Si-based conversion frequency layer for Si solar cell
Fabrice Gourbilleau
Center of investigation of ions, materials and photonics (Caen, France)
Increasing solar cell efficiency while keeping low cost process is a key issue for the Si solar industry
in the next years. One of the solutions consists in reducing the thermalization effect due to the mismatch
between the solar spectrum and the energy band gap of the solar cell. To achieve such a goal, conversion
frequency layers have been developed with the aim of converting one UV photon into two IR ones. The
layers fabricated contain the couple Tb:Yb which allows to absorb energetic photons in the 250-450 nm
range and to emit IR photons at 980 nm. The major drawback of using rare earth ions is their low
absorption cross section. This issue can be overcome by the development of a host matrix compatible
with the Si-PV fabrication and containing sensitizers that efficiently excite the rare earth ions. A previous
work on a SixOyNz matrix doped with Tb:Yb has demonstrated the feasibility of such a concept of
efficient down converter layer with a quantum efficiency of 183%.
A new approach using an oxygen-free matrix by means of sputtering technique has been considered
with the aim of fabricating a rare earth doped-SiNx layer with characteristics close to those of the
antireflective layer used in the crystalline Si solar cell. After a careful optimization of the SiN x to manage
both a wide absorption range and an efficient sensitizing role towards the Tb3+ ions, the deposition
conditions have been optimized to achieve the highest emission intensity when introducing the couple
Tb:Yb in the nitride matrix. By a careful analysis of the optical properties of this system, the excitation
mechanisms of Tb3+ and Yb3+ ions will be discussed. The absence of Tb emission for the optimized film
suggests that an internal quantum efficiency as high as 200% has been achieved. With the aim of always
improving the efficiency of such a conversion frequency layer, a multilayer approach has been developed.
It consists in depositing successively a Tb- and a Yb-doped sublayer. The thickness as well as the nature
(nitride, oxynitride or oxide) of each sublayer have been determined to optimize the absorption range and
the emission intensity at 980nm. The objective is to maximize the coupling between Tb3+ and Yb3+ ions
to achieve the highest efficiency of the down converter layer.
With the aim of further improving the efficiency of such a conversion frequency layer, a plasmonic
structure has been added. It consists in depositing a thin silver layer using the same fabrication technique
and in annealing it to form Ag nanoparticles. The effect of these later on the Tb3+, and Yb3+ emissions
will be presented.
BIODATA
Fabrice Gourbilleau graduated PhD in materials physics from the
University of Caen in 1993. He joined the CNRS in 1994 and since
then working on nanomaterials at the Centre for Res earch on Ions,
Materials and Photonics (CIMAP, UMR 6252). He is the leader of
the NIMPH team (Integrated Nanostructures for Microelectronics and
Photonics) constituting of about 20 persons, and since 2014 Deputy
Director of the CIMAP Lab as well as of the Research Group
NACRE (Nanocrystallites in dielectric). His research interests
concern the fabrication and study of thin films based on Si
nanostructure for photonics, photovoltaic and microelectronic
applications. He was and is involved in several European as
collaborator or coordinator and national projects. He is the
correspondent of Nanoscience in Normandy and is involved in
technology transfer via Normandie Incubation, which he belongs. He
has authored or co-authored over 180 publications and has 2 patents.