CH3NH3PbI3/Au/Mg0.2Zn0.8O Heterojunction Self-Powered Photodetectors with Suppressed Dark Current and Enhanced Detectivity

Interface engineering of the hole transport layer in CH3NH3PbI3 photodetectors has resulted in significantly increased carrier accumulation and dark current as well as energy band mismatch, thus achieving the goal of high-power conversion efficiency. However, the reported heterojunction perovskite photodetectors exhibit high dark currents and low responsivities. Herein, heterojunction self-powered photodetectors, composed of p-type CH3NH3PbI3 and n-type Mg0.2Zn0.8O, are prepared through the spin coating and magnetron sputtering. The obtained heterojunctions exhibit a high responsivity of 0.58 A/W, and the EQE of the CH3NH3PbI3/Au/Mg0.2Zn0.8O heterojunction self-powered photodetectors is 10.23 times that of the CH3NH3PbI3/Au photodetectors and 84.51 times that of the Mg0.2ZnO0.8/Au photodetectors. The built-in electric field of the p-n heterojunction significantly suppresses the dark current and improves the responsivity. Remarkably, in the self-supply voltage detection mode, the heterojunction achieves a high responsivity of up to 1.1 mA/W. The dark current of the CH3NH3PbI3/Au/Mg0.2Zn0.8O heterojunction self-powered photodetectors is less than 1.4 × 10−1 pA at 0 V, which is more than 10 times lower than that of the CH3NH3PbI3 photodetectors. The best value of the detectivity is as high as 4.7 × 1012 Jones. Furthermore, the heterojunction self-powered photodetectors exhibit a uniform photodetection response over a wide spectral range from 200 to 850 nm. This work provides guidance for achieving a low dark current and high detectivity for perovskite photodetectors.


Introduction
With the rapid development of two-dimensional (2D) materials, such as graphene, transition metal dichlorides (TMDs), and black phosphorus, 2D perovskites have emerged, which combine the excellent properties of 2D materials and perovskites, namely the good solution processability, molecular-scale self-assembly, and film formation of the former alongside the direct tunable band gap, high carrier mobility, and high absorption coefficient of the latter [1][2][3][4][5][6][7]. Among these, CH 3 NH 3 PbI 3 possesses excellent absorption and has been applied to photodetectors (PDs); in particular, self-powered PDs (SPPDs) have drawn considerable research interest [8]. SPPDs can detect light without the need for any power supply; furthermore, they can be miniaturized and integrated into nanodevices with remote wireless control. However, due to the complex production process, existing perovskite SPPDs show a high dark current and a low detection rate, which greatly limits their wide application. For example, a common approach to realizing perovskite SPPDs is to construct p-i-n with vertical multilayer structures, which require complex and rigorous processes [9][10][11][12]. They are important candidates in the field of renewable photovoltaics and photoelectric devices [13][14][15][16]. Since MAPbI 3 /TiO 2 PDs were reported in 2014, various PDs have been successfully fabricated using MAPbI 3 . However, it has been shown that perovskite SPPDs exhibit high dark currents and low responsivities [17]. Furthermore, perovskite heterojunction PDs are being quickly developed as well [18][19][20][21]. The built-in electric field formed at the junction interface can act as a driving force to separate the photogenerated electron-hole (e-h) pairs in the depletion region, drive the transport of the separated photogenerated carriers, and then generate the photocurrent in the PDs without an external power supply. Herein, heterojunction SPPDs are designed.
Due to their advantages of lightweight and easy processing, perovskites have attracted more and more attention, and they have shown great application potential in various fields in recent years. In particular, wideband organic photoelectric detection (OPD) has been successfully applied in many important fields, such as astronomical exploration, remote sensing, and infrared imaging. The combination of MgZnO and the advantages of organic polymers can improve the performance of PDs, so organic PDs show fascinating characteristics. Perovskite heterojunction PDs performance can be improved by using metal oxide dense films, such as MgZnO or ZnO, which are usually used as an electron-transport layer to transport electrons and holes [22][23][24][25]. MgZnO and ZnO with wide band gaps exhibit a large ultraviolet (UV)-light absorption coefficient and high carrier mobility. It is well known that intrinsic point defects, such as oxygen vacancies and metal interstitials, have an important impact on the electronic properties of metal oxides. It has been widely accepted that the presence of oxygen vacancies in Mg 0.2 Zn 0.8 O can increase the charge density of Mg 0.2 Zn 0.8 O to form a native n-type semiconductor [26][27][28][29]. Thus, it is urgently required to fabricate PDs with a simple alternative method that can also endow them with self-powered functionality. The most common method for fabricating SPPDs relies on the E b created by heterojunctions. To date, Mg 0.2 Zn 0.8 O has been rarely reported for use as an electron-transport layer in heterojunction SPPDs. Mg 0.2 Zn 0.8 O has a wide band gap and acts as the n-type layer. CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs with excellent comprehensive properties have been successfully prepared. Therefore, it is very important to develop CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs with a low dark current and high detectivity to improve their performance.
