PbI2

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Letter 3

MAPbCl Mediated Decomposition Process to Tune Cl/PbI Distribution in the MAPbI Film 2

3

Cuiping Zhang, Zhipeng Li, Juan Liu, Yunchuan Xin, Zhipeng Shao, Guanglei Cui, and Shuping Pang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00837 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 5, 2018

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ACS Energy Letters

MAPbCl3 Mediated Decomposition Process to Tune Cl/PbI2 Distribution in The MAPbI3 Film

Cuiping Zhang,[a,b] Zhipeng Li,[a,b] Juan Liu,[b, *] Yunchuan Xin,[a] Zhipeng Shao,[a] Guanglei Cui,[a] Shuping Pang,[a,*] [1] Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P.R. China. Email: [email protected] [2] Qingdao University of Science and Technology, Qingdao 266042, P.R. China. Email: [email protected]

ABSTRACT: Various methods have been developed to optimize the active layer of perovskite solar cells. Up to now, the introduction of excess PbI2 and chlorine doping are two typical means. Herein, an ion exchange/decomposition process was designed to combine these two approaches. In this process, the I-Cl exchange and MACl sublimation occurred simultaneously at elevated temperature, resulting in a PbI2-excess, Cl-doped perovskite film. This combined strategy could guarantee that as much chlorine as possible was doped into the perovskite crystal lattice, which enables the perovskite films with obvious reduced defect density and enhanced carrier diffusion length. Strikingly, the power conversion efficiency of the device is boosted from 17.17% to 20.15%. TOC GRAPHICS:

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Organic-inorganic hybrid perovskites have emerged as the unprecedentedly promising light-absorber materials for low-cost and high-efficiency perovskite solar cells (PSCs). The record power conversion efficiency (PCE) of PSCs has now exceeded 22% 1, and it has exceeded that of polycrystalline silicon solar cells. Considering the development history of perovskite solar cells, the promotion of the perovskite solar cell’s performance is attributed to eliminating the pin-hole structures via optimizing the fabrication process of device

2-8

, obtaining a wide range of

absorption and a stable phase through modifying the perovskite composition passivating the defects of the perovskite films by interface engineering

9-10

, and

11-13

, etc.

The organic/inorganic hybrid perovskites generally exhibit polycrystalline nature and it is of great significance to ensure that most photogenerated carriers can be quickly extracted from the absorber before recombination occurs. Thus, it’s essential to minimize the energetic disorders (i.e., charge traps and structural defects) in the perovskites films. In this regard, many efforts have been focused on the chemical constituent and crystallization control, such as FA\Cs\Br\Cl etc. doping and grain coarsening

14-20

. For example, PbI2-excess recipe occupying the sites at the surface

and grain boundaries is currently one of the typical approaches for the highly efficient perovskite solar cells

21-22

. The reason is that the energy barriers may be formed

between PbI2 and perovskite to prevent excitons from the surface defects and traps states, which normally induce the nonradiative recombination in the film

23

.

Furthermore, it has been found that the Cl-doped perovskite MAPbI3-xClx possesses superior optoelectronic properties by smoothing out the structural and energetic discontinuities at grain boundaries 24-29. The stronger electronegativity of chlorine (vs iodine) and the associated more rigid bonds formed with lead ions could suppress the lattice distortions and shift the density of holes and electrons far from the defect centers 30. However, there are still no effective ways to dope Cl into perovskites with controllable proportion because of its small size which against the tolerance factor of perovskite crystal structure. Colella et al. has revealed that the tolerance limit of Cl doping in MAPbI3 is of a concentration of 3~4% based on DFT calculations

31

.

