E. Fourmond

Boron-oxygen defect in Czochralski-silicon co-doped with gallium and boron

We study the boron-oxygen defect in Si co-doped with gallium and boron with the hole density 10 times higher than the boron concentration. Instead of the linear dependence of the defect density on the hole density observed in boron and phosphorus compensated silicon, we find a proportionality to the boron concentration. This indicates the participation of substitutional, rather than interstitial, boron in the defect complex. The measured defect formation rate constant is

Photonic crystal assisted ultra-thin silicon photovoltaic solar cell

A new concept of ultra-thin film photovoltaic solar cell including a planar photonic crystal is proposed. The goal is to couple the incident light into broad resonances guided in the absorbing layer. To achieve this, a periodic lattice is patterned within the active layer, for example made of holes in amorphous silicon. By adjusting the pattern dimensions, the spectral position and quality factor of these resonances can be controlled so as to optimise the global absorption. Design details will be discussed in this communication.

Impact of incomplete ionization of dopants on the electrical properties of compensated p-type silicon

This paper investigates the importance of incomplete ionization of dopants in compensated p-type Si and its impact on the majority-carrier density and mobility and thus on the resistivity. Both theoretical calculations and temperature-dependent Hall-effect measurements demonstrate that the carrier density is more strongly affected by incomplete ionization in compensated Si than in uncompensated Si with the same net doping. The previously suggested existence of a compensation-specific scattering mechanism to explain the reduction of mobility in compensated Si is shown not to be consistent with the T-dependence of the measured carrier mobility. The experiment also shows that, in the vicinity of 300 K, the resistivity of compensated Si has a much weaker dependence on temperature than that of uncompensated silicon.

Incomplete Ionization and Carrier Mobility in Compensated p -Type and n-Type Silicon

In this paper, we show through both calculations and Hall effect measurements that incomplete ionization of dopants has a greater influence on the majority-carrier density in p-type and n-type compensated Si than in uncompensated Si with the same net doping. The factors influencing incomplete ionization at room temperature are shown to be the majority-dopant concentration, its ionization energy and type, and the compensation level. We show that both the majority- and the minority-carrier mobilities are lower in compensated Si than expected by Klaassens model and that the discrepancy increases with the compensation level at room temperature. The study of the temperature dependence of themajority-carrier mobility shows that there is no compensation-specific mechanism and that the reduction of the screening in compensated Si cannot explain alone the observed gap between experimental and theoretical mobility.

Laser ablation compatible substoichiometric SiOx/SiNy passivating rear side mirror for passivated emitter and rear thin-film crystalline silicon solar cells

Fabrication of industrial thin-film crystalline silicon solar cells remains challenging because of the high level of light trapping and surface passivation required to achieve a good conversion efficiency, while reducing the process cost. This work proposes a solution of rear side reflector supplying both passivation and light trapping, and guaranteeing compatibility with a laser process for local opening in order to use the passivated emitter and rear cell architecture. The key element is the use of substoichiometric silicon oxide deposited by plasma-enhanced chemical vapor deposition with a higher silicon concentration than the usual nearly stoichiometric oxide. This material is absorbent at usual ultraviolet laser wavelengths, and thus allows laser ablation with limited substrate heat, greatly reducing substrate damage after ablation. A layer of this oxide is incorporated into a SiOx/SiNy dielectric stack, which shows the expected qualities in term of passivation and reflectivity.

Nanostructured down-converter module for photovoltaic application

In silicon-based solar cells, a substantial part of the energy losses is related to the charge carriers thermalization in the UV-blue range and the week carriers collection at these wavelenghts. To avoid this issue, we introduce a new concept which combines a rare-earths doped thin layer with a photonic crystal (PC) layer, allowing an efficient conversion from UV-blue photons to near-IR photons. We report on the feasibility of such a nanostructured down-converter module using an active rare-earth doped CaYAlO4 thin layer and a silicon nitride PC on top. By means of optical numerical simulations, the promising potentialities of the concept are demonstrated.

Optical properties of porous Si/PECVD SiNx:H reflector on single crystalline Si for solar cells

Abstract The improvement of optical confinement on the back crystalline silicon solar cell is one of the factors leading to its better performance. Porous silicon (PS) layer can be used as a back reflector (BR) in solar cells. In this work, single layers of porous silicon were grown by electrodeposition on a single crystalline silicon substrate. The measurement of the total reflectivity (RT) on Si/PS surface showed a significant improvement in optical confinement compared to that measured on Si/standard Al back surface field (BSF). The internal reflectivity (RB) extracted from total reflectivity measurements achieved 86 % for the optimized single PS layer (92 nm thick layer with 60 % porosity) in the wavelength range between 950 and 1200 nm. This improvement was estimated as more than 17 % compared to that measured on the surface of Si/BSF Al contact. To improve the stability and passivation properties of PS layer BR, silicon nitride layer (SiNx) was deposited by PECVD on a PS layer. The maximum measured total reflectivity for PS/SiNx achieved approximately 56 % corresponding to an improved RB of up to 83 %. The PS formation process in combination with the PECVD SiNx, can be applied in the photovoltaic cell technology and offer a promising technique to produce high-efficiency and low-cost c-Si solar cells.

Light trapping in advanced solar cells: photonic crystals and disordered structures

Integrating Photonic Crystals in solar cells is a powerful way to control light absorption. We will present strategies leading to high conversion efficiencies, considering both carrier recombination issues and absorption optimization by complex nanophotonic structures.

Down Converter Device Combining Rare-Earth Doped Thin Layer and Photonic Crystal for c-Si Based Solar Cell

The aim of the study is to develop ultra-compact structures enabling an efficient conversion of single high energy photon (UV) to two lower energy photons (IR). The proposed structure combines rare-earths doped thin layer allowing the down-conversion process with a photonic crystal (PhC), in order to control and enhance the down-conversion using optical resonances. On the top of the rare-earths doped layer, a silicon nitride (SiN) 2D planar PhC is synthesized. For that, SiN is first deposited by PECVD. After holographic lithography and reactive ion etching, a periodic square lattice of holes is generated on the SiN layer. The PhC topographical parameters as well as the layers thickness are optimized using Finite-Difference-Time-Domain simulations. The design and realization of such PhC-assisted down-converter structures is presented. Optical simulations demonstrate that the PhC leads to the establishment of resonant modes located in the underneath doped layer, allowing a drastic enhancement of the absorption of the rare-earth ions without disturbing the transmission in the visible and near-IR parts of the spectrum, hence demonstrating the relevance of such an approach.

Comparison between SiNx:H and hydrogen passivation of electromagnetically casted multicrystalline silicon material

Abstract This work intends to compare two different passivation methods for electromagnetically continuous pulling silicon (EMCP): remote plasma hydrogenation and remote plasma enhanced CVD of SiN followed by high-temperature sintering. All experiments are carried out on textured and non-textured EMCP samples from the same ingot. To check the effect of high-temperature diffusion on EMCP, a n+-emitter is formed on one group of the samples using POCl3 diffusion. Passivation capabilities of both techniques are checked using measurements of minority carrier lifetime by means of microwave photoconductance decay mapping. Solar cells are made to compare lifetime measurement with cell parameters.