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Abstract
CuI film was grown by thermal evaporation technology on TiO2 nanorods array synthesized using a hydrothermal method, and a p-CuI/n-TiO2 heterostructure photodetector was constructed. The structure, morphology, light absorption, and photoresponse performance of the device were investigated. The heterojunction detector is self-powered and sensitive to light in the range of 320nm∼450nm. At 0V, the on/off ratio of the device is ∼770. The peak responsivity (0V, 410nm) is about 4.5mA/W and the peak detectivity is 1.08×1011 Jones. Also, the reproducibility and stability of the heterojunction photodetector are excellent. This work provides an effective route for the study of self-powered photodetectors.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Photodetection technology, utilizing light to generate detectable electric output, has been widely used in many modern technological applications, including advanced optical communication, imaging, environmental monitoring, space exploration and other fields [1–5]. In general, most traditional photodetectors (PDs) require an external power supply, such as batteries and other power supply systems, to drive the photogenerated carriers to separate and generate photocurrent [6,7], which severely limits their applications due to their large size, irremovability and nonindependence [8–11]. Consequently, self-powered photodetectors driven by a built-in electric field have been developed and become the most widely studied photodetectors in recent years, which can decrease local perturbation of the depletion region without power supply and increase response speed, save energy and reduce device size. Based on different built-in electric fields, self-powered photodetectors are classified into three types: Schottky junction type [12–14], photoelectrochemical cell (PECC) type, and p-n junction type [15–18]. Among them, the p-n junction type has attracted more attention due to its simple structure, easy of production, and effective suppression of photogenerated electron-hole pair recombination [19,20]. To date, numerous studies had been conducted to improve the performance of self-powered p-n junction photodetectors.
In the past several years, the all-inorganic PDs based on wide bandgap semiconductors (Eg > 3.0eV in general), such as TiO2, ZnO, SnO2, ZnS and GaN [8,14,21,22], have drawn a lot of attention due to their small size, high quantum efficiency, excellent linear performance and long-term stability. Especially, TiO2 nanostructures, including nanotubes, nanowires and nanorods, have been widely studied and become the preferred material for producing highly sensitive photodetectors because of their low cost, non-toxic, stable structure, the high UV absorption coefficient. Furthermore, TiO2 single-crystal nanorods can provide a pathway for the photogenerated carriers and boost the transport rate, thus improving the performance of photovoltaic devices [23–29]. However, the TiO2 nanostructures-based photodetectors are suffering the large dark current, low sensitivity and slow photoresponse speed arising from the self-recombination of surface and defect states [30,31]. Thanks to the built-in electric field, the pn junction devices can enhance the carrier separation efficiency, and thus improve the response speed, suppress the self-recombination of carriers and reduce the dark current [32]. The heterojunction structures based on TiO2 nanostructures have been performed [33–36]. In addition, using TiO2 and suitable materials, self-powered heterojunction PDs with a large built-in field can be obtained. And γ-CuI is an excellent candidate material. Owing to the wide bandgap (3.1eV) [37–39] and large exciton binding energy (62meV) [38,40–42] at room temperature, the γ-CuI has lately emerged as a viable option for short-wavelength optoelectronic devices. Moreover, γ-CuI exhibits excellent p-type conductivity due to the point defects and has a higher hole mobility than other wide bandgap semiconductors [38,39,43,44], which makes CuI a promising candidate for the photodetectors, transparent conducting films and light-emitting diode [37,38]. Utilizing CuI's excellent hole transport performance, the p-CuI/n-TiO2 heterojunction photodetector will improve the carrier separation efficiency, and consequently response speed. Furthermore, the electron affinities of p-CuI and n-TiO2 are 2.1 eV and 4.0 eV, respectively [38,32], which could result in a large built-in voltage for the p-CuI/n-TiO2 photodetector and allow for self-powered light detection.
Herein, the p-CuI/n-TiO2 nanorods heterojunction photodetector with excellent features was constructed. Firstly, the TiO2 nanorods were grown on FTO (fluorine-doped tin oxide) conductive glass. Then, the γ-CuI layer was deposited on the TiO2 nanorods by thermal evaporation technology. At last, the Au electrode was fabricated on CuI layer and the prototype heterojunction photodetector was achieved. And then the structure and the response characteristics of the p-CuI/n-TiO2 nanorods heterojunction photodetector were investigated. Finally, the photoresponse and carriers transport mechanisms of the p-CuI/n-TiO2 nanorods heterojunction photodetector were discussed.
