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Organic Complex Materials and Devices for Near and Shortwave Infrared Photodetection

Jing Li , Weigang Zhu , Wenping Hu

Progress in Chemistry ›› 2023, Vol. 35 ›› Issue (1) : 119 -134.

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Progress in Chemistry ›› 2023, Vol. 35 ›› Issue (1) : 119-134 DOI: 10.7536/PC220603
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Organic Complex Materials and Devices for Near and Shortwave Infrared Photodetection
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Abstract

Near and shortwave infrared organic photodetector (OPDs) is extremely significant for the application as thermal imaging, night vision, agricultural inspection, biometric sensors, remote sensing and related fields. However, most commercial infrared photodetectors generally require extra deep cooling equipment and are unable to bend, which limit their applications seriously. In order to overcome these challenges, more and more researches related with organic semiconductors (OSCs) emerge. OSCs with advantages including easy and elaborate tunability of optical properties, high optical absorption coefficient, and mechanical flexible, are able to fabricate over large areas with roll-to-roll processing and be compatible with flexible substrates. Infrared photodetectors based on OSCs attract more and more attention, which are free with extra deep cooling equipment and possess many advantages beyond inorganic infrared OPDs. They are deemed as attractive candidates for next generation infrared photodetectors. Recently, infrared OPDs have attracted more and more research attention. In this review, we first introduce the basic principles of organic phototransistors and photodiodes, and present development of organic complex materials and novel device configurations. Then we summarize state-of-the-art applications such as electronic eyes, artificial synapse and wearable devices for real-time health monitoring. Finally, we discuss challenges in this field and prospect future development. We believe that this review will promote the developments in the photodetector fields.

Contents

1 Introduction

2 Architectures and fundamentals of OPDs

2.1 Organic phototransistors

2.2 Organic photodiodes

2.3 Organic photoconductors

3 Critical parameters of OPDs

4 Organic complex materials(OCMs)towards infrared

4.1 OCMs for organic phototransistors

4.2 OCMs for organic photodiodes

5 Applications

5.1 Artificial retina

5.2 Artificial synapse

5.3 Logic circuits

5.4 Photoplethysmography

5.5 Upconversion imager

6 Conclusion and prospect

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Keywords

organic semiconductors / near and shortwave infrared photodetection / organic complex materials / photodetector

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Jing Li, Weigang Zhu, Wenping Hu. Organic Complex Materials and Devices for Near and Shortwave Infrared Photodetection. Progress in Chemistry, 2023, 35(1): 119-134 DOI:10.7536/PC220603

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1 Introduction

Near-infrared (NIR: 650 ~ 900 nm) and Short-wavelength infrared (SWIR: 900 ~ 1700 nm) detection has been widely concerned. Infrared detection has a wide range of applications, such as self-driving cars, surveillance systems, military equipment, biomedical diagnostics, radar and optical communications[1⇓⇓~4]. At present, commercial near-infrared and short-wave infrared photodetectors mainly rely on inorganic semiconductor materials such as indium gallium arsenide (InGaAs)[5]. However, its preparation cost is high, the preparation process is complex, and it needs to be operated at low temperature, which will cause high energy consumption and environmental pollution[5⇓~7]. Organic semiconductors are promising next-generation photoelectric detection materials due to their light weight, flexibility, low preparation cost, and roll-to-roll processing[8⇓⇓⇓⇓⇓~14]. Because the wavelength detected by the infrared photodetector is in the infrared region, the thermal effect generated by the ground state charge transfer also needs to be taken into account, but the organic infrared detector is less affected by the infrared thermal effect, and it is possible to realize real-time portable room temperature sensing[15].
At present, organic infrared detection still faces many problems. First, because the energy of infrared light is very low, the band gap of semiconductors is required to be less than 1.77 eV, but only a small number of narrow-gap organic semiconductors can absorb this spectral range. Secondly, the intrinsic infrared light response of organic materials is very low and there is a problem of thermal noise in the detection process, which will hinder the use of light if the light absorption layer is added[16][17]. For the narrow-gap organic semiconductor itself, its adjacent energy levels will overlap, resulting in low photoelectric conversion efficiency and small photocurrent, while the high intrinsic carrier concentration will lead to large noise. In addition, the dielectric constant of organic materials is very low, and the excitons generated in organic materials are usually Frenkel excitons[18]. Its binding energy is large (0.3 ~ 1 eV), the exciton radius is small, and the average distance between electron and hole is within one lattice constant[15]. The generation of photocurrent requires the diffusion of excitons to the heterojunction interface before recombination, but the diffusion length of excitons in organic materials is usually only about 20 nm[19]. To sum up, in the same active layer, it is difficult to optimize the above performance at the same time. By preparing new materials or improving the device configuration, the performance parameters can be improved or the shortcomings of the single active layer optoelectronic device itself can be made up.
Due to the wide application of infrared detection in sensors, monitoring, biomedical diagnosis and communication, in recent years, many research teams have conducted in-depth research on the unsolved problems in the field of infrared detection, such as the synthesis of active layer materials and device construction technology. For example, nanomaterials responsive to infrared light, such as quantum dots, dyes, and perovskites, are integrated with high-mobility channel materials to achieve high detectability[17,20,21]; The generation, dissociation and transport of excitons are promoted by constructing donor-acceptor blend materials to form organic charge transfer complexes. Develop novel device structures to detect infrared light[22]. These studies have led to creative active layer design strategies, novel device architecture designs, and numerous leading-edge applications. Therefore, it is necessary to make a comprehensive summary of the latest achievements and challenges of infrared light detectors, which will contribute to the further development of this field.
In this review, we aim to provide an overview of near-infrared and shortwave infrared photodetectors from the development of active layer materials, device structure optimization, and emerging advanced applications. Firstly, the types and working principles of photoelectric detectors are summarized and introduced. Then, the key parameters of infrared photodetectors are summarized, and the calculation methods are introduced in detail. Then, novel active layer design strategies and advanced device fabrication methods are summarized. Finally, individual devices and their integration strategies are detailed, and relevant practical applications that have emerged in recent years are discussed. It is hoped that this review will combine the preparation strategies of active layer materials with novel applications, push the field of near-infrared and short-wave infrared photodetectors to a higher and deeper level, and open up new research directions.

