Quantum dot intermediate band solar cells involve nanoscale and microscale physical mechanisms that ultimately decide the operation of the cell in the intermediate band or single gap regime and must be considered for a proper description of the device behavior. In this work we discuss material- and device-level guidelines aimed at demonstrating truly intermediate band operation of quantum dot solar cells with the aid of numerical simulations built upon a hybrid modeling approach. Semiclassical transport is coupled with charge carrier transfer mechanisms involving localized states, allowing to merge the nanoscale and microscale pictures at a computational cost compatible with the simulation of a real solar cell structure.
KEYWORDS: Silicon, Transistors, Solar cells, Perovskite, Heterojunctions, External quantum efficiency, Monte Carlo methods, Thin films, Tandem solar cells, Photovoltaics
Tandem solar cells made of organometal halide perovskite and crystalline silicon cells are one of the most promising routes towards high efficiency low cost photovoltaics. Among the possible architectures, monolithic three-terminal tandems hold the promise of the highest energy/cost figure of merit, by combining the advantage of two- and four-terminal approaches. Recently, three-terminal perovskite/silicon tandems have been reported, based on interdigitated back contact heterojunction silicon cells. Alternative solutions that can be integrated with double-sided contact silicon cells are worth to be investigated in view of their higher compatibility with industrial mass production. In this work, we present a simulation-based proof-of-concept of PVK/Si threeterminal tandem cells that use a heterostructure bipolar transistor structure. The extra terminal is implemented at the common selective layer between the perovskite and silicon subcells, avoiding the use of any recombination layer or tunneling junction. We demonstrate promising device performance through physics-based simulations preliminarily validated against experimental data of other perovskite/silicon tandem technologies reported in literature.
Radiative cooling is an attractive concept for future sustainable energy strategies, as it might enable passive cooling of buildings and photovoltaic systems, hence facilitating energy savings by boosting performance and lifespan. The key idea is the adoption of materials that strongly emit thermal radiation in the atmosphere transparency window (wavelengths between 8 and 13 μm) as cooling layers. Significant progress in the field of metamaterials has enabled the realization of dielectric photonic structures with properties matching radiative cooling requirements and capable of going below ambient temperature. However, these structures are rather expensive and appear unsuitable for today’s large-scale manufacturing. In the present work, we have studied radiative cooling applied to Shockley-Queisser solar cells by exploring alternative materials, namely cementitious phases, which exhibit the required properties while being low-cost and scalable. We have determined their emission behavior by electromagnetic simulations and estimated the corresponding solar cell operating temperature by means of a detailed-balance model. The results have been benchmarked against the current state-of-the-art and hint at the possible realization of a new class of radiative coolers based on cheap and scalable cementitious materials.
Experiments on QD lasers grown on GaAs and on Si have revealed the quenching of the GS optical power as the current overcomes the ES threshold. A common technique to mitigate this quenching is the modulation p-doping, but an excessive p-doping level results in a deterioration of the GS optical power and threshold current. Theoretical models based on rate equations have ascribed the GS power quenching to the de-synchronization between the electron and hole dynamics. However, these approaches resort to phenomenological transport times. In this contribution, we study a 1.3 um QD laser grown on silicon by employing a drift-diffusion model for the transport of carriers across the SCH region. We show that the unbalance of electron and hole mobilities in the GaAs barriers is responsible for the GS quenching. The simulations also emphasize the existence of an optimum modulation p-doping level minimizing the GS threshold current, which we ascribe to electrostatic effects induced by this doping.
Quantum dots have attractive potential for multiple junction and intermediate band solar cells. However, the device level modeling of quantum dot based solar cells is a challenging task, since it inherently requires multiscale approaches combining nano- and micro-scale descriptions at an affordable computational cost. In this work quantum dot solar cells are studied by means of a multiscale model based on a semi-classical transport-Poisson framework enriched by a proper treatment of the quantum dot dynamics. The impact of a few design and physical parameters is investigated, providing better understanding of experimental results reported in literature.
Diffraction gratings have emerged as one of the main strategies for effective light trapping in thin-film solar cells. The simulation of such photonic structures requires computationally intensive 2D or 3D full-wave approaches, which are therefore unfeasible for computer-aided design purposes. This would be even more challenging in view of performing self-consistent coupling with electronic transport models to fully account for carrier collection and carrier-photon interactions. In this work this problem has been addressed by means of a novel and computationally efficient multiphysics approach for coupled electrical-optical simulations, based on the multimodal scattering matrix formalism, wherein the grating is modeled by a scattering matrix that can be easily derived from simulations performed by rigorous coupled wave analysis.
Balanced electroabsorption modulators (B-EAM) are an attractive alternative to the cross-coupled Mach-Zehnder modulator. The B-EAM enables bias independent suppression of even-order distortions, relative intensity noise (RIN), and common amplified spontaneous emission noise. By biasing the B-EAM at the 3rd order null, a 5th order distortion limited spurious free dynamic range can be achieved. We report on the experimental demonstration of the simultaneous suppression of laser RIN, 2nd and 3rd order distortions using a 300 micrometers long B-EAM.
Porous silicon (PS) has been known for quite a long time for its photoluminescence and for its usage as a sensing element. However, only in recent years this material has been proposed as a substrate for integrated optoelectronic devices and, despite the low fabrication costs and the possibility to tailor the refractive index varying the material porosity, its usage is still at the very beginning. In this paper we present the fabrication of integrated waveguides in PS and we describe our efforts to reduce the propagation losses. Different fabrication approaches have been studied: the first one uses selective anodization to obtain layers with different porosity and thus different refractive index. Another one exploits the different oxidation grades of the various porous layers to fabricate dense oxidized porous silicon waveguides. A detailed characterization of the obtained waveguides is reported. In particular, propagation losses as low as 7 dB/cm have been obtained in simple non-optimized multimode planar waveguides at the optical communication wavelength of 1.55 micrometers . This encouraging result paves the way to the next realization of porous silicon-based integrated optical devices for communication and sensing purposes. Finally, the results concerning a completely new approach, based on a laser ablation technique, to define the rib structure of porous silicon channel waveguides is presented.
In this paper we present a critical analysis on the refractive index profile reconstruction from near field measurements. We show that the near field information is not sufficient to uniquely determine the proper index parameters if the refractive index shape it is not already known a priori and then we propose a technique to refine the results found from the inversion. Experimental verifications in the case of waveguides made by ion exchange in a glass substrate confirm the results obtained with simulations.
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