Grain Boundaries Modification via F4TCNQ to Reduce Defects of

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Grain Boundaries 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 ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15031 • Publication Date (Web): 22 Dec 2017 Downloaded from http://pubs.acs.org on December 23, 2017

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ACS Applied Materials & Interfaces

Grain Boundaries Modification via F4TCNQ to Reduce Defects of Perovskite Solar Cells with Excellent Device Performance Cong Liua,b, Zengqi Huanga,b, Xiaotian Huc, Xiangchuan Menga,b, Liqiang Huanga,b, Jian Xiongd, Licheng Tan*a,b, Yiwang Chen*a,b a

College of Chemistry, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China

b

Jiangxi Provincial Key Laboratory of New Energy Chemistry/Institute of Polymers, Nanchang University, 999 Xuefu Avenue, Nanchang 330031, China c

Key Laboratory of Green Printing, Institute of Chemistry, Chinese Academy of Sciences (ICCAS), 2 Zhongguancun Beiyi Street, Beijing 100190, China

d

Guangxi Key Laboratory of Information Materials, School of Materials Science and

Engineering, Guilin University of Electronic Technology, 1 Jinji Road, Guilin 541004, China Corresponding author. Tel.: +86 791 83968703; fax: +86 791 83969561. E-mail: [email protected] (Y. Chen); [email protected] (L. Tan).

Abstract Solar cells based on hybrid organic-inorganic metal halide perovskites are developing towards the direction of high efficiency and stability. However, it is inevitable that there are defects in perovskite film, leading to a poor device performance. Here, we employ additive-engineering strategy to modify the grain boundaries (GBs) defects and crystal lattice defects by introducing a strong electron acceptor of 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane

(F4TCNQ)

into

perovskite

functional layer. Importantly, it has been found that F4TCNQ are filled in GBs and there are significant reduction of metallic lead defects and iodide vacancies in perovskite crystal lattice. The bulk heterojunction perovskite-F4TCNQ film exhibits 1

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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

Introduction Hybrid

organic-inorganic

metal halide

perovskites have

been applied as

light-absorbing materials in solar cells because of easy preparation, low cost and its excellent optoelectronic properties.1-5 However, the unwanted defects existed in perovskite film can generate charge trap states, which enhance the non-radiative 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 efficiency and stability of perovskite solar cells (PVSCs).

The defects of perovskite film are derived from pinholes, vacancies, surface roughness, grain boundaries (GBs) and anomalous crystal lattice.9-13 Morphology control of perovskite film is important in reducing defect states. GBs existed in polycrystalline perovskite film are filled with charge trap states, non-radiative recombination centers and impurities.14,26 It is detrimental for charge-carrier to pass 2


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through GBs, which can be captured or restricted in grain 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 surface could permeate moisture or oxygen into perovskite film and accelerate degradation of perovskite grain.9

Currently, the research mainly focuses on perovskite crystallization process to obtain a high quality perovskite film with larger grain and fewer GBs, aiming at reducing defects of perovskite film. 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 etc., have been attempted in perovskite precursor to increase grain size and decrease GBs with different functional mechanisms. However, the GBs cannot be completely eliminated due to the existence of polycrystalline perovskite grains. In addition to decreasing quantities of GBs, reducing intrinsic defects of GBs is also an effective method.

Herein, we employ additive-engineering strategy to modify GBs defects and crystal lattice defects in spite of without improving film morphology. The additive 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) is a fluorinated molecular p-type dopant and strong electron acceptor with a very low lowest unoccupied molecular orbital (LUMO, -5.24 eV) energy level, which has been widely 3



used in doping epitaxial grapheme30-32 and conductive polymers33-37 to improve charge transfer property. In this work, F4TCNQ molecules fill the GBs and vacancies of perovskite film to form perovskite-F4TCNQ bulk heterojunction. More importantly, the bulk heterojunction at GBs can enhance charge transfer and reduce trap-assisted non-radiative recombination losses.38 Besides, the metallic Pb defects in 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 perovskite film, which results in power conversion efficiency of 16.6%, reproducible fill factor (as high as 80%), negligible hysteresis effect and superior stability.

