Global Control of CH3NH3PbI3 Formation with Multifunctional Ionic

6 days ago - Moreover, the residual BMII provided a liquid domain to promote the coarsening of perovskite crystal during the thermal annealing process...
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Global Control of CHNHPbI Formation with Multifunctional Ionic Liquid for Perovskite Hybrid Photovoltaics Peng Chen, Yu Zhang, Jinglun Du, Yinglin Wang, Xintong Zhang, and Yichun Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01026 • Publication Date (Web): 30 Apr 2018 Downloaded from http://pubs.acs.org on April 30, 2018

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Global Control of CH3NH3PbI3 Formation with Multifunctional Ionic Liquid for Perovskite Hybrid Photovoltaics Peng Chen, Yu Zhang, Jinglun Du, Yinglin Wang*, Xintong Zhang*, Yichun Liu Center for Advanced Optoelectronic Functional Materials Research, and Key Laboratory for UV-Emitting Materials and Technology of Ministry of Education, Northeast Normal University, 5268 Renmin Street, Changchun 130024, China.

ABSTRACT: Here, a simple strategy using 1-butyl-3-methylimidazolium (BMII) as multifunctional additive was employed to globally modify the two-step deposition process of perovskite film. Morphological, structural and spectral analyses showed that the BMII additive could coordinate with PbI2 and thereby retarded the reaction of PbI2 and MAI through the ionic exchange process. Moreover, the residual BMII provided a liquid domain to promote the coarsening of perovskite crystal during the thermal annealing process. Thus, the obtained MAPbI3 film preferred low PbI2 residue, high-quality crystallization, and large-grained microstructure. Using films prepared with BMII additives, the maximum power conversion efficiency (PCE) of the solar cells were improved from 12.6% of the reference cell to 15.6%. The present study gives a reproductive and facile strategy toward high quality of perovskite thin films and efficient solar cells. 1

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INTRODUCTION Metal halide perovskites solar cells (PSCs) have attracted impressive attention on the recent scientific and commercial communities, largely ascribed to their high efficiency and solution-processed progress.1-10 As a thin-film solar cell, the photovoltaic performance of PSCs is largely dependent on the properties of perovskite

light-harvesting

film,

including

the

composition,

coverage,

crystallinity and morphology, which were proved to influence different carrier-related processes of PSCs.11-14 Thus, a number of methodologies have been developed to obtain a high-quality perovskite film, like one-step deposition and sequential (two-step) deposition.15-17 The sequential deposition method of perovskite film was first reported by Burschka and co-works, which involved depositing PbI2 film first, then converting it into perovskite through interfacial heterogeneous reaction between PbI2 and CH3NH3I (MAI), and finally thermally annealing the pre-synthesized perovskite film for grain growth.15 Nowadays, this method has been well progressed because of its good operability and reproducibility, which could be well performed through solution, vapor and solution-vapor hybrid process, respectively.15, 18-20 However,

sequential

deposition

method

still

suffers

from

several

shortcomings in each step for the preparation of high-quality perovskite film. In the first step, the unsatisfactory dissolubility of precursor PbI2 in solution limits the coverage and uniformity of the final perovskite film.21-22 In the second step, the swift nucleation of perovskite at the interface between PbI2 and MAI retards 2

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the subsequent diffusion of MAI, leading to the incomplete conversion of PbI2.23-24 Regarding the third step, the grain boundary energy on the top surface and the bottom interface of perovskite film are much lower than that inside film, leading to the abnormal exaggerated growth of grain whose low-energy crystallographic planes constitute the top surface and/or the bottom interface.25-26 As a result, numerous strategies were reported to successfully solve the respective problem in different steps. Lewis base additives, such as DMSO,

thiourea,

thioacetamide,

tetra-n-butylammonium

triiodide

and

cyclodextrin were introduced into the PbI2 solution to break the close packing of PbI2 layers, leading to the significant improvement of PbI2 dissolubility.22, 27-31

Meanwhile, extending reaction time and heating-assisted diffusion were

normally used to ensure the full conversion of PbI2.32-33 And solvent-, humidity- and vapor-assisted annealing processes were proved to be effective to increase the grain size of pre-synthesized perovskite.34-38 Herein, we reported a deliberate method for the global control of all steps in the sequential deposition process of perovskite using a single ionic liquid (IL) additive. 1-Butyl-3-methylimidazolium iodide (BMII) was added directly into the PbI2 solution in a series of concentration, whose multifunctional effects during sequential deposition process were systematically investigated. The close packing of PbI2 layer were destroyed by the intercalation effect of BMII, resulting in the remarkable improvement in the dissolubility and the stability of PbI2 solution,which enhanced the uniformity and the amorphous state of PbI2 film. Second, the added IL occupied 3

