Most crystalline silicon (c-Si) solar cells are based on high temperature–processed p-n junctions or highly doped heterojunctions. The concept of dopant-free carrier selective contact has become a research hotspot and been successfully demonstrated with n-type Si wafers, showing the great potential of simplified fabrication process and lower thermal-consuming. However, there are few successful cases on p-Si, dopant-free p-Si/CdS (cadmium sulfide)/ITO (indium tin oxide) solar cells with champion efficiency of 12.29% (device area 4 cm 2 ) have been demonstrated with DC magnetron sputtered CdS thin films working as electron-selective contact. A proper annealing treatment is found essential in improving the p-Si/CdS/ITO heterocontact and device performance. The author's preliminary results confirm the feasibility of preparation of efficient p-Si wafer–based dopant-free solar cells.
Molybdenum oxide (MoO 3− x , x < 3) has been successfully used as an efficient hole-selective contact material for crystalline silicon heterojunction solar cells. The carrier transport capability strongly depends on its work function, that is, oxygen vacancies; however, there are lack of effective methods to modulate the multiple oxidation states. Herein, the oxidation states of solution-processed MoO 3− x by doping Nb 5+ to improve its hole-selective contact performance with silicon are tuned. With the optimum doping concentration of 5%, both the reduced Mo 5+ and oxygen vacancies increase, resulting in a decrease in the contact resistivity between the MoO 3− x film and p -type silicon from 161.1 to 62.9 mΩ·cm 2 and an increase of the effective carrier lifetime from 165.4 to 391.0 μs simultaneously. Similarly, the doping of Ta 5+ or V 5+ in MoO 3− x improves the passivated contact performance with silicon, while the former increases the concentration of oxygen vacancies and the latter reduces it. The solar cell with the structure of Ag/MoO 3− x :Nb/ p -Si exhibits a conversion efficiency of 18.37%, which is the highest so far reported for the solution-processed MoO 3− x /silicon heterojunction. This work demonstrates a feasible strategy of tuning hole selectivity in MoO 3− x by doping for high-efficiency solar cells and other optoelectronic device applications.
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