Optical Characteristic Studies of Porous Silicon and Conducting Polymer (PCDM) Light Emitting Devices

For obtaining high quality LEDs, many different contacting schemes have been tried, e.g., using metals, conducting polymers and indium tin oxide. The main goal for these is to produce long life, stable and high efficient photoluminescence (PL)/electroluminescence (EL) from porous silicon at room temperature. A conducting polymer is a very promising contact for making high quality LEDs. Porous silicon-based devices produce stable electro-luminescence (EL) by the deposition of PCDM monolayer, a low band gap conducting polymer, into the nano-structure of PS. The structure of these devices is Au/PCDM/PS/Si/Al. The EL emission is bright, visible by the naked eye under normal daylight, and broad in wavelength, covering the whole visible range with a peak at 650 nm. The emission area of the devices is ~1 cm2. The EL starting voltage is in the range of 4~30V and the current is around 300mA. Since preventing oxygen molecules from approaching the PS surface, PCDM films largely increase the time stability of all the devices tested. After exposure to the air for more than 3 months, the PCDM coated samples show nearly the same emission intensity without increase of external power supplied. When a reversing bias is applied, the device can still emit stable light. There is no obvious change inI~Vcurve within 250h. The SEM image shows that the PCDM covered surface is soomther than that of bare porous silicon surface, which may imply that the PCDM may have penetrated into the nano-structure of PS. The substitution PCDM molecules for unstable molecules on the PS surface may results in theincreasing the lifetime and stability of the LEDs.