• Energy Research
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We experimentally and theoretically demonstrated the hierarchical structure of SiO(2) nanorod arrays/p-GaN microdomes as a light harvesting scheme for InGaN-based multiple quantum well solar cells. The combination of nano- and micro-structures leads to increased internal multiple reflection and provides an intermediate refractive index between air and GaN. Cells with the hierarchical structure exhibit improved short-circuit current densities and fill factors, rendering a 1.47 fold efficiency enhancement as compared to planar cells.

Abstract SiO2 nanorod arrays (NRAs) are fabricated on InGaN-based multiple quantum well (MQW) solar cells using self-assembled Ag nanoparticles as the etching mask and subsequent reactive ion etching. The SiO2 NRAs effectively suppress the undesired surface reflections over the wavelengths from 330 to 570 nm, which is attributed to the light-trapping effect and the improved mismatch of refractive index at the air/MQW device interface. Under the air mass 1.5 global illumination, the conversion efficiency of the solar cell is enhanced by ∼21% largely due to increased short-circuit current from 0.71 to 0.76 mA/cm2. The enhanced device performances by the optical absorption improvement are supported by the simulation analysis as well.

The correlation between surface profiles of nanostructures and the behavior of scattered light, including specular and diffuse components, is demonstrated in detail. The nanorod arrays with large alignment variation exhibit broadband and omnidirectional anti-reflection properties due to the gradual index profile. This work provides insight into further structural design for nanostructured optoelectronic applications.

Abstract Resistive random access memory (RRAM) is one of the most promising candidates as a next generation nonvolatile memory (NVM), owing to its superior scalability, low power consumption and high speed. From the materials science point of view, to explore optimal RRAM materials is still essential for practical application. In this work, a new material (Bi, Mn)Ox (BMO) is investigated and several key performance characteristics of Pt/BMO/Pt structured device, including switching performance, retention and endurance, are examined in details. Furthermore, it has been confirmed by high-resolution transmission electron microscopy that the underlying switching mechanism is attributed to formation and disruption of metallic conducting nanofilaments (CNFs). More importantly, the power dissipation for each CNF is as low as 3.8/20 fJ for set/reset process, and a realization of cross-bar structure memory cell is demonstrated to prove the downscaling ability of proposed RRAM. These distinctive properties have important implications for understanding switching mechanisms and implementing ultralow power-dissipation RRAM based on BMO.

A novel strategy employing core-shell nanowire arrays (NWAs) consisting of Si/regioregular poly(3-hexylthiophene) (P3HT) was demonstrated to facilitate efficient light harvesting and exciton dissociation/charge collection for hybrid solar cells (HSCs). We experimentally demonstrate broadband and omnidirectional light-harvesting characteristics of core-shell NWA HSCs due to their subwavelength features, further supported by the simulation based on finite-difference time domain analysis. Meanwhile, core-shell geometry of NWA HSCs guarantees efficient charge separation since the thickness of the P3HT shells is comparable to the exciton diffusion length. Consequently, core-shell HSCs exhibit a 61% improvement of short-circuit current for a conversion efficiency (η) enhancement of 31.1% as compared to the P3HT-infiltrated Si NWA HSCs with layers forming a flat air/polymer cell interface. The improvement of crystal quality of P3HT shells due to the formation of ordering structure at Si interfaces after air mass 1.5 global (AM 1.5G) illumination was confirmed by transmission electron microscopy and Raman spectroscopy. The core-shell geometry with the interfacial improvement by AM 1.5G illumination promotes more efficient exciton dissociation and charge separation, leading to η improvement (∼140.6%) due to the considerable increase in V(oc) from 257 to 346 mV, J(sc) from 11.7 to 18.9 mA/cm(2), and FF from 32.2 to 35.2%, which is not observed in conventional P3HT-infiltrated Si NWA HSCs. The stability of the Si/P3HT core-shell NWA HSCs in air ambient was carefully examined. The core-shell geometry should be applicable to many other material systems of solar cells and thus holds high potential in third-generation solar cells.

InGaN/GaN multiple-quantum-well solar cells were grown on (111) Si substrates. AlN/AlGaN superlattice and self-assembly SixNy masking islands were employed to alleviate the material mismatches between Si and GaN. The devices were characterized under the illumination of AM 1.5 G with different solar concentrations. As the concentration ratios increased from 1-sun to 105-sun, energy conversion efficiency was enhanced by 25%, which was noticeably greater than the enhancement reported on sapphire substrates under similar solar concentrations. The result is attributed to the superior heat sinking of Si substrates.

Abstract Surface plasmon resonance was observed to red shift in hexagonal nanohemisphere Ag array covered by double-walled carbon nanotubes. By altering the density of carbon nanotubes, the surface plasmon resonance can be tuned from 563 nm to 586 nm and the light scattering is enhanced in the spectral range from orange to red. This leads to improved performance of amorphous silicon solar cells deposited on top, which yields a short-circuit current density of 14.07 mA/cm2 and a power conversion efficiency of 6.55% under the illumination of AM 1.5G. This carbon nanotubes' density-dependent surface plasmon shift is attributed to the dielectric constant change around the periodic Ag nanostructure, which can be applied to other solar cell materials by fine-tuning the surface plasmon resonance to enhance the absorption at wavelength where the active layer is less absorptive.