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The development of thermally activated delayed fluorescent (TADF) emitters has led to the realization of 100% in internal quantum efficiencies (ηint) of organic light-emitting diodes (OLEDs). Realizing 100% in external quantum efficiencies (ηext), however, continues to be limited by only 20% in the light outcoupling efficiency (ηout). Outcoupling enhancement (ηout) typically employs structures that redirect substrate guided and organic waveguided modes, however, they often require complex and multiple fabrication steps that may induce colour shifting with increasing viewing angle. Conversely, structures that suppress the colour shifting have no effect on the Δηout. This work demonstrates a solution in using a simple, one-step Yb-doped femtosecond laser (IMRA America, Inc.) used to pattern inverted holes at the air/glass substrate interface of the OLED which are useful for simultaneously reducing spectral shifting and redirecting substrate guided modes for Δηout [1,2].
Compared to reference devices which utilize a planar glass substrate, a maximum Δηout of about 31% and 60% was achieved for patterned devices (with intrinsically high and low transmittance, respectively) having conically shaped holes of optimized periodicity of 7 μm and a depth of 10 μm. Additionally, where the EL spectra peak shifts by ~11 nm in reference devices between 0o to 70o viewing angle, all patterned devices demonstrate only a 3-5 nm shift. The Δηout and reduction in spectral shifting is attributed to the extraction of substrate guided modes into free space as a result of smaller ray contact angle made with the conical surface and suppression of interference effects due to light scattering, respectively [1,2].
This work is expected to further the attractiveness of OLED devices to the general lighting industry given high light extraction efficiency and maintenance of colour purity. Where femtosecond laser technology has primary applications in the medical (LASIK surgery) and industrial fields (lithography), their broader applicability to the field of organic opto-electronics has now been demonstrated.
[1] S. Lloyd, T. Tanigawa, H. Sakai and H. Murata, IEICE Trans. Electron. 102 (2), 180 (2019).
[2] S. Lloyd, K. Higashimine, T. Tanigawa and H. Murata, Jpn. J. Appl. Phys., 60 (SB), SBBG01 (2021).
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