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## Creator

[Xiaoyu Huang](https://orcid.org/0000-0001-5521-539X), [Hong-Tao Sun](https://orcid.org/0000-0002-0003-7941), [Naoto Shirahata](https://orcid.org/0000-0002-1217-7589)

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[Highly efficient, self-powered UV photodiodes based on leadfree perovskite nanocrystals through interfacial engineering](https://mdr.nims.go.jp/datasets/f8a55513-58d5-458a-aa1c-63cc70d1ff68)

## Fulltext

I  xd  Highly Efficient, Self-Powered UV Photodiodes based on Leadfree Perovskite Nanocrystals through Interfacial Engineering Xiaoyu Huang1,2, Hong-Tao Sun1 and Naoto Shirahata1,2,3,4 1 Research Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba 305-0047, Japan 2 Graduate School of Chemical Sciences and Engineering, Hokkaido University, Kita 13, Nishi 8, Kita-ku, Sapporo 060-0814, Japan. 3 Department of Physics, Chuo University, 1-13-27 Kasuga, Bunkyo, Tokyo 112-8551, Japan 4 CNRS–Saint-Gobain–NIMS, IRL3629, Laboratory for Innovative Key Materials and Structures, National Institute for Materials Science, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan  E-mail: SHIRAHATA.Naoto@nims.go.jp  Abstract Double perovskite crystals are promising alternatives for lead-based perovskites that has potential to address toxicity and instability issues. In this study, Cs2AgBiCl6 nanocrystals (NCs) with high absorption coefficients were synthesized by hot-injection method. The bandgap engineering was realized by tuning the halide composition in Cs2AgBiCl6 to Cs2AgBiBr6. Both NCs were used as light-absorbing layers in lead-free perovskite photodiodes that exhibit wavelength-selectivity for UV-visible light operatable even at a bias voltage of 0 V. Cs2AgBiBr6-based photodiode exhibits a characteristic detection peak at 340 nm with a responsivity of 3.21 mA/W, a specific detectivity up to 8.91×1010 Jones and a fast response speed with a rise/fall time of 30/35 msec. The excellent performance of self-driven photodiodes lights up the prospect of lead-free perovskite NCs in highly efficient optoelectronic devices. Keywords: Double perovskite, Nanocrystals, Self-powered photodiode, Electron transport layer  Introduction Recent years, metal halide perovskites (MHPs) synthesized by a simple solution process have demonstrated suitability in high-performance visible photodetectors 1–5 due to their large light absorption coefficients, high carrier mobility, long carrier diffusion lengths, and broad spectrum of light absorption.6–10 Despite those great advantages, the lead toxicity and poor stability of some compositions have encouraged researchers to find lead-free alternatives. To be free from lead, the A2M+M3+X6 chemical formula materials such as Cs2AgBiCl6,11 Cs2AgBiBr6,12 Cs2AgInCl6,13 and Cs2AgSbCl6 14 were suggested as a promising strategy to replace two divalent Pb2+ ions with one monovalent ion M+ and a trivalent ion M3+.15,16 These double perovskites possess tunable band gaps, smaller effective masses of electrons and holes, and high conductivity to efficiently transport and extract charges similar to lead-based perovskites.17–19 The excellent properties make the double perovskite applied in various research fields, such as LED, photo detection, memory, and solar cell.11,20–22 Among the suggested double perovskites, Cs2AgBiCl6 and Cs2AgBiBr6 has been recognized as favourable candidates for light-absorption layers in photovoltaic devices due to their excellent structural stability, long carrier recombination lifetime with thermal and humidity stabilities.23,24 Those research achievements further confirm that the use of double perovskites is one of the most popular directions in the lead-free solar cells. However, Cs2AgBiBr6 and Cs2AgBiCl6 have natural hindrance factors that limit the   2   performance of photoelectronic conversion due to their large exciton binding energy of 220 meV,25 and 333 meV.26 Besides, the energy band mismatch with electron transport layer materials leads to non-radiative recombination and a large energy loss at the surface, yielding a poorer photoelectronic conversion efficiency.  