"We have achieved a 10 percent increase in efficiency without yet optimizing the technology," Ågren says. "With a little more work, we estimate that a 20 to 25 percent increase in efficiency could be achieved."

Photosensitive materials used in solar cells, such as the mineral perovskite, have a limited ability to respond to infrared light. The solution, developed with KTH researchers Haichun Liu and Qingyun Liu, was to combine nanocrystals with chains of microlenses.

"The ability of the microlenses to concentrate light allows the nanoparticles to convert the weak IR light radiation to visibile light useful for solar cells," Ågren says.

Invisible infrared light accounts for half of all solar radiation on the Earth's surface, yet ordinary solar energy systems have limited ability in converting it to power.

Lanthanide photon upconversion nanoparticles (UCNPs) generally exhibit a nonlinear response to excitation light, featuring a higher quantum efficiency at a higher excitation intensity. Thus, effective excitation light concentrators, whenever feasible, are preferred, to make better use of the photon-upconverting capacity of UCNPs. Here, we explored polymer microlens arrays (MLAs) as light concentrators for irradiating UCNPs and investigated their spatial light modulation effect on the resulting upconversion luminescence (UCL).