Experimentally, a large amount of Cl doping is hardly to be achieved owing to the lower formation energy and the lower boiling point of MAPbCl3 compared with MAPbI3 24, 26, 32-34. The amount of Cl incorporation into the lattice is only of 0.05% approximately according to the current reported fabrication methods

26, 35

. Therefore,

it is essential and challenging to improve the Cl doping amount in the crystal lattice of the MAPbI3 based PSCs. 2


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ACS Energy Letters

In this paper, an ion exchange/decomposition (IED) process was developed in order to combine the positive effect of the above mentioned PbI2-excess and Cl doping in the perovskite films. It was found that during the in-situ formation of the PbI2 phase, the Cl ions could be maximally doped into the crystal lattice of MAPbI3, and the calculated content reaches up to ~ 2%. Owing to the synergetic effect of PbI2-excess and Cl doping, the defect density of as-prepared perovskite film decreased from 1.65×1015 cm-3 to 9.32×1014 cm-3 and the carrier diffusion length increased from 6.0 µm to 41.4 µm. As a result, the PCE of the devices have been boosted to 20.15%, which is much higher than the devices based on the pure MAPbI3 or the PbI2-excess devices. The IED process for the preparation of perovskite film is illustrated in Figure 1(a). The introduction of stoichiometric ratio of MACl (PbI2: MAI: MAC1 = 1: 0.7: 0.3) in the precursor solution could lead to the formation of MAPbCl3 dispersed in the MAPbI3 perovskite film due to its lower formation energy 33, shown diagrammatically in Figure 1(a). However, the MAPbCl3 will decompose into MACl and PbCl2 because of its inferior thermal stability under persistent heating environments 32. At the same time, the Cl ions in the formed PbCl2 will further exchange with the I ions from MAPbI3 transforming into MAPbI3-xClx/PbI2 composite films. To confirm the proposed mechanism in Figure 1(a), the microstructure evolution of the films during the IED process was investigated in detail. Figure 1(b-g) shows the cross-sectional and top view scanning electron microscope (SEM) images of the perovskite films after thermal annealing for 5min, 20 min and 60 min, respectively. At the initial stage (Figure 1b, c), there are some square-like structures dispersed in the perovskite film. They are speculated to be MAPbCl3, as indicated in the following X-ray diffraction (XRD) results. Meanwhile, the transmission electron microscope (TEM) measurement of MAPbI3-xClx was performed, according to the related literature36-38, as shown in Figure S1. It illustrated the close contact between MAPbI3-xClx and PbI2, and PbI2 was presented between the MAPbI3-xClx grains. Due to the short thermal annealing time, the grain boundaries are relatively fuzzy. The following annealing could enhance the crystallinity with grain boundaries becoming more and more clear. When the annealing time reaches 60 min, the square-like structure fully disappeared, replaced with strip-shape grains in relatively bright compared the adjacent grains. The bright contrast is because of its less conductive and more charges accumulation. To have an in-depth understanding of the IED process, the X-ray diffraction 3



