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

Visible light emission from porous silicon can be stimulated by applyi ng a positive bias to a formic acid/sodium formate liquid junction cel l. The emission lasts for 45 min at +2.75 V applied potential (<5 mA/c m(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 i s capable of generating the red (720 nm) emission, indicating that cur rent-induced chemical reactions aid in the generation of charge carrie rs. A mechanism involving oxidation of formic acid followed by electro n injection from a CO2- radical is proposed. Infrared spectra of the p orous silicon surface taken after anodization show formation of a stab le silyl-ester species that is thought to be responsible for the incre ase in radiative recombination efficiency through passivation of surfa ce defects. (C) 1995 American Institute of Physics.