The model of using Social Impact Bonds (SIBs) to drive innovation and better societal outcomes from government policy has been evolving over the last several years. There are clearly different reasons and benefits for undertaking them, and one of the things you must be clear on is which of them apply to your particular case. 

This is a good article from Stanford outlining what we've learned so far about the goals and how to work with them to make sure you get what you want from using SIBs.