Broadband-sensitive Ni2+–Er3+ based upconverters for crystalline silicon solar cells

Abstract

We have developed Ni²⁺, Er³⁺ codoped CaZrO₃ broadband-sensitive upconverters that significantly broaden the sensitive range, and hence overcome the shortcomings of conventional Er³⁺ doped upconverters used for crystalline silicon (c-Si) solar cells that utilize only a small fraction of the solar spectrum around 1550 nm. We have designed the combination of sensitizers and host material to utilize photons that are not absorbed by c-Si itself or Er³⁺ ions. Six coordinated Ni²⁺ ions substituted at the Zr⁴⁺ sites absorb (1060–1450) nm photons and transfer the energies to the Er³⁺ ions, and the Er³⁺ upconverts at 980 nm. Co-doping with monovalent charge compensators such as Li⁺ for high Er³⁺ solubilisation at the Ca²⁺ sites and multivalent ions (Nb⁵⁺) for stabilization of Ni²⁺ at the Zr⁴⁺ sites is essential. In addition to 1450–1600 nm (≈2 × 10²⁰ m⁻² s⁻¹) photons directly absorbed by the Er³⁺ ions, we have demonstrated upconversion of 1060–1450 nm (≈6 × 10²⁰ m⁻² s⁻¹) photons in the Ni²⁺ absorption band to 980 nm photons using the CaZrO₃:Ni²⁺,Er³⁺ upconverters. Compared with the current density gain of present c-Si solar cells (∼40 mA cm⁻²), the upconverted photons could increase this by ∼7.3 mA cm⁻², which is about 18% improvement. This architecture for broadband-sensitive upconversion may pave a new direction for the improvement in efficiency of the present c-Si solar cells to surpass the limiting conversion efficiency of single-junction solar cells.