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graphene bulk heterojunction
Optics Communications 425 (2018) 161–165
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Optics Communications journal homepage: www.elsevier.com/locate/optcom
Solution-processed graphene phototransistor functionalized with P3HT/graphene bulk heterojunction Yongli Che a , Guizhong Zhang a, *, Yating Zhang a , Xiaolong Cao a,b , Mingxuan Cao a , Yu Yu a , Haitao Dai c , Jianquan Yao a a
Key Laboratory of Opto-Electronics Information Technology (Tianjin University), Ministry of Education, School of Precision Instruments and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China b College of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao, 266590, China c Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, School of Science, Tianjin University, Tianjin, 300072, China
ARTICLE
INFO
Keywords: Phototransistor Graphene Graphene-P3HT hybrid Solution process
ABSTRACT Here we present a feasible way to fabricate a high-performance graphene phototransistor functionalized with Poly(3-hexylthiophene) (P3HT)/graphene bulk heterojunction by solution processing, which possesses the advantages of the high carrier mobility of graphene and the high visible light absorption of P3HT. The hole field-effect mobility for the device was 3.8 cm2 V−1 s−1 at room temperature. The photoresponsivity, external quantum efficiency and detectivity were 12 A/W, 3 × 103 % and 1.6 × 106 Jones at a illumination irradiance of 15 mW/cm2 respectively. The phototransistor exhibited sensitive photoresponse with a rise time of 168 ms and a decay time of 121 ms.
1. Introduction Graphene has received considerable attention for electronic and optoelectronic because of its outstanding mechanical, electrical, and optical properties [1,2]. Graphene phototransistors can realize ultrafast photodetection owing to its extraordinarily high carrier mobility [3]. However, the very low responsivity due to its weak light absorption and fast recombination rate has limited the sensitivity of graphene lightsensing devices [4]. Significant efforts have been applied to increase the absorption, among which a feasible way is to couple graphene with light-absorbing materials [5]. Poly(3-hexylthiophene) (P3HT) is known as a popular absorptive material for its large absorption coefficient in the visible range, which makes it very attractive for organic optoelectronic devices [6]. Moreover, P3HT possesses several other advantages of solution processing, low cost and good semiconducting properties [7,8]. E. H. Huisman et al. [9] reported hybrid graphene-P3HT layerheterojunction phototransistors using chemical vapor deposition (CVD) graphene on silicon/silicon dioxide, demonstrating responsivities of 10−1 A/W. The low responsivity can be enhanced by distributing graphene into the P3HT matrix, in which the graphene can act as a conducting route between the P3HT domains [10]. In this study, we report a graphene phototransistor functionalized with P3HT/graphene bulk heterojunction by solution processing, which *
combines the high carrier mobility of graphene and the high visible light absorption of P3HT. The photoresponses of the phototransistor can be greatly enhanced due to the photoexcited holes transfer from P3HT to graphene for the band alignment between graphene and P3HT [4]. Additionally, the high carrier mobility of graphene allows holes to recirculate in channel during the lifetime of the photoexcited electrons trapped in the P3HT, which can also enhance the photosensitivity of the device. 2. Experimental section 2.1. Device fabrication A heavily n-doped silicon (Si) wafer was used as the back gate electrode and a covering thermally grown silicon oxide (SiO2 ) layer of 300 nm thick was employed as the gate dielectric layer, respectively. The Si/SiO2 substrate was cleaned by using a sonication of acetone, deionized water and ethanol, sequentially. A layer of graphene was formed on the Si/SiO2 substrate by spin-coating two drops of graphene toluene solution (1 mg mL−1 ) at 2000 rpm for 10 s. Cr (20 nm)/Au (100 nm) source/drain electrodes were evaporated through shadow mask on the graphene layer deposited on the substrate, defining the channel width of 0.1 mm and channel length of 0.02 mm. Subsequently,
Corresponding author. E-mail address: [email protected] (G. Zhang).
https://doi.org/10.1016/j.optcom.2018.04.058 Received 12 January 2018; Received in revised form 23 April 2018; Accepted 23 April 2018 0030-4018/© 2018 Published by Elsevier B.V.