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apart reactive sites of PbI2, thus heterogeneous reaction rate was observed to decrease by 30% to 70% at the first 20 s reaction time, leading to the complete conversion of PbI2. Moreover, the high-polarity and high-boiling-point IL provided a liquid domain between neighbour grains to reduce the activation energy of grain-boundary motion, and facilitated the grain coarsening of perovskite crystal during the thermal annealing. Benefited from the multifunctional effects of IL, the resulting PSCs with BMII (1%, molar ratio) exhibited a high power conversion efficiency of 15.6%, as compared to that of 12.6% for the reference cell without IL additives. Our systematic investigation of different roles of IL, played during the formation process of perovskite, provided scientific clues for understanding the formation mechanism of perovskite and explored new strategies for further photovoltaic performance improvement of PSCs. RESULTS AND DISCUSSION The poor dissolubility of PbI2 is considered to be one of the most important reason to reduce the quality of perovskite materials, which mainly results from the close packing of PbI2 layer.21-22 Thus, we first systematically explored the effect of BMII on the dissolubility of PbI2 in DMF solution. The PbI2 precursor solution was obtained by dissolving BMII directly into DMF with different molar ratios between BMII and PbI2. And we named four samples as BMII-0%, BMII-1%, BMII-5%, and BMII-10% with respect to the molar ratios between BMII and PbI2 in the following discussion of this paper. As shown in the DLS results of Figure 1a, the diameters of PbI2 aggregates in DMF solution are dramatically decreased from 912 nm for BMII-0% sample to 253 4

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nm for BMII-10%. It`s worthwhile to notice that this reduction of PbI2-aggregate size leads to the improvement of solubility of PbI2 in DMF solution. The PbI2 solution with only 1% BMII remained stable without obvious precipitation after 12-hour stand, while that without BMII generated a lot of precipitation (Figure 1a). We also studied the impact of BMII on the PbI2 film and further explored the interaction between BMII and PbI2. As the XRD patterns shown in Figure 1b, adding BMII into PbI2 film attenuates the intensities of PbI2 diffraction peaks (001) obviously, and broadens these peaks. These variations of XRD patterns for PbI2 films illustrate that the BMII destroys the periodicity of the PbI2 crystal lattice and lead to the amorphous PbI2 films.39 In addition, this destruction of PbI2-layer ordering caused by the addition of BMII was also proved by the blue-shifts of absorption peak (Figure 1c) and the emission peak (Figure 1d) of PbI2 films.40-41

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Figure 1. (a) The diameter variation of PbI2 aggregates in DMF solution caused by BMII (molar ratio between BMII and PbI2: 0%, 1%, 5%, 10%). Beside: photographs of PbI2 solution (1.2 M, DMF) with 0% (a1) and 1% (a2) BMII (molar percentage), both solution were heated at 60 °C for 30 min and then remained overnight in the room temperature. (b) XRD patterns at the scanning speed of 5º/min, (c) UV-vis absorption spectra and (d) PL spectra of PbI2 films with different ratios of BMII. Inset in (b) is the normalized XRD patterns of PbI2 peak (001) at the scanning speed of 2º /min.    The PbI2 films were converted to MAPbI3 in the atmospheric condition, in order to study the effect of BMII on the interfacial chemical conversion stage of the sequential 6

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deposition method. The photographs in Figure 2a show the color evolution of PbI2 films after being placed into the isopropanol solution of MAI at 80 °C. The slower color evolution of BMII/PbI2 film, compared with that of PbI2 film without BMII, suggests that the interfacial chemical reaction between PbI2 and MAI is decelerated by BMII. We further monitored the influence of BMII on the chemical conversion of PbI2 through XRD (Figure S1). The integral peak-area ratios between PbI2 (001) and MAPbI3 (110) as a function of the immersing time of the PbI2 film in the MAI solution are showed in Figure 2b. At the beginning of the interfacial reaction, the ratio of the PbI2 film with BMII is larger than that in the pristine PbI2 film, suggesting that the rate of interfacial chemical reaction is decelerated by BMII. It is interesting to note that the residue in PbI2 film with BMII-1% is less than that in the film without BMII after 120 s reaction time, which can be observed more clearly in Figure 3b insert. From the color evolution and XRD results, we can conclude that the coordination interaction between BMII and Pb(II) changes the dynamics of the interfacial reaction between PbI2 and MAI.42 The PbI2 film is composed of a hexagonally close-packed Pb plane sandwiched between two layers of iodide ions.40, 43

