Ounsi El Daif

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.

Photonic assisted light trapping integrated in ultrathin crystalline silicon solar cells by nanoimprint lithography

We report on the fabrication of two-dimensional periodic photonic nanostructures by nanoimprint lithography and dry etching and their integration into a 1-μm-thin mono-crystalline silicon solar cell. Thanks to the periodic nanopatterning, a better in-coupling and trapping of light is achieved, resulting in an absorption enhancement. The proposed light trapping mechanism can be explained as the superposition of a graded index effect and of the diffraction of light inside the photoactive layer. The absorption enhancement is translated into a 23% increase in short-circuit current, as compared to the benchmark cell, resulting in an increase in energy-conversion efficiency.

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.

Design, fabrication and optical characterization of photonic crystal assisted thin film monocrystalline-silicon solar cells.

In this paper, we present the integration of an absorbing photonic crystal within a monocrystalline silicon thin film photovoltaic stack fabricated without epitaxy. Finite difference time domain optical simulations are performed in order to design one- and two-dimensional photonic crystals to assist crystalline silicon solar cells. The simulations show that the 1D and 2D patterned solar cell stacks would have an increased integrated absorption in the crystalline silicon layer would increase of respectively 38% and 50%, when compared to a similar but unpatterned stack, in the whole wavelength range between 300 nm and 1100 nm. In order to fabricate such patterned stacks, we developed an effective set of processes based on laser holographic lithography, reactive ion etching and inductively coupled plasma etching. Optical measurements performed on the patterned stacks highlight the significant absorption increase achieved in the whole wavelength range of interest, as expected by simulation. Moreover, we show that with this design, the angle of incidence has almost no influence on the absorption for angles as high as around 60°.

Surface emitting microlaser based on 2D photonic crystal rod lattices

2D photonic crystal (2D PC) structures consisting in a square lattice of Indium Phosphide (InP) microrods bonded on a Silicon/Silica Bragg mirror are experimentally investigated. We focus on slow Bloch modes above the light line, especially at the Gamma-point where a vertical emission can be obtained. Stimulated emission around 1.5 microm is demonstrated in such structures, at room temperature, for the first time. In addition the achieved threshold power lies within the range reported for surface emitting devices based on conventional lattices of holes. It is shown that the laser mode is laterally confined by a carrier induced refractive index change, under pulsed excitation. It is also demonstrated that this type of 2D PC is well suited for sensors integrated in microfluidic systems.

Micrometer-Thin Crystalline-Silicon Solar Cells Integrating Numerically Optimized 2-D Photonic Crystals

A 2-D photonic crystal was integrated experimentally into a thin-film crystalline-silicon solar cell of 1-μm thickness, after numerical optimization maximizing light absorption in the active material. The photonic crystal boosted the short-circuit current of the cell, but it also damaged its open-circuit voltage and fill factor, which led to an overall decrease in performances. Comparisons between modeled and actual optical behaviors of the cell, and between ideal and actual morphologies, show the global robustness of the nanostructure to experimental deviations, but its particular sensitivity to the conformality of the top coatings and the spread in pattern dimensions, which should not be neglected in the optical model. As for the electrical behavior, the measured internal quantum efficiency shows the strong parasitic absorptions from the transparent conductive oxide and from the back-reflector, as well as the negative impact of the nanopattern on surface passivation. Our exemplifying case, thus, illustrates and experimentally confirms two recommendations for future integration of surface nanostructures for light trapping purposes: 1) the necessity to optimize absorption not for the total stack but for the single active material, and 2) the necessity to avoid damage to the active material by pattern etching.

Influence of the pattern shape on the efficiency of front-side periodically patterned ultrathin crystalline silicon solar cells

Patterning the front side of an ultrathin crystalline silicon (c-Si) solar cell helps keeping the energy conversion efficiency high by compensating for the light absorption losses. A super-Gaussian mathematical expression was used in order to encompass a large variety of nanopattern shapes and to study their influence on the optical performance. We prove that the enhancement in the maximum achievable photo-current is due to both impedance matching condition at short wavelengths and to the wave nature of light at longer wavelengths. We show that the optimal mathematical shape and parameters of the pattern depend on the c-Si thickness. An optimal shape comes with a broad optimal parameter zone where experimental inaccuracies have much less influence on the efficiency. We prove that cylinders are not the best suited shape. To compare our model with a real slab, we fabricated a nanopatterned c-Si slab via nano imprint lithography.

Disordered nanostructures by hole-mask colloidal lithography for advanced light trapping in silicon solar cells

We report on the fabrication of disordered nanostructures by combining colloidal lithography and silicon etching. We show good control of the short-range ordered colloidal pattern for a wide range of bead sizes from 170 to 850 nm. The inter-particle spacing follows a Gaussian distribution with the average distance between two neighboring beads (center to center) being approximately twice their diameter, thus enabling the nanopatterning with dimensions relevant to the light wavelength scale. The disordered nanostructures result in a lower integrated reflectance (8.1%) than state-of-the-art random pyramid texturing (11.7%) when fabricated on 700 µm thick wafers. When integrated in a 1.1 µm thin crystalline silicon slab, the absorption is enhanced from 24.0% up to 64.3%. The broadening of resonant modes present for the disordered nanopattern offers a more broadband light confinement compared to a periodic nanopattern. Owing to its simplicity, versatility and the degrees of freedom it offers, this potentially low-cost bottom-up nanopatterning process opens perspectives towards the integration of advanced light-trapping schemes in thin solar cells.

Optimization of slow light one-dimensional Bragg structures for photocurrent enhancement in solar cells.

In 1D photonic crystal Bragg structures, strong localization of the light occurs in the high refractive index layers at wavelengths on the red edge of the photonic bandgap. We exploit this slow light effect for thin film solar cells in order to increase the absorption of light in silicon, as the latter has a high refractive index. Amorphous silicon and a transparent conductive oxide are chosen as high-index and low-index materials, respectively. Reference thin film cells have equivalent total thickness of amorphous silicon, plus antireflection coating and optional metallic back mirror. Through transfer-matrix calculations, we demonstrate that the spectrally integrated photon flux absorbed in active layers, hence the photocurrent, is enhanced by at least 10% with respect to reference using only a few periods. The enhancement is robust with respect to the light incidence angle. The key of such an enhancement is the tuning of the red edge to both the solar irradiance spectrum and the intrinsic material absorption spectrum, which is achieved by suitably selecting the layer thicknesses.

Enhanced absorption in thin crystalline silicon films for solar cells by nanoimprint lithography

Two dimensional (2D) periodic photonic nanostructures, fabricated by nanoimprint lithography (NIL) and dry etching on the front surface of crystalline silicon (c-Si) layers, are investigated experimentally and theoretically in order to characterize their optical properties and demonstrate their relevance to photovoltaic (PV) applications. Nanoimprint lithography is performed on c-Si wafers and ultra-thin c-Si films with various thicknesses. A comparison with state-ofthe- art front side texturing with an antireflection coating is made. The 2D periodic photonic nanostructures result in an enhanced light absorption in the photoactive material. The results are validated through simulations based on Rigorous Coupled Wave Analysis (RCWA). The nanoimprinted substrates result in a similar absorption compared to the state-ofthe- art random pyramid texturing while consuming less than a micron of photoactive material. In contrast to the random pyramid texturing, the nanopatterning exhibits a robust performance for a wide range of incident angles up to 70°. The light trapping mechanism we propose is based on the combination of a graded index effect and the diffraction of light inside the photoactive layer at high angles.