The preparation of high-quality SMOSCs with appropriate optical bandgap is of significant importance for the realization of high-performance visible photodetectors. Solution-based self-assembly has been frequently adopted for the fabrication of organic single-crystalline nanowires, nanorods, or nanoribbons.^[13] Early in 2004, Schwab and coworkers^[9] used a simple drop-casting method to prepare meso-tetrakis(4-sulfonatophenyl)porphine nanorods. Upon 488 nm light illumination, the device containing approximately 6100 nanorods showed a rapid response speed of less than 100 ms. Although the photoswitching ratio was not high due to the existence of the charge storage mechanism in the device, this work paved the way for using SMOSCs to realize visible photodetectors. In another example, Jiang et al.^[14] studied the photoconductive behavior of self-assembled 2-anthracen-9-ylmethylene malononitrile micro-/nanowires (Figs. 2(a) and 2(b)). It was found that the morphology of the as-fabricated one-dimensional (1D) structures has an influence on the absorption spectra (Fig. 2(c)), where the absorption peak was red-shifted with increasing size due to more aggregated molecules narrowing the energy bandgap. A photoswitching ratio of ∼ 100 was obtained under white light for the device with multiple microwires (Fig. 2(d)). Also, Zhou et al.^[15] reported a photoconductive device based on a single benzothiophene sub-micron ribbon (Fig. 2(e)). Since two absorption peaks were found at 414 nm and 387 nm (Fig. 2(f)), photoelectric measurements were thus performed under a violet light of 405 nm. The R was measured to be 420 A·W⁻¹ with a photoconductive gain of about 1.3 × 10³. Interfacial modification was further used to boost the photodetector performance.^[16] As illustrated in Fig. 2(g), a dielectric layer such as poly(methyl methacrylate) (PMMA) or polystyrene (PS) was pre-coated on a quartz substrate before the self-assembly of benzothiophene sub-micron ribbons. It was found that the dielectric/organic crystal interface could influence the surface morphology, polarity, as well as the density of defects. The weak PS dipoles would reduce the defect density and facilitate carrier transport, leading to an enhanced photoconductive gain of 1.3 × 10⁴ and a higher R of 4372 A·W⁻¹, respectively (Fig. 2(h)).

Fig. 2.

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Fig. 2. Scanning electron microscopy (SEM) images of self-assembled 2-anthracen-9-ylmethylene malononitrile (a) nanowires and (b) microwires. (c) Absorption spectra of 2-anthracen-9-ylmethylene malononitrile in solution, nanowire, and microwire phase. (d) Temporal response of the anthracen-based photoconductor under 5 mW·cm−2 white light with a bias voltage of 50 V. (a)–(d) Reproduced with permission.[14] Copyright 2008, American Chemical Society. (e) SEM image of a single benzothiophene sub-micron ribbon. (f) Absorption spectrum of benzothiophene sub-micron ribbons. (e), (f) Reproduced with permission.[15] Copyright 2008, Wiley-VCH. (g) Schematic diagram of a single benzothiophene sub-micron ribbon photoconductor. (h) Dependence of the responsivity on the light intensity, the devices are with different dielectric materials. (g), (h) Reproduced with permission.[16] Copyright 2013, Elsevier.

Copper phthalocyanine (CuPc) is another typical small-molecule organic material for red light detection. Due to the outstanding thermal and chemical stability, the CuPc molecules can easily form ordered stacks during a physical vapor deposition (PVD) process. A schematic illustration is shown in Fig. 3(a), where a tube furnace is used for material sublimation and deposition. The CuPc powder source is put in the high-temperature zone, where they are heated above the sublimation point. With the aid of a constant gas flow (usually Ar), CuPc molecules can easily be transported downstream and finally crystallize in the low-temperature zone. Since the whole process is in an isolated system which excludes the influence of ambient air and solvent molecules, highly purified 1D CuPc single crystals can be obtained. A typical case is shown in Fig. 3(b), where ultralong CuPc nanowires up to millimeter in length could grow along the channels of a grating template in a PVD process.^[17]

Fig. 3.

