Thermodynamic performance bounds for radiative heat engines

This paper discusses the performance limits of heat engines exchanging heat radiatively with a hot source while in thermal contact with a cold sink. Starting from solar energy conversion models, we derive power-versus-efficiency upper bounds for both reciprocal and nonreciprocal radiative heat engines. We find that nonreciprocal engines may allow significantly better performance than reciprocal ones, particularly for low emitter temperatures or when operating close to Carnot efficiency. The results give valuable guidelines for the design and optimization of thermophotovoltaic systems.Comment: 6 pages, 5 figure

Design rules for active control of narrowband thermal emission using phase-change materials

We propose an analytical framework to design actively tunable narrowband thermal emitters at infrared frequencies. We exemplify the proposed design rules using phase-change materials (PCM), considering dielectric-to-dielectric PCMs (e.g. GSST) and dielectric-to-metal PCMs (e.g. $\mathrm{VO_2}$). Based on these, we numerically illustrate near-unity ON-OFF switching and arbitrarily large spectral shifting between two emission wavelengths, respectively. The proposed systems are lithography-free and consist of one or several thin emitter layers, a spacer layer which includes the PCM, and a back reflector. Our model applies to normal incidence, though we show that the behavior is essentially angle-independent. The presented formalism is general and can be extended to \textit{any} mechanism that modifies the optical properties of a material, such as electrostatic gating or thermo-optical modulation.Comment: 9 pages, 6 figure

Deep-subwavelength Phase Retarders at Mid-Infrared Frequencies with van der Waals Flakes

Phase retardation is a cornerstone of modern optics, yet, at mid-infrared (mid-IR) frequencies, it remains a major challenge due to the scarcity of simultaneously transparent and birefringent crystals. Most materials resonantly absorb due to lattice vibrations occurring at mid-IR frequencies, and natural birefringence is weak, calling for hundreds of microns to millimeters-thick phase retarders for sufficient polarization rotation. We demonstrate mid-IR phase retardation with flakes of $\alpha$-molybdenum trioxide ($\alpha$-MoO$_3$) that are more than ten times thinner than the operational wavelength, achieving 90 degrees polarization rotation within one micrometer of material. We report conversion ratios above 50% in reflection and transmission mode, and wavelength tunability by several micrometers. Our results showcase that exfoliated flakes of low-dimensional crystals can serve as a platform for mid-IR miniaturized integrated polarization control.Comment: 8 pages, 5 figure

Optical simulations and optimization of perovskite/CI(G)S tandem solar cells using the transfer matrix method

In this work we employ the transfer matrix method for the analysis of optical materials properties to simulate and optimize monolithic tandem solar cell devices based on CuIn$_{1−x}$Ga$_x$Se$_2$, CI(G)S, and perovskite (PVK) absorbers. By finding models that fit well the experimental data of the CI(G)S solar cell, the semitransparent perovskite solar cell (PSC) and the PVK/CI(G)S monolithic tandem solar cell, we were able to perform a detailed optical loss analysis that allowed us to determine sources of parasitic absorption. We found better substitute materials for the transport layers to increase the power conversion efficiency and, in case of semitransparent PSCs, sub-bandgap transmittance. Our results set guidelines for the monolithic PVK/CI(G)S tandem solar cells development, predicting an achievable efficiency of 30%

New limits for light-trapping with multi-resonant absorption

International audienc

Resonant absorption in multilayer quantum-well and quantum-dot solar cells

Epitaxially-grown quantum well and quantum dot solar cells suffer from weak light absorption, strongly limiting their performance. Light trapping based on optical resonances is particularly relevant for such devices to increase light absorption and thereby current generation. Compared to homogeneous media, the position of the quantum layers within the device is an additional parameter that can strongly influence resonant absorption. However, this effect has so far received little attention from the photovoltaic community. In this work, we develop a theoretical framework to evaluate and optimize resonant light absorption in a thin slab with multiple quantum layers. Using numerical simulations, we show that the position of the layers can make the difference between strong absorption enhancement and completely suppressed absorption, and that an optimal position leads to an absorption enhancement twice larger than average. We confirm these results experimentally by measuring the absorption enhancement from photoluminescence spectra in InAs/GaAs quantum dot samples. Overall, this work provides an additional degree of freedom to substantially improve absorption, encouraging the development of quantum wells and quantum dots-based devices such as intermediate-band solar cells.Comment: 29 pages, 6 figure

Detailed balance calculations for hot-carrier solar cells: coupling high absorptivity with low thermalization through light trapping

Hot-carrier solar cells could enable an efficiency gain compared to conventional cells, provided that a high current can be achieved, together with a hot-carrier population. Because the thermalization rate is proportional to the volume of the absorber, a fundamental requirement is to maximize the density of carriers generated per volume unit. In this work, we focus on the crucial role of light trapping to meet this objective. Using a detailed balance model taking into account losses through a thermalization factor, we obtained parameters of the hot-carrier population generated under continuous illumination. Different absorptions corresponding to different light path enhancements were compared. Results are presented for open-circuit voltage, at maximum power point and as a function of the applied voltage. The relation between the parameters of the cell (thermalization rate and absorptivity) and its characteristics (temperature, chemical potential, and efficiency) is explained. In particular, we clarify the link between absorbed light intensity and chemical potential. Overall, the results give quantitative values for the thermalization coefficient to be achieved and show that in the hot-carrier regime, absorptivity enhancement leads to an important increase in the carrier temperature and efficiency