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Abstract
With higher detectivity, lower response time, and good mechanical flexibility, perovskite photodetectors are supposed to be a promising alternative as the next generation of photodetectors. In this work, we fabricate a low temperature-processed flexible photodetector with ITO-ZnO Schottky contact via ALD technique which has a lower dark current decreasing from 2.04×10−8 A/cm2 to 1.70×10−9 A/cm2 under -0.5 V bias voltage actuation. With 530 nm laser irradiation, the flexible device exhibits excellent performance in detectivity of 6.19×1012 Jones and LDR of 103dB. It also exhibits superior bending stability after 5000 bending circles.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photodetectors that capture light signals and convert them into electrical signals are significant components in many applications. Such devices are highly desirable for application in image sensing and optical communications. Ability in detecting weak optical signals is one of the most important requirements, especially in medical imaging technology. [1–3] Varieties of materials have been applied to photodetectors as the active layer such as Si, ZnO, quantum dots, polymers. [4–11] Nowadays, hybrid organic-inorganic perovskite has gained comprehensive attention since its prominent optical and semiconductor properties. [12] It has been observed that perovskite films show low recombination of charge carriers in the bulk films due to the properties of long charge carrier lifetime and diffusion length. [13,14] With such excellent properties, organic-inorganic hybrid perovskite has wide applications on light-emitting diode [15,16] quantum dots [17], solar cells [18–20], photodiodes [21], and many other devices, leading a huge revolution in optical devices.
In the application of photodetectors, noise current intensity is needed to be as low as possible for which extremely low optical signals can be distinguished. The insertion of carrier blocking layers is a common practice. For example, Dou et al. reported that perovskite photodetectors with hole blocking layer of PFN and BCP respectively show the detectivity of 1014 Jones under -100 mV in 2014 [22]. In 2015, Sutherland et al. reported perovskite photodetectors with Al2O3 and PCBM combined with TiO2 as electron transporting layer achieved a detectivity of 1012 Jones under -200 mV. [23] In 2020, Zhong et al. reported atomic layer deposition processed NiOx and TiO2 application in all-Inorganic perovskite photodetectors, which exhibits ultralow dark current (≈10−11 A). [24]
Nowadays, ZnO with different topography of nanoparticles [25], nanorods [8], and modified-ZnO [26] have been widely used in perovskite devices. Cao et al. modulate the band alignment and perovskite crystallization by employing ZnO and SnO2 as electron transport layer. The PCE of devices with ZnO and SnO2 reached 9.70% and 10.81%, respectively. [20] Inspired by previous reports on indium-tin-oxide(ITO) Schottky contacts to ZnO, we found that due to ZnO’s wide bandgap, the transparent ITO Schottky contacts to ZnO heterostructures resulted in excellent diode characteristics [27,28]. The effective Schottky barrier height is about 0.25 eV and could be turned by modifying the electrical properties of the ZnO layer. We proved that it also takes effect on photodetectors.
In this work, we demonstrate a flexible perovskite photodetector processed under low-temperature solution-process on a polyimide substrate. Between ITO and PTAA, a ZnO layer is inserted to decrease the dark current from 10−8 to 10−9 A·cm-2. With such a structure, photodetector could achieve higher detectivity and be able to recognize lower light density. Besides, it shows a great performance even bent under the radius of 2 mm and superior bending endurance after 5000 bending circles.
