Properties Of Spin-polarized Carriers' Injection And Transportation In Organic Light-emitting Devices

Organic light-emitting devices (OLEDs) have the merits of active emitting, low energy consumption, wide lighting spectrum, vast variety, low cost, light weight, broad visual angle, etc. In the light-emitting process of OLED, both singlet and triplet excitons are formed at the same time. Triplet excitons take up 75% of excitons while singlet excitons take up 25%. Only the 25% singlet excitons are available for luminescence because of spin-forbidden of transition from triplet to ground state. Forasmuch the luminescent efficiency is limited. It is pressing to improve the efficiency, especially for the organic semiconductor devices with sensitive properties to temperature. Spin-polarized injection is an efficient way to enhance the efficiency of OLED. On the basis of predecessors' works, the injection and transportation of spin-polarized carriers in OLEDs are studied in this work. The dissertation includes four primary parts:(1) Starting from drift-diffusion equation and generalized continuity equation, combined with Poisson equation, the differential equation of electrons and holes are deduced, dependences of spin-polarized carriers density on spin diffusion length are obtained, and influences of electrical field on the spin diffusion length are discussed. It is showed that the electrical field affects movement of spin-polarized carriers by the spin diffusion length. In terms of electrons, when direction of the electrical field is the same as that of diffusion, increasing the electrical field will lead to decrease of diffusion length, which means acceleration of diffusion. As a result, the average distance that the spin-polarized electrons go on in samples will be shorter, and thus large spin polarization will be difficult to achieve in the thin film. If direction of electrical field is reverse to that of diffusion, then, the stronger the electrical field is, the longer the diffusion length will be, so it will be easier to reach high spin polarization. When it comes to holes, the results are contrary to that of electrons.(2) Based on a "two current-channel" model of the OLED structure, the spin polarization of carrier density is obtained by analyzing electrochemical potential in electrodes and OSC. Combined with Langevin recombination theory, we calculate the singlet to triplet ratio, and analyze the influences of electrical field, spin-related interfacial conductance, bulk conductivity and polarization of electrodes on the spin polarization. It is showed that opposite spin polarization of electrons and holes are in favor of increasing singlet to triplet ratio, and the higher spin polarization of injected carrier density is, the larger singlet to triplet ratio will be. Large spin-related interfacial resistance, large polarization of electrodes, matched bulk conductivity and high electrical field under forward bias favor spin polarization of carries density in OSC.(3) Considering the up-spin polarons, down-spin polarons and non-spin bipolarons in organic semiconductors, the "two current-channel" model was developed to "three current-channel" model to discuss effects of polarons ratio, electrode's spin polarization, interfacial conductance, bulk conductivity of OSC and the electrical field. It is found that "two current-channel" model and "three current-channel" model give the same result when bipolarons vanishes in "three current-channel" model. The influence of polarons ratio on spin polarization of polarons is not simple, which is also affected by electrical field, interfacial conductance and electrode polarization. When polarons ratio is fixed, increasing electrical field favors spin polarization of polarons. Introducing a spin-dependent interfacial conductance is an effective way to overcome conductivity mismatch of OSC and electrode. When spin-dependent interfacial conductance is introduced, the discontinuity in the electrochemical potential at the interface allows electrical current to drive carriers out of the quasithermal equilibrium at the interface.(4) Considering the interaction of different spin carriers, including effects of spin coulomb drag (SCD) in the drift-diffusion equation, a spin-polarized injection into nondegenerated OSC is simulated, and the SCD effect on spin polarization is analyzed. The results show that SCD inhibits the current polarization which is affected by interfacial conductance, electrical field and current density. Large spin-related interfacial resistance, large OSC conductivity and effective mass of carriers, and high forward electrical field favor the current polarization. When OSC conductivity and electrical field are large enough, current polarization will be saturated, and the SCD effect tends to vanish.