Light Trapping in Single Elliptical Silicon Nanowires.

Wenfu Liu,Yasha Yi,Jun Song,Xiaolei Guo,Yinling Wang,Xiao Wang

doi:10.3390/nano10112121

Abstract

Light trapping in single nanowires (NWs) is of vital importance for photovoltaic applications. However, circular NWs (CNWs) can limit their light-trapping ability due to high geometrical symmetry. In this work, we present a detailed study of light trapping in single silicon NWs with an elliptical cross-section (ENWs). We demonstrate that the ENWs exhibit significantly enhanced light trapping compared with the CNWs, which can be ascribed to the symmetry-broken structure that can orthogonalize the direction of light illumination and the leaky mode resonances (LMRs). That is, the elliptical cross-section can simultaneously increase the light path length by increasing the vertical axis and reshape the LMR modes by decreasing the horizontal axis. We found that the light absorption can be engineered via tuning the horizontal and vertical axes, the photocurrent is significantly enhanced by 374.0% (150.3%, 74.1%) or 146.1% (61.0%, 35.3%) in comparison with that of the CNWs with the same diameter as the horizontal axis of 100 (200, 400) nm or the vertical axis of 1000 nm, respectively. This work advances our understanding of how to improve light trapping based on the symmetry breaking from the CNWs to ENWs and provides a rational way for designing high-efficiency single NW photovoltaic devices.

Highlights

Single nanowire (NW) solar cells have increasingly attracted attention in recent years due to the following reasons [1–8]
The Elliptical Nanowire (ENW) are illuminated perpendicularly by sunlight from the top, the wavelength range of the incident light is from 300 to 1100 nm with a step size of 5 nm considering solar radiation and the bandgap of silicon, the perfectly matched layers (PML) boundary conditions are applied to avoid any non-physical reflection with the boundaries, the total-field scattered-field (TFSF) method was adopted to ensure that a single NW interacts with an infinite plane wave
To understand the light-trapping mechanism responsible for the improved absorption of the ENW, we investigate the absorption efficiency (Qabs), ultimate photocurrent (Jph), normalized electric field (Er), absorption mode profile (Pabs) and photogeneration rate (G), respectively

Summary

Introduction

Single nanowire (NW) solar cells have increasingly attracted attention in recent years due to the following reasons [1–8]. Light trapping in single NW solar cells is very important for ensuring both high absorption and little photoactive material, which can enhance the absorption by increasing the light path length within the cell while it is possible to use less material in the cell [17–27]. Light trapping is an effective way to improve light absorption and enhance the photoelectric conversion efficiency of single NW solar cell. The overall light-trapping performance of a single NW is still far below expectations owing to the narrow resonant peaks. 22ooff1155 ltihgehtl-igtrhatp-tprianpgpipnegrfpoerrmfoarnmceanccoeucldoublde bfuerftuhretrheimr ipmrpovroevdedbybyinitnrtordoudcuicnigngaannoonn--aabbssoorrbbiningg ddiieelleeccttrriicc sshheellll ((oorr ggrraaddeedd dduuaall sshheellllss)) aass tthheeaannttiirreefflleeccttiioonn ccooaattiinngg,, wwhhiicchh wwaass eexxppeerriimmeennttaallllyy aanndd nnuummeerriiccaallllyy ddeemmoonnssttrraatteedd iinn tthhee rreecceenntt ssttuuddiieess [[4400––4444]]. RReecceennttllyy,, ssoommee nneeww ssttrraatteeggiieess hhaavveebbeeeenniimmpplleemmeenntteeddttoo iimmpprroovvee tthhee lliigghhtt--ttrraappppininggcacpapabaibliiltiytyofotfhtehNe WNsWbsasbeadseodn othne tshyemsmymetrmyebtrreyabkrinegak. TThhee ggeeoommetrical metrics used to deessccrriibbee tthheeggeeoommeettrryyoofftthheeEENNWWss,,ththeehhoorirzizoonntatal l(x(x) )axaixsisa aananddthtehevevretrictiacla(ly()ya)xaixsibs,ba,raersehsohwown.nN.

Methods

Results and Discussion

The ultimate photocurrent of Single ENWs