Study On Luminous Mechanism Of The White Organic Light-emitting Diodes Based On Exciplex

In recent years,Organic light-emitting diode has developed the characteristics of self-luminescence,large color gamut,wide viewing angle,ultra-thin and ultra-light,etc.These advantages make OLED be applied in the fields of display and lighting,including TV monitors,mobile phone screens and automotive lighting,etc.In addition,the exciplex is a new level structure generated by the interaction between different molecules.The exciplex emission spectrum is different from the original molecule spectrum,and its energy gap is lower than the original molecular energy gap.Therefore,it can reduce the driving voltage and simplify the device structure in OLED.Moreover,it can also replace phosphorescence to design white OLEDs.However,the exciplex will affect the exciton distribution,energy transfer,carrier balance and other mechanisms in the white OLED with multiple emission layers,and change the color rendering index and spectral stability of the device,which is also a problem in application for white OLED.Therefore,studying the emission mechanism of exciplex devices with high color rendering index and high spectral stability is particularly important for the development of white OLEDs.(1).The carrier balance and exciton recombination had been studied in exciplex OLED.we confirmed that 35%Liq-doped B3PYMPM can effectively improve electron transport and carrier balance in single carrier OLED.in the meantime,it was testified that the TCTA:B3PYMPM can broaden the exciton recombination area in exciplex OLED,which reduced the recombination rate of electron-hole pairs and increased the excited state of the exciplex,avoiding the concentration quenching of the TCTA/B3PYMPM interface,ultimately enhancing the device brightness and reducing the efficiency roll-off.In the end,it was determined that the doping ratio of TCTA to B3PYMPM was 3:7,which was beneficial to improve the carrier balance and exciton conversion efficiency in device.(2).The luminescence mechanism of Ir(piq)2(acac)exciton generation interface at different positions had been studied in exciplex red OLED.When the Ir(piq)2(acac)exciton interface was located on the cathode and anode sides,the carrier balance of Ir(piq)2(acac)emission region and hole trap at Ir(piq)2(acac)occupied respectively dominance.When the interface was located inside TCTA:B3PYMPM,Ir(piq)2(acac)was at the exciton recombination center of exciplex,which can enhance the energy transfer from the exciplex to Ir(piq)2(acac),ultimately improving device efficiency.In addition,the red-shift of exciplex was more obvious when Ir(piq)2(acac)was closer to the cathode,which was because the hole trap effect of Ir(piq)2(acac)gradually decreases from anode to cathode,resulting in the hole concentration gradually increased in the exciplex emission region,which increased the Coulomb potential energy of electron-hole pairs,and ultimately reduced the emission energy of exciplex.(3).The energy transfer,carrier balance,exciton drift and exciton recombination mechanism had been studied in exciplex white OLED.First,the influence of the thickness of blue-exciplex spacer TCTA on the blue energy transfer and exciton recombination was analyzed in three-color device.It was found that when the TCTA thickness was small,which was weakly influenced on the energy transfer between the blue-exciplex and the electron injection of the blue region,but it was easy to separate the excitons in the blue and exciplex emission region,resulting in the improved spectral stability.When the thickness was large,the energy transfer and electron injection was blocked,and the blocked electrons will gather in the exciplex emission region,increasing the Coulomb potential energy of electron-hole pair,leading to the red-shift of exciplex and the increase of color rendering index in device,but the spectral stability decreased.Secondly,the effects of carrier balance and emitting layer sequence on the luminescent performance were analyzed in the four-color device.It was found that the electron buffer zone can reduce the electron injection concentration on the cathode side,improving the carrier balance of the phosphorescent emission region and exciton recombination efficiency.When the emitting layer sequence was blue-red-exciplex-orange from anode to cathode,which made excitons drift to the blue region under high voltage,and the red emission received energy transfer from the blue and exciplex.At the same time,the wide TCTA:B3PYMPM recombination region between red and organge will enhance exciton recombination,ultimately improving the luminous efficiency,spectral stability and color rendering index.The power efficiency was 23.81 lm/W and color rendering index CRI reached 82 in the optimized device,simultaneously the color coordinate drift value was only(0.009,0.006)under the brightness change of 1000?10000 cd/m~2.