Dark Carrier Concentration Research Articles

Robust Fabrication of Hybrid Lead-Free Perovskite Pellets for Stable X-ray Detectors with Low Detection Limit.

X-ray detectors are widely utilized in medical diagnostics and nondestructive product inspection. Halide perovskites are recently demonstrated as excellent candidates for direct X-ray detection. However, it is still challenging to obtain high quality perovskites with millimeter-thick over a large area for high performance, stable X-ray detectors. Here, methylammonium bismuth iodide (MA3 Bi2 I9 ) polycrystalline pellets (PPs) are developed by a robust, cost effective, and scalable cold isostatic-pressing for fabricating X-ray detectors with low limit of detection (LoD) and superior operational stability. The MA3 Bi2 I9 -PPs possess a high resistivity of 2.28 × 1011 Ω cm and low dark carrier concentration of ≈107 cm-3 , and balanced mobility of ≈2 cm2 V-1 s-1 for electrons and holes. These merits enable a sensitivity of 563 μC Gyair -1 cm-2 , a detection efficiency of 28.8%, and an LoD of 9.3 nGyair s-1 for MA3 Bi2 I9 -PPs detectors, and the LoD is much lower than the dose rate required for X-ray diagnostics used currently (5.5 μGyair s-1 ). In addition, the MA3 Bi2 I9 -PPs detectors work stably under high working bias field up to 2000 V cm-1 after sensing an integrated dose >320 Gyair with continuous X-ray radiation, demonstrating its competitive advantage in practical application. These findings provide an approach to explore a new generation of low LoD, stable and green X-ray detectors based on MA3 Bi2 I9 -PPs.

Ultrasensitive and stable X-ray detection using zero-dimensional lead-free perovskites

Abstract Sensitive and reliable X-ray detectors are essential for medical radiography, industrial inspection and security screening. Lowering the radiation dose allows reduced health risks and increased frequency and fidelity of diagnostic technologies for earlier detection of disease and its recurrence. Three-dimensional (3D) organic–inorganic hybrid lead halide perovskites are promising for direct X-ray detection – they show improved sensitivity compared to conventional X-ray detectors. However, their high and unstable dark current, caused by ion migration and high dark carrier concentration in the 3D hybrid perovskites, limits their performance and long-term operation stability. Here we report ultrasensitive, stable X-ray detectors made using zero-dimensional (0D) methylammonium bismuth iodide perovskite (MA3Bi2I9) single crystals. The 0D crystal structure leads to a high activation energy (Ea) for ion migration (0.46 eV) and is also accompanied by a low dark carrier concentration (~ 106 cm−3). The X-ray detectors exhibit sensitivity of 10,620 µC Gyair−1 cm−2, a limit of detection (LoD) of 0.62 nGyair s−1, and stable operation even under high applied biases; no deterioration in detection performance was observed following sensing of an integrated X-ray irradiation dose of ~23,800 mGyair, equivalent to > 200,000 times the dose required for a single commercial X-ray chest radiograph. Regulating the ion migration channels and decreasing the dark carrier concentration in perovskites provide routes for stable and ultrasensitive X-ray detectors.

High‐Responsivity Photodetectors Based on Formamidinium Lead Halide Perovskite Quantum Dot–Graphene Hybrid

AbstractHighly performance photodetector requires a wide range of responses of the incident photons and converts them to electrical signals efficiently. Here, a photodetector based on formamidinium lead halide perovskite quantum dots (e.g., FAPbBr3 QDs)–graphene hybrid, aiming to take the both advantages of the two constituents. The FAPbBr3 QD–graphene layer not only benefits from the high mobility and wide spectral absorption of the graphene material but also from the long charge carrier lifetime and low dark carrier concentration of the FAPbBr3 QDs. The photodetector based on FAPbBr3 QD–graphene hybrid exhibits a broad spectral photoresponse ranging from 405 to 980 nm. A photoresponsivity of 1.15 × 105AW−1 and an external quantum efficiency as high as 3.42 × 107% are obtained under an illumination power of 3 µW at 520 nm wavelength. In detail, a high responsivity is achieved in 405–538 nm, while a relatively low but fast response is observed in 538–980 nm. The photoelectric conversion mechanism of this hybrid photodetector is investigated in the view of built‐in electric field from the QD–graphene contact which improves the photoconductive gain.

