To realize the potential of artificial intelligence in medical imaging, improvements in imaging capabilities are required, as well as advances in computing power and algorithms. Hybrid inorganic–organic metal halide perovskites, such as methylammonium lead triiodide (MAPbI3), offer strong X-ray absorption, high carrier mobilities (µ) and long carrier lifetimes (τ), and they are promising materials for use in X-ray imaging. However, their incorporation into pixelated sensing arrays remains challenging. Here we show that X-ray flat-panel detector arrays based on microcrystalline MAPbI3 can be created using a two-step manufacturing process. Our approach is based on the mechanical soft sintering of a freestanding absorber layer and the subsequent integration of this layer on a pixelated backplane. Freestanding microcrystalline MAPbI3 wafers exhibit a sensitivity of 9,300 µC Gyair–1 cm–2 with a μτ product of 4 × 10–4 cm2 V–1, and the resulting X-ray imaging detector, which has 508 pixels per inch, combines a high spatial resolution of 6 line pairs per millimetre with a low detection limit of 0.22 nGyair per frame.

Frequency domain techniques are useful tools to characterize processes occurring on different time scales in solar cells and solar fuel devices. Intensity-modulated photocurrent spectroscopy (IMPS) is one such technique that links the electrical and optical responses of the device. In this review, a summary of the fundamental application of IMPS to semiconductor photoelectrodes and nanostructured solar cells is presented, with a final goal of understanding the IMPS response of the perovskite solar cell (PSC) to shed light on its complex physical mechanisms of operation. The historical application of IMPS that connects its transfer function to the charge transfer efficiency of the semiconductor electrode and subsequently the considerations of diffusive transport for the dye-sensitized solar cell is summarized. These models prioritize the association of spectral features with time constants, which has led to a neglect of other absolute aspects of the spectra by the photovoltaic community. We clarify these aspects by developing the fundamental connection between the absolute value of the IMPS transfer function and the external quantum efficiency (EQEPV) of a photovoltaic cell. Basic models for the solar cell are developed using kinetic equations and equivalent circuits (EC), stressing their equivalence and the advantage of the EC representation to adequately account for different capacitances in the system. A critique of the current interpretations of the PSC IMPS spectra is performed, where time constants and their evolution are associated with characteristic transport processes of either electronic or ionic carriers within the PSC. These are clarified using the EC representation to identify that the generated characteristic processes are only related to coupling between different elements of the EC and are not reflective of transport phenomena in general. Furthermore, a general model is developed that identifies charge accumulation at the interfaces as a general feature for both low- and high-efficiency PSCs, whose charging/discharging resistances are the main factor in controlling the electrical response of the device. This model shows a separation of the photovoltage within the PSC that causes a reduction in its EQEPV at low frequencies. Further development of the PSC will involve gaining control over the low-frequency charge kinetics in the device to overcome these limitations.

Capacitive techniques, routinely used for solar cell parameter extraction, probe the voltage-modulation of the depletion layer capacitance isothermally as well as under varying temperature. Also defect states within the semiconductor band-gap respond to such stimuli. Although extensively used, capacitive methods have found difficulties when applied to elucidate bulk defect bands in photovoltaic perovskites. This is so because perovskite solar cells (PSCs) actually exhibit some intriguing capacitive features hardly connected to electronic defect dynamics. The commonly reported excess capacitance observed at low frequencies is originated by outer interface mechanisms and has a direct repercussion on the evaluation of band-gap defect levels. Starting by updating previous observations on Mott-Schottky (MS) analysis in PSCs, it is discussed how the thermal admittance spectroscopy (TAS) and the deep level transient spectroscopy (DLTS) characterization techniques present spectra with overlapping or even "fake" peaks caused by the mobile ion-related, interfacial excess capacitance. These capacitive techniques, when used uncritically, may be misleading and produce wrong outcomes.

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.