Fabrication of vertically aligned silicon nanowire arrays and its application in hybrid heterojunction solar cells

Abstract

Worldwide, numerous approaches to reduce climate change emerge every day. Photovoltaic is one of the attractive and efficient methods to generate carbon free renewable power. Reigning over the photovoltaics industry, crystalline silicon-based solar cells are efficient but expensive due to their complexity in processing. This brings the motivation for fundamental research in a quest to find of simpler and less expensive ways to harvest light in order to generate electricity. Hybrid platforms, with the help of nanotechnology, strive to include various inorganic, organic, polymer materials combined with crystalline silicon to fabricate hybrid heterojunction solar cells. The main objective of this thesis is to develop a costeffective method to fabricate silicon nanowire-based hybrid heterojunction solar cells. Employing silicon nanowires help to reduce the material usage and thus the cost by their inherently large surface to volume ratio. We found galvanic displacement method as the most cost-effective technique compared to other methods used for the synthesis of silicon nanowires. Optical and electrical properties of these nanowires have been measured to evaluate the potential of these materials for their usage in photovoltaics. We have investigated phonon confinement effect on the silicon nanowires. Being silicon an indirect band gap material; phonon confinement helps in band to band transition of electrons and holes which further increases the photovoltaic efficiency of devices built around silicon nanowires. In this thesis, we first introduce a device architecture that combines silicon nanowires and anatase TiO2, demonstrating the fabrication of bulk heterojunction solar cells using interface modification with HBr. This approach results in PV devices with up to 6.2% of power conversion efficiency (PCE) that represents 3 times enhancement compared to similar architectures with interface modification using other routes. This thesis provides a promising route for interface engineering of hybrid heterojunction based optoelectronic devices. The second device architecture described in this thesis shows the fabrication of hybrid heterojunction solar cells comprised of plasmon sensitized silicon nanowires and PEDOT:PSS. Significant plasmon induced enhancement is demonstrated by controlling the cleaning protocol during the nanowire fabrication step. This allows to leave minute amount of silver nanoparticles attached to the silicon nanowires before coating with solvent treated PEDOT:PSS. The solar cell devices produced show a PCE up to 8.4% under AM 1.5G compared to 5.5% without silver nanoparticles. This thesis yields a favorable way to achieve better performing ultra-low-cost silicon-based solar cells.

Date	29 Aug 2018
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Sylvain G. Cloutier (Supervisor)