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Boron-Doped Graphite for High Work Function Carbon Electrode in Printable Hole-Conductor-Free Mesoscopic Perovskite Solar Cells Miao Duan, Chengbo Tian, Yue Hu, Anyi Mei, Yaoguang Rong, Yuli Xiong, Mi Xu, Yusong Sheng, Pei Jiang, Xiaomeng Hou, Xiaotong Zhu, Fei Qin, and Hongwei Han ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b05689 • Publication Date (Web): 24 Aug 2017 Downloaded from http://pubs.acs.org on August 25, 2017

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ACS Applied Materials & Interfaces

Boron-Doped Graphite for High Work Function Carbon Electrode in Printable Hole-Conductor-Free Mesoscopic Perovskite Solar Cells Miao Duan†, Chengbo Tian†, Yue Hu, Anyi Mei, Yaoguang Rong, Yuli Xiong, Mi Xu, Yusong Sheng, Pei Jiang, Xiaomeng Hou, Xiaotong Zhu, Fei Qin, Hongwei Han* Michael Grätzel Center for Mesoscopic Solar Cells, Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan 430074, Hubei, P. R. China KEYWORDS: perovskite solar cells, mesoscopic, work function, boron doping, carbon electrode ABSTRACT: Work function of carbon electrodes is critical in obtaining high open-circuit voltage as well as high device performance for carbon based perovskite solar cells. Herein, we propose a novel strategy to upshift work function of carbon electrode by incorporating boron atom into graphite lattice and employ it in printable hole-conductor-free mesoscopic perovskite solar cells. The high-work-function boron-doped carbon electrode facilitates hole extraction from perovskite as verified by photoluminescence. Meanwhile, the carbon electrode is endowed with an improved conductivity due to a higher graphitization carbon of boron-doped graphite. These advantages of the boron-doped carbon electrode result in a low charge transfer resistance at carbon/perovskite interface and an extended carrier recombination lifetime. Together with the merit of both high work function and conductivity, the power conversion efficiency of hole-conductor-free mesoscopic perovskite solar cells is increased from 12.4 % for the pristine needle coke electrode based cells to 13.6 % for the boron doped cells with an enhanced open-circuit voltage and fill factor. 1 ACS Paragon Plus Environment


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INTRODUCTION The exceptional features of organometal halide perovskites including strong absorption coefficient, high charge carrier mobility and long carrier diffusion length enable power conversion efficiency (PCE) of perovskite solar cells (PSCs) to be improved from 3.8 % to certified 22.1 % in just a few years

1-3

. The remarkable boost in photovoltaic properties have

attracted worldwide attention4-6. A typical PSC utilizes organic semiconductors (e.g. spiro-OMeTAD) and a noble metal (e.g. Au) as hole transporter material (HTM) and back contact, respectively7-10. Undoubtedly, the expensive HTM and Au electrode not only increase the production cost of the devices, the most commonly used HTM, spiro-OMeTAD, has also proved to be unstable in air. As an alternative, a cost-efficient carbon-based HTM-free printable mesoscopic

perovskite

solar

cells

with

the

configuration

of

FTO/(TiO2/ZrO2

/carbon)/perovskite was developed in our group11-12. The devices avoid the use of HTM and employ cheap carbon materials as back contact to replace the Au electrode, which not only simplify the device fabrication process, but can also significantly reduce the material cost to meet the demand of commercial application13-15. For the carbon-based HTM-free PSC devices, photo-induced holes are directly extracted by carbon electrode in contact with perovskite. However, the difference between valence band maximum of perovskites and work function of carbon electrode would limit the device open-circuit voltage or introduce an undesirable carrier extraction barrier16. Therefore, it is of significance to improve work function (WF) of carbon electrode to enhance the open circuit voltage of the device or facilitate the hole extraction from the perovskite. Typically, the method to improve the WF of electrodes has been realized by coating an interface modification layer such as metal oxides16-18. Erin M. Sanehira et al. reported the influence of electrode interfaces

