Medical X-ray imaging procedures require digital flat detectors operating at low doses to reduce radiation health risks. Solution-processed organic-inorganic hybrid perovskites have characteristics that make them good candidates for the photoconductive layer of such sensitive detectors. However, such detectors have not yet been built on thin-film transistor arrays because it has been difficult to prepare thick perovskite films (more than a few hundred micrometres) over large areas (a detector is typically 50 centimetres by 50 centimetres). We report here an all-solution-based (in contrast to conventional vacuum processing) synthetic route to producing printable polycrystalline perovskites with sharply faceted large grains having morphologies and optoelectronic properties comparable to those of single crystals. High sensitivities of up to 11 microcoulombs per air KERMA of milligray per square centimetre (μC mGy cm) are achieved under irradiation with a 100-kilovolt bremsstrahlung source, which are at least one order of magnitude higher than the sensitivities achieved with currently used amorphous selenium or thallium-doped cesium iodide detectors. We demonstrate X-ray imaging in a conventional thin-film transistor substrate by embedding an 830-micrometre-thick perovskite film and an additional two interlayers of polymer/perovskite composites to provide conformal interfaces between perovskite films and electrodes that control dark currents and temporal charge carrier transportation. Such an all-solution-based perovskite detector could enable low-dose X-ray imaging, and could also be used in photoconductive devices for radiation imaging, sensing and energy harvesting.

Organic-inorganic halide perovskite is believed to be a potential candidate for high efficiency solar cells because power conversion efficiency (PCE) was certified to be more than 22%. Nevertheless, mismatch of PCE due to current density (J)-voltage (V) hysteresis in perovskite solar cells is an obstacle to overcome. There has been much lively debate on the origin of J-V hysteresis; however, effective methodology to solve the hysteric problem has not been developed. Here we report a universal approach for hysteresis-free perovskite solar cells via defect engineering. A severe hysteresis observed from the normal mesoscopic structure employing TiO and spiro-MeOTAD is almost removed or does not exist upon doping the pure perovskites, CHNHPbI and HC(NH)PbI, and the mixed cation/anion perovskites, FAMAPbIBr and FAMACsPbIBr, with potassium iodide. Substantial reductions in low-frequency capacitance and bulk trap density are measured from the KI-doped perovskite, which is indicative of trap-hysteresis correlation. A series of experiments with alkali metal iodides of LiI, NaI, KI, RbI and CsI reveals that potassium ion is the right element for hysteresis-free perovskite. Theoretical studies suggest that the atomistic origin of the hysteresis of perovskite solar cells is not the migration of iodide vacancy but results from the formation of iodide Frenkel defect. Potassium ion is able to prevent the formation of Frenkel defect since K energetically prefers the interstitial site. A complete removal of hysteresis is more pronounced at mixed perovskite system as compared to pure perovskites, which is explained by lower formation energy of K interstitial (-0.65 V for CHNHPbI vs -1.17 V for mixed perovskite). The developed KI doping methodology is universally adapted for hysteresis-free perovskite regardless of perovskite composition and device structure.

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Thermal stability of CHNHPbI (MAPbI)-based perovskite solar cells was investigated for normal structure including the mesoporous TiO layer and spiro-MeOTAD and the inverted structure with PCBM and NiO. MAPbI was found to be intrinsically stable from 85 °C to 120 °C in the absence of moisture. However, fast degradation was observed for the encapsulated device including spiro-MeOTAD upon thermal stress at 85 °C. Photoluminescence (PL) intensity and the time constant for charge separation increased with thermal exposure time, which is indicative of inhibition of charge separation from MAPbI into spiro-MeOTAD. A full recovery of photovoltaic performance was observed for the 85 °C-aged device after renewal with fresh spiro-MeOTAD, which clearly indicates that thermal instability of the normal structured device is mainly due to spiro-MeOTAD, and MAPbI is proved to be thermally stable. Spiro-MeOTAD with additives was crystallized at 85 °C due to a low glass transition temperature, and hole mobility was significantly deteriorated, which was responsible for the thermal instability. Thermal stability was significantly improved for the inverted structure with the NiO hole transporting layer, where the power conversion efficiency (PCE) was maintained at 74% of its initial PCE of 14.71% after the 80th thermal cycle (one cycle: heating at 85 °C for 2 h and cooling at 25 °C for 2 h). This work implies that the thermal stability of perovskite solar cells depends on selective contacts.

