This is because certain defects can be passivated or neutralized by other species, such as surface treatments, which can mitigate their negative impact on device performance.
Thank you Md. Ferdous Rahman Sir, can you please tell which surface treatments you are talking about. Because in simulation, there are no surface treatments, it is possible only experimentally.
It's the inverse, generally, the lower the defects density Nt the better the solar cell performs, if you find otherwise you have something wrong in your simulation unless you have another explanation. More defects means more losses and recombinations, lower charge mobilities...etc