Organic Semiconductor Devices Fabricated with Recycled Tetra Pak^®-Based Electrodes and para-Quinone Methides

Abstract

This work presents the synthesis of para-quinone methides (p-QMs), which were deposited as films using the high vacuum sublimation technique after being chemically characterized. The p-QMs films were characterized morphologically and structurally using scanning electron microscopy, atomic force microscopy, and X-ray diffraction. In addition, their optical behavior was studied by means of ultraviolet–visible spectroscopy, and the optical gaps obtained were in the range of 2.21–2.71 eV for indirect transitions, indicating the semiconductor behavior of the p-QMs. The above was verified through the manufacture and evaluation of the electrical behavior of rigid semiconductor devices, in which fluorine-doped tin oxide-coated glass slides (FTO) were used as an anode and substrate. Finally, as an original, ecological, and low-cost application, the FTO was replaced by substrates and anodes made from recycled Tetra Pak^®, generating flexible semiconductor devices. Although the electrical current transported depends on the type of p-QMs, the substituent in its structure, and the morphology, the kinds of substrate and anode also influence the type of electrical behavior of the device. This current–voltage study demonstrates that p-QM2 with 4-Cl-Ph as a radical, p-QM3 with 4-Et₂N-Ph as a radical, and p-QM6 with 5-(1,3-benzodioxol) as a radical can be used in optoelectronics as semiconductor films.

1. Introduction

In recent years, para-quinone methides (p-QMs) have been recognized as ubiquitous structural motifs, with a wide variety of uses as synthons of natural products, functional materials, and organic intermediates in organic synthesis owing to their high reactivity [1,2]. Further, they display differing biological activities, such as anti-proliferative activity in cancer cells [3], DNA alkylation [4], acting as adrenergic receptors [5], and anti-inflammatory activity [6]. The special conjugate structure between a neutral non-aromatic and zwitterionic aromatic contributing valence bonds structures a reactive intermediate in p-QMs (Figure 1). Their remarkable chemical reactivity is due to their strong dipolar character and thermodynamic driving force, which cause rearomatization [7]. The strong electron-withdrawing property of carbonyl encourages electrons to move towards it, causing the p-methylene carbon to exhibit a certain electron deficiency and thus some electrophilic properties [2]. For this reason, the electronic characteristics of this type of compound are interesting structural motifs to be studied as organic semiconductors, considering that, to our knowledge, their application in this area has not been explored.

Organic semiconductors have emerged as promising materials for a variety of modern stretchable and bendable applications, such as wearable electronics, conformable sensors, flexible displays [8,9], polymer light-emitting diodes [10], and organic thermoelectric generators [11,12,13]. Inorganic electronic devices are commonly fabricated on substrates such as rigid silicon wafers that limit their integration with unconventional forms or applications requiring conformability [8]. Devices integrated with organic semiconductors address these limitations; in this sense, the search for flexible materials, with optimal mechanical properties and high chemical resistance, to be used as substrates, has taken on great importance in organic electronics. Furthermore, to generate a circular economy, substrates that come from recycled materials, such as Tetra Pak^®, are sought after. Tetra Pak^® is one of the most used materials in the food industry thanks to its excellent ability to prevent the absorption of light, humidity, and oxygen, as well as its lightness and ease of handling [14]. Tetra Pak^® packages are composed of six layers made of three different materials: 75% rigid laminated paper, 20% low-density polyethylene (LDPE), and 5% aluminum by mass. The first, third, fifth, and sixth layers are made of LDPE, the second is cardboard, and the fourth is aluminum [15]. The cardboard provides mechanical resistance and stability to the packaging, the aluminum serves to prevent oxidation and protect food from ultraviolet light, mainly from the sun, and, finally, the LDPE layer is essential for the adherence of all other layers, and this layer protects the contents from external liquids [14]. In the latest report from the Tetra Pak^® company for the year 2022, it is mentioned that 1.2 million tons of Tetra Pak^® were collected to be sent for recycling, which is equivalent to 20% of the total material produced [16]. The rest was disposed of in landfills or incinerators, contributing to solid waste generation [14].

