Rare earth doped sulphide materials for white light generation and energy conversion.

Project leaders

Stefano Pelli, Tural Gulu Naghiyev

Agreement

AZERBAIJAN - ANAS-not in force - Azerbaijan National Academy of Sciences

Call

CNR-ANAS 2016-2017

Department

Physical sciences and technologies of matter

Thematic area

Physical sciences and technologies of matter

Status of the project

New

Research proposal

It is well known that about 20% of electricity consumption in industrial countries is devoted to lighting purposes. It is therefore obvious that any efficiency increase of lighting devices has significant positive effects both from an economical and environmental safety points of view. Actually, it is foreseeable that development of efficient lighting devices will decrease the world energy consumption, with lower need of power plants.
Presently, solid state lighting (SSL) is quickly progressing and light emission efficiency of the devices increases at a very fast rate, with costs reducing ten times every ten years (Haitz's law). Nonetheless, there is still ample room for improvements, since commercially available LED light bulbs emit on average 80-100 lumen/W, still far from 350 lumen/W, considered the theoretical limit for this class of lighting devices.
Moreover, SSL does not promise only much higher efficiency. By a proper choice of pump source and emitting phosphors species, it is relatively easy to fine-tune the colour temperature to match the desired application.
In spite of the technological advancements that have been achieved in the recent years and the fact that SSL has reached commercial grade and it is already pervading households, improvements in light quality, efficiency and cost need to be pursued. In particular, a better combination of all these factors must be investigated, both at a research level and for commercial production, since it is generally possible to obtain better results only respect to one feature at the expense of the others. An overall good level of achievement of all SSL quality defining parameters at the same time has yet to be reached.
For this reason, in this project we aim to study and define the potential for SSL of a new class of materials, namely lanthanide doped calcium thiogallates solid solutions (CaxBa1-xGa2S4). These hosts, doped with rare earths (e.g. Eu2+, Tb3+), exhibit strong photoluminescence emission in the visible range when excited in the blue-UV spectrum region, compatible with the presently available AlGaN LEDs. They can therefore find immediate application in the development of efficient SSL sources.
We therefore intend to carry out a thorough synthesis and characterization analysis of this class of compounds. As a first step, we will start with Eu2+/Ce3+ codoping and analyse the photoluminescence emission in order to find the best composition (thiogallate stoichiometry and rare earths relative concentrations) and synthesis conditions in order to optimize the material in terms of PL efficiency, CIE colour index and colour rendering index (CRI).
Other combinations of rare earths doping will then be examined, such as Ce3+/Tb3+and Ce3+/Dy3+, where Ce3+ also acts as sensitizer, thus increasing the efficiency of the system. In a later phase, depending on previous results, more complex systems such as Ce3+/Tb3+/Dy3+ may be considered. The characterisation measurements themselves will partially determine the best doping choice.
X-ray diffraction will also be employed to perform structural analysis of the samples and we will correlate these results with the optical measurements.
Regarding material synthesis, the solid solutions will be obtained by solid-state reaction technique from the BaS, CaS, Ga2S3 binary compounds in stoichiometric ratios. In a first step, CaS and BaS binary compounds will be synthesised from the corresponding metal carbonates in a H2S atmosphere. Ga2S3 binary compound will be prepared by solid state reaction at 1423 K. Finally, these materials and rare-earth elements will be placed together in quartz ampoules evacuated to 10-4 Torr pressure and put in a single-temperature furnace at 1273 K for 2 hours. The resulting compounds will then be annealed for 24 hours at 973 K after synthesis. After the synthesis, the embedding of the prepared materials in inorganic or polymer matrixes will be carried out, in order to have a good dispersion without altering the emission properties.
The emitting inorganic glasses (mainly silicates) will be prepared using sol-gel technique at low temperatures. Dispersion in organic matrices will be also considered and carried out, taking into consideration low temperature melting and UV transparent organic moieties. After a careful characterization of the spectroscopic properties, the optimal conditions in terms of host compositions, amount and type of glass and polymer will be performed, to investigate any change in the spectroscopic properties induced by the host environment.
Detailed spectroscopic characterisations will therefore be carried out both in the solid solutions as synthesised and for the materials embedded in the host. Measurements will include absorption, excitation and photoluminescence spectroscopy of the lanthanide dopants as well as lifetime measurements of the main radiative transitions. Excitation will be mainly provided by UV/blue LED sources.
The groups involved in the project have complementary expertise and equipment. In fact, synthesis of the solid solutions and structural analysis will be mainly carried out at the Institute of Physics of Azerbaijan National Academy of Sciences. The characterization tasks will be mainly performed at the Istituto di Fisica Applicata "Nello Carrara", while the embedding tests will be carried out at the Biotechnology Department, University of Verona, Italy.

Research goals

The project aims to develop a new class of materials, namely calcium thiogallates solid solutions (CaxBa1-xGa2S4).
Main goals of our proposed research are:
- Testing various thiogallates compositions and rare earth doping.
- Analysing the structural (x-ray diffraction) and spectral properties as well as the emission efficiency of the solid solutions obtained.
- Optimizing the compositions and doping of thiogallates in order to obtain efficient white light emitting solid solutions.
- Assessing the feasibility of embedding the doped thiogallates synthesised in the framework of this project in a glass or polymer host and assessing how the host environment may affect the spectral properties of the doped solid solutions.

Last update: 06/07/2025