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Cite This: ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Effects of Cyclic Tetrapyrrole Rings of Aggregate-Forming Chlorophyll Derivatives as Hole-Transporting Materials on Performance of Perovskite Solar Cells Mengzhen Li,† Na Li,† Weidong Hu,† Gang Chen,† Shin-ichi Sasaki,*,‡,§ Kotowa Sakai,‡ Toshitaka Ikeuchi,‡ Tsutomu Miyasaka,∥ Hitoshi Tamiaki,§ and Xiao-Feng Wang*,† †

Key Laboratory of Physics and Technology for Advanced Batteries (Ministry of Education), College of Physics, Jilin University, Changchun 130012, People’s Republic of China ‡ Nagahama Institute of Bio-Science and Technology, Nagahama, Shiga 526-0829, Japan § Graduate School of Life Sciences, Ritsumeikan University, Kusatsu, Shiga 525-8577, Japan ∥ Graduate School of Engineering, Toin University of Yokohama, 1614 Kurogane-cho, Aoba, Yokohama, Kanagawa 225-8503, Japan S Supporting Information *

ABSTRACT: Organic hole-transporting materials (HTMs) are essential components of high-performance perovskite solar cells (PSCs). Three zinc-coordinated chlorophyll derivatives with bacteriochlorin, chlorin, and porphyrin macrocycles, namely, ZnBChl, ZnChl, and ZnPor, respectively, were newly synthesized and employed as HTMs in PSCs. The difference in the π backbones of these HTMs causes differences in their photophysical properties, and thus different hole-extraction abilities, as revealed by steady-state photoluminescence spectra. The power conversion efficiencies (PCEs) of PSCs with a typical mesoporous structure, fluorine-doped tin oxide/compact TiO2/mesoporous TiO2/ CH3NH3PbI3/HTM/Ag, are 8.26%, 11.88%, and 0.68% for ZnBChl, ZnChl, and ZnPor, respectively. The small PCE of the ZnPor-based PSC is partially attributed to the small energy gap of the highest occupied molecular orbital (HOMO) levels between ZnPor and CH3NH3PbI3 perovskite. Therefore, we increased this energy gap slightly by shifting the HOMO level of CH3NH3PbI3 perovskite downward by incorporating formamidinium and bromide ions into the crystal lattice of CH3NH3PbI3. As a result, the PCE of the ZnPor-based PSC improved to 4.04%, and it exhibited a clearly normal current−voltage curve, indicating better energy alignment between ZnPor and the modified perovskite. In addition, the barriers both in the perovskite/ ZnPor interface and in the ZnPor layer originated from the delocalization of π-electrons on the symmetric aggregates determine the low PCE of ZnPor-based PSCs; this was deduced from measurements of atomic force microscope, ultraviolet photoelectron spectroscopy, and the electric impedance spectroscopy. KEYWORDS: chlorophyll derivatives, hole-transporting materials, photophysical properties, highest occupied molecular orbital, perovskite solar cells, power conversion efficiencies

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INTRODUCTION Organic−inorganic hybrid perovskites are ABX3-type crystals, where A represents a monovalent cation [methylammonium (MA+), formamidinium (FA+)], B represents a divalent cation (Pb2+, Sn2+), and X represents a halide anion (I−, Br−, and Cl−). Perovskite exhibits excellent photophysical properties, such as a band gap (Eg) that can be tuned by managing the chemical composition, ambipolar charge transport, and a long electron− hole diffusion length.1−3 Since perovskite was employed in solar cells by Miyasaka et al. in 2009,4 significant progress has been made in perovskite solar cells (PSCs).5−12 The state-ofthe-art PSC device has a power conversion efficiency (PCE) of 22.1%.13 Hole-transporting materials (HTMs) are essential components of PSCs that block the flow of electrons to the © XXXX American Chemical Society

anode side and efficiently extract holes generated in the perovskite layer.14,15 The most widely used HTMs are organic small molecules and polymers, such as 2,2′,7,7′-tetrakis(N,N-dip-methoxyphenylamine)-9,9′-spirobifluorene5,6 and poly(3-hexylthiophene),16−19 which are relatively expensive. The development of cost-effective HTMs is crucial.20−30 To date, considerable attention has been paid to the use of cyclic tetrapyrrole-type organic macromolecules, including phthalocyanines (Pcs), chlorins (Chls), and porphyrins (Pors), as HTMs in PSCs. Typically, a series of Pcs including Received: October 10, 2017 Accepted: December 15, 2017 Published: December 15, 2017 A

