Tunable, narrow, and enhanced electroluminescent emission from porous-silicon-reflector-based organic microcavities

Microcavity organic light-emitting diodes (MC-OLEDs) based on porous silicon distributed Bragg reflectors (PS-DBRs) have been realized, and improved structural, optical, and electrical properties have been observed. In the device, a multilayer OLED functions as the central active element, sandwiched between a top silver film and a bottom PS-DBR formed by electrochemical etching of a p⁺⁺-Si substrate. Field-emission scanning-electron-microscopy cross-sectional images show that there exist nanoscale layered structures and flat interfaces inside the cavity. Widths of green and red electroluminescent (EL) peaks emitted from the MC-OLEDs are 8 and 12 nm, respectively, greatly reduced in comparison with 85 and 70 nm measured from noncavity structures. The narrowed EL emission from the MC-OLEDs is directional and in single mode, with off-resonant optical modes highly suppressed, which is mainly due to the good optical properties of PS-DBR with high reflectivity in a wide smooth stop band. Further increases in the green and red EL intensities by factors of about 6 and 4, respectively, are achievable. The improvement is physically attributable to the spatial redistribution of the photon density of states in the cavities. In addition, current-brightness-voltage properties and lifetime-related parameters of the devices are discussed. Such device structure and emission patterns of the MC-OLEDs should be useful in silicon compatible optical interconnects and light-emitting diode array printing.