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Fig. 3. (a) Schematic illustration of a metallic nanoparticle-assisted PVD process. (b) Template-guided growth of CuPc nanowire arrays. (b) Reproduced with permission.[17] Copyright 2012, The Royal Society of Chemistry. (c) Au-induced growth of cross-aligned CuPc nanowires. (d) Schematic illustration of the measurement configuration for a image sensor: a red laser beam was projected onto the right-upper corner of the device. (e) The corresponding output current intensity mapping of the device. (a), (c)–(e) Reproduced with permission.[19] Copyright 2013, Nature Publishing Group. (f) Schematic illustration of a flexible image sensor based on CH3NH3PbI3 microwire arrays. (f) Reproduced with permission.[20] Copyright 2016, Wiley-VCH.

One of the very important advantages of photoconductive devices is that they can be easily integrated for image sensors. Jie et al.^[18,19] first demonstrated patterning of CuPc nanowire arrays as image sensors. The patterning of CuPc nanowire arrays was realized by using a metallic nanoparticle-assisted PVD method, as shown in Fig. 3(c). It was found that CuPc nanowires selectively grew on the Au nanoparticle coated areas with an average tilt angle of ∼ 70° relative to the substrate.^[18] By combining the photolithography technique, CuPc nanowires could be patterned in micro-scale pixels.^[19] Integrated image sensors with 10 × 10 pixel arrays in an area of 1.3 × 1.3 mm² were thus fabricated. When the device was exposed to a red laser beam (Fig. 3(d)), the output current intensity mapping gave an accurate sketch of the laser beam shape (Fig. 3(e)). Recently, well aligned organic/inorganic hybrid CH₃NH₃PbI₃ single crystals have been reported by Deng et al.^[20] for visible light (360–780 nm) detection. Flexible and integrated image sensors based on CH₃NH₃PbI₃ microwire arrays were constructed on a polyethylene terephthalate (PET) substrate, which showed an excellent image-sensing ability under different bending states (Fig. 3(f)). The novel image sensors can realize high-resolution, broadband and flexible detection at the same time, which are ideal to meet the needs of modern society.

The detection of UV light radiation presents a wide range of civil and military applications, such as flame detection, combustion monitoring, and missile warning.^[21] SiC, GaN, and diamond as building blocks for UV photodetectors have attracted intense attention.^[22–24] Fabrication of these devices requires costly processes, thus SMOSC-based UV photoconductors in this case provide the opportunity for device fabrication simplification.

Zhang et al.^[25] demonstrated highly responsive UV photoconductors using ris(8-hydroxyquinoline) aluminum (Alq3) microplates and nanorods. In order to fabricate device, pre-patterned photoresist hollows were constructed by photolithography. With the aid of capillary force and alternating-electric field (Fig. 4(a)), Alq3 microplate (Fig. 4(b)) or nanorods (Fig. 4(c)–4(e)) would selectively deposit in the hollows. UV–Vis absorption spectrum in Fig. 4(f) confirmed that Alq3 has an absorption peak at ∼ 250 nm with little absorption in visible light region, which is ideal for pure-UV detection. Photodetectors made from Alq3 single nanorod and microplate were with high R and D* of 14.5 A·W⁻¹, 19.9 × 10¹¹ Jones and 2.5 A·W⁻¹, 4.24 × 10¹¹ Jones under 254 nm UV light, respectively. More recently, single-crystalline C₆₀ microribbon arrays were also adopted for UV photodetectors.^[26] The growth process of C₆₀ microribbon arrays is illustrated in Fig. 4(g). The SiO₂/Si substrate was first immersed into the C₆₀ solution and then lifted up with an optimized speed. Finally, C₆₀ single crystals could grow at the contact line along the lifting direction to form ordered arrays (Fig. 4(h)). Current–voltage (I–V) characteristics of C₆₀ microribbon arrays were measured in dark and under illumination with different wavelengths ranging from 350 nm to 650 nm (Fig. 4(i)). Prominent current change was observed under UV light illumination, while the device showed slight response to visible light. The photoswitching behavior was further measured under 400 nm light (Fig. 4(j)), and a remarkable photoswitching ratio of 365 was observed at a light intensity of 1.82 mW·cm⁻².

Fig. 4.

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Fig. 4. (a) Schematic illustration of the capillary-assisted alternating electric field method for patterning Alq3 crystals. (b)–(e) SEM images of the as-fabricated Alq3 crystals in different hollows. (f) Absorption spectrum of Alq3 single crystals. (a)–(f) Reproduced with permission.[25] Copyright 2017, Wiley-VCH. (g) Schematic illustration of the dip-coating process for growing C60 microribbon arrays. (h) Optical microscopy image of the as-prepared C60 microribbon arrays. (i) I–V characteristics of the C60 microribbon arrays under different illumination conditions. Inset: optical microscopy image of a single device. (j) Photoswitching behavior of C60 microribbon arrays under 400 nm light with a bias of 10 V. (g)–(j) Reproduced with permission.[26] Copyright 2018, Elsevier.

