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Highly ordered semiconducting polymer arrays for sensitive photodetectors Xiao Wei, Hanfei Gao, Jiangang Feng, Yueyang Pi, Bo Zhang, Yu Zhai, Wen Wen, Mingqian He, James Matthews, Hongxiang Wang, Yang Li, Shimei Jiang, Lei Jiang, and Yuchen Wu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22562 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 13, 2019

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ACS Applied Materials & Interfaces

Highly Ordered Semiconducting Polymer Arrays for Sensitive Photodetectors Xiao Wei1, Hanfei Gao2,5, Jiangang Feng2,5*, Yueyang Pi8, Bo Zhang4, Yu Zhai7, Wen Wen3, Mingqian He6, James R. Matthews6, Hongxiang Wang6, Yang Li6, Shimei Jiang1,*, Lei Jiang2,4,5, Yuchen Wu2*

1State

Key Laboratory of Supramolecular Structure and Materials, College of Chemistry, Jilin

University, Changchun, 130012, P. R. China.

2Key

Laboratory of Bio-inspired Materials and Interfacial Science, Technical Institute of Physics

and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China.

3CAS

Center for Excellence in Nanoscience, CAS Key Laboratory of Standardization and

Measurement for Nanotechnology, National Center for Nanoscience and Technology, Beijing 100190, China.

4School

of Chemistry, Beihang University, Beijing 100191, P. R. China.

5University

6Corning

of Chinese Academy of Science, Beijing 100049, P. R. China.

Incorporated, One River Front Plaza, Corning, NY 14831, USA.

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

7Laboratory

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of Theoretical and Computational Chemistry, Institute of Theoretical Chemistry,

Jilin University, Changchun 130023, P. R. China.

8Key

Laboratory of Advanced Materials of Ministry of Education, School of Materials Science

and Engineering, Tsinghua University, Beijing 100084, P. R. China *E-mail: [email protected], [email protected] and [email protected]

KEYWORDS:

conjugated

polymer,

patterning,

high

crystallinity,

nanowire

arrays,

photodetector

ABSTRACT

Semiconducting conjugated polymers possess attractive optoelectronic properties, low-cost solution processability and inherent mechanical flexibility. However, the device performance is susceptible to the fabrication methods owing to their relative weak intermolecular interaction and inherent conformational and energetic disorder. An efficient fabrication technique for large-scale integration of high-quality polymer architectures is of essential importance in realizing highperformance optoelectronic devices. Here, we report an efficient method for fabrication of polymer nanowire arrays with precise position, smooth surface, homogenous size, high crystallinity and ordered molecular packing. The controllable dewetting dynamics on a template with asymmetric wettability permits the formation of discrete capillary bridges, resulting in the


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ordered molecular packing arising from unidirectional recession of the three-phase contact line. The high quality of nanowire architectures is evidenced by the morphological characteristics and hybrid edge-on and face-on molecular packing with high crystallinity. Based on these highquality nanowire arrays, photodetectors with responsivity of 84.7 A W-1 and detectivity exceeding 1012 Jones are realized. Our results provide a platform for integration of high-quality polymer architectures toward high-performance optoelectronic devices.

INTRODUCTION

Semiconducting conjugated donor-acceptor polymers experienced a boost in charge-carrier mobilities and photoexcitation quantum efficiency, arising from the delocalized π electrons along the molecular backbone,1-3 facilitating their applications toward high-performance solar cells,4,5 field-effect transistors,6,7 light-emitting diodes8 and photodetectors.9,10 However, compared to single-crystal inorganic/organic semiconductors, the conformational and energetic disorders in polymer chains restrict the coplanar π stacking of molecules and delocalized carrier transport.11,12 Disordered-free transport in polymers imposes the unique requirement of highly ordered coplanar stacking and strong intermolecular π overlap.13,14 Recently, Sirringhaus and his colleagues revealed the fundamental principles of molecular design for boosting efficient charge transport.15 Besides molecular-structure design, the relatively weak interaction between polymers introduces additional disorders in the fabrication process. For instance, the high-speed spincoating process usually generates polymer films with low packing order.16