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consisting of MoOx/Au, MoOx/Ag, MoOx/Al anode on the stability of PSCs18. Philip Schulz et al. investigated systemically the effect of high-work-function MoO3 modification layer on the photovoltaic performance of PSC devices16. However, the interface layer would complicate fabrication process and increase the electrode resistance19. It is thus essential to develop a new method for carbon electrode, which can endow carbon electrode with both high work function and improved conductivity. As previously reported, heteroatom doping is a feasible strategy to modify electronic properties of the host carbon materials such as graphene and carbon nanotube20-21. As an electron-deficient dopant, boron can efficiently enhance the work function of carbon materials by replacing partial carbon atom22. Min Chen et al. applied boron and phosphorus co-doped bilayer carbon counter electrode in PSC devices and improved the PCE of the devices from 3.72 % to 6.78 %23. Shihe Yang et al. reported that boron doping of multiwalled carbon nanotubes significantly enhanced hole extraction in PSCs24. Herein, we proposed an effective strategy to improve the work function of carbon electrode, in which boron atoms were incorporated into graphite lattice during boron doping process. The substitutional electron-deficient boron replaced partial carbon atom of graphite lattice, which not only improved work function of carbon electrode, but also decreased its resistance resulted from a higher graphitization degree. Incorporating into the PSCs, we found that the high-work-function and high conductive boron-doped graphite electrode facilitated hole extraction from perovskite to carbon electrode, leading to an improved open-circuit voltage and fill factor. As a result, the PCE of the boron-doped graphite based PSC device was improved from 12.4 % to 13.6 %. EXPERIMENTAL SECTION Preparation of boron-doped graphite



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The preparation process of boron-doped graphite was illustrated in Fig. 1. Firstly, the needle coke powder was mixed with 5 % mass ratio of boron carbide (B4C) and subsequently ground by ball milling. Then, the mixture was carbonized at 900 °C for 2 hours prior to the graphitization. Finally, the carbonized mixture was heat-treated at 2800 °C under the argon gas flow for graphitization. The obtained boron-doped graphite powder was denoted as BG. Boron-free graphite, denoted as PG, was also prepared by high temperature graphitization of needle coke without adding B4C for comparison. Noted that the PG was artificial graphite derived from needle coke of by-products of oil and was firstly used as counter electrode in PSCs, which differed from the natural graphite used in our previous literatures25-27. Preparation of carbon paste Carbon paste was then prepared as reported previously28-29. 6.5 g of PG or BG powders was mixed with 2 g of carbon black in 30 mL terpineol solution. Subsequently, 1 g of hydroxypropyl cellulose and 1 g of ZrO2 powders were added into the solution, followed by stirring vigorously via ball milling for 12 hours. This paste could be deposited by screen-printing technique. Fabrication of perovskite solar cells Fluorine-doped SnO2 (FTO) glass was firstly etched using a laser to form the desired electrode patterns. Then, the substrate was cleaned by sonication sequentially with detergent, deionized water and alcohol for 15 min, respectively. A TiO2 compact layer was deposited on the clean pre-heated FTO substrates by spray pyrolysis from a precursor solution of titanium diisopropoxide bis (acetylacetonate) solution, followed by sintering at 450 °C for 30 min. A mesoporous TiO2 layer was screen-printed on the compact layer and sintered for 30 min at 500 °C. A ZrO2 insulating layer and carbon layer was screen printed on the top of TiO2 layer


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successively and sintered at 400 °C for 30 min. After that, a 4.5 µL (5-AVA)x(MA)1-xPbI3 perovskite precursor solution was dipped on the mesoporous carbon layer, followed by annealing at 50 °C for one hour. Characterization The morphology of samples was characterized by a field-emission scanning electron microscope (SEM) (FEI Nova NanoSEM450) and the elemental microanalysis was measured with an energy dispersive X-ray spectrum (EDS) analysis system integrated into the SEM. X-ray diffraction (XRD) tests were performed on an PANalytical Empyrean X-ray diffractometer with Cu Ka radiation (k = 0.15418 nm). X-ray photoelectron spectroscopy (XPS) of the C 1s and B 1s were recorded by an X-ray photoelectron spectroscopy (Axis-Ultra DLD-600W, Shimadzu-Kratos). The film thickness was obtained by a surface profilometer (Dektak XT, Brucker). The sheet resistance of carbon films was provided by a KDY-1 four point probe analyzer. The interlayer spacing of the carbon 002 plane and graphitization degree were calculated from Bragg’s equation30 and Maire and Merings equation31, respectively. Work function of carbon films was measured in air using a Kelvin probe. The steady-state photoluminescence (PL) spectra were taken on a Horiba JobinYvon LabRAM HR800s with a 532 nm wavelength excitation source. Time resolved photoluminescence (TRPL) spectrum at 760 nm was acquired on Delta Flex Fluorescence Lifetime System (Horiba Scientific Com., Japan). Photocurrent-voltage characteristics (J-V) measurement is performed on Keithley 2400 sourcemeter under the air mass 1.5 (AM1.5) illumination at 100 mWcm-2 from Newport 91160 solar simulator (Newport, USA). Electrochemical impedance spectroscopy (EIS) was investigated by electrochemical workstations (Zahner Ennium) in the frequency range of 100 mmHz to 4MHz.



