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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2024 , Vol. 36 >Issue 2: 187 - 203

DOI: https://doi.org/10.7536/PC230526

20

Perovskite-Based Near-Infrared Photodetectors

  • Wenhuan Gao 1 ,
  • Jike Ding 1 ,
  • Quanxing Ma 1 ,
  • Yuqing Su 1 ,
  • Hongwei Song 2 ,
  • Cong Chen , 1, *
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  • 1 School of Materials Science and Engineering, State Key Lab of Reliability and Intelligence of Electrical Equipment, Hebei University of Technology, Tianjin 300130, China
  • 2 State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering, Jilin University, Changchun 130012, China.
* Corresponding author e-mail:

Received date: 2023-05-25

  Revised date: 2023-08-09

  Online published: 2023-09-10

Supported by

National Natural Science Foundation of China(62004058)

Natural Science Foundation of Hebei Province(F20202022)

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Abstract

In recent years, organo-metal halide perovskites materials with ABX3 crystal structure have shown promising application prospects in the field of photoelectric detection due to their optical and electrical properties such as adjustable bandgap engineering, high absorption coefficient and long carrier transmission distance. Especially, the hybrid perovskite prepared by pure Sn or Sn/Pb mixed cations have excellent near-infrared photoelectroresponse in the range of 760~1050 nm, showing many advantages such as high sensitivity, low dark current and high detection rate. To further broaden the near-infrared and infrared response wavelength range of perovskite, the researchers explored combining organic materials, crystalline silicon/germanium, Ⅲ-Ⅴ compounds, Ⅳ-Ⅵ compounds, upconversion fluorescent materials as complementary light absorption layers with perovskite to prepare heterostructures to construct wide-spectrum response near-infrared photodetectors. Based on the above research, this paper summarizes the current effective ways to broaden the spectrum range of perovskite photodetectors. At the same time, the future development prospect of perovskite material near infrared photodetector is prospected.

Contents

1 Introduction

2 Basic indicators of photodetectors

2.1 Device structure and working principle of photodetectors

2.2 Performance parameters of photodetectors

2.3 Strategy of broadening the spectrum response range of perovskites

3 Pb perovskite for near-infrared photodetectors

3.1 Polycrystalline perovskite materials

3.2 Single crystal perovskite materials

4 Narrow band gap Sn and Sn/Pb Mixed Perovskite- Based near-infrared photodetectors

4.1 Sn-based perovskite near-infrared photodetectors

4.2 Sn/Pb mixed perovskite near-infrared photodetectors

5 Perovskite/inorganic heterojunction near-infrared photodetectors

5.1 Silicon and other classic semiconductors

5.2 Graphene

5.3 Transition metal dichalcogenides

5.4 Ⅲ-Ⅴ compounds semiconductors

5.5 Ⅳ-Ⅳ compounds semiconductors

6 Perovskite/organic heterojunction near-infrared photodetectors

7 Perovskite/upconversion material near-infrared photodetectors

8 Application of near-infrared photodetectors

9 Conclusion and outlook

Key words： optical device; perovskite; near-infrared photodetectors; narrow-bandgap materials

Cite this article

Wenhuan Gao , Jike Ding , Quanxing Ma , Yuqing Su , Hongwei Song , Cong Chen . Perovskite-Based Near-Infrared Photodetectors[J]. Progress in Chemistry, 2024 , 36(2) : 187 -203 . DOI: 10.7536/PC230526

1 Introduction

photodetectors (PDs) are devices that can capture optical signals in a specific wavelength range in real time and convert them into electrical signals, which have been widely used in photoelectric imaging, optical communication, biosensing and other fields. In recent years, near-infrared (NIR) photoelectric detection technology, which detects the spectrum in the near-infrared (NIR, 780 ~ 2526 nm) band, has attracted much attention[1⇓⇓~4]. Compared with ultraviolet (200 ~ 400 nm) and visible light (400 ~ 780 nm), the penetration depth of NIR photons is 1 ~ 20 mm, which has the advantages of less light damage to biological samples, strong penetration ability to deep tissues, and less interference from the fluorescence of living biological systems, and is considered to be a powerful tool for medical diagnosis and surgical treatment[5]. In addition, near-infrared photodetectors (NIR-PDs) have been widely used in optical fiber communication, production, military and other fields[6]. As NIR detection technology plays a vital role in national economic and social development, it is of great practical significance to develop NIR-PDs with excellent performance, low cost, environmental protection and sustainability.
Perovskite materials are a general term for compounds with a ABX3 crystal structure, where A is located at the vertex of the cubic unit cell and is an organic or inorganic cation （CH3NH3+（MA+）, HC(NH2)2+（FA+）, or Cs+, etc.); B is located at the center of the face-centered cubic structure and is a metal cation （Pb2+ or Sn2+, etc.); X is located at the face center of the cubic unit cell and is the halide anion （Cl−, Br−, I−, or a mixture thereof). The relatively small volume of metal cations is located in the central position, each metal cation is surrounded by six anions, forming an octahedral structure, and the octahedra are connected to each other at the vertices to form a network (Fig. 1)[7]. As the most typical perovskite material, MAPbI3 has the advantages of large optical absorption coefficient, long diffusion length and high carrier mobility. In recent years, PDs based on it can achieve high sensitivity, wide dynamic responsivity, ultrafast response and other key properties[8⇓~10]. At present, organic metal halide perovskite materials have made breakthroughs in the field of photovoltaic devices, and the photoelectric conversion efficiency of solar cell devices has exceeded 26.1%, with excellent photoelectric response performance[11].
图1 具有(a)立方和(b)四方晶体结构的MAPbI3钙钛矿型晶体结构[12]

Fig.1 Atomic models of perovskite MAPbI3 nanocrystals with (a) cubic and (b) tetragonal crystal structures.[12] Copyright 2014, IOP Publishing Ltd.

In 2014, Hu et al. First applied MAPbI3 perovskite as a host material to PDs, which exhibited a broadband response range[13]. The responsivities at 365 nm and 780 nm are 3.49 A·W-1 and 0.0367 A·W-1, respectively, and the response time is less than 0.1 s. This new type of PDs with high sensitivity and fast response has undoubtedly opened up a new field of scientific research, and perovskite PDs have been developing continuously since then. However, the detection range of both single crystal and polycrystalline MAPbI3 is limited to the wavelength region within 820 nm, and the lack of NIR full-band response seriously limits their application in more fields. Wang et al. Reported that PDs based on MAPbI3 and PDPP3T/PC71BM achieved 40% external quantum efficiency (EQE) in the NIR region, but the microsecond response time of the device is difficult to meet various requirements in applications[14]. In addition, Wu et al. Reported a broad-spectrum NIR-PDs with an EQE of 70%, but the noise current of the PDs is very high and the response time is very slow, which makes it difficult to have more uses[15]. The ideal perovskite PDs should have a high EQE value in the NIR region, and have both high sensitivity and ultra-high response speed.
The study shows that perovskite is an ideal choice for a new generation of photodetectors, which is expected to develop a new generation of low-cost and high-performance NIR-PDs. However, there are still some problems such as limited response range and poor reliability. In this paper, we will focus on the topic of "perovskite-based photodetectors". Specifically, Section 2 introduces the device structure, working principle, etc. Of PDs, Sections 3 and 4 introduce the latest research progress of Pb-based, Sn-based and Sn/Pb-based perovskite NIR-PDs, respectively, and Sections 5 and 6 discuss the research progress of NIR-PDs formed by Pb-based and various narrow-gap NIR materials (including inorganic quantum dots, organic small molecules/polymers, up-conversion materials, etc.) to form heterojunction structures. Finally, the development prospect of NIR-PDs based on perovskite materials is prospected.

2 Basic specification of photoelectric detector

2.1 Device structure and working principle of photoelectric detector

There are three main structures of perovskite PDs: photoconductive, photodiode, and phototransistor (Fig. 2). Among them, the photoconductive type has received extensive attention due to its simple structure and easy integration. The photoconductor applies a bias voltage to separate the photogenerated carriers, thereby increasing the conductivity of the device. The photodiode structure consists of a transparent electrode, a hole transport layer (HTL), a perovskite active layer, an electron transport layer (ETL) and a metal electrode. Another type of phototransistor includes a dielectric layer, an active layer, and three electrodes, a source, a drain, and a gate.
图2 三种不同类型的钙钛矿PDs示意图 (a)光电二极管；(b)光电导型；(c)光电晶体管

Fig.2 Three different types of perovskite PDs schematic (a) photodiode; (b) photoconductive type; (c) phototransistor.

