Controlling the Morphology of OrganicâInorganic Hybrid Perovskites

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Controlling Morphology of Organic-Inorganic Hybrid Perovskite through Dual Additive-Mediated Crystallization for Solar Cell Application Seunghwan Bae, Jea Woong Jo, Phillip Lee, and Min Jae Ko ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b03929 • Publication Date (Web): 19 Apr 2019 Downloaded from http://pubs.acs.org on April 19, 2019

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ACS Applied Materials & Interfaces

Controlling Morphology of Organic-Inorganic Hybrid Perovskite through Dual Additive-Mediated Crystallization for Solar Cell Application Seunghwan Bae,†,§ Jea Woong Jo,‡,§ Phillip Lee,*,† and Min Jae Ko*,# †

Photo-Electronic Hybrids Research Center, Korea Institute of Science and Technology

(KIST), Seoul 02792, Republic of Korea. E-mail: [email protected]

‡ Department

of Energy and Materials Engineering, Dongguk University-Seoul, Seoul 04620,

Republic of Korea. #

Department of Chemical Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-

gu, Seoul 04763, Republic of Korea. E-mail: [email protected] §

S. Bae and J.W. Jo contributed equally to this work.

KEYWORDS: organic-inorganic hybrid perovskite, solar cells, solution-process, morphology control, crystallization

Abstract: To realize a high-efficiency perovskite solar cell (PSC), it is critical to optimize the morphology of the perovskite film for a uniform and smooth finish with large grain size during film formation. Using a chemical compound as an additive to the precursor solution has recently been established as a promising method to control the morphology of perovskite film. In this study, we propose a new method to achieve an improved

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morphology of methylammonium lead iodide perovskite film by simultaneous addition of dimethyl sulfoxide (DMSO) and methoxyammonium salt (MeO) (dual additives). We demonstrated that an appropriate amount of the MeO additive helps the precursors form a stable intermediated PbI2-DMSO-adduct during film formation and enlarges the perovskite grains by retarding the kinetics of conversion of the adduct to perovskite. Furthermore, we experimentally observed that the optical bandgaps and crystal structures of perovskite films are reasonably unaffected by the MeO additive because MeO is almost eliminated during annealing. By optimizing the amount of MeO, we achieved improved device performances of the PSCs with a high power conversion efficiency of 19.71% that is ~15% higher than that obtained for the control device (17.15%).

INTRODUCTION Solar cells using organic-inorganic hybrid perovskite (OHP) as a light absorber have been enormously explored considering that OHP exhibits high absorption coefficient, fast charge transport, high dielectric constant, and long charge carrier lifetime, among other features.1–6 In the past five years, there have been significant developments in materials, processes, and device structures leading to a rapid improvement in the power conversion efficiency (PCE) of perovskite solar cells (PSCs) that has exceeded 23%, according to recent reports.7–20 To realize a high-efficiency PSC, it is critical that the OHP layer has a large grain size apart from a uniform and smooth film. This morphology of OHP helps enhance the photo-induced charge transport and suppress the leakage current, thereby increasing the short-circuit current (JSC) and fill factor (FF) of a PSC.21–25 Several approaches such as solvent annealing and


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methylammonium iodide (MAI) gas treatments,26,27 additional precursor soaking,28,29 and modification of Pb precursor30,31 have been reported to control the morphology of OHP. These studies reveal that it is necessary to suppress undesired precipitation of perovskites and offer longer time for the diffusion of precursor ions and molecules in order to promote grain growth and reduce defects in OHP films. Recently, an additive-mediated method that modifies the precursor solution by adding a chemical compound has emerged as an alternative, simple, and direct way to control the OHP morphology.32–35 For manufacture of PSCs, OHP films are prepared using a solution-based coating method (spin-coating, spray-coating, or slot-die coating) followed by an annealing process that removes the solvent and crystallizes the OHP. Thus, the nucleation and growth of perovskites are highly influenced by the solubility of precursors and the evaporation rate of the solvents in the processing solution.22,36–38 In additive-mediated method, the role of additive is the retardation of the precipitation of precursors during crystallization of OHP. Therefore, in order to perform effectively, additive should have a strong interaction with precursors and be removed through decomposition and/or evaporation during the process.32–34 Among the various additives including 1,8-diiodooctane,39 γ-butyrolactone,2,3,40 thiourea,33 and ammonium chloride41 that have been experimented, dimethyl sulfoxide (DMSO) has attracted much attention recently. During the conversion of the precursor solution to perovskites, the DMSO interacts with Pb2+ cation forming PbI2-DMSO adduct that stabilizes the PbI2 precursor against precipitation.32,34 However, as the DMSO evaporates during the annealing of the spin-coated film at 50−200 °C, the adduct gets rapidly transformed and precipitated to PbI2, probably leading to insufficient homogeneity and grain growth in OHP film. Therefore, either a precise control of annealing or an additional post-treatment needs to be ensured in the preparation of OHP films using DMSO additive to achieve large grain