NIR photodetectors offer promising applications in passive night vision, optical communication, and bio-diagnostics.^[27–29] Current inorganic NIR photodetectors are expensive and dependent on costly and complex epitaxial growth on crystalline substrates.^[30] Contrastingly, narrow-bandgap SMOSCs offer opportunities for realizing low-cost and flexible NIR photodetectors.

Methyl-squarylium (MeSq) is an ideal organic compound for NIR light harvesting thanks to its good stability and absorption peak in the range of 700–1100 nm. Jie and coworkers reported photoconductors based on 1D MeSq micro-/nanostructures such as nanowires^[31] and microwires^[32] (Figs. 5(a) and 5(b)) using solution-based methods. Typically, dip-coating was used for ordered alignment of the MeSq microwire arrays,^[33] which showed uniform color under cross-polarized light (Fig. 5(c)), demonstrating the single-crystalline nature. The detailed dip-coating process is shown in Fig. 5(d). Photolithography was first used to construct periodic photoresist strips on a SiO₂/Si substrate. Then the exposed areas between wettable photoresist strips were modified by a layer of nonwettable octadecyltrichlorosilane (OTS). Finally, by vertically immersing the patterned substrate into MeSq/dichloromethane (CH₂Cl₂) solution and pulling it up slowly, MeSq microwires would preferentially grow along the bilateral sides of the photoresist stripes. Interestingly, study showed that MeSq/CH₂Cl₂ dilute solution only had a narrow absorption peak in the range of visible light, while the assembled MeSq nanowires had an absorption peak value at around 850 nm (Fig. 5(e)).^[31] Therefore, the MeSq microwire arrays were then tested under a 808 nm NIR laser beam. The microwire array device showed pronounced photoresponse under a light intensity of 200 mW·cm⁻², with a high photoswitching ratio up to 1600 (Fig. 5(f)). Furthermore, the MeSq microwire could be transferred to a flexible polydimethylsiloxane (PDMS) substrate.^[32] The flexible microwire device showed little degradation under 808 nm light upon being bended with bending curvatures (k) increasing from 0.5 cm⁻¹ to 1 cm⁻¹, demonstrating the application potential in flexible NIR sensors (Fig. 5(g)). Compared to SMOSC-based visible and UV photoconductors, few examples have been reported in the case of NIR photoconductors. This is mainly because small-bandgap organic semiconductors are too unstable to be synthesized effectively.

Fig. 5.

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Fig. 5. (a) MeSq nanowires obtained from the solvent exchange self-assembly method. Inset: photography of the precipitation of MeSq nanowires in a glass vial. (b) SEM images of MeSq microwires. (c) Cross-polarized optical microscopy image of the MeSq microwire arrays. (d) Schematic illustration of the patterning strategy for growing MeSq microwire arrays. (e) Absorption spectra of the MeSq nanowires and the MeSq solution with photocurrent spectral response behavior of a single nanowire device. (a), (e) Reproduced with permission.[31] Copyright 2008, Wiley-VCH. (f) Temporal response of the MeSq microwire arrays under 200 mW·cm−2 808 nm light. (c), (d), (f) Reproduced with permission.[33] Copyright 2016, American Chemical Society. (g) I–V characteristics of a flexible MeSq microwire photoconductor with different curvatures in dark and under 808 nm light irradiation. Inset: photograph of the device on a flexible PDMS substrate. (b), (g) Reproduced with permission.[32] Copyright 2015, American Scientific Publishers.

