Highly Sensitive GrapheneâSemiconducting Polymer Hybrid

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Highly Sensitive Graphene–Semiconducting Polymer Hybrid Photodetectors with Millisecond Response Time Po-Han Chang, Yi-Chen Tsai, Shin-Wei Shen, Shang-Yi Liu, Kuo-You Huang, Chia-Shuo Li, Hei-Ping Chang, and Chih-I Wu ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00626 • Publication Date (Web): 31 Aug 2017 Downloaded from http://pubs.acs.org on August 31, 2017

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Highly

Sensitive

Graphene–Semiconducting

Polymer Hybrid Photodetectors with Millisecond Response Time Po-Han Chang,† Yi-Chen Tsai,† Shin-Wei Shen,† Shang-Yi Liu,† Kuo-You Huang,† Chia-Shuo Li,† Hei-Ping Chang,† and Chih-I Wu*,† †

Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering,

National Taiwan University, Taipei 106, Taiwan (R.O.C.) KEYWORDS:

graphene, photodetectors,

polymer,

self-assembled

monolayers,

PTB7,

photogating effect

ABSTRACT: Graphene–semiconducting light absorber hybrid photodetectors have attracted increasing attention because of their ultrahigh photoconductive gain and superior sensitivity. However, most graphene-based hybrid photodetectors reported previously have shown a relatively long response time (in the order of seconds) caused by numerous long-lived traps in these hybrid systems, which greatly restricts device speed. In this work, graphene–thieno[3,4b]thiophene/benzodithiophene polymer hybrid photodetectors fabricated on self-assembled monolayer (SAM)-functionalized SiO2 substrates are demonstrated with a maximum responsivity of ~1.8×105 A W−1 and a relatively short photocurrent response time of ~7.8 ms. The fast and highly sensitive device characteristics provide great potential in low light imaging

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applications. The hybrid photodetector on the SAM-coated SiO2 substrate shows better performance in responsivities and response times as compared with those of the device on the bare SiO2 substrate. The improved responsivities are attributed to a significant increase in carrier mobility in graphene channels by introducing SAM-modified substrates. In addition, SAM functionalization is capable of effectively removing multiple surface traps and charged impurities between graphene sheets and SiO2 substrates, which prevents the long-lived trapping of photo-carriers at graphene/SiO2 interfaces and remarkably decreases device response time.

Low-light photo-responsive devices have been extensively investigated because of their numerous applications. For instance, dim-light photovoltaic cells can achieve high power conservation efficiency under weak ambient light and can be used indoors or under poor weather conditions.1,2 In addition, to satisfy low-level light detection, for example, in traditional imaging or biomedical sensing, it is crucially important to develop highly sensitive photodetectors (PDs).3,4 Graphene, a one-atom thick carbon material with outstanding electrical properties, has practical potential in high-speed electronics and optoelectronics.5–7 For application in lowintensity light detection, graphene–semiconducting light absorber hybrid photo-responsive devices have received great attention for their superior sensitivity and extremely high photoconductive gain. In 2012, Konstantatos et al. first proposed novel graphene-based phototransistors with graphene–PbS quantum dot (QD) hybrid channels, and demonstrated an ultrahigh responsivity of 5×107 A W−1 and a photoconductive gain of ∼108 under low incident optical power (< 10 fW).4 The serious drawbacks of pristine graphene PDs, such as poor optical absorption cross section (~2.3% for monolayer graphene)8 and relatively short photo-carrier lifetime (tens of picoseconds),9 have been effectively overcome by introducing graphene-QD heterostructures. This device prototype with a high sensitivity is quite promising for extremely


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low-intensity photodetection, and stimulates more novel photo-responsive devices based on graphene–semiconductor hybrid structures. Other previously reported semiconducting light absorbers, such as QDs,10–12 organic materials,13,14 two-dimensional nanosheets,15,16 or mixed organic-inorganic halide perovskites,17–21 all provided efficient light harvesting, and have been combined with graphene to achieve highly sensitive PDs or photo-memory devices. The excellent sensitivity of these hybrid PDs is attributed to the photogating effect.4,13,17,18 As the specific type of photo-carriers separated from the photo-generated electron-hole pairs at the semiconductor/graphene interface are trapped in the semiconductor layer with a relatively long lifetime, the opposite type of photo-carriers will transfer to graphene sheets. These photocarriers in graphene channels with high mobility (~103 cm2 V−1 s−1) can recirculate many times within their lifetimes, resulting in the extremely high photoconductive gain and responsivities. However, the multiple long-lived traps in graphene–semiconducting light absorber hybrid systems greatly restrict the device operating speed. For instance, colloidal PbS QDs provide a strong optical absorption cross section, solution processability, and spectral selectivity,4,10 but the numerous electron-trap states ascribed to the high surface-to-volume ratio of QDs22 are present in the QD layers. Although the incident light is switched off, some of these photo-generated electrons (photo-electrons) remain captured in QD/graphene interfaces with a relatively long trapping lifetime. The corresponding photo-generated holes (photo-holes) are still recirculating in graphene channels within this lifetime, leading to the slow photocurrent decay. The graphene– PbS QD hybrid PDs reported by Konstantatos et al. showed a slow component of photocurrent decay of approximately 2 s.4 Similarly, Shao et al. realized ultrasensitive PDs by decorating graphene with ZnO QDs, but the relatively long lifetime of the trapped photo-holes in ZnO QD


