Efficient utilization of a wide range of the solar spectrum is essential to construct ultra-high efficiency solar cells. So far, several different concepts for ultra-high efficiency have been proposed, which use multiple exciton generation, hot-carrier extraction, multi-junction and so forth. Most of them are, however, still under fundamental investigation. Although multi-junction solar cells based on III-V semiconductors are the only solar cells that have achieved a power conversion efficiency well over the single-junction limit (-30% under one-sun illumination), the solar cells rely heavily on high cost solar cell technology, which makes it difficult for the solar cells to be used widely. Organic photovoltaics made up of conjugated small molecules and/or polymers, and perovskite solar cells composed of organometal halide perovskite compounds (e.g. CH3NH3PbI3) are promising candidate for the top and/or middle cells of multi-junction solar cells. This is because the solar cells can be constructed with low-temperature and solution-based methods, and because the solar cells are able to capture visible and near-infrared photon energy efficiently. However, there are few materials to choose from for the bottom cells, as few materials harvest solar energy in the short-wave infrared. The development of low cost and efficient short-wave infrared solar cells is therefore essential to construct the bottom cell of multijunction cells. Lead chalcogenide colloidal quantum dots (CQDs), such as PbS and PbSe, appear promising for use as the middle and/or bottom cells. This is because the absorption bandgap of bulk PbS is located in the infrared region (3.1 mm) and can be readily tuned by controlling quantum dot synthesis conditions. Most importantly, CQDs are compatible with low-temperature solution-based technologies.

We have focused on CQD/ZnO nanowire (NW) structures (NW-type), with the aim of simultaneous enhancement in carrier transport and light harvesting efficiency. We also used PbS CQDs with the first exciton absorption peak locating between the near-infrared and short-wave infrared region. Our recent study revealed that NW-type solar cells give a carrier diffusion length of over 1 µm, and a record high EQE of 40% (at 1.6 mm) in the short-wave infrared region. In this presentation, we investigate the performance of PbS QD/ZnO NW solar cells using PbS CQDs absorbing photons in a wide range of solar spectrum, and discuss the potential for low-cost multi-junction solar cells with PbS CQD/ZnO NW hybrid structures.