Synthesis of Alq3 by a facile co-precipitation approach and study the impact of CNTs support on its microstructure and electronic characteristics for photodiode development

The global demand for renewable energy as an alternative to traditional fossil fuels has motivated the scientific community to develop highly efficient nanocomposite materials that can be used for enhancement to optoelectronic technology. In the present work, organometallics Alq3 and Alq3/CNTs were prepared by cost-effective chemical route. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were performed to identify the structural and morphological features. The hybrid Alq3/CNTs exhibited polycrystalline structure with a large surface area. The optical band gap (Eg) of Alq3 film was evaluated within the visible spectrum in the range of 3.047 eV which reduced to 2.979 eV by CNTs integration. Ag/Alq3/p-Si/Al and Ag/Alq3/CNTs/p-Si/Al photodiodes were fabricated using thermal evaporating technique. Current-voltage (I−V) and capacitance/conductance-voltage (C/G−V) were measured to analyze the photodiode behavior. The main electronic parameters like Rs,n,ϕbandIo were determined using different models indicating that the composite photodiode of high performance in which Rs was decreased whereas Na was increased with doping. Besides, the photocurrent sensitivity was increased from 1.45×10−8A to 1.1×10−5A due to increase of free charge carriers.


Introduction
Because of the industrial development and growing demand for clean-renewable energy, the researchers across the world are working on innovation novel strategies for manufacturing optical systems with high efficacy and low cost. Hybrid semiconductors-based nanomaterials composed of organic/inorganic or metal-organic frameworks have gained considerable attention during the current decade owing to their superior advantages including low molecular weight, flexibility, and rich carbon particles. Nowadays, organometallic compounds, 2,7-Dioctyl [1] bemzothieno [3,2-b] [1] benzothiophene (C8-BTBT), Tris(8-hydroxyquinoline) metals (Mq 3 ), metal phthalocyanine (MPc), and (C 10 -DNTT) have widely used in energy, electronics, photocatalysis, sensing and biotechnology. In particular, Tris(8-hydroxy-quinoline) aluminum (Alq 3 ) of excellent structuralmorphological merits governing by the interplay between metallic and organic ligands. In addition to, its different crystalline states; , , , , a b e d g and broad light absorption has been used as distinct electron transport in photodetectors and thermoelectric (TE) applications [1][2][3][4]. O Sevgili et al (2020) have investigated the physical properties of Alq 3 microdots for enhancement of the photosensitivity of organic photodiode [5].
Another study has reported the impact of temperature and dopant concentrations on the optical and electrical conductivity of Alq 3 /p-Si photodiode. Recently, Alq 3 nanoparticles have been used for fabricating organic photovoltaics and organic light emitting diodes (OLEDs) suggesting various photoluminescence (PL) emission spectrum anchored to their diverse crystalline phase. Despite these advantages, Alq 3 compound has displayed some drawbacks such as low charge carriers mobility, molecular aggregation, and weak charge injection. Several studies approved that incorporation of organometallic molecules with different atoms will improve their physical and chemical features. In general, organometallics can be easily doped with metals, metal Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. oxides or carbonaceous materials producing nanocomposite with promising aspects [6,7]. Dasi et al (2022) have prepared Alq 3 /ZnO thin film by sol-gel spin coating for improving the physical properties of the nanocomposite [8].
On the other side, carbonaceous materials like graphene oxide (GO), graphene nanoparticles, and carbon nanotubes (CNTs) have great importance in electronic industry. These nanomaterials can be utilized as interfacial layers for increasing the charge carriers mobility and device efficacy. Erdal et al (2019) have synthesized TiO 2 /p-Si/Ag heterojunction diode based MWCNTs and graphene nanoparticles by electrospinning technique for enhancement the photodetector behavior [9]. In the present study, carbon nanotubes (CNTs) have been demonstrated as Nobel dopant inside Alq 3 lattice. Notably, CNTs are excellent dopant based on their photoluminescence, structural, mechanical, electrical, and thermal conductivity.
Because of the weak Van der Waals weak bound interaction between Alq 3 molecules and CNTs. The presence of CNTs into the photodiode heterojunction layers led to improve the active sites and hence accelerate the free charge carriers transport to the metal contacts [9,10]. Further, the morphological architecture of two dimensional (2D) CNTs resulting in creating new energy levels within the optical band gap and enhancement the electrical conductivity of the fabricated device. Several chemical methods have been used for fabricating nanomaterials such as hydrothermal, chemical bath deposition, sol-gel, and electrodeposition. Herein, chemical co-precipitation method was utilized to prepare Alq 3 and Alq 3 /CNTs at very low temperatures. It is easy to control dopant concentrations, chemical composition, shape, and particle size of the hybrid composite using the current technique [11,12]. The findings confirmed that the hybrid Alq 3 /CNTs/p-Si diode of higher efficiency compared with Alq 3 /p-Si diode thanks to the seamless structure and high dispersion capability of CNTs.