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layers has been extracted as 2.3 s by fitting the decay trace of the transient response using a biexponential function.11 Apart from semiconducting QDs, the organometal halide perovskite, a popular material with remarkable photonic and optoelectronic properties, is regarded as one of the most promising light absorbers, and has been widely investigated in photovoltaics.23,24 Highly sensitive graphene– perovskite hybrid PDs have been achieved successfully.17–21 Nevertheless, like the aforementioned graphene–QD hybrid devices, a relatively long response time of ∼5 s was measured in perovskite nanowires/graphene PDs, which is attributed to the multiplicity of charge traps in nanowire films.19 Furthermore, other proposed graphene–perovskite hybrid devices, such as CH3NH3PbBr2I/graphene PDs18 and CH3NH3PbI3/graphene PDs fabricated on polyimide substrates,21 have presented relatively long photocurrent decay times of ∼0.75 s and ∼5.3 s, respectively. Two-dimensional transition metal dichalcogenides have been widely investigated because of their great potential in next-generation electronic and optoelectronic fields.25,26 Zhang et al. realized ultrasensitive PDs by using graphene-MoS2 heterostructures. However, the extremely long relaxation time constant of ~1350 s attributed to recombination through the defects or charged impurities in the monolayer MoS2 sheet has been extracted by using biexponential fitting.16 From the preceding discussion, it is clear that achieving highly sensitive hybrid PDs with shorter response times is of crucial importance to be practically applied in light detection. Most graphene-based electronic and optoelectronic devices, such as high-speed devices or the aforementioned hybrid PDs, were established on SiO2/Si substrates to form bottom-gate fieldeffect transistor (FET) structures. One of the crucial issues in graphene FETs (GFETs) is the substrate effect. The presence of substrates gives rise to the electron-electron and electron-


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phonon interaction originating from the underlying surface, which suggests that substrates are the major cause of limited carrier mobility.27 Although SiO2 substrates fulfill most requirements of bottom-gate FETs, in the field of graphene, their strong spatially dependent perturbation breaks the hexagonal lattice symmetry of graphene and induces mechanical strain on graphene that diminishes its carrier mobility.28,29 Moreover, the various impurities or adsorbates, such as hydroxyl groups (−OH) and dipolar molecules (e.g., water) from the ambient and the graphene wet transfer process, result in a p-type doping on graphene and charged impurity scattering, and the latter is regarded as one of the major causes that limit the electrical properties of graphene.30−35 Self-assembled monolayer (SAM) functionalization has been introduced to access the intrinsic properties of graphene, to depress charged impurity scattering, and to reduce surface phonon interaction from the underlying substrates.31−35 In this study, the octadecyltrichlorosilane (ODTS) SAM was exploited as a buffer layer to improve the interface quality between graphene and SiO2 substrates. Organic semiconductors, such as organic small molecules or polymers, are considered potential photo-active materials for photodetection applications.36 However, many previously reported phototransistors with single-layer organic channels have demonstrated limited responsivity (below 1 A W−1),36 which is attributed mainly to the relatively low carrier mobility in the organic bulks. For example, Mukherjee et al. reported F16CuPc phototransistors (mobility ~10−4 cm2 V−1 s−1) with a poor responsivity of ~1.5 mA W−1 at a drain voltage of 30 V, rise time of 10 ms, and decay time of 30 ms.37,38 Similarly, Lucas et al. investigated the photo-response of pentacene-based thin-film transistors (mobility ~10−2 cm2 V−1 s−1), and the devices showed a relatively low responsivity of ~15 mA W−1 under a source–drain bias of 30 V and a slow photocurrent rise (several seconds).39 To obtain highly photoresponsive organic PDs, Kim et al.


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reported single-crystalline organic FETs with a relatively high mobility of 1.6 cm2 V−1 s−1 and a responsivity of ~104 A W−1.40 However, extremely slow photocurrent relaxation and a photoinduced memory effect were observed in the devices owing to long-lived photo-carrier trapping in the organic bulks and at the interfaces between the device channels and dielectric layers.40,41 In this work, thieno[3,4-b]thiophene/benzodithiophene (PTB7), a low-bandgap semiconducting polymer, was employed as the photo-active material, which provides solution processability and excellent optical absorption characteristics over the full visible range42 and achieved high power conversion efficiency up to 9% applied in organic photovoltaic cells.43 We realized fast and high responsivity PDs with graphene–PTB7 hybrid channels established on ODTS-coated SiO2 substrates (ODTS substrates) with a maximum responsivity of 1.8×105 A W−1 under low-level white light illumination, which is significantly higher than that of many organic phototransistors.36–39 The photo-carriers transferred to graphene have an excellent mobility of up to ~103 cm2 V−1 s−1, which solves the problem of poor photo-carrier transport in organic bulks. Moreover, the hybrid PD on the ODTS substrate exhibits both higher photocurrents and higher responsivities compared with those on the bare SiO2 substrate; this behavior is ascribed mainly to the significant increase in carrier mobility in the graphene channels caused by introducing ODTS buffer layers. Further, the device on ODTS substrates exhibited sharp and drastic transient responses in both photocurrent rise and fall, and a photocurrent rise time of ~7.8 ms and fall time of ~16 ms were measured, which are much shorter than those of the device on SiO2 substrates (rise (fall) time of ~34.8 ms (~730 ms)). The remarkably reduced response time is attributed to the successful elimination of trap sites and charged impurities at the graphene/SiO2 interfaces by using ODTS buffer layers, which effectively depresses long-lived trapping of photo-generated carriers on substrate surfaces. The response times of our devices on the ODTS substrate are


