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Research Article Cite This: ACS Appl. Mater. Interfaces 2018, 10, 1909−1916

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Grain Boundary Modification via F4TCNQ To Reduce Defects of Perovskite Solar Cells with Excellent Device Performance Cong Liu,† Zengqi Huang,† Xiaotian Hu,‡ Xiangchuan Meng,† Liqiang Huang,† Jian Xiong,§ Licheng Tan,*,† and Yiwang Chen*,† †

College of Chemistry and Jiangxi Provincial Key Laboratory of New Energy Chemistry, Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China ‡ Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), 2 Zhongguancun Beiyi Street, Beijing 100190, China § Guangxi Key Laboratory of Information Materials, School of Materials Science and Engineering, Guilin University of Electronic Technology, 1 Jinji Road, Guilin 541004, China S Supporting Information *

ABSTRACT: Solar cells based on hybrid organic−inorganic metal halide perovskites are being developed to achieve high efficiency and stability. However, inevitably, there are defects in perovskite films, leading to poor device performance. Here, we employ an additive-engineering strategy to modify the grain boundary (GB) defects and crystal lattice defects by introducing a strong electron acceptor of 2,3,5,6-tetrafluoro7,7,8,8-tetracyanoquinodimethane (F4TCNQ) into perovskite functional layer. Importantly, it has been found that F4TCNQ is filled in GBs and there is a significant reduction of metallic lead defects and iodide vacancies in the perovskite crystal lattice. The bulk heterojunction perovskite−F4TCNQ film exhibits superior electronic quality with improved charge separation and transfer, enhanced and balanced charge mobility, as well as suppressed recombination. As a result, the F4TCNQ doped perovskite device shows excellent device performance, especially the reproducible high fill factor (up to 80%) and negligible hysteresis effect. KEYWORDS: grain boundaries, defects, perovskite solar cells, charge transfer, bulk heterojunction, fill factor

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the perovskite film and accelerate the degradation of the perovskite grain.9 Currently, the research mainly focuses on the perovskite crystallization process to obtain a high-quality perovskite film with larger grains and fewer GBs, aiming at reducing defects of perovskite films. Numerous previous studies have revealed that several types of additives, such as hydroiodic (HI) or hydrochloric (HCl) acid,15−17 fullerene derivative,18−20 organic molecules,21,22 polymers,23,24 ionic liquid,25 sulfonate-carbon nanotube,26 inorganic or ammonium salts,27−29 and so on, have been attempted in perovskite precursors to increase grain size and decrease GBs with different functional mechanisms. However, the GBs cannot be completely eliminated because of the existence of polycrystalline perovskite grains. In addition to decreasing the quantities of GBs, reducing the intrinsic defects of GBs is also an effective method. Herein, we employ the additive-engineering strategy to modify GBs defects and crystal lattice defects although it does not improve film morphology. The additive, 2,3,5,6-tetrafluoro-

INTRODUCTION Hybrid organic−inorganic metal halide perovskites have been used as light-absorbing materials in solar cells because of their easy preparation, low cost, and excellent optoelectronic properties.1−5 However, the unwanted defects existing in perovskite films can generate charge trap states, which enhance the nonradiative recombination. The reduced charge-carrier lifetime and energy loss severely deteriorate device performance. Furthermore, the defects can also cause ion migration, hysteresis effect, and device degradation,6−8 which should be addressed to enhance the efficiency and stability of perovskite solar cells (PVSCs). The defects of perovskite films can be attributed to pinholes, vacancies, surface roughness, grain boundaries (GBs), and anomalous crystal lattice.9−13 Morphology control of perovskite films is important in reducing defect states. GBs existing in polycrystalline perovskite films are filled with charge trap states, nonradiative recombination centers, and impurities.14,26 It is detrimental for charge-carriers to pass through GBs, which can be captured or restricted in the grains inside. The increase of localized charge will result in carrier recombination. Furthermore, Huang and co-workers have reported that the defects of GBs and film surfaces could permeate moisture or oxygen into © 2017 American Chemical Society

Received: October 3, 2017 Accepted: December 22, 2017 Published: December 22, 2017 1909

DOI: 10.1021/acsami.7b15031 ACS Appl. Mater. Interfaces 2018, 10, 1909−1916

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of perovskite solar cells (PVSCs) based on CH3NH3PbI3 without (w/o) and with the addition of F4TCNQ in the perovskite precursor, (b) current density−voltage (J−V) curves of the devices measured in both forward and reverse scan directions under AM 1.5 G illumination (100 mW cm−2), and (c) fill factor (FF) histogram for the devices based on the structure of ITO/NiOx/CH3NH3PbI3 (w/o and with F4TCNQ)/PCBM/BCP/Ag.

