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Flexible and Highly Photosensitive Electrolyte-Gated Organic Transistors with Ionogel/Silver Nanowire Membranes Haihua Xu, Qingqing Zhu, Ying Lv, Kan Deng, Yinghua Deng, Qiaoliang Li, Suwen Qi, Wenwen Chen, and Huisheng Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 10 May 2017 Downloaded from http://pubs.acs.org on May 11, 2017

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ACS Applied Materials & Interfaces

Flexible

and

Highly

Photosensitive

Electrolyte-Gated

Organic

Transistors with Ionogel/Silver Nanowire Membranes Haihua Xu,*,

†, ‡, §

QingQing Zhu,

†, ‡, §

Ying Lv,

†, ‡, §

Kan Deng,

†, ‡, §

Yinghua Deng,

†, ‡, §

Qiaoliang Li, †, ‡, § Suwen Qi, †, ‡, §Wenwen Chen, †, ‡, § Huisheng Zhang †, ‡, § †

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: Flexible and low-voltage photosensors with high near-infrared (NIR) sensitivity are critical for the realization of interacting humans with robots and environments by thermal imaging or night vision techniques. In this work, we for the first time develop an easy and costeffective process to fabricate flexible and ultra-thin electrolyte-gated organic phototransistors (EGOPTs) with high transparent nanocomposite membranes involved of high-conductivity silver nanowire (AgNW) networks and large-capacitance iontronic films. High responsivity of 1.5×103 A·W-1, high sensitivity of 7.5×105 and 3dB bandwidth of ~100 Hz can be achieved at very low operational voltages. Experimental studies in temporal photoresponse characteristics reveal the device has shorter photoresponse time at lower light intensity since strong interactions between photo excited hole carriers and anions induce extra long-lived trap states. The devices, benefiting

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from fast and air-stable operations, provide the possibility of the organic photosensors for constructing cost-effective and smart optoelectronic systems in the future.

KEYWORDS : transparent conductors, ionogels, phototransistors, silver nanowires, organic bulk heterojunction, flexible electronics

INTRODUCTION

The emerging field of flexible electronics has underpinned large technological innovations in displays, energy converters and sensors.1-7 Among various flexible electronic devices, solutionprocessable organic field-effect transistors (OFETs) have drawn extensive attentions because they can provide the possibility of integrating indispensable fundamental building blocks into electronics systems.8-13 Moreover, OFETs can also be used as optical sensors called as “organic phototransistors” (OPTs) which possess high photosensitivity and gate-tunable optical detections via direct manipulations of hole/electron carrier transports with gating terminals.14-16 However, in traditional OPTs, gating components such as gate dielectrics and gate electrodes are generally constructed through complicated and high-cost atomic layer depositions or thermal evaporations. A practicable solution is to adopt solution-processable materials that can be directly used to print insulating and metallic films on plastic substrates, similar with those methods for preparations of organic

semiconductor

films.

Conducting

polymer

poly

(3,4-ethylenedioxythiophene)

polystyrene sulfonate (PEDOT: PSS) is commonly used as gate electrode due to its printability and transparence.17-21 But low conductivity and poor stability have made PEDOT: PSS electrodes unlikely for future optoelectronic applications. Doped metal oxides such as indium tin oxide (ITO) are widely used for touch panels and transparent solar cells due to their high conductivity and transparence.22-24 However, costly production and brittleness prevent the practical application of doped metal oxides in flexible optoelectronics. Random network of silver


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nanowire (AgNW) is a promising candidate as a transparent and flexible electrode since it provides high conductivity, low-cost processing and excellent mechanical flexibility.25-33 One more critical challenge for OPT’s practical applications is to effectively reduce operational voltages. A feasible way is to substitute iontronic materials for traditional oxide gate dielectrics (SiO2, Al2O3 et al) and construct so-called electrolyte-gated OPTs (EGOPTs) in which a nanometer-thick Debye-Helmholtz electric double layer (EDL) is rapidly formed at the electrolyte/semiconductor interface, giving rise to an ultra-large interface capacitance (>1 µF·cm2

