Solar Cells on < 50μm Thick Epitaxial Foils Conductively Bonded to Low-Cost Si Carrier

Abstract

As PV production ramps up and numerous installed solar panels reach their end of life, potentially huge waste sources of silicon become available. Such silicon can be recovered and recycled back into the PV industry, for making solar cells directly on the recycled Si. In cases where the recycled Si is of very low purity and cannot be used directly as active material, they can instead be used to make low-cost Si supporting carriers that are highly-conductive. These supporting substrates have been made using low-cost techniques such as by sintering of low-cost Si powders. These conductive substrates can act as mechanical carriers of thin silicon (< 50 μm) and as rearcontacts in what is called a wafer-equivalent concept. In this work, thin epitaxial silicon foils from the porous siliconbased lift-off process are used. For conductive bonding, low-temperature Ag paste from INKRON (IPC-114) has been preferably used due to its robust bonding properties (low temperature curing, high conductivity and minimal volume change). As a first demonstrative step, thin (<50 μm) epifoil-based and thinned (<50 μm) Cz-based cells conductively bonded onto highly-doped p+ Cz silicon substrates have been fabricated. We chose the heterojunction technology for cell fabrication, leading to 15.7% efficiency for the best epifoil cell, partially processed after conductive bonding. It was observed that the cell performance of bonded cells are comparable to that of non-bonded epifoil cells that were processed freestanding. Furthermore, in order to reduce costs of the conductive bonding step, extensive simulations have been performed to understand the best configuration and pattern geometry for the bonding layer. This work is done in framework of H2020 EU project CABRISS.

As PV production ramps up and numerous installed solar panels reach their end of life, potentially huge waste sources of silicon become available. Such silicon can be recovered and recycled back into the PV industry, for making solar cells directly on the recycled Si. In cases where the recycled Si is of very low purity and cannot be used directly as active material, they can instead be used to make low-cost Si supporting carriers that are highly-conductive. These supporting substrates have been made using low-cost techniques such as by sintering of low-cost Si powders. These conductive substrates can act as mechanical carriers of thin silicon (< 50 μm) and as rearcontacts in what is called a wafer-equivalent concept. In this work, thin epitaxial silicon foils from the porous siliconbased lift-off process are used. For conductive bonding, low-temperature Ag paste from INKRON (IPC-114) has been preferably used due to its robust bonding properties (low temperature curing, high conductivity and minimal volume change). As a first demonstrative step, thin (<50 μm) epifoil-based and thinned (<50 μm) Cz-based cells conductively bonded onto highly-doped p+ Cz silicon substrates have been fabricated. We chose the heterojunction technology for cell fabrication, leading to 15.7% efficiency for the best epifoil cell, partially processed after conductive bonding. It was observed that the cell performance of bonded cells are comparable to that of non-bonded epifoil cells that were processed freestanding. Furthermore, in order to reduce costs of the conductive bonding step, extensive simulations have been performed to understand the best configuration and pattern geometry for the bonding layer. This work is done in framework of H2020 EU project CABRISS.