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A major thrust of medical X-ray imaging is to minimize the X-ray dose acquired by the patient, down to single-photon sensitivity. Such characteristics have been demonstrated with only a few direct-detection semiconductor materials such as CdTe and Si; nonetheless, their industrial deployment in medical diagnostics is still impeded by elaborate and costly fabrication processes. Hybrid lead halide perovskites can be a viable alternative owing to their facile solution growth. However, hybrid perovskites are unstable under high-field biasing in X-ray detectors, owing to structural lability and mixed electronic-ionic conductivity. Here we show that both single-photon-counting and long-term stable performance of perovskite X-ray detectors are attained in the photovoltaic mode of operation at zero-voltage bias, employing thick and uniform methylammonium lead iodide single-crystal films (up to 300 mu m) and solution directly grown on hole-transporting electrodes. The operational device stability exceeded one year. Detection efficiency of 88% and noise-equivalent dose of 90 pGy(air) are obtained with 18 keV X-rays, allowing single-photon-sensitive, low-dose and energy-resolved X-ray imaging. Array detectors demonstrate high spatial resolution up to 11 lp mm(-1). These findings pave the path for the implementation of hybrid perovskites in low-cost, low-dose commercial detector arrays for X-ray imaging., Nature Photonics, 17 (6), ISSN:1749-4885, ISSN:1749-4893

Since the inception of the unprecedented rise of halide perovskites for photovoltaic research, ion migration has shadowed this material class with undesirable hysteresis and degradation effects, limiting its practical implementations. Unfortunately, the localized doping and electrochemical reactions triggered by ion migration cause many more undesirable effects that are often unreported or misinterpreted because they deviate from classical semiconductor behavior. In this Perspective, we provide a concise overview of such effects in halide perovskites, such as operational instability in photovoltaics, polarization-induced abnormal external quantum efficiency in light-emitting diodes, and energy channel shift and anomalous sensitivities in hard radiation detection. Finally, we highlight a unique use case of exploiting ion migration as a boon to design emerging memory technologies such as memristors for information storage and computing., ACS Energy Letters, 7 (10), ISSN:2380-8195

CsPbBr

CsPbBr3 single crystals have potential for application in ionizing-radiation detection devices due to their optimal optoelectronic properties. Yet, their mixed ionic–electronic conductivity produces instability and hysteretic artifacts hindering the long-term device operation. Herein, we report an electrical characterization of CsPbBr3 single crystals operating up to the time scale of hours. Our fast time-of-flight measurements reveal bulk mobilities of 13–26 cm2 V–1 s–1 with a negative voltage bias dependency. By means of a guard ring (GR) configuration, we separate bulk and surface mobilities showing significant qualitative and quantitative transport differences. Our experiments of current transients and impedance spectroscopy indicate the formation of several regimes of space-charge-limited current (SCLC) associated with mechanisms similar to the Poole–Frenkel ionized-trap-assisted transport. We show that the ionic-SCLC seems to be an operational mode in this lead halide perovskite, despite the fact that experiments can be designed where the contribution of mobile ions to transport is negligible. Funding for open access charge: CRUE-Universitat Jaume I

The lifetime of a carrier is a crucial parameter for solar cell materials, and metal halide perovskite materials are promising for solar cell applications. In this study, we observed carrier recombination using time-resolved photoluminescence (TR-PL) and microwave photoconductivity decay (μ-PCD) in metal halide perovskite materials: NH3CH3PbI3 (MAPbI3), NH3CH3PbBr3 (MAPbBr3), and CsPbBr3 with single- and poly-crystalline structures. By comparing the decay curves of TR-PL and μ-PCD, we found trap levels in the band gap for all the materials. We employed two excitation wavelengths for the μ-PCD measurements, and we observed faster μ-PCD signal decays for short wavelength excitation for MAPbBr3 and CsPbBr3. Additionally, we established that the poly-crystals exhibited faster decay compared with the single crystals for MAPbBr3 and CsPbBr3. Therefore, we concluded that there are significant contributions of the interface and surface recombination on carrier recombination for MAPbBr3 and CsPbBr3, but not for MAPbI3.