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To assemble polymers into micro-/nano-architectures for practical device implementation, various patterning techniques, such as soft lithography,17,18 inkjet printing19-21 and nanoimprinting22 have been developed. However, those conventional patterning techniques are difficult to achieve ordered molecule packing, especially for polymers with high molecular weight. The fundamental issue is that the organic liquid patterned onto a solid surface experiences a disordered dewetting process driven by the isotropic capillary force,23,24 while ordered molecular packing demands a unidirectional force for regulating the assembly process.25 For instance, the solution shearing technique was demonstrated by Bao and her colleagues.26,27 They aligned the organic species with enhanced orientation through manipulating the flow of liquids. Capillary-trailing induced assembly method was also developed to possess the solution dewetting for controlling the oriented crystallization of organic small-molecule crystals.28 However, these methods are suitable to assemble small molecules with strong crystallinity instead of polymers with weak intermolecular interactions. Thus, simultaneous optimization of patterning and packing order in polymers still remains challenging.

For organic photodetectors, co-facial ordered molecular packing is crucial for the device performance. Compared to the free carriers in inorganic semiconductors, the inherent photoexcited excitons in organic molecules are mainly localized in single molecules, which easily recombine before dissociating into free carriers.29,30 The intrinsically excitonic nature of photocarriers in organic polymers yields low quantum efficiency compared to free-carrier


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systems.31 The disorder introduced in the polymers further deteriorates the efficiency of exciton dissociation, in which weakly overlapped π electrons inhibit the intermolecular charge transport.32,33 To circumvent the limited efficiency of exciton dissociation in organic semiconductors, the bulk heterojunctions are often employed in the optoelectronic devices for dissociating excitons and separating of carriers.9,34-38 Photodiode configuration can boost charge separation through a Schottky barrier and built-in electric field.39,40 For instance, Huang et al. developed a photodiode with nanoparticles embedded in polymer matrix, in which excitons generated in the nanoparticles are separated with electrons trapped in the nanoparticles and holes transferred to the polymer.41 However, those diode devices cannot provide optical gain, thus restricting the responsivity of photodetection. Photoconductor configuration with one-type trapped carriers and opposite-type free carriers can support a large optical gain and ultrahigh sensitivity,42-45 but the challenges of carrier dissociation and transport should be addressed.

In this article, we report the fabrication of polymer nanowire arrays with highly ordered molecular packing, strict regulated position and alignment by harnessing the capillary-bridge lithography

system.

A

donor-acceptor

conjugated

polymer,

poly{(3,7-bis(9-

octylnonadecyl)thieno[3,2-b]thieno[2',3':4,5]thieno[2,3-d]thiophene)-alt-(3,6-bis([2,2'bithiophen]-5-yl)-2,5-bis(8-octyloctadecyl)-2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione)} (P2TDPP2TFT4), is employed. The polymer solution confined between asymmetrically wettable (i.e. hydrophilic top and hydrophobic sidewalls) templates experience a regulated dewetting


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process, in which unidirectional continuous recession of the three-phase contact line promotes ordered packing of the polymer chains. The generated nanowire arrays exhibit strict alignment, precise position and homogeneous size. Grazing incidence X-ray diffraction measurements demonstrate the dramatically improved molecular packing order of those nanowire arrays compared to the spin-coated thin films. Owing to the ordered and co-facial molecular packing, the nanowire arrays exhibit high figures of merit for photodetection: responsivity of 84.7 A W-1, detectivity of 1012 Jones (cm Hz-1/2 W-1) and response time of ca. 18 ms. We also demonstrated that the photocurrent can be modulated by applying a back gate electric field, enabling the device to serve as a high-performance phototransistor.

RESULTS AND DISCUSSION

Controllable dewetting for the fabrication of polymer nanowire arrays.

