Exploration of Near-Infrared Organic Photodetectors | Chemistry of

Review pubs.acs.org/cm

Cite This: Chem. Mater. XXXX, XXX, XXX−XXX

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

Downloaded via GUILFORD COLG on July 19, 2019 at 06:59:51 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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 operation 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 fulfill this goal, it is extremely necessary to fabricate high-performance 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 undergo 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 However, for the highly sensitive nearinfrared (NIR, whose wavelength range generally refers to 760−3000 nm) organic photodetectors, especially for those with absorbing wavelength longer than 1000 nm, only a very limited number have 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 a 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 needed 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, and low-cost, large-scale production accessibility.17 Moreover, cooling-system-free organic NIR © XXXX American Chemical Society

photodetectors enable 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 include 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 section, we address the challenges and prospects for the future development of organic NIR OPDs.

2. FUNDAMENTALS OF OPDs OPDs refer to the devices based on organic semiconductors that can transform an optical signal into an electrical signal. In the case of 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 Special Issue: Jean-Luc Bredas Festschrift Received: March 11, 2019 Revised: June 4, 2019 Published: June 5, 2019 A

DOI: 10.1021/acs.chemmater.9b00966 Chem. Mater. XXXX, XXX, XXX−XXX

Review

Chemistry of Materials

shot noise, Johnson noise (or thermal noise), flicker noise, etc.19 (7) Specific detectivity (D*): The specific detectivity equals the NEP normalized by the device area (A). The unit of measure of D* is Jones (1 Jones = 1 cm Hz1/2/W).

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 the most commonly used architecture for OPDs. For instance, two-terminal devices, namely, photoconductors and organic 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

D* = A1/2 /NEP = R(AΔf )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 = 20 log(L light(max)/L light(min)) = 20 log(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): B is usually expressed in terms of −3 dB 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

R = Jph /L light 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 /L light) × (hc /qλ) = R × (hc /qλ)

where h is Planck’s constant, c is the speed of light, q is the elementary charge, and λ is the incident light wavelength. A photoconductive gain is obtained when the EQE exceeds 100%, which is described as the ratio of the lifetime of trapped charges 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 /Δf

1/2

Figure 1. Typical device configurations of organic photodetectors. T is short for transparent, and S and D are abbreviations for source and drain, respectively.

two-terminal devices where the photoactive layer is contacted by two metal electrodes, while the phototransistors are threeterminal devices that have three electrodes. 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

)/R

where Δf is the electrical bandwidth of the noise measurement in Hz and In is the noise current measured in the dark. There are many mechanisms that can be contributed to the sources of electronic noise inside organic photodetector devices, like the B


Review


Figure 2. continued

C


Review


Figure 2. Molecular structure of the low-bandgap polymers (red), low-bandgap small molecules (blue), and other organic materials (black) mentioned in this review.

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 carrier 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 photodetector, 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 device configuration very similar to that of 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 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 toward the cathode and the anode, respectively; and (5) the free charges are collected by the D


Review

Chemistry of Materials Table 1. Some Performance Parameters and Recent Progress of NIR Organic Photodiodesa photoactive layer structure BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ BHJ PHJ PHJ PHJ PHJ PHJ BHJ BHJ

photoactive layer PTT:PC61BM LBPP−1:P61BM PDDTT:PC61BM DDTT(P4):PC61BM PTZBTTT− BDT:PC61BM PDTTP:PC61BM PDPP−TIIG:PC61BM PDT:PC61BM CPDT(P5):PC71BM CPDT-alt-BSe:PC71BM P1:PC61BM PDAP−TPT:PC61BM PTTBAI:PC71BM PBBTPD:Tri-PC61BM PDPP−FBT:PC71BM PBDTT−DPP: PC71BM PMDPP3T:PC61BM PDPP3T:PC71BM PBD(EDOT)/PC61BM PBTI(EDOT)/PC61BM PCDTTT−C/PC61BM PCPDTBT:PC61BM FDT:PC61BM SnPc:C70 CS−DP:PC71BM DHTBTEZP:PC71BM SQ:PC61BM P3HT:O−IDTBR M1 or M2:PC61BM porphyrin-tape/C60 1−TPFB/C60 Cy7−T/poly-C60 PbPc/C60 PbPc/C70 PbS:P3HT:PCBM PbS:P3HT:PCBM:ZnO

λdet (nm)/Vbias (V)b

spectral response (nm)

Jd (A/cm2)

