Pyridine-Functionalized Fullerene Electron Transport Layer for

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 23982−23989

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Pyridine-Functionalized Fullerene Electron Transport Layer for Efficient Planar Perovskite Solar Cells Hao-Ran Liu,†,§ Shu-Hui Li,‡,§ Lin-Long Deng,*,† Ze-Yu Wang,† Zhou Xing,‡ Xiang Rong,† Han-Rui Tian,‡ Xin Li,*,† Su-Yuan Xie,*,‡ Rong-Bin Huang,‡ and Lan-Sun Zheng‡ Pen-Tung Sah Institute of Micro-Nano Science and Technology and ‡State Key Lab for Physical Chemistry of Solid Surfaces, Collaborative Innovation Center of Chemistry for Energy Materials, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China

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†

S Supporting Information *

ABSTRACT: In regular perovskite solar cells (PSCs), the commonly used electron transport layer (ETL) is titanium oxide (TiO2). Nevertheless, the preparation of a high-quality TiO2 ETL demands an elevated-temperature sintering procedure, unfavorable for fabrication of PSCs on flexible substrates. Besides, TiO2-based devices often suffer from notorious photocurrent hysteresis and serious light soaking instability, limiting their potential commercialization. Herein, a novel pyridine-functionalized fullerene derivative [6,6]-(4-pyridinyl)-C61-ethyl acid ethyl ester (PyCEE) was synthesized and applied as an ETL to replace TiO2 in n−i−p PSCs. PyCEE-based devices achieved a champion power conversion efficiency (PCE) of 18.27% with significantly suppressed hysteresis, superior to that of TiO2-based devices. PyCEE has suitable energy levels and high electron mobility, which facilitate electron extraction/transport. Besides, the pyridine moiety within PyCEE affords coordination interactions with the Pb2+ ion within CH3NH3PbI3, passivating the trap states of CH3NH3PbI3 and thus improving the device performance and suppressing hysteresis greatly. Moreover, PyCEE ETLs were applied in flexible PSCs, achieving a PCE of 15.25%. Our results demonstrated the applicability of PyCEE ETLs in flexible devices and provided new opportunity for the commercialization of PSCs. KEYWORDS: perovskite solar cells, electron transport layer, fullerene, pyridine, passivation

1. INTRODUCTION Perovskite solar cells (PSCs) have drawn great attention owing to their outstanding photovoltaic performance and are considered as next-generation photovoltaics.1−4 The power conversion efficiencies (PCEs) of PSCs have risen rapidly from 3.8 to 24.2% during the past decade.5−8 High-efficiency n−i−p structured PSCs generally employed TiO2 as the electron transport layer (ETL).9,10 Nevertheless, the TiO2 ETL needs to be sintered at around 450 °C, which is not suitable for flexible devices.7,11−13 Meanwhile, the relatively low electron mobility of TiO2 leads to severe photocurrent hysteresis in TiO2-based PSCs.14−16 Thus, it is quite necessary to explore low-temperature solution-processed, hysteresis-free, and highperformance ETLs that can effectively replace TiO2-based ETLs. Organic ETLs such as fullerenes and their derivatives are promising alternatives to TiO2-based inorganic ETLs, benefit© 2019 American Chemical Society

ing from their excellent electron transport properties, lowtemperature solution process, passivation of charge traps, and hysteresis alleviation.17 For instance, Snaith et al. adopted C60 as the ETL in regular PSCs and achieved an efficiency of 13.5% with negligible hysteresis.18 Seok et al. reported PSCs based on [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) ETLs and obtained a PCE of 15.3% on glass substrates and a PCE of 11.1% on flexible substrates, respectively.19 Cross-linkable groups,20,21 siloxane-functionalized agents,22 and n-type dopants23 have also been adopted to obtain appropriate fullerene ETLs for achieving high performance. Although the electron transport properties of fullerene ETLs could be enhanced by grafting different functional groups onto fullerene Received: February 21, 2019 Accepted: June 19, 2019 Published: June 19, 2019 23982