2 Types and working principles of infrared photodetectors

A photodetector is a device that converts incident light into an electrical signal and outputs it in order to maximize the response to light. The basic principle is that the semiconductor generates excitons under the irradiation of incident light, the excitons are separated into electrons and holes at the interface, and the current is generated when the cathode and anode are externally connected and connected. Photodetectors can be divided into three categories: Organic phototransistors (OPTs), Organic photodiodes (OPDIs), and Organic photoconductors (OPCs). In this review, we focus on the recent progress and applications of organic phototransistors and organic photodiodes.

2.1 Organic phototransistor

The construction of OPTs is similar to that of organic field effect transistors (as shown in fig. 1A), in which the current comes from the photocurrent generated by the dissociation of photogenerated excitons and the current generated by the modulation of the field effect gate. The gate of the device can help the dissociation of photogenerated electron-hole pairs more effectively, thus effectively reducing noise and enhancing photocurrent. For OPTs, the active layer should have the ability of photoelectric energy conversion, carrier generation and transport at the same time, so the photocurrent in OPTs is not only a function of the number of photogenerated excitons, but also related to the parameters related to the field effect, such as carrier mobility, capacitance, channel length-width ratio and so on.
Compared with two-terminal devices, OPTs can achieve active matrix sensing and signal amplification at the same time, so OPTs can be directly integrated into two-dimensional sensing arrays without introducing additional current amplification components, which is conducive to the development of practical applications.
The active layer of OPTs can be directly exposed to light without being blocked by electrodes, so it is easier to obtain a large linear dynamic range value, and through reasonable selection and design of the active layer, the sensitive response to weak light can be achieved. The current of OPTs changes obviously at the threshold voltage. Illumination can induce the threshold voltage shift, that is, the grating effect. The change of photocurrent can reach exponential level, and OPTs can protect the gain while suppressing the dark current to a great extent. For p (n) type transistors, the threshold voltage is preferably at a large negative (positive) value, so that the off-state current can be close to zero under illumination and gate voltage. Having a large negative (positive) threshold voltage also broadens the phototransistor's detection range, so that the curve can be shifted to positive (negative) values without causing current saturation. The trap charge assisted mechanism of OPTs can significantly prolong the lifetime of photogenerated carriers in semiconductors, but it can slow down the response time while protecting the photoconductive gain, so it is necessary to design devices reasonably to solve this key problem[23].

2.2 Organic photodiode

The construction of OPDIs is similar to that of photovoltaic devices (Figure 1b), both of which are sandwich structures, with the active layer sandwiched between two electrodes, one of which needs to be transparent. The first OPDIs were discovered and demonstrated by Japanese scientists Kudo and Moriizumi in 1987, which made people realize that organic semiconductors have the potential to realize the response function of OPDIs without filters[24]. The mechanism of photocurrent generation in photodiodes is completely different from that in phototransistors. The current in OPDIs is generated by photoelectric and photoconductive modes. Its performance depends on the collection and transport of photogenerated carriers. Hole carriers are collected at the cathode and electron carriers are collected at the anode, which requires the carrier lifetime to exceed the time they take to reach the electrode. OPDIs can be operated in both photovoltaic and photoconductive modes. In photoconductive mode, the photodiode is in a reverse bias state and is more sensitive to the generation of photocurrent, and a reverse current is output from the photodiode after the photocurrent is generated. At this time, the applied electric field promotes the separation of electrons and holes, enhances the output photocurrent, and is conducive to signal detection. When the photodiode is at zero bias, the photocurrent will be suppressed, and the internal reverse photocurrent will cause electrons and holes to accumulate at the anode and cathode, respectively, forming the photovoltaic effect[25].

2.3 Organic light guide

In addition to OPTs and OPDIs, there are also organic photoconductive photodetectors. The OPCs are configured such that the electrodes contact the semiconductor in ohmic contact on both sides of the semiconductor (Figure 1C). The mechanism of current generation in photoconductive devices is similar to that in OPDIs. If the charge generation and transport functions in a photoconductive device are realized in different layers, it is called a double-layer photoconductive device, and if they are realized in the same layer, it is a single-layer photoconductive device. In single-layer organic photoconductive devices, the generation and separation of charges are carried out in the same layer, so the absorption of light in the active layer, the separation and transmission of charges and holes all affect the detection effect. The advantage of the double-layer photoconductive device is that the photoelectric parameters of each layer can be optimized to obtain the optimal performance of the device[26].

