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Role of Evaporated Silver Nanoparticles in Organic Field-Effect Transistor: Electrical Effects and Dependence on the Size Tao Han, Linlin Liu,* Xiaoyan Wu, Lu Chen, Cong Wang, Muddasir Hanif, Linfeng Lan, Zengqi Xie, and Yuguang Ma* Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, P. R. China S Supporting Information *

ABSTRACT: The electrical effects of metal nanoparticles are determined by the nature of a single nanoparticle (shape, size, surface) and their correlation with the nanoscale electronic structure. In this work, we report that electrical properties of evaporated silver nanoparticles can be controlled by different thicknesses and thermal annealing times. The particle size and size distribution were first fully characterized by the AFM and optical extinction spectra, and then their electrical properties such as current trapping and threshold voltage were studied by the organic field-effect transistor with a device structure of Si/SiO2/Ag-NPs/PMMA/PTB7/Ag. The results show that the thickness decrease and thermal annealing are effective ways for a lower charge trapping, which corresponds to smaller particle size and homogeneous particle distribution without particle aggregates. These results would be helpful for the optoelectronic applications of metal nanoparticles.

1. INTRODUCTION Metal nanostructures (NPs) have been employed in the optoelectronic devices to get improved performance due to metal enhanced fluorescence,1−6 energy transfer,7 interface effect,8 and so on.9 At the beginning, much attention has been focused on the study of metal NPs optical effects, such as localized surface plasmon resonance (LSPR) and light scattering.10 However, many recent reports have drawn increasing attention toward optoelectronic effects of metal NPs. The metal NPs have good conductivity; however, the optoelectronic devices containing the dispersed metal NPs show a discontinuous phase and cannot form a continuous channel. Therefore, the effect of metal NPs on electricity mainly comes from their contact with the device layer through changing the work function.11 Metal NPs have been incorporated into different positions of polymer solar cells (PSC) and polymer light-emitting diodes (PLEDs) including the active layer and transport layers in order to determine their role in charge transport, conductivity, electric-field distribution, and so on. For example, Kim et al.1,12 have compared electric effects of Ag-NPs at different device positions, which indicated that the presence of Ag-NPs increases the conductivity of the PEDOT:PSS electrode. Choy et al.13,14 reported that metal NPs in the active layer can induce a change of exciton distribution and space current in PSC. Our group has also reported a Au-NPs modified cathode in the PLEDs for the effective enhancement of efficiency, which concluded that the metal NPs can induce balanced charge transport.8 However, the electrical effects in a diode device are complex, and the device incorporating metal NPs have multiple effects on hole/electron transport and space charge distribu© 2016 American Chemical Society

tion, which lead to the difficulties in quantitative characterization. An organic field-effect transistor (OFET) usually uses a single charge carrier and can be controlled by the gate voltage.15,16 Adding metal NPs into OFETs’ insulating layer is an effective way to trap charge and fabricate memory device. The direction of trapping charge mainly depends on the work function of the metal NPs and the active layer, and the trapping ability of metal NPs could reflect the strength of NPs changing the interface electrical properties. Thus, this work is aimed to study the charge trapping mechanism of metal NPs and the corresponding evolution of electrical parameters of an OFET device, where the active layer chosen as a classic hole transporting OPV polymer poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]] (PTB7). On a nanoscale,17 the electronic structure of metal NPs shows correlation with the nature of a single nanoparticle (shape, size, and surface). The electrical effects of metal NPs can depend on the nature of a single nanoparticle, similar to the optical effects. According to the previous study, silver (Ag) NPs have many advantages when compared with other metal NPs, such as low production costs, easier fabrication, and controllable morphology.18−23 Various morphologies of Ag-NPs can be tailored just via evaporation thickness and thermal annealing.18,21 In this paper, we have controlled particle size and size distribution of the Ag-NPs by preparing different Received: November 24, 2015 Revised: December 29, 2015 Published: January 2, 2016 1847

DOI: 10.1021/acs.jpcc.5b11492 J. Phys. Chem. C 2016, 120, 1847−1853

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The Journal of Physical Chemistry C evaporation thicknesses and different thermal annealing times at 200 °C separately and report the correlation between AgNPs structure and the related electrical parameters.

