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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Molecular “Flower” as the High-Mobility Hole-Transport Material for Perovskite Solar Cells Chun Kou,†,§ Shiyu Feng,†,§ Hongshi Li,‡ Wenhua Li,*,† Dongmei Li,‡ Qingbo Meng,*,‡ and Zhishan Bo*,† †
Key Laboratory of Energy Conversion and Storage Materials, College of Chemistry, Beijing Normal University, Beijing 100875, China ‡ Key Laboratory for Renewable Energy, Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *
ABSTRACT: To develop novel hole-transport materials (HTMs) with less synthetic steps is still a great challenge. Here, a small molecule hexakis[4(N,N-di-p-methoxyphenylamino)phenyl]benzene (F-1) was successfully synthesized by a relatively simple scenario. F-1 exhibits a deep highest occupied molecular orbital energy level of −5.31 eV. Notably, F-1 also features 2 times higher hole mobility of 4.98 × 10−4 cm2 V−1 s−1 than that of the mostly used 2,2′,7,7′-tetrakis(N,N-bis(4-methoxyphenyl)amino)-9,9′-spirobifluorene (spiro-OMeTAD). Consequently, F-1-based perovskite solar cells (PSCs) show markedly improved performance compared with spiro-OMeTAD-based ones. These results indicate such a material can be a promising HTM candidate to boost the overall performance of the PSC. KEYWORDS: hole-transport material synthesis, perovskite solar cell, high mobility, high efficiency, deep energy level
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INTRODUCTION Since 2009, the organic−inorganic hybrid perovskite solar cells (PSCs) have achieved a considerable progress as the power conversion efficiency (PCE) boosted from 3.81 to 22.1%.2 Substantially, such a rapid growth can be ascribed to the intrinsically unique properties of perovskite materials including small band gap, high extinction coefficients, excellent crystallinity, long electron−hole diffusion length, etc.3−6 Moreover, the dramatically improved efficiency also relies on the optimization of other function layers of solar cells, e.g., the hole-transporting layer (HTL). Appropriate HTL materials are favorable conductors, which enable generated holes be efficiently and selectively extracted by the HTL and be transported to the electrode.7,8 This requires the holetransporting materials (HTMs) to possess basically excellent charge-carrier mobility and proper energy level so as to match well with the active layer of the device. In this light, polymer, and small-molecule materials can be promising candidates for HTMs that have been intensively studied by several research groups. Poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine],9 a representative of polymer HTMs, which has advantageous characteristics of good film-forming ability, high conductivity, and high hole mobility, gave an impressive high PCE of 20.1%. However, such hole-transport polymer materials are polydisperse and exhibit poor batch-to-batch reproducibility. In contrast, small-molecule HTMs are of monodisperse, high purity, and good reproducibility, and the tuning of their energy © XXXX American Chemical Society
level, charge mobility, and conductivity can be easily achieved by molecular engineering.10−12 Among them, 2,2′,7,7′-tetrakis(N,N-bis(4-methoxyphenyl)amino)-9,9′-spirobifluorene (spiroOMeTAD) is the commonly used HTM in the state-of-the-art perovskite devices. However, spiro-OMeTAD still has some drawbacks such as the low hole mobility, poor UV instability, and the complicated multistep synthesis and purification.13,14 Another compelling example is 2′,7′-bis(bis(4-methoxyphenyl)amino)spiro[cyclopenta[2,1-b:3,4-b′]dithiophene-4,9′-fluorene] (FDT),15 which also exhibits a comparable PCE of 20.2%. Similar to spiro-OMeTAD, the synthesis of FDT is also complicated. We report here a new class of “flower-like” small-molecule HTM, hexakis[4-(N,N-di(4-methoxyphenyl)amino)-phenyl]benzene (F-1), which can be prepared by a relatively simple scenario. It has a benzene core with six “bis(4-methoxyphenyl)amino” hole-transport functional groups. Such a high density of hole-transport functional groups is helpful to obtain a higher hole-mobility material in comparison with spiro-OMeTAD comprising four bis(4-methoxyphenyl)amino hole-transport functional groups. Meanwhile, the two bis(4-methoxyphenyl)amino groups are linked by a less conjugated terphenylene spacer in F-1, which will lead to lower highest occupied Received: September 4, 2017 Accepted: November 28, 2017 Published: November 28, 2017 A
DOI: 10.1021/acsami.7b13380 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Scheme 1. Synthetic Routes of F-1
Figure 1. (a) TGA and (b) DSC curves of F-1 under N2 atmosphere. (c) UV−vis absorption (black line) and photoluminescence (PL) (red line) spectra of F-1 as film. (d) Ultraviolet photoelectron spectroscopy (UPS) spectrum of F-1. The inset figure presents detail of the onset (Eonset). The data of Ecutoff, Eonset, HOMO, and LUMO levels are shown in Table 1.
