Development of an exciplex type mixed host using a pyrrolocarbazole type material for extended lifetime in green phosphorescent organic light-emitting diodes

Accepted Manuscript Development of an exciplex type mixed host using a pyrrolocarbazole type material for extended lifetime in green phosphorescent organic light-emitting diodes Jeong Min Choi, Ji Han Kim, Yu Jin Kang, Jun Yeob Lee PII:

S1566-1199(17)30328-2

DOI:

10.1016/j.orgel.2017.07.004

Reference:

ORGELE 4196

To appear in:

Organic Electronics

Received Date: 23 January 2017 Revised Date:

26 June 2017

Accepted Date: 4 July 2017

Please cite this article as: J.M. Choi, J.H. Kim, Y.J. Kang, J.Y. Lee, Development of an exciplex type mixed host using a pyrrolocarbazole type material for extended lifetime in green phosphorescent organic light-emitting diodes, Organic Electronics (2017), doi: 10.1016/j.orgel.2017.07.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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DPhPCz DBFTrz DPhPCz:DBFTrz(50:50)

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Development of an exciplex type mixed host using a pyrrolocarbazole type material for extended lifetime in green phosphorescent organic lightemitting diodes

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School of Chemical Engineering, Sungkyunkwan University 2066, Seobu-ro, Jangan-gu, Suwon, Gyeonggi, 440-746, Korea E-mail : [email protected]

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Jeong Min Choi+, Ji Han Kim+, Yu Jin Kang+ and Jun Yeob Lee *

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+ Jeong Min Choi, Ji Han Kim, and Yu Jin Kang contributed equally.

Abstract

Lifetime improvement of green phosphorescent organic light-emitting didoes (PHOLEDs) by an exciplex type host was studied by mixing a hole transport type host and an electron

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transport type host. A pyrrolocarbazole type material was developed as the hole transport type host and a triazine type material was the electron transport type host. The exciplex type mixed host showed much longer lifetime and improved efficiency compared with each host

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material constituting the exciplex type mixed host. Hole and electron stability of the exciplex

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host was proposed as the key factor for the long lifetime of the green phosphorescent device.

Keywords: efficiency⋅lifetime⋅exciplex⋅mixed host

ACCEPTED MANUSCRIPT Introduction Lifetime and quantum efficiency (QE) are two key attributes governing the device characteristics of organic light-emitting diodes (OLEDs)[1,2]. Therefore, most developments

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in the field of OLEDs were focused on improving the lifetime and QE by device engineering or new material synthesis. From material side, phosphorescent emitters paved a way of

increasing the QE of phosphorescent OLEDs (PHOLEDs)[3], while a mixed host opened a

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way upgrading the QE of OLEDs from device side[4-7]. Particularly, the mixed host was widely applied in both fluorescent and phosphorescent OLEDs as an efficiency boosting

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method. Two types of mixed host, exciplex free mixed host[8,9] and exciplex type mixed host[4,5,7,10-12], have been reported as the mixed host systems of OLEDs. But, it often decrease an efficiency. At first, exciplex free mixed hosts were examined as the high QE and long lifetime of this sysyem, although they often decrease an efficiency[13, 14]. On the other

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hand, recently, but recently, exciplex type mixed hosts are drawing great attention because of low driving voltage of the exciplex based mixed host devices. Compared with the exciplex free mixed host, the exciplex has the shallow highest occupied molecular orbital (HOMO)

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and deep lowest unoccupied molecular orbital (LUMO) level[11,12], which lowered the driving voltage of the device[4-7]. The QE of the exciplex based mixed host devices was also

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high due to equal number of holes and electrons by efficient charge injection, which increased the power efficiency of the OLEDs in combination with the low driving voltage. However, the stability of the exciplex type mixed host devices has not been widely studied and little has been known about the lifetime of the exciplex type mixed host devices[15-18]. As the lifetime is one of the key performances of PHOLEDs[19-22], lifetime study of the exciplex mixed host based OLEDs and development of long-living OLEDs is important[21,22].