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Figure 1. Illustration of the preparation process for boron-doped graphite. RESULTS AND DISCUSSION Characteristics of boron-doped graphite

Figure 2. SEM images of (a) PG, (b) BG powders and (c) its EDS elemental mapping of C and B. SEM images of the pristine graphite (PG) and boron-doped graphite (BG) powder were exhibited in Fig. 2. It can be seen that the average size of PG sheets was ~10 µm (Fig. 2a). Size and morphology of the graphite sheets changed by boron doping, large crystalline grains and smooth layers were observed obviously presented in Fig. 2b. It was elucidated that rearrangement of the graphite crystal domains followed by the boundary disappearance might occur, which could be attributed to the incorporated boron into the hexagonal graphitic layer structure. To investigate the distribution of doped boron in graphite, EDS mapping was 6 ACS Paragon Plus Environment

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performed. As shown in Fig. 2c, C and B elements coexisted in the same regions, suggesting that B atoms were homogeneously distributed in graphite lattice without any segregation domains. Fig. 3a displayed the XRD profiles of PG and BG in the 2θ ranged from 10° to 50°. Compared with PG, new XRD peaks in the 2θ of ~35° and ~38° assigned to the 104 and 201 lattice plane of B4C were consistent with the values reported in the literature32-33. Furthermore, the separation of 100 and 101 lattice plane diffraction peaks of C became clearer after adding boron atoms, implying that the boron doped three-dimensional stacking layer structure of graphite enhanced the crystallinity34. To gain further insight into the effect of doped boron on the crystalline structure of graphite, 2θ, interlayer spacing (d002) and graphitization degree (G) of C were analyzed. As shown in Table 1, the 002 peak was shifted to a higher diffraction angle, in corresponding to the decrease of 002 lattice plane interlayer spacing, indicating the doping of boron atom affected the crystallization of graphite. According to the Maire and Merings’ equation31, the graphitization degree remarkably increased from 74 % to 85%, confirming that boron-doping accelerated the graphitization of carbon. Table 1. Crystalline parameter of PG and BG.

Samples

2θ (degree)

d002 (nm)

G (%)

PG

26.40

0.3376

74

BG

26.47

0.3367

85

The substitutional boron atom in graphite was further clarified by XPS analysis, as illustrated in Fig. 3. The detailed XPS spectra ranged from 183 to 193 eV in Fig. 3b showed the B 1s peak of BG and no peaks was detected in PG. The presence of the replaced boron atoms in graphite was confirmed by a close observation of the B 1s region in the spectrum shown in Fig.



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3c. The strong peak at 188.5 eV was originated from the substitutional boron atoms within the graphite33, 35-36, indicating the successful doping of boron. The amount of substitutional boron in graphite calculated by the curve fitted areas was 2.24 at. % (Table S1). Maximum solubility of boron in graphite has been reported to be 2.35 at.% at 2350 °C by solid-state diffusion process between B4C and graphite sheets37. Therefore, excessive boron existed in other phases. The weak peak located at 186.9 eV was assigned to boron carbide or boron cluster20. In addition, the C 1s peak come from the pristine graphite at 285.1 eV slightly shifted to lower bonding energy down to 285.0 eV as boron atoms added presented in Fig. 3d. The lowered of binding energy might be resulted from the lowered of the Fermi level by the formation of a chemical bond between carbon and electron-deficient boron33.

Figure 3. (a) XRD patterns of PG and BG. (b) C 1s and (c) B 1s XPS spectra of PG and BG. 8 ACS Paragon Plus Environment

(d) The

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magnified B 1s XPS spectra of BG. Properties of boron-doped carbon film As discussed above, within the boron doped graphite crystal lattice, the substitutional electron-deficient boron possibly lowered the Fermin level or improved work function of C38. To verify this, the work function (WF) of the PG and BG films were measured using a Kelvin probe. A highly ordered pyrolytic graphite (HOPG) sample with WF of 4.5 eV was used as a reference. From curves in Fig. 4a, the BG film displayed a high WF value of 5.10 eV, whereas the PG film had a low WF value of 4.81 eV. As a good back contact of perovskite solar cells, conductivity of carbon film was a significant parameter. Displayed in Table S2, the square resistance of PG and BG films were 35 and 16 ohm/sq, respectively. The decrease square resistance of BG film was attributed to a better graphite layer packing due to a higher graphitization degree mentioned in the XRD analysis39-40. Therefore, boron doping strategy is an effective method of improving the work function of carbon film without sacrificing the conductivity.



