Study on light outcoupling efficiency enhancement in OLEDs using microlenses fabricated with irregular 3D colloidal arrays

Abstract

Organic light emitting diodes (OLEDs) have rapidly developed over last decade due to many advantages including a low operating voltage, wide viewing angle, a fast response time, and flexibility for their potential applications in displays and solid state lighting field. However, The difference in refractive indices of several layers causes total internal reflection inside the OLEDs. Since the 80% of the emitted light are trapped and wave-guided in devices, OLEDs with conventional structure suffer from a low outcoupling efficiency. Thus, it is important to fabricate of OLED structure with enhanced outcoupling efficiency and investigate mechanisms that affect the light outcoupling characteristics for organic electroluminescent devices. Recently, for the benefit of high luminescence, microlens arrays for OLEDs have been studied in solid-state lighting field. However, microlens arrays have been generally fabricated with periodic structure leading to an optical interference effect, which causes that a spectrum of devices with microlens arrays is different from that of standard devices. In this thesis, in order to overcome the unexpected problem, templates for the fabrication of microlens arrays were prepared with irregular colloidal arrays which are bimodal mixture of polystyrene particles. It was shown that the irregular microlens arrays enhance external quantum efficiency and maintain the original spectrum of color.