Thin Films and Solar Cells Based on Semiconducting Two-Dimensional Ruddlesden–Popper (CH3(CH2)3NH3)2(CH3NH3)n−1SnnI3n+1 Perovskites

Low electrical resistivity (high dark carrier concentration) of CH3NH3SnI3 often leads to short-circuiting in solar cells, and appropriate thin-film modifications are required to ensure functional devices. The long-term durability of organic–inorganic perovskite solar cells necessitates the protection of perovskite thin films from moisture to prevent material decomposition. Herein, we report that the electrical resistivity and the moisture stability of two-dimensional (2D) Ruddlesden–Popper (CH3(CH2)3NH3)2(CH3NH3)n−1SnnI3n+1 perovskites are considerably improved compared to those of the three-dimensional (3D) CH3NH3SnI3 perovskite and subsequently show the solar cell fabrication using a simple one-step spin-coating method. These 2D perovskites are semiconductors with optical band gaps progressively decreasing from 1.83 eV (n = 1) to 1.20 eV (n = ∞). The n = 3 and n = 4 members with optimal band gaps of 1.50 and 1.42 eV for solar cells, respectively, were thus chosen for in-depth studies. We demonstrate th...

Formamidinium Lead Halide Perovskite Crystals with Unprecedented Long Carrier Dynamics and Diffusion Length

State-of-the-art perovskite solar cells with record efficiencies were achieved by replacing methylammonium (MA) with formamidinium (FA) in perovskite polycrystalline films. However, these films suffer from severe structural disorder and high density of traps; thus, the intrinsic properties of FA-based perovskites remain obscured. Here we report the detailed optical and electrical properties of FAPbX3 (where X = Br– and I–) single crystals. FAPbX3 crystals exhibited markedly enhanced transport compared not just to FAPbX3 polycrystalline films but also, surprisingly, to MAPbX3 single crystals. Particularly, FAPbBr3 crystals displayed a 5-fold longer carrier lifetime and 10-fold lower dark carrier concentration than those of MAPbBr3 single crystals. We report long carrier diffusion lengths—much longer than previously thought—of 6.6 μm for FAPbI3 and 19.0 μm for FAPbBr3 crystals, the latter being one of the longest reported values in perovskite materials. These findings are of great importance for future inte...

A ZnO-Nanowire Phototransistor Prepared on Glass Substrates

The fabrication of a phototransistor via the bridging of two prefabricated electrodes with a laterally grown ZnO nanowire is reported. It was found that the fabricated device is an n-channel enhancement-mode phototransistor with a dark carrier concentration of 6.34 × 10(17) cm(-3) when the gate voltage is biased at 5 V. With an incident-light wavelength of 360 nm and a zero gate bias, it was found that the noise equivalent power and normalized detectivity (D*) of the fabricated ZnO phototransistor were 6.67 × 10(-17) W and 1.27 × 10(13) cm Hz(0.5) W(-1), respectively. It was also found that the current in the device can be modulated efficiently by tuning the wavelength of the excitation source.

00/00912 A general multivariate qualitative model for sizing stand-alone photovoltaic systems

We report on investigations of the ambipolar diffusion process in n-i-p diodes and n-i-p-i doping superlattices performed by a new all-optical pump-and-probe technique. This new technique allows not only the determination of the ambipolar diffusion coefficient but also the spatially resolved investigation of the stationary distribution of the optically-induced excess carriers. The n-i-p-i doping superlattice exhibited an extremely large ambipolar diffusion coefficient in the range of 104cm2s−1. The ambipolar diffusion coefficient of the n-i-p and n-i-p-i structure was demonstrated not to be a constant but a function of the charge carrier density. This strongly affects the spatial distribution of the excess carriers especially in the large signal regime (density of photo-generated carriers much larger than dark carrier concentration). The spatial distribution of the carriers in the small as well as in the large signal case can be understood theoretically if the real dependence of the ambipolar diffusion coefficient on the carrier density is taken into account.

