Electrochemiluminescence from porous silicon in formic acid liquid-junction cells

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Abstract
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/cm2, 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.

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