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An Elegant Molecular Iodine/Anti-solvent Solution Engineering to Tune the Fermi Level of Perovskite CH3NH3PbI3 Ya Xu, Jianyong Feng, Tao Yu, Jiawei Li, Xiaopeng Han, Huiting Huang, Zhaosheng Li, and Zhigang Zou ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00850 • Publication Date (Web): 22 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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ACS Applied Energy Materials

An Elegant Molecular Iodine/Anti-solvent Solution Engineering to Tune the Fermi Level of Perovskite CH3NH3PbI3 Ya Xu,1,2 Jianyong Feng,5 Tao Yu,*1,2,3,4 Jiawei Li,1,2 Xiaopeng Han,1,2 Huiting Huang,5 Zhaosheng Li,5 Zhigang Zou1,2,3,4

1. National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, P.R.China 2. Ecomaterials and Renewable Energy Research Center (ERERC) at School of Physics, Nanjing University, Nanjing 210093, P.R.China 3. Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P.R.China 4. Jiangsu Provincial Key Laboratory for Nanotechnology, Nanjing University, Nanjing 210093, P.R.China 5. College of Engineering and Applied Science, Nanjing University, Nanjing 210093, P.R.China

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KEYWORDS CH3NH3PbI3, perovskite solar cell, Fermi level tuning, molecular iodine, iodine vacancies

ABSTRACT

It is important to change the semiconducting behaviors of CH3NH3PbI3 (MAPbI3) perovskites to meet the needs of different applications, which requires efficient strategies for Fermi level tuning in MAPbI3. Previous studies reported that Fermi level of MAPbI3 could be down-shifted when exposed to I2 vapor; such a down-shift of Fermi level could be rationalized by the filling of iodine vacancies (VI), a donor-type defect in MAPbI3. However, it is difficult to control the amount of incorporated I2 into MAPbI3 matrix; meanwhile, MAPbI3 degradation may also occur following I2 vapor exposure. Herein, we propose a controllable and facial method for iodine manipulation and Fermi level tuning of MAPbI3, in which molecular iodine dissolved in anti-solvent is applied as the iodine source. After iodine manipulation, the best-performing MAPbI3 solar cell exhibits a power conversion efficiency of 17.7%, as contrast to 15.8% on the pristine MAPbI3. Steady-state and time-resolved photoluminescence measurements indicate that iodine-modified MAPbI3 possesses reduced defect density and enhanced charge-carrier lifetimes. Photoelectron spectroscopy analyses reveal a 120 meV down-shift of Fermi level in iodine-modified MAPbI3, as compared to the control MAPbI3.

INTRODUCTION Organic-inorganic halide perovskite (such as MAPbI3) solar cells over the past years have attracted considerable attention because of the low-cost fabrication methods1-8 and their unique


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optoelectronic properties, such as high absorption coefficients,9 ambipolar charge transport10 and long charge carrier diffusion lengths.11 Generally, MAPbI3-based solar cells operate as p-i-n junctions, with MAPbI3 as the intrinsic (i) film.12 Moreover, MAPbI3 with both p-type and n-type conductivity can be achieved by changing the ratios of methylammonium halide (MAI) and lead iodide (PbI2).10 The tunable p-type and n-type conductivity of MAPbI3 is beneficial for various types of optoelectronic devices, such as p-n or Schottky devices. Therefore, it is important to tune the semiconducting behaviors of MAPbI3 to meet the needs of different applications. Theoretical calculations have shown that point defects of iodine vacancies (VI) are the most likely defects in MAPbI3 films because of their low formation energy.13-15 In addition, some theoretical studies have made progress in either the understanding of or the application of iodine vacancy engineering in MAPbI3.16,17 VI can promote both electronic injection and recombination rates, but overall reduces the power conversion efficiency (PCE) of MAPbI3-based solar cells.16 Moreover, previous study has presented compelling evidence that iodide ions, migrating by a vacancy mechanism, and demonstrated that MAPbI3 undergoes a transition from an extrinsic to an intrinsic ion conductor.17 As a donor-type defect, the presence of VI increases the Fermi level of MAPbI3. On the other hand, excessive VI defects in MAPbI3 may act as recombination centers, the as-assembled solar cells then exhibit largely reduced charge carrier diffusion lengths and opencurrent voltage (VOC). Therefore, the tuning of semiconducting behaviors, as well as achieving desirable PCE in MAPbI3 would require effective strategies to control the concentration of VI. Previous studies have shown that the introduction of triiodide ions (I3- ) can modify the iodide deficiency in MAPbI3, leading to further improvement in PCE values.18,19 Exposing MAPbI3 to I2 vapor, as proposed by several studies,20-22 also achieves Fermi level tuning of MAPbI3 via the reduction of VI.21,22 However, excessive exposure to I2 vapor has been observed to accelerate the



