Engineering of Tio2 Or Zno—Graphene Oxide Nanoheterojunctions for Hybrid Solar Cells Devices

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Article Engineering of TiO2 or ZnO—Graphene Oxide Nanoheterojunctions for Hybrid Solar Cells Devices

Duarte Carreira 1, Paulo A. Ribeiro 2 , Maria Raposo 2 and Susana Sério 2,*

1 Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal; [email protected] 2 CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia, Universidade NOVA de Lisboa, 2829-516 Caparica, Portugal; [email protected] (P.A.R.); [email protected] (M.R.) * Correspondence: [email protected]

Abstract: It is currently of huge importance to ﬁnd alternatives to fossil fuels to produce clean energy and to ensure the energy demands of modern society. In the present work, two types of hybrid solar cell devices were developed and characterized. The photoactive layers of the hybrid

heterojunctions comprise poly (allylamine chloride) (PAH) and graphene oxide (GO) and TiO2 or ZnO ﬁlms, which were deposited using the layer-by-layer technique and DC-reactive magnetron sputtering, respectively, onto ﬂuorine-doped tin oxide (FTO)-coated glass substrates. Scanning electron microscopy evidenced a homogeneous inorganic layer, the surface morphology of which was dependent on the number of organic bilayers. The electrical characterization pointed out

that FTO/(PAH/GO)50/TiO2/Al, FTO/(PAH/GO)30/ZnO/Al, and FTO/(PAH/GO)50/ZnO/Al architectures were the only ones to exhibit a diode behavior, and the last one experienced a decrease

in current in a low-humidity environment. The (PAH/GO)20 impedance spectroscopy study further revealed the typical impedance of a parallel RC circuit for a dry environment, whereas in a humid environment, it approached the impedance of a series of three parallel RC circuits, indicating that water and oxygen contribute to other conduction processes. Finally, the achieved devices should be Citation: Carreira, D.; Ribeiro, P.A.; encapsulated to work successfully as solar cells. Raposo, M.; Sério, S. Engineering of

TiO2 or ZnO—Graphene Oxide Keywords: hybrid solar cells; graphene oxide; zinc oxide; DC-magnetron sputtering; layer-by-layer Nanoheterojunctions for Hybrid Solar Cells Devices. Photonics 2021, 8, 75. https://doi.org/10.3390/ photonics8030075 1. Introduction The use of renewable energies is of great importance because of the increase in fossil Received: 6 February 2021 energy costs and to avoid global warming by CO reduction. Solar energy can be con- Accepted: 9 March 2021 2 Published: 12 March 2021 sidered as sustainable energy, which may effectively satisfy a part of the energy demand of future generations [1,2]. One effective way of converting solar energy into a form like

Publisher’s Note: MDPI stays neutral electricity is using solar cells, which are based on the photovoltaic (PV) effect. It has been with regard to jurisdictional claims in reported that single-junction crystalline materials such as Si and GaAs can reach power published maps and institutional affil- conversion efficiencies (PCEs) up to 30% [3]. Although advances in technology and the iations. mass production of conventional silicon-based solar modules have allowed significant price reductions, this technology presents some limitations, especially regarding material consumption, production speed, and features such as flexibility and form factor. Thus, the development of new solar cell technologies addressing the aforementioned properties is of great importance and interest to the industry. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Twenty years ago, dye-sensitized solar cells (DSSCs) were proposed as low-cost This article is an open access article alternatives to the conventional Si solar cells, particularly because of the simplicity of distributed under the terms and their fabrication processes. DSSCs correspond to the third-generation PV cells category conditions of the Creative Commons where new trends in the PV technology are applied [4,5]. In first-generation PV cells, an Attribution (CC BY) license (https:// electric interface is created between doped n-type and p-type bulk silicon, and currently, creativecommons.org/licenses/by/ this type of cell offers the highest conversion efficiency [6]. Second-generation PV cells 4.0/). are based on thin-film technology and, as these cells utilize less material, the production

