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Highly Simplified Reddish Orange Phosphorescent Organic LightEmitting Diodes Incorporating a Novel Carrier- and ExcitonConfining Spiro-exciplex-forming Host for Reduced Efficiency Roll-off Ting Xu, Ye-Xin Zhang, Bo Wang, Chen-Chao Huang, Imran Murtaza, Hong Meng, and Liang-Sheng Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b13077 • Publication Date (Web): 30 Dec 2016 Downloaded from http://pubs.acs.org on January 2, 2017

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Charge balance is one of four most important factor affecting OLEDs performance EQE = Charge balance factorîExction emission rateîOptical out-coupling factorîPL efficiency). To get the fairish OLEDs performance, it is needed to optimize the thickness of HTL and ETL to make sure that the charge carrier recombination zone is located in emission zone of the luminescent materials. Based on above-mentioned understanding of the design philosophy of OLEDs, in our reference devices, ,72 +$7 &1

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delocalization of electronic excitation over the two kinds of molecules and from partial electron transfer between the electron-donating and electron-accepting molecules. The charge transfer properties of the exciplex is mainly dependent on the strength of the electron-donating properties of the donor materials and on the electron affinity of the acceptor.50 Thus, it is reasonably inferred that the formation of exciplex emission in SAFDPA is own to the electron-donating ability of SAFDPA resulting from fluorene moiety. Although TmPyPB has more excellent electron transport capability than Bphen, BCP and TPBi, no red-shift PL peak from exciplex is observed, suggesting that the exciplex cannot be formed between SAFDPA and TmPyPB as exhibited in Fig. 3 (c). Additional exciplex peak is observed in the thin films of SAFDPA PL[HG with the electron-accepting material (BPhen, TPBi and BCP), providing possibility to get a spiro-exciplex-forming host-based device as exhibited in Figure 3. The higher LUMO level of SAFDPA (2.2 eV) lead to the possibility of acting as electron blocking layer and forming exciplex-forming host, simultaneously. Moreover, so as to discuss the carriers transport properties of SAFDPA and TAPC, we fabricate and discuss hole-only devices: ITO/MoO3 (10nm)/ TAPC or SAFDPA (100 nm)/MoO3 (10 nm)/Al (100 nm). The J-V properties of the devices are exhibited in supporting information Fig. S1. The hole mobility of SAFDPA is ~5.65î10-4 cm2/Vs, which is high enough to form good charge balance compared with Bphen (electron mobility =

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the hole mobility of SAFDPA is comparable with other SAF-series materials.51-52 In the perspective of electron-transport capability, TmPyPB is one the best commercial small-molecule electron-transport materials (ETMs) among Bphen, BCP, TPBi and TmPyPB. The electron mobilities of Bphen, BCP, TPBi and TmPyPB are



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FP 9V, respectively.53-

The distinguished electron mobility lets us choose TmPyPB as the first choice as

ETMs in designed OLEDs. Moreover, as VHHQ LQ RXU FRRSHUDWRU¶s paper,46 the higher efficiency of the devices is mainly own to the reason that TmPyPB was applied in the OLEDs promoting charge transport balance (TAPC with hole mobility of

î

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FP 9V ). Unfortunately, the PL spectra of mixed SAFDPA: TmPyPB without red shift suggest that this mixed system do not form exciplex-forming host. Although the PL spectra of mixed SAFDPA: Bphen, SAFDPA: BCP and SAFDPA: TPBi film showing more or less red shift imply existing exciplex energy interaction between SAFDPA and ETMs (Bphen, BCP and TPBi), and Bphen possesses highest electron mobility among them which helps to enhance the charge transport balance of OLEDs. Therefore, we have choosen SAFDPA: Bphen as exciplex-forming host to design and fabricate OLEDs to primely demonstrate our OLEDs design VWUDWHJ\. Moveover, we fabricate reference OLEDs with SAFDPA: TmPyPB as non-exciplex-forming co-host OLEDs showing in Figure S1 of the supporting information to effectively support our viewpoints.


