Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX
pubs.acs.org/journal/apchd5
Lateral Polymer Photodetectors Using Silver Nanoparticles Promoted PffBT4T-2OD:PC61BM Composite Tao Han,†,‡ Linlin Liu,*,† Meihua Shou,† Zengqi Xie,† Lei Ying,† Chunzhi Jiang,‡ Xiaoyi Huang,‡ Hanying Li,§ and Yuguang Ma*,† †
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Institute of Polymer Optoelectronic Materials and Devices, State Key Laboratory of Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, People’s Republic of China ‡ School of Electronic Information and Electrical Engineering, Xiangnan University, Chenzhou 423000, People’s Republic of China § State Key Lab of Silicon Materials, Zhejiang University, Hangzhou 310027, People’s Republic of China S Supporting Information *
ABSTRACT: The fabrication of lateral polymer photodetectors (L-PPDs) is rarely reported in literature. Unlike vertical photodiode or phototransistor, it would be much more difficult to experimentally improve all of the performance metrics in lateral structure under long channel without compromising any of them. The performance metrics include charge separation, photomultiplication, trapped-charge relaxation, dark current, fast response and so on. In this research, LPPDs with comparable performance to photodiodes were developed, and the photodetectors have the structure of quartz/Ag-NPs (1 nm)/ PMMA (30 nm)/PffBT4T-2OD:PC61BM (D/A ratio = 1:1.2; 140 nm)/Ag−Ag electrodes. A phase control of BHJ allows effective electrons to be trapped under high PCBM ratio, which simultaneously gives high charge separation and photomultiplication (gain = 161.5 at a current density of 73 mA/cm2). Moreover, photogenerated electrons trapped in active layer can be neutralized by photoinduced holes stored in Ag-NPs, resulting in an obvious decrease of dark current (specific detectivity, 5.26 × 1014 Jones; response time of rise and decay, 10−20 ms). The novel lateral topology of PPDs, which is similar to the architecture of inorganic photodetectors, has promising application potential. KEYWORDS: lateral polymer photodetectors (L-PPDs), silver nanoparticles (Ag-NPs), trapping effect, dark current, specific detectivity (D*) hotodetectors exhibit widespread applications in the fields of imaging sensing, spectroscopy, environmental monitoring, and so on.1−5 The photodetectors based on photovoltaic effect, which have fast response speed, are becoming one of the most important photodetectors except that derived from photoconductive effect.2,6 Polymer photodetectors (PPDs) have attracted increasingly more attention after the bulk heterojunction (BHJ) based photovoltaics were invented.7,8 The diversity of material selection in organic structures allowed more advantages, especially in obtaining wide spectral response.9−11 Gong et al.10 reported that a broad spectral response ranging from 300 to 1450 nm could be obtained by blending a small-bandgap semiconducting polymer (PDDTT) with PC61BM. By controlling the absorption of the active layer via ternary mixtures (P3HT100−x:PTB7-Thx:PC71BM1), Wang et al.11 also broadened the spectral response range from 350 to 750 nm. An ideal photodetector should, in general, entail photomultiplication for high external quantum efficiency (gain); low dark current for low background noise, nice specific detectivity, and bandwidth; and quick release of the trapped charge when the light is off for fast response.2,12−14 At the beginning, the
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© XXXX American Chemical Society
photodiode type PPDs was based on BHJ, which works in the third quadrant (the photocurrent is also very small at this quadrant) in order to keep low dark current; however, the gain is normally lower than 1.15,16 Due to the high binding energy of organic conjugated molecules, the gain in photomultiplier tubes or avalanche photodiodes is generally not desirable. In PPDs, the photomultiplication is due to one kind of the photogenerated carrier that continually passes through the active layer, which afterward is pumped to the electrode by trap-assisted charge carrier tunneling injection under ohmic contact.2 The unbalanced carrier transport in the active layer could be built by low concentration of acceptor and phase control of BHJ. Metal nanoparticles (NPs) and semiconductor quantum dots have been used to improve the electrical and optical properties of photodetectors.17−19 By changing the combination of NPs and device architecture, the addition of NPs could enhance charge trapping, carrier genaration, light absorption, and so on.12,20 In our previous work,21,22 the trapping effect Received: August 14, 2018 Published: October 16, 2018 A
