Flexible and broad spectral hybrid optical modulation transistor based

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Applications of Polymer, Composite, and Coating Materials

Flexible and broad spectral hybrid optical modulation transistor based on a polymer-silver nanoparticle blend Haihua Xu, Ying Lv, Haoxuan Zeng, Dexing Qiu, Yican Chu, and Qingqing Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06307 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 16, 2018

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ACS Applied Materials & Interfaces

Flexible and broad spectral hybrid optical modulation transistor based on a polymer-silver nanoparticle blend Haihua Xu,*, †, ‡, § Ying Lv, †, ‡, § Haoxuan Zeng, †, ‡, § Dexing Qiu, †, ‡, § Yican Chu, †, ‡, §Qingqing Zhu†, ‡, § †

Department of Biomedical and Engineering, School of Medicine, Shenzhen University,

Shenzhen, China. ‡

Guangdong Key Laboratory for Biomedical Measurements and Ultrasound Imaging, Shenzhen,

China. §

National-Regional Key Technology Engineering Laboratory for Medical Ultrasound, Shenzhen,

China.

ABSTRACT: The light-matter interplay on soft substrate is critically important for novel optoelectronic applications such as soft robotics, human-machine interfaces and wearable devices. Here, we for the first time report a flexible and efficiency-enhanced hybrid optical modulation transistor (h-OMT) in the UV-Infrared spectral range by blending polymer with silver nanoparticles (AgNPs). The h-OMT device exhibits unipolar transport and ultra-high on-off ratio of ~4.8×106 in a small voltage range of ~2 V. Using charge modulation reflection spectroscopy, we demonstrate that the h-OMT device shows a broad spectral response from 400 nm to 2000 nm and maximum optical modulation of ~15 % at λ=785 nm, 6-fold of magnitude higher than that of the device without AgNPs. Furthermore, the incorporation of AgNPs enhances by 4-fold of magnitude the extinction ratio without any complex geometry designs. We

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find the performance improvement relies on the AgNPs-induced electron trap states and electrochemical dopings in polymer. Importantly, the device exhibits pronounced mechanical flexibility, the optical modulation is remained down to a bending radius of 0.5 mm. Our data provides the possibility of organic materials for constructing novel optoelectronic systems in the future.

KEYWORDS : optical modulation, organic semiconductor, flexible device, silver nanoparticle, polymer electrolyte, electrochemical dopings

INTRODUCTION

The manipulation of light on flexible substrate is desirable for novel applications such as human-machine interfaces,1 wearable devices2 and soft robotics.3 In these systems, optical modulation device, which is regarded as a key photonic component for electrically tuning optical signals, is expected to satisfy several essential demands---mechanical flexibility, high efficiency and small footprint. A wide variety of structures such as microring/microdisk resonators,4-7 photonic crystals8 and Fabry-Perot cavities9 are introduced to achieve high modulation strength. However, all these structures require complex and high-expensive fabrication processes and are difficult to be integrated with traditional microelectronic circuits. Recently, optical modulation transistors (OMTs), with good compatibility of complementary metal-oxide-semiconductor (CMOS) integration processes, have drawn much attention. Highly-compact two-dimensional (2D) OMT devices, made from molybdenum disulfide (MoS2)10 and black phosphorus (BP)11, have been successfully demonstrated. But, extremely short interaction distance between light and 2D materials makes the modulation efficiency quite low. To solve this problem, one appropriate approach is to blend semiconductor materials with metal nanoparticles (gold, silver, etc.) which allow strong light-matter interactions and thus improve optoelectronic properties. For instance,


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the pronounced surface plasmonic resonance (SPR) effect in the metal nanoparticle can enhance optical absorption in certain wavelength light in photovoltaics, photodetectors and optical modulation devices.10,

