Anchoring Fullerene onto Perovskite Film via Grafting Pyridine toward

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Organic Electronic Devices

Anchoring Fullerene onto Perovskite Film via Grafting Pyridine toward Enhanced Electron Transport in High-Efficiency Solar Cells Bairu Li, Jieming Zhen, Yangyang Wan, Xunyong Lei, Qing Liu, Yajuan Liu, Lingbo Jia, Xiaojun Wu, Hualing Zeng, Wenfeng Zhang, Guan-Wu Wang, Muqing Chen, and Shangfeng Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 Aug 2018 Downloaded from http://pubs.acs.org on August 28, 2018

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ACS Applied Materials & Interfaces

Anchoring Fullerene onto Perovskite Film via Grafting Pyridine toward Enhanced Electron Transport in High-Efficiency Solar Cells Bairu Li,†⊥ Jieming Zhen,†⊥ Yangyang Wan,† Xunyong Lei,‡ Qing Liu,† Yajuan Liu,† Lingbo Jia,† Xiaojun Wu,† Hualing Zeng,‡ Wenfeng Zhang,§ Guan-Wu Wang,∥ Muqing Chen,*† and Shangfeng Yang*† †

Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, Synergetic Innovation Center of Quantum Information & Quantum Physics, University of Science and Technology of China, Hefei 230026, China ‡

ICQD, Hefei National Laboratory for Physical Sciences at Microscale, CAS Key Laboratory of Strongly-coupled Quantum Matter Physics, University of Science and Technology of China, Hefei 230026, China §

School of Engineering, Anhui Agricultural University, 130 West Changjiang Road, Hefei 230036, China

∥

CAS Key Laboratory of Soft Matter Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Department of Chemistry, University of Science and Technology of China, Hefei 230026, China * Corresponding Authors. E-mail: [email protected] (M. C.); [email protected] (S.Y.) ⊥

These authors contributed equally to this work.

KEYWORDS: perovskite solar cell, fullerene derivative, pyridine, alkyl chains, electron transporting layer

1 Environment ACS Paragon Plus

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ABSTRACT: Fullerene derivatives have been popularly applied as electron transport layers (ETLs) of inverted (p-i-n) planar heterojunction perovskite solar cells (iPSCs) due to their strong electron-accepting abilities, and so far [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is the most commonly used ETL, which suffers however from high cost due to the complicated synthetic route. Herein, novel pyridine-functionalized fullerene derivatives (abbreviated as C60Py) were synthesized facilely via a one-step 1,3-dipolar cycloaddition reaction, and applied as ETLs superior to PCBM in iPSCs devices. Three pyridine-functionalized fullerene derivatives with different alkyl groups including methyl, n-butyl and n-hexyl grafted onto the pyrrolidine moiety (abbreviated as C60-MPy, C60-BPy and C60-HPy, respectively) were synthesized. According to cyclic voltammogram study, the chain length of the N-alkyl group has negligible influence on the molecular energy level of C60-Py. However, the ETL performance of C60-Py is sensitively dependent on the chain length of the N-alkyl group, with C60-BPy exhibiting the highest power conversion efficiency (PCE) of 16.83%, which surpasses that based on PCBM ETL (15.87%). The PCE enhancement of C60-BPy device is attributed to the coordination interactions between the pyridine moiety with the Pb2+ ion of CH3NH3PbI3 perovskite, which anchor C60-BPy onto perovskite film and reinforce the passivation of the trap state within CH3NH3PbI3 perovskite film and suppress the nonradiative electron-hole recombinations, leading to enhanced electron transport reflected by the increase of short-circuit current density (Jsc). The ambient stability of C60-HPy-based device is much better than that based on PCBM ETL since its long N-alkyl group can function as a superior encapsulating layer protecting the CH3NH3PbI3 layer from contact with the ambient moisture.


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1. Introduction Perovskite solar cells (PSCs) based on organometal halides as light absorbers have been attracting wide-spread attention owing to the unique optical and electronic properties of organometal halide materials, including large absorption coefficient, adjustable bandgaps, high carrier mobility, and long carrier recombination lifetime.1-5 A great deal of efforts have been made during the past few years on improving the performance of PSC device by optimizing the device structure, the composition, phase and morphology of the perovskite light absorber layer as well as the perovskite/electrode interfaces.3, 6-9 As a result, an ever-increasing power conversion efficiency (PCE) reaching 23.2% has been achieved very recently,10 and this enables PSC quite promising in competing with the commercialized crystalline-Si and inorganic semiconductor thin film solar cells.11-13 Planar heterojunction structure is one of the popularly used architectures of PSC device, for which organometal halide perovskite such as CH3NH3PbI3 is sandwiched between the electron transport layer (ETL) and hole transport layers (HTL), comprising of the so-called regular (n-i-p) or inverted (p-i-n) device differentiated according to the locations of the ETL and HTL relative to the transparent electrode for light illumination.11 Specifically, the inverted (p-i-n) structure PSCs (iPSCs) with HTL clinging to the bottom transparent electrode and ETL deposited atop of perovskite layer is advantageous in terms of the low-temperature solution processibility compatible with flexible substrates and negligible hysteresis of current-voltage response.11,12 In order to improve the performance of iPSCs so as to meet the future commercialization requirement, interface engineering by means of tailoring the HTL/ETL materials has been revealed to be effective to promote charge transport and extraction, and this can be fulfilled by improving the interfacial contact between the perovskite layer and electrodes and/or adjusting the



work function of electrode.13-16 In particular, zero-dimensional nanocarbon materials such as fullerene derivatives featuring in strong electron-accepting ability have been popularly applied as ETLs of iPSCs,17 and nowadays [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) as the first effective ETL utilized in 2013 is the most commonly used ETL, which suffers however from high cost due to the complicated synthetic route.18 Therefore, it is highly desirable to develop novel fullerene derivatives as alternative ETLs of iPSCs, which can be facilely synthesized by ideally one-step reaction.19 For instance, a novel fullerene derivative, 2,5-(dimethyl ester) C60 fulleropyrrolidine (DMEC60), was synthesized by Echegoyen et al. via a one-step 1,3-dipolar cycloaddition reaction and applied as an ETL, affording a PCE of 15.2%, which was higher than that of PCBM (14.5%).20 A C60-substituted benzoic acid self-assembled monolayer (C60-SAM) synthesized initially by Jen et al. via a one-step Prato reaction was used as ETL in iPSCs showed an inferior PCE of 6% in iPSC devices,21 which was later improved to 19.5% by means of crosslinking with trichloro(3,3,3-trifluoropropyl)silane followed by doping CH3NH3I.22 More recently, N-methyl-2-pentyl-[60]fullerenepyrrolidine (NMPFP) was synthesized by Chen et al. via a similar Prato reaction, leading to a comparable PCE (13.83%) to that of PCBM-based device (13.87%).23 Note that, among them only few fullerene derivatives such as DMEC60 were reported to achieve higher PCEs than that based on PCBM ETL. Besides, the functional groups grafted onto fullerene cage are limited to alkyl, carboxyl, ester, dimethylamine and so on,24-28 which render relatively weak interactions with the bottom perovskite layer. Hence, whether fullerene derivatives grafting other functional groups can afford reinforced interfacial contact with perovskite and thus lead to increased PCE relative to PCBM remains an open question. Herein, pyridine group was grafted onto fullerene cage via a facile one-step 1,3-dipolar cycloaddition reaction, and the as-synthesized pyridine-functionalized fullerene derivatives