The fabrication of polymer nanowire arrays was performed on a template with asymmetric wettability. The templates were fabricated by photolithography followed by reactive-ion etching on silicon plates to generate periodically arranged micropillars with defined width and separation. The asymmetric wettability (hydrophilic tops and hydrophobic sidewalls) was achieved by a selective modification process. The top surface of the micropillars was protected and

the

sidewall

regions

were

modified

into

a

hydrophobic

state

by

heptadecafluorodecyltrimethoxysilane (FAS) molecules (see schematic illustration in Figure S1). The fabrication of nanowire arrays was constructed by dropping organic solutions onto templates


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and then combining with flat target substrates (see details in Experimental Section). The successful fabrication of templates with asymmetric wettability was confirmed by measuring the water contact angles (CA) in the top and sidewall regions. The low CA of 17.2° in the top regions and high CA of 105.2° in the sidewall regions demonstrate the asymmetric wettability of topographical templates after selective modification (Figure S2).

The dewetting process of assembly system was characterized in situ by fluorescence microscopy. As shown in Figure 1a, the assembly system starts from a continuous liquid film confined between the micropillars’ tops and a glass substrate. Due to the hydrophobic sidewall, the air pockets trapped between the gaps of each two micropillars can repel the liquid and enables the capture of the liquid film under the guidance of the micropillars. With the evaporation of solvents, the fission of the liquid film generates discrete capillary bridges pinned onto the tops of the micropillars (Figure 1b). The deposition of polymer starts with the saturation of polymer in solution, which generates nanowire arrays after total evaporation of the solvents (Figure 1c). As schematically illustrated in Figure 1d-f, the capillary-bridge assembly system experiences an evolution from continuous liquid film via discrete capillary bridges to patterned nanowire arrays.

In order to investigate the dewetting dynamics, Lattice Boltzmann method (LBM) fluidic dynamics simulations were used.46 As shown in the sequential snapshots of the LBM simulations (Figure 1g-i), the break of the continuous liquid film is dominated by the controllable dewetting


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process on the micropillars with asymmetric wettability. Due to the hydrophilicity of the top surfaces of the micropillars, the three-phase contact lines (TCLs) were pinned at the edges of the micropillars. With the dewetting of the polymer solution, the liquid menisci were formed. The stability of the liquid film is weakened due to the enlarged Laplace pressure with the development of liquid menisci. Consequently, a continuous liquid film was divided into discrete capillary bridges pinned on the micropillars (Figure 1h). These discrete capillary bridges not only confine the growth position of polymer architectures, but also improve the packing order of polymer molecules through regulation of the dewetting dynamics. With the pinning of TCLs on micropillars, the recession of the TCLs on the substrate is unidirectional (labeled in Figure 1h, i). The unidirectional dewetting of liquids contributes to regulation of the packing order of the polymer molecules, which has been demonstrated in our previous works. The highly ordered molecular packing in the dewetting process of the capillary bridges results in the fabrication of high-quality nanowire arrays after totally evaporation of the solvents.

Characterizations of energy levels, morphology and crystallography.

The molecular structure of P2TDPP2TF is presented in Figure 2a. As a typical semiconducting conjugated donor-acceptor (D-A) polymer, it possesses excellent photoexcitation quantum efficiency, arising from a planar rigid conjugated structure resulting in the delocalized π electrons along molecular backbone. Incorporation of the flexible side chain increases solubility of the polymer. We measured absorption spectra and conducted ultraviolet photoelectron


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spectroscopy (UPS) to determine the energy level structure of the polymer. As shown in Figure 2b, a broad spectral absorption from 1.23 to 3.5 eV is illustrated in the absorption spectrum. The bandgap of the polymer was determined to be 1.63 eV by the first exciton-resonance peak. The UPS was employed to determine the Fermi (Ef) level and highest occupied molecular orbital (HOMO) level. Figure 2c displays the secondary electron cut-off (SECO) region and HOMO region. The Ef and HOMO were determined to be -4.22 and -4.92 eV, respectively. Combined with the bandgap, the lowest unoccupied molecular orbital (LUMO) level was determined to be -3.29 eV.

