Solution Processed Inverted Quantum Dots Based Light Emitting Diodes

Quantum dots(QDs)are semiconductor nanocrystals with quantum size effect,usually in the range of 2-20 nm.QDs are widely used in optoelectronics,due to their narrow emission bandwidth,high fluorescence quantum yield and low-cost solution process.Quantum dots light emitting diodes(QLEDs)a promising candidate in the display,due to there high efficiency,high color purity and high stability.To integrate QLEDs with N-type TFT,QLEDs with inverted device structure are preferred.Integrate N-type TFT with inverted QLEDs is the most potential large-scale display driving technology,which can guarantee electron mobility and realizes large area uniform fabrication.Hole transport layer and hole injection layer of inverted QLEDs are usually deposited by vacuum evaporation technology.There are few reports about inverted QLEDs in all solution process.Compared with vacuum evaporation technology,solution process has the advantages of cost saving and large area fabricat.Therefore,inverted QLEDs with all solution process,an important direction of the development of QLEDs.The difficulty of all solution processed inverted QLEDs based on Cd-based QDs(Cd-QDs)is to eliminate the interfacial intermix caused by solvent erosion during solution process.It was found that the polar solvent 1,4-dioxane could dissolve the hole transport material PVK.The steady-state fluorescence intensity test of the QDs film and the cross-section highresolution STEM image of red QLED proved that the solvent 1,4-dioxane would not erosion the QDs film.By taking the polar solvent 1,4-dioxane to dissolve PVK,eliminated the interfacial intermix in the process of the frabrication of all solution processed inverted QLEDs.Therefore,Red QLEDs improve the maximum external quantum efficiency(EQE)record of all solution processed inverted Red QLEDs by 4 times,Blue QLEDs improve the maximum luminous efficiencies(LE)of all solution processed inverted Blue QLEDs by 25 times.To further improve the device efficiency,reduce the hole injection barrier and efficiency roll-off,solution processed PVK/TFB is introduced as Double HTL of QLEDs.Double HTL with stepped hole injection,which reduces the barrier of hole injection and improves the hole transmission ability,thus reduced the devices'turn on voltage and slowed down the devices'efficiency roll-off.The turn-on voltage of QLEDs decreases from3.4 V to 2.7 V for red,from 5.1 V to 2.7 V for green,and from 5.3 V to 4.1 V for blue.At 150mA/cm~2,the efficiency roll-off of QLEDs decreases from 32.3%to 4.0%for red,from 41.9%to 18.6%for blue,while for green the efficiency is still in improving.The peak LE reach 22.1cd/A,21.4 cd/A,and 1.99 cd/A,while the maximum EQE reach 12.7%,5.29%,and 5.99%,for red,green and blue QLEDs,respectively.Environmentally friendly InP QDs have attracted wide attention in recent years,but there are few researches of light emitting diodes based on InP QDs,and the device performance needs to be further improved.In this study,systematically studied three different hole transport layer in solution process,PVK,TFB and Poly-TPD,of inverted InP QLEDs.Devices with hogh turn-on voltage,low efficiency and luminance,when PVK as hole transport layer.While,when TFB or Poly-TPD as hole transport layer,the turn-on voltage is reduced,the efficiency and luminance is improved.Poly-TPD was chosen as hole transport layer of red InP QLEDs,TFB chosen as hole transport layer of green InP QLEDs.the peak LE reach 2.87 cd/A and 5.52 cd/A,while the maximum EQE reach 3.65%and 1.76%,for red and green InP QLEDs,respectively.Furthermore,because the exciton with long lifetime,41.88 ns and 36.76 ns,for red and green InP QDs,respectively.The longer the exciton lifetime,the higher the probability of exciton collision and quenching.In order to reduce the exciton collision and quench exciton,the broadband gap material G0 is introduced into InP QDs to separate InP QDs,which enhances the maximum luminance of InP QLEDs.Expansibility,organic-inorganic hybrid white light-emitting diodes with a color index of 92.4 were fabricated by combining red and green InP QDs with blue polymer PFSO,showing great application prospects in the field of white light illumination.