). As a result, much higher charge-carrier density at relatively low voltages (< 3V) can thus be

achieved compared with those oxide-gated OPTs.34-40 More importantly, iontronic materials possess functional properties of flexibility, transparence and good chemical and mechanical stability, and can be processed by spin coating or printing from solvents, these methods are compatible with fabrication processes of AgNW gate electrodes and can facilitate practical integrations of gating dielectrics and electrodes in organic electronic devices. In this work, we report an easy fabrication process of a flexible and low-voltage EGOPT device based on a highly transparent nanocomposite membrane involved of silver nanowire (AgNW) network and iontronic material. The AgNW network, serving as a flexible and transparent gate electrode in the EGOPT device, was directly embedded into an ionogel-type gate dielectric layer with a large capacitance and shows low sheet resistance (Rs < 50 Ω·□-1) and ultra-high optical transparency (T > 90%). Furthermore, the AgNW/ionogel nanocomposite membrane was successfully incorporated into the EGOPT device by one-step easy lamination process. Utilizing the ultra-large capacitance formed at the ionogel-semiconductor interface, sensitivity of 7.5×105, responsivity of 1.5×103 A·W-1 and 3dB bandwidth of ~100 Hz can be obtained by applying operational voltages as low as 1.5 V.


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FABRICATION AND PROPERTIES OF THE AGNW/IONOGEL NANOCOMPOSITE MEMBRANE

The AgNW/ionogel nanocomposite membrane was fabricated through simple coating and peel processes, as schematically shown in Figure 1a, the detailed information of fabrication process can be seen in Support Information. In brief, a solution of AgNW suspension in deionized (DI) water was firstly spin-coated on a pre-cleaned silicon substrate in air and dried at 100 oC for 5 minutes to form a randomly-oriented and highly-conductive network. Next, a blend solution of copolymer poly (vinylidene fluoride-co-hexafluoropro-pylene) (PVDF-HFP) and ionic liquid 1ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]) in acetone was spin-coated on the surface of the AgNW network, following with curing in a vacuum oven at 70 o

C for 24 hours. Contact angle measurements (Support Information Figure S1) show that the

AgNW/ionogel film has a larger contact angle compared to that of the AgNW/silicon substrate, suggesting that the ionogel film has a relatively lower surface free energy. Viscosity measurements (Support Information Figure S2) indicate that the iontronic solution has an ultrahigh viscosity of ~20 Pa·s at a shear rate of 10 s-1, one order higher than that of epoxy resin (typical value is ~1 Pa·s). Based on the low surface free energy and high viscosity of the iontronic material, a strong bonding force thus exists between the AgNW network and the iontronic material. This force promotes the formation of a highly cross-linked nanocomposite membrane which can be easily peeled off by hand and exhibits high tensile strength and flexibility (Figure 1b). Scanning electron microscope (SEM) images (Figure 1c) reveal that the AgNW network has average diameter and length of 115 nm and 35 µm; on the other hand, the cross-section SEM image, combining with 2D morphological and 3D topographical Atomic force microscopy (AFM) images (Figure 1d), confirms that the randomly oriented AgNW


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network is partly buried into the iontronic bulk and enables to achieve a highly cross-linked AgNW/ionogel nanocomposite.

Figure 1. (a) Schematic view showing the fabrication process of the stretchable and transparent AgNW/ionogel nanocomposite membrane. (b) Optical images of the AgNW/ionogel nanocomposite membrane showing high transparence and flexibility. (c) SEM images of the AgNW/ionogel nanocomposite membrane (left is top view and right is cross section). (d) 2D morphological AFM (left) and 3D topographical (right) images of the AgNW/ionogel nanocomposite membrane.

