Design and fabrication of flexible organic light-emitting diodes considering reliability and mechanical characteristics

Abstract

We investigated the optical properties of a dielectric-metal-dielectric (DMD) electrode in relation to the surface morphology of Ag. We intentionally modified the number of voids and the surface coverage of the Ag layer. Normal and oblique incident configurations for transmittance measurement were employed to establish the relations between the Ag morphology and optical transmittance of the DMD electrode. For the configuration of $MoO_3/Ag/MoO_3$, variation in the surface morphology of the Ag minimized the light scattering loss and the absorption of photons. This could lead to 18.6 % enhancement of the transmittance at $\lambda$ = 550 nm. The optical out-coupling was also enhanced in OLEDs, which was based on the MAM anode incorporating the continuous Ag film. The multi-layer electrode ($ZnS/Ag/MoO_3$) was also optimized by investigating the formation of a continuous Ag thin film according to the base layer. The aggregation of the Ag atom was strictly limited on the ZnS layer, which showed the best thermal stability for Ag. The thermally evaporated 7-nm-thick Ag film with surface coverage of 99.6 % was achieved on the ZnS layer. We fabricated the ZnS (25 nm)/Ag (7 nm)/$MoO_3$ (5 nm) (Z25A7M5) multi-layer electrode, optimized through the numerical calculation. The transmittance of 83 % at $\lambda$ = 550 nm and sheet resistance of 9.6 Ohm/sq were recorded from the Z25A7M5 electrode. These results were mainly attributed to the uniform film-like morphology of the Ag thin film. The flexible OLEDs, based on the Z25A7M5 anode also showed feasible I-V-L characteristics compared to those of ITO-based devices. Also, we demonstrated a high performance flexible multi-barrier containing a silica nanoparticle-embedded organic-inorganic hybrid (S-H) nanocomposite and $Al_2O_3$. The multi-barrier was prepared by low-temperature $Al_2O_3$ atomic layer deposition and with a spin-coated S-H nanocomposite. The moisture barrier properties were investigated with a water vapor transmission rate (WVTR), estimated by a Ca test at $30^\circ C$, 90% R.H.. Moisture diffusion was effectively suppressed by the sub-700 nm thick multi-barrier incorporating well-dispersed silica nanoparticles in the organic layer. A low WVTR of $1.14 \times 10^{-5}g/m^2$ day and average transmittance of 85.8% in the visible region were obtained for the multi-barrier. After bending under tensile stress mode, the moisture barrier property of the multi-barriers was retained. The multi-barrier was successfully applied to thin-film encapsulation of OLEDs. The thin-film encapsulated OLEDs showed practicable current-voltage-luminance (I-V-L) characteristics and stable real operation over 700 hours under ambient conditions. Lastly, the design and fabrication of flexible organic light-emitting diodes (FOLEDs) with the Z25A7M5 multi-layer electrode and a moisture barrier coating, which are developed in previous work are demonstrated, and their reliability and mechanical characteristics are assessed. Bending stress of the multi-layer structure is investigated based on nonlinear finite-element analysis (FEA). The neutral axis (NA) position can be strategically adjusted by the introduction of a buffer layer of UV-curable cycloaliphatic epoxy hybrid materials (hybrimer), synthesized via a sol-gel reaction. The optimized multi-layer structure, proposed as the result of FEA is validated by related experiments. Regarding the bending characteristics, the water vapor transmission rate (WVTR) of the hybrimer-coated moisture barrier is much lower than that of a non-coated sample. The encapsulated FOLEDs, which are coated by hybrimer achieve an almost identical performance to that of non-bending samples in spite of 30-days exposure to $30^\circ C$ and 90 % R.H. after a bending test with a radius of 1 cm. During this period, the occurrence of dark spots caused by moisture penetration is effectively suppressed. Collectively, these results suggest that the bending characteristics of hybrimer-coated samples are remarkably improved with the theoretical prediction of the NA position.