DOI: 10.1021/acsami.9b03304 ACS Appl. Mater. Interfaces 2019, 11, 23982−23989

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthesis of PyCEE

Figure 1. (a) ORTEP drawing of PyCEE at the 30% level (hydrogen atoms and CS2 omitted for clarity). (b) Crystal stacking of PyCEE viewed along the a-axis. Gray, carbon; white, hydrogen; red, oxygen; and blue, nitrogen. Solvent molecules are removed for clarity. (c) Schematic device structure based on PyCEE ETL. (d) The energy level diagram of various layers.

cage, including ether,24 carboxyl,25 ester,19 amino,26 etc., the interactions between these functional groups and the upper perovskite layer are relatively weak. As we all know that the perovskite layer commonly has many defects on the surfaces and grain boundaries,27,28 which significantly affect the device performance. It has been proved that Lewis bases can coordinate with undercoordinated Pb2+ ions,29 which can effectively passivate defects and greatly improve the quality of the perovskite layer. Thus, new fullerene ETLs are desperately necessary to be developed, which can afford coordination interactions with perovskite films. In this work, a new pyridine-functionalized fullerene derivative [6,6]-(4-pyridinyl)-C61-ethyl acid ethyl ester (PyCEE) was synthesized and applied as an ETL offering coordination interactions with a perovskite layer via a pyridine group, leading to enhanced efficiency and suppressed hysteresis in n−i−p PSCs. The PyCEE-based device achieved the champion PCE of 18.27% based on the fluorine-doped tin oxide (FTO)/PyCEE/CH3NH3PbI3/Spiro-OMePAD/Ag configuration, superior to that of TiO2-ETL-based device (16.91%). PyCEE has suitable energy levels and high electron mobility, which facilitate electron extraction/transport. Besides, the smooth and poor-wetting surface of PyCEE is favorable for growing high-quality and large-sized perovskite films. Meanwhile, the pyridine moiety within PyCEE offers coordination interactions with the Pb2+ ion within perovskite, which passivates the trap states of the perovskite film and leads

to enhanced PCE and suppressed hysteresis.29−31 Besides, a PCE of 15.25% has been achieved on PyCEE-based flexible devices, indicating its applicability in flexible devices.

2. RESULTS AND DISCUSSION As shown in Scheme 1, PyCEE was synthesized by the cycloaddition reaction of C60 with tosylhydrazone (see the Supporting Information for the detailed synthesis procedure). PyCEE is well soluble in commonly used organic solvents, including chlorobenzene, toluene, and o-dichlorobenzene (oDCB). The chemical structure of PyCEE was confirmed by 1H and 13C NMR and mass spectrometry (Figures S1−S3). There are several design considerations of PyCEE for the ETL. First, the [6,6] methanofullerene structure was chosen, which has a structure similar to that of PCBM, the most commonly used fullerene derivative in PSCs. Second, a flexible ethyl acetate chain was employed to provide enough solubility to ensure solution-processed preparation of the ETL. Last but not least, the pyridinyl group was introduced to passivate the defect of perovskite by coordination with undercoordinated Pb2+ ions in the perovskite layer,29−31 which will improve the device performance and suppress hysteresis. The molecular structure of PyCEE was identified by singlecrystal X-ray diffraction (Figure 1a). Single-crystal analysis indicated that PyCEE crystallizes in the monoclinic system and P21/n space group (Table S1). Each unit cell contains four fullerene molecules, as shown in Figure S4. The molecules are 23983


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ACS Applied Materials & Interfaces

Figure 2. (a) J−V curves of best-performing PSCs with different ETLs. (b) EQE spectra of the device with different ETLs. (c) Histograms of PCE measured for 50 devices based on the TiO2 or PyCEE ETL. (d) Stabilized photocurrent and power output at the maximum power point for devices with the TiO2 or PyCEE ETL.

Table 1. Photovoltaic Parameters of PSCs Based on Different ETLs ETL

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

TiO2 PCBM PyCEE

1.034 ± 0.045 1.046 ± 0.023 1.057 ± 0.016

21.87 ± 0.91 22.05 ± 0.68 22.49 ± 0.55

68.68 ± 2.80 69.46 ± 2.66 70.76 ± 2.73

15.51 ± 0.79 15.94 ± 0.73 16.81 ± 0.70

band (CB) energy of CH3NH3PbI3 (−3.9 eV).6,34 Additionally, PyCEE possesses excellent thermal stability with a decomposition temperature of 322 °C (Figure S7), indicating that it is stable enough for fabrication of PSCs. The surface property of an ETL is an important factor to determine whether it is suitable for fabricating regular perovskite solar cells. Thus, contact angles of water and N,Ndimethylformamide (DMF) on the FTO/TiO2 layer and FTO/PyCEE surface were measured (Figure S8). Water and DMF droplets exhibited good wettability on the FTO/TiO2 layer, leading to small contact angles of 11.5 and 7.8°, respectively. However, the contact angles of water and DMF on the FTO/PyCEE film increased to 89.2 and 28.1°, respectively, indicating a more poor-wetting surface of the PyCEE layer compared with the TiO2 layer. The nonwetting PyCEE surface will facilitate the formation of large-sized perovskite films.35 To evaluate the photovoltaic performance of PSCs using PyCEE as the ETL, planar n−i−p PSCs were fabricated (Figure 1c). The energy band diagram of PSCs is shown in Figure 1d. The LUMO energy level of PyCEE matches the CB of CH3NH3PbI3 perovskite well, which is favorable for charge transport from perovskite to PyCEE. The cross-section of a complete device was presented unambiguously in Figure S9. The thicknesses of PyCEE, perovskite film, Spiro-OMeTAD, and Ag are about 40, 250, 150, and 100 nm, respectively.

packed tightly. The shortest fullerene centroid-to-centroid distance is 9.89 Å (Figure S4), which is significantly less than 10.0 Å and is considered to be critical to facilitate efficient charge transport.32 PyCEE molecules form a zig-zag stack along the a-axis (Figure 1c) and b-axis (Figure S4), while they show formation of linear arrangement along the c-axis (Figure S4). Four different types of noncovalent intermolecular interactions contribute to close crystal stacking (Figure S4), including π···π interactions, CH···π interactions, and hydrogen bonds. Thus, it is expected that the well-packed PyCEE molecules show great potential as efficient ETLs. The ultraviolet−visible (UV−vis) absorption spectra of PyCEE and PCBM were measured and are exhibited in Figure S5. PyCEE and PCBM exhibit nearly identical spectra, with a strong absorption band at around 331 nm and two weak absorption bands at 431 and 698 nm, indicating PyCEE with a methano[60]fullerene pattern.33 Based on the maximum absorption onset of PyCEE, a gap value (Eg) of 1.73 eV was estimated between lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels. Besides, cyclic voltammetry (Figure S6) was carried out to study the redox properties of the PyCEE and PCBM. The onset reduction potentials together with the estimated energy levels of PyCEE and PCBM are summarized in Table S2. The calculated LUMO energy level of PyCEE was around −3.94 eV, which is well-aligned with the conduction 23984


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Figure 3. AFM images of the (a) TiO2 film and (c) PyCEE film on FTO substrates. Top view SEM images of the perovskite film on (b) FTO/ TiO2, and (d) FTO/PyCEE substrates.

mA/cm2 at 0.85 V, corresponding to the PCE of 17.70%. For comparison, the TiO2-based devices delivered a PCE of 15.62% at 0.81 V. The improved efficiency, better reproducibility, and negligible hysteresis indicate that PyCEE is an efficient ETL for planar n−i−p PSCs. To investigate the effects of the PyCEE ETL on device performance, a series of characterization methods were conducted. The surface morphology of the TiO2 or PyCEE ETL was observed by atomic force microscopy (AFM) (Figure 3a,c). The root-mean-square roughness of TiO2 was 8.1 nm, compared with 4.2 nm of PyCEE. The smooth and poorwetting surface (Figure S8) of PyCEE contributes to the formation of pinhole-free and large-size perovskite films by suppressing heterogeneous nucleation.35 Figure 3b,d exhibit the scanning electron microscopy (SEM) pictures of perovskite film deposited on the FTO/TiO2 and FTO/PyCEE substrates, respectively. Perovskite films grown on both FTO/TiO2 and FTO/PyCEE substrates were dense and uniform. Compared with the relatively small grain size of perovskite film grown on the FTO/TiO2 substrate, the average grain size is much larger for the perovskite film grown on the FTO/PyCEE substrate. Figure S12 shows the grain size distributions of perovskite layers based on different ETLs. The average grain sizes are 236 and 352 nm for perovskite films grown on FTO/TiO2 and FTO/PyCEE substrates, respectively. The influence of the ETL on the crystallization of perovskite films was further investigated by X-ray diffraction (XRD) measurements (Figure S13). Diffraction peaks appeared at 14.2, 28.5, and 32.0°, which correspond to the (110), (220), and (310) planes of tetragonal perovskite, respectively.36 Interestingly, the perovskite film grown on the FTO/PyCEE substrate showed higher peaks, suggesting improved crystallinity of the perovskite film.10,37 The enhanced crystallization of perovskite may improve the light absorption of the

