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Doping of the Metal Oxide Nanostructure and itsInfluence in Organic Electronics

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By Mi-Hyae Park, Juo-Hao Li, Ankit Kumar, Gang Li,* and Yang Yang*

Synthesizing metal oxides through the sol–gel process provides a convenient

way for forming a nanostructured layer in wide band gap semiconductors. In

this paper, a unique method of introducing dopants into the metal oxide

semiconductor is presented. The doped TiO2 is prepared by adding a Cs2CO3

solution to a nanocrystalline TiO2 solution that is synthesized via a non-

hydrolytic sol–gel process. The properties of the TiO2:Cs layer are investigated

and the results show stable nanostructure morphology. In addition to

providing morphological stability, Cs in TiO2 also gives rise to a more

desirable work function for charge transport in organic electronics. Polymer

solar cells based on the poly(3-hexylthiophene) (P3HT): methanofullerene

(PC70BM) system with the addition of a TiO2:Cs interfacial layer exhibit

excellent characteristics with a power conversion efficiency of up to 4.2%. The

improved device performance is attributed to an improved polymer/metal

contact, more efficient electron extraction, and better hole blocking

properties. The effectiveness of this unique functionality also extends to

polymer light emitting devices, where a lower driving voltage, improved

efficiency, and extended lifetime are demonstrated.

1. Introduction

Electronic devices based on organic materials (small molecules andpolymers), such as organic light emitting devices (OLEDs),[1]

organic photovoltaic cells (OPVs),[2] transistors,[3] bistable devices,and memory devices,[4] have attracted considerable attention. Themost salient attribute of polymer electronics is the potential to below-cost and versatile while having low-energy consumption andhigh-throughput processing.[5] For polymer solar cells, the polymer/fullerene based bulk-heterojunction (BHJ) system is the mostcommonly used device architecture[6–8] for which a certifiedefficiency of 5.4% for a single cell configuration was achieved.[9]

In the field of organic electronics, the metal/organic interfaceplays a critical role in influencing device performance. The

[*] Dr. G. LiSolarmer Energy, Inc.El Monte, CA 91731 (USA)E-mail: gangl@solarmer.com

Prof. Y. Yang, M.-H. Park, J.-H. Li, A. KumarDepartment of Materials Science and EngineeringUniversity of California Los AngelesLos Angeles, CA 90095 (USA)E-mail: yangy@ucla.edu

DOI: 10.1002/adfm.200801639

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interface can often be modified by aninsertion of a functional interfacial layer toimprove the device performance. Depend-ing on the characteristics of the material,the functional interfacial layer can beemployed in different configurations. Earlyprominent examples of functional inter-facial layers used in the development ofOLEDs and OPVs include: i) introductionof LiF, CsF, AlOx, etc. as an electron bufferlayer in OLEDs,[10–12] ii) application ofpolyaniline (PANI)[13] and poly(3,4-ethyle-nedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS) as a hole transport/bufferlayer,[14] iii) insertion of a TiOx layer as anoptical spacer/hole blocking layer,[15,16] andiv) combination of an n- and p-typetransport layer for tandem OLEDs (e.g.,LiF–V2O5).

[17] The role of cesium as aneffective interfacial material has beenproven. Recently, it has been shown thatthe use of salts, such as Cs2CO3 or CsF, as asource of Cs component for an n-type

interfacial layer, can improve solar cell efficiency.[18] In addition,Cs2CO3, which can be deposited either by thermal evaporation orsolution processing, can serve as an effective electron injection/buffer layer, leading to record high white and red PLEDsefficiencies with significantly reduced driving voltages andenhanced lifetimes.[19] Furthermore, combined with novelp-type interfacial layer materials, such as transition metal oxides(V2O5, MoO3, WO3, etc.), we have successfully demonstratedefficient inverted polymer solar cells.[20,21]