The working mechanism of the photodetector mainly includes the following three steps: (1) generating photogenerated carriers under illumination; (2) carrier diffusion or migration to form current; (3) The photocurrent is amplified and converted into an electrical signal in the amplifying circuit. When the detector is illuminated, if the band gap of the material is smaller than the energy of the incident photon, the valence band electrons will jump to the conduction band, thus generating photocurrent.

2.2 Performance parameters of photoelectric detector

The performance of PDs is a comprehensive evaluation index. According to the different requirements of practical applications, the emphasis on the performance parameters of devices is also different. Specifically, it includes the following important performance parameters:
Spectral response range: The spectral response range of PDs depends on the absorption spectrum of the semiconductor material, that is, on the bandwidth of the semiconductor material. For narrowband PDs, the spectral response range is affected by carrier collection, partially dependent on the absorption properties of the material itself.
Responsivity (R) is the ratio of the output voltage or current to the input optical signal power, representing the ability of PDs to convert an optical signal into an electrical signal when illuminated by incident light. It can be expressed as:
$R=I_{p}-I_{d} / P$
Where Ip is the photocurrent, Id is the dark current, P is the incident light power, and R unit is A·W-1.
EQE refers to the ratio of collected photocarriers to the number of incident photons. The EQE of PDs is generally less than 100%, which can be expressed as:
$E Q E=R h c / \lambda q$
Where H is Planck's constant, C stands for the speed of light, Q is the amount of unit charge, and is the incident light wavelength.
Specific detectivity （D*） refers to the ability of PDs to detect optical signals in noisy environments. It is often used to describe the ability of PDs to detect weak light. The higher the D*, the higher the detectability of PDs. It can be expressed as:
$D^{*}=R /\left(2 q J_{d}\right)^{1 / 2}$
Where R stands for the responsivity, Q is the amount of unit charge, Jdstands for the dark current density, and D* has units of.
The noise current is the root mean square of the random fluctuation of the dark current over the probe bandwidth. The noise current affects the sensitivity of the detector. The noise current is mainly composed of frequency-independent shot noise （ishot） and thermal noise （ithermal）, and frequency-dependent 1/f noise （i1/f） and generation-recombination noise （ig-r）.
Noise Equivalent Power (NEP) is the incident light power at which the detector output voltage is exactly equal to the output noise voltage. It represents the lowest light intensity at which PDs can recognize noise, and NEP is generally considered to be the inverse of detectivity. The smaller the NEP, the stronger the ability of PDs to detect weak light. It can be expressed as:
$N E P=(S \Delta f)^{1 / 2} / D^{*}$
Where S represents the effective area of the PDs, ∆ f represents the bandwidth, D* represents the specific detectivity, and NEP is in W.
Linear dynamic range (LDR) is the range of optical power over which the output current or voltage is linearly proportional to the input optical signal, expressed as the ratio of the strongest to the weakest optical power (irradiance) at which the PDs maintain a linear response. The wider the LDR is, the better the detection performance of PDs is. It can be expressed as:
$L D R=20 \log P_{\max } / P_{\min }$
Where Pmax and Pmin are the maximum, minimum values of the optical power in the linear range, respectively, and the LDR is in dB.
Response time: PDs The time required for the signal to rise from 10% to 90% of the maximum value and the time for the signal to fall from 90% to 10% of the maximum value under the incident light is the rise or fall time in s.

2.3 Strategy for Broadening the Spectral Response Range of Perovskites

Since perovskite materials have the advantage of tunable band gap, the spectral response range of perovskite-based PDs can be broadened by composition control of perovskites and compounding with other narrow band gap materials, including: (1) mixed Sn/Pb perovskite materials[16]. It is found that the partial substitution of Sn in Pb-based perovskites can tune the absorption to the NIR region, which will obtain an ideal semiconductor band gap (1.21 eV) close to the Shockley-Queisser limit. (2) by incorporating heterojunction materials with NIR absorption. The introduction of the heterojunction can further reduce the dark current and improve the responsivity and sensitivity. By selecting complementary materials with different band gaps and charge mobilities to realize the regulation of spectral response characteristics, the response spectrum can be effectively broadened to the NIR range, which has great potential in high-sensitivity and high-resolution imaging systems.

3 Pb-based perovskite photodetector

3.1 Polycrystalline perovskite materials.

Polycrystalline perovskites, which mainly exist in the form of thin films, are considered to be an ideal high-efficiency photovoltaic material because of their simple preparation process, compatibility with solution processing and evaporation deposition coating, as well as their high optical absorption coefficient, large carrier mobility and good grain boundary. Roqan et al. First prepared Gd-doped ZnO nanorods /MAPbI3PDs on metal substrates[17]. Fig. 3A shows that there is a good energy level matching between the metal substrate and the conduction band of Gd-doped ZnO nanorods, and the significantly improved carrier extraction efficiency helps PDs to achieve a wide spectral response in the wavelength range of 250 ~ 1357 nm. Yan et al. Prepared MAPbI3-xClx/ organic semiconductor vertical heterojunction PDs[18]. Due to its high gain, the device exhibits a high responsivity close to the 109A·W−1 at NIR wavelengths, and the polycrystalline perovskite thin film device also has good bending stability (fig. 3B). In addition to this, Yang et al. Realized NIR photodetection using surface trap States associated with the surface of MAPbI3 nanocrystals, with a spectral response range of about 1000 nm, LDR reaching 100 dB (Figure 3C), and detectivity of 1.77×1013Jones[19].
图3 (a) ZnO纳米棒/ MAPbI3 PDs能级匹配图[17];(b) MAPbI3-xClx用于柔性器件示意图[18];(c) MAPbI3纳米晶体 PDs的光电流和响应度[19];(d) MAPbI3单晶照片[21];(e) MAPbI3单晶响应速度曲线[22];(f) MAPbI3单晶空间电荷限制电流测试(SCLC)[23]

Fig.3 (a) Energy level matching diagram of ZnO nanorods/ MAPbI3 PDs[17];(b) Schematic diagram of MAPbI3-xClx for flexible devices[18];(c) Photocurrent and responsivity of MAPbI3 nanocrystalline PDs[19];(d) Photographs of MAPbI3 single crystals[21];(e) Response velocity curves of MAPbI3 single crystals [22];(f) MAPbI3 single crystal space charge limiting current test (SCLC)[23] Copyright 2017, American Chemical Society. Copyright 2017, Nature. Copyright 2020, Wiley-VCH. Copyright 2016, Wiley-VCH. Copyright 2018, Elsevier. Copyright 2022, Royal Society of Chemistry.

3.2 Ingle crystal perovskite material

Compared with polycrystalline perovskite, single crystal perovskite has wider absorption spectrum, better carrier transport properties and lower defect density. Therefore, high-quality single-crystal perovskites will be an important way to improve device performance. Zhao et al. Reported that the absorption band edge of the FAPbI3 polycrystalline film was 780 nm, while that of the single crystal film was 850 nm, proving that the single crystal could effectively broaden the response spectrum[20]. Due to the trap state absorption, the perovskite single crystal has some detection ability in the NIR range. Meredith et al. Detected the wavelength of 1064 nm on a MAPbI3 single crystal (Figure 3D), and the surface trap States of the single crystal enhanced the NIR photoresponse[21]. Liu et al. Prepared a single crystal of MAPbI3 with a length of 80 mm by the inverse temperature crystallization (ITC) method, and the single crystal PDs have the advantages of broad spectral response, fast response speed (Fig. 3e), and good stability in the range of 900 nm[22]. The MAPbI3 single crystal prepared by Yu et al. Has an absorption sideband of 840 nm, and the space-charge-limited current test SCLC (Figure 3 f) proves that its trap state density is low, and its responsivity in the near-infrared band is 1.33 A·W-1.Detectivity up to 2.18×1012Jones.Xi et al. Constructed 2D single-crystal perovskites by incorporating thermally annealed gold nanoparticles (NPs), and the local electric field induced by gold NPs enhanced the photocurrent enhancement effect while reducing the dark current[23][24]. The experimental results show that the PDs can convert the optical signal at 1310 nm into a stable electrical signal through plasma discharge. The above related work provides a new idea to promote the development of perovskite NIR-PDs and their optical detection in the field of communication.