3 Performance parameters of infrared light detector

In these infrared detectors, there are many key parameters used to measure the performance and effect of photoelectric detection, which are introduced in turn below and summarized in Table 1. The first is the spectral response range, that is, the spectral range to which each detector can respond. Photodetectors with wide spectral response are very important in modern multispectral detection and visible and mid-infrared digital cameras[27]. Responsivity (R), represents the photocurrent generated under illumination of unit power, R=Iph/Pin, where I p h represents the photocurrent, and Pin is the product of incident light power per unit area and effective detection area. External quantum efficiency (EQE) is also an important measure of a photodetector, defined as the ratio of the number of electrons collected to the number of incident photons, that is, External quantum efficiency, where H is Planck's constant, C is the speed of light, Q is the amount of charge per unit, and λ is the wavelength of incident light. For a photodiode detector, the EQE value is usually less than 100%. The output dark current (Idark) is an important indicator of the noise in organic detectors. Noise equivalent power (NEP) can describe the detectability of weak light, that is, the detection limit of a photodetector, and the Noise equivalent power is the square root of the Noise current. There are many kinds of noise in photodetectors, such as shot noise, thermal noise and flash noise[28]. For infrared detectors, the noise source mainly depends on the operating conditions of the device. At high bias, the shot noise dominates, while at low bias it is dominated by thermal noise[29,30]. Therefore, increasing the responsivity and reducing the dark current noise are effective methods to realize high sensitivity infrared organic photodiode. The specific detectivity (Specific detectivity,D*) is used to describe the detectability of the detector for the smallest light signal, which allows a direct comparison of the sensitivity of different device structures, and is calculated when the only source of noise is dark current as :D*=(S1/2R)/(2eIdark), where S is the active area, R is the responsivity, e is the amount of unit charge, and Idark is the current value in the dark state. In addition, the D* can also be calculated by the D*=R(Sƒ)1/2/ I n - 1, where is the bandwidth and In is the dark current noise. Response time τ: contains the rise time and the fall time, the rise time (τr) is the time required for the photocurrent to rise from 10% to 90%, and the decay time (τd) is the time required for the photocurrent to fall from 90% to 10%, which is related to the carrier transport mode, characteristic transport time, recombination rate, and defect state density. Linear dynamic range (LDR) represents the Linear dependence of the photocurrent of the device on the light intensity, and in general, the current signal has a linear relationship with the incident light power. The unit of LDR is decibel (dB), and the calculation formula is usually expressed as three kinds, which are LDR=20log(Iph,max/Iph,min),LDR=log2(Iph,max/Iph,min),LDR=10log(Pin,max/NEP), where Iph,max and Iph,min represent the maximum photocurrent and the minimum photocurrent following the linear region of photocurrent and light intensity, respectively. The photoconductive gain, τr, is defined as the number of collected carriers divided by the number of incident photons, where τr is the time for the carriers to undergo recombination and τt is the time for the carriers to migrate. The cut-off frequency (cut-off frequency,f-3db) is defined as the frequency corresponding to when the photoresponse value is reduced to 1/ 2 (~ 0.707) times the peak value.

4 Organic composite facing infrared light

There are usually two strategies to improve the performance of organic infrared photodetectors, one is to develop photoactive materials with absorption in the infrared region, the other is to design novel devices to optimize the detection of the infrared region. It can be estimated that the band gap of the photosensitive layer material should be less than 1. 77 eV according to the ΔEg=1240/λ(ΔEg (λ is the wavelength of the incident light), but there are very few single-component organic semiconductors with low band gap, which limits the detection and application of organic photodetectors in the long wavelength. In recent years, it has been found that there is a donor-acceptor interaction in the organic charge transfer complex formed by blending electron donor and electron acceptor materials, which is conducive to the generation, dissociation and transport of excitons, showing unique electronic characteristics that neither monomer has[12]. The absorption spectrum of organic charge transfer complex is usually affected by the new subgap formed by the superposition of the spectra of donor and acceptor molecules, and its absorption spectrum will be significantly red-shifted. That is, the Highest occupied molecular orbital (HOMO) and Lowest unoccupied molecular orbital (LUMO) of the organic charge transfer complex are significantly affected by the HOMO and LUMO of the donor and acceptor.