Herein, high-quality CH 3 NH 3 PbI 3 perovskites were prepared as hole-transport layers through the one-step spin coating method and were then used in Mg 0.2 Zn 0.8 O PDs prepared by magnetron sputtering. These devices possess light responsiveness in a broad range, from UV to near-infrared (NIR), high responsivity, and low dark current. The CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs exhibit an excellent responsivity of up to 0.58 A/W. In the self-supply voltage detection mode, it achieves a high responsivity of up to 1.1 mA/W, and the dark current is less than 1.4 × 10 −1 pA at 0 V. The best value of the detectivity is as high as 4.7 × 10 12 Jones. This work presents a new route for designing SPPDs with a low dark current and high detectivity.

Materials
All the reagents used in the experiments were of analytic grade and used without further purification. The conjugated polymers PbI 2 and CH 3 NH 3 I were purchased from Xi'an Polymer Light Technology Corp., Xi'an, China.

Preparation of the CH 3 NH 3 PbI 3 /Au PDs
Firstly, Au thin films were prepared on a 2 × 3 cm 2 polyethylene terephthalate (PET) using radio-frequency (RF) magnetron sputtering (JZ-RF600A). Subsequently, the metal semiconductor metal (MSM) structure was realized through the following steps: gluing, exposure, development, etching, and resist removal. The electrode width and spacing were both 5 µm, and the electrode length was 500 µm. We masked the PET film with 5 µm-wide Materials 2023, 16, 4330 3 of 11 channels with polyimide tape. In the next step, using a vacuum spin coater (VTC-200), the CH 3 NH 3 PbI 3 layer was deposited on the PET substrate. Briefly, 10 mg of CH 3 NH 3 I and 460 mg of PbI 2 were dissolved in 1 mL of N, N-dimethylformamide (DMF) and stirred at 70 • C for 2 h to obtain a homogeneous CH 3 NH 3 PbI 3 precursor solution. Then, the above precursor solution was spin coated on the PET substrate at an initial speed of 500 r/min for 10 s and then at a speed of 3000 r/min for 50 s. Subsequently, PET with a spin coating was heated in a vacuum oven at 70 • C for 3 min to remove the residual DMF. After cooling, the polyimide tape was removed, and the CH 3 NH 3 PbI 3 layer was obtained upon annealing at 90 • C for 10 min. Finally, the CH 3 NH 3 PbI 3 /Au PDs were obtained.

Preparation of the CH3NH3PbI3/Au PDs
Firstly, Au thin films were prepared on a 2 × 3 cm 2 polyethylene terephthalate (PET) using radio-frequency (RF) magnetron sputtering (JZ-RF600A). Subsequently, the metal semiconductor metal (MSM) structure was realized through the following steps: gluing, exposure, development, etching, and resist removal. The electrode width and spacing were both 5 µm, and the electrode length was 500 µm. We masked the PET film with 5 µm-wide channels with polyimide tape. In the next step, using a vacuum spin coater (VTC-200), the CH3NH3PbI3 layer was deposited on the PET substrate. Briefly, 10 mg of CH3NH3I and 460 mg of PbI2 were dissolved in 1 mL of N, N-dimethylformamide (DMF) and stirred at 70 °C for 2 h to obtain a homogeneous CH3NH3PbI3 precursor solution. Then, the above precursor solution was spin coated on the PET substrate at an initial speed of 500 r/min for 10 s and then at a speed of 3000 r/min for 50 s. Subsequently, PET with a spin coating was heated in a vacuum oven at 70 °C for 3 min to remove the residual DMF. After cooling, the polyimide tape was removed, and the CH3NH3PbI3 layer was obtained upon annealing at 90 °C for 10 min. Finally, the CH3NH3PbI3/Au PDs were obtained.

