Studies On The Magnetic Field Effects In Exciplex-based Organic Semiconductor Devices

Organic magnetic field effects (OMFEs) are used to describe the changes of electroluminescence or current intensity in organic optoelectronic devices without any magnetic materials under the external magnetic field. The OMFEs have received great interests in the past few years, since it will be of great value in developing the new-style organic integrated device with magnetic, optical and electrical properties and will be great scientific significance in monitoring the microphysical processes of organic light-emitting diodes (OLEDs) as a non-contact research method. Although much progress in OMFEs theoretical research have been made based on the past investigation with the scientific efforts, there are still many unsolved issues in OMFEs, including the small magnitude, the argumentative mechanism and the hardness in revealing the microscopic mechanism of OLEDs by using the OMFEs as a new non-contact measurement approach.In this M.S. Dissertation, we focused on improving the magnitude and clarifying the microscopic mechanism for the intriguing OMFEs, the exciplex-based devices were fabricated based on blends of electron-donor and electron-acceptor molecules. The ultra-large and tunable OMFEs exceeding 100% at room temperature in exciplex-based OLEDs was reported by investigating the behaviors of the singlet and triplet excited states with the external magnetic field. Meanwhile, the microscopic mechanism model for the intriguing ultra-large OMFEs was discussed. Based on the obtained mechanism, the triplet-triplet annihilation (TTA) progress in exciplex-based OLEDs was investigated by utilizing the OMFEs as a new non-contact research method. It is helpful to bring to light the reason that the low emitting efficiency in exciplex-based OLEDs.The main works in this Dissertation are listed as following:(1) In the first chart, the inter electroluminescence mechanisms in the OLEDs was discussed by introducing the basic knowledge of the material properties of organic molecules and the emission process of the organic devices. These contents provide the basis knowledge for investigating the OMFEs in the latter Chapter. In addition, to reveal the purpose and significance of this paper, the research backgrounds, contents and current issues of the OMFEs were also debated. While in the second chart, we presented the fabrication process, test methods and intruments used in testing or fabricating the exciplex-based OLEDs.(2) The exciplex-based OLEDs with the m-MTDATA molecules as donor and the 3TPYMB molecules as acceptor were fabricated by the methods of organic molecular beam deposition, co-evaporation and thermal evaporation. The structure of the exciplex-based device is ITO/m-MTDATA/m-MTDATA:3TPYMB/3TPYMB/LiF/Al. The magneto-luminescence (MEL) and magneto-conductance (MC) were measured under the different driving current at room temperature. The red-shifted emission of the spectrum suggests that the exciplex state did indeed form. The MEL results show that the ultra-large magneto-conductance can above 65% and the MEL can exceed 120% at room temperature in exciplex-based OLEDs. Based on studying the OMFE depended on the work temperature, thickness and concentration of acceptor in the emitting layer, We propose a theoretical model to explore the related physical microscopic mechanism, that is, the proportion of the singlet and triplet excited states will be affected by the different △EST under the applied external magnetic field. So the ultra-large OMFEs originate from the magnetic field modulated spin-mixing with a small value of △EST between the singlet and triplet excited states. To further demonstrate the validity of the model, the ultra-large value of OMFEs was also obtained by designing the same exciplex-based OLEDs with different electron-donor molecules. It is meaningful in developing practical process of the OMFEs based on this result.(3) The exciplex-based OLEDs with the m-MTDATA molecules as donor and the Alq3、TPBi、 PPT、PBD molecules as acceptor were prepared in the fourth Chapter. The MEL with different band-gap energies(△EST) within singlet and triplet excited states were measured under different temperature and driving current. Results indicate that the values of magneto-luminescence (MEL) decrease with the enlarging the band-gap energies in exciplex-based OLEDs. However, the line shape of the MEL in exciplex-based OLEDs exhibits increasing at low filed (B<40 mT) caused by intersystem crossing (ISC) but without decreasing at high filed (B>40 mT) caused by triplet-triplet annihilation progress (TTA) at any temperatures. It is different from the typical devices phenomena in decreasing at high field (B>40 mT) at the low temperature and high current. It suggests that ISC plays an important role in exciplex-based OLEDs with a larger value of the △EST, meanwhile the TTA process in exciplex-based devices is hardly considered to occur at all because it is a four-intermolecular interaction process. Based on this result, the values of the MEL may be tunable by selecting different electron-donor and electron-acceptor molecule with the different highest occupied molecular orbit (HOMO) and lowest unoccupied molecular orbit (LUMO) respectively.(4) The exciplex-based OLEDs based on the m-MTDATA、TTP molecules with the different triplet energy as donor were fabricated in this Chapter. The structure of devices is:ITO/TTP/TTP: TPBi/TPBi/LiF/Al and ITO/m-MTDATA/m-MTDATA:TPBi/TPBi/LiF/Al. The MEL in the devices was measured under different temperature and voltage. The red-shifted emission of the spectrum suggests that the exciplex state did indeed form. The results show that MEL does not decrease remarkably under higher magnetic fields (B>40 mT) at temperatures higher than 100 K (T>100 K). However, the decreasing MEL will only appear in the TTP device with higher donor-triplet energy and a greater energy difference (△EST) between the singlet and triplet states when the temperature is reduced to 20 K. In addition, the decreasing MEL also depends on the current density, that is, the value of MEL increase with the enlarging current density. The evolution of MEL with different voltages and temperatures confirmed the existence of the triplet-triplet annihilation process. In addition, te formation conditions of the annihilation process for exciplex-based devices were also discussed based on the experimental results. This finding not only can deepen the understanding of the internal mechanism in exciplex-based OLEDs, but also can have great significance in improving the light emission efficiency.