First example of SMOSC-based visible light phototransistors was demonstrated by Rovira et al. in 2006.^[34] Microscale tetrathiafulvalene (TTF) single crystals with a high mobility of 1 cm²·V⁻¹·s⁻¹ were adopted to fabricate OPTs. The OPTs showed a very high photoswitching ratio up to 10⁴ at a gate voltage of 10 V under white light (2.5 W·cm⁻²). In most cases, solution-phase synthesis is preferable for designing well-ordered molecular structures with face-to-face or slip-stacked π–π arrangement, where the strong π–π interaction facilitates the charge transport. For example, 1D 9,10-bis(phenylethynyl)anthracene (BPEA) nanoribbons,^[35] 6-methyl-anthra[2,3-b]benzo[d]thiophene (Me-ABT) microribbons,^[36] and N,N'-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI) nanowires^[37] have been fabricated successfully by simple solution-based self-assembly methods. Typically, the Me-ABT-based OPT had a high R of 1.2 × 10⁴ A·W⁻¹ under white light, which is comparable to commercial single-crystalline silicon thin film transistors (∼ 300 A·W⁻¹).^[38] In terms of charge-transfer complexes, they are often found in organic conjugated polymers, while Zhu’s group^[39] fabricated two-dimensional (2D) cocrystals successfully. Meso-diphenyltetrathia[22]-annulene[2,1,2,1] (DPTTA) molecules (donors) and C₆₀/C₇₀ molecules (acceptors) were segregated to form linear column networks in long-range order (Fig. 6(a)). The as-fabricated C₇₀-DPTTA microsheets have balanced ambipolar properties with an electron mobility of 0.05 cm²·V⁻¹·s⁻¹ and a hole mobility of 0.07 cm²·V⁻¹·s⁻¹. The enlarged 2D donor–acceptor interfaces facilitated charge separation and provided a highway for charge transport. With the aid of a large gate bias, the C₇₀-DPTTA microsheets have a R of 300 A·W⁻¹ under white light (Fig. 6(b)).

Fig. 6.

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Fig. 6. (a) Optical microscopy images of OPTs based on C60-DPTTA and C70-DPTTA cocrystals. (b) Transfer characteristics of the ambipolar device in dark (red line) and illumination (blue) conditions. (a), (b) Reproduced with permission.[39] Copyright 2013, American Chemical Society. (c) Schematic illustration of the capillary tube induced self-assembly of TCNQ crystals. (d) Optical microscopy image of a TCNQ-based OPT. (e) Output characteristics of a TCNQ-based OPT at low gate and source-drain voltages. (c)–(e) Reproduced with permission.[40] Copyright 2011, The Royal Society of Chemistry. (f) Optical microscopy image of a single PTCDI-C8 nanowire OPT with different channel lengths. (g) The normalized photocurrent spectra of PTCDI-C8-based OPTs with different channel lengths versus the normalized absorption spectra of PTCDI-C8. (h) Temporal response of a single PTCDI-C8 nanowire OPT with different channel lengths under 500 nm light. (f)–(h) Reproduced with permission.[42] Copyright 2018, American Chemical Society.

For large-scale device application, the capillary tube induced self-assembly was developed to fabricate ordered organic single-crystalline nanowire arrays. As shown in Figs. 6(c), 7,7,8,8-tetracyanoquinodimethane (TCNQ) solution was added drop-wise along a 3-cm-long capillary tube with an outer diameter of 1.2 mm.^[40] During continuous baking, the solvent evaporated with meniscus shrinking, which induced the formation of 1D TCNQ crystals (Fig. 6(d)) in directions normal to the tube length. Instead of the traditional Si substrate with SiO₂ gate dielectric, poly(4-vinyl phenol) (CL-PVP) with a high dielectric constant was spin-coated on an indium tin oxide (ITO) substrate. As a result, the output characteristics of TCNQ-based OPTs showed excellent saturation behavior at low gate and source-drain bias (Fig. 6(e)). The device was capable of low-voltage operation of less than 0.5 V, which was far smaller than the common high gate voltage (V_G) of ∼ 20 V. A fast response speed of ∼ 10 ms was also observed, as a trade-off, the R was only ∼ 1 mA·W⁻¹. Mukherjee et al.^[41] used the same method to construct OPTs based on N,N’-dioctyl-3,4,9,10-perylenedicarboximide (PTCDI-C8) nanowire arrays, which exhibited a R of 7 A·W⁻¹ under 7.5 mW·cm⁻² white light. Yao et al.^[42] further utilized CL-PVP as the protective layer in a photolithography process, preventing the downside PTCDI-C8 nanowires from being damaged during photoresist development. Then electrodes with different channel lengths were deposited (Fig. 6(f)), the shortest of which was downscaled to ∼ 200 nm. Short channel effect of the PTCDI-C8 nanowire-based OPTs was studied. The normalized photocurrents of different channel lengths were measured under a wavelength range from 320 nm to 690 nm (Fig. 6(g)). For OPTs with the shortest channel length, the photocurrents fit the absorption spectrum best. The temporal response of OPTs with different channel lengths is shown in Fig. 5(h). Devices with narrower channels exhibited higher current both in light on and off state, which was probably due to fewer grain boundaries and fewer trap states in a shorter region. Therefore, the OPTs with short channel lengths revealed a high R up to 4.8 × 10⁴ A·W⁻¹.

