CaTiO 3 :Er 3+ ,Ni 2+ 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 CaTiO 3 :Er 3+ ,Ni 2+ 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 Ni 2+ sensitizers absorb 1060–1450 nm photons and efficiently transfer the energies to the Er 3+ emitters. In addition to 1450–1630 nm photons that are directly absorbed by the Er 3+ ions, 1060–1450 nm photons are also upconverted to 980 nm by the Er 3+ , Ni 2+ codoped upconverter, leading to a more remarkably enhanced conversion efficiency of c‐Si solar cells compared to conventional Er 3+ ‐only doped upconverters. Codoping with Nb 5+ to compensate for the charge imbalance caused by the Ni 2+ doping at the Ti 4+ sites intensified the UC emission. Similarly, charge imbalance caused by the Er 3+ doping at the Ca 2+ 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 CaTiO 3 :Er,Ni upconverter compared to the previously reported CaZrO 3 :Er,Ni one. It is explained on the basis of higher Ni → Er energy transfer efficiency and a more efficient Er 3+ 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.