| Organic light-emitting devices (OLEDs) has drawn much attention dueto their advantages such as self-luminescence, fast response, low drivingvoltage, wide viewing angle, the all-solid-state structure. They havebroad applications in both flat panel display and solid-state lighting.In recent years, although rapid developments of OLEDs have been achieved,high efficiency is still a key issue for their applicatins. Especiallythe light extraction efficiency is too low to be further resolved. In thisthesis, laser-induced one-dimensional (1-D) and two-dimensional (2-D)grating have been introduced into the OLEDs in order to recover the powerlost to both surface plasmon and waveguide modes trapped in the OLEDs,and improve the light extraction efficiency of OLED.(1) Employ two-beam interference ablation to introduce1-D and2-Dgrating onto the functional layers of the OLEDs. PVK is chosen as ahole-transport material and it is spin-coated on the ITO glass for thefabrication of the corrugated HTL in OLEDs. Then, the sample was exposedby two beams which were split from the UV laser to fabricate1-D grating.After that, rotate the1-D grating60degree around its plane normal andexposed again for2-D grating. The atom force microscopy (AFM) images ofa laser ablated grating on the PVK film and the cathode surfaces afterdeposition of organic and metallic cathode layers by thermal evaporationfor further research were taken. As can be seen, the profile of the topsurface was essentially a replication of the underlying nanostructure,therefore, forming a periodic corrugation throughout all the layers onthe ITO substrate. The microstructure on the PVK film in a large scale is investigated by scanning electron microscope (SEM) and showshomogeneous profile. Due to the scattering and diffraction of the grating,the structured surface shows varied colors from structures of differentperiods in digital camera photos.(2) Experimentally, the groove depth is fully determined by the laserfluence at low laser energy density (<270mJ/cm~2). When the laser fluenceis higher, the linear dependence of the ablation depth on the logarithmof the fluence is deviated, and is gradually saturated to55nm.(3) Fabricate1-D corrugated green OLEDs. In case of350nm, theluminance at the current density of100mA/cm~2is5100cd/m~2and the maximumcurrent efficiency is7.2cd/A, respectively, while it is910cd/m~2, and1.8cd/A, respectively, for the planar devices. This corresponds to3times and4.6times enhancement in the maximum current efficiency andluminance, respectively. From the emission spectra at differentobservation angle, the EL spectra from the corrugated devices are morecomplex than that from the flat devices. For the devices with350nmgrating period, peaks at around520,547and595nm are observed at normaldirection and each of them splits into two peaks, which shift in wavelengthwith the increasing observation angle.(4) To establish the optical modes supported by the microstructuredOLEDs, absorption spectra are simulated. Applying the scattered matrixapproach to deal with our periodically microstructured OLED with metaland organic multilayered structure and there is excellent agreementbetween the theoretical calculation and experimentally measurement. Thefield intensity is with maximum at the Al/Alq interface and decay alongthe direction perpendicular to it at the wavelength of595nm. Concludefrom this field distribution that the emission peak at595nm originatesfrom surface plasmon-polariton (SPP) modes, since SPPs are surface waveand propagating along an interface between a metal and dielectric. On the other hand, the field at the wavelength of547and520nm is mainly confinedwithin the ITO and organic layers, which should be assigned to the TM andTE polarized waveguide (WG) modes due to the high refractive index of ITOand organic layers. So confirm that both SPP and WG modes could be coupledout by the microstructure. From the EL spectra at normal direction fromthe devices with different grating periods, it has been found that whenthe additional peaks evenly distributed on both sides of the planar OLEDpeak, the device has the highest luminance and current efficiency.(5) Because of higher efficiency in coupling SPPs to far-filedradiation of2-D grating,2-D grating OLED was fabricated. Red OLEDs with350nm and400nm of grating period have been fabricated and tested. The2-D corrugated OLEDs with350nm period show higher performance comparedto that of the device with400nm period. When the grating period is350nm, in case of2-D grating, the luminance at the current density of100mA/cm~2is5000cd/m~2and the maximum current efficiency is7.3cd/A,respectively, while it is3000cd/m~2, and5.1cd/A, respectively, forthe1-D corrugated devices. This corresponds to67%and43%enhancementin the maximum current efficiency and luminance, respectively. In caseof TM polarization, it was found the EL peak corresponding to the SPP modeat normal direction splits into4peaks for2-D corrugated device. Whilein case of TE polarization, there are2additional peaks for SPP mode.Applying the FDTD approach to deal with our periodically microstructuredOLED. It was found that surface plasmon and waveguide modes can be coupledout in only one direction for the1-D corrugation, while the surfaceplasmon and waveguide modes in all directions can be coupled out for the2-D corrugation. This results in the improvement of the couplingefficiency, and further increase the light extraction efficiency of theOLEDs.In summary, a periodic wavelength-scale corrugation has been introduced into OLEDs by one-step laser ablation of two interference beams,and enhanced outcoupling of radiation has been demonstrated. Theintroduction of this microstructure has allowed the observation of theemission originating from the SPP and WG modes, which are usually trappedwithin planar devices. An enhanced EL efficiency from the corrugated OLEDshas been observed. The microstructure has been directly formed on the HTLof the OLEDs by laser ablation without degradation of the deviceperformance. While combining with the two-beam interference, both theperiod and the groove depth of the grating can be easily and preciselycontrolled, so that it is applicable to OLEDs with different emissionwavelength. With beam expansion, an entire wafer could be structured withone-shot nanosecond laser ablation, implying a broad industrialapplication prospect of the one-step efficient large-area approaches forOLEDs with much enhanced light outcoupling efficiency.
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