400−970 300−1200 300−1450 400−1200 400−1100

NA 1.0 × 10−4/−2 10−10/−0.1 1.1 × 10−9 1.25 × 10−10

850/−5 NA 800/−0.1 800/−0.1 800/0

NA NA 2.3 × 1013 1.4 × 1012 1.75 × 1013

0.26 NA NA 0.061/−2 NA

33 35 19 36 37

400−1100 300−1200 300−1600 600−1800 600−1200 300−1700 400−1100 300−1200 350−2500 400−1000 350−900 400−1000 400−900 400−850 400−1085 300−800 400−850 300−1500 200−1000 300−1000 380−960 600−850 300−800 300−1000 900−1500 400−1460 400−900 300−1100 300−1100 300−1450 300−1100

2.64 × 10−2/−5 3.7 × 10−9/−0.1 1.96 × 10−9/−1 10−4/−1.5 10−6/−0.4 ∼10−4/−2 NA 2 × 10−7/−2 1 × 10−9 1.2 × 10−8/−0.5 3.45 × 10−10 2.98 × 10−9 10−10 8.8 × 10−10 6.7 × 10−9 2 × 10−9 1.43 × 10−6 10−6/−1 NA 4.25 × 10−10 3.44 × 10−10 2 × 10−9/−1 10−8/−5 10−9 10−5/−1 1.4 × 10−8 3.3 × 10−8 10−5 NA 10−6/−5 10−5/−4

1000/−5 800/−0.1 900/−1 600−1650 1000/0 300−1200/−0.1 800/−0.1 600−1100/−2 1500/−0.5 860/−0.5 735/−0.5 850/−0.2 840/−1 770/−0.2 830/−0.2 300−800/−2 800/−0.5 1200/o 780/−10 850/0 800/0 700 755 800/−0.1 1400/0 1140/0 850/−1 900/−6 890/0 1220/−5 930

NA 4.1 × 1011 1.9 × 1012 >109 1012 >1011 2.3 × 1010 ∼1012 2.2 × 1011 1011 3.46 × 1011 1.23 × 1013 2.2 × 1012 1.5 × 1013 1.8 × 1012 1012 2.47 × 1012 >1011 1012 5.73 × 1013 4.56 × 1012 3.4 × 1012 1012 1011 8.2 ± 0.2 × 1010 5.3 × 1010 1012 6.6 × 1010 2.7 × 1012 NA 2.26 × 1011

NA 0.018/−2 0.037 NA NA ∼0.01 NA NA 1.4 × 10−7 NA 0.07 0.37 NA NA NA NA NA NA 70 0.33 NA NA 0.42 0.05 or 0.02/−2 NA NA NA NA NA 0.16 1.24

38 39 40 41 42 43 44 45 46 47 48 49 50 51 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 27 26

D* (Jones)

R (A/W)

ref

a The molecular structure of organic materials can be found in Figure 2. bλdet represents the wavelength of the incident light and Vbias represents the bias voltage applied on the OPDs.

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.

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 (ΔELUMO) 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 (ΔEHOMO).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

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 the optical signal are two E


Review

Chemistry of Materials Table 2. Some Performance Parameters and Recent Progress of NIR OPTsa device structure (dielectric)b

semiconductor structure

single-component BGBC (SiO2, HMDS) layer BGTC (PVA, OTS) single-component layer BGTC (SiO2, OTS) single-component layer single-component BGTC (SiO2) nanowires single-component BGBC (SiO2, DDTS) layer BGTC (SiO2, OTS) nanowire network BGTC (SiO2, OTS) single-component layer submicro/ BGTC (SiO2) nanometer ribbon single-component BGTC (SiO2) films BGTC (SiO2) single layer BGTC (Al2O3/ single-component PMMA) films single-component BGBC (SiO2) films single-component BGBC (SiO2) layer BGTC (SiO2) 2D single crystal films BGBC (SiO2) BHJ BGTC (PVP− MMF) TGBC (PVA− PMMA) TGBC (PVA− PMMA) BGTC (SiO2) BGTC (SiO2, OTS or PVA) BGTC (SiO2, OTS or PVA) BGTC (SiO2) BGTC (SiO2)

mobility (cm2/(V s))

spectral response (nm)

400

1.5

76

PDVT−10

11.0 (hole)

500−1000

0.433

176

78

PBIBDF−BT

600−1100 400−1200

0.108 (p), 0.039 (n) 0.44

4552 (p), 1044 (n) 3.3 × 104

74

PBIBDF−TT

0.17 ± 0.02 (hole); 0.06 ± 0.02 (electron) 0.005 (hole)

PIBDFBTO−HH

0.16 (hole); 0.14 (electron)

500−1300

0.145

100

71

DPP−DTT pTTDPP−BT

4.0 (hole) 0.066 (hole); 0.115 (electron)

350−1000 405−950

246 NA

1000 150

91 70

F16CuPc

0.05−0.10 (electron)

