Doping is essential for the efficient upconversion process.

For more details, please refer to:

https://www.rp-photonics.com/upconversion.html

https://doi.org/10.1016/0079-6727(95)00007-0

Generally speaking, down-conversion phosphors (i.e. luminescent materials) consist in a matrix into which the activator (the emitting ion) is doped, usually at a very low concentration (e.g. 3-5% in europium red phosphors, 0.03% in cerium yellow phosphor) in order to avoid the so-called "concentration quenching" in which after excitation the energy is not released under the form of a photon but is transferred non-radiatively to a neighboring activator ion and energy migrate between several ions until reaching a defect where it is degraded.

The same prevails for upconversion phosphors (mainly erbium and thulium activators), although there are noticeable exceptions for which increasing the concentration of the emitting ion may be beneficial, depending on the excitation mode and power.