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Apr 21, 2018 - d Shenzhen Jiawei Photovoltaic Lighting Co., Ltd., New Industrial Zone No. 1-4, Fuping Road, Longgang District, Shenzhen, 518112, ...
Electrochimica Acta 275 (2018) 1e7

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Enhanced efficiency of perovskite solar cells by introducing controlled chloride incorporation into MAPbI3 perovskite films Xiaobing Cao a, Lili Zhi b, Yi Jia c, Yahui Li a, Ke Zhao a, Xian Cui a, Lijie Ci b, Kongxian Ding d, Jinquan Wei a, * a

State Key Lab of New Ceramic and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing, 100084, PR China School of Materials Science & Engineering, Shandong University, Jinan, 250061, Shandong, PR China Qian Xueshen Laboratory of Space Technology, Youyi Road No. 104, Haidian District, Beijing, 100094, PR China d Shenzhen Jiawei Photovoltaic Lighting Co., Ltd., New Industrial Zone No. 1-4, Fuping Road, Longgang District, Shenzhen, 518112, Guangdong, PR China b c

a r t i c l e i n f o

a b s t r a c t

Article history: Received 14 February 2018 Received in revised form 16 April 2018 Accepted 16 April 2018 Available online 21 April 2018

Compositional engineering of mixed halides is an effective approach to fabricate highly efficient perovskite solar cell. The mixed halide perovskite of MAPbI3-xClx attracts a great of attentions due to its excellent optoelectronic properties. However, the role of chlorine in MAPbI3-xClx is still under discussion by now. Here we introduce PbCl2 as additives into PbI2/DMF solution to study its effects on the formation of MAPbI3-xClx film, which is fabricated from Lewis acid-base adducts through a modified two step method. We demonstrate that the chloride ions have positive roles in fabrication of high quality perovskite MAPbI3-xClx films, characterized by high crystallization, strong grain orientation, large grain size and lower trap densities. Compared to perovskite solar cells without chloride incorporation, the performance of solar cells based on MAPbI3-xClx improves significantly due to its high carrier mobility, efficient carrier transportation and low recombination in device. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Perovskite films Mixed halides Lead chloride Additives Solar cell

1. Introduction Perovskite based on organiceinorganic lead halides with a formula of APbX3 (A ¼ methylammonium (MAþ), formamidinium (FAþ); X ¼ I, Br, Cl) have become a kind of star material in photovoltaic field due to its excellent optoelectronic properties and easy fabrication process. Great achievements in performance of solar cells have been made by employing organiceinorganic lead halides perovskites as light harvest layer. The power conversion efficiency (PCE) has increased from initial 3.8% to 22.1% in several years [1,2]. To improve the performance of perovskite solar cells (PSCs), compositional engineering of mixed halides/mixed cations is a widely adopted strategy to achieve chemical modification in different site of perovskites. For examples, Gr€ atzel`s group incorporated cesium and rubidium cations into perovskite to form a mixed A cation inorganiceorganic perovskite for improving stability of solar cells [3,4]. Nazeeruddin's group achieved solar cells

* Corresponding author. E-mail address: [email protected] (J. Wei). https://doi.org/10.1016/j.electacta.2018.04.123 0013-4686/© 2018 Elsevier Ltd. All rights reserved.

with efficiency beyond 20% with stable quasi-cubic FAxMA1-xPbI3 perovskite structure by introducing FAþ into MAPbI3 [5]. Yan's group improved the efficiency and stability of PSCs simultaneously by incorporating thiocyanate ions in the crystal lattice to form high quality MAPbI3-x(SCN)x films [6]. Among all mixed perovskite films, a mixed halide containing I and Cl in MA based perovskite, namely MAPbI3-xClx has attracted much attentions due to its long carrier diffusion length [7]. However, the role of chlorine in MAPbI3-xClx is still under discussion in the use of hybrid perovskite for solar cells. There are different views to explain the key role of chloride ions in improvement of performance of PSCs based on MAPbI3-xClx. Some researchers demonstrated that chloride ions plays a key role in improving the growth of perovskite grains size and crystalline orientation by changing the formation process of perovskite film [8e10]. Some researchers believed that chloride is preferentially located in close proximity to the perovskite/TiO2 interface, which improve the carries transportation between perovskite and carrier transport layer rather than within perovskite films [11,12]. However, Colella et al. found that the incorporation of Cl into MAPbI3 can improves the charge transport within the perovskite layer [13]. Some researchers demonstrated that a large band bending at grain boundaries with

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chloride incorporation into perovskite films, resulting in a formation of p-i-n type of heterojunction junction in device. The p-i-n junction facilitates the charge collection and reduces the carrier recombination [14]. Recently, Pham et al. [15] fabricated high efficient PSCs by one-step method via a modified lead chloride Lewis acidebase adduct. They demonstrated that the optimized PbCl2 in perovskite precursor solution not only helps to increase the grain size, but also shifts the fermi-level of perovskite films, resulting in longer charge carrier life and more efficient charge transportation. It needs to note that most of works about MAPbI3-xClx films were synthesized from one-step method by spin coating from a solution containing PbCl2 and MAI, in which the mole ration of MAI and PbCl2 is accurately controlled to be a “magic” ratio of 3:1 [16]. Few attentions were paid to study the effect of PbCl2 on the formation of MAPbI3-xClx fabricated through two-step method. Here, we introduce different PbCl2 as additive into PbI2/DMF solution to study its effects on the quality of MAPbI3-xClx film fabricated from an improved two-step method [17]. We find that the chloride incorporation plays multiple positive functions in fabrication of high quality perovskite films, including improving crystallization and grain orientation, enlarging grain size and reducing trap densities. As a result, the efficiency of PSCs with chloride incorporation improves significantly due to its high carrier mobility, efficient carrier transportation and low recombination in device. 2. Experimental section 2.1. Fabrication of devices Compact and microporous TiO2 layers were deposited on FTO substrates according to our previous work [18]. Perovskite films were fabricated from Lewis adducts via molecular exchange [17]. Briefly, PbI2 precursor solution was prepared by dissolving PbI2 in DMF (1.4 M) adding with different molar ratios of PbCl2 (0%, 5%, 10%), and then stirring at 100  C for 60 min. The PbI2 precursor solution was spin-coated onto the TiO2 at a speed of 5500 rpm for 30 s to form a wet PbI2 precursor films. Then, a drop of CH3NH3I solution (70 mg/mL in 2-propanol) was dipped onto the wet PbI2 precursor films at a 5500 rpm for 30 s to form a perovskite precursor film. Finally, the perovskite precursor film was annealed at 100  C for 20 min to obtain perovskite films. After cooling down to room temperature, a hole transport layer was deposited by spincoating of spiro-OMeTAD solution (72.3 mg spiro-OMeTAD dissolved in 1 mL chlorobenzene), to which 19 mL TFSI (520 mg/mL in acetonitrile), 29 mL FK102 solution (300 mg/mL in acetonitrile) and 29 mL 4-tert-butylpyridine (tBP) were added. To compete the whole solar cells, a thin layer of Au (60 nm) was deposited onto THL to form a back electrode. The effective area of each cell is fixed at 6 mm2, which was controlled by a patterned metal mask. 2.2. Characterization The perovskite films were characterized by a field-emission scanning electron microscopy (SEM, MERLIN VP Compact), a Xray diffraction (XRD, D8-Advance), and a UV/Vis absorption spectrometer (Cary 5000 UVeviseNIR). Capacity-voltage curves were recorded by an electrochemical workstation (CHI660D) in dark condition at 1000 Hz from 0 to 1 V. Impedance spectra (IS) were measured by an electrochemical workstation at dark under a bias voltage of 0.9 V with an alternative signal of 10 mV in a frequency range from 1 Hz to 106 Hz. The J-V curves of the PSCs were measured by a Keithley 4200-SCS parameter analyzer under illumination (AM 1.5G, 100 mW/cm2, 91195, Newport). Incident photon to current efficiency (IPCE) spectra were obtained from 350 nm to 950 nm by using a QEX10 solar cell quantum efficiency

