High Efficiency (18.53%) of Flexible Perovskite Solar Cells via the

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High Efficiency (18.53%) of Flexible Perovskite Solar Cells via the Insertion of Potassium Chloride between SnO2 and CH3NH3PbI3 Layers Ning Zhu, Xin Qi, Yuqing Zhang, Ganghong Liu, Cuncun Wu, Duo Wang, Xuan Guo, Wei Luo, Xiangdong Li, Haozhe Hu, Zhijian Chen, Lixin Xiao, and Bo Qu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.9b00391 • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

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ACS Applied Energy Materials

High Efficiency (18.53%) of Flexible Perovskite Solar Cells via the Insertion of Potassium Chloride between SnO2 and CH3NH3PbI3 Layers

Ning Zhua, Xin Qia, Yuqing Zhanga, Ganghong Liua, Cuncun Wua, Duo Wanga, Xuan Guoa, Wei Luoa, Xiangdong Lia, Haozhe Hua, Zhjian Chena, b, Lixin Xiaoa, b, and Bo Qua, b

a

State Key Laboratory for Artificial Microstructures and Mesoscopic Physics,

Department of Physics, Peking University, 100871, Beijing, People’s Republic of China; b

New Display Device and System Integration Collaborative Innovation Center of

the West Coast of the Taiwan Strait, 350002, Fuzhou, People’s Republic of China.

KEYWORDS: interface, modification, flexible, perovskite solar cell, potassium chloride

ABSTRACT Flexible perovskite solar cells (PSCs) were ideal candidates for wearable devices due to the merits of flexibility, high efficiency and lightweight, and they could be

* To whom all correspondence should be addressed, Bo Qu, email: [email protected], Tel.: +86-10-62766902.

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fabricated in a continuous roll-to-roll production process to achieve large-area and low cost devices. Herein, the high efficiency (up to 18.53%) and fill factor (0.81) of flexible PSCs (ITO/SnO2/KCl/MAPbI3/spiro-OMeTAD/Ag) were achieved by low-pressure assisted solution processing under low temperature (

≤ 100℃). The

surface morphology and crystallinity of perovskite films were effectively promoted by the KCl modification and the defect density of perovskite films as well as the hysteresis of the corresponding devices was reduced accordingly. In addition, the stability and bendability of the KCl-modified flexible PSCs were improved simultaneously. To the best of our knowledge, both the efficiency and fill factor are the best among all flexible PSCs reported to date. Therefore, the insertion of KCl between SnO2 and MAPbI3 layers provided a promising strategy for highly efficient flexible PSCs fabricated in low temperature (≤100℃) conditions.


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1. INTRODUCTION Perovskite solar cells (PSCs) have received much attention in recent years and the efficiency of PSCs has been increased from 3.8%1 to 22.7%2-6 within a few years. The efficiency of the PSCs was nearly comparable to that of other thin film solar cells7. Compared with the rigid and brittle PSCs, the flexible PSCs have attracted much interest because of the advantage of light weight, wearable potential, large-area production and low cost8-9. It was worthy to note that flexible solar cells could be fabricated in a continuous roll-to-roll production process10 to achieve large-area devices, which provided the possibilities for industrialization in future. Polyethylene terephthalate (PET), polyethylene naphthalate (PEN) and polyimide (PI) were usually used as the substrates11-13 for flexible perovskite solar cells. For example, Yang’s group prepared a flexible hybrid perovskite (CH3NH3PbI3-xClx) devices with an inverted planar structure, using PET as the flexible substrate to achieve a high efficiency of over 9%11. Gill et al14 reported as the electron transport layers (ETLs) of phenyl-C61-butyricacid methyl (PC61BM) and F8BT, and the corresponding flexible devices exhibited photoelectric conversion efficiency of 5.14% and 7.05%, respectively. Titanium dioxide (TiO2) was usually used as the ETL for PSCs. However, the high-temperature sintering (~500 oC) for TiO2 ETL could destroy the substrates of flexible PSCs to some extent. Heremans et al proposed the preparation of TiO2 ETL by electron beam and photoelectric conversion efficiency of 13.5% of PEN-based flexible PSCs was obtained15. Another way to avoid the high temperature annealing for TiO2 ETL was to find an alternative ETL for flexible PSCs. Recently, Liu Shengzhong Group16 prepared Nb2O5 ETL by electron beam evaporation and the highest recorded power conversion efficiency (PCE, 18.4%) of flexible PSCs was achieved accordingly. However, the roughness of flexible substrates was relatively poor, which resulted in the unevenness and low conductivity of indium tin oxide (ITO) or fluorine tin oxide (FTO) electrodes. Therefore, how to obtain highly efficient flexible PSCs with low-temperature processing was still an important issue in the research field of perovskite photovoltaics.