The current density−voltage (J−V) curves of PSCs based on the PyCEE ETL are shown in Figure 2a, together with those of the TiO2-based devices for comparison. Since PCBM is one of the most widely used ETLs in PSCs, the photovoltaic performance of PCBM-based devices was also included for comparison. The photovoltaic parameters are listed in Table 1. The TiO2-based device showed a champion PCE of 16.91% with an open-circuit voltage (Voc) of 1.06 V, a short-circuit current density (Jsc) of 22.04 mA/cm2, and a fill factor (FF) of 72.19%. The PCBM-based control device displayed the best PCE of 17.05% with a Voc of 1.05 V, a Jsc of 22.52 mA/cm2, and a FF of 72.11%. The best-performing device employing the PyCEE ETL displayed an enhanced PCE of 18.27% with a Voc of 1.05 V, a Jsc of 22.95 mA/cm2, and a FF of 75.83%. It can be concluded that the enhanced PCE is mainly due to improved Jsc and FF. The external quantum efficiency (EQE) curves of PSCs with PyCEE, TiO2, or PCBM ETLs are shown in Figure 2b. The integrated Jsc of the PyCEE-based device is 21.74 mA/ cm2, higher than that of the TiO2-based device (20.68 mA/ cm2) or the PCBM-based device (21.16 mA/cm2). To inspect the repeatability of photovoltaic performance, histograms of the device performance obtained from 50 devices based on the PyCEE or TiO2 ETL are summarized in Figure 2c. The majority of devices based on the PyCEE ETL displayed a PCE over 16.8%, whereas TiO2-based devices exhibited a PCE of 15.5%. The J−V hysteresis is further investigated by measuring the J−V curves from reverse and forward scans (Figures S10 and S11). TiO2-based devices exhibited obvious hysteresis, whereas PyCEE-based devices displayed negligible hysteresis. To avoid inaccurate estimation of the device performance by hysteresis, the steady-state photocurrent output and corresponding PCE of devices with the PyCEE or TiO2 ETL were obtained (Figure 2d). The PyCEE-based devices displayed an output current of 20.83 23985


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ACS Applied Materials & Interfaces perovskite film,38 as confirmed by the UV−vis absorption spectra shown in Figure S14, contributing to the increase of Jsc. Furthermore, X-ray photoelectron spectroscopy (XPS) characterization was used to check the coordination interactions between the pyridine moiety within PyCEE and the Pb2+ ion within CH3NH3Pbl3 (MAPbl3). As shown in Figure 4,

Figure 4. XPS spectra of Pb 4f of MAPbI3 perovskite and MAPbI3/ PyCEE films.