During the interfacial chemical reaction, the organic ammonium ions MAI have to insert into the inorganic framework, and drive PbI6 octahedrons to change from edge-sharing mode to the core-sharing one.23,

44-47

However, the coordination

interaction between PbI2 and BMII introduces a ligand-exchange process before the reaction between MAI and PbI2, during which the MAI must replace the BMII first 7

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for the further reaction.47-50 Even though this additional replacement process undoubtedly makes the interfacial chemical reaction between MAI and PbI2 more complicated, it could provide a better control of the perovskite formation by decelerating the interfacial reaction.51-52 There are numerous literatures reporting that the rapid formation of perovskite results in the PbI2 residual encapsulated inside the perovskite film.23, 51, 53-54 In contrast, the smooth reaction between PbI2 and MAI caused by BMII is beneficial to reducing the residual PbI2. It should be mentioned that excess BMII will prevent the full conversion and lead to residual PbI2 in the film in 120 s reaction time, which is proved by the XRD results of complexes with 5% and 10% BMII (Figure S1 and S2). 

Figure 2. (a) Photographs of PbI2 films collected at different times after being dipped into the isopropanol solution of CH3NH3I at 80 °C. (b) The peak-area ratio between the (001) peak of PbI2 at 12.6° and the (110) peak of MAPbI3 at 14.2°of MAPbI3 prepared without and with 1%, 5%, and 10% BMII in different immersing time.

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To figure out the influence of BMII on the perovskite crystal growth during the thermal annealing process, we compared the grains size of perovskite film before and after thermal annealing using the SEM images in Figure 3a. The thermal annealing presented negligible influence on the crystal size of pristine perovskite, however, the crystal size of perovskite with BMII-1% was enlarged from 200 nm to 500 nm by the thermal annealing. As shown in the XRD patterns (Figure 3b) of the MAPbI3 films after thermal annealing, the full-width-at-half-maximum of peaks at 14.2 º is obviously sharpened by BMII, indicating that the MAPbI3 with BMII exhibit larger grain size than the pristine perovskite.55 Moreover, the increased absorbance across the entire absorption region (Figure 3c) of MAPbI3 with BMII are due to more uniform crystal growth and film formation.56 As shown in Figure 3d, the excited state lifetime (τe) of the MAPbI3 is increased from 8.32 ns to 15.96 ns by BMII, further confirming the effect of BMII on improving the crystallization quality of perovskite, which were coincident with reported PL lifetimes of perovskite fabricated by the two-step deposition.57-59 The rise of excited state lifetime was proved to be of importance in the increase of the carrier diffusion length and photovoltaic performance of PSCs.38,60 Thus, we can conclude that the BMII additive enhances the crystallization of MAPbI3 resulting in the increasing of grain size, absorbance and excited state lifetime for the MAPbI3. Generally, the crystal growth of perovskite in the“two-step”method is considered to follow the classical coarsening mechanism of solid-state grain growth.23,61 The introducing liquid phase between grains may lead to the Ostwald ripening coarsening 9

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process during the thermal annealing.19, 62 According to the classical mechanism of liquid-mediated Ostwald ripening coarsening, the smaller crystals with higher chemical potential are easy to dissolve in the liquid phase, forming a supersaturated liquid microdomain around larger grains.63-64 The vapor of dimethyl formamide and ethanol were reported to form polar liquid phase between perovskite grains via condensation to significantly enlarge the grain size during the thermal annealing.38, 65 In this work, the nonvolatile BMII provides a liquid microdomain in the postdeposition MAPbI3 films, and thus promotes the grain growth according to the Ostwald ripening coarsening mechanism.