The use of recycled Tetra Pak^® within organic electronics has not yet been widely explored but has great potential, due to the properties of Tetra Pak^®. Among its potential uses in electronics is the one proposed by Figueroa-González et al. [17], who used graphene-covered Tetra Pak^® as an electrode for supercapacitors. Also, some authors of this work [18] used Tetra Pak^® to develop semiconductor devices. However, it would be desirable to develop a technique for creating electronics using an alternative Tetra Pak^® electronic component that is inexpensive, has good flexibility and mechanical properties, has low thermal expansion, and can reduce the accumulation of persistent waste. For this reason, this work focuses on generating new organic semiconductors based on p-QM compounds and their use as semiconductor films in rigid and flexible devices. The latter are manufactured with Tetra Pak^® anodes acting as flexible substrates. The importance of this work lies in the study of recycled Tetra Pak^® as a substrate and an electrode component of organic devices. This electrode type can favor the transport of charges inside organic semiconductor devices. Presently, the problems of the metal electrode–molecule–metal electrode devices are due to the unknown number of molecules in the semiconductor between junctions; furthermore, the molecular conformation during electrical measurement is also unknown [19,20]. This work proposes using Tetra Pak^® electrodes treated with graphitic carbon, which can generate better interactions with excellent electronic coupling properties [21]. The results are compared to those using traditional tin oxide-coated glass slides (FTO) as electrodes. It was found that the molecular assembly of the organic semiconductor film at the semiconductor/electrode interface plays important roles in determining the injection barrier, charge carrier mobility, and orientation and morphology of the later-deposited organic layers [22,23,24].

2. Materials and Methods

The following reagents were used as received: 2,6-di-tert-butylphenol, benzaldehyde, 4-chlorobenzaldehyde, 4-diethylamino benzaldehyde, 1-methylindole-3-carboxaldehyde, 9-anthracenecarboxaldehyde, 2H-1,3-benzodioxole-5-carbaldehyde, dry toluene (obtained of commercially source Sigma-Aldrich, Carlsbad, CA, USA), ethyl acetate, n-hexane, and silica gel 60 (0.063–0.200 nm, 70–230 mesh ASTM, acquired from Merck-Millipore, Steinheim, NW, Germany). ¹H and ¹³C NMR spectra were obtained in CDCl₃ using Bruker 300 Ascend equipment (Bruker, Ettlingen, BW, Germany) at 300 and 75 MHz, respectively. The IR spectrophotometry characterization was carried out using a Nicolet iS5-FT spectrophotometer (Thermo Fisher Scientific Inc., Waltham, MA, USA), within a 4000–300 cm⁻¹ region with an 8 cm⁻¹ resolution.

2.1. Synthesis and Characterization of p-QMs Compounds

The synthesis of p-quinone methides (Scheme 1), was carried out, based on a previously reported procedure [25,26,27]. 2,6-di-tert-butylphenol (0.2 g, 1.0 mmol) and 1.0 eq of corresponding aldehyde (1a–f) were added into a two-neck reaction flask; these reagents were dissolved in 20 mL of anhydrous toluene, the reaction mixture was refluxed for 30 min, 2.0 eq of piperidine was subsequently added dropwise, and the resulting mixture was stirred at 170 °C for another 8 h. Finally, 2.0 eq of acetic anhydride was slowly added. The acquired reaction mixture was then allowed to reach 25 °C before being transferred to a separatory funnel and washed with water (1 × 30 mL), brine (1 × 30 mL), and n-hexane (3 × 30 mL). The organic layers were combined and dried up with anhydrous Na₂SO₄; then, the solvent was removed by vacuum evaporation. The resulting liquid was purified by column chromatography, using n-hexane/ethyl acetate eluent in a polarity gradient 99:1. It is important to note that the obtained molecules have been previously reported; hence, their physical and spectroscopic data were appropriately correlated with literature information [25,26,27]. The structures of the target p-QMs are shown in Figure 2.