2. Experimental
2.1 Perovskite photodetector fabrication
Cleaned glass-polyimide substrates (Shanghai Huayi Corporation) coated with ITO was used in this work. Clean progress was the same as that of the glass substrate by ultrasonic cleaning in detergent, acetone, and alcohol for 20 min, respectively. ZnO (20 nm) was deposited using alternating exposures of diethyl zinc and H2O vapor at a deposition temperature of 80 oC by the atomic layer deposition (ALD) technique. Followed by UV ozone treatment processed for 10 min. ALD took place in BenQ TFS 200 Atomic layer deposition system. A solution of poly (triaryl amine) (PTAA, purchased from Xian Polymer Light Tech. Corp.) (2 mg/mL) was prepared in toluene. The layer was deposited onto ZnO via spin-coating at a speed of 6000 rpm for 30 s in nitrogen atmosphere. The substrate was heated at 110 oC for 15 min before moved into the glovebox. For the MAPbI3 perovskite active layer, the precursor solution was a mixture of 1.4 mmol/mL MAI (purchased from Tokyo Chemical Industry Co. Ltd). and PbI2 (purchased from Tokyo Chemical Industry Co. Ltd) (1:1/n:n) dispersed in γ-GBL (purchased from Tokyo Chemical Industry Co. Ltd) and DMSO (purchased from Alfa Aesar) (7:3/v:v). The perovskite precursor solution was placed on a hotplate at 60°C and stirs overnight. The film was processed via spin-coating with 3 stages: first rotating 1500 rpm for 15 s, then 6000 rpm for 15 s, and 4000 rpm for 10 s as the final stage. 600 μL of toluene (purchased from China National Pharmaceutical Group Corp. Shanghai) was dripped quickly onto the substrate at the beginning of the third stage. After the spin-coating process, the substrate was moved to hotplate annealing at 90 oC for 10 min. Then, a 20 mg/mL PCBM solution (purchased from Xian Polymer Light Tech. Corp.) was prepared in chlorobenzene (purchased from Alfa Aesar) and spin-coated on perovskite layer at a speed of 1500 rpm for 30 s. Followed by 70 oC annealing for 30 min. Finally, 100 nm Ag layer was thermally evaporated on top of the devices under a high vacuum (beneath 1×10−3 Pa). The active area of the perovskite PDs was 0.04 cm2.
2.2 Characterizations
The SEM images of MAPbI3 perovskite film was obtained by using Scanning Electron Microscope (SEM, Hitachi Regular 8100). The SEM images of ALD-ZnO layer were obtained by using Extreme-resolution Analytical Field Emission SEM (Tescan Mira 3 XH). The EDS spectra of ALD-ZnO layer was obtained by AZtec X-MaxN 80. Layer absorbance was measured by using Hitachi U-3900 ultraviolet spectrophotometer. All current density measurements were performed on the FS-380 (PDA) semiconductor characteristic analyzer. Quantum efficiency measurement was performed on 7-SCSpec solar cell measurement system. The thin-film x-ray diffraction (XRD) was detected by Rigaku SmartLab 9. The AFM topography of PTAA was detected by scanning probe microscope (Nanonavi SPA-400 SPM). Photoluminescence spectra of perovskite was detected by Fluorescence Spectrometer (FLSP920). Time-resolved PL decay curves of perovskite were detected by Photoluminescence Spectrometer (Edinburgh Instruments FLS1000).
3. Results and discussion
3.1 Photodetector design
Photograph of the device is shown in Fig. 1(a), perovskite photodetectors were fabricated on the substrate of 14 μm-thick polyimide and peeled from glass after all steps finished. The device structure is shown in Fig. 1(b) and the energy diagram is shown in Fig. 1(c). On the bottom of the device is polyimide slurry coating on a glass substrate with ITO deposited on it. On the ITO layer, an organic-inorganic hybrid MAPbI3 layer was sandwiched between PTAA (hole-transporting layer) and PCBM (electron-transporting layer), Ag was used as the top electrode. To reduce the dark current density which is of great significance among photodetectors, ZnO was used to generate a Schottky contact to receive lower noise current. The schematic diagram of Schottky contact near ZnO is shown in Fig. 1(d). Holes will be injected by the electrode, cross the Schottky barrier into the LUMO of ZnO in the presence of an electric field and transport to the HOMO of perovskite layer. This Schottky contact may inhibit hole transport under dark condition, but under the influence of the photogenerated voltage, the holes can cross the potential barrier more easily. To certify the effect of the Schottky barrier, three types of devices have been fabricated: PD1 and PD2 using PEDOT:PSS and PTAA as hole-transporting layer, respectively, PD3 using both PTAA and ZnO.
