High Performance Flexible Organic Phototransistors with Ultrashort

Article Cite This: ACS Photonics XXXX, XXX, XXX−XXX

pubs.acs.org/journal/apchd5

High Performance Flexible Organic Phototransistors with Ultrashort Channel Length Jianfeng Zhong, Xiaomin Wu, Shuqiong Lan, Yuan Fang, Huipeng Chen,* and Tailiang Guo Institute of Optoelectronic Display, National and Local United Engineering Lab of Flat Panel Display Technology, Fuzhou University, Fuzhou 350002, China

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S Supporting Information *

ABSTRACT: Organic phototransistors with high responsivity and sensitivity to light irradiance have great potential applications in environmental monitoring, space exploration, security, image sensors and healthcare systems. In this manuscript, a novel polymer bulk heterojunction field effect phototransistor with ultrashort channel length (tens of nanometers) and ultrahigh sensitivity to visible light was proposed. Due to the nanoscale channel and bulk heterojunction structure, a high-performance phototransistor with high responsivity of 750 A/W, photosensitivity of 1.0 × 106, and detectivity as high as 4.54 × 1015 Jones was demonstrated under 720 nm light illumination with 0.1 mW/cm2 intensity, which was even better that those lateral organic phototransistors. Moreover, organic phototransistors with ultrashort channel length were investigated for the first time on a flexible substrate, which exhibited outstanding mechanical flexibility due to their unique designs. Further investigation of the correlation between the morphology of bulk heterojunction blends and the device photoresponse performance indicated that the photoelectric properties of the devices could be effectively enhanced by controlling the morphology of semiconducting layers. More importantly, this work provided first clear experimental evidence that the surface area of semiconducting crystals would significantly impact dissociation of photoinduced excitons and transfer and recombination of photogenerated carriers, which is crucial for photoelectric performance of phototransistors. This work provided critical guidelines for the development of high performance flexible organic phototransistors, which opened up the doors of opportunity for organic phototransistors applications in a wide range of organic electronics. KEYWORDS: organic field-effect transistors, organic phototransistors, polymer bulk heterojunction, solvent mixtures, ultrashort channel length

W

most effective and novel strategy to enhance photosensitivity and photoresponsivity simultaneously.20,21,25,26,29 OPTs with just single semiconducting material (p-type or n-type) as channel layer, indicated that the photogenerated excitons in the channel layer can be merely separated to free carriers by the electric-field in the effective regions. Using bulk heterojunction(BHJ) composite of p-type and n-type materials as channel layers could effective enhance the dissociation of photogenerated excitons. With light illumination, the photogenerated excitons can be separated by the built-in potential in the BHJ blends, and then transferred into the electrodes. Qi et al. reported that the photoresponsivity R and the rise time of OPTs based on p-type polymer could be enhanced significantly by introducing [6,6]-phenyl C61 butyric acid methyl ester (PC61BM).20 Unfortunately, almost all the reports about the enhancement of the OPTs performance were primarily focused on selecting compatible doping materials. Nevertheless, the conventional OFETs with long channel are

ith tremendous development of conjugated polymers, organic field-effect transistors (OFETs) have gained considerable interest in organic electronics, in virtue of their promising potential applications in radio frequency-identification (RFID) tags, large-area flexible display, and sensors.1−10 Among the applications of OFETs, the organic phototransistors (OPTs), combining OFETs with photodetectors, have made great progress.11,12 Comparing with photodiodes, OPTs showed higher photosensitivity, and lower noise due to OFETs’ distinctive operation mechanism by introducing gate electric field.13,14 To obtain comparable performance with inorganic phototransistors, some valid strategies have been proposed to optimize the OPTs performance, for instance, blending semiconducting materials with dielectric polymer,15−18 and combining p-type and n-type semiconductor as donor−acceptor (D−A) heterojunction channel layer.19−31 However, for the approach of blending semiconducting materials with dielectric, the enhanced photosensitivity of OPTs usually companied with a decrease of photoresponsivity.17,18 Alternatively, blending p-type and n-type semiconductor as D−A channel layers has been found to be the © XXXX American Chemical Society

