CdSe(S) QD-LEDs Devices Using ZnSnO As Electron-Transporting Layer

Colloidal quantum dots (QDs) which have the advantage of good stability, high luminescent efficiency, color monochromaticity, and facile bandgap tunability over the visible light region, have potential to replace conventional inorganic or organic based LEDs and become a key part in high performance next-generation LEDs. Similar to traditional OLED, the lifetime of typical QDs based light-emitting diodes (QD-LEDs) is limited by the use of organic semiconductors as transport layers, because organic semiconductors are easily degraded by moisture and oxidation.Prepared by RF magnetron sputtering, the ZnSnO amorphous film is much more robust than organic semiconductors against thermal stress and degradation by oxidation or moisture, and it’s carrier concentration can be widely tuned between1012～1019cm-3while electron mobility keeps as high as3.2cm2V-1s-1under different grown parameters, making it an attractive candidate as the electron transport material for QD-LEDs. In this dissertation, QD-LEDs, which use inorganic ZnSnO as the electron transport layer, are studied. QD-LEDs with an Ag/ZnSnO/QDs/TPD/ITO structure were fabricated, in which low carrier concentration and high electron mobility ZnSnO layer is served as electron transport layer and good-thermostability material TPD is served as the hole transport layer while highly luminescent CdSe QDs is served as an emitter. Ag and ITO are used as cathode and anode electrodes, respectively. The devices present characteristics with good monochromaticity (FWHM<56nm) and low turn on voltage(-3V). When the QDs-LED is biased in atmospheric environment, it can operate brightly for several hours even been stored for several months without any encapsulation.A key factor of achieving high performance QD-LEDs is the balance between injected electrons and holes. In this dissertation, the conductivity of ZnSnO layer is varied to investigate the device’s performance. As a result, the optimized parameter of the ZnSnO layer is obtained.White light-emitting diodes based on quantum dots have been developed over the past years, however, little research on using surface-state emission of quantum dots as emitter was reported as quantum dots with surface-state emission normally display weak luminescence and not easily reproducible from one synthesis to another. To solve this problem, the dissertation explores the possibility of using highly luminescent quantum dots, in which there is no surface-state emission observable in the as-synthesized state. Tunable surface state emission is realized in EL by introducing localized surface plasmon resonance (LSPR) into the devices.The experiments demonstrated that the EL of QD-LEDs with an Ag/ZnSnO/QDs/TPD/ITO structure showed quantum dots’ surface-state emission as the thickness of ZnSnO was reduced to60nm. The experiments also demonstrated that the ratio of intensity of surface-state emission to band-band emission (SSE/B-B) experienced in a first decrease,then increase process as the bias of the devices is increased. Based on this phenomenon, a QD-LED with large Stokes shift(-200nm) and pure green-emitting CdSeS quantum dots is fabricated. The large Stokes shift allows TPD to act as an emitter and hole transport layer at the same time. This aslo facilitates the analysis of device’s working mechanism. The device’s EL exhibits wide spectrum which can be divided into three parts:blue emission from TPD, green emission from quantum dots’ band-band emission and red emission from quantum dots’ surface-state emission induced by LSPR. As SSE/B-B and injection efficiency of electrons and holes vary as a function of bias, the ratio of intensity of these three parts can be tuned, hence the color of QD-LED is tunable. The device exhibits good white light-emitting characteristic when the bias is10V, with Commission Internationale de l’Eclairage corrdinates of (0.281,0.384).