degradation of MAPbI3 to PbI2, indicating the importance of controllable incorporation of I2 into MAPbI3 matrix.20 Considering the effectiveness of molecular I2 in filling VI, it is necessary to explore a controllable and facial method for introducing molecular I2 in MAPbI3, thereby realizing Fermi level tuning and iodide deficiency modification. In this study, we propose a controllable strategy to introduce molecular I2 into MAPbI3, in which molecular I2 dissolved in anti-solvent is applied as the iodine source. By changing the concentration of molecular I2 in anti-solvent, precise iodine manipulation in MAPbI3 is realized. The optimal iodine-modified MAPbI3 solar cell exhibits a PCE value of 17.7%, as contrast to 15.8% on the pristine MAPbI3. The defect density in MAPbI3 is reduced after iodine manipulation, as indicated by time-resolved and stead-state photoluminescence measurements, yielding enhanced charge-carrier lifetimes. A 120 meV down-shift of Fermi level in iodine-modified MAPbI3, as compared to the control MAPbI3, is resolved by photoelectron spectroscopy analyses, indicating successful Fermi level tuning in MAPbI3. EXPERIMENTAL SECTION Preparation of precursor solutions Unless specifically noted, all reagents and materials were purchased commercially from Xi'an Polymer Light Technology Corp. and used as received without further purification. To prepare MAPbI3 perovskite precursor solution, 461 mg of PbI2 and 159 mg of MAI were dissolved in a mixture of 625 μL of DMF and 74 μL of DMSO (purchased from Sigma-Aldrich). Then mixture precursor solutions were stirred at room temperature until clarification in the N2-filled glove box. To prepare anti-solvent precursor solution, molecular iodine was dissolved in diethyl ether (0.50mg/mL) (purchased from Sinopharm Chemical Reagent Co., Ltd) and then anti-solvent


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precursor solution was stirred at room temperature until clarification in the N2-filled glove box. To prepare the hole-transporting-material (HTM) precursor solution, 145 mg of spiro-OMeTAD was dissolved in a mixture of 2 mL of chlorobenzene (purchased from Aladdin Reagents), 35 μL of Bis(trifluoromethane)sulfonimide lithium salt (Li-TFSI) solution (520 mg Li-TSFI in 1 mL acetonitrile) (purchased from Sigma-Aldrich) and 57 μL of 4-tert-butylpyridine (purchased from Sigma-Aldrich). Then the mixture was stirred at room temperature until clarification in the N2filled glove box. Fabrication of MAPbI3 devices First of all, FTO glass slides with sheet resistance of 14 Ω/sq were cleaned by sonication in detergent, deionized water, acetone, and absolute alcohol for 30 min successively and dried by nitrogen stream. The compact TiO2 layer was spin-coating on the pre-cleaned FTO glass and annealed in the furnace at 500℃ for 60 min. Then MAPbI3 perovskite precursor solution was dipped onto the compact TiO2 layer and then spun at 4000 rpm for 30 s. During one-step coating, iodine in diethyl ether (0.50 mg/mL) was slowly dripped on the rotating substrate within the first 5 s. The MAPbI3 film was finally heated at 110℃ for 15 min. Next, the HTM precursor solution was spin-coated on the MAPbI3 layers at 3200 rpm for 30 s. Ag electrode was deposited by using vacuum thermal evaporation method. Characterizations X-Ray diffraction (XRD) patterns were collected on a Rigaku Ultima III X-ray diffractometer (Cu-Kα). A FEI NOVA Nano SEM 230 scanning electron microscopy (SEM) was employed to characterize the morphological properties of the samples. A Varian Carry-50 and PerkinElmer LAMBDA950 Spectrophotometer were used to investigate the absorption properties