J-aggregated organic semiconductors of 1,3,6,8-tetrakis((4-hexyl phenyl)ethynyl)pyrene(PY-4(THB)) were developed by Choi’s group to detect 400 nm light.^[43] In its single-crystalline microribbon, PY-4(THB) had face-on aggregated pyrene cores which were favorable for charge transport. The PY-4(THB) microribbon-based OPTs exhibited a high field-effect mobility of 0.7 cm²·V⁻¹⋅s⁻¹ with a R of 2000 A·W⁻¹ under quite low light intensity of 5.6 μW·cm⁻². Microplates of anthracene derivative consisting of phenyl planes and acetylene groups were reported by Kim et al.^[44] The microplate-based OPT had a quite high R of 1.1 × 10⁴ A·W⁻¹ and a champion photoswitching ratio of 1.4 × 10⁵ among single-crystalline microplate-based OPTs. In recent years, concerns have focused on pure-UV detectors with visible-blind response. Well-aligned 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT) nanoribbon arrays (Fig. 7(a))^[45] and naphthalene diimides (NDIs) single nanoribbon (Fig. 7(b))^[46] have been fabricated to construct deep-UV OPTs. The photoresponse of the C8-BTBT nanoribbon-based device is shown in Fig. 7(c), which showed obvious response under UV lights of 280 nm and 365 nm. Instead, when visible light was used, almost no difference could be found between the transfer curves with light turned on and off. The absorption spectrum in Fig. 7(d) further indicated the visible light transparent feature of C8-BTBT, the C8-BTBT nanoribbons are thus promising for pure-UV detection. In the case of NDI derivatives, the nanoribbon phase showed an enhanced UV absorption compared to thin film and solution (Fig. 7(e)), which was caused by phase transition towards enhanced molecular packing order. The device was then illuminated under different wavelengths of light with a constant intensity of 100 μW·cm⁻² (Fig. 7(f)), which showed a sharp photocurrent increase with wavelengths under 450 nm both in photoconductor mode (V_G = 0 V) and phototransistor mode (V_G = 20 V). Under 365 nm UV light, the device had a maximum R of 7.2 × 10³ A·W⁻¹ with an ultrahigh detectivity of 3.1 × 10¹⁵ Jones, the D* value is one of the highest values among organic photodetectors.

Fig. 7.

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Fig. 7. (a), (b) Optical microscopy images of C8-BTBT nanoribbon arrays and a single NDI nanoribbon, respectively. (c) Transfer characteristics of the C8-BTBT device (VSD = −30 V) in dark condition (red curve) and under the illumination of 280 nm (blue curve), 365 nm (yellow curve), 405 nm (magenta curve), and visible light (green curve). (d), (e) Absorption spectra of C8-BTBT crystals and NDI derivatives in different phases, respectively. (f) Relative photocurrent and threshold voltage change of OPTs based on NDI derivatives under the illumination of light with different wavelengths. (b), (e), (f) Reproduced with permission.[46] Copyright 2018, American Chemical Society. (g) Light on/off characteristics of the C8-BTBT device, the inset indicates the rise time of the device. (h) Multiple-stage modulation of IDS by changing the gate bias: ① light-on, ② light-off, ③ VG = −30 V, ④ VG = 0 V, ⑤ VG = −50 V, ⑥ VG = 0 V, ⑦ VG = −100 V. (a), (c), (d), (g), (h) Reproduced with permission.[45] Copyright 2015, Wiley-VCH. (i) SEM image of PTCDA nanoparticles. Inset: SAED pattern of a PTCDA nanoparticle. (j) Schematic illustration of a single PTCDA nanoparticle-based OPT. (k) Three-dimensional (3D) transmission electron microscopy (TEM) tomography image of the nanoparticle device. (l) Photoswitching behavior of the nanoparticle device with different UV intensities: P1 = 1.32 mW·cm−2 (blue curve), P2 = 2.24 mW·cm−2 (red curve). (i)–(l) Reproduced with permission.[48] Copyright 2013, AIP Publishing.