Table 1. Photovoltaic Parameters for PVSCs Based on the Perovskite Layer w/o and with F4TCNQ Measured in Both Forward and Reverse Scan Directions under AM 1.5 G Illumination (100 mW cm−2)a device w/o F4TCNQ

with F4TCNQ

a

forward reverse average forward reverse average

Jsc (mA cm−2)

Voc (V)

FF (%)

PCE (%)

18.71 19.42 18.22 ± 0.70 19.29 19.57 18.94 ± 0.32

1.050 1.054 1.054 ± 0.003 1.062 1.060 1.063 ± 0.004

71.8 74.0 72.8 ± 2.7 79.6 80.0 77.8 ± 1.1

14.1 15.1 13.9 ± 0.6 16.3 16.6 15.7 ± 0.4

Average and standard deviation values for 20 cells.

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7,7,8,8-tetracyanoquinodimethane (F4TCNQ), is a fluorinated molecular p-type dopant and a strong electron acceptor with a very low unoccupied molecular orbital (−5.24 eV) energy level, which has been widely used in doping epitaxial graphene30−32 and conductive polymers33−37 to improve the charge-transfer property. In this work, F4TCNQ molecules fill the GBs and vacancies of perovskite films to form perovskite−F4TCNQ bulk heterojunction. More importantly, the bulk heterojunction at GBs can enhance charge transfer and reduce trap-assisted nonradiative recombination losses.38 In addition, the metallic Pb defects in the perovskite crystal lattice are greatly reduced with the addition of F4TCNQ. Therefore, a high electronic quality perovskite film with effective charge separation and transfer, improved and balanced charge mobility, and retarded recombination has been obtained by incorporating F4TCNQ in the perovskite film, which results in a power conversion efficiency (PCE) of 16.6%, reproducible fill factor (FF) (as high as 80%), negligible hysteresis effect, and superior stability.

RESULTS AND DISCUSSION The inverted PVSCs were fabricated according to the device configuration of indium tin oxide (ITO)/NiOx/CH3NH3PbI3 (without and with F4TCNQ)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM)/bathocuproine (BCP)/Ag, as shown in Figure 1a. The impact of perovskite precursor without and with different concentrations (wt %) of F4TCNQ on device performance was investigated, as shown in Figure S1. The best-performing PVSCs were obtained by adding 0.02 wt % F4TCNQ into the precursor solution. Further increasing the doping concentration of F4TCNQ to 0.05 wt % would deteriorate device performance. In the following measurements and characterization, we adopt the doping concentration of 0.02 wt %. Figure 1b shows the current density−voltage (J−V) curves of devices without and with F4TCNQ incorporated in the perovskite film measured in both forward and reverse scan directions under AM 1.5 G illumination (100 mW cm−2). The corresponding device parameters are summarized in Table 1, and the external quantum efficiency (EQE) results are shown in Figure S2. The reference device achieves a PCE of 15.1% with a short circuit current (Jsc) of 19.42 mA cm−2, an open-circuit 1910


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Figure 2. (a, b) Top-view scanning electron microscopy (SEM) images, (c) X-ray diffraction (XRD) patterns, and (d) ultraviolet−visible (UV−vis) absorption spectra of the perovskite films w/o and with F4TCNQ.

