Fabrication of carbon nanotube-threephase organic photodetectors with high responsivity and wide spectrum and the underlying mechanisms

Fabrication of carbon nanotube-threephase organic photodetectors with high responsivity and wide spectrum and the underlying mechanisms

Current Applied Physics xxx (xxxx) xxx–xxx Contents lists available at ScienceDirect Current Applied Physics journal homepage: www.elsevier.com/loca...

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Current Applied Physics xxx (xxxx) xxx–xxx

Contents lists available at ScienceDirect

Current Applied Physics journal homepage: www.elsevier.com/locate/cap

Fabrication of carbon nanotube-threephase organic photodetectors with high responsivity and wide spectrum and the underlying mechanisms Tao An∗, Dan Liu College of Automation and Information Engineering, Xi'an University of Technology, Xi'an, 710048, China

ARTICLE INFO

ABSTRACT

Keywords: Organic photodetector Three primary colors Carbon nanotubes Bulk heterojunction

In this work, we incorporated single-walled carbon nanotubes (SWCNTs) into organic three-phase heterojunction active layer detectors, and systematically experimentally investigated the influences of single walled carbon nanotubes (SWCNTs) on the photoabsorption and photoelectric properties of the organic three-phase heterojunction detectors. Under -1 V bias voltage, the average photoresponsivity of three primary colors detector is 475 mA/W, about 2–3 times higher than that of similar devices that without SWCNTs. The average external quantum efficiency (EQE) increases to 111%. The results show that the incorporation of single-walled carbon nanotubes in the three-phase bulk heterojunction active layer maintains the original spectral morphology, while increases a higher degree of aggregation and crystallinity of the organic conjugated polymer, and further enhances a larger capacity of light absorption, regardless of the number of active layer's mixed phases. Under -1 V bias voltage, the average photoresponsivity of three primary colors detector is 475 mA/W, about 2–3 times higher than that of similar devices. The average external quantum efficiency (EQE) can be increased to 111%. In terms of carrier transport, SWCNT can increase the exciton dissociation area, dissociation rate of the film, and provide a fast transport channel for the charge, therefore improve the charge collection of the electrode. In a concentration range of 0.75–1.75 wt% and in high-energy photon excitation, single-walled carbon nanotubes can produce a multiple exciton generation.

1. Introduction Organic/inorganic hybrid light-sensing technology has shown promising prospect due to its low-cost processing and high efficiency. In the past decade, P3HT:PCBM was widely used as a donor-acceptor pair to pursue high power conversion efficiencies (PCE) [1]. However, the overall performance of this device is still limited by the lower carrier mobility of the organic polymer in the active layer (∼2 × 10−3 cm2/V) and the shorter exciton diffusion length (∼10 nm) [2]. For example, the spectral response is relatively narrow and the morphology of interpenetrating network required for charge transport in the film is far from optimal. To break through these bottle necks, some attempts have been carried out to optimize the existing active layers, such as the addition of new components to broaden the spectral response [3], or to build an effective exciton dissociation center [4] and charge conduction pathway [5]. It has been shown that single-walled carbon nanotubes (SWCNTs) have high aspect ratios (length/diameter) (105-106) [6], large contact area and high carrier mobility (7.9 × 104 cm2/V) [7], and tunability of the bandgap energy (electronic and optical bandgaps: 0.5–5 eV) in the