size.34,35,42,43 In this study, we proposed a new method to improve the morphology (larger grain size), whereby we used an additive known as methoxyammonium chloride salt (MeO) together with the DMSO additive (Figure 1a) in the precursor solution. The use of dual additives helped modulate the elimination of the adduct during the fabrication of methylammonium lead iodide (MAPbI3)-based OHP films. We observed that the MeO participates in the adduct thereby retarding the conversion of adduct to OHP; however, the new method does not affect the bandgap and crystal structure of perovskite because MeO is removed effectively during annealing. We realized large grain size in MAPbI3 film by optimizing the amount of MeO. As a result, we improved the device performances of the PSCs achieving a higher PCE of 19.71% compared with the control device prepared without MeO (17.15%).

RESULTS AND DISCUSSION MAPbI3 OHP films were fabricated by following the one-step method reported by Park group, where N,N-dimethylformamide (DMF), DMSO, and diethyl ether were respectively used as a processing solvent to dissolve precursors, an additive to form adduct, and a dripping anti-solvent to wash DMF and freeze the constituents.32 We chose MeO as the additional additive because it is highly soluble in DMF solvents and can be eliminated by heat treatment. The thermogravimetry analysis (TGA) graph in Figure 1b shows the decomposition of MeO at 100−200 °C, which is a similar range used for the thermal treatment of OHP films.7–19 Furthermore, it has been established that more stable adduct could be formed when DMSO and ammonium salt exist simultaneously in PbI2 solution and metoxyammonium group has the interaction with various kinds of metallic elements.32,44,45 Accordingly, we hypothesized that MeO could be added for participation in the formation of PbI2 adduct thereby retarding the


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conversion from adduct to PbI2 during the film deposition by changing the stability of PbI2 adduct. To investigate the effects of MeO on the formation of OHP films, we prepared the precursor solutions by adding different amounts of MeO and denoted the variants as MeO0, MeO5, MeO10, MeO15, and MeO20 with PbI2:MeO molar ratios of 1:0, 1:0.05, 1:0.10, 1:0.15, and 1:0.20, respectively. Figure 1c shows the color change of the films of the precursor solutions spin-coated on glass substrates during annealing. All films irrespective of the MeO amount showed the color change from pale yellow (adduct) to dark brown (perovskite),32–34 indicating the formation of perovskites (OHP). Subsequently, the formation of OHP was further confirmed by UV−Vis spectroscopy measurement (Figure S1) whereby all films exhibited similar absorption spectra with an identical band gap of 1.6 eV that is typical of an MAPbI3 OHP film.46 It should be noted here that the rate of color change to dark brown slowed down gradually as the MeO amount was increased, indicating the retardation of conversion from adduct to perovskite as an effect of the addition of MeO. The microscopic morphologies of OHP films were examined by scanning electron microscopy (SEM), as shown in Figure 2a and S2, and the distributions of grain size were summarized in Figure 2b. When the amount of MeO additive was increased from 0% to 5% and 10%, the average grain size in the OHP films was observed to increase from 140 to 321 and 366 nm, respectively. This is possibly attributed to the prolonged grain growth due to the delayed conversion from adduct to OHP as an effect of MeO additive. However, higher amounts of MeO did not lead to further increase of grain size as observed from MeO15 and MeO20 that exhibited reduced average grain sizes of 200 and 126 nm, respectively. We attributed this phenomenon to the decomposition of MAI (Figure 2d) as explained here: (1) firstly, a large amount of MeO allows PbI2 to remain in the form of adduct