12-16

On the other hand, the strong electromagnetic field in the metal

nanoparticles can also confine charge to the deep energy levels and makes them act as charge trapping centers that storage charge.13,16 Although great progresses have been made to develop 2D-typed OMT devices, tedious processes such as mechanical cleavage and chemical vapor deposition (CVD) hider their large-area and cost-efficient fabrications. Polymer semiconductors possess tunable optical and electrical properties, and especially, they are intrinsically flexible and solution-processable, making them widely used for low-cost fabrications of flexible optoelectronic devices.17-24 In this work, we report the low-cost fabrication of a flexible, broad-spectral hybrid-OMT (h-OMT) device by easy exfoliation and lamination processes. The device consists of a polymer-AgNPs hybrid thin film as the active layer and an electrolytic film as the gating dielectric layer. The broad-spectral optical modulation is achieved by electrically tuning sub-gap transitions of polymer and the plasmonic absorption of the AgNPs. We introduce an in-situ technique--charge modulation reflection spectroscopy (CMRS), to evaluate the modulation response of the h-OMT device in a broad wavelength range (350-2000 nm). Using CMRS, we observe a significant enhancement in modulation efficiency of the h-OMT device. We find that the underlying mechanism is in relationship with the AgNPs-induced carrier trapping and the electrochemical doping (ECD) effect. Moreover, the device exhibits good mechanical flexibility and operational stability, providing the possibility of our flexible h-OMT device for novel optoelectronic applications.


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EXPERIMENTAL SECTION The flexible h-OMT device can be fabricated via easy lamination of two separate assemblies

by van der Waals force, as illustrated in Figure 1a (see Supporting Information for details of the device fabrication procedures). The first assembly consists of the drain/source/gate electrodes and the active layer on a polyimide (PI)-coated silicon substrate. The active layer is a ~80 nm nanocomposite

thin

film

from

blended

solution

of

copolymer

poly(diketopyrrolopyrrole-thienothiophene) (PDPP-DTT) and metal nano-material AgNPs. PDPP-DTT is adopted as the medium for carrier transporting and photon absorbing while AgNPs are used as the carrier trapping and ion attracting material.

Figure 1. Schematic view showing the fabrication process of the flexible h-OMT device.

RESULTS AND DISCUSSION The UV-vis absorption spectrum of the AgNPs in solution, PDPP-DTT film and

PDPP-DTT:AgNPs nanocomposite film can be seen in Figure 2a. The PDPP-DTT film exhibits two closely absorption peaks at ~785 nm and ~820 nm due to the repeating moieties of electron donor DPP and acceptor DTT components along the backbone structure.20, 21,25-27 Besides, an


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additional absorption peak around 415 nm is observed in the PDPP-DTT:AgNPs nanocomposite film due to the SPR effect of the AgNPs. Figure 2b shows the transmission electron microscopy (TEM) image of AgNPs solution with an average particle size of 20 nm±5 nm which was also confirmed by scanning electron microscopy (SEM) results (Figure S1). Atomic force microscopy (AFM) image (Figure 1c) and SEM image (Figure S1) on the surface of the nanocomposite film clearly reveal randomly-distributed AgNPs and fiber-like polymer chains with micro-size porous structures. Next, we carried out X-ray photoelectron spectroscopy(XPS) measurements to study the chemical compositions and bonding states of the PDPP-DTT:AgNPs nanocomposite on the silicon substrate. The wide-range XPS spectrum of the nanocomposite (Figure 1d) presents several peaks of surface chemical compositions of copolymer PDPP-DTT including O1s, N1s, C1s and S2p. Figure 1e shows the detailed XPS spectra of Ag 3d on the surface of PDPP-DTT:AgNPs nanocomposite film. Two peaks found at 369.8 eV (corresponding to Ag 3d5/2) and 375.8 eV (corresponding to Ag 3d3/2) are observed, giving evidence of the presence of silver in the form of Ag0. Furthermore, X-Ray Diffraction (XRD) measurements were carried out to investigate the crystalline nature of the PDPP-DTT:AgNPs nanocomposite. As shown in Figure 2f, four diffraction peaks labeled by blue rectangles are observed at 38.05, 44.31, 64.49 and 77.45, which can be assigned to the (111), (200), (220) and (311) reflections of the face centered cubic structure of metallic silver, respectively.28 As shown in Figure 2g, pure PDPP-DTT has two clear diffraction peaks labeled by red rectangles at 12.9 and 17.4, indicating diffraction patterns of a lamellar packing of PDPP-DTT and thus good crystallinity.29 However, for PDPP-DTT:AgNPs nanocomposite, these two peaks become relatively weaker and less obvious, revealing that the AgNPs incorporation in polymer would induce crystallinity degradation.


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Figure 2. (a) Absorption spectra of the AgNPs in solution, PDPP-DTT film and PDPP-DTT:AgNPs nanocomposite film. (b) TEM image of AgNPs with an average size of 20 nm±5 nm. (c) AFM image of the PDPP-DTT:AgNPs film. Normalized XPS of the (d) PDPP-DTT:AgNPs nanocomposite and (e) Ag 3d transitions. (f) XRD pattern of the PDPP-DTT:AgNPs nanocomposite film. (g) Comparisons of the XRD intensities between the PDPP-DTT:AgNPs and pure PDPP-DTT films.