both give champion PCE of 17.73 and 19.05%, which are markedly superior to that with spiro-OMeTAD as HTM.
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels in comparison with spiroOMeTAD and allow the HTL to contact well with perovskite active layer.16,17 As a result, F-1 was characterized with a high hole mobility and low-lying HOMO level, which matches well with those of perovskite materials. Using this material to construct HTL in different structured perovskite solar cells
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RESULTS AND DISCUSSION
Materials Synthesis and Characterization. Flower-like small-molecule F-1 was successfully synthesized via a modified method as previously reported.18 The synthetic route of F-1 is displayed in Scheme 1. More details and characterization data B
DOI: 10.1021/acsami.7b13380 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Table 1. Thermal, Optical, and Electrochemical Characteristics of F-1 and Spiro-OMeTAD
a
HTM
λmax [nm]
λPL [nm]
Tdec [°C]
Tg [°C]
Tm [°C]
EHOMO [eV]
ELUMO [eV]
Eg [eV]a
hole mobility [cm2 V−1 s−1]
F-1 spiro-OMeTAD
312 377
420 424
433 449
187 124b
236 245b
−5.31 −5.11
−2.10 −2.12
3.21 2.99
4.98 × 10−4 2.31 × 10−4
Eg was calculated from the absorption onset. bData were obtained from literature.19
Figure 2. (a) Cross-sectional image of the device based on FTO/c-TiO2/PCBA/CH3NH3PbIxCl3−x/F-1/Ag. (b) Corresponding energy level diagram of the PSC. (c) J−V characteristics for F-1- and spiro-OMeTAD-based devices. (d) External quantum efficiency (EQE) spectrum of the PSC based on F-1.
which could arise from the π−π* transition among these triarylamine moieties. Notably, the absorption onset wavelength of F-1 is located at ca. 386 nm, whereas that of perovskite is at ca. 750 nm, indicating the influence of F-1 on the light absorption of perovskite is negligible.20 It is further corroborated by the almost unchanged absorbance spectra of perovskite without and with F-1 layer as shown in Figure S1. The PL spectrum of F-1 displays a maximum emission at 420 nm, with the Stokes shift of 108 nm, suggesting that a small structural change occurs for F-1 in the excited state owing to its rigid configuration.21 Ultraviolet photoelectron spectroscopy (UPS) was applied to study the electrochemical property of F-1. In Figure 1d, the cutoff (Ecutoff) energy region of 16.56 eV and onset (Eonset) energy region of 0.65 eV were observed for F-1. Using the equation of EHOMO = 21.22 − (Ecutoff − Eonset),22 the HOMO energy level of F-1 was determined to be −5.31 eV. Similarly, the EHOMO of spiro-OMeTAD was calculated to be −5.11 eV based on the UPS spectrum shown in Figure S3. From the band gap (Eg) of F-1 (3.21 eV) and spiro-OMeTAD (2.99 eV) determined by the corresponding absorption onset, the LUMO
can be found in the Supporting Information (SI). Compound 1 was synthesized following the reported procedure. Stille coupling compound 1 with 1,2-bis(trimethylstannyl)ethyne gave 4,4′-(ethyne-1,2-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (2) in a 74% yield. Trimerization of compound 2 in the presence of Co2(CO)8 gave the final product F-1 in a 64% yield. The good solubility of F-1 in o-dichlorobenzene demonstrates it remarkable solution processability. F-1 exhibits a good thermal stability as indicated by the thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) results. Telling from Figure 1a and Table 1, the thermal decomposition temperature (Tdec) of F-1 is 433 °C, which is commensurate with spiro-OMeTAD (449 °C). The DSC thermograms shown in Figure 1b give a higher glass transition temperature (Tg) of 187 °C for F-1 than that of 124 °C for spiro-OMeTAD. Optoelectric Properties. The optical characteristics of F-1 as thin films were examined by UV−vis absorption and steadystate photoluminescence (PL) spectroscopy. The spectra are displayed in Figure 1c, and the related data are provided in Table 1. The absorption maximum is centered at ca. 312 nm, C
