Research On Quantum-dot Light-emitting Diodes Based On SnO₂ Electron Transport Layer

As a novel class of semiconductor nanomaterials,quantum dots（QDs）have many unique optical properties,such as high photoluminescence（PL）quantum yield,tunable emission spectrum,narrow emission spectrum,and good optical stability.These excellent properties are important prerequisites for the application of quantum dots in display devices.In the past three decades,with the optimization of materials and device structures,colloidal quantum-dot lightemitting diodes（QLED）have attracted wide attention in the field of display and lighting.So far,QLED has obtained excellent performance in red,green and blue primary colors.The external quantum efficiency（EQE）reached the theoretical limit value（about 20%）and the maximum luminance of red QLED could reach 614000 cd/m2.Of the high performing QLEDs reported so far,most devices adopt organic-inorganic hybrid structure: the inorganic metal oxides as electron transport layer（ETL）,the organic materials as hole transport layer（HTL）,and the QDs as emission layer（EML）.Among the common metal oxides,Zn O has excellent electron injection and transport capabilities,which makes Zn O ETL based QELD have the best performance.However,Zn O is sensitive to water and oxygen,so the stability of QLEDs using Zn O as ETL is poor,which greatly limits the commercialization of QLEDs.SnO₂,a typical n-type semiconductor,has high electron mobility and better stability.In theory,SnO₂ nanoparticles（NP）can be used as ETL to achieve better device performance.Currently,although there have been some reports of SnO₂ as a QLED ETL,the device performance is generally lower than that of Zn O ETL based QLEDs.This stems from a lack of understanding and optimisation of SnO₂ films,which prevents further development of SnO₂-based QLEDs.Therefore,the electrical properties of SnO₂ NP films and their performance as ETL QLED are studied in detail in this paper.The research work of this paper is mainly divided into the following three parts:（1）The charge transport mechanism of SnO₂ NP films was investigated.By testing the electrical properties of SnO₂ NP films at different temperatures,it is confirmed that the electron transport mechanism of SnO₂ NP films is Mott variable-range hopping,that is,the electrons are transported between different NPs through surface states rather than the conduction band of the nanocrystals,which is identical to the commonly used Zn O NP solids.However,the density of states of SnO₂ NP films is much lower than that of Zn O films.In addition,we demonstrate that SnO₂ films have similar electron mobility to Zn O films.Based on these results,we infer that the low density of states in SnO₂ NP films is a key factor for the poor performance of SnO₂-based QLED compared with Zn O.（2）QLED performance was improved by reducing the surface roughness of SnO₂ NP films.SnO₂ NP films were post treated by a facile and feasible solvent vapor annealing,which reduced the aggregation of nanoparticles,reduced the roughness of the films,and improved the efficiency of electron injection from SnO₂ to QDs.Compared with conventional annealed SnO₂,the luminance of device based on solvent vapor annealed SnO₂ is increased from 22770 cd/m2 to 30900 cd/m2 by 35%.Current efficiency increased from 6.7 cd/A to 10.8 cd/A,an increase of 61%.This makes the performance of SnO₂-based device comparable to Zn O-based devices.At the same time,some dark spots appeared in the Zn O-based device after 24 h being stored for 24 h.In contrast,SVA-SnO₂-based device can only observe some dark spots after 108 h of storage.Therefore,SnO₂-based device exhibits excellent shelf stability.（3）In view of the strong conductivity of Zn O and weak conductivity of SnO₂,we fabricated QLED using SnO₂,SnO₂/Zn O and Zn O NP films as ETL.By comparing the performance of three devices,we explored the characteristics of the double electron transport layer film.The results show that the current of SnO₂/Zn O-based QLED is between that of Zn Obased QLED and SnO₂-based QLED.This shows that the double electron transport layer can effectively solve the problem of excessive electron injection of Zn O and insufficient electron injection of SnO₂,resulting in a more balanced carrier injection in the device.Therefore,the performance of the QLED based on the SnO₂/Zn O double electron transport layer is significantly improved,and its maximum current efficiency reaches 13.40 cd/A,which is 100%higher than that of the QLED based on the SnO₂ ETL and 12.4% higher than that of the QLED based on the Zn O ETL.That means,the design of the double electron transport layer can replace the traditional ZnO ETL to achieve higher efficient QLED.