• Energy Research

[EN] In this paper we show the results of the cost model developed in LIMA project (FP7-248909). The LIMA project is titled "Improve photovoltaic efficiency by applying novel effects at the limits of light to matter interaction". The project started in January 2010 and during this year a cost model of the device developed in the project has been developed to assess the industrial viability of this innovative approach to increase the efficiency and reduce the cost of photovoltaic solar cells. LIMA project exploits cutting edge photonic technologies to enhance silicon solar cell efficiencies with new concepts in nanostructured materials. It proposes nano-structured surface layers designed to increase light absorption in the solar cell while decreasing surface and interface recombination loss. Integration in a back contact design further reduces these interface losses and avoids shading. The project improves light-matter interaction by the use a surface plasmonic nanoparticle layer. This reduces reflection and efficiently couples incident radiation into the solar cell where it is trapped by internal reflection. Surface and interface recombination are minimized by using silicon quantum dot superlattices in a passivating matrix. The distance between quantum dots ensures wave-function overlap and good conductivity. © 2010 Published by Elsevier Ltd. This work has been carried out in the framework of the LIMA Project. The EC is gratefully acknowledged for financial support under Contract number FP7-248909.

One of the ways in which the cell efficiency of solar cells may be improved by better exploitation of the solar spectrum makes use of the down-conversion mechanism, where one high energy photon is cut into two low energy photons. When energy transfer between rare earth ions is used to activate this process, high emission and absorption cross sections as well as low cutoff phonon energy are mandatory. Glass-ceramics can be a viable system to fulfill these requirements. The main advantage of the glass-ceramic is to combine the mechanical and optical properties of the glass with a crystallike environment for the rare-earth ions, where higher cross-sections of the rare earth ion can be exploited. In the case of silica-hafnia system the glass ceramic is constituted by nanocrystals of HfO2, containing the rare earth ion, imbedded in the silica-hafnia host. Hafnia nanocrystals are characterized by a cutoff frequency of about 700 cm-1, so that nonradiative transition rates are strongly reduced, thus increasing the luminescent quantum yield of the rare-earth ions. In this work we investigated the Tb3+/Yb3+ energy transfer efficiency in a 70SiO2-30HfO2 glass-ceramic waveguide in order to convert absorbed photons at 488 nm in photons at 980 nm. The energy transfer efficiency was estimated as a function of the Tb3+/Yb 3+ molar ratio as well as of the total amount of rare earth ions. A transfer efficiency of 38% was obtained for Tb3+/Yb3+ = 0.25 mol and a rare earth content [Tb+Yb]/[Si+Hf] = 5% mol.

Abstract Silicon nanocrystals show a significant shift between the strong absorption in the blue–ultraviolet region and their characteristic red–near-infrared emission as well as space separated-quantum cutting when short wavelength photons are absorbed. These two effects can be used to increase the efficiency of crystalline silicon solar cells. We fabricated high quality interdigitated back-contact crystalline silicon solar cells in an industrial pilot line and coated them with optimized silicon nanocrystals layers in a cost effective way. Here we demonstrate an increase of 0.8% of the power conversion efficiency of the interdigitated back-contact cell by the silicon nanocrystals layer. In addition, we prove that this increase is due to a combination of a better surface passivation, a better optical coating, and of the luminescent downshifting effect. Moreover we demonstrated that the engineering of the local density of photon states, thanks to the Purcell effect, is instrumental in order to exploit this effect.

The effects of a Si-rich silicon oxide (SRO) layer containing silicon nanocrystals as photoluminescence down-shifter layer on a conventional Si solar cell were investigated. Two SRO layers with different thicknesses but same composition were deposited on top of Si solar cells by plasma-enhanced chemical vapor deposition and followed by high temperature annealing to precipitate silicon nanocrystals. The SRO layers absorb efficiently high energy photons (especially higher than twice Si bandgap) and emit photons at longer wavelength, which are in turn absorbed by Si. A relative increase of about 14% to the internal quantum efficiency has been observed.

Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges.