Organic–inorganic hybrid lead halide perovskite materials have shown great achievements in the field of optoelectronic devices due to their large light absorption coefficient, low defect state density, and high carrier mobility [1–3]. At the same time, a simple solution spin-coating method can prepare high-quality films, which greatly reduces cost and promotes commercial application [4]. However, the thermal instability of organic–inorganic hybrid lead halide perovskite materials and the toxicity of lead limit their large-scale application. Therefore, seeking highly stable and environmentally friendly perovskite materials has become a research hotspot [5,6]. Different from traditional ${\mathrm{ABX}}_3$-type perovskite composed of angle-sharing [${\mathrm{BX}}_6$] octahedrons [where A is a monovalent inorganic cation (${\mathrm{Cs}}^{+}$, ${\mathrm{Rb}}^{+}$) or an organic ion group (${\mathrm{CH}}_3{\mathrm{NH}}_3^{+}$, ${\mathrm{NH}}_2{\mathrm{CHNH}}_2^{+}$), B is a divalent metal cation, and X is monovalent anions (${\mathrm{I}}^{-}$, ${\mathrm{Br}}^{-}$, and ${\mathrm{Cl}}^{-}$)], a new halide photovoltaic material based on edge sharing [${\mathrm{AX}}_6$] and [${\mathrm{CX}}_6$] octahedrons provides an idea for replacing Pb in perovskites. Monovalent cations and trivalent cations combine to form ${\mathrm{A}}_{\mathit{a}}{\mathrm{C}}_{\mathit{b}}{\mathrm{X}}_{\mathit{a}+3\mathit{b}}$ perovskite (A: Ag, Cu; C: Bi, Sb; X: I, Br), which alternately share [${\mathrm{AX}}_6$] and [${\mathrm{CX}}_6$] octahedrons (Ivan Turkevych named this structure as “rudorffites”) [7]. As a representative of the ${\mathrm{A}}_{\mathit{a}}{\mathrm{C}}_{\mathit{b}}{\mathit{X}}_{\mathit{a}+3\mathit{b}}$ perovskite, ${\mathrm{Ag}}_{\mathit{a}}{\mathrm{Bi}}_{\mathit{b}}{\mathrm{I}}_{\mathit{x}}$ ($\mathit{x}=\mathit{a}+3\mathit{b}$) is considered to be a promising material. The rapid increase in power conversion efficiency in the field of solar cells has proved their development potential [8–10], but their current device performances are not high and are restricted by fabrication methods and undesirable interfaces between the active layer and carrier transport layers. Therefore, exploring new methods and device structure to improve device performance is essential.

The performance of non-lead perovskite devices is greatly affected by the crystal quality of perovskite films [11,12]. Since most lead-free stabilized perovskite materials are all-inorganic, their thin-film crystal quality is more easily affected by the material preparation process. Among these processes, thermal evaporation is often used to fabricate all-inorganic lead-free perovskites, mainly because of the uniformity and high quality of the prepared films [13]. For example, Huabin Lan and co-authors prepared all-inorganic lead-free perovskite ${\mathrm{CsBi}}_3{\mathrm{I}}_{10}$ thin films by single-source thermal evaporation deposition; the film surface was compact and uniform, and had high homology with the crystal structure of the evaporation source material [14]. However, the use of thermal evaporation will cause great waste of materials, which is not conducive to commercialization. As a widely used solution spin-coating method in Pb-based perovskite devices, although it has the advantages of simple preparation and high utilization rate of raw materials, it is difficult to prepare all-inorganic non-lead perovskites due to the low solubility of the precursors in organic solvent and the poor film quality [15,16]. Therefore, a series of improved spin-coating methods have been developed, such as hot spin-coating, anti-solvent assisted method, and step-by-step annealing [17–19]. In addition, the use of excellent transport layer materials not only utilizes the transport and separation of photogenerated carriers, but also facilitates the growth of thin-film crystal quality, and ultimately enables the device to obtain excellent optoelectronic properties [20]. For example, according to Dong Yang and co-authors’ work, the record power conversion efficiency of planar-type perovskite solar cells with ethylene diamine tetraacetic acid-complexed ${\mathrm{SnO}}_2$ increases to 21.60% with negligible hysteresis [21].

Herein, we first introduce ${\mathrm{SnO}}_2$ as an electron transport layer (ETL) to fabricate high-quality lead-free ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskite film by a hot-spin-coating route. Compared with other ETLs, ${\mathrm{SnO}}_2$ has the following merits: (1) good energy level and high electron mobility (up to $240{\mathrm{cm}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$), which will enhance electron extraction and hole blocking, reducing the recombination loss, and improving the electron transport efficiency; (2) high transmittance over the whole visible spectra, which will allow most light to pass through ETL and be absorbed by perovskite active layer; (3) compared to ${\mathrm{TiO}}_2$ or other ETL, ${\mathrm{SnO}}_2$ displays excellent UV-resistance properties, chemical stability, and less photocatalytic activity, which will improve the stability of the device. By optimizing the fabrication process and tailoring the composition of the perovskite active layer, the crystal quality and light absorption of the ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ film were optimized, and a self-powered photodetector (PD) with the fluorine-doped tin oxide (FTO)/${\mathrm{SnO}}_2/{\mathrm{Ag}}_2{\mathrm{BiI}}_5/\text{carbon}$ structure was fabricated. Finally, our optimized ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ PDs achieved good detection capability for weak light. Under a 473 nm laser with a light intensity of $34\mathrm{nW}/{\mathrm{cm}}^2$, our PD showed detectivity ($D^{*}$) as high as $5.3\times {10}^{12}$ Jones and a linear dynamic range (LDR) of up to 138 dB. In addition, our unpackaged PDs showed good light, thermal, and storage stability in air. Our results provide a special route for the development of lead-free perovskite devices in an environmentally friendly field.

First, the FTO glass was cleaned by using deionized water, acetone, and alcohol. We mixed 15% (mass fraction) ${\mathrm{SnO}}_2$ syrup and deionized water at a volume ratio of 1:4 and ultrasonically for 10 min. After that, 100 μL of ${\mathrm{SnO}}_2$ aqueous solution was spin-coated at 3000 r/min on the surface of FTO, and then annealed at 150°C for 30 min.

We dissolved AgI powder and ${\mathrm{BiI}}_3$ powder in a 2:1 ratio in a mixed solution of dimethyl sulfoxide (DMSO) and N, N-dimethylformamide (DMF) to prepare a 0.3 M ($1\mathrm{M}=1\mathrm{mol}/\mathrm{L}$) ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ precursor solution ($\mathrm{DMF}\text{:}\mathrm{DMSO}=3\text{:}1$), and stirred completely to get a clear and transparent solution. We filtered the solution through a polytetrafluoroethylene (PTFE) syringe filter (0.45 μm). The ${\mathrm{SnO}}_2$ coated FTO substrate and ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ solution were simultaneously preheated at 100°C, and then 80 μL of the solution was spin-coated on the FTO at 5000 r/min, and finally annealed at 190°C for 10 min.

Finally, a layer of carbon paste was scraped on the surface of the ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ film and annealed at 120°C for 15 min to prepare a complete device.

High-resolution SEM images, X-ray diffraction (XRD) patterns, and absorption spectra were recorded by field emission SEM (FESEM, JEOL, JSM-6700F), a D8 FOCUS X-ray diffractometer, and a UV-Vis spectrophotometer, respectively. A Keithley 2400 source meter was used to record current–voltage (I-V) and current–time (I-t) curves. An oscilloscope was used to characterize the fast response time of our devices. A 473 nm laser was used as the light source, and its optical power was calibrated by a standard Si diode. A time-correlated single-photon counting technique (Fluo-Time 300, Pico Quant GmbH) was used to measure the time-resolved photoluminescence (TRPL) decay spectra of the samples.