400−800

13.6

4.5 × 104

83

PbPc

3.2 × 10−4 (hole)

500−1000

0.005

1.1 × 103

84

PbPc/CuPc ZnPc

8.6 × 10−5 (hole) 3.7 × 10−3 (hole)

600−1000 600−900

2.3 2679.40

82.7 933.56

92 85

squarilium dyes

10−4 (hole); 10−4 (electron)

500−1050

NA

104

93

BODIPY−BF2

0.113 (electron)

600−1000

1.14 × 104

1.04 × 104

94

TFT−CN

1.36 (electron)

500−900

9 × 104

5 × 105

95

DPP−DTT/PC61BM

350−1000

5 × 105

1.6 × 104

79

300−1100

0.25

NA

77

0.32 (p), 0.62 (n) 14

NA

73

NA

BHJ

P3HT:PDPPTTT


350−900

BHJ

PTB7:P(NDI2OD− T2) CuPc/PbPc:PTCDA C60/PTCDA:AlClPc

0.10 (electron)

500−850

HPBHJ HPBHJ

hybrid hybrid

ref

700−1500

P3HT:PEHTPPD−BT

HPBHJ

P


BHJ

HPBHJ HPBHJ

R (A/W)

PPhTQ

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

BGTC (SiO2, OTS) PHJ BGTC (SiO2) PHJ BGTC (SiO2) PHJ BGTC (SiO2) BGBC (SiO2, OTS)

semiconductor

C60/ PTCDA:AlClPc:PbPc IGZO/PBDTT− DPP:PC61BM ZnON/PBDTT− DPP:PC71BM C60/AlClPc C60/PbPc graphene/C60/ pentacene PbS/P3HT Au NRs/BPE−PTCDI NWs

−3

81

69

1.2 × 10 (hole) 1.4 × 10−4 (electron)

600−900 300−850

0.322 2.44

9.4 × 10 NA

1.43 × 10−3 (electron)

300−900

6.48

102

96

7.06 ± 0.58 (electron)

400−780

NA

1.89 × 105

75

48.57 ± 3.35 (electron)

380−940

170

NA

72

4.27 × 10−3 (electron) 3.9 × 10−2 (electron) NA

300−900 400−900 405−1550

2.65 0.109 1800

103 1.2 × 104 NA

97 98 99

0.01 (hole) 0.175 ± 0.062 (electron)

300−1200 300−1200

2 × 104 10.7

104 9.54 × 104

100 101

2

66 88

a

The molecular structure of organic materials can be found in Figure 2. bBGBC: 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)-perylene-3,4:9,10-tetracarboxylic diimide (BPE−PTCDI) nanowires (NWs).

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 low-bandgap 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 sensitization26,27 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 F


Review


[3,4-g]quinoxaline (TQ), 3,6-dithiophen-2-yl-2,5dihydropyrrolo[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 bay-annulated 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 polymer (absorption longer than 1000 nm) photodiodes. In comparison with the NIR photoresponsive low-bandgap polymers photodiodes, the NIR phototransistors based on lowbandgap polymers have been rarely 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 the 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 silicon-based phototransistors (∼300 A/W) but also was among the best single-component film phototransistors performances.80 Nevertheless, owing to this, the film phototransistor with the ambipolar field-effect behavior had the high off-current, and the value of photocurrent on/off ratio was as low as 0.5. The Qiu group also successfully fabricated the film phototransistors based on two kinds of low bandgap D−A conjugated polymers with bis(2-oxoindolin-3ylidene)-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 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 ptype channel and 3.3 × 104 and 70 mA/W for the n-type channel, respectively. Meanwhile, upon NIR illumination with an intensity of 47.1 mW/cm2, they showed higher photosensitive behavior than their thin-film counterparts. Other new D−A polymers, such as (3E,7E)-3,7-bis(2-oxoindolin-3ylidene)benzo[1,2-b:4,5-b0]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-2-yl)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

organic photodetectors in terms of device structures, semiconductor 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 0 V bias. 3.1. NIR Photoresponsive Low-Bandgap Polymer Materials. Although the low-bandgap polymers have been studied for a long time,23,24 there was no report about lowbandgap 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 the ester group as the electron withdrawing unit can stabilize the electron-rich thienothiophene, which matches the energy level of the polymer to the 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-tetraaza-cyclopenta[b]naphthalene unit bordered by electron donating thiophene units on each side.35 This D−A−D configuration results in the polymer showing a spectral response up to 1200 nm with a bandgap as low as 1 eV. Although some parameters obtained from these two low-bandgap polymer 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(5,7-bis(4-decanyl-2thienyl)-thieno(3,4-b)diathiazole-thiophene-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 electron-withdrawing 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, triphenyl-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-(2ethylhexyloxy)phenyl)-5,7-di(thiophen-2-yl)thieno[3,4-b]pyrazine] (PDTTP),38 a series of weak D−strong A polymers containing two different electron-deficient unit (diketopyrrolopyrrole and thienoisoindigo),39 a kind of D−A polymer 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]thiadiazoloG