2.2. Deposit and Characterization of Semiconductor Films

The films were deposited using four different substrates: p-type silicon wafers, glass, fluorine-doped tin oxide-coated glass slide (FTO), and recycled Tetra Pak^® (TP). Before being used as substrates, the p-type silicon was washed with hydrogen peroxide, and the glass and FTO were washed in an ultrasonic bath with chloroform, isopropanol, and acetone. The Tetra Pak^® was washed multiple times with soap, water, and ethanol, and then it was cut into long rectangles and rewashed with soap and methanol. Finally, it was dried at 90 °C for one hour, this temperature does not affect the constituents of the Tetra Pak^®, specifically the polymer with the lowest melting and decomposition temperatures of 108 °C [28]; it is worth noting that, furthermore, Tomaszewska et al. used the coupling of thermogravimetry–differential thermal analysis–mass spectrometry (TG-DTA-MS) technique to determine the degradation temperature of this polymer. Their results show that thermal and thermo-catalytic degradation occurs from 370 °C to 510 °C [29], which is the process that releases the highest quantity of greenhouse gases, such as carbon dioxide and methane [30]. The Tetra Pak^® side containing aluminum and LDPE was painted with a commercial graphitic carbon paint (which was composed of graphite microparticles dispersed in isopropanol, and a polymer was added to enhance its adherence) with a sheet resistance of 1.2 K ohms/sq. The painted pieces were air-dried for one whole day until they were completely dry. After this process, the rectangles were cut into 3 × 3 cm pieces. For the deposit of the compound films, the high vacuum sublimation technique was used, with a system (Intercovamex, S.A. de C.V., Cuernavaca, Morelos, México) of two pumps, mechanical and turbomolecular, which evaporated the compounds under a vacuum of 10⁻⁵ torr, at 200 °C, for a period of 3 min, with the deposition rate presented in

Table 1. Deposition rate and thickness of p-QM films.

Compound	Deposition Rate (Å/s)	Thickness of the Films on Glass (Å)
p-QM1	37	505
p-QM2	63	950
p-QM3	1.3	35
p-QM4	52	691
p-QM5	17.4	183
p-QM6	1.5	84

. The thickness of the films was monitored using a microbalance quartz crystal monitor, connected to a thickness sensor. For the morphological characterization of the film, a Hitachi Tabletop Microscope TM3030 (Hitachi High-Tech, HITACHI, Toyo, Japan) was used at 5 kV. The topography has been studied with an atomic force microscope (AFM) using an Ntegra platform (Nanosurf AG, Liesta, Switzerland), and the images were analyzed using Gwyddion 2.65 software. Additionally, the films were examined by X-ray diffraction analysis (XRD) using the θ–2θ technique in a Rigaku Miniflex 600 diffractometer (Rigaku Corporation, Tokyo, Japan), with Cu Kα (λ = 1.5406 Å), at 40 kV, 20 mA. The optical properties were obtained in a 200–1100 nm wavelength range, on a UV-Vis 300 Unicam spectrophotometer (Thermo Fisher Scientific Inc., Waltham, MA, USA), respectively. The FTO and TP substrates were used to measure the current–voltage (I–V) characteristics. A Keithley 4200-SCS-PK1 auto-ranging picoammeter (Tektronix Inc., Beaverton, OR, USA) was used with the collinear four-point probe method. The samples were evaluated at room temperature for their I–V electrical behavior in dark and natural lighting conditions.

3. Results and Discussion

3.1. Synthesis and Characterization of p-QM Compounds

The p-QM compounds were obtained by reacting 2,6-di-tert-butylphenol with the corresponding aldehyde in toluene as a solvent in a stochiometric ratio. The resulting molecules were obtained in an 80%–95% range yield (good to excellent yields) as yellow solids; the target molecules were characterized by IR, ¹H-NMR, and ¹³C-NMR (the Supplementary Material File includes the IR and the ¹H-NMR spectra). The spectroscopic and spectrometric data correlate with those reported in the literature [25,26,27]. Subsequently, thin films of the previously synthesized and characterized compounds were deposited. Vacuum sublimation deposition has proven to be a useful mode to improve p-QM film uniformity and stability. After depositing the films, an IR spectrophotometry evaluation was performed, confirming the chemical stability of the p-QM compounds. The films’ IR spectra for the target molecules are displayed in Figure 3. It is worth noting that the main bands at 1613 ± 4 (C=O), 2955 ± 4, 2863 ± 4, 1451 ± 7 (CH₂) and 1249 ± 6 (C–H) cm⁻¹ are in agreement with the literature [27]. It is important to note that the obtained results suggest that the employed deposition technique is suitable for manufacturing the films, since no chemical decomposition of the compounds occurs.

