Research On The Performance Enhancement Of Organic Light-emitting Devices Based On PDMS Light Extraction Films Prepared By Template Transfer Process

Organic light-emitting devices（OLEDs）with the advantages of ultra-thin,flexible,self-luminescence,and low turn-on voltage,are widely used in flat panel displays,lighting,notebook computers,AR devices,VR devices,and new energy vehicles.With the developments of organic materials,the theoretical internal quantum efficiency od OLEDs has reached 100%.However,the photons generated inside OLEDs suffer from various non-radiative modes during the transmission from the emission layer to the outside of devices,such as waveguide mode,surface plasma polarization mode,and substrate mode,which leads to the lowered external quantum efficiency of OLEDsIn OLEDs,photons are confined in the device by total reflection at the interface between glass substrate and air,which is the substrate mode.In this thesis,a method for preparing PDMS light extraction thin films based on a template transfer process is proposed.By using a two-time template transfer process,the light extraction film successfully replicates the periodic micro/nano-structure of monolayer densely stacked polystyrene（PS）nanospheres.When the prepared PDMS light extraction film is applied on the back side of the glass substrate outside OLEDs,the luminance and current efficiency are significantly improved.Theoretically simulated distribution of electromagnetic field demonstrates that the PDMS light extraction film can effectively recover and extract photons trapped in substrate mode.The work in this thesis mainly includes the following aspects:1.Proposed a method for fabricating PDMS light extraction films based on a template-transfer process:The monodisperse PS nanosphere dispersion is added dropwise in deionized water to form a monolayer dense stacked nanosphere structure with the presence of surfactant.The template transfer method is applied twice to transfer the micro/nano-structure of the monolayer dense stacked PS nanosphere to PDMS light extraction film.The surface morphology of the PDMS films is characterized by scanning electron microscopy,which demonstrates that the prepared PDMS light extraction films well replicate the periodic micro/nano-structure of the monolayer densely stacked nanospheres.2.Preparation of OLEDs devices by physical vapor deposition and optimization of the device structure:We prepared OLEDs devices with the following structure:glass substrate/ITO/Mo O₃/NPB/Ir（Bt）₂acac:CBP/TPBi/Ca/Ag.The functional layers and electrodes were deposited on the glass substrate using physical vapor deposition method.The concentration of Ir（Bt）₂acac doped with CBP,acting as the emission layer of the OLEDs,has been tuned,and a doping concentration of 10 wt%is finally determined as the optimal doping ratio.The luminance and efficiency of the prepared devices reached the best value.3.The prepared PDMS light extraction films were applied to the OLEDs and the device performance was characterized:The PDMS light extraction films has been applied to the backside of the glass substrates in OLEDs.Compared with the reference OLEDs without the light extraction film,the maximum brightness of the OLEDs device is increased from 43580 cd/m² to49840 cd/m²,and the maximum current efficiency of the OLEDs device is increased from 28.06 cd/A to 37.43 cd/A,realizing an improvement of 33.39%after the introduction of the PDMS light extraction film.Since the PDMS light extraction film is applied outside the device,there is no effect on the current-voltage characteristics of the OLEDs device as well as the electroluminescence spectra.Based on the experimental results and theoretical simulation,it can be confirmed that the proposed PDMS light extraction film can effectively extract the confined photons from the glass substrate,thus improving the luminance and efficiency of OLEDs device.