Novel Visible-Light Photodetector Based on Two-Dimensional Confined Electron Donor-Acceptor Co-Assembled Layered Double Hydroxide Ultrathin Films.

Photodetectors are a class of critical optoelectronic devices that can transform incident light into a detectable electrical signal. In this work, we develop a novel photodetector based on two-dimensional (2D) confined electron donor-acceptor co-assembled ultrathin films (UTFs). The (PCDTBT@CN-PPV/LDHs) n UTFs are composed of an organic electron donor, poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT), and an acceptor, poly(5-(2-ethylhexyloxy)-2-methoxy-cyanoterephthalylidene) (CN-PPV), within inorganic Mg2Al-layered double hydroxides (LDHs). The UTFs exhibit broad-range visible-light absorption, from 400 to 650 nm, resulting from complementary absorption of PCDTBT and CN-PPV. The fluorescence emission of the UTFs is completely quenched, implying the occurrence of photoinduced charge transfer (PCT). As a novel photodetector, the co-assembled UTFs have a high photocurrent and on/off switching ratio (300 nA/∼120), in contrast to those of the PCDTBT/CN-PPV drop-casting thin film (5.4 nA/∼1.6); a fast response; a short recovery time (lower than 0.1 s); and excellent wavelength and light-intensity dependence. The PCT mechanism can be attributed to the formation of a 2D bulk heterojunction of the two polymers within the interlayers of the LDH nanosheets. Furthermore, flexible UTFs on polyethylene terephthalate substrates are also fabricated, which exhibit excellent folding strength and electrical stability.

Introduction

Photodetectors that can transform incident light into electrical current are widely applied in industrial and scientific fields, for example, optical communications, aeronautical engineering, and biological and environmental investigations.[1] Photodetectors based on inorganic nanostructures, such as ZnO, V2O5, ZrS2, and ZnS, which exhibit excellent nanoscaled optoelectronic performances, have been widely investigated.[2] Recently, a multicolor (responding to both UV and visible light) photodetector based on the heterojunction of n-Si(111)/TiO2 nanorod arrays was developed using interface engineering, a consecutive process including chemical etching, magnetron sputtering, hydrothermal growth, and assembly.[3] However, the complicated preparation technology and requirement for high-cost equipment greatly restrict their practical applications.[4] Furthermore, most inorganic photodetector materials had fixed light absorption and a narrow spectral sensitivity,[2] which would be suitable for only fixed or narrow-band wavelength detection. For example, the first GaS-nanosheet-based photodetectors made on SiO2/Si and flexible polyethylene terephthalate (PET) substrates exhibited photoresponsivities up to 4.2 and 19.2 AW–1 at 254 nm, respectively, due to the strong absorption in the UV wavelength region.[5]

Recently, a few organic molecules with various types, a broad-band absorption range, and excellent flexibility have become suitable candidates for photodetectors. A novel kind of flexible photodetector based on perovskite/conjugated-polymer composites exhibited a wide photoresponse from 365 to 937 nm.[6] The excellent photodetector performances were ascribed to the efficient photoinduced charge transfer (PCT) and exciton dissociation between the electron donor and acceptor, with a coupled energy level. Meanwhile, organic molecules easily formed flexible films on solution processing,[7] which was the key advantage in practical application. Consequently, several types of organic photodetectors based on electron donors and acceptors generated considerable interests.[8] Liu et al. prepared an ultrathin organic photodetector that used the P3HT:PCBM blend as a photoactive layer.[9] This device exhibited a remarkable performance, with a detectivity of 1.33 × 1012 Jones at 500 nm irradiation, comparable to the detectivity of state-of-the-art inorganic photodetectors. Undeniably, organic donor and acceptor blends have great potential in the field of photodetectors, and further exploitation is highly essential to improve their abilities and expand their field of application.

