device characteristics of organic light-emitting diodes based on electronic … · 2016-08-17 ·...

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Copyright copy 2009 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 9 7485ndash7490 2009

Device Characteristics of Organic Light-EmittingDiodes Based on Electronic Structure of

the Ba-Doped Alq3 Layer

Jong Tae Lim Kyung Nam Kim and Geun Young Yeomlowast

School of Advanced Materials Science and Engineering Sungkyunkwan University Suwon 440-746 Korea

Organic light-emitting diodes (OLEDs) with a Ba-doped tris(8-quinolinolato)aluminum(III) (Alq3 layerwere fabricated to reduce the barrier height for electron injection and to improve the electron con-ductivity In the OLED consisting of glassITO44prime4primeprime-tris[2-naphthylphenyl-1-phenylamino]triphenyl-amine (2-TNATA 30 nm)44prime-bis[N-(1-napthyl)-N-phenyl-amino]-biphenyl (NPB 18 nm)Alq3

(42 nm)Ba-doped Alq3 (20 nm x x = 0 10 25 and 50)Al (100 nm) the device with the Alq3

layer doped with 10 Ba showed the highest light out-coupling characteristic However as theBa dopant concentration was increased from 25 to 50 this device characteristic was largelyreduced The characteristics of these devices were interpreted on the basis of the chemical reactionbetween Ba and Alq3 and the electron injection property by analyzing the electronic structure of theBa-doped Alq3 layer At a low Ba doping of 10 mainly the Alq3 radical anion species was formedIn addition the barrier height for electron injection in this layer was decreased to 06 eV whencompared to the pristine Alq3 layer At a high Ba doping of 50 the Alq3 molecules were severelydecomposed When the Ba dopant concentration was changed the light-emitting characteristicsof the devices were well coincided with the formation mechanism of Alq3 radical anion and Alq3

decomposition species

Keywords Organic Light-Emitting Diode Electronic Structure X-Ray Photoemission Spec-troscopy Ultraviolet Photoemission Spectroscopy Near-Edge X-Ray Absorption FineStructure

1 INTRODUCTION

One of the critical issues for fabricating high-performanceorganic semiconductors such as organic light-emittingdiodes (OLEDs) organic thin film transistors organicsolar cells and organic sensors is to improve the propertyof carrier injection in the electrodeorganic interface1ndash7

In OLEDs the exciton recombination efficiency is lim-ited by the number of injected electrons because mostorganic semiconductors are p-type and electron mobili-ties are in general lower than hole mobilities Thereforethe efficient electron-injection is the key to improving theoperation voltage and emission efficiency8 Recently theOLED with a pndashindashn-type structure in which an intrin-sic emission layer is embedded in between n- and p-typedoped wide-gap organic electron and hole transport layers(ETL and HTL respectively) have dramatically improvedthe device performance8 The OLEDs having electrically

lowastAuthor to whom correspondence should be addressed

doped carrier-injectingtransporting layers exhibit lowdriving voltages with high carrier conductivity whichis due to the formation of radical anions cations andohmic contacts at the electrode interfaces9ndash12 Kido et aldemonstrated a device with a metal-doped ETL13 Ann-type doped transporting layer was extended to devel-opments of a Li-doped 47-diphenyl-110-phenanthroline(BPhen) layer Li-doped Alq3 and Li-doped 29-dimethyl-47-diphenyl-110-phenanthroline (BCP) layer by otherresearchers14ndash15 The high electrical conductivity of a Li-doped charge transporting layer is about 3times 10minus5 Scmlayer15

In this n-doped carrier-transporting layer the two impor-tant issues are the energy level alignment and the chemicalreaction The former is important in the aspect of elec-tron injection while the latter is essential for understand-ing the chemical process that leads to the degradation ofthe device The electronic structure of the n-doped carrier-injecting layer plays an important role in determining theirluminous efficiency and lifetime in the devices