4.1 Active Layer Materials for Organic Phototransistor

Using the interaction between donor and receptor, researchers have carried out a lot of research[12,31,34]. The introduction of an electron defect layer or an exciton separation interface into the active layer can significantly improve the detectability and increase the photocurrent. The device structure is shown in Figure 2A. In 2021, Chi et al. Reported that DCV3T was introduced into the narrow-gap poly (N-alkyl diketopyrrolo-pyrrole dithienylthieno [3,2-b] thiophene) DPPDTT film as an electron trapping layer, which achieved a high response value (2000 A·W-1) to 850 nm wavelength, as well as a high detection value (1016)[12]. The electron trapping layer DCV3T not only contributes to charge separation, but also gives a strong grating effect. As a common electron acceptor material in the active layer, C60 can also act as a charge blocking layer in organic-inorganic heterojunction photodetectors, which can significantly enhance the specific detectivity of the detector[35]. In 2017, Liu et al. Also studied the NIR transistor with a multi-component bulk-phase hybrid heterojunction C60/PTCDA:AlClPc:PbPc active layer, and its maximum absorption was extended to 900 nm (Figure 2b)[33]. Wang et al. Used three molecules PbPc (lead phthalocyanine), CuPc (copper phthalocyanine) and PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) as active layers, which have different band response ranges (Figure 2b). Different active layer construction strategies such as blending and planar heterojunction were compared, and the best photoresponse (322 mA/W) and higher external quantum efficiency (50%) were obtained in the CuPc/PbPc: PTCDA hybrid planar heterojunction structure[32]. Infrared photodetectors with broadband response have also been one of the research hotspots, which can be realized by mixing several small molecules with photoresponse to different bands. In order to explore a photodetector with broadband response, Liu et al. constructed a hybrid planar-bulk heterojunction (HPBHJ), and used small molecular C60, PTCDA, and SnPc (tin phthalocyanine) as the active layer to realize a phototransistor with broadband response from deep ultraviolet to near-infrared (as shown in Figure 2C)[31]. The C60 gives the active channel layer a high carrier mobility and can respond to the ultraviolet and blue light spectral region, and PTCDA and SnPc are used for the response of the green and red-near-infrared light regions, respectively, which broadens the photoresponse range of the device. The C60/PTCDA and PTCDA/SnPc interface provide fast charge transport and exciton dissociation channels. Lu et al. Used C60/PTCDA:AlClPc to prepare HPBHJ to realize a broadband photodetector of 300 ~ 850 nm[36].
As shown in Figure 3A, in 2021, Huang et al. Used ternary semiconductor PQT-12 (poly (3,3 -dialkylquaterthiophene)/F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane) and single-walled carbon nanotubes (SWCNTs) to prepare high-performance infrared OPTs with a wide response range (400 ~ 2000 nm) and without additional refrigeration components[34]. SWCNTs are used as the charge transport layer and significantly increase the photocurrent while maintaining a high switching ratio. The electronic transition energy of the charge transfer complex is as low as 0.4 eV, which makes the detection wavelength of this OPDs reach 2600 nm. Especially for the wavelength of 2000 nm, the 2.75×106A·W-1,D* and 3.12×1014Jones,EQE of R are more than 108%, the switching ratio is 6.8×105, and the ultra-weak light detection as low as 100 nW·cm-2 is realized.
Improving the infrared photoresponse of p-type polymers has always been a hot issue, and introducing electron acceptors into p-type polymers is an effective method. For example, the complex formed by introducing (6,6) -phenyl-C61-butyric acid methylester (PCBM) into DPP-DTT showed good near-infrared detection performance[37]. Zhu et al. Demonstrated the role of n-type electron acceptors in bulk heterojunctions[38]. A fullerene derivative PC61BM([6,6]-Phenyl-C61-butyric acid methyl ester), PC71BM([6,6]-Phenyl-C71-butyric acid methyl ester), or ITIC (3,9-bis (2-methylene- (3- (1,1-dicyanomethylene) -indanone)) -5,5,11,11-tetrakis (4-hexylphenyl) -dithieno [2,3-d: 2 ′, 3 ′ -d ′] -s-indaceno [1,2-b: 5,6-b ′] dithiophene) was introduced into DPP-DTT, and the molecular structure of the response is shown in fig. 3. In the BHJ active layer, the acceptor not only reduces the contact impedance, but also can capture the injected electrons and promote the separation of excitons, thus significantly improving the responsivity and sensitivity of the device. The incorporation of n-type polymers into p-type polymers can also broaden the range of light absorption. Kim et al. Used PTB7 (oly [8-bis [ (2-ethylhexyl) -oxy] benzo [1,2-b: 4,5-b ′] dithiophene-2,6-diyl] [3-fluoro-2- [ (2-ethylhexyl) carbonyl] thieno [3,4-b] -thiophenediyl]) and P (NDI2OD-T2) (poly [ [N, N[39]. Under the same VG, VDS and illumination conditions, PTB7: P (NDI2OD-T2) shows a IDS about 8 times higher than that of P (NDI2OD-T2) single-component device, indicating that photogenerated excitons can be effectively separated in PTB7:. In addition, the threshold voltage VTH shows an obvious negative shift in PTB7: P (NDI2OD-T2), which proves the trapping effect of the interface on the photogenerated holes. In the design of the device configuration, they used a double-layer insulator gate, in which low-k PMMA (K ≈ 3.5) was used to reduce the leakage current and the charge defect density at the interface and as an insulating layer, and high-k PVA could provide high capacitance.