4


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Results and Discussion The inverted PVSCs were fabricated according to device configuration of ITO/NiOx/CH3NH3PbI3 (without and with F4TCNQ)/PCBM/BCP/Ag shown in Figure 1a. The impact of perovskite precursor without and with different concentration (wt%) F4TCNQ on device performance was investigated shown in Figure S1. The best-performing PVSCs were obtained by adding 0.02 wt% F4TCNQ into 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 perovskite film measured in both forward and reverse scan directions under AM 1.5G 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 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 enhances the device efficiency from 15.1% to 16.6%. In order to evaluate the reproducibility of the FF, 40 individual devices without and with F4TCNQ incorporated in perovskite film have been fabricated. As shown in Figure 1c, PVSCs with F4TCNQ exhibit higher FF with relatively narrow distribution compared to that 5



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 perovskite film morphology. As shown in Figure 2a and Figure 2b, there is no significant change in grain size between the two kinds of perovskite films, which is confirmed by atomic force microscopy (AFM) images shown in Figure S3. The results are likewise consistent with X-ray diffraction (XRD) patterns shown in Figure 2c, which exhibit similar diffraction intensity of (110), (220) and (310) peaks. As illustrated from ultraviolet-visible (UV-vis) absorption spectra in Figure 2d, the absorption intensity of perovskite film with F4TCNQ is similar to that of reference perovskite film. Therefore, it can be concluded that, (1) the additive F4TCNQ will not change morphology and grain size of perovskite film; (2) F4TCNQ molecules are supposed to fill into the GBs and vacancies of perovskite film. The top-view SEM with an energy dispersive spectrometer (EDS) mapping has been implemented to investigate the distribution of F4TCNQ molecules in perovskite film by tracking the characteristic fluorine (F) element. As shown in Figure S4, F element exhibits a uniform distribution inside perovskite film.

Understanding the enhancement of FF is important to develop high-efficiency, large-area PVSCs for commercialization. The PVSCs exhibit a higher FF probably on account of the effective charge separation and transfer with reduced recombination in 6


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perovskite film. To certify it, the steady-state photoluminescence (PL) spectra and time-resolved photoluminescence (TRPL) spectra of perovskite films without and with F4TCNQ were implemented shown in Figure 3a and Figure 3b, respectively. Compared with pure perovskite film, the perovskite film with F4TCNQ quenches by 25.3% in steady-state PL spectra, indicating effective charge separation.39 The similar results have been also observed when fullerene derivative is doped to perovskite functional layer.18-20 To analyze the dynamics of recombination, we conduct TPRL and evaluate the stretched exponential decay lifetimes by fitting the data with a bi-exponential decay function:

 t   t  Y = A1 exp  −  + A2 exp  −  + y0  τ1   τ2  Where A1 and A2 are the relative amplitudes and τ1 and τ2 are fast and slow decay lifetime, respectively.40 The F4TCNQ containing perovksite film exhibits fast and slow phase lifetimes of τ1=4.47 ns and τ2=59.82 ns. By contrast, the pure perovksite film gives τ1=2.69 ns and τ2=46.51 ns for corresponding lifetimes. These results indicate decreased defects concentration and reduced non-radiative 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 perovskite film are performed. As presented in Figure 3c, the reverse current density of the F4TCNQ incorporated device is about one order of magnitude lower than that of the reference device, which demonstrates enlarged shunt resistance, restrained 7



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 1.5G illumination (100 mW cm-2) and open-circuit conditions. The high-frequency 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 that the F4TCNQ molecules at GBs can provide good pathway for charge transfer with less resistance.

To further explore the influence of additive F4TCNQ on chemical properties of perovskite film, Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS) have been 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 cm-1 and 2152 cm-1 for F4TCNQ-PbI2 prepared by mixing F4TCNQ with PbI2 in a molar ratio of 0.1:1. The shift of the C≡N vibration in F4TCNQ to lower wavenumber is indicative of forming the intermediate F4TCNQ-PbI2 adduct due to the interaction between Lewis base F4TCNQ and Lewis acid PbI2. The chemical interaction can slow down crystallization rate of perovskite, which is beneficial to form high quality perovskite crystals with lower defects. The schematic diagram of crystallization process is illustrated in Figure S5. The Pb 4f XPS spectra shown in Figure 4b shifts to higher binding energies for the Pb valence electrons in 4f 7/2 and 4f 5/2 for F4TCNQ incorporated perovskite, which also proves 8