range from near infrared (NIR) to ultraviolet (UV) [8]. In addition, SWCNTs can be used as an excellent exciton dissociation center or an ultra-fast transfer carrier permeation network (0.2–10 ps) [9,10]. Therefore, it is promising to employ carbon nanotube (CNT) for improving the performances of organic optoelectronic devices. Most recently, Nicola et al. achieved 100% quantum conversion efficiency in organic-carbon nanotube solar cells [6]. Samrat Paul et al. combined polythiophene and PCBM with multi-walled carbon nanotubes to fabricate organic photovoltaic cells, of which the PCE was improved by about 40% [11]. By using PTB7:PCBM:SWCNT as the active layer structure in the organic-carbon nanotube infrared photodetector, Xu et al. further enhanced the infrared photodetection capability [12]. Through doped inorganic materials as a trap, Deng et al. exhibited a normalized detectivity (D*) over 1014 Jones [13]. Despite these achievements, the fundamental understanding of the CNT-polymerized heterojunction active layer is still incomplete. In particular, systematic studies on the doped CNT three-phase heterojunction active layer have not been reported. In order to investigate the effects of doped SWCNTs on the photoelectric properties of three-phase active layer, a red-light material

Corresponding author. E-mail address: [email protected] (T. An).

https://doi.org/10.1016/j.cap.2019.02.009 Received 21 December 2018; Received in revised form 29 January 2019; Accepted 14 February 2019 1567-1739/ © 2019 Korean Physical Society. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Tao An and Dan Liu, Current Applied Physics, https://doi.org/10.1016/j.cap.2019.02.009

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Fig. 1. Structure of the PBDTTT-F:PCBM: SWCNT device.

PBDTTT-F with narrow band gap was selected to prepare a bulk heterojunction active layer with PBDTTT-F:P3HT:PCBM:SWCNT. The influence mechanism of thin film morphology and the effect of the photoelectric properties to three-phase detectors, such as photoabsorption, active layer's exciton generation and dissociation, and charge transport, are analyzed. The SWCNT multiple exciton generation (MEG) effect is utilized in the device to achieve photomultiplier. Finally, a detector with ideal photoelectric characteristics is obtained.

Fig. 3. Optical absorption spectral of the P3HT:PCBM: PBDT-TT-F device with various mixed degree different SWCNT content.

spectrophotometer, and the fluorescence spectroscopy by LS-45/55 fluorescence spectrometer. The Keithley 2636 tester was employed to measure the volt-ampere characteristics of the device by J-V characteristic scanning. Test light source: blue, green, red LED light, of which wavelengths are 460 nm, 530 nm, 630 nm, respectively, and optical power are 5.54 mW/cm2, 2.56 mW/cm2, 0.97 mW/cm2, respectively.

2. Experimental methodologies and details As shown in Fig. 1, the basic structure of the device is ITO/PEDOT:PSS (30 nm)/PBDTTT-F:P3HT:PCBM:SWCNT/LiF (1 nm)/Al(100 nm). The corresponding energy level structure is illustrated in Fig. 2. In the sample preparation, the ITO glass substrate was firstly etched, and four ultrasonic cleanings were sequentially performed by using detergent, acetone, deionized water, and ethanol, each for 15 min. Then, in a nitrogen glove box, a 30 nm thick PEDOT:PSS layer was spin-coated at 3000 rpm for 60 s and annealed at 120 °C for 15 min; after that, a 200 nm thick active layer was spin-coated at 800 rpm for 60 s and annealed at 110 °C for 15 min. Finally, the 1 nm thick LiF layer and 100 nm thick Al electrode were sequentially deposited under a vacuum of 10−4 Pa, and subjected to thermal annealing at 120 °C. The substrate was an indium-tin-oxide (ITO) glass with area of 20 × 20 mm2 and thickness of 1.5 mm, and the sheet resistance was 15 Ω/ sq. The three-phase blend solution with SWCNTs is prepared through following approach:P3HT:PCBM:PBDTTT-F mixed solution was first dissolved in chlorobenzene (CB)(P3HT:PCBM:PBDTTTF:CB = 12 mg:8 mg:3 mg:1 ml), magnetically stirred at room temperature for 15 h; then the carboxylated SWCNTs were added and subjected to a water bath at 50 °C for 1 h, and ultrasonically shaken for 2 h. Material purchase: P3HT, PCBM was purchased from Xi'an Bao Laite Optoelectronics Technology Co., Ltd., PBDTTT-F from Suzhou Nakai Optoelectronics Technology Co., Ltd., and carboxylated single-walled carbon nanotubes from Chengdu Organic Chemical Co., Ltd. In the test, the film thickness was measured by VB-400 VASE ellipsometer. The surface morphology of the film was measured by Agilent 5500 SPM type atomic force microscope. The UV–visible absorption spectrum was measured by UV762 dual-beam UV–visible