Photonics 2021, 8, 75. https://doi.org/10.3390/photonics8030075 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 75 2 of 16

cost significantly drops. Nevertheless, they are less efficient than the bulk cells. Both first- and second-generation cells are based on opaque materials, where front-face illumination and moving supports to follow the sun position are necessary. Third-generation solar cells are based on nanostructured materials and are made of pure organic or a mixture of organic and inorganic components. The nanostructured DSSCs, invented by Michael Grätzel and Brian O’Regan in 1991 [7], is an attractive alternative to the existing solid-state p–n junction solar cells. DSSCs with photoelectric conversion efficiencies of ~12% have been achieved using a liquid electrolyte. Excellent stability data have been obtained for liquid DSSCs, but much less is known about the stability of solid-state DSSCs [8]. A volatile electrolyte will also lead to potential problems on sealing the device, although efficient sealant materials have now been developed. However, fully solid-state DSSCs are less efficient than liquid-type DSSCs, although Kanatzidis’s group recently achieved a PCE of 8.5% with a solid-state DSSC [9]. Though considerable progress has been made in improving the PCEs of fully solid-state DSSCs, these cells still suffer from low performance, mainly because of rapid carrier recombination at the various device interfaces [10–12]. Organic conducting polymers with a variety of chemical structures have promising properties, such as easy processing, possible recyclability, relatively low cost, and scalabil- ity. Inorganic semiconductors possess better electronic properties, e.g., a high dielectric constant and a high charge mobility and thermal stability [13,14]. To take advantage of both materials, hybrid nanocomposites of inorganic semiconductors and conducting polymers are of great interest, especially as candidates for PV-cell materials, where the combined absorption band of both materials can also better harvest sunlight [15]. Photoinduced charge separation mainly takes place at the interfaces between inorganic semiconductors and conducting polymers in these hybrid materials, where electrons are injected from the conducting polymers into inorganic semiconductors, and holes remain in the polymers. This interfacial charge separation can to some extent prevent the recombination of sep- arated electrons and holes [16]. Several studies have been carried out to find promising combinations of inorganic/organic materials, and optimal hybrid architectures have been attempted; significant progress has been attained in recent years [17,18]. Additionally, one of the materials that has attracted huge interest in the scientific community is graphene and its derivatives (graphene oxide (GO) and reduced graphene oxide (rGO)) [19]. These materials have been extensively studied due to their electrical, mechanical, optical, and thermodynamic properties and are presently used in several applications such as solar cells, solar fuels, lithium-ion batteries, and supercapacitors. In the case of solar cells, these compounds have been used as transparent and nontransparent electrodes, in photoactive layers and in electron transport layers and gaps [20,21]. The morphology of the active layer is another key factor for the device efficiency. In this sense, the n-type and p-type material need to be organized in a morphology that allows high exciton creation and splitting and also good charge carrier transport. Like the organic solar cells, two types of morphologies of active layers are reported in the literature, a planar and a bulk heterojunction [22]. Moreover, surface modifications of the inorganic material are of huge importance, because they can influence the acceptor conduction band, the exciton dissociation efficiency, and thus the open circuit voltage, Voc, and short circuit current, Jsc. For example, Goh et al. observed that molecular dipoles and acid–base interactions can lead to TiO2 band edge shift, inducing a change in Voc for hybrid solar cells with poly(3-hexylthiophene-2,5-diyl) (P3HT) [23]. Analogously, the conjugated polymers can be modified, as demonstrated by Krüger et al., where the functionalization of both chain ends of P3HT with cyanoacetic groups improved the charge injection efficiency and the power conversion efficiency [24]. Notwithstanding, choosing material arrangements with appropriate properties and matching work functions, loss mechanisms need to be addressed and minimized in order to achieve solar cells devices with high power conversion efficiencies. All the factors mentioned above represent an important challenge and must be considered in the development of hybrid solar cells. Photonics 2021, 8, 75 3 of 16

Thus, and within the frame of work carried out in the last years under organic conductive polymers, photoluminescent and photochromic [25–27], and also on semiconductors oxides ﬁlms, such as TiO2 and ZnO [28,29], the present work was undertaken in order to contribute solutions to capture and convert solar energy. For that, two types of hybrid solar cell devices were developed and studied: FTO/(PAH/GO)x/TiO2/Al and FTO/(PAH/GO)x/ZnO/Al, where x is the number of bilayers. The achieved results reported herein point out that the obtained devices present properties and characteristics that can be successfully applied to solar cells.

