Bilayer Luminescent Solar Concentrator with Enhanced Absorption and Efficiency for Agrivoltaic Applications

Luminescent solar concentrators (LSCs) are semi-transparent light harvesting devices, comprised of nanocrystal (NC) luminophores embedded inside optically transparent waveguides with small photovoltaic (PV) cells mounted on the edges. LSCs harvest solar energy by absorbing broadband sunlight, downshifting the light to a narrow spectrum, and concentrating the light toward the PV cells. One promising application space for LSCs is in agrivoltaics, where the LSC is mounted in a greenhouse, and simultaneously generates electricity and modifies the spectral quality of the transmitted light, promoting plant growth. The PV efficiency of LSCs can be limited by low luminophore absorption, however, and increasing the NC concentration in a polymer matrix leads to significant scattering that is detrimental to LSC performance. For agrivoltaics, the transmitted spectrum of LSCs also needs to have exceptional tunability to control the spectral quality. Therefore, we studied a bilayer LSC utilizing two different semiconductor quantum dots for enhanced absorption efficiency, and a tunable transmission spectrum.<br/><br/>A nanocomposite bilayer LSC comprised of a film of narrow band gap Si NCs embedded in poly(methyl methacrylate) (PMMA) and a film of wider band gap CdSe/CdS NCs embedded in poly(cyclohexylethylene) (PCHE) was fabricated. The Si NCs were synthesized through a non-thermal plasma synthesis method, and the film of the Si NCs embedded into PMMA was fabricated using doctor blade deposition. The CdSe/CdS NCs were synthesized using a colloidal synthesis method, then dispersed in octane. A solution of CdSe/CdS and PCHE in octane was spin-coated directly on the Si-PMMA film to create the bilayer LSC. Diffuse transmission measurements indicated low levels of haze in the bilayer devices.<br/><br/>Complementary Monte Carlo ray-tracing simulations were used to evaluate device efficiency. Compared to a single layer of Si-PMMA, the addition of the CdSe/CdS-PCHE film increased the overall absorption, which enhanced the optical efficiency of the device. Additionally, the CdSe/CdS nanocrystals act as sensitizers, increasing the absorption of the Si nanocrystals. <br/><br/>Light propagation through the LSC device was characterized experimentally by measuring the concentrated PL while changing the position of a 405 nm light source. The Si photoluminescence (PL) propagates through the bilayer LSC device with an attenuation coefficient close to zero, which is a necessary property for high efficiency LSCs. This finding emphasizes the benefit of sensitizing Si NC absorption through the incorporation of the CdSe/CdS NCs.<br/><br/>To demonstrate the tunable transmission spectrum of the bilayer device, the Soret and Q band transmission of chlorophyll was calculated while changing the optical density of the CdSe/CdS and Si NCs. The transmission through either band could be controlled by changing the optical density of either nanocomposite, thus this device can be used to design a transmission spectrum tailored for more ideal crop germination and growth.<br/><br/>This study demonstrates that incorporating two nanocrystal quantum dots into a bilayer LSC design can enhance absorption efficiency and spectral tunability, while also revealing the nature of the interactions between the luminophores.