DOI: 10.1021/acsaem.7b00018 ACS Appl. Energy Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Energy Materials

aggregation in the solid state,58 and the C172-long alkyl ester was introduced to increase the solubility in common organic solvents (synthesis of these Chl derivatives was described in the Supporting Information including Figures S1−S14).59 These different π skeletons affect their photophysical properties, including the UV/vis/near-IR absorption, intermolecular π−π stacking, aggregate morphology, and frontier molecular orbitals.60 Here we report that the ZnChl-based PSC has the highest PCE (11.88%), because it affords the most efficient hole extraction, as indicated by the steady-state photoluminescence (PL) spectra. On the other hand, the low PCE (0.68%) of the ZnPor-based PSC using methylammonium lead iodide (MAPbI3) is partially ascribed to a small energy gap between the highest occupied molecular orbital (HOMO) levels of ZnPor and MAPbI3 perovskite. The PCE can be increased to 4.04% by adjusting the Eg value of the perovskite (MAPbI3) via incorporating FA+ and Br− into the crystal lattice. UV photoelectron spectroscopy (UPS) results on these HTM films suggest that ZnPor exhibits a weaker p-type characteristic compared to ZnChl and ZnBChl owing to the large shift of the Fermi level, which is probably due to the delocalization of πelectrons over the symmetric aggregates. As a result, a large charge transfer resistance in the electric impedance spectroscopy (EIS) was observed in the ZnPor-based PSCs.

CuIIPc, ZnIIPc, and soluble peripherally substituted Pcs have been investigated recently to obtain higher PCEs.31−46 We have already studied Chls as potential HTMs in PSCs.47−49 Soon after the employment of Chls in PSCs, Por derivatives such as ZnIIPor−ethynylaniline conjugate and arylamine-substituted Por were also reported as HTMs in TiO2-based PSCs, which exhibited excellent PCEs.50−53 Bacteriochlorins (BChls), Chls, and Pors are generally structurally similar, with different degrees of π conjugation on the cyclic tetrapyrrole ring.54 The basic optoelectrical properties of synthetic aggregateforming chlorophyll derivatives are well-studied by Würthner and co-workers.55−57 The examples of successful use of Chls and Pors in PSCs suggest the importance of comprehensive investigation of these cyclic-tetrapyrrole-based molecules under parallel experimental conditions. In this work, we compare the use of three zinc chlorophyll derivatives having BChl, Chl, and Por macrocycles (ZnBChl, ZnChl, and ZnPor, respectively) as HTMs in PSCs (see Figure 1). The newly synthesized zinc

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RESULTS AND DISCUSSION Photophysical Properties of ZnBChl, ZnChl, and ZnPor. Figure 1 shows the chemical structures of the three chlorophyll derivatives used in this study. They all have the same peripheral substituents and a central zinc. The difference among them lies in the π conjugation degree of the B and D rings of the cyclic tetrapyrrole unit, which are characterized as BChl, Chl, and Por. The structural difference is expected to affect the photochemical and photophysical properties of these derivatives.58 Further, the central zinc of these derivatives’ backbones and the C31-hydroxy and C13-carbonyl groups

Figure 1. Chemical structures of ZnBChl, ZnChl, and ZnPor molecules.

chlorophyll derivatives have the same peripheral substituents and different degrees of π conjugation in the macrocycles. The characteristic feature of the C3-(1-hydroxyethyl) group, C13keto-carbonyl moiety, and central zinc along the molecular y axis of their tetrapyrrole units is expected to cause J-type self-