The morphology of polymer nanowire arrays was characterized by scanning electron microscopy (SEM). A typical SEM image (Figure 2d) illustrates the precise position, strict alignment and homogeneous size of polymer nanowire arrays. A zoom-in SEM image (Figure 2e) shows single nanowire architecture with smooth surface and, sharp edges, and free of grain boundaries, indicating the high quality of the fabricated structures. The smooth surface of the nanowire architecture was further validated by atomic force microscopy (AFM). The AFM (Figure 2f) topography and height diagram indicate the fabricated nanowire structures possess smooth surfaces with homogenous height of ca. 155 nm. As shown in typical photographs of nanowire arrays (Figure S3), the regulated dewetting dynamics of capillary bridges allows the fabrication of polymer nanowire arrays with large area (1 × 1 cm2). A representative low-magnification optical micrograph (Figure S4) presents high-quality


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nanowire arrays. The statistics of width and height of nanowire arrays were performed using AFM. The nanowire arrays fabricated with the polymer concentration of 5 mg mL-1, narrow size distribution is observed as the width of 243.8 (± 5.3) nm and height of 148.7 (± 4.1) nm (average values of 100 nanowires) (Figure S5).

The asymmetric wettability is crucial for the successful fabrication of high-quality nanowire arrays. For comparison, we performed the assembly of nanowire arrays on topographical templates without FAS modification. For micropillars free of FAS molecules, both the top and sidewall regions are hydrophilic (Figure S6a-c). Owing to the filling of polymer solutions in the gaps between each two neighboring micropillars, the divide of liquids and the formation of discrete capillary bridges cannot be achieved, resulting in the deposition of molecules in the gap regions between intended micropillars (Figure S6d, e). In addition, we also employed the other silicon template with asymmetrical wettability (hydrophobic tops and hydrophilic sidewalls), which was demonstrated to patterning organic small molecular single-crystal arrays (Figure S7, details shown in Supporting Information).28 However, polymer microbelts are grown in the gap regions between micropillars with low quality (Figure S8), possibly arising from the high viscosity of polymer solutions and the weak intermolecular interactions of polymers.

The concentration of precursor solution was optimized. At low concentrations of less than 2 mg mL-1, continuous nanowires are difficult to generate (Figure S9a). At high concentration


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exceeding 8 mg mL-1, the polymer molecules can deposit in the gap regions between micropillars (Figure S9b). The concentration-dependent formation ratio (Figure S9c) indicates the optimized concentration range is from 2 to 8 mg mL-1. The width of nanowire architectures can be tuned by controlling the concentrations of precursor solutions. The width of nanowire architectures can be tuned from ca. 180 nm to ca. 430 nm with the concentrations ranging from 2 to 8 mg mL-1 (Figure S10). The distance between nanowire architectures can be tailored by tuning the period of the micropillars. By employing micropillars with different distances, we can fabricate corresponding nanowire arrays with precise position and strict alignment (Figure S11). In addition, our technique is general for fabrication of different polymers. As optical micrographs and SEM images (Figure S12) illustrate, high-quality nanowire arrays of four typical polymers, poly(9,9-dioctylfluorene) (PFO), poly(3-hexylthiophene) (P3HT), poly(methyl methacrylate) (PMMA) and polystyrene (PS) were successfully fabricated.

The crystallography and molecular packing of polymers were characterized by X-ray diffraction (XRD) and grazing-incidence wide-angle X-ray scattering (GIWAXS). The XRD diagram of nanowire arrays presents a series of peaks, which can be assigned to the (00l) diffraction (Figure S13b). In stark contrast, the spin-coated thin film of polymer shows lowintensity diffraction signal, indicating its low crystallinity and disordered molecular packing (Figure S13a). The XRD results highlight that the order of molecular packing is susceptible to the fabrication method. The GIWAXS patterns of spin-coated thin films at room temperature