3.2. Deposit and Characterization of Semiconductor Films

The p-QM compounds were deposited simultaneously on different substrates—p-type silicon wafers, glass, fluorine-doped tin oxide-coated glass slide (FTO), and recycled Tetra Pak^® (TP)—to evaluate their morphology, structure, optical properties, and semiconductor behavior in the solid state. It is important to consider that, depending on the p-QM compound to be sublimated, even using the same temperature and pressure conditions in the vacuum system, the deposition rate during the formation of the films changed; therefore, the thickness obtained for each p-QM film was also different (see Table 1). The higher the deposition rate, the greater the thickness of the films. Vacuum deposition is a very suitable technique to manufacture semiconductor films of small molecules such as p-QMs; however, the structure of the molecule and the type of its substituents play a significant role in the type of stacking, morphology, and thickness of the deposited semiconductors films [31]. The greatest thickness was obtained for the film with p-QM2, and the smallest thickness was found for the film with p-QM3. The structure of the films was complemented by XRD and the corresponding diffraction patterns are displayed in Figure 4, demonstrating their differences. The p-QM1, p-QM2, and p-QM5 films exhibit distinct peaks at specific 2-theta values, suggesting a small degree of crystallinity. This crystallinity could be attributed to the types of substituents in these compounds, which may facilitate the molecular packing in the film; however, the diffractograms for p-QM3, p-QM4, and p-QM6 indicate an amorphous structure. It is important to consider that, in optoelectronics devices, charge transport occurs when charges skip between molecules [32]. This happens by applying a voltage between a pair of electrodes, connected to the film. The applied voltage through this simple device and the disorder in the films can impact charge mobility. Each molecule’s random orientation and position also affect charge mobility, which causes carriers to tend to localize on an individual molecule. The XDR results indicate that in this type of p-QM film, mainly amorphous, charge transport between the valence and conduction bands occurs through indirect electronic transitions.

The films deposited on glass substrates were analyzed by SEM. Figure 5 shows the microphotographs at 1000× for each p-QM film. The images show, as expected, differences due to deposition rate and nucleation and growth processes during film formation. The film of the p-QM1 compound exhibits elongated dendrite-like structures, characterized by a primary arm with secondary growth in preferential directions, forming a network across the substrate. The film’s non-uniform coverage, as evidenced by small uncoated substrate sections, leads to an incomplete film layer. In the image corresponding to the film of p-QM2, flake-like structures are observed that almost completely cover the substrate and have varied sizes. According to Table 1, this film has the greatest thickness, related to the large growth of flakes with sizes greater than 10 μm on each side. The film of p-QM3 shows poor substrate coverage, with noticeable gaps exposing the underlying material. The film is composed of elongated fiber-like structures, each with an approximate thickness of 1–2 μm. These fibers are irregularly distributed across the substrate, contributing to the incomplete coverage observed. The film of p-QM4 shows elongated structures, but they are less dendritic and more uniformly distributed compared to p-QM1. The coverage of the substrate is more uniform, with fewer exposed areas, indicating a more consistent film formation. The film of p-QM5 is also made up of elongated structures, in this case, needle-shaped, that have grown in different directions from large nucleation sites. In these sites, clusters or centers of material are observed, from which the needles emerge in different growth directions. Finally, the film of p-QM6 shows a heterogeneous morphology, characterized by particles of varying sizes and distributions, indicative of different nucleation and growth stages. They result in a thin, continuous layer covering the substrate. In subsequent stages, additional nuclei formed and grew into larger particles, some exceeding 10 μm in size. As observed in the SEM image, the initial thin film layer can be seen as a relatively smooth background, while larger particles, corresponding to later stages of nucleation, are prominently scattered across the surface.