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the existence of chemical interaction between F4TCNQ and Pb in perovskite. Besides, 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 with the existence of iodide vacancies in perovskite crystal lattice. The metallic Pb defects act a role of non-radiative recombination centers,13 which are disadvantageous to the transfer and collection of charge-carrier. Excitingly, the metallic Pb peak is greatly reduced for the perovskite film with F4TCNQ, which is ascribed to that the interaction between F4TCNQ and Pb in perovskite can effectively passivate the defects generated by uncoordinated Pb atom. 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 increasing of doping concentration shown in Figure S6, indicating that F4TCNQ is successfully doped into 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 region 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 region in which all available trap states are filled by the injected carriers. The 9



onset voltage (trap-filled limit voltage VTFL) is determined by trap density:45

VTFL =

ent L2 2εε 0

Where L is the thickness of perovskite film, ε (=32) is relative dielectric constant of CH3NH3PbI3, and ε0 is the vacuum permittivity. Consequently, the calculated electron trap density in F4TCNQ incorporated perovskite is 9.43×1015cm-3. For comparison, the electron trap density of 1.14×1016cm-3 is obtained for pure perovksite. In addition, the carrier mobility can be determined according to the Mott-Gurney law:45 V2 9 J = εε 0 µ 3 8 L

The electron mobility of 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. Besides, the hole mobility of F4TCNQ incorporated perovskite is also higher than that of the pure perovksite 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 corresponding perovskite solar cells without encapsulation, which are stored at argon glove box and tested in ambient environment with 30%-40% humidity at room temperature. The F4TCNQ incorporated device shows better stability than reference device due to the decreased defects density and the effect of preventing water from infiltrating the perovskite film by F element of 10


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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 perovskite functional layer without and with F4TCNQ is illustrated in Figure 6. It is known, the GBs are filled with charge trap states, non-radiative recombination centers and impurities.14 Some charge-carrier will be captured or restricted when passing through GBs. Importantly, the semiconductive F4TCNQ molecules filled in GBs serve as an electron-transport medium, which provide 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 heterjunction at GBs can enhance charge-carrier transfer and collection as well as reduce trap-assisted non-radiative recombination losses,38 which contribute to improve FF and PCE.

Conclusions In summary, we employ a method to reduce the grain boundaries defects and crystal lattice defects by introducing F4TCNQ into perovskite layer. The F4TCNQ additive is in favor of 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 11



heterojunction at GBs can enhance charge transfer and F4TCNQ molecules play a role as charge-transfer medium. Hence, F4TCNQ incorporated PVSCs exhibit PCE of 16.6% and FF of 80.0%, respectively, 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.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The detailed experimental section, characterizations of the work, the current density-voltage (J-V) curves, the external quantum efficiency (EQE) spectra, atomic force microscopy (AFM) images, the top-view SEM with an energy dispersive spectrometer (EDS) mapping, the schematic diagram of crystallization process, X-ray photoelectron spectroscopy (XPS) F 1s spectra, the J-V curves measured by the space-charge limited current (SCLC) model of the hole-only devices, values of carrier mobility and device statistics. This information is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *Tel.: +86 791 83968703; fax: +86 791 83969561. E-mail: [email protected] (Y. Chen), [email protected] (L. Tan). 12


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Notes Competing financial interests. The authors declare no competing financial interest.

ACKNOWLEDGMENTS L. T. thanks for the support from the National Natural Science Foundation of China (NSFC) (51663015). Y. C. thanks for 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).