Semiconducting TiO2 has been extensively studied as apromising material in a variety of applications including dye-sensitized solar cells, photocatalysts, and organic photovol-taics.[22,23] Sol–gel chemistry is widely accepted as a valuableprocess used for preparing materials with well-controlledmorphological and structural properties. However, it is widelyaccepted that the application of nanocrystalline TiO2 through thetypical sol–gel method in organic photovoltaics remains limiteddue to the hydrothermal processing or calcinations required toinduce crystallization.[24] On the other hand, a non-hydrolyticsol–gel process can provide several important advantages:i) the elimination of additional agents allows for less particleagglomeration; ii) no exposure to air is needed; and iii) theelimination of water enables the formation of homogeneousfilms.[24] The use of nanocrystalline anatase phase TiO2 producedby a non-hydrolytic sol–gel process eliminates the need for a hightemperature sintering process (400–500 8C), which would

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otherwise inhibit the application of crystalline TiO2 in regularOPV structures. In addition, the introduction of dopants in sol–gel chemistry offers a convenient method for producingfunctional materials. It has been demonstrated that the propertiesof metal oxides can be enhanced and tuned through the additionof various dopants and processing methods.[25,26] The design andsynthesis of functional materials formed by the sol–gel methodvia doping are being widely investigated and possess a greatpotential for the development of nanoscale technology.

In this study, we present amethod for dopingmetal oxides via anon-hydrolitic sol–gel method and demonstrate an approach tomake efficient organic electronic devices. A functional interfaciallayer prepared by mixing solution processable semiconductingmetal oxides and salts is inserted into the polymer electronicdevice. It is found that introducing a nanoscale Cs doped TiO2

layer can enhance the solar cell performance. The effectiveness ofthis unique approach was also expanded to polymer LEDs, wherea lower driving voltage, improved efficiency and extended lifetimeare demonstrated once again. The properties of the TiO2:Cs layerwere investigated and a discussion of the source of improvementin device performance is presented.

2. Results and Discussion

2.1. Synthesis and Characterization

Synthesis of crystalline TiO2 nanoparticles follows a previouslypublished method.[27] A Cs doped TiO2 was obtained by mixingthe individual solutions of Cs2CO3 and TiO2 together. Transmis-sion electron microscopy (TEM) images of TiO2 and the TiO2:Csare shown in Figure 1a and b, respectively. An overview image of

Figure 1. a) TEM images of TiO2 and b) a Cs doped TiO2 (TiO2:Cs).

c) X-Ray powder diffraction patterns for TiO2 (bottom), and TiO2:Cs

overview (middle) and zoomed in (top). d) XPS profiles of TiO2 (dot line)

and TiO2:Cs (solid line) samples for Ti peak.

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the TiO2 nanoparticles illustrates that the material is entirelycomposed of nanosized particles that are homogeneouslydistributed throughout the material. As we continue to blendwith Cs2CO3, the TEM images show that the product consists ofmarkedlymoremonodispersed shapes. A comparison of the TEMimages (not shown) of the TiO2 and TiO2:Cs (taken 1 week afterbeing exposed to air) shows that the mixture is stable and that theproduct has not agglomerated upon addition of Cs2CO3. Thismaybe explained by Cs2CO3 having a stabilizing effect on the solution,which prevents the three-dimensional titania network fromshrinking.

The crystalline phase evolution of these two samples wasmonitored with an X-ray powder diffractometer (XRD data shownin Fig. 1c). The X-ray powder diffraction pattern for TiO2,obtained by the sol–gel method, confirms the existence ofnanocrystalline TiO2 in the anatase phase, which agrees with theliterature.[24,27] All the peaks are ascribed to the anatase crystalstructure without any secondary reaction impurities. The indexedbroad peaks indicate the nanocrystalline nature of TiO2 with sizesbetween 7 and 8 nm. The XRD spectrum of TiO2:Cs, along withan enlarged spectrum, are shown. When Cs2CO3 is added toTiO2, the peak patterns for both the anatase phase of TiO2 as wellas the CsCl cubic structure can be assigned to the XRD spectrum.The CsCl can be formed through the reactions of the residualbenzyl chloride, which is a by-product from the stock TiO2

solution with cesium in Cs2CO3. The narrow peak width of CsClshows highly ordered crystalline characteristics of CsCl ascompared to TiO2. On the other hand, the existence ofnanocrystalline anatase TiO2 in the TiO2:Cs sample is evidentfrom the similar peak width and intensity from the enlarged XRDdata.