4 Narrow-gap Sn-based and Sn/Pb perovskite photodetectors.

4.1 Pure Sn-based perovskite NIR-PDs.

Compared with Pb-based perovskites, nontoxic Sn-based perovskites with pure substitution of Sn2+ for Pb2+ have narrower band gap and can be used as photoactive layers of NIR-PDs. However, the natural oxidation of the Sn2+ to Sn4+ state in Sn-based perovskites induces p-type doping, which changes the semiconducting character to a metal-like one[25⇓~27]. This behavior will lead to too high carrier concentration, reduced carrier lifetime and diffusion length, and thus reduce the performance of devices containing Sn2+. In pursuit of efficient NIR-PDs, researchers have conducted a series of methods to inhibit the oxidation of Sn2+ in Sn-based perovskites to improve the photoresponsiveness in the NIR band (as shown in Table 1)[28,29].
表1 常见Sn基和 Sn/Pb基钙钛矿NIR-PDs

Table 1 Common Sn and Sn−Pb Perovskite NIR-PDs

Perovskite Wavelength range (nm) Responsivity (mA·W−1) Detection rate (Jones) EQE
(%)		 LDR (dB) Response time [trise/tdecay] Ref.
MASnI3 300~1000 470 8.8×1010 1.5 s/0.4 s 30
CsSnI3 475~940 54 @940 nm 3.85×105 83.8 ms/243 ms 31
CsSnI3 400~900 257 1.5×1011 0.35 ms/1.6 ms 32
FASnI3 300~1000 33
FASnI3 300~1000 1.1×108 1.9×1012 180 s/360 s 34
FASnI3 300~1000 2×108 @850 nm 3.2×1012 117 s/206 s 35
PEA0.15FA0.85SnI3 450~850 0.39 8.29 × 1011 0.78 μs 50
MA0.975Rb0.025Sn0.65Pb0.35I3 300~1100 400 @910 nm >1012 110 40 ns/468 ns 38
MASnxPb1-xI3 300~1100 200 @940 nm >1011 >20% @780-970
nm		 100 0.09 μs /2.27 μs 39
FA0.85Cs0.15Sn0.5Pb0.5I3
 600~1000 530 @940 nm 6 ×1012 ≈80% @
760-900 nm		 103 58.3 ns/0.86 μs 40
(FASnI3)0.6(MAPbI3)0.4 300~1000 400 @950 nm 1.1 × 1012 >65% @350-900
nm		 167 6.9 μs/9.1 μs 14
Cs0.05MA0.45FA0.5Pb0.5Sn0.5I3 300~1050 530 @910 nm 2.01 × 1011 0.035 μs 41
CsPb0.5Sn0.5I3
(5% (PEA)2Pb0.5Sn0.5I4)		 700~900 270 @850 nm 5.42×1014 42
MA0.5FA0.5Pb0.5Sn0.5I3 (2.5% (PEA)2Pb0.5Sn0.5I4) 700~900 ≈100 @800 nm ≈1.6 × 1012 ≈14% @800 nm 10 μs /10 μs 51
(MAPbI3)0.5(FASnI3)0.5 300~1050 410 2.91×1012 >60 @808 nm 10.9 ms/8.9 ms 43
MA0.5FA0.5Pb0.5Sn0.5I3 350~1000 >200 @
800~950 nm		 >1012 ≈10% @800 nm 44
MA0.3FA0.7Pb0.5Sn0.5I3 470~910 600 1.5 × 1012 85% @850 nm 45
FA0.5MA0.45Cs0.05Pb0.5Sn0.5I3 300~1050 350 @950 nm 2.21 × 1011 75% @800 nm 185 42.9 ns 46
Cs0.15FA0.85Pb0.5Sn0.5I3 300~1050 520 @850 nm 5.34 × 1012 75% @850 nm 224 39.68 ns 47
FA0.7MA0.3Sn0.5Pb0.5I3 450~900 510 1.8 × 1012 75.4% @840 nm 94 ns/97 ns 48
FA0.85Cs0.15Sn0.5Pb0.5I3 400~900 570 8.48 × 1012 80 @910 nm 67.5 ns/0.72 μs 49
MASn0.25Pb0.75I3 500~900 510 1.1 × 1013 192.6 52
The use of SnX2（X=F,Cl,Br,I） as an additive can stabilize the phase and form a Sn2+ rich environment, which can reduce the Sn vacancy density. Waleed et al. Deposited Sn at the bottom of the porous alumina template, which effectively blocked the diffusion of water and oxygen molecules[30]. Thus, three-dimensional MASnI3 nanowires (NWs) with a band gap of 1.3 eV were fabricated, and Figure 4A is the growth schematic of the perovskite NWs, and the device exhibited broad spectral absorption in the wavelength range of 300 – 1000 nm, with responsivity up to 0.47 AW-1 and detectivity of 8.8×1010Jones. Yang et al. Prepared CsSnI3NWs with a band gap of 1.34 eV by chemical vapor deposition, which reduced the density of Sn vacancies due to the reconstructed reciprocal lattice relationship, as shown in Figure 4 B. The rise and fall times of the device were 83.8 and 243.4 ms, respectively[31]. This work applies CsSnI3 perovskite NWs to room-temperature NIR detection, which lays the foundation for the development of novel and efficient low-dimensional all-inorganic perovskite photovoltaic devices.
图4 (a) Sn钙钛矿NWs的样品示意图[30] ;(b) CsSnI3钙钛矿PDs上升/下降时间[31];(c) 在空气中暴露6 h后的CsSnI3样品XPS曲线[32];(d) 有无KHQSA修饰的FASnI3薄膜的SEM图像[33];(e) FASnI3/PEDOT:PSS异质结的PDs的探测率曲线[35]

Fig.4 (a) Sample schematic of Sn perovskite NWs[30];(b) Rise/fall time of CsSnI3 perovskite PDs[31];(c)XPS curves of CsSnI3 samples after exposure to air for 6 h[32];(d) SEM images of FASnI3 thin films with and without KHQSA modification[33];(e) Detectivity curves of FASnI3/PEDOT:PSS perovskite PDs [35] Copyright 2016, American Chemical Society. Copyright 2019, American Chemical Society. Copyright 2020, Wiley-VCH. Copyright 2019, Wiley-VCH. Copyright 2020, American Chemical Society.

In order to improve the stability of Sn-based perovskite materials and detection devices, Cao et al. Used ascorbic acid to promote the growth of CsSnI3 crystals and inhibit the oxidation of Sn2+ to Sn4+. XPS in Fig. 4 C shows that ascorbic acid was successfully applied to the sample, and broadband PDs with a spectral range of 350 – 1000 nm were obtained. The responsivity of the PDs at 850 nm wavelength was 0.257 A·W−1, and the detectivity was 1.5×1011Jones[32]. The introduction of antioxidant hydroxybenzene sulfonic acid (KHQSA) as an additive is also effective for controlling the rapid growth of Sn-based perovskites and inhibiting Sn oxidation[33]. The SEM image of the FASnI3 film with and without KHQSA modification is shown in Fig. 4D, the interaction between sulfonic acid groups and Sn2+ makes the SnCl2 additive composite layer coat the perovskite particles in situ, which prevents the oxidation of tin and makes the surface of perovskite film more flat. The device fabricated after Liu et al. Introduced KHQSA into the crystallization process of FASnI3 showed high responsivity in a wide wavelength range of 300 – 1000 nm, which significantly improved the oxidation resistance of Sn-based perovskite[34]. Subsequently, they reported a FASnI3/PEDOT:PSS heterojunction of PDs and found that shorter response time and higher detectivity could be achieved by reducing the thickness of PEDOT: PSS (Figure 4E)[35].