Y. Che et al.
Optics Communications 425 (2018) 161–165
after being exposed to air for two weeks, revealing the 𝐼DS level is slightly reduced after two weeks, confirming the good air stability of the device. Therefore, as an effective strategy, the PMMA protective layer can enhance the ambient stability of devices, which is beneficial to many aspects of device fabrication. In order to confirm the photoresponse characteristics of the phototransistor based on the graphene/PGH hybrid, we investigated the electrical variations of the device at different incident laser power intensities under the illumination of 𝜆 = 532 nm. Fig. 3(a) shows the output characteristics of the device under different illumination irradiance (𝐸e ) at a fixed 𝑉GS of −8 V. The 𝐼DS under illumination (𝐼ill ) of the device was composed of the dark current (𝐼dark ) and the photocurrent (𝐼ph ). The 𝐼ph (𝐼ph = 𝐼ill - 𝐼dark ) increased with the increase of the 𝐸e , consistent with the increasing number of the photo-generated carriers. Under light exposure, photogenerated electron–hole pairs were produced in P3HT and separated at the P3HT/graphene interface, due to the built-in electric field near the interface between P3HT and graphene. The photogenerated holes were transferred from P3HT to graphene and the photogenerated electron were trapped inside P3HT, which were caused by the field effect gating. Increasing hole concentration in the p-type channel leaded to an increase of its conductance, which greatly enhanced the current. For a phototransistor, photoresponsivity (R) is a key figure of merit for investigating photosensitivity and is defined as photocurrent generated per incoming optical power. It can be calculated by the following relation [15]:
P3HT-graphene hybrid (PGH) absorption film was deposited by spincoating the PGH solutions using layer-by-layer approach at 2000 rpm for 15 s. The P3HT-graphene hybrid solutions were prepared by mixing three parts of P3HT chloroform solution (3 mg mL−1 ) and one part of graphene chloroform solution (1 mg mL−1 ). Graphene powder was purchased from The Sixth Element, Inc. P3HT was purchased from Tixiai Industrial Development Co., Ltd. Finally, a layer of poly (methyl methacrylate) (PMMA) with about 20 nm thickness was coated as the protective layer on top of the channel by spin-coating PMMA ethyl acetate solution (4 mg mL−1 ) at 2000 rpm for 15 s. 2.2. Sample measurements The electrical properties of the phototransistor were analyzed using the Keithley 2400 semiconductor parameter analyzers. The photoresponses of the device were measured using the green laser source with 532 nm wavelength, where the optical power was tunable. A Zolix Omni-𝜆300 spectrometer was used to analyzed the absorbance of the PGH and P3HT. Scanning electron microscopy (SEM) was used to investigate the structure of the phototransistor. 3. Results and discussion Fig. 1(a) illustrates the bottom-gate and top-contact schematic structure of the phototransistor based on the graphene/PGH hybrid. A graphene/PGH hybrid film used as the channel was deposited on the Si/SiO2 substrate with Cr/Au source–drain electrodes. Fig. 1(c) shows the cross-sectional SEM image of the device, where the PGH layer were clearly observed on the Si/SiO2 substrate with the thickness of 90 nm. Fig. 1(d) illustrates the absorption spectrum of the PGH, exhibiting absorption peaks at 540 nm and 610 nm, which is similar to that of the pristine P3HT (Figure S1) As shown in Fig. 1(b), the drain voltage (𝑉DS ) was connected between the drain and the source (i.e., ground connection) electrodes; the gate voltage (𝑉GS ) was applied to the gate and the source electrodes; and the channel current (𝐼DS ) flowed from the drain to the source. The electronic characteristics of the phototransistor were measured in the dark under ambient conditions. Fig. 2(a) shows the output characteristics of the phototransistor at different back-gate