Perovskite-based Tandem Photovoltaics
The rise of perovskite semiconductors affords a novel opportunity to advance the power conversion efficiency of the market-dominating photovoltaic technologies based on c-Si and CIGS in so-called tandem solar cells. Moreover, perovskite semiconductors of suitable bandgaps can be combined in all-perovskite tandem devices. Advancing these technologies is a focus of our team’s research activities  . The key advantage of such a tandem photovoltaics solar module lies in the efficient exploitation of the solar spectrum  . The high energy light is harvested at high efficiency in the perovskite top solar cell while the low energy light is harvested in the c-Si, CIGS or perovskite bottom solar cell  . With our research partners, we research, develop and prototype highly efficient multijunction perovskite/c-Si solar cells and perovskite/CIGS photovoltaic solar cells.
Figure 1: The perovskite-based tandem photovoltaics efficiently uses the AM1.5G solar spectrum. The high energy light is harvested in the top perovskite solar cell, the low energy light is harvested in the bottom solar cell (e.g., c-Si, CIGS or perovskite).
Perovskite-based tandem solar cells with c-Si or CIGS bottom solar cells.
Our team is involved in research and development of novel materials, processes and device architectures of high-efficiency perovskite-based tandem photovoltaics. We demonstrated state-of-the-art tandem solar cells with high power conversion efficiencies in collaborations with our partners. 
Figure 2: Schematic illustration and secondary electron micrsocopy (SEM) image of a 4-terminal perovskite/CIGS tandem solar cell. The current-voltage characteristics demonstrate a measured power conversion efficiency of 27.3%.
All-perovskite tandem solar cells.
All-perovskite tandem solar cells combine wide-bandgap and low-bandgap perovskite semiconductors. While research on wide-bandgap perovskite top solar cells demonstrated advances in stability and performance in recent years, the low bandgap candidates lag behind. In a recent study, we developed a strategy to control the morphology of low bandgap perovskite thin films, enabling all-perovskite tandem solar cells with power conversion efficiencies above 23% . Future research will need to address the challenges of developing more stable materials and tandem architectures with minimum optical and electrical losses .
Figure 3: Photograph of the external quantum efficiency (EQE) measurement of an perovskite-based tandem solar cell.