measurement system (QEX10, PV measurements, USA). Steadystate and time-resolved photoluminescence (PL) decay spectra were obtained by using an FLS920 transient optical spectrometer. The samples in steady PL measurements were excited by a monochromatic xenon lamp source (central wavelength lexc ¼ 460 nm). Time resolved PL measurements were photoexcited by a laser beam with a wavelength of 405 nm in frequency range from 10 to 20 MHz in a time-correlated single photon counting (TCSPC) system. 3. Results and discussion Fig. 1 is SEM images of the perovskite films fabricated from different concentrations of PbCl2 in PbI2/DMF solution, showing that chloride has evident effects on morphology, grain size, and microstructure of the perovskite films. Compared to the perovskite films fabricated from 0% PbCl2, the mean grain size increases from 223 nm to 353 nm when 5% PbCl2 is introduced into PbI2/DMF solution (see Fig. 1a and c). Meanwhile, grain microstructure also evolves from small grains to large columnar grains (see Fig. 1b and d), which is perpendicular to substrate, benefiting reducing the surface area of the grain boundary [19]. The columnar grains enable photogenerated carriers transport freely along thickness direction without interruption of grain boundary [20]. It needs to note that there are some faceted precipitates on the surface of the perovskite films with chloride incorporation, which might be chlorine-rich areas [21]. The faceted precipitates become more obvious when the amount of PbCl2 further increases to 10%, which leads to a blurry distribution of grains (see Fig. 1e), as well as destruction of the desired large columnar grains for high efficient PSCs (see Fig. 1f). Fig. 2a is XRD patterns of the perovskite films with different chloride concentrations. The main characteristic peaks located at 14.3 , 28.6 and 32.0 are separately assigned to (110), (220) and (330) planes of the tetragonal MAPbI3 perovskite [22]. It is interesting that the intensity of characteristic diffraction peaks increases by multi fold with chloride incorporation, which indicates the crystallinity of the perovskite film improves significantly. Here, we introduce the intensity ratios of I110/I310 and I220/I310 to study the effects of chloride incorporation on grain orientation, which are calculated by the intensity of (110), (220) separately divided by the intensity of (310) [23]. For the sample fabricated from 0% PbCl2, the values of I110/I310 and I220/I310 are 3.15 and 1.70, respectively. However, they separately increases to 15.13 and 6.49 when 5% PbCl2 is introduced to PbI2/DMF solution. The enhancements of the I110/ I310 and I220/I310 indicate that it exhibits better preferred (110) grain orientation in perovskite film fabricated 5% PbCl2 than those fabricated from 0% PbCl2. However, for the sample fabricated from 10% PbCl2, the values of I110/I310 and I220/I310 reduce to 10.6 and 5.06, respectively, which indicate that the preferred orientation grains is weakened at high concentration of PbCl2. In order to estimate the doping concentration, we performs MotteSchottky analysis basing on complete solar cells (see Fig. 2b). In the MotteSchottky curves, it describes the relationship between capacity and applied voltage as the following formula [24e26]:

1 2ðVbi  VÞ ¼ εε0 eAN C2

(1)

where, C is capacity, Vbi is built-in potential, V is applied voltage, ε is relative dielectric constant (ε ¼ 21.2 for MAPbI3) [27], ε0 is the permittivity of vacuum, e is the elementary charge, A is active area, N is the doping concentration of the donor. From formula (1), one can see that the square capacity (1/C2) versus applied voltage V yields to a straight line with its slope inversely to proportional to the doping concentration (N), and the intercept represents the in-

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Fig. 1. The surface (left column) and cross-sectional (right column) SEM images of the perovskite films fabricated from different amount PbCl2 in PbI2/DMF solutions (a), (b) 0%, (c), (d) 5%, (e), (f) 10%. The inset is the grain distribution of the corresponding perovskite film.