Preparation of the Au/Mg0.2Zn0.8O PDs
Mg0.2Zn0.8O thin films were prepared on a 2 × 3 cm 2 PET using RF magnetron sputtering (JZ-RF600A). The vacuum chamber was initially evacuated to 5 × 10 −4 Pa, and O2 and Ar were introduced into the chamber in a flow ratio of 10:40. PET was washed with acetone, absolute ethanol, and deionized water for 10 min. The Mg0.2Zn0.8O films were sputtered on the PET substrates at a total pressure of 4 Pa and an RF power of 150 W for 30 min to obtain the Mg0.2Zn0.8O thin films, as shown in Figure 1a,b. Next, Au was RF sputtered on the Mg0.2Zn0.8O thin films to realize the MSM structure, as shown in Figure 1c,d; the process included gluing, exposure, development, etching, and resist removal. The electrode width and spacing were both 5 µm, and the electrode length was 500 µm. The obtained Au/Mg0.2Zn0.8O UV PDs are shown in Figure 1d.  Firstly, we masked the Mg 0.2 Zn 0.8 O film with 5 µm-wide channels with the polyimide tape. In the second step, using a vacuum spin coater (VTC-200), the CH 3 NH 3 PbI 3 layer was deposited on the Mg 0.2 Zn 0.8 O thin film. 10 mg of CH 3 NH 3 I and 460 mg of PbI 2 were dissolved in 1 mL of DMF and stirred at 70 • C for 2 h to obtain a homogeneous CH 3 NH 3 PbI 3 precursor solution. Then, the above precursor solution was spin coated on the Mg 0.2 Zn 0.8 O thin film at an initial speed of 500 r/min for 10 s and then at a speed of 3000 r/min for 50 s. Then, a Mg 0.2 Zn 0.8 O thin film with spin coating was heated in a vacuum oven at 70 • C for 3 min to remove the residual DMF. After cooling, the polyimide tape was removed, and a CH 3 NH 3 PbI 3 layer was obtained upon annealing at 90 • C for

Device Characterization
The crystal structures of CH 3 NH 3 PbI 3 /Au PDs and CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs were characterized using a Rigaku Ultima VI X-ray diffractometer (XRD). The morphology was characterized by scanning electron microscopy (SEM) using a JEOL JSM-7600F microscope. The UV-visible (Vis)-NIR absorption spectra of the CH 3 NH 3 PbI 3 /Au PDs and CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs were measured using a PerkinElmer Lambda 950 spectrophotometer. The dark and photocurrentvoltage (I-V) curves of the CH 3 NH 3 PbI 3 /Au PDs and CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs were measured using an Agilent 16442A test fixture. The responsivity spectra of the CH 3 NH 3 PbI 3 /Au PDs and CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs were measured using a Zolix DR800-CUST testing system.

Results and Discussion
As shown in Figure 2a, the XRD pattern of the CH 3 NH 3 PbI 3 /Au PDs exhibits diffraction peaks at 2θ = 14.1 • , 28.4 • , 31.9 • , and 40.8 • , which are associated with the (110), (220), (310), and (224) planes, respectively; the strongest diffraction peaks are (110) and (220), which shows that the materials grow preferentially along the (110) direction, which is consistent with previous studies [30][31][32]. The still-remaining diffraction peak at 12.65 • suggests the level of the PbI 2 impurity phase [33]. The XRD pattern of the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs shows a peak at 34.51 • , which corresponds to the (002) planes of Mg 0.2 Zn 0.8 O [34,35]. The other peaks are coincident with those of CH 3 NH 3 PbI 3 , indicating the diffraction peak of Mg 0.2 Zn 0.8 O film. The Mg 0.2 Zn 0.8 O thin film has no effect on the crystallinity of the CH 3 NH 3 PbI 3 layer. Figure 2b shows the normalized absorption spectra of the Mg 0.2 Zn 0.8 O/Au PDs, CH 3 NH 3 PbI 3 /Au PDs, and CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs. For the Mg 0.2 Zn 0.8 O/Au PDs, there is an absorption peak at a wavelength of 330 nm. A broad absorption band from 330 to 780 nm is observed for the CH 3 NH 3 PbI 3 /Au PDs. It is worth noting that the absorption of the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs is in the range from 330 to 780 nm, and the absorption performance is better than that of the CH 3 NH 3 PbI 3 /Au PDs. Overall, the excellent light absorption properties make CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs promising candidates for highperformance PDs. It can be seen from Figure 2c,d that the cuboid-shaped CH 3 NH 3 PbI 3 grains are uniformly distributed on the substrate. Through the double-layer film of Mg 0.2 Zn 0.8 O and CH 3 NH 3 PbI 3 , it can be seen that the upper layer of the CH 3 NH 3 PbI 3 polycrystalline film is more dense; at the same time, the crystal grains of CH 3 NH 3 PbI 3 can be enlarged. Larger CH 3 NH 3 PbI 3 grains mean fewer perovskite grain boundaries. This result suggests that the Mg 0.2 Zn 0.8 O film can passivate the surface defects of the perovskite and that a uniform and flat CH 3 NH 3 PbI 3 layer is formed. Figure 3a shows the responsivity (R) spectra of the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs in the UV-Vis-NIR range. It can be seen that in the range of 250-850 nm, the R value is significantly higher than that of the Mg 0.2 Zn 0. 