Fig. 1. Design of hybrid perovskite photodetectors. (a) Photograph of the device fabricated on the basis of polyimide. (b) Device architecture of the photodetector. (c) Energy diagram of the perovskite photodetector under a slight reverse bias. (d) Band edge curvature within Schottky contact.
3.2 Characterization of the Schottky contact
For Schottky contact, high material quality of ZnO is important. The defects in the ZnO surface layer, especially the oxygen vacancies and zinc interstitials, which may pin the ZnO Fermi level below the conduction band minimum are a well-known hinder to achieving high-quality contacts. [28] Thus, the ALD technique was applied to receive a planer ZnO layer which is more compact than that of solution-based ZnO layers which will efficiently suppress charge recombination inside the layer. [29] UV ozone treatment processed for 10 min after ALD process to reduce oxygen vacancies inside the layer. [30–32]
A top-view SEM image of the ALD-ZnO layer is shown in Fig. 2(a). It clearly indicates that the ZnO performed above the ITO surface is a dense island-like structure with no pinholes appearing. EDS spectra of the ALD-ZnO layer is shown in Fig. 2(b). The characteristic peak of Zn appears in 6.85 eV compared to the glass substrate, which indicates the existence of ZnO. Then we studied the carrier dynamic of the devices. Fig. 2(c) shows the photoluminescence (PL) spectra of perovskite performed on PTAA/ITO with or without ZnO. It shows the same emission peak in the device with or without hole transport layer. And perovskite with ZnO presents a stronger PL density in comparison to that without ZnO. Typically, a significant PL quenching effect means efficient charge transfer from the photoactive layer to the hole transport layer. [33,34] This phenomenon indirectly explains the role of Schottky contact in blocking carrier transport. Fig. 2(d) shows the time-resolved PL decay curves of perovskite fabricated on PTAA/ITO with or without ZnO. The TRPL curve has the best results with second-order fitting. The time-resolved curve is fitted using the following function Eq. (1)
And the effective PL lifetime is calculated as Eq. (2)
Fig. 2. Characterization of the Schottky contact. (a) A top-view SEM image of the ALD-ZnO layer. (b) The EDS spectra of the ALD-ZnO layer deposited on glass. (c) Photoluminescence spectra and (d) Time-resolved PL decay curves of perovskite, perovskite fabricated on PTAA/ITO, and perovskite fabricated on PTAA/ZnO/ITO.
Fitting to bi-exponential decay function, both perovskite device with or without ZnO exhibited fast and slow time constants which are: τf = 29.87 ns and τs = 63.48 ns for device without ZnO and τf = 29.43 ns, τs = 59.08 ns for device with ZnO while τf and τs of the perovskite layer are 28.77 ns and 71.11 ns, respectively. Fast time is assigned to the presence of a surface component while the slow time is assigned to the presence of a bulk component. [35] Such phenomenon reveals the lifetime of carriers propagating deeper in the material and carrier lifetime shows a slight decrease after the insertion of ALD-ZnO layer. PL lifetime of perovskite layer is calculated to be 53.89 ns, while that of perovskite performed on PTAA/ITO with or without ZnO are 45.02 ns and 48.86 ns, respectively.
