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Photoelectron spectroscopy and work-function measurements have been used to study a ${\mathrm{K}}_{2}{\mathrm{O}}_{2}/\mathrm{S}\mathrm{i}(100)$ surface as a function of annealing temperature. The results show that the features for ${\mathrm{K}}_{2}{\mathrm{O}}_{2}$ gradually move to high binding energy, and the work function decreases with increasing temperature up to 460 K. As the species of ${\mathrm{K}}_{2}{\mathrm{O}}_{2}$ decomposes at 540 K, there exist two kinds of oxygen bonding states. A thin K-O layer is formed on the top surface, while oxygen bonded to silicon is located at subsurface, leading to an extremely low value (0.3 eV) of the work function. For further annealing, the oxygen bonded to potassium gradually transfers to Si due to the desorption of potassium, and Si-O bonds appear on the top layer, which is related to increases of the work function.