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(abbreviated as C60-Py) were applied as effective ETLs in iPSCs devices for the first time, affording PCE superior to device based on PCBM ETL. Three different alkyl groups including methyl, n-butyl and n-hexyl were attached onto the pyrrolidine moiety of C60-Py, and the effects of the substitution group on the molecular energy level and ETL performance of C60-Py were investigated, revealing that the ETL performance of C60-Py is sensitively dependent on the length of N-alkyl group although its influence on the molecular energy level is negligible. The grafting of the pyridine moiety within C60-Py leads to anchoring of C60-Py onto perovskite layer via the coordination interactions between pyridine with the Pb2+ ion of CH3NH3PbI3 perovskite, which contribute to the enhanced electron transport property of C60-Py relative to PCBM.

2. Results and Discussion 2.1. Synthesis and characterization of pyridine-functionalized fullerene derivatives (C60-Py). The novel pyridine-functionalized fullerene derivatives (C60-Py) were synthesized facilely via a one-step 1,3-dipolar cycloaddition reaction (i.e., Prato reaction) as shown in Scheme 1. The detailed synthetic procedures are given in the experimental section. In order to investigate the effect of the substitution group on the yield and solubility of C60-Py, we rationally designed and synthesized three derivatives with different alkyl groups including methyl, n-butyl and n-hexyl grafted onto the pyrrolidine moiety (abbreviated as C60-MPy, C60-BPy and C60-HPy, respectively) by choosing different amino acids with variable N-alkyl groups as reactant. The yields of C60MPy, C60-BPy and C60-HPy purified via silicon column are 37.5%, 34.6% and 35.2%, respectively, which are comparable. The solubilities of C60-MPy, C60-BPy and C60-HPy in chlorobenzene are ca. 15 mg/ml, 16 mg/ml and 18 mg/ml, respectively, indicating that longer alkyl chain leads to higher solubility of C60-Py.



Scheme 1. The synthetic routes of C60-MPy, C60-BPy and C60-HPy.

The chemical structures of C60-MPy, C60-BPy and C60-HPy were characterized by 1H and 13C NMR spectroscopy, Fourier transform infrared spectrometry (FT-IR) and matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry. According to the MALDI-TOF mass spectra (see Supporting Information Figure S1), C60-MPy, C60-BPy and C60HPy show dominant ionic peaks at m/z=854, 896 and 924 respectively along with peaks assigned to C60 as decomposed compound due to laser irradiation, coinciding with their expected chemical forms as mono-adducts. In the 1H NMR spectra of C60-MPy, C60-BPy and C60-HPy , the proton signals of the pyrrolidine and pyridine groups are clearly observed as well as those of the N-alkyl groups (see Supporting Information Figure S2), confirming their proposed structures. Moreover, according to the

13

C NMR spectra of C60-MPy, C60-BPy and C60-HPy, in addition to the sp2-

carbon signals of the C60 cage and pyridine rings found in the range of 124.1-155.9 ppm, the sp3carbon signals in the range of 66.9-82.4 ppm were assigned to the -CH2- and -CH- moieties in pyrrolidine ring and sp3-carbon atoms at the addition sites of C60 cage, while the sp3-carbon


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signals of the N-alkyl groups are observed in the range of 14.5 - 53.3 ppm (see Supporting Information Figure S3). We further performed FT-IR spectroscopic study to confirm the successful grafting of the pyridine group in C60-Py, for which the vibrational signals are found at 2781/2787, 1593/1595 and 767/769 cm-1 (see Supporting Information Figure S4). Besides, the vibrational signals at 1177/1179 cm-1 are assigned to the stretching vibrations of the C-N bonds within the pyrrolidine moiety of C60-Py, and those at 2916-2952, 1412, 824-834 cm-1 were assigned respectively to the C-H stretching vibration, in-plane blending vibration and out-of-plane blending vibration of the N-alkyl chains attached onto the pyrrolidine moiety. On the other hand, the characteristic vibrational peaks of C60 at 1457/1463 and 526 cm-1 are retained, consistent with the chemical structure of C60-Py. Thermal gravimetric analysis (TGA) was used to evaluate the thermal stability of C60-Py (see Supporting Information Figure S5). Two weight-loss processes in the TGA curves of C60-Py are observed, including, 1) In the first weight-loss step (348.1-400.3 oC, 399.8-452.2 oC, 378.5-469.1 o

C for C60-MPy, C60-BPy and C60-HPy, respectively), 4.7% - 5.1% weight losses are observed,

which can be assigned to the detaching of the functional groups of C60-Py; 2) The second weightloss at the higher temperatures (400.3-575.9 oC, 452.2-592.8 oC, and 469.1-606.3oC for C60-MPy, C60-BPy and C60-HPy, respectively) are correlated to the decomposition of the C60 cage within C60-Py. Therefore, the excellent thermal stability of C60-Py fulfills the requirement of ETL materials used in PSC devices with thermal annealing process often implemented. We then studied the effect of the N-alkyl group on the molecular energy levels of C60-Py, which is crucial for charge transport in PSC device. The energy levels of the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of C60-MPy, C60-



BPy and C60-HPy were estimated by a cyclic voltammetric study. As shown in Figure 1a, the onset reduction potentials (Eredonset) of C60-MPy, C60-BPy and C60-HPy were estimated to be 1.00, -0.99, and -0.97 V vs Fc+/Fc, respectively. Thus, the LUMO energy levels of C60-MPy, C60-BPy and C60-HPy were calculated to -3.80, -3.81 and -3.83 eV (see Supporting Information Figure S6 and Table S1 for details). Likewise, based on the optical bandgap measured by UV-vis absorption onset of C60-MPy, C60-BPy and C60-HPy (1.71 eV), the HOMO energy levels of C60MPy, C60-BPy and C60-HPy are calculated to be -5.51, -5.52 and -5.54 eV, respectively. Noteworthy, both the LUMO and HOMO energy levels of C60-MPy, C60-BPy and C60-HPy are comparable to those of PCBM (see Supporting Information Table S1), revealing that the length of N-alkyl group has negligible influence on the molecular energy level of C60-Py. This is understandable since the electronic structure of fullerene derivative is predominantly determined by the fullerene cage and hardly perturbed by the addend group.29 This feature makes the UV-vis absorption spectra of C60-MPy, C60-BPy and C60-HPy quite similar to that of PCBM as well (Figure 1b). Given that PCBM has been commonly used as ETL of iPSC devices, the comparable energy levels of C60-MPy, C60-BPy and C60-HPy suggest their suitability as alternative ETLs from the viewpoint of energy level matching with CH3NH3PbI3 perovskite.