Id =

(1)

Id is the drain−source current, μ is the field-effect mobility. W (1000 μm) and L (50 μm) are the device’s channel width and length, respectively. Ci (10.4 nF/cm2) is the capacitance per unit area of the gate dielectric, which is determined by the permittivity and thickness of SiO2 and PMMA. Vg and Vth0 are the gate voltage and threshold voltage, respectively.

2. EXPERIMENTAL SECTION The schematic of the bottom-gate OFET device structure used in this work is shown in Scheme 1. OFETs were fabricated in a Scheme 1. Schematic of Charge Distribution Mode of a Bottom-Gate/Top-Contact OFET with Embedding Ag-NPs into Insulating Layera

a

W ·μCi(Vg − Vth0)2 2L

3. RESULTS AND DISCUSSION 3.1. Controlling the Surface Morphology of Ag-NPs. In order to achieve different Ag-NPs structures, different thicknesses of Ag-NPs were deposited on the SiO2 surface of a silicon wafer by thermal vacuum evaporation. The AFM morphology is shown in Figure 1a−c for the different thermal evaporation thicknesses of the nontempered Ag island layer on the SiO2 surface. The apparent thickness from 1 to 4 nm is measured during evaporation by the Filtech. The real morphology under AFM images is composed of several Ag islands with 6−13 nm thickness and average diameter = 11.3− 28.1 nm. The particle size distribution of Ag-NPs shown in the AFM morphology has been statistically plotted by the Image J software (Figure S1). The average diameter and Ag-NPs coverage density are summarized in Figure 1g. The average diameter of particles increases with the thermal evaporation thickness of Ag-NPs, while the number of Ag-NPs per unit area decreased. Figure 2a presents the optical extinction spectra of the different thermal evaporation thicknesses on the quartz substrates. Because of the nontransparent silicon wafer substrate, we have used quartz substrates for the spectroscopic measurements. We think that the morphology on the SiO2 surface of the silicon wafer and quartz substrates remains the same because both the substrates have the same chemical components. The Ag island substrates showed an extinction maximum at 444 nm for 1 nm thick Ag-NPs. The extinction maximum is progressively red-shifted with the increasing particle size, and the corresponding bandwidth increases for the increase in size distribution. After evaporation, the Ag island containing substrates with different thicknesses were annealed at 200 °C under N2. At this temperature, the Ag-NPs with small sizes can melt and recrystallize, which enhances the quality of NPs. As an example, Figure 1d−f presents the AFM morphology of a Ag island with 1 nm thickness as a function of thermal annealing time. The XY diameter and Z thickness of Ag islands with different annealing times are compared with that of nontempered 1 nm thick AgNPs, as presented in Figure 1a by AFM cross section. It can be seen that, with increasing annealing time, the actual thicknesses of Ag islands decreases from 6 to 3.5 nm and the average size of particles increases from 11.3 to 14.5 nm (Figure 1 and Figure S1). The corresponding optical extinction spectra (Figure 2b) showed that the extinction maximum blue-shifts from 444 to 415 nm. Especially, the bandwidth of annealed Ag-NPs is narrow and the long wavelength extinction is lowered when compared with a nontempered Ag island layer. These phenomena are similar for all the thicknesses (Figure S2 shows the result of 2 nm thick Ag island), suggesting that the particle aggregate is well restrained and the Ag-NPs can become more homogeneous through thermal annealing. 3.2. Electrical Effect of Ag-NPs in OFETs. The electrical effects of evaporated Ag-NPs are achieved by comparing

“+” = free holes.

