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Abstract
Ultraviolet (UV) photodetector plays an important role in military, civilian and people's daily life, and is an indispensable part of spectral detection. However, photodetectors target at the UVB region (280-320 nm) are rarely reported, and the devices detected by medium-wave UV light generally have problems such as low detection rate, low sensitivity, and poor stability, which are difficult to meet the market application needs. Herein, Cs-Cu-I films with mixed-phase have been prepared by vacuum thermal evaporation. By adjusting the proportion of evaporation sources (CsI and CuI), the optical bandgaps of mixed-phase Cs-Cu-I films can be tuned between 3.7 eV and 4.1 eV. This absorption cut-off edge is exactly at both ends of the UVB band, which indicating its potential application in the field of UVB detection. Finally, the photodetectors based on Cs-Cu-I/n-Si heterojunction are fabricated. The photodetector shows good spectral selectivity for UVB band, and has a photoresponsivity of 22 mA/W, a specific detectivity of 1.83*1011 Jones, an EQE over 8.7% and an on/off ratio above 20.
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1. Introduction
Electromagnetic radiation in the X-ray, ultraviolet and visible range is widely utilized in various fields such as industrial, agriculture, medicine and daily life. Ultraviolet radiation (UV) consists of three wave bands with different wavelengths and energies: UVC (100-280 nm), UVB (280-320 nm), and UVA (320-390 nm) [1–3]. Among them, UVC is the most dangerous part of UV radiation with the highest energy. Fortunately, it is fully absorbed by atmospheric oxygen (O2) and stratospheric ozone (O3), while UVB radiation (also named medium-wave UV radiation) is mostly absorbed by O3 and UVA is largely completely transmitted to the Earth's surface [4]. The ozone layer has been declining due to gaseous pollutants such as chlorofluorocarbons, chloroform, hydrochlorofluorocarbons, carbon tetrachloride, methyl bromide and reactive nitrogen (nitric oxide, nitrous oxide, etc.) for nearly four decades [5,6]. This significant destruction of O3 reduces UV absorption efficiency causing more radiation reaches the Earth's surface (1% reduction in O3 results in 1.3-1.8% increase in UVB on the Earth's surface) [7–9]. UV radiation have a far-reaching impact on human survival and development. Moderate exposure of the skin to natural or artificial UV radiation is beneficial to health by promoting the synthesis of vitamin D, killing bacteria, and treating or preventing rickets. In contrast, excessive UVB will lead to various diseases such as cataracts and skin cancer, and can even accelerate the aging process. Besides that, the yield of crops is also strongly affected by UV radiation. Therefore, the detection of medium-wave UV radiation is of great significance in the fields of health and life sciences [10].
With the development of new generation semiconductors, newly developed materials, improved manufacturing processes, and the invention of nanotechnology provide huge opportunities for the development of next generation UV detectors. Although UVB detectors have been explored through alloy semiconductor manufacturing technology, semiconductor materials with a bandgap range precisely in the UVB band are scarce [11]. This leads to the problems such as low detection rate, low sensitivity, and weak stability in the devices that currently used for UVB detection, which are difficult to meet the market application requirements. Recently, metal halide perovskites have received extensive attention and research as an emerging semiconductor material. It has become a candidate material for new generation optoelectronic devices owing to its large absorption coefficient, high carrier mobility, adjustable band gap, and high photoluminescence quantum yield. The progress of perovskites in the field of detectors has been very rapid. Huang et al. reported a two-dimensional perovskite detector with responsivity up to 923 A/W [12], Nurul et al. reported a long-lifetime organic-inorganic hybrid perovskite detector [13], Chaohao Chen et al. reported a perovskite photodetector-based single pixel color camera for artificial vision [14], Shou Liu et al. reported a perovskite photodetector for encrypted optical communication [15], and Qingshan Fan et al. reported a perovskite photodetector for RGB detection application [16]. However, the previously reported devices have mainly focused on visible/infrared light detection until now, while the perovskite photodetectors targeting the UV region have rarely been reported. In addition, most previous studies have used traditional lead halide perovskites, whose environmental instability and toxicity limit their further commercialization [17–20]. The development of new environmentally friendly materials that do not contain organic lead halide compounds is essential for medium-wave UV detection technology. Z. Zang’s group reported a series of inorganic copper halide perovskite devices with excellent stability, which have great application potential in the field of detection and imaging [21–23]. All-inorganic lead-free perovskites Cs-Cu-I have two phases, CsCu2I3 and Cs3Cu2I5, both of which have direct bandgaps [24,25]. The optical bandgaps obtained in the experiment are 3.7 eV and 4.1 eV, and the corresponding absorption edges are located at 330 nm and 280 nm, respectively [26,27]. This absorption cut-off edge is exactly at both ends of the UVB band, indicating its potential application in the field of UVB detection. Compared to oxide semiconductor, the preparation of Cs-Cu-I perovskites is convenient and the process is simple. We can obtain high-quality thin films under simple experimental conditions, which is conducive to the realization of industrialization.
