Simplified processing of SHJ-IBC solar cells

Abstract

High conversion efficiencies and low production costs have placed Silicon heterojunction (SHJ) solar cells as one of the top contenders in the photovoltaic market right now. When combined with the interdigitated-back-contacted (IBC) architecture, SHJ-IBC cells set world record efficiencies for silicon cells: in 2024, the record stands at 27.3% efficiency. However, the IBC architecture introduces complex processing steps. The goal of this project is the simplification of processing steps of SHJ-IBC solar cells, obtained with the use of hard masks for the direct deposition of patterned layers.

The first part of the project is focused on the development of hard masks. First, silicon based hard masks have been fabricated using wet and dry etching methods. Their fabrication process has been optimized until high quality masks with low defect rates have been achieved. In addition to these masks, quartz mask samples provided by a third party have been tested. Deposition of 50 and 100nm of (n) nc-Si:H have been made with all masks to evaluate their potential based on their resolution, quality, reusability and price. While quartz masks have demonstrated to be very promising and industry orientated, the best trade-off of qualities in a research environment has been found in dry etched silicon masks.

In the core of the project, dry hard masks have been used to substitute two photolithography steps in the fabrication flowchart of IBC-SHJ solar cells in order to achieve a significant process simplification. The use of hard masks has brought relevant impacts to the SHJ-IBC cell fabrication, like the removal of wet etching steps that can potentially bring uncertainty to process flowchart and the reduction of the processing time to half of what it was before the use of hard masks. Regarding the depositions made through the hard masks, the challenges of obtaining a precise alignment and accurate feature sizes been assessed. The implication of those challenges in the performance of the solar cells has been then evaluated.

Lastly, the thesis introduces a simplified fabrication process of SHJ-IBC solar cells with the implementation of hard masks. With the new fabrication process, efficiencies of up to 5.47% have been demonstrated. Through the introduction of the SHJ-IBC flowchart based on hard mask patterning, the project contributes to the ongoing effort to bring SHJ-IBC cells closer to the industry.