Synthesis of Alq 3 and Alq 3 /CNTs nanomaterials
The Alq 3 was prepared from the reaction of 8-hydroxyquinoline, and aluminum nitrate as indicated in Scheme 1. Firstly, solution A (HQ solution) was prepared by dissolving 6.97 grams 8-hydroxyquinoline in 100 ml absolute ethanol. Besides, solution B was prepared by dissolving 6.00 grams Al(NO 3 ) 3 .9H 2 O in 100 ml double distilled water. Consequently, solution A was added to solution B with full stirring at constant speed 800 rpm for 15 min. After that, 7.2 grams NaOH was dissolved in 60 ml double distilled water for 30 min then added drop by drop to the mixture until pH 6. The whole mixture was refluxed for 1 h then left on stirring for 24 h at room temperature. The resulting precipitate filtered off under vacuum, washed with distilled water then dried in a furnace at 150°C overnight. The hybrid nanocomposite Alq 3 /CNTs was prepared by dissolving 0.5 g Alq 3 in 30 ml dimethyl sulfoxide for 2 h. A stock solution was prepared by dissolving 0.020 g CNTs into 20 ml deionized water/ethanol for 2 h followed by ultrasonic bath for 1 h. Thereafter, 10 ml CNTs were added drop wise to the Alq 3 solution with continuous stirring for 5 h. Then, the mixture was heat treated in a microwave furnace for 1 h. The resulting precipitate powder was separated by filter paper, washed many times using distilled water, and left for drying in a furnace at 80°C for 12 h.

Characterization of the prepared nanomaterials
The crystal structure of Alq 3 and Alq 3 /CNTs was identified using x-ray diffraction (XRD, Bruker D8 Discovery diffractometer) at wavelength (λ) = 1.540 Å (CuKα radiation) through 2θ changed between 5°-70°. Scanning electron microscopy (SEM, Helios Nanolab. 400) was examined to visualize the morphological properties of the nanoparticles. The nanomaterials were deposited on glass and p-type Si substrates using a spin coater (SpinNXG-P1AC). The average thickness of the coated films was measured using profilometer (Dektak 8 Stylus). The optical properties and the energy gap were investigated from the absorbance spectrum measured by the spectrophotometer Jasco (V-570).
2.4. Fabrication of Ag/Alq 3 /p-Si/Al and Ag/Alq 3 /CNTs/p-Si/Al diode Alq 3 and Alq 3 /CNTs were separately coated onto p-type silicon substrates using a spin coater. Before the deposition process, the silicon wafers have been etched by dilute HF then cleaned with deionized water, methanol, and acetone bath of ratios 3:1:1 respectively, and finally washed by deionized water. Silver (Ag) metal top contact was evaporated via thermal evaporation technique giving diode contact area 3.00 × 10 −2 cm 2 . The current-voltage I V -( ) characteristics were analyzed using a programmable (Keithley 6517b) electronic device. The photocurrent sensitivity was examined using a 200 W halogen lamp and the power intensity controlled by solar power meter (TM-206). In the frequency ranging from 10 Hz 1 MHz, a computerized HIOKI 3531-Hitester LCR meter was utilized for

Microstructure analysis
The crystal structure of Alq 3 and Alq 3 /CNTs was examined by XRD at room temperature through the diffraction 2θ angle ranging from 5°-70°. As illustrated in figure 1(a), the pattern of Alq 3 exhibited numerous diffraction peaks with a well-defined sharp intensity at 2θ = 10.50°. This dominant peak was indexed to the pure phase of polymorphs Alq 3 crystal phase according to the standard card (JCPDS 26-1550) [13,14]. The XRD diffraction of hybrid Alq 3 /CNTs is relatively different in which the intensity of the diffraction peaks was decreased by doping concentrations suggesting the small crystalline size. Further, a weak peak was observed at 2θ = 26.90°attributed to the presence of CNTs into the host Alq 3 framework [15]. Moreover, the morphological nature was examined using SEM micrographs. Figure 1(b) shows the particles of Alq 3 possess irregular granular shapes, in nanoscale, agglomerated together with high density. The SEM image of Alq 3 /CNTs described the granules particles decorated with CNTs in large surface area, besides the surface appears in high porosity, (figure 1(c)) [15,16]. The microstructure analysis revealed that the hybrid composite of porosity surface, quantum confinement size, and large surface area. These special structural-morphological aspects play an efficient role in fabricating high performance photodiode [16,17].

