Study On Spectral Stability Of White Organic Light-emitting Diodes Based On Non-doped Emissive Layers

WOLEDs have become a new display product,due to its small-size,light-weight,high-efficiency,low-power,full-color,large area flexible display and green environmental protection.After nearly 30 years of development,the life time,efficiency and other performances can meet the market and commercial requirements.However,in the actual display and lighting applications,WOLED still has some disadvantages,such as color drift.Because of the waste of cost by doping technology,in this paper,the spectral stability mechanism of WOLED is systematically studied based on ultrathin non-doped phosphorescent emitting layers.The main contents in this paper are listed as follows.1.We insert an interlayer into the undoped emitting layer,and we can realize white light emission by adjusting energy transfer and exciton distribution.In this paper,the mixture of hole transport layer TCTA and electron transport layer tmpypb is introduced as the bipolar hybrid interlayer,and a high color stable white OLED is obtained.The wide and uniform exciton distribution in the bipolar hybrid compartment plays an important role in the spectral stability of the device.By changing the doping ratio of TCTA and Tm Py PB in the mixed interlayer,the spectral stability can be effectively controlled.When the doping ratio of Tm Py PB in the bipolar interlayer is more than 1:1,the spectral stable WOLED with ? CIE less than 0.01 is realized.2.Based on the 1:1 mixed interlayer structure of TCTA and Tm Py PB,we studied the influence of the thickness of ultra-thin light-emitting layer on the spectral stability of the device.Through different emitters orders: blue/green/orange(named B-G-O)and orange/green/blue(named O-G-B).We find that the thickness ratio of the EML at the anode and cathode side is proved to be the core factor on spectral stability.At the same time,green layer,located between blue layer and orange layer,acts an indispensable role in keeping the color stability by alleviating triplet–triplet annihilation and maintaining sequential energy transfer.3.We have studied the mechanism of spectral stability of WOLED.The results of transient photoluminescence lifetime test show that the energy transfer efficiency of firpic affects the change of color stability.The current measurement results of single electron and single hole devices show that the trapping effect of PO-01 at low voltage also plays an important role in the change of spectral shape.Therefore,in the three band undoped ultra-thin WOLED,we attribute the mechanism of spectral stability to the combined effect of hole capture and effective energy transfer at low voltage.This discovery provides a very meaningful theoretical support for the realization of high efficiency and high color stability WOLED.