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Figure 3. (a) SEM images are the top surfaces of perovskite film before (Top row) and after (Bottom row) thermal annealing at 90 °C for 30 min formed without (left) and with (right) BMII-1% additive. (b) XRD patterns, (c) UV-vis absorption spectra and (d) transient photoluminescence decay (excitation at 405 nm and emission wavelength at 778 nm) of MAPbI3 films without and with BMII additive. The photovoltaic performance of PSCs with and without BMII was compared for the further investigation of the effect of BMII. The fabrication processes of BMII-0% and BMII-1% solar cells were carefully optimized by controlling the immersing time of PbI2 in CH3NH3I solution (Figure S3 and Table S1) and the heating time of perovskite films (Figure S4 and Table S2). And the high-performances of both solar cells were obtained at the immersing time of 120 s and the heating time of 30 min. The corresponding the current density-voltage (J-V) curves (Figure 4a), as well as the open-circuit voltage (Voc), short-circuit current density (Jsc), fill factor (FF), and PCE of devices are summarized in Table 1. The reference device without BMII showed a PCE of 12.6% with a Voc of 0.96 V, a Jsc of 19.4 mA/cm2 and a FF of 0.68, while that with 1% BMII exhibited a significantly enhanced PCE of 15.6% with a Voc of 1.04 V, a Jsc of 21.6 mA/cm2 and a FF of 0.70. The PSCs containing 1% BMII showed a PCE rise of ∼20% compared to the reference solar cell. However, the devices with BMII-1% exhibit more significant hysteresis behavior than the BMII-0% one (Figure S5), which may be caused by the aggravated migration of iodide ions due to the excess iodide ions provided by BMII.66-67 The incident photon-to-electron conversion efficiency 11

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(IPCE) in Figure 4b confirms the Jsc increase, in which the integrated Jsc for the devices with and without BMII-1% are 19.3 and 17.4 mA/cm2, respectively. The efficiency distributions of devices with and without BMII-1% are shown in Figure 4c and summarized in Table 1. The average efficiency of devices with BMII-1% is much higher than those without additive, which reveals that the reproducibility of BMII greatly improves the performance of PSCs. In order to further investigate the effect of the additive on the charge-recombination processes of the PSCs, the open-circuit photovoltage decay measurement was performed. And the electron lifetime of the cell with BMII-1% is longer than that of BMII-0% one (Figure 4d), suggesting the efficient effect of BMII on suppressing the charge recombination of PSCs.56 This increased electron lifetime can be well ascribed to the improved quality of perovskite film resulted from the multifunctional effects of BMII. In addition, we investigated another methylimidazolium iodide IL with different alkyl chain lengths as the additives, including 1, 3-dimethylimidazolium iodide (DMII) and 1-hexyl-3-methylimidazolium iodide (HMII). The IL additives with shorter or longer alkyl chains could not improve the PCE of solar cells as efficiently as BMII (Figure S6 and Table S3). We deduced that the HMII with long alkyl chains exhibited more difficulty in breaking the close packing of PbI2 layers compared with DMII and BMII with short alkyl chains. Moreover, the HMII may cause more difficulty in ligand exchange and larger deceleration of interfacial chemical conversion, due to its larger molecular size compared with DMII and BMII.

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However, the DMII with short alkyl chains can`t facilitate the grain coarsening of perovskite crystal effectively.

Figure 4. (a) Current density-voltage curves of PSCs measured under standard AM 1.5G illumination with input solar power of 100 mW cm-2, (b) incident photon-to-electron conversion efficiency (IPCE) spectra and IPCE date-based integrated Jsc, (c) the distribution of the PCE and (d) open-circuit photovoltage decay curves from devices fabricated with and without BMII additive.

Table 1. Photovoltaic properties for PSCs fabricated with and without 1% BMII additives.

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Voc/V

Jsc/mA cm-2

FF

PCE/%

BMII-0%

0.96

19.40

0.68

12.58

BMII-1%

1.04

21.58

0.70

15.60

Cells

The results and analysis above systematically proves that the BMII acts as an additive to globally control three stages of the perovskite fabrication via “two-step” method, including the preparation of PbI2 precursor film, interfacial chemical conversion, and grain growth. As described in the scheme 1, in the first stage, BMII breaks the van der Waals interaction between PbI2 layers, and thus significantly improves the solubility of PbI2. This solubility improvement was proved to promote the fabrication of PbI2 film with high uniformity and coverage. During the second stage of interfacial chemical reaction, BMII remaining between PbI2 layers benefits the diffusion of MAI into PbI2 layer. Besides, MAI has to replace BMII through ligand exchange before react with PbI2, which reduces the reaction rate to improve the controllability of interfacial chemical reaction. Therefore, the effects of intercalation and ligand exchange provided by BMII can reduce the PbI2 residual in the post-deposited MAPbI3 film. In the third stage of grain growth by thermal annealing, BMII with high boiling point remains inside the perovskite film and forms high-polarity liquid microdomain between grains. The liquid-mediated grain growth resulted from BMII drives small grains of perovskite to disappear and large grain to further coarsen. As a result, the addition of BMII with the optimized amount was 14

