Challenges for the future of tandem photovoltaics on the path to terawatt levels: A technology review

As the photovoltaic (PV) sector approaches 1 TW in cumulative installed capacity, we provide an overview of the current challenges to achieve further technological improvements. On the raw materials side, we see no fundamental limitation to expansion in capacity of the current market technologies, even though basic estimates predict that the PV sector will become the largest consumer of Ag in the world after 2030. On the other hand, recent market data on PV costs indicates that the largest cost fraction is now infrastructure and area-related, and nearly independent of the core cell technology. Therefore, additional value adding is likely to proceed via an increase in energy yield metrics such as the power density and/or efficiency of the PV module. However, current market technologies are near their fundamental detailed balance efficiency limits. The transition to multijunction PV in tandem configurations is regarded as the most promising path to surpass this limitation and increase the power per unit area of PV modules. So far, each specific multijunction concept faces particular obstacles that have prevented their upscaling, but the field is rapidly improving. In this review work, we provide a global comparison between the different types of multijunction concepts, including III-Vs, Si-based tandems and the emergence of perovskite/Si devices. Coupled with analyses of new notable developments in the field, we discuss the challenges common to different multijunction cell architectures, and the specific challenges of each type of device, both on a cell level and on a module integration level. From the analysis, we conclude that several tandem concepts are nearing the disruption level where a breakthrough into mainstream PV is possible.