A research team led by Hans Ågren, professor in theoretical chemistry at KTH Royal Institute of Technology, has developed a film that can be applied on top of ordinary solar cells, which would enable them to use infrared light in energy conversion and increase efficiency by 10 percent or more.
“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.
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). It was found that a piece of MLA can potentially concentrate excitation light by orders of magnitude, subject to its structure and optical properties, and lead to a very significant enhancement of the UCL. MLAs can be easily incorporated into different types of UCNP-enhanced photonic devices, such as dye-sensitized solar cells, and bring further performance improvement in the near infrared range. A test on a dye-sensitized solar cell proved this contention, however, there is much room for optimizing a variety of parameters both for the solar cells and for the light concentrating upconverting layers to make the combined effects even more significant.
The dearth of high upconversion luminescence (UCL) intensity at low excitation irradiance hinders the prevalent application of lanthanide-doped upconversion nanoparticles (UCNPs) in many fields ranging from optical bioimaging to photovoltaics. In this work, we propose to use microlens arrays (MLAs) as spatial light modulators to manipulate the distribution of excitation light fields in order to increase UCL, taking advantage of its nonlinear response to the excitation irradiance. We show that multicolored UCL from NaYF4:Yb3+,Er3+@NaYF4:Yb3+,Nd3+ and NaYF4:Yb3+,Tm3+@NaYF4:Yb3+,Nd3+ core/shell UCNPs can be increased by more than one order of magnitude under either 980 or 808 nm excitation, by simply placing a polymeric MLA onto the top of these samples. The observed typical green (525/540 nm) and red (654 nm) UCL bands from Er3+ and a blue (450/475 nm) UCL band from Tm3+ exhibit distinct enhancement factors due to their different multi-photon processes. Importantly, our ray tracing simulation reveals that the MLA is able to spatially confine the excitation light (980 and 808 nm) by orders of magnitude, thus amplifying UCL by more than 225-fold (the 450 nm UCL band of Tm3+) at low excitation irradiance. The proposed MLA method has immediate ramifications for the improved performance of all types of UCNP-based devices, such as UCNP-enhanced dye sensitized solar cells demonstrated here.
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