M. Lemiti

Absorption enhancement using photonic crystals for silicon thin film solar cells

We propose a design that increases significantly the absorption of a thin layer of absorbing material such as amorphous silicon. This is achieved by patterning a one-dimensional photonic crystal (1DPC) in this layer. Indeed, by coupling the incident light into slow Bloch modes of the 1DPC, we can control the photon lifetime and then, enhance the absorption integrated over the whole solar spectrum. Optimal parameters of the 1DPC maximize the integrated absorption in the wavelength range of interest, up to 45% in both S and P polarization states instead of 33% for the unpatterned, 100 nm thick amorphous silicon layer. Moreover, the absorption is tolerant with respect to fabrication errors, and remains relatively stable if the angle of incidence is changed.

Optical properties of silicon nanocrystal LEDs

Abstract In this work, we describe how to fabricate good quality 3 nm nc-Si with low size distribution in thermal SiO2 oxides. Photoluminescence, excited photoluminescence, and photocurrent measurements are discussed on the basis of theoretical calculations of the quantified levels in nc-Si. The impact of shape and size in quantum dots on transition energies has been highlighted, thanks to 2D symmetrical self-consistent Poisson–Schrodinger simulations. Both direct and indirect gaps in silicon have been considered in order to carry out a better comparison between simulations and optical measurements. A good agreement is found between simulations and experimental data for the indirect gap of 3 nm dots which show a threshold energy around 2 eV . However, the optical recombinations seems to be related to lower energy states probably due to interfacial radiative defects around 1.58 eV . On the basis of highly luminescent nc-Si, we have fabricated an optimized light emitting device (LED) with a calculated design in order to favour both electron and hole injection. Stable red electroluminescence has been obtained at room temperature and the I–V measurements confirm that the current is related to a pure tunnelling process. A modelling of I–V curves confirms a Hopping mechanism with an average trap distance between 1.4 and 1.9 nm . The Fowler–Nordheim process is not observed during light emission for electric fields below 5 MV / cm . Finally, we have not hot carrier injection and thus it seems possible to develop Si-based LEDs with a good reliability.

Absorbing one-dimensional planar photonic crystal for amorphous silicon solar cell

We report on the absorption of a 100nm thick hydrogenated amorphous silicon layer patterned as a planar photonic crystal (PPC), using laser holography and reactive ion etching. Compared to an unpatterned layer, electromagnetic simulation and optical measurements both show a 50% increase of the absorption over the 0.38-0.75micron spectral range, in the case of a one-dimensional PPC. Such absorbing photonic crystals, combined with transparent and conductive layers, may be at the basis of new photovoltaic solar cells.

Two-dimensional photonic crystal for absorption enhancement in hydrogenated amorphous silicon thin film solar cells

We propose a photovoltaic solar cell design based on a 100 nm thick absorbing layer made of hydrogenated amorphous silicon and patterned as a two-dimensional planar photonic crystal (PPC). After scanning the parameters of the PPC within the patterned cell, optical simulations performed on the best configuration obtained reveal that a relative increase in the integrated absorption inside the active layer of 28% can be expected between 300 and 720 nm compared to an equivalent but nonpatterned cell under normal incidence. Besides, this integrated absorption is found to be robust toward the angle of incidence. Incident light is efficiently coupled to leaky mode resonances of the PPC provided an appropriated tuning of its parameters. The effects of the reflectance of the back contact coupled to a conductive optical spacer on the absorption are also discussed.

Light harvesting by planar photonic crystals in solar cells: the case of amorphous silicon

In this paper, we discuss on light management in silicon thin film solar cells, using photonic crystals (PhC) structures. We particularly focus on photovoltaic devices including amorphous silicon absorbers patterned as 2D PhCs. Physical principles and design rules leading to the optimized configuration of the patterned cell are discussed by means of optical simulations performed on realistic thin film solar cell stacks. Theoretically, a maximum 40%rel increase of integrated absorption in the a-Si:H layer of the patterned cell is expected compared to the unpatterned case. Moreover, both simulation and optical characterization of the fabricated cells demonstrate the robustness of their optical properties with regards to the angle of incidence of the light and to the fabrication induced defects in the PhCs. Finally, the impact of the surface recombination due to the generation of new free surfaces with higher defect densities is addressed. We demonstrate that patterning still induces a substantial increase of the conversion efficiency, with a reasonable surface recombination velocity.

Electronic properties of highly-doped and compensated solar-grade silicon wafers and solar cells

Compensation effects are intensively studied on two highly doped ingots grown from solar-grade silicon feedstocks purified using metallurgical routes, through a comparison of the electrical properties at iso-carrier densities. Working at given carrier densities enables a clearer extraction of the compensation effects, at the wafer and solar cell levels. At the wafer level, the majority carrier mobility and the carrier lifetime are investigated. Regarding the mobilities, it was found that current models may underestimate the amount of incomplete ionization of boron leading to underestimated mobilities. In addition, the majority carrier mobility was found to be strongly affected at high compensation level. Regarding the carrier lifetimes, our results show that after a phosphorus diffusion step, dopants alone — and especially boron — can limit the lifetime in highly doped solar-grade silicon. At the cell level, I-V characteristics under standard illumination were studied. In particular, the observed reductio...

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

Effect of total pressure on the formation and size evolution of silicon quantum dots in silicon nitride films

The size of silicon quantum dots (Si QDs) embedded in silicon nitride (SiN(x)) has been controlled by varying the total pressure in the plasma-enhanced chemical vapor deposition (PECVD) reactor. This is evidenced by transmission electron microscopy and results in a shift in the light emission peak of the quantum dots. We show that the luminescence in our structures is attributed to the quantum confinement effect. These findings give a strong indication that the quality (density and size distribution) of Si QDs can be improved by optimizing the deposition parameters which opens a route to the fabrication of an all-Si tandem solar cell.

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.

Plasmon-enhanced nonlinear optical properties of SiC nanoparticles

An original plasmonic nano-Ag/SiN(x) substrate was elaborated to strongly enhance the nonlinear response of SiC NPs for the first time. A plasmon-induced two order of magnitude increase of second-harmonic generation and two-photon excited photoluminescence was experimentally achieved. The measured enhancement factors were correlated with local field intensities theoretically estimated by finite-difference time-domain calculations. The obtained plasmon-enhanced nonlinear response of the SiC nanostructures make them promising in nonlinear optics applications.