It is noteworthy that the photomemory phenomenon was observed in the C8-BTBT nanoribbon-based phototransistors.^[45] The photoswitching behavior of C8-BTBT at zero gate bias is shown in Fig. 7(g), and the inset indicated a rise time of ∼ 0.45 s upon UV light illumination. However, when UV light was turned off, the device showed a persistent current, which was ∼ 10⁵ larger than the previous dark current. The device was then applied as a simple memory unit. As shown in Fig. 7(h), the first step was the light-on process, which could be considered as “writing”, the second light-off step was the “reading” process, in the third/fourth step, V_G was turned from 0 V to −30/−50 V and then to 0 V rapidly, which represented the “rereading and rewriting”, finally in the “erasing” process, a quite large V_G = −90 V was applied, and the off-state current recovered to the initial level. An appropriate explanation is that for p-channel OPTs, after the generation of photo-induced carriers, holes drift toward the conductive channel, while most of electrons are trapped. When no V_G is applied, the trapped electrons can function as the negative gate voltage, causing a persistent increase of source-drain current (I_SD). Once V_G with negative bias is applied, I_SD increases rapidly due to more accumulation of holes at the insulator/semiconductor interface. Meanwhile, the recombination rate of electrons and holes increases with the increase in the absolute value of V_G, leading to a decrease in I_SD when the negative V_G is turned off. Trials have also been done to avoid trapping of electrons in p-type SMOSCs. Mathews et al.^[47] applied a 600 μs pulsed laser to a rubrene-based OPT, which did not allow enough time for electrons to move into the bulk, and thus led to a fast recombination rate. Instead of the normal long photomemory that persisted for 2 min, a quite short photocurrent decay time of 100 μs was obtained.

Besides conventional OPTs based on 1D and 2D nanostructures, the UV response behavior of single-crystalline nanoparticles has been studied by Nguyen et al.^[48] Perylene tetracarboxylic dianhydride (PTCDA) nanoparticles with diameters of ∼ 80 nm (Fig. 7(i)) were deposited onto a Si₃N₄ membrane through a simple thermal evaporation process. The selected area electron diffraction (SAED) pattern (inset of Fig. 7(i)) indicated the single-crystalline nature of PTCDA nanoparticles. Novel nanoparticle-based OPTs were constructed by growing PTCDA nanoparticles inside bowl-shaped pores on Si₃N₄ and depositing volcano-shaped Al gate electrodes with Al₂O₃ insulating layers (Figs. 7(j) and 7(k)). The device showed stable switching behavior with increasing UV light intensity (Fig. 7(l)), and the EQE reached a high value of 3.5 × 10⁶%. The single nanoparticle device was thus able to reveal the intrinsic properties of SMOSCs without the influence of grain boundaries.

The existence of many applications of highly sensitive NIR photodetectors in the field of remote control, fire and airborne early warning, and biomedicine, pushed the development of NIR phototransistors.^[27–29] Recently, novel NIR OPTs based on ultrathin SMOSCs have been reported by Wang and coworkers.^[49] 2D single-crystalline films based on a furan-thiophene quinoidal compound called TFT-CN were fabricated successfully on water surface (Fig. 8(a)). The single-crystalline TFT-CN films had an average thickness of 4.8 nm in a large area of around 10 × 20 μm² (Figs. 8(b) and 8(c)), which was ideal building block for high-performance OPTs. As illustrated in Fig. 8(d), the left figures represent the accumulation regime. In thicker SMOSCs, there is a gradient of the carrier density from the electrodes to the gate dielectric. However, for ultrathin films, the carrier density remains almost constant. The right figures are of the depletion regime, where only ultrathin films are fully depleted, which means that rare leakage current exists. Therefore, the as-prepared TFT-CN ultrathin film OPTs had a quite low dark current of 0.3 pA with an ultrahigh D* of 6 × 10¹⁴ Jones under 808 nm NIR light. Moreover, the EQE reached 4 × 10⁶% along with a high R of 9 × 10⁴ A·W⁻¹, showing the great application potential in NIR detection. In order to enlarge the detection range of SMOSC-based photodetectors, hybrid phototransistors were constructed by combining BPE-PTCDI single crystals with Au nanorods (Fig. 8(e)).^[50] As simulated by finite difference time domain (FDTD) in Fig. 8(f), with the existence of light-scattering plasmonic Au nanorods, the electric field around Au surface was intensified under NIR light and additional hot electrons could be injected into the adjacent BPE-PTCDI nanowires upon light illumination. As a result, the hybrid phototransistor was capable of wide-spectrum detection from 350 nm to 980 nm (Fig. 8(g)). A maximum R of 7.7 × 10⁵ A·W⁻¹ with a maximum EQE of 1.42 × 10⁸% was obtained under 2.5 μW·cm⁻² red light. The hybrid system demonstrated a feasible way to construct high-performance and broadband photodetectors, however, the major shortcoming was the quite slow response speed of several seconds.

