1. Introduction

Organic materials hold the great promise for the development of flexible electronics, such as the folding displays, flexible sensing, and radio-frequency identification devices on plastic substrates [1–3]. One of the most attractive prospects is their incomparable light absorption over their inorganic counterparts, which seemingly enables the promising photon-electron energy conversion and photon sensing. To achieve the practical aspirations as organic photodetector, however, there are a lot of challenges for the organic materials [4,5].

The binding energy of Frenkel excitons in organic matrix is usually about hundred meV, which is much higher than that of the thermal energy, confining the exciton spontaneous dissociation into the free carriers [6–8]. Therefore, external driving force, such as blended donor/acceptor interfaces, is necessary to overcome the binding energy for the spontaneous dissociation of excitons. Unfortunately, the exciton diffusion length L_ex for most disordered organic materials is usually short (10-50 nm), much smaller than the light absorption length (∼ 1 µm) in the visible spectral range. In addition, the carrier mobility of most organic matrix is inferior. As a result, vast excitons within the organic layer would be recombined before realizing charge dissociation, restricting the quantum efficiency of organic photodetector.

Previous works have shown that the L_ex becomes longer as the crystallinity becomes higher. For some highly ordered organic single crystals, the maximum L_ex can reach up to the order of micrometers (µm), comparable to the light absorption length [9]. For the efficient exciton separation goal, donor/acceptor heterojunction are still required. However, the electron or hole doping is still difficult for organic single crystals. Organic single crystal semiconductors constructed by π-conjugated molecules exhibit the long-range periodic order, and this well-defined molecular packing enables their highly thermal stability carrier mobility. Impertinent molecule doping would induce the extrinsic impurities and structural defects, which inversely leads to the low carrier mobility, compromising the charge-extract efficiency and operation speed of organic photodetector [10,11].

An effective structure is the planar bilayer heterojunction with electron donor and acceptor layers, where the excitons can efficiently generate and then dissociate within the L_ex of the interfaces under light illuminations, contributing to the photocurrent. For advanced organic photodetectors, the higher carrier mobility of channel material is highly desired, while the mobility of the organic crystals is finite compared to the inorganic semiconductors. Graphene is an attractive and promising material for the ultrabroadband photodetection due to high mobility and gapless band. However, limited by its ultrafast carrier recombination and quite low light absorption, the photoresponse of individual graphene-based photodetectors is extremely low. Here, there is an idea is to interface this organic crystal with graphene sheet. In these kinds of heterojunction devices, the photoinduced electron-hole pairs are generated in this organic crystal, dissociated at the interface, and collected by graphene to product photocurrent. Utilizing the long exciton diffusion length of organic single crystals, coupling them with a high mobility material is a promising strategy. In this work, we construct a planar organic photodetector by integrating n-type perylene-3, 4, 9, 10-tetracarboxylic dianhydride (PTCDA) single crystal with the monolayer graphene. The photo sensitivity of this hetrostructure device are boosted by the long-range exciton diffusion and efficient interfacial charge transfer. The photoresponsivity and specific detectivity of the device is about 8 × 10⁴ A W⁻¹ and >10¹¹ Jones for the 532 nm laser illumination, with a fast response speed of 220 µs. With the aid of the defect absorption and ultrahigh photogain, the device exhibits also the NIR response for 980 and 1550 nm. Finally, we demonstrate several visible and NIR image sensing in this device. Our work suggested that the high quality organic crystal hetrointerface is promising in the field of sensitive photon sensor and image recognition.