4.2 Sn/Pb mixed perovskite NIR-PDs

The partial substitution of Sn for Pb in Pb perovskites has also been shown to be an effective way to reduce the perovskite band gap. The band gap of Sn/Pb mixed perovskite is lower than that of pure Pb-based perovskite （MAPbI3≈1.55 eV） and Sn-based perovskite （MASnI3≈1.30 eV）, which can be as low as 1.17 eV and show good absorption properties at 1060 nm[36]. The optical band gap of the hybrid Sn/Pb perovskite can be controlled by engineering its composition, and Figure 5A demonstrates the band gap variation (0 < X < 1) of the MASnxPb1-xI3 perovskite[37]. In addition, compared with pure Sn-based perovskites, mixed Sn/Pb perovskites have higher stability, while they can also reduce the use of Pb and reduce the damage to the environment.
图5 (a) MASnxPb1-xI3钙钛矿薄膜的带隙(0 < x < 1) [37] ;(b) 有无铷离子掺杂下的钙钛矿薄膜的XRD图像[38] ;(c) 不同结晶时间下钙钛矿薄膜的SEM图像[39];(d) 不同厚度下(FASnI3)0.6(MAPbI3)0.4钙钛矿薄膜的SEM图像[14];(e) 利用PEAI双面钝化Sn/Pb钙钛矿PDs EQE 光谱曲线[41] ;(f) 有无偶氮苯衍生物下PDs的光电流和暗电流J-V曲线[46]

Fig.5 (a) Band gap of MASnxPb1-xI3 perovskite films (0 < x < 1)[37] ;(b) XRD images of perovskite films doped with or without rubidium ions[38] ;(c) SEM images of perovskite films at different crystallization times[39];(d) SEM images of (FASnI3)0.6(MAPbI3)0.4 perovskite films with different thicknesses [14];(e) Double-sided passivation of Sn/Pb perovskite PDs EQE spectral curve by PEAI[41] ;(f) Photocurrent and dark current J-V curves of PDs with or without azobenzene derivatives[46] Copyright 2018,Wiley-VCH. Copyright 2018, Wiley-VCH Copyright 2019, American Chemical Society. Copyright 2017, Wiley-VCH. Copyright 2020, Wiley-VCH. Copyright 2021，Elsevier.

Fabrication of smooth, uniform, and pinhole-free films is the key to achieving high-performance PDs with low dark current, low noise, and high photoelectric voltage. Zhu et al. achieved 300 – 1100 nm wide spectrum detection by doping rubidium ions into the Sn/Pb perovskite system, and XRD (Fig. 5B) showed that the doping of Rb effectively adjusted the orbital interaction between Sn/Pb and I atoms and the octahedral tilt of the (Sn/Pb)I2 framework.As a result, the energy disorder is reduced, the crystallinity of the film is increased, and the preferred orientation is enhanced, so that the linear dynamic response range of the detector reaches 110 dB[38]. In addition, they explored the growth of Sn/Pb perovskites on CMOS-compatible metal substrates to advance the integration of perovskites with silicon-based electronic devices[39]. The effect of annealing on the morphology and grain size of the Sn/Pb film was investigated, and the density and position of the nanocrystals in the perovskite precursor were successfully adjusted by controlling the annealing time, and a dense perovskite film was obtained (as shown in Figure 5C), with a LDR of 100 dB and a rapid drop time of 2.27 μs for this PDs.
The film thickness is another important parameter to control the responsivity and dark current. In order to prepare MA-free FA-Cs-based Sn/Pb perovskite with better thermal stability, Liu et al. Proposed a low-temperature annealing method to achieve NIR detection, which effectively optimized the perovskite crystallization at the top and bottom of the thick precursor film, and finally achieved high-quality Sn/Pb perovskite thick film[40]. Similarly, Wang et al. Reported PDs with high sensitivity and better stability based on (FASnI3)0.6(MAPbI3)0.4 as the active layer, Fig. 5d shows the SEM images of perovskite films with different thicknesses, the average grain size increases with the increase of perovskite film thickness, due to the decrease of grain boundary density.Favoring the suppression of carrier recombination, the increase in perovskite thickness promotes good light absorption in the NIR range, with an EQE increase of 65% in the range of 300 – 1000 nm, while exhibiting detectivity of 1.1×1012Jones comparable to that of commercial inorganic photodetectors[14].
After optimizing the morphology and thickness of the films, the surface defects of Sn/Pb perovskites can be effectively passivated by appropriate Lewis base additives. Zhao et al. Used PEAI to passivate the surface defects at the interface. The presence of PEAI at the bottom of the Sn/Pb perovskite film greatly promoted the growth of a smooth and uniform film on top of the perovskite, significantly improving its stability in the atmospheric environment. The PDs showed a smooth EQE of about 80% in the range of 300 ~ 1050 nm (Fig. 5e)[41]. Cao et al. Observed many pinholes in the pure CsPb0.5Sn0.5I3 film, in contrast, the film with the addition of a small amount of two-dimensional (PEA)2Pb0.5Sn0.5I4 obviously became uniform and dense[42]. This pinhole-free film can effectively improve the performance by inhibiting the permeation of oxygen and moisture, thereby inhibiting Sn2+ oxidation. The halogenated derivative of PEA, 2F-PEA (2-fluorophenethylamine), also has great advantages in passivating defects and inhibiting tin oxidation. The device incorporating 2 F-PEA has a high responsivity of 0.41 AW−1, and the detectivity in the range of 800 – 1000 nm exceeds 1012Jones[43].
In order to passivate the perovskite defects and improve the oxidation resistance and stability of Sn-containing perovskite, Xu et al. Treated the MA0.5FA0.5Pb0.5Sn0.5I3 with ascorbic acid to effectively enhance the oxidation resistance of the film, thus significantly inhibiting the generation of leakage current. The spectral range of the PDs extended to 1100 nm, and the detectivity exceeded that of 1012Jones[44]. Avoiding the oxidation of Sn2+ in the precursor solution by using reducing agents such as tin powder is also an effective way to reduce the Sn4+ content in Sn/Pb perovskite films. Morteza Najarian et al. Fabricated MA0.3FA0.7Pb0.5Sn0.5I3 perovskite PDs based on tin powder with 85% EQE at 850 nm, dark current less than 10-8A·cm-2, and response time faster than 100 ps[45]. In addition, Ma et al. Added azobenzene derivative (TBAAzo) as an additive to Sn/Pb perovskite PDs, and the long carbon chain has hydrophobicity, which improves the stability[46]. The N = N in TBAAzo can effectively passivate the uncoordinated Pb2+ on the surface of perovskite film and suppress the non-radiative recombination. Figure 5 f shows that the dark current is reduced by nearly two orders of magnitude, resulting in low noise current and fast response, with a linear dynamic range of 185 dB, which is nearly three times higher than that of the commercial photodetector InGaAs (66 dB). He et al. Introduced ATFBA passivator to passivate surface defects and inhibit the oxidation of Sn2+ through hydrogen bonding and chelating coordination of terminal amino and carboxyl groups, and the perfluorophenyl ring structure can act as a hydrophobic protective barrier to prevent the invasion of moisture[47]. The addition of ATFBA improves the conduction band position of Sn/Pb perovskite, which is beneficial for effective electron extraction and transport at the perovskite/electron transport layer interface.
The growth of perovskite crystals is also crucial to improve the performance and stability of photodetectors. Liu et al. Introduced a stronger perovskite surface passivator, thiophene-2-carbohydrazide (TAH), in which the carbonyl and thiophene in the TAH molecule can interact with the undercoordinated Pb2+/Sn2+ through coordination bonds, and can also interact with the FA+ in the perovskite through hydrogen bonds[48]. The hydrazine group can reduce the oxidation of Sn2+ and interact with I- in perovskite through hydrogen bonding. With the addition of TAH, the average grain size of the pristine film increases significantly from 477.69 nm to 756.09 nm, and the dark current density is 3 times lower than that of the pristine film. Similarly, tin thiocyanate (Sn(SCN)2) acts as an antioxidant to control crystallinity and growth orientation, and Sn(SCN)2 tends to form a unique bifacial surface preferential distribution within the perovskite film, mainly located at the bottom and top surfaces of the FA0.85Cs0.15Sn0.5Pb0.5I3 perovskite, with very little inside the film[49]. The unique distribution structure helps to improve the morphology and oxidation resistance of the material. The stability of the device is greatly improved, and the longest time is up to 2300 hours.
Benefiting from low cost, ease of fabrication, and superior optoelectronic properties, perovskites have proven to be ideal photodetection materials for efficient NIR-PDs. Some key parameters of Sn/Pb hybrid perovskite-based PDs are even better than those of commercial devices. For example, Sn/Pb mixed perovskite-based PDs achieve a high response of 0.53 A·W−1 at a wavelength of 940 nm, which is much higher than the responsivity of ordinary silicon image sensors. In addition, the detectivity of Sn/Pb-based perovskite PDs exceeds 1012Jones at a wavelength of 1100 nm, and the linear dynamic range exceeds 213 dB, which is four times higher than that of commercial photodetector GaN (50 dB). We believe that Sn/Pb mixed perovskite PDs will have a broader development prospect in the future.

