Exploration of Near-Infrared Organic Photodetectors

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Exploration of Near-Infrared Organic Photodetectors Qingyuan Li, Yunlong Guo, and Yunqi Liu Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.9b00966 • Publication Date (Web): 05 Jun 2019 Downloaded from http://pubs.acs.org on June 5, 2019

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Chemistry of Materials

Exploration of Near-Infrared Organic Photodetectors Qingyuan Li,†,‡ Yunlong Guo,*,† and Yunqi Liu*,† †Beijing

National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China ‡University

of Chinese Academy of Sciences, Beijing 100049, P. R. China

ABSTRACT: Near-infrared organic photodetectors (NIR OPDs) own some unique properties such as tailorable optoelectronic properties, ease of processing, compatibility with flexible substrates and operating at room temperature. Therefore, NIR OPDs are attractive candidates for future electronic products due to the increasingly desired for wearable electronic devices and biomedical applications. In order to fulfil this goal, it is extremely necessary to fabricate highperformance NIR OPDs. In this review, we present a broad overview of advances in NIR OPDs from the perspective of material selection and device performance optimization over the past decade, and we also summarize the potential applications of NIR OPDs. At last, we are under a deep discussion about the challenges and prospects for the future development of organic NIR OPDs.

1.

INTRODUCTION

Organic photodetectors (OPDs) have attracted much attention due to their tremendous potential applications in flexible and wearable electronics. Significant research efforts in material synthesis and device engineering have allowed for realizing high-performance ultraviolet (UV) and visible light (Vis) photodetectors.1,2 While for the highly sensitive near-infrared (NIR, whose wavelength range generally refers to 760–3000 nm) organic photodetectors, especially for those with absorbing wavelength longer than 1000 nm, there are only a very limited number been reported.3–5 For example, owing to the longer propagation distance with low attenuation within biological tissues of NIR light, the NIR photodetectors are highly desirable in scientific and industrial applications. The hot topics are health monitoring,6–8 artificial vision,9,10 optical communication networks,11 night vision,12 spectroscopy,3,13 biomedical imaging,4 etc. However, commercialized NIR photodetectors rely primarily on epitaxially grown crystalline inorganic semiconductors such as silicon or III–V compounds (InGaAs or HgCdTe). They are not just inherently rigid and fragile but also costly, require complex processing and impose low-temperature cooling system during operation. All of these characteristics determine that inorganic NIR photodetectors have a fundamental limitation for application in the fields of flexible and wearable electronics.14–16 Therefore, alternative semiconductors that are highly responsive to the NIR region are a need urgently. Among the emerging materials, organic small molecules and polymers are most appealing. They show inherent softness, light-weight, adjustable optoelectronic properties by designing the molecular structure, flexibility, low-cost, large-scale production accessibility.17 Moreover, cooling-system-free

organic NIR photodetectors enable the application in future wearable electronic products under real-time monitoring of health conditions. After years of unremitting exploration and research, remarkable achievements have been made in NIR OPDs, which includes designing new materials and optimizing device structures. In this review, we attempt to present a brief overview of recent advances in NIR OPDs from materials selection, device performance optimization and their prospects for application in the flexible and wearable electronics. Section 2 is mainly focused on the fundamentals of OPDs, including the key parameters and the typical device configurations. In the next two sections, we will offer a broad overview of developments in NIR OPDs from the perspective of both materials selection and device performance optimization over the past decade. Following, we outline the potential applications of NIR OPDs in advanced electronics. In the concluding part, we address the challenges and prospects for the future development of organic NIR OPDs.

2.

FUNDAMENTALS OF OPDS

The OPDs refer to the devices based on organic semiconductors that can transform an optical signal into an electrical signal. In case of the OPDs, the main mechanism is based on the photoelectric effect, which means that organic semiconductors absorb incident photons with energy higher than the band gap of semiconductors and subsequently generate electron−hole pairs (i.e. excitons). Different device configurations are utilized to effectively separate strongly bound excitons. In this part, we begin by introducing the performance metrics for evaluating photodetectors, and then summarize the operation mechanisms of most commonly used architecture for OPDs. For instance, two-terminal devices, namely photoconductors and organic

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A photoconductive gain is obtained when the EQE exceeds 100%, which is described as the ratio of the lifetime of trapped charge to the transit time of free carriers. This scenario often occurs in photoconducting systems, where one type of charge carrier is trapped and the other type can traverse the channel freely. When the trapping time is greater than the free carrier transit time, the photoconductive gain is generated.18 (6) Noise Equivalent Power (NEP): The noise equivalent power is the minimum optical power that is used to distinguish a signal from the noise by a photodector. It is used to evaluate the sensitivity of the device. NEP = (In/∆ƒ1/2)/R Where ∆ƒ is the electrical bandwidth of the noise measurement in Hz, and In is the noise current measured in the dark. There are many mechanisms can be contributed to the sources of electronic noise inside organic photodetector devices, like the shot noise, Johnson noise (or thermal noise) and flicker noise, etc.19 (7) Specific Detectivity (D*): The specific detectivity equals to the NEP normalized by the device area (A). The unit of D* is Jones (1 Jones = 1 cm Hz1/2/W). D* = A1/2/NEP = R(A∆ƒ)1/2/In When it comes to specific detectivity, the value of specific detectivity is generally overestimated. Because the dark current is usually used instead of the noise current in calculating specific detectivity. Some reports have argued that the contribution of thermal noise to the total noise current becomes significant in organic NIR photodetector devices (especially based on low-bandgap materials).3 (8) Linear Dynamic Range (LDR): Linear dynamic range means that within a certain range, the photocurrent is linearly proportional to the incident optical power. LDR = 20log(Llight(max)/Llight(min)) = 20log(Jph(max)/Jph(min)) Where Llight(max) is the maximum incident optical power above which the response of the photodetectors deviates from the linear region and Llight(min) is the minimum detectable optical power. Jph(max) and Jph(min) correspond to the maximum and minimum current respectively. (9) Response Speed: The response speed is characterized by the rise time and fall time of a photodetector under the stimulation of a pulsed optical signal. (10) Frequency Response or Bandwidth (B: Hz): It is usually expressed in terms of −3dB bandwidth. 2.2. Operation Mechanisms for OPDs. According to the different mechanisms or device architecture, the organic photodetectors can be roughly categorized into photoconductors, photodiodes, and phototransistors,20 as shown in Figure 1. Among them, photoconductors and photodiodes are two-terminal devices where the photoactive layer is contacted by two metal electrodes. While the phototransistors are three-terminal devices that have three electrodes.

