Upconversion of 1.54 μm radiation in Er³⁺ doped fluoride-based materials for c-Si solar cell with improved efficiency

¹ Laboratoire de Chimie de la Matière Condensée de Paris LCMCP UMR7574 CNRS/UPMC/Chimie ParisTech, Chimie ParisTech, 11 rue Pierre et marie Curie, 75235 Paris, France
² Center for Information and Optics Technology, University of Information Technology, Mechanics and Optics, 199034, St. Petersburg, Russia
³ IRDEP, UMR 7174CNRS/EDF/Chimie ParisTech, 6 quai Watier, 78401 Chatou, France

^a e-mail: fabienne-pelle@chimie-paristech.fr

Received: 3 January 2011
Accepted: 1 June 2011
Published online: 6 October 2011

Abstract

Upconverted emission from erbium ions in fluoride materials (glass and disordered crystal of the system CaF₂-YF₃) are observed in a wide spectral range (from the visible to the near infrared) under infrared excitation at 1.54 μm. In both cases, the upconverted emission in the near infrared (~1 μm) dominates the spectrum. Absolute UC efficiency defined as the ratio between the UC luminescence power and the absorbed pump power has been experimentally measured. The NIR (~1 μm) luminescence energy yield for the glass and the disordered crystal varies from 2.4 to 11.5% for the glass and from 7.7 to 16% for the crystal for an infrared excitation power density ranging from 2 W/cm² to 100 W/cm². This is of a particular interest for their use as upconverter to improve the c-Si cells quantum efficiency since the energy of the excitation lies below the c-Si absorption edge (1.12 eV at 300 K) and is well located compared to the AM1.5G solar spectrum, outside of the absorption lines due to different atmospheric gases. Furthermore, the most efficient upconverted emission recorded in the investigated materials occurs at an energy just above the gap. A current generated in a bifacial c- Si solar cell is observed when the Er³⁺ doped material (1.55 mA and 2.15 mA for the glass and the crystal respectively), placed at the rear face of the cell, is excited at 1.54 μm. The current dependence as a function of the sub-bandgap excitation power has been measured and modelled. Finally the EQE of the complete device is deduced from the measured short-circuit current and the incident photon flux on the cell. An increase of the cell quantum efficiency of 2.4% and 1.7% is obtained at 1.54 μm with adding the glass and the crystal respectively at the rear face of the c-Si cell. The results are compared to those already obtained with Er: NaYF₄ known as the most efficient upconverter.