ACCEPTED MANUSCRIPT In this work, a stable exciplex type mixed host was developed using a hole transport type host with pyrrolocarbazole moiety and an electron transport type host with triazine moiety. The pyrrolocarbazole based host material, 3,3'-bis(1-phenylpyrrolo[3,2-b]carbazol-5(1H)-yl)-1,1'-

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biphenyl (DPhPCz), was newly synthesized as the stable hole transport type host material and it was mixed with a triazine based host material, 2,8-bis(4,6-diphenyl-1,3,5-triazin-2-

yl)dibenzo[b,d]furan (DBFTrz). The two host materials formed exciplex by mixing and the

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exciplex type host provided enhanced QE and elongated lifetime compared with each host material. Therefore, it was established that the exciplex type mixed host can behave as the

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long lifetime host system as well as a high QE host system.

Experimental

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General information

3,3'-Dibromo-1,1'-biphenyl, tris(dibenzylideneacetone)dipalladium(0) (P&H tech), sodium tert-butoxide, tri-tert-butylphosphine (Sigma Aldrich Co.) and toluene (Samchun pure

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chemical Co. Ltd) were purchased and used without purification. The 1H and 13C nuclear magnetic resonance spectra were observed using an Avance-500

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(Bruker, 500MHz) at room temperature. Fluorescence and phosphorescence spectra were recorded on PerkinElmer LS-55 fluorescence spectrometer. The mass spectra were observed using an Advion, Expression LCMS spectrometer in APCI mode. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels were determined using a cyclic voltammetry (CV, Ivium Tech., Iviumstat). Tetrabutylammonium perchlorate was dissolved in acetonitrile at 0.1M concentration to prepare an electrolyte. Silver, platinum, and carbon electrodes were used as reference, counter, and working electrodes, respectively.

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Synthesis 3,3'-Bis(1-phenylpyrrolo[3,2-b]carbazol-5(1H)-yl)-1,1'-biphenyl (DPhPCz) (0.5

g,

1.60

mmol),

1-phenyl-1,5-dihydropyrrolo[3,2-

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3,3'-Dibromo-1,1'-biphenyl

b]carbazole (1.17 g, 3.52 mmol), tris(dibenzylideneacetone)dipalladium(0) (0.15 g, 0.08 mmol) and sodium tert-butoxide (0.77 g, 8.01 mmol) were dissolved in toluene (50 ml) and

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the mixture was stirred for 20 min in the N2 purged round-bottomed flask. Tri-tertbutylphosphine (0.10g, 0.48 mmol) was slowly added to the mixture by dropping and the

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reactant mixture was refluxed for 24 h under N2. After cooling to room temperature, the mixture was extracted with dichloromethane and distilled water several times. The dichloromethane layer was separated and dehydrated with magnesium sulfate. After filtering off the magnesium sulfate, a crude compound was obtained by evaporating dichloromethane

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solvent. The mixture was purified by column chromatography (dichloromethane:n-hexane = 1:4) and sublimation. A white powder was obtained as a pure product. Yield 63% (0.72 g), 1H NMR (500 MHz, DMSO-d6) : δ 8.16 – 7.93 (m, 8H), 7.84 – 7.81 (t, 13

C NMR (125 MHz,

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2H), 7.67 (d, 2H, J=7.0 Hz), 7.54 – 7.23 (m, 20H), 5.97 (s, 2H).

DMSO-d6): δ 140.50, 139.88, 139.05, 138.59, 135.11, 133.59, 130.58, 129.84, 129.71, 126.96,

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126.69, 126.44, 124.44, 124.25, 123.89, 120.08, 119.89, 119.29, 115.15, 113.62, 109.61, 104.17, 104.02, 100.84. MS (APCI) m/z 715.28 [(M + H) +]. Anal. Calcd. for C52H34N4: C, 87.37; H, 4.79; N, 7.84. Found: C, 88.07; H, 4.47; N, 7.52.

Device fabrication Basic electrical performances of the exciplex host device and single host devices were tested by depositing 4,4'-(cyclohexane-1,1-diyl)bis(N,N-di-p-tolylaniline) (TAPC, 20 nm), 1,3di(9H-carbazol-9-yl)benzene (mCP, 10 nm), emitting layer (25 nm), diphenylphosphine

ACCEPTED MANUSCRIPT oxide- 4-(triphenylsilyl)phenyl (TSPO1, 5 nm), 2,2′,2″-(1,3,5-benzinetriyl)-tris(1-phenyl-1H-benzimidazole) (TPBi, 40 nm), LiF (1.5 nm), and Al (200 nm) on 120 nm thick indium tin oxide