X-Ray photoemission spectroscopy (XPS) was performed tofurther investigate the surface characteristics of TiO2 and theTiO2:Cs interfacial layer. The data is shown in Figure 1d. Thesamples were prepared by spin casting the films on an Ag-coatedSi wafer, where the instrument was calibrated using an internalAg standard. The atomic ratio of oxygen to titanium wasestimated to be 1.99 based on the integrated area under theelement peak and the sensitivity factor, with commerciallyavailable crystalline TiO2 powder used as a reference (Sigma–Aldrich, used as received). The data imply that titanium dioxideprepared from the non-hydrolytic sol–gel method is chemicallystoichiometric, which is also in good agreement with previouslyreported Ti 2p3/2 peak position for TiO2.

[28] We observe that the Ti2p3/2 spectra for TiO2:Cs shifts toward a lower binding energy by0.78 eV in comparison to the value for TiO2. We suspect that thischange is attributed to the creation of partially reduced Ti ions,which is consistent with previous reports that the Ti (2p) peaksshift considerably to a lower binding energy upon Cs or Kadsorption.[29,30] The Cs 3d5/2 peak position of TiO2:Cs shiftedtoward a lower binding energy, compared to that of Cs2CO3,which also supports the idea of charge transfer between TiO2 andCs. Metal ions in an organic/inorganic matrix can act as a dopingcomponent. This discrepancy in the XPS survey spectra may beexplained by the possible formation of Cs-doped TiO2 materials.The energy levels of both TiO2 and TiO2:Cs samples weredetermined through electrochemical cyclic voltammetry (C-V)and the energy offset wavelength from the UV–Vis absorptionspectra. The energy level diagram is shown in Figure 2.

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Figure 2. Corresponding energy level diagram of a device based on a

TiO2:Cs interfacial layer.

Figure 3. a) J–V characteristics of a P3HT:PC70BM based photovoltaic cell

with an evaporated Al cathode and different interfacial layers (none;

Cs2CO3; TiO2; TiO2:Cs), and b) external quantum efficiencies (EQE) of

the device with and without the TiO2:Cs interfacial layer.

Table 1. Summarized photovoltaic performance characterisitcs of corre-sponding regular configuration devices with different interfacial layers.

Device Voc [V] Jsc [mA/cm2] PCE [%] FF [%]

None 0.42 9.64 2.0 48

Cs2CO3 0.36 5.34 0.7 37

TiO2 0.46 10.48 2.4 50

TiO2:Cs 0.58 10.76 4.2 67

2.2. Photovoltaic Device Performance

The photovoltaic devices were fabricated using a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C71-butyric acid methylester (PC70BM) with Al as the cathode. Cs2CO3, TiO2, and theTiO2:Cs were inserted individually as an interfacial layer betweenthe active layer and the cathode. The current density–voltage (J–V)characteristics under AM 1.5G one-sun illumination condition isshown in Figure 3a. Table 1 summarizes the characteristics of thedevice performance. A comparison of devices with an Al electrodeto those with a Cs2CO3/Al electrode shows a decreasing opencircuit voltage (Voc) and short circuit current density ( Jsc) uponinsertion of the Cs2CO3 spin-casted film. This implies that aCs2CO3-only interfacial layer does not provide the appropriatefunction in terms of charge extraction and charge transport to theelectrode. The insertion of a TiO2 layer between the active layerand the evaporated Al cathode layer leads to an increase in Voc upto 0.46V. It is known that the open circuit voltage is generallydetermined by the difference between the highest occupiedmolecular orbital (HOMO) of the donor and the lowestunoccupied molecular orbital (LUMO) of the acceptor in thecase of an Ohmic contact between the active layer and thecathode.[31] Thus, the increase in Voc may arise from the workfunction of TiO2. The conduction band level of TiO2 is 4.3 eV, asdetermined from the C-V experiments, which is slightly higherthan the work function of 4.2 eVof the Al electrode. This results inunfavorable electron charge extraction from the active layer to theelectrode and an S-shape J–V curve is observed. On the otherhand, we clearly see an improvement inVoc, Jsc, and fill factor (FF)for the devices fabricated with a functional TiO2:Cs layer,resulting in efficient device performances. The Voc increasesfrom 0.42 V (for the device with no interlayer) to 0.58Vand the FFimproves dramatically up to 67%. This yields the average powerconversion efficiency (PCE) of 4.0% and the highest PCE