Poly[N-9′-heptadecanyl-2,7-carbazole-alt-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)] (PCDTBT) has been widely used in photovoltaic applications, with outstanding chemical stability, good organization, excellent solubility, and high photon-to-electron conversion efficiency.[10] Meanwhile, poly(5-(2-ethylhexyloxy)-2-methoxy-cyanoterephthalylidene) (CN-PPV) usually acts as an electron acceptor in photoelectric devices, such as organic light-emitting diodes and organic solar cells, due to its lower electron energy levels, with electron withdrawing groups as substituents.[11] However, to the best of our knowledge, PCDTBT and CN-PPV, individually or combination, have not been investigated until now in the field of photodetectors. Layered double hydroxides (LDHs) are a kind of important two-dimensional (2D) layered solid solution with unique versatility due to their adjustable anionic composition in galleries,[12] and importantly, LDHs can confine intercalated guests and modulate their electronic properties.

In this work, 2D confined polymer electron donor (PCDTBT)/acceptor (CN-PPV) ultrathin films ((PCDTBT@CN-PPV/LDHs) UTFs) were fabricated by the layer-by-layer assembly technique, which was different from the traditional simple blending method. The fluorescence emission of the UTFs was quenched when the mass ratio of PCDTBT to CN-PPV was 1:1, indicating the occurrence of PCT between PCDTBT and CN-PPV within the LDH interlayers.[13] Furthermore, PCDTBT and CN-PPV exhibited complementary visible absorption spectra, which played a crucial role in enhancing the sensitivity of the photodetector devices.[14] The co-assembled photodetectors on both rigid substrates and flexible PET substrates had enhanced photoresponsive properties compared to those of drop-casted PCDTBT/CN-PPV thin films, which were highly appreciated for their electronic applications. Therefore, the 2D confined electron donor–acceptor systems fabricated as novel photodetectors had the following advantages: (1) the 2D confined nanoscale interlayers of the UTFs were highly favorable for PCT and electron transport due to the well-organized distribution of PCDTBT and CN-PPV within the Mg2Al-LDH nanosheets, something like the 2D bulk heterojunction; (2) the positively charged LDH layers could attract photogenerated electrons and repel photogenerated holes, which contributed to improving the charge separation and diffusion in the nanospace; (3) the stability and durability of the novel co-assembled photodetectors were enhanced due to the immobilization of the electron donor/acceptor within Mg2Al-LDH nanosheets.

Results and Discussion

Structural and Morphology of (PCDTBT@CN-PPV/LDHs) UTFs

(PCDTBT/LDHs), (CN-PPV/LDHs), and (PCDTBT@CN-PPV/LDHs) UTFs were prepared by the layer-by-layer assembly method. First, the influence of PCDTBT/CN-PPV weight ratios on fluorescence emission was investigated. It was noted that UTFs with a 1:1 weight ratio showed completely quenched luminescence (Figure S1a), a direct reflection of the occurrence of PCT between PCDTBT and CN-PPV.[13] For simplicity, the co-assembled films with 1:1 weight ratio have been denoted as (PCDTBT@CN-PPV/LDHs) UTFs. As shown in Figure S2, the characteristic UV–vis absorption peaks of PCDTBT (0.05 g/L) and CN-PPV (0.05 g/L) solutions in toluene were located at 400 and 550 nm and 450 nm, respectively. Concerning the UV–vis absorption of (PCDTBT/LDHs) UTFs and (CN-PPV/LDHs) UTFs, however, no significant changes were observed compared to those in toluene solutions (Figures S2 and S3), indicating that the single-assembled polymers kept their original structures as those in solutions. In other words, the 2D confined individual polymer, PCDTBT or CN-PPV, achieved a homogeneous interlayer distribution within LDHs.

The multilayered co-assembly process for the formation of (PCDTBT@CN-PPV/LDHs) UTFs was monitored by UV–vis absorption spectroscopy. The results showed that the absorption intensities at 400, 450, and 550 nm increased linearly with increasing co-assembled cycle number (n) (Figure a), implying that the film growth was uniform and regular and encapsulation of PCDTBT and CN-PPV into LDH interlayers occurred. It was noted that the co-assembled UTFs exhibited broadened visible light absorption from 400 to 650 nm due to the complementary absorption of PCDTBT and CN-PPV, which was highly favorable for light absorption. Although PCDTBT and CN-PPV were neutral polymers without any net charge, they were co-assembled within LDH nanosheets through hydrogen-bond interactions, N···H–O/O···H–O, which originated from the carbazole and benzothiadiazole groups in PCDTBT, ethylhexyloxy and methoxy groups in CN-PPV, and hydroxyl groups in the LDH layers.[15] Consequently, (PCDTBT@CN-PPV/LDHs) UTFs with a cycle number (n) of up to 20 can be obtained, and it can be expected that the two neutral polymers can interact with each other through π–π interactions within the LDH interlayers.