(ITO)

coated

with

poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

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(PEDOT:PSS), stepwisely. Lifetime test was carried out using a device structure consisting of N,N'-diphenyl-N,N'-bis-[4-(phenyl-m-tolyl-amino)-phenyl]-biphenyl-4,4'-diamine (DNTPD) instead of PEDOT:PSS and N,N,N’N’-tetra[(1,1’-biphenyl)-4-yl]-(1,1’-biphenyl)-4,4’-

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diamine (BPBPA) instead of TAPC. mCP was replaced with mCBP and TSPO1 was eliminated from the electron transport layer. The device structure was ITO (120 nm)/DNTPD

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(60 nm)/BPBPA (20 nm)/mCBP (10 nm)/ emitting layer (30 nm)/TPBi (35 nm)/LiF (1.5 nm)/Al (200 nm). The emitting layer was 10 wt% fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) doped DPhPCz, DBFTrz and DPhPCz:DBFTrz. Single carrier device structures for stability test were ITO (120 nm)/DNTPD (60 nm)/BPBPA (20 nm)/mCBP (10 nm)/host (25

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nm)/DNTPD (5 nm)/Al for hole only devices and ITO (120 nm)/TPBi (5 nm)/host (25 nm)/TPBi (35 nm)/LiF (1.5 nm)/Al for electron only devices. Constant current driving method was used to evaluate the lifetime of the green devices and

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voltage sweep method was utilized to test the current density (J), luminance (L), and QE of

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the devices. Detailed device evaluation methods were reported in other papers[23,24].

Results and discussion

Stable mixed host for long lifetime can be developed by mixing stable exciplex forming hole transport type and electron transport type hosts. Proper selection of each host material is critical to the lifetime of the mixed host device because unstable host materials would decrease the lifetime by decomposition of the host material. As the stable host materials, two host materials made up of only stable hole transport or electron transport type moieties were chosen for the exciplex type mixed host device. The hole transport type host was DPhPCz

ACCEPTED MANUSCRIPT having pyrrolocarbazole as the hole transport moiety and the electron transport type host was DBFTrz possessing diphenyltriazine as the electron transport moiety. The pyrrolocarbazole moiety is a new donor moiety in the host and the diphenyltriazine was already proven as the

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stable chemical moiety. Although the pyrrolocarbazole moiety was not proven as the stable functional unit, but the aromatic character of the pyrrolocarbazole is expected to stabilize the pyrrolocarbazole. Therefore, pyrrolocarbazole-derived DPhPCz and diphenyltriazine-derived

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DBFTrz were adopted to as the exciplex forming host materials.

The pyrrolocarbazole type DPhPCz host material was newly synthesized in this work and

DPhPCz

host

was

derived

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DBFTrz was reported in our previous work as an electron transport material[25]. The from

3,3'-dibromo-1,1'-biphenyl

and

1-phenyl-1,5-

dihydropyrrolo[3,2-b]carbazole which was reported as a strong donor moiety[20]. Synthetic scheme of DPhPCz is presented in Scheme 1.

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Basic photoluminescence (PL) characteristics of DPhPCz were analyzed at room temperature (fluorescence) and at 77 k in liquid nitrogen (phosphorescence). The fluorescence and phosphorescence spectra in Figure 1 obtained using a dilute toluene solution of DPhPCz

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suggested fluorescent emission at 410 nm and the first phosphorescent emission at 426 nm.

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The phosphorescent emission peak wavelength was converted into triplet energy of 2.91 eV, indicating that the DPhPCz host can behave as a high triplet energy host material. In order to judge whether the DPhPCz host can form an exciplex with DBFTrz, the HOMO and LUMO levels was evaluated by electrochemically driven CV method using ferrocene as the standard material. CV based oxidation and reduction measurement of DPhPCz in Figure 2 estimated the HOMO of -5.77 eV and the LUMO of -2.66 eV by converting onset voltage of oxidation and reduction. From the HOMO/LUMO of -6.64/-3.07 eV of DBFTrz, it can be predicted that DPhPCz and DBFTrz can form an exciplex in the mixed host structure because

ACCEPTED MANUSCRIPT of large HOMO offset of 0.87 eV and LUMO offset of 0.35 eV[21]. The strong donor character of the pyrrolocarbazole moiety lead to the shallow HOMO level in the DPhPCz. Exciplex formation between DPhPCz and DBFTrz hosts was identified by PL emission