DOI: 10.1021/acsami.7b13380 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Table 2. Photovoltaic Parameters of the Devices Based on Spiro-OMeTAD and F-1
PCE [%] HTM
Voc [V]
Jsc [mA cm−2]
FF [%]
average
max
spiro-OMeTAD F-1
1.06 (±0.01) 1.10 (±0.03)
22.11 (±0.03) 22.83 (±0.07)
62.2 (±3.6) 66.2 (±2.1)
14.48 (±1.26) 16.67 (±1.06)
15.74 17.73
Figure 3. (a) SCLC measured in dark of the devices with the configuration of ITO/PEDOT:PSS/HTM/Au. (b) Steady-state PL and (c) TRPL spectra of the perovskite film with or without capping different HTMs. The inset figure presents detail of PL spectrum for F-1- and spiro-OMeTADcoated perovskite film.
cm−2 under identical condition. Obviously, a general promotion for these three parameters has been observed when F-1 substitutes the conventional spiro-OMeTAD, and the overall PCE enhancement is mainly attributed to the improvement in Voc and FF. For Voc, it is reasonable to understand its increase from 1.07 to 1.13 V in view of the lower HOMO energy level of F-1 compared to that of spiro-OMeTAD as we mentioned above. To elucidate and illustrate the FF enhancement, we utilized the space charge limitation of current (SCLC) method to study the hole-transporting properties of HTM. The devices were constructed in the hole-only structure of indium tin oxide (ITO)/PEDOT:PSS/HTM/Au. The hole mobility of F-1 and spiro-OMeTAD were calculated to be 4.98 × 10−4 and 2.31 × 10−4 cm2 V−1 s−1, respectively, according to the Mott−Gurney law26 shown in Table 1. The results demonstrate that F-1 can conduct the hole and retard charge recombination more efficiently due to its higher hole mobility. The superior holetransporting property of F-1 can be further verified by the steady-state photoluminescence (PL) and the time-resolved photoluminescence (TRPL) measurements. Figure 3b demonstrates the normalized steady-state PL spectra of pristine perovskite film and that with two kinds of HTMs. At the excitation wavelength of 550 nm, the pristine perovskite film exhibits a strong emission in the range from 700 to 850 nm. Taking spiro-OMeTAD as an overlayer on it, the intensity of PL is greatly diminished. Using F-1 as the HTM, a much more marked drop in the intensity is observed and the PL quenching efficiency reaches almost extremum, manifesting less charge recombination and more efficient hole extraction and charge transfer occurring in the perovskite/F-1 interface than in the perovskite/spiro-OMeTAD interface.27 TRPL decay transient spectra are displayed in Figure 3c. Fitting the curves with exponential diffusion model results in the decay time of 284.05, 2.14, and 1.97 ns for the pristine perovskite, perovskite/spiroOMeTAD, and perovskite/F-1 films, respectively. Notably, the PL lifetime for the F-1-coated perovskite film is mostly substantially shortened. This signifies a stronger chargeextraction capability of F-1 than spiro-OMeTAD, which induces a faster charge-carrier transfer from the perovskite film to F-1 layer.28,29 Both the higher hole mobility and
energy levels are calculated by the equation ELUMO = EHOMO + Eg and exhibited in Table 1. Photovoltaic Properties. PSC were constructed with the configuration of fluorine-doped tin oxide (FTO)/c-TiO2 (130 nm)/PCBA/CH3NH3PbIxCl3−x (230 nm)/HTM/Ag, where PCBA is [6,6]-phenyl-C61-butyric acid reported in our previous work23 and HTM is either homemade F-1 (140 nm) or commercial spiro-OMeTAD (150 nm). The fabrication details are described in the Experimental Section (SI), and all of the devices studied in the context were fabricated under the optimal conditions. As shown in Figure 2a, from the scanning electron microscopy image, each functional layer of the prepared device could be clearly distinguished, and their respectively energy levels are shown in Figure 2b. Wherein, the energy levels of TiO2, PCBA, CH3NH3PbIxCl3−x, and Ag were obtained from literatures.23−25 In comparison