Synergetic effects of solution-processable fluorinated graphene and PEDOT as a hole-transporting layer for highly efficient and stable normal-structure perovskite solar cells

Jae-Hun Yu a, Cheol-Ho Lee b, Han-Ik Joh c, Jun-Seok Yeo *d and Seok-In Na *a
aProfessional Graduate School of Flexible and Printable Electronics and Polymer Materials Fusion Research Center, Chonbuk National University, Jeonju-si, Jeollabuk-do 561-756, Republic of Korea. E-mail: nsi12@jbnu.ac.kr
bCarbon Convergence Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology, Jeollabuk-do 565-905, Republic of Korea
cDepartment of Energy Engineering, Konkuk University, Seoul, 143-701, Republic of Korea
dSchool of Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju, 500-712, Republic of Korea. E-mail: jsyeo8610@gmail.com

Received 4th June 2017 , Accepted 22nd July 2017

First published on 25th July 2017


We demonstrate that a bi-interlayer consisting of water-free poly(3,4-ethylenedioxythiophene) (PEDOT) and fluorinated reduced graphene oxide (FrGO) noticeably enhances the efficiency and the stability of the normal-structure perovskite solar cells (PeSCs). With simple and low temperature solution-processing, the PeSC employing the PEDOT + FrGO interlayer exhibits a significantly improved power conversion efficiency (PCE) of 14.9%. Comprehensive investigations indicate that the enhanced PCE is mostly attributed to the retarded recombination in the devices. The minimized recombination phenomena are related to the interfacial dipoles at the PEDOT/FrGO interface, which facilitates the electron-blocking and the higher built-in potential in the devices. Furthermore, the PEDOT + FrGO device shows a better stability by maintaining 70% of the initial PCE over the 30 days exposure to ambient conditions. This is because the more hydrophobic graphitic sheets of the FrGO on the PEDOT further protect the perovskite films from oxygen/water penetration. Consequently, the introduction of composite interfacial layers including graphene derivatives can be an effective and versatile strategy for high-performing, stable, and cost-effective PeSCs.


Introduction

Compared to the earliest attempts at developing perovskite solar cells (PeSCs), dramatic progress on their performances has been made,1–3 and nowadays, the power conversion efficiencies (PCEs) of the PeSCs reached the certified value of 22.1%.4 A major breakthrough in the performance of the early version of the PeSCs, which were based on the dye-sensitized solar cells, was the replacement of liquid-type hole-transport layers (HTLs) with solid HTLs. In 2012, Park et al. first employed a solid HTL of 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9′-spirobifluorene (spiro-OMeTAD) in the PeSCs, realizing a PCE of 9.7% with a reasonable device-operation time.5 This result demonstrated that the choice of adequate HTLs in the PeSCs is as important as the optimization of the perovskite films for the progress of the PeSCs.6,7 Although the breakthrough with the spiro-OMeTAD has triggered high-level research studies on the PeSCs, the spiro-OMeTAD suffered from several limitations that made its application unfeasible, from an industrial point of view: first, its synthesis is too expensive, due to the required multi-step synthetic procedures and complex purification. Second, additional dopants are necessary due to the low conductivity of the pristine spiro-OMeTAD, which degrades the reproducibility of the PeSC performance and leads to a short standing-time of the spiro-OMeTAD solution.8–11 Thus, further research on alternatives is required to develop the HTL materials with high performance, low-cost, simple and reproducible solution processing without doping.

To address this issue, various p-type materials including CuSCN,8 CuI,9 and conjugated organic semiconductors11,12 have been employed and investigated. Among them, the poly(3,4-ethylenedioxythiophene) (PEDOT) has been considered as a promising alternative HTL due to its cost-effective, highly conductive, and simply solution-processable properties. However, the water-based dispersion with poly(styrene sulfonate) (PSS) templates, which could dissolve the underlying perovskite films,13 has restricted the application of the PEDOT into the normal structure of the PeSCs. From this point of view, Snaith et al. and Brabec et al. recently introduced a water-free PEDOT, which is dispersed in non-polar toluene, into the normal PeSCs, and demonstrated the improved device-stability by a low temperature and facile solution-processing.14,15 Although these results proved the potential of the PEDOT HTL in the normal PeSCs in terms of processing and device-stability aspects, the relatively low performance of the PEDOT HTL remains an issue.