top contact geometry using silver as the source and drain electrodes. The control device was fabricated with a structure composed of a Si/SiO2 (300 nm)/PMMA (30 nm)/PTB7 (80 nm)/Ag (90 nm). Highly n-doped silicon and thermally grown 300 nm silicon dioxide (Hefei Kejing Materials Technology Co., Ltd.) were selected as the back gate and first inorganic gate dielectric, respectively. As to the second polymer dielectric, we choose cross-linked PMMA (220 °C, 30 min for cross-link), which can provide an excellent interface bond with PTB7, high dielectric constant (3.6 at 60 Hz), and suppressed the leakage current from source and drain to gate electrodes.24 The PMMA (1 wt %, 2000 r/min) in butyl-acetate was spin-coated on SiO2, and PTB7 (2 wt %, 3000 r/min) in chlorobenzene spin-coated on cross-linked PMMA. Then, the silver electrode (90 nm) was deposited under vacuum as the source and drain electrodes. The embedded Ag-NPs OFET device’s structure was composed of a Si/SiO2 (300 nm)/Ag-NPs/PMMA (30 nm)/ PTB7 (80 nm)/Ag (90 nm). Different thicknesses of Ag NPs (1 nm/2 nm/4 nm measured by the Filtech during evaporation, depositing rate controlled at 0.1 Å/s) were deposited on the SiO2 surface by thermal vacuum evaporation, and then thermal annealing at 200 °C in the glovebox with different annealing times (0 min/20 min/40 min/60 min). After deposited AgNPs, the cross-linked PMMA, the PTB7, and the silver electrode were fabricated with the same procedure as the control device. All of the commercially available chemicals were used without further purification. PMMA (Mw = 350 000), butylacetate, and chlorobenzene were purchased from Aldrich, and PTB7 was purchased from the company 1-Material. The atomic force microscopy (AFM) images were obtained using a MultiMode 8 (Bruker), and the radius of the tip of the AFM cantilever is 2 nm (A tip of SNL-10). UV−vis spectra were recorded by a UV-3600 (SHIMADZU UV-3600). The OFET characterizations were performed in air using a Cascade probe station (DPP210-M-L/R) and a four semiconductor parameter analyzer (Agilent 4155 C). The OFET output curves in the saturation region are obtained by the following equation25 1848

DOI: 10.1021/acs.jpcc.5b11492 J. Phys. Chem. C 2016, 120, 1847−1853

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Figure 1. AFM morphology of Ag island layer on SiO2 surface: (a) Ag-NPs (1 nm), 200 °C, 0 min; (b) Ag-NPs (2 nm), 200 °C, 0 min; (c) Ag-NPs (4 nm), 200 °C, 0 min; (d) Ag-NPs (1 nm), 200 °C, 20 min; (e) Ag-NPs (1 nm), 200 °C, 40 min; (f) Ag-NPs (1 nm), 200 °C, 60 min. Variation of the average diameter and Ag-NPs density (δ) as a function of evaporation thickness with nontempered Ag-NPs devices (g) or annealing time of different thicknesses of Ag-NPs devices after 200 °C annealing (h).

Figure 2. Optical extinction spectra of Ag island layer on quartz substrates: (a) different thermal evaporation thicknesses of nontempered Ag-NPs devices, (b) different thermal annealing times of Ag-NPs devices after 200 °C annealing with Ag-NPs (1 nm).

electrical parameters of OFETs between that with Ag-NPs and the control device. The device structure of OFETs is shown in Scheme 1. For the presence of the Ag island structure usually induces a charge trapping and hysteresis effect in OFETs,19 the systemic comparison of the electrical parameters requires a very high quality control device. Here, SiO2/PMMA is chosen as the

dielectric layer and PTB7 as the active layer. As exhibited above, Ag-NPs were evaporated on the SiO2 surface of the silicon wafer. Additional PMMA dielectric layers were introduced to modify the defects of SiO2. The PMMA layer can also be regarded as a buffer layer between metal NPs and the active layer, in order to restrain strong optoelectric 1849