Figure 3. (a) Steady-state photoluminescence (PL) and (b) time-resolved photoluminescence (TRPL) spectra of the perovskite films w/o and with F4TCNQ deposited on the glass substrate. (c) Dark J−V curves and (d) electrical impedance spectra (EIS) of the devices based on the structure of ITO/NiOx/CH3NH3PbI3 (w/o and with F4TCNQ)/PCBM/BCP/Ag.

voltage (Voc) of 1.054 V, and a fill factor (FF) of 74.0%. In contrast, the F4TCNQ incorporated device shows PCE of 16.6% (16.3%) with Jsc of 19.57 (19.29) mA cm−2, Voc of 1.060

(1.062) V, and FF of 80.0% (79.6%) measured in reverse (forward) scan, suggesting negligible hysteresis effect. The high FF resulting from efficient charge transfer and reduced defects 1911


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Figure 4. (a) Fourier transform infrared (FTIR) spectroscopy of F4TCNQ and F4TCNQ−PbI2 prepared by mixing F4TCNQ with PbI2 at a molar ratio of 0.1:1. (b) X-ray photoelectron spectroscopy (XPS) of the perovskite films w/o and with F4TCNQ. (c) J−V curves measured by the spacecharge limited current (SCLC) model of the electron-only devices based on the structure of ITO/SnOx/CH3NH3PbI3 (w/o and with F4TCNQ)/ PCBM/BCP/Ag. (d) Stabilities of the devices measured under ambient environment based on the structure of ITO/NiOx/CH3NH3PbI3 (w/o and with F4TCNQ)/PCBM/BCP/Ag.

tion. The PVSCs exhibit a higher FF probably on account of the effective charge separation and transfer with reduced recombination in the perovskite film. To certify it, the steadystate photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra of perovskite films without and with F4TCNQ were implemented, as shown in Figure 3a,b, respectively. Compared with the pure perovskite film, the perovskite film with F4TCNQ quenches by 25.3% in the steady-state PL spectra, indicating effective charge separation.39 Similar results have been also observed when fullerene derivative is doped to the perovskite functional layer.18−20 To analyze the dynamics of recombination, we conducted TPRL and evaluated the stretched exponential decay lifetimes by fitting the data with a biexponential decay function

enhances the device efficiency from 15.1 to 16.6%. To evaluate the reproducibility of the FF, 40 individual devices without and with F4TCNQ incorporated in the perovskite film have been fabricated. As shown in Figure 1c, PVSCs with F4TCNQ exhibit higher FF with relatively narrow distribution compared to that of the reference devices, indicating a good reproducibility of FF. The top-view scanning electron microscopy (SEM) was carried out to explore the effect of F4TCNQ doping strategy on the perovskite film morphology. As shown in Figure 2a,b, there is no significant change in grain size between the two kinds of perovskite films, which is confirmed by the atomic force microscopy (AFM) images shown in Figure S3. The results are likewise consistent with the X-ray diffraction (XRD) patterns shown in Figure 2c, which exhibit similar diffraction intensities of (110), (220), and (310) peaks. As illustrated from the ultraviolet−visible (UV−vis) absorption spectra in Figure 2d, the absorption intensity of the perovskite film with F4TCNQ is similar to that of the reference perovskite film. Therefore, it can be concluded that (1) the additive F4TCNQ will not change the morphology and grain size of the perovskite film and (2) F4TCNQ molecules are supposed to fill into the GBs and vacancies of the perovskite film. The top-view SEM with an energy dispersive spectrometer (EDS) mapping has been implemented to investigate the distribution of F4TCNQ molecules in the perovskite film by tracking the characteristic fluorine (F) element. As shown in Figure S4, F element exhibits a uniform distribution inside the perovskite film. Understanding the enhancement of FF is important to develop high-efficiency, large-area PVSCs for commercializa-

⎛ t ⎞ ⎛ t⎞ Y = A1 exp⎜ − ⎟ + A 2 exp⎜ − ⎟ + y0 ⎝ τ2 ⎠ ⎝ τ1 ⎠

where A1 and A2 are the relative amplitudes and τ1 and τ2 are fast and slow decay lifetimes, respectively.40 The F4TCNQ containing the perovskite film exhibits fast and slow phase lifetimes of τ1 = 4.47 ns and τ2 = 59.82 ns, respectively. In contrast, the pure perovskite film gives τ1 = 2.69 ns and τ2 = 46.51 ns for the corresponding lifetimes. These results indicate decreased defects concentration and reduced nonradiative recombination, consistent with the negligible hysteresis effect of PVSCs. To further prove that the carrier recombination is suppressed, the dark current−voltage (J−V) characteristics for the devices without and with F4TCNQ incorporated in the 1912