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(a)

(b) 

PCBM C60-MPy

Absorbance (a.u.)



Current (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


 

PCBM C60-MPy -2.5

-2.0

-1.5

-1.0

C60-BPy C60-HPy -0.5

+

0.0

0.5

400

300

400

Potential (V vs Fc/Fc )

500

500

600

600

C60-BPy C60-HPy

700

700

800

Wavelength (nm)

Figure 1. (a) Cyclic voltammograms of C60-MPy, C60-BPy, C60-HPy and PCBM in o-dichlorobenzene with 0.1 M tetrabutylammonium perchlorate (TBAP) as the supporting electrolyte at a scan rate of 50 mV/s. The asterisks label the oxidation peaks of ferrocene. (b) UV-vis absorption spectra of C60-MPy, C60-BPy, C60-HPy and PCBM in toluene. The inset shows the expanded range of 400-750 nm. All curves are shifted vertically for clarity.

2.2. ETL performance of C60-Py in iPSC devices. Using C60-MPy, C60-BPy or C60-HPy as an ETL, we fabricated BHJ-iPSC devices in glove box with the structure of FTO/NiOx/CH3NH3PbI3/C60-Py/bathocuproine (BCP)/Ag (Figure 2a), in which CH3NH3PbI3 perovskite film with a thickness of ~300 nm was prepared by one-step method, and appears very compact and dense without obvious pinholes (Figure 2b). A thin NiOx film (~20 nm thick, see Figure 2c) prepared by a sol-gel process30,31 was used as HTL, while bathocuproine (BCP) was incorporated between C60-Py ETL and Ag electrode as a cathode buffer layer to block hole transport.32,33 For comparison, similar devices based on the conventional PCBM ETL were also fabricated as control devices. The thickness of C60-Py film has been optimized by controlling the spin-coating speed of the saturated C60-Py solution in chlorobenzene based on its solubility discussed above (see Supporting Information Figures S7, S8 and Table S2). Under the optimized condition (spin-coating speed of 2000 rpm), the



optimized thicknesses of for C60-MPy, C60-BPy, and C60-HPy films determined by surface profilometer are about 37.8, 40.8, and 41.7 nm, respectively, which are slightly smaller than that of PCBM film (44.2 nm). The current density-voltage (J-V) curves of the iPSC device based on C60-Py ETL measured under one sun illumination are shown in Figure 2d, which includes also that of the control PCBM-based device for comparison. The measured photovoltaic parameters including opencircuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), PCE, series resistance (Rs), shunt resistance (Rsh) are summarized in Table 1. The control device based on PCBM ETL (device A) shows a Voc of 1.03 V, Jsc of 19.38 mA/cm2, FF of 69.56%, and an average PCE of 14.02%, which is comparable to the reported values of iPSC devices fabricated under similar conditions.34,35 Upon substituting PCBM with C60-MPy ETL, the device (device B) achieves a comparable PCE of 14.28% calculated from a Voc of 1.00 V, a Jsc of 20.24 mA·cm-2, and a FF of 70.37%. Interestingly, when C60-BPy with N-butyl group was used as ETL, the average PCE of device C increases obviously to 15.50%, and the champion PCE reaches 16.83%, which is much higher than that based on PCBM ETL (15.87%). With the further increase of the length of the Nalkyl group from butyl to hexyl, an apparent decrease of PCE of device D is observed, which is even lower than that based on PCBM ETL (see Table 1). These results indicate that C60-MPy, C60-BPy and C60-HPy can all function as effective ETL, while the length of the N-alkyl group sensitively affects their ETL performance.


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(a) Ag

Ag +

BCP ETL Perovskite NiOx FTO

(b)

(e)

24

90

70

16 12

PCBM C60-MPy

8

C60-BPy C60-HPy

4 0 -4

20

80

20

EQE (%)

Current density (mA/cm2)

(d)

(c)

0.0

0.2

0.4

0.6

0.8

1.0

15

60 50 40 30

PCBM C60-MPy

20

C60-BPy

10

C60-HPy

0 300

400

500

600

700

10

5

800

Integrated Jsc (mA/cm2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0

Wavelength (nm)

Voltage (V)

Figure 2. (a) Schematic structure of the iPSC device based on fullerene derivative ETL and the chemical structures of C60-Py and PCBM. (b) Top-view SEM image of CH3NH3PbI3 film. (c) Cross-sectional SEM image of iPSC device base on C60-Bpy ETL. (d) J-V curves of the control and C60-Py ETL-based devices measured under illumination of an AM 1.5 solar simulator (100 mW•cm-2) in air. The scanning direction is from open-circuit voltage to short circuit (reverse), and the scan speed is 100 mV/s. (e) EQE spectra of the control PCBM and C60-Py ETL-based devices measured in air.



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Table 1. Device parameters of perovskite solar cells using PCBM and C60-Py as ETLs.

device

ETL

PCBM A C60-MPy B C60-BPy C C60-HPy D a Averaged over 30

Voc (V)

Jsc (mA/cm2)

PCE (%)

Rs b

Rsh b

(Ω•cm2)

(Ω•cm2)

FF (%) Average a

Best

1.03±0.02 19.38±0.98 69.56±2.95 14.02±0.71 15.87 5.3 1034.4 1.00±0.02 20.24±1.03 70.37±3.81 14.28±0.87 16.29 5.7 2345.9 1.00±0.01 21.59±1.09 71.63±1.82 15.50±0.83 16.83 5.3 1601.6 1.00±0.02 20.03±0.65 65.43±2.47 13.10±0.67 14.55 7.1 1216.3 b devices fabricated independently. Rs and Rsh are obtained by the PCE measurement

system.