Received: May 31, 2018

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DOI: 10.1021/acsphotonics.8b00729 ACS Photonics XXXX, XXX, XXX−XXX

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EXPERIMENTAL SECTION Preparation of Materials. The p-type highly π-extended organic D−A copolymer poly[2,5-bis(alkyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-5,5′-di(thiophen-2-yl)-2,2′-(E)2-(2-(thiophen-2-yl)vinyl)-thiophene] (PDVT-8) (Mw = 50K, polymer dispersity index (PDI) = 2.4) was purchased from 1Materials with a concentration 10 mg/mL and [6,6]-phenyl C61 butyric acid methyl ester (PC61BM) was acquired from Solenne BV. The mixture of PDVT-8/PC61BM (at a weight ratio of 20:1) was dissolved in a mixture solvent of chloroform (CF)/chlorobenzene (CB). In this work, the mixture solvent was mixed with CF and CB at various volume ratio 10:0, 9:1, 8:2, 7:3, 6:4, and 5:5, respectively. The solution of PDVT-8/ PC61BM was heated at 60 °C for 2 h on a hot plate. Conductive Ag nanowires (Ag NWs; a diameter of 40 ± 5 nm and a length of 30 ± 5 μm) solution with a concentration 5 mg/mL in isopropanol was received from Suzhou ColdStones Technology Co., Ltd. and then was diluted to 0.5 mg/mL with isopropanol. The silver nanoparticles-based ink (JET-605C) was obtained from Hisense Electronics, Kunshan, China, and used as source and drain electrodes. Phototransistors (OPTs) Fabrication. Heavily n-type doped Si wafers with 100 nm SiO2 as dielectric layer were used as substrates, which were cleaned with acetone, isopropanol, and DI water, respectively, and dried with a nitrogen blower. The diluted Ag NWs solution was spin-coated on the dielectric layer at 2000 rpm for 1 min and annealed at 110 °C for 5 min as network source electrodes. Subsequently, PDVT-8/PC61BM solutions with various CF and CB ratio were spin-coated on the Ag NWs network source electrodes at 1000 rpm for 2 min as semiconductor layers and were annealed at 150 °C for 10 min. They were then immersed in chloroform partially to remove the excess semiconductor to expose out the Ag NWs network source electrode to form source contact on it later. Finally, the silver nanoparticles-based ink was inkjet-printed on the semiconductor layer and Ag NWs network source electrodes respectively as drain electrodes and source contact electrodes by a piezoelectric inkjet printing system (Microfab, Jetlab II) with a 60 μm diameter nozzle. The inkjet-printed electrodes were subsequently sintered at 120 °C for 10 min in ambient conditions. 35 V driving voltage and 1000 Hz frequency with 170 pL droplet size were selected for inkjet printing and the substrate was kept at room temperature. All of these fabrication processes were in ambient conditions. The schematic diagrams of device preparation process were illustrated in Figure S1. For the flexible OPTs, 25 μm flexible polyimide (PI) substrate was deposited by blade coating on clear glasses and subsequently annealed at 100 °C for 12 h, 180 °C for 1 h, and 300 °C for 0.5 h in vacuum oven. And then 100 nm Al2O3 buffer layer was deposited by atomic layer deposition (ALD; Beneq TFS 200−211) on the PI substrate at 180 °C deposition temperature. After that, 100 nm Al gate electrode was prepared by thermal evaporation through a sophisticated shadow mask, and 100 nm Al2O3 as dielectric layer was deposited with the same condition as mentioned above. The following Ag NWs network source electrodes, PDVT-8/ PC61BM semiconducting layer and source contact-drain electrodes were fabricated with the same process as mention before. Characterization. The electrical characteristics of the OPTs devices were carried out in ambient conditions using a