of the samples. The PL spectra were conducted at the room temperature on a fluorescence spectrophotometer with the excitation wavelength of 405 nm. XPS analysis was conducted on Thermo Scientific K-Alpha XPS. The J-V measurements of the fabricated MAPbI3 solar cells were carried out on a Keithley 2400 source measurement unit under AM 1.5 illuminations (standard 100 mW/cm2) cast by an Oriel 92251A-1000 sunlight simulator calibrated by the standard reference of a Newport silicon solar cell. RESULTS AND DISSCUSSION Figure 1 shows schematic illustration of the experimental process. The MAPbI3 precursor solution containing MAI and PbI2 was prepared in a mixture of N, N-Dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). Then the precursor solution was spin-coated on the compact TiO2/FTO glass substrates via one-step spin-coating method.8 During one-step spin-coating, I2 in diethyl ether was dripped on the MAPbI3 precursor films. Then the films were annealed at 110℃ for 15 min. It has been reported that iodine dissolves in diethyl ether in the form of molecular I2.23-25 The intensity of absorption peak at 460 nm in UV-vis absorption spectra increases gradually with higher concentrations of molecular I2 in diethyl ether (Figure S1), and can be used to determine the quantity of molecular I2. By simply changing the concentration of molecular I2 in diethyl ether, we can achieve controllable introduction of molecular I2 in MAPbI3. The statistical PCE values for the MAPbI3 solar cells with different I2 concentration in diethyl ether are presented in Figure 2a. The highest PCE is attained when 0.50 mg/mL of molecular I2 in anti-solvent diethyl ether is applied. For simplicity, 0.50 mg/mL of molecular I2 treated MAPbI3 solar cell is labeled as the target device. The current-voltage characteristics of the target device are compared with those of the control device without molecular I2 in diethyl ether.


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As shown in Figure 2b, the target device exhibits a PCE of 17.7% with a short-circuit current density (JSC) of 22.3 mA/cm2, VOC of 1.09 V and a fill factor (FF) of 72.7%. The control device shows a JSC of 22.8 mA/cm2, VOC of 1.06 V and FF of 65.6%, for a PCE of 15.8%. In addition, some statistical photovoltaic parameters for each case from 20 devices are summarized in Figure S2. To further distinguish the improvement of the PCEs, the J-V curves for the control and the target devices were recorded at reverse scan and forward scan under the illumination of 100 mW/cm2, as shown in Figure S3. The detailed performance parameters including JSC, VOC, FF and PCE are shown in Table S1. For the target devices, we find that J-V hysteresis has been improved. Furthermore, we investigated the stabilized current density and PCE for the control and the target MAPbI3 devices, as shown in Figure S4. For the control devices, the photocurrent density stabilized within seconds to approximately 18.32 mA/cm2 at 0.794 V, yielding a stabilized PCE of 14.5%. For the target devices, the JSC increases rapidly to the stabilized value of 19.93 mA/cm2 with the stabilized PCE of 17.5%. To investigate the influence of I2 treatment on the MAPbI3 films, scanning electron microscope (SEM), X-ray diffraction (XRD) and UV-vis absorption spectroscopy measurements were carried out on the control and the target MAPbI3 films. As shown in Figure 3a and 3b, there are no morphological structure and grain size changes observed on the control and the target MAPbI3 films. And there is no obvious change in the orientation of control and target MAPbI3 films (Figure 3c and Figure 3d). Figure 3e shows the XRD patterns of the control and the target MAPbI3 films. For the target MAPbI3 films, the dominant diffraction peaks at 14.6°, 28.9° and 32.3° corresponding to (110), (220) and (310) lattice planes are higher than those of the control ones, which implies better crystallinity. The target MAPbI3 films show similar absorption profiles with the control ones, as given in Figure 3f. Besides, we have not discerned obvious difference in