Figure 1. Typical device configurations of organic photodetectors. T is short for Transparent, S and D are abbreviations for Source and Drain, respectively.

photodiodes, are discussed in Section 2.2.1 and Section 2.2.2, respectively; three-terminal devices, namely organic phototransistors (OPTs), are presented in Section 2.2.3. 2.1. Performance Metrics for OPDs. The critical parameters that evaluate the photodetector performance include responsivity (R: A/W), photocurrent on/off ratio (P), spectral response region or wavelength (nm), dark current (Id: A) or dark current density (Jd: A/cm2), specific detectivity (D*: Jones), linear dynamic range (LDR: dB), frequency response or bandwidth (B: Hz), and so on, are summarized as below. (1) Spectral Response Region: Each photodetector only responds to a specific wavelength range. In this review, we only focus on the OPDs whose spectral response range can reach the NIR region. (2) Dark Current (Id)/Dark Current Density (Jd): Dark current refers to the current flowing without illumination. Dark current density is dark current per unit area. (3) Responsivity (R): Responsivity is determined by the ratio of photocurrent generated to the incident optical power at a given wavelength λ. It is expressed as R = Jph/Llight Where Jph is the photocurrent density (A/cm2) and Llight is the incident optical power (W/cm2). (4) Photocurrent on/off ratio (P): The ratio of photocurrent generated to the dark current. P = Iph/Id Where Iph is the photocurrent (A) and Id is the dark current (A). (5) External Quantum Efficiency (EQE): External quantum efficiency is described as the ratio of the number of charge carriers collected to the number of incident photons. The EQE is unitless and often expressed as a percentage. EQE = (Jph/Llight) × (hc/qλ) = R × (hc/qλ) Where h is the Planck’s constant, c is the speed of light and q is the elementary charge, λ is the incident light wavelength.

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Figure 1. The molecular structure of the low-bandgap polymers (red), low-bandgap small molecules (blue) and other organic materials (black) mentioned in this review.

2.2.1. Organic Photoconductors. Photoconductors, also known as photoresistors or light-dependent resistors, are based on photoconductive phenomena. In the dark, they exhibit large resistance and then become more conductive under suitable illumination. For the organic photoconductors, a typical structure of the device is an organic semiconductor layer contacted by two symmetrical electrodes with ohmic contacts. One type of charge carriers can be recirculated across symmetrical contacts until they recombine with the opposite charge. Because of this multiple carrier recirculation, they can achieve high photoconductive gain and high responsivity. Nevertheless, since the lifetime of trapped carriers determines the response time and the photoconductive

gain simultaneously, there exists a trade-off between them. The high photoconductive gain mostly shows rather slow response speed. For this kind of photodetectors, they usually have relatively high operation voltages, slow response speed, small linear dynamic range, and large dark current. 2.2.2. Organic Photodiodes. Organic photodiodes have a very similar device configuration to organic solar cells (OSCs), the typical device of which is a photoactive layer sandwiched between asymmetrical electrodes, and at least one of them is transparent. In the most organic photodiodes, the photoactive layer is generally composed of a mixture of a donor (D) component and an acceptor (A) component, which can be roughly divided into two


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Chemistry of Materials the optical signal are two representative aspects. In the past decades, researchers have devoted great efforts to extend the spectral response range of OPDs to the NIR region. The design of low-bandgap materials is an effective method, and several excellent reviews have also described the design principles of the low-bandgap NIR conjugated small molecules and/or polymers (absorbing in the NIR range and typically having a bandgap smaller than 1.6 eV) in detail.23–25 However, when reducing the optical bandgap of materials, how to maintain a highly efficient photoinduced charge transfer between lowbandgap donors and suitable electron acceptors (such as fullerene derivatives) is still a great challenge. In addition to the use of low-bandgap absorbing materials mentioned above, other methods have also been applied to extend the spectral response to the NIR region include inorganic quantum dot sensitization,26,27 and the utilization of the intermolecular charge transfer (CT) absorption.13,28–32 In this review, we consider just recent advances in materials applied to NIR organic photodetectors. Figure 2 displays the molecular structure of organic materials and their optical bandgap mentioned in this review. In Table 1 and Table 2, we summarized the recent progress in the development of NIR organic photodetectors in terms of device structures, semiconductors structures, photoactive materials, and some performance parameters, including their spectral response wavelength, responsivity (R), dark current density (Jd), specific detectivity (D*), and photocurrent on/off ratio (P). Whereas due to the use of different evaluation criteria, there are discrepancies in the literature regarding performance parameters. It should be noted that since almost OPDs are generally operated with an applied bias voltage, it seems unreasonable to use values of dark current that are actually measured near 0V bias. 3.1. NIR Photoresponsive Low-Bandgap Polymers Materials. Although the low-bandgap polymers have been studied for a long time,23,24 there was no report about low-bandgap polymers based NIR OPDs until 2007. In 2007, Yao et al. reported the first low-bandgap polymer based NIR photodiode with a new type of ester group modified polythieno[3,4-b]thiophene (PTT).33 The absorption onset of PTT film appears at 970 nm with an optical band gap (Eg) of 1.3 eV, due to the fact that the fused thiophene moieties can stabilize the quinoid structure of the backbone.34 The introduction of ester group as the electron withdrawing unit can stabilize the electron-rich thienothiophene, which matches the energy level of the polymer to PC61BM part. In the same year, Perzon et al. also reported a NIR photodiode based on a new conjugated polymer, LBPP–1, with a central electron accepting 2-thia-1,3,5,8-tetraazacyclopenta[b]naphthalene unit bordered by electron donating thiophene units on each side.35 This D-A-D configuration results in the polymer to show a spectral response up to 1200 nm with a bandgap as low as 1 eV. Although some parameters obtained from

