Photodetector Based on CsPbBr 3 /Cs 4 PbBr 6 Composite Nanocrystals with High Detectivity

: High-quality, all-inorganic CsPbBr 3 /Cs 4 PbBr 6 composite perovskite nanocrystals (NCs) were obtained with all-solution-processing at room temperature


Introduction
All-inorganic lead halide perovskites have shown excellent photoelectrical properties with high absorption coefficients, wide spectra, high carrier mobility values, and long charge diffusion lengths.Because of these advantages, they are widely used in PDs [1][2][3][4], solar cells [5][6][7][8], lasers [9][10][11], light-emitting diodes [12][13][14], and so on.In addition, compared with the organic-inorganic hybrid perovskite, all-inorganic lead halide perovskites have higher chemical stability to the environment because there are no organic ions in them.Among many kinds of all-inorganic lead halide perovskites, CsPbBr3 NCs demonstrate strong competitiveness for their excellent photoelectric characteristics, such as adjustable photoluminescence (PL) with narrow full-width at half maximum (FWHM) in visible light, an easy preparation process for their devices, and small size which causes a quantum size effect.Moreover, the CsPbBr3 NCs can be stored in air for 30 days with no obvious decrease in their photoelectric performance [15].Some research on optoelectronic devices based on CsPbBr3 NCs has been conducted.For example, Li et al. reported an interesting recyclable dissolution-recrystallization phenomenon of CsPbBr3 NCs and its application on the room temperature self-healing of compact and smooth carrier channels for PDs with D* of 6.1 × 10 10 Jones [16].Zeng et al. demonstrated a high-performance selfactuation PD, for which CsPbBr3 NCs were synthesized using room temperature saturation recrystallization, and the NC films were prepared by a centrifugal casting method.Compared with the drip-coating method, the photocurrent of the device increased threefold, and the optimized device had a high on/off ratio (>10 5 ) [15].However, the D* of these all-inorganic perovskite NC PDs is not high enough.Generally, the detectivity can be improved by increasing the bright current and suppressing the dark current.Additionally, improving the quality of nanocrystalline films to reduce defect states is vital for the suppression of the dark current.Quan et al. have found that embedding CsPbBr3 NCs in the Cs4PbBr6 matrix can prevent the agglomeration of CsPbBr3 NCs [17].This means that the presence of Cs4PbBr6 NCs could improve the quality of NC films.Thus, we prepared CsP-bBr3/Cs4PbBr6 composite NCs by controlling the proportion of PbBr2 in the synthesis process.Cs4PbBr6 is an indirect band gap semiconductor with a wide band gap (3.8 eV) [18], and Cs4PbBr6 NC is a zero-dimensional material, which absorbs light yet does not emit it [19].This non-luminous property can effectively reduce radiation recombination.In addition, it has been proved that the presence of Cs4PbBr6 will enhance the absorption of the dual-phase perovskite NCs in the UV spectrum [17,20,21].This is helpful for improving the detection performance of the PD in the UV spectrum.
Taking the above-mentioned issues into account, the saturation recrystallization method was utilized to synthesize the dual-phase NCs at room temperature.The dualphase NC film was spin-coated on the customized substrate and Au electrode.Moreover, the planar metal-semiconductor-metal (MSM) PDs based on CsPbBr3/Cs4PbBr6 composite NCs were realized.The proposed PD exhibited a high D* of 4.24 × 10 12 Jones, with an LDR of 115 dB under 1 V bias.To explain the improvement of D*, the role of Cs4PbBr6 NCs was investigated.Our work might provide a guidance for developing a high-performance PD based on CsPbBr3/Cs4PbBr6 perovskite NCs.
Synthesis of CsPbBr3/Cs4PbBr6 NCs: The saturation recrystallization method was utilized to synthesize the NCs at room temperature without any noble gases, and the specific synthesis steps are shown in Figure 1.Firstly, the beaker and the small glass bottle were each cleaned with deionized water, isopropanol, and absolute ethanol for 15 minutes, and then dried with high-purity nitrogen.After that, they were placed on a hot table at 80 °C to accelerate solvent evaporation and ensure no residual water.Then, 0.4 mmol of CsBr and 0.4 mmol of PbBr2 were dissolved in 10 mL of DMSO.A clear and transparent solution was obtained by ultrasonic cleaning, and no water entered the solution in this process.Then, 0.5 mL of OAm and 1 mL of OA were added as ligands of NCs to prepare a precursor solution.After that, 1 mL of the above precursor solution was dribbled into the 10 mL of toluene solution with a fast speed of 800 r/min.In this way, various inorganic ions were transferred from the benign solvent to the undesirable solvent, and the NCs were deposited due to the much higher solubility of NCs in DMSO than in DMSO mixed with toluene.