not conducive to the dissociation of excitons and carries transport. Bao et al. has proved that the responsivity decreased with increasing channel length, as the long channel would rise the probability of recombination and result in a lower collection efficiency.32 Therefore, the device photoelectric performance was supposed to be enhanced if ultrashort channel length could be achieved by structure design. Hence, there is an urgent requirement to design a new device architecture that enables downscaling of channel length to improve the performance of OPTs. Unfortunately, there have not relative reports to enhance the photoelectric properties in OPTs by rational structure design to achieve ultrashort channel length. Moreover, a tremendous body of literature has demonstrated that the morphology of semiconductor film has significant effect on the performance of OFETs, which is ascribed to the improvement of microstructural ordering or crystalline structure of semiconductor, resulting in improved charge mobility for OFETs.33−35 Meanwhile, learning from polymer bulk heterojunction solar cells, the morphology of polymer/fullerene composites are crucial for the exciton dissociation, charge transfer, and then the photovoltaic properties due to the limited exciton diffusion length of organic materials (about 10 nm).36−39 Therefore, there is a stringent demand to develop subsequent processing, for instance, thermal annealing, solvent annealing, and solvent additives to enable the morphology of the blends to evolve toward a target structure that ameliorates device performance. Unfortunately, little work has been done on the impact of the nanoscale morphology of polymer BHJ blends on OPTs performance. Herein, in this work, novel organic field effect phototransistors (OPTs) with ultrashort channel length based on bulk heterojunction (BHJ) mixture was invented. Due to the nanoscale channel length and BHJ structure, the photogenerated excitons could be efficiently separated into free carriers and vertically transferred to electrodes quickly. The devices have exhibited high photoresponsivity of more than 750 A/W, ultrahigh detectivity of 4.54 × 1015 Jones, and high photosensitivity 1 × 106 under light illumination, which is even better that those conventional lateral organic phototransistors. Moreover, we attempt to examine the impact of the morphology of BHJ mixture on the photoelectric properties in OPTs by the addition of second solvent. The evolution of the morphology of BHJ mixture is examined by grazing incidence wide-angle X-ray scattering (GIWAXS), and the development of exciton quenching efficiency and charge transfer process are observed by photoluminescence (PL) and time-resolved photoluminescence (TRPL). The results demonstrated that there is a distinct correlation between the morphology and photoelectric properties for polymer blends with the second solvent addition in OPTs. Importantly, the enhancement of photoresponse performance is dominated by the increased of the surface area of polymer crystals, which facilitates the dissociation of photoinduced excitons and transfer of photogenerated carriers and decreases the probability of bimolecular recombination. Hence, with a combination of the device architecture design and the monitoring of semiconductor morphology, the OPTs exhibit excellent photoelectric properties and mechanical flexibility, which have great potentials for the application in a wide range of organic electronics. B


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Figure 1. (a) Schematic diagram of the device structure of PDVT-8/PC61BM OPTs. The channel length (L) is defined by the thickness of the active layers. (b) Schematic of the excitation and charge-transfer process in PDVT-8/PC61BM blend. (c) Transfer characteristic curves of OPTs in dark and under illumination at various VDS. (d) Output characteristic curves of OPTs. The light intensity was 0.1 mW/cm2, and the VDS or VGS bias were −10, −20, −30, and −40 V, respectively.

PC61BM BHJ layer and the Ag drain electrode from bottom to top. The channel length L was determined by the thickness of the PDVT-8/PC61BM BHJ layer with a nanoscale thickness (135 nm). Figure 1b depicted the energy levels of PDVT-8/ PC61BM BHJ and the photogenerated charge transfer processes.20,40 With light illumination, PDVT-8 absorbed photons to form excitons, which were dissociated into free carriers at PDVT-8/PC61BM P−N junction interface driven by the built-in potential and the source-drain bias. Holes were then transferred to drain, and electrons were trapped by PC61BM quickly.21,32 To investigate the OPTs performance, the PDVT-8/PC61BM blend, which was dissolved in 100% chloroform, was first examined, and the absorption spectrum was shown in Figure S3. Figure 1c,d presented the OPTs transfer and output characteristics under various source-drain voltages. The devices exhibited typical P-type transistors characteristics in dark and high current density over 22 mA/ cm2 at source-drain bias VDS = −40 V. With light illumination of 0.1 mW/cm2, the IDS current density was dramatically increased to 38 mA/cm2, and a more than 40 V positive shift in threshold voltage Vth was observed, which was attributed to the increase of hole concentration in PDVT-8 domains. With light illumination, the photogenerated excitons were separated in the P−N junction interface and effectively transferred to the electrodes due to the ultrashort channel length, resulting in the increase of hole concentration in PDVT-8 domains. The increase in hole concentration resulted in the Fermi-level closer to highest occupied molecular orbital HOMO, which

