, regardless of CH3NH3Br concentration in the methylammonium halide mixture solutions as shown in Fig. 7(e). The reason was that the DMSO vapor could promote the film fully grown and enhanced crystallinity of the perovskite films, which resulted in performance enhancement of the devices. In particular, when the CH3NH3Br concentration was 0 mg m l-1, the efficiency of devices increased from 8.6% (N2) to 9.7% (DMSO). When the CH3NH3Br concentration increased to 2.5 mg m l-1, the PCE increased from 9.9% (N2) to 11.2% (DMSO). Furthermore, when the CH3NH3Br concentration equaled 5 mg m l-1, the PCE increased by 23% from 10.2% (N2) to 12.5% (DMSO).When the CH3NH3Br concentration increased to 7.5 mg m l-1, the efficiency increased from 11.0% (N2) to 13.6% (DMSO), showing a relative improvement of 24%. However, when the CH3NH3Br concentration further increased to10 mg m l-1, the efficiencies increased slightly from 10.5% (N2) to 11.8%, respectively. The most remarkable improvement was observed when the CH3NH3Br concentration increased to 7.5 mg ml-1, which was consistent with the SEM images in Fig. 1, where the S7.5 sampler annealed under DMSO had a better film morphology quality than others. A best PCE of 13.6% was obtained with an open-circuit voltage VOC of 1.01 V, a short-circuit current density Jsc of 19.8 mAcm-2 and a fill factor（FF）of 68.1%. Beyond that, we also studied the photovoltaic performances of perovskite solar cells fabricated using DMSO solvent and annealing under N2 atmosphere. As shown in Table 2, the device samples S0 based on the CH3NH3PbI3 film using DMSO solvent and annealing under N2 atmosphere exhibited open circuit voltage (VOC) of 0.84 V, short circuit current (Jsc) of 15.0 mAcm-2, fill factor (FF) of 62.7 %, corresponding to PCE of 7.9%. By comparison, for other samples S2.5，S5.0，S7.5，S10, the VOC were 0.89V, 0.90V, 0.93V, 0.96V, the Jsc were 16.6 mAcm-2, 17.0 mAcm-2, 17.5mAcm-2, 16.1 mAcm-2, the FF were 62.9%, 63.3% ,63.9%, 63.4% and PCE were 9.3%, 9.7%, 10.4%, 9.8%, respectively. Results showed that with the increase of CH3NH3Br concentration in the methylammonium halide mixture solutions, the efficiency of the device samples were gradually increased. When the CH3NH3Br concentration was 7.5 mg ml-1, the PCE reached a maximum followed by a decrease with higher CH3NH3Br concentration, suggesting a proper CH3NH3Br concentration was required. Furthermore, the perovskite solar cells based on the CH3NH3PbI3-xBrx film using DMSO solvent showed lower efficiency than that of DMF solvent under N2 annealing atmosphere, which was consistent with the previous report result [44]. This was because the use of DMSO as a precursor solvent could make the film grain sizes increase, but the film surface became rough and cracks occurred, resulting in poor film quality, which could be identified from the SEM results as showed in the insets of Fig. 7 (b), (d).

ACCEPTED MANUSCRIPT Table 2.Photovoltaic parameters of devices annealed under N 2 annealing condition (CH3NH3PbI3-xBrx prepared with DMF and DMSO solvents, respectively.) and DMSO annealing condition (CH3NH3PbI3-xBrx prepared with DMF solvent)

S2.5

S5

S7.5

S10

JSC(mAcm-2)

FF(%)

PCE(%)

N2 (DMF) DMSO (DMF)

0.89 0.90

15.4 16.8

62.9 63.5

8.6 9.7

N2 (DMSO)

0.84

15.0

62.7

7.9

N2 (DMF)

0.91

17.1

63.7

9.9

DMSO (DMF)

0.95

18.4

64.2

11.2

N2 (DMSO)

0.89

16.6

N2 (DMF)

0.92

17.3

DMSO (DMF)

0.98

19.0

N2 (DMSO)

0.90

17.0

N2 (DMF)

0.95

18.1

DMSO (DMF)

1.01

19.8

N2 (DMSO)

0.93

17.5

N2 (DMF)

0.98

16.8

DMSO (DMF)

1.03

17.3

N2 (DMSO)

0.96

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VOC(V)

16.1

62.9

9.3

64.0

10.2

67.1

12.5

63.3

9.7

64.2

11.0

68.1

13.6

63.9

10.4

63.7

10.5

66.1

11.8

63.4

9.8

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annealing solvent

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Previous studies have demonstrated that N2 blow drying techniques are beneficial for the formation of a high quality and smooth perovskite layer, which have an effect on improving the performance of solar cells [41-43]. Therefore, perovskite solar cells with and without employing N2 blow drying during the CH3NH3PbI3-xBrx annealing process were fabricated to investigate the influence of static and dynamic N2 annealing on the device performance. Fig.8 showed the J-V

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curves of solar cells with CH3NH3PbI3-xBrx perovskite layers made from solution of the optimal precursor concentration deriving from our above experiments. The photovoltaic results were summarized in the inset of Fig. 8. The device with sample S7.5 annealed under static N2 condition without N2 drying showed the open circuit voltage (VOC) of 0.95 V, short circuit current (Jsc) of

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18.1 mAcm-2, fill factor (FF) of 64.2 %, corresponding to PCE of 11.0 %. When the N2 blow drying was introduced during N2 annealing process, the PCE of the solar cell annealed at 10m/s N2 flow speed significantly improved from 11.0 % to 12.6 % with an increase in the VOC (i.e. from