Thus, it is crucial to select and adopt proper electron transfer layers (ETLs) with high mobility, low-defect state, and a suitable energy level arrangement for accelerating injection and transport of electrons to enhance the device performances of perovskite-based photodetectors. Until now, the ETL materials in perovskite photodetectors were generally metal oxides such as ZnO, TiO2, and SnO2.27–29 Zhang et al. reported that modifying the number of layers ZnO to change the hydrophobicity, thereby fabricated the pinhole-free high efficiency perovskite solar cells.30 Although ZnO is usually used as an ETL material due to its high electron mobility and low fabrication temperature, the low hydrophilic property of ZnO surface gives rise to a poor coverage film by creating pinholes.31,32 In addition, the mismatched energy levels between the perovskite film and the ZnO ETL caused a large energy loss.33,34 To overcome the shortcomings, the interface engineering at the heterojunction involves the construction of a bilayer or multi- layered ETL as a very critical technique.35,36 For strategically constructed double ETLs with ZnO, we selected [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM), which has been reported to have a good band alignment with organic-inorganic lead perovskite films and to improve the power conversion efficiency of solar cells.33  In this study, we report on a self-powered ultraviolet (UV) photodiode where Cs2AgBiBr6 nanocrystals (NCs) are used as an active layer. In particular, the double ETL layers of PCBM/ZnO structure are investigated to reveal the effects of double ETLs on the enhancement of device performance. The insertion of polymer PCBM between ZnO and active layers allows formation of films with more well-matched energy levels for efficient electron transport. The Cs2AgBiBr6 NCs-based photodiode with PCBM/ZnO double ETLs exhibited outstanding detection performance compared to that of the single ETL device. The optimized photodiode exhibited a good self-powered device performance which acted as the responsivity of 3.1 mA/W when detectivity approached 8.91 × 1010 Jones, and a large light/dark current ratio near 200 was obtained. More importantly, spectral wavelength selectivity of Cs2AgBiBr6 NCs-based photodetector manifested the potential utilization in ultraviolet detection. Experimental Synthesis of Cs2AgBiCl6 and C2AgBiBr6 nanocrystals Cesium acetate (CsOAc, 99.99%, Aldrich), silver acetate (AgOAc, 99%, Aldrich), bismuth(III) acetate (Bi(OAc)3, 99.99%, Aldrich), 1-octadecene (ODE, 90%, TCI), oleylamine (OLA, 70%, Aldrich), oleic acid (OA, 90%, Aldrich), chlorotrimethylsilane (TMCS, 98%, Aldrich), trimethylbromosilane (TMBS, 97%, Aldrich), hexane (96%, Wako) and ethyl acetate (99.5%, wako) were used without further purification. The colloidal synthesis of NCs was performed using the reported protocol.37,38 1 mmol CsOAc, 0.45 mmol AgOAc and 0.5 mmol Bi(OAc)3 were mixed with 10 mL ODE, 0.7 mL OLA and 2.8 mL OA on a 50 mL three-neck flask. The flask was connected to Schlenk line, then the mixture was heated to 110 oC and degassed for an hour. Then the heating temperature was raised by the speed of 6 oC/min, and when the temperature reached 165 oC, 0.4 mL TMCS was injected into the precursor solution. After 2 min of continuous heating, the reaction was stopped by immersing the flask into an ice water bath. When the temperature reached 30 oC, the reaction mixture was transferred into a centrifuge tube and centrifuged at 9000 rpm for 20 min. The supernatant was removed. Then the precipitate was dispersed in 5 mL hexane and centrifuged at 6000 rpm for 15 min, then the precipitate was discarded and the supernatant was mixed with ethyl acetate at the volume ratio of 1:1. Following a centrifugation process at 10000 rpm for 5 min. Finally, the precipitate was redispersed in hexane at the concentration of 25 g/mL for device fabrication. All the centrifugatios were conducted at 25 oC. Materials characterization The crystallinity was investigated by powder X-ray diffraction (XRD, Rigaku, Cu Kα1), where the liquid sample was dropped on the silicon sample holder. High-resolution transmission electron microscopy (TEM) characterization was performed using a JEM-2100F2 instrument (JEOL) at 200 kV acceleration voltage. Scanning transmission electron microscopy coupled with energy