different structures, namely planar heterojunction (PHJ) and bulk heterojunction (BHJ), according to their different mixed forms, as shown in Figure 1. The working mechanism of organic photodiodes is also akin to the OSCs: (1) the organic semiconductors absorb the photon energy to create excitons (bounded electron−hole pairs); (2) these excitons diffuse to a lower energy state, typically at a D–A interface or an impurity; (3) the excitons dissociate into free charge carriers (electrons and holes) assisted by the built-in potential or an extra reverse bias; (4) the electrons and holes transfer through the organic semiconductors layer towards the cathode and the anode, respectively; (5) the free charges are collected by the external electrode to generate photocurrent eventually. Note that organic semiconductors typically have large the electron–hole binding energies (e.g., 0.3–0.5 eV), a sufficient energy offset between the highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO/LUMO) levels of the acceptor and the HOMO/LUMO levels of the donor is required to overcome this binding energy and dissociate excitons into free charge carriers efficiently. It is generally considered that the LUMO offset (∆ELUMO) between the donor and acceptor in the range of 0.3−0.4 eV is required for optimal charge separation in systems where the donor material is typically the primary absorber. For these systems where the acceptor material absorbs significant light, the similar requirements are needed for the HOMO offset (∆EHOMO).21 Unlike OSCs, organic photodiodes attach importance to the output of photocurrent signals rather than the transmission of electric power to a load. Compared with the photoconductors, photodiode detectors usually have low dark current, fast response, and wide linear dynamic range.22 2.2.3. Organic Phototransistors (OPTs). OPTs essentially have the exact same three-terminal configurations of organic field effect transistors (OFETs) (Figure 1). Moreover, the three electrodes refer to the source, drain, and gate, respectively, where the channel resistance between the source and drain electrodes can be modulated by means of the third gate electrode. Different regular OFETs, a gate-controlled semiconductor transporting channel can be also modulated by the incident light in OPTs. Thus, a single OPT can make both the signal amplification function of a transistor and the photodetecting ability of a photodiode, which usually shows high sensitivity and low noise.

3.

FUNCTIONAL MATERIALS FOR NIR OPDS

In order to develop high-performance NIR OPDs that can meet the need of a practical application, several important factors should be considered. Among them, the application of appropriate materials that can achieve effective absorption in the NIR region and the utilization of optimal device structure to transfer the stimulation of



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Table 1. Some performance parameters and recent progress of NIR organic photodiodes. (The molecular structure of organic materials can be found in Figure 2.) Photoactive layer structure BHJ

Photoactive layer

Jd (A/cm2)

λdet(nm)/Vbias(V)a

PTT:PC61BM

Spectral Response (nm) 400–970

R (A/W)

ref.

850/−5

D* (Jones) NA

NA

0.26

33

BHJ

LBPP–1:P61BM

300–1200

1.0 × 10−4/−2

NA

NA

NA

35

BHJ

PDDTT:PC61BM

300–1450

10−10/−0.1

800/−0.1

2.3 × 1013

NA

19

BHJ

DDTT(P4):PC61BM

400–1200

1.1 × 10−9

800/−0.1

1.4 × 1012

0.061/−2

36

BHJ

PTZBTTT– BDT:PC61BM

400–1100

1.25× 10−10

800/0

1.75 × 1013

NA

37

BHJ

PDTTP:PC61BM

400–1100

2.64 × 10−2/−5

1000/−5

NA

NA

38

BHJ

PDPP–TIIG:PC61BM

300–1200

3.7 × 10−9/–0.1

800/−0.1

4.1 × 1011

0.018/−2

39

BHJ

PDT:PC61BM

300–1600

1.96 × 10−9/−1

900/−1

1.9 × 1012

0.037

40

BHJ

CPDT(P5):PC71BM

600–1800

10−4/−1.5

600–1650

> 109

NA

41

BHJ

CPDT-alt-BSe:PC71BM

600–1200

10−6/−0.4

1000/0

1012

NA

42

BHJ

P1:PC61BM

300–1700

~10−4/−2

300–1200/−0.1

> 1011

~0.01

43

BHJ

PDAP–TPT:PC61BM

400–1100

NA

800/−0.1

2.3 × 1010

NA

44

BHJ

PTTBAI:PC71BM

300–1200

2 × 10−7/−2

600–1100/−2

~1012

NA

45

BHJ

PBBTPD:Tri-PC61BM

350–2500

1 × 10−9

1500/−0.5

2.2 × 1011

1.4 × 10−7

46

BHJ

PDPP–FBT:PC71BM

400–1000

1.2 × 10−8/−0.5

860/−0.5

1011

NA

47

BHJ

PBDTT–DPP: PC71BM

350–900

3.45 × 10−10

735/−0.5

3.46 × 1011

0.07

48

BHJ

PMDPP3T:PC61BM

400–1000

2.98 × 10−9

850/−0.2

1.23 × 1013

0.37

49

BHJ

PDPP3T:PC71BM

400–900

10−10

840/−1

2.2 × 1012

NA

50

BHJ

PBD(EDOT)/PC61BM

400–850

8.8 × 10−10

770/−0.2

1.5 × 1013

NA

51

BHJ

PBTI(EDOT)/PC61BM

400–1085

6.7 × 10−9

830/−0.2

1.8 × 1012

NA

51

BHJ

PCDTTT–C/PC61BM

300–800

2 × 10−9

300–800/−2

1012

NA

52

BHJ

PCPDTBT:PC61BM

400–850

1.43 × 10−6

800/−0.5

2.47 × 1012

NA

53

BHJ

FDT:PC61BM

300–1500

10−6/−1

1200/o

> 1011

NA

54

BHJ

SnPc:C70

200–1000

NA

780/−10

1012

70

55


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Photoactive layer structure BHJ

Jd (A/cm2)

λdet(nm)/Vbias(V)a

CS−DP:PC71BM


R (A/W)

ref.

850/0

D* (Jones) 5.73 × 1013

4.25 × 10−10

0.33

56

BHJ

DHTBTEZP:PC71BM

380–960

3.44 × 10−10

800/0

4.56 × 1012

NA

57

BHJ

SQ:PC61BM

600–850

2 × 10−9/−1

700

3.4 × 1012

NA

58

BHJ

P3HT:O−IDTBR

300–800

10−8/−5

755

1012

0.42

59

BHJ

M1 or M2:PC61BM

300–1000

10−9

800/−0.1

1011

0.05 or 0.02/−2

60

PHJ

Porphyrin-tape/C60

900–1500

10−5/−1

1400/0

NA

61

PHJ

1−TPFB/C60

400–1460

1.4 × 10−8

1140/0

8.2 ± 0.2 × 1010 5.3 × 1010

NA

62

PHJ

Cy7−T/poly-C60

400–900

3.3 × 10−8

850/−1

1012

NA

63

PHJ

PbPc/C60

300–1100

10−5

900/−6

6.6 × 1010

NA

64

PHJ

PbPc/C70

300–1100

NA

890/0

2.7 × 1012

NA

65

BHJ

PbS:P3HT:PCBM

300–1450

10-6/−5

1220/−5

NA

0.16

27

BHJ

PbS:P3HT:PCBM:ZnO

300–1100

10-5/−4

930

2.26 × 1011

1.24

26

aλ

det

Photoactive layer

represents the wavelength of the incident light and Vbias represents the bias voltage applied on the OPDs.