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Additive to Reduce the Hysteresis and Boost the Fill Factor of Planar Perovskite Solar Cells. Adv. Mater. 2016, 28, 5214-5221. [30] Kumar, A.; Banerjee, K.; Dvorak, M.; Schulz, F.; Harju, A.; Rinke, P.; Liljeroth, P. Charge-Transfer Driven Nonplanar Adsorption of F4TCNQ Molecules on Epitaxial Graphene. ACS Nano 2017, 11, 4960-4968. [31] Chen, W.; Chen, S.; Qi, D. C.; Gao, X. Y.; Wee, A. T. S. Surface Transfer p-Type Doping of Epitaxial Graphene. J. Am. Chem. Soc. 2007, 129, 10418-10422. [32] Maccariello, D.; Garnica, M.; Niño, M. A.; Navío, C.; Perna, P.; Barja, S.; Vázquez de Parga, A. L.; Miranda, R. Spatially Resolved, Site-Dependent Charge Transfer and Induced Magnetic Moment in TCNQ Adsorbed on Graphene. Chem. Mater. 2014, 26, 2883-2890. [33] Cochran, J. E.; Junk, M. J.; Glaudell, A. M.; Miller, P. L.; Cowart, J. S.; Toney, M. F.; Hawker, C. J.; Chmelka, B. F.; Chabinyc, M. L. Molecular Interactions and Ordering in Electrically Doped Polymers: Blends of PBTTT and F4TCNQ. Macromolecules 2014, 47, 6836-6846. [34] Kang, K.; Watanabe, S.; Broch, K.; Sepe, A.; Brown, A.; Nasrallah, I.; Nikolka, M.; Fei, Z.; Heeney, M.; Matsumoto, D.; Marumoto, K.; Tanaka, H.; Kuroda, S.;Sirringhaus, H. 2D Coherent Charge Transport in Highly Ordered Conducting Polymers Doped by Solid State Diffusion. Nat. Mater. 2016, 15, 896-902. [35] Pingel, P.; Zhu, L.; Park, K. S.; Vogel, J.-O.; Janietz, S.; Kim, E.-G.; Rabe, J. P.; Brédas, J.-L.; Koch, N. Charge-Transfer Localization in Molecularly Doped Thiophene-Based Donor Polymers. J. Phys. Chem. Lett. 2010, 1, 2037-2041. 18


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a

h+ h+

ee-

b

c

Figure 1 (a) Schematic illustration of perovskite solar cells (PVSCs) based on CH3NH3PbI3 without (w/o) and with addition of F4TCNQ in perovskite precursor, (b) current density-voltage (J-V) curves of the devices measured in both forward and reverse scan directions under AM 1.5G 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.

21



a

b w/o F4TCNQ

with F4TCNQ

500 nm c

Page 22 of 28

500 nm d

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.

22


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a

b

c

d

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 glass substrate. (c) Dark J-V curves and (d) electrical impedance spectra (EIS) of devices based on the structure of ITO/NiOx/CH3NH3PbI3 (w/o and with F4TCNQ)/PCBM/BCP/Ag.

23



a

b

c

d

Figure 4 (a) Fourier transform infrared (FTIR) spectroscopy of F4TCNQ and F4TCNQ-PbI2 prepared by mixing F4TCNQ with PbI2 in a molar ratio of 0.1:1. (b) X-ray photoelectron spectroscopy (XPS) of the perovskite films w/o and with F4TCNQ. (c) The J-V curves measured by space-charge 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) The stabilities of the devices measured under ambient environment based on the structure of ITO/NiOx/CH3NH3PbI3 (w/o and with F4TCNQ)/PCBM/BCP/Ag.

24


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a

b

c

d

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.

25



PCBM e- e- ee- h+ e-

h+

e-

h+

h+

e-

PCBM

e- e- - ee eh+ h+

e-

h+ h+ h+ h+

e-

ee-

Page 26 of 28

e-

e-

h+ h+ e- h +

ee-

h+

h

h+

NiOx

h+ h+ h+ h+

e- ee-

h+

e- e- h+ h+

e- e- +h+ h e-

e- e- - + h+ e h

e-

e- eh+ +

h+ h+ e- h+ h+ + h h+ h+

e- e- - e e

h+ h+ + h+ h+ h

NiOx

Perovskite

F4TCNQ

charge trap state

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

26


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Table 1 Photovoltaic parameters for PVSCs based on perovskite layer w/o and with F4TCNQ measured in both forward and reverse scan directions under AM 1.5G illumination (100 mW cm-2). Jsc

Voc

FF

PCE

(mA cm-2)

(V)

(%)

(%)

Forward

18.71

1.050

71.8

14.1

Reverse

19.42

1.054

74.0

15.1

Average

18.22±0.70

1.054±0.003

72.8±2.7

13.9±0.6

Forward

19.29

1.062

79.6

16.3

Reverse

19.57

1.060

80.0

16.6

Average

18.94±0.32

1.063±0.004

77.8±1.1

15.7±0.4

Device

w/o F4TCNQ

with F4TCNQ

a)

Average and standard deviation values for 20 cells.

27



Page 28 of 28

Graphical abstract

h+ h+

ee-

28


Grain Boundaries Modification via F4TCNQ to Reduce Defects of

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