The axial ligand (X) in SubPc is determined by the boron trihalide (BX₃) used in the templated cyclotrimerization reaction with 4-nitrobenzonitrile. Chlorine (Cl) and bromine (Br) were selected to study the influence of halogen electronegativity on the material properties. The synthetic pathways for SubPc a and b are shown in Fig. 1. Weigh 2.22 g (12.6 mmol) of 4-nitrobenzonitrile and transfer it to a three-necked, round-bottomed flask. Add 30 mL of o-dichlorobenzene as the reaction solvent, followed by 5 mL of 1M BCl₃ toluene solution. Conduct the entire reaction in a nitrogen atmosphere. Stir and heat to 180 °C, maintaining the reaction for 5 hours. After the reaction has finished, remove the solvent and any excess boron trichloride by vacuum distillation. Separate the product using silica gel column chromatography to obtain the red solid SubPc a with a yield of 63.21%. ¹H nuclear magnetic resonance (¹H NMR) (400 MHz, CDCl₃): $\delta$ 8.69 (d, J = 2.2 Hz, 3H), 8.62 (dd, J = 8.6, 2.2 Hz, 3H), 8.11 (d, J = 8.6 Hz, 3H); Fourier-transform infrared (FT-IR) (KBr): $\upsilon$ (cm^-1): 1620 (C=C), 1529 (C=N), 1512 (N-O), 698, 740, 794, 856; electrospray ionization-mass spectrometry (ESI-MS): calcd for C₂₄H₉BN₉O₆Cl, [M + H]⁺, 566.3620, found: 566.4282.

Fig. 1.

The synthetic route of the SubPc a and b. SubPc, subphthalocyanine.

The synthesis method for SubPc b is identical to that for SubPc a. The reagents used were 4-nitrobenzonitrile (2.00 g, 11.5 mmol) and boron tribromide (0.49 mL, 57.5 mmol). The yield is 53.46%. ¹H NMR (400 MHz, CDCl₃): $\delta$ 8.69 (d, J = 2.2 Hz, 3H), 8.62 (dd, J = 8.6, 2.2 Hz, 3H), 8.11 (d, J = 8.6 Hz, 3H); FT-IR (KBr): $\upsilon$ (cm^-1): 1604 (C=C), 1589 (C=N), 1541 (N-O), 738, 802, 858, 929, 1074; ESI-MS: calcd for C₂₄H₆BN₉O₆Br, [M + H]⁺, 610.3389, found: 610.1292.

4-nitrobenzonitrile, boron trichloride and boron tribromide were obtained from Ascender Chemical Technology Company (Shanghai, China). o-Dichlorobenzene was purchased from Tokyo Chemical Industry (Shanghai, China) Chemicals Company (Shanghai, China). The Fourier-transform infrared (FT-IR) spectra were collected in the range 4000–400 cm^-1 on a Nicolet Fourier spectrophotometer (Thermo Fisher Scientific, Waltham, MA, USA) using KBr pellets. The mass spectra were measured with a matrix-assisted laser desorption/ionization time-of-flight/time-of-flight (MALDI-TOF/TOF Mass spectrometer) (Bruker, Karlsruhe, Baden-Württemberg, Germany). The NMR spectra were recorded using a Bruker Ultrashield Spectrometer (Bruker, Karlsruhe, Baden-Württemberg, Germany).

