Solution-processed tandem solar cells, that stack two or more single junction subcells with different band gaps to harvest photons in the full solar spectrum more efficiently, have attracted increasing attention recently. Organic photovoltaics and perovskite solar cells are promising candidates for the top and/or middle subcells of tandem solar cells because the solar cells are able to capture visible and near-infrared photon energy. However, there are few materials to choose from for the infrared bottom subcell. While PbS and PbSe colloidal quantum dots (CQDs) have been gaining much attention for short-wave infrared solar cells owing to their wide-range bandgap tunability and solution process compatibility. Among various CQD-based solar cells, PbS/ZnO depleted heterojunction solar cells showed a power conversion efficiency (PCE) over 13% in 2020 (Nat. Commun. 2020, 11, 103). But there is a limited number of literatures reporting PbS-QD-based solar cells working in the low photon energy region (< 1.0 eV) where optical absorption of PbS QDs is weak. Thus, we have focused on PbS QD/ZnO nanowire (NW) structures with the aim of achieving efficient carrier transport and light absorption simultaneously (J. Phys. Chem. Lett. 2013, 4, 2455). We then investigated the performance of PbS QD/ZnO NW solar cells using PbS CQDs with the first exciton absorption peak locating in the short-wave infrared region. (ACS Energy Lett. 2017, 2, 2110). Here, we develop high efficiency infrared PbS CQDs solar cells aimed at the bottom subcell of tandem solar cells, and discuss the potential for the bottom subcell.
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