SPIE Invited Talk 11:15 am, 6th August 2017. In this presentation we outline, how using a low refractive index electron transport layer can suppress evanescent loss and increase optical out-coupling. To cite this presentation please cite this publication: Salehi A, Ho S, Chen Y, Peng C, Yersin H, So F. Highly Efficient Organic Light‐Emitting Diode Using A Low Refractive Index Electron Transport Layer. Advanced Optical Materials. 2017 Jun 1;5(11).

1. Suppressing Evanescent Loss in
OLEDs Using Low Refractive Index ETL
Amin Salehi1, Szuheng Ho2, Ying Chen2, Cheng Peng2, Hartmut
Yersin3 and Franky So2
1Department of Physics, North Carolina State University, USA
2Department of Material Science and Engineering, North Carolina State University, USA
3Institut für Physikalische Chemie, Universität Regensburg, Germany
SPIE Invited Talk
6th August 2017

2. Acknowledgment
Prof. Franky So Szuheng Ho
Cheng Peng
Ying Chen
Prof. Hartmut Yersin
Universität Regensburg,
Germany
2

3. Outline
• Principles of refraction
• Light interaction in OLED: Dipole model
• Effect of ETL refractive index
• OLED device studied in this work
3
To cite this presentation please cite this publication:
Salehi A, Ho S, Chen Y, Peng C, Yersin H, So F. Highly Efficient Organic
Light‐Emitting Diode Using A Low Refractive Index Electron Transport
Layer. Advanced Optical Materials. 2017 Jun 1;5(11).

4. Principles of Refraction
|𝒌| =
𝟐𝝅𝒏 𝒎𝒆𝒅𝒊𝒖𝒎
𝝀
Yue, Qingyang, et al. Advances in
Materials Science and Engineering 2012
4
𝒌 𝒂𝒊𝒓 =
𝟐𝝅𝒏 𝒂𝒊𝒓
𝝀
𝒌 𝒈𝒍𝒂𝒔𝒔 =
𝟐𝝅𝒏 𝒈𝒍𝒂𝒔𝒔
𝝀
𝒌 𝒈𝒍𝒂𝒔𝒔 > 𝒌 𝒂𝒊𝒓
𝒌 𝟐 = 𝒌 𝒙
𝟐 + 𝒌 𝒛
𝟐
𝑩𝒐𝒖𝒏𝒅𝒂𝒓𝒕𝒚 𝒄𝒐𝒏𝒅𝒊𝒕𝒊𝒐𝒏: 𝒌 𝒙 𝒈𝒍𝒂𝒔𝒔
= 𝒌 𝒙 𝒂𝒊𝒓
= 𝒌 𝒙
• If 𝒌 𝒙 > 𝒌 𝒂𝒊𝒓 , Total Internal Reflection (TIR)
happens.

5. Dipole model
𝑅𝑒𝑔𝑖𝑜𝑛 1 𝐴𝑖𝑟 : 𝑘𝑖𝑛 <
2 𝜋 𝑛 𝑎𝑖𝑟
𝜆
∶ 𝑘𝑖𝑛 < 0.0119 𝑛𝑚−1
𝑅𝑒𝑔𝑖𝑜𝑛 2 𝑆𝑢𝑏𝑠𝑡𝑟𝑎𝑡𝑒 ∶
2 𝜋 𝑛 𝑎𝑖𝑟
𝜆
≤ 𝑘𝑖𝑛 <
2 𝜋 𝑛 𝑔𝑙𝑎𝑠𝑠
𝜆
: 0.0119 ≤ 𝑘𝑖𝑛 < 0.0179 𝑛𝑚−1
𝑅𝑒𝑔𝑖𝑜𝑛 3 𝑊𝑎𝑣𝑒𝑔𝑢𝑖𝑑𝑒 :
2 𝜋 𝑛 𝑔𝑙𝑎𝑠𝑠
𝜆
≤ 𝑘𝑖𝑛 <
2 𝜋 𝑛 𝐸𝑀𝐿
𝜆
∶ 0.0179 ≤ 𝑘𝑖𝑛 ≤ 0.0209 𝑛𝑚−1
𝑅𝑒𝑔𝑖𝑜𝑛 4 𝐸𝑣𝑎𝑛𝑒𝑠𝑐𝑒𝑛𝑡 : 𝑘𝑖𝑛 >
2 𝜋 𝑛 𝐸𝑀𝐿
𝜆
∶ 𝑘𝑖𝑛 > 0.0209 𝑛𝑚−1
5