dispersive spectroscopy (STEM-EDS) was utilized for the element analysis. Samples for the TEM analysis were prepared by drop-casting the 0.1 mg/mL hexane solution of NCs onto carbon-coated copper grids. Elemental analysis and chemical shift were studied by X-ray photoelectron spectroscopy (XPS, ThermoFisher), the samples for the XPS analysis were drop-casted on the silicon substrate. The absorption spectra was acquired with a JASCO V-650 UV-visible spectrometer. The LUMO and HOMO of nanocrystals were measured by ultraviolet photoelectron spectroscopy (UPS, ThermoFisher), the samples were prepared same with the XPS samples. Device fabrication A 10×20 mm2 rectangle soda-lime glass covered with 150 nm thick ITO with a sheet resistance of 10~14 Ω/sq was used as the substrate. The ITO film was patterned to into a narrow strip about 2 mm wide and 20 mm long. ITO-covered glasses were cleaned by the detergent, deionized water, isopropanol, and acetone in ultrasonic cleaner successively (30 min for each   3   step). Next, organic contaminations on the substrate surface were removed exposure to vacuum ultraviolet (VUV) light (Ushio Inc., Japan, λ = 172 nm and 10 mW/cm2) for 30 min under a reduced pressure of 1 kPa and N2 flow, resulting in a super hydrophilic surface due to oxidation.39 Then the substrates were transferred into the Ar-filled glove box where the oxygen and water levels are continuously monitored to maintain both O2 < 1 ppm and H2O < 1 ppm. 70 μL of an PEDOT:PSS solution was spin-coated on ITO glassed at a rate of 3000 rpm for 45 s and annealed at 150 oC for 30 min. The NC solution was spin-coated onto PEDOT:PSS films at 1500 rpm for 45 s and annealed at 180 oC for 10 min. PCBM solution was spin-coated onto the perovskite-coated films at 2500 rpm for 45 s and heated at 120 oC for 10 min. Next, ZnO solution was spin-coated at 2500 rpm for 45 s and heated at 140 oC for 30 min. Finally, Al electrode with a thickness of 170 nm was thermally evaporated onto ZnO films under the pressure of 5×10-5. Device measurement The electrical characterization was done using Keithly 2425 Source Meter and KickStart software. Compact Xenon light source (300 W, Asahi Spectra Co., Ltd, MAX-301) combined with bandpass filters (310, 340, 365, 380, 410, 440, 470 and 490 nm) was used as light sources for different wavelengths. The peak linewidths of the incident light passing through the bandpass filters were 10 nm, respectively. The power of UV light at different intensities was recorded using a Laser power & energy meter (Ophir Optronics Solutions Ltd, NOVA II). All the light current density−voltage (I-V) curves were measured in ambient air at various conditions. For the I-V measurement, the dark current was measured without illumination for each device, the light current was measured after each scan of dark current under the illumination of a specific wavelength (310, 34, 365, 380, 410, 440, 470 and 490 nm) at a specific power intensity (range from 2~20 mW/cm2) as needed. There was no obvious difference when we compared the dark current which was measured before the light current to that measured after the light current for same device. For the dynamic response measurement, the anode and cathode of the device were connected to a 1 GΩ load resistor and a digital oscilloscope (Iwatsu Electric Co., Ltd, DS-5624A) combined with an optical chopper (#55-783, Edmund Optics). The dynamic response displayed on the oscilloscope and the response time was extracted according to the record. All the device measurements were conducted in air and at room temperature. Results and discussion Figure 1(a) shows XRD patterns of Cs2AgBiCl6 and Cs2AgBiBr6 NCs. The presence of multiple diffraction peaks exhibited the structural characteristics of the nanocrystals. For Cs2AgBiCl6 NCs, domain diffraction peaks at 23.28°, 33.21°, 40.96°, and 47.64° were assigned to (022), (004), (224) and (044) diffraction planes of standard cubic double perovskite structure with a space group of Fm-3m. The estimated lattice parameter was 10.78 Å. Other peaks were consistent with the simulated values of Cs2AgBiCl6_11523 in the literature,40 confirming the formation of double perovskite structure via the modified hot-injection method. For Cs2AgBiBr6 