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spectrum of the mixed film of DPhPCz and DBFTrz. It has been well known that the PL spectrum of the exciplex is red-shifted compared to that of each compound constituting the exciplex[22-24,26]. The same phenomenon was observed in the mixed film of DPhPCz and

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DBFTrz. Solid PL peak wavelength of the DPhPCz:DBFTrz (50:50 wt%) mixed film (in Figure 3) was 489 nm, which was red-shifted by more than 74 nm relative to that of each

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material. No intense emission peak below 420 nm assigned to DPhPCz and DBFTrz emission was not detected, which explains efficient exciplex formation between DPhPCz and DBFTrz. The singlet energy and triplet energy of DPhPCz:DBFTrz were 2.86 and 2.84 eV from the onset wavelength of fluorescence and phosphorescence, respectively.

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As the PL emission peak of the DPhPCz:DBFTrz (50:50)(w%) exciplex fell on the ultraviolet-visible (UV-vis) absorption range of green triplet emitter Ir(ppy)3, green phosphorescent OLEDs were developed using the DPhPCz:DBFTrz exciplex as the host.

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Two devices with the DPhPCz and DBFTrz single host were compared as reference devices. To confine triplet excitons, high triplet energy mCP and TSPO1 were inserted in the device

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structure. Basic electrical performances represented by J and voltage (V) of the DPhPCz, DBFTrz, and DPhPCz:DBFTrz devices are shown in Figure 4. In the V sweep data of the green devices, the J of the exciplex devices was higher than that of DPhPCz and DBFTrz single host devices. This result is in agreement with the data reported in other exciplex devices[4-7]. As described in other literatures, the DPhPCz:DBFTrz exciplex devices inject holes via hole carrying DPhPCz host and electrons via electron carrying DBFTrz host. Therefore, both holes and electrons are easily carried by the exciplex host, which increased

ACCEPTED MANUSCRIPT the J of the exciplex devices. The low current density of the DPhPCz device is due to low electron current density in spite of good hole transport property and hole trapping effect of Ir(ppy)3 which reduces hole mobility of the DPhPCz host. Whereas, the low current density

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of the DBFTrz device is caused by low hole current density. Spectroradiometric measurement of the green phosphorescent OLEDs in Figure 5 revealed that the exciplex host also

increased luminance (L) of the devices because the L is determined by number of excitons in

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the emitting layer which is dependent on J and recombination efficiency. Both J and recombination efficiency (in Figure 6) were high in the exciplex device.

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External quantum efficiency (EQE) of the green devices is shown in Figure 6, which states that the exciplex device is more efficient than the single host device in terms of electron to photon conversion. As carrier balance is the most important parameter for high EQE, the exciplex device which can carry both holes and electrons was better than other single host

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devices. In the case of the single host devices, hole or electron injection is relatively difficult as projected from the energy level diagram (Figure 7) showing the energy barrier for carrier injection, which was the reason for the relatively low EQE of the DPhPCz and DBFTrz

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devices. The exciplex formation between Ir (ppy)3 and DBFTrz was not observed because the LUMO difference between Ir (ppy)3 and DBFTrz is not as large as the LUMO difference

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between DPhPCz and DBFTrz.

As the main intention of developing the DPhPCz:DBFTrz exciplex host was to improve lifetime of the single host devices, the operational stability of the exciplex device was examined by driving the green devices at the same initial L of 5,000 cd/m2. The device structure and energy level diagram are in Figure 8. Additionally, device performances of the green phosphorescent OLEDs for lifetime test are summarized in Figure 9. The time dependent L change of the green devices in Figure 10 indicates the lifetime improving effect of the exciplex host. The lifetime of the exciplex device up to 80% of initial L was more than

ACCEPTED MANUSCRIPT 15 times longer than that of the single host device. The dramatic increase of lifetime by the exciplex host is related with the light-emission mechanism of the exciplex device. It was reported that main light-emission mechanism of the exciplex device is energy transfer from

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the exciplex host to triplet emitter[27,28]. Exciplex is created by recombination of holes in the DPhPCz host and electrons in the DBFTrz host, and the exciton energy is transferred to the triplet emitter. In the exciton generation process, holes are dominantly present only in the

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DPhPCz host and electrons are mainly present in the DBFTrz host. The separation of hole and electron carrier path is advantageous for long lifetime because hole carrying DPhPCz is