3.3 Characterization of the hybrid perovskite layer
Figure 3(a) shows the typical thin-film XRD pattern of a spin-coated polycrystalline film of the hybrid perovskite CH3NH3PbI3 on PD3 with the crystal structure shown in the inset. It confirms a high degree of crystallinity and orientation, from which we can find the diffraction peaks at 13.88°, 28.3°, 31.68° (assigned to the (110), (220), (222) respectively). The absence of characteristic peaks of PbI2 verifies the complete reaction of all the PbI2 into MAPbI3. Figure 3(b) shows the absorption spectrum of perovskite with a thickness of 240 nm. In the wavelength range of 300 nm to 760 nm, perovskite shows strong absorption in a broad range. The morphology of MAPbI3 films plays a vital role in photodetector performance. Therefore, MAPbI3 films deposited on PEDOT: PSS and PTAA with or without ZnO layer were examined via SEM as shown in Fig. 3(e). The surface of the MAPbI3 films is covered by dense and homogenous grains and there are no voids or pinholes on the surface of the perovskite film. However, the size and uniformity of the crystals were very different. The crystal sizes were analyzed by using Nano measurer and the statistical values were summarized in Table 1. The mean perovskite grain size values of PD1, PD2, and PD3 are 107 nm, 162 nm, and 148 nm, respectively. As is shown in Table 1, crystal size increased when fabricated on PTAA and became more homogeneous after the insertion of ZnO and device performances were in accordance with the previous reports that device with the most homogeneous MAPbI3 crystal had the best photoelectric performance, while MAPbI3 with the largest but non-uniform crystal size destroyed device performance. [36] Figs. 3(c) and 3(d) show that the surface morphologies of PTAA thin films fabricated on ITO with or without ALD-ZnO. The roughness of the PTAA surface increases after the insertion of ALD-ZnO which increased from 1.60 nm to 1.98 nm. The morphological changes of the perovskite layer mean that the roughed surface of PTAA thin films eliminates the available sites for the nucleation of MAPbI3 thin films and thereby improves the crystallinity of MAPbI3 thin films which matches the previous reports. [37]
Fig. 3. Characterization of the hybrid perovskite layer. (a) XRD pattern and crystal lattice of perovskite. (b) The absorbance of the perovskite layer with a thickness of 240 nm. (c, d) The AFM topography of PTAA deposited on (c) ITO and (d) ZnO/ITO (e) A top-view SEM image of perovskite layer on PD1, PD2, and PD3, respectively.
Table 1. The detailed crystal size values of perovskite films.
3.4 Detectivity characterization
Photodetectors are evaluated based on responsivity and detectivity. Responsivity, the specific ratio of the output photocurrent (Iout) and incident optical power (Pin), indicates how efficiently the detector responds to an optical signal. It is expressed as Eq. (3),
Detectivity indicates the ability to recognize weak light signal when the dark current is dominated by the shot noise, it is given by Eq. (4),
A green light-emitting diode (λ=530) has been used as a monochromatic light source. The current density–voltage (J–V) diagram characterized both in the dark and under illumination (power density = 2mW/cm2) shows in Fig. 4(a) and the detectivity of PD1, PD2, and PD3 calculated by current density–voltage curves are shown in Fig. 4(b). PD1 and PD2 show dark current densities of 3.36×10−8 A·cm-2 and 2.04×10−8 A·cm-2 (at -0.5V), respectively. The specific detectivity in 530nm was calculated to be 1.40×1012 Jones and 1.79×1012 Jones (cm·Hz1/2·W-1), respectively. In a specific experiment, a ZnO layer was injected between ITO and PTAA. Given that the electron affinity of ZnO is 4.35 eV [38], the Schottky barrier height of ZnO/ITO is 0.25 eV. Therefore, ITO/ZnO can form a Schottky contact with which PD3 shows a better electrical property. On one hand, the perovskite film above ZnO/PTAA became more homogeneous which is beneficial to the enhancement of photocurrent; on the other hand, Schottky contact partially inhibits the transport of carriers, which is unbeneficial to the enhancement of photocurrent. Considering the above two reasons, the photocurrent of the device does not change much. However, a smaller dark current and a higher detectivity are shown in PD3. When PD3 is under bias of -0.5V, the dark current comes to 1.71×10−9 A·cm-2 and the detectivity is calculated to be 6.19×1012 Jones. If calculated at -0.25 V, the detectivity is as high as 5.50×1013 Jones (Jd at -0.25 V is lower and getting closer to the noise current). The high detectivity of perovskite photodetectors is mainly due to its extremely low dark current under reverse bias. Interface engineering ensures a good Schottky contact to avoid the leakage current.
