| In recent years, although efficiency of organic light emitting diodes (OLED)has achived a great breakthrough, but light extraction efficiency of devices isstill very low, for large fraction of spontaneous emission is lost within the devicein forms of surface plasmon polariton (SPP) mode, waveguide mode andsubstrate mode, and as a result only about20%of the emitting light is extractedoutside the device. So many studies recently focused on improving theextraction efficiency of OLED. The periodic grating structures insub-wavelength scale have been utilized to increase light extraction efficiency ofOLED but this method requires complicated nanofabrication techniques, such aslithography or nanoimprint, bringing challenges in cost management. Besides,the Bragg condition must be satisfied if the periodic structures are used toimprove light extraction efficiency, therefore the enhanced efficiencies onlypresent at particular wavelengths and angles. In order to solve the aboveproblems, we propose a randomly patterned structure in sub-wavelength scale toenhance light extraction efficiency of OLED, Such a patterned structure can be prepared by a simple, easy and cost-effective method. Main studies in this paperare as follows:(1) Studies on fabrication technique of randomly patterned PEDOT:PSS film.First a mixture of the aqueous solution of PEDOT:PSS and polystyrene (PS)nanospheres with an optimized volume ratio of36:1is spin-coated onto ITOglass at the spin rate of3000rpm for60s. Next, the removal of PS nanospheresis performed by dipping the sample into THF with sonification for5min,followed by annealing at120°C for30min in the air ambient. As a result, arandom concave-convex structure of PEDOT:PSS is formed on ITO glass. Weinvestigate the surface morphologies by the step profiler, atomic forcemicroscopy (AFM) and scanning electron microscopy (SEM). The height of thefeatures in the patterned PEDOT:PSS is approximately30-60nm. The surfacefeatures after depositing a30nm film of NPB and a40nm film of Alq3onPEDOT:PSS film are also characterized by AFM. The concave-convex featuresare extended to Alq3layer, but the height decrease to10nm-40nm.(2) Studies on the optical properties of randomly patterned ITO/PEDOT:PSSand ITO/PEDOT:PSS/Alq3films. The reflection, transmission and absorptionspectra of the abovementioned patterned films were tested. The results indicatethat the reflection and transmission of the randomly embossed films are reducedand the absorptivity is improved comparing with the corresponding planar film.By the absorption and photoluminescence spectra of concave-convex film, it isdeduced that the improvement of light outcoupling efficiency could be due to the randomly embossed structure recovering some optical trapping modes likewaveguide mode and SPP mode.(3) The performance optimization of OLED based on NPB/Alq3byrandomly embossed sub-wavelength structure. The planar device of ITO/PEDOT:PSS/NPB/Alq3/LiF/Al is optimized firstly by adjusting spin rate andtime of PEDOT:PSS to control the thickness. As a result, when the spin rate andtime are4000rpm and60s, the devices has the highest efficiencies. Next, theembossed devices of ITO/patterned PEDOT:PSS/NPB/Alq3/LiF/Al is optimizedmainly from two aspects: the spin rate of PEDOT:PSS and the concentration ofPS nanospheres. PS nanospheres with differnt diameters of200nm,300nmand500nm are chosen in our experiments. Lots of experiments indicate thatwhen PS nanospheres with the diameter of200nm are used, the concentration ofPS nanospheres is1.5%and the spin rate of PEDOT:PSS is3000rpm, theperformance of the obtained device are optimal; when PS nanospheres with thediameter of300nm are used, the spin rate of3000rpm for PEDOT:PSS and theconcentration of3%for PS nanospheres are the optimal conditions; when PSnanospheres with the diameter of500nm are used, the spin rate of PEDOT:PSSis3000rpm and the concentration of PS nanospheres is2%, optimal device isobtained. Among them, the performances of device fabricated by PSnanospheres with the diameter of200nm are the best.The experimental results demonstrated that the introduction of embossedPEDOT:PSS into the typical NPB/Alq3device, not only making the driving voltage of the device to be lower than the corresponding planar structure, butalso making the current and power efficiencies are increased. For devicefabricated by PS nanospheres with the diameter of200nm the maximum currentefficiency is3.34cd/A with the enhancement factor of18.0%and the maximumpower efficiency is2.16lm/W with the enhancement factor of67.9%compared with the typical planar Alq3-based OLED (with noPEDOT:PSS layer). Two reasons are responsible for the improvment ofefficiency:①PEDOT:PSS as the hole injecting layer reduces the hole injectionbarrier and improve the hole transport property, so the driving voltage of thedevice is reduced;②The embossed structure could extract the optical modeslimited within devices. |
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