In this work, the molten mixture of CsI and CuI powder is evaporated onto the substrate to form uniform and dense thin films by vacuum thermal evaporation. Here, we realize the composition changes of CsCu2I3 and Cs3Cu2I5 phases in thin films by adjusting the molar ratio of CsI and CuI in raw powder. Based on the obtained high quality films, we designed the Cs-Cu-I/n-Si detector and the influence of phase ratio changes on its photoelectric detection performance was studied. UV detector with good spectral selectivity for UVB band was obtained in this study. The experimental results show that CsCu2I3 and Cs3Cu2I5 are reliable candidate for UV photodetectors.
2. Materials and methods
2.1 Materials
Cesium iodide (CsI, Alfa Aesar, 99.998%), Copper iodide (CuI, Alfa Aesar, 99.998%). They were used without further purification.
2.2 Preparation of Cs-Cu-I thin films
CsI and CuI powders were weighed according to the molar ratio, and ground in an agate mortar for 30 minutes to ensure thorough mixing. The mixed powder was then placed in a crucible and heated until all the powders melted, followed by further heating to evaporate the molten mixture onto the substrate. The growth conditions were as follows: vacuum pressure of 4.5-3.5*10−4 Pa, evaporation rate of 0.2-0.3 Å/s, and nominal thickness of 400 nm.
2.3 Construction of Cs-Cu-I/n-Si heterojunction photodetector
Au electrodes were deposited on the Cs-Cu-I film and n-Si by thermal evaporation to form ohmic contacts.
2.4 Characterization
XRD images were collected using a Bruker AXS D8 ADVANCE; SEM images were collected using a Nova Nano-SEM 450 field emission scanning electron microscope; photodetection and electrical performance tests were conducted using a 150W Xe lamp (RF5301PC, SHIMADZU Corporation, 150W) with a grating monochromator as the light source, and current-voltage data were collected using a KEITHLEY 2612B digital source meter connected to a probe station.
3. Results and discussion
Figure 1(a) illustrates the preparation process of Cs-Cu-I perovskite films by vacuum thermal evaporation. In this work, mixed powders of CsI and CuI with molar ratios of 0.5:1, 0.6:1, 0.8:1, 1:1, 1.2:1, 1.4:1, and 1.5:1 were selected as the evaporation sources. They were evaporated onto the substrate to produce uniform and dense perovskite films after heating the mixed powders to a molten state in vacuum.
Fig. 1. (a) Schematic diagram of preparation of all-inorganic lead-free Cs-Cu-I perovskite films. (b) XRD spectrum of Cs-Cu-I perovskite films grown with different molar ratios. SEM image of Cs-Cu-I films grown with different molar ratios: (c) CsI:CuI = 0.5:1, (d) CsI:CuI = 0.8:1, (e) CsI:CuI = 1.2:1, (f) CsI:CuI = 1.5:1.
X-ray diffraction (XRD) patterns are used to study the phase changes in Cs-Cu-I thin films. In Fig. 1(b), the main diffraction peaks at 26.189°, 26.457° and 29.395° correspond to (221), (131) and (002) planes of CsCu2I3. With the increase of CsI molar ratio in raw materials, the diffraction peaks of Cs3Cu2I5 appear. The main diffraction peaks at 24.734°, 26.345° and 27.059° correspond to (400), (222) and (130) planes of Cs3Cu2I5. It can be seen that in the case of CsI:CuI molar ratio of 0.5:1, there are diffraction peaks of γ-CuI and CsCu2I3 in the XRD pattern. When the CsI:CuI molar ratio increased to 1.5:1, the diffraction peak of CsCu2I3 is almost absent. The XRD pattern clearly reflects the variation of diffraction peak intensity of CsCu2I3 and Cs3Cu2I5 in the mixed-phase films with the change of CsI:CuI molar ratio. Fig. S4 shows the variation of diffraction peak intensity of CsCu2I3-(221) and Cs3Cu2I5-(222) with the change of CsI:CuI molar ratio.
The morphology of the Cs-Cu-I mixed-phase films with different CsI:CuI molar ratios was characterized by scanning electron microscope (SEM). Figure 1(c-f) and Fig. S1-S3 shows the Cs-Cu-I films with CsI:CuI molar ratio of 0.5:1, 0.8:1, 1.2:1, 1.5:1, 0.6:1, 1:1, 1.4:1, respectively. The Cs-Cu-I films prepared in this work are dense, uniform, smooth and without obvious pinholes, which can meet the needs of manufacturing semiconductor optoelectronic devices.