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and those after anneal show poor diffraction signal, which further validates their poor crystallinity (Figure 3a, S14). In contrast, the ordered molecular packing and high crystallinity of nanowire arrays are evident from the sharp diffraction spots in the GIWAXS pattern (Figure 3b). A series of diffraction spots ranging from 0.3 to 0.8 Å-1 is parallel with the outof-plane momentum transfer vector (qz), which can be assigned to the (00l) planes. With the (00l) planes parallel to the qz axis, we demonstrate the presence of edge-on stacked molecules in nanowire arrays. In addition, a broad diffraction spot centered at 1.7 Å-1 on qz axis is contributed by the π-π stacking of polymers, which indicates the existence of face-on molecular packing. To have a further insight into the crystallographic orientation of films and nanowire arrays, we extracted the X-ray scattering line profiles (Figure S15) from GIWAXS patterns. Compared with the spin-coated film, the first-order diffraction peak of nanowire arrays significantly enhances and shifts from 0.165 Å-1 to 0.187 Å-1, indicating that the polymer chains packing becomes compact with the increase of crystallinity. Consistent with the GIWAXS results, the packing of molecules is schematically illustrated in Figure 3c. The nanowire architectures consist of mixed face-on and edge-on packing of polymer chains with greatly improved ordering and crystallinity, which benefits the realization of high-performance optoelectronic devices.

To further understand how P2TDPP2TFT4 features with delocalized π electrons, theoretical computation was performed (details shown in Supporting Information). In this work, we chose


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the backbone of the monomer to investigate the geometry and electronic structural properties. The calculation shows that the backbone of monomer in the gas phase loses its planarity (Figure S16a). However, bithiophene was observed to be planar in crystal, where π-π stacking interaction pulls every single molecule in a plane. As shown in Figure S16c, when three units stack together, the planarity structure is established. Therefore, the planarity of the conjugated backbone becomes better with the increase of the crystallinity of P2TDPP2TFT4.

Photodetectors based on nanowire arrays.

Based on the high-quality polymer nanowire arrays, we measured the figures of merit of photodetectors. The nanowire arrays were fabricated on 300-nm SiO2/Si substrates, in which the SiO2 layer serves as gate dielectric insulator. A pair of metal electrodes (10/100 nm Ag/Au) was deposited directly onto the nanowire arrays (see schematically illustration in Figure 4a). The Ag was chosen as contact metal considering the match of the Fermi level of the semiconductor and the work function of the metal, which ensures good contact between semiconductor and electrode. A typical SEM image of the fabricated device presents nanowire structures between two electrodes with conductive channel length of ca. 10 μm (Figure 4b). A series of I-V curves were recorded in the dark state and under different light irradiances with sweeping voltage bias from -30 V to 30 V (Figure 4c). At voltage bias of V = 30 V, the nanowire arrays show a dark current of 5.44 × 10-10 A and increased currents with the photon input. The photocurrents, Iphoto, were calculated by, Iphoto = Ilight - Idark, where Idark and Ilight are currents in dark states and under


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light illumination, respectively. We obtained the responsivity, R, of the photodetectors at 30 V voltage bias by, R = Iphoto/P, where P is the illumination power. The irradiance-dependent photocurrents and responsivities are shown in Figure 3d. Both the photocurrents and responsivities show linear dependence on the irradiances. The champion responsivity of 84.7 A W-1 was obtained at a light irradiance of 0.035 mW cm-2. The specific detectivity (D*) at 30 V voltage bias is calculated by, D* = R (A)1/2/(2e Idark)1/2, where e denotes the elementary charge and A is the operating area of device. The D* was calculated to be 2.27 × 1012 Jones. Compared to the high-quality nanowire arrays with smooth surfaces (the root mean square roughness, Rq = 1.35 nm) and ordered molecular packing, the spin-coated thin films present high-roughness surfaces (Rq = 7.28 nm), low crystallinity and disordered molecular packing (Figure S17). We also measured the photodetection performance based on devices fabricated from spin-coated thin films. Compared to their nanowire arrays counterparts, the spin-coated films exhibit much lower responsivities with a maximum value of 0.49 A W-1 (Figure S18). These results, in good agreement with the theoretical computation, highlight the importance of ordered molecular packing and high crystallinity in realizing efficient charge transport and high-performance optoelectronic devices. Our method provides an efficient approach not only for the large-scale integration of organic semiconducting polymers into practical devices, but also for improving the device performance.