polymers containing two different electron-deficient unit (diketopyrrolopyrrole and thienoisoindigo),39 a kind of DA polymers containing a novel donor unit dithienobenzotrithiophen (DTBTT) and the thienoisoindigo-based acceptor,40 a series of D-A conjugated polymers consisting of an exocyclic olefin substituted cylopentadithiophene (CPDT) donors,41,42 three strong D−strong A polymers combining [1,2,5]thiadiazolo[3,4-g] quinoxaline (TQ), 3,6-dithiophen2-yl-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione (DPP), and benzobisthiadiazole (BBT) with dithienopyrrole (DTP),43 polymers containing a new diazapentalene (DAP) building block as the acceptor unit,44 a group of conjugated polymers with the nature-inspired bayannulated indigo (BAI) dye as a building block,45 and so on. Although many low-bandgap materials have been synthesized by utilizing the D–A strategy and successfully applied to NIR OPDs, the EQE values in the NIR region of those devices are still quite low due to the problem of a mismatching energy level of the electron acceptors. Hence, the precise energy level control remains a huge challenge in designing and synthesizing materials, which have an appreciable EQE of the NIR photoresponsive lowbandgap polymers (absorption longer than 1000 nm) photodiodes. In comparison with the NIR photoresponsive lowbandgap polymers photodiodes, the NIR phototransistors based on low-bandgap polymers have been rarely

these two low-bandgap polymers based NIR photodiodes are still inferior to those of commercial inorganic detectors, these two works have opened the way for narrow bandgap polymers in the field of NIR light detection. Later on, Gong et al. reported a high-detectivity polymer photodiode with a spectral response range of up to 1450 nm.19 The polymer used in this OPD was poly(5,7bis(4-decanyl-2-thienyl)-thieno(3,4-b)diathiazolethiophene-2,5) (PDDTT), a small bandgap (~0.8 eV) conjugated polymer with an absorption onset at 1450 nm. In this polymer, the introduction of the electronwithdrawing unit thienopyrazine (TP) not only stabilizes the electron-rich thiophene chain but also accelerates the transfer of charge along with the conjugated skeleton. Lately, the spectral response range of NIR OPDs has been successfully extended to 2500 nm.46 The photoactive layer was comprised of a low bandgap D-A copolymer PBBTPD with a fullerene derivative, tri-phenyl-C61-butyric acid methyl ester (Tri-PC61BM), and the low bandgap D–A copolymer which consists of a donating group dithienopyrrole and a strong accepting unit benzobisthiadiazole contributes to the NIR photoresponse. In addition to the molecules mentioned above, other D-A low-bandgap polymers have been also introduced into NIR photodiodes, such as poly[2,3-bis(4(2-ethylhexyloxy)phenyl)-5,7-di(thiophen-2-yl)thieno[3,4b]pyrazine] (PDTTP),38 a series of weak D−strong A



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NIR region. Two types of NIR photodetectors (phototransistors and photodiodes) based on phthalocyanines both have been reported. For instance, Tang et al. first reported a phototransistor based on single-crystalline sub-micro/nanometer ribbons of F16CuPc in 2007. This type of phototransistors displayed a good photoresponse under 785 nm wavelength light.83 Besides, Su et al. demonstrated two kinds of structures of panchromatic organic photodiodes, either PbPc/C70 planar heterojunction (PHJ) structure or PbPc/PbPc:C70/C70 hybrid planar-mixed molecular heterojunction (PM–HJ) structure.65 The spectral response ranges of the PHJ and PM–HJ devices both covered wavelengths from 300 to 1100 nm. Among them, the PHJ device showed EQE higher than 10% and D* on the order of 1012 Jones in the wavelength region from 400 to 900 nm, this D* was among the highest detectivities reported for organic small-molecule photodiodes. Compared with the PHJ devices, the PM–HJ photodiodes exhibited an even higher D* in the NIR region, which was also indicated that the bulk heterojunction structure could increase the efficiency of exciton dissociation. Porphyrin-based compounds are another class of representative materials that can be used for NIR organic photodetectors. A few solution processed NIR photodiodes based on porphyrin-based small molecules had demonstrated the high performances.57,61,90 A good example was reported by Li et al., they realized the solution-processed organic NIR photodiodes based on a porphyrin small molecule (DHTBTEZP).57,61 By optimizing the thickness of the photoactive layer, the device performance can achieve an optical response in the NIR region with a high EQE of around 20% at a bias of 0 V, a low dark current density in the nA/cm2 range and high detectivities over 1012 Jones from 380 to 930 nm. By introducing porphyrin dimeric small molecule CS–DP as the donor component of BHJ, their group recently reported another high performance organic NIR photodiodes.56 For the reason that the porphyrin dimers CS–DP shows longer wavelength absorption range than porphyrin monomers, the CS–DP photodiodes with optimized photoactive layer thickness can achieve the highest detectivity of 1013 Jones in a broad spectral region from 360 to 950 nm. In addition to the two categories of small molecules mentioned above, other NIR photoresponsive small molecules for NIR photodetectors include squarilium dyes,58,93 4,4-Difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY–BF2),94 organic heptamethine salts,62 rhodanine-benzothiadiazole-coupled indacenodithiophene (IDTBR),59 an organic diradicaloid molecule based on stable benzotriazinyls (FDT),54 etc. Recently, Wang et al. reported a 2D ultrathin organic single-crystal semiconductor TFT–CN based NIR phototransistor.95 This NIR phototransistor exhibited an extremely low dark current of ~0.3 pA and an ultra high