J Nanosci Nanotechnol 2009 Vol 9 No 12 1533-4880200997485006 doi101166jnn20091774 7485


Copyright American Scientific Publishers

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Device Characteristics of OLEDs Based on Electronic Structure of the Ba-Doped Alq3 Layer Lim et al

In this study a new n-type carrier-transporting layer ofBa-doped Alq3 was applied to OLEDs The correlationbetween the device performance and the electronic fea-tures of a Ba-doped Alq3 layer was investigated over a Badoping range of 0 to 50 In addition the decompositionmechanism of the Alq3 molecule composing the Ba-dopedAlq3 layer was studied as a function of Ba doping level

2 EXPERIMENTAL DETAILS

The OLED was composed of glasstin-doped indiumoxide (ITO about 10 )44prime4primeprime-tris(2-naphthyl-phenyl-1-phenylamino)triphenylamine (2-TNATA 30 nm)44prime-bis(N-(1-napthyl)-N-phenyl-amino)-biphenyl (NPB18 nm)tris(8-quinolinolato)aluminium(III) (Alq3 42 nm)Ba-doped Alq3 (20 nm x)Al (100 nm) with Ba dop-ing levels of 0 10 25 and 50 for devices 1ndash4respectively The emissive active area of all devices was2times2 mm2 All devices were vacuum-fabricated by thermalevaporation on glassITO substrates In the fabrication ofthe Alq3 layer doped with Ba at 10 25 and 50 thedeposition rate for the Alq3 matrix molecule was fixed at02 nms while the deposition rates for the Ba dopant werevaried as 002 005 and 01 nms respectively Dopingwas achieved by co-evaporation where the evaporationrates of Alq3 and Ba were controlled independently bymeasuring with separate quartz thickness monitors Thecurrent densityndashvoltagendashluminance characteristics werealso measured using a Keithley 2400 electrometer a pho-todiode (Oriel 71608) and a Keithley 485 picoammeterThe electronic structure of the Ba-on-Alq3 layer was

examined by using X-ray photoemission spectroscopy(XPS) ultraviolet photoemission spectroscopy (UPS) andnear-edge X-ray absorption fine structure (NEXAFS) spec-troscopy at the 4B1 beam line in the Pohang AcceleratorLaboratory (Korea) All the measurements and the deposi-tion of the Ba-doped Alq3 layer (10 nm) were performedin an ultra-high vacuum system consisting of a main anal-ysis chamber (approximately 5times 10minus10 Torr) and a sam-ple preparation chamber (approximately 5times 10minus9 Torr)All samples were prepared in-situ by sequential thermalevaporation on a Si wafer and all thicknesses were deter-mined by timed depositions calibrated using a quartz-crystal microbalance In the XPS studies incident photonenergies of 650 and 550 eV were used to obtain the corelevel spectra of O 1s and N 1s respectively For the UPSmeasurements the He I (212 eV) line from a UV sourcewas used The photoemission onset reflecting the vacuumlevel at the surface of all samples was measured by bias-ing the samples at minus20 V The incident photon energy wascalibrated by measuring the Au 4f level of a clean Au sur-face In the NEXAFS analyses N K-edge (387sim417 eV)and O K-edge (520sim560 eV) spectrum were measured ata photon incident angle of 45

3 RESULTS AND DISCUSSION

Figure 1(a) shows the current densityndashvoltagendashluminancecharacteristics for devices1ndash4 The device performanceresults are summarized in Table I In Figure 1(a) the turn-on voltages (voltage at 01 cdm2 VT of devices 2ndash4 were26 26 and 28 V respectively However the Ba-freedevice 1 showed the highest VT value of 54 V Mean-while at the luminance of about 100 cdm2 (V100 thebias voltages of devices 2ndash4 were 40 (71) 46 (74) and62 V (89 mAcm2 respectively as seen in Figure 1(a)When the Alq3 layer doped with 10 Ba was insertedbetween the cathode and Alq3 the lowest power con-sumption (PC was observed among the devices with aBa-doped Alq3 layer and the Ba-free device Figure 2(b)shows the external quantum efficiencyndashcurrent densityndashpower efficiency characteristics for devices 2ndash4 At theluminance of 100 cdm2 (L100 device