voltages, exhibiting a clear transition from the linear to the saturation region when 𝑉DS increases negatively, demonstrating the dominance of holes in the channel under field-effect gating which is attributed to the p-type semiconductor P3HT. The phototransistor shows an ambipolar behavior with a sharp rise of 𝐼DS at low 𝑉GS , which is attributed to the ambipolar characteristic of the graphene. Fig. 2(b) shows the U-shaped transfer curves of the phototransistor, which confirms the ambipolar properties of the device. Due to the Dirac point is obtained upon a negative gate bias, the phototransistor displays a n-type behavior [11]. When 𝑉GS < 𝑉dirac , it indicates a p-channel phototransistor. The carrier fieldeffect mobility was then calculated from the linear regime of the transfer curve according to the following equation [12,13]: 𝜇=
𝜕𝐼 𝐿 ⋅ DS 𝑊 𝐶𝑖 𝑉DS 𝜕𝑉GS
𝑅=
𝐼ph 𝐴𝐸e
(2)
where A is the effective illuminated area. The external quantum efficiency (EQE) represents the ratio of photo-generated carriers to incident photon and is related to the R by the expression [16]: 𝐸𝑄𝐸 =
ℎ𝑐 𝑅 × 100% 𝑞𝜆
(3)
where h is the Plank’s constant, c is the speed of light, q is the electron charge, and 𝜆 is the incident wavelength. Fig. 3(b) shows the photoresponsivity (black line) and EQE (red line) as a function of the 𝐸e at 𝑉DS = −10 V and 𝑉GS = −8 V. The phototransistor exhibits a R of 12 A/W and an EQE of 3 × 103 % for the 𝐸e of 15 mW/cm2 , due to their values increase as the 𝐸e decreases. The R in this case is higher than that of the previous work [9] (10−1 A/W), due to the photo-generated electron–hole pairs can be easier to separate in P3HT-graphene bulkheterojunction than in P3HT-graphene layer-heterojunction, which lead to a larger photogenerated current. Another important parameter of a phototransistor is the noise equivalent power (NEP), which is can be expressed as [17]: 𝑖𝑛 (4) 𝑅 where 𝑖𝑛 is noise current (Fig. 3(d)). The dominate noise source is Flicker noise. The detectivity (𝐷∗ )characterizes the minimum incident light power to generate 𝐼ph and is associated with NEP according to the equation below [18]: √ 𝐴𝛥𝑓 𝐷∗ = (5) 𝑁𝐸𝑃 ∗ where 𝛥𝑓 is bandwidth. Fig. 3(c) shows the NEP and 𝐷 values of the phototransistor as a function of the 𝐸e . As the 𝐸e decreases, the value of NEP increases and the value of 𝐷∗ decreases. NEP was found to be 1.2 × 10−10 WHz−1∕2 and 𝐷∗ was 1.6 × 106 Jones at 𝐸e of 15 mW/cm2 shown in Fig. 3(c). The photoswitching characteristics of the device was measured by periodically turning light on and off under the irradiance of 45 mW/cm2 at 𝑉GS = −8 V, 𝑉DS = −8 V, revealed in Fig. 4(a). As seen, the device exhibits good on–off switching performance with excellent sensitivity, stability and reproducibility. Fig. 4(b) shows the time-resolved response of photocurrent rise and decay according to the laser being switched
𝑁𝐸𝑃 =
(1)
where 𝐶𝑖 is the gate capacitance per unit area. The hole mobility calculated from the transfer characteristic of the p-type phototransistor is 3.8 cm2 V−1 s−1 . This hole mobility is higher than that of the previously reported P3HT-graphene bulk-heterojunction phototransistor [14] due to the high mobility of bottom graphene layer, and lower than that of the reported P3HT-CVD graphene layer-heterojunction phototransistor [9] due to the higher mobility of CVD graphene. To enhance the stability of the phototransistor, we coated a layer of PMMA on the channel to isolate it from atmospheric oxygen and water. Fig. 2(c) shows the output curves of the device in ambient condition after two weeks, exhibiting good FET behavior is retained. Fig. 2(d) shows the output curves of the device as soon as being prepared and 162
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Optics Communications 425 (2018) 161–165
Fig. 1. (a) Schematic of the phototransistor. (b) The measurement setup of the device. (c) Cross-sectional SEM images of the device. (d) Light absorbance of PGH.