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comparable to those of the F16CuPc phototransistors reported by Mukherjee et al.37,38 and are much shorter than most previously reported graphene-based hybrid PDs, also revealing that there are few or no long-lived carrier traps at the PTB7/graphene interfaces. The fast and ultrasensitive hybrid PDs we achieved in this work are easy to fabricate and potentially suitable in low-light imaging sensors in the visible region.

RESULTS and DISCUSSION The contact angle measurement was employed to investigate the hydrophobicity of the substrate surface. Figure 1 presents the lateral contact angle images captured by a charge-coupled device. The contact angle of a deionized (DI) water droplet on the bare SiO2 and ODTS substrate approaches 44 ± 1° and 100 ± 2°, respectively, indicating a considerably hydrophobic surface is formed on the ODTS-functionalized substrate. The three-dimensional structure and side views of a graphene–PTB7 hybrid PD are illustrated in Figure 2a. The 300-nm SiO2 was grown via a thermal oxidation process on heavily doped p-type Si wafers. Before graphene transfer, substrates were coated with ODTS SAMs on SiO2 as buffer layers with a thickness of 2.2 nm.32 Chemical vapor deposition (CVD)-grown graphene sheets on the copper foil were transferred onto the substrates by a traditional poly(methyl methacrylate) (PMMA)-assisted wet transfer process. To verify the quality of the graphene, the Raman spectra are shown in Figure S1. The intensity ratio of the 2D and G bands (I2D/IG) is ~1.8, indicating the monolayer characteristics of our CVD-grown graphene (I2D/IG > 1.6).44 The absence of the D band peak located at ~1340 cm−1 also reveals high quality and fewer defects in the graphene sheets.45 Optical microscope images of graphene transferred to SiO2 substrates also demonstrate excellent uniformity and coverage, proving that the CVD-grown graphene can occupy the entire device area (see Figure


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S2). After graphene transfer, the interdigital electrodes were deposited on graphene to form back-gate GFETs. The channel length and width of the devices were ~40 and ~5710 µm, respectively. PTB7 thin films (~40 nm) were spin-coated on graphene channels as the device active regions. The thickness of the PTB7 films on graphene was estimated by a surface profiler (refer to Figure S3). The surface morphology is bumpy near the boundaries of the graphene– PTB7 hybrid films, attributed to locally incomplete or fragmented coverage of the graphene sheets. The surface roughness and morphologies of two samples, PTB7/graphene/SiO2 and PTB7/graphene/ODTS/SiO2, were studied using atomic force microscopy (AFM) (Figure S4). The results show the low root mean square roughness (~2 nm) of both samples, revealing that PTB7 can be spin-coated on graphene to form uniform and compact heterostructures, which is advantageous for efficient light harvesting. Figure 2b shows the absorption spectrum of the graphene–PTB7 hybrid films. The result indicates that the optical absorption cross section of the PTB7-coated graphene sheet is over the full visible range. Furthermore, the profile of this spectrum shows stronger light absorption on the longer-wavelength side, which is consistent with the reported absorption feature of PTB7 polymer.42 Figure 2c shows the channel conductances as a function of gate voltage for the pristine GFETs with interdigital electrodes fabricated on bare SiO2 and ODTS substrates. The drain bias was set to be 1.5 V. Both curves exhibit ambipolar-transport properties originating from the gapless nature of graphene.46,47 The Dirac points (DPs) of GFETs on bare SiO2 and ODTS substrates are approximately 75 V and 40 V, respectively, indicating that the charge transfer phenomenon between the graphene and substrates is significantly restrained. Fewer charged impurities and residues from the graphene transfer process are intercalated at the graphene/SiO2 interfaces because of the hydrophobic surface built by the ODTS treatment, significantly reducing the DPs


