High‐Performance Organic Phototransistors Based on Single‐Crystalline Microwire Arrays

High‐performance organic phototransistors (OPTs) have attracted considerable attention owing to their high photoresponse and low‐cost solution‐processing manufacturing. To meet the increasing demand for integrated optoelectronic circuits, organic single‐crystalline micro‐/nanowire arrays for OPTs construction are prominently anticipated. However, the manufacturing and patterning of organic single‐crystalline arrays have hit a bottleneck due to the uncontrollable dewetting dynamics. In this work, a capillary‐bridge lithography strategy is proposed to guide ordered nucleation and unidirectional dewetting of microfluid, thus enabling the large‐scale preparation of highly aligned organic single‐crystalline microwire arrays. Taking advantage of efficient carrier transport, a competitive average field‐effect hole mobility (μ) of 6.64 cm2 V−1 s−1 is obtained, and the high‐throughput one‐dimensional (1D) arrays based OPTs also exhibit excellent optoelectrical performance with photosensitivity (P) of 1.36 × 106, responsivity (R) of 3.18 × 104 A W−1, and specific detectivity (D*) of 9.22 × 1014 Jones. This work provides a guide for the designing and patterning of high‐throughput OPTs toward multifunctional integrated optoelectronics.

Generally, high  and low dark current (I dark ) are required to achieve the high R, P, and D * for high-performance OPTs, which is not only affected by the intrinsic physicochemical property of activelayer organic material itself, but also the morphology and crystallinity of the photoactive layer. [4,7,9,19,20][23] In contrast, organic single-crystalline 1D microarrays show a promising advantage for highly sensitive photodetection.][31][32][33] Furthermore, microarray phototransistor possessing suppressed pixel crosstalk can be integrated monolithically with external components to realize high-throughput active-matrix sensor arrays.[36][37] However, for the common solutionprocessing strategies to pattern organic single crystals (e.g., inkjet printing and dip-pen lithography), [32,[38][39][40][41][42][43] it is difficult to control the capillary flow in the dewetting process due to the coffee-ring effect, yielding the uncontrollable crystallization with inevitable random crystal-growth orientation and morphology of crystal domains.Therefore, it is imperative to develop an efficient strategy for controllable manufacturing and patterning of organic 1D microarrays with high crystallinity and pure crystallographic orientation toward high-performance OPTs.
Herein, we present a capillary-bridge lithography strategy for efficient large-scale patterning of 6,13-bis (triisopropylsilylethynyl)pentacene (TIPS-PEN) single-crystalline microwire arrays.With the assistance of asymmetric-wettability micropillar templates to manipulate the nucleation and orientated growth of organic crystal, high-quality single-crystalline 1D microarrays with strict alignment, homogeneous size, and ordered crystallographic packing can be successfully fabricated.The synergy of single-crystalline nature and the optimal packing orientation resulted in high average hole mobility of 6.64 cm 2 V −1 s −1 and low dark current of organic field effect transistors (OFETs) based on the TIPS-PEN single-crystalline 1D microarrays.As a result, microwire array OPTs showed a superior optoelectrical characteristics with P of 1.36 × 10 6 , R of 3.18 × 10 4 A W −1 , and D * of 9.22 × 10 14 Jones compared to their polycrystalline film counterparts.Our work provides a new platform for constructing high-performance phototransistors, and accelerating the development of integrated optoelectronics.