Fabrication of PD:
The fabrication of the device was mainly based on the gold interdigital electrode and the inorganic perovskite NCs.The schematic diagram of the fabrication process is illustrated in Figure 1.The interdigital space of the gold interdigital electrode is 3 μm and the thickness of the Au electrode is about 100 ± 10 nm.The substrate is made up of silicon (Si) wafer, SiO2, and chromium (Cr) plating.A 300 nm-thick SiO2 layer covers the surface of the silicon wafer.Additionally, there is a Cr plating layer with a thickness of 10 nm between the SiO2 layer and the Au electrode.The planar PD was fabricated in a glove box by spin-coating.The NC solution was spin-coated on the substrate with the rotational speed of 2000 r/min.The device was then placed on a hot platform at 90 °C to vaporize the methylbenzene and DMSO.

Characteristics of nanocrystals:
The photoelectric property of NCs was tested by a light-emitting diode (LED) light source (375 nm, Thorlabs, Shanghai, China).The optical image of NCs was characterized using an optical microscope (Nikon, LV150, Shanghai, China).The high-resolution transmission electron microscopy (HRTEM) images of NCs were collected using a JEOL JEM-2100 microscope (Nippon Electronics Corporation, Shanghai, China) at an accelerating voltage of 100 kV.The X-ray diffraction (XRD) spectrum of the NCs was measured by a diffractometer (Haoyuan Instrument Co., Ltd., DX-2700, Dandong, China).The absorption spectrum of NCs was determined by an ultraviolet-visible absorption spectrometer (Shimadzu, UV-2600, Shimadzu, Japan).The photoluminescence spectrum at room temperature was carried out by a home-built fluorescence spectrophotometer system using a 343 nm femtosecond laser (Light Conversion, Carbide 5W, Vilnius, Lithuania) as the excitation light source.

Photoelectric characteristics:
The PD was placed on the probe table (Prcbe, Mini), and the external shielding box was used to isolate the external optical signals throughout the test.The current-voltage (I-V) curves and the transient photo responses of the PD based on inorganic perovskite NCs were tested using a semiconductor analyzer (Agilent, B1500, Shanghai, China).For completing the I-V measurements, the light intensity was tuned by adjusting the LED light source (375 nm, Thorlabs, Shanghai, China) at a fixed driving voltage.The transient photo responses were acquired with an LED lamp, which emitted light faster than the response of the measured PDs.

Composition of NCs
To determine the composition of NCs, XRD measurements were carried out, as shown in Figure 2a.The peaks at 15.185°, 21.551°, 30.644°, 34.371° and 37.767° can be indexed to the (100), ( 110), ( 200), ( 210) and (211) lattice planes, respectively, which are the characteristic peaks of the CsPbBr3 phase.On the other hand, the peaks at 12.885°, 20.079°, 22.413°, 25.427°, 27.510°, 28.603° and 30.268° can be identified as the (110), ( 113), (300), (024), ( 131), ( 214) and (223) reflections, respectively, which are the characteristic peaks of Cs4PbBr6 [18,21].In addition, the intensity of the diffraction peaks of Cs4PbBr6 is much higher than those of CsPbBr3, which means that the main component of the NCs is Cs4PbBr6.To further investigate the composition of the NCs, the HRTEM images were obtained and shown in Figure 2b, and the selected area corresponding to Figure 2b is illustrated in Figure 2c.It can be found that the lattice fringes of 0.68 nm can be indexed to the (110) lattice planes of orthorhombic Cs4PbBr6, while the lattice fringes of 0.29 nm can be indexed to the (200) lattice planes of cubic CsPbBr3.Thus, the dual-phase was also evidenced by the selected area TEM of dual-phase NCs.

