Thin Cost-Effective Silicon Wafers for Heterojunction Based Photovoltaic Devices: High Throughput Surface Passivation

The research relates to developing a solar cell technology that is in line with the PV roadmap moving towards unconventional substrates. A key objective is the reduction in cost without sacrificing much with respect to the overall performance of the solar cell. So far, one technology that has seen considerable success and achieved top solar cell efficiency is the HIT (Heterojunction with Intrinsic Thin-layer) based technology developed by Sanyo Electric Corp. It was shown to perform at 23.9 % in R&D with 1 to 2% less absolute efficiency in production. This technology also has the advantage of a minimal thermal budget compared to conventional Si based technologies. The lower processing temperature leads to reduced diffusion of contaminants or other undesirable elements (1,000C processes compared to 200-250C ones) and hence relaxes the requirements on the purity of the base Si wafer material. Other technologies, requiring high temperature process steps, will create major challenges as we move towards thinner wafers, resulting in wafer bowing and enhanced recombination losses. The low temperature process, characteristic of the heterojunction cell, will alleviate this major problem.

The HIT technology, coupled with thin Si wafers, is not without challenges. This includes: passivation of the c-Si wafers, the creation of an emitter and back surface field with good electrical qualities, yet not affecting the passivation of the surface is a challenging matter. Likewise, is the critical need to prepare optimized transparent conducting oxide (TCO) layers to enable efficient current extraction. To push this technology beyond the present limitation requiring thick Si substrates (e.g. high quality CZ wafers 180 to 200 micrometers thick), we plan to explore the use of the HIT cell approach on unconventional substrates.

The research responds to MI’s vision to become the leader in renewable and sustainable technologies and develop future energy systems. In this project Prof. Gougam, in collaboration with MIT, a leader in the field of innovative technologies, and in particular with the groups of: Prof. Tuller whose record in the field of ceramics and conducting oxides for electronic and renewable energy applications is impressive and Prof. Buonassisi who has an extensive record in the application of materials defect engineering towards PV applications, would constitute a very strong partnership enabling and strengthening the establishment of an intellectual capacity at MI in terms of exposing our students and researchers to top rated scientific work. This is in line with the other target of MI and the Abu Dhabi region to build a knowledge-based society. This project will constitute an excellent platform for training future researchers and leaders that will feed “next technology hubs” in this region. The number of commonalities between the silicon based solar cells and silicon based IC devices will also contribute to innovative approaches in elaborating advanced materials for energy applications.

One of the major issues still impeding solar cell technology’s ability to compete with conventional power generating technologies is cost with a major cost component being the silicon wafer used in mainstream PV devices. Significantly reducing wafer thickness, while increasing efficiency, represents a path toward cost competitiveness with fossil fuel-based energy sources.