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achieved is 4.2%, which is comparable to a device with a Ca/Alelectrode. As a result, TiO2:Cs is a promising candidate forreplacing Ca, as it has been shown that inorganic oxides are quitestable to oxygen and moisture.[32] One possible reason forthe increased performance of the devices with TiO2:Cs is theformation of a better Ohmic contact that is created by thedecreased conduction band level of the TiO2:Cs layer (3.93 eV)such that the interfacial layer facilitates electron transport fromthe active layer to the cathode. Under dark conditions, therectification ratio is on the order of �106, the serial resistance is

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Figure 4. a) CELIV extraction peaks and b) conductance data for the ITO/

PEDOT/P3HT:PC70BM/interfacial layer/Al device with Cs2CO3; TiO2;

TiO2:Cs interfacial layers.

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considerably decreased to 1–2V � cm2, while the shunt resistanceremains as high as �107V � cm2, making it ideal for photo-voltaics. It is believed that the TiO2:Cs layer can keep the hot Alelectrode from diffusing into the active layer during evaporationand can offer good contact morphology between the active layerand the electrode. This is also supported by the dark currentcharacteristics of the device with no interlayer having a similarshunt resistance as that of the TiO2:Cs layer but a higher serialresistance of several tens of V � cm2. In addition, the highlynegative valence band level of the interfacial layer serves as anefficient hole blocking layer, which is confirmed by the smallleakage current for the TiO2:Cs based device. The externalquantum efficiency (EQE) of both the reference device using nointerlayer and the device with a TiO2:Cs interfacial layer is shownin Figure 3b; the EQE is consistent with J–V characteristics. Wenote that the device utilizing only a CsCl interfacial layer does notdisplay any of the improved device characteristics, including highVoc, high FF, and small serial resistance. This indicates thatalthough the CsCl seems to be a major component in the XRDdata, the CsCl layer does not play a direct role in improving theefficiency. Instead, the nanocrystalline anatase phases derivedfrom TiO2, such as doped TiO2, is a possible contributor to theenhancement in efficiency. The related improvements in Voc andFF were observed with a TiO2:CsF interlayer, where CsF acts asanother source of Cs component, and also with several otherpolymer systems. However, further investigation is required toclarify the mechanism.

We used the charge extraction by linearly increasing voltage(CELIV) method to investigate the charge carrier transportcharacteristics of the TiO2:Cs layer for some of the representativeregular configuration devices. In CELIV, the initial rise speedprovides information on the bulk conductivity of the sample andthe time of extraction current maximum, tmax, is used forestimating the drift mobility of equilibrium charge carriers.[33]

Under a ramping speed of 105 V cm�1, CELIV extractionpeaks were obtained as shown in Figure 4a. The change in tmax

is negligible, indicating the fairly consistent mobility valuesobtained from the different devices, all of which have mobilitieson the order of 10�4 cm2 V�1 � s�1. Impedance spectroscopy wasused to measure the bulk conductivity of the samples. All of ourdevices fitted well to the Rp–Cp (resistor–capacitor in parallel)model wherein the conductance (G¼ 1/Rp) should be indepen-dent of the frequency and the susceptance ½B ¼ jð2pfCpÞ� shouldvary linearly with frequency. The conductance data derived withthis method is shown in Figure 4b. The conductance for thedevice with a TiO2:Cs layer was at least three orders of magnitudehigher than the corresponding Cs2CO3/Al or TiO2/Al devices.Since the number of charges extracted is directly proportional tothe ratio of the conductivity divided by the mobility, we concludedthat the devices with TiO2:Cs improve charge extraction from thepolymer active layer. Additional support for this argument can beobserved from the CELIV data. The area under the currentdensity–time curve is the sum of the capacitive charges and theequilibrium charges extracted from the device. Subtracting thecapacitive charges [initial current rise j(0)], we see that the area forthe TiO2:Cs/Al device is larger than that for the TiO2/Al device.Hence, more equilibrium charge carriers are extracted under noillumination for the TiO2:Cs/Al devices than for the TiO2/Aldevices.

