Yes we can vary the solar spectrum by using luminescent solar concentrator like downconversion, downshifting and upconversion. Downconversion and downshifting is straight forward compared to upconversion. But they have their limitations. I am also working in this field. I have prepared few luminescent materials which absorbs in the UV-Blue part of the solar spectrum and emit in the near infrared region where the efficiency of c-Si solar cell is high.

While working with this, we come across various difficulties like

1. Rare earth like Yb3+ emit at around 980 nm whereas c-Si solar cell external quantum efficiency is maximum at around 850-900 nm. For this type of luminescent materials c-Si solar cell must have EQE around 1000 nm then only you may get change in the EQE of Solar cell due to luminescent materials.

2. Because of this luminescent materials, there is an effect called shadow effect which is hammering the solar cell efficiency because the luminescent material is not transparent.

We have deposited luminescent materials which absorb UV light and emit in NIR region on c-Si solar cell, while taking the EQE of solar cell coated with luminescent material, we observe that the EQE curve in the short wavelength (UV) region is similar to the excitation spectrum of the coated luminescent materials. which means luminescent material is playing its role in the UV region. While we have not observed the improvement in the efficiency of solar cell. In conclusion, this negative result is surely due to the above two reasons.