Recently, for the better interfacial engineering, it has been reported that two other materials could exhibit synergetic effects on the charge-transporting ability when simultaneously used as composite interfacial layers.16–20 For example, Yang et al. showed that the introduction of self-assembled monolayers on conventional metal oxide interlayers could reduce trap sites and facilitate the charge transfer between interlayers and perovskites.16 Also, Yip et al. developed tailored conjugated copolymers to compensate for PEDOT:PSS properties as a HTL, and the use of the resulting copolymers/PEDOT:PSS bi-layer HTL realized enhanced PCEs of the PeSCs, mainly caused by reduced interfacial trap-assisted recombination around the PEDOT:PSS interface.20 Very recent reports have shown the effectiveness of composite interfacial layers based on graphene derivatives.17,18 The graphene derivatives, represented by graphene oxides (GOs) and reduced graphene oxides (rGOs), show excellent features as promising HTLs such as solution-processability, high conductivity, non-diffusivity, and chemical inertness.21,22 Benefiting from these properties, Rizzo et al. and Kim et al. improved the device-performance and the device-stability by using graphene derivative composite HTLs to overcome the drawbacks of the single PEDOT:PSS HTL.17,18 However, unlike various outstanding research studies improving the conventional interlayers with the composite interlayer concepts, these strategies have not been applied to the water-free PEDOT in the normal-structure PeSCs despite its various advantages as mentioned above. Furthermore, to date, no attempt has been made to explore the effects of graphene derivative HTLs, especially GO and rGO, in the normal-structure PeSCs.

Herein, we report the effect of the water-free PEDOT/rGO double HTL on the efficiency and stability of the normal-structure PeSCs. We found that the HTL performance of PEDOT could be maximized with the aid of recently developed fluorinated rGO (FrGO), which possesses a high work-function (WF), hydrophobicity, and a chemically stable structure.18 In addition, the orthogonal solubility between toluene-dispersed PEDOT (Clevios HTL Solar 3) and 2-propanol-dispersed FrGO allows a simple solution-processing without high-temperature post-treatments. Consequently, with facile fabrication processing, the PeSCs based on the PEDOT + FrGO HTL showed a significantly higher PCE of 14.9%, compared to a PCE of only 10.3% for the control device based on the single PEDOT HTL. More importantly, the device-stability of the PeSCs with the PEDOT + FrGO was further enhanced as a result of the capping of the PEDOT surface with more hydrophobic FrGOs. To reveal detailed origins of the enhanced device-efficiency and -stability, we investigated device quantum efficiencies, device impedance, surface chemistry, surface potentials, and nanoscale morphologies of HTLs.

Results and discussion

Fig. 1(a) shows the device-configuration, and the chemical structure of the FrGO used in this study. The normal PeSCs were fabricated with the configuration of indium tin oxide (ITO)/ZnO/C60/CH3NH3PbI3 (MAPbI3)/HTL/MoO3/Ag. In our previous reports, the use of the FrGO HTL in optoelectronic devices enhanced both the device-performance and the device-stability, assisted by its high conductivity, WF, and hydrophobicity.22,23 Furthermore, the high dispersibility of the FrGO in various organic solvents guarantees a diverse processing window for solution-based layer-stacking. Thus, by taking these fascinating features of the FrGO, it is expected that the HTL capability of the PEDOT in the normal PeSCs can be improved by simple methods. Due to the orthogonal solubility of the PEDOT in 2-propanol, which is a solvent for the FrGO, the FrGO layer could be sequentially deposited onto the PEDOT film via facile spin-coating without any post-treatments. To optimize the performance of the bi-layer HTLs, the number of spin-coating cycles for the FrGO was controlled from 1 cycle to 5 cycles.
image file: c7nr03963h-f1.tif
Fig. 1 (a) Schematic PeSC structure and chemical structure of the FrGO. (b) Representative JV curves of the PeSCs with different HTLs, and (c) the corresponding statistical data of PCEs.