Xie et al. Reported a charge transfer complex composed of TTF-CA (tetrathiafulvalene-chloranil) and graphene, and the phototransistor device constructed was very sensitive to short-wave near-infrared light (Figure 4A)[40]. The charge transfer complex has an ultra-low energy intramolecular charge transport as low as 0.5 eV, and achieves efficient short-wave absorption at wavelengths longer than 2 μm. The grating effect at the interface between the organic compound and graphene achieves an ultra-high optical gain detectability of about 1013Jones at room temperature, with a response time of only 8 ms (Figure 4 B). The grating effect plays a very important role in this device. By comparison, it is found that the photoresponse value decreases significantly with the increase of illumination intensity. The power-dependent photoresponse behavior proves the influence of the grating effect on the device. In addition, a significant increase in current was observed under illumination in the negative gate voltage region (Fig. 4C), and the device has significant gate selectivity, that is, the gate can control the Fermi level of graphene, which is more conducive to the transmission of holes between TTF-CA and graphene. Using Lewis acid small molecule doping is also an effective way to reduce the energy level of the active layer. In 2021, Kim et al. Reported that by doping a proper amount of small molecule BCF (tris (pentafluorophenyl) borane) into the triarylamine-based polymer PolyTPD (poly [N, N '-bis (4-butylphenyl) -N, N' -bis (phenyl) benzidine]) (Figure 4D), the obtained polymer film appeared at the incident wavelength of 1000 ~ 3000 nm[41]. In the device construction, P3HT and PMMA are used as the charge transport layer and the insulating layer, respectively, and they have no absorption to the short-wave infrared, so the short-wave infrared light can reach the photosensitive layer without damage. Moreover, the n-butyl acetate of the PMMA coating has poor solubility to PolyTPD, so the composite film is not damaged. Spectroscopic characterization proved that BCF produced free radicals in PolyTPD after doping, which led to the continuous shift of HOMO energy level. With the PolyTPD: BCF film as the gate-sensitive layer, the device can effectively detect the SWIR, and its maximum responsivity reaches 583.4 mA·W-1(λ=1500 nm), 695.4 mA·W-1(λ=2000 nm), and 829.4 mA·W-1(λ=2500 nm), respectively. In addition, a high turn-on voltage shift can be achieved in a phototransistor by controlling the minority carrier trap, but if the carrier trap concentration is too high, carrier recombination will occur. Liu et al. Introduced spatially separated hole/electron carrier double traps into the phototransistor, and demonstrated the possibility of combining electron traps and hole traps under illumination and non-illumination conditions, and obtained a large on-state voltage shift. At the same gate voltage (VG=0 V), the double trap can achieve low noise current, high photoresponse (5.26×103A·W-1) and ultra-high detectability ( D 1 / f * =1.88×1015Jones; D s h o t * =8.21×1016Jones )[23]。 Ultra-high detectability can also be achieved by constructing an organic-inorganic heterojunction, Tao et al. Reported a phototransistor device with ZnO as an electron extraction layer on graphene and an organic ternary bulk phase heterojunction as a photosensitive layer, which achieved ultra-high responsivity (6.1×106A·W-1) at 850 nm wavelength 1.75 nw·cm-2 power and achieved ultra-high detectability (2.4×1013Jones)[42].
In addition, the development of flexible devices has always been the difficulty and focus in the field of organic photoelectric detection, and improving the stability of flexible devices has always been a difficult problem to overcome. In 2021, Zhu ET al. Prepared a poly(5,5'-bis[5-bis(thienyl)phenyl]-2,2'-bithiophene-3-ethylesterthiophene])(PTPBT-ET)/In2O3 heterojunction through a solution process (Figure 5A), which combines the good photoresponse properties of narrow band gap PTPBT-ET and the excellent electron transport ability of In2O3.The In2O3/PTPBT-ET heterojunction transistor not only gives high mobility (7.0 cm2·V-1·s-1) and switching ratio, but also the same photoresponse performance is achieved in this system, the responsivity reaches 200 A·W-1, and the specific detectivity reaches 1.2×1013Jones[43]. They found that the introduction of PTPBT-ET changed the electron concentration of the In2O3 channel, resulting in a decrease in the surface potential and a negative shift in the threshold voltage. The interface charge density extracted from the calculation also proves that the semiconductor and insulator interface is not damaged. According to the energy level structure (Fig. 5B), it is found that the electrons in the heterojunction can be effectively transferred from the LUMO energy level of PTPBT-ET to the conduction band of the In2O3, but the charges are confined at the interface, which further enhances the grating effect under near-infrared light and realizes the regulation of the channel conductance in the phototransistor.
Vertical field-effect phototransistors (VFEPTs) have also received considerable attention in optimizing the device structure. VFEPTs have the characteristics of high current density, low operating voltage and low energy consumption. Compared with the planar structure, the shorter transmission path and thinner active layer of the vertical structure are conducive to reducing the channel length to nanometer scale and precisely controlling the geometric configuration of the device[45,46]. Musselman et al. Designed and fabricated VFEPTs with efficient infrared response (Figure 6). In this device structure, charge generation and charge transport are carried out in separate active layers[44]. The PbSe CQDs are used as an infrared responsive active layer, and the PCBM is used as a good electron acceptor layer for charge carrier transport under field effect control.

