Quantum-dot Light-emitting Diodes Based On The Charge-generation Layer And Therir Working Mechanism

Quantum-dot light-emitting diode(QLED)has excellent characteristics,such as wide color gamut,low fabrication cost,flexibility and self-luminescence,which can meet people's needs for high-quality terminal displays.The performance of QLED has improved significantly since the 1990s.The external quantum efficiency(EQE)of the red,green and blue devices has been improved from less than 0.01%to 21%,the maximum brightness has reached 357000,614000 and 88900 cd/m~2 respectively,and the working lifetime under 100 cd/m~2 has reached 1800000,1760000 and 15850 h.Nonetheless,the comprehensive performance of QLED can't be satisfied by the commercial demand at present.Generally speaking,there are two ways to improve QLED performance:(1)design functional layer materials with excellent performance;(2)optimize the device structure.In this paper,relevant researches are carried out on the optimization of device structure.The charge generaton layer(CGL)was used to construct conventional device,tandem device and alternative-current(AC)device,and the carriers distribution and the working mechanism of the devices are explored by analying the photoelectric properties of the devices.First,we investigated the electrical and optical properties of the CGL.Experiments show that the CGL composed by poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate)(PEDOT:PSS)and Zn O has good charge generation ability and optical transmittance.The charge generation rate is as high as 100%,and the transmittance of the CGL film in the visible light range is higher than 95%.Afterwards,it was applied in conventional QLED device,which replaced the electron transport layer(ETL)in conventional device.Compared with the ETL-based device,the performance of the CGL-based device is effectively improved,and its maximum current efficiency is increased by 9.4%,which is attributed to the more balanced carriers in the CGL-based device.We further analyzed the carrier distribution in the device with transient EL(Tr EL)and found that the CGL-based device responds faster to voltage.The fast responsiveness of the device facilitates the application of QLEDs to visible light communication devices.The results confirm that the CGL composed by PEDOT:PSS and Zn O can effctively generate holes and electrons,and is feasibility to replace the charge injection layer of QLED.The second,to further boosting the performance of the device,we constructed the tandem device with the CGL as the interconnecting layer,which shows high transmittance and high charge generation We constructed the tandem QLED structure with two light-emitting units,and the structure is consisting of ITO/Zn O/QDs/Poly(9-vinylcarbazole)(PVK)/PEDOT:PSS/Zn O/QDs/4,4'-bis(9-carbazolyl)-2,2'-biphenyl(CBP)/Mo Ox/Al.We investigated the photoelectric properties of single and tandem devices,and characterized films properties.The experimental results show that the performance of the device composed by bottom light-emitting unit is poor,which is due to the functional layers may be damaged in the bottom device made by all-solution-process.The experimental results that the PVK film may be damaged by the PEDOT:PSS rinsing,resulting in poor hole injection and the carrier imbalance in the device;at the same time,part of PEDOT:PSS will contact with QDs,which will cause quenching of QDs and reduce the luminous efficiency of QDs.Compared with single devices,the performance of the device with the tandem structure has been effectively improved.Compared with the devices composed by the bottom light-emitting unit and the top light-emitting unit,the maximum current efficiency of the tandem device is increased by 180%and 35%,respectively.Meanwhile,the turn-on voltage of the tandem QLED device is about 3.2 V,which is the lowest turn-on voltage since tandem QLEDs were reported.We further use transient EL to analyze the distribution and transport of carriers in single and tandem devices,respectively.We find that the transient EL of the device composed by the bottom light-emitting unit shows the slowest rise and fall,which is due to the film in the bottom device is poor,which affects the transport of carriers and the recombination of excitons;on the contrary,the response process of the tandem device is the fastest,because the PEDOT:PSS constituting the CGL can store some holes.The storage charges will influence the arrangement of residual carriers in the device after the pulse is turned off,which indirectly affects the response of the device to voltage.Finally,in view of the phenomenon that the AC-driven light-emitting devices can only work in the positive or negative AC cycle,we successfully constructed a real AC QLED using the CGL,and the electroluminescence is realized in the whole AC cycle.The device structure is consisting of ITO/bottom CGL/green-QDs/red-QDs/top CGL/Al.We demonstrated an color-tunable AC-QLED,and the AC-QLED can be driven in the manner of in-planar-electrode mode or with a vertically stacked.The turn-on voltage of the device is 5.6 V,which is the lowest turn-on voltage since AC-QLED was reported.Moreover,the electroluminescence work mechanism of the device is described in detail.When the device is driven by effective electric field intensity,the QDs in the device undergo an ionization process,electrons in the valence band of QDs will be injected into the conduction band of the next layer of QDs,and the hole produced by the ionization form excitons with the electron produced by the ionization or injected from the CGL,and then exciton radiative recombination.