(XRD) measurements of the perovskite films after different thermal annealing period (5, 20, 40, 60, 80, 100 min) were performed (Figure 2(a)). As shown in Figure S2, besides the MAPbI3 perovskite phase, PbI2 and MAPbCl3 phase are also confirmed by the appearance of two new peaks at around 12.7o (001) and 15.6o (100) 35. Figure 2(b) is the evolution tendency of the characteristic peaks of PbI2 (001) and MAPbCl3 (100) with prolonging the annealing time. We can intuitively see that, at the beginning, there is a linear decrease of the diffraction peak intensity of MAPbCl3 and it fully disappears when the film heated at 100 °C up to 60 min. Correspondingly, from 0 to 60 min, there is also a linear increase of the intensity of PbI2 (001) peak. While from 60 to 100 min, the generation rate of PbI2 phase obviously reduced with the reduced slope compared to the initial stage of the curve which is indicative of the different decomposition thermodynamics of MAPbCl3 and MAPbI3-xClx, providing the feasibility of employing MAPbCl3 as an intermediate to tune the film’s composition. Figure 2(c) displays the magnified (110) diffraction peak of the control MAPbI3 film and MAPbI3-xClx samples with various annealing time. It is obviously that the (110) peak initially occurs positive-shift with prolonging the annealing time and reaches the maximum positive shift at 60 min, with the diffraction angle from 14.11o to 14.17o, the similar shifts also occur at the (220), (116), (114) peaks (Figure S3). The gradually positive-shift indicates the shrinking of the crystal lattice as the increasing Cl content doping into the MAPbI3 lattice. Further prolonging the annealing time to 80 min or 100 min, however, the peak position of (110) presents a reverse shift and gradually closing to the pristine film owing to the removal of Cl from the MAPbI3-xClx film. It also indicates that the Cl doped MAPbI3 film is not thermally stable, which is the main barrier for realizing maximum Cl doping in the perovskite crystal lattice 26, 32, 34. During the IED process, supposing that the formed MACl could be instantaneously volatilized from the perovskite films, the Cl in the film could be divided into two parts, one is from the MAPbCl3 and the other is from MAPbI3-xClx. The amount of Cl in the MAPbI3 crystal lattice could be calculated through the shift of the diffraction peak according to the Bragg equation 2dsinθ=nλ. Here, it is assumed that the lattice is a cubic structure and the composition engineering follows the Vegard's law, where the lattice parameter is linearly proportional to relative composition of each species

16, 39-41

. The diagonal line in Figure 2(d) is the

dependence of the pseudocubic lattice parameter as a function of the content of Cl in MAPbI3-xClx according to the Vegard’s law. Therefore, given the XRD pattern, the 4


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ACS Energy Letters

content of doped Cl could be obtained directly from this line. The total Cl content could be measured via energy dispersive spectroscopy (EDS, Table S1). As shown in Figure 2(d), on the right side of the diagonal line, the Cl ions are mainly distributed in MAPbCl3 phase. While on the left side of the diagonal line, the Cl ions are mainly as the doped Cl in MAPbI3-xClx phase. For the sample annealed exceed 60 min, the detected Cl ions only come from the MAPbI3-xClx phase because there is no MAPbCl3 phase detected in the films. And when the sample annealed for about 60 min, the Cl content reaches ~2.0%. For the mixed perovskite films, UV-Vis spectroscopy is a very effective means to study the optical properties of the main MAPbI3-xClx phase because the absorption edges of PbI2 and MAPbCl3 are at the range of short wavelengths

32, 42

. The sample

annealed for 60 min shows the largest band gap (Figure S4), which is in good consistent with the above XRD results (Figure 2). In order to explore the influence of the Cl doping and PbI2 passivation on the photophysical properties of perovskite films, time-resolved photoluminescence (TPRL) and photoluminescence (PL) measurements were performed as shown in Figure 3. The PL measurements we performed with all the films under the same conditions without encapsulated. The exponential fitting gives time-resolved PL decay time of 27 ns, 48 ns and 72 ns for MAPbI3, MAPbI3/PbI2 and MAPbI2.94Cl0.06/PbI2 films, respectively. The results showed that the introduction of Cl doping and PbI2 passivator were effective in inhibiting the recombination of carriers and more powerful than that of only PbI2 passivation. The time-resolved PL measurements of the samples with different annealing time were also demonstrated in the Figure S5. It was indicative that the Cl doping into the lattice is the dominated reason for the enhancement of the PL decay time. The more efficient the carrier recombination is suppressed as the more content of Cl doping into the crystal lattice. When the annealing time is prolonged from 60 min to 100 min, the Cl doping content was reduced from 2% to 0% (Figure 2) and the PL decay time decreased to 31 ns correspondingly. As shown in Figure 3(b), the PL intensity of Cl-doped perovskite films is about 5 times higher than that of MAPbI3 films, 2 times higher than that of the film with excess PbI2. The enhancement of PL intensity further confirms the synergistic effect of Cl doping and excess PbI2 on the suppressed nonradiative recombination associated with a lower trap density, consistent with the previous reports 43. Based on the pure MAPbI3, MAPbI3/PbI2 and MAPbI2.94Cl0.06/PbI2 films, the 5