Figure 2. (a) UV/vis/near-IR absorption spectra of ZnBChl, ZnChl, and ZnPor in the forms of aggregate (solid film spin-coated on a glass) and monomer (a THF solution), (b) XRD patterns of the three solid films deposited on glass, (c) SEC spectra, and (d) valence spectra of their aggregate films on ITO. B


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ACS Applied Energy Materials induce them to self-assemble to form well-ordered Jaggregates61 via a simple spin-coating process, making them suitable for use as HTMs in PSCs.48 Figure 2a shows the UV/vis/near-IR absorption spectra of ZnBChl, ZnChl, and ZnPor in the forms of aggregates spincoated on glass as a solid film and monomers dissolved in tetrahydrofuran (THF). The corresponding electronic absorption parameters are listed in Table 1. Several characteristic

direction of molecule alignment is the same as that of the carrier transport, the high crystallinity of molecular assembly would be beneficial to the performance of photovoltaic devices, and vice versa. On the other hand, when the crystalline structure of molecules is not aligned with the carrier transport direction, the high crystallinity may carry out a negligible or even negative effect on the performance of photovoltaic devices. The frontier molecular orbitals of these HTMs are key parameters determining the efficiency with which holes generated in perovskite are extracted. We employed ultraviolet photoelectron spectroscopy (UPS) to determine the HOMO levels and work functions (Φ) of the HTMs.62−64 Figure 2c shows the secondary electron cutoff (SEC) spectra of ZnBChl, ZnChl, and ZnPor. The values of Φ from the SEC onsets are 4.47, 4.39, and 4.26 eV for ZnBChl, ZnChl, and ZnPor, respectively. The negative Φ corresponds to the Fermi level (Ef) distribution of these HTMs. From the valence spectra (Figure 2d), we estimated the onset of valence band maximum (EB) to be at 0.37, 0.65, and 0.83 eV for ZnBChl, ZnChl, and ZnPor, respectively. The above energy level distributions were shown in Figure S15. Clearly, the Ef of ZnPor lies far from the HOMO of ZnPor indicating a weaker p-type characteristic of ZnPor films as compared with ZnBChl and ZnChl.65 The high EB value of ZnPor results from the delocalized π-electrons on the symmetric core structure. The Φ, EB, and Eg values were used to determine the HOMO and lowest unoccupied molecular orbital (LUMO) levels of the HTMs as follows:48

Table 1. Summary of the Electronic Absorption Parameters and HOMO, LUMO levels of ZnBChl, ZnChl, and ZnPor monomer

aggregates

HTM

λ(Soret) (nm)

λ(Qy) (nm)

λ(Soret) (nm)

λ(Qy) (nm)

HOMO (eV)

LUMO (eV)

ZnBChl ZnChl ZnPor

347 424 428

725 647 608

369 434 415

794 704 650

−4.84 −5.04 −5.09

−3.41 −3.38 −3.29

features appear in the absorption spectra: (1) all the absorption spectra of the solid films show broadened and red-shifted Qy bands compared to those of the corresponding THF solutions, which indicate the formation of J-type aggregates in the solid state; (2) the Qy absorption bands of these derivatives appear at longer wavelengths in the order of ZnPor < ZnChl < ZnBChl, which is consistent with their Eg decreases in both the aggregates and monomers, in good agreement with previous observations of chlorophyll derivatives;54,58 (3) the UV/vis/ near-IR absorption spectrum of the ZnPor aggregate exhibits a broader Soret band between 300 and 500 nm than those of ZnBChl and ZnChl, which overlap the main absorption peaks of MAPbI3 perovskite. To obtain additional structural information about these solid aggregates, X-ray diffraction (XRD) measurements were conducted at room temperature. Figure 2b shows the XRD patterns of ZnBChl, ZnChl, and ZnPor solid films deposited on glass. The XRD patterns of ZnBChl and ZnChl show typical reflection peaks at 2θ = 7.47° and 5.47°, respectively. However, ZnPor exhibited two strong diffraction peaks at 2θ = 4.97° and 9.39°. The crystallite diameters of the aggregates are calculated using Scherrer’s formula 1, and the results are summarized in Table 2:

D=

kλ β cos θ

HOMO = −(Φ + E B) LUMO = HOMO + Eg

(2)

These energy levels are summarized in Table 1. The HOMO energies of these derivatives decrease in the order of ZnBChl (−4.84 eV) > ZnChl (−5.04 eV) > ZnPor (−5.09 eV). Fabrication of PSCs Using TiO2 as an ElectronTransporting Material and ZnBChl, ZnChl, or ZnPor as an HTM: Factors limiting the PCE. We fabricated PSCs using TiO2 as an electron-transporting material (ETM) and ZnBChl, ZnChl, or ZnPor as an HTM. Figure 3a shows cross-

(1)

Table 2. Indexing of the Small Angle Reflections in the Film of XRD Pattern and Grain Diameters (Ds) of ZnBChl, ZnChl, and ZnPor Aggregates at Room Temperature Deposited on a Glass Substrate materials

2θ (deg)

D (nm)

ZnBChl ZnChl ZnPor

7.47 5.47 4.79, 9.39

15.88 11.16 11.78, 12.34

where k is a constant (0.89), λ is the wavelength of the X-rays, β is the half-peak width of the corresponding reflection, and θ is the corresponding diffraction angle.48 The calculated size D of these Chl aggregates increases in the order of ZnChl (11.16 nm) < ZnPor (11.78 or 14.14 nm) < ZnBChl (15.88 nm). The XRD patter of ZnPor exhibits strong reflection peaks in two directions that are clearly different from the others.48 When the

Figure 3. (a) Cross-sectional SEM images of the device consisting of FTO/cm-TiO2 + me-TiO2/MAPbI3/HTM and (b) energy level alignment of the devices based on ZnBChl, ZnChl, or ZnPor as HTMs. C


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Figure 4. (a) UV/vis/near-IR absorption spectra and (b) steady-state PL spectra (excitation at 403 nm) of glass/MAPbI3 and glass/MAPbI3 on which ZnBChl, ZnChl, or ZnPor was deposited.

Figure 5. (a) Typical J−V curves, (b) EQE spectra of the PSCs-based ZnBChl, ZnChl, and ZnPor as HTMs, and (c) time evolution of photocurrent density spectra for the PSC employing ZnBChl and ZnChl as HTMs at 0.9 V forward bias.

RMS values for MAPbI3, MAPbI3/ZnBChl, and MAPbI3/ ZnChl are 13.1, 11.3, and 8.18 nm, respectively. Obviously, the surface roughness of MAPbI3 film is lowered after interfacing with ZnBChl and ZnChl. However, the presence of ZnPor delivers the roughest film surface with a RMS value of 32.9 nm among these films, suggesting that the interface between ZnPor and MAPbI3 is not smoothly aligned, which may bring a negative effect to the hole transfer through this surface.66 Figure 4a presents the UV/vis/near-IR absorption spectra of bare glass/MAPbI3 and glass/MAPbI3 on which ZnBChl, ZnChl, or ZnPor were deposited. The absorption spectra of MAPbI3/ZnChl and MAPbI3/ZnPor exhibit obvious absorption peaks at longer wavelengths compared to their monomers in solution, indicating the formation of J-aggregates on the perovskite surface. On the other hand, the absorption spectrum of MAPbI3/ZnBChl shows distinct absorption peaks of both the monomer (750 nm) and aggregate (794 nm), suggesting lower self-assembly ability. Figure 4b shows the steady-state PL spectra of glass/MAPbI3, glass/MAPbI3/Zn(B)Chl, and glass/ MAPbI3/ZnPor under excitation from the glass side at 403 nm. The PL intensity of MAPbI3 at 770 nm is greatly reduced, by