2. Experiments
The TiO2 nanorods were synthesized on the FTO conductive glass substrates (Advanced Election Technology Co. Ltd) using the hydrothermal method. Firstly, 15ml hydrochloric acid (36%) was diluted with 15ml deionized water before adding 0.5ml C16H36O4Ti (Macklin). In the ultrasonic cleaner, the solution was uniformly mixed for 30 minutes. In the meantime, the FTO wafers were cleaned using an ultrasonic cleaner in deionized water and acetone for 15 minutes, sequentially. And then the FTO wafers were dried in a high purity nitrogen flow. At last, the mixed solution was transferred into a high-pressure reactor, and the cleaned FTO wafer was fixed on the reaction holder in the mixed solution with FTO facing down. The high-pressure reaction kettle was securely sealed and placed in a constant temperature oven (HASUC, DHG-9205A) at 150°C for 6 hours. After cooling down, the FTO wafer with TiO2 nanorods was cleaned with deionized water and blown dry with high purity nitrogen. Then the FTO wafer with TiO2 nanorods was transferred into the thermal evaporation vacuum chamber, serving as substrate.
Copper iodide (CuI, Alfa Aesar, 99.998%) powder served as the evaporator source. The vacuum chamber was exhausted to 1.0×10−4 Pa before the vacuum thermal evaporation growth of CuI films. The CuI was deposited on the top of the TiO2 nanorods with a growth rate of 0.1Å/s at the substrate temperature of 100°C. The nominal thickness of CuI was 700nm. At last, ∼30nm ohmic contact Au electrode was deposited on p-CuI to fabricate the p-CuI/TiO2 nanorods heterojunction photodetector. The procedure for preparing the device was given in Fig. 1. And the I-V curves of Au-CuI and FTO-TiO2, as shown in the center of Fig. 1, demonstrated that good ohmic contacts were established.
Fig. 1. The schematic diagram for the preparation process of the p-CuI and n-TiO2 nanorods self-powered ultraviolet photodetector.
Crystal structures of the TiO2 nanorods and p-CuI/n-TiO2 nanorods heterojunction photodetector were examined by X-ray diffractometer (XRD, Bruker AXS D8 ADVANCE) with Cu-Kα radiation (λ=0.15418 nm). The surface and cross-sectional morphology, as well as EDS mapping, of the TiO2 nanorods and p-CuI/n-TiO2 nanorods heterojunction photodetector were obtained with field-emission scanning electron microscope (FE-SEM, HITACHI SU8010) equipped with an energy dispersive X-ray spectroscopy (EDS) detector. The current-voltage (I-V) data were measured using a Keithley 2612B source meter. The PL spectra were collected using a grating spectrometer (SR-500i, ANDOR) stimulated with 325 nm laser (He-Cd laser, Kimmon Koha Co., Ltd). The absorbance spectrum of CuI was gathered using an ultraviolet-visible spectrophotometer (UV-2600i, SHIMADZU). The photoresponse characteristics of the photodetector were examined using a monochromator (RF5301PC, SHIMADZU) equipped with a 150 W Xe lamp.
3. Results and discussion
The XRD result of the TiO2 nanorods grown on FTO conductive glass was shown in Fig. 2(a). As shown in the XRD patterns, besides three diffraction peaks of FTO substrate(JCPDS No. 71-0652), there was mainly one strong diffraction peak and two weak diffraction peaks located at 27.6°, 36.2°, and 41.4°, corresponding to (110), (101) and (111) planes of rutile TiO2 (JCPDS No. 88-1172). There were no more peaks of other TiO2 phases or impurity peaks. In addition, only the diffraction peak of (101) plane is strong and narrow, whereas the diffraction peaks of (110) and (111) planes were very weak, indicating that the TiO2 nanorods grew preferentially along with the (101) crystal orientation. Furthermore, the small FWHM (full width at half maximum) of 0.34° of the (101) peak demonstrated the high crystal quality of TiO2 nanorods. Figure 2(b) showed the surface and cross-sectional SEM images of the TiO2 nanorods grown on FTO. As can be observed, the TiO2 nanorods were quadrangular prisms and grew approximately vertically on the substrate with a highly ordered regular tetrahedron shape and a rough top surface, which provided better binding for the subsequently grown CuI layer. While the interspace between TiO2 nanorods provided space for CuI growth and greatly increased the contact area between CuI and TiO2 nanorods. The average length of the TiO2 nanorods was about 1.6 microns with an average width of about 130 nanometers, achieving a length-width ratio of 12.