2. Materials and Methods All chemicals used in this work were of analytical grade or chemical grade (Sigma-Aldrich).

2.1. Materials In this work, two types of solid supports were used depending on the type of electrical characterization to be performed, namely ﬂuorine-doped tin oxide (FTO)-coated glass substrates (TEC15, 12−14 Ω/ ) purchased from Solaronix (Aubonne, Switzerland, dimensions 39 mm × 15 mm) and glass substrates with deposited interdigitated electrodes (IDE) purchased from DropSens (Oviedo, Asturias, Spain). The IDE glass substrates were formed by BK7 glass with deposited gold interdigitated electrodes (G-IDEAU10) with dimensions 22.8 × 7.6 × 0.7 mm, and each ‘ﬁnger’ was 10 µm in width, which had the same spacing between them. The used gases argon, oxygen, and nitrogen had a ≥99.9% purity.

2.2. Devices Preparation The development of devices involved three phases, described below, in order to obtain the desired architecture (see Figure1a). Figure1b depicts a schematic illustration of the energy band diagram of the developed devices [30].

Figure 1. (a) Representative scheme of the developed hybrid solar cell architectures. (b) Schematic representation of the energy band diagram of the developed devices.

2.2.1. Organic Layer Deposition Thin ﬁlms of the polyelectrolyte poly (allylamine hydrochloride) (PAH) (average Mw = 50,000–65,000 g/mol) and graphene oxide GO (CAS number 777676), both purchased from Sigma-Aldrich (Steinheim, Germany), were prepared by the layer-by-layer Photonics 2021, 8, 75 4 of 16

technique [25,27]. This technique consists of the alternating adsorption of cationic and anionic polyelectrolytes on solid substrates. For the present work, aqueous solutions of the PAH polyelectrolyte were prepared with a concentration of 10−2 M by dissolving 0.0234 g of PAH granules in 25 mL of ultra- pure water (Milli-Q, with a resistivity of 18.2 MΩ cm at 25 ◦C). This water was used in all solutions and also as washing solution. The GO solutions were obtained by dissolving 0.5 mL of GO aqueous solution with a concentration of 4 mg/mL in 24.5 mL of ultra-pure water to obtain a concentration of 0.5 × 10−2 M. In the preparation of PAH/GO LbL films, the solid supports (FTO-coated glass substrates or IDE glass substrates) were first immersed in the PAH aqueous solution during a period of 1 min, thereby adsorbing a positively charged polyelectrolyte layer. After this adsorption, the substrate was washed with ultra-pure water (washing solution) to remove excess molecules or aggregates that have not been completely adsorbed. Subsequently, the substrate was also immersed in the anionic GO aqueous solution for 1 min. The adsorption times were chosen, considering studies of adsorption kinetics carried out by the group. After adsorption of the anionic layer, the substrate was again washed with ultra-pure water to remove the nonadsorbed molecules and was then dried with a stream of nitrogen (99.9% purity, Air Liquide). To note, drying with nitrogen was decisive in obtaining the desired films, as studies performed without drying with nitrogen at the end of each bilayer showed that after a few bilayers (~5), the film began to desorb and the solution of GO precipitated. Therefore, the drying nitrogen step was introduced in the layer-by-layer technique. After the aforementioned sequence, a film with a bilayer was obtained. The procedure described above was repeated according to the number of bilayers, x, that was intended in the film, with the LbL film designated by (PAH/GO)x. In this work, devices with 20, 30, and 50 bilayers of PAH/GO were prepared using the procedure described above. Before the deposition of the LbL films, the FTO and IDE glass substrates were washed with isopropanol, acetone, and ultra-pure water in this order, and then dried with a stream of nitrogen just before use. This procedure, besides guaranteeing the cleaning of the substrates, also allow them to be negatively charged, thus allowing adsorption of the PAH (positive charge) layer. In addition, to prevent contact between the electrodes of the devices in the case of the FTO glass substrates, a portion of the FTO film—approximately 10 mm × 15 mm —was removed on all substrates using a solution of zinc (Zn) with hy- drochloric acid (HCl). The solution was applied to the desired area during a period of about 24 h.