sectional scanning electron microscopy (SEM) images of the fabricated PSCs without a silver electrode. The SEM images show that the devices contain ∼100 nm thick compact TiO2 and mesoporous TiO2 (cm-TiO2 + me-TiO2) as an ETM, ∼427 nm thick MAPbI3 deposited on the cm-TiO2 + me-TiO2 as a light-absorption layer by a one-step deposition method, and ∼70 nm thick ZnBChl, ZnChl, or ZnPor as an HTM.48,49 Figure 3b depicts the energy alignment of the devices. The sufficiently high HOMO levels of these HTMs could allow holes generated in perovskite to travel to the silver electrode. Among these HTMs, ZnBChl, which has the highest HOMO level, is expected to show a lower Voc than ZnChl and ZnPor. To survey the aggregate morphology of HTMs deposited on perovskite, the solid film surfaces of ZnChl, ZnBChl, or ZnPor deposed on the MAPbI3 have been characterized by atomic force microscopy (AFM). We measured AFM images of these devices not on FTO but ITO substrate due to the smaller surface roughness of the latter compared to the former that allows reliable observation of the morphology. As shown in Figure S16, the root-mean-square (RMS) values were obtained by computational analysis to determine the film roughness. The D


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AFM images is not smooth, which may result in the recombination of charge carriers;66(2) ZnPor exhibits delocalized π-electrons over the symmetric aggregates resulting in a broad EB and a weak p-type characteristic to prevent efficient hole transfer.64,65 To further investigate the above possibilities, we conducted the EIS measurements of the PSC devices in the dark; panels a and b of Figure 6 show the corresponding equivalent electrical circuit. R1 and C1 in the first shunt pair represent the recombination resistance and chemical capacitance of these devices, respectively. R2 and C2 in the second shunt pair are ascribed to the contact resistance and the selective contact capacitance, respectively, owing to the charge buildup at the interface of perovskite/contact layers. R3 is mainly related to the resistance of the external wires and devices. The EIS results compose of three parameters, i.e., frequency (Hz), impedance, and angel. The values on x axis are obtained from the formula “impedance × cos(angle/180π)”, which corresponds to the real part of the complex impedance. The values of y axis are disposed from the formula “− impedance × sin(angle/180π)”, which represents the imaginary part of the complex impedance. These EIS results reveal that the shape of impedance spectra is a semicircle, which is corresponds to the series resistance (Rs) of these devices. Apparently, ZnPor-based PSCs show bigger Rs compared with ZnChl and ZnBChl owing to charge transfer barriers in the device.69,70 Moreover, the ZnPor-based device gives the smallest dark current (Figure S18), indicating the leakage of charges at the interface may not be severe in such devices. The good PCEs of ZnChl-based PSCs can be explained as follows: (1) ZnChl has a compatible HOMO level for mediating charge transfer between perovskite and Ag; (2) ZnChl forms a less rough film surface, offering an advantage for hole transport between perovskite and ZnChl;66 (3) the wellordered J-aggregate formation of ZnChl with enhanced intermolecular π−π stacking enables smooth hole transport in the dye layers; (4) ZnChl exhibits the lowest series resistance in EIS among these chlorophyll derivatives indicating its most excellent charge transfer property. The time evolution of the photocurrent density spectra (Figure 5c) of the PSCs employing ZnBChl or ZnChl as an HTM show that the current density maintains a stable output up to 2400 s (40 min) under one sun illumination (100 mW· cm−2, AM 1.5G) and a 0.9 V forward bias. In addition, Figure S19 shows the stability of the PCE, Jsc, Voc, and fill factor (FF) values of the devices employing ZnChl as an HTM. The device maintains 66.7%, 86.6%, 96.8%, and 74.6% of its initial PCE, Jsc, Voc, and FF values, respectively, after 936 h (39 days) under N2

factors 16, 30, and 8, when it is interfaced with ZnBChl, ZnChl, and ZnPor, respectively. The difference in PL quenching could be ascribed to the effectiveness of hole transfer at the interface. Thus, the hole-extraction ability of these HTMs can be ordered as follows: ZnChl > ZnBChl > ZnPor. Figure 5 shows (a) typical current−voltage (J−V) curves and (b) external quantum efficiency (EQE) spectra of the PSCs employing ZnBChl, ZnChl, or ZnPor as an HTM. Table 3 lists Table 3. Photovoltaic Performance Parameters of the PSCs Based on ZnBChl, ZnChl, and ZnPor as HTMs HTM