Fig. 4. Electrical properties of the device. (a) Current density–voltage curves of photodetectors with different hole-transporting layer. PD1, with PEDOT:PSS as hole transport layer; PD2, with PTAA as hole transport layer; PD3, with PTAA as hole transport layer and ITO-ZnO Schottky contact. (b) Detectivity of PD1, PD2, and PD3 calculated under different biases. (c) The linear dynamic range of three devices. (d) External quantum efficiency and detectivity of the hybrid perovskite photodetector at different wavelengths (under 0 V bias). (e) Transient photocurrent response of PD3 at a pulse frequency of 1 Hz.
Another figure-of-merit for photodetectors is the LDR, or photo-sensitivity linearity (typically quoted in dB). Linear dynamic range is given by Eq. (5),
where J*ph is the photocurrent, measured at a light intensity of 10−3 W·cm-2. It indicates that within a certain range, the photocurrent has a linear response as the incident light intensity changes. This is important because beyond this range the intensity of the light signal cannot be detected and calculated precisely.Fig. 4(c) shows photocurrent vs light intensity of PD3 in comparison with PD1 and PD2. When illuminated under light, PD3 shows a linear response within the power density range from 2×10−3 W·cm-2 to 1×10−8 W·cm-2 and a linear dynamic range of 103 dB. While the linear dynamic ranges of PD1 and PD2 are calculated to be 84 dB and 82 dB, respectively. The EQE and detectivity of PD3 at different wavelengths is shown in Fig. 4(d). The device has photoresponse from 300 to 700 nm and a flat EQE peak among this range, which is mainly ascribed to the absorption of MAPbI3 in the visible range corresponding to the absorption in Fig. 3(b). Furthermore, the current density as a function of time is given in Fig. 4(e) for PD3, in order to ensure the response ability of devices. It was measured the on/off switching of the device performed under a 530 nm pulse light from a light-emitting diode at lighting on/off modulation frequency of 0.5 Hz. The results indicate that under light conditions, the current of the photodetector increases when the light is on and returns back to its original state after turning off. The transient response for the device with ZnO/PTAA shows both rise and decay time within 1ms, as is displayed in Fig. 4(e) (right), which is the detection limit of our facility. As is shown in the figure, the response/recovery characteristics of these photodetectors are stable and repeatable, which is very important for practical applications in medical diagnosis.
3.5 Flexibility and bending stability
The flexibility and bending stability were presented by performing various bending radius and cycles. Fig. 5(a) depicts the transient photocurrent response after 1000 and 5000 bending cycles at a radius of 5mm and a frequency of 0.5 Hz. The results were measured under bias of -0.5 V and a light density of 2 mW·cm-2. After bending for 1000 cycles, light current fell to 63% of the original device. And after 5000 cycles of bending, the device remains stable. Fig. 5(b) indicates dark current and light current bent under different radius by attaching the device onto cylinders [Fig. 5(c)] We can find that flexible photodetectors exhibited promising mechanical stability, even under the bending radius of 4 mm. In comparison with previous papers [39,40], our device shows stable detection performance at smaller bending radii and competitive performance after a greater number of bends.
Fig. 5. Flexibility and bending stability (a) Normalized transient photocurrent response versus bending cycle number of flexible MAPbI3 photodetectors (b) Current density tested with/without illumination versus bending radius. (c) Images of flexible MAPbI3 photodetectors bending under different radius.
4. Conclusion
We produced hybrid perovskite photodetectors with ZnO Schottky contact to reduce the dark current of the device. We compare modified devices to devices without ZnO, a considerable decrease of dark current occurs with no decrease in light current enables lower light signal detecting, which exhibits low dark current density of 1.71×10−9 A·cm-2 high detectivity of 6.19×1012 Jones under the bias of –0.5 V. It is meaningful in x-ray detectors which allow lower dose rates to be recognized. Such devices can also support applications in unique situations like dental clinics for its superior bending stability at a bending radius of 4 mm. As is exhibited in this report, the formation of Schottky contact may polish up the performance of photodetector and our results should inspire new studies on the modification of structure on similar devices.
Funding
National Key Research and Development Program of China (2017YFB0404703); National Natural Science Foundation of China (51725505); Shanghai Industrial foundation project (GYQJ-2018-2-04).
Disclosures
The authors declare no conflicts of interest.
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