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compared to GFETs on bare SiO2 substrates. Moreover, the effective elimination of these undesirable contaminants that restrict carrier ballistic transport on graphene because of dielectric surface polar phonon, surface roughness, and Coulomb impurity scattering is beneficial to increasing carrier mobility in graphene channels.31−35 The hole mobility of pristine GFETs fabricated on SiO2 and ODTS substrates is measured to be ~1500 cm2 V−1 s−1 and ~3200 cm2 V−1 s−1, respectively, at room temperature. Even though the GFETs were fabricated on ODTS substrates, the positive value of the DP is probably ascribed to the p-type doping effect from unintentional PMMA residues.32,48,49 Figure 2d shows the channel conductances with respect to gate voltages for graphene–PTB7 hybrid PDs on bare SiO2 and ODTS substrates at the identical drain bias as in Figure 2c. The devices on ODTS substrates exhibit both higher channel conductivities and lower DPs, which is similar to the results of pristine GFETs. The carrier mobilities of GFETs with interdigital electrodes on SiO2 (ODTS) substrates were reduced by a factor of ~1.8 (~1.4) after they were coated with PTB7 thin films, because some disorder or defects were probably introduced on the graphene surface during active material deposition by the solution process.4,22 The surface band bending at the PTB7/graphene interfaces is the key to separate the photoexcited excitons and to enhance photocurrents through the photogating effect. The work functions (WFs) of graphene transferred on SiO2 and ODTS substrates have been reported to be 4.5 eV and 4.25 eV, respectively, by using ultraviolet photoemission spectroscopy.32 The lower WF of ODTS samples indicates that the p-type doping effect from the water or oxygen adsorbates at the graphene/SiO2 interfaces can be effectively depressed by intercalating ODTS buffer layers between graphene and SiO2 substrates.32 In addition, the p-type nature and band structure of the PTB7 polymer have also been studied previously using field-effect


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measurements50 and photoemission spectroscopies,51 respectively. The highest occupied molecular orbital (HOMO) energy level and lowest unoccupied molecular orbital (LUMO) energy level of PTB7 have been measured to be −5.01 eV and −2.76 eV, respectively.51 The reported HOMO and LUMO values are both relative to the vacuum level (Evac). Figure 2e shows schematic diagrams of the individual band structures of graphene and PTB7 according to the previous studies.32,51 It is expected that the charge transfer from PTB7 to graphene takes place because of the Fermi-level difference between these two materials, and thus an internal electric field will be built at the PTB7/graphene interfaces, which plays a crucial role for photo-excited exciton separation. The electron transfer behavior between graphene and PTB7 can be confirmed by the DP shift of the devices. It can be observed that the DP of PTB7-coated GFETs (65 (25) V for SiO2 (ODTS)) is smaller than that of pristine GFETs (75 (40) V for SiO2 (ODTS)) (see Figure 2c and 2d). The negative shift of the DP implies an n-type doping effect to graphene due to electrons transferred from PTB7, leading to the band bending at the PTB7/graphene interfaces. However, the charges accumulated at the graphene/PTB7 interfaces caused by Fermi-level alignment give rise to Coulomb scattering in the graphene channels, which is another possible factor that may decrease the carrier mobility of graphene. Schematic illustrations of the photoinduced charge transfer (i.e., the photogating effect) in graphene–PTB7 hybrid systems on SiO2 and ODTS substrates are shown in Figure 2f. The photo-generated excitons near the PTB7/graphene interface are separated by the internal electric field, and these photo-holes transfer into graphene to change its conductivity, while the corresponding photo-electrons are trapped in PTB7 layers. The photo-response characteristics were studied on two devices, the graphene–PTB7 hybrid PD on SiO2 substrates and that on ODTS substrates, namely the SiO2 device and ODTS device,


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as described in the following section. White light-emitting diodes (WLEDs) were utilized as the light source in all photo-response measurements. The photocurrents of SiO2 and ODTS devices with respect to drain voltages at several white light intensities are shown in Figure 3a and 3b, light light dark dark respectively, which are calculated by I ph = I drain − I drain , where I drain and I drain are the drain current

of a PD under light and dark conditions, separately. The gate bias was set to be zero. That is, the devices were operated in the hole conduction region (gate voltage < DP). While these two hybrid PDs are under light exposure, the photo-holes from PTB7 films transfer into the graphene, resulting in the increased channel conductance and positive photocurrents. The positive DP shift is approximately 1 V in both SiO2 and ODTS devices under incident light intensity of 7.7 mW cm−2 (see Figure S5). The photocurrent intensities show a nearly linear correlation with the drain voltage in both devices, and the ODTS device exhibits significantly higher photocurrents than the SiO2 device under all selected light intensities. Responsivity (R) is a crucial parameter in characterizing the performance of a PD and can be defined as Iph×P−1, where Iph and P are the photocurrent and incident optical power, respectively. The photocurrents and responsivities of the SiO2 and ODTS devices with respect to incident light intensities at a given drain voltage of 3.5 V and gate voltage of 0 V are demonstrated in Figure 3c and 3d, separately. The ODTS device shows remarkably larger photocurrents and responsivities compared with those of the SiO2 device, regardless of incident light intensities. The maximum responsivity of the ODTS device is 1.8×105 A W−1 achieved at a drain voltage of 3.5 V, zero gate bias, and low optical intensity of 133 nW cm−2, which is significantly higher than many organic-based phototransistors reported previously.36−39 This is attributed to the aforementioned photogating effect.