DOI: 10.1021/acs.jpcc.5b11492 J. Phys. Chem. C 2016, 120, 1847−1853

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Figure 3. Contrast output characteristics curves (a) and transfer I−V characteristics curves (b) of control device and embedded nontempered 2 nm thick Ag-NPs into the OFET. In the transfer curves, the drain−source voltage (Vd) maintained at −60 V. Vth0 is defined as the threshold voltage when the gate voltage varies between +35 and −60 V, and Vth1 is defined as the threshold voltage when the gate voltage varies between −60 and +35 V.

curves;19 we suggest an important role of the active and dielectric layers in this work for low hysteresis effect. The scheme for the charge distribution mode of the OFET device (Scheme 1) helps us to understand the evolution of the charge trap and related electrical parameters in the presence of Ag-NPs. The transfer characteristics curves of the control device were measured at a constant Vd = −60 V, which becomes more negative than the gate voltage (+35 to −60 V), and the channel accumulated hole density decreases from the source to drain contact.26 Horowitz et al.27 deduced the formula about the variation of the space charge density with the gate voltage changes, while the formula neglects the threshold voltage. The amended formula is applicable to the condition under which the threshold voltage is not 0 V, and it abides by the following equation.

quenching (similar motivation with LSPR). For the active layer, PTB7 is used as an amorphous conjugated polymer with high purity, acceptable hole mobility, and good experimental repeatability. The output characteristics and transfer I−V characteristics curves of the control device are shown in Figure 3. We prepared the high performance control device with hole mobility (3 × 10−3 (cm2/(V·s))) and on/off current ratio (8 × 104). Especially, there is no hysteresis effect in transfer I−V characteristics curves. Here, the difference between two threshold voltages of different sweeping directions for the same device is defined as ΔVth = Vth0 − Vth1. The good coincidence between the trace and retrace of transfer I−V characteristics curves with a very small hysteresis effect (ΔVth ≅ 0.5 V) indicates that both the transporting layer (PTB7) and the dielectric layer (SiO2/PMMA) have very low defects. Thus, when Ag-NPs are introduced into the OFETs, the threshold voltage shift and hysteresis effect can be fully attributed to the presence of Ag-NPs. This can be used as the basis for the systemic comparison. Figure 3a compares the contrast output characteristics curves of a control device and the one with nontempered Ag-NPs (2 nm). The saturation current decreases when the SiO2 surface is modified by the Ag-NPs at the same gate voltage. Figure 3b presents the transfer curves of the control device and that with nontempered Ag-NPs (2 nm), with the drain−source voltage (Vd) maintained at −60 V. Compared with the control device as shown in Figure 3b, it also reveals that the nontempered AgNPs (2 nm) device has the lower hole mobility (1.6 × 10−3 (cm2/(V·s))), on/off current ratio (2.2 × 104), and more negative threshold voltage (Vth0 = −13 V). These results indicated that embedded Ag-NPs into OFET can change the electrical parameters, which mainly comes from Ag-NPs’ contact with the device layer through changing the work function.11 In the presence of Ag-NPs, OFET devices still have a very small ΔVth, which indicates that the storage capacity of evaporated Ag-NPs itself (without any modification on nanoparticle surface) is low and the trapped charge can be released very soon. We tested the source−drain current as a function of time for the OFET devices based on nontempered Ag-NPs (2 nm) (Figure S3). The currents at the programming and erasing are both very low, which indicates that the charge storage capacity of Ag-NPs is very poor. Previously, it has been reported that metal NPs based OFETs have a memory window in the transfer

ρz = q

Ci 2(Vg − Vth0)2 ⎛ ⎜1 + 2kTεs ⎝

z ⎞ ⎟ 2 LD ⎠

2

(2)