Figure 1

(a) UV–vis absorption spectra of (PCDTBT@CN-PPV/LDHs) UTFs; (b) PL emission spectra of (PCDTBT/LDHs)20 UTFs, (CN-PPV/LDHs)20 UTFs, and (PCDTBT@CN-PPV/LDHs)20 UTFs; (c) photographs of (PCDTBT/LDHs) UTFs (up), (CN-PPV/LDHs) UTFs (middle), and (PCDTBT@CN-PPV/LDHs) UTFs (bottom) (from left to right, n = 4, 8, 12, 16, 20) under irradiation at 364 nm UV light; (d) XRD patterns of (PCDTBT@CN-PPV/LDHs) UTFs.

Compared with the PCDTBT/CN-PPV solution (Figure S1b), the single-assembled UTFs displayed the same luminescence peaks at 652 and 556 nm, respectively, without any shift or broadening with increasing n (Figure S4), indicating that there were no distinct changes in the microenvironment of the polymers throughout the assembly process. In contrast, the co-assembled UTFs showed significant luminescence quenching due to the occurrence of PCT (Figure b). Correspondingly, photographs of the (PCDTBT/LDHs) UTFs and (CN-PPV/LDHs) UTFs under 365 nm UV illumination showed visible red and yellow luminescence, respectively, with the brightness increasing with increasing n (Figure c), whereas the co-assembled (PCDTBT@CN-PPV/LDHs) UTFs did not show any fluorescence.

The periodic structure of the UTFs was evaluated through XRD patterns (Figure d). The intensity at about 2θ = 1.5° increased with increasing n, suggesting a well-ordered periodic stacking of the UTFs in the normal direction, with a period of about 5.89 nm, which was similar to that of the layer spacing of one LDH layer (0.48 nm) plus the lateral dimension of PCDTBT (2.37 nm) and CN-PPV (2.18 nm) (Figure S5). It can be speculated that PCDTBT and CN-PPV were arranged as entangled bilayers within LDH nanosheets.

The surface morphologies of (PCDTBT/LDHs)20 UTFs, (CN-PPV/LDHs)20 UTFs, and (PCDTBT@CN-PPV/LDHs)20 UTFs were observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The top-view SEM images (Figure S6) showed that all three UTF surfaces were continuous and homogeneous, with an average film thickness of ca. 100 nm. The AFM images (scan = 2 μm × 2 μm) (Figure S7) also indicated that the surfaces of the films were uniform and smooth, with an average roughness root-mean-square of less than 4 nm (n = 20).

Estimation of the Energy Level of (PCDTBT@CN-PPV/LDHs) UTFs

The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of the polymers can be calculated from the onset potentials of cyclic voltammogram (CV) measurements (Figures S8 and S9).[16] The energy levels of PCDTBT and CN-PPV within LDH interlayers were evaluated from the CV curves of (PCDTBT/LDHs)20 UTFs and (CN-PPV/LDHs)20 UTFs, respectively, provided that there were no significant differences between single-assembly and co-assembly processes. The measured energy-level values are listed in Table S1 for comparison. As shown in Figure S8, the CV curves of PCDTBT, CN-PPV, (PCDTBT/LDHs)20 UTFs, and (CN-PPV/LDHs)20 UTFs show one oxidation process and one reduction process each, implying their excellent ambient stabilities.[17] It was noted that the HOMO/LUMO levels of PCDTBT and CN-PPV within LDHs were −5.35/–3.85 and −5.60/–3.95 eV, respectively, which was in good accordance with the required energy levels for the PCT process.[18] Meanwhile, the slight difference in energy levels between the co-assembled PCDTBT (or CN-PPV) and pristine PCDTBT (or CN-PPV) was ascribed to the 2D confined effect of LDHs.

Furthermore, all of the Eg values estimated from the absorption edge were very close to those calculated from the CV measurements, implying the reliability of the energy levels from the CV measurements.[19] The Eg’s of PCDTBT and CN-PPV within LDHs estimated from the absorption edge were also narrowed compared to those of pristine PCDTBT and CN-PPV, corresponding to the systematically red-shifted absorption onset by ∼40/10 nm.