investigated. One possible reason is that the carrier mobilities of infrared light sensitive organic materials are generally too low to be suitable for use as the active materials of channels in phototransistors.66,67 Accordingly, there are few low-bandgap polymers based phototransistors.7,68–79 As an example, Li et al. presented a novel phenanthrene condensed thiadiazoloquinoxaline D–A conjugated polymer, PPhTQ.76 The maximum absorption wavelength of PPhTQ film can reach 1500 nm, and the optical bandgap is approximately 0.80 eV estimated from the absorption onset of the solid film. The film phototransistor with PPhTQ as a photoactive layer showed a maximum photoresponsivity of 400 A/W, this value was not only higher than single-crystal siliconbased phototransistors (~300 A/W) but also was among the best single-component film phototransistors performances.80 Nevertheless, owing to this film phototransistor with the ambipolar field-effect behavior had the high off-current, the value of photocurrent on/off ratio was as low as 0.5. Qiu group also successfully fabricated the film phototransistors based on two kinds of low bandgap D–A conjugated polymers with bis(2oxoindolin-3-ylidene)-benzodifuran-dione (BIBDF) as an acceptor unit.74,81 The maximum absorption wavelength of the polymer films of PBIBDF–BT and PBIBDF–TT can reach 1100 nm and 1200 nm, respectively. Moreover, both the polymers exhibited a hole and electron carrier transport response to incident light at different intensities. Their group further fabricated NIR phototransistors made up of BIBDF-based low bandgap polymer (PBIBDF–TT) nanowires.81 The nanowires phototransistors based on PBIBDF–TT showed photocurrent on/off ratios and photoresponsivities as high as 1.3 × 104 and 440 mA/W for the p-type channel, and 3.3 × 104 and 70 mA/W for the ntype channel, respectively. Meanwhile, upon NIR illumination with an intensity of 47.1 mW/cm2, they showed higher photosensitive behavior than their thinfilm counterparts. Other new D–A polymers, such as (3E,7E)-3,7-bis(2-oxoindolin-3-ylidene)benzo[1,2-b:4,5b0]difuran-2,6(3H,7H)-dione (IBDF)-based small bandgap polymer (PIBDFBTO–HH)71 and poly(3-(5(benzo[c][1,2,5]thiadiazol-4-yl)thieno[3,2-b]thiophen-2-yl)2,5-bis(2-octyldodecyl)-6-(thieno[3,2-b]thiophen-2yl)pyrrolo-[3,4-c]pyrrole-1,4(2H,5H)-dione) (pTTDPP– BT)70 are also studied as photoactive layers in phototransistors. 3.2. NIR Photoresponsive Small Molecule Materials. In the case of small molecules, phthalocyanines are a kind of classical material that has been widely used for NIR organic photodetectors. The absorption spectra of these molecules, such as copper phthalocyanine (CuPc ),82 copper hexadecafluorophthalocyanine (F16CuPc),83 lead phthalocyanine (PbPc),64,84 zinc phthalocyanine (ZnPc),85 phthalocycanine (Pc),86 vanadylphthalocyanine (VOPc),87 Tin phthalocyanine (SnPc),55 chloroaluminum phthalocyanine (AlClPc),88 and octabutoxy tin naphthalocyanine dichloride  (OSnNcCl2),89 can reach


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Chemistry of Materials make up their photoactive layers are not narrow band-gap materials, they can also be used to detect NIR light. For example, Rauch et al. used the PbS nanocrystalline quantum dots sensitized poly(3-hexylthiophene-2,5-diyl)

D* exceeding 6 × 1014 Jones when manipulating in the depletion region. 3.3. Other Non-Narrow Bandgap Materials. For some NIR OPDs, although the organic materials that

Table 2. Some performance parameters and recent progress of NIR OPTs. (The molecular structure of organic materials can be found in Figure 2.) Device structure (Dielectric)a

Semiconductor Structure

Semiconductors

Mobility (cm2/Vs)


R (A/W)

P

ref.

BGBC (SiO2, HMDS)

Single-component layer

PPhTQ

BGTC (PVA, OTS)


PDVT–10

0.09 (hole) 0.06 (electron) 11.0 (hole)

400

1.5

76

500–1000

0.433

176

78

BGTC (SiO2, OTS)


PBIBDF–BT

0.17 ± 0.0 2(hole) 0.06 ± 0.02 (electron) 0.005 (hole)

600–1100

0.108 (p), 0.039 (n)

4552 (p), 1044 (n)

74

BGTC (SiO2)

Single-component nanowires

PBIBDF–TT

400–1200

0.44

3.3 × 104

81

BGBC (SiO2, DDTS)


PIBDFBTO–HH

0.16 (hole) 0.14 (electron) 4.0 (hole)

500–1300

0.145

100

71

BGTC (SiO2, OTS)

Nanowire network

DPP–DTT

350–1000

246

1000

91

BGTC (SiO2, OTS)


pTTDPP–BT

0.066 (hole) 0.115 (electron) 0.05–0.10 (electron)

405–950

NA

150

70

BGTC (SiO2)

Submicro/nanometer ribbon Single-component films

F16CuPc

400–800

13.6

4.5 × 104

83

PbPc

3.2 × 10−4 (hole)

500–1000

0.005

1.1 × 103

84

BGTC (SiO2)

Single-layer

PbPc/CuPc

8.6 × 10−5 (hole)

600–1000

2.3

82.7

92

BGTC (Al2O3/PMMA)

Single-component films

ZnPc

3.7 × 10−3 (hole)

600–900

2679.40

933.56

85

BGBC (SiO2)

Single-component films

Squarilium dyes

500–1050

NA

104

93

BGBC (SiO2)


BODIPY–BF2

10−4 (hole) 10−4 (electron) 0.113 (electron)

600–1000

1.14 × 104

1.04 × 104

94

BGTC (SiO2)

2D single crystal films

TFT–CN

1.36 (electron)

500–900

9 × 104

5 × 105

95

BGBC (SiO2)

BHJ

DPP–DTT/PC61BM

350–1000

5 × 105

1.6 × 104

79

BGTC (PVPMMF)

BHJ

P3HT:PEHTPPD–BT

0.3 (hole) 8 × 10−6 (electron) 3 × 10−4 (hole)

300–1100

0.25

NA

77

TGBC (PVAPMMA)

BHJ

P3HT:PDPPTTT

350–900

0.32 (p), 0.62 (n)

NA

73

TGBC (PVA– PMMA)

BHJ

PTB7:P(NDI2OD–T2)

0.14 (hole) 0.06 (electron) 0.10 (electron)

500–850

14

NA

69

BGTC(SiO2)



Device structure (Dielectric)a

Semiconductor Structure

Semiconductors

Mobility (cm2/Vs)

BGTC (SiO2)

HPBHJ

CuPc/PbPc:PTCDA

1.2 × 10−3 (hole)

BGTC (SiO2, OTS or PVA)

HPBHJ

C60/PTCDA:AlClPc

BGTC (SiO2, OTS or PVA)

HPBHJ

BGTC (SiO2)

Page 10 of 27 Spectral Response (nm) 600–900

R (A/W)

P

ref.