PCE (%)

Voc (V)

Jsc (mA·cm−2)

FF (%)

ZnBChl ZnChl ZnPor

8.26 11.88 0.68

0.79 0.96 1.14

18.94 19.63 1.94

57.4 63.1 30.8

the relevant photovoltaic performance parameters extracted from the J−V curves. The corresponding PCE and short-circuit current (Jsc) values are ordered as follows: ZnChl (11.88%, 19.63 mA·cm−2) > ZnBChl (8.26%, 18.94 mA·cm−2) > ZnPor (0.68%, 1.94 mA·cm−2), in good agreement with the order of the hole-extraction ability of the HTMs. However, the opencircuit voltage (Voc) shows a different order: ZnPor (1.14 V) > ZnChl (0.99 V) > ZnBChl (0.85 V), in agreement with the HOMO levels of the HTMs. The lower Voc of ZnBChl-based PSCs can be explained by the higher HOMO level of ZnBChl. In addition, ZnBChl formed an imperfect J-aggregate film on the perovskite surface, which could limit the PCE of the PSCs. Further, the lower PCE of the ZnPor-based PSCs is attributable to its lower HOMO level, which produces a higher hole injection barrier from perovskite to ZnPor compared with those of ZnChl and ZnBChl. To overcome this problem, we increased the energy gap slightly by shifting the HOMO level of MAPbI3 perovskite downward by incorporating FA+ and Br− into the crystal lattice of MAPbI3.67,68 Consequently, the PCE of the modified ZnPor-based PSC is improved to 4.04%, and it clearly has a normal J−V curve, indicating better energy alignment between ZnPor and the modified perovskite. The corresponding XRD patterns, UV/vis/near-IR absorption spectra, and photovoltaic performance parameters are presented in Figure S17 and Table S1. Given that the energy levels of HOMO orbitals in both ZnChl and ZnPor are quite similar, the following factors other than the interfacial energy alignment may contribute to the large discrepancy in the resulting PSCs. (1) The interface alignment between perovskite and ZnPor deduced from the

Figure 6. (a) Complex impedance spectra for devices employed ZnBChl, ZnChl, and ZnPor as HTMs, and (b) fitting model of an equivalent circuit. E


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F.W.) and JSPS KAKENHI Grant Number JP16K05826 in Scientific Research (C) (to S.S.), and JSPS KAKENHI Grant Numbers JP24107002 and JP17H06436 in Scientific Research on Innovative Areas (to H.T.). This work was also supported by natural science foundation of Jilin Province (No. 20160101303JC)

atmosphere. These results indicate that these chlorophyll derivatives have excellent durability. Moreover, Figure S20 shows the statistical data on the PCE, Voc, Jsc, and FF values of the PSCs using ZnBChl and ZnChl as HTMs. Reasonable repeatability was obtained in these devices.