Photoresponse Properties of (PCDTBT@CN-PPV/LDHs) UTFs

The photoresponse performances were explored by a prototype photodetector that was fabricated by casting the silver electrode onto UTFs. Figure a shows the current–voltage (I–V) curves of the devices under white light and dark environments. For the co-assembled UTFs, a photocurrent of 300 nA was detected at a voltage of 3.0 V, whereas only 45 and 34 nA were observed for single-assembled UTFs. Furthermore, the logarithmic relationships of the current and voltage for co-assembled UTFs under light and dark conditions were replotted in Figure b, suggesting the prominent photoconductive behaviors.

Figure 2

(a) I–V curves and (b) logarithmic I–V curves of the (PCDTBT@CN-PPV/LDHs)20 UTF co-assembly photodetector illuminated with white-light irradiation (150 mW cm–2) or under dark conditions on a silicon substrate; (c) cyclic on/off switching; and (d) response and recovery times of the photocurrent for the (PCDTBT@CN-PPV/LDHs)20 UTF photodetector on a silicon substrate.

The crucial factors for determining the capability of a photodetector were its sensitivity and repeatability.[20]Figure c shows that the photocurrent of the UTFs rise and fall with on/off switching of the incident light. For the co-assembled UTFs, dark current was only 2.5 nA, whereas photocurrent approached 300 nA at a bias voltage of 3.0 V under an incident light intensity of 150 mW cm–2, giving an on/off switching ratio of 120. Furthermore, the co-assembled UTFs exhibited an outstanding symmetric photocurrent, with no obvious degradation during seven cycles, indicating excellent photosensitivity of the device, which is close to performance of state-of-the-art organic–inorganic hybrid photodetectors.[20] In contrast, devices based on CN-PPV-only and PCDTBT-only UTFs exhibited relatively low photocurrents of 50 and 36 nA under the same illumination conditions, about 7 and 5 times enhancement compared to those under dark currents of 7.1 and 7.0 nA, respectively. The light on/off current also changed slowly due to the weak sensitivity. This could be ascribed to the fact that the charge transportation efficiency between PCDTBT and CN-PPV within the LDH nanosheets was much higher than that in the single-assembled UTFs under light illumination.[21] For comparison, the photocurrents and on/off ratios of pristine PCDTBT, CN-PPV, and PCDTBT@CN-PPV (1:1) drop-casting films on silicon substrates were measured to be 2.5 nA/1.5, 2.2 nA/1.2, and 5.4 nA/1.6, respectively (Figure ). The improvements in the photocurrent and on/off ratio for LDH-based co-assembled UTFs were ascribed to the effective PCT process between the 2D confined PCDTBT and CN-PPV within the LDH interlayers and the nanometer dimension of the polymer@LDH nanocomposites, which was favorable for photogenerated carrier transport.

Figure 3

(a) I–V characteristics and (b) reproducible on/off switching of the PCDTBT, CN-PPV, and PCDTBT@CN-PPV (1:1) films dropped on the silicon substrate at 150 mW cm–2 illumination.

Usually, the time at which the initial current increased to 90% or decreased to 10% of the peak value was defined as the rise or decay time of the photodetector. More detailed transient photocurrents of the UTF device are presented in Figure d. It was observed that both the rise and decay times for co-assembled UTFs were shorter than 0.1 s, which was comparable to those for P3HT and CdSe nanowire heterojunctions.[22] Consequently, the rapid reversibility response and high stability of the co-assembled UTFs allowed their use in promising high-quality photodetector applications.