0.322

9.4 × 102

66

1.4 × 10−4 (electron)

300–850

2.44

NA

88

C60/PTCDA:AlClPc:PbPc

1.43 × 10−3 (electron)

300–900

6.48

102

96

HPBHJ

IGZO/PBDTT– DPP:PC61BM

7.06 ± 0.58 (electron)

400–780

NA

1.89 × 105

75

BGTC (SiO2)

HPBHJ

ZnON/PBDTT– DPP:PC71BM

48.57 ± 3.35 (electron)

380–940

170

NA

72

BGTC (SiO2, OTS)

PHJ

C60/AlClPc

4.27 × 10−3 (electron)

300–900

2.65

103

97

BGTC (SiO2)

PHJ

C60/PbPc

3.9 × 10−2 (electron)

400–900

0.109

1.2 × 104

98

BGTC (SiO2)

PHJ

Graphene/C60/pentacene

NA

405–1550

1800

NA

99

BGTC (SiO2)

Hybrid

PbS/P3HT

0.01 (hole)

300–1200

2 × 104

104

100

BGBC (SiO2, OTS)

Hybrid

Au NRs/BPE–PTCDI NWs

0.175 ± 300–1200 10.7 9.54 × 101 0.062 104 (electron) BGBC: bottom gate bottom contact; BGTC: bottom gate top contact; TGBC: top gate bottom contact; Poly(methyl methacrylate) (PMMA); hexamethyldisilazane (HMDS); n-octadecyltrichlorosilane (OTS); poly(vinyl alcohol) (PVA); Poly(4-vinylphenol) (PVP), methylated poly(melamine-co-formaldehyde) (MMF); gold nanorods (Au NRs); N,N’-bis(2-phenylethyl)-perylene3,4:9,10-tetracarboxylic diimide (BPE–PTCDI) nanowires (NWs)

photoactive layer is generally up to several microns. Recently, some groups have further used this intermolecular CT absorption to fabricate organic narrowband NIR photodetectors.13,30,31 We will cover this part in detail later. In this review, we also mention some organic materials, whose absorption spectra cannot reach the NIR region, but frequently appear in the photoactive layer of the NIR organic photodetector. Their molecular structures are also given in Figure 2.

Figure 3. The optimized device configurations of photodetectors. (a) Photodiodes. (b) Phototransistors. HTL, ETL and BHJ are abbreviations for Hole Transporting Layer, Electron Transporting Layer and Bulk Heterojunction, respectively. T is short for Transparent. S and D are short for Source and Drain.

4.

EXPLORATION OF NIR EXCELLENT PERFORMANCE

OPDS

WITH

In order to achieve high-performance NIR OPDs toward practical applications, many efforts have been made. For example, attempting to find suitable ways to effectively reduce the impact of the drawbacks inherent in device structures on the device performance, thereby achieving the optimal device performance. Through the innovation of the device structures to overcome the limitations of material systems, some application-oriented functional devices have been fabricated. 4.1. Construction of NIR OPDs with Optimized Performance. Photodiodes and phototransistors are the two most common structures. How to achieve the optimal performance of these two types of devices is critical to the development of NIR OPDs. According to

(P3HT):PC61BM BHJ as the photoactive layer to fabricate photodiodes, which achieved a corresponding NIR detection up to 1.8 µm, and a responsivity and specific detectivity of 0.5 A/W and 2.3 × 109 Jones, respectively.27 The intermolecular CT absorption is a very weak intermolecular absorption that exists universally in organic D–A blends, which is determined by the energy difference between the HOMO of the donor and the LUMO of the acceptor. Yang et al. made a high sensitivity infrared photodetector covering 650 to 1000 nm spectral region by using CT state absorption between P3HT donor and PC61BM acceptor.28 In the case of the CT exciton infrared photodetector structure, the thickness of the


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Figure 4. (a) Photocurrent, dark current, and calculated detectivity at λ = 800 nm for each of the OPDs structures biased at −100 mV. (b) Detectivities of Si photodetector, InGaAs photodetector, and polymer photodetector versus wavelength. The high detectivities (1012 Jones) of the InGaAs photodetectors require cooling the devices to 4.2 K. The detectivities of the OPDs (ITO/PS−TPD−PFCB/PDDTT:PC60BM/C60/Al) were calculated at λ = 500 nm (point A) and λ = 800 nm (point B) biased at −100 mV. The solid blue curve was obtained from the measured photoresponsivity data with absolute magni-tude determined by points A and B. (c) LDR of the polymer photodetectors with ITO/PS−TPD−PFCB/PDDTT:PC60BM/C60/Al architecture. Reproduced with permission from Ref. 19. Copyright 2009, American Association for the Advancement of Science.

For example, Wu et al. investigated the influence of photoactive layer thickness ranging from 110 to 360 nm on the photoresponse and the dark J–V characteristics of two different structures of photodiodes, ITO/PEDOT:PSS (40 nm)/PBDTTT−C−T:PC71BM (1:1.5) (110−360 nm)/PFN (5 nm)/Al and ITO/PFN (5 nm)/PBDTTT−C−T:PC71BM (1:1.5) (110−360 nm)/MoO3/Ag.50 Both device structures indicated that the use of thick active layer could effectively suppress the leakage current in the reverse bias region, and an ultra low dark current density of 0.3 nA/cm2 was achieved at a reverse bias of −1 V from the structure of latter device. However, note that the increase in the thickness of the photoactive layers has the adverse effect on EQE due to an increase in charge recombination loss. Consequently, it is not an ideal choice to reduce dark current simply by increasing the thickness of photoactive layers. Interface Engineering. A few reports have addressed that introducing a transport hole transporting/electronblocking layer and/or an electron transporting/holeblocking layer into the sandwich-like photodiode is a good method to effectively minimize the dark current without damaging the photocurrent,19,29,37,50,52,58,106,108–111 as shown in Figure 3a. A classic example is Gong et al. successfully suppressed the dark current density of the photodetectors based on low-bandgap polymer PDDTT (~0.8 eV) to be 1 × 10−9 A/cm2 biased at −0.1 V by inserting a high-conductivity, cross-linkable, hole transporting/electron-blocking polymer interlayer (PS−TPD−PFCB) at the anode interface and an electron transporting/hole-blocking layer (C60) into the cathode interface.19 While the photodetector exhibited higher photodetectivity than 1012 Jones from 1150 to 1450 nm, which was comparable to or even better than those from inorganic photodetectors based on Si and InGaAs, as shown in Figure 4. Furthermore, Saracco et al. also found

two different devices, we will respectively introduce the effective methods introduced to obtain high-performance NIR OPDs. 4.1.1. Performance Optimization of NIR Photodiodes. As an important criterion for evaluating the performance of photodetectors, the level of dark current density would affect some fundamental aspects of photodetectors like power consumption and the effectiveness of photocurrent readout process. Organic photodiodes often employ the structure of a bulk heterojunction (BHJ) layer sandwiched between two electrodes to improve their responsivity, owing to it has been demonstrated that the efficiency of excitons separation can be improved by the use of a bulk heterojunction layer.102–104 However, since the simultaneous presence of both donor and acceptor components near the metal interface in the BHJ, the situation of charge injection from the metal contacts under external bias becomes quite serious. The existence of this charge injection phenomenon will directly lead to a high dark current density. For the NIR organic photodetectors based on low-bandgap materials with lower HOMO and higher LUMO, this injection phenomenon appears to be more prominent. Therefore, it is critically needed to explore suitable methods to effectively reduce the dark current density without the sacrifice of the photocurrent for NIR organic photodetectors. In accordance with the earlier research, there are many factors that can affect the dark current and some measures have been taken to effectively reduce dark current to some extent. However, unfortunately it seems that no matter which method is adopted, it will inevitably bring some negative effects. Thickness of The Active Layer. Some works have already illustrated that the dark current density is correlated with the thickness of the photoactive layer.105–