Due to the advantages of wide band gap, high transparency and high conductivity17-19, SnO2 was selected as the electron transport layer for flexible PSCs in this work. SnO2 ETL exhibited the superior electron transportation and hole blocking capabilities under low temperature conditions20. In addition, the conventional PSCs usually employed the anomalous hysteresis21, which could be improved by the modification of the interface between the perovskite and ETLs22-24. The previous work pointed out that the chlorine atoms at the interface of ETL/perovskite favored higher electron injection of devices and lead to a conduction band energy upshift of ETL22. Moreover, Cl- ions could effectively inhibit the formation of deep trap states at the interface of ETL/perovskite, which promoted the surface passivation and the suppressing of carrier recombination in PSCs23. K+ ion as an additive of perovskite could occupy interstitial sites in the perovskite lattice and then the surface defects of perovskite were decreased23. Therefore, the carrier transportation in perovskite layer was imporved and the elimination of hysteresis of PSCs was realized accordingly24. In this work, KCl was introduced into flexible PSCs to modify the interface of SnO2/perovskite. And the flatness of the SnO2 ETL and the crystallinity of the MAPbI3 were improved simultaneously. With the modification of KCl, the interface defects between SnO2/perovskite were decreased and the hysteresis of flexible PSCs was reduced. In addition, the conduction band minimum (ECBM) of ETL (SnO2/KCl) was increased with the modification of KCl, which facilitated higher VOC and PCE of flexible PSCs. An average PCE of 17.50% was achieved for the KCl-based flexible PSCs, which was 10.55% higher than that of the control device without KCl. Moreover, the champion device in this work exhibited a PCE of 18.53%, a VOC of 1.11 V, a JSC of 20.69 mA/cm2 and a FF of 0.81, respectively. All the experimental data implied that KCl modification for the interface of SnO2/perovskite effectively improved the photovoltaic properties of the flexible PSCs.

2. EXPERIMENTAL SECTION


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2.1.

Materials. PET/ITO flexible substrates, with the visible transmittance of 80% and the

square resistance of 60 ohms, were purchased from Sigma-Aldrich. Tin (IV) oxide (SnO2, 15% in H2O colloidal dispersion), anhydrous N,N-dimethylformamide (DMF), anhydrous dimethyl sulfoxide (DMSO, 99.5%) and chlorobenzene were obtained from

Alfa

Aesar.

Lead

iodide

(PbI2,

>99.99%)

and

2,2’,7,7’-tetrakis-(N,Ndi-4-methoxyphenylamino)-9,9’-spirobifluorene (spiro-OMeTAD, >99.0%) were purchased from Xi’an Polymer Light Technology Co., Ltd. Potassium chloride (KCl, >99.5%) was purchased from Tianjin Chemical Reagents Institute. Methyl ammonium iodide (MAI, 99.5%) was purchased from Borun New Material Technology Co. Ltd. Bis (trifluoromethane) sulfonamide lithium salt (Li salt, 99.95% trace metals basis) and 4-tert-butylpyridine (tBP, 96%) were obtained from Sigma-Aldrich. All the materials were not further purified before use.

2.2.

Device Fabrication. The PET/ITO substrates were treated by oxygen plasma for 15 minutes directly.

SnO2 colloid was diluted with deionized water (1:6 volume ratio of SnO2:H2O) and stirred for 60 min before use. The acquired SnO2 solution was spin-coated onto PET/ITO substrates (3000rpm, 30s) and then annealed at 100 oC for 30 min. KCl (aqueous solution) was spin-coated onto the PET/ITO/SnO2 substrates (3000rpm, 30s), followed by being annealed at 100 oC for 10 min. After cooling down, the MAPbI3 precursor (1 mol/L), containing methyl ammonium iodide and PbI2 (1:1 molar ratio) in 1 mL DMF mixed with 71μL DMSO, was spin-coated (4000rpm, 6s) on the acquired substrates and treated with the flash method as reported in the previous literature25. The obtained films were then annealed at 100 oC for 10 min to form the perovskite active layers. The spiro-OMeTAD precursor solution was prepared by dissolving 72.3 mg spiro-OMeTAD, 30 μL tBP and 20 μL Li salt acetonitrile solution (520 mg/mL) in 1 mL chlorobenzene (CB) and spin-coated onto



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perovskite films (4000 rpm, 30 s) as the hole-transporting layer. To complete the fabrication of the flexible PSCs, 100 nm Ag as the anode was thermal evaporated through a shadow mask in a vacuum chamber. The size of the devices was controlled to be 0.04 cm2.

