Silicon-based tandem solar cells are rapidly opening pathways to advance Si solar cell performance that greatly exceed the single-junction limit of standard Si solar cells. High efficiency without large increase in cost at the manufacturing scale, combined with outstanding long-term device reliability and compatibility with existing Si manufacturing infrastructure are critical to increase PV penetration into traditional and non-traditional energy markets. The ideal bandgap profiles for highest efficiency enabled by III-V/Si integration, coupled with the proven and outstanding long-term reliability of the constituent technologies therefore make III-V/Si tandems very attractive. Furthermore, the ability to access mature and proven III-V and Si devices and circuits through clever processing and integration schemes could open active, on-wafer power management, and other functions, on the same PV wafer platform in the future. This presentation focuses on the critical role and impact of defect and interface engineering at the materials science level for advancing monolithically-integrated III-V/Si tandem materials and solar cells. While a brief review of the status of III-V/Si tandem solar cells will be provided for context, the primary focus is on GaAsP/Si tandem solar cells produced by metalorganic chemical vapor deposition (MOCVD) with ideal 1.7 eV/1.1 eV bandgap profiles for maximum efficiency. Device designs that account for the presence of elevated dislocation densities on transport properties in early stage monolithic GaAsP/Si tandem solar cells have already resulted in the current certified record efficiency of 23.4%, for this cell type even at dislocation densities that are greater than 1x107 cm-2. While this dislocation density value has appeared to have generally reached a plateau for the past several years, we have made a recent breakthrough in the growth and design of metamorphic GaAsP/GaP buffers on Si that have resulted in TDD values 10x lower through the insertion of novel dislocation glide enhancing structures during GaP growth. At these improved dislocation densities in the low 106 cm-2 range, a near term pathway to achieve stable Si tandem efficiencies >28% and up to 31% for two junction tandems is presented.

Dr. Steven A. Ringel is a Distinguished University Professor at The Ohio State University, where he also holds the Neal Smith Endowed Chair in Electrical and Computer Engineering and serves as both the Associate Vice President for Research for Ohio State and the Executive Director of Ohio State's Institute for Materials and Manufacturing Research (IMR). The IMR is the gateway for Ohio State Materials and Manufacturing Research, manages major research centers and facilities, including Ohio State's major semiconductor efforts. Dr. Ringel is internationally recognized for his research contributions in the field of semiconductor materials and devices. His research focuses on photovoltaics and wide bandgap semiconductors, emphasizing compound semiconductor - silicon heterogeneous integration, crystal defect characterization and device reliability. His research has led to record performance solar cells, has helped to guide the worldwide development of wide bandgap semiconductors now used in applications such as energy-efficient power electronics, space-based communication technologies and satellite solar power systems. Dr. Ringel has authored/co-authored > 450 technical articles and conference presentations, edited several books and contributed many book chapters. He is a Fellow of IEEE, Fellow of AAAS, Associate Fellow of AIAA, former NSF National Young Investigator, and with his students have together received 11 best paper/presentation awards. In addition to being an Ohio State Distinguished University Professor, Ohio State's highest faculty title, Dr. Ringel has been honored with the OSU Distinguished Scholar Award - the university's highest recognition for scholarly achievement. Ringel holds or has held leadership positions at organizations in the U.S., Singapore, Ireland and India. He is a Distinguished Visiting Professor at IIT Bombay and was recently named as the first Distinguished Visiting Fellow of the Tyndall National Institute in Ireland.