Preparation Of Quantum-dot-based Light Emitting Diodes And Stability

For thin-film light emitting diodes (LEDs), considerable research interest has now been aroused in thequantum-dot-based LEDs (QD-LEDs), due to the facile solution processable materials, advantageous pureand saturated colors, tunable emissions, and feasibility of large scale synthesis of colloidal quantum dots.Such characteristics are much superior to the organic light emitting diodes (OLED). The core-shellstructured quantum dots exihibit good photostability and photoluminescence quantum yield, which directlybenefit the external quantum efficiency (EQE) of QD-LEDs containing core-shell QDs. Currently, thepublished brightness and EQE has reached104-105cd/m~2and18%, respectively, which satisfiedapplications such as solid-state lighting and flat panel displays. However, the performance of QD-LEDs arestill being limited by bad device stability in air and limited understanding of the fundamental operatingmechanisms, which must be addressed if they are to become a commercial reality.Based on the aforementioned problems, spin-coating process was adopted to fabricate QD-LEDsunder ambient conditions. QD-LEDs with different device structures and QD-LEDs with different transportlayer treatment process were tested and analyzed to improve their brightness of electroluminescence (EL),stability, and efficiency. For the choice of hole transport materials, solution-processablePoly(N,N’-bis(4-butylphenyl)-N,N’-bis(phenyl)benzidine)(Poly-TPD) andPoly[(9,9-dioctylfluorenyl-2,7-diyl)-co(4,4’-(N-(4-sec-butylphenyl)diphenylamine)](TFB) were used ashigh efficient hole transport layers, and corresponding device performance were tested and compared witheach other. The results indicate that QD-LED utilizes poly-TPD exihits a better performance due to itshigher HOMO level of~5.4eV compared with that of TFM (with a HOMO level of about5.3eV). Such adifference in HOMO level may lead to a better hole injection efficiency for QD-LEDs with poly-TPD.For the choice of electron transport material, ZnO seems to be a good candidate. ZnO nanocrystalswere synthesized using a sol-gel method and the optimization of synthesis were also studied. By comparingthe results obtained under different reaction parameters such as molar ratio of raw materials, reactiontemperatures, and the sample store time, we find that non-uniform ZnO nanocrystals obtained at20~8oCwith stoichiometric ratio of raw materials and succeeding aging process can improve the performance ofQD-LEDs remarkably. After further calculation and analysis, we propose that the smaller size of these non-uniform ZnO nanocrystals can fill the space between the QD emitter layer during the spin coatingprocess, and increase the charge transport efficiency, while somel aggregated ZnO nanocrystals producedafter the aging treatment can provide a better contact with the aluminum cathode. Finally, the optimizedQD-LEDs devices were obtained with the maximum luminance and luminous efficiency of17,550cd/m~2and3.55cd/A,11,830cd/m~2and1.63cd/A,90cd/m~2and0.17cd/A, for red, green, and blue QD-LEDs,respectively.The use of ZnO ETL greatly improved the performance of devices. However, the hole injection barrierbetween HTL and QDs is still too large to achieve an efficient hole injection, and the nearly insulate surfacecapping ligand contributed a great part to this barrier. Therefore, we carried out a simple ligand exchangeprocess by replacing the long chain alkane with short chain thiols. The QDs modified with thiols of shorterchain lengths did not improve the EL performance of QD-LEDs obviously. However, the turn-on voltage ofdevices decreased along with decreased ligand chain, which indicated that short chain ligand on QDs couldpromote the efficiency of charge transportation.We futher studied the EL stability of QD-LEDs. With a H2SO4-treated PEDOT:PSS layer, the turn-onvoltage of QD-LEDs decreased by nearly0.9V as compared with the untreated counterparts. A slightdecrease of28%of luminance with a consistent current density for the QD-LEDs with a H2SO4-treatedPEDOT:PSS layer were observed, while a rapid loss of73%and98%for current density and brightnesswithin a day for the untreated devices were recorded. The improved performance for QD-LEDs withH2SO4-treated PEDOT:PSS film was probably originated from the conformational change of the polymerchains and the loss of the insulating and hydrophilic PSS-bases from PEDOT:PSS layer. Moreover, westudied the different stages in the device fabrication process using the surface photovoltage measurement.The results indicate that H2SO4-treated process have significant effects on the local energies ofPEDOT:PSS and Poly-TPD films.