Can't add \raisebox to ToC in XeTeX

You need to protect the fragile command \PoETeX:

 \addcontentsline{toc}{section}{\protect\PoETeX}

If you plan to often use \PoETeX in moving arguments, it's better to use

\DeclareRobustCommand*{\PoETeX}{%
  P\kern -.15em\raisebox{-0.21em}{O}\kern -.05em E\TeX}

so you won't have to put \protect before the command in many places.

Roughly speaking, defining a command with \DeclareRobustCommand is like having \protect before it whenever it's needed. Don't take the habit of defining all commands like this: use the technique only where it proves necessary.

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Alternative definition:

\protected\def\PoETeX{%
  P\kern -.15em\raisebox{-0.21em}{O}\kern -.05em E\TeX}

This is actually more efficient than \DeclareRobustCommand, but has the disadvantage of using low level commands.