5 Perovskite/inorganic heterojunction photodetector

Although pure Pb-based perovskites can be detected slightly in the NIR range (~ 850 nm) due to their band gap, their weak optical absorption leads to a narrow NIR detection range and low responsivity. The photoelectric properties of heterojunction PDs depend not only on the respective contributions of the two components, but also on the charge transfer between the two components[53,54]. Many studies have proved that the carrier lifetime can be effectively extended by inhibiting the recombination of photogenerated charges, which is helpful to improve the photoelectric performance. Therefore, fast-response NIR-PDs can be prepared by compositing various inorganic narrow-gap semiconductor materials with Pb-based perovskites (as shown in Table 2).
表2 常见Pb基钙钛矿NIR-PDs

Table 2 Common Pb Perovskite NIR-PDs

Perovskite Wavelength range (nm) Responsivity
(mA·W−1)		 Detection rate (Jones) EQE (%) LDR (dB) Response time [trise/tdecay] (µs) Ref.
MAPbI3/Gd-doped ZnO nanorods 250~1357 220 @1357 nm 9.3×109@1357 nm 4 × 105 /5 × 105 17
MAPbI3-xClx 1012 @1100 nm 5.6 × 1013 @895 nm 18
MAPbI3 400~1064 150 @820 nm 22% @820 nm 1.2 × 105/8 × 104 21
MAPbI3 400~1000 4 × 103 @800 nm 600% @800 nm 39/1.9 22
CsPbBr3/GeSn 450~2200 4.7 @2200 nm -/26 55
Si/MAPbBr3 single crystal 405~1064 5 @1064 nm 2×1010 @1064 nm 0.52/2.44 56
MAPbI3/Si-NPA 400~1050 8.13 @780 nm 9.74 × 1012 @780 nm 253.3/230.4 57
MAPbI3-x(SCN)x/Si-NWs
 350~1100
 1.3 × 104
@800 nm		 1.0 × 1013 @800 nm 22.2/17.6 58
Cs-doped FAPbI3/Si nanowire array 300~1200 14.86 @850 nm 2.04 × 1010 @850 nm 4/8 59
PVP-modified MAPbIxCl3-x/Si 405~988 ≈1250 @988 nm ≈5.3 × 1011 @808 nm ≈275% @808 nm 44 645/560 60
Si/MAPbI3 300~1150 50.9 @815 nm 2.23 × 1012 @815 nm <10% 1.3×104/1.46×
104		 61
MAPbIxCl3-x/Si 300~1150 870 @800 nm 6 × 1012 @800 nm 5×104
/1.5×105		 62
MAPbI3/Si 400~1200 18.4 @970 nm 1.8 × 1012 @970 nm 23.5% 97
graphene/CH3NH3PbI3 400~800 180 >1015 5×104% 87 ms/540 ms 63
(PEA)2(MA)2Pb3I10/GaAs NWs 400~800 75 1.49×1011 568 ms/785 ms 74
FA0.85Cs0.15PbI3/PtSe2 300~1200 117.7 @808 nm 2.91 × 1012 @808 nm 14.9% @808 nm 0.078/0.060 68
FA0.85Cs0.15PbI3/PtSe2 200~1550 313 @808 nm 2.72 × 1013 @808 nm 50% @808 nm 3.5/4 69
MAPbI3/MoS2 500~850 1.11×105 @850 nm 2.39 × 1010 @850 nm 6.17×106/4.5×
106		 71
graphene /(PEA)2SnI4/MoS2/ graphene 300~900 121 8.09 × 109 38.2 34 ms/38 ms 70
MAPbI3/PbS QDs layer 375~1100 132 @900 nm 5.1 × 1012 @900 nm 18.2% @900 nm 100 80
MAPbI3/PbS-SCN QDs layer 365~1550 1.58×103 @940 nm 3.0 × 1011 @940 nm <4.2×104 81
MAPbI3:PbS QDs 400~1000 3.30 × 1011 @900 nm 6% @900 nm <5 × 105 83
MAPbI2.5Br0.5PbS QDs 400~1400 99 @975 4 ×1012 @1240 nm 40% @1240 nm 60 <10 79
MAPbI3/PbSe QDs layer
300~1500 700 @1200 nm 7×107@1200 nm 2.5×103/3×103 82
MAPbI3-xClx:PbS QDs 300~1500 350 @1300 nm 9 × 1010@1300 250/500 98
MAPbI3/PDPP3T 300~940 154 @835 nm 8.8 × 1010 @835 nm 1% @937 nm 3×104/1.5×105 88
MAPbI3/PDPPTDTPT 350~1050 1 × 1011 @900 nm 10%~20% @800~950 nm 95 6.1 × 10−3 89
MAPbI3/PTB7-Th/IEICO-4F 340~940 518 >1010 @340~940 nm >70% 500/510 92
MAPbI3/SWCNTs/NDI-DPP 375~1400 150 @1064 nm 2×1012 @920~940 nm 20% @920~940 nm 4.32/12.16 93
MAPbI3/F8IC:PTB7-Th 300~1000 370 @870 nm 2.3 × 1011 @870 nm 54% @850 nm 191 35/20 94

5.1 Classical semiconductors such as silicon

Silicon has a band gap of about 1. 1 eV and mature processing technology, which has broad application prospects in high sensitivity and broadband photoelectric detection. Geng et al. Prepared PDs with Si/MAPbBr3/Au heterojunction with wider spectral range and shorter response time by depositing MAPbBr3 on silicon[55]. Zhang et al. Deposited MAPbI3 on SiO2/Si substrate, and the responsivity of MAPbI3/ silicon heterojunction PDs was 18.4 mA·W-1 and the specific detectivity was 1.8×1012Jones under 970 nm illumination[56].
Silicon nanoporous pillar array (Si-NPA) and silicon nanowire (Si-NW) are used as substrates to grow perovskite, which can realize NIR detection[57][58]. The Si-NPA structure has the characteristics of low light reflectance, low resistivity, high hole mobility and large specific surface area, and the nanopore morphology enhances the light harvesting ability and increases the transport and extraction paths of carriers, which is beneficial to the growth of pinhole-free and dense perovskite films. The SEM image of the perovskite layer covering the Si-NPA substrate is shown in Figure 6A, and the responsivity of the fabricated silicon NPA/MAPbI3/ZnO heterojunction photodiode at 780 nm is 8.13 mA·W−1, and the detectivity reaches 9.74×1012Jones[57]. Asuo et al. Deposited Pb(SCN)2 doped MAPbI3 perovskites directly on Si-NWs, and both Si-NWs and halide perovskites acted as light absorbers, generating electrons and holes upon illumination[58]. The MAPbI3/SiNW heterojunction device has a responsivity of 13 A W-1, detectivity up to 1013Jones, and fast rise/fall times of 22.2/17.6 μs (Fig. 6 B). The device showed very good stability under ambient conditions even after 30 days of storage. Liu et al. uniformly coated a Cs-doped FAPbI3 perovskite layer on a vertical Si-NW, and the photovoltaic effect provided the heterojunction with the ability to detect NIR without voltage supply[59]. The device has good photosensitivity and stability, and the dark current and photocurrent are almost unchanged after 3 weeks of storage under unencapsulated conditions. By introducing PVP to control the morphology and crystallization of MAPbIxCl3-x polycrystalline perovskite films, Zhao et al. Prepared films with uniform surface morphology and high crystallinity[60]. The broadband photodiode with doped p-type silicon /PVP:MAPbIxCl3-x/Austructure has a spectral response extending to 980 nm and a good spectral uniformity in the range of 450 ~ 808 nm.
图6 (a) 钙钛矿层覆盖Si-NPA衬底的SEM图像[57];(b) MAPbI3/SiNW 异质结器件的上升下降曲线[58];(c) Si/SnO2 /MAPbI3/MoO3异质结能带示意图[61]

Fig.6 (a) SEM images of Si-NPA substrate covered with perovskite layer[57];(b) The ascending and descending curves of MAPbI3/Si-NW heterojunction device[58]; (c) diagram of Si/SnO2 /MAPbI3/MoO3 heterojunction energy bands [61] Copyright 2019,Elsevier. Copyright 2021, Wiley-VCH. Copyright 2020, The Japan Society of Applied Physics.