Single-crystal halide perovskites exhibit photogenerated-carriers of high mobility and long lifetime, making them excellent candidates for applications demanding thick semiconductors, such as ionizing radiation detectors, nuclear batteries, and concentrated photovoltaics. However, charge collection depreciates with increasing thickness; therefore, tens to hundreds of volts of external bias is required to extract charges from a thick perovskite layer, leading to a considerable amount of dark current and fast degradation of perovskite absorbers. However, extending the carrier-diffusion length can mitigate many of the anticipated issues preventing the practical utilization of perovskites in the abovementioned applications. Here, single-crystal perovskite solar cells that are up to 400 times thicker than state-of-the-art perovskite polycrystalline films are fabricated, yet retain high charge-collection efficiency in the absence of an external bias. Cells with thicknesses of 110, 214, and 290 µm display power conversion efficiencies (PCEs) of 20.0, 18.4, and 14.7%, respectively. The remarkable persistence of high PCEs, despite the increase in thickness, is a result of a long electron-diffusion length in those cells, which was estimated, from the thickness-dependent short-circuit current, to be ≈0.45 mm under 1 sun illumination. These results pave the way for adapting perovskite devices to optoelectronic applications in which a thick active layer is essential.

Non-fullerene-based organic solar cells (OSCs) have recently proven to perform with efficiencies above 18%. This is an important milestone for one of the most promising technologies in the fields of flexible and transparent/semitransparent photovoltaics. However, the stability of OSCs is still a challenging issue to meet the industry requirements. Herein, several devices with IT-4F:PM6 as the active layer with and without 1,8-Diiodooctane (DIO) additive are characterized before and after a 1400 h degradation test under 1 sun white light-emitting diode (LED) illumination intensity. The optoelectronic study via impedance spectroscopy under illumination at quasi-open-circuit correlates the use of DIO as an additive with a retarded degradation behavior and an overall improved device performance. In dark conditions, the Mott–Schottky analysis suggests that samples without DIO develop self-doping during degradation, changing the p-i-n doping profile into a p–n type, most likely related to the evolution of the blend demixing. These mechanisms are further confirmed by drift-diffusion simulations. Space-oriented redistribution of shallow trap levels (self-doping) and homogeneous increase in deep-trap levels (nonradiative recombination) are shown to be hindered by the use of the DIO additive.

Aerosol deposition (AD) is a promising additive manufacturing method to fabricate low‐cost, scalable films at room temperature, but has not been considered for semiconductor processing, so far. The successful preparation of cesium lead tribromide (CsPbBr3) perovskite films on interdigitated indium tin oxide (ITO) electrodes by means of AD is reported here. The 20–35 µm thick layers are dense and have good adhesion to the substrate. The orthorhombic Pnma crystal structure of the precursor powder was retained during the deposition process with no signs of defect formation. The formation of electronic defects by photoluminescence spectroscopy is investigated and found slightly increased carrier recombination from defect sites for AD films compared to the powder. A nonuniform defect distribution across the layer, presumably induced by the impact of the semiconducting grains on the hard substrate surface, is revealed. The opto‐electronic properties of AD processed semiconducting films is further tested by electrical measurements and confirmed good semiconducting properties and high responsivity for the films. These results demonstrate that AD processing of metal halide perovskites is possible for opto‐electronic device manufacturing on 3D surfaces. It is believed that this work paves the way for the fabrication of previously unimaginable opto‐electronic devices by additive manufacturing.

Stability of perovskite-based photovoltaics remains a topic requiring further attention. Cation engineering influences perovskite stability, with the present-day understanding of the impact of cations based on accelerated ageing tests at higher-than-operating temperatures (e.g. 140°C). By coupling high-throughput experimentation with machine learning, we discover a weak correlation between high/low-temperature stability with a stability-reversal behavior. At high ageing temperatures, increasing organic cation (e.g. methylammonium) or decreasing inorganic cation (e.g. cesium) in multi-cation perovskites has detrimental impact on photo/thermal-stability; but below 100°C, the impact is reversed. The underlying mechanism is revealed by calculating the kinetic activation energy in perovskite decomposition. We further identify that incorporating at least 10 mol.% MA and up to 5 mol.% Cs/Rb to maximize the device stability at device-operating temperature (, Current view of the impact of A-site cation on the stability of perovskite materials and devices is derived from accelerated ageing tests at high temperature, which is beyond normal operation range. Here, the authors reveal the great impact of ageing condition on assessing the photothermal stability of mixed-cation perovskites using high-throughput robot system coupled with machine learning.