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The temporal response of photodetectors was measured by chopping the light into 5-s pulses. Figure 4e presents the temporal response of photodetectors based on nanowires. The fast rise and decay of the device indicate the polymer nanowire arrays have relatively high mobility. The response time is determined as current increasing from 10% to 90% of maximum for rise and decreasing from 90% to 10% for decay. As shown in Figure 4f, 17.6 ms and 6.2 ms can be achieved for rise and decay, respectively.

The carrier concentration in semiconductors can be modulated by applying a gate electric field, thus permitting the modulation of photodetection performance. The carrier mobility of the polymer films and the nanowire arrays is 5.13 × 10−3 cm2 V−1 s−1 and 6.48 × 10−2 cm2 V−1 s−1, respectively (Figure S19). The carrier mobility of the nanowire arrays was greatly improved due to the smooth surfaces and the higher crystallinity. To evaluate the device performance under gate modulation, we measured the dark currents and photocurrents of nanowire arrays under sweeping gate bias. The transfer curves of devices under dark and light illumination are presented in Figure 5a. This polymer exhibits p-type charge transport. Both dark currents and photocurrents can be modulated by the gate electric field. Under the applied gate electric field, the Fermi level is approaching the LUMO level of the polymer molecules, which produces a higher concentration of thermally activated carriers in the semiconductor channel. The corresponding responsivities at different gate bias are shown in Figure 5b. With the varying gate bias, nearly two orders of magnitude modulation of responsivity can be achieved.


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CONCLUSIONS

We have demonstrated an efficient technique for the large-scale assembling of high-quality polymer nanowire arrays with high crystallinity and ordered molecular packing. The templates with asymmetric wettability can effectively manipulate the organic polymer solutions, resulting in the regulated position and unidirectional dewetting of discrete capillary bridges. Owing to the high-quality nanowire arrays with eliminated grain boundaries, smooth surfaces, ordered molecular packing and high crystallinity, the photodetectors exhibit high responsivity of 84.7 A W-1 at the wavelength of 532 nm, which is much higher than that of spin-coated thin films. Therefore, our method based on controlled dewetting on the templates with asymmetric wettability not only provide a high-efficiency platform for patterning and integrating organic semiconductors for practical implementation, but also facilitates the realization of highperformance optoelectronic devices.

EXPERIMENTAL SECTION

Fabrication of polymer nanowire arrays. The selective modification of the templates with asymmetric wettability is shown in Figure S1. To fabricate polymer nanowire arrays, the solution was prepared by dissolving P2TDPP2TFT4 in p-xylene. 15 μL of the solution was carefully dropped onto the topographical template with asymmetric wettability followed by combining with a target substrate (silicon or 300-nm SiO2/Si). The assembled system was placed in 80 °C environment for 5 h to totally evaporate the organic solvents. Aside from the controlled trials


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(Figure S9-11), nanowire arrays were prepared with polymer concentration of 5 mg mL-1 through the templates with silicon micropillars of 2 μm in width, 1 cm in length, 20 μm in height, and 5 μm in distance.

Characterization. The dewetting process of the assembled system was characterized in situ by fluorescence microscopy (Vision Engineering Co., UK). To obtain clear pictures through fluorescent microscopy, we added red fluorescent dye into the polymer solution. Fermi and HOMO levels were measured using ultraviolet photoelectron spectroscopy (ThermoFisher ESCALAB 250Xi) in high vacuum of 10-8 torr with a 21.2 eV photon source. The morphology was characterized by SEM (Hitachi, S-4800, Japan) at an accelerating voltage of 5 kV. The AFM measurements were carried out on a Brüker MultiMode 8 Atomic Force Microscope. The crystallinity of the polymer samples was evaluated by XRD (Brüker D8 diffractometer) with monochromatized Cu Kα radiation (λ = 1.5406 Å). The GIWAXS patterns of the nanowire arrays and thin films were recorded on XEUSS SAXS/WAXS system with incidence the angle of 0.2°. The direction of X-ray incidence is parallel to the polymer nanowire arrays.