Organic-inorganic halide perovskite solar cells have attracted great attention because of their superb efficiency reaching 22 % and low-cost, facile fabrication processing. Nevertheless, stability issues in perovskite solar cells seem to block further advancements toward commercialization. Thus, device stability is one of the important topics in perovskite solar cell research. In the beginning, the poor moisture resistivity of the perovskite layer was considered as a main problem that hindered further development of perovskite solar cells, which encouraged engineering of the perovskite or protection of the perovskite by a buffer layer. Soon after, other parameters affecting long-term stability were sequentially found and various attempts have been made to enhance intrinsic and extrinsic stability. Here we review the recent progresses addressing stability issues in perovskite solar cells. In this report, we investigated factors affecting stability from material and device points of view. To gain a better understanding of the stability of the bulk perovskite material, decomposition mechanisms were investigated in relation to moisture, photons, and heat. Stability of full device should also be carefully examined because its stability is dependent not only on bulk perovskite but also on the interfaces and selective contacts. In addition, ion migration and current-voltage hysteresis were found to be closely related to stability.

To solve the stability issues of perovskite solar cells (PSC), here a novel interface engineering strategy that a versatile ultrathin 2D perovskite (5‐AVA)2PbI4 (5‐AVA = 5‐ammoniumvaleric acid) passivation layer that is in situ incorporated at the interface between (FAPbI3)0.88(CsPbBr3)0.12 and the hole transporting CuSCN is reported. Surface analysis using X‐ray photoelectron spectroscopy confirms the formation of 2D perovskite. Hysteresis is reduced by the interfacial 2D layer, which could be ascribed to improvement of interfacial charge extraction efficiency, associated with suppression of recombination. Moreover, introduction of the interface passivating layer enhances the moisture stability and photostability as compared to the control perovskite film due to hydrophobic nature of 2D perovskite. The unencapsulated device retains 98% of the initial power conversion efficiency (PCE) after 63 d under moisture exposure of about 10% in the dark. A PCE of the control device is boosted from 13.72 to 16.75% as a consequence of enhanced open‐circuit voltage (Voc) and fill factor along with slightly increased short‐circuit current density (Jsc), which results from reduced trap states of (FAPbI3)0.88(CsPbBr3)0.12 as evidenced by enhanced carrier lifetimes and charge extraction. The perovskite/hole transport material interface engineering gives insight into simultaneous improvements of PCE and device stability.

Recently, organic-inorganic halide perovskite (OHP) has been suggested as an alternative to oxides or chalcogenides in resistive switching memory devices due to low operating voltage, high ON/OFF ratio, and flexibility. The most studied OHP is 3-dimensional (3D) MAPbI. However, MAPbI often exhibits less reliable switching behavior probably due to the uncontrollable random formation of conducting filaments. Here, we report the resistive switching property of 2-dimensional (2D) OHP and compare switching characteristics depending on structural dimensionality. The dimensionality is controlled by changing the composition of BAMAPbI (BA = butylammonium, MA = methylammonium), where 2D is formed from n = 1, and 3D is formed from n = ∞. Quasi 2D compositions with n = 2 and 3 are also compared. Transition from a high resistance state (HRS) to a low resistance state (LRS) occurs at 0.25 × 10 V m for 2D BAPbI film, which is lower than those for quasi 2D and 3D. Upon reducing the dimensionality from 3D to 2D, the ON/OFF ratio significantly increases from 10 to 10, which is mainly due to the decreased HRS current. A higher Schottky barrier and thermal activation energy are responsible for the low HRS current. We demonstrate for the first time reliable resistive switching from 4 inch wafer-scale BAPbI thin film working at both room temperature and a high temperature of 87 °C, which strongly suggests that 2D OHP is a promising candidate for resistive switching memory.