DOI: 10.1021/acsphotonics.8b01134 ACS Photonics XXXX, XXX, XXX−XXX
ACS Photonics
Article
was observed by adding Ag-NPs into the organic field-effect transistors (OFETs) insulating layer. A new phenomenon defined as photoinduced charge trapping was observed in evaporating naked Ag-NPs. Given that the charge trapping process is strongly related to the photoirradiation, this method have big potential in the application of photodetector.22 The fabrication of L-PPDs is rarely reported compared with the device topology of vertical photodiode or phototransistor, although the lateral topology makes the incident light irradiate on the channel directly and benefit the sensitivity of the device.23 Unlike vertical photodiode or phototransistor, it would be much more difficult to experimentally improve all of the performance metrics in lateral structure having long channel without compromising any of them. The performance metrics include charge separation, photomultiplication, trapped-charge relaxation, dark current, fast response, and so on.2,24,25 First, the method of low-concentration acceptor cannot be extend to lateral structure, because it would induce low charge separation and current density. Second, when the light is off, the trapped charges would still be stored in active layer and relax slowly for the long channel.22 Therefore, it is difficult for L-PPDs to maintain high gain, low dark current, and fast response simultaneously. In this work, a phase control of BHJ allowed effective electrons to be trapped under high PCBM ratio, which gave high charge separation and photomultiplication simultaneously. And photogenerated electrons trapped in active layer were neutralized by photoinduced holes storage of Ag-NPs, which obviously decreased the dark current. In this work, the PPDs based on PffBT4T-2OD:PC61BM composite with lateral-structural:quartz/Ag-NPs/PMMA/ PffBT4T-2OD:PC61BM/Ag−Ag electrodes have been fabricated successfully. PffBT4T-2OD:PC61BM was selected as the active layer, which is high-efficient (power conversion efficiency >10%) donor/acceptor materials of polymer solar cell (PSC).26 Herein, the experimental results are presented.
Absorption measurement was performed on a Shimadzu UV-3100 spectrophotometer. The PPDs characterizations were performed in air using a Cascade RF1 manual probe station and a semiconductor parameter analyzer (Keithley 2636B). The light illumination was provided by a diode pumped crystal laser (Newport CL-2000) with a wavelength of 405 nm, and the light intensity was tested by a laser power meter (header Ophir NOVA II and probe PD300-UV). The different light wavelength was provided by a continuous spectrum light source at 20 Hz (Opolette 355 LD). Two different modes have been chosen to test the dark current and photocurrent, one was the I−V characteristics at different light intensities, and the other was the on/off switching properties at different light intensities. The PPDs without Ag-NPs (control device) and with Ag-NPs (Ag-NPs device) were exposed with different light intensities at a bias of −20 V. In the on/off switching mode, the I represents on-switching, the II represents off-switching, and the a−f represent 0.0031, 0.015, 0.089, 0.25, 1.56, and 10.42 mW/cm2, respectively. Test was successive and every light condition involved one on-switching (I) and off-switching (II). While the device was off-switching (I), the light intensity was adjusted until on-switching (II) of the next light intensity. The 405 nm laser spot focused on tens micrometers, matching with the channel length between the two electrodes (Figure S1a, 1′) and its light intensity fitted the Gaussian distribution. In this test, all devices were irradiated by the laser on the active layer between