A multiband NIR upconversion core-shell design for enhanced light harvesting of silicon solar cells

Exploring lanthanide light upconversion (UC) has emerged as a promising strategy to enhance the near-infrared (NIR) responsive region of silicon solar cells (SSCs). However, its practical application under normal sunlight conditions has been hindered by the narrow NIR excitation bandwidth and the low UC efficiency of conventional materials. Here, we report the design of an efficient multiband UC system based on Ln³⁺/Yb³⁺-doped core-shell upconversion nanoparticles (Ln/Yb-UCNPs, Ln³⁺= Ho³⁺, Er³⁺, Tm³⁺). In our design, Ln³⁺ ions are incorporated into distinct layers of Ln/Yb-UCNPs to function as near-infrared (NIR) absorbers across different spectral ranges. This design achieves broad multiband absorption withtin the 1100 to 2200 nm range, with an aggregated bandwidth of ~500 nm. We have identified a synthetic electron pumping (SEP) effect involving Yb³⁺ ions, facilitated by the synergistic interplay of energy transfer and cross-relaxation between Yb³⁺ and other ions Ln³⁺ (Ho³⁺, Er³⁺, Tm³⁺). This SEP effect enhances the UC efficiency of the nanomaterials by effectively transferring electrons from the low-excited states of Ln³⁺ to the excited state of Yb³⁺, resulting in intense Yb³⁺ luminescence at ~980 nm within the optimal response region for SSCs, thus markedly improving their overall performance. The SSCs integrated with Ln/Yb-UCNPs with multiband excitation demonstrate the largest reported NIR response range up to 2200 nm, while enabling the highest improvement in absolute photovoltaic efficiency reported, with an increase of 0.87% (resulting in a total efficiency of 19.37%) under standard AM 1.5 G irradiation. Our work tackles the bottlenecks in UCNP-coupled SSCs and introduces a viable approach to extend the NIR response of SSCs.