semiconductor parameter analyzer (Keysight B2902A) in dark and under illumination. The light source system consists of a xenon lamp (Solar-500, NBeT Group Corp.) and a monochromator (Omno501, NBeT Group Corp.). The light intensity was 0.1 mW/cm2 with a wavelength of 720 nm. Flexible performance of the OPTs were characterized while the devices were tightly bent around the cylinders with various bending radius (r = 54, 27, and 18 mm). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific ESCALAB 250 Xi spectrometer. The thickness of semiconducting films was obtained from the Stylus Profiler (Dektak XT, Bruker) and the thickness of all the films were 135 ± 5 nm. The grazing incidence X-ray scattering (GIWAXS) experiments were performed at BL14B1 beamline at Shanghai Synchrotron Radiation Facility. The photoluminescence (PL) spectra were obtained from Hitachi F4600 fluorescence spectrophotometer. The time-resolved photoluminescence (TRPL) measurements were performed by fluorescence lifetime measurement system (HORIBA scientific). The image of Ag NWs network source electrodes was carried out by scanning electron microscope (Quanta 250) and shown in Figure S2.

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RESULTS Figure 1a presented the architecture of the organic field-effect phototransistors with ultrashort channel length (OPTs) schematically. The OPTs device consisted of the gate, SiO2 dielectric layer, Ag NWs network source electrodes, PDVT-8/ C


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Figure 2. Device characteristics of PDVT-8/PC61BM OPTs under various intensity illumination at VDS = −40 V. (a) Transfer characteristic curves as a function of the gate voltage, (b) responsivity R vs gate voltage, (c) photosensitivity (Ilight/Idark) as a function of gate voltage, (d) detectivity vs gate voltage. The light intensity was 0.02 (pink), 0.05 (blue), and 0.1 mW/cm2 (red), respectively.

0.02 mW/cm2 light illumination. Interestingly, R first increased and then decreased with a decrease of VGS. In addition, the maximal R as a function of VGS decreased with an increase of light intensity, which was attributed to the filling of the lowestlying, longest lived trap states that provide the highest photoconductive gain at low intensity.42 Both P and D were enhanced significantly with an increase of light intensity, resulting in the maximal P = 1 × 105 and D = 9.4 × 1014 Jones. The high performance of OPTs is ascribed to the unique vertical structure with nanoscale channel length and the PDVT-8/PC61BM BHJ system. To further investigate the impact of morphology on OPTs performance, PDVT-8/PC61BM films were prepared by a mixed solvent containing chloroform (CF) and chlorobenzene (CB) at various volume ratios. Figure 3a−f showed the OPTs transfer characteristics of PDVT-8/PC61BM blend BHJ films with various CB concentrations in dark and under illumination. All devices presented a typical P-type transistor behavior with good current saturation in the dark. The drain current density was dramatically enhanced with an increase of CB concentration up to 30%, and then gradually decreased. A maximum current density of 33 mA/cm2 was observed for the device with 30% CB. With light illumination fixed at 0.1 mW/cm2, the maximal current density of 85 mA/cm2 was obtained for the device with 30% CB, which had 124% increment than the device without CB. The increase of current density was associated with improved morphology induced by addition of

indicates that less negative gate voltage was required to align the Fermi-level to the HOMO to switch on the devices.32 The responsivity R, photosensitivity P, and detectivity D are the three critical parameters of OPTs14,41 and defined as follow. R=

P= D=

Ilight − Idark PIN Ilight − Idark Idark R (2qJd )1/2

(1)

≈

Ilight Idark

(2)