2.3.

Characterization. The X-ray diffraction (XRD) of MAPbI3 films was collected by using the X-ray

diffraction system (PANalytical Inc.) with monochromatic Cu Kα irradiation (l = 1.5418 Å) at a scan rate of 6°/min. Scanning electron microscope (SEM) and atomic force microscope (AFM) were conducted on a Hitachi S-4800 and Agilent Series 5500, respectively. The XPS (Kratos, Axis Ultra) analysis was performed at room temperature, equipped with a monochromatic Al Kα X-ray source (1486.7 eV). UV–visible

absorption

spectra

were

measured

by

using

a

UV–vis–NIR

spectrophotometer (UV3600Plus). The electrochemical impedance spectroscopy (EIS) was performed on a Zahner Zennium electrochemical workstation in the dark. A 10-mV ac-sinusoidal signal source was employed over the constant bias with the frequency ranging from 1 MHz to 1 Hz. Photoluminescence (PL) was measured with a Naonlog infrared fluorescence spectrometer (Nanolof FL3-2lhr). Time-resolved photoluminescence (TRPL) was carried out using ultra-Fast Lifetime Spectrometer (Delta flex). The current density-bias voltage (J-V) curves of the flexible PSCs were recorded with a scan rate of 0.1 V/s under 100 mW/cm2 AM 1.5G simulated illumination of Newport solar simulate. The incident photon-to-current conversion efficiency (IPCE) spectra were measured using a lock-in amplifier (model SR830 DSP) coupled with a 1/4 m monochromator (Crowntech M24-s) and 150 W tungsten lamp (Crowntech). All the measurements for the solar cells without encapsulation were carried out under ambient conditions at room temperature.

3. RESULTS AND DISCUSSIONS Low-temperature processed flexible PSCs with the n-i-p structure of


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PET/ITO/SnO2/KCl/MAPbI3/spiro-OMeTAD/Ag were fabricated (Figure 6a) and studied in this work. The XPS spectrum of ITO/SnO2/KCl (2 mg/ml) was recorded in Figure S1 and the peaks assigned to K 2s, K 2p and Cl 2p demonstrated the existence of KCl. The surface morphology of the SnO2 layer coated onto the flexible PET/ITO substrate was investigated by atomic force microscopy (AFM) as shown in Figure 1a and the root-mean-square roughness (rms) of SnO2 film was ~1.75 nm. Meanwhile, the rms of SnO2/KCl film was reduced to be ~0.77 nm, which was about the half value of that of the pristine SnO2 film. Then, the morphology of SnO2 film was smoothed after the spin coating of KCl aqueous solution. As shown in Figure 1b, the ultrathin KCl layer was not a continuous film and KCl clusters were filled into the voids of SnO2 layer. The defect states of KCl-modified SnO2 ETL would be decreased accordingly. Electrochemical impedance spectrometry (EIS) was carried out to investigate the charge transfer dynamics at the ETL/perovskite interface. The Nyquist plots of the control and KCl-based (2mg/mL) devices measured at 1.0 V in the dark were presented in Figure S2 and two semicircles were observed in the EIS spectra. The first semicircle at high frequency was related to the charge transport resistance (Rct) in the interface and ETL26-27. The second semicircle at middle frequency was assigned to a recombination resistance (Rrec)28 of the devices. Compared with the control device, the KCl-based PSC was employed a lower Rct, which implied the enhanced charge transport by using the modified ETL of SnO2/KCl (2 mg/ml). Then, the improved morphology and decreased defect density of SnO2/KCl (2 mg/ml) were realized accordingly. The flat morphology of SnO2/KCl film facilitated superior conductivity between perovskite and ETL, and the crystallinity of the MAPbI3 might be optimized yet.



(a)

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(b)

Figure 1. The AFM topography of SnO2 (a) and SnO2/KCl films (b).