Fig. 2. (a) Output characteristics and (b) transfer characteristics of the phototransistor. (c) Output characteristics of the phototransistor after two weeks in air. (d) Output characteristics of the phototransistor measured as prepared (black curve) and after two weeks in air (red curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
on and off. The rise and decay time values can be extracted by fitting the temporal photoresponse curves to the exponential function. The rise response can be fitted by the following equation: 𝛥𝐼DS
[ ] = 𝐼dark + A1 e−𝑡∕𝜏1
the following equation: 𝛥𝐼DS = 𝐼dark + 𝐴2 [𝑒−(𝑡−𝑡0 )∕𝜏2 ]
(6)
(7)
The time constant 𝜏2 is 121 ms, which represents the recombination process of the photogenerated holes and electrons. The response time in this case is similar to the response time obtained for the previously reported device [9], indicating their response speed is similar.
where 𝜏𝑖 is the time constant, 𝐴𝑖 .is the amplitude weights. The time constant 𝜏1 is 168 ms, which represents the photogenerated holes transfer from P3HT to graphene. The decay response can be fitted by 163
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Optics Communications 425 (2018) 161–165
Fig. 3. (a) Output characteristics of the phototransistor under different radiation powers. (b) Responsivity (black line) and external quantum efficiency (red line) as a function of the irradiance. (c) Detectivity and the noise equivalent power (inset) of the phototransistor as a function of the irradiance. (d) A noise level of the device in 1 s with sample rate of 5 ms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. (a) Time-dependent photocurrent of the device. (b) The temporal photocurrent response in one period.
The fast photoresponse characteristics of the device can be explained by its carrier transport properties. Under light exposure, photoinduced excitons were generated in P3HT and separated at the P3HT/graphene interface. This charge separation was induced by the built-in electric field near the interface between P3HT and graphene. Due to the high carrier mobility of graphene, photogenerated holes quickly transmitted in the channel and greatly enhanced the photocurrent. When the light was turned off, the number of photogenerated holes in the channel rapidly decreased following the recombination of the photogenerated carriers, and thus greatly decreased the photocurrent.
a rise time of 168 ms and a short decay time of 121 ms. The device exhibited good air stability by using the PMMA protective layer. In this way, it provides an easy fabrication of high mobility, sensitivity and stability phototransistor, which effectively paves a way to employ solution-processed graphene to other photoelectronic applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 61675147, 61605141, and 11674243) and the Foundation of Independent Innovation of Tianjin University (No. 0903065043).
4. Conclusions Notes In conclusion, we have fabricated a phototransistor consisting of solution-processed graphene/PGH hybrid channel. The device exhibited good photoelectric properties due to an increase in the charge transfer and optical absorption of the hybrid system. Electrical studies showed that the device exhibited the high hole mobility of 3.8 cm2 V−1 s−1 due to the high charge transport of the graphene. The R, EQE, NEP and 𝐷∗ were 12 A/W, 3 × 103 %, 1.2 × 10−10 WHz−1∕2 and 1.6 × 106 Jones at a illumination irradiance of 15 mW/cm2 , respectively. We observed that R, EQE and 𝐷∗ were strongly dependent upon the incident optical irradiance. The phototransistor exhibited a sensitive photoresponse with
The authors declare no competing financial interest. References [1] S. Kim, P. Zhao, S. Aikawa, E. Einarsson, S. Chiashi, S. Maruyama, Highly stable and tunable n-type graphene field-effect transistors with poly(vinyl alcohol) films, ACS Appl. Mater. Interfaces. 7 (2015) 9702–9708. [2] H.S. Song, S.L. Li, H. Miyazaki, S. Sato, K. Hayashi, A. Yamada, N. Yokoyama, K. Tsukagoshi, Origin of the relatively low transport mobility of graphene grown through chemical vapor deposition, Sci. Rep. 2 (2012) 337.
