| Organic Light-Emitting Diode has raised focus in scientific research since it encountered many new problems both in chemical synthesis and device physics, it also created much new knowledges. On the other hand, as a new generation display techonology after liquid crystal display and new technology in white light illumination, it shows attractive prospect. For example, Sony has its flat panal display product based on OLED and Lumiotec has its white light source product based on OLED. However, there still have many things to be improved, such as the lifetime and the efficiency of the devices.PLEDs are "dual injection" devices with electrons and holes injecting from the cathode and anode, respectively, under forward bias. Then, the carriers move in the emissive layer toward the opposite electrode, and form exciton with pairs of electron and hole. The radiant decay of exciton gives out light. So it is a key element wheather the carriers are balance for the efficiency of the devices. Based on this mechanism, people can enhance the efficiency of the devices by increasing the minority carriers or decreasing the majority carriers.Polyfluorene and its derivatives have relatively lower triplet level, when it is used as the host in phosphorescent OLED, if the gust materials has triplet level higher than the host, emission quench happened in the form of back energy transfer from the guest to host. This is why Polyfluorene and its derivatives are not suitable to be the host of the green phosphorescent materials. In chapter3 of this thesis, we successfully inhibit the energy back transfer from the green phosphorescent material to our polyfluorene derivative host by inserting a buffer layer between the hole injection layer PEDOT:PSS and the active layer and fabricated a highly efficient white polymer light emitting diode. By further inserted a water-acohol soluable electron injection layer PFN, we got white emission with CIE of (0.262,0.310) at the current density of 10mA/cm2, and the highest efficiency of 15.1 cd/A.There are mainly two proces to fabricate OLED i.e solution processing and vacuum deposition. The former can achieve an arbitrary ratio of precice doping and the disadvantage is that it is difficult to form multi-layer structure and difficult to control the film thickness accurately. The vacuum deposition allows precise control of thickness and formation of multi-layer structure, but it is difficult to achieve accurate trace dopping. In chapter 4 of this thesis, we first ly spin coated the high efficiency blue material dopped with red emission material MEH-PPV, then we deposited the green emission layer Alq3, by controlling the thickness of the Alq3, we successfully tuned the color of the devices and got white emission closed to CIE of (0.33,0.33). By insertin buffer layer TPBI and PVK, we realized the highest current efficiency of 7cd/A, and CIE of (0.35,0.37) white emission at the current density of 10mA/cm2.It is an effective way to enhance the efficiency of PLED by dopping. To dope one dimentional nanowires into the emitting materials is a relatively small area involved. Following the work of Jian Wang, in chapter5 of this thesis, we use PFO-BT with different BT ratios, and dopped with a soluable nanowire material in different ratios. We found that with dopping ratio of 1%, devices based on different BT ratio got the highest efficiency. We further characterize the devices with ic impedance spectro. From the Cole-Cole diagram, we infer that the devices can be equivalent to be a low resistance and a parallel RC circuit in series. The low resistance corresponds to the sum resistance of the substrate and the contact resistance, RC parallel circuit corresponds to the emitting layer. We found that the R in RC parallel increased 2-3 orders with small increase of the dopping ratio of nanowires, which means that the nanowires in the system enhanced the efficiency by stopping the electron crrurent in the devices and with good agreement with the J-V-L curves.We can enhance the efficiency of the OLED by increasing the minority carriers. In chapter 6 of this thesis, based on the work of Xuihui Zhu, we made 3 new small molecular soluable electrolytes as electron injection material by replacing the anion. By compareing with the bare Al and the reported electrolyte, we found that the new electrolytes also have good electron injection ability. Among them, the one with tetrafluoroborate as anion shows the best injection ability, the devices based on it remains highly efficient even at high current density. Further more, we carried the OPV measurement to determine the buit-in potential, we found the one with tetrafluoroborate as anion has the largest built-in potentials, which means the best electron injection ability.
|
PDF Full Text Request