Studies On White Organic Light-emitting Diodes With High Color Rendering Index

The white organic light-emitting diode (OLED) with high color rendering index (CRI) is an important category in OLEDs. It has drawm much attention in recent years. In this thesis, a series of studies have done on the white OLEDs with high CRI.We have fabricated the white OLED with high CRI using undoped technology. The white behaves a maximum luminance of 19096 cd/m2 and a current efficiency of 4.12 cd/A, as well the CIE-1931 xy coordinates shift slightly under biases from 8 to 12 V. A CRI of 90.2 and the corresponding CCT of 5804 K were obtained as well. Because these are no exciton blocking layer in the device, the efficiency is low. To enhance the efficiency, we add the exciton blocking layer into the device. A white OLED with a maximum brightness of 21010 cd/m2 and a current efficiency of 5.17 cd/A were observed, respectively. as well the CIE coordinates locate near the point (0.333, 0.333) under biases from 7 to 12 V. A CRI of 91.2 and the corresponding CCT of 6145 K were obtained.A type of white OLED using four exciplexes was fabricated. A very broad white EL band with half-width of about 270 nm was obtained by four emission bands that strongly overlap. No monomer emission is responsible for the EL. A CRI of 94.1, CIE coordinates of (x=0.33, y=0.35), and CCT of 5477 K were obtained at bias voltage of 10 V.Firstly, we investigated a type of cohost system. The cohost system can be used in many different red and green dopants. The maximum current efficiencies are increased by 54% and 104% for green and red devices comparing to the conventional Alq3 single host devices. Then we fabricated a white OLED with high CRI using the cohost system above. As a sesult, a maximum brightness of 27853 cd/m2 and a current efficiency of 9.58 cd/A were observed, respectively. The maximum efficiency is increased by 73.6% compared to the control device. The CIE coordinates shift from (0.3432, 0.3397) to (0.3243, 0.3218), the CCT varies from 5035 K to 5915 K and all the CRIs exceed 90 under the biases from 8 to 14 V.