Super Broadband-Sensitive Upconversion in Tm and Ni Codoped Perovskites

Hom Nath Luitel,Yasuhiko Takeda,Shintaro Mizuno

doi:10.1155/2019/7679182

Abstract

We investigated broadband-sensitive upconversion (UC) processes in a series of Tm- and Ni-sensitized ABO3 (A = Ca/Sr/Ba and B = Ti/Zr/Hf) perovskites. We have designed combinations of the sensitizers and host cations such that super broad solar radiation ranging from 900 nm to nearly 2000 nm can be efficiently upconverted to 800 nm and shorter wavelengths. The Ni2+ ions located at the center of O2− octahedra absorbed photons in the 900–1500 nm range and transferred those energies to the nearby Tm3+ ions. The Tm3+ ions upconverted those energies at 800 nm, along with the energies absorbed by themselves in the 1100–1250 and 1550–2000 nm ranges, exhibiting super broadband sensitivity. Among the ABO3:Tm, Ni (A = Ca/Sr/Ba and B = Ti/Zr/Hf) upconverters, CaTiO3:Tm, Ni exhibited the best performance due to its most distorted crystal structure, which intensified the emission and absorption extents by increasing the optical transition probabilities of Tm3+ and Ni2+ ions. Introduction of alkali ions at the Ca2+ sites and Nb5+ ions at the Ti4+ sites intensified the UC emission by many folds, mainly due to a charge balance mechanism. At the same time, bigger and smaller codoped alkali ions created an asymmetric crystal field around the active ions and further enhanced the UC emission. Importantly, the upconverted photons are within the absorption edges of GaAs, Cu2ZnSnS4, and dye-sensitized solar cells making wider applications of these upconverters besides crystalline Si solar cells.

Highlights

Solar cells, which convert sunlight into a usable form of energy, are the most useful renewable energy devices for the present sustainable society
An upconverter layer is placed at the back face of a solar cell such that it absorbs low-energy NIR photons transmitted through the solar cell and emits high-energy photons, which are efficiently absorbed by the solar cells
Ni2+ ions located at the center of the TiO6 octahedra (Ti4+ sites) absorb 900–1500 nm photons and transfer the energies to the Tm3+ ions

Summary

Introduction

Solar cells, which convert sunlight into a usable form of energy, are the most useful renewable energy devices for the present sustainable society. Present solar cells suffer severely from their low conversion efficiencies that hardly reach 25% even in the case of optimized single-junction solar cells [1]. The low conversion efficiency is mainly due to the mismatch of photon energies below and above the bandgap of the semiconductor used in solar cell devices. Photon upconversion (UC), a process of using two or more low-energy photons to generate a high-energy photon, can increase conversion efficiency of such solar cells. In this technique, an upconverter layer is placed at the back face of a solar cell such that it absorbs low-energy NIR photons transmitted through the solar cell and emits high-energy photons, which are efficiently absorbed by the solar cells

Methods

Results

Conclusion