AppleScript’s support for Unicode seems to be somewhat limited. Here’s one approach that seems to work, passing the Unicode text via a file:
set txtUnicode to "???"
put txtUnicode into file "/tmp/UnicodeToAppleScript"
set txtScript to merge of {{
set f to a reference to posix file "/tmp/UnicodeToAppleScript"
open for access f
set s to read f as ?class utf8?
close access f
display dialog s
return s
}}
set result to doAppleScript(txtScript)
put result
Hopefully that will work for you. If not, there may be other options we can explore.