3. Results and discussions First, we studied the photoabsorption properties of the three-phase blend active layer without SWCNTs. In order to broaden the spectral response range, three different kinds of polymers (PBDTTT-F, P3HT and PCBM) were mixed at a certain proportion to prepare the active layer. As shown in Fig. 3, the absorption spectral of the active blend layer covers almost the entire visible light band(350–750 nm), and the increase of the proportion of PBDT-TT-F would enhance the absorption ability for red light. Besides, according to the previous research results, the photocurrent of the three-phase blend active layer was reached the maximum at 15 wt% (i.e. the ratio of P3HT:PCBM:PBDT-TT-F is 12:8:3). Therefore, the elementary active layer of P3HT:PCBM:PBDTTT-F with the ratio 12:8:3 was selected in this work, which performs a broad absorption spectral and a high photocurrent. Moreover, we dispersed SWCNTs into the P3HT:PBDTTT-F:PCBM blend active layer to investigate its influence on the photoabsorption properties of the devices. The selected SWCNTs with high aspect ratio can be classified into two types: metallic ones (1/3) and semiconducting ones (2/3), of which the diameter varies in the range of 0.7–2 nm. Fig. 4 shows the absorption spectral of films with different SWCNTs concentration. It can be found that both the intensity of light

Fig. 2. Energy level diagram of the PBDTTT-F: PCBM:SWCNT device.

Fig. 4. Optical absorption spectral of the P3HT:PCBM:PBDT-TT-F device. 2

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Fig. 5. AFM images of the P3HT:PBDT-TT-F:PCBM films with different concentration of SWCNTs (a) 0 wt%, (b) 0.5 wt%, (c) 1 wt%, (d) 1.5 wt%, (e) 2 wt%. (f)–(j) are 3D-AFM image of (a)–(e), respectively.

Fig. 6. AFM images of the PBDT-TT-F films with different concentration of SWCNTs (a) 0 wt%, (b) 1 wt%, (c) 2 wt%, (d) 3 wt%, (e) 4 wt%. (f)–(j) are 3D-AFM image of (a)–(e), respectively.

absorption and the degree of red shift are increased as the concentration of SWCNTs grows. To be more specific, the original absorption peaks of P3HT centered at 525 nm, 552 nm and 597 nm are red-shifted to 552 nm, 576 nm, and 621 nm, respectively, when the concentration of SWCNTs is 2 wt%. This is because SWCNTs, acting as a nucleating agent for P3HT, promote the localized ordering of molecular chains along the axial direction of nanotubes. In addition, through the noncovalent interaction with the nanotubes, the inter-molecular conjugation length of P3HT is increased, and the delocalization length of electrons along the molecular chain is increased [14,15]. Similar phenomena can be found at the absorption peaks of PBDT-TT-F and PCBM, originally centered at 670 nm and 350 nm, respectively. Besides, the inset of Fig. 4 displays another absorption peak at 980 nm, which is contributed by the semiconducting SWCNTs and consistent with the related E11s (V1→C1) transition absorption (absorption peak 800–1200 nm) [16]. Fig. 5 shows the surface morphology of the three-phase layers incorporating with different concentration of SWCNTs. It can be found that the chains of the polymers are arranged along the nanotubes and aggregate locally. When the concentration of SWCNTs is relatively low (less than 1 wt%), a great amount of bright worm-shaped areas are formed on the layer surface (see e.g. Fig. 5(b) and (c)). Consequently, the surface roughness increases as the concentration is raised. Moreover, as the concentration of SWCNTs continues to be increased, the worm-shaped area on the surface evolves into large-area sheet