Despite the intense research on photovoltaic lead halide perovskites, reported optical properties as basic as the absorption onset and the optical band gap vary significantly. To unambiguously answer the question whether the discrepancies are a result of differences between bulk and "near-surface" material, we perform two nonlinear spectroscopies with drastically different information depths on single crystals of the prototypical (CH3NH3)PbI3 methylammonium lead iodide. Two-photon absorption, detected via the resulting generation of carriers and photocurrents (2PI-PC), probes the interband transitions with an information depth in the millimeter range relevant for bulk (single-crystal) material. In contrast, the transient magneto-optical Kerr effect (trMOKE) measured in a reflection geometry determines the excitonic transition energies in the region near (hundreds of nm) the surface which also determine the optical properties in typical thin films. To identify differences between structural phases, we sweep the sample temperature across the orthorhombic-tetragonal phase transition temperature. In the application-relevant room-temperature tetragonal phase (at 170 K), we find a bulk band gap of 1.55 ± 0.01 eV, whereas in the near-surface region excitonic transitions occur at 1.59 ± 0.01 eV. The latter value is consistent with previous reflectance measurements by other groups and considerably higher than the bulk band gap. The small band gap of the bulk material explains the extended infrared absorption of crystalline perovskite solar cells, the low-energy bands which carry optically driven spin-polarized currents, and the narrow bandwidth of crystalline perovskite photodetectors making use of the spectral filtering at the surface.

Potentiostatic impedance spectroscopy (IS) is a well-established characterization technique for elucidating the electric resistivity and capacitive features of materials and devices. For solar cells, by applying a small voltage perturbation the current signal is recorded and the recombination processes and defect distributions can be accessed. In this work, a photo-impedance approach, named “light intensity modulated impedance spectroscopy” (LIMIS), is first time tested in all-solid-state photovoltaics by individually recording photocurrent (IMPS) and photovoltage (IMVS) responsivity signals due to a small light perturbation at open-circuit (OC), and combining them: LIMIS = IMVS/IMPS. The experimental LIMIS spectra from silicon, organic, and perovskite solar cells are presented and compared with IS. Our theoretical analyses, including equivalent circuit models, show a correction to the lifetimes values by obtaining the total differential resistances and capacitances combining IS and LIMIS. This correction addresses some discrepancies among different techniques, as also shown with photo-induced transient photovoltage. The experimental differences between IS and LIMIS proves the unsuitability of the superposition principle and suggest a bias-dependent photo-current correction to the empirical Shockley equation of the steady-state current at different illumination intensities around OC. In addition, new features are reported for the low-frequency capacitance of perovskite solar cells.

The ubiquitous hysteresis in the current-voltage characteristic of perovskite solar cells (PSCs) interferes in a proper determination of the diode ideality factor ( n ), a key parameter commonly adopted to analyze recombination mechanisms. An alternative way of determining n is by measuring the voltage variation of the ac resistances in conditions of negligible steady-state dc currents. A reliable analysis of n based on the determination of the resistive response, free of hysteretic influences, reveals two separated voltage exponential dependences using different perovskite absorbers (3D perovskites layer based on CH3NH3PbI3 or mixed Cs0.1FA0.74MA0.13PbI2.48Br0.39) and a variety of interlayers (2D perovskite thin capping). The dominant resistive element always exhibits an exponential dependence with factor n ≈ 2 , irrespective of the type of perovskite and capping layers. In addition, a non-negligible resistive mechanism occurs at low-frequencies (with voltage-independent time constant ~ 1 s) which is related to the kinetic properties of the outer interfaces, with varying ideality factor ( n = 2 for CH3NH3PbI3, and n = 1.5 for Cs0.1FA0.74MA0.13PbI2.48Br0.39). Our work identifies common features in the carrier recombination mechanisms among different types of high-efficiency PSCs, and simultaneously signals particularities on specific architectures, mostly in the carrier dynamics at outer interfaces.

Mixed-halide perovskites have emerged as promising materials for optoelectronics due to their tunable band gap in the entire visible region. A challenge remains, however, in the photoinduced phase segregation, narrowing the band gap of mixed-halide perovskites under illumination thus restricting applications. Here, we use a combination of spatially resolved and bulk measurements to give an in-depth insight into this important yet unclear phenomenon. We demonstrate that photoinduced phase segregation in mixed-halide perovskites selectively occurs at the grain boundaries rather than within the grain centers by using shear-force scanning probe microscopy in combination with confocal optical spectroscopy. Such difference is further evidenced by light-biased bulk Fourier-transform photocurrent spectroscopy, which shows the iodine-rich domain as a minority phase coexisting with the homogeneously mixed phase during illumination. By mapping the surface potential of mixed-halide perovskites, we evidence the higher concentration of positive space charge near the grain boundary possibly provides the initial driving force for phase segregation, while entropic mixing dominates the reverse process. Our work offers detailed insight into the microscopic processes occurring at the boundary of crystalline perovskite grains and will support the development of better passivation strategies, ultimately allowing the processing of more environmentally stable perovskite films.