Figure 2(a). Two obviously optical absorption edges are observed at 311 nm (3.99 eV) and 284 nm (4.37 eV), these peaks are originated from the band transition from the valence band maximum to the conduction band minimum of CsCu2I3 and Cs3Cu2I5, respectively. In addition, a low-energy absorption peak is also found at 322 nm (3.85 eV), which would be assigned to the exciton absorption of CsCu2I3 [28]. This is consistent with the reported results by Li et al [29]. With the increase of CsI, the intensity of the absorption peak of CsCu2I3 gradually decreases and that of Cs3Cu2I5 gradually increases, which is consistent with the results of the XRD pattern. When the CsI:CuI mole ratio is 1.2:1, the absorption peak intensity of Cs3Cu2I5 exceeds that of CsCu2I3. The optical band gap changes of Cs-Cu-I films with different components were calculated by Tauc equation. It can be seen in the inset of Fig. 2(a) that with the increase of CsI molar ratio, the optical band gap of the film increases.
Fig. 2. (a) The absorption spectra of Cs-Cu-I films with different molar ratios. (b) The photoluminescence (PL) spectra of Cs-Cu-I films. (c) The transmittance spectra of Cs-Cu-I films.
Figure 2(b) shows the photoluminescence (PL) spectra of Cs-Cu-I films excited by a 310 nm xenon lamp. CsCu2I3 exhibits a yellow-green emission peak centered around 552 nm, while Cs3Cu2I5 shows a blue emission peak centered around 445 nm. With the increase of CsI molar ratio, the emission peak of CsCu2I3 gradually weakens, and that of Cs3Cu2I5 increases, which is consistent with the results shown in the XRD spectra and absorption spectra. The transmittance spectra of the Cs-Cu-I films was shown in Fig. 2(c), with a transmittance exceeds 75% in the visible range.
Based on the above high-quality Cs-Cu-I films, we have fabricated the Cs-Cu-I/n-Si heterojunction detector. And Fig. 3(a) shows the schematic diagram of the device structure. The current-voltage (I-V) curves of the Cs-Cu-I/n-Si heterojunction detector under dark conditions are shown in Fig. 3(b-h). The detector exhibits significant rectification characteristics and has a high on/off ratio in the negative bias section. For heterojunction photodetectors, the responsivity and detectivity can be expressed as [30–32]:
Fig. 3. (a) The schematic diagram of the Cs-Cu-I/n-Si heterojunction detector device structure. (b)-(h) The current-voltage (I-V) curves of the Cs-Cu-I/n-Si heterojunction detector with different molar ratios under different lighting conditions.
R represents the responsivity, Iph is the photocurrent, Plight is the optical power, D* is the detectivity, q is the elementary charge (1.602*10−19 C), Idark is the dark current, and S is the effective area, h is Planck’s constant (6.626*10−34 J·s), c is the speed of light (2.998*108 m/s), and λ is the wavelength of incident light. According to the formula, the responsivity and detectivity curves of Cs-Cu-I/n-Si heterojunction detectors with different molar ratios are shown in Fig. 4(a-b).
Fig. 4. (a) The responsivity and (b) detectivity curves of Cs-Cu-I/n-Si heterojunction detectors with different molar ratios. (c) The EQE curves and (d) time-dependent photoresponse curves at 2 V of Cs-Cu-I/n-Si heterojunction detector with CsI:CuI = 1.2:1.
CsCu2I3 is sensitive to light centered at 330 nm, while Cs3Cu2I5 is sensitive to light centered at 280 nm. With the molar ratio of CsI increases, the Cs-Cu-I/n-Si heterojunction detector shows a decrease in sensitivity to 330 nm light and an increase in sensitivity to 280 nm light. As shown in Fig. 4(a-c), the detector has good spectral selectivity for 280-330 nm light, with a response above 22 mA/W, a EQE over 8.7% and a detectivity above 1.83*1011 Jones when the CsI:CuI molar ratio is 1.2:1. Considering the overall responsivity and detection rate of the UVB band are higher than those of other bands and better uniformity, we believe that the photodetector performance is optimal when the CsI:CuI molar ratio is 1.2:1. In Fig. 4(d), the Cs-Cu-I/n-Si heterojunction detectors exhibits good response to 280-330 nm light.
On the other hand, we found that CsCu2I3 gradually decreasing and Cs3Cu2I5 gradually became the main phase with the increase of CsI. As shown in Fig. S5, the (221) crystal plane of CsCu2I3 has a higher lattice compatibility with the Si (100) crystal plane. The reduction of CsCu2I3 phase may lead to the generation of interface defects. It can be clearly seen that there are some pores at the interface in Fig. S6. We also plotted the ideal factors of the device under different wavelengths of irradiation. As shown in Fig. S7, the ideal factors for all devices are between 3.3 and 3.5, which also indicates the presence of interface defects while the effects of the interface defects are manageable. These interface imperfections lead to significant recombination of charge carriers, reducing the response and detection rates of the device, leading to instability under high current conditions, and generating a large amount of heat during long-term operation.