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Figure 4. (a) Work function curves of PG and BG film. (b) Steady-state PL spectra and (c) TRPL decays of perovskite on bare glass, PG film and BG film. (d) The magnified curves of dashed rectangle in (b). To evaluate the hole extraction ability of high WF carbon film, the stead-state PL and TRPL were measured under the same condition. Fig. 4b showed steady-state PL spectra of perovskite on the insulating glass and two kinds of carbon films fabricated by PG and BG, respectively. When perovskite was formed in carbon films, the steady-state PL was quenched because of fast charge transfer from the perovskite to carbon. The magnified curves in Fig. 4d presented that BG film had a weaker PL peak, suggesting that BG film has a better hole extraction ability than PG. The TRPL for the samples was also measured as depicted in Fig. 4c. The perovskite film deposited on the glass exhibited a lifetime of about 41 ns. When the carbon film was deposited to extract photo-induced holes, the lifetime of the perovskite on PG and BG 10 ACS Paragon Plus Environment

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films was obviously decreased to about 1.7 ns and 0.9 ns, respectively. The result revealed that the high WF carbon film exhibited a more efficient charge transport and interfacial transfer than the low WF carbon film. Performance of perovskite solar cells

Figure 5. (a) Device architecture of a hole-conductor-free mesoscopic PSC based on carbon electrode and (b) its corresponding energy band diagram. (c) J-V curves of PSCs with PG and BG as carbon electrode measured at reverse scan. (d) The steady-state power output at the maximum power point for BG electrode devices. (e) Photovoltaic characteristics for 10 randomly selected PG and BG electrode based PSCs devices. To investigate the effect of different WF of carbon electrode based on PG and BG, the mesoscopic perovskite solar cells were fabricated. The schematic structure of MPSCs with carbon electrode was presented in Fig. 5a. Mesoporous TiO2, ZrO2 and carbon film were screen-printed layer by layer on a compact TiO2 deposited on a patterned FTO glass, respectively. The cross-sectional image of the device was presented in Fig. S1, suggesting a ~600-nm-thick TiO2 layer, a ~2-µm-thick ZrO2 layer and a ~10-µm-thick carbon layer. The 11 ACS Paragon Plus Environment


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band alignment of the device components was represented in Fig. 5b. Because of the matching band structure, photo-induced electrons from conduction band (-3.9 eV) of the perovskite was injected into conduction band TiO2 (-4.0 eV) and the holes from the perovskite valence band (-5.4 eV) will be extracted by carbon electrode. Fig. 5c gave the density-voltage (J-V) curves of the best performed cells under illumination of AM1.5G 100 mW cm-2. The device with PG electrode displayed an open-circuit voltage (VOC) of 900 mV, a short-circuit current (JSC) of 22.82 mA/cm2 and a fill factor (FF) of 0.6, yielding a PCE of 12.4 %. Noted that the PG was artificial graphite and firstly used as carbon electrode in PSCs, formed by rapid crystallization via high temperature heat treatment of the carbonaceous precursor, which differed from the natural graphite used in our previous literatures25-27. Instead by high WF BG carbon electrode, the device gives an enhanced VOC of 940 mV and FF of 0.63, a close JSC of 22.87 mA/cm2, yielding an improved PCE of 13.6 %. The improved VOC of BG-based device can be reasonably attribute to the higher work function of BG41. The enhanced FF was related to the facilitated charge transfer between perovskite and carbon electrode, which will be discussed in the EIS result. The J-V curves for the PG and BG based PSC devices measured by forward and reverse scans were given in Fig. S2. The BG based devices exhibited a reduced hysteresis in their J-V curves, which was probably attributed to a well-matched band alignment at the perovskite/BG interface42. To verify the reliability of the J-V measurement, the steady-state photo-current output and their corresponding power output of the BG based cells were measured at 680 mV bias was shown in Fig. 5d. It can be seen that a steady-state current density of about 19.5 mA/cm2 and cell efficiency of 13.3 %were obtained, in accordance well with the J-V results. To check the reproducibility of the device performance, the statistical photovoltaic parameter data of one batch of ten devices screen-printed on a 10 cm×10 cm FTO glass showed in Fig. S3 were


