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1. Introduction

Photodetectors—the vital components of modern imaging and communication systems—have been playing an increasingly important role in modern industrial production, basic scientific research, space development, ocean exploration, military and national defense, environmental protection, medical diagnosis, transportation, and other fields. For example, ultraviolet photodetectors can be used in ozone sensing, flame sensing, etc. ^[1][2][3]. The visible photodetectors can be used in biological sensing, video imaging, and convert communications ^[4][5][6][7]. Infrared photodetectors can be used as infrared night vision. ^[8][9][10] The THz photodetectors can be used in the security detection of customs, airports, and other special occasions ^[11][12][13]. Therefore, the further research of photodetectors is of great significance. A semiconductor, which is essential for a photodetector, can absorb the incident photons and generate electron and hole pairs. In the presence of a built-in or applied electric field, electric current is generated when the electrodes extracted and collected photogenerated carriers. In order to obtain a high-performance photodetector with high sensitivity and fast response, the semiconductor needs to have effective charge collection, low trap state density, and high carrier mobility. Till now, a large variety of semiconductor materials have been used for constructing photodetectors, including Si ^[14][15][16], carbon nanotubes ^[17][18], group II–VI and III–V compounds ^[19][20][21], and remarkable progress has been made in improving the detection performance. Epitaxial growth technology with stringent conditions, which is the most commonly used for synthesis, hampers their commercial application ^[22]. Therefore, it is of great significance to exploit candidates to reduce production cost and improve photodetector performance.

₃ has attracted wide research interest in the photodetector field ^[23][24][25]. For ABX

₃

₃

₃⁺

⁺

²⁺

²⁺

⁻

⁻

⁻). Many properties of perovskite make it an ideal material for photodetectors. For example, ambipolar transport, low density of defects and traps within bandgap can effectively reduce the charge recombination, and thus improve the performance of photodetectors. ^[26] The absorption spectra can cover the UV–Visible–Infrared region by facile halide substitution, which is desirable for broadband photodetector applications ^[27][28]. High absorption coefficient and direct bandgap result in a fast photoresponse in a very thin perovskite layer because of short transmission distance of photongenerated carriers ^[29][30]. In addition, low cost and easy preparation remove obstacles in future mass preparation. For example, MAPbI

₃

⁻¹

³% ^[31]. However, there is an inherent paradox to simultaneously possess both low dark currents and high photocurrents. The former requires a large number of defects or barriers that appear in polycrystalline film to inhibit the transmission of thermally excited carriers ^[32][33][34], while the latter requires single crystals with good crystallinity for effective charge transfer ^[35][36][37]. Compared with photodetectors based on polycrystalline film and bulk crystals, nanostructured photodetectors exhibit superior performances. The large surface-to-volume ratios of nanostructures result in longer photocarrier lifetime, which is conducive to higher sensitivity and responsivity. In addition, the reduced dimension shortens the carrier transmission time and improves the response speed ^{[19][38][39][40][41]}. Therefore, the performance of photodetector based on nanostructured perovskites will be better. For example, a high responsivity of 1294 A W

⁻¹

¹⁴

₃

₃

⁷

_3-x

_x

₂

Table 1

Table 1.

The key parameters of photodetectors.

Parameters	Definition
Photoresponsivity (R)	the ratio of the photocurrent to the incident power on the active area: R = (Ip − Id)/(PA), where Ip is the photocurrent, Id is the dark current, P is the light intensity, A is the active area
EQE	Photoelectric conversion efficiency. EQE = Rhc/e, where h is the Planck’s constant, c is the light velocity, e is the electronic charge.
Gain (G)	The number of charge carriers through external circuit for per incident photon: G = τl/τt = τl(µV)/d2, where τl is the carrier lifetime, τt is the carrier transit time, µ is the carrier mobility, V is bias voltage, d is the channel length
Detectivity (D*)	D* = (A△ f)1/2/NEP, where A is the active area of the detector, △f is the electrical bandwidth, NEP is the noise equivalent power.
LDR	LDR usually stands for “Linear Dynamic Range”, defined as the range in which the current response of the photodetector is linearly proportional to the light intensity. LDR = 20 log (Ip*/Id), Where Id is the dark current.
Response speed (rise/decay time)	The ability of devices to track the incident light signal.

2. Photodetectors

Perovskite-based photodetector devices can be divided into two categories, photovoltaic and photoconductive photodetectors. According to the spatial layout of the photoactive medium and electrodes, perovskite-based photodetector devices can be further divided into vertical type and lateral type. In general, vertical photodetectors provide fast response and low driving voltage because of the small electrode spacing with a short carrier transit length; in contrast, lateral photodetectors show slow response and high driving voltage due to their large electrode spacing. For photovoltaic photodetector, or photodiode (

Figure 1

a), the device structure is similar to that of solar cell configuration. Photodiodes based on perovskite polycrystalline films or single crystals are widely reported, but those based on nanostructured perovskites are rarely reported. Photodiodes typically rely on PN junction, which can provide a built-in electrical field at the junction interface to aid the electrons and holes to transport in opposite directions toward electrodes. Owing to the junction barrier at the interface, photodiodes exhibit low dark current and large detectivity. However, they suffer from low responsivity and external quantum efficiency (EQE ≤ 100%). As for photoconductive photodetector, it can be further divided into photoconductor (

Figure 1

b) and phototransistor (

Figure 1c). Compared with photovoltaic photodetector, photoconductive photodetector exhibits high responsivity, EQE (beyond 100%) and large gain. External voltage leading to multiple electrical carriers recycling per single incident photon should be responsible for the large gain ^{[47][48][49][50]}. However, large gain, in turn, usually results in a slow response speed because both the response time and the gain are determined by the carrier lifetime. Therefore, the intrinsic contradictions between the responsivity and response speed always exist. One solution is to fabricate phototransistor by adding gate electrode (Si) and dielectric layer (SiO

c). Compared with photovoltaic photodetector, photoconductive photodetector exhibits high responsivity, EQE (beyond 100%) and large gain. External voltage leading to multiple electrical carriers recycling per single incident photon should be responsible for the large gain [47,48,49,50]. However, large gain, in turn, usually results in a slow response speed because both the response time and the gain are determined by the carrier lifetime. Therefore, the intrinsic contradictions between the responsivity and response speed always exist. One solution is to fabricate phototransistor by adding gate electrode (Si) and dielectric layer (SiO

₂

) to the photoconductor. The charge transport can be controlled by applying a gate voltage. It is demonstrated that phototransistor can simultaneously enhance the photoresponsivity and exhibit an ultrafast photoresponse speed [51].

Figure 1.

Schematic and characteristics of perovskite-based photodetectors. (

a

) photodiode, (

b

) photoconductor, and (

c

) phototransistor.