Optical, morphological, structural, electrical, molecular orientation, and electroluminescence characteristics of organic semiconductor films prepared at various deposition rates (original) (raw)
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Chemical Physics Letters, 2009
The authors report a systematic study on the influence of the film deposition rate on the performance of organic light-emitting diodes (OLEDs) based on planar bis(10-hydroxybenzo[h]qinolinato) beryllium. The lower the deposition rate is, the lower the electroluminescence efficiency is. The observation is attributed to easier formation of ordered Bebq 2 aggregates for slower deposition rates, which result in higher electron mobility and lower photoluminescence (PL) efficiency. The faster degradation in both PL efficiency and electron injection accounts for the decreased device lifetime with the decrease in the deposition rate. The role of molecular packing in device performance is discussed.
International Journal of Photoenergy, 2014
We demonstrated a fabrication technique to reduce the driving voltage, increase the current efficiency, and extend the operating lifetime of an organic light-emitting diode (OLED) by simply controlling the deposition rate of bis(10-hydroxybenzo[h]qinolinato) beryllium (Bebq 2 ) used as the emitting layer and the electron-transport layer. In our optimized device, 55 nm of Bebq 2 was first deposited at a faster deposition rate of 1.3 nm/s, followed by the deposition of a thin Bebq 2 (5 nm) layer at a slower rate of 0.03 nm/s. The Bebq 2 layer with the faster deposition rate exhibited higher photoluminescence efficiency and was suitable for use in light emission. The thin Bebq 2 layer with the slower deposition rate was used to modify the interface between the Bebq 2 and cathode and hence improve the injection efficiency and lower the driving voltage. The operating lifetime of such a two-step deposition OLED was 1.92 and 4.6 times longer than that of devices with a single deposition rate, that is, 1.3 and 0.03 nm/s cases, respectively.
High performance organic polymer light-emitting heterostructure devices
Applied Physics Letters, 1999
We report a high performance electroluminescence device based on bi-layer conjugated polymer structures consisting of a hole transporting ͑amine-fluorene͒ and an emissive ͑benzothiadiazole-fluorene͒ polymer layers prepared by the spin-coating technique on the glass substrate. Devices showed green emission with an electroluminescence peak located at around 545 nm and a full width at half maximum of about 80 nm. Our devices have also shown a high brightness (ϳ10 000 cd/m 2 at 0.84 mA/mm 2 ), good emission efficiency (ϳ14.5 cd/A͒ and luminous efficiency ͑2.26 lm/W͒, a large external quantum efficiency ͑3.8%͒, and a reasonable forward-to-reverse bias current rectification ratio (Ͼ10 3 at Ϯ25 V͒.
Efficient multilayer organic light emitting diode
Synthetic Metals, 2001
Some organic light emitting diodes with Alq 3 as emission layer, multilayer composed of m-MTDATA, NPB, TPD or SA as hole transport layer and lithium doped Alq 3 as electron injection assistant layer have been investigated. The current voltage characteristics and the electroluminescent (EL) output voltage characteristics have been investigated. It is found that the m-MTDATA is suitable as a hole injection layer close to the hole injection electrode ITO, NPB is suitable in contact with the emission layer, SA and TPD can be inserted between them to form an energy ladder structure, with which the ef®ciency of device has been increased. When thin lithium doped Alq 3 layer is inserted between Alq 3 and the aluminium electrode, the driving voltage of the devices has been clearly decreased while the current density and the EL output increased.
Improving the performance of organic light-emitting diodes
SPIE Newsroom, 2008
Organic light-emitting diodes (OLEDs) are promising candidates for large-area full-color flat panel displays due to their ease of fabrication and convenience for many applications. 1 OLEDs work through the passage of an electric current across a fluorescent or phosphorescent organic layer resulting in an excitation/emission profile of the material used. With OLEDs, the injection efficiency of electrons is a critical parameter and depends to a great extent on the work function (the minimum energy needed to move an electron out of a substance) of the electrode. A thin hole-injection layer (HIL) or an anode buffer layer (ABL) between the indium tin oxide (ITO) anode and the organic emitting layer are usually adopted to enhance the performance of the hole-injection process. 2-6 Thus, current electroluminescent devices typically have the following layered configuration: ITO anode/HIL or ABL/organic emitting layer/tris(8-hydroxyquinoline) aluminum (Alq 3 )/lithium fluoride (LiF)/aluminum cathode. Our recent work suggests that either an HIL composed of metal phthalocyanine (MPc) or an ABL of Li-doped zinc oxide (LZO) should improve the holeinjection efficiency. The organic, inorganic, and Al layers of our test device were successively deposited using vacuum vapor evaporation at room temperature. The LZO powders with a doped concentration of 5% Li were prepared by sintering a mixture of ZnO and Li 2 CO 3 powders in air. Various MPc layers were tested for their effect on injection efficiency (see ). The turn-on voltage of the devices decreases from 5.3V to 4.3V when CoPc or CuPc layers are inserted: see (b). Compared to the non-MPc device, higher emission efficiency was observed in all MPc devices. The CuPc device achieved the highest efficiency as shown in . For the same emission intensity, the higher efficiency suggests that a much lower current density is required. Figure 1.
Novel approach for deposition of thin films from low molecular weight compounds by pulverization is presented. The method was supplied for preparation of flexible organic light emitting device with tris(8-quinolinolato)-aluminum (Alq3) emissive layer. Additional film of N-N′-diphenyl-N-N′-bis (1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) was also spray deposited as a hole transporting layer (HTL) to increase the injection efficiency of the organic electroluminescent structure. Suitable substrate temperature was set to avoid dissolving and damage of both layers, caused by solvent penetration from NPB in Alq3. After optimization of the deposition conditions and because of the energy level alignment with introduction of NPB, it was measured reduction of the turn-on voltage with approximately 2 V. Current-voltage characteristics show 6 mA higher current at given voltage for the structure with HTL and the brightness-voltage characteristics show that the emission intensity is 300 cd/m2 hi...
Structural Analysis of Enhanced Performance Organic Light Emitting Diodes (OLEDs
International Journal of Computer Networks and Communications Security, 2020
We present a detailed study on structure of Organic LEDs (OLEDs) that promise flexibility and enhanced performance. Ordinary LEDs fail when it comes to need of ultra-smart size, thin, flexible smart screens and high efficiency light sources. With electroluminescent layer made of organic compounds, OLEDs promise all such features. We did a comprehensive analysis to find what structural features distinguish OLEDs from semiconductor LEDs. We found that it is the special six layered structure with organic emissive layer and delocalized charges due to weak pi bonds that enable OLEDs to perform better. We discuss a few limitations related to production and life of these LEDs and suggest possible solutions to overcome these challenges. A rigorous, in-depth analysis of this structure is imperative to further comprehend the working of this device in order to make future devices cheaper and more efficient.