normal

perovskite

solar

cells

were

fabricated

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in

the

structure

of

FTO/c-TiO2/m-TiO2/Perovskite/Spiro-OMeTAD/Au. The J-V curves were presented in Figure 4(a) and the detailed photovoltaic parameters were given in Table 1. Figure S6 demonstrated the excellent photoelectric performance and the high reproducibility of MAPbI2.94Cl0.06/PbI2, compared with MAPbI3 and MAPbI3/PbI2. To confirm the high reproducibility of Voc, Figure S7 presented histograms of the Voc parameter for the MAPbI2.94Cl0.06/PbI2 systems. The champion perovskite device prepared based on the MAPbI2.94Cl0.06/PbI2 exhibits short-current density (Jsc) of 22.6 mA/cm2, fill factor (FF) of 76.6% and open-circuit voltage (Voc) of 1.17 V, thus boosting the PCE up to 20.15%, which is much higher than that of pure MAPbI3 and MAPbI3/PbI2 solar cells. Also, Figure S9 demonstrated a small J-V hysteresis of MAPbI2.94Cl0.06/PbI2. The external quantum efficiency (EQE) was given inset in Figure 4(a), showing an integrated current density of 21.4 mA/cm2, which is close to the Jsc reported in the J-V curve. Due to the dramatic enhancement of the Voc from 1.05 V to 1.17 V, the transient photovoltage (TPV, Figure 4b) was measured to estimate the recombination kinetics in different films. The devices were soaked under AM 1.5 simulated illuminations, and laser pulses (337 nm, 4 ns) were applied to disturb the open circuited devices and trigger

a

small transient

A1exp(-x/τ1)+A2exp(-x/τ2))

photovoltaic gives

the

signal44-45.

Exponential fitting

charge-recombination

lifetimes

of

(y= the

MAPbI2.94Cl0.06 device is of 110 µs, which is much higher than that of the pure MAPbI3 (46 µs) and MAPbI3/PbI2 (72 µs) solar cells (using average values τr= A1τ1+A2τ2). It strongly suggests that the trap defect induced recombination centers in the active layer have been efficiently suppressed with the introduction of excess PbI2 and the Cl doping. Subsequently, the hole and electron defects in the perovskite films has been studied separately via constructing the single carrier devices according the space-charge limited current (SCLC) theory

46

. The increased charge-recombination

life is induced by the reduction of trap density (nt), which could be calculated from the relation VTFL=entd2/2εε0, where VTFL represents the onset voltage of trap filled limit, e is electric charge, d is thickness of the active layer, ε0 is the vacuum permittivity (8.85×10-12 F/m) and ε is the dielectric constant (ε = 34 for MAPbI3). It is obviously shown in Figure 4(c, d) that the improving only happens in the hole transportation in the perovskite film. The hole trap density nt was calculated to be 9.32×1014 cm-3 for MAPbI2.94Cl0.06/PbI2 film, 1.10×1015 cm-3 for MAPbI3/PbI2 film and 1.65×1015 cm-3 for pure MAPbI3 film. It is obviously that the PbI2 has the more 6