(3)

where Ilight is the drain current under illumination, Idark is the drain current in the dark, PIN is the incident illumination power on the channel area, q is the charge of an electron, and Jd is the dark current density. The transfer characteristics of OPTs in dark and under light illumination of 0.02, 0.05, and 0.1 mW/ cm2 were shown in Figure 2a, with a fixed VDS = −40 V. With light intensity increased from 0.02 to 0.1 mW/cm2, on current density was enhanced from 28 to 38 mA/cm2 and larger positive shift in threshold voltage Vth was observed. Figure 2b− d showed responsivity R, photosensitivity P, and detectivity D as a function of VGS under various light intensities. The maximal R = 820 A/W was achieved at VGS = −19 V under D


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Figure 3. Transfer characteristics curves of PDVT-8/PC61BM OPTs in dark and under illumination (a) without CB additive, and with (b) 10 vol % CB additive, (c) 20 vol % CB additive, (d) 30 vol % CB additive, (e) 40 vol % CB additive, and (f) 50 vol % CB additive, respectively. The light intensity was 0.1 mW/cm2, and the VDS bias were −10, −20, −30, and −40 V, respectively.

Table 1. Photoelectric Properties of PDVT-8/PC61BM OPTs with Various CB Concentrations (0, 10, 20, 30, 40, and 50 vol%) under 0.1 mW/cm2 Light Illumination photosensitivity (Ilight/Idark) 0% CB 10% CB 20% CB 30% CB 40% CB 50% CB

1.0 2.3 2.5 1.0 5.2 1.1

× × × × × ×

5

10 105 105 106 105 105

responsivity (A/W)

detectivity (Jones)

gain

328 534 633 750 452 315

× × × × × ×

× × × × × ×

9.1 1.7 2.1 4.5 2.5 1.3

E

14

10 1015 1015 1015 1015 1015

5.7 9.4 1.1 1.3 7.9 5.5

rise/fall time (s) 2

10 102 103 103 102 102

1.13/0.03 1.08/0.03 0.78/0.03 0.72/0.03 1.30/0.03 1.67/0.03


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Figure 4. Devices characteristics of PDVT-8/PC61BM OPTs. (a) Photosensitivity Ilight/Idark (black) and responsivity (blue) as a function of CB concentration. (b) Detectivity (black) and gain (blue) vs CB concentration. (c) Switch on (black) and switch off (blue) time as a function of CB concentration.

extracted in Figure S6 and shown as a function of CB content in Figure 4c. The results were also listed in Table 1. The switch on time was first decreased from 1.13 to 0.72 s and then gradually increased. The lowest switch on time 0.72 s was obtained at 30% CB concentration, which showed that with an appropriate amount of CB addition, the OPTs photoresponse speed was dramatically enhanced. The switch off time showed no obvious difference with 0.03 s decay time for all devices, which is ascribed to the effective elimination of the photomemory effect.16,20 More importantly, the switch on and off time is much shorter than planar OPTs, which is associated with ultrashort channel length.16,17,20 The results above clearly showed that the device performance is significantly impacted by the addition of the second solvent, which must be correlated with the morphology change with addition of cosolvent. Therefore, to understand the impact of mixture solvent on the morphology of the BHJ films, grazing incidence wide-angle X-ray scattering (GIWAXS) measurements were performed. Figure 5a showed the out-ofplane X-ray scattering profiles extracted from GIWAXS. As depicted in Figure 5a, a (100) peak at Q ≈ 3.5 nm−1 was exhibited in all curves, which is relevant to lamellar distance of PDVT-8 with two alkyl side chains. In addition, the area and the sharpness of PDVT-8 (100) peak varied with CB content, which indicated a transformation in crystalline area and crystal size with mixture solvent. The information on crystalline area, which is proportional to the area of PDVT-8 (100) peak, and crystal size possessed by Scherrer’s equation are determined by analyzing these scattering curves and depicted in Figure 5b. With an increasing of CB concentration, the crystalline area of (100) peak (A) and crystal size of PDVT-8 (ξ) were