NCs, domain diffraction peaks at 15.49°, 22.06°, 27.32°, 31.60°, 39.00°, and 45.32° were assigned to (002), (022), (222), (004), (224) and (044) diffraction planes of standard cubic double perovskite structure with a space group of Fm-3m. The estimated lattice parameter was 11.31 Å. These diffraction peaks were consistent with the Cs2AgBiBr6_18989 in the literature.41  The results of XRD indicated that the diffraction pattern of Cs2AgBiBr6 shifts to a low 2θ angle side compared to Cs2AgBiCl6, wherein the lattice expansion could be resulted from the larger ionic radius of Br- (1.96 Å) compared to Cl- (1.81 Å). HR-TEM images of Cs2AgBiCl6 and Cs2AgBiBr6 NCs are shown in Figure 1(b)-(c). The mean diameters of the NCs were 10.83 ± 3.60 nm and 9.88 ± 2.40 nm for Cs2AgBiCl6 and Cs2AgBiBr6 samples, respectively. The inset shows the enlarged images of the representative NCs. It could be seen that the interplanar spacing of (022) is consistent with the XRD results which indicate the lattice expansion when the halide ion in double perovskite changed from Cl- to Br-.  Figure 1. (a) X-ray diffraction (XRD) patterns of Cs2AgBiCl6 and Cs2AgBiBr6 NC samples. HR-TEM images of (b) Cs2AgBiCl6 NCs and (c) Cs2AgBiBr6 NCs. The inset figures are enlarged images that show the lattice spacing of the individual NCs.   4   Quantitative analyses of the electronic structures and chemical properties of Cs2AgBiCl6 and Cs2AgBiBr6 NCs were performed by the combination of XPS and STEM-EDS analysis. The binding energies of XPS spectra were calibrated according to the signal of the C1s spectrum. Figure 2 shows the survey and narrow scan XPS spectra of NCs. The spectra color-coded with red and blue represent Cs2AgBiCl6 and Cs2AgBiBr6 NCs, respectively. Figure 2(a) confirms the elemental composition of Cs2AgBiCl6 with the presence of peaks of Cs, Ag, Bi, Cl, and Cs2AgBiBr6 NCs with the presence of peaks of Cs, Ag, Bi, and Br. In the case of Cs2AgBiCl6 NCs, the Cs3d doublet peaks for Cs2AgBiCl6 NCs detected at 738.0 eV (Cs3d3/2) and 724.0 eV (Cs3d5/2) correspond to the standard Cs element, as shown in Figure 2(b). The two peaks were disjoint with an energy value of 14 eV. Figure 2(c) exhibits the Ag3d XPS spectra of Cs2AgBiCl6 NCs with peaks at 373.8 eV (Ag3d3/2) and 367.8 eV (Ag3d5/2) associated with a monovalent oxidation state of Ag. The two peaks have an energy difference of 6 eV. The Bi4f5/2 and Bi4f7/2 peaks were located at 164.5 eV and 159.1 eV, as seen in Figure 2(d). Additional Bi peaks with lower binding energy were observed at 161.6 eV and 156.5 eV corresponding to the low valence state Bi(3-x)+, suggesting the formation of chlorine vacancies.42 At the binding energy of 199.1 eV and 197.6 eV, the Cl2p peak can be divided into two peaks originating from Cl2p1/2 and Cl2p3/2, respectively (Figure 2e). In the case of Cs2AgBiBr6 NCs, Figure 2(f) shows the Br3d doublet peaks at 68.9 eV and 68.0 eV which are attributed to Br3d3/2 and Br3d5/2, respectively. Compared the Cs2AgBiBr6 to the Cs2AgBiCl6, the peak of Cs3d and Ag3d shift 0.2 eV toward higher binding energy, while the shift of Bi4f is smaller (0.15 eV), which indicates that the X-Br (X = Cs, Ag, Bi) interactions in Cs2AgBiBr6 are stronger than the X-Cl (X = Cs, Ag, Bi) interactions in Cs2AgBiCl6, meanwhile, the Bi-Cl interactions are weaker than Cs-Cl and Ag-Cl. The UV-VIS spectroscopy was utilized to evaluate the optical properties and light absorption capability of the Cs2AgBiCl6 and Cs2AgBiBr6 NCs as shown in Figure 3(a). The absorption peaks account for the effective absorption of photons, which is potential for the capture of photocurrents of the photodiodes based on NCs. The sharp absorption peaks at around 368 nm and 425 nm are originated from the hybrid orbital of Bi6s/Ag4d defect level.43 The optical bandgaps were estimated from the absorption spectra by using Tauc’s plot method,44,45 as shown in the inset of Figure 3(a), red and blue dash lines are the tangent lines of the (αhv)1/n, where α is the absorption coefficient, h is Plank’s constant, v is the frequency of light, and the index n = 2 is related to the bandgap of indirect allowed transition semiconductors. The estimated bandgaps of the Cs2AgBiCl6 and Cs2AgBiBr6 NCs are 3.17 eV and 2.66 eV, respectively. In