Here is what you want to put in the project’s Post-build event command line:
copy /Y "$(TargetDir)$(ProjectName).dll" "$(SolutionDir)lib\$(ProjectName).dll"
EDIT: Or if your target name is different than the Project Name.
copy /Y "$(TargetDir)$(TargetName).dll" "$(SolutionDir)lib\$(TargetName).dll"