Solution-Processed Inorganic Perovskite Flexible Photodetectors with High Performance

All inorganic CsPbI3-xBrx perovskites have been widely used in photodetectors due to their excellent optoelectronic properties and simple preparation processes. Here, high-performance flexible photodetectors based on inorganic CsPbI3-xBrx perovskites are demonstrated, which are achieved by a modified solution-processed method. When biased at a low voltage of 10 mV, the device yielded fast response speeds (90 μs /110 μs for CsPbI2Br PDs and 100 μs/140 μs for CsPbIBr2 PDs), a high on/off ratio of 104, and a high detectivity about 1012 Jones. Meanwhile, the devices showed outstanding environmental stability and mechanical flexibility. The periodic I-t curves had negligible fluctuation (< 5%) after storing in air atmosphere for 30 days or bending for 100 times. The results indicate that CsPbI3-xBrx perovskites have great potential in photodetection areas and pave the way to achieve high-performance flexible PDs.


Introduction
In the past few decades, a variety of inorganic semiconductor materials have drawn most attention of photodetectors research, such as InGaAs, GaN, ZnO, and Si [1][2][3][4][5][6]. Benefited by their excellent optical and electrical properties, the devices based on these materials exhibit high detectivity and fast response for visible light. However, such materials are usually obtained by extremely complex approaches or using expensive equipment [7][8][9], which is a bottleneck on the way to their commercial application. Hence, seeking for more promising substitute materials to reduce the cost, and simplify the preparation process is very important.
In this work, we prepared CsPbI 3-x Br x (x = 1, 2) films for flexible perovskite photodetectors (PDs). The flexible perovskite PDs based on CsPbI 3-x Br x exhibited fast response speed (90 μs/110 μs for CsPbI 2 Br PDs and 100 μs/140 μs for CsPbIBr 2 PDs), a high on/off ratio (10 4 ), and a high detectivity (10 12 Jones) under 520 nm lamp with 10 mV bias. Meanwhile, it exhibited outstanding mechanical flexibility and environmental stability. After keeping the device for 30 days in ambient air at 35-45% relative humidity, the periodic I-t curves of the device are only slightly declined (~3%). In addition, after bending the flexible PDs for 100 times under the bending radius of 9.12 mm, the periodic I-t curves of the device showed negligible change (< 3% decrease). The results indicate the potential of CsPbI 3-x Br x perovskites for flexible PDs.
The perovskite solution was prepared as follows. The CsPbI 2 Br perovskite precursor solutions were prepared by mixing the 1 M (mol L −1 ) CsBr, 1 M PbBr 2 , 2 M CsI, and 2 M PbI 2 in mixed anhydrous solvents of DMSO and DMF (9:1 in volume). The CsPbIBr 2 perovskite precursor solutions were prepared by mixing the 2 M CsBr, 2 M PbBr 2 , 1 M CsI, and 1 M PbI 2 in mixed anhydrous solvents of DMSO and DMF (9:1 in volume). Then, the mixed perovskite solutions were stirred at 75°C more than 2 h. All solutions must be were prepared in a nitrogen glovebox.

Preparation
Flexible substrates [polyimide (PI)] were cleaned consecutively with acetone, absolute ethanol, and deionized water for 15 min, respectively. And then the substrates were dried in an oven. Subsequently, interdigitated golden electrode (80 nm) was evaporated onto the flexible substrates by thermal evaporation. Before use, the patterned substrates were treated with UV-ozone for 20 min. Then the flexible substrates were transferred into glovebox for perovskite films deposition. The CsPbI 3-x Br x films were prepared using anti-solvent (DEE) one-step spin-coating method. Eighty-microliter precursor solution was spin-coating at a speed of 2000 rpm for 60 s. Then, 0.5 mL diethyl ether was poured on the samples at 10 s before the end of the spinning-coating processes. Then, the samples were annealed at 65°C for 5 min and 135°C for 15 min.

Measurements and Characterizations
Scanning electron microscope (SEM) images were obtained by using field emission SEM (FEI-INSPECT F50, Holland). X-ray diffraction (XRD) was carried out using a Bede D1 system with Cu Kα radiation. The ultraviolet-visible (UV-vis) absorption spectrum was measured by an UV-vis spectrophotometer (Schimadzu UV-3101 PC). The current-voltage (I-V) curves were carried out by Keithley 2636 semiconductor parametric analyzer under the illumination of a LD light source. Photocurrent was measured with an oscilloscope (Agilent DOS5012A) and an optical chopper modulating the light illuminated on the device. All measurements were carried out under ambient conditions at room temperature.

