Photonic crystal-driven spectral concentration for upconversion photovoltaics

International audienceThe main challenge for applying upconversion (UC) to silicon photovoltaics is the limited amount of solar energy harvested directly via erbium-based upconverter materials (24.5 W m(-2)). This could be increased up to 87.7 W m(-2) via spectral concentration. Due to the nonlinear behavior of UC, this could increase the best UC emission by a factor 13. In this paper, the combined use of quantum dots (QDs)for luminescent down-shiftingand photonic crystals (PCs)for reshaping the emissionto achieve spectral concentration is shown. This implies dealing with the coupling of colloidal QDs and PC at the high-density regime, where the modes are shifted and broadened. In the first fabricated all-optical devices, the spectral concentration rises by 67%, the QD emission that matches the absorption of erbium-based upconverters increases by 158%, and the vertical emission experiences a 680% enhancement. Remarkably, the PC redshifts the overall emission of the QDs, which could be used to develop systems with low reabsorption losses. In light of this, spectral concentration should be regarded as one of the main strategies for UC photovoltaics

Crystalline-Size Dependence of Dual Emission Peak on Hybrid Organic Lead-Iodide Perovskite Films at Low Temperatures

In this work, we have investigated the crystalline-size dependence of optical absorption and photolumines-cence emission of CH3NH3PbI3 films, which is necessary to identify the potencial practical applications of the gadgets based on perovskite films. This study was carried out at low temperatures to minimize the extra complexity induced by thermal effects. The purpose was clarifying the origin of the dual emission peak previously reported in literature. We have found that the grain-size is responsible of the appearance or disappearance of this dual emission on CH3NH3PbI3 at low temperatures, whereas we have inferred that the thickness of the perovskite layer is a much more important factor than the size of the grains in the location of the energy of the bandgap. Moreover, the increase in the grain size allows slowing down the phase transition. Additionally, we evidence a decrease in the effective Rydberg energy of the exciton in several samples, from 23-25 meV at 7 K to 12-13 meV at 165 K, by fitting to Elliot-Toyozawa theory. We have extracted other im-portant physical parameters of perovskites from the photoluminescence-data deconvolution, such as bandgap, exciton-phonon interaction and exciton binding energy. A new phase transition at 45.5 K was determined by the temperature dependence of full width at half maximum and integrated intensity of the photoluminescence, and it was confirmed by the radiative lifetime obtained from the time-resolved photoluminescence emission by mean of time-correlated single photon counting at different temperatures, excitation fluencies and emission energies

Recycled Photons Traveling Several Millimeters in Waveguides Based on CsPbBr3 Perovskite Nanocrystals

Reabsorption and reemission of photons, or photon recycling (PR) effect, represents an outstanding mechanism to enhance the carrier and photon densities in semiconductor thin films. This work demonstrates the propagation of recycled photons over several mm by integrating a thin film of CsPbBr3 nanocrystals into a planar waveguide. An experimental set-up based on a frequency modulation spectroscopy allows to characterize the PR effect and the determination of the effective decay time of outcoupled photons. A correlation between the observed photoluminescence redshift and the increase of the effective decay time is demonstrated, which grows from 3.5 to near 9 ns in the best device. A stochastic Monte Carlo model reproduces these experimental results and allows the extraction of the physical mechanisms involved. In the waveguide under study recycled photons follow a drift (directional enhancement) velocity ≈5.7 × 105 m s−1, dominating over the diffusive regime observed in a standard thin film (D ≈ 420 m2 s−1). This means that recycled photons propagate mm-distances in shorter traveling times in the waveguide (≈5 ns) as compared to the film (>20 ns). These results are expected to pave the road for exploiting the PR effect in future optoelectronic and photonic devices

Single-Exciton Amplified Spontaneous Emission in Thin Films of CsPbX3 (X = Br, I) Perovskite Nanocrystals

CsPbX3 perovskite nanocrystals (PNCs) have emerged as an excellent material for stimulated emission purposes, with even more prospective applications than conventional colloidal quantum dots. However, a better understanding of the physical mechanisms responsible for amplified spontaneous emission (ASE) is required to achieve more ambitious targets (lasing under continuous wave optical or electrical excitation). Here, we establish the intrinsic mechanisms underlying ASE in PNCs of three different band gaps (CsPbBr3, CsPbBr1.5I1.5, and CsPbI3). Our characterization at cryogenic temperatures does not reveal any evidence of the biexciton mechanism in the formation of ASE. Instead, the measured shift toward long wavelengths of the ASE band is easily explained by the reabsorption in the PNC layer, which becomes stronger for thicker layers. In this way, the threshold of ASE is determined only by optical losses at a given geometry, which is the single-exciton mechanism responsible for ASE. Experimental results are properly reproduced by a physical model