2 showed the high-est device performance with an external quantum effi-ciency (ext of 39 lmW and a power efficiency (PEof 20 However as the Ba dopant concentration was

Fig 1 (a) Current densityndashvoltagendashluminance and (b) external quan-tum efficiencyndashcurrent densityndashpower efficiency curves for devices1ndash4 The devices were composed of glassITO2-TNATA (30 nm)NPB (18 nm)Alq3 (42 nm)Ba-doped Alq3 (20 nm x)Al(100 nm) (x the weight percentage concentration of Ba for devices1ndash4= 0 [diamond symbol] 10 [square symbol] 25 [circle symbol] and50 [triangle symbol])

7486 J Nanosci Nanotechnol 9 7485ndash7490 2009



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Lim et al Device Characteristics of OLEDs Based on Electronic Structure of the Ba-Doped Alq3 Layer

Table I Current densityndashvoltagendashluminance characteristics for devices 1ndash4

Doping concentrationDevices of Ba () ext () PE (lmW) V100 (V) VT (V) Lmax (cdm2

1 0 mdash mdash mdash 52 mdash2 10 20 39 40 (71 mAcm2 26 47850 at 90 V3 25 15 26 46 (73 mAcm2 26 43140 at 106 V4 50 07 12 62 (89 mAcm2 28 28350 at 122 V

ext PE and V100 are the values at 100 cdm2 VT and Lmax are the turn-on voltage at a luminance of 01 cdm2 and the maximum luminance respectively

increased from 10 to 50 ext and PE were abruptlyreduced The results of PC ext and PE demonstrated thedependence of the device performance on the characteris-tics of the Ba-doped Alq3 layer The device performancewas maximized with the 10 Ba-doped Alq3 layer butwas deteriorated with further increase in the Ba dopingTo elucidate the mechanism involved in the enhancementof the device performance the electronic structures of theBa-doped Alq3 layer were investigated as a function of Badopant concentrationFigure 2(a) shows the evolution of the O 1s electron

density curves (EDCs) as a function of the weight per-centage of Ba in the Ba-doped Alq3 layer as measuredby XPS For the pristine Alq3 layer the O 1s core-levelEDC was symmetrical in shape and was composed of asingle component indicating a clean Alq3 film The sin-gle component of the O 1s EDC was initially shifted toa higher binding energy of 5322 eV (Alq3 radical anionformed by electron charge transfer (ECT) from Ba) untilthe Ba dopant concentration of 10 compared with pris-tine Alq3 of 5316 eV From the Ba dopant concentrationof 25 the O 1s peak of 5306 eV (decomposition byAlndashO bond breaking) began to appear with low intensity ata lower binding energy and the intensity of the 5306 eV

Fig 2 The evolution of XPS O 1s (a) and N 1s (b) core level EDCsas a function of the weight percentage of Ba in the Ba-doped Alq3 layerThe values in the figures indicate the weight percentage of Ba

peak was further increased at the dopant concentration of50 The peak shift to a lower binding energy was causedby a decrease in the Coulomb potential between the nucleiand the electrons of the O 1s core level as a result of theincreased electron density at the valence band of O atomsdue to an electron provided by Ba2