Fig. 2. (a) XRD patterns of TiO2 nanorod. (b) Cross-sectional SEM image of the TiO2 nanorod, the inset is the surface morphology.
Then the CuI layer with a nominal thickness of 700nm was successfully deposited on the TiO2 nanorods by vacuum thermal evaporation technology. The XRD result of CuI/TiO2 nanorods heterostructure was shown in Fig. 3(a). It can be found that besides the diffraction peaks of TiO2 and FTO substrate as shown in Fig. 2(a), there were four additional diffraction peaks corresponding to the (111), (200), (220) and (311) planes of γ-CuI (JCPDS No. 82-2111). Taking into account of the split of Kα1 and Kα2 of X-ray in high angle region, all the diffraction peaks of CuI are very sharp (FWHM < 0.18°), demonstrating that the CuI deposited by vacuum thermal evaporation has good crystal quality.
Fig. 3. (a) XRD patterns of p-CuI/n-TiO2 nanorods heterojunction. (b) The surface and cross-section of the p-CuI/n-TiO2 nanorods heterojunction photodetector. (c) EDS mapping of p-CuI/n-TiO2 nanorods heterojunction photodetector.
Figure 3(b) showed the surface and cross-sectional SEM images of 700nm thick CuI on TiO2 nanorods. According to the cross-sectional image, CuI deposited on the surface of TiO2 nanorods and filled the gap between them in the beginning, and later grew into flake crystals at the top of TiO2 nanorods. The thickness of CuI crystal flakes was between 40 and 120 nm, and their lateral size can reach 1.1 µm, based on the surface morphology of CuI layer, as shown in the inset of Fig. 3(b). Moreover, the CuI crystal flakes accumulated densely on the top of TiO2 nanorods with random orientation, which resulted in several diffraction peaks of CuI in XRD patterns. The EDS mapping of the heterojunction cross-section, as shown in Fig. 3(c), further demonstrated the interlaced growth of CuI and TiO2 nanorods heterostructure. The elements were uniformly distributed throughout the as-fabricated p-CuI/p-TiO2 nanorods heterojunction photodetector. It should be noticed that there was iodine element in FTO substrate. We believe the reason is the evaporated iodine atoms pass through the interspaces among the TiO2 nanorods to reach the root of the nanorods, thus penetrating into the FTO (fluorine-doped tin oxide) layer. Furthermore, fluorine and iodine are both halogen elements, so the diffusion of iodine in FTO is very easy. Therefore, there was iodine element distribution in FTO substrate.
Because CuI is a direct bandgap semiconductor, its optical bandgap can be calculated from absorption spectrum using Tauc plot [45–47]. The absorption spectrum of the CuI film with a nominal thickness of 700nm grown on fused silica was measured, as shown in Fig. 4(a). The optical bandgap of the CuI in this work was calculated to be ∼3.0 eV, as shown in the inset of Fig. 4(a), which is consistent with previous studies [48–51].
Fig. 4. (a) Absorption spectrum of CuI film, the inset is optical band gap of the CuI film calculated by Tauc Plot. (b) PL spectrum of CuI thin film, the inset is the schematic diagram of the PL mechanism of CuI.
Figure 4(b) showed the photoluminescence (PL) spectrum of CuI film excited with a 325nm laser. It was obvious that the PL spectrum consisted of two emission peaks centered at approximately 410 nm (peak1) and 421 nm (peak2), respectively. The emission peak located at 410 nm was attributed to the transition of an interband exciton, while the emission peak centered at 421 nm was assigned to the trap level induced by Cu vacancies [49,52], as seen in the inset of Fig. 4(b). The PL result was accord with the Tauc plot result. From the Gaussian fitting curves of the PL spectrum shown in Fig. 4(b), the peak2 was stronger than peak1 indicating that there were many Cu vacancies in CuI due to the inherent characteristics of the vacuum thermal evaporation technology.
After the ohmic contact Au electrode was prepared on CuI, the p-CuI/n-TiO2 nanorods heterojunction prototype device was obtained. And its photoelectric properties, including I-V characteristics, on/off ratio, and spectral response characteristics were studied in depth. The I-V curves of the p-CuI/n-TiO2 heterojunction under dark and 410nm illumination (∼870µW/cm2), as shown in Fig. 5(a), exhibited obvious rectification characteristics. At 0V, the dark current and photocurrent were 4.10×10−10A and 3.15×10−7A, respectively, hence the on/off ratio of the photodetector was ∼770 at 0V, as shown in the inset of Fig. 5(a).