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Figure 5. Histograms of device parameters: (a) Voc, (b) Jsc, (c) FF, and (d) PCE for two batches of cells based on the structure of ITO/NiOx/ CH3NH3PbI3 (with F4TCNQ)/PCBM/BCP/Ag, composed of 20 separate devices.

perovskite film are performed. As presented in Figure 3c, the reverse current density of the F4TCNQ incorporated device is about 1 order of magnitude lower than that of the reference device, which demonstrates enlarged shunt resistance, restrained leakage current, and higher rectification ratio.41 Electrical impedance spectroscopy (EIS) is another effective method to understand the charge-transfer process and carrier recombination. It is measured under AM 1.5 G illumination (100 mW cm−2) and open-circuit conditions. The highfrequency region of the EIS curve represents the series resistance, and the low-frequency region is ascribed to charge-transfer resistance.42,43 As shown in Figure 3d, the charge-transfer resistance is reduced in the F4TCNQ incorporated device, which is attributed to the fact that the F4TCNQ molecules at GBs can provide a good pathway for charge transfer with less resistance. To further explore the influence of additive F4TCNQ on the chemical properties of perovskite films, Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) were conducted. As shown in Figure 4a, the stretching vibration of cyanogroup (CN) appeared at 2222 cm−1 for pure F4TCNQ, which was shifted to 2186 and 2152 cm−1 for F4TCNQ−PbI2 prepared by mixing F4TCNQ with PbI2 at a molar ratio of 0.1:1. The shift of the CN vibration in F4TCNQ to lower wavenumbers is indicative of the formation of the intermediate F4TCNQ−PbI2 adduct because of the interaction between Lewis base F4TCNQ and Lewis acid PbI2. The chemical interaction can slow down the crystallization rate of perovskite, which is beneficial to form highquality perovskite crystals with lesser defects. The schematic diagram of the crystallization process is illustrated in Figure S5. The Pb 4f XPS spectra, shown in Figure 4b, shift to higher binding energies for the Pb valence electrons in 4f7/2 and 4f5/2

for F4TCNQ incorporated perovskite, which also proves the existence of chemical interaction between F4TCNQ and Pb in perovskite. In addition, two small peaks in Pb 4f XPS spectra located at 136.1 and 141.1 eV for pure perovskite can be observed, which are attributed to the presence of metallic Pb.44 A large amount of atomic Pb is bound to accompany the existence of iodide vacancies in the perovskite crystal lattice. The metallic Pb defects play the role of nonradiative recombination centers,13 which are disadvantageous to the transfer and collection of charge-carriers. Interestingly, the metallic Pb peak is greatly reduced for the perovskite film with F4TCNQ, which is ascribed to the fact that the interaction between F4TCNQ and Pb in perovskite can effectively passivate the defects generated by uncoordinated Pb atoms. The greatly reduced perovskite crystal lattice defects by the addition of F4TCNQ can decrease the probability of carrier recombination, which agrees with the TRPL results. The peak intensity of F 1s is enhanced with the increase of doping concentration, as shown in Figure S6, indicating that F4TCNQ is successfully doped into the perovskite film. Furthermore, the trap-state density and carrier mobility of the perovskite film without and with F4TCNQ have been measured using a space-charge limited current (SCLC) model. Figure 4c shows the J−V curves of the electron-only devices based on the structure of ITO/SnOx/CH3NH3PbI3 (w/o and with F4TCNQ)/PCBM/BCP/Ag. Three regions can be identified according to different values of the exponent n (J ∝ Vn relation): n = 1 is the Ohmic region and n = 2 is the SCLC region. n > 3 is the trap-filled limited (TFL) region in which all available trap states are filled by the injected carriers. The onset voltage (trap-filled limited voltage VTFL) is determined by trap density45 1913


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VTFL

assisted nonradiative recombination losses,38 which contribute to improve FF and PCE.