107



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Figure 5. (a) and (b) J–V characteristics of the polymer photodetectors in dark and under AM 1.5 G (100 mW/cm2) illumination. (a) PBT(EDOT) and PBT(TH) based photodetectors; (b) PBD(EDOT) and PBD(TH) based photodetectors. (c) The external quantum effi ciency (EQE) spectra and (d) the detectivity of the polymer photodetectors aquired at −0.2 V bias. Reproduced with permission from Ref. 51. Copyright 2015, John Wiley and Sons.

fluorene)-alt-2,7-(9,9-bis(3-ethyl(oxetane-3-ethyloxy)hexyl)fluorene)] (PFN−OX),113 2,9-dimethyl-4,7-diphenyl1,10-phenanthroline (BCP),36 ZnO nanowire arrays,114 and so on. It should be noted that by introducing blocking layer to reduce leakage current, an additional injection barrier is often introduced, resulting in lower photocurrent especially at higher light intensities and lower forward current. Molecular Side-Chain Engineering. Several groups have reported that the morphology of the photoactive layer had an effect on the magnitude of leakage current in the photodetectors.36,47,105,106,115 Side-chain, as an important factor to influence the morphology of the polymer,116 is expected to be available in the NIR polymer photodetectors to optimize the dark current. Zhang et al. significantly depressed the dark current of the lowbandgap polymer photodetectors by modifying the polymer structures with 3,4-ethylenedioxythiophene (EDOT) side chains conjugated to the semiconducting polymer backbone.51 Figure 5 suggested that the introduction of EDOT side-chain could effectively reduce

that by addition of a very thin layer of ethoxylated polyethylenimine (PEIE) between the photoactive layer and the electrode in inverted photodiodes based on lowbandgap polymer PBDTTT−C (~1.6 eV) could also obtain very low dark currents (2 nA/cm2 at −2 V bias).52 Liu et al. further demonstrated that the water-soluble CdTe quantum dots could be also used as an anode interlayer to lower leakage currents of the NIR polymer photodetectors.112 Additionally, other materials commonly used as a hole transport layer in NIR organic photodetectors include poly[N,N’-bis(4-butylphenyl)-N,N’bis(phenyl)-benzidine] (poly–TPD),109 poly[2-methoxy-5(2’-ethyl-hexyloxy)-1,4-phenylene-vinylene]  (MEHPPV),58 poly(N-vinyl-carbazole) PVK,19,50 MoO337 and poly(3-hexylthiophene) (P3HT).29 These materials have been evidenced to effectively reduce the dark current. Other electron transporting layer could be used in NIR photodetectors as follows: sol-gel processed TiOx,106 poly[(9,9-bis(3’-(N,N-dimethylamino)propyl)-2,7fluorene)-alt-2,7-(9,9-dioctylfluor-ene)] (PFN),37 poly[(9,9-bis(60-(N,N-dimethylamino)-propyl)-2,7-


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Figure 6. (a) J–V characteristics of glass/ITO/PEIE/PMDPP3T:PC61BM/PEDOT:PSS in the dark and under simulated 1 Sun. The red line is the J–V characteristics curve of light incidence from the PEDOT:PSS polymer top electrode and the green line is the J‒V characteristics curve of light incidence from the ITO bottom electrode. (b) J–V characteristics of glass/ITO/PEIE/PMDPP3T:PC61BM/MoO3/Ag. (c) The detectivity of glass/ITO/PEIE/PMDPP3T:PC61BM/PEDOT:PSS versus the wavelength. The blue line is the detectivity of light incidence from the PEDOT:PSS polymer top electrode and the red line is the detectivity of light incidence from the ITO bottom electrode. (d) Current density of glass/ITO/PEIE/PMDPP3T:PC61BM/PEDOT:PSS as a function of light intensity at 850 nm. Reproduced from Ref. 49 with permission from The Royal Society of Chemistry.

the Jd of the photodetector devices by about two orders of magnitude compared to devices based on the polymers without the EDOT side-chain, while EQE hardly decreased. Therefore, the photodectivity increased by more than one order of magnitude. Authors conjectured that the reduction of the dark current at reverse bias was a consequent of the increase of the interaction between the polymer and the PEDOT layer, accordingly affording favorable vertical phase separation and forming polymerrich layer near the PEDOT surface. Those also indicated that the method of side-chain engineering had some pertinence to the device structure. Transfer-printed Electrode. It has been proved that during thermally depositing a metal electrode under high

vacuum, the metal would diffuse into the pre-deposited organic semiconductor layer, which produced remarkable effects on the performance of the semiconductor device.117–119 In order to prevent the evaporation metal from diffusing to the photoactive layer, thus affecting the dark current density of the NIR organic photodiodes, Xiong et al. employed a transfer-printed conducting polymer (tp-CP) as the top electrode of the NIR photoresponsive low-bandgap polymers (PMDPP3T) based organic photodiodes.49 The device structure is expressed as glass/ITO/PEIE/PMDPP3T:PC61BM/PEDOT:PSS. By contrast, they also fabricated a device with thermally vacuum-deposited MoO3/Ag electrodes, which is



Page 14 of 27

Figure 7. (a) and (b) The working principles of charge collection narrowing (CCN) OPDs. (c) The normalized EQE of the narrowband CCN OPDs comprising thick junctions of PCDTBT (red) and DPP−DTT (NIR) blended with PC71BM. The normalized absorption of PCDTBT:PC71BM and DPP−DTT:PC71BM blends are also shown (dashed lines). (d) The tunability of the CCN OPDs. The normalized EQE of PCDTBT:PC70BM photodiodes measured at 120 Hz and −1 V with different thicknesses allowing for fine-tuning the charge carrier photogeneration profile and thereby the EQE. As the active layer thickness increases from 1,500 to 3,000 nm the photogeneration in the volume occurs at longer wavelengths and the EQE peak red shifts from red to the NIR. The dashed line represents the absorption coefficient of the blend. Reproduced with permission from Ref. 30. Copyright 2015, Springer Nature.