Meanwhile Choong et al reported that the bindingenergy difference between an Alq3 radical component anda decomposition component in the CaAlq3 interface was20 eV in the XPS spectra of O 1s16 The large peakshift of 16 eV between the Alq3 radical species and Alq3decomposition species as shown in Figure 2(a) indicatesbond breakage between the Al and O atoms in the Alq3molecule Therefore the peak at a low binding energy of5306 eV was attributed to the appearance of a metallicAlq3 component that was decomposed by the large ECTfrom Ba at the higher Ba concentration Figure 2(b) showsthe N 1s core level EDCs as a function of the Ba dopantconcentration ranging from 0 to 50 Even with the low-est Ba dopant concentration of 10 Alq3 decompositionspecies (around 3976 eV) reacting with Ba was observedwith Alq3 radical species (around 3995 eV) This indicatesthat Ba interacts with N before O which is reasonablegiven that the AlndashN bond is the weakest as the coordinatedbonding17 in the Alq3 molecule Considering the depen-dence of the device characteristics shown in Figure 1 onthe Ba dopant concentration the formation of the decom-position component in the N XPS spectra did not affect thedecomposition of Alq3 molecules at any of the Ba dopantconcentrations However the emergence of the decomposi-tion component leading to the breakage of the AlndashO ionicbonding17 in the O XPS spectra provides a direct cause ofthe poor device performance at the Ba dopant concentra-tion of above 10Figure 3(a) shows the NEXAFS spectra at the N K-edge

of the Ba-doped Alq3 layer over the Ba dopant concentra-tion range of 0 to 50 NEXAFS spectroscopy shows theresonance features induced by a transition from the ini-tial state (core level) to the final state (vacant level) andcan be used to obtain information on the unoccupied stateCurioni et al18 reported peaks at 3929 4005 and about4030 eV in pristine Alq3 due to the transitions of electronsfrom the N 1s level to the lowest unoccupied molecularorbital (LUMO) LUMO+2 (and LUMO+3) and manylowast respectively The spectrum from pristine Alq3 was in

J Nanosci Nanotechnol 9 7485ndash7490 2009 7487



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Fig 3 Total electron yield (TEY) near-edge X-ray absorption finestructure (NEXAFS) spectra at the N K-edge (a) and O K-edge (b) ofthe Ba-doped Alq3 layer over a weight percentage concentration of Bafrom 0 to 50 The lowastrea and lowastunrea indicate the transitions from Ncore level to lowast levels of nitrogen species reactive with Ba and thoseunreactive with Ba respectively

good agreement with the results of Curioni et al18 Inter-estingly new resonance peaks appeared at 3929 eV uponBa adsorptionThe peaks at 3929 and 4005 eV which were induced

by the transition to LUMO levels consisting of lowast unoc-cupied states were attributed to lowastrec (species of N atomreacted with Ba) and lowastunrec (species of N atom unre-acted with Ba) respectively The difference (76 eV) inthe large photon energy between lowastrec and lowastunrec wasinterpreted as being caused by the change in the finalstate because the N 1s core level does not show anystrong feature related with the chemical modification asshown in Figure 2(b) The appearance of a lowastrec peakeven at the Ba concentration of 10 on Alq3 indicatedthat the orbitals involved with ECT were mainly localizedat the pyridyl ring17 rather than the phenoxide ring17 onthe 8-quinolinolato ligand composing the Alq3 moleculeFurthermore no lowastrec peak appeared in the interfacesof KAlq3

19 and CsAlq320 in the NEXAFS spectra

These results indicated that electron charge injection couldbe more improved by Ba than by K (or Cs) becauseBa could provide more electrons than K (or Cs) to the

orbitals involved with the pyridyl ring composing the Alq3molecules Therefore Alq3 incorporating Ba is expectedto show superior device performance than K (or Cs)Meanwhile the ratio of lowastreclowastunrec in Figure 3(a) wasincreased with increasing Ba dopant concentration Thisresult was in agreement with the increasing tendency ofthe lowastreclowastunrec ratio with increasing Ba dopant con-centration as shown in Figure 2(b)Figure 3(b) shows the NEXAFS spectra at the O K-edge