Fabrication of 1D Organic Single-Crystalline Microwire Arrays
To pattern large-scale 1D organic arrays toward the highperformance phototransistors, a capillary-bridge lithography strategy was employed to steer the dewetting and crystallization process by introducing topographical template composed of periodically arranged micropillars with hydrophilic tops and hydrophobic sidewalls (Figures S1 and S2, Supporting Information).Briefly, the capillary-bridge assembly system consists of TIPS-PEN solution drop, target substrate, and micropillar template, in which the asymmetric-wettability micropillar template facilitates the formation of discrete capillary bridges pinned on the micropillars enabling directional capillary flows for mass transportation, crystal nucleation, and further crystal growth, thus yielding the long-range-ordered assembly of microwires.To gain further insights into the dewetting dynamics and crystallization process, in situ observation of assembly processes was carried out utilizing a fluorescent microscope (Figure S3, Supporting Information).The organic solution first formed thin liquid film and was trapped between the hydrophilic pillar tops and the target substrate, giving rise to a sandwiched configuration assembly system.With the evaporation of solvent, the continuous liquid film split into discrete bridges pined by micropillars, which is driven by the capillary force and Laplace pressure, thus confining the organic crystal growth site.With further evaporation, capillary bridges receded along the micropillars and became supersaturated, steering the nucleation and directional growth of organic crystal.Consequently, 1D organic microwire arrays were achieved with highly aligned, controlled position, and high crystallinity after the complete evaporation of solvents.Considering the significant role of the topographical template, we further investigated the impact of the micropillar template wettability on the quality of the fabricated 1D organic microarrays.The results reveal that TIPS-PEN molecules can only assemble into disordered pattern using the symmetricwettability micropillar templates with hydrophilic or hydrophobic tops and sidewalls (Figures S4 and S5, Supporting Information), demonstrating the critical role of the asymmetric wettability of micropillars in dominating the controllable fluid dynamics and long-range-ordered assembly of microwires.Furthermore, as shown in Figure S6 (Supporting Information), the width of the TIPS-PEN microarrays is tunable by changing the template parameter.

Morphological and Crystallographic Characterization
To evaluate the quality of the fabricated TIPS-PEN (Figure 1a) 1D microwire arrays, a systematic characterization of morphology and crystallography were performed.As shown in Figure 1b and Figure S7 (Supporting Information), the optical microscopy images present highly ordered organic microwire arrays with homogeneous morphology over the large area.The scanning electron microscope (SEM) was employed to further demonstrate the high crystallinity of ordered organic microwire arrays with strict alignment, controlled position, and homogeneous size (Figure 1c).Zoom-in SEM image further manifests that the single microwire possesses a smooth surface and straight boundary (Figure S8, Supporting Information).The corresponding energy dispersive spectrum (EDS) mappings of the TIPS-PEN microwire arrays present the homogeneous distribution of C and Si elemental elements.A typical atomic force microscopy (AFM) in Figure 1d illustrates the ordered organic microwire with smooth surfaces and sharp edges, 190.2 nm in height, and 1.9 μm in width, confirming high-quality organic microwire arrays.Figure 1e displays the parallel-polarized and cross-polarized optical microscopy of microwire arrays observed by polarization microscopy.As the polarization angle rotates from 0°to 45°, all 1D crystals extinguished cross-polarized fluorescence simultaneously, reflecting the single-crystalline property of the long-range ordered microwire arrays with pure crystallographic orientation.The ultraviolet-visible (UV-vis) absorption spectrum of TIPS-PEN single crystal is displayed in Figure 1f, showing an intense absorption peak at 365 nm.