Optical Property of NCs
In order to investigate the optical property of NCs, its absorption and PL spectrum were tested, as shown in Figure 3a,b.We found that there were two absorption peaks in the absorption spectrum.One peak was located at 505 nm, and the other one was located at 316 nm.According to the existing research on CsPbBr3, we know that the absorption peak at 505 nm is attributed to CsPbBr3.Thus, it can be inferred that the absorption peak at 316 nm was caused by Cs4PbBr6.In addition, the absorption band edges of CsPbBr3 and Cs4PbBr6 NCs are 360 nm and 547 nm, respectively.Based on this, the band gaps of CsP-bBr3 and Cs4PbBr6 NCs were calculated, which were 2.3 eV and 3.8 eV, respectively.This is in agreement with the literature [18].For the PL spectrum (shown in Figure 3b), a narrow emission peak was observed at 520 nm with a FWHM of 26 nm, which is 3.1 nm lower that of pure CsPbBr3 NCs (29.1 nm) [22].This suggests that the quality of NC films was improved by the introduction of Cs4PbBr6 NCs.It is worth noting that there was only one PL peak that originated from CsPbBr3 NCs, which is consistent with the previous studies' finding that Cs4PbBr6 does not emit light [23].Moreover, the PL peak was in the green light emission band, which echoes the luminescence of the sample shown in the inset of Figure 3b.CsPbBr3/Cs4PbBr6 NCs in toluene (illustration) were yellow under sunlight.However, when exposed to ultraviolet light, the CsPbBr3/Cs4PbBr6 NCs in toluene emitted bright green light.This is also consistent with the reported emission band of CsP-bBr3/Cs4PbBr6 NCs [21].

Performance of the Composite NC PD
The dual-phase inorganic perovskite NC film was spin-coated on the gold interdigital electrodes, and the planar MSM PDs were prepared.The I-V curves of the proposed PD under 375 nm LED illumination with different light intensities are shown in Figure 4a.It can be found that the photocurrent increased with the rise in the optical power density.Moreover, the dark current at 1 V bias was 6.67 pA, which was significantly suppressed compared with that of the PD based on pure CsPbBr3 NCs (1.5 nA at 2 V bias) [15].It suggests that the quality of NC film was improved by the introduction of Cs4PbBr6 NCs, which could prevent the agglomeration of CsPbBr3 NCs.The light current with a light intensity of 10.2 mW/cm 2 at 1V bias was 19.32 nA and the ratio of light current to dark current was 2894 at 1 V bias.This value was much lower than that of the PD based on CsPbBr3 NCs (10 5 under 4.65 mW/cm 2 at 2 V bias), which meant that the bright current was weakened by the introduction of Cs4PbBr6 NCs.The external quantum efficiency (ηEQE), responsivity (R), and D* of the proposed PD were calculated and the results are shown in Figure 4c-e.As the power density of the incident light increased from 13 μW/cm 2 to 2689 mW/cm 2 , the ηEQE decreased continuously.At the lowest detectable illumination of 13 μW/cm 2 at a bias of 1V, the ηEQE was only 22.1%.This value is also lower than the PDs based on CsPbBr3 NCs (41%) [16].These phenomena may be due to the fact that the main component of dual-phase NC film is Cs4PbBr6.Moreover, the wide band gap of Cs4PbBr6 determines that the absorption wavelength of Cs4PbBr6 must be lower than 360 nm.However, due to the limitation of the experimental conditions, our test wavelength could only reach 375 nm.Thus, the light absorption intensity and the amount of photogenerated carriers of the PD based on CsPbBr3/Cs4PbBr6 composite NCs were correspondingly reduced under the illumination of 375 nm.This also led to low R, which was only 0.094 A/W under the optical power of 13 μW/cm 2 .The expression of R was (IL is photocurrent, Id is dark current, Pin is incident optical power density, S is effective area).Although the PD had an ultra-low dark current, the light current was not satisfied, which led to a small R.However, compared with the previous reported MSM PDs based on CsP-bBr3 NCs (summarized in Table 1), the D* of the proposed PD was among the highest level, which reached 4.24 × 10 12 Jones under the optical power of 13 μW/cm 2 .Due to the great improvement of D*, it could be expected that the LDR would also be improved.
Here, the LDR of the proposed PD under 1 V bias with 532 nm continuous laser illumination were shown in Figure 4b.It can be found that the proposed PD was capable of detecting incident light as low as 13 μW/cm 2 under 1 V bias, and its LDR was 115 dB.In order to explain the reason for the improvement of D*, the type of contact between the semiconductor and the metal electrode was analyzed.Due to the high work function of Au, it can be expected that the interface between the inorganic perovskite NCs and the Au electrode will form a good ohmic contact [15].This prediction was proved by the experiment.The I-V curve at different biases were measured, and Figure 4f shows the change trend of the dark current after taking the logarithm of the x-axis and y-axis.It is clear that the current increased significantly with the increase in bias, and the slope (n1) of the curve is 1.07, which is almost linear.This means that a good ohmic contact was formed between the dual-phase NCs and Au electrode, providing a guarantee for the rapid transfer of the carrier.On the other hand, the band gaps of CsPbBr3 and Cs4PbBr6 were 2.3 eV and 3.8 eV, respectively, and both the VBM and CBM of Cs4PbBr6 were higher than those of CsPbBr3.Thus, by combining CsPbBr3 NCs with Cs4PbBr6 NCs, the energy band of the composite NCs would be moved up as a whole relative to the Au electrode.Moreover, the height difference between the VBM of the composite NCs and the Fermi energy of the Au electrode would be reduced, which facilitates the transmission of electrons.This is also helpful for the improvement of D*.In addition, both the strongly suppressed dark current and the narrowed FWHM of the PL spectrum proved that the quality of the NC film was indeed greatly improved with the introduction of Cs4PbBr6 NCs.Furthermore, the radiation recombination could also be suppressed due to the non-luminous property of Cs4PbBr6 NCs.All these behaviors led to the improvement of D*.
To study the photoresponse of the composite NC PD, the transient response performance under 375 nm with a light intensity of 10.2 mW/cm 2 at 1 V bias were measured (shown in Figure 5a), which indicated that the proposed PD could respond stably when the illumination was turned on and off.Moreover, response speed was another important index of the photodetector, and the transient photocurrent response relationship (I-T) curve of the proposed PD under 1 V bias is shown in Figure 5b.According to the definitions of the rise time (Trise) and fall time (Tfall) for the time when the photocurrent increased from 10% to 90% (declined from 90% to 10%) in the on and off cycles under light, Trise and Tfall were 10.85 ms and 2.25 ms, respectively.The sum of the rise time and the fall time was considered the response time of the PD, which was 13.1 ms.