with spiroOMeTAD of −5.11 eV, F-1 possesses a depressed HOMO level of −5.31 eV shifting toward the perovskite of −5.5 eV. Such a low-lying HOMO level matches better with that of CH3NH3PbIxCl3−x, indicating that the driving force for holes injection from perovskite to F-1 is predictably more powerful. Charge separation and transfer occurs more easily at the interface of perovskite/F-1 than at that of perovskite/spiroOMeTAD. Usually, several key photovoltaic parameters, as exemplified by open-circuit voltage (Voc) and short-circuit current density (Jsc), are highly dependent on these physical processes. The current density−voltage (J−V) characteristics of devices were measured both in the forward and reverse directions with the scan speed of 0.02 V s−1 under the simulated AM 1.5 G, 100 mW cm−2 solar illumination. In the text, the data collected in reverse scan are exemplified for our following discussions, and meanwhile data collected in forward scan are all presented in SI, Table S1. The J−V curves in reverse scan are exhibited in Figure 2c and the related photovoltaic data are summarized in Table 2. The best device with F-1 as HTM shows a fill factor (FF) of 68.5%, a Voc of 1.13 V and a Jsc of 22.90 mA cm−2, which makes PCE as high as 17.73%. As for the spiroOMeTAD-based device, it exhibits a relatively low PCE of 15.74% with FF of 65.8%, Voc of 1.07 V, and Jsc of 22.14 mA D
DOI: 10.1021/acsami.7b13380 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
ACS Applied Materials & Interfaces
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excellent hole-transporting property of F-1 rationalize the enhancive Jsc as well. It is worth pointing out that the measured Jsc value is in good agreement with that obtained from integrating the external quantum efficiency (EQE) curves shown in Figure 2d, which indicates good reasonability of measurements. Moreover, we have to say that even though the commonly observed hysteresis phenomenon is not easily negligible in our case, the beneficial effect of F-1 can also be visually reflected by the data collected in forward scan, as shown in the SI. Significantly, as a new HTM for perovskite solar cells, it works well not only for one system. For the solar cells having hybrid (FAPbI3)0.85(MAPbBr3)0.15 as perovskite material, the final PCE can be improved from 18.32 to 19.04% when replacing spiro-OMeTAD with F-1, suggesting that F-1 is potentially more favorable for the construction of highly efficient perovskite solar cells. More detailed photovoltaic parameters can refer to in the SI. Besides the PCE, the stability of F-1-based devices is also improved because more than 60% of their initial efficiency is still maintained when these devices were exposed to an ambient environment for 8 days without any encapsulation, whereas it only keeps less than 50% for the control spiro-OMeTAD-based devices as shown in the SI, Figure S5. We speculate that this preferable stability can be mainly ascribed to the differential hindrance to water infiltration between F-1 and spiroOMeTAD. The measured contact angles (α) of deionized water drop on different HTM surfaces give a strong support to our speculation. As shown in Figure S4, the contact angles of 80.4° for spiro-OMeTAD and 85.7° for F-1 are obtained. This slightly enhanced hydrophobicity of F-1 film makes it more effective to prevent the penetration of water into the perovskite layers, thus leading to better durability against degradation.30,31
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (W.L.). *E-mail:
[email protected] (Q.M.). *E-mail:
[email protected] (Z.B.). ORCID
Wenhua Li: 0000-0003-1189-5287 Qingbo Meng: 0000-0003-4531-4700 Zhishan Bo: 0000-0003-0126-7957 Author Contributions §
C.K. and S.F. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank the financial support by the NSF of China (21574013 and 91233205) and the Program for Changjiang Scholars and Innovative Research Team.