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collected in Fig. 5e. It should be noted that the enhanced VOC and FF of the PSC devices with BG electrode was reproducible. Furthermore, we have fabricated a series of BG-based PSC devices using more mass ratios of B4C ranged from 0.1% to 8% except for 5% mass ratio. As shown in Table S4 and Table S5, it was found that the 5% mass ratio of B4C was the most favorable to achieve a balance between high work function and conductivity, thereby achieving a high PCE. Additionally, some supplementary experiments were performed to further investigate the effect of residual B4C on the carbon film and the performance of the PSCs. Based on the 2.24 at. % doped boron concentration, the mass ratio of residual B4C was approximately calculated 2 %. Hence, 2 wt. % B4C was mixed with the heat-treated graphite, which was denoted as MG. The WF, conductivity and TRPL results of PG and MG films were shown in Table S3. The WF of the MG film was approximate to that of PG, which suggested that B4C did not affect the energy band of carbon. Further, the TRPL curve of the MG film shown in Fig. S4 exhibited an approximate lifetime compared to that of PG. The result indicated that the hole extraction ability of the carbon electrode was not affected by the addition of a small quantity of B4C. Besides, the conductivity of the MG film was slightly lower than PG, which was explained that the B4C with much lower conductivity brought down the conductivity of the carbon film. Finally, according to the J-V curves in Fig. S5 and the results in Table S4, it was found that the VOC, JSC, FF and PCE of the MG based PSC was not improved in contrast with that of PG. Therefore, the residual B4C has almost no effect on the carbon film and the performance of the cells.



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Figure 6. (a) Nyquist plots of the PSC devices based on PG and BG electrode. (b) the plot of the charge recombination lifetime τe for PG and BG based PSC devices. Electrical impedance spectroscopy (EIS) measurements were conducted to investigate the interfacial charge transport and recombination behavior of perovskite solar cell devices. Fig. 6a showed the Nyquist plots in the frequency range from 4 MHz to 100 mmHz for PG and BG based devices at a bias of 600 mV under dark condition. The semicircle at high frequency region was attributed to the charge transfer resistance at carbon/perovskite interface (Rct). It was found that the device based on BG electrode exhibited a relatively smaller radius of the semicircle, indicating a lower Rct at carbon/perovskite interface, which could be responsible for the improvement FF of the device. Fig. 6b showed charge recombination times of the devices as a function of VOC. Generally, the long lifetime of excited electrode indicated a slower recombination rate, which was related to a high value of VOC43. This clearly showed the long recombination lifetime for BG based device, which contributed to an improved VOC.

CONCLUSIONS In summary, we successfully incorporated boron atom into graphite lattice by solid-state diffusion during graphite formation and thereby improved work function of carbon electrode 14 ACS Paragon Plus Environment

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from 4.81 to 5.10 eV. The high-work-function carbon electrode facilitated hole extraction from perovskite and shorten its PL lifetime from 1.7 to 0.9 ns, accompanying a longer recombination lifetime. Meanwhile, in contrast to the PG, the conductivity of BG carbon electrode was improved due to a higher graphitization. The result led to a lower charge transfer resistance at carbon/perovskite interface. Correspondingly, the power conversion efficiency of the boron-doped graphite based PSC device was improved from 12.4 % to 13.6 % by enhancement in VOC and FF. This work indicates that heteroatom-doping strategy to improve the work function of the carbon electrode is a promising method to enhance the open circuit voltage for the carbon electrode based mesoscopic PSCs. ASSOCIATED CONTENT Supporting Information The supporting information is available free of charge via the Internet at http://pubs.acs.org. Elemental abundances determined by XPS in PG and BG; Square resistance of PG and BG films; Cross-sectional SEM image of the BG based PSC device; J-V curves for the PG and BG based PSC devices with forward and reverse scanning direction; An array of mesoscopic solar cells array printed on 10 cm×10 cm FTO glass; Work function, square resistance and TRPL data of MG and PG films; TRPL decays of perovskite on PG and MG films; J-V curves of PSCs with PG and MG as carbon electrode measured at reverse scan; Device parameters of the PSCs based on of MG and PG; Work function and square resistance of BG based carbon films prepared by different mass ratio of B4C; Device parameters of the BG based PSCs prepared by different mass ratio of B4C. AUTHOR INFORMATION Corresponding Author *Email: [email protected] 15 ACS Paragon Plus Environment