As shown in Fig. 1(b), the current–density and voltage (JV) curves of respective PeSCs with pristine PEDOT, PEDOT + FrGO1, PEDOT + FrGO3, and PEDOT + FrGO5 HTLs were compared. The corresponding photovoltaic parameters and their statistical data, which were obtained from 50 different cells, are provided in Table 1 and Fig. 1(c), respectively. The device with only PEDOT as the HTL showed a moderate PCE of 10.3% with an open-circuit voltage (VOC) of 0.94 V, a current–density (JSC) of 17.5 mA cm−2, and a fill factor (FF) of 62.7%. Encouragingly, the insertion of the FrGO even with 1 spin-coating cycle (PEDOT + FrGO1) apparently increased all photovoltaic parameters, thus leading to a better device-efficiency as follows: an VOC of 0.97 V, a JSC of 17.6 mA cm−2, a FF of 73.2%, and a PCE of 12.5%. The optimal number of the spin-coating cycles for the FrGO layers was 3, and the PeSC employing the PEDOT + FrGO3 HTL exhibited the highest PCE of 14.9% with an VOC of 1.04 V, a JSC of 18.5 mA cm−2, and a FF of 77.1%. A further increase in the cycles of the FrGOs began to lower the PCE of the PeSC with the PEDOT + FrGO5: an VOC of 1.01 V, a JSC of 18.2 mA cm−2, a FF of 70.7%, and a PCE of 13.0%. The above observations allow us to conclude that the introduction of the bi-layer HTL consisting of PEDOT and FrGO is an effective strategy to raise the efficiency of the normal-structure PeSCs without compromising processing advantages.

Table 1 Representative photovoltaic characteristics of the PeSCs with different HTLs
HTL V OC (V) J SC (mA cm−2) FF (%) PCE (%) R S (Ω cm2) R Sh (kΩ cm2)
PEDOT:PSS 0.94 17.5 62.7 10.3 1.42 609
PEDOT:PSS + FrGO1 0.97 17.6 73.2 12.5 1.45 757
PEDOT:PSS + FrGO3 1.04 18.5 77.1 14.9 1.48 1119
PEDOT:PSS + FrGO5 1.01 18.2 70.7 13.0 1.52 819


To gain insight into the enhanced performance of the PeSC based on the PEDOT + FrGO HTL, we performed device-analyses by investigating quantum efficiencies, impedance spectra, and device-resistances of the optimal PEDOT + FrGO3 devices in comparison with the reference PEDOT devices. As shown in Fig. 2(a), the PEDOT + FrGO3 device exhibited a broad photo-response from 300 nm to 800 nm wavelength and a high external quantum efficiency (EQE) reaching 80%. More importantly, the higher EQE of the PEDOT + FrGO3 device mainly originated from the higher internal quantum efficiency (IQE) than that of the PEDOT device as shown in Fig. 2(b). Considering the identical conditions between the PEDOT + FrGO3 and the PEDOT devices except for the HTL types, it is reasonable to deduce that the superior charge-transfer ability of the PEDOT + FrGO3 HTL was attributed to the enhanced quantum efficiency of the PEDOT + FrGO3 device, thereby leading to its higher JSC.22,24 In order to further explore the charge-transfer abilities of the respective HTLs, we compared impedance spectra between the two types of PeSCs. Impedance spectroscopy, which has been recently introduced to evaluate interfacial engineering in the perovskite devices, enables us to separate contact resistance (RC) in the higher frequency region and recombination resistance (RRec) in the lower frequency region.25,26Fig. 2(c) shows the Nyquist plots of the different PeSCs under illumination with an applied voltage of 0.90 V. By replacing the PEDOT HTL with the PEDOT + FrGO3 HTL, the RRec values were significantly increased from 143 Ω to 237 Ω. On the other hand, the RC for the PEDOT + FrGO3 device slightly increased compared to that of the PEDOT device. These observations were consistent with the device-resistances, series resistance (RS) and shunt resistance (RSh), of the respective devices. The RS and RSh were calculated from the JV curves shown in Fig. 1(b), and are listed in Table 1. It was observed that the PeSC with the PEDOT + FrGO3 exhibited a significantly improved value of the RSh (1119 Ω cm2) compared to that of the PeSC with the pristine PEDOT (609 Ω cm2). Also, the changes in the RS for both devices were not noticeable. The above device-analyses indicate that the improvement in the PEDOT + FrGO performance could be due to the retarded carrier-recombination in the PeSC rather than the reduced contact resistances. Also, it is responsible for the highly improved VOC and FF of the PEDOT + FrGO devices.


image file: c7nr03963h-f2.tif
Fig. 2 (a) External and (b) internal quantum efficiencies (EQE and IQE) of the PeSCs with PEDOT and PEDOT + FrGO3. (c) Impedance spectra of the corresponding PeSCs under the white light at 0.90 V bias.

