Are Mobilities in Hybrid Organic-Inorganic Halide Perovskites Actually "High"?

The outstanding performance of hybrid organic-inorganic perovskites (HOIPs) in photovoltaic devices is made possible by, among other things, outstanding semiconducting properties: long real charge-carrier diffusion lengths, L, of up to 5 and possibly even 10 μm, as well as a lifetime, τ of ~1 μs or more in single crystal and polycrystalline films. 1–9 Top electronic transport materials will have a high “μτ” product, the product of the charge carrier mobility, μ, and lifetime. This is directly related to the diffusion length (L=√(Dτ), where D is the carrier diffusion coefficient given as D=(μq/kBT), where q is the electron charge, kB is the Boltzmann constant, and T is the absolute temperature. Long lifetimes, which imply slow recombination and low trapping probabilities, do not automatically imply high mobilities, which are limited by scattering. Charge carrier mobilities in HOIPs are often described as “high”, but this statement warrants some scrutiny. HOIP mobilities are often compared to those of charge carriers in organic semiconductors (see Table I) and are then indeed much higher. 10 But in our opinion, carrier mobilities in HOIPs need to be placed in the context of typical inorganic semiconductors, for several reasons: First, HOIPs exhibit a band structure resembling that of a good inorganic semiconductor, with the conduction and valence band dominated by the inorganic cations and anions, respectively. 11–14 This leads to small computed charge-carrier effective masses 11,13,14 : a reduced effective mass on the order of 0.1 electron mass, close to that of Si (0.08) or GaAs (0.03) 15 , a value confirmed by magneto-absorption measurements. 16,17 Second, material disorder, which often lowers mobilities dramatically, is low: HOIPs exhibit sharp x-ray diffraction peaks, 18