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.