CaTiO₃:Er³⁺,Ni²⁺ broadband-sensitive upconverter: An effective way to harvest unused NIR solar irradiation for crystalline silicon solar cells

We have realized broadband-sensitive upconversion (UC) in CaTiO₃:Er³⁺,Ni²⁺ that can harvest 1060–1630 nm photons, which are not utilized by present crystalline silicon (c-Si) solar cells, and upconvert to 980 nm. The Ni²⁺ sensitizers absorb 1060–1450 nm photons and efficiently transfer the energies to the Er³⁺ emitters. In addition to 1450–1630 nm photons that are directly absorbed by the Er³⁺ ions, 1060–1450 nm photons are also upconverted to 980 nm by the Er³⁺, Ni²⁺ codoped upconverter, leading to a more remarkably enhanced conversion efficiency of c-Si solar cells compared to conventional Er³⁺-only doped upconverters. Codoping with Nb⁵⁺ to compensate for the charge imbalance caused by the Ni²⁺ doping at the Ti⁴⁺ sites intensified the UC emission. Similarly, charge imbalance caused by the Er³⁺ doping at the Ca²⁺ sites was compensated by codoping with alkali ions (Li⁺, Na⁺, K⁺), resulting in a significant improvement in the UC emission intensity. Nearly 6.5-fold intense UC emission was realized in the newly developed CaTiO₃:Er,Ni upconverter compared to the previously reported CaZrO₃:Er,Ni one. It is explained on the basis of higher Ni → Er energy transfer efficiency and a more efficient Er³⁺ UC emission. If these upconverters are applied at the rear-face of a bifacial c-Si solar cell, ∼4.8% absolute efficiency improvement is possible.