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ACS Energy Letters

significant effect on the passivation of the hole trap density. Beside the defect density of the films, the hole diffusion length is extremely important to realize highly efficient collection of the photo-generated carriers especially for the normal structured perovskite solar cells46. The carrier mobility µ is obtained according to the Mott-Gurney law J= (9εε0µV2)/(8d3). Using the formula LD= (kBTµτr/e)1/2, where τr is the carrier lifetime, the diffusion length of the hole in MAPbI3 (6.0 µm) and MAPbI3/PbI2 (7.4 µm) films is little difference. Strikingly, the hole diffusion length of the Cl doped perovskite MAPbI2.94Cl0.06/PbI2 films is significantly increased to 41.4 µm. Considering the sensitivity of the doped Cl content in the crystal lattice, the devices prepared from different annealing time were also measured to demonstrate the dependence of device performance on the Cl doping content in the perovskite films (Figure S10). The consistent conclusion is that the high Cl doping content in the perovskite film is beneficial to achieving high performance PSCs. The stability of the MAPbI2.94Cl0.06/PbI2 device was further performed by exposing into a fixed humidity ambience of 20% for 30 days, and its PCE retains 80% of the initial value (Figure S11). The excellent performance of device confirmed the advantages of the IDE process for the preparation of the highly quality PSCs. In summary, we innovatively report here an ion exchange/decomposition process to construct the MAPbI3-xClx/PbI2 structured perovskite solar cells. With MAPbI3/MAPbCl3 as the raw materials, the I-Cl exchange and MACl sublimation could be occurred simultaneously. On the one hand, the excess PbI2 passivator could be in situ formed in the perovskite films. On the other hand, the ion-exchange at elevated temperature could realize a high Cl doping content of ~2%, which improves the hole diffusion length in the Cl doped perovskite MAPbI2.94Cl0.06/PbI2 films. With the synergetic effect of PbI2-excess and Cl doping, the PCE of the device has been boosted to 20.15%. These findings highlight the crucial rule of both the interface passivation and carrier diffusion length for achieving highly efficient PSCs. ASSOCIATED CONTENT * Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXX Experimental section and additional characterization and analysis data for the perovskite films, and PSCs. ACKNOWLEDGMENT 7



This work was supported by the National Natural Science Foundation of China (51672290, 21671196), the Outstanding Youth Foundation of Shandong Province (JQ201813), DICP&QIBEBT (UN201705), the Youth Innovation Promotion Association of CAS (2015167) and the Qingdao Key Lab of Solar Energy Utilization and Energy Storage Technology.

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ACS Energy Letters

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ACS Energy Letters

Perovskite Solar Cells: The Relevance of Non-Stoichiometric Precursors. Energy Environ. Sci. 2015, 8 (12), 3550-3556. (22) Jiang, Q.; Chu, Z.; Wang, P.; Yang, X.; Liu, H.; Wang, Y.; Yin, Z.; Wu, J.; Zhang, X.; You, J. Planar-Structure Perovskite Solar Cells with Efficiency beyond 21%. Adv. Mater. 2017, 29 (46), , 1703852. (23) Chen, Q.; Zhou, H.; Song, T. B.; Luo, S.; Hong, Z.; Duan, H. S.; Dou, L.; Liu, Y.; Yang, Y. Controllable Self-Induced Passivation of Hybrid Lead Iodide Perovskites toward High Performance Solar Cells. Nano Lett 2014, 14 (7), 4158-4163. (24) Dong, Q.; Yuan, Y.; Shao, Y.; Fang, Y.; Wang, Q.; Huang, J. Abnormal Crystal Growth in CH3NH3PbI3−xClx Using a Multi-Cycle Solution Coating Process. Energy Environ. Sci. 2015, 8 (8), 2464-2470.. (25) Gao, P.; Grätzel, M.; Nazeeruddin, M. K. Organohalide Lead Perovskites for Photovoltaic Applications. Energy Environ. Sci. 2014, 7 (8), 2448-2463. (26) Song, T.-B.; Chen, Q.; Zhou, H.; Luo, S.; Yang, Y.; You, J.; Yang, Y. Unraveling Film Transformations and Device Performance of Planar Perovskite Solar Cells. Nano Energy 2015, 12, 494-500. (27) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-Hole Diffusion Lengths Exceeding 1 Micrometer in an Organometal Trihalide Perovskite Absorber. Science 2013, 342 (6156), 341-344. (28) Yang, B.; Keum, J.; Ovchinnikova, O. S.; Belianinov, A.; Chen, S.; Du, M. H.; Ivanov, I. N.; Rouleau, C. M.; Geohegan, D. B.; Xiao, K. Deciphering Halogen Competition in Organometallic Halide Perovskite Growth. J. Am. Chem. Soc. 2016, 138 (15), 5028-5035. (29) Yang, B.; Brown, C. C.; Huang, J.; Collins, L.; Sang, X.; Unocic, R. R.; Jesse, S.; Kalinin, S. V.; Belianinov, A.; Jakowski, J.; et al. Enhancing Ion Migration in Grain Boundaries of Hybrid Organic-Inorganic Perovskites by Chlorine. Adv. Funct. Mater. 2017, 27 (26), 1700749. (30) Nan, G.; Zhang, X.; Abdi-Jalebi, M.; Andaji-Garmaroudi, Z.; Stranks, S. D.; Lu, G.; Beljonne, D. How Methylammonium Cations and Chlorine Dopants Heal Defects in Lead Iodide Perovskites. Adv. Energy Mater. 2018, 1702754. (31) Colella, S.; Mosconi, E.; Fedeli, P.; Listorti, A.; Gazza, F.; Orlandi, F.; Ferro, P.; Besagni, T.; Rizzo, A.; Calestani, G.; et al. MAPbI3-xClx Mixed Halide Perovskite for Hybrid Solar Cells: The Role of Chloride as Dopant on the Transport and Structural Properties. Chem. Mater. 2013, 25 (22), 4613-4618. 11