In order to study the effect of KCl modification on the crystallinity of perovskite layer, XRD patterns and SEM images of CH3NH3PbI3 films were collected as shown in Figure 2. When KCl layers (different concentrations) were inserted between SnO2 and perovskite layer, the intensity at 14.1° and 28.4°, corresponding to the (110) and (220) planes of the MAPbI3 crystal29 respectively, was increased simultaneously as shown in Figure 2c. Furthermore, when the concentration of KCl solution was 2 mg/mL, the (110) diffraction peak (14.1o) of MAPbI3 reached nearly twice that of the control specimen without KCl modification as shown in Figure 2d. The KCl-modified (2 mg/mL) perovskite film exhibited small full width at half maximum (FWHM) (0.10° as listed in Table S1) of the (110) diffraction peak, which indicated the superior crystallinity of the MAPbI3 with the effective KCl modification.


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Figure 2. SEM images of MAPbI3 fabricated onto PET/ITO/SnO2 (a) and PET/ITO/SnO2/KCl (2mg/mL) substrates (b). XRD patterns [(c) and (d)] of MAPbI3 films fabricated onto KCl layers (0 mg/mL, 1 mg/mL, 2 mg/mL, 5 mg/mL).

The surface topography of MAPbI3 layers fabricated onto PET/ITO/SnO2 and PET/ITO/SnO2/KCl (2mg/mL) substrates was studied by the SEM as shown in Figure 2a, b. With the modification of KCl (2 mg/mL), the size of perovskite grains was increased to be 300-500 nm, which was larger than that of the control specimen without KCl (100-200nm). The existence of Cl- ion slowed down the crystallization rate of the perovskite and the formation of larger grains of MAPbI3 was realized accordingly30-31. It was reported that perovskite active layer with 300-500 nm grains was mainly advantageous to the charge transportation of the corresponding perovskite solar cells32-33. In addition, the single crystal size (D) of the perovskite was estimated



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to be 50-100 nm by using the formula 𝐷 = 𝜆/(FWHM × cos 𝜃) (λ =1.5418 Å, 2𝜃 = 14.1°)34, which indicated that the perovskite grains were composed of several single crystals of MAPbI3.

Table 1. The average photovoltaic data of PSCs with and without KCl modification. KCl

VOC (V)

JSC (mA cm-2)

FF

PCE (%)

w/o 0.5mg/mL 1 mg/mL 2 mg/mL 5 mg/mL

1.05 1.04 1.08 1.08 1.11

21.21 21.58 21.70 21.50 21.12

0.71 0.71 0.73 0.76 0.65

15.83 15.94 17.00 17.50 15.16

Figure 3. The average photovoltaic data: (a) VOC, (b) JSC, (c) FF, and (d) PCE of PSCs w/o and with KCl modification (2 mg/mL) from 30 devices for each group.


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In order to investigate the photovoltaic properties of flexible PSCs with KCl modification, KCl layers (concentrations of 0.5 mg/mL, 1 mg/mL, 2 mg/mL and 5mg/mL) were inserted between the SnO2 and perovskite layers in this work. The average photovoltaic data of open-circuit voltage (VOC), short-circuit current density (JSC), fill factor (FF) and PCE were summarized in Table 1. When the concentration of KCl solution reached 2 mg/mL, the best flexible PSC in this work was achieved and the average PCE, VOC, JSC and FF were evaluated to be 17.50%, 1.08V, 21.50 mA/cm2 and 0.76, respectively. In addition, the average PCE of the flexible PSCs without KCl modification was only 15.83%. The average photovoltaic parameters (the statistics of photovoltaic data) of the devices were depicted in Figure 3 and the significant promotion of VOC, JSC, FF and PCE was realized when the flexible PSCs were modified by KCl layer (2 mg/mL). The enhanced FF was ascribed to the better morphology and the less defect of KCl-modified perovskite layers, which could also improve the hysteresis of corresponding flexible PSCs. Furthermore, the champion device based on KCl layer (2mg/mL) exhibited the maximum photovoltaic data, a PCE of 18.53%, a VOC of 1.11V, a JSC of 20.69 mA/cm2 and a FF of 0.81. An obvious hysteresis of the control device without KCl was shown in the J-V curves (Figure 4a) and the PCE under reverse and forward scan directions (0.1 V/s) was evaluated to be 15.84% and 14.79% respectively. However, the hysteresis of the device with KCl modification (2 mg/mL) was reduced as shown in Figure 4b, which was attributed to the existence of K+ ion (passivation effect) in the interface of ETL/perovskite35-37.



Figure 4. J-V curves of PSCs without (a) and with KCl modification (2 mg/mL) (b) under different scanning directions with a scan rate of 10 mV/s. Steady-state photocurrent output and PCE of PSCs without (c) and with KCl modification (2 mg/mL) (d) measured at the maximum power point.