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proved to efficiently improve the crystallinity of perovskite. In addition, this improved crystallinity of perovskite correspondingly increased the carrier lifetime of PSCs as indicated by the results of Voc decay measurement, and thus enhanced photovoltaic parameters of PSCs. Our idea based on the global modification of each stage during the fabrication of perovskite should be essential for the further exploration of novel method for the high-quality perovskite materials and high-performance PSCs.

Schema 1. (a) Schematic of the proposed mechanism of perovskite formation controlled by the BMII. CONCLUSIONS In summary, we successfully modified each step in sequential deposition utilizing a single IL BMII as a multifunctional-effect additive, in order to obtain high-quality MAPbI3 film for the perovskite photovoltaic. BMII was confirmed to intercalate into the PbI2 layers, and disturb the close packing in the lamellar PbI2. Thus, the dissolubility of PbI2 in polar solvent was largely increased, which further improved 15

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the uniformity of PbI2 precursor film. And BMII could slow down the rate of interfacial chemical reaction, which was associated to the additional ionic exchange process between BMII and MAI precursor due to the occupation of reaction site of PbI2 by IL. The decelerated reaction promoted the complete conversion of PbI2 to perovskite. Furthermore, the BMII brought a liquid domain for the grains growth of perovskite, generating significant increase of crystalline size. We finally improved PCE of PSCs from 12.6% of the reference cell to 15.6% of the BMII-assisted cells, providing a simple and efficient strategy to improve the performance of PSCs. Our idea about the global control of the perovskite fabrication provides a scientific guide on rational improvement of perovskite-film quality and could also be utilized in other perovskite optoelectronic devices. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge. Figures and images as detailed in the text. References. (66,67) AUTHOR INFORMATION

Corresponding Author * E-mail address: [email protected] (X.Zhang), [email protected] (Y.Wang).

Notes 16

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The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the Natural Science Foundation of China (Grant No. 51602047, 51372036, 91233204), the Key Project of Chinese Ministry of Education (No. 113020A), and the 111 project (No. B13013). REFERENCES 1 Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskites as Visible-Light Sensitizers for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050-6051. 2 Jeon, N. J.; Noh, J. H.; Yang, W. S.; S.

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30 Mabrouk, S.; Dubey, A.; Zhang, W.; Adhikari, N.; Bahrami, B.; Hasan, M. N.; Yang, S.; Qiao, Q.Increased Efficiency for Perovskite Photovoltaics via Doping the PbI2 Layer. J. Phys. Chem. C 2016, 120, 24577-24582. 31 Colella, S.; Masi, S.; Aiello, F.; Listorti, A.; Balzano, F.; Altamura, D.; Giannini, C.; Caliandro, R.; Uccello-Barretta, G.; Rizzo, A. Connecting the Solution Chemistry of PbI2 and MAI: a Cyclodextrin-Based Supramolecular Approach to the Formation of Hybrid Halide Perovskites. Chem. Sci. 2018, Accepted Manuscript, DOI: 10.1039/C7SC05095J. 32 Zhou, Y.; Yang, M.; Vasiliev, A. L.; Garces, H. F.; Zhao, Y.; Wang, D.; Pang, S.; Zhu, K.; Padture, N. P. Growth Control of Compact CH3NH3PbX3 Thin Films via Enhanced Solid-State Precursor Reaction for Efficient Planar Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 9249-9256. 33 Bi, C.;

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35 Numata, Y.; Kogo, A.; Udagawa, Y.; Kunugita, H.; Ema, K.; Sanehira, Y.; Miyasaka, T. Controlled Crystal Grain Growth in Mixed Cation−Halide Perovskite by Evaporated Solvent Vapor Recycling Method for High Efficiency Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 18739-18747. 36 Wu, W. -Q.; Chen, D.; McMaster, W. A.; Cheng, Y. -B.; Caruso, R. A. Solvent-Mediated Intragranular-Coarsening of CH3NH3PbX3 Thin Films toward High-Performance Perovskite Photovoltaics. ACS Appl. Mater. Interfaces 2017, 9, 31959-31967. 37 Xiao, S.;

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