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(32) Liu, Y.; Yang, Z.; Cui, D.; Ren, X.; Sun, J.; Liu, X.; Zhang, J.; Wei, Q.; Fan, H.; Yu, F.; et al. Two-Inch-Sized Perovskite CH3NH3PbX3 (X=Cl, Br, I) Crystals: Growth and Characterization. Adv. Mater. 2015, 27 (35), 5176-5183. (33) Mosconi, E.; Amat, A.; Nazeeruddin, M. K.; Grätzel, M.; De Angelis, F. First-Principles Modeling of Mixed Halide Organometal Perovskites for Photovoltaic Applications. J. Phys. Chem. C 2013, 117 (27), 13902-13913. (34) Liao, H.-C.; Guo, P.; Hsu, C.-P.; Lin, M.; Wang, B.; Zeng, L.; Huang, W.; Soe, C. M. M.; Su, W.-F.; Bedzyk, M. J.; et al. Enhanced Efficiency of Hot-Cast Large-Area Planar Perovskite Solar Cells/Modules Having Controlled Chloride Incorporation. Adv. Energy Mater. 2017, 7 (8), 1601660.. (35) Rao, H.; Ye, S.; Sun, W.; Yan, W.; Li, Y.; Peng, H.; Liu, Z.; Bian, Z.; Li, Y.; Huang, C. A 19.0% Efficiency Achieved in CuOx-Based Inverted CH3NH3PbI3−xClx Solar Cells by an Effective Cl Doping Method. Nano Energy 2016, 27, 51-57. (36) Ji, F.; Pang, S.; Zhang, L.; Zong, Y.; Cui, G.; Padture, N. P.; Zhou, Y. Simultaneous Evolution of Uniaxially Oriented Grains and Ultralow-Density Grain-Boundary Network in CH3NH3PbI3 Perovskite Thin Films Mediated by Precursor Phase Metastability. ACS Energy Letters 2017, 2 (12), 2727-2733. (37) Zhou, Y.; Game, O. S.; Pang, S.; Padture, N. P. Microstructures of Organometal Trihalide Perovskites for Solar Cells: Their Evolution from Solutions and Characterization. J. Phys.Chem. Lett. 2015, 6 (23), 4827-4839. (38) Zong, Y.; Zhou, Y.; Zhang, Y.; Li, Z.; Zhang, L.; Ju, M.-G.; Chen, M.; Pang, S.; Zeng, X. C.; Padture, N. P. Continuous Grain-Boundary Functionalization for High-Efficiency Perovskite Solar Cells with Exceptional Stability. Chem 2018, 4 (6), 1404-1415. (39) Xu, J.; Tang, Y.-B.; Chen, X.; Luan, C.-Y.; Zhang, W.-F.; Zapien, J. A.; Zhang, W.-J.; Kwong, H.-L.; Meng, X.-M.; Lee, S.-T.; Lee, C.-S. Synthesis of Homogeneously

