Calculation Of Ideal Total External Quantum And Power Efficiencies:

Management of singlet and triplet excitons for

efficient white organic light emitting devices

Yiru Sun*, Noel C. Giebink*, Hiroshi Kanno*‡, Biwu Ma†, Mark E. Thompson†, Stephen R. Forrest*+

Supplementary Information

Table: Selected electrophosphorescent WOLED architectures with their corresponding performance characteristics.

Architecture ηext [%] ηp [lm/W] CRI Refs Exciton managed fluorescent/ 11.0, 11.0†, 7.8‡ 22.1, 16.1†, 5.8 ‡ 85 phosphorescent This work

Phosphorescent triple-doped 12*, 8.0†, 3.2‡ 26*, 13.3†, 1.8 ‡ 80 1 Two phosphor doped layers 12*, 9.3†, 4.1‡ 10*, 4.6†, 1.3 ‡ <60 2 Single dopant phosphorescent 6.4 12 67 3 Multiple phosphor doped layers 5.2 6.4 83 4 Phosphorescent triplet excimers 4 4.4 78 5 Blue-fluorescent/ Red- 1.4 0.92 <60 phosphorescent 6 Phosphorescent sensitizer 1.8 4.1 <60 7

Here, ηext and ηp are the forward viewing external quantum and power efficiencies,

respectively. Where ηext or CRI is not reported, a value is calculated from the

luminous efficiency and spectral data provided in the reference. *Values at 10-3 mA/cm2; †Values at 1mA/cm2; ‡Values at 102 mA/cm2; Values without specific notation are the maximum reported.

Luminance characteristics of the fluorescent/phosphorescent white organic light emitting device.

104 ) 2 103 m /

c 1 n 10 a n i 0

m 10 u L 10-1

10-2 10-3 10-2 10-1 100 101 102 103 Current Density (mA/cm2)

Figure. 1. Forward viewing luminance–current density of the fluorescent- phosphorescent WOLED

Non-Radiative exciton transfer in the white fluorescent/phosphorescent organic light emitting device To determine if triplets are lost due to formation directly on BCzVBi molecules or otherwise wasted through non-radiative channels, we compare two structures: Device III, which is similar to the same device in the text, has structure: [NPD(30nm)/5wt.-%

BCzVBi:CBP(5nm)/CBP(4nm)/5wt.-%Ir(ppy)3:CBP(20nm)/CBP(4nm)/5wt.- %BCzVBi:CBP(5nm)/BCP(40nm)], and Device IV with structure: [NPD(30nm)/CBP (9nm)/5wt.-%Ir(ppy)3:CBP (20nm)/CBP (9nm)/BCP (40nm)] fabricated at the same time. Here, Device IV omits the BCzVBi doping at the edges of the emission layer, but is otherwise identical to Device III. If charge trapping and significant triplet losses on the blue dopant do in fact occur, then the electrophosphorescent emission should be stronger in Device IV than in Device III.

However, the unnormalized EL spectra of these two devices show that Ir(ppy)3 emission intensity decreases, instead of increases, for Device IV when the EML is not doped with BCzVBi, as shown in Fig. 2. The external quantum efficiency (EQE) is 5.8% for Device III, and 2.8% for Device IV. The lower EQE of Device IV is arises since excitons form in the NPD layer (broad NPD emission is observed at a wavelength of 430nm in Device IV), and hence this portion of triplets cannot diffuse into the CBP layer due to the intervening energy barrier. The doping of BCzVBi at the emission layer edges results in increased the Ir(ppy)3 emission while eliminating that from NPD, strongly suggesting that the presence of the blue dopant does not result in a loss in triplets.

5 Device III (BCzVBi /Irppy /BCzVBi) Device IV (CBP /Irppy /CBP)

u J=100mA/cm . 4 a (

3 d e z i

a 2 m r o n

- 1 n o N 0

400 500 600 700 800 Wavelength (nm)

Figure 2. Unnormalized electroluminescence (EL) spectra of Device III and IV.

One further consideration is that triplet excitons might be formed by direct trapping on the BCzVBi since it has a lower triplet energy than CBP. To conclusively demonstrate this mechanism does not occur with BCzVBi as the dopant, we fabricated a further device (Device V structure: NPD (30nm)/ 3wt.-% DCM2: CBP

(10nm)/ CBP (4nm)/3wt.-% Ir(ppy)3: CBP (20nm)/ CBP (4nm)/ 3wt.-% DCM2: CBP (10nm)/ BCP (40nm)) using the same device structure as in Device III, except that BCzVBi is replaced with the red dopant DCM2, which has a HOMO energy of 5.26eV.8 DCM2 is excited primarily by direct recombination at the dopant molecules resulting from both electron and hole trapping,9 instead of energy transfer from the host. In Device V, the Ir(ppy)3 emission at =511nm is negligible (see Fig. 3, below) since excitons are directly formed on, and triplets are then trapped on the DCM2, and cannot subsequently diffuse to the Ir(ppy)3-doped region. However, the unnormalized

EL spectra in Fig. 3 show that Ir(ppy)3 emission is >50 times that in Device III. That is, we can conclude that BCzVBi does not trap, and hence waste triplets as does

DCM2. Given the large difference in Ir(ppy)3 emission intensity from Device III and Device V, the origin of highly efficient phosphorescent emission in Device III and the WOLED can only be attributed to CBP triplet diffusion from the BCzVBi:CBP exciton formation region.

Device III 0.20 Device VI ) . u . a ( 0.15 L E

i 0.10 l a m r o

n 0.05 n U 0.00

300 400 500 600 700 800 Wavelength (nm)

Fig. 3: Comparsion of emission from Devices III and V. Note the lack of Ir(ppy)3 emission in Device V. No direct transfer can occur from DCM to Ir(ppy)3 in Device V due to their considerable spatial separation. * Department of Electrical Engineering, Princeton Institute for the Science and Technology of Materials (PRISM), Princeton University, Princeton, New Jersey 08544, USA † Department of Chemistry, University of Southern California, Los Angeles, California 90089, USA ‡ Currently on leave from Sanyo Electric Co., Ltd., Osaka Japan 573-8534 + Current address: Departments of Electrical Engineering and Computer Science, Physics, and Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109. email: [email protected]

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