Haoting Shen

Radial junction silicon wire array solar cells fabricated by gold-catalyzed vapor-liquid-solid growth

The fabrication of radial junction silicon (Si) solar cells using Si wire arrays grown by Au-catalyzed vapor-liquid-solid growth on patterned Si substrates was demonstrated. An important step in the fabrication process is the repeated thermal oxidation and oxide etching of the Si wire arrays. The oxidation cleaning process removes residual catalyst material from the wire tips and exposes additional Au embedded in the material. Using this cleaning process and junction formation through POCl3 thermal diffusion, rectifying p-n junctions were obtained that exhibited an efficiency of 2.3% and open circuit voltages up to 0.5 V under Air Mass 1.5G illumination.

Vapor-liquid-solid growth of 〈110〉 silicon nanowire arrays

The epitaxial growth of <110> silicon nanowires on (110) Si substrates by the vapor-liquid-solid growth process was investigated using SiCl4 as the source gas. A high percentage of <110> nanowires was obtained at high temperatures and reduced SiCl4 partial pressures. Transmission electron microscopy characterization of the <110> Si nanowires revealed symmetric V-shaped {111} facets at the tip and large {111} facets on the sidewalls of the nanowires. The symmetric {111} tip faceting was explained as arising from low catalyst supersaturation during growth which is expected to occur given the near-equilibrium nature of the SiCl4 process. The predominance of {111} facets obtained under these conditions promotes the growth of <110> SiNWs.

Effect of c-Si doping density on heterojunction with intrinsic thin layer (HIT) radial junction solar cells

Radial junction Si pillar array solar cells based on the heterojunction with intrinsic thin layer (HIT) structure were fabricated from p-type crystal Si (c-Si) wafers of different doping densities. The HIT structure consisting of intrinsic/n-type hydrogenated amorphous Si (a-Si:H) deposited by plasma-enhanced chemical vapor deposition (PECVD) at low temperature (200°C) was found to effectively passivate the high surface area of the p-type Si pillar arrays resulting in open circuit voltages (Voc>0.5) comparable to that obtained on planar devices. At high c-Si doping densities (>1018 cm-3), the short-circuit current density (Jsc) and energy conversion efficiency of the radial junction devices were higher than those of the planar devices demonstrating improved carrier collection in the radial junction structure.

Parametric study of micropillar array solar cells

Micro/nano pillar arrays are a promising architecture for high-efficiency solar cells that employ inexpensive photovoltaic materials with short minority carrier diffusion lengths (Ln, p). To investigate design tradeoffs of the radial junction array solar cells, we fabricated 25 μm tall c-Si pillar array devices having different diameters and pillar filling ratios. The high-aspect-ratio radial n+-p+ junctions were formed by gas phase diffusion of an n-type dopant into etched p-type Si pillars. The c-Si pillar arrays showed clear rectifying properties. The spectral reflectance decreased as the pillar filling ratio increased from 0.2 to 0.5, and no subsequent decrease was observed above a filling ratio of 0.5. Approximately two times higher cell efficiency was obtained with an 8 μm diameter (<1 Ln) pillar array than with a 32 μm diameter (>3 Ln) pillar array having the same pillar filling ratio.

Radial Junction Silicon Nanowire Photovoltaics With Heterojunction With Intrinsic Thin Layer (HIT) Structure

Single-wire radial p-i-n heterojunction with intrinsic thin layer nanowire solar cell devices were fabricated by plasma-enhanced chemical vapor deposition of thin intrinsic and n-type hydrogenated amorphous silicon (a-Si:H) shell layers on p-type silicon nanowires synthesized by vapor-liquid-solid growth. The thin intrinsic a-Si:H layer provided an effective passivation of the crystalline Si nanowire surface, and the corresponding device exhibits a dark current density of 1.6 × 10^-7 A/cm². Under one-sun illumination, the device shows a short-circuit current (I_sc) of 2.66 × 10^-7 mA, open-circuit voltage (V_oc) of 0.46 V, fill factor (ff) of 0.52, and energy conversion efficiency (η) of 6.3%. Indium tin oxide (ITO) contacts were also deposited on the n-type shell as an antireflection layer to improve light absorption in the nanowire device. After ITO deposition, the device exhibited a larger I_sc of 2.75 × 10^-7 mA, but the overall efficiency decreased due to a larger leakage current and lower V_oc.

Uniform p-type doping of silicon nanowires synthesized via vapor-liquid-solid growth with silicon tetrachloride

Boron-doped silicon nanowires (SiNWs) grown by the vapor-liquid-solid growth mechanism using silicon tetrachloride (SiCl4) as the silicon precursor and trimethylboron (TMB) as the boron source were studied to understand the axial and radial doping uniformity. TMB-doped SiNWs with diameters up to 400 nm and lengths > 7.5 μm were integrated into a global back-gated test structure with multiple electrodes for electrical characterization. From gate modulated measurements, the SiNWs were confirmed to be heavily doped p-type. Multiple four point resistivity measurements across a total length of 7.5 μm were taken on as-grown SiNWs. Resistivity, corrected for surface charge, was determined to be 0.01 +/− 0.002 Ω cm along the entire length of the as-grown boron doped SiNWs. This was also observed in the axial direction for etched SiNWs, with corrected resistivity of 0.01 +/− 0.003 Ω cm, therefore confirming the uniform p-type doping of SiNWs using TMB and SiCl4 as precursors.

Infrared dielectric functions of hydrogenated amorphous silicon thin films determined by spectroscopic ellipsometry

Amorphous hydrogenated silicon (a-Si:H) thin films have found use in photovoltaic, transistor, and microbolometer applications. Routine optical metrology of a-Si:H is generally performed in the visible range but is not directly sensitive to hydrogen bonding. Infrared spectroscopic ellipsometry (IR-SE) allows a direct measurement of the relative absorption strength of various hydrogen-related modes, giving some insight into the hydrogen content and relative disorder of films. IR-SE is used here to develop a parameterization of ε=ε1+iε2 for several thin (<; 300 nm) hydrogenated amorphous germanium (a-Ge:H) and a-Si:H films deposited onto silicon nitride or titanium-coated crystalline silicon substrates.