Organic semiconductors have attracted increasing attentions in the application of modern electronics and optoelectronics because of their outstanding optoelectronic properties，flexibility，low cost and solution-processable advantages^［1-6］. In comparison with inorganic materials，organic semiconductors are lightweight and easily fabricated over large areas，showing unique potential in the fields of wearable electronics^［7-10］，flexible display screens^［11-15］，photodetectors^［16-19］，and photovoltaic devices^［20-25］.

Recent advancements in the design of narrow-bandgap conjugated small molecules and polymers have led to the development of high-performance organic semiconductor materials，broading their application spectrum further^［26-30］. For instance，the emergence of Y-series non-fullerene acceptors has significantly boosted the performance of organic solar cells，whose power conversion efficiency is approaching to 20%^［31-37］. As one of the most representatives，Y6 processes banana-curved molecule structure，and by replacing one outer alkyl chain in Y6 with an alkoxy chain leading to the structure of Y6-1O. It has a tighter molecular packing in thin film and can be easily grown into single crystals，leading to the excellent electronic transport properties. Y. Chen et al. has shown the fabrication of the Y6-1O single crystal on silicon substrate through the droplet casting method. And the resulting n-type organic field effect transistors based on Y6-1O has shown high performance with high on/off ratio and extraordinary electron mobility^［38］. Moreover，the ultraviolet-vis-near infrared（UV-vis-NIR） absorption spectra has shown that Y6-1O has a high absorption efficiency in the near-infrared（NIR）^{［39，40］}，with a bandgap estimated to be approximately 1.33 eV^［41］，indicating its high potential for NIR photodetection applications. However，studies on the photodetection application of Y6-1O single crystal are still absent.

In this work，we fabricated a phototransistor based on the Y6-1O single crystal and had systematically studied its photoresponse from visible to the NIR. We found that the photodetector based on Y6-1O shows excellent photoresponsivity with low dark current. And the maximum optical photoresponse occurs at 785 nm in the NIR region，which aligns well with the photoluminescence spectroscopy of the Y6-1O. Furthermore，we observed that the photoresponsivity increases with the thickness of Y6-1O single crystals. These results shown the organic semiconductor single crystal Y6-1O could function as a promising infrared photodetector in the future organic optoelectronics. It can not only contribute to the advancements in infrared detection technology，but also paves the way for novel application of organic semiconductor materials in the field of photodetection.

The Y6-1O single crystals were made through the droplet casting method on the heavily p-doped silicon substrate with a 285 nm-thick oxidation layer. In this process，20 μL of 0.5 mg/ml of Y6-1O xylene solution was dispensed onto the clean substrate，and placed in the N₂ glove box for 12 hours to allow solvent evaporation. Subsequently，Y6-1O single crystal with suitable length and thickness were selected through the optical microscope，and micro sized metal mask were used to fabricate the electrode. Au electrodes of 60 nm-thick was deposited through a thermal evaporation system at a rate of 2 angstroms per second. The photoluminescence（PL） spectra were measured using the laser micro confocal spectrometer（Renishaw，532 nm excitation laser）. Optoelectrical characterization of the Y6-1O based phototransistor was conducted using a Keithley 4200A-SCS parameter analyzer in the lakeshore probe station. Laser diode operating at different wavelengths were used as the optical lightsource to measure the photoresponse of the device.

The molecule structure of Y6-1O is shown in Fig. 1（a），it smartly combines the electron-pulling core and the electron-pushing side chain，and by adjusting the delicate balance of these structures，leading to a highly efficient conversion of the photons. The single crystal of Y6-1O grown on Si/SiO₂ substrates is typically in a nanoribbon shape，with a length of tens micrometers. The photograph of Y6-1O nanoribbon is taken with a polarized optical microscopy. As it can be seen in Fig. 1（b），it demonstrates the alternating between light and dark while changing the polarization of the incident light. This is because the birefringence nature of the Y6-1O single crystal，revealing the single crystalline structure of the nanoribbon.

The phototransistors based on the Y6-1O single crystal with different thicknesses were fabricated on the Si/SiO₂ substrates. Fig. 1（c） and 1（d） the scanning electron microscopy（SEM） and atomic force microscopy（AFM） of the device，respectively. The channel length of Y6-1O phototransistor is approximately 15 μm. The thickness profile of the devices with different thicknesses，108 nm，660 nm and 1060 nm are shown in Fig. 1（e）. The AFM tomography shows the Y6-1O single crystals process flat surface with minimum roughness，indicating their highly crystal quality. It’s worthy to note that as the thickness of the Y6-1O nanoribbon changes，the width of single crystal also varies. To study the optoelectronic properties of the Y6-1O single crystal，we firstly measured the PL properties of the single crystal in different thicknesses. As depicted in Fig. 1（f），the PL spectra of the Y6-1O show a high emission peak at approximately 910 nm，indicating its bandgap falls within the NIR region. Another weak emission peak locates in the visible light region，about 610 nm. These results suggests the Y6-1O could be employed in the visible to NIR photodetection. Moreover，the PL intensity increases with the thickness of single crystal，which is likely due to the higher absorption efficiency in thicker materials.