structure, which leads to the decline of roughness. This proves that the incorporation of SWCNTs promotes the orderly arrangement of conjugated organic molecules, resulting in increased crystallinity and increased π-π* intensity, also inhibiting charge recombination [14,17]. In order to eliminate the interference of other polymers, we chose a single material PBDTTT-F doped SWCNTs, the AFM of which is shown in Fig. 6. It can be seen from the figures that as the SWCNTs doping ratio is increased, the film roughness is increased, and the distribution of worm morphology with SWCNTs as the core is formed. The micron size of the SWCNT interpenetrating network structure facilitates charge transfer [18]. As shown in Fig. 7, we further measured the optical absorbance of single-phase films (P3HT, PBDTTT-F and PCBM), two-phase mixed films (P3HT:PCBM and PBDTTT-F:PCBM) as well as the corresponding films incorporating SWCNTs. Compared with the results of the films without SWCNTs, the spectral of the single-phase or the mixed-phase films incorporating nanotubes remain unchanged on the morphology, but display evident absorption enhancement and red shift, similar as that of three-phase active layers. Fig. 7(c) not only shows the enhancedπ-π* absorption of PCBM at 350 nm, but also a new absorption peak appearing at 550 nm, which is related to the E22s (V1→C1) transition absorption of semiconducting SWCNTs [16]. Therefore, the influences of SWCNTs on the photoabsorption and aggregation configuration of the active layers are independent of the number and type of the mixed phases. 3

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Fig. 7. Optical absorption spectral of the blend films based on (a) PBDT-TT-F, (b) P3HT, (c) PCBM, (d) P3HT:PCBM, (e) PBDT-TT-F:PCBM without SWCNTs and with different concentration of SWCNT.

Furthermore, we investigated the influence of SWCNTs on the electrical properties of the three-phase blend active layer. According to the energy band structure illustrated in Fig. 2, we can predict that in the three-phase organic film, second type of bulk heterojunctions between SWCNTs and each single polymer phase are generated under the action of van der Waals force, in which SWCNTs act as the core and polymer as the outer shell. Since the carriers in the SWCNTs electronic structure are weakly combined with the phonons, the SWCNTs exhibit a “ballistic” charge transport along the main nanotube axis [6]. When the metallic and the semiconducting SWCNTs are incorporated into the organic film, a blended interpenetrating permeation network structure is formed. These networks are responsible for efficient charge transport of the carrier [19]. In order to prove the existence of these three heterojunctions, we performed fluorescence (PL) tests on the single-phase, two-phase and three-phase organic films as well as the respective SWCNTs-incorporated films. The PL spectra are shown in Fig. 8. The

effective exciton dissociation and charge transfer occurring at the interface between SWCNTs and organic matter are clearly observed, which indicates that SWCNTs can form effective heterojunction with the three conjugated organic compounds no matter whether it is singlephase, two-phase or three-phase film. Finally, we studied the photoelectric properties of the three-phase active layer devices of ITO/PEDOT:PSS/PBDTTTF:P3HT:PCBM:SWCNT(12:8:3:X %)/LiF/Al. The concentration of SWCNTs therein was selected to be 0 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%. The power density of the blue, green and red light sources in the test were 5.54, 2.56, 0.97 mW/cm2, respectively. According to the results displayed in Fig. 9, it can be observed that all photocurrents of the three primary colors increase as the SWCNTs concentration increases (0–1 wt%). At relatively low concentration, the SWCNTs network is not fully formed, and there are a large number of dead arms [20]. In addition, the spacing between SWCNTs is large, and 4

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Fig. 8. Photoluminescence spectral of the blend films based on (a) PBDT-TT-F, (b) P3HT, (c) PCBM, (d) P3HT:PCBM, (e) PBDT-TT-F:PCBM, (f) P3HT:PCBM:PBDT-TTF without SWCNT and with different concentration of SWCNT.