As this study proposes using TP as a substrate in flexible devices, and the charge transport mechanism changes, concerning rigid devices on glass, the SEM study was carried out on the films on TP. In the SEM images at 1000×, as shown in Figure 6, it is evident that the nucleation and growth processes are different during the formation of the films on the two different substrates. The p-QM1 film exhibits dendritic structures with growth in preferential directions on both substrates. On the glass substrate, the dendritic structures are present but smaller and less defined. In contrast, on the TP substrate, the dendritic structures are larger, more defined, and prominently display growth in preferential directions. Additionally, the film coverage on the TP substrate is more uniform compared to the glass substrate, where significant substrate areas remain exposed. In the p-QM2 film, the morphology changes drastically, and on the glass substrate, the film maintains a flake-like structure with numerous uncoated areas, indicating an irregular coverage. The flakes appear thin and fragmented. Conversely, on the TP substrate, the morphology seems much thicker and more robust, although the flake-like structure is still present. The coverage on TP is more uniform compared to the glass; however, there are still visible uncoated regions. Another important change is observed for the p-QM3 compound, which, unlike the deposit on glass, presents complete coverage of the TP and a uniform morphology in this case. On the other hand, the film with the p-QM4 compound is the only one that presents the greatest similarity when deposited on glass. In TP, its morphology is uniform, with almost complete substrate coverage and elongated structures with growth in preferential directions. However, on the TP substrate, the elongated structures that make up the film are thicker. The morphology of p-QM5 differs significantly between the glass and TP substrates. Large nucleation and growth sites are evident on the glass substrate, appearing as prominent protrusions on the surface, resulting in an irregular and non-homogeneous film. In contrast, the film on the TP substrate is much more homogeneous, with no large nucleation and growth sites observed. Instead, the TP film is composed of elongated grains with clearly defined borders between them, indicating a more consistent and uniform coverage. Finally, in the film of p-QM6, the morphology also changes drastically with respect to the film on glass. As shown in Figure 6, material segregation occurs, leaving large substrate areas uncoated. From the previous results, it is observed that, although the film deposition process is carried out simultaneously within the same vacuum sublimation system, and under the same operating parameters, the type of substrate, in addition to the structure of the corresponding p-QM, exerts an important influence on the film morphology. AFM was performed to study the topography of the films, and 3D images for each p-QM film deposited on TP are shown in Figure 7. This substrate was considered for AFM analysis, due to the purpose of using it as a support in flexible electronic devices. As observed in the 3D images, the films present a heterogeneous topography formed by prominent ridges and valleys.

The UV-VIS spectrophotometry of each film allowed us to determine their optical behavior and the semiconductor capacity through their band gap. Figure 8 shows the absorbance and transmittance spectra of the films, and it is evident that each compound’s chemical composition and structure are responsible for its optical behavior. The p-QM1, p-QM2, and p-QM5 films that present small crystallinity have similar absorbance and transmittance spectra, while the amorphous films, p-QM3, p-QM4, and p-QM6, also have similarities in their spectra. Regarding the absorbance (Figure 8a), the spectra of the p-QM1 and p-QM2 films show a band in the range between 325 and 440 nm, and this band is shifted towards longer wavelengths (410–500 nm) for the p-QM5 film. It is also evident that in the spectrum of the rest of the p-QM3, p-QM4, and p-QM6 films, two bands are present the first band between 340 and 430 nm and in the second band between 500 and 600 nm. These films have a greater radiation absorption capacity, and a greater semiconductor capacity would also be expected. The presence of the bands is due to the type of substituent in the p-quinone methide molecules, which also gives rise to the different morphology in each of the deposited films. On the other hand, concerning transmittance (Figure 8b), it is observed that from 500 nm, transparency increases significantly in all films, with the highest being in the p-QM5 film with 89%, followed by p-QM4 with 76%, p-QM6 with 44%, p-QM3 with 32%, p-QM1 with 11%, and p-QM2 with 6%. From these results, it can be deduced that the p-QM5 film has the potential to be used as a transparent anode in devices such as organic solar cells. An important aspect to highlight is the optical behavior of the p-QM3 film, which, especially at wavelengths λ > 400 nm, presents alternating absorption–transmission behaviors. This behavior is a result of the structure of the substituent, since apparently, the substituent 4-Et₂N-Ph favors this optical behavior.

The fundamental absorption edge is analyzed within the one-electron theory of Barden et al. [33]. This theory has been used to obtain information about the interband transitions because the absorption coefficient α is related to the photon energy hν for the indirect interband transition by the relation [33,34]:

α = α₀(hν − E_g ± E_phonon)²

(1)

where h is Planck’s constant, α₀ is also a constant, E_phonon is phonon energy assisting the indirect transition, and E_g is the optical energy gap. The absorption coefficient α is obtained according to the following expression:

α = (1/d) ln (1/T)

(2)

d is the thickness of the film and T is the transmittance. The inverse of α is the depth of radiation penetration into the semiconductor and is equal to the average penetration distance of the photon before being absorbed. Concerning photon energy hν, ν is the frequency, which is given by:

ν = c/λ

(3)

where c is the speed of light, and λ is the wavelength. The indirect band gap was determined by plotting α^1/2 as a function of photon energy hν, as shown in Figure 9. The extrapolation of the straight-line graphs at α^1/2 = 0 will give the optical energy gap [34]. Due to the predominantly amorphous structure in p-QM films, the electronic transitions are of an indirect type, and the values obtained are within the same order of magnitude between 2.21 and 2.71 eV. The growth in the optical band gap follows the order p-QM4 ˂ p-QM5 ˂ p-QM6 ˂ p-QM1 ˂ p-QM2 ˂ p-QM3, so in terms of this optical band gap, the p-QM4 (R = 3-(1-methylindole)) film is the one that would present a smaller energy gap between the valence and conduction bands, which can promote charge transport more efficiently. It is important to consider that when a semiconductor is in thermal equilibrium, its electrons tend to occupy the lowest energy levels in each band. Consequently, free electrons reside in the “valleys” of the conduction band, and holes reside at the peaks of the valence band. During the recombination process, a conduction electron moves into the place of a hole. In the indirect gap semiconductor, a phonon (heat) is normally emitted; in a semiconductor with indirect electronic transitions, the transitions consist of the simultaneous absorption/emission of a photon and a phonon. As mentioned above, the structure of each p-QM substituent is responsible for the E_opt values obtained. The molecular structure of p-QM compounds displays two carbon bonds, alternating between a single carbon–carbon bond and a double carbon–carbon bond (see Figure 1). The electrons of their π orbitals are delocalized over the molecules and they move with greater or lesser ease depending on the substituent in its structure. If the p-QM films are expected to function as an active layer in organic optoelectronic devices, they must have interconnected delocalized conduction bands and valence bands in the structure. From the calculation of the optical band gap, it is observed that all the values obtained are within the range of organic semiconductors, lower than 3 eV [32,35,36]. However, it is required to evaluate the electrical behavior in the films to establish the charge transport mechanism that occurs in p-QM films.

3.3. Electrical Study with FTO and TP Anode

To study the electrical behavior of p-QM compounds, simple single-layer devices were built, with the following conformation: glass/FTO/p-QM/Ag (see Figure 10a). In these devices, a joint union assembly is formed, where multiple p-QM molecules stack between the FTO and Ag electrodes, with one electrode on the top and the other on the bottom. The results of their I–V studies for natural lighting and dark conditions are presented in Figure 11. The fact of measuring in these two lighting conditions is because devices such as organic photodiodes or organic solar cells present different electrical and photovoltaic responses, respectively, in service conditions in natural light or darkness. Except for the device with p-QM1, where the transported current is very low and its behavior is similar in light and dark conditions, there are differences in the rest of the devices. This fact is due to a probable photoconductive or photovoltaic behavior in the films; in natural lighting conditions, the electrical current transported is greater than in dark conditions, which is due to an excitation and transport of the charges by ultraviolet radiation from the sun. From the notable differences in the I–V plots, it is evident that the chemical composition of each film plays a decisive role in its electrical behavior. The behavior is ohmic in devices with p-QM1, p-QM5, and p-QM6 films. In contrast, in devices with p-QM2 and p-QM3 films, the behavior is ohmic at low voltages and changes to the Space Charge Limited Current mechanism (SCLC) at high voltages. The above occurs because by increasing the voltage, to improve the current transported, a situation is reached in which the charge carriers do not move fast enough and accumulate in a region of the p-QM film. Under these conditions, this thin film is no longer homogeneous, since the charge density would have a different value depending on the region of the film. The device enters a regime that Ohm’s Law does not govern and the SCLC mechanism becomes important. However, these same devices are the ones that generate the largest electric current, which is two orders of magnitude greater than the current carried in the devices with p-QM5 and p-QM6 and three orders of magnitude greater than the electrical current in the device with p-QM1. The p-QM1 film can be considered an electrical insulator due to the low electrical current transported, and its low electrical response in light and dark conditions. On the other hand, the p-QM4 film conducts more current in dark conditions than in natural lighting, so it does not behave like a photosensitive semiconductor, and although it carries a current of the same order of magnitude as in the two previous devices, apparently the natural lighting conditions do not generate the electron–hole pairs required for the photocurrent. This study aimed to determine the electrical behavior of p-QM films as organic semiconductors. From the above results, it is observed that the compounds p-QM2, p-QM3, p-QM5, and p-QM6 have the potential to be used in optoelectronics as semiconductor films, and even their photoconductive characteristics can be studied. To our knowledge, there are no studies on the electrical properties of compounds of this type; however, in devices with p-QM2 and p-QM3 films, electric currents of 10⁻² A are generated, which is of the same order of magnitude as in devices with consolidated organic semiconductors such as metallic phthalocyanines (MPcs). Some examples are the Au/CoPc/p-Si device prepared by H.S. Soliman et al. [37] or the ITO/SiPc(OH)₂/Ag device with the same ITO and Ag electrodes as in this study [38]. Other organic semiconductor films with ITO and Ag as electrodes that generate electric currents of the same order of magnitude as p-QM2 and p-QM3 are the indanones [39]. These examples give an indication of the broad field of study that p-QMs have in optoelectronics.

