Building-integrated photovoltaics are drawing much attention to make the built environment self-sufficient in terms of electricity. Luminescent solar concentrators (LSCs) can provide this electricity generation when used as photovoltaic windows. Paramount to an efficient LSC are non-overlapping absorption and emission spectra, to avoid self-absorption. This non-overlap can be achieved by absorbing incoming UV light and emitting in the red to infrared. In this article, we present a technique for optimizing LSCs without self-absorption, using Eu³⁺-doped AlN as a model system. The parameters affecting light absorption, emission and transport are extracted from a combinatorially sputtered gradient material library. This library results from a single deposition, with a gradient in thickness and Eu concentration. AlN:Eu³⁺ absorbs strongly until 450 nm, with a peak solar absorption of 499 cm⁻¹ at%⁻¹ at 350 nm due to a charge transfer band. The strongest emission is at 622 nm, thereby exhibiting no self-absorption. The presented optimization model strikes a balance between concentration quenching and absorptivity of Eu dopants by using the parameters extracted from the material library. For thicker films, concentration quenching can be avoided by using a lower dopant concentration, while still outperforming thinner films due to fast increasing absorption. The results demonstrate that, while AlN:Eu³⁺ itself should only be viewed as a model system, thin films doped with rare earths can yield industry-compatible, high efficiency LSCs because of their high absorption coefficients and lack of self-absorption.