8 Figure 3a shows the responsivity (R) spectra of the CH3NH3PbI3/Au/Mg0.2Zn0.8O heterojunction SPPDs in the UV-Vis-NIR range. It can be seen that in the range of 250-850 nm, the R value is significantly higher than that of the Mg0.2Zn0.8O/Au PDs or CH3NH3PbI3/Au PDs under 1 V. For the CH3NH3PbI3/Au/Mg0.2Zn0.8O heterojunction SPPDs, the highest R value is up to 0.58 A/W, which is 11.09 times higher than that of the best CH3NH3PbI3/Au PDs and is 161 times higher than that of the best Mg0.2Zn0.8O/Au PDs. It can be seen that the CH3NH3PbI3/Au/Mg0.2Zn0.8O PDs have a higher R than the Mg0.2Zn0.8O/Au PDs and the CH3NH3PbI3/Au PDs alone.  Figure 3b shows a comparison of the external quantum efficiency (EQE) measured under 1 V. The EQE curve is similar to the absorbance curve of the CH 3 NH 3 PbI 3 /Au PDs. The EQE is one of the most important performance parameters of perovskite PDs; the EQE represents the number of electron-hole pairs generated for a single incident photon. The EQE of the two groups of devices was calculated using formula (1). In the case of a certain bias, the EQE of the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs is 10.23 times that of the CH 3 NH 3 PbI 3 /Au PDs and 84.51 times that of the Mg 0.2 ZnO 0.8 /Au PDs. The EQE is defined as: The I-V curves of the CH 3 NH 3 PbI 3 /Au PDs and CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs under dark and light conditions are shown in Figure 3c,d. The light-dark current ratio of the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs is 100 times that of the CH 3 NH 3 PbI 3 /Au PDs. Additionally, the nonlinear I-V curves indicate that Schottky metal-semiconductor contacts were formed. By comparing Figure 3c,d, it is found that this is mainly attributed to the fact that the heterojunction inhibits the rise of the dark current. From Figure 3d, it can be seen that the dark current of the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs is very low, and the light-dark current ratio is about 10 3 . The dark current of the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs is less than 1.4 × 10 −1 pA at 0 V, which is more than 10 times less than that of the CH 3 NH 3 PbI 3 PDs. Furthermore, the dark current is very low, which can effectively reduce the lowest detectable optical power and enhance the capability of detecting weak light [36]. When the CH 3 NH 3 PbI 3 thin film is spin coated on Mg 0.2 Zn 0.8 O, the dark current of the device decreases, the pho-tocurrent increases, and the photoresponsivity gain is enhanced for the heterojunction. There are few carrier-donating defects in the bilayer, and the interfacial charge transfer reduces the carrier concentration in the dissipative region. The higher the light current-dark current ratio, the better the device detection performance. At the same time, the recombination of the electron pairs in the Mg 0.2 Zn 0.8 O thin film is reduced in the gap between the CH 3 NH 3 PbI 3 grains, which is beneficial for the hole separation and transport in the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction thin film; this enables the realization of a high response and a very low dark current.  The EQE is one of the most important performance parameters of perovskite PDs; the EQE represents the number of electron-hole pairs generated for a single incident photon. The EQE of the two groups of devices was calculated using formula (1). In the case of a certain bias, the EQE of the CH3NH3PbI3/Au/Mg0.2Zn0.8O heterojunction SPPDs is 10.23 times that of the CH3NH3PbI3/Au PDs and 84.51 times that of the Mg0.2ZnO0.8/Au PDs. The EQE is defined as: The I-V curves of the CH3NH3PbI3/Au PDs and CH3NH3PbI3/Au/Mg0.2Zn0.8O heterojunction SPPDs under dark and light conditions are shown in Figure 3c,d. The lightdark current ratio of the CH3NH3PbI3/Au/Mg0.2Zn0.8O heterojunction SPPDs is 100 times that of the CH3NH3PbI3/Au PDs. Additionally, the nonlinear I-V curves indicate that Schottky metal-semiconductor contacts were formed. By comparing Figure 3c,d, it is found that this is mainly attributed to the fact that the heterojunction inhibits the rise of the dark current. From Figure 3d, it can be seen that the dark current of the CH3NH3PbI3/Au/Mg0.2Zn0.8O heterojunction SPPDs is very low, and the light-dark current  . Clearly, the photocurrent density increases gradually with the incident light wavelength. The main reason is that the dark current is ultimately limited by the recombination current, which is an inherent property of semiconductor materials and