Hybrid organic–inorganic perovskites have emerged as cost-effective and high-performance semiconductors for optoelectronic applications. Precise knowledge of charge carrier mobility and especially the temperature dependence of mobility is therefore of utmost relevance for advancing high-performance materials. Here, the charge carrier mobility in methylammonium lead iodide single crystals is investigated with time of flight technique from 290 to 100 K. A nondispersive transport with an electron mobility of 135 (±20) cm2/V s and a hole mobility of 90 (±20) cm2/V s is obtained at room temperature. A power-law temperature dependence of mobility, μ ∝ Tm, with an exponent m = −2.8 and −2.0, is measured for electrons and holes in the tetragonal phase. The highest electron and hole mobilities measured are 635 (±70) and 415 (±20) cm2/V s, respectively. Our results indicate that the scattering of charge carriers with phonons is the limiting factor for carrier mobilities at room temperature.

Recently, AgBiS2 has been demonstrated to be a promising non-toxic, earth-abundant absorber material for solar energy application. In this work, a novel route to deposit AgBiS2 thin films from single molecular precursor ink is presented. It is found that the amount of thiourea has a crucial impact on the formulation of a stable molecular ink. Understanding the coordination chemistry of the molecular precursor ink is important for getting better control over the thin film processing. With the assistance of Raman spectroscopy, possible complexation mechanisms and general coordination states within the molecular ink are studied. In addition, the influence of ink composition as well as the annealing temperature on the structure and morphology of resulting AgBiS2 films is systematically investigated. It is found that the crystallinity and particle size increase with higher annealing temperature. The obtained AgBiS2 thin films show a cubic structure with a preferred orientation in [111] direction. Optical and electrical measurements demonstrate that the obtained AgBiS2 is an indirect band gap material, which features two transition mechanisms with an indirect band gap of around 0.87 eV and a direct one of around 1.21 eV. The high absorption coefficient, low Urbach energy and fast transient photoconductivity confirm its potential as an absorber for photovoltaic applications.

Simultaneously enhancing device performance and longevity, as well as balancing the requirements on cost, scalability, and simplification of processing, is the goal of interface engineering of organic solar cells (OSCs). In our work, we strategically introduce antimony (Sb3+) cations into an efficient and generic n-type SnO2 nanoparticles (NPs) host during the scalable flame spray pyrolysis synthesis. Accordingly, a significant switch of conduction property from an n-type character to a p-type character is observed, with a corresponding shift in the work function (WF) from 4.01 ± 0.02 eV for pristine SnO2 NPs to 5.28 ± 0.02 eV for SnO2 NPs with 20 mol. % Sb content (ATO). Both pristine SnO2 and ATO NPs with fine-tuned optoelectronic properties exhibit remarkable charge carrier extraction properties, excellent UV resistance and photo-stability being compatible with various state-of-the-art OSCs systems. The reliable and scalable pristine SnO2 and ATO NPs processed by doctor-blading in air demand no complex post-treatment. Our work offers a simple but unique approach to accelerate the development of advanced interfacial materials, which could circumvent the major existing interfacial problems in solution-processed OSCs.

Self‐healing of defects imposed by external stimuli such as high energy radiation is a possibility to sustain the operational lifetime of electronic devices such as radiation detectors. Cs3Bi2Br3I6 polycrystalline wafers are introduced here as novel X‐ray detector material, which not only guarantees a high X‐ray stopping power due to its composition with elements with high atomic numbers, but also outperforms other Bi‐based semiconductors in respect to detector parameters such as detection limit, transient behavior, or dark current. The polycrystalline wafers represent a size scalable technology suitable for future integration in imager devices for medical applications. Most astonishingly, aging of these wafer‐based devices results in an overall improvement of the detector performance—dark currents are reduced, photocurrents are increased, and one of the most problematic properties of X‐ray detectors, the base line drift is reduced by orders of magnitude. These aging induced improvements indicate self‐healing effects which are shown to result from recrystallization. Optimized synthetic conditions also improve the as prepared X‐ray detectors; however, the aged device outperforms all others. Thus, self‐healing acts in Cs3Bi2Br3I6 as an optimization tool, which is certainly not restricted to this single compound, it is expected to be beneficial also for many further polycrystalline ionic semiconductors.

Methyl ammonium lead triiodide perovskite wafers for application in direct conversion X-ray detectors are fabricated by a room-temperature sintering process. A conversion efficiency of 2,527 mC Gyaircm–2 under 70 kVp X-ray exposure is obtained.