of the Ba-doped Alq3 layer over the Ba dopant concen-tration range of 0 to 50 The peaks at 528 and 534 eVin pristine Alq3 were due to the transitions of electronsfrom the O 1s level to LUMO (and LUMO+2) and manylowast respectively18 As shown in the NEXAFS spectra ofFigures 3(a and b) the shape of each atom-resolved elec-tronic spectrum that is mainly composed of Alq3 radicalanion species (10 doping) was very similar to that of thepristine Alq3 (0 doping) However the transition of theAlq3 decomposition species (50 doping) showed a largedifference to that of pristine Alq3 (0 doping) due to thedestroyed Alq3 molecular structureFigure 4(a) shows the UPS spectra around the highest

occupied molecular orbitals (HOMOs) of the pristine Alq3layer and the Alq3 layer doped with 10 Ba The HOMOlevel in the Alq3 layer doped with 10 Ba was shiftedto a binding energy 06 eV higher than that of the pris-tine Alq3 layer This lowering of the HOMO level impliedthat the barrier height for electron injection was lowered2

In addition as shown in Figure 4(a) the metal-inducedgap state (MIGS)21 which is roughly a free-electron-likemetal wave function penetrating into the organic semicon-ductor side was observed at the forbidden energy gap ofapproximately 16 eV above the HOMO of Alq3 The bandof the Ba-doped Alq3 layer was realigned due to the intro-duced MIGS and work function (WF) difference22 In facta new gap state has also been observed in several inter-faces between a low WF metal (eg Li Na K Ca andMg) and Alq3

23 The inset of Figure 4(a) shows the pho-toemission onset measured at the pristine Alq3 layer andthe Alq3 layer doped with 10 Ba with a sample bias ofminus20 V reflecting the WF shifts of Alq3 achieved by BaadsorptionFigure 4(b) shows the proposed energy band diagram

for the pristine Alq3 layer and the Alq3 layer doped with10 Ba The WF ( = Alq3minusBa-doped Alq3 andHOMO level (HOMO=HOMOAlq3minusHOMOBa-dopedAlq3 shifts were obtained from Figures 4(a) and (b)respectively The energy band gap (Eg of pristine Alq3was 29 eV2 The Alq3 layer doped with 10 Ba showeda low barrier height for an electron injection of 02 eVdue to the lowering of the HOMO level This loweringof 06 eV in the HOMO level in the Alq3 layer dopedwith 10 Ba was due not only to band bending by Fermilevel pinning but also to the chemical modification of theAlq3 molecules by the excess electrons of the Ba metal




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Fig 4 (a) Ultraviolet photoemission spectrum (UPS) around theHOMO of the pristine Alq3 layer and the Alq3 layer doped with 10Ba The inset shows the onset of photoemission indicating the vacuumlevel of the Ba-doped Alq3 layer (B) Proposed energy band diagram forthe pristine Alq3 layer and the Alq3 layer doped with 10 Ba

Moreover the presence of MIGS in the Ba-doped Alq3layer as shown in Figure 4(a) is further evidence for thechemical reaction between Ba and Alq3

4 CONCLUSIONS

In conclusion we investigated the mechanism of theimproved device performance achieved by inserting theBa-doped Alq3 layer between the cathode and the ETLThe performance of the device comprising the Alq3 layerdoped with 10 Ba was enhanced by the reduction of theHOMO level by 06 eV as shown in the UPS spectra This

reduction was due to the MIGS formed by chemical reac-tion as well as the band bending formed by Fermi levelpinning Although the electron-injecting barrier height ofthe Ba-doped Alq3 at all dopant concentrations used in thisstudy was almost identical the interface chemistry largelydepended on the Ba dopant concentration as shown in thereactive species in the XPS spectra (O 1s and N 1s) andthe NEXAFS spectra at the N and O K-edge The for-mation of stable Alq3 radical anion species at the low Baconcentration of 10 enhanced the OLED device perfor-mance but the appearance of Alq3 decomposition speciesfrom the Ba concentration of 25 degraded the device per-formance due to AlndashO rather than AlndashN bond breakingin the Ba-doped Alq3 layer

