Molecular Interdiffusion between Stacked Layers by Solution and Thermal Annealing Processes in Organic Light Emitting Devices

Abstract

In organic light emitting devices (OLEDs), interfacial structures between multilayers have large impacts on the characteristics of OLEDs. Herein, we succeeded in revealing the interdiffusion in solution processed and thermal annealed OLEDs by neutron reflectometry. We investigated interfaces between a polymer under layer and small molecules upper layer. The small molecules diffused into the swollen polymer layer during the interfacial formation by the solution process, but the polymer did not diffuse into the small molecules layer. At temperatures close to the glass transition temperatures of the materials, asymmetric molecular diffusion was observed. We elucidated the effects of the interdiffusion on the characteristics of OLEDs. Partially mixing the interface improved the current efficiencies due to suppressed triplet-polaron quenching at the interface. Controlling and understanding the interfacial structures of the miultilayers will be more important to improve the OLED characteristics.

57‐3: Invited Paper: Solution‐Processed Electron Transporting Layer and Interface Characterization in Organic Light Emitting Diodes

“…The mixed and unclear interface such as the device 1 has been considered as disadvantage to the OLED performance to date, however, the mixed interface having controled chemical composition by the solution-process would give positive effect to the OLED efficiency and possibly to the lifetime of the devices. NR is a nondestructive and highly quantitative analytical method that enables precise evaluation of interface structures with sub-nanometer-order resolution 4,[6][7][8][9][10][11][12][13][14] . In NR, deuteration of samples substantially increases the scattering length densities (SLDs) within layers, which enhances SLD contrast to an adjacent nondeuterated compound layer and enables analysis of the interface structure between the two organic layers.…”

Inhibition of solution-processed 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile crystallization by mixing additives for hole injection layers in organic light-emitting devices

“…Poly-TPD shows a high resistance to the coating solvent. 36 The energy diagram of the fabricated devices is shown in Figure 3. Here, we used the averages of the reported energy level values for HAT-CN.…”

Sputter Depth‐Profile Study of Accelerated Interface Mixing by Thermal Annealing in Solution‐Processed Organic Light‐Emitting Diodes

“…The organic materials at such temperature turn into liquid‐like phase and mix with each other relatively easily. But in solution‐processed OLEDs the interfacial mixing can occur at temperatures below the T g . The exact mechanism for this is not well known and the effect of this phenomenon on device performance is also not clear.…”