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Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Solution-processed graphene phototransistor functionalized with P3HT/graphene bulk heterojunction Yongli Che a , Guizhong Zhang a, *, Yating Zhang a , Xiaolong Cao a,b , Mingxuan Cao a , Yu Yu a , Haitao Dai c , Jianquan Yao a a
Key Laboratory of Opto-Electronics Information Technology (Tianjin University), Ministry of Education, School of Precision Instruments and Opto-Electronics Engineering, Tianjin University, Tianjin 300072, China b College of Mechanical and Electronic Engineering, Shandong University of Science and Technology, Qingdao, 266590, China c Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology, School of Science, Tianjin University, Tianjin, 300072, China
ARTICLE
INFO
Keywords: Phototransistor Graphene Graphene-P3HT hybrid Solution process
ABSTRACT Here we present a feasible way to fabricate a high-performance graphene phototransistor functionalized with Poly(3-hexylthiophene) (P3HT)/graphene bulk heterojunction by solution processing, which possesses the advantages of the high carrier mobility of graphene and the high visible light absorption of P3HT. The hole field-effect mobility for the device was 3.8 cm2 V−1 s−1 at room temperature. The photoresponsivity, external quantum efficiency and detectivity were 12 A/W, 3 × 103 % and 1.6 × 106 Jones at a illumination irradiance of 15 mW/cm2 respectively. The phototransistor exhibited sensitive photoresponse with a rise time of 168 ms and a decay time of 121 ms.
1. Introduction Graphene has received considerable attention for electronic and optoelectronic because of its outstanding mechanical, electrical, and optical properties [1,2]. Graphene phototransistors can realize ultrafast photodetection owing to its extraordinarily high carrier mobility [3]. However, the very low responsivity due to its weak light absorption and fast recombination rate has limited the sensitivity of graphene lightsensing devices [4]. Significant efforts have been applied to increase the absorption, among which a feasible way is to couple graphene with light-absorbing materials [5]. Poly(3-hexylthiophene) (P3HT) is known as a popular absorptive material for its large absorption coefficient in the visible range, which makes it very attractive for organic optoelectronic devices [6]. Moreover, P3HT possesses several other advantages of solution processing, low cost and good semiconducting properties [7,8]. E. H. Huisman et al. [9] reported hybrid graphene-P3HT layerheterojunction phototransistors using chemical vapor deposition (CVD) graphene on silicon/silicon dioxide, demonstrating responsivities of 10−1 A/W. The low responsivity can be enhanced by distributing graphene into the P3HT matrix, in which the graphene can act as a conducting route between the P3HT domains [10]. In this study, we report a graphene phototransistor functionalized with P3HT/graphene bulk heterojunction by solution processing, which *
combines the high carrier mobility of graphene and the high visible light absorption of P3HT. The photoresponses of the phototransistor can be greatly enhanced due to the photoexcited holes transfer from P3HT to graphene for the band alignment between graphene and P3HT [4]. Additionally, the high carrier mobility of graphene allows holes to recirculate in channel during the lifetime of the photoexcited electrons trapped in the P3HT, which can also enhance the photosensitivity of the device. 2. Experimental section 2.1. Device fabrication A heavily n-doped silicon (Si) wafer was used as the back gate electrode and a covering thermally grown silicon oxide (SiO2 ) layer of 300 nm thick was employed as the gate dielectric layer, respectively. The Si/SiO2 substrate was cleaned by using a sonication of acetone, deionized water and ethanol, sequentially. A layer of graphene was formed on the Si/SiO2 substrate by spin-coating two drops of graphene toluene solution (1 mg mL−1 ) at 2000 rpm for 10 s. Cr (20 nm)/Au (100 nm) source/drain electrodes were evaporated through shadow mask on the graphene layer deposited on the substrate, defining the channel width of 0.1 mm and channel length of 0.02 mm. Subsequently,
Corresponding author. E-mail address: [email protected] (G. Zhang).
https://doi.org/10.1016/j.optcom.2018.04.058 Received 12 January 2018; Received in revised form 23 April 2018; Accepted 23 April 2018 0030-4018/© 2018 Published by Elsevier B.V.
