Off-Resonant Absorption Enhancement in Single Nanowires via Graded Dual-Shell Design.

Wenfu Liu,Xiaolei Guo,Yuxiao Zhang,Liang Zhang,Shule Xing,Xiao Wang,Haizi Yao,Yinling Wang,Liuyang Bai,Dachuan Wu,Qi Wang,Yasha Yi

doi:10.3390/nano10091740

Abstract

Single nanowires (NWs) are of great importance for optoelectronic applications, especially solar cells serving as powering nanoscale devices. However, weak off-resonant absorption can limit its light-harvesting capability. Here, we propose a single NW coated with the graded-index dual shells (DSNW). We demonstrate that, with appropriate thickness and refractive index of the inner shell, the DSNW exhibits significantly enhanced light trapping compared with the bare NW (BNW) and the NW only coated with the outer shell (OSNW) and the inner shell (ISNW), which can be attributed to the optimal off-resonant absorption mode profiles due to the improved coupling between the reemitted light of the transition modes of the leak mode resonances of the Si core and the nanofocusing light from the dual shells with the graded refractive index. We found that the light absorption can be engineered via tuning the thickness and the refractive index of the inner shell, the photocurrent density is significantly enhanced by 134% (56%, 12%) in comparison with that of the BNW (OSNW, ISNW). This work advances our understanding of how to improve off-resonant absorption by applying graded dual-shell design and provides a new choice for designing high-efficiency single NW photovoltaic devices.

Highlights

Single nanowire (NW) solar cells have attracted more and more research interests as nanoelectronic power sources in recent years due to their unique characteristics, such as enhanced light-harvesting capability, efficient carrier collection, ultra-compact volume, large surface area and convenience of integrating with optoelectronic nanosystems [1,2,3,4,5,6,7,8]
Comparing to the bare NWs (BNWs), semiconductor core-dielectric shell NWs (CSNWs) provides the possibility to tune the position of the resonant peak and enhance the light-harvesting capability at the off-resonant wavelengths by adjusting the thickness and the refractive index of the shell, which is attributed to the high nanofocusing effect [30,38]
Note here that r = 100 nm, t = 180 nm, R = r + t = 180 nm, t2 = 0 → 180 nm, where t = 0, t2 = 0 and t2 = 180 nm denote the cases of the BNW, OSNW and ISNW, respectively; m3 is the complex refractive index of the Si core, m2 = 2.0, m1 = 1.5 and m0 = 1.0, as shown in the insets of Figure 1

Summary

Introduction

Single nanowire (NW) solar cells have attracted more and more research interests as nanoelectronic power sources in recent years due to their unique characteristics, such as enhanced light-harvesting capability, efficient carrier collection, ultra-compact volume, large surface area and convenience of integrating with optoelectronic nanosystems [1,2,3,4,5,6,7,8]. Comparing to the bare NWs (BNWs), semiconductor core-dielectric shell NWs (CSNWs) provides the possibility to tune the position of the resonant peak and enhance the light-harvesting capability at the off-resonant wavelengths by adjusting the thickness and the refractive index of the shell, which is attributed to the high nanofocusing effect [30,38]. Some new strategies have been employed to improve the light-trapping capability of the CSNW, including the off-axial core-shell design [39] and partially capped design [40]. To the best of our knowledge, the dual shells with the graded refractive index have not been applied to improve the light absorption of single NWs

Methods

Results

Conclusion