Currently, the most prevalent direct X-ray detection application is photoconductive-type flat-panel X-ray detectors, they generally require dark current densities well below 1 nA cm-2 to maintain high detective quantum efficiency and dynamic range.
However, the high dark current of perovskite or other material systems photoconduction detectors largely blocks their utilization in practical X-ray imagers. First, a high dark current can quickly fill up the storage capacitance of TFT or CMOS pixels prior to X-ray illumination, which deteriorates the image contrast and dynamic range. Also, a large dark current significantly increases the shot noise and results in a poor signal-to-noise ratio (SNR).
In our work recently published in Nature Communications (https://doi.org/10.1038/s41467-023-36313-6), we report a very common detector architecture with a unique shunting electrode working as a blanking unit to suppress dark current, and it theoretically can be reduced to zero. This method is called the dark-current-shunting (DCS) strategy. We experimentally fabricate the DCS X-ray detector, which exhibits a record-low dark current of 51.1 fA at 5 V mm-1, a detection limit of 7.84 nGyair s-1, and a sensitivity of 1.3 × 104 μC Gyair-1 cm-2. The signal-to-noise ratio of their polycrystalline perovskite-based detector is even outperforming many previously reported state-of-the-art single crystal-based X-ray detectors by serval orders of magnitude. This work provides a device strategy to fundamentally reduce dark current and enhance the signal-to-noise ratio of X-ray detectors and photodetectors in general.
In conventional photoconduction devices (Fig. 1a), the dark current and photocurrent are conducted in the same path and collected by the same electrodes, reducing the dark current is equivalent to increasing the resistance, which in turn also largely cuts the X-ray photocurrent.
In the DCS device structure (Fig. 1b), the electrons under dark are emitted from the source electrode and most of them can be collected by the top DCS electrode, or at least they will no longer be received by the drain, which gives the zero source-drain dark current.In principle, there is a critical voltage (CV), under which the mobile electrons under dark can be completely shunted by the top DCS electrode.Under X-rays (Fig. 1c), the excited electrons, are firstly generated from X-ray-sensitive materials and transported to a lateral conduction channel with higher carrier mobility, thus, the captured carriers can transport rapidly in the conduction channel, reinject and recirculate swiftly between source and drain contacts, due to the long lifetime of the conduction channel, the captured carriers can transport longer before the recombination. The amount of charges passing through the cross section of a conductor per unit of time is much greater. Therefore, these carriers in the conduction channel can offer much stronger photocurrent than the case with the mere X-ray sensitive materials. Even some of the X-ray induced electrons will be attracted by DCS electrode, with the high photoconductive gain effect of the conduction channel, the photocurrent is still quite strong. Then the signal current is collected by the drain under an electric field applied from drain to source. In this scenario, the drain only receives X-ray-generated electrons, simultaneously gaining nearly-zero dark current and relatively high X-ray photocurrent.
Fig. 1: Working mechanism of dark-current-shunting (DCS) detector and conventional photoconduction detector. a, Working mechanism of conventional photoconduction detector. The dark current and photocurrent are conducted in the same path and collected by the same electrodes. b, Working mechanism of dark-current-shunting (DCS) detector. The electrons in the dark are emitted from the source and collected by the DCS electrode. c, The X-ray induced electrons are generated from X-ray sensitive material and part of them drifted into a conduction channel with high carrier mobility under a built-in electric field between X-ray sensitive material and electron transport layer (ETL), then collected by drain electrode under an electric field applied in the lateral conduction channel. Even some of the X-ray induced electrons will be attracted to DCS electrode, with the high photoconductive gain effect of the conduction channel, the photocurrent is still strong.