order to obtain information on the energy band for further application, the valence band minimum (VBM) and conduction band maximum (CBM) of the NCs were determined by using UPS. The UPS spectra of the secondary electron cutoff and valence band edge regions of both thin films are shown in Figure 3(b). The work function  Figure 2. XPS spectra of (a) wide scan, (b) Cs 3d, (c) Ag 3d, (d) Bi 4f, (e) Cl 2p, (f) Br 3d core level obtained for Cs2AgBiCl6 (red line/scatter) and Cs2AgBiBr6 (blue line/scatter) NCs. All the spectra are calibrated to 284.8 eV, corresponding to C 1s peak.   5   is calculated by the difference in energy between the secondary electron cutoff and the the excitation photon energy (hʋ = 21.22 eV). The low and high binding energy cutoff regions of the UPS spectra are -0.09 eV (Consest) and 15.44 eV (cutoffhigh) for the Cs2AgBiCl6 and 0.33 eV (Conset) and 15.34 eV (cutoffhigh) for the Cs2AgBiBr6, respectively. The energy level of Cs2AgBiCl6 NCs are -2.52 ~ -5.69 eV, and that of Cs2AgBiBr6 NCs is -3.55 ~ -6.21 eV, as shown in Figure 3(c). To examine the optoelectronic properties of photodiodes based on Cs2AgBiCl6 and Cs2AgBiBr6 NCs in our work, devices based on Cs2AgBiCl6, Cs2AgBiBr6, Cs2AgBiBr6/PCBM were fabricated, and their photo-response were measured using the same procedure. Figure 4(a)-(b) showed the device configuration and the energy band diagram of photodiodes based on Cs2AgBiCl6 NCs under voltage unbiased conditions. Electrons and holes as separated charge carriers are promoted by built-in electric field offered by the energy difference between the functional layers. In the multilayer, by utilizing the type-II band alignment between ZnO and the perovskite NC layers, electrons and holes could  Figure 3. (a) UV-VIS spectra and Tauc plot of Cs2AgBiCl6 and Cs2AgBiBr6 NCs. (b) UPS spectra of Cs2AgBiCl6 and Cs2AgBiBr6 thin film. The left shows the cutoff of energy, and the right figure shows the enlarged spectral parts near the Fermi edge. (c) Energy level diagrams showing the results for Cs2AgBiCl6 and Cs2AgBiBr6 NCs on an ITO substrate. All energy values are scaled to eV, EA: electron affinity, IE: ionization energy, EF: fermi level, Φ: work function.  Figure 4. (a) Device configuration of photodiodes based on Cs2AgBiCl6 NCs. (b) The energy level which is corresponding to the device structure. The green and blue arrows indicate the moving direction of electron and hole, respectively. (c) Current versus applied voltage of photodiodes irradiated by incident light with different wavelength. (d) Current versus applied voltage of photodiodes irradiated by 365 nm incident light with different power intensities. (e) Responsivity and detectivity versus the wavelength of incident light under a bias voltage of 0 V. (f) Responsivity and detectivity versus incident light intensity of photodiode at a bias voltage of 0 V under 365 nm illumination. Irradiated by Incident light of 365 nmIrradiated by Incident light of 4 mW1.8x1091.6x1091.4x1091.2x1091.0x1098.0x1086.0x108  6   separate on ultrafast time scales for detection. Figure 4(c) shows the I-V characteristics of the Cs2AgBiCl6-based PDs while varying the wavelength of incident light between 310 nm and 380 nm. The incident light intensity was fixed at 4 mW par a same irradiated area. As shown in the figure, the device featured a pronounced photo-response at both forward and reverse bias of voltage. Obviously, photocurrent was detected under light irradiation. The generated photocurrent enhanced the built-in electric field and separate the electron-hole pairs. In the figure, the photocurrent was the highest at 365 nm light irradiation but tended to decrease with irradiation of even longer wavelengths. This wavelength dependency was consistent with the optical absorption behaviour in Figure 3(a). Figure 4(d) shows the I-V characteristics under 365-nm light irradiation while varying the photon power from 1 mW to 4 mW. It was observed that the photocurrent was dependent of the photon power by almost same increment. The photocurrent under a bias voltage of 0 V was about twice larger than the dark current under the 365 nm illumination. As the typical figure-of-merits, responsivity (R) and specific detectivity (D*) identify the ability of a photodetector responding to a light radiation, and which level of light intensity can be detected, respectively.46 R is expressed by R = (Ilight - Idark)/Pin, where Pin represents the incident light power.47,48 The specific  Figure 5. Dynamic current-time response of the photodiode under bias voltages of 0 V (a) and 0.5 V (b). (c) Enlarged spectra of the rise time and fall time of the photodiode under the illumination of 365 nm.  