(Invited) Light Management in Efficient Luminescent Si-Based Layers for Photovoltaic Applications

Abstract

Energy harvesting and saving are the key issues for this century. Many approaches, developments and efforts have been devoted these last years to propose innovative solutions that can offer a way to decrease the CO2production as well as the energy consumption, or to increase the green energy use. Among the different solutions, such as energy storage, energy conversion, fuel cells, the development of luminescent materials with high efficiency in a low cost process is one of the challenges to face. Indeed, luminescent materials are widely used in sensors, scintillators, light emitting diodes as well as photovoltaic applications. In most cases, the photon emission is achieved by means of rare earth ion excitation mechanism that offers a wide range of emission wavelengths ranging from the UV to the infrared. For the Silicon PV industry, increasing Si solar cell efficiency with the aim of always keeping a low cost process is a key issue to continuously decrease the cost of the power generation. This is the condition to remain a major player in the provision of power generation solutions in the forthcoming years. Among the different loss paths in a Si solar cell, the thermalization effect due to the mismatch between the solar spectrum energy range (UV region) and the solar cell band gap energy (1.1 eV of the Si solar cell) can be overcome. To achieve such a goal, frequency conversion layers so-called Down Conversion (DC) layers have been developed in order to convert one UV incident photon into two IR ones that can be absorbed by the Si cell. To reach this objective many systems using a couple of trivalent ions such as Pr-Yb, Tb-Yb have been developed. Unfortunately, their major drawbacks are either a non Si-compatible process due to the nature of the host matrix or a low absorption cross section of the rare earth ions that limits their excitability in the solar spectrum range. This problem can be solved by the development of a Si-PV compatible host matrix containing sensitizers that efficiently excite rare earth ions. This paper details the development a Si-PV compatible host matrix that can either contain sensitizers that efficiently absorb the solar spectrum for exciting rare earth ions and/or favors the incorporation of a high content of rare earth ions without the detrimental clustering effect. We will describe the fabrication and study of Tb:Yb and Ce:Yb doped Si-based thin films deposited by reactive co-sputtering technique. First Tb- or Ce-doped systems have been optimized to get the maximum emission intensity under UV range excitation prior to incorporating the Yb3+ions. Quantum efficiency as high as 200% has been achieved for these systems either in a composite or a multilayer structure. The fabrication parameters have been optimized to improve the coupling rate between the rare earth ions to get the maximum of emitted photon at 980 nm. The use of metallic nanoparticules has been developed to increase the optical path length of pumping photons and thus improve the efficiency of the system. External Quantum Efficiency measurements on a solar cell are carried out and will be presented. This work has been supported the French Research National Agency through the project GENESE (N° ANR-13-BS09-0020-01).