Figure 1(a) shows the electronic structure of ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskite, which indicates AgI and ${\mathrm{BiI}}_3$ regular octahedrons alternately share edges. To optimize the surface morphology and quality of the ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskite film, we first study the influence of the precursor concentration on the morphology of the perovskite film, as shown in Figs. 1(b)–1(f). The 0.2–0.6 M ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ precursor solution is spin-coated at 5000 r/min, and the corresponding SEM images are shown in Figs. 1(b)–1(f). It can be seen that as the concentration of the precursor increases, the grain size of the perovskite first increases and then decreases; the largest grain size of the perovskite can reach as big as 500 nm and no holes can be seen, which is better than other reports [22,23] and may be attributed to the ${\mathrm{SnO}}_2$ ETL. In addition, there are large particles among the perovskite grains, and as the concentration increases (0.3–0.5 M), the particles increase in size and quantity; these particles should be ${\mathrm{BiI}}_3$, which can be verified by X-ray photoelectron spectroscopy (XPS), shown in Figs. 1(g)–1(i). From the figures, we can calculate a ratio of $\mathrm{Ag}\text{:}\mathrm{Bi}\text{:}\mathrm{I}=19.04\text{:}19.65\text{:}61.31$, indicating that many Bi and I elements exist on the surface of the perovskite.

Figure 2(a) shows the XRD patterns of the ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskite films, and all of the curves indicate a strong (003) diffraction peak with few weak peaks, which is consistent with the reported ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskite films in the literature [22,23], and further illustrates that the perovskite films prepared on ${\mathrm{SnO}}_2$ have good crystal quality. There are weak peaks located at 25.7° and 41.6°, which can further verify the existence of ${\mathrm{BiI}}_3$ grains. In addition, with the increasing concentration of the precursor solution, the (003) peak intensity increases, which may be attributed to the larger grain size and thicker film resulting from the larger concentration of the precursor. However, when the precursor concentration exceeds 0.5 M, the XRD peaks of the film decrease. It may be because that the larger solution concentration leads to poor perovskite film quality, which can be seen from the SEM results. The absorption spectrum curves of the ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskite films are shown in Fig. 2(b). We can clearly see the light absorption increases with the increase in concentration of the precursor, and when the concentration of the solution is oversaturated ($>0.5\mathrm{M}$), its bad film quality results in reduced light absorption. Figure 2(c) shows the TRPL curves, which further confirm the above results. From the curves, when the precursor concentration increases from 0.2 to 0.6 M, the average carrier lifetime of the film is 6.17, 30.45, 31.36, 35.40, and 29.24 ns, showing a trend of increasing first and then decreasing. Considering the above comparison of surface morphology and film quality, we choose the ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskite film fabricated by 0.3 M precursor concentration, which results in the cleanest and densest surface morphology, pure and appropriate XRD peaks, and long average carrier lifetime.

Figure 3(a) shows the device structure diagram in which the FTO, ${\mathrm{SnO}}_2$, and ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskites are the bottom electrode, ETL, and active layer, respectively, and the carbon is used as the hole transport layer and the top electrode. Based on the cost consideration, a PD with an hole transport layer (HTL)-free structure is fabricated. In conventional devices, a hole transport layer and a gold electrode are necessary. For our HTL-free structure devices, replacing the conventional hole transport layer and gold electrode with thicker carbon not only reduces the fabrication cost of the device and simplifies the fabrication process, but also helps to isolate moisture and oxygen from the air. The cross-sectional SEM image of the device is shown in Fig. 3(b), which demonstrates very clear ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskite film with thickness of about 400 nm.

