  Space-charge-limited current (SCLC) measurements are used to estimate charge
carrier mobilities and electronic trap densities of semiconductors by analysing
the current density-voltage (JV) relationship for unipolar devices predicted by
the Mott-Gurney (MG) law. However, the interpretation of SCLC measurements for
metal-halide perovskites is problematic due to mobile ionic defects which
redistribute to screen electrostatic fields in devices during measurements. To
overcome this, an SCLC measurement protocol was recently suggested that
minimises ionic charge redistribution by probing the current during millisecond
voltage pulses superimposed on a background bias. Here, we use drift-diffusion
simulations with mobile ions to assess the validity of the MG law for analysing
both the standard and new protocol JV measurements. We simulated idealised
perovskite devices with differing mobile ion densities and compared them with
simulations and measurements of devices with typical contact materials. We
found the validity region for the MG law is limited to perovskites with mobile
ion densities lower than the device's equilibrium charge carrier density
(<10^17 cm-3 for 400 nm thick methylammonium lead iodide films) and contacts
with injection/extraction barriers <=0.1 eV. The latter limitation can be
partially overcome by increasing the device thickness, whereas the former
limitation cannot. This restricts the range of perovskite layer compositions
and viable contact materials that can be reliably analysed with the MG law.
Approaches such as estimating trap densities from the apparent voltage onset to
trap-free SCLC regime should also be critically reviewed since they rely on the
same potentially invalid assumptions as the MG law. Our results demonstrate
that extracting meaningful and accurate values for metal halide perovskite
material properties from SCLC maybe challenging, or often not possible.