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PC τ transit , where The photoconductive gain of graphene-based hybrid PDs is proportional to τ life PC τ life and τtransit are the photo-carrier lifetime in light absorbers and the device transit time,

respectively,11,18,22 and the responsivity is proportional to the photoconductive gain.11,22 The transit time in the device channels is given by the expression τtransit = L2(µVd)−1, where L, µ and Vd are the channel length (~40 µm), carrier mobility, and applied drain voltage, respectively.11,18,22 The hole mobility of PTB7 is ~5.8×10−4 cm2 V−1 s−1 measured from the space-charge limited current model.42 The presence of graphene channels under PTB7 films provides a remarkably fast carrier transport with excellent mobility higher than 103 cm2 V−1 s−1, which dramatically decreases the transit time (< 4.6×10−9 s at a drain voltage of 3.5 V) of the photo-carriers, and these transferred photo-holes can recirculate more times within their lifetime, leading to the high photocurrent gain and responsivity. Moreover, the ODTS device exhibits larger photocurrents and responsivities than the SiO2 device, mainly ascribed to the higher carrier mobility (shorter transit time) in the graphene channels on ODTS substrates. In addition, it is noteworthy that lower responsivities are found as the light intensity increases. While more photo-holes are transferred into graphene, an additional building field established by these photo-holes and the corresponding photo-electrons trapped in PTB7 films attenuates the internal electric field near the PTB7/graphene interface originating from the band alignment, which causes reduced capability in exciton separation and decreased responsivity as the illumination intensity increases. To show the power law of the optical-intensity-dependent responsivity, the responsivity of the ODTS device is fitted by this equation: R = A × I light B , where Ilight, R, A, and B are the light intensity, the responsivity, and two fitting constants, respectively.10 A and B were found to be 0.21158 and −0.86261, respectively (refer to Figure S6). In addition, the photoresponses of a test device without graphene are demonstrated in Figure S7, and it is observed that


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its photocurrent is several orders of magnitude lower than that of the ODTS device even under a high applied bias of 50 V, which is attributed to poor carrier mobility in the PTB7 channels (~10−4 cm2 V−1 s−1).42 Time-dependent photo-response measurements were carried out under a fixed drain bias of 1.5 V and zero gate voltage. The same WLEDs were utilized as the light source, and the illumination intensity was selected to be 7.7 mW cm−2 consistently. Figure 4a shows the normalized temporal photocurrent responses of the SiO2 and ODTS devices under an optical pulse train composed of off-and-on half cycles for 10 s. The photocurrents in both of these devices exhibit excellent reproducibility in the three cycles under a fixed illumination intensity. In addition, the photocurrent responses of the pristine GFETs fabricated on SiO2 and ODTS substrates under an identical light intensity pulse sequence are shown in Figure S8. It is observed that the photoresponse cannot be resolved in these two devices without the PTB7 coating because of the low optical absorption of monolayer graphene, which is consistent with previously reported results for pristine GFETs.19 To invetigate the substrate effect on the response time of graphene–PTB7 hybrid PDs, the photocurrent decay transient responses of the SiO2 and ODTS devices are demonstrated in Figure 4b and its inset, respectively, and the time that the light source is removed is set to be zero. The photocurrent decay of the SiO2 device contains both fast and slow components, which is similar to the decay traces of previously reported graphene-based hybrid PDs.4,10–12,14,16–21 The fast one represents the recombination of photo-holes in graphene and photo-electrons trapped in PTB7 films;10,14,52 the slow one is associated with the multiplicity of carrier traps in the hybrid system4,16,19 and the charge transfer in the light absorber.10,14,52 The ODTS device, by contrast, exhibits a sharp and drastic transient response of the photocurrent decay, and the slow component (several seconds) is effectively eliminated (refer to the inset in


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Figure 4b). In this study, the rise (fall) time of a PD is defined by the time taken for a 70% rise (drop) in photocurrent.18,19 The transient photocurrent rise and fall of ODTS device and the corresponding rise and fall times are presented in Figure 4c and 4d, respectively. In addition, the rise and fall times of the SiO2 device are indicated in Figure S9 and 4b, respectively. The relatively very short photocurrent rise time of ~7.8 ms and fall time of ~16 ms were measured for the ODTS device, which is much shorter than those of the SiO2 device (rise time of ~34.8 ms and fall time of ~730 ms). The mechanism of the significantly reduced response time of graphene–PTB7 hybrid PDs by introducing ODTS buffer layers is qualitatively illustrated in the following section. According to previous studies, numerous unintentional impurities30−35 and charge trap sites33−35 will be nonuniformly located at the graphene/SiO2 interfaces, and cause the doping effect and pervasive charge density ﬂuctuations in graphene.53 It has been reported that the SAM functionalization is capable of improving the interface quality between graphene and SiO2. The hydrophobic surface built by ODTS SAMs prevents water or other possible charged impurities from being intercalated between graphene and substrates, and successfully depresses the charge trapping at the surface of SiO2.31−35 The conductance hysteresis of pristine GFETs with interdigital electrodes on bare SiO2 and ODTS substrates is demonstrated in Figure S10 at a drain voltage of 1.5 V. This effect is related to the characteristics of charge injection at the graphene/substrate interfaces.34 The GFET on the SiO2 substrate exhibits obvious hysteresis phenomena. On the contrary, the device on the ODTS substrate displays only slight distortion between the forward and backward sweep, and no hysteresis behaviors are observed, which is consistent with previous results of GFETs on SAM-functionalized dielectric31,34 and indicates that the trapping effect at the graphene/SiO2 interfaces can be diminished by the SAM treatment.34 Exploiting


