Reliability Study of a Fluorescent Blue Organic Light-Emitting Device

Abstract

In this paper, we measured and analyzed the operation lifetime of a high efficiency blue OLED which consists of N,N' –Vdiphenyl -N,N'-bis(1-napthyl) -1,1'-biphenyl-4,4'- diamine (NPB) as the hole-transport layer (HTL), 4,4'-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi) doped in 9,10-bis(2';-naphthyl) anthracene (ADN) as the emitting layer (EML), and bis(10-hydroxyben-zo[h]quinolinato)beryllium (Bebq2) as the electron-transport layer (ETL). Due to the high electron mobility of the ETL (one order of magnitude higher than Alq3), the carrier balance is achieved and a blue OLED with a high external quantum efficiency of 8.32% is obtained. The device structure of our blue OLED device is ITO /HTL (40nm)/EML (45nm, 4% dopant)/ETL (15nm)/ LiF(1.2nm)/Al (100nm). In our operation lifetime measurement, we fixed the initial luminescence of the blue OLEDs at 12500, 10000, 7000, 5000 cd/m² with a constant current driving. The resulting half-lifetime are 5.58, 16.56, 27, 109.819 hours, respectively. To estimate the half-lifetime of this device, we use a well-known relation in our fitting: L*t1/2n= constant where n is the acceleration coefficient, and t1/2 is the half-lifetime. In our blue OLED, the n value is 3.088. By using the equation, we can calculate that the estimated half lifetime at an initial luminance of 1000 cd/m² achieves 15611 hours in our device. For further investigating the lifetime mechanism in our blue OLED, we fit all the luminance versus time curves obtained under different driving condition. We found that luminance is inversely proportional to the square of the time, rather than a typically stretched exponential decay which means the luminance decay is a second-order reaction in our blue OLED.