Conclusions
In summary, we have successfully demonstrated all-solution-processed PDs based on CsPbBr3/Cs4PbBr6 composite NCs.A good ohmic contact was formed between the gold electrode and CsPbBr3/Cs4PbBr6 composite NCs, which provided a guarantee for the rapid transfer of the carrier.In addition, both the strongly suppressed dark current and the narrowed FWHM of the PL spectrum proved that the quality of the NC film was greatly improved with the introduction of Cs4PbBr6 NCs.Moreover, the non-luminous property of Cs4PbBr6 NCs could inhibit the radiation recombination.Thus, the D* of the PD based on CsPbBr3/Cs4PbBr6 composite NCs was improved, reaching 4.24 × 10 12 Jones.Our work might provide guidance for developing a high-performance PD based on CsP-bBr3/Cs4PbBr6 perovskite NCs.

Figure 1 .
Figure 1.Schematic diagram of the fabrication process of the PD based on CsPbBr3/Cs4PbBr6 NCs.

Figure 2 .
Figure 2. (a) XRD patterns of the dual-phase CsPbBr3/Cs4PbBr6 NCs; (b) HRTEM image of the dualphase inorganic perovskite NC film, inset: Fast Fourier transform (FFT) image of the dual-phase inorganic perovskite NC film; (c) HRTEM corresponding to Figure 2b.

Figure 3 .
Figure 3. (a) Absorption spectrum of CsPbBr3/Cs4PbBr6 NCs; (b) PL spectrum of CsPbBr3/Cs4PbBr6 NCs, insets show NC solution illuminated by a fluorescent lamp and a 375 nm LED lamp, respectively.

Figure 4 .
Figure 4. (a) I-V curves of CsPbBr3/Cs4PbBr6 NC PD with different optical power densities under the irradiation of a 375 nm LED lamp; (b-e) LDR, ηEQE, R, and D* measured using a 532 nm laser as a light source; (f) Change trend of the dark current after taking the logarithm of the x-axis and yaxis, the red line is the fitting curve.

Figure 5 .
Figure 5. (a) Photocurrent-time response measured under 375 nm with a light intensity of 10.2 mW/cm 2 ; (b) Rise and fall time of the proposed PD.

Table 1 .
Summary of the performance of inorganic perovskite NC PDs.