2.2.2. Inorganic Layer Deposition

The next phase consisted of the deposition of the inorganic layer (TiO2 or ZnO), for the FTO glass substrates with an organic layer, which was carried out by DC-reactive magnetron sputtering. Titanium and zinc discs (Goodfellow, Huntingdon, England, 99.99% purity) of 64.5 mm in diameter and 4 mm in thickness each were used as the sputtering targets. A turbomolecular pump (Pfeiffer TMH 1001, Pfeiffer Vacuum GmbH, Asslar, Germany) was used to attain a base pressure of 10−4−10−5 Pa (before introducing the sputtering gas). Before the sputter-deposition step of the films, a movable shutter was interposed between the target and the substrates. The target was pre-sputtered in Ar atmosphere for 2 min to clean the target surface. The target-to-substrate distance was kept constant at 100 mm. Gases in the system were pure Ar and O2 (both 99.99%, Air liquide) and their pressures were separately controlled by needle valves. TiO2 and ZnO depositions were both carried out in 100% O2 atmosphere. For the TiO2 film, the total pressure was kept constant at 2 Pa, the sputtering power was 530 W, and the deposition time was 25 min. In the case of the ZnO film, the total pressure was fixed at 4.8 Pa, the sputtering power was 300 W, and the deposition time was 30 min. The target- to-substrate distance was kept constant at 100 mm. No external substrate heating was Photonics 2021, 8, 75 5 of 16

used during the depositions. The substrate temperature was measured by a thermocouple passing through a small hole in a copper piece, which was placed in contact with the substrate. During the deposition process, the sample temperature increased up to 60 ◦C due to the bombardment of plasma particles of the substrate.

2.2.3. Aluminum Electrode Deposition Finally, to obtain the desired solar cell architecture (see Figure1), an aluminum electrode was deposited by thermal evaporation over an area of approximately 0.95 cm2. The Al electrodes’ deposition was carried out with Al wires of high purity (Advent Research Materials, Oxford, England, 99.5%) placed on a spiral tungsten resistor in a vacuum chamber, and the ﬁlms were mounted on a vertical support at about 10 cm from the tungsten resistor. The deposition was performed with a pressure of 10−5−10−6 mbar assured by a turbomolecular pump, an applied current of 50 A, and a deposition time of 1 min.

2.3. Characterization Techniques The characterization of the thin film’s morphology was carried out by a field emission scanning electron microscope (FEG-SEM JEOL 7001F) operating at 15 keV. A chromium thin film was coated on the films surface before SEM analysis to prevent charge build-up. The device’s architecture was evaluated from the SEM cross-sectional images. The electric measurements (I–V characteristic curve) of the final devices were carried out using a programmable DC power supply model Rigol DP811A (programmable DC power supply, LX1, Telonic Instruments, Berkshire, UK) in the absence of light, at ambient light, and with light from a 250 W halogen lamp positioned at 40 cm from the device. All the I–V measurements were performed by changing the voltage between 0 and ~1.5 V, with an increment of 50 mV at room temperature, which was guaranteed by a vent placed in the measurement system. The electrical analysis of (PAH/GO)20 onto BK7 solid support with gold interdigitate electrodes was carried out by measuring the impedance spectra of this device in air or in argon atmosphere, both at ambient light with a Solartron 1260 Impedance Analyzer (Solartron Analytical, AMETEK scientific instruments, Berwyn, PA, USA) in the frequency range of 1 Hz–1 MHz, applying an AC voltage of 25 mV. All measurements were performed at room temperature 25 ◦C. All devices were stored in the dark inside a desiccator with silica gel to control the humidity.

3. Results and Discussion 3.1. Build-Up of PAH and GO LbL Films PAH and GO LbL films were prepared with different numbers of layers, namely, 20, 30, and 50 bilayers. The growth of these films was previously characterized by measuring the UV-vis absorbance spectra as a function of the number of bilayers, and a linear film growth was detected, as is reported in detail elsewhere [31]. Although the aforementioned linearity is easily perceived, it should be noted that in the classic LbL technique, a linear film growth is not commonly detected in the adsorption of the first bilayers. This phenomenon can be explained by the roughness of the used substrate and uneven distribution of charges over the surface substrate [32].

