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The efficiency of crystalline silicon solar cells is reaching its practical limit in R&D as well as in high-volume manufacturing. To further drive down the cost of electricity produced by photovoltaics, increasing the efficiency beyond the limit for single-junction crystalline silicon is strongly desirable. 2-terminal perovskite-silicon tandem solar cells represent an appealing option: they benefit from the installed manufacturing capacity and process maturity of the silicon bottom cell, and achieve the high efficiency potential by adding a thin-film top cell. The first lab results with efficiencies of more than 30% have recently been announced [1]. Naturally, such champion devices are initially produced on small areas of the order of 1 cm2 and to date not all of the fabrication methods used in the laboratory meet the challenge of scaling up to industrial production. In this paper we discuss recent advances in the field, with a focus on potential device designs and manufacturing processes.

Within the NextTec project, a German consortium of equipment and metrology manufacturers as well as R&D institutes successfully developed the next generation of high-throughput processes and equipment for gigawatt-scale Si solar cell production. The project goal was the development of technical concepts (proof of concept) for the realization of an increase in throughput by a factor of 2 to 3 compared to currently used industrial production technology, ideally with an equivalent equipment footprint and only moderately increased equipment costs. Consequently, system throughputs of up 15,000-20,000 wafers per hour are addressed. Based on these values, a production capacity of approximately 1 GW per system and year can be achieved.

In 1997, a visionary study addressed limitations of the cost reduction potential for high efficiency solar cell devices. The results of this study triggered increased efforts for the development of production technology at Fraunhofer ISE. Many new approaches with substantial impact have been developed to the demonstration level. Economic analysis and a technology readiness terminology have been established to aid in assessing technological approaches, while the set-up of the PV technology evaluation center (PV-TEC) allowed for testing equipment, processes and materials on a higher maturity level. Reference processes building the backbone of the technology have been developed consistently further and device efficiencies follow a roughly 0.5%/a progress gradient since 2005, which is in good agreement with reports for an industrial pilot line. Today, we can rely on reference processes for PERC, TOPCon and SHJ in the PV-TEC with total and large area efficiencies of up to 22.7%, 24.0% and 24.1%, respectively. Further learning includes that for production technology R&D for PV devices high average power conversion efficiencies must be targeted and that scaling should be combined with thorough testing of products along the value chain. All cost and value determining parameters like production efficiency distribution, yield, capital and operational expenditure as well as degradation and performance under field conditions have to be monitored.