The crystal morphology prediction is obtained by BFDH (Bravais-Fridel and Donnay-Harker) algorithm.The predicted crystal morphologies reveal the preferred orientation [010] in the crystal shape (Figure 2a).To verify this prediction, we carried out X-ray diffraction (XRD), transmission electronic microscopy (TEM), selected area electron diffraction (SAED) and grazingincidence wide-angle X-ray scattering (GIWAXS) to examine the in-plane and out-of-plane orientations of TIPS-PEN microwire arrays.Figure 2b displays the out-of-plane XRD pattern with smooth baseline and sharp diffraction peaks, indicating the high crystallinity of microwire arrays.According to the data simulated from the bulk crystal, the primary diffraction peak at 5.33°is well assigned to (001) diffraction peak, while the multi-order diffraction peaks at 10.67°, 16.03°, and 26.86°corresponding to (002), (003), and (005), respectively, reflecting that the ab plane of microwire arrays is parallel to the substrate (Figure 2b,d). [44]As shown in Figure 2c, TEM image reveals a smooth surface and sharp edge of individual microwire, and the sharp and discrete diffraction spots can be observed in the corresponding SAED pattern, verifying the single-crystalline nature of microwire.Combining the TEM and SAED results with the crystal structure data of TIPS-PEN, it is clear that the microwires grow along with the [010] orientation, coinciding with the direction of - molecular packing, which is favorable for the hole transport (Figure 2d). [45,46]As presented in Figure 2e, the sharp and discrete diffraction Bragg spots can be clearly seen in GIWAXS pat-tern, which is further confirmed the high crystallinity of highly ordered microwires and the out-of-plane direction of [001] crystal orientation.Therefore, the TIPS-PEN 1D arrays fabricated by capillary-bridge lithography strategy were demonstrated with intrinsically defect-free single-crystalline nature, which would facilitate the in-plane carrier transport and quality contact between the microwires and the electrodes for high-performance devices with high mobility.
To evaluate the electrical properties of the TIPS-PEN 1D singlecrystalline array, we constructed typical bottom-gate top-contact configuration OFETs on SiO 2 /Si substrates with Au (80 nm) as source and drain electrodes.The corresponding SEM image was shown in the Figure S9 (Supporting Information).Figure 3a shows the typical transfer curve of a representative TIPS-PEN OFET measured in the dark with a gate voltage (V GS ) modulated from +20 to −40 V.It exhibits the typical p-channel transistor characteristic with a saturation hole mobility and a current on−off ratio determined to be 6.78 cm 2 V −1 s −1 and 1.3 × 10 7 , respectively.As shown in Figure 3b, the output curves further confirm the p-channel transistor behaviors of the present OFET, and reveal the good contact between the semiconductors and electrodes as the source−drain currents (I DS ) increase linearly with the increase of source−drain voltages (V DS ) at the low V DS stage.As statistical histogram presented in Figure 3c, the average hole mobility among 25 devices is calculated to be 6.64 cm 2 V −1 s −1 , and the highest mobility is up to 8.32 cm 2 V −1 s −1 , indicating the efficient carrier transport in high-quality TIPS-PEN 1D singlecrystalline arrays.Note that the mobility of these devices falls within a narrow range of 5.38 to 8.32 cm 2 V −1 s −1 , which further demonstrates the high uniformity of 1D single-crystalline arrays fabricated by our capillary-bridge lithography strategy.As a comparison, further investigation of OFETs based on their spincoated film counterparts was carried out.The SEM result shows that the spin-coated thin film features a rough surface, large grain boundary (Figure S10, Supporting Information), which is in notable contrast with the TIPS-PEN 1D single-crystalline arrays with high crystallinity.The poor crystallinity may increase the undesirable scattering and nonradiative recombination of carriers, thus resulting in significant degradation of device performance.Figure 3d,e exhibit typical transfer and output characteristics of the polycrystalline film based OFET.Additionally, the average hole mobility of only 0.98 cm 2 V −1 s −1 is demonstrated among 25 thin film devices with the mobility varying from 0.53 to 1.36 cm 2 V −1 s −1 (Figure 3f), which is one order of magnitude lower than that of corresponding 1D single-crystalline array based device.The TIPS-PEN 1D array OFETs exhibit higher mobility compared with their polycrystalline film, which is attributed to the optimal hole transport along with the - stacking direc-tion in high-quality TIPS-PEN 1D microcrystals fabricated by our capillary-bridge lithography strategy.
