I just need someone to check my proof and provide me feedback:
Since $R[x]$ is a PID, then the ideal $I = (x-1)$ generated by the polynomial $x-1$ is maximal because it is of degree 1 added to a constant.
So $R[x]/I\simeq R$ is a field, so $R$ is a field.
Is this proof enough/correct?
Thanks.