We had reported the synthetic procedures of TTN2 previously (Sun, et al., 2016). To grow its crystals, TTN2 powder was dissolved in a mixed solution with a m-xylene: toluene: dichloromethane volume ratio of 2 : 1: 1 at a concentration of 0.5 mg ml⁻¹. Glass weighing bottles (40 mm × 25 mm) were used as the containers to grow the microcrystals. The bottles were cleaned by sonification in ethanol for 30 min. SiO₂ (300 nm)/Si substrates were successively cleaned by sonification in deionized water (DI), acetone, and isopropanol. The substrates were then treated with oxygen plasma at 80 W for 10 min followed by immediate modification with octadecyltrichlorosilane (OTS) by a vapor phase method. The OTS-modified SiO₂/Si substrates were successively cleaned by sonification in chloroform, n-hexane and isopropanol. The cleaned OTS-modified SiO₂/Si substrates were then placed flat at the bottom of the glass weighing bottles. 20 μL of as-prepared TTN2 solution was slowly dropped on the surfaces of the substrates. The bottles were placed in an oven and kept at 60°C for 2 h. After completer evaporation of the solvents, microcrystals of TTN2 were obtained.

Bottom-gate/top-contact OFETs were constructed with OTS-modified SiO₂ (300 nm) as the gate dielectric layer. Source and drain electrodes were fabricated by stamping Au (80 nm) stripes on TTN2 microcrystals (Tang, et al., 2008a). Field-effect mobility (μ) in the saturation regime was calculated from the following equation: I _DS = (W/2L)µ C _i (V _GS−V _TH)², where I _DS is the source-drain current, μ is the field-effect mobility, V _TH is the threshold voltage, V _GS is the applied gate voltage, L is the channel length, W is the channel width and the C _i is the specific capacitance (10 nF cm⁻²).

Optical and cross-polarized optical microscope (OM and POM) images were obtained with Nikon ECLIPSE Ci-POL polarized optical microscope. Tapping mode atomic force microscopy (AFM) images were measured using a Bruker Dimension Icon. X-ray diffraction (XRD) measurements were carried out in reflection mode at 45 kV and 200 mA with monochromatic Cu Kα radiation utilizing a Rigaku Smartlab diffractometer. Ultraviolet-visible and near-infrared (UV-Vis-NIR) absorption spectrum of TTN2 microcrystals was measured with a Agilent Technologies Cary Series UV-Vis-NIR Spectrophotometer. OFETs were characterized using a Keithley 4200 SCS in ambient environment at room temperature. The photo responses of the OFETs were measured with lasers with tunable power intensity. The laser power intensity was measured in situ with a PM100 digital power meter.

Tetrathiafulvalene (TTF) has been extensively investigated as a strong electron donor in organic electronics (Canevet, et al., 2009; Wudl, et al., 1970). Among TTF derivatives, DB-TTF has attracted much attention (Figure 1A). It is a symmetrical, planar, and completely conjugated molecule, which resembles the prevailing structure of common OSCs (Wang, et al., 2018; Zhang, et al., 2018). DB-TTF shows a herringbone packing motif in the solids with a mean molecular plane distance of 3.60 Å (Figures 1A–C) (Emge, et al., 2007). It has been studied as a high performance OSC in OFETs (Mas-Torrent and Rovira, 2005, 2006b; Naraso, et al., 2005; Del Pozo, et al., 2016). However, in the solid state UV-Vis-NIR spectra, DB-TTF exhibits an absorption peak at 476 nm and the optical bandgap estimated from the absorption edge of the spectra was 2.36 eV (Mas-Torrent, et al., 2006a). The photoabsorption in VNIR range and the narrow optical bandgap indicate that OFETs based on DB-TTF are potentially unstable when irradiated by visible light and they are not suitable for applications that require visible-blind photoresponse (Mas-Torrent, et al., 2006a).