Alloyed

Cu2−x(SySe1−y)

Nanowire

Bundles

with

Tunable

Compositions and Bandgaps. Adv. Funct. Mater. 2010, 20 (23), 4190-4195. (40) Ji, F.; Wang, L.; Pang, S.; Gao, P.; Xu, H.; Xie, G.; Zhang, J.; Cui, G. A Balanced Cation Exchange Reaction Toward Highly Uniform and Pure Phase FA1−xMAxPbI3 Perovskite Films. J. Mater. Chem. A 2016, 4 (37), 14437-14443.

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Page 13 of 17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(41) Yang, Z.; Chueh, C.-C.; Liang, P.-W.; Crump, M.; Lin, F.; Zhu, Z.; Jen, A. K. Y. Effects of Formamidinium and Bromide Ion Substitution in Methylammonium Lead Triiodide Toward High-Performance Perovskite Solar Cells. Nano Energy 2016, 22, 328-337. (42) Kim, Y. C.; Jeon, N. J.; Noh, J. H.; Yang, W. S.; Seo, J.; Yun, J. S.; Ho-Baillie, A.; Huang, S.; Green, M. A.; Seidel, J.; et al. Beneficial Effects of PbI2 Incorporated in Organo-Lead Halide Perovskite Solar Cells. Adv. Energy Mater. 2016, 6 (4), 1502104. (43) Dequilettes, D. W.; Vorpahl, S. M.; Stranks, S. D.; Nagaoka, H.; Eperon, G. E.; Ziffer, M. E.; Snaith, H. J.; Ginger, D. S. Impact of Microstructure on Local Carrier Lifetime in Perovskite Solar Cells. Science 2015, 348 (6235), 683-686. (44) O'Regan, B. C.; Barnes, P. R.; Li, X.; Law, C.; Palomares, E.; Marin-Beloqui, J. M. Optoelectronic Studies of Methylammonium Lead Iodide Perovskite Solar Cells with Mesoporous TiO2: Separation of Electronic and Chemical Charge Storage, Understanding Two Recombination Lifetimes, and the Evolution of Band Offsets during J-V Hysteresis. J. Am. Chem. Soc. 2015, 137 (15), 5087-5099. (45) Zhu H.; Miyata K.; Fu Y.; Wang J.; P. Joshi P.; Niesner D.; W. Williams K.; Jin S.; Zhu X. Screening in Crystalline Liquids Protects Energetic Carriers in Hybrid Perovskites[J]. Science, 2016, 353(6306): 1409-1413. (46) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Solar cells. Electron-Hole Diffusion Lengths>175 µm in Solution-Grown CH3NH3PbI3 Single Crystals. Science 2015, 347 (6225), 967-970

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Figure 1. (a) Schematics of ion exchange/decomposition process. (b, c), (d, e), (f, g) the cross-sectional and top view of perovskite films annealed for 5 min, 20 min, 60 min, respectively. The scale bar in all the images is 500 nm.

14


Page 14 of 17

100 min

(a)

(b)

1.0 0.8

PbI2

XRD Intensity (a.u.)

80 min 60 min 40 min 20 min

0.6 0.4 0.2

5 min

MAPbCl3

MAPbI3 0

5

10

20

30

0.0 0

40

20

2 Theta (degree)

100 min 80 min 60 min 40 min 20 min

(d)6.26 Lattice Constant (‫)إ‬

(c)

5 min MAPbI3 13.9

14.0

14.1

14.2

40

60

80

100

Annealing time (min)

XRD Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

XRD Intensity (norm.)