In the past few years, hybrid organic–inorganic and all-inorganic metal halide perovskite nanocrystals have become one of the most interesting materials for optoelectronic applications. Here, we report a facile and rapid room temperature synthesis of 15–25 nm formamidinium CH(NH2)2PbX3 (X = Cl, Br, I, or mixed Cl/Br and Br/I) colloidal nanocrystals by ligand-assisted reprecipitation (LARP). The cubic and platelet-like nanocrystals with their emission in the range of 415–740 nm, full width at half-maximum (fwhm) of 20–44 nm, and radiative lifetimes of 5–166 ns enable band gap tuning by halide composition as well as by their thickness tailoring; they have a high photoluminescence quantum yield (up to 85%), colloidal and thermodynamic stability. Combined with surface modification that prevents degradation by water, this nanocrystalline material is an ideal candidate for optoelectronic devices and applications. In addition, optoelectronic measurements verify that the photodetector based on FAPbI3 nanocrystals...

Controllably manipulating the spectral response of broadband-absorbing semiconductors is crucial for developing wavelength-selective optoelectronic devices. In this article, we report for the first time, the bias-dependent spectral responses for a metal-halide perovskite photodiode. Tunable external quantum efficiencies in the short- and long-wavelength regimes, and the full spectral range (ca. 300–800 nm) are observed when the device is operated under short-circuit, and forward and reverse bias conditions, respectively. This observation is understood by the interplay of wavelength-dependent penetration depth and barrier formation within the photodiode device stack. The general applicability of this concept is confirmed by a systematic study on a series of mixed-halide perovskite devices. These results suggest that the proposed concept allows as a promising platform and should inspire further exploration of multispectral responsive optoelectronic devices.

Potentiostatic impedance spectroscopy (IS) is a well-known tool for characterization of materials and electronic devices. It can be complemented by numerical simulation strategies relying on drift-diffusion equations without any equivalent circuit-based assumptions. This implies the time-dependent solutions of the transport equations under small perturbation of the external bias applied as a boundary condition at the electrodes. However, in the case of photosensitive devices, a small light perturbation modulates the generation rate along the absorber bulk. This work then approaches a set of analytical solutions for the signals of IS and intensity modulated photocurrent and photovoltage spectroscopies, intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS), respectively, from one-sided p-n junction solar cells at the open-circuit. Subsequently, a photoimpedance signal named “light intensity modulated impedance spectroscopy” (LIMIS = IMVS/IMPS) is analytically simulated, and its difference with respect to IS suggests a correlation with the surface charge carrier recombination velocity. This is an illustrative result and the starting point for future more realistic numerical simulations. We acknowledge the funding support from the Ministerio de-Ciencia, Innovación y Universidades of Spain under project (No. MAT2016-76892-C3-1-R). O.A. acknowledges the financial support from the VDI/VD Innovation + Technik GmbH (Project-title: PV-ZUM) and the SAOT funded by the German Research Foundation (DFG) in the framework of the German excellence initiative.

We investigate the photoinduced absorption (PIA) spectra of the prototypical donor acceptor polymer [2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b']dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (C-PCPDTBT) and its silicon bridged variant Si-PCPDTBT over a spectral range from 0.07 to 1.5 eV. Comparison between time-dependent density functional theory simulations of the electronic and vibrational transitions of singlet excitons, triplet excitons, polarons, and bipolarons with the experimental results proves that the observed features are due to positive polarons delocalized on the polymer chains. We find that the more crystalline Si-bridged variant gives rise to a red-shift in the transition energies, especially in the mid-infrared (MIR) spectral range and furthermore observe that the pristine polymers' responses depend on the excitation energy. Blending with PCBM, on the other hand, leads to excitation independent PIA spectra. By computing the response properties of molecular aggregates, we show that polarons are delocalized in not only the intra-but also the interchain direction, leading to intermolecular transitions which correspond well to experimental absorption features at the lowest energies.

Here the fabrication of an inorganic metal‐halide perovskite CsPbBr3 based X‐ray detector is reported utilizing a simple, scalable, and cost‐sensitive melt processing directly on substrate of any size. X‐ray diffraction analysis on the several 100 mm thick melt processed films confirms crystalline domains in the cm2 range. The CsPbBr3 film features a resistance of 8.5 GΩ cm and a hole mobility of 18 cm2 V−1 s−1. An X‐ray to current conversion rate of 1450 µC Gyair−1 cm−2 at an electric field of 1.2 × 104 V cm−1 and a detection limit in the sub µGyair s−1 regime is demonstrated. The high crystallinity and chemical purity of the melt processed CsPbBr3 films are suggested to be responsible for a performance which is on par to current state‐of‐the‐art Cd(Zn)Te based X‐ray detector technology.