for an extended time without conversion to perovskite or original PbI2; (2) the consumption of MAI through perovskite formation by combining with PbI2 slows down; (3) this exposes the MAI to heat and air for a longer time during annealing thereby decomposing it partially;47–50 (4) the grain growth of perovskite is limited due to non-stoichiometric condition with a reduced amount of MAI, and an excess amount of PbI2 is precipitated on the grain boundary. In this context, we performed an X-ray diffraction (XRD) analysis to identify the crystal structure and PbI2 in the OHP films prepared using different amounts of MeO additive. As shown in Figure 2c, all OHP films exhibited similar diffraction patterns that are usually observed in a tetragonal perovskite structure of MAPbI3.7,18 It implies that MeO does not affect the lattice parameter and crystal structure by replacing the site of MA cation. Moreover, any peaks shown in the XRD pattern of MeO powder (Figure S3a) were not observed in OHP films, indicating the effective removal of MeO during the process for OHP formation. On the other hand, an intensive and sharp PbI2 peak at 12.7° appeared in the diffraction patterns of MeO15 and MeO20 films, whereas this peak was not observed for MeO0, MeO5, and MeO10. Thus, we confirmed that large amounts of MeO generate a non-stoichiometric condition causing excessive precipitation of PbI2 in the OHP films. For an in-depth understanding of the role of MeO additive, we carried out Fourier transform infrared spectroscopy (FT-IR) of the intermediate phase powders (Figure 3a and S3b). The stretching vibration of S=O (ν(S=O)) of MeO0 was shifted to lower wavenumber of 1020 cm–1 compared with that of DMSO (1040 cm–1), indicating that the bond strength of S=O is decreased due to the formation of adduct. Further decrease of bond strength of S=O was observed in MeO10, and its ν(S=O) peak appeared at 1018 cm–1. This observation is suggestive of an increased interaction between DMSO and PbI2 in the presence of MeO additive.32 The formations and structures of adducts were investigated by measuring XRD of intermediate phase powders (Figure 3b). The sample from MeO0 showed three diffraction peaks at 6.49,


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7.16, and 9.13°, which are similar to the previously reported adduct with DMSO.32–34 Interestingly, as the contents of MeO was increased, the peaks of adduct were gradually shifted to higher angles (Figure 3c). It means that a more compact adduct is formed when MeO additive is added. The thermal stability of the adduct was identified by TGA (Figure S3c). The temperature at 95% residual weight was increased from 90 °C for MeO0 to 107, 114, 118, and 120 °C for MeO5, MeO10, MeO15, and MeO20. It was observed that the degradation was delayed increasingly with the amount of MeO. This phenomenon revealed that more stable adduct is developed with dual DMSO and MeO additives, attributed to the increased interaction between DMSO and PbI2 and the improved structure (more compact). Consequently, we suggested that the retardation of conversion from adduct to perovskites, when MeO is added, originates from the formation of more stable adduct. Additionally, the MeO additive did not lead to the improvement in the device performances when DMSO did not exist in the processing solvent due to the absence of the intermediate state (Figure S5). The MeO salt used in this study consists of methoxyammonium and chloride ions, and it is possibly suggested that the increase of MAPbI3 crystallite size is attributed to not only MeO but also chloride content. However, the variation of morphologies in OHP films affected by chloride ions have been typically shown when large enough amounts (molar ratio of chloride salt to PbI2 > ~0.3) of chloride was introduced in precursor solution and the small contents of chloride have been demonstrated not to induce the significant increase of OHP crystallite size.51–55 In our study, the increased grain size in the OHP film was observed when the small amounts of MeO (MeO/PbI2 ≤ 0.10) were used and therefore we carefully suggested that the improvements of OHP morphology mainly arise from methoxyammonium cation rather than chloride anion. Additionally, when secondary ion mass spectrometry (SIMS) was measured, the only few amounts of chloride ions (Cl/Pb < 1/1000) were identified in the OHP films, indicating that the chloride ions were almost eliminated during the deposition process of OHP