A careful analysis of the three photovoltaic parameters indicates that the fluctuation of PCE upon substituting PCBM with C60-Py ETL is primarily due to the change of both Jsc and FF, while Voc exhibits negligible changes (from 1.03 to 1.00 V, 3% variation) (see Table 1). This phenomenon is confirmed further by comparing the statistical photovoltaic parameters (PCE, FF, Jsc and Voc) based on 30 devices fabricated independently (see Supporting Information Figures S9 and S10 for the box plots and histograms of the photovoltaic parameters). For C60-BPy with the best ETL performance, Jsc (from 19.38 to 21.59 mA/cm2) and FF (from 69.56% to 71.63%) increase simultaneously relative to PCBM ETL, contributing to the overall enhancement of the average PCE by ~10.5%. Even for C60-HPy ETL with deteriorated PCE relative to PCBM ETL, the PCE drop originates mainly from the decrease of FF whereas Jsc even increases slightly. In order to confirm the increase of Jsc, we carried out the external quantum efficiency (EQE) measurements for the devices based on different ETLs. According to the comparison of the EQE values of different ETL-based devices illustrated in Figure 2e, all curves show that EQE values exceed 75% in the broad visible light region of 400-700 nm, guaranteeing the efficient utilization of the absorbed photons. The overall EQE responses of the devices based on C60-Py ETL are higher than that of the control PCBM-based device, resulting in the increases of the integrated


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photocurrent densities. This result agrees well with the increase of Jsc obtained in J-V measurements. To examine the hysteresis of J-V curves, we measured the J-V curves in different scan directions. While an obvious hysteresis of J-V curves has been usually observed in n-i-p PSC devices due mainly to the existence of numerous traps on ETL (typically TiO2) surface and ion migration,36-39 p-i-n-structure iPSC devices with PCBM ETL rarely exhibit obvious hysteresis of J-V curves.40-43 Similar to the case of PCBM ETL, no obvious hysteresis of J-V curves is observed for devices based on C60-Py ETL as well (see Supporting Information Figure S11 and Table S3). These results confirm the suitability of C60-Py as potentially commercial ETL. 2.3. Effect of the N-alkyl group on the ETL performance of C60-Py in iPSC devices. A series of morphological and spectroscopic characterizations were then carried out to understand the effect of the N-alkyl group on the ETL performance of C60-Py. We first compared the uniformity and roughness of CH3NH3PbI3/fullerene derivative films by atomic force microscopy (AFM) in tapping mode. The pristine CH3NH3PbI3 perovskite film looks uniform with a root mean-square (RMS) roughness of 8.30 nm. After depositing the fullerene derivative layer, the CH3NH3PbI3/C60-Py film looks more compact with a dramatic decrease of the RMS roughness to 3.21, 2.71 and 2.18 nm for C60-MPy, C60-BPy and C60-HPy, respectively, which are comparable to that for PCBM (2.19 nm, see Supporting Information Figure S12). The relatively small changes of the RMS roughness between PCBM and C60-Py films suggests that their film formation abilities are comparable, thus the morphological change seems not a major factor responsible for their difference on the ETL performance. Given that the major structural discrepancy between PCBM and C60-Py lies in the grafted conjugated ring (phenyl vs pyridine), it is intriguing to investigate whether the grafted



conjugated ring affects the interactions between the PCBM/C60-Py layer with the underneath CH3NH3PbI3 perovskite layer. We carried out X-ray photoelectron spectroscopic (XPS) characterization to monitor the change of the binding energy of Pb2+ ion within CH3NH3PbI3 after depositing different fullerene derivative layer. According to the comparison of the highresolution Pb 4f XPS spectra of pristine CH3NH3PbI3 and CH3NH3PbI3/PCBM films (Figure 3a), both of Pb 4f7/2 and Pb 4f5/2 signals at 143.3 and 138.4 eV keep almost unshifted.44,45 For CH3NH3PbI3/C60-Py films, however, both signals shift obviously towards the lower core-level binding energies of 142.8 and 138.0 eV, respectively. Such negative shifts of Pb 4f7/2 and Pb 4f5/2 binding energies upon C60-Py substituting PCBM is likely due to the existence of pyridine moiety within C60-Py which donates the lonely electron pair on N atom to the empty 6p orbital of Pb2+.46-48 Similar phenomena had been reported when polyurethane (PU) were incorporated in perovskite precursor solution, and the negative shifts of Pb 4f7/2 and Pb 4f5/2 binding energies were interpreted by the coordination interactions between PU with the Pb2+ ions in CH3NH3PbI3 perovskite.49,50 To solidify this statement, we synthesized another fullerene derivative (C60-MPh) analogous to C60-MPy via the similar one-step 1,3-dipolar cycloaddition reaction, in which the pyridine group was substituted by a phenyl group. Interestingly, quite similar to the case of PCBM, no discernible shift of Pb 4f7/2 and Pb 4f5/2 signals was observed for CH3NH3PbI3/C60MPh film (see Supporting Information Figure S13). Since C60-MPy differs only from C60-MPh in the pyridine group, this result confirms the crucial role of the pyridine group in the coordination interactions between C60-Py with the Pb2+ ion of CH3NH3PbI3 perovskite, anchoring C60-Py onto CH3NH3PbI3 perovskite film. Moreover, interestingly the I 3d3/2, 3d5/2 signals at 630.2 eV and 618.7 eV for CH3NH3PbI3/C60-Py films also exhibit similar negative shifts relative to those for CH3NH3PbI3/PCBM film (630.9 eV and 619.3 eV) (Figure 3b). This is understandable since


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electron donation of C60-Py to Pb2+ ion would disturb the static interactions between Pb2+ and Iions within CH3NH3PbI3 perovskite. (a)

(b) Pb

4f 5/2

I

4f 7/2

3d3/2

Intensity (a.u.)

MAPbI3/C60-BPy MAPbI3/C60-MPy MAPbI3/PCBM

MAPbI3/C60-BPy MAPbI3/C60-MPy MAPbI3/PCBM

MAPbI3 148

3d5/2

MAPbI3/C60-HPy

MAPbI3/C60-HPy

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MAPbI3 146

144

142

140

138

136

635

Binding energy (eV)

(c)

630

625

620

615

Binding energy (eV)

(d) C60-BPy (-72.17 kJ/mol )

C60-MPy (-74.58 kJ/mol)

(f)

(e) C60-HPy

PCBM (-54.70 kJ/mol)

(-71.69 kJ/mol)

Figure 3. (a-b) XPS spectra of Pb 4f and I 3d signals of MAPbI3 perovskite, MAPbI3/C60-Py, MAPbI3/PCBM films. The vertical dotted lines show the shifts of signal peaks. (c-e) Schematic diagrams of anchoring C60-Py ETL onto CH3NH3PbI3 perovskite film via absorption on the CH3NH3PbI3 (110) surface. The DFT-calculated absorption energy (Eabs.) is given in the bracket. (f) Schematic diagram of absorption of PCBM on CH3NH3PbI3 perovskite (110) surface, showing the absorption energy (Eabs.) is much smaller than that of C60Py ETL.