In view of the basic transistor performances with high mobility facilitating the exciton separation, we further investigated the photoelectrical properties of TIPS-PEN 1D single-crystalline arrays by the construction of OPTs.As shown in Figure 4a, the typical transfer characteristic of the p-type OPTs was measured under dark condition and 365 nm light illumination with tunable intensities.By increasing the illumination intensity, I DS noticeably increase, and the threshold voltages (V th ) shift toward a more positive direction, implying a prominent photoresponse.This could be ascribed to the effective reduction of the hole injection barrier from the source electrode, which is caused by the rapid separation of photogenerated holes and electrons and accumulation in the source/leakage electrodes under the inner electric field.Therefore, the increasing hole concentrations occur within the TIPS-PEN channel as compared with the case without illumination, giving rise to the increasing in I DS and the positive shift in V th under illumination.As an important figure of merit of phototransistors, the P related to the sensitivity can be calculated as  S11a (Supporting Information), it is observed that the P trends to increase first and then decline as the V GS sweeps from −40 to +20 V. Additionally, P increases significantly with the increase of illumination intensity, and reaches a value up to 1.55 × 10 4 even with illumination intensity as low as 58.9 μW cm −2 .Figure 4b depicts the P of the phototransistor as a function of incident illumination intensity, and the maximum value of P up to 1.36 × 10 6 at V GS of 12 V and illumination intensity of 1321.5 μW cm −2 could be obtained, which can be attributed to the suppressed defect and eliminated grain boundary in high-quality 1D arrays contributing to the low I dark and high I light .The other important parameter R refers to the photoelectric conversion efficiency of OPTs, which is defined as R = (I light − I dark )/(P inc S), where P inc and S are the incident illumination intensity and the channel area exposed to light, respectively.When V GS decreases toward the negative bias direction, R increases at each light intensity, because the photogenerated electron−hole pairs can be effectively separated with the assistance of the vertical electric field (Figure S11b, Supporting Information).Note that the R is gradually reduced as the incident light intensity increased at V GS = −40 V, which is attributed to increasing exciton recombination at higher incident light intensity.Particularly, TIPS-PEN 1D arrays show a maximum value of R as high as 3.18 × 10 4 A W −1 at V GS of −40 V and the light power of 58.9 μW cm −2 .The excellent photoresponsivity can be attributed to the high mobility and strong electron-trapping capability of the crystallographicordering structure of TIPS-PEN 1D single-crystalline arrays.The most critical parameter for evaluating the photodetection, D * , describes the photosensitivity and thus distinguishes the minimum optical signal from the background noise.Assuming that the shot noise from the dark current mainly contributes to the noise current, D * is given by D * = RS 1/2 /(2eI dark ) 1/2 , where e is the elementary charge.The D * of the OPTs as functions of V GS are plotted in Figure S11c (Supporting Information), exhibiting a similar trend with P. Owing to the high photocurrent and ultralow noise current, a maximum D * up to 9.22 × 10 14 Jones is obtained at V GS = 12 V and the light power of 1321.5 μW cm −2 (Figure 4c), which is comparable to that of most reported OPTs.As a result, our photodetector shows significant improvement over reported device performances (Table S1, Supporting Information).
To verify the high-sensitivity photoresponse of high-quality TIPS-PEN 1D array, we also carefully measured the photoelectrical characteristics of OPTs based on spin-coated polycrystalline film counterparts under the same condition.Film based OPTs show a similar transfer characteristic with 1D arrays, that is, the I DS increases with increasing illumination intensity, along with a positive shift of threshold voltage (Figure 4d).Besides, the trend of critical parameters P, R, D * with illumination intensity or V GS in film based OPTs has similar characteristics to that of 1D array based OPTs, as plotted in Figure 4e,f and Figure S12 (Supporting Information).It is worth noting that film based OPTs exhibit a lower I light and higher I dark compared to that of 1D arrays.As a result, the lower R of 504.4A W −1 , and D * of 3.04 × 10 13 Jones are observed in film based OPTs, which is much lower than that of array based OPTs.We consider the following two features contributing to the higher mobility of TIPS-PEN 1D array based OPTs to achieve the excellent photoelectronic performances (Figure 4g,h): i) Single-crystalline arrays feature long-range periodic ordered structures with eliminated defects and grain boundaries, which significantly suppress the carrier scattering and nonradiative recombination.ii) TIPS-PEN -conjugate cores adopt a slipped face-to-face packing structure along the long axis of 1D microwire arrays, which increases the overlap of the electronic wave functions between the packed molecules, facilitating the photocarrier transport along the microwires.By taking into account large-area organic 1D single crystal arrays with accurate positioning, uniform size and alignment, in conjunction with high-performance organic 1D array photodetectors, we con-ducted a thorough investigation into imaging applications.The photodetectors based on 15 × 16 pixels showed high accuracy and successfully achieved the imaging of letters, including "J", "H", and "L" (Figure S13, Supporting Information).