the two electrodes. Figure S1b shows that the PMMA layer is not sensitive to light, having no contribution to photocurrent (=1.0 × 10−13 A), indicating PMMA can provide indeed an excellent organic interface bonding with active layer and ensuring the device to obtain the low dark current. While the light was on the whole electrode (2′ as shown in Figure S1a) or on the active layer outside electrodes (3′ as shown in Figure S1a), this photocurrent is 2− 3% of the photocurrent produced with light on the active layer between electrode (Figure S1b). To sum up, the conditional tests mentioned above showed that photocurrent of the device was mainly provided by the excitation of light at the active layer between the electrodes (1′ as shown in Figure S1a). There are three important parameters to characterize the performance of the photodetector: the responsivity (R), the photo gain (G), and the specific detectivity (D*). In calculating the above parameters, it should be considered that for the quartz-based device, a large part of the light will be transmitted and not be absorbed effectively by the active layer. So the transmission coefficient (T) should be introduced. At the same time, for the L-PPDs device, the current direction is vertical to the radiation direction of the light, so it is necessary to introduce the current density of the corresponding cross section. The calculation of cross section is demonstrated in the Supporting Information. Moreover, the D* of the device is related to the noise current. The noise current arises from the shot noise, thermal noise, 1/f noise, and so on.2 In this work, we choose the lateral structure, which leads to the apparent current value (∼100 nA) that is obviously lower than that of the vertical photodetector. So, the thermal noise is very weak. On the other hand, at the testing process, the direct current (DC) was provided by the semiconductor parameter analyzer (Keithley 2636B), making the 1/f noise neglectable. Besides, the shot noise from the dark currents as a major factor for the detector noise is reported by much literature.2,7,28 So, we mainly consider the effect of dark current density (Jd) on D*.
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EXPERIMENTAL SECTION The PMMA (Mw = 350000), butyl-acetate, dichlorobenzene, and chlorobenzene were purchased from Aldrich, and PC61BM were purchased from 1-Material. All of these commercially available chemicals were used without further purification. The cross-linked PMMA was selected as the polymer dielectric layer,27 which can effectively restrain Ag-NPs from quenching carriers in the active layer and also provide an excellent organic interface bonding with PffBT4T-2OD:PC61BM. The PMMA (1 wt %, 2000 r.p.m.) in butyl-acetate was spin-coated on quartz and cross-linked at 220 °C for 30 min. To completely dissolve the polymer, the active layer solution PffBT4T2OD:PC61BM (D/A ratio = 1:1.2, 0.9 wt % in chlorobenzene/ dichlorobenzene 1:1 volume ratio) was stirred on a hot plate at 110 °C for 24 h. The quartz substrate was preheated on a hot plate at 110 °C, and then the active layer was spin-coated on cross-linked PMMA. The PffBT4T-2OD:PC61BM films were annealed at 110 °C for 10 min afterward. The film needs to be put in a vacuum chamber (at least 2 h) for removing the solvents. Then the silver film (110 nm) was deposited as the electrodes under vacuum. Particularly, different thicknesses of Ag-NPs (1 nm/2 nm/4 nm measured by the Filtech during evaporation, depositing rate controlled in 0.1 Å/s) were deposited on the quartz surface by thermally vacuum evaporation, and the AFM of different thicknesses of Ag-NPs were reported in our previous work.21 B
DOI: 10.1021/acsphotonics.8b01134 ACS Photonics XXXX, XXX, XXX−XXX