Poor energy level matching at the perovskite/silicon interface hinders carrier transport and extraction, resulting in performance degradation. The introduction of metal oxides （SnO2, TiO2, etc.) can solve the problem of energy level mismatch and promote the separation of interface carriers. Qu et al. Inserted a SnO2 layer into the perovskite/silicon interface, and the energy band diagram of PDs with and without SnO2 and MoO3 layers is shown in Figure 6 C. The energy barrier of the valence band at the SnO2/MAPbI3 interface prevents the transfer of holes in the MAPbI3 layer to the Si side, thereby reducing the interface recombination and inhibiting the generation of dark current, thus enhancing the detection capability[61]. Cao et al. Inserted a TiO2 layer at the interface between n-type silicon wafer and perovskite layer, which reduced the energy mismatch between silicon and perovskite and realized the carrier transfer in the heterojunction[62]. This PDs extends the spectral response to 1150 nm wavelength with responsivity reaching 0.87 A·W-1 at 800 nm and detectivity exceeding 1012Jones in the NIR region.

5.2 Graphene

Monolayer graphene is composed of sp2 hybridized carbon atomic layers, which has high specific surface area and carrier mobility, good thermal conductivity and light transmittance. These unique properties make graphene widely used in photovoltaic devices. The zero band gap of graphene makes it have great potential in UV-THz band. On this basis, the combination of single-layer graphene and perovskite materials is expected to greatly improve the photoelectric properties of graphene/perovskite PDs by using the wide spectral response characteristics of graphene.
The carrier transport mechanism from graphene to perovskite is the key to achieve efficient photoelectric conversion of perovskite/graphene stacked PDs. Lee et al. First reported novel hybrid PDs composed of graphene and MAPbI3 perovskite layers[63]. The trapped carriers produce an effective photovoltaic effect, and the presence of the charge also changes the conductivity of the graphene channel through capacitive coupling, resulting in a broad spectral response of the device in the range of 400 ~ 850 nm. Wang et al. Prepared perovskite/graphene heterojunction high-performance broadband PDs with a responsivity of 6.0×105A·W-1 by rapid crystallization deposition method, due to the effective photogating effect on graphene, which increased the lifetime of carriers trapped in perovskite and enhanced its photoconductive gain[64]. In addition, Spina et al. Prepared MAPbI3/ graphene PDs by chemical vapor deposition, and the device showed 2.6×106A·W-1 photoresponsiveness due to the strong light harvesting ability of perovskite with nanowire morphology[65].
The properties of perovskite/graphene-based PDs can also be regulated by introducing alternative organic bonds or halide groups to meet specific application needs. Graphene quantum dots (QDs), a new zero-dimensional material, not only have the excellent properties of graphene, but also show significant quantum confinement effect and boundary effect. Qian et al. Prepare PDs with nitrogen-doped graphene quantum dot-perovskite-light reduced graphene oxide[66]. The fast and efficient carrier transport between the perovskite layer and the light reduced graphene oxide layer and the high carrier mobility of the light reduced graphene oxide layer enable the detection wavelength of the PDs to reach 940 nm, while the photoresponsivity is as high as 1.92×104A·W-1, and the fast response capability is about 10 ms. In addition, Feng et al. Prepared new broadband PDs with good stability by combining single-layer graphene and Au square nanoarrays, and the hybrid system of graphene and Au square nanoarrays effectively improved carrier mobility and light absorption.The light harvesting and photoinduced carrier extraction are simultaneously maximized, thus greatly enhancing the photocurrent extraction ability of PDs in the visible and near-infrared range, and the responsivity and detectivity are about two orders of magnitude higher than those of PDs based only on perovskite[67].

5.3 Two-dimensional transition metal sulfide

Two-dimensional transition metal sulfides (TMDs, including PtSe2, PdSe2, MoSe2, and WSe2), as an emerging material family, have been widely used in solar cells, photodiodes, and sensors in recent years due to their advantages such as high carrier mobility and stability, and have also been proved to be ideal materials for fabricating high-performance NIR-PDs. Due to the strong built-in electric field generated by the contact between TMDs and perovskite heterojunction, as well as the NIR absorption and high carrier mobility of TMDs, Pb-based perovskites combined with TMDs can achieve efficient detection capability.
Wu et al. Constructed a heterojunction composed of multiple layers of PtSe2 and Cs/FAPbI3. Because of the high carrier mobility of PtSe2, the light response speed is very fast. Meanwhile, the heterojunction PDs can detect the NIR region with a responsivity of 117.7 mA·W-1 and a detectivity of 2.91×1012Jones（ Figure 7 a)[68]. Combining FA0.85Cs0.15PbI3 with multilayer PdSe2 has three advantages: (I) Compared with other 2D nanomaterials such as black phosphorus, dense and uniform PdSe2 films can be synthesized on any substrate at low temperature, which is easy to fabricate large-area devices. The （ii）PdSe2 has a high carrier mobility exceeding that of the 1000 cm2·V−1·s−1, and has a fast light response speed; (iii) The low-symmetry crystal structure makes the device based on PdSe2 have good polarized light signal detection ability, and the PdSe2/FA0.85Cs0.15PbI3 heterojunction PDs prepared by Zeng et al. Can detect the wavelength range of 200 – 1550 nm with responsivity of 313 mA·W−1 and detectivity up to 1013Jones[69]. Due to the high-quality and strongly anisotropic crystal structure of the two-dimensional PdSe2, it can be clearly seen from Fig. 7 B that the output photocurrent is highly dependent on the polarization angle. When the polarization angle changes from 0 ° to 360 °, the normalized photocurrent changes periodically, reaching a maximum at 0 ° (180 °) and a minimum at 90 ° (270 °), and the polarization sensitivity is as high as 6.04, which indicates that it has a good ability to detect polarized light signals. The PDs of the MoS2/2D perovskite heterostructure reported by Fang et al. Are capable of sensing the entire UV-Vis-NIR wavelength range by using few-layer graphene flakes as electrical contacts, thereby enabling the PDs performance to be improved[70]. The responsivity of perovskite /MoS2 heterojunction PDs prepared by Park et al. Is 104.24 A·W−1 at 850 nm wavelength, which reduces the recombination rate of photogenerated charges due to the reduced scattering between photoexcited electrons, thus enhancing the photocurrent[71].
图7 (a) PtSe2/钙钛矿异质结PDs的波长响应度和探测率[68]; (b) PdSe2 /钙钛矿异质结PDs光电流随不同偏振角度的函数变化[69]

Fig.7 (a) Wavelength responsiveness and detection of PtSe2/ perovskite heterojunction PDs[68]; (b) The photocurrent of PdSe2/perovskite heterojunction PDs varies as a function of different polarization angles[69] Copyright 2018, American Chemical Society. Copyright 2019, Wiley-VCH.

