Recently, I have rediscovered a program/app which a classmate of me once told me about a couple of years ago.
BOINC (Berkeley-whatever, just DuckDuckGo it ).
If installed and set up, the BOINC Android app* will run if your phone is in standby and charging. It uses the unused computing power of your phone to do calculations for scientific projects. I signed up for World Community Grid (www.worldcommunitygrid.com), where you can donate computing power to e.g. Ebola or AIDS research, but there are plenty other projects as well, like decrypting Enigma-messages from WWII.
I think it would be a good idea if Fairphoners would support this!
PS: If you sign up at WCG with this link, I get some kind of bonus points (I don’t know what they are used for (yet), probably some kind of highscore system): https://join.worldcommunitygrid.org?recruiterId=1063739
*BOINC is also available for Windows, macOS and Linux