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3k μm−2. The high coverage density of Ag-NPs with small particle size and large superficial area would support effective charge trapping and detrapping. PffBT4T-2OD:PC61BM ratio is 1:1.2, which fast spin coated with chlorobenzene/ dichlorobenzene (1:1 volume ratio) solvent at 110 °C. Crystallization and phase separation were observed in the TEM image of PffBT4T-2OD:PC61BM, as presented in Figure 1c. The lighter areas are rich in PffBT4T-2OD phase (red arrow), which is similar to that morphology of PffBT4T-2OD device without doping PC61BM (Figure S4a). Darker areas were also observed after doping PC61BM. This can be assigned as PC61BM-rich domains or areas (orange circle) due to the higher electron density of PC61BM. The PffBT4T-2OD-rich area (hole transporting) is obviously converged, leading to the formation of PC61BM-rich discontinuous domain (electron transporting; Figure 1c). In addition, we also add the TEM of PffBT4T-2OD:PC61BM (D/A ratio = 1:1.2) with 3% DIO as shown in Figure 1d. Compared with Figure 1c, the TEM of PffBT4T-2OD:PC61BM (D/A ratio = 1:1.2) with 3% DIO has lesser light areas (red arrow) but more dark areas (orange circle), indicating that the phase separation of donor and acceptor improves significantly by adding DIO. Different from the result in the PSC, the PPDs performance of directly spin coated films is better than that obtained by adding diiodooctane (DIO), indicating that PDDs favor unbalanced transport of electrons and holes,26 which will be discussed in detail below. By comparing the results at some typical light intensity, the current (I)/voltage (V) characteristics of the PPDs without Ag-NPs (control device) and with Ag-NPs (Ag-NPs device) are shown in Figures 2a and S5. Here, the current in dark at a bias of −20 V is defined as Idark, and the current under illumination at a bias of −20 V is defined as Iph. The I−V curves in the dark for all the devices are linear and quasisymmetric (Figure S5d). The linear current dependence of the applied voltage under both positive and negative biases suggests an ohmic carrier-transporting process inside the organic semiconductor.29,31 The dark current is of premium importance for the performance of PPDs (specific detectivity and response time), thus, attention was paid to it at the beginning of this research. Figure 2c displays a function of dark current and time, where the average dark current is chosen from the offswitching (II; Figure 2b) . In the control device, the original dark current is small (3.6 × 10−11 A) which benefits from laterally long channel. However, after several switch-on and switch-off cycles of the photodetector, the dark current started to increase step by step. Consequently, a 20-fold increase was obtained in the dark current of control device, reaching 7.5 × 10−10 A at 120 s (corresponding device is from switch-on to switch-off at the light intensity of 10.42 mW/cm2). The added dark current is induced by the trapped photogenerated electrons, because the hole mobility is much larger than electron mobility. A stable low dark current is required to ensure high Iph/Idark value and excellent repeatability of photodetector before its application. Normally this problem can be solved by decreasing the ratio of electron transporting materials (such as PC61BM), which will, in turn, decrease the separation of excitons dramatically (low photocurrent). In this work, this problem has been solved by inserting evaporated AgNPs in dielectric layer, and L-PPDs have been successfully fabricated. As shown in Figure 2c, the original dark current of Ag-NPs device becomes one fiftieth of that in control device,
The R, G, and D* are calculated according to the following equations:7,29 Jph R= LI × (1 − T ) (1) G=R×
D* =
hυ q
R 2q × Jdark
(2)
(3)
Here, Jph and Jdark are the photocurrent density and dark current density, respectively; hυ is the photon energy; LI is the incident light intensity; q is the absolute value of electron charge; and T is the transmission of light through the film. According to the Lambert−Beer law, the device’s transmissivity with and without Ag-NPs are Twith = 0.612 and Twithout = 0.65 (Figure S2a), respectively.
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RESULTS AND DISCUSSION PPDs were fabricated with a lateral-structure: quartz/Ag-NPs/ PMMA/active layer/Ag−Ag electrodes, as shown in Figure 1a.
Figure 1. (a) Schematic of the PPDs exposed to the 405 nm laser, (b) the AFM morphology of nontempered Ag island layer (1 nm), (c) PffBT4T-2OD:PC61BM (D/A ratio = 1:1.2) thin film, and (d) PffBT4T-2OD:PC61BM (D/A ratio = 1:1.2) thin film with 3% diiodooctane (DIO).