Furthermore, we also performed density functional theory (DFT) calculations to provide theoretical understanding on the coordination interactions between C60-Py with the Pb2+ ion.43,51,52 According to our calculation results, the absorption energies (Eabs.) of C60-MPy, C60BPy and C60-HPy on the surface of CH3NH3PbI3 perovskite layer are -74.58, -72.17 and -71.69 kJ/mol, respectively (Figures 3c-e), which is much larger than that of PCBM (-54.70 kJ/mol, Figure 3f). This reveals that the interactions of C60-Py with CH3NH3PbI3 perovskite is much stronger than PCBM.53 For C60-MPy, C60-BPy and C60-HPy, their comparable calculated absorption energies suggest that the length of the N-alkyl group has negligible effect on the coordination interactions between C60-Py with the Pb2+ ion. This is understandable since the Nalkyl group is far away from the surface of CH3NH3PbI3 perovskite layer according to the optimized configurations (Figures 3c-e), imposing little influence on the distance between C60Py with CH3NH3PbI3 perovskite layer. Since surface morphology and charge state of the perovskite layer affect largely the charge transport within the perovskite layer, the effect of the N-alkyl group on the charge recombination dynamics within the CH3NH3PbI3 perovskite layer was then investigated by steady-state and time-resolved photoluminescence (PL) spectroscopy. Steady-state PL spectra of the CH3NH3PbI3 perovskite films with different fullerene derivative ETLs measured under an excitation wavelength of 460 nm are compared in Figure 4a. The pristine CH3NH3PbI3 perovskite film exhibits a strong PL peak centered at 765 nm. Upon depositing C60-Py, such a characteristic PL peak of CH3NH3PbI3 shows a considerable quenching by 80.8%, 82.8%, and 79.0% for C60-MPy, C60-BPy and C60-HPy, respectively. These quenching ratios are all higher than that observed for PCBM (74.0%), suggesting that the charge transfer from CH3NH3PbI3 to C60-Py at the interface is more efficient than that at CH3NH3PbI3/PCBM interface. These results demonstrate that the


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coordination interactions between C60-Py with Pb2+ ion via the pyridine group, which is absent for PCBM ETL, can facilitate the charge transfer from CH3NH3PbI3 to fullerene derivative ETL. Among three C60-Py ETLs, the highest quenching ratio of C60-BPy-based film suggests that the charge transfer from perovskite to C60-BPy is the most efficient, and this is one factor responsible for the largest Jsc value obtained for C60-BPy-based device. In addition, we further compared the time-resolved photoluminescence (TRPL) spectra of the CH3NH3PbI3 perovskite films with different fullerene derivative ETLs (Figure 4b), from which the charge recombination dynamics within the perovskite layer can be obtained.54,55 The pristine CH3NH3PbI3 perovskite film shows a carrier lifetime of τ = 16.7 ns by fitting the spectra with a bi-exponential function. After depositing C60-MPy, C60-BPy, C60-HPy and PCBM onto the surface of perovskite layer, the decay lifetimes are 1.25, 1.15, 1.66 and 1.82 ns, respectively (see Supporting Information S11 for detailed analyses). The smallest carrier lifetime obtained for C60-BPy suggests the most rapid electron extraction from CH3NH3PbI3 perovskite to C60-BPy,55 contributing to the largest Jsc value obtained for C60-BPy-based device as well. We further investigated the effect of C60-Py on the trap state density within CH3NH3PbI3 perovskite film by using the space charge limited current (SCLC) method.56,57 Electron-only devices with structures of ITO/TiO2/CH3NH3PbI3/ETL/Ag were constructed, and the corresponding J-V curves in dark are shown in Figure 4c. The electron trap state density (nt) of CH3NH3PbI3 perovskite are 1.06×1016, 8.95×1015, 1.43×1016 and 1.64×1016 cm-3 for C60-MPy, C60-BPy, C60-HPy and PCBM, respectively (see Supporting Information S12 for detailed analyses). This reveals that C60-MPy, C60-BPy, C60-HPy all have lower nt than PCBM, while C60-BPy exhibits the lowest nt value. Therefore, the grafted pyridine moiety within C60-Py anchors C60-Py onto CH3NH3PbI3 perovskite film via its coordination interactions with the Pb2+



ion, reinforcing the interactions between fullerene derivative with perovskite and consequently passivating the trap state within CH3NH3PbI3 perovskite film effectively. In this sense, the lowest nt value obtained for C60-BPy suggests the maximum degree of passivation of the trap state within CH3NH3PbI3 perovskite film, due presumably to the most optimized molecular orientation and packing of C60-BPy at the perovskite/C60-Py interface, for which further systematic studies are needed. Using a similar SCLC method based on another type of electron-only device structure of ITO/CH3NH3PbI3/ETL/Ag, we also investigated the effect of the N-alkyl group on the electron mobility (µe).58,59 While the PCBM-based device exhibits a µe value of 5.24×10-4 cm2V-1S-1, the µe values of the C60-Py-based devices increase to 1.97×10-3 cm2V-1S-1, 3.51×10-3 cm2V-1S-1 and 1.04×10-3 cm2V-1S-1 for C60-MPy, C60-BPy and C60-HPy, respectively (see Supporting Information S13 for detailed analyses). Noteworthy, the increase of µe value for C60-Py-based devices relative to that for PCBM-based device reveals that C60-Py affords enhanced electron transport than PCBM, contributing directly to the increase of Jsc. In this sense, the largest Jsc value obtained for C60-BPy-based device can be interpreted by its highest µe value, originated from the lowest trap states within the CH3NH3PbI3 layer.


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(b)

MAPbI3

Normalized PL Intensity

(a) PL intensity (a.u.)

MAPbI3/PCBM MAPbI3/C60-MPy MAPbI3/C60-BPy MAPbI3/C60-HPy

650

700

(c)

750

800 Wavelength (nm)

MAPbI3 MAPbI3/PCBM

1

MAPbI3/C60-MPy MAPbI3/C60-BPy MAPbI3/C60-HPy

0.1

0.01

850

0

10

20

30

40

50

60

70

80

Time (ns)

(d) 160 Rs

Rco

140 CPE

0.01

120

1E-3

PCBM C60-MPy

1E-4

1E-5

-Z" (Ω)

Current (mA)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100

60

C60-BPy

40

C60-HPy

20 0

0.1

1

PCBM C60-MPy

80

C60-BPy C60-HPy

0

50

100

150

200

250

300

Z' (Ω)

Voltage (V)

Figure 4. (a) Steady-state and (b) time-resolved photoluminescence (PL) spectra of the CH3NH3PbI3, CH3NH3PbI3/PCBM, CH3NH3PbI3/C60-MPy, CH3NH3PbI3/C60-BPy, CH3NH3PbI3/C60-HPy films on glass substrate. (c) Dark J-V responses of the electron-only devices with C60-Py or PCBM ETL, from which the trap-filled limit voltage (VTFL) is determined as the kink point. (d) Nyquist plots of the C60-Py- and PCBMbased devices measured in the dark under a reverse potential of 1.0 V. The points show the experimental data and the solid lines show the fitted curves. The inset is the equivalent circuit model used for fitting the impedance spectra.

