Rolf Brendel studied in Freiburg and Heidelberg, Germany, and at the University of Sussex, Brighton, U.K. He received the Physics Diploma degree in 1987 on nuclear fusion research, as well as the Mathematics degree in 1988. In 1992, he received the Ph.D. degree in infrared spectroscopy from the University of Erlangen, Erlangen, Germany. He then joined the Max-Planck-Institute for Solid State Research in Stuttgart to study the physics of thin-film silicon solar cells and the thermodynamics of photovoltaic power conversion. In 1997 he became head of the Division for Thermosensorics and Photovoltaics at the Bavarian Center for Applied Energy Research, where he worked on imaging diagnostics of solar cells. In 2004 he joined the Institute of Solid State Physics of the Leibniz University of Hannover as a full professor and became director of the Institute for Solar Energy Research in Hamelin (ISFH).
The high quality of today’s Si wafers makes the contacts the decisive element for the processing cost, the reliability and the efficiency of Si solar cells. The contacts have to be selective. This means they shall have a small resistance for the transport of majority carriers and a large resistance against the transport and subsequent recombination of minority carriers. We define contact selectivity quantitatively. In a simplifying model it is possible to analytically express the optimal area fraction of either contact, the optimal cell efficiency and the optimal light concentration analytically as a function of the contacts’ selectivities. With these theoretical results screening of various contact combinations is an easy task. We find that combinations of n-type polycrystalline Si on oxide (POLO) junctions in combination with screen-printed Al contacts are an attractive compromise for achieving high efficiencies with cost effective processes that are proven in today’s PERC+ production lines. The presentation discusses the current ISFH roadmap that is based on POLO junctions in combination with Al screen-printed contacts. Among various experimental achievements along this roadmap we highlight achieving 24.2% efficiency with a two sides-contacted, p-type, CZ-grown, Ga-doped, M2-sized, screen-printed Si solar cell that uses half the amount of Ag than other next-generation solar cells. We also report progress on fully avoiding Ag in future PV module fabrication.
