Rav Ovadia Yosef permits putting them on Bein Hashmashos with a Bracha (source), which is interesting given that the Rambam implies it is an issur dioraisa to put them on at night (Tefillin 4:11), but Shulchan Aruch (O.C. 30:2) does not pasken that way, and leaves it as equally rabbinic to leave them on or to put them on (or actually more of a problem to put them on).
If, however, one has already davened Maariv, even if it is still before Shekia, Shulchan Aruch (O.C. 30:5) says to not put on Tefilin, and Rav Ovadia Yosef paskens to put them on without a Bracha (during Bein Hashmashos), apparently agreeing with the Magein Avraham there as well as the Shulchan Aruch HaRav (O.C. 30:5).
So from all of this you see that actually putting them on at actual night time is not permitted. There is a dissenting opinion in Rishonim (I have seen Rabbeinu Yona quoted as one) - that the idea of הלכה אין מורין כן (discussed in the above sources) applies even to putting on Tefilin at night, so the whole rabbinic prohibition - according to that opinion - isn't that strict, it is more about how people in general should be trained rather than a strict prohibition.
Here Rav Ovadia Yosef is quoted as saying that a soldier in a non-Jewish army that is simply not allowed the opportunity to put on Tefilin during the day light hours should put them on at night without a Bracha.