an appropriate amount of CB. Moreover, a more than 40 V positive shift in Vth was observed of all devices, which is associated with the increase of hole concentration. R, P, and D as a function of gate voltage under a fixed light intensity of 0.1 mW/cm2 and VDS = −40 V were depicted in Figures S4 and S5 in the Supporting Information, and the results were tabulated in Table 1. All the devices showed the same trend with an increase of VGS, which first increased and then decreased. Figure 4a,b showed R, P, and D as a function of CB content. The R, P, and D all first increased with an increase of CB concentration and reached the maximum value at 30% CB, and then gradually decreased. The highest value of R, P, and D obtained at 30% CB content under 0.1 mW/cm2 were 750 A/W, 1.0 × 106, and 4.5 × 1015 Jones, respectively. Photogain G is expressed by the following equation, G = Rhν/ q, where R is the responsivity, hν is the incident photon energy, and q is the elementary charge.41 G as a function of CB content was depicted in Figure 4b, which was extracted in Figure S5. G showed the same trend as R, and the highest responsivity of 750 A/W corresponded to the highest G above 1.5 × 103 at 30% CB content, which indicated excellent photoelectric properties of the ultrashort channel OPTs. In addition, the real-time photoresponse was investigated by the change of IDS with repeated photo on/off switching. Due to the persistent photoconductivity (PPC) effect, an additional negative voltage pulse −40 V was applied to erase the PPC effect.20 As shown in Figure S6, all devices exhibited reproducible photosensing behavior. The switch on time (defined as the time for the current to rise to 80% of the maximum value) and the switch off time (defined as the time for the current to decay to 20% of the maximum value) were F


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Figure 5. (a) Out-of-plane GIWAXS profiles for all the PDVT-8/PC61BM blend films. (b) Crystalline area (A) of (100) peak (black) and crystal size (ξ) of PDVT-8 (blue) vs CB concentration, acquired from GIWAXS profiles. (c) The surface area of crystal A/ξ as a function of CB concentration.

Figure 6. (a) Photoluminescence PL spectra and (b) time-resolved photoluminescence TRPL spectra of the PDVT-8/PC61BM blend films with various CB concentrations (0, 10, 20, 30, 40, and 50 vol %).

crystals (A/ξ) gives a clear representation of the area of interface between the crystalline phase and amorphous mixtures. As shown in Figure 5c, A/ξ was first increased with an increase of CB concentration and reached maximum value at 30% CB.

significantly increased, which indicated that the high boiling point CB additive increased drying time of deposited BHJ films, allowing the formation of more and larger PDVT-8 crystals, which were also verified by the atomic force microscopy (AFM) images in Figure S7. The surface area of G


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Figure 7. (a) Schematic of flexible PDVT-8/PC61BM OPTs. (b) Responsivity (left) and photosensitivity Ilight/Idark (right) and (c) detectivity (left) and gain (right) vs bending times under various bending radii (r = 54, 27, and 18 mm).

separation, which enhanced the photoelectric properties of OPTs.36 Moreover, to meet the growing demands for the nextgeneration organic electronics with light weights, excellent implantability and portability, highly flexible OPTs were fabricated. As shown in Figure 7a, flexible OPTs were fabricated on the polyimide (PI) substrate. As shown in Figure 7b,c, the flexible devices exhibited excellent photoelectric properties with R = 718 A/W, Ilight/Idark = 4.5 × 105, and D as high as 4.1 × 1015 Jones before bending. More importantly, the photoresponse performance of devices only slightly decreased as the bending radius decreased from 54 to 18 mm and only a slight decrease was found for the devices even with 1000 bending cycles, indicating great flexibility and excellent mechanical stability of the devices.

To examine the influence of CB addition to the vertical phase profiles of PDVT-8/PC61BM BHJ layers, X-ray photoelectron spectroscopy (XPS) measurements were carried out and the normalized S 2p spectrum of the surface of the semiconducting layers were presented in Figure S8. All the samples showed a similar S 2p spectrum with similar peak area, which indicated that similar material composition was on the top surfaces and the CB additions did not impact the vertical phase profiles of PDVT-8/PC61BM BHJ layers.43 Steady state and time-resolved PL were performed to further examine the improvement of device performance enabled by the mixture solvent strategy. The PL spectrum of all the samples was presented in Figure 6a, and the PL behavior was compared at their PL peak at about 827 nm. As CB content increased, PL intensity first decreased significantly and then gradually increased. The lowest PL intensity was measured at 30% CB concentration, which indicated that 30% CB addition could deliver most efficient exciton dissociation and charge transfer at the P−N junction interfaces. The time-resolved PL (TRPL) was employed at the peak wavelength of PL spectrum to further investigate the dissociation of photogenerated excitons and charge transfer as depicted in Figure 6b. The lifetime of thin films without CB was 4.07 ns, gradually decreased to the lowest value of 0.72 ns for the sample with 30% CB addition, and then gradually increased. The detailed decay time of all blend films could be obtained in Table S1 in Supporting Information. The shorter decay time showed the higher exciton quenching efficiency and efficient charge

