By Dr Teodor Todorov, IBM T. J. Watson Research Center, USA.
Photovoltaic technologies have come a long way since 1883 when the first solar cell was made of elemental selenium coated with a layer of gold. After generations of research, silicon and thin film photovoltaics have finally enabled energy production at costs competitive, and often lower, than traditional power sources. As continued efficiency improvements asymptote towards the practical limits of single-junction devices, it is of particular interest for future growth of PV to develop low-cost, efficient high-bandgap solar cells that can be fabricated at low temperature. This will allow the development of planar multi-junction solar cells with efficiencies superior to state-of the art single junction cells. The ideal top cell would allow for compatibility with existing commercial photovoltaic materials and fabrication processes, such as the relatively low band gap crystalline silicon. Another application of low-temperature, high band gap devices is the exponentially growing industry of wearable and Internet of Things (IoT) applications. This requires powering billions of small devices, many of them operating in indoor lighting, which is significantly blue-shifted in comparison to standard AM1.5 conditions. In LED and fluorescent light, high-bandgap devices outperform even the best low-bandgap devices optimized for outdoor performance, and provide additional benefit of higher voltage reducing series resistance, as well as interconnection or conversion losses needed for charging batteries. Selenium is an attractive candidate in this category because of its very low processing temperature (<200C) and a band gap of 1.95eV ideally suited for indoor spectrum as well as a top absorber in tandem or triple junction solar cells. We will present our most recent developments including ultrathin record-perfomance Se devices reaching 1000mV, and semitransparent solar cells for tandem and wearable applications.
Teodor Todorov*, Douglas M. Bishop
Teodor Todorov completed his PhD in Materials Science at the Jaume-I University (Spain). He has always been attracted to low-cost and scalable solar energy technologies and explored different non-vacuum routes for PV films at Hahn-Meitner Institut (now Helmhotz-Zentrum, Berlin), IRDEP (l’Institut de Recherche et Développement sur l’Énergie Photovoltaïque, Paris) and IEC (Institute of Energy Conversion, Delaware). In 2008 he joined IBM T. J. Watson Research Center where his World-record results with CZTSSe photovoltaic technology motivated the launch of many research projects at the company and around the globe. He invented numerous thin film device structures, methods and processing equipment that lead to more record results including CZTSSe, solution-processed CIGS, monolithic tandem perovskite solar cells and ultrathin high-bandgap selenium devices. He is currently expanding the area of his research towards other thin-film materials for photovoltaic and energy storage applications.