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unstable under electron injection and electron carrying DBFTrz is unstable under hole injection. This can be confirmed by the bond dissociation energy (BDE) calculation results of DPhPCz and DBFTrz materials in Figure 11. In the BDE calculation, positive polaron, neutral and negative polaron formation in the pyrrolocarbazole and diphenyltriazine was

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assumed. B3LYP 6-31G basis set of Gaussian 09 Software was used to estimate the BDE of the host materials. The BDE of C-N bond between a biphenyl core and pyrrolocarbazole of DPhPCz in positive polaron state was much higher than that in negative polaron state. The

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BDE of C-C bond between dibenzofuran and triazine of DBFTrz in negative polaron state was much higher than that in positive polaron state. This supports the lifetime enhancement

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effect by the exciplex type host in the green phosphorescent OLEDs. The BDE calculation results supporting the improved lifetime of the green phosphorescent OLEDs were confirmed by analyzing material stability under positive and negative polarons. Hole only and electron only devices of DPhPCz, DBFTrz, and DPhPCz:DBFTrz hosts were fabricated to measure the stability of the hosts. The hole only and electron only devices were driven at 10 mA/cm2 and the voltage change of the single carrier devices was recorded according to driving time of the single carrier devices. Figure 12 shows the voltage change of the single carrier devices according to operation time of the devices. In the driving test of the

ACCEPTED MANUSCRIPT hole only devices, the voltage change was rather large in the DBFTrz device, but it was relatively small in the DPhPCz and DPhPCz:DBFTrz hosts. The DPhPCz:DBFTrz mixed host followed the trend of the DPhPCz, indicating the stability of the exciplex host under

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positive polarons. However, the difference of the driving voltage increase of the exciplex host device compared to the DPhPCz device is not clear, yet. In the driving test of the electron only devices, the voltage change was significant in the DBFTrz device, but it was relatively

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small in the DPhPCz and DPhPCz:DBFTrz hosts. This result indicates that the exciplex type mixed host is stabilized under negative polarons by DBFTrz constituting the mixed host. The

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DBFTrz in the exciplex host endures electron stress during device driving, which stabilized the DPhPCz:DBFTrz exciplex host under negative polarons. Other than the mechanisms explained, broad recombination zone of the DPhPCz:DBFTrz exciplex devices can be regarded as the origin of the improved lifetime[16,24]. Therefore, the exciplex type host

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could enhance both EQE and lifetime of green phosphorescent OLEDs.

Conclusions

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A newly synthesized hole carrying DPhPCz and electron carrying DBFTrz were used to develop an exciplex host and the DPhPCz:DBFTrz (50:50) exciplex host enhanced the EQE

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and lifetime of the green phosphorescent OLEDs. Particularly, the lifetime of the green phosphorescent OLEDs was elongated by more than 15 times by separating hole and electron path. Therefore, the exciplex host may become practical due to reduced driving voltage, increase EQE and elongated lifetime.

Acknowledgements This work was supported by Basic Science Research Program (2016R1A2B3008845) and

ACCEPTED MANUSCRIPT Nano Material Technology Development Program (2016M3A7B4909243) through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT, and

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Future Planning.

References

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ACCEPTED MANUSCRIPT List of figures Scheme 1. Synthetic scheme DPhPCz.

Figure 2. CV oxidation and reduction data of DPhPCz.

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Figure 1. Fluorescent and phosphorescent spectra of DPhPCz.

Figure 3. PL emission spectra of DPhPCz, DBFTrz and DPhPCz:DBFTrz hosts.

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Figure 4. Current density plots against driving voltage of DPhPCz, DBFTrz and

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DPhPCz:DBFTrz devices.

Figure 5. Luminance plots against driving voltage of the DPhPCz, DBFTrz and DPhPCz:DBFTrz devices.

Figure 6. Quantum efficiency plots against current density of the DPhPCz, DBFTrz and

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DPhPCz:DBFTrz devices.

Figure 7. Energy level diagram of DPhpCz, DBFTrz and DPhPCz:DBFTrz devices.

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lifetime devices.

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Figure 8. Energy level diagram of DPhpCz, DBFTrz and DPhPCz:DBFTrz devices for

Figure 9. Current density-voltage-luminance (a) and quantum efficiency-luminance plots of the DPhPCz, DBFTrz and DPhPCz:DBFTrz devices for lifetime test. Figure 10. Lifetime plot against driving time of the DPhPCz, DBFTrz and DPhPCz:DBFTrz devices at an initial luminance of 1,000 cd/m2 Figure 11. The bond dissociation energy (BDE) calculation results of DPhPCz and DBFTrz materials

ACCEPTED MANUSCRIPT Figure 12. The change of voltages of (a) hole only devices, (b) electron only devices

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according to aging time.

