Thermal emission of macroporous silicon chirped photonic crystals

Postprint (author's final draft

Enabling silicon-on-silicon photonics with pedestalled Mie resonators

High-refractive-index Mie resonators are regarded as promising building blocks for low-loss all-dielectric nanophotonic applications. To avoid the otherwise excessive damping and loss of symmetry such devices typically need to be implemented over a low-index substrate, which hampers their integration in many practical applications. In this paper we propose a new photonic structure consisting of silicon-on-silicon spheroidal-like resonators, each one supported by a slim silicon pedestal that makes the micro-cavities stand optically separated from the substrate while providing both mechanical stability and electrical contact with the substrate. These structures are produced in high-quality monocrystalline Si and their size and arrangement can be precisely controlled through standard lithography. We demonstrate that such structures present an optical performance similar to the one achieved with low-index substrates, opening new avenues for developing novel hybrid photonic/electronic devices.Postprint (author's final draft

Three-dimensional metallo-dielectric selective thermal emitters with high-temperature stability for thermophotovoltaic applications

Selective thermal emitters concentrate most of their spontaneous emission in a spectral band much narrower than a blackbody. When used in a thermophovoltaic energy conversion system, they become key elements defining both its overall system efficiency and output power. Selective emitters' radiation spectra must be designed to match their accompanying photocell's band gap and simultaneously, withstand high temperatures (above 1000 K) for long operation times. The advent of nanophotonics has allowed the engineering of very selective emitters and absorbers; however, thermal stability remains a challenge since nanostructures become unstable at temperatures much below the melting point of the used materials. In this paper we explore a hybrid 3D dielectric-metallic structure that combines the higher thermal stability of a monocrystalline 3D silicon scaffold with the optical properties of a thin platinum film conformally deposited on top. We show experimentally that these structures exhibit a selective emission spectrum suitable for TPV applications and that they are thermally stable at temperatures up to 1100 K. These structures are ideal in combination with HI-V semiconductors in the range E-g=0.4-0.55 eV such as InGaAsSb (E-g=0.5-0.6 eV) and InAsSbP (E-g=0.3-0.55 eV). (C) 2014 Elsevier B.V. All rights reserved.Peer ReviewedPostprint (author’s final draft

Controlling Plateau-Rayleigh instabilities during the reorganization of silicon macropores in the Silicon Millefeuille process

The reorganization through high-temperature annealing of closely-packed pore arrays can be exploited to create ultra-thin (<20 µm) monocrystalline silicon layers that can work as cheap and flexible substrates for both the electronic and the photovoltaic industries. By introducing a periodic diameter modulation along deep etched pores, many thin layers can be produced from a single substrate and in a single technological process. Besides the periodicity, the exact shape of the modulation also has a profound impact on the process and subtle profile changes can lead to important differences on the process outcome. In this paper we study both theoretically and experimentally the effect of the initial profile on the pore reorganization dynamics and the morphology of the thin layers obtained through annealing. We show that process reliability, annealing time and final layer characteristics, all can be engineered and optimized by precisely controlling the initial pore profile.Postprint (published version

Textured PDMS films applied to thin crystalline silicon solar cells

© 2020 IEEE. Personal use of this material is permitted. Permission from IEEE must be obtained for all other uses, in any current or future media, including reprinting/republishing this material for advertising or promotional purposes,creating new collective works, for resale or redistribution to servers or lists, or reuse of any copyrighted component of this work in other works.Front surface texturization is a standard procedure used to improve optical properties of photovoltaic devices. In some particular cases, such as when dealing with ultrathin substrates, common texturization techniques can become unpractical or even unfeasible. Texturized polymer films applied on top of such devices may be used as an alternative. In this article, we report on the development of textured polydimethylsiloxane (PDMS) films to be placed on top of planar crystalline silicon solar cells based on thin substrates (=40 µ m). The PDMS polymer is deposited onto a rough surface (conventional random pyramid textured silicon), cured and detached from it. By scanning electron microscope images, we demonstrate that the dilution of PDMS into toluene helps in a better replica of the master surface. Next, we apply the optimized PDMS films on top of dummy samples based on 10, 20, and 40 µm thick crystalline silicon (c-Si) substrates whose reflectance is significantly reduced after placing the PDMS films. Accurate optical simulations indicate that the optical improvement comes from three mechanisms: higher light transmission into the device, lower reflectance at the c-Si surface, and better light trapping properties at the thin c-Si absorber. Experimental verification of the optical improvement with texturized PDMS films is reported based on 40 µ m thick solar cell, where a short-circuit current density gain of 1.7 mA/cm 2 is observed.This work was funded by MINECO from Spanish government under projects TEC2017-82305-R, ENE2016-78933-C4-1-R, ENE2017-87671-C3-2-R. The work was also supported in part by project REFER COMRDI15-1-0036 funded by ACCIÓ and the European Regional Development Fund (FEDER).Peer ReviewedPostprint (published version

Impact of doping and silicon substrate resistivity on the blistering of atomic-layer-deposited aluminium oxide