The second assembly is an ultra-thin, transparent and flexible electrolytic film as the gating dielectric layer, which was prepared by gelation of copolymer poly (vinylidene fluoride-co-hexafluoropro-pylene)

(P(VDF-HFP))

in

ionic

liquid

of

1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) (Figure 3a) on an additional silicon wafer. The electrolytic film, with high transparency and flexibility (Figure 3b), was then directly laminated onto the first assembly after peeling-off from the substrate,


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which is similar with our previous work.21 Finally, a flexible and ultra-thin h-OMT device with thickness of approximately 3 µm was obtained by entirely exfoliating the two parts from the supporting silicon wafer. For comparisons, we also fabricated a pure OMT (p-OMT) device which contains a PDPP-DTT active layer. The electrolytic film plays a crucial role in modulation performance of the OMT devices: firstly, the high transparence can avoid loss of incident light. Secondly, the soft nature of the electrolytic film provide the possibility of obtaining a flexible OMT device. Thirdly, the large electric double capacitance (>1 µF·cm-2, Figure S2) at the electrolyte/semiconductor interface ensures low-voltage operation. More importantly, the existence of a large number of ions in the electrolytic film will induce a strong ECD effect in PDPP-DTT:AgNPs nanocomposite and enhance modulation efficiency which will be discussed next. A spectroscopic technique CMRS was then introduced to evaluate the modulation response of the two OMT devices. In the CMRS measurement (Figure 1f), a broad spectral light of 350-2000 nm was generated from a halogen lamp and coupled to the OMT device via a fiber. Application of a small voltage on the gate enables carrier injections with high concentration in polymer. These carriers will couple with the modulated electric field, and thus introduce changes in cross section of optical absorption in the sub-gap region. The reflected light on charge modulation was then recorded from two fiber-optic spectrometers with different wavelength ranges. Therefore, the applied electrical signal on the OMT device can be converted to the optical signal, this is basically opposite to our previously reported electrolyte-gated organic phototransistor that is used for optical-to-electrical translations.21 As illustrated in the inset of Figure 3c, the CMRS technique can provide direct evidence of the AgNPs-induced carrier


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trapping and

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the ECD effect for modulation enhancements, which will be discussed in details

next.

Figure 3. (a) Molecular structures of P(VDF-HFP) and [EMIM][TFSI] used to form the electrolytic film.(b) Transmittance spectra of the electrolytic film, inset: the optical image of the film. (c) Schematic of the CMRS measurement on the OMT device, inset: schematic configurations of the electrolyte/nanocomposite interfaces.

To confirm the AgNPs incorporation in modulation improvement, we firstly compared electrical properties between the h-OMT and p-OMT devices. It is noted that all the following measurements were performed in atmosphere unless otherwise stated. Figure 4a shows how the drain-source current (Ids) evolves in response to the gating voltage (Vgs), revealing a large on/off ratio of Ids (>104) within a small voltage difference (-1.5V to 1.5 V), owing to the large electric double capacitance at the electrolyte/semiconductor interface. The p-OMT device follows a clear ambipolar behavior (µp= ~1.2 cm2 V-1s-1 and µn =~1.4×10-2 cm2V-1s-1, Support Information), quite similar to the results of reported copolymer-based oxide-gated organic transistors,20,26,27 this is because PDPP-DTT belongs to the conjugated copolymer which has repeating donor and acceptor components. However, the h-OMT device exhibits almost unipolar transport (µp= ~


