Ultra-low reflection porous silicon nanowires for solar cell applications

A Najar,P Pirasteh,R Sougrat,J Charrier

doi:10.1364/oe.20.016861

Abstract

High density vertically aligned Porous Silicon NanoWires (PSiNWs) were fabricated on silicon substrate using metal assisted chemical etching process. A linear dependency of nanowire length to the etching time was obtained and the change in the growth rate of PSiNWs by increasing etching durations was shown. A typical 2D bright-field TEM image used for volume reconstruction of the sample shows the pores size varying from 10 to 50 nm. Furthermore, reflectivity measurements show that the 35% reflectivity of the starting silicon wafer drops to 0.1%, recorded for more than 10 μm long PSiNWs. Models based on cone shape of nanowires located in a circular and rectangular bases were used to calculate the reflectance employing the Transfert Matrix Formalism (TMF) of the PSiNWs layer. Using TMF, the Bruggeman model was used to calculate the refractive index of PSiNWs layer. The calculated reflectance using circular cone shape fits better the measured reflectance for PSiNWs. The remarkable decrease in optical reflectivity indicates that PSiNWs is a good antireflective layer and have a great potential to be utilized in radial or coaxial p-n heterojunction solar cells that could provide orthogonal photon absorption and enhanced carrier collection.

Highlights

Semiconductor nanostructures are promising building blocks for generation solar cells with higher energy conversion efficiencies and lower cost [1,2]
The main improvement in the performance of the porous silicon (PS) layers is the rough surface and low effective refractive index compared with crystalline silicon (c-Si), which can decrease the reflective loss of solar radiation and lead to an increase of the short-circuit current density of solar cells [7,8,9]
The three successive longitudinal slices obtained from the reconstructed volume revealed a distribution of irregular size/shape vacuum spaces and silicon frame, constituting the Porous Silicon Nanowires (PSiNWs)

Summary

Introduction

Semiconductor nanostructures are promising building blocks for generation solar cells with higher energy conversion efficiencies and lower cost [1,2]. Vertically-aligned Silicon Nanowires (SiNWs) arrays exhibit low reflection and strong broadband absorption [10,11] and may be used as antireflection coatings or as the active layer in solar cells [10,12,13]. Optical properties of SiNW arrays over the solar spectrum have previously been calculated [18] They studied arrays with varying nanowire diameters for a fixed lattice constant of 100 nm, typical of the structures reported in Refs [19,20]. Their results showed that SiNW arrays could have much lower reflectance than silicon thin films. It is of interest to investigate the optical properties, including reflectance for different length of PSiNWs structures based on TMF (Transfert Matrix Formalism) and Bruggeman model

Experimental conditions

Results and discussion

Modeling and discussion

Conclusion