To correlate the enhanced HTL performances and the characteristics of the PEDOT + FrGO, we investigated the surface chemistry, surface potentials, and surface morphologies of PEDOT, PEDOT + FrGO1, PEDOT + FrGO3, and PEDOT + FrGO5 layers on ITO substrates. First, the surface chemical compositions of the respective films were studied via X-ray photoelectron spectroscopy (XPS), and Fig. 3(a) shows the corresponding XPS survey profiles. In contrast to the general PEDOT:PSS, which exhibits the intense S 2p peaks, the water-free PEDOT showed negligible S 2p signals, as a result of the absence of the PSS templates.13,15 Meanwhile, with an increase in the spin-coating cycles of the FrGO, the atomic ratios of fluorine to carbon ([F]/[C]) were gradually raised: 0.07 for PEDOT + FrGO1, 0.12 for PEDOT + FrGO3, and 0.15 for PEDOT + FrGO5. The enriched fluorine content on the surfaces of the PEDOT + FrGO films stems from the trifluoromethylphenyl groups on the FrGO,22 and as shown in Fig. S2, the increase in the spin-cycles of the FrGO did not deteriorate the underlying PEDOT film as a result of the low solubility of the PEDOT to the FrGO solvent, i.e. 2-propanol. These results imply that the coverage of the FrGO can be readily modulated by the spin-cycles of the FrGO. The resulting fluorinated surfaces of the films resulted in more hydrophobic and higher WFs of the HTLs. As evidenced in Fig. 3(b), the incorporation of the FrGOs exhibited improved contact angles of water-droplets of 96.1° for the PEDOT + FrGO1, 104.1° for PEDOT + FrGO3, and 108.4° for PEDOT + FrGO5, respectively, whereas the contact angle of the pristine PEDOT was 95.2°. In addition, the WFs of the PEDOT + FrGO films, which were obtained from ultraviolet photoelectron spectroscopy (UPS), were enhanced by ca. 0.2 eV compared to the pristine PEDOT film. According to the literature, fluorine moieties in the interfacial layers play a role in blocking electrons and in recombination, which is due to their high ionization potential and inert feature.22,27–29 Consequently, we can infer that the severe recombination in the single PEDOT-based device could be effectively alleviated by the incorporation of the fluorinated graphene derivative, i.e., the FrGO.


image file: c7nr03963h-f3.tif
Fig. 3 (a) Survey profiles of X-ray photoelectron spectroscopy (XPS), (b) contact angles of water droplets, and (c) ultraviolet photoelectron spectroscopy (UPS) spectra for different HTLs.

For a more detailed study of the FrGO effects on the bi-layer HTL, the surface morphologies and the corresponding surface potentials of the different HTLs were investigated by atomic force microscopy (AFM) and scanning kelvin probe microscopy (SKPM). As can be seen in Fig. 4(a), the surface coverage of the FrGOs on the PEDOT layer increased with the spin-cycles of the FrGO, which was accompanied by an increase in the surface roughness of the films: 2.73, 5.21, 6.47, and 6.67 nm for the pristine PEDOT, PEDOT + FrGO1, PEDOT + FrGO3, and PEDOT + FrGO5, respectively. More importantly, the presence of the FrGO sheets on the PEDOT changed the surface potentials of the respective films as shown in Fig. 4(b) and (c). The surface potential adjacent to the FrGO area is more negative than that of the pristine PEDOT area, which is well-correlated with the WF changes of the films as shown in Fig. 3(c). Also, it is worth mentioning that the differences in the surface potentials decreased with the increase in the coverage of the FrGOs, indicating that the more uniform surface potential could be achieved by increasing the spin-cycles of the FrGOs. These different surface potentials between the PEDOT and the FrGO induce the interfacial dipoles at the PEDOT/FrGO interface pointing outward from the FrGO as provided in Fig. S4.30,31 The interfacial dipoles at the PEDOT/FrGO could prevent undesirable recombination by providing sufficient electron-blocking ability, which is consistent with the analytical results of the device-resistance as shown in Fig. 2. Also, the interfacial dipoles at the PEDOT/FrGO could reinforce the built-in potential across the device because of the identical direction between the interfacial dipoles and the built-in potential.30,32,33 Thus, the enhanced VOC and FF observed in Fig. 1, which are highly related to the reduced recombination phenomena, could be attributed to the interfacial dipoles at the PEDOT/FrGO interface with a favorable direction for the electron-blocking and the built-in potential.


