UNPAYWALL

The Unpaywall database has a very simple structure: we have one record for every article with a Crossref DOI (that's about 95 million records all told). We harvest from lots of sources to find Open Access content, and then we match it to these DOIs using content fingerprints. So, for any given DOI, we know about any OA versions that exist anywhere (at least that's the idea).

Licence: 
Free