Optical characterization
The optical absorbance of Alq 3 and Alq 3 /CNTs thin films deposited on glass substrates was measured through the ultraviolet-visible (UV-vis) region. As demonstrated in figure 2(a), Alq 3 has a fundamental absorption edge of intense peak at 395 nm shifted to higher wavelength by CNTs dopants. The hybrid Alq 3 /CNTs depicted strong optical absorption inside the visible region attributed to the interaction between the incident photon energy and the particles on the thin film surface. Further, the energy gap E g ( ) is an important optical parameter used for determining the optical properties of semiconductor materials and the type of electron transition.
The energy gap of the fabricated films was defined from Tauc's relation expressed by the following equation [18,19]: where, A is the optical absorbance, hn is the photon energy, K is a constant, t is the thickness of the films was 145 ± 10 and 178 ± 10 for Alq 3 and Alq 3 /CNTs, respectively, and a is the optical absorption coefficient. From  h h 2 a n -n ( ) plot in figure 2(b), the E g was estimated by extending the straight line of h 2 a n ( ) to the h n axis at h 0.   ) curve at the voltage axis. From the straight-line part in the forward biased region, the ideality factor n ( ) was calculated with values greater than unity suggesting that the fabricated diodes of non-ideal behavior.
It is worthy to note that, the deviation in the ideality factor attributed to several factors like irregular coated layers, organic crystal structure defects, inhomogeneity of organic/inorganic materials, and series resistance. Besides, the reverse saturation current I o ( ) was calculated from equation (4) which suddenly decreased in case of Alq 3 /CNTs/p-Si composite diode. Moreover, the series resistance R s has a significant impact on the device behavior however, the shunt resistance R sh provides its quality [26,27]. The values of R s and R sh were determined from R V jplot where, R j V I = ¶ ¶ [27,28]. As depicted in figure 3(b), R s was decreased whereas R sh was increased by loading CNTs. The obtained electronic parameters were summarized in table 1. As presented, R , s , b f n, I o were decreased while R sh was sharply increased dependence of structural-morphological modulation by CNTs addition. The seamless-hexagonal honeycomb nature enable the CNTs to easily interact with Alq 3 molecules producing new interfacial states at the heterojunctions and consequently enhancement the photodiode performance. Furthermore, Cheung-Cheung model was given by the following relations [26][27][28]: In this model, R s was calculated from the slope of dv dln I ( ) -I plot and n was identified from the intercept on the current axis nkT q, (figure 4(a)). The b f was obtained from the intercept of the relation H I -( ) I equal to n b f whereas R s was defined again from the slope of figure 4(b). On the other side, Nord functions were applied using the formulas [25][26][27]:  ) Additionally, it is better to apply Nord function for ideal diode of ideality factor equals one [27,28]. Figures 6(a), (b) demonstrates the I V -(

Photocurrent sensitivity
) characteristics under various illumination intensities. As illustrated in figure 6(a), the photoconductivity of Alq 3 /p-Si diode was increased, when the diode subjected to illumination, the trapped charge carriers absorb the light energy and transfer to the conduction band [29,30]. The photosensitivity of hybrid Alq 3 /CNTs/p-Si diode was increased from 1.45 10 A 8 in dark to 1. 1 10 A