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REFERENCES
(1) Kojima, A.; Teshima, K.; Shirai, Y.; Miyasaka, T. Organometal halide perovskites as visible-light sensitizers for photovoltaic cells. J. Am. Chem. Soc. 2009, 131, 6050−6051. (2) Yang, W. S.; Park, B.-W.; Jung, E. H.; Jeon, N. J.; Kim, Y. C.; Lee, D. U.; Shin, S. S.; Seo, J.; Kim, E. K.; Noh, J. H.; Seok, S. I. Iodide management in formamidinium-lead-halide−based perovskite layers for efficient solar cells. Science 2017, 356, 1376−1379. (3) Dong, Q.; Fang, Y.; Shao, Y.; Mulligan, P.; Qiu, J.; Cao, L.; Huang, J. Electron-hole diffusion lengths >175 μm in solution-grown CH3NH3PbI3 single crystals. Science 2015, 347, 967−970. (4) Savenije, T. J.; Ponseca, C. S., Jr.; Kunneman, L.; Abdellah, M.; Zheng, K.; Tian, Y.; Zhu, Q.; Canton, S. E.; Scheblykin, I. G.; Pullerits, T.; et al. Thermally activated exciton dissociation and recombination control the carrier dynamics in organometal halide perovskite. J. Phys. Chem. Lett. 2014, 5, 2189−2194. (5) Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J. Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 2013, 342, 341− 344. (6) Luo, W.; Wu, C.; Sun, W.; Guo, X.; Xiao, L.; Chen, Z. High Crystallization of Perovskite Film by a Fast Electric Current Annealing Process. ACS Appl. Mater. Interfaces 2017, 9, 26915−26920. (7) Wang, Y. K.; Yuan, Z. C.; Shi, G. Z.; Li, Y. X.; Li, Q.; Hui, F.; Sun, B. Q.; Jiang, Z. Q.; Liao, L. S. Dopant-Free Spiro-Triphenylamine/ Fluorene as Hole-Transporting Material for Perovskite Solar Cells with Enhanced Efficiency and Stability. Adv. Funct. Mater. 2016, 26, 1375−1381. (8) Mane, S. B.; Sutanto, A. A.; Cheng, C.-F.; Xie, M.-Y.; Chen, C.-I.; Leonardus, M.; Yeh, S.-C.; Beyene, B. B.; Diau, E. W.-G.; Chen, C.-T.; Hung, C.-H. Oxasmaragdyrins as New and Efficient Hole-Transporting Materials for High-Performance Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2017, 9, 31950−31958. (9) Yang, W. S.; Noh, J. H.; Jeon, N. J.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. High-performance photovoltaic perovskite layers fabricated through intramolecular exchange. Science 2015, 348, 1234−1237. (10) Hu, Z.; Fu, W.; Yan, L.; Miao, J.; Yu, H.; He, Y.; Goto, O.; Meng, H.; Chen, H.; Huang, W. Effects of heteroatom substitution in spiro-bifluorene hole transport materials. Chem. Sci. 2016, 7, 5007− 5012. (11) Chen, H.; Fu, W.; Huang, C.; Zhang, Z.; Li, S.; Ding, F.; Shi, M.; Li, C.-Z.; Jen, A. K. Y.; Chen, H. Molecular Engineered HoleExtraction Materials to Enable Dopant-Free, Efficient p-i-n Perovskite Solar Cells. Adv. Energy Mater. 2017, 7, No. 1700012. (12) Huang, C.; Fu, W.; Li, C.-Z.; Zhang, Z.; Qiu, W.; Shi, M.; Heremans, P.; Jen, A. K. Y.; Chen, H. Dopant-Free Hole-Transporting