heterojunctions. The built-in electric field of the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction with the increased Fermi level of Mg 0.2 Zn 0.8 O provides a strong driving force to separate and transfer the photogenerated carriers, and the depletion layer becomes wider, which decreases the recombination of the carriers and then reduces the dark current. This endows the heterojunction devices with a high photoresponse performance under external bias. The high detection rate of the perovskite PDs is mainly due to their very low dark current under reverse bias. Such a small dark current explains the reason for its high photodetection and also shows that perovskite-based PDs can exhibit very good detection capability. Moreover, the device shows an apparent photovoltaic behavior under illumination, and the offset voltage is 0 V, which exhibits a self-powered characteristic behavior, as shown in Figure 4c. In the self-supply voltage-detection mode, the device achieves a high responsivity of up to 1.1 mA/W. A large number of electron-hole pairs are generated in the film due to the internal electric field at the surface. The space charges separate and drift in opposite directions, generating photocurrent at the electrode to produce zero bias. dark current. This endows the heterojunction devices with a high photoresponse performance under external bias. The high detection rate of the perovskite PDs is mainly due to their very low dark current under reverse bias. Such a small dark current explains the reason for its high photodetection and also shows that perovskite-based PDs can exhibit very good detection capability. Moreover, the device shows an apparent photovoltaic behavior under illumination, and the offset voltage is 0 V, which exhibits a self-powered characteristic behavior, as shown in Figure 4c. In the self-supply voltagedetection mode, the device achieves a high responsivity of up to 1.1 mA/W. A large number of electron-hole pairs are generated in the film due to the internal electric field at the surface. The space charges separate and drift in opposite directions, generating photocurrent at the electrode to produce zero bias.  Figure 5 shows a schematic of the carrier transport mechanism of the CH 3 NH 3 PbI 3 / Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs. Such a mechanism can be explained by the band diagram. As can be seen from the figure, the band gaps of CH 3 NH 3 PbI 3 and Mg 0.2 Zn 0.8 O are 1.45 and 3.89 eV, respectively. When the Mg 0.2 Zn 0.8 O film forms a heterojunction with the CH 3 NH 3 PbI 3 film, the electrons in the former diffuse into the latter. Furthermore, the pores in the CH 3 NH 3 PbI 3 film diffuse into the Mg 0.2 Zn 0.8 O film. When they are in equilibrium, a self-built electric field is formed at the heterojunction interface, as shown in Figure 5b. Under illumination, Mg 0.2 Zn 0.8 O absorbs UV light to produce photogenerated electrons and holes, while the CH 3 NH 3 PbI 3 film absorbs UV, Vis, and NIR light to produce electrons and holes. Under the action of this self-established electric field, the photogenerated electrons and holes, which are diffused from the dissipative region of the heterojunction, rapidly separate; the electrons drift to the Mg 0.2 Zn 0.8 O film, and the holes drift to the CH 3 NH 3 PbI 3 film; thus, a stable external current is generated. Therefore, self-driven CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O devices have a low dark current, a high light current, and high responsiveness. Due to the high recombination probability of the photogenerated electrons and holes in CH 3 NH 3 PbI 3 , the photocurrent enhancement of the CH 3 NH 3 PbI 3 /Au PDs is not very significant. However, when the n-Mg 0.2 Zn 0.8 O layer and the p-CH 3 NH 3 PbI 3 perovskite material are introduced, a p-n junction with a well-matched band structure is formed, from p-CH 3 NH 3 PbI 3 with a high Fermi level to n-Mg 0.2 Zn 0.8 O with a low Fermi level. The Fermi level of p-CH 3 NH 3 PbI 3 in the p region gradually increases, while that of Mg 0.2 Zn 0.8 O in the n region gradually decreases, thus increasing the Fermi level [37]. Therefore, when the recombination probability of photogenerated electron-hole pairs in the CH 3 NH 3 PbI 3 layer decreases, the photocurrent of the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs increases significantly [38][39][40][41][42]. Thus, lowering the contact barrier on the surface results in a high photocurrent in the device. Therefore, the heterostructure PD can not only reduce the recombination probability of electrons and holes but also increase the depletion layer width, as shown in Figure 5c; the dark current decreases, so that the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs have a high optical gain.