Fig. 8.

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Fig. 8. (a) Schematic illustration of transferring 2D TFT-CN films onto an arbitrary substrate. (b) Optical microscopy image of a TFT-CN film, the inset is the corresponding SAED pattern. (c) Atomic force microscopy (AFM) image of a TFT-CN film with an average roughness of ∼ 0.36 nm and thickness of ∼ 4.8 nm. (d) Schematic diagram of OPTs with different active layer thicknesses under different VG in dark. (a)–(d) Reproduced with permission.[49] Copyright 2018, Wiley-VCH. (e) High-resolution transmission electron microscopy (HRTEM) image of a hybrid BPE-PTCDI nanowire. Inset: TEM image of Au nanorods. (f) FDTD simulation of the electric field enhancement in a hybrid BPE-PTCDI nanowire under 980 nm light. (g) Absorption spectra of BPE-PTCDI nanowires, Au nanorods, and a hybrid BPE-PTCDI nanowire dispersion in ethanol. (e)–(g) Reproduced with permission.[50] Copyright 2016, Wiley-VCH.

A breakthrough of fabricating p–n junctions constructed by all SMOSCs was made by Zhang and coworkers in 2010.^[51] Bilayer nanoribbons consisting of p-type CuPc and n-type copper hexadecafluorophthalocyanine (F₁₆CuPc) were successfully fabricated by using a two-step PVD method. The two kinds of SMOSCs are similar in molecular structure and lattice constants, ensuring highly selective crystallization (Figs. 9(a)–9(d)). Then photodiodes were constructed by depositing asymmetric Au and Al electrodes at two sides of the nanoribbons, a pronounced photocurrent was observed at AM 1.5. Core-shell coaxial p–n junctions (Fig. 9(e)) consisting of p-type CuPc and n-type 5,10,15,20-tetra(4-pyridyl)-porphyrin (H₂TPyP) nanowires were fabricated by Cui et al.^[52] The p–n junction device (Fig. 9(f)) had a photoswitching ratio of around 100 when white light with an intensity of 5.51 mW·cm⁻² was turned on and off, and the performance was better than that of each single component device. Instead of the PVD method, Park et al.^[53] reported the fabrication of cross-stacked organic p–n junctions via solution-based transfer printing for the first time. The detailed process is shown in Fig. 9(g), where electron beam lithography (EBL) was used to pattern nanolines down to 100 nm in width onto a polyurethane acrylate (PUA) mold. The PUA mold was then coated with C₆₀ ink solution to form C₆₀ nanowires in channels. After contact printing, C₆₀ nanowires could be transferred from the mold to a flexible poly(ethersulfone) (PES) substrate. With the consecutive alignment and contact printing of 6,13-bis-(triisopropylsilylethynyl) pentacene (TIPS-PEN) nanowires, cross-aligned single-crystalline C₆₀/TIPS-PEN heterojunctions were formed (Figs. 9(h) and 9(i)). The single heterojunction device showed diode characteristics with a rectification ratio of 13.5 at 50 V. However, due to the small junction area (∼ 1 × 10⁻¹⁰ cm²), the photoresponse was quite poor. In order to improve the poor device performance caused by small junction area, single-crystalline 2D layered heterojunctions were fabricated by Wang’s group.^[54] It was found that C8-BTBT (or PTCDA) molecules tend to epitaxially grow on graphene substrate to form a uniform monolayer (∼ 0.9 nm) in a PVD process (Figs. 9(j)–9(k)). Heterojunction photodetectors with vertically stacked Au/C₈-BTBT/PTCDA/graphene structures were thus constructed (Fig. 9(l)), which showed ideal diode characteristics with a rectification ratio of over 10³ (Fig. 9(m)). Under 514 nm light illumination, a R of 0.37 mA·W⁻¹ was obtained.

Fig. 9.