Y. Che et al.
Optics Communications 425 (2018) 161–165
after being exposed to air for two weeks, revealing the 𝐼DS level is slightly reduced after two weeks, confirming the good air stability of the device. Therefore, as an effective strategy, the PMMA protective layer can enhance the ambient stability of devices, which is beneficial to many aspects of device fabrication. In order to confirm the photoresponse characteristics of the phototransistor based on the graphene/PGH hybrid, we investigated the electrical variations of the device at different incident laser power intensities under the illumination of 𝜆 = 532 nm. Fig. 3(a) shows the output characteristics of the device under different illumination irradiance (𝐸e ) at a fixed 𝑉GS of −8 V. The 𝐼DS under illumination (𝐼ill ) of the device was composed of the dark current (𝐼dark ) and the photocurrent (𝐼ph ). The 𝐼ph (𝐼ph = 𝐼ill - 𝐼dark ) increased with the increase of the 𝐸e , consistent with the increasing number of the photo-generated carriers. Under light exposure, photogenerated electron–hole pairs were produced in P3HT and separated at the P3HT/graphene interface, due to the built-in electric field near the interface between P3HT and graphene. The photogenerated holes were transferred from P3HT to graphene and the photogenerated electron were trapped inside P3HT, which were caused by the field effect gating. Increasing hole concentration in the p-type channel leaded to an increase of its conductance, which greatly enhanced the current. For a phototransistor, photoresponsivity (R) is a key figure of merit for investigating photosensitivity and is defined as photocurrent generated per incoming optical power. It can be calculated by the following relation [15]:
P3HT-graphene hybrid (PGH) absorption film was deposited by spincoating the PGH solutions using layer-by-layer approach at 2000 rpm for 15 s. The P3HT-graphene hybrid solutions were prepared by mixing three parts of P3HT chloroform solution (3 mg mL−1 ) and one part of graphene chloroform solution (1 mg mL−1 ). Graphene powder was purchased from The Sixth Element, Inc. P3HT was purchased from Tixiai Industrial Development Co., Ltd. Finally, a layer of poly (methyl methacrylate) (PMMA) with about 20 nm thickness was coated as the protective layer on top of the channel by spin-coating PMMA ethyl acetate solution (4 mg mL−1 ) at 2000 rpm for 15 s. 2.2. Sample measurements The electrical properties of the phototransistor were analyzed using the Keithley 2400 semiconductor parameter analyzers. The photoresponses of the device were measured using the green laser source with 532 nm wavelength, where the optical power was tunable. A Zolix Omni-𝜆300 spectrometer was used to analyzed the absorbance of the PGH and P3HT. Scanning electron microscopy (SEM) was used to investigate the structure of the phototransistor. 3. Results and discussion Fig. 1(a) illustrates the bottom-gate and top-contact schematic structure of the phototransistor based on the graphene/PGH hybrid. A graphene/PGH hybrid film used as the channel was deposited on the Si/SiO2 substrate with Cr/Au source–drain electrodes. Fig. 1(c) shows the cross-sectional SEM image of the device, where the PGH layer were clearly observed on the Si/SiO2 substrate with the thickness of 90 nm. Fig. 1(d) illustrates the absorption spectrum of the PGH, exhibiting absorption peaks at 540 nm and 610 nm, which is similar to that of the pristine P3HT (Figure S1) As shown in Fig. 1(b), the drain voltage (𝑉DS ) was connected between the drain and the source (i.e., ground connection) electrodes; the gate voltage (𝑉GS ) was applied to the gate and the source electrodes; and the channel current (𝐼DS ) flowed from the drain to the source. The electronic characteristics of the phototransistor were measured in the dark under ambient conditions. Fig. 2(a) shows the output characteristics of the phototransistor at different