Charge carrier trapping processes in lanthanide doped La-, Gd-, Y-, and LuPO4

Various methods for deliberate design electron and hole trapping materials are explored with a study on double lanthanide doped rare earth ortho phosphates. Cerium acts as recombination center while lanthanide codopants as electron trapping centers in LaPO4:0.005Ce3+,0.005Ln3+. The electron trap depth generated by lanthanide codopants can be tailored by the choice of lanthanide, and for fixed set of lanthanide dopants like in Gd1-xLaxPO4:0.005Ce3+,0.005Ho3+ solid solutions by changing x leading to conduction band (CB) engineering. Here, the electrons liberated from Ho2+ recombine through the conduction band at Ce4+ to yield Ce3+ 5d-4f emission. In contrast, samarium, europium and ytterbium are recombination centers, while Tb3+ and Pr3+ act as hole trapping centers in double lanthanide doped YPO4. For Tb3+ and Pr3+ codopants recombination is realized via hole release rather than the more common reported electron release. The holes recombine via the valence band with the electrons trapped at Yb2+, Sm2+, or Eu2+ to generate 4f-4f luminescence from Yb3+, Sm3+, or Eu3+. Lu3+ was introduced in YPO4 to tailor the valence band (VB) energy and to tune the hole trap depths of Tb3+ and Pr3+ in Y1-xLuxPO4:0.005Ln3+ solid solutions. Our results promote the deliberate design electron and hole trapping materials from deep understanding of trap level locations and on the transport and trapping processes of charge carriers.