Lead halide perovskites are used in thin-film solar cells, which owe their high efficiency to the long lifetimes of photocarriers. Various calculations find that a dynamical Rashba effect could significantly contribute to these long lifetimes. This effect is predicted to cause a spin splitting of the electronic bands of inversion-symmetric crystalline materials at finite temperatures, resulting in a slightly indirect band gap. Direct experimental evidence of the existence or the strength of the spin splitting is lacking. Here, we resonantly excite photocurrents in single crystalline ( C H 3 N H 3 ) P b I 3 with circularly polarized light to clarify the existence of spin splittings in the band structure. We observe a circular photogalvanic effect, i.e., the photocurrent depends on the light helicity, in both orthorhombic and tetragonal ( C H 3 N H 3 ) P b I 3 . At room temperature, the effect peaks for excitation photon energies Δ E = 110 meV below the direct optical band gap. Temperature-dependent measurements reveal a sign change of the effect at the orthorhombic–tetragonal phase transition, indicating different microscopic origins in the two phases. Within the tetragonal phase, both Δ E and the amplitude of the circular photogalvanic effect increase with temperature. Our findings support a dynamical Rashba effect in this phase, i.e., a spin splitting caused by thermally induced structural fluctuations which break inversion symmetry.

Lead halide perovskites often suffer from a strong hysteretic behavior on their j–V response in photovoltaic devices that has been correlated with slow ion migration. The electron extraction layer has frequently been pointed to as the main culprit for the observed hysteretic behavior. In this work three hole transport layers are studied with well‐defined highest occupied molecular orbital (HOMO) levels and interestingly the hysteretic behavior is markedly different. Here it is shown that an adequate energy level alignment between the HOMO level of the extraction layer and the valence band of the perovskite, not only suppresses the hysteresis, avoiding charge accumulation at the interfaces, but also degradation of the hole transport layer is reduced. Numerical simulation suggests that formation of an injection barrier at the organic/perovskite heterointerface could be one mechanism causing hysteresis. The suppression of such barriers may require novel design rules for interface materials. Overall, this work highlights that both external contacts need to be carefully optimized in order to obtain hysteresis‐free perovskite devices.

Medical X-ray imaging requires cost-effective and high-resolution flat-panel detectors for the energy range between 20 and 120 keV. Solution-processed photodetectors provide the opportunity to fabricate detectors with a large active area at low cost. Here, we present a disruptive approach that improves the resolution of such detectors by incorporating terbium-doped gadolinium oxysulfide scintillator particles into an organic photodetector matrix. The X-ray induced light emission from the scintillators is absorbed within hundreds of nanometres, which is negligible compared with the pixel size. Hence, optical crosstalk, a limiting factor in the resolution of scintillator-based X-ray detectors, is minimized. The concept is validated with a 256 × 256 pixel detector with a resolution of 4.75 lp mm−1 at a MTF = 0.2, significantly better than previous stacked scintillator-based flat-panel detectors. We achieved a resolution that proves the feasibility of solution-based detectors in medical applications. Time-resolved electrical characterization showed enhanced charge carrier mobility with increased scintillator filling, which is explained by morphological changes.

Nature Photonics, 9, ISSN:1749-4885, ISSN:1749-4893

The evolution of real-time medical diagnostic tools such as angiography and computer tomography from radiography based on photographic plates was enabled by the development of integrated solid-state X-ray photon detectors, based on conventional solid-state semiconductors. Recently, for optoelectronic devices operating in the visible and near infrared spectral regions, solution-processed organic and inorganic semiconductors have also attracted immense attention. Here we demonstrate a possibility to use such inexpensive semiconductors for sensitive detection of X-ray photons by direct photon-to-current conversion. In particular, methylammonium lead iodide perovskite (CH3NH3PbI3) offers a compelling combination of fast photoresponse and a high absorption cross-section for X-rays, owing to the heavy Pb and I atoms. Solution processed photodiodes as well as photoconductors are presented, exhibiting high values of X-ray sensitivity (up to 25 µC mGyair-1 cm-3) and responsivity (1.9×104 carriers/photon), which are commensurate with those obtained by the current solid-state technology.