Following this design principle, we successfully fabricated DCS X-ray detectors with a structure as shown in Fig. 2a. As expected, firstly, the drain current decreased with the increased voltage applied to the DCS electrode (Fig. 2b). At a CV of ~0.56V, the drain current was almost fully suppressed to be nearly-zero. With further elevated voltage at the DCS electrode, the drain current turned to be negative and its absolute value continuously increased. As shown in Fig. 2b, with varied drain-source voltages (0.25V, 0.5V and 0.75V), we can always find a critical DCS voltage (0.30 V, 0.56 V and 0.85 V respectively) to shunt the dark current completely. Those detectors are operating with the matched DCS and drain-source voltages under X-rays, and the photocurrent is always positive under this working condition as shown in Fig. 2c. It can be seen that when a CV was applied to the DCS electrode, the drain current was decreased to nearly-zero, and the X-ray photocurrent was only slightly diminished, compared with the case of the disabled DCS electrode.
Fig. 2: Schematic, current-voltage characteristics and sensitivity of DCS X-ray detectors. a, Schematic of DCS X-ray detectors. b, Current-voltage curves in terms of DCS electrode voltages and drain currents in dark conditions. The drain current decreases with increased DCS biases. c, Current-voltage curves in terms of DCS electrode voltages and drain currents when exposed to X-ray. d, X-ray generated pulse signals were illustrated when the DCS electrode was disabled, applied with a CV (0.56 V) and applied with a voltage larger than CV (1 V). The dose rate of X-ray was 233 μGyair s-1. e, The sensitivity of the DCS detectors was calculated when the DCS electrode was disabled and applied with a CV (0.56 V).
After applying CV to the DCS electrode, the dark current and noise current are as low as 51.1 fA and 152 fA, respectively, approaching the lowest measurable current of a typical semiconductor analyzer or source-meter. As a result, the SNR was improved by two orders of magnitude, showing an excellent signal output under the X-ray pulse train (Fig. 3). It is shown that our DCS detector delivers very low dark current, low detection limit, and ultrahigh SNR, even outperforming many previously reported state-of-the-art single crystal-based photoconductors. The DCS method is a universal device strategy and can be applied in other perovskite composition and conduction channel materials.
Fig. 3: Characterizations of pulse-train response, signal-to-noise ratio (SNR) and lowest detection limit. a, X-ray pulse-train response. By applying CV (0.56 V) to the DCS electrode, the signal-to-noise ratio (SNR), the pulse quality and stability are obviously improved. There is almost no dark current drifting. b, Characterization of dark current drifting. When the DCS electrode is disabled, the dark current gradually shifts from 0.615 nA to 0.986 nA. When the DCS electrode is biased with CV, the dark current reveals almost no shift during the pulse-train measurement. c, The noise current and the SNR are 22.5 pA and 147.6, respectively, when the DCS electrode is disabled. d, The noise current and the SNR are 0.152 pA and 12500, respectively, when DCS electrode is applied with a CV (0.56 V). e, With the DCS electrode biased with a CV (0.56 V), the SNRs are 36.18 and 21.97 under respective X-ray dose rate of 166 nGyair s-1 and 83 nGyair s-1. f, With the DCS electrode disabled, the SNRs are 0.22 and 0.112 under respective X-ray dose rate of 166 nGyair s-1 and 83 nGyair s-1. g, Characterizations of detection limit. The detection limit (SNR = 3) of the DCS detector is as low as 7.84 nGyair s-1, which is 350 times lower than the control photoconductor detector. The lowest detection limit was estimated by the reverse extension line to where the SNR = 3. h, The comparison with respect to the dark current density and the detection limit between this work and other state-of-the-art photoconduction detectors is demonstrated. The applied electric fields are noted along with the detector materials. i, The sensitivity/Jdark, a figure of merit to evaluate SNR, is illustrated to compare our DCS detector and other reported state-of-the-art photoconduction detectors.
Finally, the proof-of-concept X-ray imaging of a 64 × 64 pixels dark-current-shunting detector array is successfully demonstrated.
Jin, P., Tang, Y., Li, D. et al. Realizing nearly-zero dark current and ultrahigh signal-to-noise ratio perovskite X-ray detector and image array by dark-current-shunting strategy. Nat. Commun. 14, 626 (2023).