ρz is the space charge density, k is the Boltzmann constant, T is the Kelvin temperature, εs is the dielectric constant of organic semiconductor, z is the perpendicular coordinates of interface, and LD is the Debye length. The insulating layer of PMMA is thin enough to make Ag-NPs trap the holes in the channel. The space charge density increases with the increasing gate voltage, as obtained from eq 2. At the high gate voltage, the space charge density increases in the channel, which eases hole diffusion into the Ag-NPs. From eq 2, we know that the space charge density decreases with the drifting of Vth0, and it is equal to the increase of the trap charge ability. Adding Ag-NPs is an effective way to shift the Vth0, as shown in Figure 3b, then leading to the space charge density decreasing, so the electrical parameters (such as hole mobility and on/off ratio) of the AgNPs device would be decreased. 3.3. Relationship between Ag-NPs Surface Morphology and Device Electrical Effect. The Ag-NPs with different sizes and size distributions have the different charge trapping behaviors, as summarized in Figure 3, Figures S4 and S5, and Tables S1 and S2, which lead to the space charge density decreases to different levels. Therefore, we can establish a relationship between electrical parameters and the particle size and particle distribution, and it would be helpful for the electrical behavior control of metal NPs in optoelectronic applications. 1850

DOI: 10.1021/acs.jpcc.5b11492 J. Phys. Chem. C 2016, 120, 1847−1853

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Figure 4. Variation of the ΔI as a function of gate voltage: (a) different thermal evaporation thicknesses of nontempered Ag-NPs devices, (b) different thermal annealing times of Ag-NPs devices after 200 °C annealing with Ag-NPs (1 nm).

Figure 5. Variation of the basic parameters as a function of evaporation thickness with nontempered Ag-NPs devices (a, c) or annealing time of different thicknesses of Ag-NPs devices after 200 °C annealing (b, d). Ntrap/δ represents the trapping ability of one Ag-NP.

systematic comparison of ΔI as a function of gate voltage and Ag-NPs structures is shown in Figure 4. At each Ag-NPs thickness, the ΔI increases with the increase in gate voltage, while the increasing velocity of ΔI (slope of ΔI−Vg curves) also increased with the increase in gate voltage. It indicates that the charge trap process is accelerated at higher gate voltage. As the Ag-NPs thickness increases (Figure 4a), the ΔI also increases at the same gate voltage and shows a stronger dependence behavior than the gate voltage from 1 to 2 nm. After 2 nm thickness, the trap ability tends to saturation. Annealing with different thermal annealing times at 200 °C for the same Ag island layer, as shown in Figure 4b (results of 2 nm Ag-NPs in Figure S6), the ΔI increases with the increase in gate voltage at

In the output curves (Figure 3a), the current difference in the saturation region at the same gate voltage between the control device and Ag-NPs devices is defined as ΔI = Istandard − IAg, where Istandard and IAg are the drain−source current of the control device and Ag-NPs devices, respectively. ΔI > 0 means that the Ag-NPs trap charge, which makes the current decrease at the same gate voltage. During the initial rising stage of gate voltage, as shown in Figure 3a, ΔI enhances more quickly when the gate voltage varies from 0 to −60 V. As a result, the Ag-NPs can easily trap charge at a higher gate voltage, as a signal amplifier for trapping charge. The comparison of nontempered, Ag-NPs devices (thickness: 1 nm (Figure S4) and 2 nm) shows that a bigger total mass of Ag-NPs will obtain a larger ΔI. A 1851

DOI: 10.1021/acs.jpcc.5b11492 J. Phys. Chem. C 2016, 120, 1847−1853

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The Journal of Physical Chemistry C each annealing time, while the ΔI decreases with the increasing annealing time. The big total mass and surface area of Ag-NPs can drift the Vth0 (Tables S1 and S2), which decreases the channel space charge density at the same gate voltage, eventually leading to an increase in the ΔI. Combined with the particle size and coverage density in Figure 1, we conclude that a bigger particle size induces lower charge trapping when the amount of Ag-NPs was kept the same. At the same time, a drifting threshold voltage (ΔVth0) is defined in order to determine the trap density, which is given by the difference between Vth0 of the control device and that with Ag-NPs, i.e., ΔVth0 = Vsth0 − VAth0. The Vsth0 and VAth0 represent the threshold voltage of the control device and that with Ag-NPs, when the gate voltage is varied between +35 and −60 V, respectively. Trap density (Ntrap) represents the charge trapping ability of Ag-NPs, which is obtained by the following equation28 Ntrap = Ci·