The L, W, and d represent the electrode length, wide, and active layer thickness, respectively. In order to obtain the best device performance, L, W, and d are set as 1000 μm, 40 μm, and 140 nm, respectively. Quartz was selected as substrate due to its slight doping, which will bring a symmetrical I−V characteristics (if the heavily doped Si/SiO2 substrate is used, I−V characteristics is obviously nonsymmetrical).29,30 The morphological characterization focuses on Ag-NPs and active layer that determine the device performance. The AFM morphology of thermal evaporated Ag island layer (1 nm thick measured by the Filtech during evaporation) on quartz surface is shown in Figure 1b. The average XY diameter and Z thickness of Ag islands under AFM images are 14.2 and 6 nm, respectively. The particle size distribution of Ag-NPs in AFM morphology was calculated by ImageJ software (Figure S3), and the results show that Ag-NPs coverage density is around C
DOI: 10.1021/acsphotonics.8b01134 ACS Photonics XXXX, XXX, XXX−XXX
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Figure 2. (a) I−V characteristics of PPDs both in dark and under 405 nm UV light illumination, (b) the on/off switching properties of Ag-NPs devices and control devices exposed to the 405 nm laser with different light intensity at a bias of −20 V, (c) the dependence of the dark current (off-switching current) on time at the bias of −20 V, (d) the dependence of the photocurrent (on-switching current) density on the light intensity at the bias of −20 V. Here, the inset image of (a) presents the I−V characteristics of PPDs under 10.42 mW/cm2. The I represents on-switching, the II represents off-switching, and the a−f represent 0.0031, 0.015, 0.089, 0.25, 1.56, and 10.42 mW/cm2, respectively. The full line and the dotted line in (d) represent the linear fit curve of the control device and Ag-NPs device, respectively. The ∂ is the exponent of the exponential function fitting, which meets the equation log(J) = A + ∂·log(LI). J stands for the current density, LI stands for the light intensity, and A stands for the coefficient.
but the lowest dark current (∼1.2 × 10−12 A) remains unchanged after several switch-off cycles of the photodetector at high light intensity. This phenomena is related to the photoinduced stored charge of naked Ag-NPs, which neutralize accumulated photogenerated electrons in the active layer,22 which will be discussed in detail below. The photocurrent is the most important part of PPDs, which is theoretically determined by interaction of carrier photogeneration, recombination and trapping/detrapping at the localized states, carrier transport at the extended states, and so on.29 In regard to the low charge mobility of the conjugated polymer and the long channel of L-PPDs, the photocurrent and current density are the key points of fabricating L-PPDs. As shown in Figure 2d, the photocurrent (on-switching current) density of the control device increases with higher light intensity, which indicates a good response of the irradiation. Under 10.42 mW/cm2, the photocurrent becomes 4.4 × 10−8 A, which is 4−5 orders of magnitude bigger than the dark current. Besides, the corresponding photocurrent density reaches about 73 mA/cm2, which is in the similar order of magnitudes with vertical photovoltaic and photodiode.8,32 The photocurrent density of L-PPDs would be a remarkable result because the channel length (40 μm) is 400-fold of the vertical device (normally 100 nm). Considering that the previous results in other BHJ system (such as P3HT:PCBM) are very low,8 the contribution of great efficient PffBT4T2OD:PC61BM composite is highly recognized.
The introduction of Ag-NPs have dramatically promoted the device performance. The photocurrent (on-switching current) density increase of Ag-NPs device is more than that of control device as shown in Figure 2d. The Ion/Ioff of PffBT4T2OD:PC61BM composite layer increases from 1.88 × 104 to 1.42 × 105 (Figure S5). With the increase in light intensity, the exponent of the increase in the photocurrent density of AgNPs device (∂ = 0.93) is higher than that of the control device (∂ = 0.68). ∂ = 1 means the photocurrent has a linear relationship with light intensity.29 In other words, the ∂ < 1 signifies the photocurrent turns saturated easily under high light. Thereby, with the exponent of Ag-NPs device closing to 1, it shows an approximate linear response, which demonstrates that the device added with Ag-NPs has much better performance especially under high light intensity. Apart from the decreased dark current, as mentioned above, the photocurrent has a 32% enhancement under illumination (10.42 mW/cm2; inset of Figure 2a). As shown in Figure S2a, only slight absorption enhancement was observed by UV−vis spectroscopy. This can be explained by the fact that the distance between Ag-NPs and active layer is too far (30 nm) to generate localized surface plasma resonance (LSPR) effect (