4.2 Active layer materials for organic photodiodes

OPDIs have the advantages of relatively low manufacturing cost, light weight and flexibility, and have great potential in the field of photosensitive applications. Moreover, the fabrication of organic photodetector arrays at lower temperatures (typically below 150.degree. C.) can be achieved if industrial wide range coating techniques such as slit coating are used. There are many feasible strategies to improve the performance of near-infrared photodiode detectors, such as using inorganic quantum dot sensitization, adding narrow optical gap donor organic semiconductors to traditional heterojunction structures, and charge collection narrowing[47⇓~49].
Kim et al. Used small molecule lead phthalocyanine (PbPc) and C60 as the active layer to achieve near-infrared absorption and regulated the crystallinity of the PbPc/C60 blend film (Figure 7 A)[50]. In order to obtain a triclinic crystal phase that absorbs near-infrared light, they heated the substrate and used C60 as a template layer and an electron extraction layer to construct an inversion device. The Iph is significantly improved after the heat treatment, as shown in fig. 7B. The responsivity and external quantum efficiency of the final photodiode for 970 nm light at a bias voltage of-3 V are 244 mA·W-1 and 31.1%, respectively.
An urgent problem to be solved in photodetectors is the suppression of dark current. The direct contact between the active layer and the cathode and anode in bulk organic photodiodes will cause a large noise current, and the narrow band gap materials are more affected. In order to provide a feasible strategy for suppressing dark current, Nguyen et al. Prepared PTB7-Th: CO1-4 Cl bulk heterojunction photodetector by solution method in 2019 (Fig. 8A), and proposed a strategy of modulating junction thickness to suppress leakage current and delay the starting position of space-limited current[51]. They compared photodetectors with different active layer thicknesses, in which the thin active layer is about 87 nm. Compared with the thin active layer, the starting position of space charge current limitation of the thick active layer (about 300 nm) is significantly delayed and the leakage current is effectively suppressed (Fig. 8 B, C), achieving a high specific detection. As shown in Fig. 8 a, the detector adopts a reverse structure with a ZnO layer as a hole blocking layer, a bulk heterojunction active layer coated on an ITO substrate, and then an electron blocking layer MoO3 and an Ag electrode are evaporated. The maximum response value in the near infrared (920 ~ 960 nm) region exceeds the 0.5 A·W-1, and the near infrared spectral region reaches 1010 nm, and the specific detection value is about 1012Jones. Similar strategy proves that the addition of narrow band gap fullerene acceptor is more beneficial to the energy level regulation between donor and acceptor[53]. In order to reduce the dark current, Heremans et al. Proposed a charge transport layer to reduce reverse charge injection and interface defect-induced current generation[52]. The cross-linked PolyTPD is used as an electron blocking layer to effectively prevent charge injection under reverse bias, so that the device shows good reproducibility; In addition, they co-evaporated C60 and LiF to form (C60/LiF), which not only acted as a hole blocking layer for the anode, but also reduced the trap-induced current under reverse bias (Fig. 8d, e). The device dark current can be as low as 0.2 nA·cm-2 at -2 V bias. At the wavelength of 940 nm, the detection degree reaches the 1013Jones.
In order to broaden the spectral response range, Jacoutot et al. Used the donor-acceptor blending strategy to combine the ultra-low band gap polymer (TQ-T) with the non-fullerene acceptor Y6 to achieve a spectral response range of 1800 nm, and the specific detectivity reached 1010Jones[54]. In 2019, Ng et al. Constructed a bulk phase heterojunction and adjusted the donor-acceptor ratio through the donor-acceptor strategy of preparing polymers and fullerenes[54,55]. The lowest density of deep defects and the highest external quantum yield were demonstrated at a donor-acceptor ratio of 1:3. After testing the optimal composition, it is proved that the device can achieve 1 ~ 1.3 μm imaging. In addition to the need to expand the detection range, maintaining the stability of the device is also a key technical problem in practical applications. In 2019, Lin et al. Introduced a non-fullerene acceptor COi8DFIC and blended it with fullerene derivatives and donor molecules to form a ternary heterojunction[56]. The photodetector exhibits a response to visible light and near infrared light. Stability performance can be further improved by interface engineering, building thick junctions, and heat treatment. The photocurrent can still maintain 90% after 11 H of irradiation at a light intensity of 100 mW·cm-1.
Polarization photoelectric detection is also one of the difficulties in the field of photoelectric detection[58]. Yang et al. Fabricated a photodetector with Bi2Te3/PbPc and Bi2Te3/CuPc as the active layer, and this three-dimensional topological insulator/organic thin film heterojunction can realize the detection of linearly polarized light (Figure 9 a)[57]. The film has an anisotropic mobility ratio of up to 2.56 (fig. 9 B) and is responsive to a wide range of visible to mid-infrared wavelengths (fig. 9 C, d). Three-dimensional topological heterojunction structure and organic compounds play an important role in the generation of photocurrent. Photon energy above 520 nm can excite both Bi2Te3 and organic films to produce photocurrent, while photon energy below 980 nm exceeds the energy range that organic compounds can absorb. However, compared with the pure Bi2Te3 thin film, the absorption of visible light to near-infrared light is significantly enhanced due to the presence of the organic thin film.
In addition, the photoconductive gain can be achieved by reasonable design, and the EQE value of the device will greatly exceed 100%. The photoelectric multiplication effect was first proposed by Hiramoto in glass/Au/Me-PTC/Au constructed by organic semiconductor Me-PTC in 1994[59]. There are two modes in which high EQE can be achieved in such a device structure, namely, hole trap-assisted electron tunneling injection and electron trap-assisted hole tunneling injection. By trapping the minority carriers so that the majority pass through the device multiple times before recombination, a photoconductive gain and an external quantum efficiency of more than 100% can be achieved[60]. In 2015, Wang et al. Reported a single-layer infrared photodetector with a device structure of Au/Y-TiOPc @ PC/Au, with a detection range of 940 nm[61]. Because of its electron tunneling effect, its external quantum efficiency reaches 35600% at the incident wavelength of 830 nm. Hirsch et al. Proved the mechanism of obtaining high gain and external quantum efficiency in PCDTBT:PC60BM vertical photoconductive organic detectors, that is, in some specific cases, for p-type devices, excessive charges will lead to additional carrier injection and lead to photoelectric multiplication effect, in which electron traps play a key role[62].
In terms of device construction, wide-gap semiconductors in photodiodes can also respond to near-infrared light by constructing optical microcavities and using microporous electrodes to create Surface plasmon resonances (SPR). Hou et al. Demonstrated the use of porous metal electrodes to fabricate SPR effects and optical microcavities (Figure 10 a), and the wide-bandgap (3.3 eV) organic semiconductor 2,2 ', 7,7' -tetrakis (N, N '-di-p-methylphenylamino) -9,9' -spiro-bifluorene OPDI device also showed significant photoresponse behavior under 830 nm illumination (Figure 10 B)[22]. It provides an effective structural reference for the manufacture of plasma photodetectors and a feasible reference for the improvement of photoelectric conversion efficiency.
To sum up, narrow band gap polymers and small molecules have irreplaceable advantages in organic near-infrared photodetection. The major narrow band gap polymers, small molecules, and other organic molecules mentioned herein are summarized in fig. 11.