The steady-state power output was measured at the maximum power point under the simulated solar illumination. The device modified with KCl (2 mg/mL) showed a stabilized PCE of 18.53% at a bias of 0.95 V (Figure 4d), while the device without KCl showed a PCE of 15.87% at a bias of 0.86 V (Figure 4c). The experimental data of the steady-state power output were matched well with the PCE evaluated from the J−V curves of the PSCs. Therefore, the stable power output of the devices with KCl modification was realized, which implied the promising application of the flexible PSCs . Moreover, the IPCE spectra were recorded as shown in Figure S4 and the integrated Jsc obtained from IPCE spectra of the control and KCl-based (2mg/mL) devices was 20.70 mA/cm2 and 20.43 mA/cm2, respectively.


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The steady-state PL spectra were measured to investigate the charge separation and transportation of the perovskite films without and with KCl modification (2 mg/mL). As shown in Figure 5a, an obvious attenuation of PL intensity was observed when KCl was inserted between SnO2 and perovskite layer, indicating the efficient electron injection from perovskite into SnO2 layer38. Moreover, the time-resolved photoluminescence (TRPL) measurement (Figure S3) was carried out to evaluate the electron extraction capacities of devices with and without KCl. The stretched exponential decay lifetimes were obtained by fitting the data with a biexponential decay function39. A=B1exp (t/τ1) + B2exp (t/τ2) +A0, where A was the intensity of TRPL, B1 and B2 were the relative amplitudes, τ1 and τ2 were the lifetimes for the fast and slow recombination, respectively. The ITO/SnO2/KCl/perovskite films showed the short-lived and long-lived lifetimes of 𝜏1 = 4.8 ns and 𝜏2 = 33.3 ns (1 mg/mL of KCl solution), 𝜏1 = 5.3 ns and 𝜏2 = 33.3 ns (2 mg/mL of KCl solution), respectively. By contrast, ITO/SnO2/perovskite films exhibited the short-lived and long-lived lifetimes of 𝜏1 = 2.1 ns and 𝜏2 = 34.0 ns as summarized in Table S2. In the biexponential PL decay process, the short-lived lifetime (𝜏1) was correlated with surface property and/or non-radiative recombination40. And

𝜏1

value of

ITO/SnO2/KCl/perovskite was effectively prolonged compared with that of the specimen without KCl modification, which implied the reduced defect concentration and non-radiative recombination of ITO/SnO2/KCl/perovskite films41.

Figure 5. (a) Steady-state photoluminescence (PL) of perovskite film without and



with KCl modification (2 mg/mL). (b) Current-voltage characteristics of the electron-only devices without and with KCl modification (2 mg/mL).

The electron-only devices of PET/ITO/SnO2/(KCl)/MAPbI3/PCBM/Ag were fabricated to investigate the trap density in the MAPbI3 film via the space-charge limited current (SCLC) model42 and the current-voltage characteristics of the devices were shown in Figure 5b. The SCLC curves could be devided into three regions according to the exponent n (J ∝ 𝑉𝑛): n=1 was the ohmic region, n=2 was the trap-free SCLC region and the section in between was the trap-filled limited region where n was over 3

43.

The SCLC curve showed a sharp increase when the bias voltage

exceeded the trap-filled limited voltage (VTFL), demonstrating that the trap states in MAPbI3 films were completely filled. VTFL for the electron-only devices without and with KCl modification (2 mg/mL) was 0.43 V and 0.22 V, respectively. Correspondingly, the trap density could be estimated by the following equation 44 : 𝑁𝑡𝑟𝑎𝑝𝑠 =

2𝜀0𝜀𝑟𝑉𝑇𝐹𝐿 𝑒𝐿2

where 𝜀0 was the vacuum permittivity, 𝜀 was the relative dielectric constant45 (~18) and L was the thickness of the perovskite film (~230 nm). The trap density of the devices without and with KCl modification (2 mg/mL) was calculated to be 1.65 × 1016cm-3 and 8.71 × 1015 cm-3, respectively. Therefore, the insertion of KCl between SnO2 and perovskite resulted in the low trap density of perovskite layer, which facilitated the high PCE and FF of the corresponding flexible PSCs. The ultraviolet photoelectron spectra (UPS) of SnO2 and SnO2/KCl films were carried out to study the energy levels of the films as shown in Figure 6c and 6d, respectively. The valence band maximum (EVBM) of the films was calculated according to the method described in the previous work46. The EVBM of SnO2 and SnO2/KCl was evaluated to be -8.35 eV and -7.98 eV, respectively. The absorption edge of the SnO2 and SnO2/KCl films was both 348 nm as shown in Figure 6e and the energy band gap of the films was evaluated to be 3.56 eV accordingly47. Therefore, the conduction band minimum (ECBM) of SnO2 and SnO2/KCl films was calculated to