image file: c7nr03963h-f4.tif
Fig. 4 Images of (a) surface morphologies and (b) surface potentials for different HTLs with (c) cross-sectional line profiles of surface potential images.

Finally, to further verify the feasibility of the PEDOT + FrGO HTL for the practical purpose, we examined the operational stability of the PEDOT + FrGO. Because perovskite films are prone to be degraded by moisture and oxygen, the top-lying HTL on the perovskite film in the normal-structure PeSCs can act as a protection layer against moisture and oxygen penetration. Fig. 5 shows changes in the PCEs by comparing the best performing PEDOT + FrGO3 and the reference PEDOT devices as a function of exposure time, which were measured under ambient conditions. As expected, the device based on the bi-layer HTL with the FrGO showed an improved device-stability compared to that of the PEDOT only device. The PCE of the PEDOT-based cell was gradually deteriorated and exhibited very poor photovoltaic characteristics with 36% of the initial PCE after 30 days of exposure. In contrast, the PEDOT + FrGO3 device showed a greatly stable device-performance retaining 70% of the initial PCE over the same period of air exposure. The enhanced operational stability of the PeSC with the PEDOT + FrGO could arise from its significant hydrophobicity as confirmed by the contact angle measurement and the graphitic 2-dimensional structures of the FrGO.22,34 Thus, the incorporation of additional FrGO layers not only improved the interfacial characteristics of the PEDOT, but also improved the protection degree of the HTL against oxygen/water ingress into the perovskite films.


image file: c7nr03963h-f5.tif
Fig. 5 Changes in PCEs of the PeSCs as a function of exposure time to ambient atmosphere without encapsulation.

Experimental section

Preparation of FrGO

Graphene oxides (GOs) were prepared via the previously reported method.23 For the reduction process, the resulting GOs were dispersed in 100 ml of deionized (DI) water by using an ultrasonic bath with a concentration of 4 mg ml−1. Then 2 ml of reductant (4-(trifluoromethyl)phenyl hydrazine, Sigma-Aldrich) was added into the GO colloidal solution, and stirred for 6 h. Subsequently, the colloidal solution was filtered by using a vacuum filter, and then, washed with alcohol. The filtered materials were dried in an oven at 60 °C. Finally, the FrGO was dispersed in 2-propanol at a concentration of 1 mg ml−1 for device-fabrication.