The indium tin oxide (ITO) glass first underwent sequential ultrasonic cleaning to remove surface organic matter and particles, followed by ultraviolet (UV)-ozone treatment for 20 minutes to enhance the hydrophilicity of the ITO surface. A 40 nm thick layer of poly(3,4-ethylenedioxythiophene) (PEDOT) was spin-coated onto the ITO glass at 2000 rpm for 30 seconds, followed by heat treatment at 100 °C for 20 minutes to remove residual solvents. The thickness of the active layer was controlled by adjusting the concentration of the solution and the spin speed, and was consistently measured to be 40 $\pm$ 3 nm using a DektakXT surface profilometer (Bruker, Billerica, MA, USA). Subsequently, SubPc a or SubPc b was mixed with poly N-vinylcarbazole (PVK) at a weight ratio of 1:4, dissolved in 1,2-dichlorobenzene, spin-coated onto the PEDOT layer under a nitrogen atmosphere, and finally annealed at 100 °C for 15 minutes to form an active layer 40 nm thick. This temperature was chosen as it effectively removes residual solvent and improves active layer morphology without approaching the decomposition temperature of the materials (see thermogravimetric analysis, TGA). Finally, a 30 nm C₆₀ electron transport layer, a 1 nm LiF buffer layer, and a 120 nm aluminum cathode layer were thermally evaporated. Photoresponse characteristics, such as on-off switching curves and response times, were acquired on a Keithley 2400 source meter (Tektronix, Beaverton, OR, USA) at 0 V bias. The response times (rise and decay) were extracted from the switching curves as the time taken for the photocurrent to rise from 10% to 90% ($\tau$_rise) and to fall from 90% to 10% ($\tau$_decay) of its maximum value.

SubPc a or b was blended with PVK at various weight ratios (1:2, 1:4, 1:6). A preliminary device screening indicated that the 1:4 ratio yielded the optimal balance between film quality and photocurrent response, and was thus selected for all devices reported herein.

PEDOT: PSS (Baytron P VP Al 4083) was purchased from Heraeus (Hanau, Hessen, Germany). PVK was purchased from Lumtec Technology Corp. (Hsinchu, Taiwan). C₆₀ was purchased from Sigma-Aldrich (St. Louis, MO, USA). All other reagents, solvents, and chemicals were purchased from commercial sources and used without further purification unless otherwise specified.

The service life and environmental stability of organic materials are strongly correlated with their thermal stability. A TG1093F1 thermogravimetric analyzer (NETZSCH Company, Selb, Bavaria, Germany) is employed to analyze the thermal stability of SubPcs. The testing instrument is carried out under nitrogen, and the rate of temperature increase is 10 °C/min. The test results are shown in the figure below.

As shown in Fig. 4, both SubPc a and SubPc b exhibit good thermal stability below 100 °C, with a mass loss of less than 10%. A marked decrease in mass, corresponding to thermal decomposition, occurs at approximately 233 °C and 246 °C ($\pm$3 °C), respectively.

Fig. 4.

TGA curves of SubPc a and b. TGA, thermogravimetric analysis.

Cyclic voltammetry (CV) curves are a commonly used research method for electrochemical testing to determine the electrochemical activity of compounds as well as to determine their lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) positions [17]. In this paper, a CHI640E electrochemical workstation (Shanghai, China) was used with three electrodes, including a reference electrode, a working electrode, and a counter electrode.

Fig. 5 shows the cyclic voltammetric curves, from which the energy levels of SubPc materials could be obtained using Eqns. 4,5,6 [18, 19]. E_OX denotes the starting oxidation potential. E_g values can be obtained from the absorption spectra. Table 3 lists the calculated results. Multiple scanning tests show deviations of $\pm$0.05 eV in oxidation potential and bandgap values.

(4)$$\mathrm{HOMO}(\mathrm{eV})=-4.74\mathrm{eV}-{\mathrm{eE}}_{\mathrm{ox}}$$

(5)$$\mathrm{E}(\mathrm{eV})=1240/{\lambda }_{\text{onset}}$$

(6)$$|\mathrm{LUMO}|=|\mathrm{HOMO}|-{\mathrm{E}}_{\mathrm{g}}$$

Fig. 5.

CV curves of SubPc derivatives in THF. CV, cyclic voltammetry.

The conformational optimization of two subphthalocyanines was carried out using Gaussian 16W software (Wallingford, CT, USA) based on Density Functional Theory (DFT) with the B3LYP method and the 6-31(G*) basis set [11]. The optimization results are shown in Fig. 6.

Fig. 6.