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DISCUSSION The present research on OPTs are primarily concentrated on selecting compatible doping materials to improve their photoelectric properties, but overlooked the effect of the long channel length of lateral OPTs on the photosensing mechanism. It has been proved that the long channel length rises the bimolecular recombination and results in a lower collection efficiency, which dramatically decreases the photoresponse performance.32 Hence, there is an urgent requirement to design a new device architecture that enables downscaling of channel length to improve the performance of OPTs. In this work, novel polymer BHJ phototransistors with nanoscale channel length (OPTs) were reported. Due to the ultrashort H


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Table 2. Summary of the Key Parameters of the OPTs materials

responsivity (A/W)

Ilight/Idark

gain

detectivity (Jones)

ref

ZnPc/C60 PTB7:P(NDI2OD-T2) P3HT:THBT-4ht CuPc C8-BTBT BPE-PTDCI pentacene/PC61BM DNTT/PCDTBT:PCBM pentacene/PTCDI-C8 PDVT-8/PCBM

11 14.2 14 430 393 2.5 0.33 480 1 × 10−2 750

1 × 104 50 1 × 103 2.2 × 104 1 × 104 20 1 × 102 1 × 102 5 × 104 1 × 106

27.5 30 28 1420 1400 600 0.8 1200 N/A 1.5 × 103

N/A N/A N/A N/A N/A 1.9 × 1013 N/A N/A 1.26 × 109 4.5 × 1015

27 24 23 46 17 11 31 19 26 this work

PDVT-8 crystal phase and the mixing phase of amorphous PDVT-8 and PCBM. The existence of mixing phase would provide ample P−N junction interfaces, which would facilitate the exciton to dissociate into individual electron and hole. Meanwhile, the coexistence of hole and electron in the mixing phase would enhance the probability of bimolecular recombination, which deteriorated the photoelectric performance. Therefore, the presence of PDVT-8 crystal phase would provide pathway for effective hole transport to electrode and reduce the probability of bimolecular recombination. More importantly, an increase of the surface area of PDVT-8 crystals, in other words, the interfacial area between PDVT-8 crystals and mixing phase, would provide more opportunities for holes to effectively transport from the “dangerous and slow” mixing phase to “safe and fast” PDVT-8 crystal phase, resulting in an enhanced hole transport and reduced bimolecular recombination, leading to the enhanced photoelectric performance. Moreover, the findings were further verified by PL and TRPL results. Both PL and TRPL spectrum displayed a pronounced quenching with CB additives, which indicated the occurrence of a charge transfer process from the excited PDVT-8 to PC61BM.20,44,45 With the appropriate amount of CB addition, the PL and decay time intensity decreased dramatically, which indicated the dissociation of photogenerated excitons at the PDVT-8/PC61BM interfaces was enhanced. The weakest PL intensity and fastest decay time was observed with 30% CB addition, indicating highest exciton quenching efficiency and fastest charge transfer at this condition. With light illumination, the photoinduced electrons were trapped in the disperse PC61BM domains and the holes were transferred from mixing phase to PDVT-8 crystalline phase and then to electrode. An increase of surface area of PDVT-8 crystals provided more pathway for exciton dissociation and efficient hole transport, resulting in improved photoelectric performance.