In the past few years, hybrid organic-inorganic and all-inorganic metal halide perovskite nanocrystals have become one of the most interesting materials for optoelectronic applications. Here, we report a facile and rapid room temperature synthesis of 15-25 nm formamidinium CH(NH

The performance of organic solar cells is determined by the delicate, meticulously optimized bulk-heterojunction microstructure, which consists of finely mixed and relatively separated donor/acceptor regions. Here we demonstrate an abnormal strong burn-in degradation in highly efficient polymer solar cells caused by spinodal demixing of the donor and acceptor phases, which dramatically reduces charge generation and can be attributed to the inherently low miscibility of both materials. Even though the microstructure can be kinetically tuned for achieving high-performance, the inherently low miscibility of donor and acceptor leads to spontaneous phase separation in the solid state, even at room temperature and in the dark. A theoretical calculation of the molecular parameters and construction of the spinodal phase diagrams highlight molecular incompatibilities between the donor and acceptor as a dominant mechanism for burn-in degradation, which is to date the major short-time loss reducing the performance and stability of organic solar cells., Li et al. study degradation in organic photovoltaics from a morphological perspective. They find that donor and acceptor phases undergo excessive demixing via spinodal decomposition resulting in a reduction of charge generation. Demixing is due to the inherently low miscibility of both materials.

Flexible OPV modules, based on P3HT:PCBM as absorber layer, were manufactured with a power conversion efficiency over 3% and for a total area of 3500 mm2 consisting of 14 in series interconnected cells. The modules utilize the excellent mechanical and the outstanding optical properties of sputtered transparent ITO-Metal-ITO (IMI) electrodes deposited on the PET foil on the one hand, and the combination of large area slot-die coating with high resolution ultrafast laser patterning on the other hand. The manufacturing of modules with outstanding performance was found to be reproducible. The right combination of innovative electrodes and smart roll-to-roll compatible processing technologies demonstrates a viable path towards high efficient industrial module technology.

In colloidal nanoparticle (NPs) devices, trap state densities at their surface exert a profound impact on the rate of charge carrier recombination and, consequently, on the deterioration of the device performance. Here, we report on the successful application of a ligand exchange strategy to effectively passivate the surface of cuprite (Cu2O) NPs. Cu2O NPs were prepared by means of a novel synthetic route based on flame spray pyrolysis. FTIR, XRD, XPS, and HRTEM measurements corroborate the formation of cubic cuprite Cu2O nanocrystals, excluding the possible presence of undesired CuO or Cu phases. Most importantly, steady-state emission and transient absorption assays document that surface passivation results in substantial changes in the intensity of emissive excitonic states--centered at copper and oxygen vacancies--and in the lifetime of excitons near the band edge. To shed light onto ultrafast processes in Cu2O nanocrystals additional pump probe experiments on the femtosecond and nanosecond time scales were carried out. Two discernible species were observed: on one hand, an ultrafast component (~ps) that relates to the excitons; on the other hand, a long-lived component (~μs) that originates from the defects/trap states.

Two different types of aluminum-doped zinc oxide (AZO) thin films were fabricated using low cost sol–gel technique. By applying damp heat testing, the optical and electrical properties of those films were investigated under the influence of accelerated degradation from moisture or moisture vapor. Complementary measurements of optical transmittance, work function, and conductivity allowed exploring the degradation of AZO thin films and the corresponding OPV devices. We found that optical properties like transmittance, absorption coefficient, and band gap are not influenced by temperature and moisture. However, an increase in the work function, and a decrease in the conductivity of AZO films were observed upon damp heat exposure indicating the formation of a barrier or depletion layer at the metal oxide semiconductor interface.

Metal halide perovskites (MHPs) changed the world of solution processed semiconductors, previously dominated by organic semiconductors, toward predominantly inorganic materials with a relatively high electron/hole mobility. A series of devices benefit from their optoelectronic properties, including X-ray detectors. After the introduction of MHP X-ray detectors in 2013, they have achieved significant improvements in the form of single crystals, polycrystalline materials, and pixelated imaging devices. In addition, MHPs in the form of colloidal nanocrystals act as excellent scintillators. We see the bright future of MHPs in energy resolved X-ray detection, either achieved in the single counting mode, or in nanocrystal multilayer stacked devices, acting as a combination of selective X-ray filters and scintillators.