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An inverted structure was investigated for polymer solar cellsusing Cs2CO3 to modify the ITO electrode as a cathode and usinga transition metal oxide V2O5 as a hole buffer layer. We havereported inverted solar cells with an efficiency of 2.25% due tonon-optimized interfacial layer and active layer processes.[20] Athick buffer layer on top of the active materials can be applied ininverted cells, so that the structure is more robust to transparentelectrode deposition, e.g., ITO sputtering. A lamination fabrica-tion process of semitransparent and flexible solar cells based onthe same interface modification approach was recently shown.[34]

Here, we apply a TiO2:Cs to replace the Cs2CO3 layer anddemonstrated fabrication of highly efficient inverted polymersolar cell based on the P3HTand PCBM system. The structure ofthe inverted device is as follows: ITO/TiO2:Cs/P3HT:PC70BM/V2O5/Al. The dark and photo (AM1.5G, 100mA cm�2) J–V curvesof the device with a TiO2:Cs interfacial layer are shown inFigure 5. For the device with a TiO2:Cs layer, optimization of thedevice fabrication process again leads to improvements inVoc andFF. Subsequently, this results in a device efficiency of 3.9%, withthe Voc, Jsc, and FF being 0.60V, 11.5mA cm�2, and 57%,respectively. The high rectification ratio is also attributed to theimproved injection current under forward bias as shown fromthe dark current.

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Figure 5. J–V curve of an inverted solar cell with TiO2:Cs in the dark and

under illumination of AM 1.5.

2.3. Polymer Light Emitting Devices (PLEDs) Performance

In an effort to explore the effectiveness of the TiO2:Cs layer,green-polyfluorene based PLEDs were constructed with thestructure ITO/PEDOT:PSS(40 nm)/light emitting polymer (LEP)(80 nm)/interfacial layer/Al, where the interfacial layer is i) TiO2,ii) Cs2CO3, and iii) TiO2:Cs, all in 2-ethoxyethanol. To excludethe solvent effects on the device performance, the solvent itselfwas spun-cast between the LEP and Al to make a referencediode. Figure 6a shows a comparison of the current-density–

Figure 6. a) Current-density–voltage–brightness characteristics and

b) current efficiency for the ITO/PEDOT/LEP/EIL/Al device with different

interfacial layers (none; Cs2CO3; TiO2; TiO2:Cs).

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voltage–brightness (J–V–L) characteristics of the devices withdifferent interfacial layers and its effect on device performance.The Cs2CO3 interfacial layer has been shown to be an effectiveelectron-injection layer, which leads to white and red emissionPLEDs reaching record highs in power efficiencies.[35,36] Thesignificant improvements in device performances have beenattributed to the formation of a low work-function complex andsurface dipole, which can facilitate electron injection from thecathode.[19] Surprisingly, with the use of the TiO2:Cs interfaciallayer, further improvements in both the current density andbrightness were observed in comparison to devices with only aCs2CO3 or TiO2 interfacial layer. As shown in Figure 6b, thedevice with a TiO2:Cs interfacial layer has a current efficiency of11.5 cd A�1 or power efficiency of 14 lm w�1 at a bias of 2.8 V. Theturn-on voltage (around 2.3 V) does not change, which impliesthat the PLEDs with the TiO2:Cs interfacial layer may not furtherlower the electron injection barrier as compared to the referencedevices. However, the increase in current density and brightnesssuggests that the better charge balance should be responsible forthe efficiency enhancement. As discussed above, a nearly Ohmiccontact is observed with the interfacial layer of TiO2:Cs, supportedby the conductivity and energy level alignment between theorganic materials and the metal cathode. Moreover, the valenceband level (7.6 eV) of the interface layer is lower than the HOMO(5.4 eV) level of the organic active layer, providing a hole-blockingeffect in our device structure. By combining the Ohmic contactand the hole-blocking effects, a better charge balance andenhanced device performance can be achieved. Therefore,compared to PLEDs containing only a Cs2CO3 or TiO2 interfaciallayer, the TiO2:Cs layer exhibits the advantageous characteristicsof both the lower work function and the hole-blocking effect fromCs2CO3 and TiO2, respectively.