-
under light intensity, 100 mW cm −2 ( figure 6(b)). The conduction mechanism was identified from the following equation [31,32]: Ph m = ( ) I Ph is the photocurrent, A is constant, P is the illumination intensity and m is an exponent that provides information about the trapped centers and the ability of charge carriers to transfer to the conduction band. The exponent m was estimated from the slope of ln I lnP Phplot (figure 7) to be 1.32 and 0.823 for pure and hybrid diode, respectively suggesting that the performance of the photodiode dependence of the presence of trapped centers between the interfacial states [30,32].
)analysis The influence of voltage combined with alternating current frequency on the photodiode capacitance was examined. As demonstrated in figure 8(a), Alq 3 /p-Si diode has two capacitance peaks (C p ) at −0.80 and −2.30 V which significantly decreased at higher frequencies. The capacitance reduction is associated with the insufficiency of trapped charge carriers that follow high applied frequencies. The trapped charges at the interface states rapidly response to the AC signals at low values from 10 kHz-300 kHz which is less than the dielectric relaxation frequency of Alq 3 [33,34]. Figure 8(b), depicts a prominent capacitance peak at −1.63 V, this peak was regularly decreased, broadened, and shifted to higher bias with applied frequencies attributed to the variation of depletion width by CNTs integration. Moreover, the capacitance was corrected by study the influence of the series resistance given by the equations [34,35]: C adj is the corrected capacitance, 2 f w p = the angular frequency, C p the measured capacitance and G p the measured conductance. It was observed from figures 8(c), (d) that, the corrected capacitance sharply decreased with the applied frequencies associated with the presence of series resistance or the insulator layers between the interface states. Further, the regular response of the trapped charge carriers at low frequencies may be owing to the impact of CNTs, figure 8(d). On the other side, the conductance -voltage G V p -( ) and corrected conductance-voltage G V adj -( ) of the photodiode were examined as a function of frequency. The conductance of Alq 3 /p-Si and Alq 3 /CNTs/p-Si diode was increased with applied frequencies, figures 9(a), (b). The conductance was corrected by series resistance from the relation [35]: As illustrated in figure 9(c), there is no specific behavior attributed to the G adj of Alq 3 /p-Si diode This behavior can be explained by the influence of series resistance on the response of the trapped centers at various alternating current signals. However, the G adj of Alq 3 /CNTs/p-Si was increased with applied frequencies until 600 kHz affected by the presence of CNTs inside the Alq 3 framework, figure 9(d) which result in improvement the charge carriers mobility and improvement the diode conductivity [34,36]. The deviation in the photodiode conductivity at the frequencies 700 kHz and 900 kHz can be explained by the change in the trapped charge carriers response to applied AC signals. This response strongly dependence of the dielectric relaxation frequency as well as the change in thermal emission rate of Alq 3 . It is worthy to note that, the charge carriers would follow the Ac signals when the applied frequency is less than the relaxation frequency which happened in the composite diode with CNTs addition until 600 kHz. [37,38]. The behavior of the series resistance was investigated using the relation below [39,40]: Figures 10(a), (b) depicts the reduction of R s with applied frequencies. As shown in figure 10(b), the hybrid diode has lower series resistance owing to the high response of the interface states to Ac signals and increase the free charge carriers by CNTs integration [41,42].
The capacitance-voltage C V - o e N C are the electronic charge, built-in potential, applied DC bias, static dielectric constant, space charge capacitance, permittivity of vacuum, and density of states, respectively. From Mott Schottky 1 C V 2 -/ ( ) plot at low voltage, the slope of the straight line gives N a which represents the density of motivated charges, and the intercept defines the built-in potential V , bi (figure 11) [41,42]. The N a value was used to calculate b f using equation (17). The results in table 2 demonstrate that the hybrid composite diode has greater acceptor concentration which clearly confirms the significant impact of CNTs on enhancement of the photodiode quality. As presented in table 2, the composite diode has revealed low V bi at the interface state which is attributed to the small potential across the depletion region.

Conclusions
Alq 3 and Alq 3 /CNTs nanomaterials were successfully synthesized by scalable chemical technique. The microstructure and optical analysis revealed that Alq 3 /CNTs composite of crystalline structure, large surface area and wide optical band gap 2.97 eV.
The photodiode performance was evaluated from the I V -( ) characteristic curve using different models. The main electronic parameters comprising R , , R s b s h F and n were calculated in dark suggesting that the hybrid diode of higher efficiency. Under the impact of illumination intensity, the photosensitivity was increased attributed to the presence of trapped charge carriers. Further, the C G V -/ measurements indicated that the photodiode behavior was strongly affected by the interfacial states located at the heterojunction layers. The findings approved the important role of CNTs for improvement the photodiode efficacy thanks to their exceptional physical and chemical properties.

Data availability statement
The data that support the findings of this study are openly available at the following URL/DOI: https://doi.org/ 0000-0002-4032-5462.