Whether the change of ETL affects the interfacial charge transport behavior of the iPSC device was investigated by the electrochemical impedance spectroscopy (EIS), carried out in dark under a reverse potential of 1.0 V which is near the open circuit potential. The Nyquist plots of devices based on different ETLs are presented in Figure 4d, which includes also the corresponding fitted curves using an equivalent circuit employed for fitting (see the inset). Only one arc was observed in high frequency regime for all devices, which is typically associated with the charge transfer process at the perovskite/ETL interface. According to the comparison of the 19 Environment ACS Paragon Plus


fitted parameters from Nyquist plots summarized in Supporting Information Table S4, the series resistance (Rs) values for different devices are similar (14.7 to 19.0 Ω·cm2). However, the charge transfer resistance (Rct) value shows distinct differences. The PCBM-based device exhibits a Rct value of 276.4 Ω·cm2, which decreases dramatically to 76.0, 43.3, and 130.1 Ω·cm2 for C60-MPy, C60-BPy and C60-HPy, respectively. The lowest Rct value for C60-BPy ETL indicates that the unfavorable electron-hole recombination at perovskite/ETL interface can be prohibited at most relative to other ETLs. This contributes directly to the highest Jsc and FF value obtained for C60BPy-based device.

2.4. Effect of the N-alkyl group on the stability of iPSC device based on C60-Py ETL. In order to verify the reliability of the device performance, we carried out steady-state photocurrent output measurements at the maximum power point. While the PCE of the control device based on PCBM ETL can be stabilized at 15.06 %, the C60-BPy-based device exhibits a higher stabilized PCE of 16.68 % after 300 s of illumination at the maximum power point of 0.802 V (see Supporting Information Figure S15). This tendency is consistent with that obtained from J-V curves (see Table 1), confirming the reliability of the measured PCE. We further monitored the ambient stabilities of the iPSC devices by storing the devices without any encapsulation in ambient and dark condition (temperature: 20 ºC, relative humidity: 25%) for 1500 h. As seen in Figure 5, the control device based on PCBM ETL is deteriorated with a PCE drop of ca. 20% after 1500 h storage. This phenomenon is similar to those reported extensively in literatures, mainly due to the decomposition of CH3NH3PbI3.60 By using C60-MPy or C60-BPy ETL, the devices exhibits comparable stability in overall. Interestingly, however, C60HPy-based device shows much better stability without obvious degradation, with PCE even increasing at the beginning and keeping almost unchanged after 1500 h storage. These results 20 Environment ACS Paragon Plus

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indicate that the length of the N-alkyl group affects sensitively the stability of the C60-Py-based device. A plausible explanation is that the hydrophobic C60-HPy with the longest N-alkyl group can function as a superior encapsulating layer protecting the CH3NH3PbI3 layer from contact with the ambient moisture.33,61,62 This assumption is supported by comparing the hydrophobicity of different fullerene derivatives based on water contact angle measurements. The water contact angles of CH3NH3PbI3/PCBM, CH3NH3PbI3/C60-MPy, CH3NH3PbI3/C60-BPy, CH3NH3PbI3/C60HPy are 76.1°, 71.5°, 73.6°and 80.3°, respectively (see Supporting Information Figure S16), revealing that the hydrophobicity of fullerene derivative is sensitively affected by the length of the N-alkyl group. The highest hydrophobicity of C60-HPy deduced from its largest water contact angle renders the best encapsulating effect, thus the best stability of C60-HPy-based device is achieved.

Figure 5. Stabilities of devices stored in ambient condition without encapsulation (temperature: 20 ºC, relative humidity: 25%). Device parameters (a) PCE, (b) Voc, (c) Jsc, (d) FF vary with a function of storage time. The photovoltaic parameters are based on the mean values averaged over 5 devices.



3. Conclusion

In summary, three novel pyridine-functionalized fullerene derivatives C60-Py with different Nalkyl groups grafted onto the pyrrolidine moiety were synthesized facilely via a one-step 1,3dipolar cycloaddition reaction. The molecular structures of C60-Py were confirmed by 1H and 13C NMR spectroscopy, FT-IR spectroscopy and MALDI-TOF mass spectrometry. The chain length of the N-alkyl group has negligible influence on the molecular energy level of C60-Py according to cyclic voltammogram study. However, the ETL performance of C60-Py is sensitively dependent on the chain length of N-alkyl group, with C60-BPy exhibiting the highest PCE of 16.83%, which is superior to that based on PCBM ETL (15.87%). The grafting of pyridine moiety leads to its coordination interactions with the Pb2+ ion of CH3NH3PbI3 perovskite, which anchor C60-BPy onto perovskite film and consequently reinforce the passivation of the trap states within CH3NH3PbI3 perovskite film and suppress the nonradiative electron-hole recombinations at perovskite/ETL interface, leading to enhanced electron transport and consequently enhanced PCE. The ambient stabilities of iPSC devices based on C60-MPy and C60-BPy ETL are comparable to that based on PCBM ETL, whereas C60-HPy-based device shows much better stability since its longest N-alkyl group can function as a superior encapsulating layer protecting the CH3NH3PbI3 layer from contact with the ambient moisture. Our method for anchoring fullerene derivative onto perovskite film via the pyridine group is facile and effective in enhancing electron transport at perovskite/ETL interface, potentially promising for achieving high-efficiency iPSC device.

4. Experimental Section


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Materials. FTO-coated glass substrates with a sheet resistance of 13±1.5 Ω sq-1 were purchased from NSG Group, Japan. CH3NH3I was bought from Xi’an Polymer Light Technology

Corp.

PbI2

was

obtained

from

Tokyo

Chemical

Industry

Co.,

Ltd.