Fig. 5. (a) I-V characteristic curves of p-CuI/n-TiO2 heterojunction. The inset is on/off ratio curve. (b) Spectral responsivity curve (red curve) and specific detectivity curve (blue curve in the inset) of p-CuI/n-TiO2 heterojunction at 0 V. (c) Time-dependent photoresponse curve of the photodetector at 0 V. (d) The response and recovery time of the photodetector.
Receptivity (R in A/W) may be expressed as the ratio of photocurrent to the intensity of incident light [53],
where Iph is the photocurrent of the photodiode, Plight is incident light intensity.For the heterojunction photodetector, the shot noise from the dark current is the major contribution to the noise that limits specific detectivity D* [54], the specific detectivity can be calculated by the following equation [55,56]:
According to Eqs. (1) and (2), the spectral responsivity and specific detectivity under 0V were obtained, as shown in Fig. 5(b). It can be seen that the photodetector had a very good spectral selectivity and was highly sensitive to light within the range 320 to 450 nm. Under 0V, the peak responsivity was 4.5mA/W, located at ∼410nm, and the peak specific detectivity was 1.08×1011 Jones. As for the tiny response peak near 800 nm, we believe it originated from the working concept of the grating monochromator.
In order to further study the stability and reproducibility of the detector, the current-time response curve was measured under 0V by periodically illuminating the photodetector with 410nm light (∼870µW/cm2). As illustrated in Fig. 5(c), p-CuI/n-TiO2 nanorods heterojunction photodetector exhibited outstanding stability and reproducibility at 0V. In general, the rise time (τr) is defined as the time required for the photocurrent to increase from 10% to 90% [57,58]. Similarly, the fall time (τf) is defined as the time required for the photocurrent to fall from 90% to 10%. In this work, the rise time and fall time of the heterojunction photodetector were 329 ms and 220 ms, as shown in Fig. 5(d). Table 1 compared the performance of TiO2-based and CuI-based self-powered photodetectors over the last few years.
Table 1. Comparison of the key parameters for TiO2 and CuI based self-powered photodetectors
The photoresponse and carriers transport mechanisms of the p-CuI/n-TiO2 nanorods heterojunction photodetector can be understood with the band diagram. The bandgaps (Eg) of γ-CuI and rutile TiO2 are 3.1 eV and 3.2eV, and the electron affinities of them were taken as 2.1 eV [37] and 4.0 eV [31], respectively. Using measured carrier concentrations of p(CuI) = 1.22×1018/cm3 and n(TiO2) = 4.46×1015/cm3 and effective masses of 0.3m0 for CuI [59] and 0.095m0 for TiO2 [60], the Fermi levels of p-CuI and n-TiO2 nanorods can be calculated to be −5.168eV and −4.13eV, achieving a built-in voltage of 1.038eV. The work functions of FTO and Au were taken as 4.5eV and 5.1eV according to previous reports [61,62], and the band diagram was obtained, as shown in Fig. 6. When the surface of the photodetector was illuminated, the light was absorbed by CuI and the electrons and holes were generated in the CuI/TiO2 interface region. Due to the filter effect [63,64], TiO2 nanorods with larger bandgap can’t absorb the light passing through CuI layers. The large band offsets (ΔEC = 1.9eV, ΔEV = 2.0eV) and built-in voltage prevented the self-recombination of photogenerated carriers, and the photogenerated carriers separated quickly, and then the holes flowed to the Au electrode, the electrons flowed to FTO via TiO2 nanorods. As a result, the photocurrent was produced without any applied bias, thanks to the large built-in voltage.
4. Conclusion
In summary, 700nm CuI film was deposited on the TiO2 nanorods array with thermal evaporation technology, and the p-CuI/n-TiO2 heterojunction photodetector was constructed. The XRD results demonstrated that TiO2 is in the rutile phase, and CuI is in the γ-phase. The SEM images and EDS mappings demonstrate that the CuI wraps the upper part of the nanorods and covers the nanorods with a sheet-like microcrystal layer. The self-powered p-CuI/n-TiO2 nanorods heterojunction photodetector has good spectral selectivity and is sensitive to light in the range of 320nm∼450nm. The peak responsivity and peak detectivity (0V, 410nm) are 4.5mA/W and 1.08×1011 Jones, respectively. Also, the reproducibility and stability of the photodetector are very good. This work provides a simple and effective route to design advanced self-powered photodetectors.
Funding
Natural Science Foundation of Shandong Province (ZR2021MF121); National Natural Science Foundation of China (62075092); Yantai City-University Integration Development Project (2020XDRHXMP11, 2021XKZY03).
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.
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