3.2. Morphological Characterization The developed devices were characterized by scanning electron microscopy to study the morphology and to evaluate if the desired architecture was obtained. By the SEM cross-sectional images analysis, images not shown, it was veriﬁed that all the developed devices exhibited the desired architecture. On the other hand, the surface SEM images reveal for all samples a uniform surface with no apparent defects, such as the presence of cracks, which could limit or prevent the normal functioning of devices. Photonics 2021, 8, x FOR PEER REVIEW 6 of 15

The developed devices were characterized by scanning electron microscopy to study the morphology and to evaluate if the desired architecture was obtained. By the SEM Photonics 2021, 8, 75 cross-sectional images analysis, images not shown, it was verified that all the developed 6 of 16 devices exhibited the desired architecture. On the other hand, the surface SEM images reveal for all samples a uniform surface with no apparent defects, such as the presence of cracks, which could limit or prevent the normal functioning of devices. For the TiOFor2-based the TiOdevices2-based (Figure devices 2), the (Figure formation2), the formationof aggregates of aggregates of grains can of grainsbe can be noticed, rangingnoticed, from ranging 260 to from 400 260nm tofor 400 the nm devices for the with devices 20 bilayers with 20 bilayersand from and 200 from to 340 200 to 340 nm nm for the devicesfor the deviceswith 30 bilayers. with 30 bilayers.In the case In of the the case device of the with device 50 bilayers, with 50 much bilayers, smaller much smaller aggregates aggregateswere detected were (of detected the order (of theof 20–30 order ofnm). 20–30 Moreover, nm). Moreover, the SEM the images SEM imagesreveal reveal that that TiO2 filmsTiO2 followfilmsfollow the underlying the underlying rough roughFTO morphology, FTO morphology, although although they present they present a a more more granulargranular morphology morphology due to due the toorganic the organic layer deposited layer deposited just below. just below.The surface Thesurface of of the the aluminumaluminum electrodes electrodes was also was analyzed also analyzed and, as and, can as be can observed be observed in Figure in Figure 2, the2, the surface surface waswas also also homogeneous homogeneous with with no nosigns signs of cracks. of cracks. The The morphology morphology was was like like the the one one detected for the TiO films, with changes also being observed with the number of bilayers of the detected for the TiO2 films,2 with changes also being observed with the number of bilayers of the organicorganic film. film.

Device Magnification—×80,000

TiO2 Surface Al Surface

FTO/(PAH/GO)20/TiO2/Al 200 nm 200 nm

FTO

FTO/(PAH/GO)30/TiO2/Al 200 nm 200 nm

FTO/(PAH/GO)50/TiO2/Al 200 nm 200 nm

Figure 2. SEM images for devices with the architecture (FTO/PEI/GO/TiO2/Al)x, with x = 20, 30, and 50 bilayers. Inset Figure 2. SEM images for devices with the architecture (FTO/PEI/GO/TiO2/Al)x, with x = 20, 30, and 50 bilayers. Inset depicts the FTO SEM image taken with the same magnification. depicts the FTO SEM image taken with the same magniﬁcation.

In the caseIn of the the case devices of the deviceswith ZnO with as ZnO the asinorganic the inorganic layer, layer, for forall allsamples, samples, a a homoge- homogeneousneous surface surface is isobserved observed with with no no sign sign of of cracks (Figure(Figure3 )3) for for the the aluminum aluminum electrode. In electrode. Inaddition, addition, for thesefor these devices, devices, the SEM the imagesSEM images evidenced evidenced the presence the ofpresence aggregates, of which do aggregates,not which present do not significant present differencessignificant indifferences their dimensions in their betweendimensions the analyzedbetween the devices, being analyzed devices,independent being of independent the number ofof thethe organicnumber bilayers. of the Theirorganic dimensions bilayers. rangedTheir between dimensionsapproximately ranged between 50 andapproximately 200 nm and, 50 in and comparison, 200 nm and, these in valuescomparison, are considerably these lower values are thanconsiderably those observed lower forthan the those TiO2 films.observed The surfacefor the morphologyTiO2 films. ofThe the surface Al electrode was also like the one detected for the corresponding ZnO film, as can be observed in Figure3.

Photonics 2021, 8, x FOR PEER REVIEW 7 of 15

morphology of the Al electrode was also like the one detected for the corresponding ZnO film, as can be observed in Figure 3. It should be noted that the surface morphology is completely different for the TiO2 and ZnO devices. For TiO2 films, the agglomerates of grains are distributed over the substrate surface with a ‘blooming flower-like’ appearance. In the case of ZnO films, the surface presents a pronounced cone-like morphology. Photonics 2021, 8, 75 The results of the morphological characterization performed for both inorganic films 7 of 16 (TiO2 and ZnO) are in good agreement with the ones reported recently elsewhere, which were produced in similar conditions [33].

