encountered in solar cell engineering. One of them is caused by the presence of mobile ions and how these species alter the internal electrical field, interact with the contact materials, or modulate electronic properties. [3-6] Upon biasing, charged moving ions accumulate in the vicinity of the outer interfaces causing electrical field partial shielding. [7-9] It has also been reported how intrinsic defects chemically react with the electrodes giving rise to losses in performance and device instabilities. [10,11] The occurrence of polarized interfaces in hybrid perovskite-based electronic devices was proposed [12] as an explaining mechanism for the measured excess capacitance at low frequencies. In dark conditions, mobile ions pile up at outer interfaces forming double layer-like structures in the vicinity of the perovskite/contact interface. [13,14] Excess dark capacitance of order 1-10 μF cm −2 can be readily explained in this way. In addition to purely electrostatic approaches for the interfacial phenomena, it is known that chemical reactions between mobile ions and contacting materials might give rise to the formation of dipole-like structures. [15,16] Also, deviations from stable electrical characteristics (i.e., hysteresis in current density-voltage J-V or non-ohmic response) have previously been correlated with the dynamics of migrating ions that interact with the contacts. [14,15,17] A survey about the chemical reactivity of the perovskite/contact materials can be found elsewhere. [14] In this sense, the kinetics of electrode charging may be understood not only Metal halide perovskite single crystals are being explored as functional materials for a variety of optoelectronic applications. Among others, solar cells, field-effect transistors, and X-and γ-ray detectors have shown improved performance and stability. However, a general uncertainty exists about the relevant mechanisms governing the electronic operation. This is caused by the presence of mobile ions and how these defect species alter the internal electrical field, interact with the contact materials, or modulate electronic properties. Here, a set of high-quality thick methylammonium lead tribromide single crystals contacted with low-reactivity chromium electrodes are analyzed by impedance spectroscopy. Through examination of the sample resistance evolution with bias and releasing time, it is revealed that an interplay exists between the perovskite electronic conductivity and the defect distribution within the crystal bulk. Ion diffusion after bias removing changes the local doping density then governing the electronic transport. These findings indicate that the coupling between ionic and electronic properties relies upon a dynamic doping effect caused by moving ions that act as mobile dopants. In addition to electronic features, the analysis extracts values for the ion diffusivity in the range of 10 −8 cm 2 s −1 in good agreement with other independent measurements.

The optoelectronic properties of halide perovskite materials have fostered their utilization in many applications. Unravelling their working mechanisms remains challenging because of their mixed ionic–electronic conductive nature. By registering, with high reproducibility, the long-time current transients of a set of single-crystal methylammonium lead tribromide samples, the ion migration process was proved. Sample biasing experiments (ionic drift), with characteristic times exhibiting voltage dependence as ∝ V –3/2 , is interpreted with an ionic migration model obeying a ballistic-like voltage-dependent mobility (BVM) regime of space-charge-limited current. Ionic kinetics effectively modify the long-time electronic current, while the steady-state electronic currents’ behavior is nearly ohmic. Using the ionic dynamic doping model (IDD) for the recovering current at zero bias (ion diffusion), the ionic mobility is estimated to be ∼10 –6 cm 2 V –1 s –1 . Our findings suggest that ionic currents are negligible in comparison to the electronic currents; however, they influence them via changes in the charge density profile.

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The study and development in recent years of hybrid (organic–inorganic) halide perovskite materials have given them an unprecedented opportunity for direct ionizing radiation detection, given their large attenuation coefficient and sufficient charge carrier mobility lifetime product. The use of single crystals, considered as model materials, allows us to investigate their intrinsic properties. Characterizations under X-ray illumination of detector devices based on methylammonium lead tribromide (MAPbBr3) single crystals, obtained by optimized growths, show good sensitivity but high dark current density. To improve this critical parameter, while using MAPbBr3 as the base material, we employ anion engineering within the halide elements. We present here mixed halide perovskite crystals, with bromide partially replaced with chloride, obtained through optimized growths using modified inverse temperature crystallization in dimethylformamide, leading to high-quality single crystals of the general formula MAPb­(Br1–x Cl x )3. Six chlorine contents are targeted and carefully determined experimentally via energy-dispersive X-ray analysis and X-ray powder diffraction. For each composition, several crystals are synthesized and used to prepare X-ray detection devices. Their optoelectronic properties are determined under standard X-ray medical conditions and hint at the existence of an optimal composition. MAPb­(Br0.85Cl0.15)3 exhibits the best sensitivity with a value of S ≈ 3 μC mGyair –1 cm–2 for RQA5 spectral quality and the lowest dark current density with a value of J dark ≈ 22 nA mm–2, both recorded at a 50 V mm–1 electric field. This sensitivity value doubles our own MAPbBr3 single crystal device and is higher than that of CsI­(Tl)- or a-Se-based flat panels. The present work broadens the benefits and drawbacks of employing halide engineering in perovskite materials to improve the optoelectronic performance under high-energy radiation.

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