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Fig. 9. (a)–(d) Optical, SEM, AFM characterizations and the height profile of a CuPc/F16CuPc p–n junction nanowire, respectively. (a)–(d) Reproduced with permission.[51] Copyright 2010, American Chemical Society. (e), (f) SEM images of the CuPc/H2TPyP p–n junction and the corresponding device, respectively. (e), (f) Reproduced with permission.[52] Copyright 2012, Wiley-VCH. (g) Schematic illustration of the transfer and patterning procedure for the fabrication of cross-stacked organic p–n junctions. (h) SEM image of the cross-stacked C60/TIPS-PEN p–n junction. (i) Photograph of integrated C60/TIPS-PEN photodiodes on a flexible substrate. (g)–(i) Reproduced with permission.[53] Copyright 2014, American Chemical Society. (j) AFM images of a graphene layer before (left) and after (right) epitaxial growth of C8-BTBT. Scale bar: 2 μm. (k) Raman spectrum of the C8-BTBT layer grown on graphene. Inset: Raman mapping of the C8-BTBT signal. Scale bar: 2 μm. (l) The device structure illustration of a PTCDA/C8-BTBT p–n junction photodiode. (m) I–V characteristics of the PTCDA/C8-BTBT p–n junction in linear scale (black) and log scale (blue), the red line is the fitting curve of a standard diode. (j)–(m) Reproduced with permission.[54] Copyright 2016, American Chemical Society.

Besides all SMOSC-based p–n junction photodiodes, single-crystalline organic semiconductor/metal Schottky junction photodiodes and organic/inorganic semiconductor p–n junction photodiodes have also been reported so far. Jie’s group^[55] reported the in situ self-assembly of 2,4-bis[4-(N, N-dimethylamino)phenyl]squaraine (SQ) nanowire arrays directly on a SiO₂/Si substrate with pre-deposited Au/Ti electrodes (Fig. 10(a)). Schottky-type photodetectors (Fig. 10(b)) were constructed due to the large work function difference between SQ nanowires and the Ti electrode (Fig. 10(c)). Under white light illumination, the Schottky-type device exhibited obvious rectification behavior with a photoswitching ratio 60 times higher than that of the ohmic-type device. The integrated photodiodes showed uniform rectification characteristics with small pixel-to-pixel variation under 0.1 mW·cm⁻² white light (Figs. 10(d) and 10(e)). The applications of self-assembled SQ nanowires were further broadened by the same group.^[56] SQ solution was dropped onto a crystalline Si (c-Si) substrate with pre-patterned SiO₂ layer and Au electrodes (Fig. 10(f)). The device took the advantages of both organic SQ nanowires and inorganic c-Si, leading to a high rectification ratio of 10⁴ as well as the broadband photoresponse from UV to NIR. It is noteworthy that the photoresponse within the detection wavelength range was quite even with R values greater than 1 A·W⁻¹ and D* values greater than 6 × 10⁹ Jones (Fig. 10(g)), the R (1.3–9.8 A·W⁻¹) was much better than those of organic heterojunction broadband photodetectors.^[57,58] Recently, Wang and coworkers^[54,59] have reported the optoelectronic applications of 2D C8-BTBT single crystals by van der Waals epitaxial growth. Due to the relatively strong van der Waals interactions between the deposited organic layers and the 2D atomic crystal substrates (such as graphene, BN, and MoS₂), ordered molecular packing can occur near the interface without lattice mismatch. Few layers of C8-BTBT crystals were then deposited onto MoS₂ substrate via van der Waals epitaxial growth.^[59] A R of 22 mA·W⁻¹ under white light at zero bias was obtained, along with a power conversion efficiency of 0.31%, paving the way for organic/inorganic self-driven photodetectors.

Fig. 10.

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Fig. 10. (a) Schematic illustration of an SQ nanowires/Ti Schottky photodiode. (b) Optical microscopy image of the integrated SQ nanowires/Ti Schottky photodiode. (c) Energy band diagram of the SQ nanowires/Ti Schottky junction. (d) Distribution of the rectification ratio for each Schottky-type device in dark. (e) 2D contrast map of the integrated Schottky-type photodiode under 0.1 mW·cm−2 white light. (a)–(e) Reproduced with permission.[55] Copyright 2013, American Chemical Society. (f) SEM image of a single SQ nanowire/c-Si device. (g) Wavelength-dependent R and D* of the SQ nanowire/c-Si device. (f), (g) Reproduced with permission.[56] Copyright 2014, American Chemical Society.