To evaluate the role played by the AgNW/ionogel nanocomposite membrane in photosensing applications, we firstly compared light transmittance properties of the AgNW network with the AgNW/ionogel nanocomposite membrane. As shown in Figure 2a, the AgNW network exhibits high transparence of ~90% in the wavelength range from 400 nm to 900 nm while the transmittance of the AgNW/ionogel nanocomposite membrane is almost unchanged. Next, conductive characteristics of the AgNW network were firstly investigated under different bending radius (Figure 2b). As embedded into the iontronic material, the unbent AgNW network


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shows low square resistance (R0 = ~45 Ω·□-1 at T=300 K). Additionally, the value of square resistance slowly increases on bending radius because of the existing large contact resistance between the test probe and AgNW network when bent at a small radius. Even so, the value of square resistance remains as low as 200 Ω·□-1 as bent to 2.33 cm. Temperature-dependent conductivities of the AgNW network were further examined by fixing the AgNW/ionogel nanocomposite membrane inside a cryostat with a high vacuum of ~2 ×10-6 mbar and measuring current-voltage curves from 130 K to 390 K. Since a low temperature induces weak thermal vibration energy inside metal silver atoms, conductive properties of the AgNW network thus degrade with the increased temperature (Figure 2c). It is noted that in the high temperature range (300 K to 390 K), such trend becomes much more serious which might be due to the relatively high thermal expansion of the iontronic material. 27 Frequency-dependent capacitance measurements were carried out by sticking the membrane onto a flexible ITO electrode with different bending radius. A nanometer-thick EDL is formed at the electrode/ionogel interface, leading to an ultra-large interfacial capacitance (Figure 2d). As shown, in the low frequency range (< 80Hz), the interfacial capacitance under unbent condition is ~ 2 µF·cm-2, one or two orders of magnitude larger than values of oxide dielectric material (typically ~ 10 nF·cm-2). It is noted that the interfacial capacitance rapidly decays with the increased frequency because of slow polarization time of ions.33 It is observed that the membrane under bending conditions has slightly small specific capacitance compared to that in the unbending status, this is might be caused by the reduced area at the ionogel/electrode interface.


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Figure 2. (a) Transmittance spectra of the AgNW network and the AgNW/ionogel nanocomposite membrane. (b) Square resistance versus reciprocal bending radius (R-1). (c) Rate of square resistance versus temperature (∆R=RTR0, R0 is the square resistance at T=300 K). (d) Frequency-dependent specific capacitance of the ionogel film under different bending radius (R).

PHOTOSENSOR MADE BY LAMINATION PROCESS

The AgNW/ionogel nanocomposite membrane was then introduced to construct a flexible EGOPT device. Firstly, an ultra-thin (3 µm thickness) and flexible polyimide (PI) film was coated in advance with a photosensitive OBHJ film and source-drain interdigital electrodes. Then, an AgNW/ionogel nanocomposite membrane peeled off from silicon wafer was directly laminated onto the substrate by van der Waals bonding force to form a full EGOPT device (Figure 3a).The detailed fabrication process can be seen in Support Information. The total thickness of the device is about 10 µm, thus it can be closely attached onto skin (Figure 3c). The OBHJ layer contains recently developed high hole mobility and narrow-bandgap polymer


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poly(diketopyrrolopyrrole-thienothiophene) (PDPP2T) (Figure 3d). It exhibits high hole mobility (>1 cm2 V-1 s-1) and high on/off ratio (~106) in OFETs.15, 41- 43 After blending PDPP2T with wide bandgap electron acceptor material PC61BM, a broad spectral response covering ultraviolet (UV) to near infrared(NIR) can be achieved (Figure 3d). Output characteristics of the device are depicted in Figure 3e, exhibiting a typical field-effect behavior with both clear linear and saturation regimes. It is noted that all of the following measurements were carried out in ambient air unless otherwise stated. Transfer curves were obtained by setting the drain voltage (Vds) as a constant value of -0.5 V and sweeping the gate voltage (Vgs) with a speed of 50 points per second. As shown in Figure 3f, the device exhibits typical asymmetric ambipolar transports with hole and electron mobilities of µp=0.06 cm2 V-1 s-1 and µn=1.3 ×10-4 cm2 V-1 s-1. The large mobility difference contributes to photoresponse enhancement; this will be discussed later. Considering the existing electron-trapping and holetrapping states induced by strong interactions of electron/cation and hole/anion, threshold voltage of hole(Vthh) and electron(Vthe) are -0.3 V and 0.24 V, respectively. When Vgs is set to Vthe, small amounts of hole and electron carriers are accumulated at the ionogel/OBHJ interface, resulting in ultra-low drain current (Ids= ~20 pA) and ultra-high on-off current ratio of ~106.