films. The influences of the MeO additive on solar cell performances were investigated by fabricating PSCs with a normal structure of F-doped SnO2 (FTO)/compact-TiO2 layer/mesoporous-TiO2 layer/MAPbI3/2,2′,7,7′-tetrakis(N,N-p-dimethoxyphenylamino)-9,9′spirobifluorene (spiro-MeOTAD)/Au. Averages of the device properties were calculated from the data pertaining to more than 40 devices (Table 1), and the current−voltage (J−V) curves of the champion PSCs prepared using various amounts of MeO are shown in Figure 4a. The control device without MeO (MeO0) showed an average PCE of 15.59% with an open circuit voltage (VOC) of 1.07 V, a short-circuit current density (JSC) of 21.42 mA cm−2, and a fill factor (FF) of 0.68. When small amounts of MeO were added (MeO5 and MeO10), all device parameters were enhanced, especially the average PCEs were improved to 16.51% (VOC = 1.08 V, JSC = 21.53 mA cm−2, and FF = 0.71) and 16.86% (VOC = 1.10 V, JSC = 21.59 mA cm−2, and FF = 0.71) for MeO5 and MeO10, respectively, resulting from the enlarged perovskite grain that is beneficial for facilitating charge transport and for suppressing charge recombination owing to the reduced grain boundaries.26 However, higher amounts of MeO affected the device performance as observed from MeO15 and MeO20 that exhibited lower average PCEs of 14.33% (VOC = 1.05 V, JSC = 20.37 mA cm−2, and FF = 0.67) and 13.46% (VOC = 1.02 V, JSC = 19.69 mA cm−2, and FF = 0.67), respectively, because of smaller grain size and the precipitation of PbI2 in the OHP films. Therefore, the highest PCE of 19.71% with a VOC of 1.13 V, a JSC of 22.89 mA cm−2, and an FF of 0.76 was achieved using MeO10, which was ~15% higher than the best PCE obtained in the control device, MeO0 (17.15%). This PSC using MeO10 also showed the reduction in hysteresis compared with the control device (Figure S4). Figure 4b shows the external quantum efficiency (EQE) spectra of PSCs. MeO10 PSC showed higher EQEs than the control device (MeO0) in the wavelength range 350–750 nm, which contributes to the higher JSC of MeO10 PSC.


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The different recombination properties of PSCs using MeO0 and MeO10 were confirmed using electrochemical impedance spectroscopy (EIS), as shown in Figure 4c and Table S1. To fit the Nyquist plots, the equivalent circuit including series resistance (RS), resistance associated with interfacial contacts (R1), and resistance associated recombination in OHP layer (R2) was introduced.56,57 R1 of PSCs using MeO0 and MeO10 exhibited the similar resistances of 6.51 and 5.55 kΩ, indicating the comparable interfacial charge transfer between OHP layer and selective contacts in the devices. However, MeO10 had ~6 times higher R2 resistance than MeO0 and it demonstrated the OHP layer of MeO10 is more resistant against the charge recombination than that of MeO0. This improved charge recombination characteristics was attributed to the increased grain size of OHP (Figure 2a) and would lead to the enhanced photovoltaic performance of MeO10 (Figure 4a). The reduction of charge recombination during the device operation was also supported by the shunt resistance of PSCs and MeO10 showed higher shunt resistance than the control MeO0 device (Table S2).

CONCLUSIONS In this study, we presented that adding DMSO and MeO simultaneously (dual additives) to a precursor solution is a promising strategy to control the morphology of OHP. We found that the MeO participates in the adduct during film formation and enlarges the OHP grains by retarding the conversion of adduct to perovskite. The experiments confirmed that MeO is almost eliminated during annealing while the optical bandgap and crystal structure of OHP are not affected by the addition of MeO additive. For an optimized amount of MeO (MeO10), the PSC showed a highly improved PCE of 19.71% compared with the control device prepared without the MeO additive (17.15%). The nature of MeO, which enhances the interaction between DMSO and PbI2 and decomposes during annealing, is suggestive of implementing an



additive-based concept to control the morphology of OHP films in solar cell applications. Overall, we believe that this study will provide a new insight into the film-forming mechanisms and morphology controls in perovskite-based systems universally.