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CONCLUSIONS In this work, three chlorophyll derivatives, ZnBChl, ZnChl, and ZnPor, were newly synthesized for use as HTMs in PSCs, and the photophysical properties and performance of the PSCs were characterized. The structural differences among these chlorophyll derivatives determined their characteristic photophysical properties and consequently led to a great disparity in their PCEs, which are ordered as follows: ZnChl (11.88%) > ZnBChl (8.26%) > ZnPor (0.68%). The PCE of the ZnBChlbased PSC were limited by the higher HOMO energy level distribution and by imperfect J-aggregate formation compared with ZnChl. The small PCE of the ZnPor-based PSC is partially attributed to its deeper HOMO level, which produces a small energy gap between ZnPor and MAPbI3 perovskite, and thus a higher hole injection barrier from perovskite to ZnPor compared with ZnChl and ZnBChl. Thus, we successfully tuned this energy gap by shifting the HOMO level of MAPbI3 perovskite downward by incorporating FA+ and Br− into its crystal lattice and improved the PCE of the ZnPor-based PSC from 0.68% to 4.04%. In addition, the ZnPor/perovskite interface is not well aligned, with possible inner barriers owing to the large surface roughness and delocalized aggregate structure of ZnPor; this hinders the hole transfer at the interface.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsaem.7b00018. Experimental details including synthesis of chlorophyll derivatives, 1H NMR and 13C NMR spectra, fabrication of device, materials characterization, and devices performance characterization (PDF)

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ABBREVIATIONS AFM = atomic force microscopy BChls = Bacteriochlorins Chls = chlorins cm-TiO2 + me-TiO2 = compact TiO2 and mesoporous TiO2 EB = binding energy Eg = band gap EQE = external quantum efficiency ETM = electron-transporting material FA+ = formamidinium FF = fill factor HOMO = highest occupied molecular orbital HTMs = hole-transporting materials Jsc = short-circuit current J−V = current−voltage LUMO = lowest unoccupied molecular orbital MA+ = methylammonium MAPbI3 = methylammonium lead iodide Pcs = phthalocyanines PL = photoluminescence Pors = porphyrins PSCs = perovskite solar cells SEM = scanning electron microscopy SEC = secondary electron cutoff THF = tetrahydrofuran UPS = ultraviolet photoelectron spectroscopy Voc = open-circuit voltage XRD = X-ray diffraction Φ = work function REFERENCES

(1) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-Range Balanced Electron- and Hole-Transport Lengths in Organic-Inorganic CH3NH3PbI3. Science 2013, 342, 344−347. (2) Zhang, W.; Saliba, M.; Moore, D. T.; Pathak, S. K.; Horantner, M. T.; Stergiopoulos, T.; Stranks, S. D.; Eperon, G. E.; Alexander-Webber, J. A.; Abate, A.; Sadhanala, A.; Yao, S. H.; Chen, Y. L.; Friend, R. H.; Estroff, L. A.; Wiesner, U.; Snaith, H. J. Ultrasmooth OrganicInorganic Perovskite Thin-Film Formation and Crystallization for Efficient Planar Heterojunction Solar Cells. Nat. Commun. 2015, 6, 6142. (3) Guo, Z.; Wan, Y.; Yang, M.; Snaider, J.; Zhu, K.; Huang, L. LongRange Hot-Carrier Transport in Hybrid Perovskites Visualized by Ultrafast Microscopy. Science 2017, 356, 59−62. (4) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal Halide Perovskite as Visible-Light Sensitizer for Photovoltaic Cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (5) 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.; Grätzel, M.; Park, N.-G. Lead Iodide Perovskite Sensitized AllSolid-State Submicron Thin Film Mesoscopic Solar Cell with Efficiency Exceeding 9%. Sci. Rep. 2012, 2, 591−597. (6) 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, 338, 643−647.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X.-F.W.). *E-mail: [email protected] (S.S.). ORCID

Hitoshi Tamiaki: 0000-0003-4797-0349 Xiao-Feng Wang: 0000-0002-8388-7019 Author Contributions

X.-F.W. generated the idea and designed the experimental plan. M.L., N.L., and W.H. conducted the corresponding device fabrication and basic characterization. S.S., K.S., and T.I. contributed to the synthesis of Chl-1 and Chl-2. H.T., G.C., T.M., and X.-F.W. provided related experimental conditions and participated in the technical discussions. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. Daobin Yang of Yamagata University for measuring AFM images. This work was partially supported by the Natural Science Foundation of China (No. 11574111 to XF


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