In this paper, we demonstrate that laser patterning of organic solar cells by ultrafast laser systems (pulse length

The understanding and control of nanostructures with regard to transport and recombination mechanisms is of key importance in the optimization of the power conversion efficiency (PCE) of solar cells based on inorganic nanocrystals. Here, the transport properties of solution-processed solar cells are investigated using photo-CELIV (photogenerated charge carrier extraction by linearly increasing voltage) and transient photovoltage techniques; the solar cells are prepared by an in-situ formation of CuInS2 nanocrystals (CIS NCs) at the low temperature of 270 °C. Structural and morphological analyses reveal the presence of a metastable CuIn5S8 phase and a disordered morphology in the CuInS2 nanocrytalline films consisting of polycrystalline grains at the nanoscale range. Consistent with the disordered morphology of the CIS NC thin films, the CIS NC devices are characterized by a low carrier mobility. The carrier density dynamic indicates that the recombination kinetics in these devices follows the dispersive bimolecular recombination model and does not fully behave in a diffusion-controlled manner, as expected by Langevin-type recombination. The mobility–lifetime product of the charge carriers properly explains the performance of the thin (200 nm) CIS NC solar cell with a high fill-factor of 64% and a PCE of over 3.5%.

Recently, the concept of near-infrared sensitization is successfully employed to increase the light harvesting in large-bandgap polymer-based solar cells. To gain deeper insights into the operation mechanism of ternary organic solar cells, a comprehensive understanding of charge transfer-charge transport in ternary blends is a necessity. Herein, P3HT:PCPDTBT:PCBM ternary blend films are investigated by transient absorption spectroscopy. Hole transfer from PCPDTBT-positive polarons to P3HT in the P3HT:PCPDTBT:PCBM 0.9:0.1:1 blend film can be visualized. This process evolves within 140 ps and is discussed with respect to the proposed charge-generation mechanisms.

In this paper we report on the replacement for the commonly used ITO electrode material by a low temperature solution processed silver nanowire/(doped) metal oxide composite. Devices employing silver nanowires (AgNWs)/buffer layer electrodes with a photoactive layer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) are showing a comparable performance to the ITO reference cell with fill factors (FF) of over 62% and a power conversion efficiency of ∼2.7%. Zinc oxide (ZnO) and highly conductive Al doped ZnO (AZO) are used as buffer layer. AgNW devices without a buffer layer have a high open circuit voltage (VOC) but the FF and the short circuit current density (jSC) are substantially lower. Overall it is demonstrated that AgNWs and the low temperature solution process of the buffer layer are an attractive device concept towards an indium free organic solar cell.

Perovskite solar cells based on CH3NH3PbBr3 with a band gap of 2.3 eV are attracting intense research interests due to their high open-circuit voltage (V-oc) potential, which is specifically relevant for the use in tandem configuration or spectral splitting. Many efforts have been performed to optimize the V-oc of CH3NH3PbBr3 solar cells; however, the limiting V-oc (namely, radiative V-oc:V-oc,V- rad) and the corresponding Delta V-oc (the difference between V-oc,V- rad and V-oc) mechanism are still unknown. Here, the average V-oc of 1.50 V with the maximum value of 1.53 V at room temperature is achieved for a CH3NH3PbBr3 solar cell. External quantum efficiency measurements with electroluminescence spectroscopy determine the V-oc,V- rad of CH3NH3PbBr3 cells with 1.95 V and a Delta V-oc of 0.45 V at 295 K. When the temperature declines from 295 to 200 K, the obtained V-oc remains comparably stable in the vicinity of 1.5 V while the corresponding Delta V-oc values show a more significant increase. Our findings suggest that the V-oc of CH3NH3PbBr3 cells is primarily limited by the interface losses induced by the charge extraction layer rather than by bulk dominated recombination losses. These findings are important for developing strategies how to further enhance the V-oc of CH3NH3PbBr3-based solar cells.

In recent years the concept of ternary blend bulk heterojunction (BHJ) solar cells based on organic semiconductors has been widely used to achieve a better match to the solar irradiance spectrum, and power conversion efficiencies beyond 10% have been reported. However, the fill factor of organic solar cells is still limited by the competition between recombination and extraction of free charges. Here, we design advanced material composites leading to a high fill factor of 77% in ternary blends, thus demonstrating how the recombination thresholds can be overcome. Extending beyond the typical sensitization concept, we add a highly ordered polymer that, in addition to enhanced absorption, overcomes limits predicted by classical recombination models. An effective charge transfer from the disordered host system onto the highly ordered sensitizer effectively avoids traps of the host matrix and features an almost ideal recombination behaviour. Carrier recombination in organic solar cells usually limits their optoelectronic performance, in particular their fill factor. Gasparini et al show that adding an ordered polymer to a ternary blend reduces carrier recombination, achieving a fill factor of 77%.