Charge transport materials for monolithic perovskite-based tandem solar cells: A review

Developing perovskite-based tandem solar cells (TSCs) is a promising technique to surpass the Shockley-Queisser limit set for single-junction solar cells. Encouragingly, all the perovskite-based TSCs, including perovskite/silicon, perovskite/perovskite, perovskite/copper indium gallium selenide and perovskite/organic tandems have demonstrated higher efficiency than the corresponding single-junction solar cells, showing great potential for further breakthroughs. In tandem devices, charge transport materials (CTMs) are vital components of perovskite sub-cells that directly determine the charge transportation and energy loss. Generally, high conductivity and transmittance, favorable energy-level alignment and chemical stability are crucial for CTMs for tandem applications. To date, various CTMs including conductive metallic oxides, organic molecules, polymers, fullerenes, and self-assembled materials have been extensively employed in highly efficient TSCs. In this review, we first summarize the recent progress of CTMs for different types of monolithic perovskite-based TSCs, in which the electrical and optical properties of CTMs and their influence on device performance are carefully discussed. Then we put forward the challenges and outlook for the further development of CTMs for tandem applications. This comprehensive review will provide effective guidance for device design for different perovskite-based TSCs.