Halide perovskites show extensive application potential in fields such as solar cells, light-emitting diodes, and photodetectors due to their high light absorption coefficient, high carrier mobility, adjustable band gap, as well as low cost and simple synthesis methods^[1−5]. Perovskite photodetectors (PDs) with self-driven characteristics can not only leverage the excellent properties of perovskites to endow the device with a fast response time, high responsivity, and high detectivity, but also show low energy consumption as they do not require an external voltage for biasing and only drive the photoelectric conversion process through the built-in electric field^[6−9]. These advantages make self-driven perovskite PDs have broad application prospects in multiple fields such as environmental monitoring, forest fire warning, medical diagnosis, and life sciences^[10−12].

High-performance self-driven perovskite PDs usually have electron/hole transport layers to achieve efficient extraction of carriers^[13−15]. However, the existence of these transport layers often leads to a poor perovskite/transport layer interface, restricting the performance and stability of the device. Zhao et al. reported an interface engineering based on surface passivation strategy to achieve high-quality CsPbBr₃ perovskite films by introducing thioacetamide (C₂H₅NS, abbreviated as TAA) as interface materials, and the self-powered photodetector based on the passivated CsPbBr₃ film prefers the responsivity of 0.26 A/W and detectivity of 8.39 × 10¹² Jones with excellent stability^[13]. Yang et al. introduced 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) to modify the NiO_x/perovskite interface to enhance the hole extraction and charge carrier mobility, resulting in good thermal stability and photostability of the PD^[15]. Therefore, it is very necessary to design a PD with a simpler structure, fewer interfaces, and self-driven performance.

Here, perovskite nanowires are in-situ prepared on PbI₂, which not only simplifies the preparation of the device but also enables its detector to achieve excellent self-driven performance. In this structure, PbI₂ not only acts as the reaction raw material for perovskite but also realizes efficient extraction of carriers, so that our device shows excellent self-driven performance. For example, the responsivity (R) reaches 0.33 A/W, and the detectivity (D*) is as high as 3.52 × 10¹³ Jones. Furthermore, the device can detect the light with its optical power lowered to 0.1 nW/cm². This research provides a new method for preparing perovskite devices with simple structure but excellent performance.

Fig. 1 shows the preparation process of CsPbBr₃ nanowires. Before spin coating, the ITO substrate is placed in an ultraviolet ozone instrument for 50 min of treatment. The substrate and PbI₂ solution are preheated on a heating plate at 70 °C for 5 min so that the solution can be better and evenly coated on the substrate. Take 90 μL of PbI₂/DMF solution and spin coat it on the ITO substrate at 3000 r/min for 30 s, and then anneal it at 70 °C for 15 min to obtain a PbI₂ film. Immerse the PbI₂ film in a petri dish containing 30 mL of CsI (8 mg/mL) methanol solution and grow for 0.2/0.5/1/2/4/6/9/12/15 h respectively. Immerse the substrate in isopropanol solution to wash off the surface CsI methanol solution to obtain a non-perovskite phase CsPbI₃ (N-CsPbI₃) nanowire film. Then immerse the N-CsPbI₃ nanowires in a petri dish containing 30 mL of CsBr (8 mg/mL) methanol solution for 0.2/0.5/1/2/4 h respectively. Finally, anneal it on a hot plate at 200 °C for 15 min to obtain perovskite phase CsPbBr₃ (P-CsPbBr₃) nanowires. Finally, polymethyl methacrylate (PMMA) is used to fill the gap of nanowires and passivate the surface defects of namowires^[6], and the green and low-cost carbon paste is coated on the CsPbBr₃ nanowire film through a screen-printing template with the area of 0.2 × 0.2 cm². All preparation processes are carried out in air with a humidity of 40% to 60% and a temperature of 25 ± 5 °C.

In this work, the thickness is the key for efficient devices. A thickness that is too thin will be completely consumed to generate nanowires, thereby losing the function of carrier extraction. Being too thick results in the transport layer being too thick to effectively transport charge carriers, so the appropriate thickness of PbI₂ determines the high performance of the device. To obtain the PbI₂ hole extraction layer with the optimal thickness, we will study it from three aspects: PbI₂ thickness, nanowire growth time, and halogen ion exchange time. First, we investigate the PbI₂ thickness by tailing the rotation speed, as shown in Fig. 2. From Fig. 2(a), as the rotation speed decreases, a thicker PbI₂ film can bring a lower dark current. When the rotation speed is 2000 r/min, the dark current reaches the lowest. When the rotation speed is lower than 2000 r/min, the dark current suddenly increases. This is because at this time the rotation speed is too low, and PbI₂ cannot uniformly adhere to the ITO substrate to form a film. For a more intuitive understanding, we also calculated the on/off ratio of devices at different rotation speeds, as shown in Fig. 2(b). The on/off ratio is consistent with that of dark current. Besides, the XRD patterns of CsPbBr₃ nanowire films at different rotation speeds (Fig. 2(c)) show that they are the CsPbBr₃ film^{[16, 17]}. Based on the above discussion, we think 2000 r/min is the optimal rotation speed for spin-coating PbI₂.

