In this paper, We have demonstrated a blue fluorescent organic light-emitting device (OLED) with a current efficiency
of 19.2 cd/A at 100 cd/m2, an estimated half-lifetime of 15611 hours at an initial luminance of 1000 cd/m2, and a voltage
of 4.9 V at 20 mA/cm2 with a high electron mobility electron transport layer (ETL) material and high efficiency dopant
material. The external quantum efficiency (EQE) in this optimized OLED is 8.32%, which is very close to the
theoretical limit. Carrier balance condition is achieved due to the incorporation of the high mobility ETL, bis(10-
hydroxyben-zo[h]quinolinato)beryllium (Bebq2), which can not only effectively increase the current efficiency and
elongate the operation lifetime, but also reduce the driving voltage and increase the power efficiency. The EML
consisted of 4,4'-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi) as the blue dopant and 9,10-bis(2'-
naphthyl) anthracene (ADN) as the matrix. We found that the dopant concentration of DPAVBi did not affect the
mobility value of the EML which is consistent with the J-V characteristics. Besides, although it is believed the bulk
ADN is a kind of HTL materials, we found the electron mobility of ADN is one order of magnitude higher than its hole
mobility in our blue OLEDs.
In this paper, we demonstrated methods for determining the recombination zone in a mixed-host (MH) organic light-emitting device (OLED). The host of the emitting layer material in this device consists of a hole transport layer and an electron transport layer fabricated by co-evaporation. By comparing the spectra shift between bilayer and MH OLEDs, the recombination position with different mixing concentration can be determined. It showed the recombination zone shifts from the anode to the cathode side with increasing NPB mixing ratio.
In this paper, we report a high efficiency organic light-emitting device (OLED) with a high electron mobility electron transport layer (ETL) material and high efficiency blue dopant material. Typically, the mobility of a hole transport layer (HTL) material is much higher than that of an electron transport layer (ETL) material. Here we used the bis(10-hydroxyben-zo[h]quinolinato)beryllium (Bebq2) as the ETL material that exhibits superior electron mobility. It effectively reduced the driving voltage and increased the power efficiency. The blue dopant material was 4,4'-bis[2-(4-
(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi) in the 9,10-bis(2’-naphthyl) anthracene (BNA), that was the host material of the emitting layer (EML). At 100 cd/m2, the current efficiencies with different dopant concentration can be as high as 19.2 cd/A with the CIE coordinate at (0.154, 0.238). Driving voltage at 20mA/cm2 was 4.94 V of this device. With increasing the doping concentration, the drive voltage variation did not vary much and was within 0.6 V at 20 mA/cm2. The emitting mechanism in such a device may be mainly due to energy transfer rather than carrier trapping. CIE coordinate of such a device shifted toward blue with increasing current density due to intense light emission from the host material of the EML. The highest efficiency was achieved when doping concentration is 3%.
In this paper, we present the device performance of N4,N4'-Di-naphthalen-2-yl- N4, N4'-di-naphthalen-1-yl-biphenyl-4,4'-diamine (TNB) as the HTL material and bis(10-hydroxyben-zo[h]quinolinato) beryllium (Bebq2) as the ETL material. The mobility of TNB and Bebq2 is at the same order of magnitude from our time of flight (TOF) measurement. Therefore, a device with more balanced carrier transport leads to better device performance. At 10 mA/cm2, the drive voltage of the devices is as low as 3.16 V since the use of the high mobility ETL, Bebq2. The voltage variation when changing HTL thickness is nearly the same as that when changing ETL thickness. That shows the voltage drop is higher on HTL than ETL due to the use of high mobility ETL material. That also leads to the more balanced carrier transport than that in a conventional OLED. We also observed that a thinner device has longer operation lifetime that may be due to fewer traps in such a device.
In this paper, we study device performance and carrier dynamics of an organic light-emitting device (OLED) with an emitting layer (EML) based on a mixed-host (MH) structure. Such a structure is composed of two different host and one dopant materials. It exhibits longer operation lifetime as compared with a conventional heterojunction (HJ) device. In such a MH layer structure, carrier transport characteristic is modified and emission zone position is changed. Energy transfer from the two hosts to one dopant is studied by EL, PL and TRPL measurements. We observe spectrum shift from the EL measurement under different current injection. Incompletely energy transfer from NPB to DPAVBi is shown in cw PL measurement. Time constant at different probe wavelengths with different mixing concentration suggests that different energy transfer in such a MH structure.
In this paper, we conducted photoluminescence (PL) and time-resolved photoluminescence (TRPL) measurements for the organic films that was composed of tris(8-hydroxyquinoline) aluminum (Alq3) as the host and 10-(2-benzothiazolyl)-1, 1, 7, 7-tetramethyl-2, 3, 6, 7-tetrahydro-1H, 5H, 11H, [l] benzo-pyrano [6,7,8-ij] quinolizin-11-one
(C545T) as the green dopant with different concentration. Typical quench behavior was observed by typical PL measurements when doping concentration exceeds 2% and the carrier lifetime decreased monotonically with increasing doping concentration in TRPL measurements. Time constant for energy transfer from host to dopant decreased with increasing dopant concentration and saturated above 2% dopant concentration. An anomalous blue shift at the initial probe time-interval was observed when dopant concentration was over 4%. It indicated a fast event energy absorption and/or relaxation process which had a time constant less than two picosecond. Two physical mechanisms with different time constant was observed those accounted for the concentration quench behaviors in the Alq3/C545T system.