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0.5 cm2 V-1s-1 and ~ 4.7 × 10-5 cm2 V-1 s-1) and ultra-high on-off ratio of ~ 4.8 × 106 , revealing three important features that should be thereby noticed. Firstly, a decrease in mobility is observed in the h-OMT device, consistent with the AgNPs-induced structural disorders which limit carrier hopping in adjacent molecules. Secondly, the electron transport in the h-OMT device is greatly suppressed, this is possibly because that the existence of AgNPs in polymer induces deep trap states easily occupied by electron carrier, as illustrated in Figure 4c; and inefficient percolation of electrons through polymer chains thus occur, resulting in extremely low switch-off current of ~4×10-11 A, two orders of magnitude lower compared to that of the p-OMT device.The inefficient percolation in the h-OMT device was confirmed by the relatively larger contact resistance (Figure S3). Thirdly, even though the relatively weaker charge transport, the switch-on current of the h-OMT device remains almost unchanged, suggesting that much more carriers are injected to compensate the transport limitation. These additional carriers are most likely to be originated from Coulombic attactions by exisiting opposite ions in the semiconductor bulk. Considering that the ECD effect will induce mass transport of ions across the electrolyte/polymer interface and into the semiconductor bulk,30,31 it provides evidence of the existence of the ECD effect in the h-OMT device. Figure 4b illustrates the hole mobility (µp) as a function of Vgs , which gives further confirmation of the existence of the ECD effect in the h-OMT device. Both devices exhibit positive µp-Vgs dependence as similar with the reported results.31 This can be explained that charge transport in organic semiconductors is limited by the most difficult hopping regions where there exist amounts of trap states; hence, charge carriers will firstly occupy these trap states until fully filled, more carriers will be then unaffected with the increased negative Vgs. Interestingly, for the h-OMT device, µp is nearly independent of Vgs. Such phenomenon can be explained by the Vgs-dependent ion-induced trap states which need to


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be filled before efficient transport can take place. The ion-induced trap states arise from the ECD effect which is critically important for realizing enhanced optical modulation for our h-OMT device which will be discussed next.

Figure 4. (a) Transfer characteristics of the p-OMT and h-OMT devices at Vds= -50 mV. (b) Extracted hole mobility (µp) versus Vgs curves of the p-OMT and h-OMT devices.(c) Energy level diagrams of the h-OMT device in the hole accumulation mode (left side where the existing ECD effect will attract more holes at the electrolyte/polymer interface) and in the hole depletion mode (right side where the existing AgNPs-induced electron trap states will limit electron transports).

The optical modulation of our OMT device fundamentally relies on the strong couplings between the lower excitonic level and the higher forbidden level with the injection of charge carrier in organic semiconductors.32 As a consequence, the Stark shift of Frenkel exciton


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absorptions occurs, which means the neutral absorption of organic semiconductors will disappear while the charge-induced sub-gap absorption will emerge.33-38 Theoretical and experimental results have proven that as carriers are trapped in an isolated molecular chain, strong carrier-phonon couplings in polymer produce localized polarons (corresponding to C1 and C2 transitions); but, when carriers are delocalized (or hopping) over adjacent molecular chains, these polaronic levels will be further splitted with newly emerging sub-gap optical transitions such as C3 and charge transfer (CT) transitions which are closer and far away to the neutral absorption peak, respectively, as shown in Figure 5a.33 These carrier-modulated sub-gap transitions, which determine the broad-spectral modulation performance of our OMT device, can be clearly reflected by the spectroscopic information in the CMRS. The CMRS in-situ probes the change of the optical reflection of the OMT device upon electrically tuning carrier density at the semiconductor/dielectric interface. The modulation level can thus be represented by the change in reflection upon gating: ∆R / R0 = - Vgs ⋅ C ⋅ σ (e ⋅ A) , with C the dielectric capacitance, σ the optical absorption cross section, e the unit charge and A the area of the device. Figure 5b plots the CMRS spectra of the p-OMT and h-OMT devices in the hole accumulation region (Vgs= -1.5 V). Both devices exhibit clear wavelength-dependent ∆R/R0 with positive (∆R/R0 >0, bleaching of short-wavelength neutral absorption) and negative (∆R/R0>0, emerging of long-wavelength charge-induced absorption) modulations. In the positive modulation region, the h-OMT device exhibits much stronger optical modulations compared to the p-OMT device, the maximum value is ~ 15 % at λ= 785 nm, 6-fold of magnitude higher. This is consistent with the remarkable ECD effect of the h-OMT device which allows much more carriers injected in polymer and therefore a much higher value of σ.


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As discussed above, the large interfacial capacitance of the OMT device allows carrier accumulation with a high concentration at low voltage while the optical modulation is determined by the charge-induced sub-gap transitions, and therefore, the reflected light in the CMRS is expected to be effectively modulated by applying a slight Vgs. Figure 5c displays the normalized reflection of 785 nm light, as a function of Vgs. Both p-OMT and h-OMT devices exhibit clear Vgs-dependent optical modulations within a small voltage range (-1.5 V

Flexible and broad spectral hybrid optical modulation transistor based

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