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graphene–PTB7 hybrid structures provides a large photoconductive gain through the photogating effect and photo-carrier recirculation with high drift velocity in the graphene channels. It is worth noting that these greatly enhanced photocurrents are concentrated in the one-atom-thick graphene sheet, and the contribution of photocurrent in PTB7 bulks can be reasonably neglected (refer to Figure S7). That is, nearly all photocurrents are in close proximity to the substrate surface and are susceptible to interacting with the underlying substrate. The photo-carrier behaviors in both SiO2 and ODTS devices under light switching on are illustrated in Figure 5, and Figure S11 expresses the corresponding band structures of these two devices. The photo-electrons left in the PTB7 layers and the corresponding photo-holes transfer into the graphene to generate photocurrents by the applied drain voltage. Meanwhile, a part of these excess photo-holes will directly cross the graphene sheet and be captured in these trap states and charged impurities at the graphene/SiO2 interfaces with a relatively long lifetime ( SiO 2 τ life ) because of the one-atom-thick nature of graphene that makes these holes easy to tunnel

through it (see the left panel of Figure 5 and S11). The charge transfer in these surface trap sites and charged impurities results in the relatively slower photocurrent rise. On the other hand, introducing ODTS buffer layers depresses the trapping effect at the graphene/SiO2 interfaces. Therefore, as the ODTS device is illuminated (refer to the right panel of Figure 5 and S11), nearly all the photo-holes are immediately transferred into the graphene channels within their transfer time (several ms), leading to the faster transient response. These transferred photo-holes will be rapidly collected by the biased voltage within a short transit time (~10−9 s) and generate photocurrents. For the ODTS device, the rise time is mainly dominated by the transfer time of the photo-holes from PTB7 to graphene.


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Similarly, Figure 6 schematically illustrates the photo-carrier relaxation process in both the SiO2 and ODTS devices. For the latter (see the right panel of Figure 6), as the light source is removed, the photo-holes in graphene channels directly recombine with the corresponding photoPC electrons trapped in the PTB7 films within their lifetime ( τ life ). The fall time of the ODTS device

PC and the charge transfer in PTB7 films. From the inset in Figure 4b, the is dominated by τ life

sharp and drastic transient photocurrent decay measured from the ODTS device also reveals that few or no long-lived charge traps are located at the PTB7/graphene and graphene/substrate interfaces. Nevertheless, for the SiO2 device (refer to the left panel of Figure 6), certain photoholes continue to be trapped at the graphene/SiO2 interface after the light has been switched off. A part of these photo-holes will gradually be released from trapping during the trapping lifetime SiO 2 ), return to the graphene channels, and be collected by the applied voltage, resulting in the ( τ life

slow component of the photocurrent decay (the so-called residual photocurrents; refer to Figure 4b). The capture-emission process and the charge transfer in these interface traps and charged impurities are the principal factor that slows the temporal response of graphene−PTB7 hybrid SiO 2 PDs. To estimate this long trapping lifetime ( τ life ), the slow component of the photocurrent

(

)

SiO2 decay of the SiO2 device was fitted by a simple exponential function: ∆I exp − t / τ life , where ∆I is

a constant. We find that the lifetime of photo-carriers trapped at graphene/SiO2 interfaces is ~5.38 s (see Figure S12). Like the numerous charge trap states at the light absorber/graphene interfaces that restrict the device operating speed,16,19 it is predictable that the surface traps, charged impurities, and residues non-uniformly distributed at the graphene/SiO2 interfaces will be the influential factors that cause a slower device transient response. Moreover, the capture of these photo-holes will probably reduce the number of photo-carriers recirculating in the graphene channels, leading to the decreased photocurrent gain and responsivity.


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To ensure that the slow component of the photocurrent temporal response can be effectively suppressed by using ODTS-modified substrates, 10 additional graphene−PTB7 hybrid PDs on bare SiO2 and ODTS substrates were compared. The transient photocurrent rise and decay traces of these devices are demonstrated in Figure S13, S14, S15, and S16, separately. It is observed that most temporal photocurrent traces (both rise and fall) of the hybrid PDs on SiO2 substrates show two different transient responses. A sharp response is followed by a slower one which can be attributed to the non-uniform distribution of surface trap sites and charged impurities at the graphene/SiO2 interfaces. In contrast, all hybrid PDs on ODTS substrates exhibit sharp and drastic photocurrent rise and fall, and the slow components of these transient responses of several seconds are effectively eliminated. Introducing ODTS buffer layers not only enhances the responsivities of graphene−PTB7 hybrid PDs because of higher carrier mobility in graphene channels, but also decreases the response time by significantly restraining carrier traps and charged impurities between the graphene and substrates. The time-dependent photocurrent responses of the ODTS device along with its fitting curves are shown in Figure S17. The photocurrent rise is fit with this exponential function: rise I ph = ∆I r1 (1 − exp(− t / τ r1 )) . The response time constant τr1 relating to the time duration of photo-

holes transferring from PTB7 films to graphene is 6.6 ms. Similarly, the photocurrent fall is fall = ∆I f1 exp(− t / τ f1 ) + ∆I f2 exp(− t / τ f2 ) . The short decay time fitted with the biexponential function: I ph