Dimethylformamide (DMF), dimethylsulfoxide (DMSO), chlorobenzene (CB), isopropanol, and acetonitrile were purchased from Sigma-Aldrich. PCBM was purchased from 1M Company. 2,9dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) was purchased from Alfa Aesar. C60 was purchased from Suzhou Dade Carbon Nanotechnology Co, Ltd. All reactants and solvents were used as received without further purification. Synthesis of C60-Py: C60-MPy. To a solution of C60 (144 mg, 0.2 mmol) in chlorobenzene (25 mL) sarcosine (36 mg, 0.4 mmol) and 4-pyridinecarboxaldehyde were added. Then, the mixture was heated at 80 °C under a nitrogen atmosphere for 5 h and monitored with TLC. After cooling to room temperature, the solution was removed under reduced pressure. The crude product was purified by silica gel column chromatography, affording 64 mg (37.5%) of C60-MPy. 1H NMR (400 MHz, CDCl3) δ 8.66 (d, J = 5.1 Hz, 2H), 7.73 (s, 2H), 4.99 (d, J = 9.5 Hz, 1H), 4.92 (s, 1H), 4.28 (d, J = 9.5 Hz, 1H), 2.81 (s, 3H).

13

C NMR(101 MHz, CS2/CDCl3, ppm): δ 155.68, 153.52, 152.47,

152.00, 150.34, 147.39, 146.41, 146.38, 146.30, 146.25, 146.20, 146.13, 146.05, 145.74, 145.69, 145.63, 145.51, 145.48, 145.41, 145.32, 144.78, 144.64, 144.48, 144.40, 143.25, 143.13, 142.82, 142.73, 142.71, 142.68, 142.30, 142.24, 142.15, 142.13, 142.06, 142.03, 141.95, 141.89, 141.79, 141.67, 140.37, 140.10, 139.66, 137.15, 136.44, 136.14, 135.68, 124.15, 82.41, 76.47, 70.11, 69.09, 40.11. MALDI-TOF m/z: calculated for C68H10N2 [M]+ 854.68; found 854.46. C60-BPy. The experimental process of C60-BPy is similar with that of C60-MPy. To a solution of C60 (144 mg, 0.2 mmol) in chlorobenzene (25 mL), the N-butyl glycine (52.5 mg, 0.4 mmol)



and 4-pyridinecarboxaldehyde were added. The resulted product purified via silica gel column chromatography afforded 62 mg (34.6%) of C60-BPy. 1H NMR (400 MHz, CDCl3) δ 8.65 (d, J = 5.1 Hz, 2H), 7.74 (s, 2H), 5.12 (d, J = 9.5 Hz, 1H), 5.04 (s, 1H), 4.13 (d, J = 9.4 Hz, 1H), 3.17 (dt, J = 11.9, 8.3 Hz, 1H), 2.59 (ddd, J = 12.1, 8.3, 4.2 Hz, 1H), 1.91–1.46 (m, 4H), 1.08 (t, J = 7.3 Hz, 3H). 13C NMR (101 MHz, CDCl3/CS2) δ 155.94, 153.75, 152.58, 152.15, 150.22, 147.39, 146.71, 146.41, 146.37, 146.29, 146.25, 146.20, 146.05, 146.03, 145.74, 145.72, 145.65, 145.52, 145.47, 145.43, 145.39, 145.38, 145.33, 145.30, 145.27, 144.79, 144.65, 144.49, 144.40, 143.25, 143.12, 142.81, 142.72, 142.69, 142.66, 142.33, 142.28, 142.22, 142.15, 142.11, 142.05, 141.93, 141.88, 141.78, 141.65, 140.37, 140.08, 139.61, 137.13, 136.43, 136.10, 135.64, 124.32, 81.55, 75.92, 68.98, 66.92, 53.25, 30.85, 21.13, 14.57. MALDI-TOF m/z: calculated for C71H16N2 [M]+ 896.71; found 896.41. C60-HPy. The experimental process of C60-HPy is similar with that of C60-MPy. To a solution of C60 (144 mg, 0.2 mmol) in chlorobenzene (25 mL), the N-hexyl glycine (63.7 mg, 0.4 mmol) and 4-pyridinecarboxaldehyde was added. The resulted product purified via silica gel column chromatography affords 65 mg (35.2%) of C60-HPy. 1H NMR (400 MHz, CDCl3) δ 8.65 (d, J = 4.8 Hz, 2H), 7.73 (s, 2H), 5.11 (d, J = 9.4 Hz, 1H), 5.04 (s, 1H), 4.13 (d, J = 9.4 Hz, 1H), 3.16 (dt, J = 11.7, 8.3 Hz, 1H), 2.63–2.53 (m, 1H), 2.06–1.82 (m, 2H), 1.73–1.48 (m, 2H), 1.48–1.35 (m, 4H), 0.97 (dd, J = 9.1, 4.9 Hz, 3H).

13

C NMR (101 MHz, CDCl3/CS2) δ 155.92, 153.74,

152.59, 152.15, 150.27, 147.39, 146.59, 146.42, 146.37, 146.36, 146.29, 146.25, 146.20, 146.05, 146.03, 145.75, 145.72, 145.67, 145.65, 145.51, 145.49, 145.43, 145.39, 145.37, 145.33, 145.30, 145.27, 144.79, 144.66, 144.49, 144.41, 143.25, 143.12, 142.81, 142.73, 142.70, 142.67, 142.32, 142.28, 142.23, 142.16, 142.12, 142.06, 141.94, 141.89, 141.79, 141.66, 140.38, 140.09, 139.62,


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137.13, 136.45, 136.10, 135.64, 124.29, 81.56, 75.91, 68.97, 66.97, 53.59, 32.28, 28.72, 27.59, 23.22, 14.57. MALDI-TOF m/z for C73H20N2 [M]+ 924.73; found 924.42. DFT calculations. Our first-principles calculations are based on density functional theory (DFT) as implemented in Vienna ab initio simulation package (VASP) with exchangecorrelation functional of Perdew, Burke, and Ernzerhof (PBE) of generalized gradient approximation (GGA). Surface slabs with a vacuum thickness of 20 Å were modeled as (110)terminated slabs of tetragonal structure. In our structure, 2 layers supercell of 2×3 (110) surface with PbI2 as terminal with the bottom layer fixed was used, the nitrogen atom in functionalized C60 binds to Pb2+ ion. Plane-wave cutoff energy of 400 (450) eV and 1×1×1 Γ point were used for structure optimization (self-consistent). The convergence threshold for self-consistent iteration is 10-5 eV, and all the atomic positions are fully optimized until all components of the residual forces are smaller than 0.1 eV/Å. The absorption energies (Eabs., eV) for C60-MAPbI3 are calculated from: Eabs.=E(C60-X@CH3NH3PbI3)-E(CH3NH3PbI3)-E(C60-X), X=MPy, BPy, HPy, where E(structure) is the calculated energy of the corresponding structure. Device fabrication. The FTO glass was cleaned by sequential ultrasonic treatment in deionized water, acetone, and isopropanol, and then treated in an ultraviolet-ozone chamber for 20 min. The sol-gel process NiOx films are prepared according to following steps. The NiOx precursor solution (0.1 mmol dissolved in 10 ml ethanol and 60 μL ethanolamine, stirring for 4 hours at 70 o