6. Effect of ETL refractive index
𝑘𝑖𝑛_𝑆𝑃𝑃 =
2π
𝜆
(
𝜀 𝑐𝑎𝑡ℎ𝑜𝑑𝑒 . 𝜀 𝐸𝑇𝐿
𝜀 𝑐𝑎𝑡ℎ𝑜𝑑𝑒 + 𝜀 𝐸𝑇𝐿
)
1
2
1.5 1.6 1.7 1.8 1.9 2.0
30
35
40
45
50
55
60
65
Evanescently coupled
Coupled to substrate and air
RelativeModeContribution(%)
ETL refractive index (n)
Percentage of Radiation that
couples to evanescent and to
Air+Substrate:
6
• More extraction at the expense of less evanescent loss.

7. ETL thickness vs ETL refractive index
𝑑 𝐸𝑇𝐿
∝
𝜆 𝑜
4𝑛 𝐸𝑇𝐿
7
• Lower refractive index ETL requires larger thickness to achieve
maximum electroluminescence.

8. ETL thickness effect on SPP coupling
8
50 60 70 80 90
30
35
40
45
50
55
60
RelativeModeContribution(%) ETL thickness (nm)
Coupled to substrate and air
Evanescently coupled
Percentage of Radiation that
couples to evanescent and to
Air+Substrate:
• Thicker ETL results in less evanescent loss.

9. 9
Emitter used in this study
Emitter
Cul-iBuPyrPHOS
TADF green emitter:
• Solution process-able.
• No concentration quenching.
• S1/T1 = 2.9eV/2.8eV.
• PLQY: 70%.
• A maximum EQE of 14% is
expected assuming 20% light
extraction.

10. HTL and host materials used
10
Hole-transport material Host
PLEXCORE UT-314 PYD2
Electron-transport material

11. EmitterHole-transportmaterial
Cul-iBuPyrPHOSPLEXCOREUT-314
Electron-transportmaterial
T2TNBphen
Device using T2T/NBPhen
• Maximum EQE of 12% achieved. Close to the 14% expected
EQE based on 70% PLQY and 20% light outcoupling.
11
1 10 100 1000
0
5
10
15
20
25
30
ExternalQuantumEfficiency(%)
Luminance (cd m-2
)

12. -transportmaterial
Host
PLEXCORE
UT-314
PYD2
Electron-transportmaterial
NBphen
3TPYM
B
Device using 3TPYMB
• A maximum EQE of 21% was achieved using 3TPYMB.
Nearly 75% higher than the T2T/NBPhen device.
12
1 10 100 1000
0
5
10
15
20
25
30
ExternalQuantumEfficiency(%)
Luminance (cd m-2
)

13. Comparing two devices
13
0 1 2 3 4 5 6
10-5
10-4
10-3
10-2
10-1
100
101
Voltage (V)
CurrentDensity(mAcm-2
)
100
101
102
103
104
105
106
T2T/NBPhen
3TPYMB
Luminance(cdm-2
)
1 10 100 1000
0
5
10
15
20
25
30
T2T/NBPhen
3TPYMB
ExternalQuantumEfficiency(%)
Luminance (cd m-2
)

14. Refractive indices of ETLs
14
• 3TPYMB has the lowest refractive index.

15. Optical Mode Profile
15
T2T/NBPhen 3TPYMB
Waveguide~5%

16. Low waveguide loss
• Solution-processed EML has a low refractive index ~1.6.
16
𝟐 𝝅 𝒏 𝑬𝑴𝑳
𝝀
𝟐 𝝅 𝒏 𝒈𝒍𝒂𝒔𝒔
𝝀
Waveguide

17. 3TPYMB: Lowest refractive index
*HOD: Horizontally Oriented
Dipoles
**PLQY: Photoluminescence
Quantum Yield
17

18. Conclusion
• Low refractive index ETL increases outcoupling and
decreases evanescent loss:
1. By shifting radiation toward the outcoupled region.
2. By requiring a larger ETL thickness.
• 3TPYMB has the lowest refractive index among
common ETLs and leads to significant enhancement.
18

19. Thank you
19
To cite this presentation please cite this publication:
Salehi A, Ho S, Chen Y, Peng C, Yersin H, So F. Highly Efficient Organic
Light‐Emitting Diode Using A Low Refractive Index Electron Transport
Layer. Advanced Optical Materials. 2017 Jun 1;5(11).