Acknowledgment This research was supported by agrant (PAD-4) from the Information Display RampD Centerone of the 21st Century Frontier RampD Programs funded bythe Ministry of Knowledge Economy of the Korean gov-ernment This work were also supported by Pohang Accel-erator Laboratory in Korea and by the WCU program ofKorea Science and Engineering Foundation funded by theKorean government (MOEHRD) (Grant No R32-2008-000-10124-0)

References and Notes

1 L L Chua J Zaumseil J-F Chang E C W Ou P KH Ho H Stirringhaus and R H Friend Nature 434 194(2005)

2 H Ishii K Sugiyama E Ito and K Seki Adv Mater 11 605(1999)

3 K Y Wu Y T Tao and H W Huang Appl Phys Lett 99 241104(2007)

4 C W Tang Appl Phys Lett 48 183 (1986)5 L Wang D Fine D Sharma L Torsi and A Dodabalapur Anal

Bioanal Chem 384 310 (2006)6 J H Lee H Y Chu J I Lee K I Song and S J Lee J Nanosci

Nanotechnol 8 5185 (2008)7 B C Kwack K S Lee D C Choo T W Kim J H Seo and

Y K Kim J Nanosci Nanotechnol 8 5532 (2008)8 L C Palilis M Uchida and Z H Kafafi IEEE J Sel Top Quantum

Electron 10 1 (2004)9 J S Huang M Pfeiffer A Werner J Blochwitz K Leo and S Y

Liu Appl Phys Lett 80 139 (2002)10 J M Bharathan and Y Yang J Appl Phys 84 3207 (1998)11 A Yamamori C Adachi T Koyama and Y Taniguchi Appl Phys

Lett 72 2147 (1998)12 X Zhou J Blochwitz M Pfeiffer A Nollau T Fritz and K Leo

Adv Funct Mater 11 310 (2001)13 J Kido and T Matsumoto Appl Phys Lett 73 2866 (1998)14 J Huang M Pfeiffer A Werner J Blochwitz K Leo and S Liu

Appl Phys Lett 80 139 (2002)15 G Parthasarathy C Shen A Kahn and S R Forrest J Appl Phys

89 4986 (2001)16 V E Choong M E Mason C W Tang and Y Gao Appl Phys

Lett 72 2689 (1998)17 C H Chen and J Shi Coordi Chem Rev 171 161 (1998)18 A Curioni W Andreoni R Treusch F J Hinpsel E Haskal




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P Seidler C Heske S Kakar T van Buuren and L J TerminelloAppl Phys Lett 72 1575 (1998)

19 T Yokoyama H Ishii N Matsuie K Kanai E Ito A FujimoriT Araki Y Ouchi and K Seki Synth Metals 152 277 (2005)

20 J T Lim J H Lee Y S Kim and G Y Yeom J Korean PhysSoc 52 476 (2008)

21 M Kiguchi R Arita G Yoshikawa Y Tanida S Ikeda S EntaniI Nakai H Kondoh K Saiki and H Aoki Phys Rev 72 75446(2005)

22 J Tersoff Phys Rev Lett 52 465 (1984)23 C Shen I G Hill A Kahn and I Schwartz J Am Chem Soc

122 5391 (2000)

Received 9 October 2008 Accepted 17 March 2009




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In this study a new n-type carrier-transporting layer ofBa-doped Alq3 was applied to OLEDs The correlationbetween the device performance and the electronic fea-tures of a Ba-doped Alq3 layer was investigated over a Badoping range of 0 to 50 In addition the decompositionmechanism of the Alq3 molecule composing the Ba-dopedAlq3 layer was studied as a function of Ba doping level

