In recent years, quantum dots (QDs) solar cells have been studied as one of the next generation solar cells. They have high theoretical conversion efficiency (44%) ^[1] and ease of fabrication compared to silicon solar cells. As a device structure of QDs solar cells, PbS QDs are used for a light absorption layer and a hole transport layer, and ZnO nanowires (ZnO NWs) are used for an electron transport layer. Since ZnO NWs are useful for smooth electron transfer due to its one-dimensional structure, PbS QDs/ZnO NWs solar cells are attracting attention ^[2]. However, the maximum conversion efficiency of PbS QDs/ZnO NWs solar cells are still 8.5 % ^[3], which is lower compared to the theroretical efficiency. One reason for it may be because of an increase in interface recombination due to an increase in the interface area of PbS QDs/ZnO NWs. Therefore, in this study, ZnO doped Mg (Zn1-XMgXO: X = 0.10, 0.20) was applied to reduce interfacial recombination by passivating the PbS QDs /ZnO NWs interface. The change of charge recombination at the interface of PbS QDs / ZnO NWs @ZnMgO heterojunction solar cells due to the passivation with varying Mg content was investigated.

We measured the transient open-circuit voltage (Voc) responses of the solar cells using ZnO NWs and ZnO NWs @Zn1-XMgXO (X = 0.10, 0.20). The relaxation time of Voc (that is, the charge recombination time in a solar cell) was the longest for ZnO NWs @Zn1-XMgXO (X = 0.10) and the shortest for ZnO NWs. ZnO has been reported to have surface defects due to oxygen vacancies ^[4]. From our experimental results, the surface defect density of NWs can be reduced as the Mg content in Zn1-XMgXO increases. Then, we measured photovoltaic performance of the PbS QDs/ZnO NWs @ZnMgO solar cells. Comparing ZnO NWs @ Zn1-XMgXO (X = 0.10) and ZnO NWs, the open-circuit voltage (Voc) increased systematically, and the photoelectric conversion efficiency (PCE) was about 1.7 times larger in the former case. This is presumed that the surface defect density of the NWs decreased, and the suppression of PbS QDs / ZnO NWs interface charge recombination occured. From the above results, it was found that Zn1-XMgXO passivation is effective for suppressing the interfacial charge recombination and improving open-circuit voltage and photoelectric conversion efficiency of the PbS QDs/ZnO NWs solar cells.