Device and electrical measurement. The 300-nm SiO2/Si was cleaned as substrate. The polymer nanowire arrays were fabricated by controlling dewetting on the template with asymmetric wettability. For the fabrication of devices, the solution concentration was optimized to 5 mg mL-1. Due to good stability of the polymer, thin films and nanowires are both fabricated in air. The devices were fabricated based on nanowire arrays by deposition of 10/100 nm Ag/Au


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electrodes using a shadow mask. The effective area was calculated as 30.8 μm2 by the length and width of channel, which is measured as 3.08 μm and 10 μm from SEM image, respectively. The control experiments of thin-film devices were prepared by spin-coating at 3000 rpm for 30s. The device measurements were carried out on a manual probe station (Lake Shore) in a vacuum of 10-5 torr at room temperature. A 532-nm continuous-wave laser was employed to illuminate the devices. The light power was calibrated by a silicon photodiode (Thorlabs, S130C). For the temporal measurement, the 5-s light pulses were provided by chopping the 532-nm laser with a function generator (Tektronix AFG1062). The I-V curves were measured on a Keithley 4200 semiconductor characterization system.

FIGURES


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Figure 1. Fabrication of polymer nanowires by controlling dewetting on a template with asymmetric wettability. a-c) In situ fluorescence microscope observations of the dewetting process. d-f) Schematic illustration of dewetting of organic liquids and growth of nanowire arrays. g-i) Lattice Boltzmann method simulation of the dewetting process.


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Figure 2. Energy level and morphology of polymer nanowire arrays. a) Molecular structure of P2TDPP2TFT4. b) Absorption spectrum of nanowire arrays. c) UPS of polymer nanowire arrays. The Fermi level is determined as -4.22 eV. d) A representative SEM image of polymer arrays, indicating the strict alignment, precise position and smooth boundary of nanowires. e) A zoom-in SEM image shows the high quality of a nanowire. f) AFM topography and height diagram present a nanowire architecture with smooth surface.


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Figure 3. Crystallographic orientation of polymer nanowire arrays. GIWAXS patterns of a) polymer films and b) nanowire arrays. c) Schematic representation of the polymer molecular packing in nanowire arrays. The structural formula of polymer backbone is illustrated for highlighting the direction of π-π stacking.


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Figure 4. Photodetectors of polymer nanowire arrays. a) Schematic illustration of photodetectors based on polymer nanowires. b) SEM image of photodetectors with polymer nanowires in the photoconductive channels. c) I-V curves of polymer photodetectors under dark and light illumination with different irradiance. d) Irradiance-dependent photocurrent and responsivity of a device. e) Temporal response of photodetectors under light irradiance of 39.6 mW cm-2. f) Response time of polymer photodetectors shows 17.6 ms for rise and 6.2 ms for decay.


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Figure 5. Phototransistors of polymer nanowire arrays. a) Transfer curves of polymer photodetectors under dark and light illumination. b) Gate-modulated responsivity of photodetectors.


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ASSOCIATED CONTENT Supporting Information The details of fabrication of polymer structures on templates with selective modification; Tunable position and width of polymer nanowires; Patterning of nanowires with different polymers; XRD diagrams, GIWAXS line profiles and AFM images of polymer films and nanowires; Calculated structures of backbone of monomer; Photodetectors based on polymer films; Typical transfer curves of devices based on polymer films and nanowires.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] and [email protected]

Author Contributions Xiao Wei and Hanfei Gao contributed equally to this work. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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This work was financially supported by the National Natural Science Foundation (21703268, 21633014), the Beijing Natural Science Foundation (2182081), and the Ministry of Science and Technology (MOST) of China (2017YFA0204504).


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