Sylvain Nicolay

Current status of AlInN layers lattice-matched to GaN for photonics and electronics

We report on the current properties of Al1-x InxN (x approximate to 0.18) layers lattice- matched ( LM) to GaN and their specific use to realize nearly strain- free structures for photonic and electronic applications. Following a literature survey of the general properties of AlInN layers, structural and optical properties of thin state- of- the- art AlInN layers LM to GaN are described showing that despite improved structural properties these layers are still characterized by a typical background donor concentration of ( 1 - 5) x 10(18) cm(-3) and a large Stokes shift (similar to 800 meV) between luminescence and absorption edge. The use of these AlInN layers LM to GaN is then exemplified through the properties of GaN/ AlInN multiple quantum wells ( QWs) suitable for near- infrared intersubband applications. A built- in electric field of 3.64MVcm(-1) solely due to spontaneous polarization is deduced from photoluminescence measurements carried out on strain- free single QW heterostructures, a value in good agreement with that deduced from theoretical calculation. Other potentialities regarding optoelectronics are demonstrated through the successful realization of crack- free highly reflective AlInN/ GaN distributed Bragg reflectors ( R > 99%) and high quality factor microcavities ( Q > 2800) likely to be of high interest for short wavelength vertical light emitting devices and fundamental studies on the strong coupling regime between excitons and cavity photons. In this respect, room temperature ( RT) lasing of a LM AlInN/ GaN vertical cavity surface emitting laser under optical pumping is reported. A description of the selective lateral oxidation of AlInN layers for current confinement in nitride- based light emitting devices and the selective chemical etching of oxidized AlInN layers is also given. Finally, the characterization of LM AlInN/ GaN heterojunctions will reveal the potential of such a system for the fabrication of high electron mobility transistors through the report of a high two- dimensional electron gas sheet carrier density ( n(s) similar to 2.6 x 10(13) cm(-2)) combined with a RT mobility mu(e) similar to 1170 cm(2) V-1 s(-1) and a low sheet resistance, R similar to 210 Omega square.

22.5% efficient silicon heterojunction solar cell with molybdenum oxide hole collector

Substituting the doped amorphous silicon films at the front of silicon heterojunction solar cells with wide-bandgap transition metal oxides can mitigate parasitic light absorption losses. This was recently proven by replacing p-type amorphous silicon with molybdenum oxide films. In this article, we evidence that annealing above 130 °C—often needed for the curing of printed metal contacts—detrimentally impacts hole collection of such devices. We circumvent this issue by using electrodeposited copper front metallization and demonstrate a silicon heterojunction solar cell with molybdenum oxide hole collector, featuring a fill factor value higher than 80% and certified energy conversion efficiency of 22.5%.

Multiscale transparent electrode architecture for efficient light management and carrier collection in solar cells

The challenge for all photovoltaic technologies is to maximize light absorption, to convert photons with minimal losses into electric charges, and to efficiently extract them to the electrical circuit. For thin-film solar cells, all these tasks rely heavily on the transparent front electrode. Here we present a multiscale electrode architecture that allows us to achieve efficiencies as high as 14.1% with a thin-film silicon tandem solar cell employing only 3 μm of silicon. Our approach combines the versatility of nanoimprint lithography, the unusually high carrier mobility of hydrogenated indium oxide (over 100 cm(2)/V/s), and the unequaled light-scattering properties of self-textured zinc oxide. A multiscale texture provides light trapping over a broad wavelength range while ensuring an optimum morphology for the growth of high-quality silicon layers. A conductive bilayer stack guarantees carrier extraction while minimizing parasitic absorption losses. The tunability accessible through such multiscale electrode architecture offers unprecedented possibilities to address the trade-off between cell optical and electrical performance.

