Kirchhoff's law of thermal radiation imposes a constraint on photon-based energy harvesting processes since part of the incident energy flux is inevitably emitted back to the source. By breaking the reciprocity of the system, it is possible to overcome this restriction and improve the efficiency of energy harvesting. Here, we design and analyze a semitransparent emitter that fully absorbs normally incident energy from a given direction with zero backward and unity forward emissivity. The nearly ideal performance with wavelength-scale thickness is achieved due to the magneto-optical effect and the guided-mode resonance engineered in the emitter structure. We derive the general requirements for the nonreciprocal emitter using the temporal coupled mode theory and the symmetry considerations. Finally, we provide a realistic emitter design based on a photonic crystal slab consisting of a magnetic Weyl semimetal and silicon.

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The quantum dot (QD) light emitting diode has emerged as one of the candidates for the next generation display technology with advantages such as tunable wavelength of emitted light and narrow bandwidth. For QD pixel patterning, inkjet-printing is superior to other methods in terms of material loss and process time. However, inkjet-printing has difficulty in controlling the uniformity of the QD layer. This is a serious issue considering that proper thickness and uniformity of the QD layer are important factors for high efficiency in displays. The main reason behind this problem is known to be the coffee ring effect (CRE), in which differential evaporation across a sessile droplet leads to an edge-ward flow of the fluid inside and causes suspending QDs to be concentrated at the perimeter of the droplet. Here, the possibility of improvement in layer uniformity by the reduction of the CRE is demonstrated. Mathematical simulation of the evaporation process of the QD colloidal solution droplet is conducted by solving partial differential equations (PDEs) numerically. The Navier-Stokes equation, continuity condition, and mathematical expressions of physical parameters including the evaporative flux are used for setting up the PDEs, which are then solved with the finite difference method. A filter is included in the process to suppress unwanted instability. Using this simulation, the whole evaporation process is analyzed by observing time evolution of parameters. As a result, various conditions for reducing the CRE are found: sufficient initial concentration, proper solvent type, small contact angle, and fast evaporation rate. The outcome appears to agree with experimental data.

The Landsberg limit represents the ultimate efficiency limit of solar energy harvesting. Reaching this limit requires the use of nonreciprocal elements. The existing device configurations for attaining the Landsberg limit, however, are very complicated. Here, we introduce the concept of a nonreciprocal multijunction solar cell and show that such a cell can reach the Landsberg limit in the idealized situation where an infinite number of layers are used. We also show that such a nonreciprocal multijunction cell outperforms a standard reciprocal multijunction cell for a finite number of layers. Our work significantly simplifies the device configuration required to reach the ultimate limit of solar energy conversion and points to a pathway toward using nonreciprocity to improve solar energy harvesting.