nanoscale channel, the carriers only vertically traveled an extremely short distance to reach the electrode, which enabled photogenerated carriers to circulate in channel many times before recombination and reduced the scattering induced by structural defects and gain boundaries. In addition, the ultrashort channel could effectively enhance the electric field across the channel, which increased the carriers drift velocity and consequently the photocurrent, resulting in the improvement of OPTs performance.32 Meanwhile, PDVT-8/PC61BM BHJ structure provided ample P−N junction interfaces for the dissociation of photogenerated excitons. Therefore, high photoelectric performance OPTs were obtained, which were even better than those traditional lateral organic phototransistors summarized in Table 2. The excellent performance of OPTs was attributed to the unique architecture with nanoscale channel length and the PDVT-8/PC61BM BHJ structure, which facilitated exciton dissociation, carriers transfer, and thus, enhanced photoresponse properties. Moreover, the devices exhibited excellent mechanical flexibility along with outstanding photoelectric properties. The superior mechanical stability is attributed to the unique structure, for which the cracks formed during bending has negligible effect on the carriers transfer. As illustrated in Figure S9a,b, comparing with the lateral transport of carriers in conventional OFETs, the carriers in this device vertically transfer along the semiconducting polymer, which is inconsiderably affected by the cracks and dislocations inside the channel induced by mechanical bending. Further investigation of the impact of semiconducting morphology on the photoresponse performance of OPTs clearly demonstrated that the photoresponse performance of OPTs could be effectively enhanced by the mixture solvent, as summarized in Table 1. The further investigation demonstrated that the photoresponse performance of OPTs was significantly impacted by BHJ morphology, which is tuned by the mixture solvent. The best device performance including detectivity, photogain, photosensitivity, and responsivity, was achieved with addition of 30% CB. The GIWAXS results indicated that the crystalline area and crystal size of PDVT-8 increased with addition of CB. The addition of CB prolongs the drying process of film formation, which provides more time for self-assembly process of semiconducting polymer and crystallization, leading to the formation of larger crystals along with enhanced crystallinity. A change in crystalline area and crystal size with the addition of CB leaded to a transformation of surface area of crystals. It increased with addition of CB and reached maximum with addition of 30% CB, which showed similar trend with photoelectric performance of BHJ devices. There are at least two phases in PDVT-8/PCBM mixture: the

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CONCLUSION In conclusion, a novel flexible organic phototransistor with ultrashort channel length was invented for high performance OPTs, which exhibited outstanding photoelectric properties. The unique device architecture and BHJ structure can readily enable OPTs devices with nanoscale channel length to ensure high photoresponsivity and excellent Ilight/Idark and detectivity, which are better than the reported conventional lateral organic phototransistors. Moreover, this structure is more compatible for highly flexible electronics, as the carrier transport is inappreciable affected by the in-plane cracks inside the semiconducting layer, providing excellent mechanical stability. I


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More importantly, the impact of semiconducting morphology on the photoresponse performance of OPTs was investigated. The morphology of BHJ mixture would be efficiently modified during film deposition by the judicious addition of second solvent, leading to improved photoelectric properties. Furthermore, this work fist demonstrated that surface area of semiconducting crystals in BHJ blends are crucial for the dissociation of photogenerated excitons and efficient hole transport, resulting in improved photoelectric performance. Hence, this work demonstrated a novel flexible organic phototransistor (OPTs) with excellent responsivity, photosensitivity, and detectivity along with outstanding mechanic stability, which met the practical requirement of photodetectors applications. Moreover, the judicious addition of a cosolvent provides a level of morphological control for BHJ mixtures, especially the surface area of semiconducting crystals, which offers guides for the morphology evolution in polymer blends OPTs and a promising simple process for high performance phototransistors.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.8b00729.

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Further information relating to schematic illustration of the fabrication process of the OPTs, the responsivity, photosensitivity, detectivity, photogain, and the real-time response characteristics of OPTs, the XPS spectrum of semiconducting films, and the schematic illustration of carrier transport and AFM images (PDF).

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Huipeng Chen: 0000-0003-1706-3174 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are grateful for financial support from National Key Research and Development Program of China (2016YFB0401103) and National Natural Science Foundation of China (51503039). The GIWAXS was granted by BL14B1 station of Shanghai Synchrotron Radiation Facility. We gratefully thanked the staff members of BL14B1 for their help with experiments and data reduction.

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ACS Photonics

Article

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High Performance Flexible Organic Phototransistors with Ultrashort

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