back-gate voltages, exhibiting a clear transition from the linear to the saturation region when 𝑉DS increases negatively, demonstrating the dominance of holes in the channel under field-effect gating which is attributed to the p-type semiconductor P3HT. The phototransistor shows an ambipolar behavior with a sharp rise of 𝐼DS at low 𝑉GS , which is attributed to the ambipolar characteristic of the graphene. Fig. 2(b) shows the U-shaped transfer curves of the phototransistor, which confirms the ambipolar properties of the device. Due to the Dirac point is obtained upon a negative gate bias, the phototransistor displays a n-type behavior [11]. When 𝑉GS < 𝑉dirac , it indicates a p-channel phototransistor. The carrier fieldeffect mobility was then calculated from the linear regime of the transfer curve according to the following equation [12,13]: 𝜇=
𝜕𝐼 𝐿 ⋅ DS 𝑊 𝐶𝑖 𝑉DS 𝜕𝑉GS
𝑅=
𝐼ph 𝐴𝐸e
(2)
where A is the effective illuminated area. The external quantum efficiency (EQE) represents the ratio of photo-generated carriers to incident photon and is related to the R by the expression [16]: 𝐸𝑄𝐸 =
ℎ𝑐 𝑅 × 100% 𝑞𝜆
(3)
where h is the Plank’s constant, c is the speed of light, q is the electron charge, and 𝜆 is the incident wavelength. Fig. 3(b) shows the photoresponsivity (black line) and EQE (red line) as a function of the 𝐸e at 𝑉DS = −10 V and 𝑉GS = −8 V. The phototransistor exhibits a R of 12 A/W and an EQE of 3 × 103 % for the 𝐸e of 15 mW/cm2 , due to their values increase as the 𝐸e decreases. The R in this case is higher than that of the previous work [9] (10−1 A/W), due to the photo-generated electron–hole pairs can be easier to separate in P3HT-graphene bulkheterojunction than in P3HT-graphene layer-heterojunction, which lead to a larger photogenerated current. Another important parameter of a phototransistor is the noise equivalent power (NEP), which is can be expressed as [17]: 𝑖𝑛 (4) 𝑅 where 𝑖𝑛 is noise current (Fig. 3(d)). The dominate noise source is Flicker noise. The detectivity (𝐷∗ )characterizes the minimum incident light power to generate 𝐼ph and is associated with NEP according to the equation below [18]: √ 𝐴𝛥𝑓 𝐷∗ = (5) 𝑁𝐸𝑃 ∗ where 𝛥𝑓 is bandwidth. Fig. 3(c) shows the NEP and 𝐷 values of the phototransistor as a function of the 𝐸e . As the 𝐸e decreases, the value of NEP increases and the value of 𝐷∗ decreases. NEP was found to be 1.2 × 10−10 WHz−1∕2 and 𝐷∗ was 1.6 × 106 Jones at 𝐸e of 15 mW/cm2 shown in Fig. 3(c). The photoswitching characteristics of the device was measured by periodically turning light on and off under the irradiance of 45 mW/cm2 at 𝑉GS = −8 V, 𝑉DS = −8 V, revealed in Fig. 4(a). As seen, the device exhibits good on–off switching performance with excellent sensitivity, stability and reproducibility. Fig. 4(b) shows the time-resolved response of photocurrent rise and decay according to the laser being switched
𝑁𝐸𝑃 =
(1)
where 𝐶𝑖 is the gate capacitance per unit area. The hole mobility calculated from the transfer characteristic of the p-type phototransistor is 3.8 cm2 V−1 s−1 . This hole mobility is higher than that of the previously reported P3HT-graphene bulk-heterojunction phototransistor [14] due to the high mobility of bottom graphene layer, and lower than that of the reported P3HT-CVD graphene layer-heterojunction phototransistor [9] due to the higher mobility of CVD graphene. To enhance the stability of the phototransistor, we coated a layer of PMMA on the channel to isolate it from atmospheric oxygen and water. Fig. 2(c) shows the output curves of the device in ambient condition after two weeks, exhibiting good FET behavior is retained. Fig. 2(d) shows the output curves of the device as soon as being prepared and 162
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Fig. 1. (a) Schematic of the phototransistor. (b) The measurement setup of the device. (c) Cross-sectional SEM images of the device. (d) Light absorbance of PGH.
