(Invited) Quantum Dot/Wide-Bandgap Semiconductor Hetero-Junction Interface Engineering for Efficient Infrared Solar Cells
Authors: Kubo Takaya , Wang Haibin , Segawa Hiroshi 
Published: February 2020

Fulfilling the potential of solar power is essential toward the realization of a low-carbon society. In doing so, ultra-high efficiency solar cells play a crucial role, and several different concepts for ultra-high efficiency have been proposed: e. g. multiple exciton generation, hot-carrier extraction, intermediate band, multi-junction (M-J). Among these concepts, M-J solar cells based on III-V semiconductors are the only solar cells that have exceeded the single-junction limit (approximately 30% under one-sun illumination). However, the solar cells rely heavily on high cost solar cell technology, which makes a wide implementation of the solar cells difficult.
We have been exploiting the potential of solution-processed M-J solar cells. In constructing M-J solar cells, OPVs, and perovskite solar cells can be used for the top and/or middle cells of M-J solar cells. However, there are few materials to choose from for the bottom cells that can operate in the short-wave infrared. Therefore, the development of low cost and efficient short-wave infrared solar cells is required. One of the promising candidates for use as the middle and/or bottom cells is lead chalcogenide colloidal quantum dots (CQDs) because the bandgap of bulk PbS is located in the infrared region (3.1 μm) and can be readily tuned by controlling quantum dot synthesis conditions. CQDs are compatible with low-temperature solution-based technologies. Although we see recent progress in solar cell performance of PbS CQD-based solar cells, there are many problems to be solved. The active layer thickness of the solar cells is needed to be thick enough to absorb solar energy in a wide range of the solar spectrum, particularly in the near-infrared and short-wave infrared region. The typical exciton diffusion length of the solar cells is approximately 200 nm in PbS QD/ZnO planar solar cells (Fig. 1 insert (a)).
We then focused on PbS CQDs and ZnO nanowire (NW) to construct heterojunction structures to achieve simultaneous enhancement in carrier transport and light harvesting efficiency ((Fig. 1 insert (b)). Unlike PbS QD/ZnO planar structures, PbS QD/ZnO NW hybrid structures forming bulk-heterojuntion allow almost all the photo-generated carriers to reach the PbS QD/ZnO NW interface even when the active layer gets thicker than the carrier diffusion lengths [J. Phys. Chem. Lett., 4, 2455 (2013)]. Our recent study revealed that NW-type solar cells give an effective carrier diffusion length of over 1 μm [J. Phys. Chem. C, 119, 27265 (2015)]. The charge recombination at the interface is a major issue that degrades solar cell performance. Therefore, passivation of the surface of ZnO NWs is an effective way to suppress the recombination process at the interface. In fact, polyethylenimine treatment was successfully performed to decrease the intensity of the PL from defect states of ZnO NWs, which results in increasing the Voc
of the NW-type solar cells by approximately 10%, compared to the untreated solar cells.
Narrowing the bandgap is necessary to efficiently utilize the sun light. We synthesized different-sized PbS CQDs showing absorption bands in the wavelength region ranging from the visible to short-wave infrared (from 300 nm to 2000 nm).
The external quantum efficiency (EQE) spectra of the ZnO NW type solar cells give an EQE peak originating from the first exciton absorption. From the EQE spectra, we confirmed that the solar cells were able to convert photon energy to electricity from 300 nm to 2000 nm (Fig. 1) [ACS Energy Lett., 2, 2110 (2017)]. The solar cells using PbS CQDs having the first exciton absorption peak locating at approximately 1510 nm produced an EQE of 30% at the peak, which is the highest value ever reported on the solution-processed solar cells.
These features of PbS QD/ZnO NW solar cells show the high potential for the subcells of solution-processed M-J solar cells. The solar cells that can operate in the infrared region have other interesting features such as low Voc loss as well as relatively high Voc, which is as high as that of Ge solar cells. PbS QD/ZnO NW solar cells have been confirmed to have long-term stability in the air and under illumination [Phys. Stat. Sol. rrl, 8 (12), 961 (2014)]. 5-year air stability was also verified [Sci. Technol. Adv. Mater., 19, 336 (2018)].

Figure 1