5.4 Ⅲ-Ⅴ compound

Inorganic Ⅲ-Ⅴ compound semiconductors such as gallium arsenide (GaAs) have shown considerable potential in optoelectronic devices due to their direct band gap (1.42 eV), high electron mobility （≈8500 cm2·V−1·s−1）, and good thermal stability.
Guo et al. Fabricated ligand-free GaAs nanocrystals (NCs), and GaAs NCs modulated perovskites exhibited better charge carrier transport compared with pristine perovskite devices[72]. Jang et al. Proposed grating resonant InGaAs narrowband multispectral detection PDs in the wavelength range of 1300 ~ 1700 nm, and the device showed low dark current, clear narrowband spectrum, and narrow half-peak width[50].
Due to the lack of suitable integration techniques, there are challenges in transferring and integrating these nano/micro structures on flexible substrates and large areas. Zumeit et al. Proposed laterally aligned doped GaAs microstructures to develop high-performance flexible broadband PDs, which exhibited excellent performance in the ultraviolet and near-infrared, including ultrafast response (2.5 ms) and high responsivity （>104A·W-1）, detectivity （>1014Jones）[73]. Hou et al. Developed a new type of PDs by combining GaAs nanowires and organic perovskite materials, and the responsivity and detectivity were significantly improved, reaching 75 A·W−1 and 1.49×1011Jones, respectively[74]. Therefore, Ⅲ-Ⅴ compounds such as GaAs and the demonstrated integration technology have great potential to achieve the next generation of high-performance and broadband PDs.

5.5 IV-VI compound

Inorganic quantum dots of IV-VI compounds, such as PbS and PbSe, are ideal materials for Pb-based perovskite PDs to realize near-infrared absorption complementation. As a conventional infrared-responsive semiconductor with a band gap of 0.6 eV, bulk PbS has been used in NIR-PDs as early as the 20th century[75]. Quantum dot/perovskite heterojunction optoelectronic devices benefit from the tunable band gap of quantum dots, the high absorption coefficient of perovskite, and the passivation of quantum dots on the perovskite surface to achieve efficient broadband detection and fast response[76,77]. Since Konstantatos et al. Reported the first infrared PDs based on PbS QDs in 2005, PDs based on PbS QDs have been developed vigorously[78]. Its key performance parameters are not only comparable to those of traditional Ⅲ-Ⅴ semiconductor PDs, but also surpass them in some fields. In addition, they also show the potential of selective photoelectric detection and near-infrared detection that traditional semiconductor PDs do not have.
The structural defects on the surface of PbS QDs passivated perovskite can effectively reduce the carrier recombination and suppress the generation of leakage current[79]. Liu et al. Proposed ultrasensitive broadband PDs fabricated using MAPbI3/PbS QDs, which effectively reduced the recombination loss and thus improved the photoresponse[80]. The detectivity of the device in the NIR region exceeds 5×1012Jones, and the responsivity exceeds 130 mA·W−1. The performance parameters of these devices are comparable to those of pristine inorganic devices. Similarly, thiocyanate anion (SCN−) was introduced into PbS quantum dot passivation to tune the energy level and enhance the conductivity[81]. Among them, the PbS-SCN/MAPbI3 complex PDs have broadband light detection ability at 365 – 1550 nm wavelength, and the detectivity at 940 nm is 3.0×1011Jones. In addition, Yu et al. Prepared PDs based on MAPbI3/PbSeQDs and realized broadband detection at 300 ~ 1500 nm wavelength and showed stable photocurrent response[82].
The anti-solvent process can disperse the PbS QDs between perovskite grain boundaries and control the crystallinity and morphology of perovskite. To this end, Zhao et al. Dispersed PbS QDs in different volumes of anti-solvent toluene to adjust the crystallinity and grain morphology of perovskite films, thus preparing perovskite/PbS QDs films with good crystallinity and uniform grain size. The anti-solvent can reduce the crystallization time of perovskite in the spin-coating process, thus making the perovskite grain size smaller and the film denser[83]. In addition, Pan et al. Designed a high-gain NIR-PDs based on perovskite-coupled PbS colloidal quantum dots based on this method, with a linear dynamic range of 200 dB and detectivities up to 1.3×1013Jones and 2.6×1012Jones at about 800 nm and 1200 nm, from which it can be seen that the introduction of PbS quantum dots greatly improves the performance of PDs[84].

6 Perovskite/organic heterojunction NIR-PDs

Perovskite/organic hybrid PDs combine the advantages of perovskite materials, such as high charge carrier mobility and tunable band gap, and show excellent performance, which is expected to achieve NIR wavelength detection[85,86].
Organic narrow band gap polymers are capable of absorbing light at near-infrared wavelengths and have tunable energy levels and efficient charge separation. The polymer interface layer as a dipole layer can provide an additional electric field to prevent the entry of holes, while enhancing the entry of electrons. The composite PDs demonstrated by Yang et al. Have a detectivity of 1014Jones, a linear dynamic range of more than 100 dB, and a fast photoresponse capability[87]. The PDs (Figure 9a) obtained by Shi et al. Using MAPbI3 and PDPP3T processing showed high photoresponse in the UV-Vis-NIR region. The PDPP3T heterojunction greatly promoted the dissociation of excitons into more separated free carriers, thus enhancing the photocurrent, and the device had sensitive photoresponse and high detectivity at 937 nm[88]. Shen et al. Reported composite PDs based on MAPbI3 and PDPPTDTPT/PC61BM with a probe wavelength of 950 nm. The introduction of organic materials reduces the direct contact between perovskite and metal electrodes, thereby reducing the dark current. The response time of the PDs is 5 ns (Figure 9 B), and the detectivity at 900 nm exceeds that of 1011Jones. This work provides an effective way to realize the next generation of PDs with wider and faster response[89].
图9 (a) 用MAPbI3/PDPP3T复合光敏层构建柔性PDs的示意图[88] ; (b) 钙钛矿/PDPPTDTPT/PC61BM 复合PDs的EQE和TPC图[89] ; (c) 钙钛矿/PC61BM/C60 PDs 暗电流密度-电压(J-V)图像[90]; (d)引入双电子传输层IEICO-4F和PTB7-Th PDs能带图[92] ; (e) 引入NDI-DPP/钙钛矿PDs的探测率与波长的关系[93]

Fig9 (a) Schematic diagram of manufacturing flexible PDs using MAPbI3/PDPP3T composite photosensitive layer[88]; (b) EQE and TPC of perovskite /PDPPTDTPT/PC61BM composite PDs[89]; (c) Perovskite /PC61BM/C60 PDs dark current density-voltage (J-V) image[90]; (d) The dual-electron transport layer IEICO-4F and PTB7-Th PDs band map are introduced[92]; (e) detectivity versus wavelength of introduced NDI-DPP/perovskite PDs[93]. Copyright 2016, Wiley-VCH. Copyright 2017, Royal Society of Chemistry. Copyright 2015, Wiley-VCH. Copyright 2018, Wiley-VCH. Copyright 2017, Wiley-VCH.

To improve the photocurrent extraction of organic NIR materials, researchers have explored hybrid device architectures in which perovskite/organic NIR semiconductors are mixed with fullerenes. Meredith et al. Demonstrated perovskite/organic PDs with a double fullerene PC61BM/C60 layer, demonstrating comparable performance metrics to commercial inorganic PDs, and the PC60BM/C60 interlayer suppressed the dark current by forming a conformal coating on the perovskite surface and limiting hole reinjection (Figure 9C), enhancing the extraction of photocurrent and achieving a dynamic response of 170 dB for this device[90]. Ma et al. Demonstrated ITO/PEDOT:PSS/MAPbI3/PDPP3T:PC71BM/Al broadband perovskite/organic PDs with a vertical structure, and the composite PDs showed a wide spectral response range of 390 – 950 nm. Although this PDs can achieve NIR wavelength detection, the low energy level at the interface of perovskite/organic heterojunction layer usually leads to a decrease in the responsivity in the Vis-NIR region[91].
In order to promote the extraction and transport of photogenerated charges between perovskite and organic bulk heterojunction layers and improve the responsivity of PDs in the NIR region, Wu et al. Introduced IEICO-4F with PTB7-Th heterojunction double electron transport layer to transport the electrons transferred from MAPbI3, while PC61BM also promoted the extraction and transport of electrons from MAPbI3 to the cathode[92]. The energy level structure is shown in Figure 9D, and the integrated PDs reaches the highest responsivity 0.444 A·W−1 and 0.518 A·W−1 in the vis and NIR regions, respectively, due to the double electron transfer channels of PC61BM and IEICO-4F. Subsequently, Gong et al. Combined MAPbI3:SWCNTs with NDI-DPP:PC61BM to make the energy levels more matched, and the near-infrared absorption of NDI-DPP was effectively utilized to promote the photoresponse of the PDs in the near-infrared range, and the detectivity exceeded 2×1012Jones（ Fig. 9 e)[93]. Li et al. Prepared PDs by combining organic bulk heterojunction (BHJ) F8IC/PTB7-Th with perovskite, which successfully extended the detection range to 1000 nm and realized broad spectrum detection in the NIR region. The EQE value of perovskite/BHJ PDs at 850 nm reached 54%, which has great development prospects in realizing the next generation of high-performance and broadband PDs[94].
Fig. 8 shows the molecular structures of PDPP3T, PDPPTDTPT, F8IC, PTB7-Th, IEICO-4F and NDI-DPP mentioned above.
图8 采用(a) PDPP3T[88];(b) PDPPTDTPT[89]; (c) F8IC[94]; (d) PTB7-Th[92]; (e) IEICO-4F[92]; (f) NDI-DPP[93]有机材料与钙钛矿复合，拓宽钙钛矿PDs的光谱响应范围