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Author Contributions †

M.D. and C.T. contributed equally in this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS The authors acknowledge financial support from the National Natural Science Foundation of China (51502141, 91433203, 61474049), the Ministry of Science and Technology of China (863, 2015AA034601) and the 111 Project (No. B07038). We thank the Analytical and Testing Center of Huazhong University of Science and Technology (HUST) for FESEM testing. REFERENCES 1. Pazos-Outón, L. M.; Szumilo, M.; Lamboll, R.; Richter, J. M.; Crespo-Quesada, M.; Abdi-Jalebi, M.; Beeson, H. J.; Vrućinić, M.; Alsari, M.; Snaith, H. J.; Ehrler, B.; Friend, R. H.; Deschler, F. Photon Recycling in Lead Iodide Perovskite Solar Cells. Science 2016, 351, 1430-1433. 2. Li, X.; Bi, D.; Yi, C.; Décoppet, J.-D.; Luo, J.; Zakeeruddin, S. M.; Hagfeldt, A.; Grätzel, M. A Vacuum Flash–Assisted Solution Process for High-Efficiency Large-Area Perovskite Solar Cells. Science 2016, 353, 58-62. 3. Liu, M.; Johnston, M. B.; Snaith, H. J. Efficient Planar Heterojunction Perovskite Solar Cells by Vapour Deposition. Nature 2013, 501, 395-398. 4. Tan, H.; Jain, A.; Voznyy, O.; Lan, X.; García de Arquer, F. P.; Fan, J. Z.; Quintero-Bermudez, R.; Yuan, M.; Zhang, B.; Zhao, Y.; Fan, F.; Li, P.; Quan, L. N.; Zhao, Y.; Lu, Z.-H.; Yang, Z.; Hoogland, S.; Sargent, E. H. Efficient and Stable Solution-Processed Planar Perovskite Solar Cells Via Contact 16 ACS Paragon Plus Environment

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Passivation. Science 2017, 355, 722-726. 5. Saliba, M.; Matsui, T.; Domanski, K.; Seo, J.-Y.; Ummadisingu, A.; Zakeeruddin, S. M.; Correa-Baena, J.-P.; Tress, W. R.; Abate, A.; Hagfeldt, A.; Grätzel, M. Incorporation of Rubidium Cations into Perovskite Solar Cells Improves Photovoltaic Performance. Science 2016, 354, 206-209. 6. 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. 7. Jeon, N. J.; Noh, J. H.; Yang, W. S.; Kim, Y. C.; Ryu, S.; Seo, J.; Seok, S. I. Compositional Engineering of Perovskite Materials for High-Performance Solar Cells. Nature 2015, 517, 476-480. 8. Ahn, N.; Son, D.-Y.; Jang, I.-H.; Kang, S. M.; Choi, M.; Park, N.-G. Highly Reproducible Perovskite Solar Cells with Average Efficiency of 18.3% and Best Efficiency of 19.7% Fabricated Via Lewis Base Adduct of Lead(II) Iodide. J. Am. Chem. Soc. 2015, 137, 8696-8699. 9. Yan, K.; Long, M.; Zhang, T.; Wei, Z.; Chen, H.; Yang, S.; Xu, J. Hybrid Halide Perovskite Solar Cell Precursors: Colloidal Chemistry and Coordination Engineering Behind Device Processing for High Efficiency. J. Am. Chem. Soc. 2015, 137, 4460-4468. 10. Shao, Y.; Yuan, Y.; Huang, J. Correlation of Energy Disorder and Open-Circuit Voltage in Hybrid Perovskite Solar Cells. Nat. Energy 2016, 1, 15001. 11. Mei, A.; Li, X.; Liu, L.; Ku, Z.; Liu, T.; Rong, Y.; Xu, M.; Hu, M.; Chen, J.; Yang, Y.; Gratzel, M.; Han, H. A Hole-Conductor-Free, Fully Printable Mesoscopic Perovskite Solar Cell with High Stability. Science 2014, 345, 295-298. 12. Rong, Y.; Hou, X.; Hu, Y.; Mei, A.; Liu, L.; Wang, P.; Han, H. Synergy of Ammonium Chloride and Moisture on Perovskite Crystallization for Efficient Printable Mesoscopic Solar Cells. Nat. Commun. 2017, 8, 14555. 13. Rong, Y.; Liu, L.; Mei, A.; Li, X.; Han, H. Beyond Efficiency: The Challenge of Stability in