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0.95 to 0.99 V), Jsc (i.e. from 18.1 to 19.6 mAcm-2) and FF (i.e. from 64.2% to 64.7% ).When the N2 flow speed increased to 20 m/s, a PCE of 11.2 % was achieved with a VOC of 0.96 V, Jsc of 18.7 mAcm-2, and FF of 62.2% suggesting that a high speed N2 flow applied on the CH3NH3PbI3-xBrx film could result in decreased power conversion efficiency (PCE).The variation in the photovoltaic performance of the planar perovskite solar cells made with employing N2 blow drying during the CH3NH3PbI3-xBrx annealing process is rarely mentioned and needs to be further systematically investigated in our group. 4. Conclusions In summary, we studied the effects of annealing treatment of CH3NH3PbI3-xBrx films under N2 and DMSO conditions on the microstructure of films and photoelectric properties of the solar cells. Moreover, we further studied the photovoltaic performances of perovskite solar cells fabricated using DMSO solvent and annealing under N2 atmosphere. The relationship of the

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component ratio of RBr/I= CH3NH3PbI3-xBrx/CH3NH3PbI3 in the resulting perovskite versus CH3NH3Br contents in the methylammonium halide mixture solutions also was invested. The SEM, XRD and UV-Vis characterization indicated that the films annealed under DMSO had larger grain size, better crystallinity, and higher light absorption than that of N2 condition. The defect density of CH3NH3PbI3-xBrx was effectively decrease, which could suppress non-radiative recombination and thus resulted in enhanced photovoltaic performance of devices. The component ratio of RBr/I linearly increased in the resulting perovskite. Furthermore, the R Br/I obtained under DMSO annealing atmosphere were relatively high compared with that of N2 annealing. The PCE of the photovoltaic devices based on the CH3NH3PbI3-xBrx with the 7.5 mgml-1 CH3NH3Br treatment could be optimized to 13.7% in DMSO annealing. Acknowledgments

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This study was supported by the National Natural Science Foundation of China (Grant no. 51572008) References

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Fig.1. Experimental diagram for preparing the perovskite solar cells based on CH3NH3PbI3-xBrx films annealed under N2 and DMSO atmospheric conditions (SC: Spin-coating, TA: thermal-annealing, SA: solvent annealing, TE: thermal evaporation, LI: light).

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Fig.2 Surface SEM images of perovskite (CH3NH3PbI3-xBrx) films on ITO-coated substrates after annealing under N2 (upper row) and DMSO (lower row) atmospheric conditions: the corresponding film prepared with the CH3NH3Br concentrations to be 0 mg ml-1（a1 and a2）, 2.5 mg ml-1（b1 and b2）, 5 mg ml-1（c1 and c2）, 7.5 mg ml-1(d1 and d2）, 10 mg ml-1（e1 and e2）, respectively. All the scale bars represent 1 um.

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TA(N2) S7.5 S5.0

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Fig.3. XRD patterns of perovskite films with different Br doping content annealed under N2 atmospheric conditions (a, b) and DMSO (c, d) atmospheric conditions. The fit of XRD spectrum for perovskite film with CH3NH3Br concentration of 5 mg m L-1 annealed under N2 atmosphere (e), the change of ratio of two components (CH3NH3PbI3-xBrx, CH3NH3PbI3) in perovskite films versus CH3NH3Br concentration (f).

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Fig. 4 (a) UV-vis absorption of the CH3NH3PbI3-xBrx films with different Br doping content annealed under N2 and DMSO atmospheric conditions, respectively. (b) Plot of the band-gaps of the films versus CH3NH3Br concentration annealed under N2 and DMSO conditions.

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Fig. 5.PL spectrum of perovskite films with different Br doping content annealed under N2 atmospheric condition (a) and DMSO (b) atmospheric condition. The fit of PL spectrum for perovskite film with CH3NH3Br concentration of 2.5 mg m l-1 annealed under DMSO atmosphere (c).The relationship of the ratio of two components (CH3NH3PbI3-xBrx, CH3NH3PbI3) in perovskite films versus CH3NH3Br concentration (d).

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Fig. 7.A cross-sectional SEM image of the device (a).The J-V characteristic curves of the devices fabricated (b) using DMF solvent and annealing under N2 atmosphere, (c) using DMF solvent and annealing under DMSO atmosphere, (d) using DMSO solvent and annealing under N2 atmosphere. A comparison of PCE between the perovskite films with different Br doping content prepared under N2 (with DMF and DMSO solvents, respectively) and DMSO annealing conditions (e). The insets of (b), (c) and (d) are the corresponding SEM images of CH3NH3PbI3 films, respectively. All the scale bars represent 1 um.

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Fig. 8．J-V cures of devices annealed under static N2 annealing condition (without N2 blow drying) and dynamic N2 annealing condition (with N2 blow drying at 10m/s and 20m/s N2 flow speed, respectively).The inset shows the detailed performance parameters.

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1. Preparation of the CH3NH3PbI3-xBrx films with various Br-doping contents 2. The effects of annealing treatment of CH3NH3PbI3-xBrx films were systematically investigated

annealing under DMSO showed PCE of 13.7%.

The ratio of two components RBr/I (RBr/I=CH3NH3PbI3-xBrx/CH3NH3PbI3) linearly

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increased in the resulting perovskite.

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3. Planar perovskite solar cell based on CH3NH3Br concentration of 7.5 mg ml-1