Fig.8 The structural formula of (a) PDPP3T[88]; (b) PDPPTDTPT[89]; (c) F8IC[94]; (d) PTB7-Th[92]; (e) IEICO-4F[92]; (f) NDI-DPP[93]; which are adopted to combine with perovskite to broaden the spectral response range of Pb perovskite-based PDs.

7 Perovskite/upconversion material photodetector

Through multiphoton absorption of multiple excited States, the upconversion material can absorb NIR light and then emit UV-Vis-NIR light[95]. Therefore, the combination with upconversion materials can effectively enable Pb-based perovskite PDs to acquire NIR response ability.
Rare earth erbium ions allow the capture of NIR photons due to their rich discrete energy level structure to achieve upconversion light emission capability and be used to detect the infrared band. He et al. Used upconversion nanoparticles to achieve efficient spectral absorption of perovskite solar cells at NIR wavelengths. At the same time, waveguide-based hybrid optoelectronic devices have also attracted much attention and interest in the industry due to the significant enhancement of optical absorption by waveguide cavities. Zhang et al. First proved that the introduction of erbium-ytterbium silicate (EYS) nanosheets into perovskite PDs can effectively broaden the spectral response range to the NIR band, and EYS can produce strong up-conversion luminescence and firmly fix it in its cavity.The PDs are effectively coupled to an adjacent perovskite photosensitive layer to excite the perovskite photosensitive layer so as to realize the detection of infrared light, and the PDs have excellent optical switching characteristics, and the response time is about 900 μs, which is five orders of magnitude faster than that of silicon-based PDs[96]. Among these optical devices, the efficient light confinement and propagation ability of the waveguide cavity and the multiple energy transfer at the interface significantly improve the responsivity, which guides the way for the development of photovoltaic devices such as PDs for detection in the NIR band.

8 NIR-PDs application

Image sensors composed of broadband PDs are widely used in various applications such as image sensing, optical communications, optical couplers, and infrared laser rangefinders. The PbS-SCN/MAPbI3 composite PDs were integrated into a 10 × 10 array as a 10 × 10 pixel sensor for image sensing[81]. As shown in Figure 10 a, the letters "U", "E", and "I" can be clearly identified under the illumination of 365 nm, 520 nm, and 1310 nm. Demonstrated the great potential of Pb-based perovskite PDs for image sensing applications in the UV-vis-NIR region.
图10 (a) 用于图像传感的PbS-SCN/MAPbI3 PDs阵列的设计和演示图[81] ;(b) OIHP PDs的图像扫描系统示意图和实际成像图[99] ;(c) 近红外上转换检测系统示意图[41] ;(d) 光电探测器集成近红外声光通信系统示意图[46] ;(e) 6×6像素Sn/Pb钙钛矿器件的光电流分布和捕获图像[39]

Fig10 (a) Design and demonstration of a PbS-SCN/MAPbI3 photodetector array for image-sensing application[81] ;(b) Schematic of the image scanning system and actual imaging for the OIHP photodetector[99] ;(c) Schematic diagram of the NIR up-conversion detection system with pictures of experimental detection under weak light and darkness to avoid the effects of indoor lighting[41] ;(d) Schematic diagram of the integrated NIR acousto-optic communication system with mixed perovskite photodetector[46] ;(e) Photocurrent distribution and capture images of 6 × 6 pixel Sn/Pb chalcogenide devices[39] Copyright 2019,American Chemical Society. Copyright 2020, Nature. Copyright 2020, Wiley-VCH. Copyright 2021，Elsevier. Copyright 2021, American Chemical Society.

In the field of imaging, the photosensitive element is usually a charge-coupled device (CCD), which can sense light and convert the image into a digital signal. Functionally, perovskite/heterojunction PDs can be used to replace CCD elements to some extent. Fig. 10B shows the effect of the photodiode prepared by Li et al. In a single-pixel vis-NIR imaging system. Due to its good high dynamic range imaging capability, a "SITP" pattern was designed on the LED screen to verify the imaging capability of Pb-based PDs for complex objects[99].
Infrared up-conversion system is of great significance for near-infrared visualization in the conversion of infrared light to visible light. Zhao et al. Used PEA+ to passivate Sn/Pb perovskite on both sides, integrated the perovskite photodetector into the infrared up-conversion system, and designed an amplifier circuit to amplify weak signals, as shown in Figure 10c.The circuit system ensures that the electrical signal obtained by NIR-PDS drives the white LED to emit light, which proves that the Sn/Pb perovskite photodetector can completely convert the near-infrared signal into the visible signal[41]. Ma et al. Integrated Sn/Pb perovskite as an optical signal receiver into near-infrared optical communication to realize encrypted information transmission in different media[46]. A perovskite photodetector as shown in Figure 10 d is used to receive the near-infrared light signal and convert it into an electrical signal that is collected by a loudspeaker. Audio signals can be accurately transmitted without significant distortion using Sn/Pb perovskite photodetectors, demonstrating the potential of Sn/Pb perovskite photodetectors in the field of optical communications. The high-performance Sn/Pb perovskite photodetectors can also be used for near-infrared imaging applications. Zhu et al. Used a Sn/Pb perovskite photodetector based on a metal/silicon substrate, integrating 6 × 6 pixels with an active area of 1 mm × 1 mm per pixel, as shown in Figure 10 e, to demonstrate the ability of the photodetector to capture images[39]. They observed excellent photocurrent uniformity in high-quality perovskite films. The success of pixel integration and image capture potentially facilitates further development of commercial applications of NIR-PDs.

9 Conclusion and prospect

NIR-PDs based on perovskite and its composites have the advantages of high sensitivity, which can be widely used in near-infrared communication and optical imaging. By regulating the spectral absorption range of perovskite materials, it can open up a new way for the application of NIR-PDs. In this paper, the research progress of Pb-based, Sn-based and Sn/Pb-based perovskite near-infrared photodetectors is reviewed from the aspects of photoelectric properties, device structure, working mechanism and performance parameters of photodetectors. It can be predicted that NIR-PDs based on perovskite materials will continue to influence the development of optoelectronic devices. However, at this stage, there are still several problems to be solved.
For Pb-based perovskite materials, researchers should have a correct understanding of the mechanism of Pb-based devices for near-infrared absorption and detection in future work.Secondly, more efficient and stable NIR-PDs can be developed from the perspective of forming heterojunctions between Pb-based perovskites and narrow band gap semiconductor materials (including Si, graphene, TMDs, IV-VI compounds, III-V compounds, organic small molecules/polymers and upconversion nanocrystals). At the same time, issues such as energy level matching of heterojunction devices should be studied in depth, and on this basis, the potential of narrow gap semiconductor materials in perovskite PDs should be explored. For Sn-based and Sn/Pb-based perovskite materials, due to the problems of easy oxidation, rough surface and low coverage, most of the research should start from avoiding the oxidation of Sn2+ into Sn4+ leading to p-type self-doping.In addition, how to design it to suppress the dark current and improve the device performance needs further exploration, such as composition engineering, reducing additives, crystal tuning, packaging technology and so on.
With the deepening of research, we have reason to believe that perovskite materials, as the most promising semiconductor materials, will be widely used in our daily life, and perovskite-based NIR-PDs will also have a bright future in the field of optoelectronic devices.

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