the Tauc plots of the control and the target MAPbI3 films. The bandgap values extracted from Tauc plots show the same value of 1.60 eV for the control and the target MAPbI3, as shown in Figure S5, which are in a good agreement with previous report.26 To further elucidate the photophysical properties of the control and the target MAPbI3 films, steady-state photoluminescence (PL) measurements were conducted. Clear emission peaks for the control and the target MAPbI3 films appear at ~770 nm (Figure 4a), which are in good agreement with their bandgap values. The target MAPbI3 film exhibits higher PL intensity than that of the control one, indicating lower defect density (such as VI) in the target MAPbI3 films. We also measured the time-resolved PL decay dynamics for the control and the target MAPbI3 films, as shown in Figure 4b. The carrier dynamics derived from the transient PL behavior provides information of charge recombination via nonradiative recombination related to the defect density. The decay curves were fitted to a biexponential equation: Y=A1exp(-t/τ1)+A2exp(-t/τ2).27 The fast decay of PL intensity corresponds to defect-induced nonradiative recombination, while the slow component is attributed to radiative recombination.28 The control MAPbI3 films exhibit a PL lifetime with a fast component of ~4.01 ns and a slow component of ~52.21 ns. Comparatively, the target MAPbI3 films display a PL lifetime of ~4.26 ns and ~78.73 ns. These measurements collectively indicate that the target MAPbI3 films possess less nonradiative recombination and enhanced charge-carrier lifetime, which may be ascribed to reduced VI density. X-ray photoemission spectroscopy (XPS) measurements were carried out for MAPbI3 films to derive chemical composition information. Because of the contamination of the adventitious carbon, the C 1s spectra of MAPbI3 films consist of two peaks. The obtained results of XPS are calibrated using C 1s (284.60 eV) as the reference，as shown in Figure S6. The higher


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binding energy peak is the CH3NH3+ component in MAPbI3 films, while the lower binding energy peak is the adventitious carbon.20,29 For the control MAPbI3 film, the Pb 4f spectrum consists of two peaks at 143.19 and 138.29 eV, these two peaks appear at 143.09 and 138.19 eV in the target MAPbI3 films (Figure S7a). The I 3d spectrum of the control MAPbI3 film consists of two peaks at 630.39 and 618.89 eV. For the target MAPbI3 film, these two peaks occur at 603.49 and 618.99 eV (Figure S7b). For the control and the target MAPbI3 films, the spin-orbit split of the Pb 4f is 4.90 eV while the spin-orbit split of the I 3d is 11.50 eV. Both of the spin-orbit split values are ascribed to Pb-I in the MAPbI3 films.30-33 These XPS spectra indicates all the elements in the control and target MAPbI3 films are in similar chemical environment. The stoichiometric information of MAPbI3 films can also be obtained from XPS measurements. As derived from the XPS spectra, the atomic ratio of I to Pb in the control and the target MAPbI3 films is 2.65 and 2.90, respectively (Table 1). These results suggest that the target MAPbI3 films have a lower VI density. To gain deeper insight into the Fermi level tuning of MAPbI3 by molecular I2 in antisolvent of diethyl ether, XPS-based valence band measurements were carried out. By deducing the photoelectron distribution curves acquired on the control and target MAPbI3 films shown in Figure 5a, the VBM position is at 1.41 eV (Figure S8a) and 1.29 eV (Figure S8b), for the control and the target MAPbI3 films respectively, with respect to the Fermi level. Consequently, the Fermi level of the target MAPbI3 has been down-shifted by 120 meV towards the valence band when comparing with the control one, indicating the successful iodine manipulation and Fermi level tuning in MAPbI3. A schematic illustration of energy level diagram in MAPbI3 is given in Figure 5b, showing a 120 meV downward moving of Fermi level after iodine manipulation in MAPbI3. For the MAPbI3 films with I2 in diethyl ether (1.00mg/mL), we find that the VBM position is at 1.39 eV (Figure S9). In order to understand the position of the Fermi level varied between the