To explore the effect of different precursor solution concentrations on the performance of PDs, we spin-coated ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ films fabricated by different precursor concentrations at a speed of 5000 r/min, and nine devices with a size of $2\mathrm{mm}\times 2\mathrm{mm}$ were used to count the data of light and dark currents, which were measured by using a 473 nm laser with an intensity of $314{\mathrm{\mu W}/\mathrm{cm}}^2$. The statistical results of the light and dark currents are shown in Fig. 3(c), and the corresponding photo-dark current switch data of the devices are shown in Fig. 3(d). It can be seen that when the precursor concentration increases from 0.2 to 0.3 M, the light (dark) current of PDs increases (decreases) significantly, and the corresponding switching ratio of each device also increases significantly [Fig. 3(d)]. When the precursor concentration continues to increase, the light (dark) current of PDs shows a decreasing (increasing) trend, leading to a reduced on–off ratio. Finally, the thin-film device spin-coated with 0.3 M precursor solution shows the best device performance.

We further study the influence of different speeds on the performance of PDs, and the results are shown in Fig. 4(a). We find that the light–dark currents of the devices prepared at low speeds (3000–6000 r/min) have little difference, and the switching ratios are almost the same [Fig. 4(b)]. At higher speeds (7000–8000 r/min), the dark current of the ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskite device increases and the switching ratio decreases. In addition, we fabricate the ${\mathrm{Ag}}_{\mathit{a}}{\mathrm{Bi}}_{\mathit{b}}{\mathrm{I}}_{\mathit{a}+3\mathit{b}}$ perovskite film PDs by tailoring the ratio of AgI and ${\mathrm{BiI}}_3$ to find the most suitable content of ${\mathrm{Ag}}_{\mathit{a}}{\mathrm{Bi}}_{\mathit{b}}{\mathrm{I}}_{\mathit{a}+3\mathit{b}}$, and the results are shown in Figs. 4(c) and 4(d). It can be seen that the device with AgI: ${\mathrm{BiI}}_3=2\text{:}1$ shows the best performance. Finally, the 0.3 M ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ precursor solution and 5000 r/min are chosen for follow-up research.

Figure 5(a) shows the I-V characteristics of the ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskite self-powered PD, and the irradiation light intensity is $277\mathrm{mW}/{\mathrm{cm}}^2$. The device exhibits an open circuit voltage of about 0.45 V, indicating the good self-powered capacity of our perovskite PDs. Figure 5(b) shows the I-t curves of the device under different light intensities at a bias of 0.1 mV. These curves show that the photocurrent increases distinguishably with the increase in light intensity, indicating that the device has good light intensity dependence, which is extremely beneficial for PDs to work in different light and dark environments. In addition, we find that the top three I-t curves above are not square pulses. The reason is that when the light is turned off, a small fraction of the charge carriers will remain in the active layer due to the large number of carriers under strong light intensity, which results in the phenomenon that the top three I-t curves are not square pulses in Fig. 5(b). Nonetheless, under strong light intensity, the photocurrent of the PDs drops very fast, and the value of the photocurrent drops by three orders of magnitude within 40 μs, showing a fast response characteristic. Furthermore, it can be seen that our device can show a dark current as low as $4\times {10}^{-10}\mathrm{A}$, and under strong light ($277\mathrm{mW}/{\mathrm{cm}}^2$), the photocurrent reaches $2.5\times {10}^{-4}\mathrm{A}$. In addition, under light intensity of $34\mathrm{nW}/{\mathrm{cm}}^2$, our device can still show good photoresponse [Fig. 5(c)], indicating that our PDs have excellent weak-light detection capabilities.

As a quality factor of PDs, the LDR is a state of the art that characterizes the linear response of the device, usually expressed as [24] $$\mathrm{LDR}=20\log \frac{P_{\mathrm{sat}}}{P_{\mathrm{low}}},$$(1)where, $P_{\mathrm{sat}}$ ($P_{\mathrm{low}}$) is the strongest (weakest) light intensity of incident light when the photocurrent starts to deviate from linearity. The LDR result is shown in Fig. 5(d), in which the current values are obtained by recording the current at a voltage of 0.1 mV under 473 nm laser irradiation, and the optical power is adjusted by the attenuator. By linearly fitting the data, the R-square determination coefficient of the device is 0.99168, which is infinitely close to one, showing that the device has good linear response when the light intensity is changed from $34\mathrm{nW}/{\mathrm{cm}}^2$ to $277\mathrm{mW}/{\mathrm{cm}}^2$. Finally, a large LDR value of 138 dB is achieved.