PeSC fabrication and characterization

The configuration of the PeSCs studied in this report was ITO/ZnO/C60/MAPbI3/PEDOT/FrGO/MoO3/Ag. The pre-patterned ITOs (10 Ohm sq−1, Samsung Corning) on glass-substrates were sequentially cleaned with acetone, deionized water, and 2-propanol in an ultrasonic bath for 20 min, respectively, and dried in an oven at 80 °C for 10 min. Then, the substrates were exposed in the UV-ozone for 30 min. For the ZnO layer, zinc oxide nanopowder (ZnO nanopowder, <100 nm particle size, Sigma-Aldrich) dissolved in ammonium hydroxide solution (NH4OH, 50% v/v aq. soln, Alfa Aesar) at a concentration of 8 mg ml−1 was spin-coated on the ITO-coated glass at 5000 rpm for 40 s, and subsequently dried at 200 °C for 20 min in air. Then, C60 (fullerene, 99.5%, Nano-C) dissolved in 1,2-dichlorobenzene (15 mg ml−1) was spin-coated on the ITO/ZnO substrates at 2000 rpm for 40 s, and annealed at 100 °C for 10 min in an N2 glove box. Next, for the perovskite layer, PbI2 (lead(II) iodide, 99.9985%, Alfa Aesar) of 0.759 g and MAI (Dyesol) of 0.262 g were dissolved in DMF (anhydrous 99.8%, Sigma-Aldrich) solvent of 1.612 ml, and then, 120 ml of N-cyclohexyl-2-pyrrolidone (CHP, Sigma-Aldrich) was added into the solution. The prepared perovskite precursor solution was spin-coated on the ZnO/C60 at 4000 rpm for 40 s, followed by annealing at 100 °C for 4 min in an N2 glove box. For the HTLs, PEDOT (Clevios HTL Solar 3) and FrGO solutions were spin-coated on the active-layer at 5000 rpm for 40 s, and then dried at 100 °C for 5 min in an N2 glove box, respectively. Finally, the MoO3 (2.5 nm)/Ag (80 nm) electrodes were deposited via a thermal evaporator with a shadow mask to define an active-area of 4.64 mm2.

Device measurement and characterization

The photocurrent density–voltage (JV) characteristics were measured by Keithley 2400 source measurement under AM 1.5 G illumination (100 mW cm−2) using a solar simulator. Both external and internal quantum efficiency (EQE and IQE) of PeSCs were analyzed using a certified IPCE measurement system (IQE-200, Oriel Instruments). The integrated values of the JSC obtained from the IPCE data are in close agreement (within nearly 9%) with the values obtained from the JV curves. To analyze interface characteristics, the impedance measurements of PeSCs were performed under white light conditions by using a Solartron 1260 Impedance/gain-phase analyzer, with a frequency range of 100[thin space (1/6-em)]000 Hz to 0.1 Hz and an AC oscillating voltage of 0.90 V. A Scribner ZView 2 software was used as the fitting program of the spectra. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) (AXIS-NOVA, Kratos Inc.) were carried out with monochromatic Al-Kα (1486.6 eV) for XPS and He 1 ( = 21.2 eV) for UPS. The surface morphology and surface potential of the HTLs were detected by atomic force microscopy (AFM) and scanning Kelvin probe microscopy (SKPM) (Dimension 3100, Veeco), respectively. In order to compare optical transmittance according to spin-coating cycles of FrGO solution, UV-vis (LAMBDA 750 UV/Vis/NIR spectrophotometer, PerkinElmer Inc.) was used. The contact-angles of water-droplets on the HTLs were measured by Phoenix 300 (SEO Inc.). Scanning electron microscopy (SEM, Quanta 3D-FEG/FEI) was used to obtain the cross-sectional image of the PeSCs.

Conclusion

In conclusion, we demonstrated that the combination of the water-free PEDOT and the FrGO HTL noticeably elevated the efficiency and the stability of the normal-structure PeSCs that were obtained via simple and low temperature solution-processing. The PeSC employing the PEDOT + FrGO HTL exhibited a significantly improved PCE of 14.9%, which is a 45% enhancement over that of the PeSC with the single PEDOT HTL. Comprehensive investigations revealed that the enhancement in the PCE mostly originated from the retarded recombination in the devices. Also, mitigated recombination phenomena were related to the interfacial dipoles at the PEDOT/FrGO interface, which facilitated the electron-blocking and the higher built-in potential in the devices. In addition to the PCE enhancements, the PEDOT + FrGO device showed a better stability by retaining 70% of the initial PCE over the 30 days of air exposure. This is because the more hydrophobic graphitic sheets of the FrGO on the PEDOT act as additional protection layers against oxygen/water penetration into the perovskite layer. All these results confirm that the introduction of composite interfacial layers including graphene derivatives can be an effective and versatile strategy realizing high-performing, stable, and cost-effective PeSCs.

Acknowledgements

This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2016R1D1A1B04933887) and funded by Korea Electric Power Corporation (KEPCO) under the project entitled by “Development of Semi-Transparent Perovskite Solar Cells for BIPVs” (CX72170050).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nr03963h

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