Conclusion
In summary, we have developed a capillary-bridge lithography strategy for patterning TIPS-PEN 1D single-crystalline microwire arrays with controlled geometry, position, and size in large area.The fabricated 1D arrays feature high crystallinity and ordered crystallographic alignment, promoting the charge transport ef-ficiency.The corresponding 1D array based OFETs show an excellent electrical performance with high average hole mobility of 6.64 cm 2 V −1 s −1 .Encouraged by the significantly improved mobility and reduced dark current, high-performance OPTs were successfully achieved with high photoresponse with P 1.36 × 10 6 , R of 3.18 × 10 4 A W −1 , and D * of 9.22 × 10 14 Jones, which are superior to the corresponding film-based devices.We believe these results are likely to provide a promising platform for single-crystalline array phototransistor in future developments of multifunctional integrated optoelectronics.

Experimental Section
Materials: TIPS-PEN was purchased from Sigma-Aldrich.All materials were used without further purification.
Fabrication of Topographical Templates: N-doped silicon wafers with diameter of 10 cm, thickness of 525 μm, and <100> orientation were used as template carriers.To fabricate periodic micropillar-structure substrates, a direct laser writing apparatus (Heidelberg DWL200) was adopted to transfer the computer predefined design onto the photoresist (Shipley Microposit S1800 series) coated wafers with ≈1 μm precision, then followed by UV irradiation and etching with fluorine-based reagents using deep reactive ion etching (Alcatel 601E).After resist stripping (Microposit Remover 1165) and cleaning with acetone and ethanol, the topographical templates possessed periodic micropillar structures with width of 2 μm, height of 20 μm, and adjacent distance of 5 μm were obtained.
Selective Modification of Topographical Template with Asymmetric Wettability: The template was first cleaned with acetone, ethanol, and deionized water, followed by oxygen plasma for ≈10 min.Then a flat silicon substrate with spin-coated thin film of SU-8 was pressed onto the micropillar template.After SU-8 curing at 95°C, the silicon substrate was peeled off, leaving a SU-8 film covered on the lyophilic micropillar tops.The top-protected micropillar template was further modified by heptadecafluorodecyltrimethoxysilane molecules with low surface energy, giving rise to the lyophobic sidewalls of micropillars.After removing the SU-8 protection film, the asymmetric-wettability topographical template was fabricated.
Fabrication of Single-Crystalline TIPS-PEN 1D Arrays: A asymmetricwettability topographical template was utilized to manipulate the unidirectional dewetting process to fabricate single-crystalline TIPS-PEN 1D microarrays.10 μL TIPS-PEN solution was dropped onto the template and then covered with a cleaned silicon substrate.The substrate-solutiontemplate system was then heated in a vacuum drying oven at 60°C until all the solvent evaporates.After removing the template, highly aligned single-crystalline TIPS-PEN 1D microarrays were fabricated on the target substrate.
