Short-Wave Infrared PbS Colloidal Quantum Dot ZnO Nanowire Solar Cells Aimed at Tandem Solar Cells
Authors: Wang Haibin , Kubo Takaya , Nakazaki Jotaro , Segawa Hiroshi 
Published: December 2020

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.
We constructed two different solar cells using infrared PbS QDs with the first exciton absorption peak at 1550 nm, which is nanowire-type solar cell (NW-cell) and planar-type solar cell(P-cell). The ZnO nanoparticles layer (P-cell) or ZnO nanowire (NW-cell) as an electron acceptor formed on highly infrared transparent 1% Ta-doped SnO2 (TTO) conductive glass substrates. PbS CQD were deposited on the ZnO layers from a PbS QD octane solution via a layer-by-layer dip coating method. The PbS-QD layer were treated with a tetrabutylammonium iodide (TBAI) methanol solution to replace the insulating oleic acid ligands with iodide anions (PbS-I). The external quantum efficiency (EQE) spectra and the structures of the P-cell and NW-cell are shown in the figure. In the almost entire wavelength region ranging from 380 to 1700 nm, the EQE gave higher in the NW-cell than in the P-cell. The best performing planar-type solar cell exhibited a PCE of 2.68% (Jsc= 18.9 mA/cm2; Voc=0.316 V; FF=0.451) under 100 mW/cm2 simulated AM 1.5G illumination. On the other hand, ZnO nanowire-type solar cells gave a PCE of 5.38% (Jsc=38.7 mA/cm2; Voc=0.335 V; FF=0.415), and showed almost twice as high Jsc value (38.7 mA/cm2) as that of the planar-type solar cells. The integrated Jsc of the nanowire-type solar cells obtained from the EQE spectrum 38.5 mA/cm2 is approximately the measured Jsc value. To investigate the potential for the bottom subcell of the tandem solar cell, infrared-performance of nanowire-type solar cells were studied using different short-cut filters. The solar cells exhibited 2.50 % PCE (Jsc=19.2 mA/cm2; Voc=0.307 V; FF=0.424) and 1.11% PCE (Jsc=8.6 mA/cm2; Voc=0.282 V; FF=0.460) under a long-pass filtered one-sun illumination (l >810 nm (perovskite solar cells equivalent), and l>1100 nm (Si solar cells equivalent)), respectively. All these results indicate that the ZnO nanowire PbS QD solar cells are promising candidates to construct tandem devices with perovskite and c-Si solar cells.

Figure 1