Development of multi-junction solar cells using low-cost technologies such as solution-based methods is one ultimate goal of solar energy uses. To do so, utilization of photon energy in a wide range of solar spectrum is a pivotal issue. There are many options for solution-based solar cells that can operate in the visible region. However, there are few materials to choose from for solar cells that can operate in the near and short-wave infrared region (referred to as IR region, for short). In this sense, PbS colloidal quantum dots (CQDs) are a promising material for IR solar cells (0.8 - 2 mm). We constructed PbS CQD solar cells by employing ca 1 mm long ZnO nanowires (NWs) as a good electron pathway to achieve high light harvesting efficiency and good carrier transportation [H. Wang et al., J. Phys. Chem. Lett., 4, 2455 (2013)].
We then synthesized nine different PbS QDs, which give the first exciton peak in the IR region, and carried out systematic investigation into the performance of PbS QD/ZnO NW solar cells made up of these PbS QDs. The solar cells were confirmed to convert a wide range of solar energy (3.54 - 0.62 eV, corresponding to 0.35 - 2.0 μm). We also found that the solar cells working in the IR region (Eg (eV) = 0.75, 0.70, 0.63) gave open circuit voltage (Voc) of 0.39，0.35，0.27 V, respectively. The energy loss (Eloss) defined by Eg - qVoc is ca 0.4 V independent of Eg. The Voc and energy loss are comparable to those of Ge solar cells (Voc = 0.27 V, Eg = 0.67 eV, Eloss = 0.4 V), which are widely used for III-V compound semiconductor triple-junction solar cells [Wang et al., ACS Energy Lett., 2, 2110 (2017)]. However, EQE values in the IR region were not higher than we expected. This is mainly because the free carrier absorption of transparent conductive oxide (TCO) layers such as F-doped SnO2 (FTO) causes non-negligible energy loss of the incident solar energy in the IR region. Therefore, highly transparent IR conductive oxides were required to increase the efficiency of the solar cells. We then focused on high-mobility Ta-doped SnO2 (TTO) layers grown on polycrystalline anatase TiO2 seed layers by pulsed laser deposition, which show high transmittance in the IR region as well as low resistivity. The PbS QD/ZnO NW solar cells were fabricated by using TTO films together with PbS QDs that give the first exciton peak at 1.6 mm. The external quantum efficiency (EQE) of the exciton peak (1.6 mm) obtained on the solar cells with FTO substrates was 20%, whereas the solar cells fabricated with the TTO substrates reached 40% EQE; this EQE value is the highest EQE ever reported on solution-processed solar cells (to be submitted). The TTO films were confirmed to be promising TCO materials for efficient IR solar cells.
These results indicate that solution-processed PbS QD / ZnO NW solar cells are promising candidates for the middle and/or bottom subcells of multi-junction solar cells.