Fig. 2. (a) Output characteristics and (b) transfer characteristics of the phototransistor. (c) Output characteristics of the phototransistor after two weeks in air. (d) Output characteristics of the phototransistor measured as prepared (black curve) and after two weeks in air (red curve). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
on and off. The rise and decay time values can be extracted by fitting the temporal photoresponse curves to the exponential function. The rise response can be fitted by the following equation: 𝛥𝐼DS
[ ] = 𝐼dark + A1 e−𝑡∕𝜏1
the following equation: 𝛥𝐼DS = 𝐼dark + 𝐴2 [𝑒−(𝑡−𝑡0 )∕𝜏2 ]
(6)
(7)
The time constant 𝜏2 is 121 ms, which represents the recombination process of the photogenerated holes and electrons. The response time in this case is similar to the response time obtained for the previously reported device [9], indicating their response speed is similar.
where 𝜏𝑖 is the time constant, 𝐴𝑖 .is the amplitude weights. The time constant 𝜏1 is 168 ms, which represents the photogenerated holes transfer from P3HT to graphene. The decay response can be fitted by 163
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Fig. 3. (a) Output characteristics of the phototransistor under different radiation powers. (b) Responsivity (black line) and external quantum efficiency (red line) as a function of the irradiance. (c) Detectivity and the noise equivalent power (inset) of the phototransistor as a function of the irradiance. (d) A noise level of the device in 1 s with sample rate of 5 ms. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. (a) Time-dependent photocurrent of the device. (b) The temporal photocurrent response in one period.
The fast photoresponse characteristics of the device can be explained by its carrier transport properties. Under light exposure, photoinduced excitons were generated in P3HT and separated at the P3HT/graphene interface. This charge separation was induced by the built-in electric field near the interface between P3HT and graphene. Due to the high carrier mobility of graphene, photogenerated holes quickly transmitted in the channel and greatly enhanced the photocurrent. When the light was turned off, the number of photogenerated holes in the channel rapidly decreased following the recombination of the photogenerated carriers, and thus greatly decreased the photocurrent.
a rise time of 168 ms and a short decay time of 121 ms. The device exhibited good air stability by using the PMMA protective layer. In this way, it provides an easy fabrication of high mobility, sensitivity and stability phototransistor, which effectively paves a way to employ solution-processed graphene to other photoelectronic applications. Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 61675147, 61605141, and 11674243) and the Foundation of Independent Innovation of Tianjin University (No. 0903065043).
4. Conclusions Notes In conclusion, we have fabricated a phototransistor consisting of solution-processed graphene/PGH hybrid channel. The device exhibited good photoelectric properties due to an increase in the charge transfer and optical absorption of the hybrid system. Electrical studies showed that the device exhibited the high hole mobility of 3.8 cm2 V−1 s−1 due to the high charge transport of the graphene. The R, EQE, NEP and 𝐷∗ were 12 A/W, 3 × 103 %, 1.2 × 10−10 WHz−1∕2 and 1.6 × 106 Jones at a illumination irradiance of 15 mW/cm2 , respectively. We observed that R, EQE and 𝐷∗ were strongly dependent upon the incident optical irradiance. The phototransistor exhibited a sensitive photoresponse with
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