the transport efficiency of electrons between SWCNTs is low. In this case, the conductive network is primarily determined by polymer conductance. However, as the concentration of SWCNTs increases, the conductive network gradually forms, which leads to the enhancement of the electron transfer efficiency and the decrease of the probability of recombination between charges [21]. Additionally, the decrease of tube spacing and the occurrence of non-resonant tunneling with the increasing of SWCNTs concentration further improve the transport efficiency of electrons by the combination of disordered hopping transport (67%) and non-resonant tunneling (33%) [6]. Meanwhile, the increasingly aggregation of the PBDTTT-F and P3HT (see Fig. 5(a–c) and (f-h)) promoted by SWCNTs strengthens the interaction between molecular chains and alters hole transfer behavior from one-dimensional to three-dimensional. When the SWCNTs concentration reaches a certain level, the conductivity of the active layer is mainly determined by the conductance of the metallic and the semiconducting SWCNTs network [20]. As shown in Fig. 5, a uniform random porous network structure can be observed when the concentration reaches 1 wt%. At this point, the series resistance of the device reaches minimum as

shown in Fig. 10 (a), and photocurrent comes to its maximum. When the concentration of SWCNTs is further increased, larger than 1 wt%, the results show that each photocurrent of the three primary colors declines under the same bias voltage (When bias voltage is −1 V, blue current density Jb: 2.62–0.584 mA/cm2; green current density Jg: 1.17–1.01 mA/cm2; red light current density Jr: 0.482–0.396 mA/cm2). In this stage, the independent SWCNTs are easily entangled into columns under the action of non-covalent bonds. As a result, the original random interpenetrating network structure is destroyed (see Fig. 5(c–e) and (h-j)), which leads to the elimination of the electron transport channel. At the same time, the excessive aggregation of PBDTTT-F and P3HT also destroys its original random network, causing the damage of partial transport channel of the hole and the reduction of collection channel [22]. Hence, the series resistance of the device would increase with the increase of SWCNTs concentration, as shown in Fig. 10 (a). The holes and electrons are easily transported to the lower energy recombination center by the aggregated conjugated polymer or SWCNT aggregate beam [23]. What's more, Fig. 9 (d) shows that the dark current of the detector also tends to increase first and then decrease as 5

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Fig. 9. J-V curves of P3HT:PCBM:PBDT-TT-F devices with 0 wt%, 0.5 wt%, 1 wt%, 1.5 wt%, 2 wt% SWCNT under different light (a) red, (b) green, (c) blue, (d) dark. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

the concentration of SWCNTs increases. When the concentration is 1 wt %, the dark current under -1 V bias is nearly an order of magnitude higher than that of the non-SWCNTs concentration device. To avoid the negative effect of the increasing dark current, the concentration of the incorporated SWCNTs should not be too high. Fig. 10 show the curves of the characteristic photoelectric parameters of the detector as functions of SWCNTs concentration. The optical responsivity R, specific detection rate D∗, linearity LDR, and external quantum efficiency EQE of the detector are calculated as follows:

R=

D =

Jph Pin

=

Jlight

Jdark Pin

R 2eJdark

EQE = 1240

(1) (2)