Structural Characterization: For the characterization of sample morphology, optical and cross-polarized optical microscope images were carried out on Nikon ECLIPSE Ci-POL polarized optical microscopes with a blue filter.SEM images and EDS maps were collected on Hitachi S-4800 scanning electron microscope.AFM images were collected on Bruker Dimension Icon.To characterize the optical properties of samples, the UV−vis absorption spectra were measured on Agilent Cary 7000 spectrophotometer.To clarify the crystallinity and crystallographic orientation of samples, TEM and SAED characterizations were carried out on Tecnai G2 F20 S-TWIN.In order to keep the integrity of the microwires during the TEM observation, the thermal release tape was used to transfer the sample from Si substrate to the amorphous carbon film-coated Cu grid.XRD measurements were performed on Rigaku Smartlab diffractometer with monochromatized Cu K radiation.GIWAXS patterns were measured by XEUSS SAXS/WAXS system with an incidence angle of 0.2°.All measurements were carried out in the air at room temperature.
OFETs and OPTs Fabrication and Characterization: Top-contact bottom-gate OFETs were constructed on SiO 2 (300 nm)/Si substrates by evaporation of Au (80 nm) source and drain electrodes on the TIPS-PEN arrays with the assistance of shadow mask.The electrical and photoelectric measurements of OFETs and OPTs were performed on semiconductor characterization system (Keithley, 4200) connected to manual probe station (Everbeing, BD4) at room temperature.The field-effect mobility was calculated from the saturation regime by plotting the square root of the drain current versus the gate voltage, and followed the formula I DS = (WC i /2L) ×  × (V GS − V th ) 2 , where C i is the specific capacitance of the gate dielectric layer (C i = 10 nF cm -2 ), W is the width of channel, and L is the length of channel, repectively.In addition, a 365 nm light-emitting diode (LED) with tunable power density modulated by LED controller was used as the light source to explore the photoresponse characteristics of OPTs.
Statistical Analysis: To obtain an average value of mobility (Figure 3c,f), 25 devices based on 1D single-crystalline arrays and spin-coated polycrystalline films were tested, respectively.The fitting of data was achieved by fitting a normal distribution curve.To obtain contact angle of sidewall of pillars (Figures S2c,d, S4b,c, and S5b,c, Supporting Information), 6 samples were tested to get the value, respectively.

Figure 1 .
Figure 1.a) The molecular structure of TIPS-PEN.b) In situ optical microscopy image of TIPS-PEN 1D microwire arrays.c) SEM image and d) AFM image and height diagram of 1D arrays.e) The cross-polarized optical microscopy of 1D arrays, indicating the single-crystalline nature.f) The UV-vis absorption spectra of TIPS-PEN single crystal.Scale bars in (b,c,e): 5 μm.

Figure 2 .
Figure 2. a) Calculated crystal growth morphology of the TIPS-PEN.b) XRD pattern of 1D arrays, revealing the (001) out-of-plane direction.c) TEM image of an individual microwire and the corresponding SAED pattern, suggesting a [010] growth direction of 1D single-crystalline microwire.Scale bar: 1 μm.d) Out-of-plane molecular packing of TIPS-PEN crystal with ab plane parallel to the substrate.e) GIWAXS image of 1D arrays, verifying the (001) out-of-plane direction.

Figure 3 .
Figure 3. a,d) Typical transfer characteristics, b,e) output characteristics, and c,f) mobility distribution of 25 OFETs based on 1D single-crystalline arrays and spin-coated polycrystalline films, respectively.

Figure 4 .
Figure 4. a) Transfer characteristics of OPTs based on 1D single-crystalline arrays under dark condition or different illumination intensities.Illumination intensity-dependent b) photosensitivity and c) photoresponsivity and specific detectivity of array based OPTs.d) Transfer characteristics of OPTs based on spin-coated polycrystalline film under dark condition or different illumination intensities.Illumination intensity-dependent e) photosensitivity and f) photoresponsivity and specific detectivity of film based OPTs.g) Scheme of carrier dynamics in the photodetector of single-crystalline arrays and polycrystalline film.h) The performance comparison of single-crystalline arrays and film.