Single Step Deposition of an Interacting Layer of Perovskite Matrix with Embedded Quantum Dots

Hybrid lead halide perovskite (PS) derivatives have emerged as very promising materials for the development of optoelectronic devices in the last few years. At the same time, inorganic nanocrystals with quantum confinement (QDs) possess unique properties that make them suitable materials for the development of photovoltaics, imaging and lighting applications, among others. In this work, we report on a new methodology for the deposition of high quality, large grain size and pinhole free PS films (CH3NH3PbI3) with embedded PbS and PbS/CdS core/shell Quantum Dots (QDs). The strong interaction between both semiconductors is revealed by the formation of an exciplex state, which is monitored by photoluminescence and electroluminescence experiments. The radiative exciplex relaxation is centered in the near infrared region (NIR), ≈1200 nm, which corresponds to lower energies than the corresponding band gap of both perovskite (PS) and QDs. Our approach allows the fabrication of multi-wavelength light emitting diodes (LEDs) based on a PS matrix with embedded QDs, which show considerably low turn-on potentials. The presence of the exciplex state of PS and QDs opens up a broad range of possibilities with important implications in both LEDs and solar cells.Generalitat Valenciana PROMETEOII/2014/020 PROMETEOII/2014/059 ISIC/2012/008 Spanish MINECO (Ministry of Economy and Competitiveness) MAT2013-47192-C3-1-R MAT2015-70611-ERC TEC2014-53727-C2-1-R Spanish MINEC

Microconcrete with partial replacement of Portland cement by fly ash and hydrated lime addition

[EN] The reduction in Portland cement consumption means lower CO2 emissions. Partial replacement of Portland cement by pozzolans sucha as fly ash has its limitations due to the quantity of calcium hydroxide generated in the mix. In this work we have studied the contribution of the addition of hydrated lime to Portland cement + fly ash systems. We have also studied several levels of cement replacement, ranging from the 15% to 75%. The best mechanical results were obtained replacing 50% of Portland cement by the same amount of fly ash plus the addition of hydrated lime (20% respect to the amount of fly ash). In these systems, an acide-base self-neutralization of the matrix has occurred through a pozzolanic reaction of fly ash with portlandite liberated in the hydration of Portland cement and the added hydrated lime. It has been identified for these mixtures a significant amount of hydrated gehlenite, typical reaction product from rich-alumina pozzolans.Lorca, P.; Calabuig Pastor, R.; Benlloch Marco, J.; Soriano Martinez, L.; Paya Bernabeu, JJ. (2014). Microconcrete with partial replacement of Portland cement by fly ash and hydrated lime addition. Materials and Design. 64:535-541. doi:10.1016/jmatdes.2014.08.022S5355416

Lead-free FASnI3 laser amplifiers integrated in flexible waveguides

Proceedings of the Optica Advanced Photonics Congress. Maastricht, Netherlands, 24-28 July, 2022A FASnI3 lead-free perovskite is integrated in a flexible waveguide to demonstrate Amplified Spontaneous Emission. An extremely low threshold, 1 µJ/cm2, is observed together with the formation of narrow random lasing lines (< 1 nm)

Directional and Polarized Lasing Action on Pb-free FASnI3 Integrated in Flexible Optical Waveguide

In this work, high-quality FASnI3 (FA, formamidinium) lead-free perovskite thin films are successfully incorporated in a flexible polyethylene terephthalate (PET) substrate to demonstrate amplified spontaneous emission (ASE) and lasing. The waveguide (WG) consists of polymethylmethacrylate(PMMA)/FASnI3 bilayer deposited on a PET substrate and is properly designed to allow single-mode propagation at the photoluminescence wavelength. This geometry optimizes the excitation of the emitting FASnI3, enhances the light−matter interaction in the semiconductor thin film, provides a preferable direction for the emitted light and allows its direct outcoupling for on-chip or fiber-optic applications. As far as the authors know, ASE and random lasing are obtained for the first time in a flexible-based WG integrating a highly efficient lead-free perovskite. The high quality of the deposited films and the optimized design of the structure result in an extremely low ASE/lasing threshold in the range of 1 µJ cm−2, which is only ten times higher than that measured in the same PMMA/FASnI3 structure deposited on a rigid substrate (Si/SiO2). More interestingly, these WGs exhibit a strong polarization anisotropy for the outcoupled ASE/lasing light with a preferable transverse electric polarization. This work is the base for the future development of ecofriendly, flexible, and efficient photonic devices.This project received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 862656 (project DROP-IT) and the European Research Council (ERC) via Consolidator Grant (724424, No-LIMIT) and by the Spanish MINECO through projects no. PID2020-120484RB-I00 and PID2019-107314RB-I00 (Stable)