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Figure 3. (a) Schematic structure of the EGOPT with an AgNW/ionogel gating structure. (b) Molecular structures of P(VDF-HFP) and [EMIM] [TFSI]. (c) Photograph of the device showing flexible and transparent features. (d) Absorption spectra of the OBHJ film (inset: molecular structures of PDPP2T and PC61BM). (e) Output curves under low gate voltage (Vgs is scanned from 0 V to -1.5 V). (f) Transfer characteristics of the device measured (the drainsource voltage is set to -0.5 V).

Typical photosensing characteristics were evaluated by measuring transfer curves of the EGOPT device in the dark and under light illuminations with a near infrared (NIR) laser of λ= 808 nm at different light intensities from 50 mW·cm-2 to 5 µW·cm-2 (Figure 4a). Owing to the ultra-large EDL capacitance at the ionogel/OBHJ interface, a small negative Vgs can repulse large amounts of anions to the surface of the ionogel film, giving rise to a high surface carrier density at the ionogel/OBHJ interface. In a typical electrolyte-gated organic transistor, the carrier density would be in excess of 1013 cm-2; 44-46 in combination with the nanometer-thick of the EDL, we can thus simply estimate that the carrier concentration at the ionogel/OBHJ interface in our device would be higher than 1020 cm-3. Even so, large photocurrent multiplication can be achieved as the light intensity becomes as weak as 5 µW·cm-2 which is much lower than the carrier density. To investigate the origination of the photocurrent enhancement, we firstly carried out photoresponse measurements under continuous irradiation (Support Information Figure S3).


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Considering the cumulative effects of light irradiation, the photoresponse characteristic was enhanced with the increased irradiation time, even though the effect was not obvious (after continuous irradiation of > 1 hour, the photoresponse is raised up to ~13%). It is noted that the scanning time of transfer curves is in the level of several seconds, the cumulative effect thus become quite weak. However, the generation of secondary photocurrent in the EGOPT device would be considered as the key origination of the photocurrent enhancement. The secondary photocurrent is originated from injection and transit processes of charge carriers from the drain-source electrodes. 47-48 For instance, in a hole accumulation region, since there are amounts of electron/ hole trapped states in the bandgap, holes (majority carriers) are extracted with a transmit time (τtransit time) while electrons (minority carriers) are localized in trap states with a lifetime (τlifetime). Charge neutrality rules allow extra holes to be injected from the electrode; giving rise to secondary photocurrent until new charge equilibrium is rebuilded. The photoconductive gain (G) thus equals the ratio of electron trapping time over hole drifting time and can be expressed as the following equation: 49

G = ∙ =

=

(1)

where R is responsivity, λ is the wavelength of incident light, q is electron charge, h is Planck’s constant, c is light speed, and L is the channel length. Therefore, the large mobility difference between hole and electron in the EGOPT device will generate obvious hole-transporting/electron-trapping characteristics which means that photoinduced holes transport along the PDPP2T molecules with fast transit time while electrons remain trapped in PC61BM molecules with much longer lifetime. Consequently, multiplying holes circulate in the PDPP2T molecules during a single photoexcitation process of electron-hole pair, giving rise to generation of a large photoconductive gain.


