Thiyagu Subramani

Functionalization of Silicon Nanostructures for Energy-Related Applications

Silicon (Si) is used in various application fields such as solar cells and electric devices. Functionalization of Si nanostructures is one way to further improve the properties of these devices such as these. This Review summarizes recent results of solar cell and Li-ion battery applications using Si-related nanostructures. In solar cell applications, the light trapping effect is increased and the carrier recombination rate is decreased due to the short carrier collection path achieved by radially constructed p-n junction in Si nanowires, resulting in higher power conversion efficiency. The nonradiative energy transfer effect created by nanocrystalline Si is a novel way of improving solar cell properties. Si-related nanostructures are also anticipated as new anode materials with higher capacity in Li-ion batteries. Si-related nanocomposite materials which show densely packed microparticle structures agglomerated with small nanoparticles are described here as a promising challenge. These unique structures show higher capacity and longer cycle properties.

Diffused back surface field formation in combination with two-step H2 annealing for improvement of silicon nanowire-based solar cell efficiency

Silicon nanowire (SiNW)-based solar cell fabrication was developed using a combination of diffused back surface field (BSF) formation and two-step H2 annealing for greater efficiency enhancement. Two different n-SiNW structures synthesized by metal-catalyzed electroless etching and nanoimprinting followed by Bosch process were used as substrates to fabricate single-junction solar cells with chemical vapor deposition grown p-Si shell layer. Without BSF formation, shell layer coverage of nanoimprinted SiNW solar cells fabricated using two-step H2 annealing was optimized and the H2 annealing effect on the shell layer crystallinity was investigated using transmission electron microscopy. Then, the effects of combined H2 annealing process for BSF formation together with shell layer treatment were observed under various annealing times. Enhancement of ~1.5% absolute efficiency of both SiNW-based solar cells was achieved. Reduction of carrier recombination by BSF layer accompanied with the reduction of the defect density on n-SiNW surfaces and inside p-Si layer could be accomplished.

Pencil-shaped silicon nanowire synthesis and photovoltaic application

Pencil-shaped silicon nanowires (SiNWs) were synthesized by colloidal lithography and inductively coupled plasma reactive ion etching for photovoltaic application. Light reflectance of below 10% could be obtained by the asymmetric pencil-shaped SiNW array. The pencil-shaped SiNW structure and properties were investigated and compared with metal catalyzed electroless etching (MCEE) and nanoimprinted SiNWs. Single-junction solar cells consisting of pencil-shaped n-SiNW substrates and p-Si shell layer grown by chemical vapor deposition were fabricated. A combination of two-step H2 annealing and back surface field formation was applied to improve the solar cell properties. Good crystal quality and surface of pencil-shaped SiNWs provided an excellent solar cell junction interface. The great light trapping of the pencil-shaped SiNW solar cell could enhance the conversion efficiency by more than 0.6% compared to the solar cells using MCEE and nanoimprinted SiNWs.

Control of grain size and crystallinity of poly-Si films on quartz by Al-induced crystallization

The crystalline properties of poly-Si films on quartz grown using Al-induced crystallization (AIC) were investigated. The orientation fraction and grain size were controlled by modulating the annealing temperature and sample thickness. The results confirmed the enhancement of (111)-orientation fractions and grain size by lowering the annealing temperature and reducing the thickness. The effects of both parameters, annealing temperature and thickness, on the growth process were investigated, as was the role of the Al layer. We successfully formed (111)-oriented grains up to 384 μm in size at a rate of 99% in a 50 nm-thick sample annealed at 400 °C. Furthermore, the real applications of AIC poly-Si as a growth template were demonstrated through silicon thin-film and nanowire formation.

Improvement of silicon nanowire solar cells made by metal catalyzed electroless etching and nano imprint lithography

Silicon nanowires were fabricated by metal catalyzed electroless etching (MCEE) and nano imprint lithography (NIL), then a shell p-type layer was grown by thermal chemical vapor deposition (CVD) techniques. To reduce back surface recombination and also to activate the dopant, we used two techniques, back surface field (BSF) treatment and rapid thermal annealing (RTA), to improve device performance. In this study, we investigated BSF and RTA treatments in silicon nanowire solar cells, and improved the device performance and efficiency from 4.1 to 7.4% (MCEE device) and from 1.1 to 6.6% (NIL device) after introducing BSF and RTA treatments. Moreover, to achieve better metal contact without sacrificing the reflectance after the shell formation, the selective-area etching method was investigated. Finally, after combining all processes, silicon nanowire solar cells fabricated via the MCEE process exhibited 8.7% efficiency.