Tetrathianaphthalene (TTN) is an isomer of TTF which differs structurally from the latter only in the arrangement of the two ethyne bridges. In contrast to the planar molecular configuration of TTF, TTN adopts an armchair-like configuration with a dihedral angle between the terminal ethene and the central tetratia-enthene of about 137° (Varma, et al., 1988). It has been well-known that planarity of molecules plays a major role in determining the shape and wavelength position of the absorption spectra of materials (Nijegorodov and Downey, 1994). The decrease in planarity from TTF to TTN can cause blue shift of the absorption edge, making TTN a possible skeleton for stable OSCs. While aryl substituted or fused TTF derivatives have been intensively investigated in OFETs (Naraso, et al., 2005; Gao, et al., 2007; Yamashita, 2009; Mas-Torrent and Rovira, 2011, 2004; Yang, et al., 2013), the charge transport properties of TTN derivatives are still unknown. In this study, the optoelectronic properties of TTN2, a molecule with an armchair-like configuration, are uncovered. Figures 1D–F show the molecular structure and packing of TTN2, whose skeleton is an isomer of DB-TTF. As can be seen in Figure 1E, the dihedral angle between the central moiety (C₂S₄) and the terminal group in TTN2 was about 128° (Sun, et al., 2016). Compared with DB-TTF, TTN2 changed from a planar structure to an armchair configuration. UV-Vis-NIR absorption spectra of TTN2 solution and microcrystals (Figure 1G; Supplementary Figure S1) show that there is no absorption after 400 nm and the maximum wavelength absorption peak of TTN2 microcrystals is at 398 nm. Compared with DB-TTF with a maximum wavelength absorption peak at 476 nm (Mas-Torrent, et al., 2006a), the absorption of TTN2 shows a dramatical blue shift. Figure 1H shows the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of DB-TTF and TTN2. The HOMO levels of DB-TTF and TTN2 calculated by cyclic voltammetry (CV) are −4.88 eV and −5.38 eV, respectively (Mas-Torrent, et al., 2005; Sun, et al., 2016). The optical bandgaps of DB-TTF and TTN2 estimated from the solid state UV-Vis-NIR absorption spectrum are 2.36 eV (Mas-Torrent, et al., 2006a) and 3.02 eV, respectively. Therefore, the LUMO levels calculated from the HOMO levels and the optical bandgaps are −2.52 eV and −2.36 eV, respectively. We also obtained the frontier molecular orbitals (FMOs) and the HOMO and LUMO energy levels of DB-TTF, TTN2 and 1,4-benzodithiino[2,3-b][1,4]benzodithiine (TTN1) by theoretical calculations (B3LYP/6-31G (d,p)) based on their crystal structure (Supplementary Figure S2). The results showed that the increased optical bandgap of TTN2 was due to the change of its molecular configuration and the four methoxy groups showed little influence on the energy levels. By changing the molecular configuration from planar to armchair, the LUMO level becomes higher and the HOMO level becomes lower, indicating that controlling molecular configuration is an effective strategy to tune the bandgaps of OSCs.

In order to investigate the charge transport properties, TTN2 microcrystals were prepared by a simple solution drop-casting method (Figure 2A, see the Experimental method for details). Figure 2B shows an OM image of a TTN2 microcrystal, which exhibits a smooth surface without any notable cracks or steps. Under a POM, when the sample was rotated 45°, the color of the whole microcrystal changed uniformly and significantly from bright to dark (Figures 2C,D), indicating a crystalline nature of the microcrystal. The microcrystals were very stable—no morphology changes were observed by OM after the crystals being placed in ambient air and light for more than two years (Supplementary Figure S3). Figures 3A,B show typical AFM images of TTN2 microcrystals. The surface is atomically flat with a root-mean-square (RMS) roughness as low as 0.873 nm. Typical thickness of the as-grown TTN2 microcrystal is 55.9 nm (Figure 3C). The crystalline property of the TTN2 microcrystals was further assessed by out-of-plane XRD (Figure 3D). The smooth baseline and sharp diffraction peaks in the XRD spectra indicate that the TTN2 microcrystals are highly crystalline. The strong diffraction peak at 7.01°, which corresponds to a d-spacing of 12.6 Å, can be assigned to (100) plane. The corresponding second, fourth, and fifth order peaks at 14.1°, 28.4°, and 35.8° are also observed, indicating that (100) plane of the TTN2 microcrystals are parallel to the substrates. The sharp diffraction peaks in the XRD spectra of TTN2 microcrystals were unchanged after being placed in ambient air and light for more than two years, further indicating the high stability of the microcrystals (Supplementary Figure S4). Judging from the aforementioned results, TTN2 microcrystals possess high crystallinity and excellent stability.

The charge transport properties and optoelectronic performances of TTN2 microcrystals were investigated by the construction OFETs with a bottom-gate top-contact configuration (Figure 4). The device structure is shown in Figure 4A. The transfer and output characteristic curves were tested in ambient air under dark conditions (Figures 4B,C). All devices showed typical p-type behaviors operating in accumulation mode. Judging from the dual sweep curves under dark conditions (Supplementary Figure S5), the devices exhibited a small hysteresis. High on/off ratio (I _on/off) of approximately 10⁷ was obtained. The highest mobility reached 6.64 × 10⁻² cm² V⁻¹ s⁻¹ and the average mobility of 32 devices was 5.40 × 10⁻³ cm² V⁻¹ s⁻¹ (Supplementary Figure S7A). Note that the results represent the first exploration of OFETs based on TTN derivatives. For the same device, after being placed in ambient air and light for more than two years, it still showed good field-effect performance (Supplementary Figure S6) and the average mobility of 32 devices could still reach 1.33 × 10⁻³ cm² V⁻¹ s⁻¹ (Supplementary Figure S7B).

Judging from the UV-Vis-NIR absorption spectra and the optical bandgap of TTN2, OFETs based on TTN2 microcrystals are visible-blind. Figure 4D shows the photoresponse of the OFETs under laser irradiation of different wavelengths from 405–1,020 nm. No shift of the transfer characteristic curves was observed, indicating that both mobilities and threshold voltages of the OFETs were unaffected by VNIR light irradiation. In addition, under laser irradiation of different intensities, the curves also remained unchanged (Supplementary Figure S8). After that, we also tested the bias-stress stability of the devices. The transfer characteristic curves of the device showed negligible shifts during a continuous application of gate bias voltage for one hundred times (Supplementary Figure S9), which proved that the device had good bias-stress stability. Judging from the aforementioned results, OFETs based on TTN2 showed excellent environmental and operational stability.