en L2 = t 2εε0

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where L is the thickness of the perovskite film, ε (=32) is the relative dielectric constant of CH3NH3PbI3, and ε0 is the vacuum permittivity. Consequently, the calculated electron trap density in the F4TCNQ incorporated perovskite is 9.43 × 1015 cm−3. For comparison, the electron trap density of 1.14 × 1016 cm−3 is obtained for pure perovskite. In addition, the carrier mobility can be determined according to the Mott−Gurney law45 J=

CONCLUSIONS In summary, we employ a method to reduce the grain boundaries defects and crystal lattice defects by introducing F4TCNQ into the perovskite layer. The F4TCNQ additive helps in obtaining a high electronic quality perovskite film with efficient charge separation and transfer, improved and balanced charge mobilities, and suppressed recombination. More importantly, the existence of perovskite−F4TCNQ bulk heterojunction at GBs can enhance charge transfer, and the F4TCNQ molecules act as the charge-transfer medium. Hence, F4TCNQ incorporated PVSCs exhibit PCE of 16.6% and FF of 80.0%, as well as negligible hysteresis effect and better stability. This work provides a simple route to fabricate PVSCs with excellent performance based on improved charge transfer and reduced defect states.

9 V2 εε0μ 3 8 L

The electron mobility of the perovskite film with F4TCNQ is higher than that of pure perovskite based on a qualitative comparison of the SCLC regions from the corresponding J−V curves. In addition, the hole mobility of F4TCNQ incorporated perovskite is also higher than that of the pure perovskite, as shown in Figure S7. The quantitative comparison of the carrier mobility is summarized in Table S1. The additive F4TCNQ not only improves electron and hole mobilities simultaneously, but also makes them more balanced. Figure 4d shows the stability of the corresponding perovskite solar cells without encapsulation, which are stored in an argon glovebox and tested in ambient environment with 30−40% humidity at room temperature. The F4TCNQ incorporated device shows better stability than that of the reference device because of the decreased defects density and the effect of preventing water from infiltrating the perovskite film by the F element of F4TCNQ molecules. The histograms shown in Figure 5 exhibit the distribution of device parameters for two batches of cells (Table S2) based on the structure of ITO/ NiOx/CH3NH3PbI3 (with F4TCNQ)/PCBM/BCP/Ag. The average PCE is 15.7% with good reproducibility. The charge-transfer mechanism in the perovskite functional layer without and with F4TCNQ is illustrated in Figure 6. It is

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15031. Detailed experimental section, characterizations of the work, current density−voltage (J−V) curves, external quantum efficiency (EQE) spectra, atomic force microscopy (AFM) images, top-view SEM with energy dispersive spectrometer (EDS) mapping, schematic diagram of the crystallization process, X-ray photoelectron spectroscopy (XPS) F 1s spectra, J−V curves measured by the space-charge limited current (SCLC) model of the hole-only devices, values of carrier mobility, and device statistics (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.T.). *E-mail: [email protected]. Tel: +86 791 83968703. Fax: +86 791 83969561 (Y.C.). ORCID

Yiwang Chen: 0000-0003-4709-7623 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS L.T. thanks for the support from the National Natural Science Foundation of China (NSFC) (51663015). Y.C. thanks for the support from the National Science Fund for Distinguished Young Scholars (51425304). We also thank for support from Natural Science Foundation of Jiangxi Province (20165BCB19003 and 20171BCB23011) and Graduate Innovation Fund Projects of Nanchang University (CX2017048).

Figure 6. Schematic illustration of the charge-transfer mechanism in the perovskite functional layer w/o and with F4TCNQ.

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known that the GBs are filled with charge trap states, nonradiative recombination centers, and impurities.14 Some charge-carriers will be captured or restricted when passing through GBs. Importantly, the semiconductive F4TCNQ molecules filled in GBs serve as an electron-transport medium, which provides a good pathway for electron transfer with less resistance. Furthermore, perovskite and F4TCNQ serve as electron donor and electron acceptor, respectively, to form bulk heterojunction. The bulk heterojunction at GBs can enhance charge-carrier transfer and collection as well as reduce trap-

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