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Scheme 1

ACCEPTED MANUSCRIPT 1.2 Fluoroscence Phophorescence

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0.8 0.6 0.4 0.2 0 400

500 Wavelength (nm)

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Figure 1

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300

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Intensity (arb. unit)

1

600

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-4

-3

-2

-1

0

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Voltage (V)

1

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Current (arb. unit)

DPhPCz

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Figure 2

3

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400

500

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Wavelength (nm)

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300

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Intensity (arb. unit)

DPhPCz DBFTrz Exciplex(Fluorescent) Exciplex(Phosphorescent)

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Figure 3

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DPhPCz DBFTrz DBFTrz:DPhPCz(50:50)

35 30 25

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20 15 10 5 0 0

2

4

6

8

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Voltage (V)

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Current density (mA/cm2)

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Figure 4

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DPhPCz DBFTrz DBFTrz:DPhPCz(50:50)

8000

RI PT

6000 4000 2000 0 0

2

4

6

M AN U

Voltage (V)

8

AC C

EP

TE D

Figure 5

10

SC

Luminance (cd/m2)

10000

ACCEPTED MANUSCRIPT

16 14 12

RI PT

10 8 6 DPhPCz DBFTrz DBFTrz:DPhPCz(50:50)

4 2 0 1

10

100

10000

M AN U

Luminance

1000

(cd/m2)

SC

Quantum efficiency (%)

18

AC C

EP

TE D

Figure 6

ACCEPTED MANUSCRIPT

-2.40

-2.52

-2.66

-2.80

TPBi

RI PT

TSPO1

-2.90

SC

-5.20

DBFTrz

-4.70 -5.10

-3.07

Ir(ppy)3

DPhPCz

TAPC

mCP

ITO

PEDOT:PSS

-2.80

LiF/Al

-2.00

-5.50 -5.77

-6.10

M AN U

-6.10

AC C

DBFTrz

mCP

EP

TAPC

TE D

-6.64

Figure 7

DPhPCz

-6.79

TSPO1

TPBi

Ir(ppy)3

AC C

EP

TE D

M AN U

SC

RI PT

ACCEPTED MANUSCRIPT

Figure 8

ACCEPTED MANUSCRIPT

100000 DPhPCz

35

DBFTrz

30

10000

DBFTrz:DPhPC(5:5)

25

1000

20 100

15 10

10

5 0

1 0

2

4

6

8

10

SC

Voltage (V)

(b) DPhPCz

M AN U DBFTrz

14

DBFTrz:DPhPC(5:5)

12 10 8 6 4 2 0 1

10

TE D

Quantum efficiency (%)

18 16

100

1000

AC C

EP

Luminance (cd/m2)

Figure 9

RI PT

Current density (mA/cm2)

40

Luminance (cd/m2)

(a)

10000

ACCEPTED MANUSCRIPT 105

DPhPCz DBFTrz DPhPCz:DBFTrz(50:50)

RI PT

95 90 85 80 10

20 Time (h)

30

M AN U

0

SC

Luminance (%)

100

AC C

EP

TE D

Figure 10

40

ACCEPTED MANUSCRIPT

Neutral -charge +charge

RI PT

4.0

2.0

SC

Bond dissociation energy (eV)

6.0

0.0

DBFTrz

M AN U

DPhPCz

AC C

EP

TE D

Figure 11

ACCEPTED MANUSCRIPT

(a) 5

DPhPCz DBFTrz DPhPCz:DBFTrz(50:50)

3

RI PT

△ V (V)

4

2

0 0

2

4

6

(b) 5

10

DPhPCz DBFTrz DPhPCz:DBFTrz(50:50)

4 3 2

TE D

△ V (V)

8

M AN U

Time (h)

SC

1

1

2

AC C

0

EP

0

Figure 12

4

6

Time (h)

8

10

ACCEPTED MANUSCRIPT Hole transport type host derived from pyrrolocarbazole for exciplex host

-

Lifetime improvement of green phosphorescent organic light-emitting diodes using exciplex host

-

Exciplex host of a pyrrolocarbazole based hole type host and triazine based electron type host

AC C

EP

TE D

M AN U

SC

RI PT

-