Figure 6. (a) Device configuration of photodiodes based on Cs2AgBiBr6 NCs. (b) The energy level which is corresponding to the device structure. (c) Current versus applied voltage of photodiodes irradiated by 380 nm incident light with different power intensities. (d) Current versus applied voltage of photodiodes irradiated by incident light with different wavelength. (e) Responsivity and detectivity versus incident light intensity of photodiode under a bias voltage of 0 V. (f) Responsivity and detectivity versus the wavelength of incident light under a bias voltage of 0 V. Irradiated by Incident light of 4 mWIrradiated by Incident light of 380 nm9.0x1098.0x1097.0x1096.0x1095.0x109  7   detectivities were calculated on account of  measured photocurrent, dark current, and incident light intensity which extracted from the equation D* = Rλ(S/(2qIdark))1/2, where Rλ is the responsivity at a specific wavelength, the absolute charge value of electron is q, and S represents the effective irradiation area. 49 As shown in Figure 4(e)-(f), RA and D* of PD were achieved, it showed 0.22 mA/W of responsivity and 1.7 × 109 Jones of detectivity under 365 nm illumination at a bias voltage of 0 V. As shown in Figure 5, the wavelength-dependent time-resolved response characteristics demonstrate on/off switching under radiation at 310, 340, 365, and 380 nm, respectively, at bias voltages of (a) 0 V (a) and (b) 0.5 V. Rise and fall times are regarded as the period time for dark current increase to the 90% of the maximal photocurrent and photocurrent drop to 10% of the maximal value. The response speed of device was 25 and 25 msec of rise and fall time, respectively. When the active layer changed from Cs2AgBiCl6 NCs to Cs2AgBiBr6 NCs, the device exhibits different performance. Figure 6(a)-(b) shows the device configuration and the energy band diagram of photodiodes based on Cs2AgBiBr6 NCs under voltage unbiased conditions. Figure 6(c) shows the I-V characteristics of PDs while varying  Figure 7. (a) Dynamic current-time response of the photodiode under a bias voltage of 0 V irradiated by the incident light with different wavelength. (b) Dynamic current-time response of the photodiode under a bias voltage of 0 V irradiated by the 380 nm light with different power intensity. (c) Enlarged spectra of the rise time and fall time of the photodiode irradiated by the 0.14 mW of 380 nm light measured under a bias voltage of 0 V.  Figure 8. (a) Device configuration of photodiodes based on Cs2AgBiBr6 NCs with PCBM inserting layer. (b) The energy level which is corresponding to the device structure. (c) Current versus applied voltage of photodiodes irradiated by 380 nm incident light with different power intensities. (d) Current versus applied voltage of photodiodes irradiated by 4 mW incident light with different wavelength. (e) Responsivity and detectivity versus incident light intensity of photodiode under a bias voltage of 0 V. (f) Responsivity and detectivity versus the wavelength of incident light under a bias voltage of 0 V. (b)6.0x10107.0x10108.0x10109.0x10101.0x10111.1x10101.2x10101.3x1011Detectivity (Jones)Irradiated by Incident light of 380 nmIrradiated by Incident light of 4 mW  8   incident light intensity from 1 to 4 mW. It was observed that the light current intensity is in scale with the incident power density, consistent with the case of PD with Cs2AgBiCl6. The device showed significant photovoltaic effects with a maximum open circuit voltage (Voc) of ∼0.6 V. Figure 6(d) shows I-V characteristics measured at 4 mW while varying incident light wavelength between 310 nm and 490 nm. The highest photocurrent was obtained under 365 nm illumination whereas the photocurrent was the lowest under 490 nm illumination. The photocurrent difference between 365 nm and 490 nm