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

The maximum absorption wavelength of silicon solar cells (SSCs) is approximately 1100 nm, due to the inherent bandgap limitations of silicon materials. This constraint results in significant optical energy loss. It is estimated that unabsorbed near-infrared (NIR) light energy accounts for about 20% of the total solar energy, contributing to the large gap between the current photovoltaic efficiency of SSCs and their theoretical limit. To address this, the development of efficient NIR fluorescent conversion materials is crucial for enhancing the performance of silicon-based photovoltaic cells.

 

Traditional upconversion materials typically have a single excitation wavelength, predominantly governed by the absorption bands of Er³⁺ and Yb³⁺ ions. Additionally, lanthanide-doped upconversion nanoparticles (Ln-UCNPs, Ln³⁺ = Ho³⁺, Er³⁺, Tm³⁺) excited by long-wavelength NIR light generate strong multiphoton (≥3 photons) upconversion emission and downshifted luminescence at wavelengths >1100 nm, both of which result in significant incident light energy loss and low upconversion quantum yield, hindering the enhancement of photovoltaic efficiency.

 

To overcome these limitations, this work introduces Yb³⁺ ion doping in Ln-UCNPs. As shown in Fig. 1, the emission spectra of Ln/Yb-UCNPs, along with the integral intensity statistics of each band, demonstrate a marked improvement. After Yb³⁺ doping, fluorescence emission within the SSC responsive region (400–1100 nm) is significantly enhanced, while multiphoton upconversion-induced visible emission and long-wavelength downshifted luminescence are rapidly suppressed (Fig. 1a). This phenomenon can be explained by the synergistic energy transfer process (SEP) mechanism, wherein cross-relaxation and energy transfer between Yb³⁺ and Ln³⁺ ions form a positive feedback loop. This interaction continuously feeds energy from the Ln³⁺ ion’s visible and long-wavelength emission levels to the Yb³⁺ ion's ²F₅/₂ level, leading to super-intense emission (Fig. 1f).

 

In summary, Yb³⁺ doping effectively suppresses inefficient luminescence, directing incident photons into the 980 nm emission of Yb³⁺ ions via a two-photon upconversion process, which aligns closely with the optimal response band of SSCs. Furthermore, a core-shell structure design was implemented to integrate three Ln/Yb-UCNP fluorescence conversion layers within a unified core-multi-shell (CSSS) structure (Figs. 2a and 2b). This design achieves broad multiband absorption in the 1100–2200 nm range, with an effective bandwidth of about 500 nm (Figs. 2c and 2d). This structure expands the SSC response range and enhances photovoltaic conversion efficiency.

 

By self-assembling a CSSS film onto commercial SSCs, the photovoltaic efficiency under AM 1.5G irradiation increased by 0.87%, with 0.67% experimentally attributed to the upconversion contribution of the CSSS film (Figs. 3b and 3c). Incident photon-to-electron conversion efficiency (IPCE) tests revealed that CSSS-coated SSCs achieved a response range spanning 350–2200 nm (Fig. 3e). Additionally, the CSSS structure exhibits excellent stability, making it highly suitable for commercial applications (Fig. 3f).

 

In conclusion, this work addresses the challenges of low multi-photon upconversion efficiency and excessive long-wavelength emission in traditional upconversion materials through Yb³⁺ doping and an innovative core-shell structure. This approach achieves approximately 500 nm of NIR wavelength conversion, significantly enhancing SSC performance under standard solar conditions. These findings provide new insights for improving the efficiency of photovoltaic cells.

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