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be -4.79 eV and -4.42 eV, respectively. The energy level diagram of the flexible PSCs without and with KCl modification was shown in Figure 6b. The ECBM of SnO2/KCl film was increased by ~0.37 eV compared with that of the pristine SnO2 film, which facilitated the high VOC and PCE of the corresponding flexible PSCs46.

Figure 6. (a) The device structure of the flexible PSCs with KCl modification; (b) Energy level diagram of the flexible PSCs w/o and with KCl modification; UPS spectra of SnO2 (c) and SnO2/KCl films (d); Absorbance spectra (e) of SnO2 and SnO2/KCl films.



The stability and bendability of devices were also key issues for flexible PSCs48-49. When the unencapsulated flexible PSCs modified with KCl (2 mg/mL) were stored in the air at a relative humidity of 40% for 120 days, the PCE of the devices was retained ~90% of the initial value, which was ascribed to the superior crystallinity and reduced defect density of the perovskite film. In contrast, the PCE of the control device without KCl was remained only ~75% of the initial value in the same experimental condition as shown in Figure 7a, which was attributed to the degradation of perovskite via the vacancy-assisted decomposition mechanism.50-52 Therefore, the stability of flexible PSCs was effectively improved by the insertion of KCl (2 mg/mL) between SnO2 and perovskite layers.

Figure 7. (a) The stability of the flexible PSCs without and with KCl (2 mg/mL) at a relative humidity of 40%. (b) The bendability of the flexible PSCs at a radius of 5 mm.

The bendability of the flexible PSCs was studied by being bended at a radius of 5 mm. When the devices were treated for 100 bending cycles, the PCE of the PSC modified with KCl (2 mg/mL) was remained ~80% of the initial value, which was higher than that of the control device without KCl as shown in Figure 7b. The repeated bending process would produce cracks in the ETL and perovskite layers and then the flexible PSCs were degraded. However, inserting KCl layer between SnO2


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and perovskite could facilitated the promoted morphology and reduced defect density of ETL(SnO2) and perovskite layers. As the cracks were initiated at defects and propagated more across the whole surface53-54, the formation of cracks in the flexible PSCs was decreased via the KCl modification. Therefore, the insertion of KCl between SnO2 and perovskite layers promoted the bendability of the flexible devices, which implied a promising strategy for the applications of flexible PSCs.

4. CONCLUSIONS Low-temperature processed ( ≤ 100℃) flexible PSCs were fabricated with the n-i-p structure of ITO/SnO2/KCl/MAPbI3/spiro-OMeTAD/Ag. With the modification of KCl between SnO2 and MAPbI3, the high ECBM of ETL (SnO2/KCl) was achieved, which promoted the VOC and PCE of the flexible PSCs. In addition, the hysteresis of the PSCs modified with KCl was reduced by the passivation effect of K+ ion. The strong ionic bonding between KCl and MAPbI3 induced the superior crystallinity and reduced defect density of perovskite films. The stability and bendability of the flexible PSCs with KCl modification (2 mg/mL) were simultaneously improved. The champion device with KCl modification (2mg/mL) exhibited a PCE of 18.53%, a VOC of 1.11V, a JSC of 20.69 mA/cm-2 and a FF of 0.81, respectively. It is worthy to note that both the PCE and FF are the highest values reported so far for flexible PSCs. Therefore, the insertion of KCl between SnO2 and perovskite was an effective and low-cost strategy for highly efficient flexible PSCs.

ASSOCIATED CONTENT Supporting Information



AUTHOR INFORMATION Corresponding Author *Email: [email protected] Conflicts of interest There is no conflict to declare. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China under grant Nos 11574013, U1605244, 11527901, 61575005 and 61775004.

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Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J., Efficient Hybrid

Solar Cells Based on Meso-Superstructured Organometal Halide Perovskites. Science 2012, 1228604. 3.

Liu, M.; Johnston, M. B.; Snaith, H. J., Efficient planar heterojunction perovskite solar

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High Efficiency (18.53%) of Flexible Perovskite Solar Cells via the

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