Usually in a cover letter you mainly explain why you think that you are a good fit to a position. Your detailed skills and experience you have gained during your PhD should be in your CV.
The following is a good summary on what your cover letter should include
- Introduce yourself, identify the position you are applying for, and explain how you found out about the position.
- Explain how you are qualified for the position and why you would be a great fit for the job.
- Request an interview and thank the employer