Long-range charge carrier mobility in metal halide perovskite thin-films and single crystals via transient photo-conductivity

Abstract

Charge carrier mobility is a fundamental property of semiconductor materials that governs many electronic device characteristics. For metal halide perovskites, a wide range of charge carrier mobilities have been reported using different techniques. Mobilities are often estimated via transient methods assuming an initial charge carrier population after pulsed photoexcitation and measurement of photoconductivity via non-contact or contact techniques. For nanosecond to millisecond transient methods, early-time recombination and exciton-to-free-carrier ratio hinder accurate determination of free-carrier population after photoexcitation. By considering both effects, we estimate long-range charge carrier mobilities over a wide range of photoexcitation densities via transient photoconductivity measurements. We determine long-range mobilities for FA_0.83Cs_0.17Pb(I_0.9Br_0.1)₃, (FA_0.83MA_0.17)_0.95Cs_0.05Pb(I_0.9Br_0.1)₃ and CH₃NH₃PbI_3-xCl_x polycrystalline films in the range of 0.3 to 6.7 cm² V^-1 s^-1. We demonstrate how our data-processing technique can also reveal more precise mobility estimates from non-contact time-resolved microwave conductivity measurements. Importantly, our results indicate that the processing of polycrystalline films significantly affects their long-range mobility.

Fig. 1. Measurement and calculation of transient photo-conductivity (TPC).

a Schematic illustration of the experimental setup and sample structure with in-plane electrodes used in this study. b Photo-induced transient change of conductivity and exponential decay fitting for FACs polycrystalline perovskite thin-films. Photo-conductivity (σ_Photo) at t = 0 is extrapolated from the fitted plot to calculate mobility. Inset shows σ_Photo decays from the experiment and fitting with various excitation densities (cm⁻³).

Fig. 2. Evaluation of excitation density-dependent optoelectronics properties of FACs perovskite thin films.

a ${\sigma }_{\mathrm{Photo}\left(t=0\right)}$ calculated from the extrapolation as a function of excitation density (cm⁻³). b Charge carrier mobility (ϕ∑μ, sum-of-mobilities of electron and hole) determined by TPC and TRMC (for circle and diamond, respectively, lines are only to guide the eyes) as a function of excitation density (cm⁻³). For TPC, the ${\sigma }_{\mathrm{Photo}\left(t=0\right)}$ is used for Eq. 4, (${\sigma }_{\mathrm{Photo}\left(t=0\right)}=e{I}_{\mathrm{Tot}}\varphi \mu$). The charge carrier mobilities (ϕ∑μ) from both measurements are in a good agreement for the entire range of excitation densities.

Fig. 3. Elucidating the effect of free-carrier density on optoelectronics properties of FACs perovskite thin films.

a Photo-induced carrier population changes as a function of time: Assuming all absorbed photons lead to free-carriers (black solid line, also represents Supplementary Fig. 6a) and assuming free-carrier density reduced by early-time recombination (blue dashed line, also represents Supplementary Fig. 6b), and assuming free-carrier density reduced by early time recombination and exciton-to-free-carrier-ratio (red dotted line, also represents Supplementary Fig. 6c). Star symbol is a corresponding time (ns) for peak free-carrier density. b Photo-conductivity before (${\sigma }_{\mathrm{Photo}\left(t=0\right)}$, black square) and after (${\sigma }_{\mathrm{Photo}\left(t=\mathrm{peak}\right)}$, red diamond) correction as a function of free-carrier density. c Charge carrier mobility before ($\varphi {\mu }_{\left(t=0\right)}$, black square) and after (${\mu }_{\left(t=\mathrm{peak}\right)}$, red diamond) correction as a function of carrier density, respectively, determined by TPC. All lines (3b–c) are only to guide the eyes.

Fig. 4. Evaluation of carrier density invariant lateral mobility of metal halide perovskite.

a Mobility of metal halide perovskite thin-films prepared via different methodologies as a function of free-carrier density (cm⁻³); CH₃NH₃PbI_3-xCl_x from dimethyl formamide solvent (DMF route, black line), CH₃NH₃PbI₃ from the acetonitrile solvent with the addition of methylamine (ACN/MA route, red line), CH₃NH₃PbI₃ from DMF as the solvent employing lead-acetate as the lead source precursor with the addition of hypophosphorous acid (HPA) (DMF/HPA route, blue line), FA_0.83Cs_0.17Pb(I_0.9Br_0.1)₃ (FACs, green line) and (FA_0.83MA_0.17)_0.95Cs_0.05Pb(I_0.9Br_0.1)₃ (FAMACs, yellow line). For comparison, we reproduce mobilities of CH₃NH₃PbI_3-xCl_x (evaporation route) determined through non-contact OPTP (brown dashed line). b Mobility of CH₃NH₃PbI₃ (MAPbI₃) thin single crystals as a function of fluence (photon cm⁻²). Average mobility of three devices is 45 ± 3 cm² V⁻¹ s⁻¹ (grey dashed line). The inset illustration shows device structure. All errors shown are statistical errors from four different devices.