Photoresponsive Wavelength Dependence of (PCDTBT@CN-PPV/LDHs) UTFs

The co-assembled (PCDTBT@CN-PPV/LDHs)20 UTFs also exhibited excellent wavelength dependence. Figure S10 shows full I–V sweeps of the co-assembled and single-assembled UTFs when illuminated with visible light from 400 to 650 nm (150 mW cm–2). Figure a summarizes the I–λ relationships of the devices at a voltage of 3.0 V. For (PCDTBT@CN-PPV/LDHs)20 UTFs, the photocurrent increased gradually from 400 to 500 nm and then decreased at 500 nm, which was well in accordance with the UV–vis absorption spectra (Figure b). It was observed that the photoconductance exhibited a quick response to the excitation wavelength, which was ascribed to the various concentrations of photoexcited electron–hole pairs under different wavelengths of light (400–650 nm). The strong absorption could stimulate the electrons from the HOMO to the LUMO and provide more carriers for the photocurrent, a fact that the photogenerated electrons and holes were proportional to the absorbed photon flux, leading to excellent wavelength dependence.[22] It should be pointed out that the highest photocurrent was observed at 500 nm, whereas the maximum absorption of (PCDTBT@CN-PPV/LDHs)20 UTFs was at about 400 nm. The lower photocurrent at 400 nm was possibly attributed to the enhanced absorption of high-energy photons at or near the surface region of the hybrid semiconductors,[23] which was a common phenomenon in other broad-band photodetectors.[22] The single-assembled UTFs also exhibited photocurrent–wavelength dependence. However, the photocurrents were much lower due to the lower absorption and single-component with lower electron/hole efficiency.[24]

Figure 4

(a) Photocurrent of the UTF device under various wavelengths, at a bias voltage of 3.0 V; (b) UV–vis absorption spectra of (PCDTBT/LDHs)20 UTFs, (CN-PPV/LDHs)20 UTFs, and (PCDTBT@CN-PPV/LDHs)20 UTFs.

Photoresponsive Intensity Dependence of (PCDTBT@CN-PPV/ LDHs) UTFs

The photosensitivities (i.e., light response ability) of the co-assembled UTFs were evaluated under different incident light intensities from 0.5 to 150 mW cm–2. As shown in Figure a, the I–V curves at different illumination intensities exhibited typical photoconductive behavior, with increased electric conductivity at a higher light intensity. It was noted that a slight change in light intensity could result in an obvious response in photocurrent, which could be attributed to the different photon densities.[25] The co-assembled UTF device exhibited a nearly linear photocurrent–light intensity relationship, with a power law of θ = 0.88 (i.e., I ∼ P0.88) (Figure b), well in accordance with the previously reported value (0.5 < θ < 1), suggesting a powerful light-intensity dependence.[26] These results confirmed the great potential of 2D confined electron donor–acceptor UTFs in the field of highly sensitive photodetectors.

Figure 5

(a) I–V relationships of the (PCDTBT@CN-PPV/LDHs)20 UTFs excited by light with different intensities, ranging from 0.5 to 150 mW cm–2; (b) corresponding fitting power law curve of photocurrent and light intensity.

Mechanism of Photoresponsive Properties of (PCDTBT@CN-PPV/LDHs)20 UTFs

A schematic description of the UTF photodetector has been presented in Figure a, which is composed of highly smooth and homogeneous UTFs, silver electrodes, and a rigid SiO2/Si substrate. The coupled and matched energy levels of PCDTBT and CN-PPV within LDH nanosheets allowed the occurrence of PCT under irradiation, thus producing photoexcited electrons and holes (Figure b). Then, the electrons could be accepted by the Ag positive electrode, with a relatively lower energy level compared to that of the LUMO of CN-PPV; meanwhile, the holes could be caught by the Ag negative electrode, with a relatively higher energy level compared to that of the HOMO of PCDTBT.[27] In our system, the polymers within LDH interlayers exhibited a relatively orderly and dense arrangement, which was highly favorable for exciton dissociation and carrier transport.[28] Meanwhile, the positively charged LDH nanosheets could attract photogenerated electrons and repel holes, which played a dominating role in the exciton dissociation and carrier transport within the interlayers (Figure c). Effective exciton separation and smooth carrier transport were achieved within the 2D confined interlayers upon illumination, leading to the highest photocurrent in the co-assembled UTFs (Figure a). Therefore, greatly enhanced photoresponse for the co-assembled UTFs was achieved, compared to that for the single-assembly or drop-casting devices.

Figure 6

(a) Schematic illustration of the (CN-PPV@PCDTBT/LDHs)20 UTF photodetector; (b) energy-level alignment of PCDTBT and CN-PPV within the Mg2Al-LDH nanosheets under light irradiation; and (c) the proposed 2D PCT mechanism scheme.