Page 13 of 30

(b)

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The PL spectra of SAFDPA, BPhen and SAFDPA: BPhen = 1: 1 (w: w) codeposited films are exhibited in Fig. 3 (a). The mixture of SAFDPA: BPhen with 1: 1 (w: w) exhibits violet blue emission with a peak at 438 nm. The energy of this peak (~2.8 eV) nearly corresponds to the offset between the LUMO of BPhen (~3.0 eV) and the HOMO of SAFDPA (~5.6 eV), verifying that the luminescence arises from exciplex emission. The mixture of SAFDPA: BCP exhibits violet blue emission with a peak at 413 nm as exhibited in Figure 3 (b). This peak energy (~3.0 eV) hardly corresponds to the offset between the LUMO of BCP (~3.0 eV) and the HOMO of SAFDPA (~5.6 eV). The PL spectra of BCP (Fig. 3 (b)) show secondary peaks at 520 nm own to the effect of excimers, dimers or other aggregates58 which destroy by doping SAFDPA. The PL



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Page 14 of 30

spectra of mixture films of SAFDPA: TmPyPB with 1: 1 (w: w) do not show the red shift relative to the PL spectrum of SAFDPA as exhibited in Fig. 3 (c). The mixture of SAFDPA: TPBi exhibits violet blue emission with a peak at 413 nm as exhibited in Fig. 3 (d). The peak energy (~3.0 eV) hardly corresponds to the energy offset between the LUMO of TPBi (~3.0 eV) and the HOMO of SAFDPA (~5.6 eV). The PL spectra of SAFDPA: BCP and SAFDPA: TPBi imply that the mixture may not form exciplex emission. Fig. 4. VKRZV delayed PL emission intensity of mixed SAFDPA: TmPyPB (1:1, Z Z ¿OPV DQG PL[HG 6$)'3$ %SKHQ

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exciton lifetime of SAFDPA: Bphen co-host is remarkably longer than the exciton lifetime of SAFDPA: TmPyPB films supporting the formation of SAFDPA: Bphen exciplex emission. One leaf is not as same as another (exciplex-forming systems)5961

in the world, even so, we find a similar and comparable known exciplex-forming

systems: TCTA: B3PYMPM (), to our SAFDPA: Bphen, which can be used to design high efficiency exciplex emission OLEDs.62 Moreover, SAFDPA: Bphen own suitable energy state following three rules proposed in the Introduction part:

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ASSOCIATED CONTENT

AUTHOR INFORMATION Corresponding Author * Hong Meng. Email: [email protected]

* Liang-Sheng Liao. Email: [email protected]



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Page 24 of 30

Supporting Information: Experimental section; Performance of reference OLEDs with SAFDPA: TmPyPB as non-exciplex-forming co-host showing in Figure S1; The current density-voltage (J-V) curves for the hole-only devices showing charge transporting properties of the SAFDPA in Figure S2.

ACKNOWLEDGMENT This work is supported by the National Natural Science Foundation of China (61575136, 51035008, 51373075), National Basic Research Program of China (973 Program, 2015CB932200). This project was also funded by the Collaborative Innovation Center of Suzhou Nano Science and Technology, by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), and by Shenzhen Key Laboratory of Organic Optoelectromagnetic Functional Materials of Shenzhen Science and Technology Plan (ZDSYS20140509094114164).

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À v

Page 29 of 30

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òôX dÇ vU zXr^XU KŒP v] o]PZšr u]šš]vPr ]} òõX ]& ^ o

]v }ŒP v] o]PZšr u]šš]vP ]}

•X :}µŒv o }( ‰‰o]

WZÇ•] • îììòU õõ ~î•U ìîðõìíX



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents


Page 30 of 30

Highly Simplified Reddish Orange Phosphorescent ... - ACS Publications

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