The preparation of CsPbI₃ nanowires from PbI₂ film in CsI solution is top-down. Therefore, by controlling the growth time, a layer of PbI₂ film can be formed at the bottom of the nanowires. We explored the growth time of CsPbI₃ in CsI solution, as shown in Fig. 3. According to Fig. 3(a), as the growth time gradually increases, the dark current first gradually decreases, and reaches the lowest value when the growth time is 1 h. Then, the dark current gradually starts to rise. The photocurrent gradually increases as the growth time lengthens until the growth time reaches 2 h. For a more intuitive understanding, we calculated the on/off ratio of nanowire devices with different growth times, as shown in Fig. 3(b). The on/off ratio first gradually increases with the growth time of nanowires. When the growth time exceeds 1 h, the on/off ratio gradually decreases, which we think the PbI₂ film is too thin to effectively block electron transport. When the growth time exceeds 4 h, the on/off ratio has a significant decrease. We think that the PbI₂ film is completely consumed due to excessive soaking time in the solution, resulting in the bad on/off ratio. From the XRD patterns of CsPbBr₃ nanowires shown in Fig. 3(c), CsPbBr₃ films can be successfully formed under different growth times without other impurity peaks. Based on the above discussion, the nanowire growth time of 1 h is the best.

The SEM images further prove that when the nanowire growth time is 2 h, the nanowire films show good uniform surface morphology (Figs. 4(a) and 4(b)), and an obvious PbI₂ film at the bottom can be seen (Fig. 4(c)). When the time is 4 h, the morphology of the nanowires becomes poor, and there is no obvious PbI₂ film at the bottom.

For CsPbI₃ nanowires, although Br⁻ ions preferentially replace the I⁻ ions during the reaction, but an excessively long exchange time will also consume the underlying PbI₂ film, resulting in the invalidity of PbI₂ as a carrier blocking layer/transport layer. So investigating the exchange time of halogen element in CsPbI₃ nanowires is also important. From Fig. 5(a), as the exchange time increases, the dark current gradually increases and the photocurrent decreases. When the exchenge time is 0.2 h, the dark current reaches the lowest and the photocurrent reaches the maximum, resulting in a biggest on/off ratio (Fig. 5(b)). Besides, the XRD patterns show the exchange time of 0.2 h is sufficient to obtain CsPbBr₃ nanowires with few impurity peaks (Fig. 5(c)). Based on the above discussion, the optimal halogen exchange time is 0.2 h.

Based on the excellent photophysical properties of CsPbBr₃ nanowires, the performance of inverted photovoltaic photodetectors with ITO/PbI₂/CsPbBr₃/carbon structure are performed, as shown in Fig. 6. The current−time (I−t) curves of the device under variable illumination intensity (Fig. 6(a)) show stable and significant transient current response at all illumination intensities. Even when the light intensity is as low as 1.14 × 10⁻¹⁰ W/cm², the device still has a light response ability (Fig. 6(b)), showing excellent weak light detection ability. By extracting the data of the I−t curves, the linear dynamic range (LDR), R and D* of the device are calculated with the values of 215 dB (Fig. 6(c)), 0.33 A/W (under the light intensity of 1.14 × 10⁻¹⁰ W/cm², Fig. 6(d)) and 3.52 × 10¹³ Jones (under the light intensity of 1.14 × 10⁻¹⁰ W/cm², Fig. 6(d)), respectively. These results are the best for the reported perovskite nanowires PDs^{[4, 6, 18−22]}. Besides, the f_−3dB of the device reaches 379 Hz (Fig. 6(e)), and the response time: t_rise and t_fall are as low as 148.72 and 372.19 μs, respectively (Fig. 6(f)).

In addition to device performance, the stability of the device is also investigated. First, we tested the I−t curve of the unencapsulated device after heating at 86 °C for 6 h. As shown in Fig. 7(a), the performance of the unencapsulated device is around 80% of the initial performance after heating. Then we recorded the I−t curve of the unencapsulated device when it was irradiated with a big light intensity of 1.39 mW/cm² for up to 3600 s. As shown in Fig. 7(b), the performance of the unencapsulated device only drops to 75% of its original value. These results show that our device behaves good light- and thermal stability.

In summary, by in-situ fabricating perovskite nanowires on PbI₂, high-performance CsPbBr₃ nanowire PD with self-driven characteristic was obtained. In our device structure, the PbI₂ not only serves as the reaction raw material but also realizes efficient carrier extraction, avoiding the problem of poor perovskite/transport layer interface caused by traditional electron and hole transport layers. Finally, our device showed the R of 0.33 A/W, the D* of as high as 3.52 × 10 ¹³ Jones. Furthermore, the device can detect the light with its optical power lowered to 0.1 nW/cm². These results provide a research direction for further improving the performance of the device in the future.