Photodetectors Based on (PCDTBT@CN-PPV/LDHs)20 Flexible UTFs

Recently, the preparation of flexible electronic and optoelectronic devices has gained great interest due to their prosperity in the fields of portable and hand-held electronics.[29] In this work, we developed a flexible photodetector by co-assembling neutral polymers and LDH nanosheets onto a flexible PET substrate to explore its photoresponse performance. It was noted that the flexible photodetector exhibited excellent photocurrent stability and sensitivity (Figure S11), with a photocurrent of 15 nA and an on/off ratio of 118 for at least five cycles. The flexible device also showed excellent wavelength dependence (Figure S12) and light-intensity dependence (Figure S13). Compared to the photocurrent on the rigid substrate, the much lower value for the flexible substrate could be attributed to the lower charge-carrier-transport efficiency on the PET substrate.[30]

For practical application of photodetectors, information on their durability and endurance is commonly desired. It was worth noting that the I–V curves of the flexible (PCDTBT@CN-PPV/LDHs)20 UTF devices remained almost constant even after 150 cycles of bending compared to those of the original flexible one (Figure a,b). No obvious degradation was detected for a continuous photocurrent of ∼15 nA under fixed voltage and light illumination over 1800 s, implying excellent photocurrent stability, which was mainly attributed to the immobilization of LDH nanosheets. As shown in Figure S14, the photocurrent of the flexible UTFs was almost not influenced by the external bending stress under five different bending states. All of these suggested a high folding strength, excellent flexibility, and electrical stability of the co-assembled flexible UTFs, implying their great potential in photodetector applications.

Figure 7

I–V relationships of the co-assembled flexible UTFs (a) after 0, 30, 60, 90, 120, and 150 cycles of bending; (b) corresponding enlarged figure (illumination of visible light, 150 mW cm–2).

Comparison among Various Organic–Inorganic Hybrid Photodetectors

Until now, there have been few reports on 2D confined organic electron donor–acceptor thin films for high-performance photodetector applications. Table shows a comparison among various organic–inorganic hybrid photodetectors related to this work. It was noted that only our co-assembled films have a broadened spectral response with excellent spectral dependence. Although some devices with an organic–inorganic structure showed higher on/off ratios than those of the co-assembled films, their response/recovery times were systematically longer than those of our co-assembled photodetector. Moreover, the co-assembled 2D confined systems in this work have the advantages of easy fabrication compared with the traditional complicated preparation technology and enhanced photosensitivity due to the 2D confined effect of LDH nanosheets.

Table 1

Comparison among Various Organic–Inorganic Hybrid Photodetectors

composites	response/recovery time	on/off switching ratio	spectral selectivity	ref
CuInSe2/P3HT		113		(21)
ZnO NW/CuPc	2.4 s/3.0 s	104	UV	(31)
TiO2 NPs/mCP	0.21 s/0.23 s	104	240–400 nm	(32)
PbS QD/P3HT	≤0.16 s/≤0.11 s	310	365 nm	(33)
	≤0.16 s/≤0.12 s	550	625 nm
	0.58 s/ca.0.48 s	14	850 nm
SnO2/PEDOT:PSS	several seconds/several seconds		365–500 nm	(34)
CdTe/P3HT:PCBM	0.22 s/0.34 s		<317 nm	(35)
QDs/SAM		103	600–800 nm	(36)
(PCDTBT@CN-PPV/LDHs)n UTFs	<0.1 s/0.1 s	120	400–700 nm	present work

Experimental Section

Materials

PCDTBT ((C43H47N3S3)C12H10, Mw = 20 000–100 000) and CN-PPV ((C36H46N2O4)) were both provided by Sigma Chemical Co., Ltd. Mg(NO3)2·6H2O, Al(NO3)3·9H2O, urea, formamide, and toluene were all of analytical grade and obtained from Beijing Chemical Factory, China. SiO2 (300 nm)/Si wafer and silver paste were provided by J&K Co., Ltd.