5 Applications of Infrared Optoelectronic Devices

5.1 Artificial retina

The retina of the human eye can be used to absorb and distinguish photons of different wavelengths to construct images. Imitating the retina and extending the photodetector to the near-infrared region is of great significance for night vision, retinal repair, and medical imaging applications. At present, most infrared organic photoresponsive devices need to add filters to reduce the interference of visible light. To solve this problem, Liu et al. Developed a near-infrared organic sensor without a filter[63]. By integrating a photosensitive device and a floating gate, the device exhibits accurate color resolution. As shown in Fig. 12A and B, they controlled the thickness of ROT300 in the heterojunction ROT00/VOPc to make the heterojunction exhibit a larger photocurrent under near-infrared light than under green light (Fig. 12c). That is to say, under the same irradiation, the voltage divider will give a larger output voltage under 850 nm irradiation than under 550 nm irradiation. The heterojunction is used as an optical voltage divider sensor, which is connected in series with a load resistor to realize the voltage signal conversion related to the incident infrared light. When it is applied to a floating gate transistor, the "write" process is realized under near-infrared light irradiation at a sufficiently large gate voltage. When the floating gate PIID-TT bipolar active layer is replaced by PIID-BT unipolar active layer, the green light conversion process is forbidden because the floating gate transistor hardly transmits electrons, while the dynamic range of the near infrared is hardly affected, and the green light (550 nm) is not enough to trigger the write process, but it can be used as a dynamic optical switch. As shown in fig. 12d, the prepared pixel array can be attached to the eyeball of a doll. Under the irradiation of near-infrared light, the spatial position distribution and incident light intensity of near-infrared light can be indicated, which reflects the imaging process of mammalian eyes (Fig. 12e, f). Therefore, the high optical band selectivity of green light total blindness and near-infrared writable is realized.

5.2 Artificial synapse

Near-infrared photodetectors can simulate artificial synapses by properly controlling the dielectric layer and the device interface, and realize the potential application of information storage and encryption. Huang et al. fabricated a phototransistor device on a Si/SiO2 with BBT2T 4,8-di (thiophen-2-yl) -1l2,3l2-benzo [1,2-c: 4,5-c0] bis ( [1,2,5] thiadiazole) and tellurophene copolymer as active layers, which has artificial synaptic photoresponse characteristics to short-wave infrared (Fig. 13A)[64]. After being replaced by a flexible polyethylene terephthalate (PET) substrate, due to the high dielectric constant of PBTT and the strong local electric field of PET, excitons will be spontaneously separated and transmitted under the irradiation of short-wave infrared light, and current will be generated without an external electric field. They fabricated a phototransistor array on a flexible substrate and realized the application of encryption mode (Fig. 13 B). As shown in Fig. 13, the device not only achieves the memory effect similar to the neural synapse, but also the current gradually increases after two times of continuous irradiation under the 915 nm(100 mW·cm-2,1 Hz), and the current value slowly decreases after removing the light source, which reflects the perception and recording effect of short-wave infrared. Because the photocurrent of the device can be controlled by different wavelengths at the same time, it is expected to realize the potential application of information encryption.

5.3 Optoelectronic logic circuit

Near-infrared photodetectors can also be applied to optoelectronic logic circuits and implement large-area near-infrared sensing. The In2O3/PTPBT-ET heterojunction phototransistor prepared by Zhu et al. Has a response time as low as 5 ms, and the fast response time is very beneficial to the construction of optoelectronic logic circuits[43]. They fabricated NOT and OR gates based on logic gates with near-infrared response, as shown in Figure 14A, with a heterojunction transistor and a 2 MΩ impedance in the NOT gate circuit. When the near-infrared light is irradiated, the impedance of the phototransistor is smaller than the load resistance, so the voltage drop across the phototransistor is negligible, thereby obtaining a low output voltage.
In contrast, the or gate consists of an independent phototransistor, and the circuit can be controlled by electrical signals or near-infrared light signals. In a NOT gate, any input signal gets a high output signal. This is expected to achieve application possibilities in integrated circuits and near-infrared sensing arrays. Transparent also integrated a 10 × 10 array (100 pixels) on a silicon wafer of 2×2 cm2 and extracted the mobility and photocurrent demonstrating the possibility of spatial light imaging distribution (Figure 14 B).

5.4 Photoplethysmography

One of the functions of photople thysmography (PPG) is to monitor heart rate and pulse. As a non-invasive optical technology, its basic working principle is that the light emitted from LEDs is absorbed, reflected or sputtered by human tissues and detected by optical sensors. In each cardiac cycle, the pulse signal will be superimposed with various low-frequency signals, which can be extracted to assess the heart rate[65].
The PTB7-Th: CO1-4 Cl bulk heterojunction prepared by Nguyen et al. Shows a good real-time heart rate monitoring function[51]. In order to exclude background signal interference, the experimental setup was tested in the dark state. In the work of Jacoutot et al., as shown in Figure 15A, the inversion near-infrared photodetector with TQ-T: IEICO-4F as the active layer still has a high detection degree at wavelengths longer than 1000 nm, which can be used for infrared detection of deeper tissues (Figure 15b)[54]. Wearable flexible near-infrared detection PPG sensing can display diastolic and systolic signals in real time through an oscilloscope without additional complex signal amplification and filtering circuits. Zhao et al. integrated ultra-sensitive OPT and high-efficiency inorganic LEDs (Figure 15C) to realize the potential application of flexible wearable PPG sensors[66]. As shown in fig. 15d, OPT has good stability and mechanical flexibility, and can be attached to the skin for physiological signal monitoring. A Al2O3/SiO2 double dielectric layer is used in the OPT for low voltage (< 3 V) operation. PTFE (polytetrafluoroethylene) and polychloroparaxylylene are used as a coating layer to form a water molecule and oxygen molecule barrier, and the monitoring of heart rate, heart rate variability and blood pressure is realized.