2. Device fabrication and characterization

The fabrication process of the graphene/ PTCDA single crystal hybrid device is illustrated in Fig. 1(a). Here, the graphene was prepared by mechanical stripping method. The PTCDA single crystals in experiment were grown onto SiO₂/Si substrate by the microspacing in-air sublimation, and the growth temperature and time are about 400 °C and 20 min. (see Figure S1, Supplement 1). For obtain a clean graphene-organic material interface, two prefabricated Au patches were mechanically transferred onto an exfoliated graphene sheet by using the tungsten micro-probe tip under the optical microscope as source/drain contact electrodes to form a graphene transistor. Then PTCDA single crystal was transferred onto the channel of graphene transistor by using dry transfer technique. The typical device area is 14 × 18 µm², the thickness of PTCDA crystal is 80 nm (Figure S2, Supplement 1). Raman finger of graphene on SiO₂/Si substrate is shown in Figure S3, indicating a defect-free monolayer nature. Figure 1(b) shows the polarized optical microscopy (POM) images of the PTCDA single crystal under a 530 nm LED as the light source. The obvious brightness variation dependence on the polarization plane of incident light testified to the macroscopic molecular ordering and absence of polycrystalline domains [12–15]. In addition, the discernible birefringence and the uniform brightness indicate PTCDA as a pristine anisotropic single-crystal structure. We performed atomic force microscopy (AFM) to obtain the surface morphology of the PTCDA single crystal, as shown in Fig. 1(c). The root-means-square (RMS) roughness is only 78 pm, indicating its atomic scale flatness. Figure 1(d) shows the steady-state photoluminescence spectrum (PL) characterization from the ab face of the PTCDA sample. The PL spectrum of the individual PTCDA single crystal sample on SiO₂/Si substrate shows a broad peak, which can be deconvoluted as arising at 680 to 720 nm due to charge transfer (CT) exciton and excimer channels. The PL intensity at the Graphene/PTCDA heterostructure exhibits the obvious PL quenching (decreased to about 40%), implying that the abundant excitons dissociated near the Graphene/PTCDA interface.

 

Fig. 1. Structure and characterization of the heterostructure device and the PTCDA single crystal. (a) The fabrication process of the proposed device structure. (b) Cross-polarized optical microscopy images of a PTCDA single crystal. (c) The surface morphology of the PTCDA single crystal. (d) Photoluminescence spectra of PTCDA single crystal with (red) and without (black) graphene.

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3. Optoelectronic performance and image sensing

Transfer curves of the graphene/PTCDA device and the individual graphene transistor are compared in Fig. 2(a). It is observed that the Dirac point of the graphene transistor shifted from 20 to 25 V after covering PTCDA single crystal, the Dirac point of the heterostructure device shifts to right as compared with pristine graphene device, indicating a p-type doping (hole-doping) for graphene sheet from the PTCDA single crystal, which is accordance with the PL results. The inset of Fig. 2(a) shows an optical microscope image of the graphene/PTCDA device. Figure 2(b) shows the transfer curves of this heterojunction device with the back-gate bias V_G under different optical power densities (532 nm). Under illumination, a lot of Frenkel excitons would be produced within the PTCDA single crystal. Some excitons will diffuse to the graphene/PTCDA interface and then occurs the exciton dissociation. The photogenerated electron transfer into graphene from PTCDA single crystal, while holes remain in the PTCDA matrix, causing the left-shift of Dirac point forming a photocurrent. Figure 2(c) shows the photocurrent as a function of the gate voltage at V_DS = 50 mV. In Fig. 2(d), we calculated the responsivity (defined R = I_ph/P_in), and the maximum R of the device can reach up to 7 × 10⁴ A W⁻¹ under low light power at V_DS = 50 mV, which reaches up to in first-class as compared to the previously reported organic photodetector at similar excitation intensity [7,16–18]. With the increase of incident illumination, the R exhibits a reduced trend, which is related to the saturated absorption, and increased recombination probability in the higher carrier density. Figure 2(e) described the output curves of the device under different incident light power at V_G= 0 V. Under illumination, the device exhibits a decreased current with ascending the power of light illumination. Generally, the graphene is p-doping due to the noncovalent polymer residual and physisorbed oxygen molecules in the transfer processes. The photogenerated electrons in PTCDA are transferred into graphene, quenching the p-type holes and leading to the decrease of source-drain current. In order to analysis easily, the corresponding photocurrent (I_ph =∣I_light-I_dark∣) as a function of the source-drain voltage (V_DS) under different illumination levels are shown in Fig. 2(f). The linear scaling of the photocurrent with the bias is clearly observed, implying we can obtain a higher responsivity by raising the drain bias. Furthermore, we have found that the device has response in NIR wavelength and tested the photoelectric detection characteristics of the typical device under 980 nm and 1550 nm, respectively. (Figure S4 and S5, Supplement 1).