Device Magnification ×80,000 ZnO Surface Al Surface

FTO/(PAH/GO)20/ZnO/Al 200 nm 200 nm

FTO/(PAH/GO)30/ZnO/Al 200 nm 200 nm

FTO/(PAH/GO)50/ZnO/Al 200 nm 200 nm

Figure 3. SEM images for three devices with the architecture (FTO/PEI/GO/ZnO/Al)x, with x = 20, 30, and 50 bilayers. Figure 3. SEM images for three devices with the architecture (FTO/PEI/GO/ZnO/Al)x, with x = 20, 30, and 50 bilayers.

3.3. Influence of theIt shouldNumber be of notedBilayers that on the the surface Electric morphology Properties of isFTO/(PAH/GO) completely differentx/TiO2/Al for the TiO2 and and FTO/(PAH/GO)ZnO devices.x/ZnO/Al For (x TiO = 220films, and 30). the agglomerates of grains are distributed over the substrate To analyzesurface the withinfluence a ‘blooming of the surrounding flower-like’ environment appearance. on In the the electrical case of ZnO properties films, the surface of the devices,presents samples a pronounced with 20 cone-likeand 30 morphology.PAH/GO bilayers were analyzed at room temperature, at aThe temperature results of theranging morphological from 20 °C characterization to 25 °C, with a performedrelative humidity for both (RH) inorganic films between 50%(TiO and2 and 60%, ZnO) for both are intypes good of agreement devices. with the ones reported recently elsewhere, which In the casewere of produced TiO2 devices, in similar the characterization conditions [33]. was performed only in ambient light, in an environment without light interaction, and is reported in detail in [31]. For both 3.3. Influence of the Number of Bilayers on the Electric Properties of FTO/(PAH/GO) /TiO /Al devices, a linear dependence of the current with the applied voltage was observed, andx 2 and FTO/(PAH/GO)x/ZnO/Al (x = 20 and 30) thus, a resistive behavior, which was not expected. This can be attributed to the high To analyze the influence of the surrounding environment on the electrical properties of resistance at the photoactive electrode layer interface, thus blocking a large portion of the the devices, samples with 20 and 30 PAH/GO bilayers were analyzed at room temperature, charge carriers and/or the existence of an inefficient donor–acceptor junction, which will at a temperature ranging from 20 ◦C to 25 ◦C, with a relative humidity (RH) between 50% lead to an inefficient transport of charges to the electrodes. Based on these results, both and 60%, for both types of devices. devices were not analyzed when exposed to a light spot. In the case of TiO2 devices, the characterization was performed only in ambient light, in an environment without light interaction, and is reported in detail in [31]. For both devices, a linear dependence of the current with the applied voltage was observed, and thus, a resistive behavior, which was not expected. This can be attributed to the high resistance at the photoactive electrode layer interface, thus blocking a large portion of the charge carriers and/or the existence of an inefficient donor–acceptor junction, which will lead to an inefficient transport of charges to the electrodes. Based on these results, both devices were not analyzed when exposed to a light spot. On the other hand, for ZnO-based devices, the results evidenced for the device with 20 bilayers a typical resistive behavior, where the current depends linearly on the applied Photonics 2021, 8, x FOR PEER REVIEW 8 of 15