Review


also be used to detect NIR light. For example, Rauch et al. used the PbS nanocrystalline quantum dots sensitized poly(3hexylthiophene-2,5-diyl) (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 the 650 to 1000 nm spectral region by using CT state absorption between the P3HT donor and the PC61BM acceptor.28 In the case of the CT exciton infrared photodetector structure, the thickness of the photoactive layer is generally up to several micrometers. Recently, some groups have further used this intermolecular CT absorption to fabricate organic narrow-band 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.

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 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 submicro/nanometer ribbons of F16CuPc in 2007. This type of phototransistor 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 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 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 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 photodiode.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 4,4difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY−BF2),94 organic heptamethine salts,62 rhodanine−benzothiadiazolecoupled indacenodithiophene (IDTBR),59 an organic diradicaloid molecule based on stable benzotriazinyls (FDT),54 etc. Recently, Wang et al. reported a 2D ultrathin organic singlecrystal semiconductor TFT−CN based NIR phototransistor.95 This NIR phototransistor exhibited an extremely low dark current of ∼0.3 pA and an ultrahigh 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 make up their photoactive layers are not narrow band gap materials, they can

4. EXPLORATION OF NIR OPDs WITH EXCELLENT PERFORMANCE 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 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 the 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 that it has been demonstrated that the efficiency of excitons separation can be improved by the use of a bulk heterojunction layer.102−104 However, because of 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 H


Review


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[2methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene-vinylene] (MEHPPV),58 poly(N-vinyl-carbazole) PVK,19,50 MoO3,37 and poly(3-hexylthiophene) (P3HT).29 These materials have been evidenced to effectively reduce the dark current. Other electron transporting layers could be used in NIR photodetectors as follows: sol−gel processed TiOx,106 poly[(9,9bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9dioctylfluor-ene)] (PFN),37 poly[(9,9-bis(60-(N,N-dimethylamino)-propyl)-2,7-fluorene)-alt-2,7-(9,9-bis(3-ethyl(oxetane3-ethyloxy)-hexyl)fluorene)] (PFN−OX),113 2,9-dimethyl-4,7diphenyl-1,10-phenanthroline (BCP),36 ZnO nanowire arrays,114 and so on. It should be noted that by introducing a 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 The 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 low-bandgap 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 the EDOT side chain could effectively reduce the Jd of the photodetector devices by about 2 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 1 order of magnitude. The authors conjectured that the reduction of the dark current at reverse bias was a consequence of the increase of the interaction between the polymer and the PEDOT layer, accordingly affording favorable vertical phase separation and forming a polymer-rich 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 predeposited 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 vacuumdeposited MoO3/Ag electrodes, which is expressed as glass/ ITO/PEIE/PMDPP3T:PC61BM/MoO3/Ag. The NIR organic photodiodes with the tp-CP electrode exhibited over 2 orders of magnitude lower dark current density than the device with the vacuum-deposited metal electrode, while a consid-

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 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 a thick active layer could effectively suppress the leakage current in the reverse bias region, and an ultralow 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 hole transporting/electron-blocking layer and/or an electron transporting/hole-blocking 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

Figure 3. 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.

example is Gong et al., who 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 that by addition of a very thin layer of ethoxylated polyethylenimine (PEIE) between the photoactive layer and the electrode in inverted photodiodes based on low-bandgap polymer PBDTTT−C (∼1.6 eV) they could also obtain very low dark currents (2 nA/cm2 at −2 V bias).52 Liu et al. further I


Review


Figure 4. (a) Photocurrent, dark current, and calculated detectivity at λ = 800 nm for each of the OPD 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 magnitude 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.

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 efficiency (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.

erable photocurrent is shown in Figure 6a,b. The D* and LDR of the NIR organic photodiodes with the tp-CP electrode also displayed a high value seen from Figure 6c,d. Unfortunately, when compared with Figure 6a,b, we can find that this tranferprinted 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 photocharge 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 the mobilities of most NIR photosensitive organic materials are low. To fabricate high-performance low-bandgap 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 J


Review


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 with permission from ref 49. Copyright 2016 The Royal Society of Chemistry.

order of 105.79 The high photoconductive gain was obtained, which is because the BHJ structure could effectively capture one type of carrier 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 transporting occurred in the 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 narrow-band (

Exploration of Near-Infrared Organic Photodetectors | Chemistry of

Recommend Documents