Study On Optoelectronic Performance For Tandem OLEDs Based On Organic Charge Generation Layer Of C₆₀/ZnPc

A tandem organic light-emitting diode?tandem OLED?consists of two or more individual electroluminescent?EL?units in series via a charge generation layer?CGL?.Under an applied external bias,the electrons and holes are generated within the CGL and injected into each emission layer?EML?of individual EL units.The tandem OLEDs,with high luminance,high efficiency and long lifetime under low current density,have been extensively studied in recent years.Up to date,the achievement of performance improvement in both current efficiency and power efficiency for tandem OLEDs is still a challenge.To overcome these issues in tandem devices,fullerene?C60?/zinc phthalocyanine?Zn Pc?with photovoltaic properties was used as charge generation layer within tandem OLEDs,and the main research contents of this thesis are as follows:1.To obtain a CGL structure with an efficient charge generation,separation,and injection capacities,the optical and electrical properties for the bilayer structure of C₆₀/ZnPc were researched by fabricating a variety of thin films with different thicknesses and a series of non-light-emitting devices with inverted structures.It shows that the C₆₀/ZnPc possesses obvious absorption bands in the visible region,and its absorption in different bands mainly caused by the C60 and Zn Pc,respectively.The absorption level of C₆₀/ZnPc for three lights including red emitter of Ir?piq?2acac,green emitter of Ir?ppy?3,and blue emitter of Firpic is Ir?piq?2acac>Firpic>Ir?ppy?3.Moreover,since the huge barriers between the C₆₀/ZnPc and its adjacent charge transport layers,the CGL without charge injection layers exhibits rather weak charge generation,separation,and injection abilities in the non-light-emitting devices,whereas the charge generation unit?CGU?composed of the CGL and the appropriate charge injection layers has relative high charge generation,separation,and injection abilities.The optimized CGU structure in non-lightemitting devices is Li F?0.5 nm?/Al?2 nm?/C60?5 nm?/Zn Pc?5 nm?/Mo O3?3 nm?.2.In order to validate the above results and to further optimize the CGU,the green tandem OLEDs based on the C₆₀/ZnPc were constructed.It displays that the results obtained by using the non-light-emitting devices and green tandem OLEDs are almost identical.The C₆₀/ZnPc with weak charge generation abilities is unsuitable in the tandem OLEDs,only the CGU of Li F/Al/C₆₀/ZnPc/Mo O3 with adequate charge generation,separation,and injection abilities is suitable in the tandem devices.When the structure of the CGU is Li F?0.5 nm?/Al?1 nm?/C60?5 nm?/Zn Pc?5 nm?/Mo O3?3 nm?,the green tandem OLED possesses the best photoelectric performances.In addition,when the thin film of Al exceeds 2 nm within the CGU,two emission units in a tandem device do not emit light simultaneously.Furthermore,a remarkable enhancement in both current efficiencies and power efficiencies for red,green,and blue tandem OLEDs was achieved by employing the optimal CGU with the structure of Li F?0.5 nm?/Al?1 nm?/C60?5 nm?/Zn Pc?5 nm?/Mo O3?3 nm?,which effectively overcame the issues that the improvement in both the current efficiency and power efficiency is difficult in a tandem device.The current efficiencies for red,green,and blue tandem OLEDs are 3.3-,2.1-,and 2.9-fold than those of corresponding single-unit devices.Simultaneously,the power efficiencies of trichromatic colors tandem OLEDs were increased by 72.2%,13.6%,and 37.2%,respectively,when the luminance reached 1 000 cd×m-2.These excellent results main thanks to the fact that the C₆₀/ZnPc with the photovoltaic property generates additional charges within the CGU leads to the improvement in charge balance within the tandem OLEDs.3.The working mechanism of the tandem OLED based on the C₆₀/ZnPc with the photovoltaic property was explored for obtaining a highly efficient blue tandem device.It is indicated that the charge generation ability of the CGU could be enhanced by the photovoltaic property of C₆₀/ZnPc in tandem OLEDs.Under high luminance,a cycle composed of the CGU and emission layers in tandem OLEDs could convert some of the internal radiation into external radiation for improving performances.Simultaneously,the optimal blue tandem OLED exhibits a high external quantum efficiency of 42.9% at 100 cd·m-2 and its efficiency roll-off is only 3.5% at 100-1 000 cd·m-2.