PC constant τf1 indicating the lifetime of the photo-electrons trapped in PTB7 layers ( τ life ) is 6.6 ms;

the long relaxation time constant τf2 representing the time duration for charge transfer in PTB7 films is 72.5 ms.10,14,52 The graphene−PTB7 hybrid PD on the ODTS substrate we achieved in this work exhibits a relatively very short rise time of ~7.8 ms and fall time of ~16 ms, ascribed to little or no long-lived trapping at the PTB7/graphene interfaces and the effective elimination of


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trap states and charged impurities between the graphene and substrates. Compared to most previously reported graphene–light absorber hybrid PDs utilizing QDs, organic materials, ZnO nanorods, MoS2, compound semiconductor films, ruthenium complex, or perovskites as the active materials, our device demonstrates even several orders of magnitude shorter response time. These device performances are listed in Table S1 for comparison.

CONCLUSION In this work, graphene–PTB7 hybrid PDs fabricated on ODTS substrates were demonstrated with an high responsivity of up to ~1.8×105 A W−1. This hybrid PD exhibits a relatively very short photocurrent rise time of ~7.8 ms and fall time of ~16 ms, mainly attributed to little or no long-lived photo-carrier trapping at PTB7/graphene interfaces and the effective elimination of surface traps and charged impurities between graphene and substrates by introducing ODTS buffer layers. The fast and ultrasensitive device characteristics provide a great latent capacity to realize low-light imaging sensors in the visible region.

METHODS Substrate preparation. The 300-nm SiO2 was thermally grown on heavily-doped p-type silicon wafers. Piranha solution, a 35:15 mixture of concentrated sulfuric acid (95–97%, SigmaAldrich) with hydrogen peroxide (30%, Sigma-Aldrich), was prepared. The SiO2 substrates were soaked in the Piranha solution for 30 min twice to remove organic residues and to hydroxylate the SiO2 surfaces. The substrates were cleaned in DI water in an ultrasonic bath to remove Piranha solution residues for 10 min three times. ODTS (≥ 90%, Sigma-Aldrich) was dissolved in anhydrous toluene (≥ 99.9%, Merck) (0.3 M) within a nitrogen-filled glove box, and the as-


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cleaned substrates were immediately immersed in the solution for 24 h. Finally, the substrates were cleaned in fresh anhydrous toluene and ethanol (≥ 99.8%, Merck) in an ultrasonic bath for 15 min each sequentially to remove unbound ODTS molecules. Device Fabrication. CVD-grown graphene on a copper foil54 was transferred to the substrates as the channels of GFETs using the traditional PMMA-assisted method. The substrates were soaked in acetone over 12 h to wash out the PMMA, and were rinsed by clean isopropanol after being lifted from the PMMA. The interdigital electrodes (Cr 5 nm/Au 80 nm) with channel length of ~40 µm and width of ~5710 µm were deposited on the graphene by thermal evaporation through a shadow mask. Before deposition of the PTB7 films, the substrates were annealed at 110 °C in a glove box to remove water absorbates on graphene surfaces. After cooling, PTB7 (99.5%, 1-Material) dissolved in anhydrous chlorobenzene (99.8%, SigmaAldrich) (10 mg/ml) was spin-coated on the graphene channels at 1500 revolutions per minute (rpm) for 60 s in a glove box. A test PD without graphene channels was fabricated by a similar process. The same interdigital electrodes were first thermally evaporated on clean SiO2 substrates, and then PTB7 was spin-coated on the target substrates as the channels of the devices. Material Characterizations. Optical microscope images of the CVD-grown graphene on SiO2 were captured by an upright microscope (Eclipse LV150N, Nikon). The absorption spectra of the graphene–PTB7 hybrid films were investigated using a UV-Vis-NIR spectrophotometer (V-670, JASCO). Graphene was transferred onto glass substrates, and then PTB7 layers were spin-coated on graphene substrates at 1500 rpm for 60 s in a glove box for absorption characteristics measurements. The thicknesses and surface morphologies of PTB7 films deposited on graphene/SiO2 and graphene/ODTS/SiO2 using the identical spin-coating process were obtained by a surface profiler (Dektak 150, Veeco) and AFM (B1022, NT-MDT).


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Device Measurements. High power WLEDs (LSD-1025, Taiwan Fiber Optics) were utilizing as the light sources for the measurements of PDs. The white light intensities were controlled by neutral-density filters (NDC-100C-4M, Thorlabs), and were collected using a power meter (Nova II, Ophir) with a measurement sensor (PD300-BB, Ophir). The periodic optical pulses for time-dependent photo-response measurements were generated by utilizing a diaphragm shutter (SHB1T, Thorlabs) controlled by a function generator (33250A, Agilent). The PDs were loaded into a high-vacuum probe system (∼10−5 Torr), and the device characteristics were measured utilizing a semiconductor parameter analyzer (Keithley 4200).