might appear due to the absorbance difference between two wavelengths as shown in Figure 3(a). For the Cs2AgBiBr6 NCs-based PD, the photocurrent under a bias voltage of 0 V was around 50 time larger than the dark current under the 365 nm illumination. As shown in Figure 6(e)-(f), R and D* of PD were achieved, it showed 0.27 mA/W of responsivity and 9.94 × 109 Jones of detectivity under 380 nm illumination at a bias voltage of 0 V. These values were higher than those of Cs2AgBiCl6 NCs-based PD, which might be attributed to the less defect density of Cs2AgBiBr6 NCs and more efficient separation of electron-hole pairs. The wavelength dependent time-resolved response characters are shown in Figure 7(a), which indicates on/off switched properties under different illumination wavelength ranging from 310 to 490 nm at a bias voltage of 0 V. The photon power of the light was controlled to 20 mW/cm2.The wavelength-dependent response was consistent with the result shown in Figure 6(f). The on/off switched characteristics shown in Figure 7(b) were measured at a bias voltage of 0 V while varying photon power of incident light of 380 nm from 0.14 mW to 4 mW. Figure 7(c) exhibits a portion extracted from Figure 7(b). The response rise/fall time of device were 50/45 msec. The responsivity and detectivity were improved when compared to the device performance of Cs2AgBiCl6 NCs-based PD, but the response was a little slower. Generally, the NCs has the potential of quenching resulted from the interface defects, thus generating the trap which captures the carriers and slow the response. Further study is needed to determine the possible structural parameters that are responsible for defect formation. Interface engineering is an effective strategy to enhance the carrier transportation between adjacent nanocrystals. Here the PCBM was used as an interface modifier, 50 and was inserted between the active layer and ZnO layer. In this work, we used the PD with Cs2AgBiBr6 that exhibited the better performance than the device based on Cs2AgBiCl6. Figure 8(a)-(b) shows the device configuration and energy band diagram of photodiodes based on the Cs2AgBiBr6/PCBM under voltage unbiased conditions. The I-V curves of the device under various incident power densities were shown in Figure 8(c). The incident light wavelength was fixed at 380 nm. As expected, the photogenerated current density increased with incident light intensity. Clearly, the device exhibited the photo voltaic characteristics and the Voc has almost saturated at the measured minimum intensity of 1 mW. Figure 8(d) shows the I-V curves of PDs while varying the incident light wavelength between 310 nm and 490 nm. For the PCBM-modified PD, the photocurrent under a bias voltage of 0 V was around 150 time larger than the dark current under the 365 nm illumination. As shown in Figure 8(e)-(f), we obtained a 3.21 mA/W of responsivity and an 8.91 × 1010 Jones of detectivity when irradiated with 340-nm light at a bias voltage of 0 V. These values were much higher than those of non-modified Cs2AgBiBr6 NCs-based PD, possibly because the presence of PCBM creates a continuous gradient band structure as shown in Figure 8(b). Figure 9(a) exhibited the dynamic responses, which indicates on/off switched properties when irradiated with light of wavelengths from 310 to 490 nm at a bias voltage of 0 V. The photon power of incident light was tuned to 20 mW/cm2. The wavelength-dependent response was consistent with the result shown in Figure 8(f). Figure 9(b) shows the time dependent photo responses of the Cs2AgBiBr6/PCBM- Figure 9. (a) Dynamic current-time response of the photodiode under a bias voltage of 0 V irradiated by the incident light with different wavelength. (b) Dynamic current-time response of the photodiode under a bias voltage of 0 V when irradiated by the 380 nm light with different power intensity. (c) Enlarged spectra of the rise time and fall time of the photodiode irradiated by the 0.14 mW of 380 nm light measured at a bias voltage of 0 V.   9   based PD at a bias voltage of 0 V. Clearly the reproducible photocurrent response with a good cycling stability is observed. In addition, the PD responses to the UV-light with photon power as small as 0.14 mW. The