EXPERIMENTAL SECTION Materials. Methoxyamine hydrochloride (MeO, 98%), DMSO (anhydrous, ≥ 99.9%), bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI, 99.95%), 4-tert-butylpyridine (tBP, 96%), and chlorobenzene (CB, anhydrous, 99.8%) were purchased from Sigma Aldrich, and methylammonium iodide (MAI, 99.98%) was purchased from Dyesol. Lead(II) iodide (PbI2, 99.9985%) and DMF (anhydrous, 99.8%) were purchased from Alfa Aesar, and spiroMeOTAD (SOLARPUR® SHT-23, ≥ 99.9%, Merck) was purchased from Merck. Ethanol (anhydrous, 99.8%) was purchased from Carlo Erba, and TiO2 blocking solution was purchased from ShareChem. All materials were used without any purification. Perovskite precursor solutions were prepared by mixing x mmol of MeO (x= 0, 0.05, 0.1, 0.15, and 0.2), 1 mmol of MAI, and 1 mmol of PbI2 in a vial of 10 ml, and the mixture was dissolved in a DMSO/DMF cosolvent (71 L of DMSO and 636 L of DMF). The solutions were stirred for 3 h and filtered with 0.45-m PTFE syringe filter prior to their usage. Spiro-MeOTAD solution was prepared using 56 mg of spiro-MeOTAD, 5.6 mg of Li-TFSI, 30 L of tBP, and 1 mL of CB. Device fabrication. Pre-patterned FTO glass (Pilkington, TEC8) was cleaned by stepwise sonication in detergent, deionized water, and isopropanol for 30 min in each case followed by drying in an oven. After complete drying, FTO glass was treated with UV-ozone for 20 min. TiO2 blocking solution was diluted to 0.15 M using ethanol and spin-coated on the substrate at 3000 rpm for 30 s. For mesoporous TiO2 layer, TiO2 paste (Dyesol, 30NR-D) was diluted in ethanol (150 mg ml−1) and spin-coated on top of TiO2 blocking layer at 4000 rpm for 20 s. TiO2


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blocking layer and mesoporous layer were sintered at 500 °C for 1 h, respectively. MAPbI3 perovskite was fabricated by solvent engineering.32 Perovskite precursor solution was spincoated at 1000 rpm for 5 s followed by 4000 rpm for 15 s, and 0.5 ml of diethyl ether was dropped over the substrate when 7 s was left for the completion of the second stage. After the spin-coating, the substrate was heat-treated at 60 °C for 1 min followed by 100 °C for 5 min. Spiro-MeOTAD solution was spin-coated on the perovskite layer at 2500 rpm for 20 s after the substrate was cooled to room temperature. Gold electrode (80 nm) was deposited in a highvacuum chamber (< 10–6 torr) using thermal evaporator. Characterization. The morphologies of the MAPbI3 films were characterized by SEM (Nova NanoSEM 200, FEI), and the optical properties of the films were examined using a UV−Vis spectrometer (Lambda 35, PerkinElmer). The crystal structures of MAPbI3 and its adduct were characterized by an X-ray diffractometer (Dmax 2500, Rigaku) using Cu-K radiation (= 1.5418 Å) at a scan rate of 2° min–1. The adduct films were prepared by the spincoating process that was used for device fabrication, but these samples were not thermally treated in order to obtain the intermediated state. The infrared spectra were characterized using an FT-IR spectrometer (FT-IR, Spectrum 100, PerkinElmer). The thermogravimetric analysis was performed using TGA Q50 (TA Instruments), whereby the heating was at a ramp rate of 10 °C min–1. The photocurrent–voltage curves were collected using Keithley 2400 source meter under AM 1.5G (1 sun, 100 mW cm–2) which was calibrated with an NREL-calibrated Si solar cell with a KG-1 filter. The light source was a Xenon lamp of 180 W in a solar simulator (Denso YSS-50A, Yamasita). The active area was determined by a mask having an aperture (0.12 cm2). MAPbI3 solar cells were measured at a scan rate of 10 mV s–1. EQE was measured using an incident-photon-to-current conversion efficiency equipment (PV measurement, Inc). Electrochemical spectra were measured for frequencies ranging from 1.0 MHz to 100 mHz



under dark condition with short circuit condition using an impedance analyzer (Solartron 1287).