the mobilities of most NIR photosensitive organic materials are low. To fabricate high-performance lowbandgap organic phototransistors, an effective strategy is developed a BHJ constituted by an electron donor and an electron acceptor as a photoactive layer, just like the photodiodes, to achieve high photoexciton dissociation efficiency. A good example is Xu et al. used a bulk heterojunction composed by a NIR photoresponsive lowbandgap polymer PDPP−DTT and PC61BM as a photosensitive layer of phototransistors. The resulting phototransistors showed excellent performances with responsivity up to 5 × 105 A/W and photoconductive gain tunability on the order of 105.79 The high photoconductive gain was obtained, which is because the BHJ structure could effectively capture one type of carriers in one component while allowing rapid transport of carriers with opposite charges in the other component. Peng et al. reported another type of NIR organic phototransistor with a hybrid planar-bulk heterojunction (HPBHJ) structure (Figure 3b), in which the photoexcitons generation and dissociation occurred mainly in the bulk heterojunction PbPc:PTCDA, while the current

expressed as glass/ITO/PEIE/PMDPP3T:PC61BM/MoO3/Ag. The NIR organic photodiodes with the tp-CP electrode exhibited over two orders of magnitude lower dark current density than the device with the vacuum-deposited metal electrode, while a considerable photocurrent shown in Figure 6a and 6b. The D* and LDR of the NIR organic photodiodes with the tp-CP electrode also displayed a high value seen from Figure 6c and 6d. Unfortunately, when compared with Figure 6a and 6b, we can find that this tranfer-printed electrode method also can lead to the lower forward current. 4.1.2. Performance Optimization of NIR Phototransistors. The fundamental physical processes of phototransistors are the generation of photo charge carriers and the transportation of field-effect current simultaneously under light illumination. For a traditional phototransistor with a single-component organic semiconductor layer, the exciton generation and the carrier transport take place in the same photoactive layer. When it comes to low band-gap organic molecules, it becomes more difficult, which stems from the fact that


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Figure 8. (a) and (b) Working principle of the original cavity enhanced OPD. (a) Simplified scheme of the device architecture with a sketch of the optical field distribution for the resonance wavelength in the NIR; (b) Simplified energy diagram at open circuit. (c) NIR detectors based on ZnPc:C60 CT absorption. Spectrally resolved EQE or absorption on linear (top) or logarithmic (bottom) scale. The green, crossed line indicates the EQE of a ZnPc:C60 solar cell with minimal optical cavity effect. The grey lines are scaled to the previous curve and represent the absorption of neat C60 (marked as I) or ZnPc (marked as II). The remaining curves show the EQE of several cavity-enhanced ZnPc:C60 detectors, measured at short circuit. (d) NIR detectors based on TPDP:C60 CT absorption. Normalized EQE spectra of several tetraphenyl dipyranylidene:fullerene (TPDP:C60) detectors at short circuit. Reproduced with permission from Ref. 31. Copyright 2017, Springer Nature.

noise, visible-blind NIR OPDs, the authors introduced a new concept of charge collection narrowing (CCN) in thick bulk heterojunctions (DPP−DTT:PC71BM).30 This was the first report of visible-blind sub-100 nm full-widthat-half-maximum (FWHM) NIR photodetectors without the additional use of input filtering. The working principles of CCN OPDs were presented in Figure 7a, 7b. In this thick bulk heterojunction photoactive layer, the shorter wave length photons (A and B) are absorbed close to the indium tin oxide (ITO) side, accordingly the extraction of photogenerated electrons is hindered. Under the circumstances, the charge collection efficiency is narrowed to merely the wavelengths in the spectra that are much close to the absorption edge (C), in which the heterojunction extinction coefficient is low. Therefore, the EQE presents a sharp peak at the onset of the absorption. Generally speaking, the spectral selectivity is achieved by adjusting the internal quantum efficiency (IQE) by means of extraordinarily unbalancing the charge carrier transport. This CCN is a new way to control the spectral response, and the detection window can be chosen depending on the absorption onset of the used bulk heterojunctions materials system (see Figure 7c).

transporting occurred in high mobility channel layer based on CuPc.66 This phototransistor based on the HPBHJ structure exhibited a high photoresponsivity of 322 mA/W, a high external quantum efficiency of around 50%, and maximal photosensitivity of 9.4 × 102. Subsequently, Yang’s group further developed the hybrid planar-bulk heterojunction of phototransistors by using the bulk heterojunctions comprising of low-bandgap polymers and PC61BM as the NIR photosensitive layers and oxide semiconductors as charge transport layers to fabricate the oxide-semiconductor based phototransistors.72,75 Those organic-inorganic hybrid phototransistors presented a broad bandwidth response from ultraviolet (UV) to the NIR region, and the detectivity and a linear dynamic range exceeded 1012 Jones and 100 dB, respectively. 4.2. Optimization Concepts for NIR OPDs. For better satisfying functional requirements for NIR OPDs and overcoming the limitation of material factors on the choice of device structure, some novel concepts are also proposed. Charge Collection Narrowing Photodetectors. In order to fabricate a truly narrowband (< 100 nm), low



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Figure 9. (a) Illustration for the device structure of NIR-sensing organic phototransistors with gate-sensing layer (GSL). (b) Transfer characteristics of devices in the dark according to the different GSL thickness at VD = −60 V (inset: output characteristics at VG = −10 V). (c) Output characteristics of devices: Output curves for organic phototransistors with GSL according to the different PEHTPPD−BT (GSL) thickness (t = 0–600 nm). The dashed lines denote the dark current, while the solid lines represent represent the photocurrent upon illumination with the visible (= 555 nm) and NIR (= 780 and 1000 nm) light at VG = −10 and −60 V. The incident light intensity was 0.79 μW/cm2. Reproduced with permission from Ref. 68. Copyright 2018, John Wiley and Sons.

The NIR CCN OPDs based on the optimized CCN with thick junction OPD structures: ITO (80 nm)/PEDOT:PSS (20 nm)/DPP−DTT:PC71BM (2 mm)/C60 (50 nm)/Al (100 nm), showed FWHM

Exploration of Near-Infrared Organic Photodetectors

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