built electric filed (Vbi). As fitted in Fig. 2b, the built-in potential is about 0.8 V for all samples. For the sample fabricated from 0% PbCl2, the fitted doping concentration is 2.45*1017 cm3. This value increases to 3.62*1017, 5.92*1017 cm3 when the amount of PbCl2 increases to 5% and 10%, respectively. Those values are comparable to previous report [24]. The increase of doping concentration indicates that chloride has embedded into the lattice of MAPbI3. Fig. 2c is normalized steady PL spectra of the perovskite films. The PL spectra show a slight blue-shift with chloride incorporation, which is consistent with previous report [28,29]. Compared to the MAPbI3, the blue-shift phenomenon in mixed perovskite of MAPbI3-xClx may be related to incorporation of chloride into the MAPbI3 crystal lattice, which reduces the lattice symmetry and thus increases the material bandgap [15]. However, the UV absorption spectra of the perovskite films show that chloride incorporation almost have no effects on the absorption ability in the whole wavelength range (see Fig. 2d). Fig. 3a is a SEM image of a complete PSC with a configuration of FTO/TiO2/MAPbI3/sipro-OMeTAD/Au. By employing this configuration, we achieves a best PCE of 17.0% with additional 5% PbCl2 into PbI2/DMF solution. Other photovoltaic parameters are follows: short-circuit current density (Jsc) ¼ 21.53 mAcm2, open circuit voltage (Voc) ¼ 1.04 V, and fill factor (FF) ¼ 0.759. The best PSC fabricated from 10% PbCl2 exhibit performance of PCE ¼ 15.5%, Jsc ¼ 20.67 mA cm2, Voc ¼ 1.01 V, FF ¼ 0.744. All performance are better than those fabricated from 0% PbCl2 (PCE ¼ 13.2%, Jsc ¼ 19.65 mAcm2, Voc ¼ 0.99 V, FF ¼ 0.676) (see Fig. 3b). The

photovoltaic parameters are listed in Table 1. The statistic results of the photovoltaic performance are also shown in Fig. 4. IPCE spectra reconfirm the superior performance of PSCs fabricated with 5% PbCl2 compared to those fabricated from 0% PbCl2 (see Fig. 3c), which is consistent with the light J-V curves. The integrated Jsc based on IPCE spectra is 17.70, 19.88 mAcm2, which is slight lower (175 mm in solution-grown CH3NH3PbI3 single crystals, Science 347 (2015) 967e970. [36] M. Abdi-Jalebi, M.I. Dar, A. Sadhanala, S.P. Senanayak, M. Franckevi cius, €tzel, R.H. Friend, N. Arora, Y. Hu, M.K. Nazeeruddin, S.M. Zakeeruddin, M. Gra Impact of monovalent cation halide additives on the structural and optoelectronic properties of CH3NH3PbI3 perovskite, Adv. Energy Mater. 6 (2016), 1502472. [37] X.B. Cao, C.L. Li, Y.H. Li, F. Fang, X. Cui, Y.W. Yao, J.Q. Wei, Enhanced performance of perovskite solar cells by modulating the Lewis AcideBase reaction, Nanoscale 8 (2016) 19804e19810. [38] X.B. Cao, C.L. Li, L.L. Zhi, Y.H. Li, X. Cui, Y.W. Yao, L.J. Ci, J.Q. Wei, Fabrication of high quality perovskite films by modulating the Pb-O bonds in Lewis acidbase adducts, J. Mater. Chem. A 5 (2017) 8416e8422. [39] X. Cao, L. Zhi, Y. Li, X. Cui, L. Ci, K. Ding, J.Q. Wei, Enhanced performance of perovskite solar cells by strengthening a self-embedded solvent annealing effect in perovskite precursor films, RSC Adv. 7 (2017) 49144e49150. [40] J.A. Christians, R.C.M. Fung, P.V. Kamat, An inorganic hole conductor for organo-lead halide perovskite solar cells. Improved hole conductivity with copper iodide, J. Am. Chem. Soc. 136 (2014) 758e764. [41] H.S. Ko, J.W. Lee, N.G. Park, 15.76% Efficiency perovskite solar cells prepared under high relative humidity: importance of PbI2 morphology in two-step deposition of CH3NH3PbI3, J. Mater. Chem. A 3 (2015) 8808e8815. [42] Q. Dong, Z. Wang, K. Zhang, H. Yu, P. Huang, X. Liu, Y. Zhou, N. Chen, B. Song, Easily accessible polymer additives for tuning the crystal-growth of perovskite thin-films for highly efficient solar cells, Nanoscale 8 (2016) 5552e5558. [43] X. Zhao, H. Shen, Y. Zhang, X. Li, X. Zhao, M. Tai, J.F. Li, J.B. Li, X. Li, H. Lin, Aluminum-doped zinc oxide as highly stable electron collection layer for perovskite solar cells, ACS Appl. Mater. Interfaces 8 (2016) 7826e7833.