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CONCLUSIONS A flower-like small-molecule F-1 was synthesized by a relatively simple scenario and used as an effective hole-transporting material to fabricate a high-efficiency perovskite solar cells. TGA and DSC measurements revealed its good thermal stability. The UPS results gave a low-lying HOMO energy level of −5.31 eV, which could match better with CH3NH3PbIxCl3−x than with spiro-OMeTAD. The hole mobility of F-1 as high as 4.98 × 10−4 cm2 V−1 s−1 was highlighted, which is beneficial to the improvement of Jsc and FF. Such superior optical−electrical properties endow F-1 with suitable energy level and fast chargeextraction/transfer ability. Consequently, the best PCE of 17.73% was achievable for the corresponding devices, which was higher than that using state-of-the-art spiro-OMeTAD as the HTM. Besides, F-1-based devices are more stable than spiro-OMeTAD-based ones. The results suggest that F-1 is a competitive candidate as the HTM to replace the classical spiro-OMeTAD in the fabrication of perovskite solar cells.
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Research Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b13380. Synthesis and characterization of F-1, UV, UPS, water contact angles, stability measurement, device fabrication, and photovoltaic parameters of the solar cells (PDF) E
DOI: 10.1021/acsami.7b13380 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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ACS Applied Materials & Interfaces
(31) Peng, S.-H.; Huang, T.-W.; Gollavelli, G.; Hsu, C.-S. Thiophene and diketopyrrolopyrrole based conjugated polymers as efficient alternatives to spiro-OMeTAD in perovskite solar cells as hole transporting layers. J. Mater. Chem. C 2017, 5, 5193−5198.
Material with a C3h Symmetrical Truxene Core for Highly Efficient Perovskite Solar Cells. J. Am. Chem. Soc. 2016, 138, 2528−2531. (13) Murray, A. T.; Frost, J. M.; Hendon, C. H.; Molloy, C. D.; Carbery, D. R.; Walsh, A. Modular design of SPIRO-OMeTAD analogues as hole transport materials in solar cells. Chem. Commun. 2015, 51, 8935−8938. (14) Ganesan, P.; Fu, K.; Gao, P.; Raabe, I.; Schenk, K.; Scopelliti, R.; Luo, J.; Wong, L. H.; Grätzel, M.; Nazeeruddin, M. K. A simple spirotype hole transporting material for efficient perovskite solar cells. Energy Environ. Sci. 2015, 8, 1986−1991. (15) Saliba, M.; Orlandi, S.; Matsui, T.; Aghazada, S.; Cavazzini, M.; Correa-Baena, J.-P.; Gao, P.; Scopelliti, R.; Mosconi, E.; Dahmen, K.H.; et al. A molecularly engineered hole-transporting material for efficient perovskite solar cells. Nat. Energy 2016, 1, No. 15017. (16) Torres, A.; Rego, L. G. Surface effects and adsorption of methoxy anchors on hybrid lead iodide perovskites: Insights for spiromeotad attachment. J. Phys. Chem. C 2014, 118, 26947−26954. (17) Jeon, N. J.; Lee, H. G.; Kim, Y. C.; Seo, J.; Noh, J. H.; Lee, J.; Seok, S. I. o-Methoxy substituents in spiro-OMeTAD for efficient inorganic−organic hybrid perovskite solar cells. J. Am. Chem. Soc. 