Silicon-based spectrally selective emitters with good high-temperature stability on stepped metasurfaces

Solar thermophotovoltaic (STPV) systems have attracted increasing attention due to their great prospects for breaking the Shockley-Queisser limit. As a critical component of high-performance STPV systems, fabrication of a spectrally selective emitter with good stability at high temperature is one of the main research challenges. In this study, we developed a hybrid silicon-based metasurface emitter with spectral selectivity and high temperature stability using a simple fabrication process by introducing a controlled silicon nitride (SiN_x) layer on a silicon stepped nanopillar substrate coated with molybdenum (Mo). Owing to the cooperative effect of cavity mode resonance and the interference effect of the SiN_x dielectric layer, our proposed silicon-based metasurface emitter achieves a broadband optical absorption of ∼95% in the wavelength range of 220-2000 nm, while effectively suppressing the heat radiation to ∼19% in the long wavelength range (>5 μm). Moreover, polarization-independence and angle-insensitivity behaviors are demonstrated in the emitters. Additionally, due to the presence of a SiN_x protection layer, this silicon-based metasurface emitter is experimentally proved to sustain its excellent spectral properties after ultra-high temperature treatments, including annealing at 1273 K under an Ar atmosphere for 6 h, even at 1073 K in air for 1 h, which makes it an alternative candidate for application in actual STPV systems.

Vertically Arranged Zinc Oxide Nanorods as Antireflection Layer for Crystalline Silicon Solar Cell: A Simulation Study of Photovoltaic Properties

This paper describes the unique antireflection (AR) layer of vertically arranged ZnO nanorods (NRs) on crystalline silicon (c-Si) solar cells and studies the charge transport and photovoltaic properties by simulation. The vertically arranged ZnO NRs were deposited on ZnO-seeded c-Si wafers by a simple low-temperature solution process. The lengths of the ZnO NRs were optimized by changing the reaction times. Highly dense and vertically arranged ZnO NRs were obtained over the c-Si wafer when the reaction time was 5 h. The deposited ZnO NRs on the c-Si wafers exhibited the lowest reflectance of ~7.5% at 838 nm, having a reasonable average reflectance of ~9.5% in the whole wavelength range (400–1000 nm). Using PC1D software, the charge transport and photovoltaic properties of c-Si solar cells were explored by considering the lengths of the ZnO NRs and the reflectance values. The 1.1 μm length of the ZnO NRs and a minimum average reflectance of 9.5% appeared to be the optimum values for achieving the highest power conversion efficiency of 14.88%. The simulation study for the vertically arranged ZnO NRs AR layers clearly reflects that the low-temperature deposited ZnO NRs on c-Si solar cells could pose a greater prospect in the manufacturing of low-cost c-Si solar cells.

Inverted Silicon Nanopencil Array Solar Cells with Enhanced Contact Structures

Although three-dimensional nanostructured solar cells have attracted extensive research attention due to their superior broadband and omnidirectional light-harvesting properties, majority of them are still suffered from complicated fabrication processes as well as disappointed photovoltaic performances. Here, we employed our newly-developed, low-cost and simple wet anisotropic etching to fabricate hierarchical silicon nanostructured arrays with different solar cell contact design, followed by systematic investigations of their photovoltaic characteristics. Specifically, nano-arrays with the tapered tips (e.g. inverted nanopencils) are found to enable the more conformal top electrode deposition directly onto the nanostructures for better series and shunt conductance, but its insufficient film coverage at the basal plane would still restrict the charge carrier collection. In contrast, the low-platform contact design facilitates a substantial photovoltaic device performance enhancement of ~24%, as compared to the one of conventional top electrode design, due to the shortened current path and improved lateral conductance for the minimized carrier recombination and series resistance. This enhanced contact structure can not only maintain excellent photon-trapping behaviors of nanostructures, but also help to eliminate adverse impacts of these tapered nano-morphological features on the contact resistance, providing further insight into design consideration in optimizing the contact geometry for high-performance nanostructured photovoltaic devices.

Effective light absorption and its enhancement factor for silicon nanowire-based solar cell

Although nanowire (NW) antireflection coating can enhance light trapping capability, which is generally used in crystal silicon (CS) based solar cells, whether it can improve light absorption in the CS body depends on the NW geometrical shape and their geometrical parameters. In order to conveniently compare with the bare silicon, two enhancement factors E(T) and E(A) are defined and introduced to quantitatively evaluate the efficient light trapping capability of NW antireflective layer and the effective light absorption capability of CS body. Five different shapes (cylindrical, truncated conical, convex conical, conical, and concave conical) of silicon NW arrays arranged in a square are studied, and the theoretical results indicate that excellent light trapping does not mean more light can be absorbed in the CS body. The convex conical NW has the best light trapping, but the concave conical NW has the best effective light absorption. Furthermore, if the cross section of silicon NW is changed into a square, both light trapping and effective light absorption are enhanced, and the Eiffel Tower shaped NW arrays have optimal effective light absorption.

Ultrathin planar hematite film for solar photoelectrochemical water splitting

Hematite holds promise for photoelectrochemical (PEC) water splitting due to its stability, low-cost, abundance and appropriate bandgap. However, it suffers from a mismatch between the hole diffusion length and light penetration length. We have theoretically designed and characterized an ultrathin planar hematite/silver nanohole array/silver substrate photoanode. Due to the supported destructive interference and surface plasmon resonance, photons are efficiently absorbed in an ultrathin hematite film. Compared with ultrathin hematite photoanodes with nanophotonic structures, this photoanode has comparable photon absorption but with intrinsically lower recombination losses due to its planar structure and promises to exceed the state-of-the-art photocurrent of hematite photoanodes.