 

Fig. 2. Optical electrical performance in the graphene/PTCDA interface. (a) Transfer characteristics of the pristine graphene transistor and graphene/PTCDA device in dark. (b) Transfer curves of the graphene/PTCDA phototransistor under various light powers (λ = 532 nm, V_DS = 50 mV). (c) The photocurrent (Iph =|Ilight – Idark|) as a function of the gate voltage and light illumination powers (λ = 532 nm, V_DS = 50 mV). (d) Responsivity of the device versus light power at different gate voltages. (e) Output curves at different light powers. (f) Photocurrent of the graphene/PTCDA device as a function of drain-source voltage (V_DS) under different optical powers at V_G = 0 V, showing a linear dependence on V_DS bias.

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To accurately assess specific detectivity D* (units cm Hz^1/2W⁻¹), a significant indicator that reflects the sensitivity of photodetector, we implemented the spectral noise density S(f) in Fig. 3(a), to analyze the noise current: $\left\langle {i_n^2} \right\rangle = \int_0^{\Delta f} {S(f)} df$ [19,20]. All the low-noise spectra exhibit a typical 1/f power density from low frequency to high frequency range. At small measurement bandwidth of 1 Hz, the specific detectivity can be equivalent to $D\ast{=} \frac{{R{A^{1/2}}}}{{\sqrt {\int_0^{\Delta f} {S(f )df/\Delta f} } }} \approx \frac{{R{A^{1/2}}}}{{\sqrt {S(f )} }}$, where A is the device area. Using the experimental noise density spectra, we calculated that the value of D* can reach to >10¹¹ Jones at V_DS = 50 mV (Fig. 3(b)). Even at ambient condition, the device keeps a good responsivity without any degradation after 500 cycles, as shown in Fig. 3(c). Response speed of the device is another key parameter for the advanced phototetectors. The rise time (τ_r) and decay time (τ_d), are defined as the time required to increase from 10% to 90% of the maximum photocurrent and decrease from 90% to 10% of the maximum photocurrent. From the amplifying temporal photoresponse in Fig. 3(d), we seen that the τ_r and τ_d of the device for 532 nm illumination are estimated to be 220 and 290 µs, respectively. The fast response of the device is attributed to the low defect states within PTCDA single crystal, as well as the low charge trapping states at graphene/PTCDA interface. As shown in Table S1 (Supplement 1), compared with other typical photodetectors, the performances of our photodetector have reached up to in first-class and have significant potential for electronic and optoelectronic applications. And we have tested the response time of the device for NIR wavelength illumination (see Figure S6, Supplement 1). At a low power intensity, these shallow trap states would be filled first, leading to a slow response speed. At a higher light power, the shallow states get saturated, resulting in a fast response speed. This infrared response is mainly originated from the defect absorption of PTCDA and high photogating gain. To further verify the photoresponse generation mechanism of the graphene/PTCDA photodetector, high-resolution spatial photocurrent mapping measurement of a typical device (Fig. 3(e)) under a confocal optical microscope was carried out. As shown in Fig. 3(e), the photocurrent signals region is mainly restricted to the overlapped region of PTCDA single crystal and graphene sheet, implying a significant role for this sensitive photodetector. This photocurrent generation physical mechanism can be explained by using a concise band diagram in Fig. 3(f). Under the light illumination, a great deal of excitons are generated in PTCDA single crystal, then they diffuse into the graphene/PTCDA heterointerface to separate into the free carriers. The photogenerated electrons transfer into the graphene channel through the energy transfer effect, forming the photocurrent. To confirm the temporal photoresponse characteristic of our device, the photocurrent with periodically switched illumination was measured under a small bias voltage of 50 mV. Such organic heterostructure device exhibits robust switching behavior and excellent reproducibility.

 

Fig. 3. Performance metrics of the graphene/PTCDA device. (a) Noise current spectral density as a function of frequency under different gate voltages. (b) The specific detectivity (D*) as a function of gate voltage (V_G) at finite illumination (λ = 532 nm, P = 36 pW). (c) Multicycle temporal photocurrent response at atmosphere environment (V_G = 0, V_DS = 50 mV). (d) Photoresponse time of the device under dark and light illumination (532 nm). (e) Photocurrent mapping of conductive channel at V_DS = 50 mV for the device. (f) The band diagram of photocurrent generation under 532 nm wavelength laser.