C before used) was spin-coated on the FTO at 6000 rpm for 30s, and then heated at 280 °C for

40 min at ambient atmosphere. Next, The perovskite precursor solution (CH3NH3I and lead(II) iodide (PbI2) with a molar ratio of 1:1 were dissolved in DMF and DMSO with a volume ratio of 7:3) was then spin-coated onto FTO/NiOx substrate at 1000 rpm for 10 s and 4000 rpm for 45 s. During the last 25 s of the second spin-coating step, the substrate was treated with toluene drop-



casting (80 μL). The as spun films were annealed at 100 °C for 10 min. Then, the PCBM (20 mg/ml in chlorobenzene) and C60-Py (saturated solution in chlorobenzene, 15 mg/ml, 16 mg/ml, 18 mg/ml for C60-MPy, C60-BPy, C60-HPy ) were deposited by spin coating at 2000 rpm for 30 s. The devices were finished by thermally evaporating BCP (8 nm) and silver (Ag) in vacuum under a base pressure of about 10-5 Torr. The device working area was 0.1 cm2, as defined by the overlap of FTO and the Ag cathode. Measurements and characterization. 1H NMR spectrum was recorded on a Bruker AV 400 MHz NMR spectrometer. Tetramethylsilane (TMS) was used as internal standard. Mass spectra were collected on a Bruker Autoflex Speed mass spectrometer. The electrochemical cyclic voltammetry was conducted on a CHI630D Electrochemical Workstation. A Pt disk was taken as the working electrode, Pt wire as the counter electrode, and saturated calomel electrode as the reference electrode. A solution of o-dichlorobenzene with 0.1 M tetrabutylammonium perchlorate (TBPA) was used as electrolyte, and the scan rate was 50 mV s−1. UV-Vis spectroscopy was recorded on a UV-vis-NIR 3600 spectrometer (Shimadzu, Japan). The current density-voltage (J-V) characterizations were conducted using a Keithley 2400 source measurement unit under simulated AM 1.5 irradiation (100 mWcm2) with a standard xenon-lamp-based solar simulator (Oriel Sol 3A, USA). The solar simulator illumination intensity was calibrated with a monocrystalline silicon reference cell (Oriel P/N 91150 V, with KG-5 visible color filter) calibrated by the National Renewable Energy Laboratory (NREL). The EQE measurements were carried out on an ORIEL Intelligent Quantum Efficiency (IQE) 200TM Measurement system equipped with a tunable light source. XPS measurements were conducted on a Thermo ESCALAB 250 instrument with a monochromatized Al Kα X-ray source. FTIR spectra were performed on a TENSOR 27 spectrometer (Bruker, Germany) at room temperature.


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The film thickness were determined by a KLA-Tencor P6 surface profilometer. AFM measurements were performed on a XE-7 scanning probe microscope in non-contact mode (Park systems, Korea). SEM images were obtained with a field-emission scanning electron microscope (FEI Quanta 200). The steady-state photoluminescence (PL) spectra were acquired using an Edinburgh Instruments FLS920 fluorescence spectrometer with an excitation wavelength of 460 nm. The time-resolved photoluminescence (TRPL) spectra were measured via the timecorrelated single-photon counting method with a Picoquant Gmbh Solea Supercontinuum Laser. A picosecond pulsed diode laser at 543 nm with a pulse width of 104 ps is used as the excitation source. The decays were recorded with Timeharp 260 software. Impedance spectroscopic measurements (EIS) were performed using an electrochemical workstation (Autolab 320, Metrohm, Switzerland) with a frequency range from 1 Hz to 1 MHz under 1.0 V in the dark. AC 20 mV perturbation was applied with a frequency from 1 MHz to 1 Hz. The obtained impedance spectra were fitted with Z-View software (v2.8b, Scribner Associates, USA). Contact angles were measured using a CAM instrument (Data-Physics, Germany). ASSOCIATED CONTENT Supporting Information Available: MALDI-TOF mass spectra, 1H NMR and 13C NMR spectra, FTIR spectra, TGA analysis, energy levels estimation, optimization of the film thickness of C60-Py ETL, statistic photovoltaic parameters, photovoltaic parameters in different scan direction, analysis of TRPL spectra, trap state densities measurement, electron mobilities characterization, water contact angles, etc. This material is available free of charge via the Internet at http://pubs.acs.org.



ACKNOWLEDGMENT This work was partially supported by the National Key Research and Development Program of China (2017YFA0402800, 2016YFA0200602), National Natural Science Foundation of China (51572254), the Major/Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (No. 2016FXZY003), and Anhui Initiative in Quantum Information Technologies. REFERENCES (1) Wehrenfennig, C.; Eperon, G. E.; Johnston, M. B.; Snaith, H. J.; Herz, L. M., High Charge Carrier Mobilities and Lifetimes in Organolead Trihalide Perovskites. Adv. Mater. 2014, 26, 1584-1589. (2) Yin, W.-J.; Shi, T.; Yan, Y., Unusual Defect Physics in CH3NH3PbI3 Perovskite Solar Cell Absorber. Appl. Phys. Lett. 2014, 104, 063903. (3) Kim, H. S.; Lee, C. R.; Im, J. H.; Lee, K. B.; Moehl, T.; Marchioro, A.; Moon, S. J.; Humphry-Baker, R.; Yum, J. H.; Moser, J. E.; Gratzel, M.; Park, N. G., Lead Iodide Perovskite Sensitized All-Solid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591. (4) Wu, W.-Q.; Chen, D.; Caruso, R. A.; Cheng, Y.-B., Recent Progress in Hybrid Perovskite Solar Cells Based on N-Type Materials. J. Mater. Chem. A 2017, 5, 10092-10109. (5) Correa-Baena, J.-P.; Abate, A.; Saliba, M.; Tress, W.; Jesper Jacobsson, T.; Grätzel, M.; Hagfeldt, A., The Rapid Evolution of Highly Efficient Perovskite Solar Cells. Energy Environ. Sci. 2017, 10, 710-727. (6) Xiao, Z.; Bi, C.; Shao, Y.; Dong, Q.; Wang, Q.; Yuan, Y.; Wang, C.; Gao, Y.; Huang, J., Efficient, High Yield Perovskite Photovoltaic Devices Grown by Interdiffusion of SolutionProcessed Precursor Stacking Layers. Energy Environ. Sci. 2014, 7, 2619-2623.


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Table of Contents (TOC) image

16.83% 21.59

C60-Py

PCE Jsc (mA/cm2) 15.87% 19.38 CH3NH3PbI3 perovskite

CH3NH3+ Pb2+ I-

PCBM

0

4

8

12

16


20

Anchoring Fullerene onto Perovskite Film via Grafting Pyridine toward

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