On the other hand, for ZnO-based devices, the results evidenced for the device with 20 bilayers a typical resistive behavior, where the current depends linearly on the applied voltage to the device terminals [31]. For all the measurements performed, no significant hysteresis effect was observed, and no change in its electrical behavior was detected when exposed to ambient light. In turn, the sample with 30 PAH/GO bilayers exhibited a behavior very close to what would be expected for a diode. In comparison with the other Photonics 2021, 8, 75 8 of 16 devices, the behavior of this device seemed to change when exposed to the light spot from a halogen lamp. Nevertheless, based on the analysis of its current–voltage characteristic curve, it was concluded that the slight increase in current in the presence of light was not voltagesignificant to the enough device to terminalsbe considered [31]. a For photosensitivity all the measurements phenomenon. performed, no significant hysteresis effect was observed, and no change in its electrical behavior was detected when exposed3.4. Comparison to ambient of the light.Electric In Properties turn, the of sampleFTO/(PAH/GO) with 3050 PAH/GO/TiO2/Al and bilayers exhibited a behaviorFTO/(PAH/GO) very close50/ZnO/Al to what would be expected for a diode. In comparison with the other devices,To theunderstand behavior why of this the device previously seemed presente to changed devices when exposedexhibited to a thepoor light performance, spot from aa halogendevice with lamp. 50 Nevertheless,bilayers was produced. based on The the analysisincrease in of the its current–voltagenumber of bilayers characteristic is expected curve,to prevent it was the concluded aluminum that migration the slight inside increase the indevice, current which in the occurred presence in ofprevious light was works not significantcarried out enough by the togroup be considered [34], although a photosensitivity the SEM characterization phenomenon. did not reveal such an effect. Moreover, this device was analyzed inside a desiccator, where the temperature 3.4. Comparison of the Electric Properties of FTO/(PAH/GO)50/TiO2/Al and FTO/(PAH/GO)50 /ZnO/Alranged from 20 °C to 23 °C, with a very low relative humidity, varying from 10% and 15%, because these parameters can also influence the characterization of the sample. The obtainedTo understand current–voltage why the curves previously are depicted presented in Figure devices 4 for exhibited both types a poor of devices. performance, Though a device with 50 bilayers was produced. The increase in the number of bilayers is expected this experimental setup enabled a more accurate control of temperature and humidity, it to prevent the aluminum migration inside the device, which occurred in previous works did not allow the characterization of the device under the action of a light spot. carried out by the group [34], although the SEM characterization did not reveal such an For the TiO2 device, the analysis of the results evidenced a characteristic curve of a effect. Moreover, this device was analyzed inside a desiccator, where the temperature diode and hysteresis effect. This phenomenon is particularly evident from the first to the ranged from 20 ◦C to 23 ◦C, with a very low relative humidity, varying from 10% and second measurement for both illumination conditions, which suggests that the 15%, because these parameters can also influence the characterization of the sample. The degradation of the organic films most likely occurred, leading to a less conductive device. obtained current–voltage curves are depicted in Figure4 for both types of devices. Though It should be noted that this device was more conductive when positively polarized (see this experimental setup enabled a more accurate control of temperature and humidity, it Figure 4). did not allow the characterization of the device under the action of a light spot.

(a) (b)

FigureFigure 4. 4.Comparison Comparison of of the the electrical electrical properties properties of (aof) (FTO/PAH/GO/TiO(a) (FTO/PAH/GO/TiO2/Al)2/Al)50 50and and (b )(b (FTO/PAH/GO/ZnO/Al)) (FTO/PAH/GO/ZnO/Al)5050 forfor differentdifferent light light conditions, conditions, in in a a low-relative-humidity low-relative-humidity environment. environment. Adapted Adapted from from [ 27[27].].

ForFor thethe TiOZnO2 device,device, thea similar analysis behavior of the resultsis observed evidenced to the aone characteristic expected for curve a diode- of a diodetype device. and hysteresis effect. This phenomenon is particularly evident from the first to the secondThe measurement analysis of for Figure both illumination 4 further conditions,revealed that which between suggests the that first the degradationand second ofmeasurements the organic films and mostbetween likely the occurred, third and leading fourth, to the a lesshysteresis conductive effect device. is reduced, It should with beno notedsignificant thatthis current device decrease was more between conductive them. Still, when this positively effect is polarizedparticularly (see evident Figure between4). the secondFor the ZnOand third device, measurements. a similar behavior Again, is observedfor this device, to the oneno expectedcurrent increase for a diode- was type device. The analysis of Figure4 further revealed that between the first and second measurements and between the third and fourth, the hysteresis effect is reduced, with no significant current decrease between them. Still, this effect is particularly evident between the second and third measurements. Again, for this device, no current increase was observed in the presence of light, and therefore, it is not photosensitive. Considering the quotient between the applied voltage values and the obtained current values, this device was much less conductive than the others. It is important to note that this device was analyzed in an environment with 10% relative humidity, and this parameter can be a key factor on the Photonics 2021, 8, x FOR PEER REVIEW 9 of 15