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

Page 18 of 21

Mesoscopic Perovskite Solar Cells. Adv. Energy Mater. 2015, 5, 1501066-1501081. 14. Ku, Z.; Rong, Y.; Xu, M.; Liu, T.; Han, H. Full Printable Processed Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cells with Carbon Counter Electrode. Sci. Rep. 2013, 3, 3132. 15. Xu, M.; Rong, Y.; Ku, Z.; Mei, A.; Liu, T.; Zhang, L.; Li, X.; Han, H. Highly Ordered Mesoporous Carbon for Mesoscopic CH3NH3PbI3/TiO2 Heterojunction Solar Cell. J. Mater. Chem. A 2014, 2, 8607-8611. 16. Schulz, P.; Tiepelt, J. O.; Christians, J. A.; Levine, I.; Edri, E.; Sanehira, E. M.; Hodes, G.; Cahen, D.; Kahn, A. High-Work-Function Molybdenum Oxide Hole Extraction Contacts in Hybrid Organic– Inorganic Perovskite Solar Cells. ACS Appl. Mater. Interfaces 2016, 8, 31491-31499. 17. Zhao, Y.; Nardes, A. M.; Zhu, K. Effective Hole Extraction Using Moox-Al Contact in Perovskite CH3NH3PbI3 Solar Cells. Appl. Phys. Lett. 2014, 104, 213906. 18. Sanehira, E. M.; Tremolet de Villers, B. J.; Schulz, P.; Reese, M. O.; Ferrere, S.; Zhu, K.; Lin, L. Y.; Berry, J. J.; Luther, J. M. Influence of Electrode Interfaces on the Stability of Perovskite Solar Cells: Reduced Degradation Using Moox/Al for Hole Collection. ACS Energy Lett. 2016, 1, 38-45. 19. Yan, L.; Song, Y.; Zhou, Y.; Song, B.; Li, Y. Effect of Pei Cathode Interlayer on Work Function and Interface Resistance of ITO Electrode in the Inverted Polymer Solar Cells. Org. Electron. 2015, 17, 94-101. 20. Kim, Y. A.; Fujisawa, K.; Muramatsu, H.; Hayashi, T.; Endo, M.; Fujimori, T.; Kaneko, K.; Terrones, M.; Behrends, J.; Eckmann, A.; Casiraghi, C.; Novoselov, K. S.; Saito, R.; Dresselhaus, M. S. Raman Spectroscopy of Boron-Doped Single-Layer Graphene. ACS Nano 2012, 6, 6293-6300. 21. Carroll, D. L.; Redlich, P.; Blase, X.; Charlier, J. C.; Curran, S.; Ajayan, P. M.; Roth, S.; Rühle, M. Effects of Nanodomain Formation on the Electronic Structure of Doped Carbon Nanotubes. Phys. Rev. Lett. 1998, 81, 2332-2335.


Page 19 of 21

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


22. Panchakarla, L. S.; Subrahmanyam, K. S.; Saha, S. K.; Govindaraj, A.; Krishnamurthy, H. R.; Waghmare, U. V.; Rao, C. N. R. Synthesis, Structure, and Properties of Boron- and Nitrogen-Doped Graphene. Adv. Mater. 2009, 21, 4726-4730. 23. Chen, M.; Zha, R.-H.; Yuan, Z.-Y.; Jing, Q.-S.; Huang, Z.-Y.; Yang, X.-K.; Yang, S.-M.; Zhao, X.-H.; Xu, D.-L.; Zou, G.-D. Boron and Phosphorus Co-Doped Carbon Counter Electrode for Efficient Hole-Conductor-Free Perovskite Solar Cell. Chem. Eng. J. 2017, 313, 791-800. 24. Zheng, X.; Chen, H.; Li, Q.; Yang, Y.; Wei, Z.; Bai, Y.; Qiu, Y.; Zhou, D.; Wong, K. S.; Yang, S. Boron Doping of Multiwalled Carbon Nanotubes Significantly Enhances Hole Extraction in Carbon-Based Perovskite Solar Cells. Nano Lett. 2017, 17, 2496-2505. 25. Zhang, L.; Liu, T.; Liu, L.; Hu, M.; Yang, Y.; Mei, A.; Han, H. The Effect of Carbon Counter Electrodes on Fully Printable Mesoscopic Perovskite Solar Cells. J. Mater. Chem. A 2015, 3, 9165-9170. 26. Sheng, Y.; Hu, Y.; Mei, A.; Jiang, P.; Hou, X.; Duan, M.; Hong, L.; Guan, Y.; Rong, Y.; Xiong, Y.; Han, H. Enhanced Electronic Properties in CH3NH3PbI3 Licl Mixing for Hole-Conductor-Free Printable Perovskite Solar Cells. J. Mater. Chem. A 2016, 4, 16731-16736. 27. Hou, X.; Hu, Y.; Liu, H.; Mei, A.; Li, X.; Duan, M.; Zhang, G.; Rong, Y.; Han, H. Effect of Guanidinium on Mesoscopic Perovskite Solar Cells. J. Mater. Chem. A 2017, 5, 73-78. 28. Chen, J.; Rong, Y.; Mei, A.; Xiong, Y.; Liu, T.; Sheng, Y.; Jiang, P.; Hong, L.; Guan, Y.; Zhu, X.; Hou, X.; Duan, M.; Zhao, J.; Li, X.; Han, H. Hole-Conductor-Free Fully Printable Mesoscopic Solar Cell with Mixed-Anion Perovskite CH3NH3PbI(3−X)(BF4)X. Adv. Energy Mater. 2016, 6, 1502009-1502014. 29. Liu, T.; Rong, Y.; Xiong, Y.; Mei, A.; Hu, Y.; Sheng, Y.; Jiang, P.; Hou, X.; Duan, M.; Guan, Y.; Hong, L.; Han, H. Spacer Improvement for Efficient and Fully Printable Mesoscopic Perovskite Solar