different MAPbI3 films, a full comparison on the XRD and SEM were conducted, as shown in Figure S10. As I2 concentration increased from 0 to 0.50 mg/mL, the intensity of MAPbI3 peaks gradually increased and no peaks corresponding to PbI2 were observed; when I2 concentration exceeded 0.50 mg/mL, the intensity of MAPbI3 peaks decreased, meanwhile the intensity of PbI2 peak increased (Figure S10). The XRD results suggest that excessive I2 incorporation into MAPbI3 can lead to the appearance of PbI2 impurity phase, consistent with a previous study that overexposure of MAPbI3 to I2 vapor would lead to a detrimental transformation of MAPbI3 to PbI2.20 Therefore, appropriate concentrations of molecular iodine in anti-solvent (≤0.50 mg/mL) can be used to shift down the Fermi level of MAPbI3 without the formation of undesirable PbI2; while high concentrations of molecular iodine in anti-solvent (>0.50 mg/mL) would promote the formation of PbI2 impurity phase in MAPbI3. The VBM position of 1.00 mg/mL I2 treated MAPbI3 is 1.39 eV below the Fermi level, showing a 0.1 eV upshift of Fermi level as compared with 0.50 mg/mL I2 treated one (1.29 eV below the Fermi level). This result could be due to the n-type doping behavior in PbI2-rich MAPbI3 films as reported previously.10 The SEM images of MAPbI3 films are shown in Figure S11. As I2 concentration increased from the optimal concentration 0.50 mg/mL to 1.00 mg/mL, the grain sizes decrease. Hence, 0.50 mg/mL is the optimal concentration for the PCE of MAPbI3-based solar cells in our expertiement. All in all, we can precisely control the amount of incorporated I2 into MAPbI3 via solution process in avoid of the degradation of MAPbI3 films, as contrast to I2 vapor treatment ( Table S2). CONCLUSION In conclusion, a controllable and facial method for iodine manipulation and Fermi level tuning in MAPbI3 is developed, in which molecular I2 dissolved in anti-solvent is applied as the iodine source. After iodine manipulation, part of the VI defects have been filled, producing 10 Environment ACS Paragon Plus

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MAPbI3 with the I : Pb ratio closer to stoichiometric 3:1, as well as a 120 meV down-shift of Fermi level. The iodine treated MAPbI3 possesses less nonradiative recombination and enhanced chargecarrier lifetimes, which may be related with reduced VI density. As a result, the optimal iodinemodified MAPbI3 solar cell exhibits a PCE of 17.7%, as contrast to 15.8% on the pristine MAPbI3. This work represents an effective strategy to finely tune the Fermi level and semiconducting behaviors of MAPbI3, which is valuable for solar cell and other optoelectronic applications of halide perovskites.

Figure 1. Schematic illustration of the experimental process for preparing MAPbI3 films via I2 in diethyl ether as anti-solvent during one-step spin-coating.



a

b

Figure 2. (a) Statistical PCE values with different iodine concentration for each case from 20 devices. (b) J-V characteristics for the control and the target MAPbI3 solar cells.


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Figure 3. (a) Surface SEM images of the control MAPbI3 films. (b) Surface SEM images of the target MAPbI3 films. (c) Cross-sectional SEM images of the control MAPbI3 films. (d) Crosssectional SEM images of the target MAPbI3 films. (e) XRD patterns of the control and the target MAPbI3 films. (f) UV-vis absorption spectra for the control and the target MAPbI3 films.



Figure 4. (a) Steady-state PL spectra of the control and the target MAPbI3 films deposited on glass. (b) The time-resolved PL curves for the control and the target MAPbI3 films deposited on glass.


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Figure 5. (a) Valence levels distribution curves acquired on the control and target MAPbI3 films using XPS. (b) The energy level diagram of the control and the target MAPbI3 films. Table 1. Atomic percent of the control and the target MAPbI3 films. C 1s

N 1s

I 3d

Pb 4f

I/Pb

Control film

60.62%

7.81%

22.92%

8.65%

2.65

Target film

43.67%

9.81%

34.58%

11.94%

2.90

ASSOCIATED CONTENT



Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Absorption spectra of iodine, Tauc plots and XPS valence band spectra of the control and the target MAPbI3 films. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported primarily by the National Key Research and Development Program of China (2018YFA0209303) and the National Natural Science Foundation of China (61377051). We thank Zhi Zhu, Wenjing Su, Qingxiao Meng, Jincheng Li, Qian Li and Zhaodong Wang in our SPC group for their informative discussions and experimental and technical assistances. REFERENCE (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.


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Table of Content

We propose a controllable strategy to introduce molecular I2 into MAPbI3, in which molecular I2 dissolved in anti-solvent is applied as the iodine source. There is a 120 meV down-shift of Fermi level in target MAPbI3, as compared to the control MAPbI3.


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Anti-solvent Solution Engineering to

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