Page 15 of 17

14.3

6.25

100 min 80 min 60 min

5 min 20 min 40 min

6.24

6.23

6.22

0

14.4

2 Theta (degree)

2

4

6

8

10

Cl concentration (mol%)

Figure 2. (a) X-ray diffraction pattern of pure MAPbI3 and MAPbI3-xClx /PbI2 perovskite films. (b) The intensity evolution tendency of PbI2 (001) diffraction peak and MAPbCl3 (100) diffraction peak along with the ion exchange/decomposition process. (c) X-ray diffraction pattern of pure MAPbI3 and MAPbI3-xClx /PbI2 perovskite films magnified around (110) diffraction peak. (d) The measured Cl content in the films and calculated amount of Cl doped in the crystal lattice.

15


ACS Energy Letters

(a)

1

(b)

MAPbI3 MAPbI3/PbI2

MAPbI3 MAPbI3/PbI2

5

MAPbI2.94Cl0.06/PbI2

PL Intensity (a.u.)


PL (Norm.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1

4

3

2

1

0.01 0

100

0 700

200

Time (ns)

750

800

850

Wavelength (nm)

Figure 3. (a) Time-resolved photoluminescence (TRPL) and (b) photoluminescence (PL) measurements for the MAPbI3, MAPbI3/PbI2 and MAPbI2.94Cl0.06/PbI2 films, respectively.

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Page 17 of 17

25

1

(b)

20

MAPbI3 MAPbI3/PbI2

0.8


100 2

Jsc=21.4 (mA/cm )

10

60

15

40

10

5

20

Integrated Jsc (mA/cm2 )

15

0

0 400

500

600

700

0.6

Normalized (∆V)

20

80

EQE (%)

Current density (mA/cm2)

(a)

800

0.4

Voltage (V)

5

MAPbI3

MAPbI3/PbI2

MAPI2.94Cl0.06/PbI2 0 0.0

0.2

0.4

0.6

0.8

1.0

0.2

1.2

0

100

200

Voltage (V)

(d)

MAPbI3 1E-4

1E-5

1E-6

MAPbI3/PbI2 MAPbI2.94Cl0.06/PbI2

n>3

J∝Vn

300

400

Time (µs)

Current density (mA/cm2)

(c) Current density (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

n=2

n=1

MAPbI3 1E-4

MAPbI3/ PbI2 MAPbI2.94Cl0.06/ PbI2

n=2 1E-5

n>3

J∝Vn

n=1 1E-6

VTFL 1E-7 0.01

0.1

1

0.01

0.1

1

Voltage (V)

Voltage (V)

Figure 4. (a) The current-voltage (J-V) curves and (b) transient photovoltage (TPV) of solar cells based on the MAPbI3, MAPbI3/PbI2 and MAPbI2.94Cl0.06/PbI2 films. The inset in (a) is the EQE curves and integrated current density of the MAPbI2.94Cl0.06/PbI2 solar cell. (c) J-V curves of hole-only device based on the structure of FTO/NiO/Perovskite/Spiro/Au and (d) J-V curves of electron-only device based on the structure of FTO/TiO2/Perovskite/ PCBM/Au. Table 1 Summary of the photovoltaic data of the champion perovskite device with the structure of FTO/c-TiO2/m-TiO2/Perovskite/Spiro-OMeTAD/Ag shown in Figure 4(a) and Figure S5. Time

Voc

Jsc 2

FF

PCE

(min)

(V)

(mA/cm )

(%)

(%)

5

1.11

21.6

72.5

17.46

20

1.13

21.3

72.8

17.49

40

1.13

22.1

73.8

18.47

60

1.17

22.6

76.3

20.15

80

1.11

21.2

70.7

16.62

100

1.07

20.9

70.0

15.65

MAPbI3/PbI2

1.11

21.8

71.3

17.17

MAPbI3

1.05

21.5

68.4

15.44

17


PbI2

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