Responsivity ($R$) and $D^{*}$ are two important parameters to characterize the photoelectric response capability of a PD, and their values are determined by the following formulas [25,26]: $$R=\frac{I_{\mathrm{ph}}}{S{L}_{\mathrm{light}}},$$(2)$$D^{*}=\frac{R}{\sqrt{2q{J}_d}}.$$(3)

$I_{\mathrm{ph}}$, $S$, $L_{\mathrm{light}}$, and $J_d$ are the photogenerated current, effective illumination area of the device, light intensity, and dark current density, respectively. The $R$ and $D^{*}$ curves are shown in Fig. 5(e), which indicates that as the light intensity increases, both $R$ and $D^{*}$ show a downward trend. When the light intensity is $34\mathrm{nW}/{\mathrm{cm}}^2$, the measured $R$ and $D^{*}$ are the largest, with values of $0.3\mathrm{A}{\mathrm{W}}^{-1}$ and $5.3\times {10}^{12}$ Jones, respectively, which are significantly higher than those of reported high-performance lead-free perovskite PDs, and can be comparable to high-performance lead-based perovskite PDs (detailed comparison shown in Table 1). Figure 5(f) shows the response time of the device measured by using an oscilloscope at a frequency of 100 Hz. We can see that our device exhibits a faster response with the rise/fall time of 0.34/0.26 ms.Table 1.

Preparation Method and Device Performance of Metal Halide Perovskite Photodetectors

The stability of the device is very important for commercialization. Here, we investigate the light, thermal, and storage stability in air for the unpackaged device, as shown in Fig. 6. Figure 6(a) shows the I-t curves of the device measured at $314\mathrm{\mu W}/{\mathrm{cm}}^2$ light intensity for 1 h. After 3600 s of periodic illumination, the photocurrent of the device does not decrease, showing good light stability of the PD. Figure 6(b) shows thermal and storage stability characteristics. For the thermal stability test, we perform the following operations on the same sample: first, the sample is placed on a hot stage at 100°C for 30 min, and then its I-t curves are obtained. Its photocurrent decays to 93% of the initial value after it is heated at 100°C for 0.5 h, and the performance of the device returns to the initial state after being stored for a certain period of time, showing the good heat stability of our PDs. The small drop in photocurrent value during heating is attributed to the accumulation of ions and charge carriers at the interface when the device is operated at moderately high temperature [27]. In addition, after our unpackaged device is placed in air (stored in air at room temperature with humidity $50\%\pm 10\%$) for 25 days, its photocurrent shows no decrease, indicating excellent storage stability. Moreover, we perform the XRD to further study these treated samples, and find that after heating at 100°C for 30 min and storage for 25 days, the intensity of the diffraction peak does not decrease [Fig. 6(c)], which further proves that our device shows good thermal and storage stability.

In summary, we have prepared a non-toxic ${\mathrm{Ag}}_2{\mathrm{BiI}}_5$ perovskite PD by a low-cost, anti-solvent-free, and solution-processed method. After optimizing the concentration of the precursor solution and the rotation speed of the spin coating, a high-performance lead-free perovskite self-powered PD is fabricated. Under weak light ($34\mathrm{nW}/{\mathrm{cm}}^2$), our devices show good self-powered photoelectric response with $D^{*}$ as high as $5.3\times {10}^{12}$ Jones. In addition, the performance of the unpackaged device does not deteriorate after 25 days of storage, and good light and thermal stability is achieved. This work is helpful for fabricating lead-free perovskites and lays the foundation for the application of perovskites in an environmentally friendly field.