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at http://pubs.acs.org. UV−Vis absorption spectra and SEM images of perovskite films, IR spectra and TGA spectra of PbI2-adducts, J–V hysteresis of PSC, Fitting parameters for EIS

AUTHOR INFORMATION Corresponding Author *Email: [email protected] (P. Lee) *Email: [email protected] (M. J. Ko)

Author Contributions §

S. Bae and J.W. Jo contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work was supported by the Technology Development Program to Solve Climate Changes (2015M1A2A2056824 and 2017M1A2A2087353), the Global Frontier R&D Program on Center for Multiscale Energy System (2012M3A6A7054856), and Research Program (2018R1A2B2006708) funded by the National Research Foundation under the Ministry of Science and ICT, Korea; This work is also supported by the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20173010013200 and No. 2018201010636A) and KIST institutional program.

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Table 1. Photovoltaic parameters of PSCs prepared with various contents of MeO HTL

VOC,best (VOC,ave) (V)

JSC,best (JSC,ave) (mA cm–2)

FFbest (FFave)

PCEbest (PCEave) (%)

MeO0

1.10 (1.07 ± 0.03)

21.95 (21.42 ± 1.24)

0.71 (0.68 ± 0.05)

17.15 (15.59 ± 1.29)

MeO5

1.10 (1.08 ± 0.02)

22.93 (21.53 ± 0.88)

0.73 (0.71 ± 0.02)

18.41 (16.51 ± 0.93)

MeO10

1.13 (1.10 ± 0.02)

22.89 (21.59 ± 1.05)

0.76 (0.71 ± 0.03)

19.71 (16.86 ± 1.13)

MeO15

1.10 (1.05 ± 0.07)

21.60 (20.37 ± 1.24)

0.72 (0.67 ± 0.08)

17.22 (14.33 ± 2.63)

MeO20

1.08 (1.02 ± 0.05)

22.68 (19.69 ± 1.84)

0.70 (0.67 ± 0.07)

17.03 (13.46 ± 2.78)



Figure 1. MeO additive for controlling the crystallization of MAPbI3 OHP. (a) The chemical structure and (b) TGA curves of MeO salt. (c) Different color changes of perovskite films depending on the amounts of MeO additive during annealing at 60 °C after spin-coating of precursor solutions on glass substrates.


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Figure 2. The effects of MeO additive on the morphologies of MAPbI3 OHP films. (a) SEM images of perovskite films prepared using MeO0, MeO10, and MeO20. (b) Grain size distribution and (c) XRD spectra of perovskite films prepared using different amounts of MeO additive. (d) Schematic representation for the formation of MAPbI3 perovskite films using different amounts of MeO additive during annealing.



Figure 3. Characteristics of intermediated phases observed during the conversion from the precursors to MAPbI3 OHP films. (a) FT-IR spectra of DMSO and PbI2-adducts prepared using MeO0 and MeO10. (b) XRD spectra and (c) the lattice parameter changes of adduct films prepared using different contents of MeO additive.


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Figure 4. Enhanced performances of PSCs by introducing MeO as an additive. (a) J–V curves of PSCs using five different amounts of MeO additive. (b) EQE spectra and (c) electrochemical impedance spectra of PSCs using MeO0 and MeO10. The calculated JSC values from EQE spectra were 20.1 and 21.4 mA cm–2 for MeO0 and MeO10 devices. The inset of Figure 4c shows the equivalent circuits for fitting Nyquist plots.



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Controlling the Morphology of OrganicâInorganic Hybrid Perovskites

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Controlling the Morphology of OrganicâInorganic Hybrid Perovskites