Figure 1. Contact angles of a DI water droplet on (a) bare SiO2 and (b) ODTS substrates.


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Figure 2. (a) Schematic illustration of a graphene–PTB7 hybrid PD. Inset shows the side view of the device. (b) The optical absorption spectrum of the PTB7 thin film deposited on graphene. Channel conductances of (c) pristine and (d) PTB7-coated GFETs with interdigital electrodes


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fabricated on SiO2 and ODTS substrates. (e) Band diagrams of graphene and PTB7. The schematic diagram of the surface band bending at the PTB7/graphene interface is shown below. (f) Schematic illustration of the photo-carrier transfer in graphene–PTB7 hybrid systems established on SiO2 and ODTS substrates under light illumination.

Figure 3. Photocurrents of (a) SiO2 and (b) ODTS devices with respect to drain voltages at several selected light intensities. (c) Photocurrents and (d) responsivities of the SiO2 and ODTS devices with respect to light intensities at a fixed drain voltage of 3.5 V and zero gate voltage.


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Figure 4. (a) Temporal photocurrent responses of the SiO2 and ODTS devices under an optical pulse train composed of on and off half cycles for 10 s at a drain voltage of 1.5 V and zero gate voltage. (b) Time-dependent response of photocurrent decay and fall time of the SiO2 device. The transient photocurrent decay of the ODTS device is shown in the inset. (c) Rise time and (d) fall time of the ODTS device.


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Figure 5. Schematic illustration of the photo-carrier response process in the SiO2 and ODTS devices.

Figure 6. Schematic illustration of the photo-carrier relaxation process in the SiO2 and ODTS devices.


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ASSOCIATED CONTENT Supporting Information. Raman spectra of CVD-grown graphene; Optical microscope images of graphene on SiO2 substrates; Thickness measurements and AFM images of PTB7 films deposited on graphene/SiO2 and graphene/ODTS/SiO2; Channel conductances of the SiO2 and ODTS devices under dark and illuminated conditions; The fitting curve of the responsivity of the ODTS device as a function of light intensity; Photocurrent responses of the test device without graphene; Temporal photocurrent responses of pristine GFETs on SiO2 and ODTS substrates under a light pulse train composed of off and on half cycles for 10 s at a fixed drain voltage of 1.5 V and zero gate bias; Rise time of the SiO2 device; Channel conductances of pristine GFETs with interdigital electrodes on SiO2 and ODTS substrates for forward and backward sweeps; Energy band diagrams of graphene–PTB7 hybrid systems fabricated on SiO2 and ODTS substrates; The fitting curve of the residual photocurrent of the SiO2 device; Transient photocurrent rise and fall responses of 10 additional graphene–PTB7 hybrid PDs established on SiO2 substrates; Transient photocurrent rise and fall responses of 10 additional graphene–PTB7 hybrid PDs established on ODTS substrates; Temporal photocurrent responses and fitting curves of the ODTS device; Comparison of response times and fitting time constants of the previously reported graphene–light absorber hybrid PDs (PDF). The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions


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P.-H. C. designed the experiments. P.-H. C., Y.-C. T., and S.-W. S. fabricated the devices. P.-H. C. and Y.-C. T. performed the optical absorption, electrical, and photo-response measurements. S.-Y. L. developed the ODTS modification process and measured the contact angles. K.-Y. H., C.-S. L., and H.-P. C. carried out the surface morphology and film thickness measurements. P.H. C. and C.-I. W. analyzed the data and wrote this manuscript. Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGMENT This work is supported by Ministry of Science and Technology (MOST 104-2112-M-002-014MY3). REFERENCES (1)

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For Table of Contents Use Only Insert Table of Contents Graphic and Synopsis Here Highly Sensitive Graphene–Semiconducting Polymer Hybrid Photodetectors with Millisecond Response Time Po-Han Chang, Yi-Chen Tsai, Shin-Wei Shen, Shang-Yi Liu, Kuo-You Huang, Chia-Shuo Li, Hei-Ping Chang, and Chih-I Wu* Graphene–PTB7 hybrid photodetectors fabricated on ODTS-modified SiO2 substrates are demonstrated with a high responsivity of ~1.8×105 A W−1 and a relatively very short photocurrent response time of ~7.8 ms. The excellent sensitivity of the hybrid PDs is attributed to the photogating effect. The photo-generated excitons near the PTB7/graphene interface are separated by the internal electric field, and these photo-holes transfer into graphene to change its conductivity. These photo-carriers in graphene channels with high mobility (~103 cm2 V−1 s−1) can recirculate many times within their lifetimes, resulting in the high photoconductive gain and responsivity. The shore response time is mainly attributed to little long-lived photo-carrier trapping at PTB7/graphene interfaces and the effective elimination of surface traps and charged impurities between graphene and substrates by ODTS functionalization.


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Highly Sensitive GrapheneâSemiconducting Polymer Hybrid

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Highly Sensitive GrapheneâSemiconducting Polymer Hybrid