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In view of the anisotropic crystal structure of the PTCDA single crystal, the polarization sensitization sensitivity of the graphene/PTCDA heterojunction photodetector is further investigated. The optical microscope image of the graphene/PTCDA photodetector in shown in Figure S7. In the measurement, the linearly polarized light is achieved through a linear polarizer and a half-wave plate, the device is fixed on the probe station, and the polarization angle is varied by hand operated rotating the half-wave plate with an interval of 10°. With the gradual variation of the polarization angle, the schematic of the polarized light current test setup is shown in Fig. 4(a). The photocurrent reaches the maximum value at the angle of 0° (180°) and the minimum value at 90° (270°), showing an obvious 180° periodic variation yielding two-lobed shapes (Fig. 4(b)). The photocurrent image is displayed under a constant illumination wavelength of 532 nm as a function of the polarization angle, with the bias voltage of 0.05 V. We have calculated the photocurrent anisotropic ratio is 1.4, and we compared the anisotropic ratio and responsivity of previous advanced devices in recent years (Figure S8), indicating its relatively good overall performances [21–30]. Figure 4(c) suggesting the polarization photocurrent mapping originating from the device was for the whole overlapped area of the graphene/PTCDA photodetector [31–34].

 

Fig. 4. Polarization detection characteristics of the graphene/PTCDA photodetector. (a) Schematic diagram of the polarization sensitivity measurement. (b) The relationship of the photocurrent of the graphene/PTCDA heterojunction photodetector with the polarization angle in polar coordinates with the bias voltage of 0.5 V. The corresponding photocurrent mapping images of the device under illumination of 532 nm with different light polarizations in (c).

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In now information age, high-speed imaging plays a critical role in civil and military applications such as medicine diagnosis, environmental monitoring, space exploration and enemy surveillance [35–37]. To check the potential application of this organic heterostructure photodetector for image information collection, the single-pixel imaging capability was further explored employing 405, 532, 980 and 1550 nm lasers by a home-built system, as illustrated in Fig. 5(a). A patterned metal mask as the target object is placed between the light source and the device as a single-pixel sensor, which could be moved linearly in the X-Y plane controlled by two piezo tubes, and the incident light is modulated by a signal generator source. When the light signal shined into the device terminal by passing through the metal mask, the position-dependent real-time current could be recorded. The image consisting of 100 × 100 scanned points was formed by controlling the movement of a two-dimensional control platform and recording the value of photocurrent. Figure 5(b) shows the optical images of the patterned masks. Figure 5(c) are corresponding high-resolution imaging by employing the different lasers with different wavelengths. These results suggested that such organic heterostructure photodetector is promising as an imaging block building in future advanced optoelectronic system and video-frame-rate imaging applications [38].

 

Fig. 5. High-speed imaging application. (a) Schematic illustration of the measurement system of the single pixel imaging. (b) Optical microscope images of several under-test objects. (c) Photocurrent imaging results under different wavelengths (405, 532, 980 and 1550 nm laser). Scale bar: 20 µm.

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4. Conclusion

In conclusion, we constructed a sensitive organic phototransistor based on PTCDA single crystal and monolayer graphene sheet. The long-range exciton diffusion of PTCDA crystal and efficient charge transfer at graphene/PTCDA interface enable a high responsivity of 8 × 10⁴ AW⁻¹and the satisfied specific detectivity of > 10¹¹ Jones. Moreover, the device exhibits a fast response speed with the rise/decay time of ∼220/290 µs, which arise from the high-quality organic crystal and lower charge trap density and also shows a significant role in performance enhancement of high sensitivity to polarized visible light illumination photodetection. Finally, high-resolution single-pixel imaging is demonstrated in such organic heterostructure photodetector as their imaging functionality. Our work suggested that the organic single crystal is promising for high-performance photodetector, but also offered an efficient method for boosting the sensitivity of optoelectronic device.