Results and Discussion
Anti-solvents, which are often used in the preparation of perovskite films to obtain a high-quality surface morphology. Herein, we used one-step spin-coating method with anti-solvent (DEE) to improve the morphology of CsPbI 3-x Br x films. Figure 1 exhibits the plan-view SEM images of CsPbI 3-x Br x films with or without DEE treatment. As shown in Fig. 1a, b, the CsPbI 2 Br and CsPbIBr 2 film without DEE-treatment possess enormous pinholes and small crystal grain size. In contrast, after DEE-treatment in the fabrication of perovskite, continuous film and larger grains are observed in the SEM images (Fig. 1c, d). The results of SEM images indicated that the morphology of the CsPbI 3-x Br x films was significantly improved by the treatment of DEE. Therefore, in the subsequent work, the anti-solvent treatment process was adopted to prepare CsPbI 3-x Br x films.
In order to examine the crystal structure of the prepared CsPbI 3-x Br x films, XRD patterns were carried out. As shown in Fig. 2a, the red and the blue lines indicate the XRD patterns of CsPbIBr 2 films and CsPbI 2 Br, respectively. It was clear to observe, in the red lines, the main peaks located at 14.75°, 20.94°, 29.96°, and 34.93°, which are assigned to the (100), (110), (200), and (210) facts of CsPbIBr 2 , respectively [41][42][43]. In the blue lines, we note two main peaks centered at 14.44°and 20.3°corresponding to the (100) and (200) planes of the pure CsPbI 2 Br α phase [44,45]. In addition, to certify that the CsPbIBr 2 and CsPbI 2 Br films were successfully prepared, UV-vis absorption curves and Tauc plots were measured (Fig. 2b-d). As shown in Fig. 2b, a small blue shift is visible in the absorption spectrum of the CsPbIBr 2 film compared to the CsPbI 2 Br film, which attributed to the bandgap difference between CsPbI 2 Br and CsPbIBr 2 . To explore the optical bandgap of CsPbI 3-x Br x film, we calculated the energy band (Eg) according to the reflection and transmission through the Tauc plots, as shown in Fig. 2c, d. It was observed from the Tauc plots that Eg of CsPbI 2 Br and CsPbIBr 2 were 1.91 eV and 2.05 eV, respectively, which is consistent with the previous report [46][47][48][49][50]. The results indicated the CsPbI 3-x Br x films were successfully fabricated.
After confirming the properties of the treated CsPbI 3-x Br x films, we prepared the flexible PDs based on CsPbI 3-x Br x films. In Fig. 3a, the flexible PDs with the device structure of PI/Au interdigitated electrodes/CsPbI 3-x Br x are shown. When it is irradiated on the perovskite layer under a bias voltage, the charge carrier transport is formed in PDs, as shown in Fig. 3b. Finally, electrons (holes) are collected by electrodes and circulated through an external circuit to generate photocurrent. To characterize the performance of CsPbI 3-x Br x PDs, we measured the photocurrent under 520 nm LD source at low bias voltages as shown in Fig. 3c, d. The result showed that the maximum photocurrent of CsPbI 2 Br PDs and CsPbIBr 2 PDs were beyond 180 μA and 120 μA under the illumination intensity of 8.23 mW/cm 2 , respectively. Importantly, the photocurrent of the device was measured at a bias voltage of 10 mV. To evaluate the response speed of the device, we investigated the transient photocurrent of the device. As shown in Fig. 3c, d, the rise and decay time of CsPbI 2 Br PDs are extracted to be around 90 μs and 110 μs, respectively. The rise and decay time of CsPbIBr 2 PDs were found to be about 100 μs and 140 μs, respectively. Compared to the same structural devices previously reported [12], the results manifested that the device  based on DEE-treated CsPbI 3-x Br x perovskite layer showed excellent performance.
To further investigate the photoelectrical characteristics of the PDs, the dark current and photo-current, responsivity, and detectivity were measured and plotted, as shown in Fig. 4. As showed in Fig. 4a, b, the dark current and photocurrent curves have approximate symmetry when the voltage was changed from − 5 to 5 V. This result indicated the formation of an ohmic contact between the metal and the perovskite layer. Owing to the contact barrier being very low for ohmic contact, carriers can be easily transferred under low bias. At low bias voltages, the CsPbI 3-x Br x flexible PDs possess a high on/off ratio of approximately 10 4 . Responsivity (R) and detectivity (D*) of the PDs were showed in Fig. 4c [51]. Therefore, the CsPbI 3-x Br x -based flexible PDs working under low bias voltages exhibit outstanding performances.
Environmental stability and mechanical flexibility are two important factors that influence the practical application of photodetectors based on perovskite. As described in Fig. 5, the environmental stability and mechanical flexibility of the PDs were tested by measuring the changes in periodic I-t curves of the device. It was clearly observed that the I-t curves of the device changed slightly (~3% decrease for CsPbI 2 Br PDs and~3% decrease for CsPbIBr 2 PDs). The changes of the devices' currents could be ignored after being exposed for 30 days in ambient conditions at 35-45% relative humidity (Fig. 5a, b). As showed in Fig. 5c, d, compared with the initial periodic I-t curves, a negligible oscillation was observed (~2% decrease for CsPbI 2 Br PDs and~3% decrease for CsPbIBr 2 PDs) after bending the device for one hundred times under the bending radius of 9.12 mm. The results demonstrate that our device has a great stability as well as good mechanical flexibility.

Conclusion
In summary, we present the flexible photodetectors based on CsPbI 3-x Br x (x = 1, 2) films which were treated with DEE in this work. The devices showed excellent performance, which were comparable to the same configuration devices. At a bias of 10 mV, the photodetectors showed a  high on/off ratio of 10 4 under 520 nm illumination, fast response speed (90 μs/110 μs for CsPbI 2 Br PDs and 100 μs/140 μs for CsPbIBr 2 PDs), and an excellent detectivity (10 12 Jones). Furthermore, the flexible PDs exhibited outstanding environmental stability and mechanical flexibility. After storing the device for 30 days in air at 35-45% relative humidity, the I-t curves of the device are only slightly declined (~3%). In addition, performance of the PDs showed a negligible change after bending the flexible PD a hundred times with a bend radius of 9.12 mm. This work demonstrates the enormous potential of CsPbI 3-x Br x perovskites in photoelectron detection and provides a promising approach to achieving high performance.