The band diagram based on the Anderson model will help to understand the photo response and carrier transport mechanism of CsCu2I3 and Cs3Cu2I5 with n-Si. The band gap-electron affinity of CsCu2I3 and Cs3Cu2I5 are 3.71 eV-2.52 eV and 4.06 eV-1.85 eV, respectively [33–35]. n-Si substrates used for the devices have a band gap of 1.12 eV and an electron affinity of 4.05 eV [36,37]. According to the Anderson model, the energy band diagrams for the CsCu2I3/n-Si and Cs3Cu2I5/n-Si heterojunction detectors are shown in Fig. 5. It can be seen that the heterojunctions formed by CsCu2I3 and Cs3Cu2I5 with n-Si are both of type-I band alignment (straddling gap) with a large band offset. For electronic devices such as photodetectors, type-I band alignment is commonly used. This band alignment can effectively confine photogenerated charge carriers and reduce the leakage current, which is beneficial for the detection of weak signals. When the surface of the photodetector is illuminated by light, photo-generated electrons and holes are generated in the Cs-Cu-I/n-Si heterojunction. The large energy band offset and built-in electric field prevent the free recombination of photo-generated carriers. Driven by the built-in electric field, photo-generated carriers can quickly separate and flow to the electrode to form a photocurrent.
For a long time, the instability of halide photo detectors component and structure limited its practical application. Therefore, a series of tests were conducted on stability for the Cs-Cu-I films and Cs-Cu-I/n-Si heterojunction detectors. As shown in Fig. 6(a), after stored in the air for 120 days, in the XRD patterns there are no diffraction peaks appear or disappear, which indicating that the film has a good structural stability. In Fig. 6(b), the time-dependent photoresponse curves of the photodetector is still relatively stable after 120 days of storage in the atmospheric environment. In Fig. 6(b), the current intensity increases over time. It is reported that there is serious ion migration phenomenon in CsCu2I3 [38]. Meanwhile, we found that the temperature of the device significantly increased after continuous operation for a period of time, which may lead to an increase in the internal carrier concentration of the semiconductor and exacerbate ion migration phenomena. The continuous increase of photocurrent intensity may be the result of the combined effect of the above factors.
Fig. 6. (a) The XRD spectrum of Cs-Cu-I perovskite films and (b) time-dependent photoresponse curves of Cs-Cu-I/n-Si heterojunction detectors at 0 V-300 nm with CsI : CuI = 1.2:1 after exposure to ambient conditions for 120 days.
Figure 7(a) shows the schematic diagram of the ultraviolet imaging array system. Under the irradiation of 300 nm UVB light, the pattern on the mask was projected onto the surface of the photodetector array (pixel array is 5 × 5). Then, the photodetector will send the current corresponding to all positions to the computer for counting. Figure 7(b) shows the image based on photocurrent value. The pattern and the difference caused by light intensity can be observed clearly, indicating that the photodetectors are suitable for UVB light image sensing applications.
Fig. 7. (a) The schematic diagram of the ultraviolet imaging array system. (b) The simulated image based on photocurrent value.
4. Conclusion
In summary, Cs-Cu-I films with different phase ratios were prepared by vacuum thermal evaporation and Cs-Cu-I/n-Si heterojunction photodetectors were constructed by adjusting the molar ratio of CsI and CuI powders in this paper. The Cs-Cu-I films prepared in this work are dense, uniform, smooth and without obvious pinholes. CsCu2I3 gradually decreasing and Cs3Cu2I5 gradually became the main phase with the increase of CsI. The optical and electrical properties of mixed-phase Cs-Cu-I films were systematically studied. When CsI:CuI molar ratio was 1.2:1, the detector with good spectral selectivity for UVB band was obtained. The photodetector exhibits good environmental stability with a photoresponsivity of 22 mA/W, a specific detectivity of 1.83*1011 Jones, an EQE over 8.7% and an on/off ratio above 20. Meanwhile, the heterojunction photodetectors exhibit excellent reproducibility and stability. These all indicate that Cs-Cu-I is a highly promising candidate in the field of UVB detection and imaging.
Funding
Taishan Scholar Foundation of Shandong Province (tsqn202306239); National Natural Science Foundation of China (No. 12274189, No. 62075092); Natural Science Foundation of Shandong Province (No. ZR2021MF121, No. ZR2022MA045, No. ZR2022QF052, No. ZR2023QE197).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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