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As shown in Figure 4b, a maximum R of ~1.5×103 A·W-1 was obtained at light intensity of 5 µW·cm-2 when the gate and drain voltages are set to -1.5 V and -0.5 V, respectively. It is noted that the responsivity monotonously decreases with the increased light intensity. This phenomenon is quite similar with that in oxide-gated OFETs15 which can be simply explained that at the ionogel/OBHJ interface, photo-induced charge carriers are firstly filled into long-lived trap states which are relative to high photoconductive gain, at high light intensity, long-lived trap states are fully occupied thus charge transports start to be affected by the shorter-lived trap states, leading to lower responsivity. Accordingly, the validity of the explanation above can be further confirmed by the effect of the responsivity over the gate voltage. As expected, the slope of the intensity-responsivity curve becomes more gradual when the gate voltage was positively changed to 0.2 V. Considering broad-spectrum absorption of the OBHJ layer, such pronounced photoresponse features can be also obtained under green (λ= 532 nm) and IR (870 nm) light illuminations (Support Information Figure S4). Furthermore, temperature-dependent photoresponse measurements were carried out by placing the device inside a cryostat with a high vacuum of ~2 ×10-6 mbar. The normalized photocurrent (IT/Io, Io is the value of photocurrent at T=300 K) under different gate voltages are plotted in Figures 4c. It is demonstrated that photoresponse is weaken with the deceased temperature, which is due to the subdued charge carrier hopping as current conduction of the device in the low temperature. Noted that when applying a higher gate voltage, a higher concentration of photo excited carrier cannot efficiently transport through the channel, resulting in more serious decline of photoresponse.


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Figure 4. (a) Transfer curves of the EGOPT devices under dark and light illumination conditions. The light wavelength is 808 nm. (b) Light-intensity dependence of responsivity in the depletion (Vgs= 0.2 V) and accumulation (Vgs= -1.5 V) regions. (c) Normalized temperature-dependent photocurrent curves at different gate voltages.

To assess the utility of the EGOPT device for flexible electronic applications, the device was bent to different radius while transfer curves involving of photocurrent (Ids) and leakage current (Igs) were measured at the same time (Figure 5a). To eliminate strain effects in bending conditions, the device was fixed onto the test platform without any supporter (Support Information Figure S5). Indeed, bending actions bring about slightly photosensing degradations due to the increased leakage current at a small bending radius. A key metric of the EGOPT device--sensitivity (light-to-dark current ratio) was further extracted under different bending radius to evaluate the device’s flexibility; the results are summarized in Figure 5b. To achieve high sensitivity, the gate voltage was set to Vthe to enable extremely-low dark current (~20 pA). As unbent, the device shows an ultra-high sensitivity of 7.5×105, even the bending radius is reduced as small as 2 mm, the sensitivity remains in the level of 105.


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Figure 5. (a) Photocurrent and leakage current curves of the EGOPT devices at different bending radius. The light wavelength is 808 nm and the drain voltage is set to -0.5V. (b) Sensitivity versus bending radius. (inset: photographs of the device measured with bending radius of 5 mm and 2 mm, respectively).

Furthermore, transient behaviors to a square optical signal of 5 Hz with different light intensities were measured, and the results are shown in Figure 6a. The gate and drain voltages were set to 0.2 V and -0.5 V, respectively. Remarkably high sensitivity enables the EGOPT device for temporal detection on optical signal as weak as 5 µW·cm-2 with fast rise and decay time of ~8 ms and ~15 ms. Notably, both rise and decay time become shorter when decreasing light intensity (Figure 6b). Such phenomenon is in contrast to that of traditional organic phototransistors.37This phenomenon can be explained that in electrolyte-gated transistors, strong interactions between photo excited hole carriers and negatively ions induce considerably additional long-lived trap states which can be called as “ionic trap states”. When the light intensity increases, much more negatively ions residing on the side of the AgNW/ionogel membrane migrate into the OBHJ bulk thus cause electrochemical doping effects. In the light of the energy level diagram of the EGOPT device (Figure 6c), it can be understood that the existing ions will induce trap states in the charge transport channel and give rise to longer response time.