Aluminium oxide (Al2O3) thin films grown at low temperatures using atomic layer deposition (ALD) are known to often suffer from local delamination sites, referred to as "blisters", after post-deposition annealing during device processing. In this work, we report our observation that doping of the silicon substrate has an effect on blister formation. The introduction of a highly doped layer by diffusion or implantation is found to significantly reduce blistering, compared to the non-doped regions in the immediate vicinity. Similar behavior is observed for both phosphorus and boron doping. Further investigation of this phenomenon using substrates with different resistivities reveals that even when introduced already during silicon crystal growth, doping affects the blistering of aluminium oxide films. Changes in several properties of silicon affected by doping, most importantly surface terminating groups, native oxide growth, and passivation of defects with hydrogen, are discussed as potential reasons behind the observed effect on blistering.Peer reviewe

Emissive properties of SiO2 thin films through photonic windows

Postprint (published version

Empirical demonstration of CO2 detection using macroporous silicon photonic crystals as selective thermal emitters

This study describes the detection of CO2 using macroporous silicon photonic crystals as thermal emitters. It demonstrates that the reduction of structural nonhomogeneities leads to an improvement of the photonic crystals’ emission. Narrow emission bands (¿~120) located within the R-branch of carbon dioxide were achieved. Measurements were made using a deuterated triglycine sulfate photodetector and the photonic crystals, heated to 400°C, as selective emitters. A gas cell with a CO2 concentration between 0 ppm and 10,000 ppm was installed in the center. Results show high sensibility and selectivity that could be used in current nondispersive infrared devices for improving their features. These results open the door to narrowband emission in the mid-infrared for spectroscopic gas detection.Postprint (author's final draft

Black ultra-thin crystalline silicon wafers reach the 4n2 absorption limit–application to IBC solar cells

Cutting costs by progressively decreasing substrate thickness is a common theme in the crystalline silicon photovoltaic industry for the last decades, since drastically thinner wafers would significantly reduce the substrate-related costs. In addition to the technological challenges concerning wafering and handling of razor-thin flexible wafers, a major bottleneck is to maintain high absorption in those thin wafers. For the latter, advanced light-trapping techniques become of paramount importance. Here we demonstrate that by applying state-of-the-art black-Si nanotexture produced by DRIE on thin uncommitted wafers, the maximum theoretical absorption (Yablonovitch's 4n2 absorption limit), that is, ideal light trapping, is reached with wafer thicknesses as low as 40, 20, and 10 µm when paired with a back reflector. Due to the achieved promising optical properties the results are implemented into an actual thin interdigitated back contacted solar cell. The proof-of-concept cell, encapsulated in glass, achieved a 16.4% efficiency with an JSC = 35 mA cm-2, representing a 43% improvement in output power with respect to the reference polished cell. These results demonstrate the vast potential of black silicon nanotexture in future extremely-thin silicon photovoltaics.Peer ReviewedPostprint (published version

Hole selective contacts based on transition metal oxides for c-Ge thermophotovoltaic devices

© 2022 Elsevier. This manuscript version is made available under the CC-BY-NC-ND 4.0 license http://creativecommons.org/licenses/by-nc-nd/4.0/Thermophotovoltaics has become a very attractive solution for heat-to-electricity conversion due to its excellent conversion efficiencies. However, further research is needed to reduce the device cost which is typically based on III-V semiconductors. To tackle this limitation, crystalline germanium (c-Ge) has been proposed as an excellent substrate for low-cost devices. One of the key advances behind high system efficiencies is the excellent reflectance of the out-of-band photons at the rear surface of the photovoltaic device. These photons with lower energy than the absorber bandgap are reflected back to the thermal emitter reducing its thermal losses. In this work, we explore the performance of hole selective contacts based on evaporated transition metal oxides (MoOx, VOx, WOx) to be introduced at the rear surface of c-Ge devices. Regarding electrical properties, we characterize the selectivity of the contact by measuring effective surface recombination velocity (Seff) and contact resistivity (¿C). Best results are obtained with MoOx contacted by Ag/ITO with Seff = 588 cm/s and ¿C = 55.6 mO cm2 which can be improved by using gold as a metal contact leading to Seff = 156 cm/s and ¿C = 60.9 mO cm2. Regarding out-of-band reflectance, it is better for the case of Ag/ITO/MoOx contact with 87.5% compared to 78.9% for Au/MoOx when a 1473 K black body spectrum is used. Device simulations show potential system efficiencies in the range of 18–19% which are comparable to the best reported efficiencies using c-Ge thermophotovoltaic devices.This work has been supported by the Spanish government under projects PID2019-109215RB-C41 (SCALED), PID2020-116719RB-C41 (MATER ONE) and PID2020-115719RB-C21 (GETPV) funded by MCIN/ AEI/10.13039/501100011033. The authors would like to thank the master student Oscar Llados ´ and Guillem Ayats for their help in processing the samples, Dr. Alejandro Datas from Instituto de Energía Solar (IES) in Madrid for providing the c-Ge wafers and fruitful discussions.Peer ReviewedPostprint (published version