photocurrent increases successively upon increasing the light irradiance power from 0.14 mW to 4 mW. Figure 9(c) exhibits on/off switched properties under radiation of 380 nm with different power intensity under a bias voltage of 0 V. The response rise/fall times of device were improved to 30/35 msec, respectively, which are faster than non-modified PDs (response speed of 50/45 msec). Table 1 shows a summary of the device performances of photodetectors using different forms of crystalline Cs2B(I)B’(III)X6 (B=Ag or Na; B’=Bi or In; X= Cl or Br) as optical absorption layers. There are some advantages of using perovskite crystals as the optical absorption layer for optoelectronic applications instead of semiconductor quantum dots (QDs). First, there is no need to create a core-shell structure that requires skillful synthesis. Second, for QDs it has been common replace the long ligands used for synthesis with short ligands or ions to increase the packing density of QDs in the active layer, thereby appearing the photoresponse property.54,55 In our case, the nanocrystals’ surfaces were terminated with long ligands such as OA and OLA for colloidal stability. Nevertheless, it should be noted that our active layer exhibits good photoresponsive features. Compared to the reported performance of devices operated at a bias voltage of 0 V shown in Table 1, our device was somewhat less responsivity and detectivity, with a response time that is an order of magnitude slower. This is probably due to the large interparticle distance created by the organic ligands, which reduces carrier mobility. To enhance the device performance, the carrier mobility must be increased based on the strategy of interface engineering. Furthermore, the energy loss that occurs before the incident light reaches the light absorbing layer must be minimized. For example, the use of quarts glass covered with transparent electrode with a large bandgap such as SnO2 is one of the procedures. Conclusion The photodiodes based on Cs2AgBiCl6 lead-free double perovskite NCs were prepared in our work which peculiarities proved to be promising by advantages of the simple composition engineering for halogen atoms to be replaced by the Br atoms from Cl atoms. The Cs2AgBiBr6 NCs with narrower bandgap were successfully synthesized which achieved the wavelength-selectivity when applied in photodiodes, additionally enhance the device performance. On the other hand, PCBM thin layer was inserting as an electron transport layer, the optimized photodiode exhibited a good self-powered performance which acted as the responsivity of 3.21 mA/W when the specific detectivity approached 8.91 × 1010 Jones, and a large light/dark current ratio near 200 was obtained. More importantly, spectral wavelength selectivity of Cs2AgBiBr6 NCs-based photodetector manifested the potential utilization in ultraviolet detecting, which point out potential application of lead-free double perovskite nanocrystals in next-generation photodiodes.  Acknowledgements  The authors thank Dr. OHSAWA Takeo of NIMS for technical support of the UPS measurements. This work was supported by JSPS KAKENHI Grant-in-Aid for Scientific Research (B) (Grant Numbers 21H01910 and 21H01743).    Table 1. Comparison of the device performance of UV photodetectors based on Cs2B(I)B’(III)X6 crystals reported in literatures (B=Ag or Na; B’= Bi or In; X= Cl or Br). Material Shape Responsivity R (mA/W) Detectivity D* (Jones) Response speed (msec) References Cs2AgBiBr6 2D nanosheet 110 at 0 V 2×1010 at 0 V 3 51 Cs2AgBiCl6 2D nanosheet 9.68 at 0 V 1.11×1012 at 0 V - 11 Cs2AgInCl6 Single crystal 13 at 5V 9.6×1011 at 5 V ~1.0 52 Cs2NaBiCl6 2D nanosheet 67.98 at 1.5 V 4.17×1011 at 1.5 V 16.82 53    10   References [1] Dou L, Yang Y (Micheal), You J, Hong Z, Chang W-H, Li G and Yang Y 2014 Solution-processed hybrid perovskite photodetectors with high detectivity Nat. Commun. 5 5404 [2]  Zhang W, Eperon G E and Snaith H J 2016 Metal halide perovskites for energy applications Nat. Energy 1 16048 [3]  Chen Q, Zhou H, Hong Z, Luo S, Duan H-S, Wang H-H, Liu Y, Li G and Yang Y 2014 Planar Heterojunction Perovskite Solar Cells via Vapor-Assisted Solution Process J. Am. Chem. 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