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

Page 20 of 21

Cells. RSC Adv. 2017, 7, 10118-10123. 30. Wang, R.; Gao, H.; Zhu, D.; Yang, T.; Zhang, X.; Li, L.; Yin, G.; Zhu, L.; Feng, Z.; Li, X. Dehydration Behaviour and Structural Evolution of Graphene Oxide Membranes on Silicon Substrate. Carbon 2017, 114, 23-30. 31. Dasgupta, K.; Sathiyamoorthy, D. Disordered Carbon–Its Preparation, Structure, and Characterisation. Mater. Sci. Technol. 2003, 19, 995-1002. 32. Tanaka, U.; Sogabe, T.; Sakagoshi, H.; Ito, M.; Tojo, T. Anode Property of Boron-Doped Graphite Materials for Rechargeable Lithium-Ion Batteries. Carbon 2001, 39, 931-936. 33. Fujimoto, H.; Mabuchi, A.; Natarajan, C.; Kasuh, T. Properties of Graphite Prepared from Boron-Doped Pitch as an Anode for a Rechargeable Li Ion Battery. Carbon 2002, 40, 567-574. 34. Sogabe, T.; Nakajima, K.; Inagaki, M. Effect of Boron-Doping on Structure and Some Properties of Carbon-Carbon Composite. J. Mater. Sci. 1996, 31, 6469-6476. 35. Shirasaki, T.; Derré, A.; Ménétrier, M.; Tressaud, A.; Flandrois, S. Synthesis and Characterization of Boron-Substituted Carbons. Carbon 2000, 38, 1461-1467. 36. Cermignani, W.; Paulson, T. E.; Onneby, C.; Pantano, C. G. Synthesis and Characterization of Boron-Doped Carbons. Carbon 1995, 33, 367-374. 37. Lowell, C. E. Solid Solution of Boron in Graphite. J. Am. Ceram. Soc. 1967, 50, 142-144. 38. Lin, T.; Huang, F.; Liang, J.; Wang, Y. A Facile Preparation Route for Boron-Doped Graphene, and Its Cdte Solar Cell Application. Energy Environ. Sci. 2011, 4, 862-865. 39. Sevilla, M.; Fuertes, A. B. Catalytic Graphitization of Templated Mesoporous Carbons. Carbon 2006, 44, 468-474. 40. Mattia, D.; Rossi, M.; Kim, B.; Korneva, G.; Bau, H.; Gogotsi, Y. Effect of Graphitization on the Wettability and Electrical Conductivity of CVD-Carbon Nanotubes and Films. J. Phys. Chem. B 2006,


Page 21 of 21

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


110, 9850-9855. 41. Behrouznejad, F.; Shahbazi, S.; Taghavinia, N.; Wu, H.-P.; Wei-Guang Diau, E. A Study on Utilizing Different Metals as the Back Contact of CH3NH3PbI3 Perovskite Solar Cells. J. Mater. Chem. A 2016, 4, 13488-13498. 42. Snaith, H. J.; Abate, A.; Ball, J. M.; Eperon, G. E.; Leijtens, T.; Noel, N. K.; Stranks, S. D.; Wang, J. T.-W.; Wojciechowski, K.; Zhang, W. Anomalous Hysteresis in Perovskite Solar Cells. J. Phys. Chem. Lett. 2014, 5, 1511-1515. 43. Yang, Y.; Ri, K.; Mei, A.; Liu, L.; Hu, M.; Liu, T.; Li, X.; Han, H. The Size Effect of TiO2 Nanoparticles on a Printable Mesoscopic Perovskite Solar Cell. J. Mater. Chem. A 2015, 3, 9103-9107.

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