Finally, to determine the functionality of Tetra Pak^® treated with graphitic carbon (TP), as a substrate and an anode in organic electronics, devices with the TP/p-QM/Ag conformation (see Figure 10b) were manufactured and analyzed. The top and bottom electrodes’ size, uniformity, and material properties affect the ensemble device measurement [22,40]. It is for this reason that, in this work, it was decided to use an electrode made with recycled TP, to determine the functionality of this electrode, and to compare it with the I–V behavior generated by the FTO. Graphite on the surface of the Tetra Pak^® allows the construction of molecular networks with greater tunability and flexibility [22]. Figure 12 shows the I–V results obtained for natural lighting and dark conditions, and mainly in the devices with the p-QM1, p-QM3, p-QM4, and p-QM6 films, the effect of the change in lighting conditions is evident. It is also observed that in the devices with the p-QM1 and p-QM5 films, the behavior is ohmic and the transported current has such low orders of magnitude that it could fall within the category of insulating materials. The devices with p-QM2, p-QM3, p-QM4, and p-QM6 films carry electrical currents that are several orders of magnitude higher. In the device with the p-QM2 film, the ohmic behavior drops after 0.6 V, while in the device with the p-QM3, the ohmic behavior is maintained. In the device with p-QM4 in lighting conditions, the transport current is ohmic at low voltages and remains constant from 0.6 V onwards. This may be because, as mentioned above, this compound is not photosensitive to exposure to radiant energy. Finally, in the device with the p-QM6 film, ohmic behavior occurs up to 0.5 V and subsequently changes to SCLC. The changes in the electrical behavior of these devices regarding devices on glass with FTO anode are due to the presence of the TP-based substrate, which also acts as an electrode and changes the energy barrier for charge transport, at its interface with the p-QM film. Taking into consideration the morphology of the p-QM films, it would be expected that the devices with p-QM3, p-QM4, and p-QM5 would be the ones that carry the most electrical current. However, it is observed in the curves in Figure 12 that the highest current transported occurs in the device with p-QM2, followed in an order of magnitude smaller by the devices with p-QM3 and p-QM6. In this case, it is verified that the use of TP as a substrate and electrode has different effects, such as the change in morphology of the films, and with this, the change in the electrical behavior of the films of the p-QM compounds. The latter is associated with the presence of graphitic carbon.

4. Conclusions

A series of para-quinone methide semiconductors were synthesized and characterized. Due to their high thermal stability, the semiconductor films of p-QMs were deposited by high vacuum sublimation. This technique generated heterogeneous films with an amorphous structure in the case of p-QM3, p-QM4, p-QM5, and p-QM6, and with a slightly crystalline structure in the case of p-QM1 and p-QM2. The optical gap is between 2.21 and 2.71 eV, which places these compounds within the organic semiconductors. With the p-QM films, rigid devices were manufactured using glass and FTO as a substrate and anode, respectively. Their electrical behavior was compared to that obtained using a substrate and anode made from recycled TP. The change in electrical behavior is evident; however, the values of electrical current transported in the devices demonstrate that p-QM2, p-QM3, and p-QM6 films can be used in optoelectronics. Moreover, the first two exhibit apparent photoconductive behavior. Finally, given the obtained results with the target p-QMs, this work could be the beginning of further studies related to their electrical behavior and their integration as thin films for organic optoelectronic devices.