3. Conclusions

In summary, we have demonstrated a novel approach forfabricating efficient organic electronic devices by introducingdopants into solution processable metal oxides as an interfaciallayer. The nanocrystalline TiO2 was synthesized using a non-hydrolytic sol–gel approach and was mixed with a Cs2CO3

solution. Polymer solar cells based on the P3HT:PC70BM systemwith a TiO2:Cs interfacial layer reached a PCE of 4.2% in regularconfigurations. Significant improvements in PLED performanceshave also been obtained. We anticipate that this study willstimulate further research on metal oxides and salts as materialsfor combined functional layers to achieve efficient chargetransport properties.

4. Experimental

Material: All chemicals were purchased from Sigma–Aldrich and usedas received. TiO2 was synthesized from a non-hydrolytic sol–gel approachdescribed as follows: After stirring a solution of TiCl4, ethanol, and benzylalcohol for 9 h at 80 8C, it was washed with diethyl ether. The white TiO2

precipitate was obtained by centrifuging the crude product. The final TiO2

solution was prepared by dispersing it in ethanol. A solution of TiO2:Cs wasobtained by blending 0.2wt % of Cs2CO3 in 2-ethoxyethanol solution withthe TiO2 solution (0.2wt %) at a 1:1 volume ratio.

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Characterization: The nanotextures of TiO2 and TiO2:Cs were char-acterized by TEM (JEOL JEM-2000FX). XRD analysis was performed with aPANalytical X’Pert Pro Powder Diffractometer on finely powdered samplesusing CuKa radiation. The powder samples containing TiO2 and TiO2:Csfor XRD analysis were prepared by evaporating off the solvent at 110 8C inan oven. The XPS experiment was performed in an Omicron Nanotechnol-ogy system with a base pressure of 2� 10�10 Torr. A MgKa radiationsource was used for the XPS measurements. For CELIV analysis, the datawere taken with a Tektronix TDS-430A digital oscilloscope and a WavetekDatron 195 waveform generator. The impedance measurements werecarried out using a HP4284A Precision LCR meter.

Device Fabrication: The polymer blend of P3HT:PC70BM at a 1:1 weightratio was spin-casted at 800 rpm on top of a layer of PEDOT:PSS depositedon ITO-coated glass. This was followed by thermal annealing at 110 8C. Theinterfacial layer was spin-casted from each solution and that film wasannealed at 80 8C. The device fabrication was completed by thermalevaporation of 100 nm of Al as the cathode. For an inverted configurationdevice, the TiO2:Cs layer was spin-casted on ITO-coated glass and thermalannealing was performed at 150 8C for 30min. After spin-casting a polymerblend solution of P3HT:PC70BM, another thermal annealing step wasperformed at 110 8C for 10min. The device fabrication was completed bythermal evaporation of 5 nm of V2O5 and 80 nm of Al as the anode.

For OLED device fabrication, 1% green polyfluorene in p-xylene (as aLEP) was spin-casted on a layer of PEDOT:PSS deposited on ITO-coatedglass. The cathode was formed by spin coating from each interfacial layersolution, followed by thermal deposition of Al.

Acknowledgements

This work was financially supported by Solarmer Energy, Inc. (grant no.20061880) and UC-Discovery Grant (no. GCP05-10208). The authors thankMr. Hyun Cheol Lee for recording TEM images.

Received: November 7, 2008

Published online: February 25, 2009

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