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Figure 6. (a) Temporal responses of the EGOPT device under near-infrared light illumination (λ=808 nm) at different light intensities, inset: rise time (trise) and decay time (tdecay) at light intensity of 5 µW·cm-2. (b) Rise time and decay time of photocurrent versus optical intensity. (c) Energy level diagram of the EGOPT device in the hole accumulation region under light illuminations.

The operation stability of the EGOPT device was further investigated via light pulse excitations with a frequency of 5 Hz (Figure 7a). Temporal operation maintains reasonably stable after more than 2000 cycles, which confirms the reliability of AgNW/ionogel membrane as the gating materials. A slight drift is most likely due to bias-stress effects in EGOPTs which can be reasonably explained referred to the interpretations of oxide-gated OFETs, of which redox reactions create an equilibrium between hole carriers in the organic semiconductors and protons at the dielectric/semiconductor interface induced by electrolytes: 41, 50 2

!" ⇄ 2 $

2!

2 $

2! ⇄ 2

!"

1⁄2 "

(2) (3)


14

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Positive protons on the side of the AgNW/ionogel membrane will drift to the ionogel/OBHJ interface, accompanying with electric-field screens of hole accumulations. Accordingly, longterm bias operations will convert large amounts of hole carriers to protons, resulting in a threshold voltage shift. It is noted that the oxidization of the AgNW electrode in air might also cause the device’s bias-stress effect. Finally, frequency response of the device was estimated by modulating light pulse signals to different frequencies, and the normalized photocurrent as a function of frequency was then measured in the range from 1Hz to 10 kHz (Figure 7b). The results indicate that the EGOPT device has a 3dB bandwidth of ~100 Hz, satisfying various low-speed photosensing applications including detections of pulse signals of human body and infrared thermal imaging.

Figure 7. (a) Dynamic output photovoltage versus cycles with 5 Hz illumination square wave light of 5 mW·cm-2. (b) Frequency-dependent photoresponse of the EGOPT devices (Vds= -0.3V, Vgs= 0.3V).

CONCLUSION

Summarily, a highly photosensitive, low-voltage and flexible electrolyte-gated organic transistor with an AgNW/ionogel nanocomposite gating structure has been successfully developed through a simple and cost-effective lamination process. Transient photoresponse measurements demonstrate that the presence of strong interactions between photo generated hole carriers and


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negatively ions induces additional long-lived trap states, leading to fast optical response characteristics under weak light illumination. The device also enables fast and air-stable operations, offering the opportunity of organic field-effect optical sensors for rapidly developing wearable electronics and human-robot interactions.

ASSOCIATED CONTENT

Supporting Information Details for fabrications of ionogel/silver nanowire membranes and EGOPT devices, contact angle measurements, viscosity characteristics of the iontronic material, photoresponse versus irradiation time, photoresponse characteristics under light illuminations with different lasers, setup of bending photoresponse measurement.

AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

ACKNOWLEDGMENTS

We thank Prof. Ni Zhao and Prof. Jianbin Xu from the Chinese University of Hong Kong for providing the capacitance measurements. We gratefully acknowledge for financial support from National Natural Science Foundation of China (61505110, 61401285, 81401750), Guangdong Science and Technology Research Project, China (2014A010103035), Guangdong Medical Research Foundation, China (A2016421), Shenzhen Basic Research Project, China (JCYJ20150324141711627,CYJ20140415093052190,CYJ20160307114925241,CYJ201404181 82819179,CXZZ20140418182638764), Shenzhen Overseas High-level Talents Key Foundation for Innovation and Entrepreneurship, China (KQJSCX20160226200541) and Natural Science


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Foundation of Shenzhen University, China(grant no. 201577, 827000122, 2016079, 80100036104 and 0000240655).

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Flexible and Highly Photosensitive Electrolyte ... - ACS Publications

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