Minimum doping densities for p–n junctions

In their Article, Cui et al.1 describe the fabrication and characterization of planar p–n junction solar cells based on lead-halide perovskites. The formation of a p–n junction is noteworthy given the doping densities, measured using the Hall effect, which were reported to vary from ND = 1 × 1012 cm−3 to 8 × 1012 cm−3 for the solution-processed n-type layer and to equal NA = 8 × 109 cm−3 for the evaporated p-type layer. Although these devices outperform their counterparts that are supposedly undoped, the results raise three important questions. Are the reported doping densities high enough to change the electrostatic potential distribution in the device from that of undoped ones? Are the doping densities high enough for the p–n junction to remain intact under typical photovoltaic operation conditions? Is a p–n junction beneficial for photovoltaic performance given the typical properties of lead-halide perovskites.