Analysis Of Light Out-coupling Efficiency In Polymer Light-emitting Devices

Because of the advantages such as ultra thin, light weight, low cost, low energyconsumption, active illuminating, wide viewing angle and fast response speed and soon, organic light emitting diodes(OLEDs) have a great potential application prospectin display and lighting area. It has been paid great attentions attention in both theacademic and industrial circles. Also it is expected to become the next generation offlat panel display and light source. Over the years, the luminous efficiency improvingis thought to be the key factor of improving the performance of OLEDs. There for, thedevelopment of new materials has been widely concerned and studied. On the otherhand, the device structure optimizing is another way to improve the deviceperformance. In recent years, there is more and more research work on light outputprocess of the OLEDs. How to reduce the total reflection effects in the OLEDs, andhow to improve optical coupling efficiency have become the key problems whichmust be solved.Generally, the traditional structure of OLEDs has been used in most occasion.But when it’s applied in thin film transistor(TFT) active driver display screen, there iscompetition between the display area and driving circuit of the light pixels. Thedriving circuits cover30%-50%light area of OLEDs. Applying the top emittingstructure of OLEDs can make up for traditional OLEDs’lower aperture opening ratio,and greatly improve the optical coupling efficiency.The light out-coupling efficiency of polymer light-emitting devices（PLED）wassimulated based on transfer matrix method by using MATLAB software. Theinfluence of emitting layer, PEDOT layer, ITO layer on the performance of thePLED’s out-coupling efficiency was studied. The influence of three different microshapes on glass surface such as hemispherical shape, conical shape are simulated byusing TRACEPRO software. The results show that the light out-coupling efficiency isthe largest and the uniformity of diodes light output is the best when the conical shapeon the glass surface is employed. To overcome the deficiency of the traditional OLED device structure, the lightout-coupling efficiency of top-emitting polymer light-emitting devices（TEPLED）based on MEH-PPV was simulated by applying the transfer matrix method andsoftware MATLAB. The influence of anode materials, cathode materials, cappinglayers and observing angle on the performance of the TEPLED’s out-couplingefficiency was studied. Compared with Ag film anode device, the one based ondistributed Bragg reflector（DBR）presented higher out-coupling efficiency andreached47.43%. To decrease the light absorption and reflection by sub-transparentmetal, transparent materials cathode was designed so that the light-coupling efficiencyraised about10%compared with Ag anode devices which reached67.11%. Theinfluence of DBR on the observe angles of total TEPLED was studied. The resultsshow that it has little effect on light-coupling efficiency of TEPLED by DBR whenthe observe angle changes in a narrow range(30°). It was bigger when the observeangle with more than30°. The research of OLED structures and the application ofDBR provided a theoretical basis for the optimization of out-coupling efficiency ofTEPLED.