We firstly investigated the optoelectronic properties of the Y6-1O with a thickness of 108 nm. As shown in Fig. 2（a） and 2（b），the output and transfer curves of Y6-1O transistor revealed its n-type charge transport characteristics in its single crystal form，consistent with the previous report^［38］. The resistance modulation through SiO₂ could reach as large as 10⁴. The field effect electron mobility is calibrated to be 0.88 cm2V-1s-1，which is much higher than that of thin film devices^［38］. The enhanced carrier mobility is attributed to the long-range ordering of molecular arrangement within the single crystal. Fig. 2（c） shows the transfer characteristic curves under 638 nm laser illumination with different power intensities. A distinct photoresponse was evident in the Y6-1O single crystal phototransistor due to the photoconducting effect. As illustrated in Fig. 2（d），photons with energy higher than the bandgap（1.33 eV^［41］） of the single crystal elicited an intrinsic photoresponse. The time resolved photoresponse is further measured by alternatively switching the laser on and off periodically. Fig. 2e shows the photoresponse for 405 nm and 638 nm laser under different intensity. The highest on/off ratio of the photocurrent could reach approximately 10² under strong incident light. The relationship between the photocurrent（I_ph） and incident intensity of light（P） follows a power law function，Iph∝Pα， where α equals to 0.64 and 0.63 for 405 nm and 638 nm light，respectively. The relationship highlights the influence of photogenerated carrier generation and recombination dynamics^［42-45］. According to previous reports^［46］，the coefficient α is near to one for the pure photoconducting effect in semiconductors. The deviation from the ideal value indicates the involvement of defects in the photoresponse process.

The photoresponse for different wavelengths from visible light to NIR（405 nm - 980 nm） of Y6-1O phototransistor has been further studied under different backgate voltages，as shown in Fig. 3（a）-（c）. The dark current of the device decreases when backgate voltage changes from 40 V to -40 V，which is consistent with the transfer curves. Interestingly，the maximum photoresponse wavelength also changes with the backgate voltage（V_bg）. The maximum photoresponse locates at 630 nm when V_bg = 40 V，but it changed to 785 nm when V_bg = -40 V. These results are thought to be related to the band diagram changes under gate voltages，as shown in Fig. 3（d）-（f）. On one hand，the gate voltage effectively modulate the electron density of the Y6-1O channel，resulting in the modulation of the dark current as well as the photocurrent. On the other hand，the fermi level of the Y6-1O is also regulated by the gate voltage，thereby adjusting the metal-semiconductor barrier at the electrode contacts. This is presumably related to the change of the maximum photoresponse wavelengths. The spectral response and responsivity under different backgate voltages are summarized in Fig. 3（g）. As it can be seen，the net photocurrent and responsivity can be modulated three orders of magnitude by gate voltages. The maximum photoresponsivity is approximately 61.5 mA/W when V_bg = 40 V.

As aforementioned，the optical properties of Y6-1O are also influenced by the thickness of single crystal. Hence，we investigated the optoelectronic photoresponse for the device with thicker Y6-1O. Fig. 4（a） shows the photoresponse of Y6-1O with a thickness of 660 nm. We found the device shows higher photocurrent for wavelength from 405 nm to 850 nm when V_bg = 0 V comparing to the device made with 108 nm single crystal channel，while the dark current is still low. In particular，the photoresponse to the NIR（785 nm） gets enhanced. The photocurrent measured under different intensity of light are summarized in Fig. 4（b）（i）. The photocurrent maitains a power law relationship with the intensity of incident light from 405 nm to 850 nm. The power coefficient α is estimated to be 0.87，which is superior comparing with the device made with thinner Y6-1O crystal，indicating the better crystal quality for thick nanoribbons. The maximum photoresponsivity is approximately 16.6 mA/W for 785 nm light（Fig. 4（b）（ii））. The comparison of the spectral photoresponse for both devices with different thicknesses（108 nm and 660 nm） is shown in Fig. 4（c）. Firstly，we found that the photoresponse of the device made with 660 nm Y6-1O single crystal was significantly improved. And the peak photoresponse occurring in the NIR region，consistent with the absorption and PL properties of the Y6-1O single crystal. Additionally，a secondary peak photoresponse is observed at 520 nm in the visible region. In contrast，the device made with thinner（108 nm） Y6-1O single crystal exhibits a maximum photoresponse only in the visible region. Therefore，these findings suggest that Y6-1O single crystal hold promise for sensitive visible to NIR photodetection，with thicker samples show better performance in the NIR region.

In summary，we made the organic semiconductor single crystal Y6-1O through the convenient droplet casting method. Polarized optical microscopy revealed the high-quality single crystal structure of Y6-1O，while PL properties suggest its suitability for use in NIR optoelectronic devices. Phototransistors based on Y6-1O with different thicknesses were thoroughly investigated. These devices exhibit low dark current and high photoresponsivity across the visible to NIR spectrum，with the peak photoresponse observed in the NIR region. Moreover，the spectral response of the Y6-1O photodetector could be tuned by adjusting both the gate voltage and the thickness of the sample. In particular，the device made with 660-nm-thick Y6-1O single crystal shows photoresponsivity of 16.6 mA/W for 785 nm light，and shows strong potential in application in the visible to NIR photodetection. Ultimately，this work highlights an alternative approach of developing novel NIR photodetectors using the organic semiconducting single crystals. Such devices hold promise for various applications including night vision monitoring，biomedical imaging，environmental monitoring，and wearable devices owing to their low cost，scalability，and flexibility characteristics.