5.5 Up-conversion imaging

The development of shortwave infrared imaging technology is limited by the integration of complex and expensive reading circuits. If the photons absorbed by the detection layer can be up-converted into high-energy photons in the light-emitting layer to emit visible light in the device, the photosensitive and display functions can be realized in the same device.
Li et al. Used a polymer donor-acceptor mixture as the active layer, and NPB (N, N '-bis (napthalen-1-yl) -N, N' -bis (phenyl) benzidine) and tris (8-hydroxyquinoline) aluminum (III) (Alq3) as the electron blocking layer and the light emitting layer, respectively, to construct a short-wave infrared up-conversion imaging device (fig. 16A)[67]. The introduction of the NPB layer limits the injection of electrons into the active layer, thereby significantly reducing the dark current, improving the EQE value of the device, and improving the up-conversion efficiency of short-wave infrared photons to visible photons by an order of magnitude. They used this near-infrared upconversion device for blood vessel imaging, because the blood stream absorbs near-infrared light and therefore appears relatively darker compared to the dorsal blood vessel, as shown in Figure 16b. The device can also be used for environmental monitoring, such as when visible light is strongly scattered in the presence of smoke, but when the near-infrared upconversion imaging device can be used for letter indications such as "EXIT" (Figure 16c).

6 Conclusion and prospect

In the past decade, the field of organic infrared detectors has made rapid progress. This review summarizes the latest and influential research results of near-infrared phototransistors and photodiodes in active layer control, device structure optimization, and exploration of practical applications. For the optimization of the active layer, the development of material systems such as low band gap polymers and non-fullerene acceptors has brought many new opportunities for near-infrared detection, and the device manufacturing processes such as the preparation of optical cavities, porous electrodes, and the construction of vertical structures have also been greatly optimized in the past few years[68].
For OPTs, the detectable wavelength has reached 2600 nm, and they have high specific detectivity and on-off ratio[34]. The optimization strategies for OPTs mainly include: improving the detection and response values by introducing an electron capture layer; Small molecule blending strategy broadens the detection range; Development of Electron Transfer Complexes for Ultra-Weak Light Detection; In terms of device structure, the development of vertical field effect phototransistors effectively reduces the thickness of the active layer, shortens the transmission path of carriers, and improves the sensitivity of OPTs. For OPDIs, the detection range has been extended to 2500 nm by using appropriate donor-acceptor active layers and constructing novel device structures, and OPDIs have very high specific detection D*(1011Jones)[69]. The suppression of dark current can be achieved by modulating the thickness of the heterojunction. Blending with non-fullerene acceptors broadens the spectral response range. The stability of the device can be well maintained by selecting a suitable fullerene acceptor. Polarized light detection can be realized by regulating the structure of the active layer; Ultra-high external quantum efficiency can be achieved by hole/electron-assisted tunneling. Based on the SPR effect, semiconductor molecules without low band gap can also achieve infrared response.
Based on the above great progress in OPTs and OPDIs, potential applications are gradually being developed, such as artificial retinal systems, artificial synapses, optoelectronic logic circuits, health monitoring, and up-conversion imaging. Flexible OPTs with near-infrared response can be used for wearable health monitoring, and it is also one of the hot topics in infrared detection application research. The great achievements and progress made so far strongly prove the great potential and application prospects of near-infrared OPTs and OPDIs.
Although considerable progress has been made in organic near-infrared detection, there are still many important issues to be solved so far. For the further development of organic near-infrared and short-wave infrared detection in the future, the following aspects may need to be focused on. First, the small molecule or polymer material systems with narrow band gap and infrared response characteristics are still limited, which limits the development of infrared detection to a certain extent. The development of stable, visible-light-blind, high-mobility narrow-band gap organic materials with precisely tunable energy levels remains the key in this field. Similarly, the development of non-fullerene acceptor materials is also helpful to optimize the photoresponse performance of devices because fullerene acceptors have general light absorption and are not conducive to film formation. Secondly, in terms of device structure construction, the physical mechanisms of high dark current noise, low EQE, long carrier transport path and long response time in OPTs are still unclear, the characterization methods are still limited, and there are still no new theories and solutions. Thirdly, at present, most of the infrared detectors are still single devices, and the work of integration and array is still less. The development of arrayed devices can effectively simplify the complex process problems caused by the introduction of additional circuits, reduce energy consumption and improve resolution. Finally, compared with commercial detectors, organic infrared detectors have the advantages of flexibility, light weight, large-scale preparation, and compatibility with flexible substrates. The development of flexible devices is also a difficult point in this field, which can facilitate the development of wearable photoelectric detection devices and medical-compatible health monitoring applications.
To sum up, near-infrared and short-wave infrared light detection devices based on organic semiconductors have many excellent characteristics, and show great potential in photoelectric detection, health monitoring, night vision, imaging and logic circuits. Although there are still many challenges, every challenge is an opportunity. It is believed that with the further development of organic synthesis methods and preparation processes and a deeper understanding of their inherent physical mechanisms, organic infrared photodetectors will have bright prospects for development, and their application potential will be fully exploited.

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Funding

Discretionary Fund of Tianjin University(2104)

National Natural Science Foundation of China(U21A6002)

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