Extremely fast ambipolar diffusion in n-i-p-i doping superlattices investigated by an all-optical pump-and-probe technique

We report on investigations of the ambipolar diffusion process in n-i-p diodes and n-i-p-i doping superlattices performed by a new all-optical pump-and-probe technique. This new technique allows not only the determination of the ambipolar diffusion coefficient but also the spatially resolved investigation of the stationary distribution of the optically-induced excess carriers. The n-i-p-i doping superlattice exhibited an extremely large ambipolar diffusion coefficient in the range of 104cm2s−1. The ambipolar diffusion coefficient of the n-i-p and n-i-p-i structure was demonstrated not to be a constant but a function of the charge carrier density. This strongly affects the spatial distribution of the excess carriers especially in the large signal regime (density of photo-generated carriers much larger than dark carrier concentration). The spatial distribution of the carriers in the small as well as in the large signal case can be understood theoretically if the real dependence of the ambipolar diffusion coefficient on the carrier density is taken into account.

The detailed balance principle and the reciprocity theorem between photocarrier collection and dark carrier distribution in solar cells

It is shown that a recently described general relationship between the local collection efficiency of solar cells and the dark carrier concentration (reciprocity theorem) directly follows from the principle of detailed balance. We derive the relationship for situations where transport of charge carriers occurs between discrete states as well as for the situation where electronic transport is described in terms of continuous functions. Combining both situations allows to extend the range of applicability of the reciprocity theorem to all types of solar cells, including, e.g., metal-insulator-semiconductor-type, electrochemical solar cells, as well as the inclusion of the impurity photovoltaic effect. We generalize the theorem further to situations where the occupation probability of electronic states is governed by Fermi–Dirac statistics instead of Boltzmann statistics as underlying preceding work. In such a situation the reciprocity theorem is restricted to small departures from equilibrium.

Long-term relaxation of the photovoltage in a heteroepitaxial structure

The distribution of the transverse photovoltage has been investigated in samples of a Ge-GaAs heterostructure with an oxide layer (∼15 A) at the interface. A method of simultaneous excitation with modulated and unmodulated radiation is used to observe the long-term relaxation of the photovoltage in these heterostructures. The energy barriers responsible for this effect are localized at the surface of the Ge film and the GaAs substrate, adjacent to the interlying oxide. It is shown that the gradients of the dark carrier concentration in the film and in the substrate are in opposite directions. A transition layer is formed during the growth process and not as a result of the classical heterodiffusion of the components in the film and the substrate. It is noted that the photoconductivity signal observed at the modulation frequency in the field formed at the sample as a result of the unmmodulated illumination must be taken into account in the recorded voltage signal.

Transient Photoconductivity in GaAs Films Grown by Molecular Beam Epitaxy.

AbstractHahn-Meitner-Institut, D-1000 Berlin 39, W. Germany Abstract The transient photoconductivity, in MBE grown silicon doped GaAs films, was studied by a contactless microwave conductivity technique. The dark carrier concentration of the investigated films varies from 4 x 1014 cm-3 to 6 x 1017 cm-3. At least two decay channels can be distinguished. An initial fast decay process (t<20 ns), which is excitation intensity dependent, followed by a slower one. The fast decay is tentativly identified with a direct electron-hole recombination and is more pronounced at high excitation intensities and in low impurity materials. The slower decay component is characterized by effective electron lifetimes ranging from 2 µs for low impurity fims up to 100 µs for high impurity samples. It can be concluded that an increase of the Si-concentration leads to an increase in the concentration of hole traps, which quenches the initial electron-hole recombination and decreases the electron decay rate in the microsecond region.

Photocurrent gain in a.c. biased photoconductors

Analytical expressions are derived for the gain-bandwidth product of photoconductors under high-frequency bias. The results of this approach are applied to describe the performance of a photoconductor mounted in a microwave cavity. The polarization of the dark carrier concentration under the influence of the applied electric field and the influence of capacitative coupling on the distribution density of the photoexcited carriers are both considered.The current gain G of a photoconductor in a microwave cavity is expressed in terms of the photoconductor properties and the parameters of the microwave field. The form of the analytic expression for G is related to the physical processes in the system.