I've seen a few approaches to this

my dad did work on up-jump phosphors in the 80's. They worked great except they were terrible at pooling energy absorbed by radiation and the expelled radiation was omni-directional.

One design I came across was using a metallic surface that was struck by radiation and on the other side a vacuum and a potential field to draw expelled electrons out to a cathode.

Now the problem lies in finding a metal that can be excited enough to expel an electron at very low photon energies....

Well I do believe you can get hydrogen to be in a metal state, and its valence shell is the first shell so the required energy of the photon is going to be the lowest (practically speaking)

Perhaps tho instead of a pure metal a ligand bound metal could do the trick. Instead of directly exciting the valence electrons one could set the ligand vibrating, this can easily pool and maybe when the  potential in that vibration is high enough then (how ever ligand bonding works...some kind of crystal field thing) im hoping some kind of effect could excite the metal component to expel an electron