ΔVth0 q

systemic conclusion on the size dependence of charge trapping. It is well controlled by the thickness and thermal annealing time, corresponding to different particle sizes, size distributions, and particle aggregates. These results are helpful for the optoelectronic applications of metal nanoparticles. The amorphous active layer used in the OFETs showed welldefined experimental repeatability and demonstrated its benefits in the device physics.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b11492.

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(3)

Characterization data of Ag-NPs, and detailed electrical parameters of control device and Ag-NPs devices (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Fax: +86-20-87110606. Tel: +8620-22237036 (L.L.). *E-mail: [email protected] (Y.M.).

where q is the unit charge with a value equal to 1.6 × 10−19 C. Ntrap/δ corresponds to the number of trapped charges for each nanoparticle, which helps us to estimate the trapping ability for different sizes and distributions of nanoparticles. Figure 5a presents the variation of the ΔVth0 as a function of evaporated Ag-NPs thickness, and the variation of the Ntrap/δ as a function of average diameter with nontempered Ag-NPs devices. By increasing the thickness of Ag-NPs, the ΔVth0 value increased from 3 to 14.5 V. By increasing the Ag-NPs average diameter from 11.3 to 28.1 nm, the Ntrap/δ value increased from 54 to 943, indicating that Ag-NPs with bigger average diameters have larger trapping ability. Figure 5b shows the variation of the ΔVth0 as a function of different annealing times of Ag-NPs devices, and the variation of the Ntrap/δ as a function of average diameter under different annealing times. Compared with the 1 nm thick Ag-NPs, the ΔVth0 value of 2 nm thick Ag-NPs decreases very fast after 200 °C thermal annealing. By increasing the Ag-NPs average diameter, the Ntrap/δ value decreases from 54 to 20 after thermal annealing at 200 °C. The results from transfer curves are similar to those from the output curves, indicating that the thickness decrease and thermal annealing process is good for low charge trapping. Some other electrical parameters such as mobility and Ion/Ioff were investigated as a function of Ag-NP thickness and thermal annealing time. Figure 5c presents the variation of mobility and Ion/Ioff as a function of evaporation thickness with nontempered Ag-NPs devices. As the thickness of Ag-NPs increases, the mobility value decreases from 3.3 × 10−3 to 1.4 × 10−3 cm2/(V· s) and the Ion/Ioff value also decreases from 8.1 × 104 to 2.3 × 104, respectively. The space charge density decreases to different levels when Ag-NPs were embedded into the device, resulting in the lower mobility values. After 200 °C thermal annealing with different annealing times of Ag-NPs devices, the mobility and Ion/Ioff value returns close to original values, as shown in the Figure 5d.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their thanks to the Natural Science Foundation of China (51303057, 91233113, 21334002, 51373054, 51473052, 21174042), the Ministry of Science and Technology of China (2013CB834705, 2015CB655003), the Fundamental Research Funds for the Central Universities (2015ZZ010), and the Introduced Innovative R & D Team of Guangdong (201101C0105067115) for their support.

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REFERENCES

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4. CONCLUSIONS In conclusion, we have controlled the electrical effects in evaporated silver nanoparticles by size and size distribution dependence. For a single charge carrier (hole) and effective current amplification by the gate voltage, addition of metal nanoparticles into the OFET’s insulating layer which achieved a 1852

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DOI: 10.1021/acs.jpcc.5b11492 J. Phys. Chem. C 2016, 120, 1847−1853