2014, 136, 7837−7840. (18) Lambert, C.; Nöll, G. One- and Two-Dimensional Electron Transfer Processes in Triarylamines with Multiple Redox Centers. Angew. Chem., Int. Ed. 1998, 37, 2107−2110. (19) Nishimura, H.; Ishida, N.; Shimazaki, A.; Wakamiya, A.; Saeki, A.; Scott, L. T.; Murata, Y. Hole-transporting materials with a twodimensionally expanded π-system around an azulene core for efficient perovskite solar cells. J. Am. Chem. Soc. 2015, 137, 15656−15659. (20) Kwon, Y. S.; Lim, J.; Yun, H.-J.; Kim, Y.-H.; Park, T. A diketopyrrolopyrrole-containing hole transporting conjugated polymer for use in efficient stable organic−inorganic hybrid solar cells based on a perovskite. Energy Environ. Sci. 2014, 7, 1454−1460. (21) Choi, H.; Cho, J. W.; Kang, M.-S.; Ko, J. Stable and efficient hole transporting materials with a dimethylfluorenylamino moiety for perovskite solar cells. Chem. Commun. 2015, 51, 9305−9308. (22) Li, Z.; Zhu, Z.; Chueh, C.-C.; Jo, S. B.; Luo, J.; Jang, S.-H.; Jen, A. K.-Y. Rational design of dipolar chromophore as an efficient dopant-free hole-transporting material for perovskite solar cells. J. Am. Chem. Soc. 2016, 138, 11833−11839. (23) Dong, Y.; Li, W.; Zhang, X.; Xu, Q.; Liu, Q.; Li, C.; Bo, Z. Highly efficient planar perovskite solar cells via interfacial modification with fullerene derivatives. Small 2016, 12, 1098−1104. (24) Mihailetchi, V. D.; Xie, H.; de Boer, B.; Koster, L. A.; Blom, P. W. Charge transport and photocurrent generation in poly(3hexylthiophene): methanofullerene bulk-heterojunction solar cells. Adv. Funct. Mater. 2006, 16, 699−708. (25) 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. (26) Xu, Q.; Lu, Z.; Zhu, L.; Kou, C.; Liu, Y.; Li, C.; Meng, Q.; Li, W.; Bo, Z. Elimination of the J−V hysteresis of planar perovskite solar cells by interfacial modification with a thermo-cleavable fullerene derivative. J. Mater. Chem. A 2016, 4, 17649−17654. (27) Xing, G.; Mathews, N.; Sun, S.; Lim, S. S.; Lam, Y. M.; Grätzel, M.; Mhaisalkar, S.; Sum, T. C. Long-range balanced electron-and holetransport lengths in organic-inorganic CH3NH3PbI3. Science 2013, 342, 344−347. (28) Lin, Y.; Shen, L.; Dai, J.; Deng, Y.; Wu, Y.; Bai, Y.; Zheng, X.; Wang, J.; Fang, Y.; Wei, H.; et al. π-Conjugated Lewis Base: Efficient Trap-Passivation and Charge-Extraction for Hybrid Perovskite Solar Cells. Adv. Mater. 2017, 29, No. 1604545. (29) de Quilettes, D. W.; Vorpahl, S. M.; Stranks, S. D.; Nagaoka, H.; Eperon, G. E.; Ziffer, M. E.; Snaith, H. J.; Ginger, D. S. Impact of microstructure on local carrier lifetime in perovskite solar cells. Science 2015, 683−686. (30) Cao, J.; Liu, Y.-M.; Jing, X.; Yin, J.; Li, J.; Xu, B.; Tan, Y.-Z.; Zheng, N. Well-defined thiolated nanographene as hole-transporting material for efficient and stable perovskite solar cells. J. Am. Chem. Soc. 2015, 137, 10914−10917. F
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