  Traditional inorganic semiconductors can be electronically doped with high
precision. Conversely, there is still conjecture regarding the assessment of
the electronic doping density in metal-halide perovskites, not to mention of a
control thereof. This paper presents a multifaceted approach to determine the
electronic doping density for a range of different lead-halide perovskite
systems. Optical and electrical characterisation techniques comprising
intensity-dependent and transient photoluminescence, AC Hall effect,
transfer-length-methods, and charge extraction measurements were instrumental
in quantifying an upper limit for the doping density. The obtained values are
subsequently compared to the charge on the electrodes per unit volume at
short-circuit conditions, which amounts to roughly $10^{16}$ cm$^{-3}$. This
figure equals the product of the capacitance $C$ and the built-in potential
$V_\mathrm{bi}$ and represents the critical limit below which doping-induced
charges do not influence the device performance. The experimental results
demonstrate consistently that the doping density is below this critical
threshold ($<10^{12}$ cm$^{-3}$ which means $<CV_\mathrm{bi}$ per unit volume)
for all common lead-based metal-halide perovskites. Nevertheless, although the
density of doping-induced charges is too low to redistribute the built-in
voltage in the perovskite active layer, mobile ions are present in sufficient
quantities to create space-charge-regions in the active layer, reminiscent of
doped pn-junctions. These results are well supported by drift-diffusion
simulations which confirm that the device performance is not affected by such
low doping densities.