CH3NH3PbI3/Au PDs is not very significant. However, when the n-Mg0.2Zn0.8O lay the p-CH3NH3PbI3 perovskite material are introduced, a p-n junction with a well-ma band structure is formed, from p-CH3NH3PbI3 with a high Fermi level to n-Mg0.2 with a low Fermi level. The Fermi level of p-CH3NH3PbI3 in the p region gra increases, while that of Mg0.2Zn0.8O in the n region gradually decreases, thus incr the Fermi level [37]. Therefore, when the recombination probability of photogen electron-hole pairs in the CH3NH3PbI3 layer decreases, the photocurrent o CH3NH3PbI3/Au/Mg0.2Zn0.8O heterojunction SPPDs increases significantly [38][39][40][41][42]. lowering the contact barrier on the surface results in a high photocurrent in the d Therefore, the heterostructure PD can not only reduce the recombination probabi electrons and holes but also increase the depletion layer width, as shown in Figure  dark current decreases, so that the CH3NH3PbI3/Au/Mg0.2Zn0.8O heterojunction S have a high optical gain.  D* is a measurement of the detector's sensitivity. Assuming that shot noise from the dark current is the major contributor to the total noise, it can be written as [43,44]: where R is the responsivity, A is the area of the detectors, q is the unit charge, and I d is the dark current. Due to the suppressed dark current and the enhanced responsivity of the CH 3 NH 3 PbI 3 / Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs, as shown in Figure 4d, the best value of D* is as high as 4.7 × 10 12 Jones at 780 nm, while it is only 3.0 × 10 10 Jones in CH 3 NH 3 PbI 3 /Au devices. Notably, the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction PDs exhibit a pronounced response in the vis-light range as well as in the UV-and NIR-light ranges. It is found that the detectivity of the prepared perovskite SPPDs is greatly improved in the range of 300-800 nm, and a detectivity exceeding 2.34 × 10 12 Jones is achieved in most of the range (320-780 nm). These results are comparable with those of other reports, and a detailed comparison is provided in Table 1. The table shows that the detection rate of the device prepared in this paper is higher than that of other devices, which have a detection rate of 4.7 × 10 12 Jones and obtain ultra-low magnitude-dark currents compared to other devices, which have a dark current of 1.4 × 10 −1 pA. The significant increase in detectivity indicates that the modification on both sides of the active layer results in a reduced trap density as well as improved carrier transport and extraction.

Conclusions
In summary, a CH 3 NH 3 PbI 3 film was prepared and combined with an Mg 0.2 Zn 0.8 O film fabricated via vacuum magnetron sputtering to realize CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs. The results show that due to the addition of the Mg 0.2 Zn 0.8 O layer, the recombination probability of the photogenerated electron-hole pairs is reduced, and the optical gain is enhanced. Compared with the CH 3 NH 3 PbI 3 /Au PDs, the photoresponsivity is increased by nearly 53.17 times across the entire spectral range, and the EQE of the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs is increased from 12.45% to 120%. Remarkably, in the self-supply voltage-detection mode, the SPPDs achieve a high responsivity of up to 1.1 mA/W. The dark current of the CH 3 NH 3 PbI 3 /Au/Mg 0.2 Zn 0.8 O heterojunction SPPDs is less than 1.4 × 10 −1 pA at 0 V. The best value of the detectivity is as high as 4.7 × 10 12 Jones.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding author upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.