ELECTROCHEMILUMINESCENCE FROM POROUS SILICON IN FORMIC-ACID LIQUID-JUNCTION CELLS

Visible light emission from porous silicon can be stimulated by applying a positive bias to a formic acid/sodium formate liquid junction cell. The emission lasts for 45 min at +2.75 V applied potential (<5 mA/cm(2), power conversion efficiency >10(-2)%) and is reliably generated from n- or p-type porous silicon. An applied voltage as low as 1.3 V is capable of generating the red (720 nm) emission, indicating that current-induced chemical reactions aid in the generation of charge carriers. A mechanism involving oxidation of formic acid followed by electron injection from a CO2- radical is proposed. Infrared spectra of the porous silicon surface taken after anodization show formation of a stable silyl-ester species that is thought to be responsible for the increase in radiative recombination efficiency through passivation of surface defects. (C) 1995 American Institute of Physics.