2 EXPERIMENTAL DETAILS

The OLED was composed of glasstin-doped indiumoxide (ITO about 10 )44prime4primeprime-tris(2-naphthyl-phenyl-1-phenylamino)triphenylamine (2-TNATA 30 nm)44prime-bis(N-(1-napthyl)-N-phenyl-amino)-biphenyl (NPB18 nm)tris(8-quinolinolato)aluminium(III) (Alq3 42 nm)Ba-doped Alq3 (20 nm x)Al (100 nm) with Ba dop-ing levels of 0 10 25 and 50 for devices 1ndash4respectively The emissive active area of all devices was2times2 mm2 All devices were vacuum-fabricated by thermalevaporation on glassITO substrates In the fabrication ofthe Alq3 layer doped with Ba at 10 25 and 50 thedeposition rate for the Alq3 matrix molecule was fixed at02 nms while the deposition rates for the Ba dopant werevaried as 002 005 and 01 nms respectively Dopingwas achieved by co-evaporation where the evaporationrates of Alq3 and Ba were controlled independently bymeasuring with separate quartz thickness monitors Thecurrent densityndashvoltagendashluminance characteristics werealso measured using a Keithley 2400 electrometer a pho-todiode (Oriel 71608) and a Keithley 485 picoammeterThe electronic structure of the Ba-on-Alq3 layer was

examined by using X-ray photoemission spectroscopy(XPS) ultraviolet photoemission spectroscopy (UPS) andnear-edge X-ray absorption fine structure (NEXAFS) spec-troscopy at the 4B1 beam line in the Pohang AcceleratorLaboratory (Korea) All the measurements and the deposi-tion of the Ba-doped Alq3 layer (10 nm) were performedin an ultra-high vacuum system consisting of a main anal-ysis chamber (approximately 5times 10minus10 Torr) and a sam-ple preparation chamber (approximately 5times 10minus9 Torr)All samples were prepared in-situ by sequential thermalevaporation on a Si wafer and all thicknesses were deter-mined by timed depositions calibrated using a quartz-crystal microbalance In the XPS studies incident photonenergies of 650 and 550 eV were used to obtain the corelevel spectra of O 1s and N 1s respectively For the UPSmeasurements the He I (212 eV) line from a UV sourcewas used The photoemission onset reflecting the vacuumlevel at the surface of all samples was measured by bias-ing the samples at minus20 V The incident photon energy wascalibrated by measuring the Au 4f level of a clean Au sur-face In the NEXAFS analyses N K-edge (387sim417 eV)and O K-edge (520sim560 eV) spectrum were measured ata photon incident angle of 45

3 RESULTS AND DISCUSSION

Figure 1(a) shows the current densityndashvoltagendashluminancecharacteristics for devices1ndash4 The device performanceresults are summarized in Table I In Figure 1(a) the turn-on voltages (voltage at 01 cdm2 VT of devices 2ndash4 were26 26 and 28 V respectively However the Ba-freedevice 1 showed the highest VT value of 54 V Mean-while at the luminance of about 100 cdm2 (V100 thebias voltages of devices 2ndash4 were 40 (71) 46 (74) and62 V (89 mAcm2 respectively as seen in Figure 1(a)When the Alq3 layer doped with 10 Ba was insertedbetween the cathode and Alq3 the lowest power con-sumption (PC was observed among the devices with aBa-doped Alq3 layer and the Ba-free device Figure 2(b)shows the external quantum efficiencyndashcurrent densityndashpower efficiency characteristics for devices 2ndash4 At theluminance of 100 cdm2 (L100 device

2 showed the high-est device performance with an external quantum effi-ciency (ext of 39 lmW and a power efficiency (PEof 20 However as the Ba dopant concentration was

Fig 1 (a) Current densityndashvoltagendashluminance and (b) external quan-tum efficiencyndashcurrent densityndashpower efficiency curves for devices1ndash4 The devices were composed of glassITO2-TNATA (30 nm)NPB (18 nm)Alq3 (42 nm)Ba-doped Alq3 (20 nm x)Al(100 nm) (x the weight percentage concentration of Ba for devices1ndash4= 0 [diamond symbol] 10 [square symbol] 25 [circle symbol] and50 [triangle symbol])




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device characteristics of organic light-emitting diodes based on electronic … · 2016-08-17 ·...

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