R

LDR = 20 log

is because the red light of unit intensity contains more photons than the other two lights. Table 1 shows the characteristic parameters of the detector under a bias of −1 V. It is shown that the R, EQE and Jlight of the detector for three primary colors are significantly improved after the incorporation of SWCNTs, which is about 2–4 times higher when the concentration is 1 wt%, but the increases of LDR and D∗ are not obvious due to the increase of dark current (Jdark). For the detectors with SWCNTs concentration of 0.75–1.75 wt%, the results shows that the EQE of blue and green light is greater than that of red light, and the maximum value of blue light is surprisingly larger than 100%. It has been reported [16] that after absorbing a single highenergy photon, the excitation energy becomes two times more than the energy gap of the transition absorption, and the MEG would occur in SWCNT. Since the wavelength of the excitation blue light is 460 nm, the photon energy is much larger than the energy level of the E11s (V1→C1) transition of the SWCNTs with PL spectrum about 950–1300 nm [16]. Therefore, the MEG effect would be remarkable. Likewise, MEG effect can also be produced in the case of green light (520 nm) but much weaker than that of blue light. However, for red light with wavelength of 630 nm, it is difficult for the detector to produce MEG effect. When the SWCNT concentration is small, the MEG of blue and green light is not obvious, and the EQE of red light is larger. As the concentration of SWCNTs increases, the MEG of blue and green light will increase dramatically. Consequently, the EQE of blue and green light exceeds that of red light, even over 100%. Moreover, as the SWCNTs concentration continues to increase, the dispersed SWCNTs are entangled into columns, which leads to the increase of the concentration of the charge and the constraint weakening. As a result, the Auger recombination tends to increase while the MEG decreases [24], and then the photocurrent decreases.

(3)

Jph Jdark

(4)

where Jlight, Jdark, and Jph are the current, dark current, and photocurrent in the light, respectively, Pin is the incident light power, e the electron power, and λ the incident light wavelength. It can be seen that the photoresponsivity R, the external quantum efficiency EQE, the linear dynamic range LDR and the specific detectivity D∗ of the detector increase significantly after the incorporation of SWCNT. After reaching the maximum values at the concentration of 1 wt%, these parameters decrease as the SWCNT concentration further increases. Interestingly, both the optical responsivity and the specific detectivity of red light are greater than that of blue and green light. This

6

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Fig. 10. Curves of (a) the series resistance, (b) responsivity, (c) EQE, (d) average specific detectivity, (e) LDR of the detector under the bias of -1 V varies with the concentration of SWCNT.

Table 1 Summary of the characteristic parameters of three primary colors detectors with 0 wt%, 1 wt% SWCNT under −1 V bias. Jdark (mA/cm2)

Parameter items SWCNT (0 wt%)

SWCNT (1 wt%)

red green blue average red green blue average

−4

5.72 × 10

1.65 × 10−3

Jlight (mA/cm2) 0.213 0.302 0.584 0.366 0.482 1.17 2.62 1.42

R (mA/W) 220 118 105 148 496 456 473 475

7

D* (Jones) 11

5.1 × 10 2.8 × 1011 2.5 × 1011 3.4 × 1011 6.8 × 1011 6.3 × 1011 6.5 × 1011 6.5 × 1011

EQE (%)

LDR

43 28 28 33 97 109 127 111

51 54 60 55 49 57 64 57

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4. Conclusions [7]

The effects of the incorporation of SWCNTs on the photoabsorption and photoelectric properties in the P3HT:PBDTTT-F:PCBM three-phase bulk heterojunction active layer are investigated systematically. The results show that the addition of SWCNTs does not change the electronic structure of the organic conjugated polymer, and the spectral morphology of the original film remains unchanged. The existence of SWCNTS increases the aggregation and crystallinity of the conjugated organic matter, and causes the enhancement of the absorption intensity and red shift of the spectrum, regardless of the number of mixed phases of the film and the type of the conjugated organic substance. The heterojunction formed by the combination of SWCNTs with conjugated organics (PBDTTT-F, P3HT and PCBM) can increase the exciton dissociation area and enhance the dissociation rate of the film. When the concentration of SWCNTs is 1 wt%, the film will form a uniform interpenetrating network of fast charge transport channel, which improves the charge collection efficiency of the electrode. When the concentration of SWCNTs is in the range of 0.75–1.75 wt%, a multiple exciton generation process (photoelectric carrier multiplication) would be produced for the blue and green light by high-energy photon excitation. In addition, the variation of LDR and D∗ with respect to the SWCNTs concentration is relatively not significant due to the increase of Jdark, and this problem will be considered by studying the buffer layer in the future.

[8] [9]

[10]

[11] [12] [13] [14]

[15] [16] [17]

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