The sun works by fusing hydrogen to helium (at the moment). While hydrogen is a single proton (I'm ignoring electrons here... not sure if they are relevant since it's probably a plasma anyway) helium consists of two protons and two neutrons.
To get fusion to work on earth we therefore use deuterium and tritium (getting us one extra neutron in the process, but it's easier to fuse them).
I don't think that the hydrogen in the core of the sun is mostly deuterium (or tritium for that matter). So where do the neutrons in the sun come from?