Optimization of ZnO Front Electrodes for High-Efficiency Micromorph Thin-Film Si Solar Cells

The quest for increased performances in thin-film silicon micromorph tandem devices nowadays requires an increase of current density. This can be achieved with thin cells by combining both robust cell design and efficient light management schemes. In this paper, we identify three key requirements for the transparent conductive oxide electrodes. First, strong light scattering into large angles is needed on the entire useful wavelength range: A front electrode texture with large enough features is shown to grant a high total current (typically >26 mA/cm2 with a 2.4-μm-thick absorber material), while sharp features are reported to allow for high top cell current (>13 mA/cm2) and reduced reflection at the ZnO/Si interface. Second, sufficiently smooth substrate features are needed to guarantee a high quality of the silicon active material, ensuring good and stable electrical properties (typically Voc around 1.4 V). Third, conduction and transparency of electrodes must be cleverly balanced, requiring high transparent conductive oxide mobility (∼50 cm

Laser-Scribing Patterning for the Production of Organometallic Halide Perovskite Solar Modules

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On the Interplay Between Microstructure and Interfaces in High-Efficiency Microcrystalline Silicon Solar Cells

/V/s) to maintain the sheet resistance below 30 Ω/sq while keeping absorption as low as possible. Optimization of these three key requirements using ZnO electrodes allowed us to realize high-efficiency micromorph devices with 13.5% initial and 11.5% stabilized efficiency.

Lattice-Matched GaN–InAlN Waveguides at

Efficiencies of solar cells based on organometallic halide perovskite absorber material have dramatically increased over the past few years. Most of efficiencies reported so far have, however, been obtained on solar cells with very small lab-scale area of less than 0.3 cm2. Only a handful of studies addressed the performances of minimodules based on perovskite, and all of them showed relatively large dead areas between the solar cell segments. In this study, we used laser-scribing techniques to pattern the module segment, reduce the dead area, and optimize the aperture area efficiency. The fraction of the dead area in the module is less than 16%, which proves that the laser-scribing technology can be adopted for monolithic serial interconnected perovskite modules and paves the way to improving module efficiency.

\lambda=1.55\ \mu

This paper gives new insights into the role of both the microstructure and the interfaces in microcrystalline silicon (μc-Si) single-junction solar cells. A 3-D tomographic reconstruction of a μc-Si solar cell reveals the 2-D nature of the porous zones, which can be present within the absorber layer. Tomography thus appears as a valuable technique to provide insights into the μc-Si microstructure. Variable illumination measurements enable to study the negative impact of such porous zones on solar cells performance. The influence of such defective material can be mitigated by suitable cell design, as discussed here. Finally, a hydrogen plasma cell post-deposition treatment is demonstrated to improve solar cells performance, especially on rough superstrates, enabling us to reach an outstanding 10.9% efficiency microcrystalline single-junction solar cell.

m Grown by Metal–Organic Vapor Phase Epitaxy

We report on the demonstration of low-loss, single-mode GaN-InAlN ridge waveguides (WGs) at fiber-optics telecommunication wavelengths. The structure grown by metal-organic vapor phase epitaxy contains AlInN cladding layers lattice-matched to GaN. For slab-like WGs propagation losses are below 3 dB/mm and independent of light polarization. For 2.6-mum-wide WGs the propagation losses in the 1.5- to 1.58-mum spectral region are as low as 1.8 and 4.9 dB/mm for transverse-electric- and transverse-magnetic-polarization, respectively. The losses are attributed to the sidewall roughness and can be further reduced by the optimization of the etching process.

Nanometer- and Micrometer-Scale Texturing for High-Efficiency Micromorph Thin-Film Silicon Solar Cells

Optimized transparent conductive oxide front electrodes are vital to further increase the efficiency of thin-film silicon solar devices. We report details on the fabrication of multiscale textured zinc oxide substrates and their implementation in amorphous silicon/microcrystalline silicon tandem (micromorph) devices. Such substrates allow separate optimization of light trapping in the top and bottom cells, and efficient decoupling of transparency and conduction. We show in particular the need for sharp, nanoscale texturing for antireflection and light trapping in the top cell. We also show that smooth, micrometer-scale texturing can efficiently improve large-wavelength light management without degrading the quality of the silicon material grown on the substrate. By combining the appropriate morphologies, high currents can be reached in both the top and bottom subcells, while conserving the optimal electrical properties of the solar cells.