the Pb 4f XPS spectrum of pristine CH3NH3PbI3 films exhibits two characteristic Pb 4f5/2 and Pb 4f7/2 signals at 143.7 and 138.8 eV, respectively. For CH3NH3PbI3/PyCEE films, both signals show lower binding energies of 143.5 and 138.6 eV, respectively. The reason is probably that the Pb2+ ion accepts a lone pair of electrons from the nitrogen atom of the pyridine moiety within PyCEE via coordination bonding.30 The coordination interactions effectively passivate the trap states of perovskite, which contributes to the enhancement of PCE and suppression of hysteresis.10 The electron extraction behavior between the perovskite and ETL was investigated by steady-state and time-resolved photoluminescence (PL) measurements. The PL peak of CH3NH3PbI3 at around 770 nm shows a significant quenching effect for the perovskite film deposited on the TiO2 or PyCEE substrate (Figure 5a). Furthermore, the perovskite film on the PyCEE ETL exhibits more efficient PL quenching than that on the TiO2 ETL, indicating faster electron transfer between CH3NH3PbI3 and PyCEE.27 In addition, time-resolved photoluminescence (TRPL) spectroscopy was used to confirm the improved charge transfer from perovskite to PyCEE (Figure 5b). A biexponential function was used to fit the data. The fitting parameters are listed in Table S3. The average recombination lifetime (τave) was 18.79 ns for perovskite films on the FTO/TiO2 substrate, which was further decreased to 9.98 ns by replacing TiO2 with PyCEE. The smaller carrier lifetime obtained for PyCEE indicates more effective charge transfer from perovskite to PyCEE,20,39 leading to enhanced Jsc values of PyCEE-based devices. To understand why PyCEE-based devices exhibit better device performance and smaller J−V hysteresis, the direct current conductivities (σ0) of TiO2 and PyCEE were compared. The σ0 can be calculated from the slopes of the current−voltage (I−V) plots of FTO/TiO2/Ag and FTO/ PyCEE/Ag devices,37,40,41 respectively (Figure S15). The corresponding conductivity of TiO2 was 5.5 × 10−3 mS/cm. In contrast, the conductivity of PyCEE was 3.4 × 10−2 mS/cm, which is around sixfold higher than that of TiO2. The higher conductivity of PyCEE indicates that it has better charge

Figure 5. (a) Steady-state PL and (b) TRPL spectra of perovskite films deposited on glass, FTO/TiO2, and FTO/PyCEE substrates.

transport properties and is beneficial for the balance of electron and hole transport. Electrochemical impedance spectroscopy (EIS) was employed to further inspect the interfacial charge transport behavior of PSCs. The Nyquist plots of PSCs are shown in Figure S16. The high frequency feature represents the contact resistance (Rco) at the perovskite/ETL interface, and the lower frequency feature corresponds to the systematic recombination resistance (Rrec) and chemical capacitance (CPE2).42,43 The fitted parameters obtained from Nyquist plots are tabulated in Table S4. Compared to the TiO2-based device, the Rrec of the PyCEE-based device increased from 1094 to 2595 Ω·cm2, suggesting a lower electron−hole recombination rate at the interface between perovskite and the ETL. This result indicates that PyCEE suppresses charge recombination, contributing to higher Jsc and FF values of the PyCEE-based device. In addition to photovoltaic performance, long-term stability is also a significant aspect for the commercialization of PSCs. Thus, the ambient stability of the unencapsulated devices with the TiO2 or PyCEE ETL was compared (Figure S17). The control devices based on the TiO2 ETL lost ca. 30% of their initial PCEs after 350 h, whereas the PyCEE-based devices maintained ca. 90% of their initial performance. The better stability of the PyCEE-based devices is likely attributed to the hydrophobic nature of PyCEE, which can alleviate the intrusion of moisture into the perovskite layer.44 Thus, PyCEE is a promising alternative to the conventional TiO2 ETL in terms of device stability. To prove the superiority of PyCEE in flexible PSCs, we fabricated PSCs with the PyCEE ETL on indium tin oxide (ITO)-coated poly(ethylene naphthalate) (PEN) substrates. Since the maximum temperature that these PEN/ITO substrates can withstand is around 150 °C,45,46 the two-step deposition method used above is not suitable for fabrication of the MAPbI3 film on these flexible substrates. In the meantime, the PEN substrates are easily penetrated by water vapor 23986


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Figure 6. (a) J−V curves of flexible device with the PyCEE ETL. The inset shows the image of a practical flexible device. (b) The normalized PCE values of the flexible device as a function of bending cycles at a curvature radius of 10 mm.

compared to the glass substrates,47 which is harmful for the stability of the MAPbI3 film. Hence, we used a one-step method to deposit more stable mix-ion perovskite films, CsFAMAPbI3−xBrx,48 on the PEN/ITO substrates for flexible devices. As shown in Figure 6a, the J−V curve of a champion flexible device based on the PyCEE ETL presented a PCE of 15.25% with a Voc of 1.09 V, a Jsc of 20.11 mA/cm2, and a FF of 69.55%. Apart from the photovoltaic performance, durability is also of great importance. Herein, the mechanical stability of the flexible devices was inspected by repeatedly bending at a curvature radius of 10 mm. As shown in Figure 6b, the flexible cells maintained 94% of original PCE after 1000 bending cycles, which exhibited excellent device stability against mechanical bending. The degraded performance of the flexible devices was mainly due to the degraded ITO conductive film that is brittle during bending cycles.49,50

acetonitrile). The Spiro-MeOTAD solution was spun on top of the perovskite film at 3000 rpm for 30 s. Finally, Ag (∼100 nm) was deposited by using a thermal evaporator at 2 × 10−4 Pa through a shadow mask. 4.2. Device Characterization. The J−V characteristics of PSCs were measured under AM1.5G simulated solar irradiation (100 mW/ cm2 from a Newport Oriel 92192 Solar Simulator). The area of each device is ∼0.06 cm2 for all of the photovoltaic devices in this work. The EQE was recorded on a Newport Oriel QE/IPCE measurement kit. The SEM images were taken on a field emission SEM (HITACHI S-4800). XRD patterns were recorded with a Rigaku Ultima IV diffractometer. AFM measurements were carried out on an XE-7 atomic force microscope (Park systems, Korea). XPS was measured by a PHI Quantum-2000 electron spectrometer. TRPL spectra were measured using an Edinburgh Instruments FLS980 spectrometer. EIS measurements were performed with a CHI660E electrochemical workstation.

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3. CONCLUSIONS In summary, a novel pyridine-functionalized fullerene derivative PyCEE was successfully synthesized, which can act as an excellent ETL for efficient PSCs. The molecular structure of PyCEE was confirmed by 1H NMR, 13C NMR, APCI mass spectrometry, and X-ray single-crystal analysis. Benefiting from its suitable energy levels, high electron mobility, and coordination interactions with the Pb 2+ ion within CH3NH3PbI3, PyCEE-based PSCs achieved the champion PCE of 18.27% with dramatically suppressed hysteresis. Therefore, PyCEE can be considered as a competitive alternative to the TiO2 ETL for PSCs. Moreover, a PCE of 15.25% has been obtained on a PyCEE ETL based flexible PSC, which demonstrates the applicability of PyCEE in flexible PSCs and provides new opportunity for large-scale production and commercialization of PSCs.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03304. Synthesis of PyCEE; 1H NMR, 13C NMR, and APCIMS spectra; crystal structure; UV−vis absorption spectra; cycle voltammograms; TGA analysis; water and DMF contact angles; cross-sectional SEM image; J− V curves from reverse and forward scans; grain size distributions; XRD patterns and UV−vis absorption spectra of perovskite films; I−V characteristics of TiO2 and PyCEE devices; Nyquist plots of TiO2 and PyCEE devices (PDF)

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4. EXPERIMENTAL SECTION

Crystallographic data and parameters for PyCEE (CIF)

AUTHOR INFORMATION

Corresponding Authors

4.1. Device Fabrication. Patterned FTO glasses were washed with ultrasonic baths of detergent, deionized water, acetone, and 2propanol and further treated with ultraviolet-ozone for 10 min. Then, TiO2, PCBM, or PyCEE ETLs were deposited onto the FTO substrates. The TiO2 ETL was prepared according to a previously reported method.51 The fullerene (PCBM or PyCEE) ETL (∼40 nm) was spin-coated from chlorobenzene solution (20 mg/mL) at 3000 rpm for 30 s. The perovskite layer was then deposited onto FTO/ TiO2 or FTO/fullerene substrates referring to a modified gas−solid crystallization process.51 For flexible devices, the perovskite film was prepared by a one-step method.48 Spiro-OMeTAD solution was prepared by mixing 72.3 mg of Spiro-OMeTAD with additives in 1 mL of chlorobenzene. Additives were composed of 28.8 μL of 4-tertbutylpyridine and 17.5 μL of Li-TFSI solution (520 mg/mL in

*E-mail: [email protected] (L.-L.D.). *E-mail: [email protected] (X.L.). *E-mail: [email protected] (S.-Y.X.). ORCID

Lin-Long Deng: 0000-0002-8588-1825 Xin Li: 0000-0003-2036-1367 Su-Yuan Xie: 0000-0003-2370-9947 Author Contributions §

H.-R.L. and S.-H.L. contributed equally.

Notes

The authors declare no competing financial interest. 23987


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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21721001, 51572231, 51502252, and 21701134), the Natural Science Foundation of Fujian Province of China (2016J01264), and the China Postdoctoral Science Foundation (2016M602067).

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