Effects of two compounds on molecular orbital energy levels.

From Fig. 6, the HOMO energy levels of SubPc a and b are –6.71 eV and –6.69 eV, respectively, and their HOMO orbitals span the entire subphthalocyanine ring. In contrast, the LUMO orbitals are primarily distributed across the benzene rings of the two isoindole units and the nitro groups.

Following material characterization, photodetectors were fabricated. Key parameters, including PVK:SubPc ratio, annealing temperature, and layer thicknesses, were optimized as described in the Experimental Section. Both subphthalocyanine derivatives exhibit strong green light absorption and high HOMO energy levels, which enable them to be used as donor materials in high-performance photodetectors [20, 21]. Two green light photodetectors were prepared using the two SubPc derivatives as photoactive layers in Fig. 7. Fig. 7b presents a schematic energy level diagram of ITO/PEDOT:PSS/C₆₀/Al. The favorable alignment between the ITO/PEDOT:PSS work function and the SubPc HOMO level facilitates hole injection, while the ‘staircase’ alignment of the SubPc LUMO, C₆₀ LUMO, and Al work function promotes efficient electron extraction and blocks hole leakage, contributing to high D*.

Fig. 7.

Device structure and energy level diagrams of SubPc-based photodetectors. (a) Device with SubPc. (b) Energy levels in the absence of SubPc. PEDOT, poly(3,4-ethylenedioxythiophene); PSS, polystyrene sulfonate; ITO, indium tin oxide; PVK, poly (9-vinylcarbazole).

To further characterize the photodetectors, key performance parameters such as R, D* and EQE were calculated using the following formulas [22, 23, 24]. These parameters are summarized in Table 4.

(7)$$R=\frac{\mathrm{\Delta }I}{A\times P}$$

(8)$$D^{*}=\frac{R}{\sqrt{2q{J}_d}}$$

(9)$$EQE=\frac{R\times hc}{e\lambda }$$

In the formula above, $\mathrm{\Delta }$I represents the disparity between the photocurrent and the dark current, which is derived from Fig. 8. A signifies this effective device area, P represents the intensity of incident light, q stands for the charge of an electron, J_d indicates the density of dark current, h is Planck’s constant, c is the speed of light, e is the elementary charge, and $\lambda$ corresponds to the incident light wavelength.

Fig. 8.

Switching characteristics of the SubPc-based photodetectors.

As presented in Table 4, the device based on SubPc a demonstrated superior photodetection performance under 520 nm light illumination (0.5 mW/cm²), with a specific detectivity (D*) of 1.65 $\times$ 10¹² Jones and an EQE reaching 28.34 %. In comparison, the SubPc b-based device showed slightly lower metrics, with a D* of 1.52 $\times$ 10¹² Jones and an EQE of 27.13 %. The reported parameters are the average values with standard deviations obtained from over three devices, indicating good reproducibility. Preliminary operational stability testing showed the photocurrent remained above 95% of its initial value after 30 minutes of continuous illumination, suggesting promising short-term stability.

Response time ($\tau$) is one of the key parameters that determines whether the photodetectors can quickly track the changing optical signals [25, 26]. Fig. 9 shows the response times of these two photodetectors and the results show that both photodetectors can detect green light and respond quickly within about 32 ms. The response speed, characterized by the rise time ($\tau$_rise = 31 $\pm$ 4 ms) and decay time ($\tau$_decay = 34 $\pm$ 4 ms) as defined in the OPDs Preparation and Characterization section, yields an average response time ($\tau$) of approximately 32 ms.

Fig. 9.

The diagrams of the light response times of the two photodetectors based on SubPcs.

We have synthesised novel phthalocyanine semiconductor materials and conducted in-depth investigations into their optoelectronic properties and photogenerated charge separation capabilities. We have also achieved preliminary results in their application within organic photodetector devices. However, the transport mechanism of photogenerated carriers at semiconductor heterojunction interfaces remains to be thoroughly explored. This will be our primary research focus in the next phase.