Fabrication of (PCDTBT@CN-PPV/LDHs) UTFs

The fabrication and exfoliation of Mg2Al-LDHs have been described in our previous work.[37] Mg2Al-LDH (0.1 g) was stirred in 100 mL of formamide solution for 24 h to produce a stable exfoliated Mg2Al-LDH suspension. The quartz glass substrate was first washed in a mixed solution of concentrated NH3 (30%) and H2O2 (volume ratio 7:3) and in concentrated H2SO4 for 30 min each. Thereafter, the substrate was washed thoroughly with deionized water. The substrate was dipped in an LDH suspension (1 g/L) for 15 min. After washing with deionized water, the substrate was then dipped in PCDTBT toluene solution (0.05 g/L), CN-PPV toluene solution (0.05 g/L), or PCDTBT@CN-PPV toluene solution (PCDTBT 0.05 g/L and CN-PPV 0.05 g/L) for 15 min and washed with ethanol. (PCDTBT/LDHs), (CN-PPV/LDHs), or (PCDTBT@CN-PPV/LDHs) UTFs were prepared by alternately depositing the LDH suspension and PCDTBT toluene solution, CN-PPV toluene solution, or PCDTBT@CN-PPV toluene solution for n cycles. The fabricated UTFs were dried under N2 flow for 2 min. A quartz substrate and indium tin oxide (ITO)-coated glass were used for optical and CV optoelectronic measurements, respectively. An SiO2 (300 nm)/Si wafer substrate and flexible ITO-coated PET were applied for photoresponse measurements. Silver paste was coated onto the hybrid film (SiO2 (300 nm)/Si wafer substrate), with a spacing of 1 mm. The device was then dried in a vacuum oven for 2 h to firm the silver paste. Flexible photodetectors were constructed on flexible ITO-coated PET using a similar preparation technique as that for the rigid ones. Comparison samples of a drop-casted PCDTBT film, CN-PPV film, and PCDTBT@CN-PPV film were fabricated by the traditional solvent evaporation technique.

Characterization

The surface morphology of the UTFs was observed by SEM (Hitachi S4800) at an accelerating voltage of 5 kV. AFM (Nano Scope Analysis, Bruker, Germany) was carried out to measure the surface roughness and thickness of the UTFs. UV–vis absorption spectra were obtained on a spectrophotometer (Shimadzu U-3600), ranging from 200 to 900 nm. Fluorescence spectra of PCDTBT and CN-PPV were collected on a Hitachi F-7000 fluorospectrophotometer, with excitation wavelengths of 460 and 490 nm, respectively. The excitation and emission slits were both set as 5 nm.

Photoresponse Measurements

Electronic and optoelectronic measurements were carried out in a three-electrode system on an electrochemical workstation (CHI660E; CH Instruments Inc., China). The ferrocene–ferrocenium (Fc/Fc+) redox couple (4.8 eV below the vacuum level) was applied as the internal standard, and Ag/Ag+ (0.1 M AgNO3 in acetonitrile) was applied as the reference electrode. The different wavelengths (from 365 to 700 nm) of light emitted from a 300 W Xe lamp were focused by a monochromator. The incident power light was tested by a power meter (CEL-NP2000). All of the tests were carried out in air at room temperature.

Conclusions

In summary, a novel 2D confined (PCDTBT@CN-PPV/LDHs) UTF comprising neutral polymer molecules PCDTBT and CN-PPV within LDH interlayers was constructed via the layer-by-layer assembly method. The (PCDTBT@CN-PPV/LDHs) UTFs exhibited broad-band UV–vis absorption, from 400 to 650 nm. The fluorescence emission of the (PCDTBT@CN-PPV/LDHs) UTFs was quenched, which was the essential requirement for the PCT process. The photocurrent of the co-assembled photodetector was highly improved due to the 2D confined effect of the LDH layers, which was favorable for the PCT and carrier-transport processes. As a novel photodetector, (PCDTBT@CN-PPV/LDHs)20 UTFs exhibited a high on/off switching ratio, excellent photoresponse sensitivity, a short recovery time, and outstanding wavelength and light-intensity dependences both on rigid and flexible substrates. The excellent photocurrent stability and durability of the co-assembled UTFs were attributed to the effective 2D confined effect and anti-UV properties of Mg2Al-LDH nanosheets, which constructed a nanoscaled space for the formation of the 2D PCDTBT/CN-PPV bulk heterojunction, facilitating the PCT process and charge transport. All of these were highly appreciated for high-frequency and high-speed photodetectors in practical applications. Further exploration concerning other 2D confined electron donor and acceptor photoelectronic devices is in progress.