In this work, ultrasonic spray pyrolysis has been used to produce bifacial ITO/SiOx/(nn(+))Cz-Si heterojunction solar cells by growing an ITO layer on monocrystalline n-type silicon. We have examined the effect of wet-chemical treatment before ITO growth on the Suns-Voc characteristics of heterojunctions and the properties (thickness and stoichiometry) of the SiOx layer, which were studied by spectroscopic ellipsometry. Varying wet-chemical treatment conditions, we have produced a bifacial solar cell with front/rear conversion efficiency under 1-sun illumination as high as 16.6%/14.6% referred to the total area of the cell, which is the highest reported value for ITO/Si heterojunction solar cells. The equivalent efficiency of this solar cell at 1-sun front illumination and 20% albedo (0.2-sun rear illumination) is equal to 19.6%.