observed in the presence of light, and therefore, it is not photosensitive. Considering the quotient between the applied voltage values and the obtained current values, this device was much less conductive than the others. It is important to note that this device was Photonics 2021, 8, 75 analyzed in an environment with 10% relative humidity, and this parameter can be9 a of key 16 factor on the conduction of the developed solar cells. In fact, according to the literature [35], the presence of moisture and/or oxygen plays a major role in the conduction of organic films, making them more conductive. conduction of the developed solar cells. In fact, according to the literature [35], the presence of moisture and/or oxygen plays a major role in the conduction of organic films, making 3.5. Electric Characterization of (PAH/GO)20 onto IDE Gold Electrodes Glass Substrates them more conductive. 3.5.1. (PAH/GO)20 onto IDE Gold Electrodes Glass Substrates: I–V Curves 3.5. ElectricTo understand Characterization the influence of (PAH/GO) of the20 enviroonto IDEnmental Gold Electrodesconditions Glass on the Substrates performance of 3.5.1.the organic (PAH/GO) layer20 and,onto consequently, IDE Gold Electrodes on the pr Glassoduced Substrates: hybrid solar I–V Curves cells, devices with 20 bilayersTo understand of PAH/GO thewere influence developed. of the The environmental choice of this number conditions of bilayers on the performancewas based on ofpreliminary the organic studies layer and,carried consequently, out by the ongroup the producedthat showed hybrid that solarthis number cells, devices leads withto an 20adequate bilayers thickness of PAH/GO in order were to developed.avoid short Thecircuit choice of the of devices, this number besides of requiring bilayers was less basedmaterial on and preliminary lower production studies carried time compared out by the with group the that other showed developed that thisdevices. number The leadsstudy toof an this adequate organic thickness solar incell order involved to avoid several short circuitphases, of addressing the devices, different besides requiringenvironmental less material conditions. and lower production time compared with the other developed devices.In this The sense, study the of thisfirst organicmeasurements solar cell were involved carried several out in phases,ambient addressing light, with differenta relative environmentalhumidity of approximately conditions. 55% and at a temperature of 21 °C. As can be seen from Figure 5a, inIn these this sense,conditions, the first the measurements device has a werepurely carried ohmic out conduction in ambient mechanism. light, with aIt relative should ◦ humiditybe noted ofthat approximately the hysteresis 55% effect and is at quite a temperature noticeable, of with 21 C. an As increase can beseen in the from conductivity Figure5a, inof thesethe sample conditions, being thedetected, device from has athe purely first to ohmic the second conduction measurement. mechanism. After It a should period be of notedabout that24 h, the new hysteresis measurements effect is were quite performed noticeable, sequentially, with an increase with inthe the first conductivity measurement of thecarried sample out beingunder detected,similar conditions from the firstthan tothe the ones second mentioned measurement. before. On After the aother period hand, of aboutthe following 24 h, new measurements measurements were were carried performed out in sequentially,a beaker with with argon the atmosphere, first measurement and the carriedresults are out depicted under similar in Figure conditions 5b. The thanobtain theed ones results mentioned corroborate before. what On was the expected: other hand, The theabsence following of moisture measurements and oxygen were carriedleads to out the in aless beaker conductive with argon devices. atmosphere, It is also and easily the resultsobserved are that depicted the slope in Figure of the5 fourthb. The and obtained fifth measurements results corroborate is considerably what was lower expected: than Thethe absenceone observed of moisture in the and third oxygen measurement, leads to the which less conductive is indicative devices. of an Itincrease is also easilyin its observed that the slope of the fourth and fifth measurements is considerably lower than the resistivity. one observed in the third measurement, which is indicative of an increase in its resistivity.

(a) (b)

FigureFigure 5.5.( a(a)) Current–voltage Current–voltage curvescurves ofof a a (PAH/GO) (PAH/GO)2020 ﬁlmfilm inin air,air, inin thethe presencepresenceof of ambient ambient light. light. (b(b)) Current–voltage Current–voltage curvescurves of of a a (PAH/GO) (PAH/GO)2020 filmﬁlm in air, in the presence ofof ambientambient lightlight andand inin aa beakerbeaker withwith argon.argon.

ToTo understandunderstand whether whether these these changes changes in conductivityin conductivity were were reversible, reversible, two newtwo mea-new surementsmeasurements (sixth (sixth and seventh)and seventh) were were performed, performed, in an in environment an environment with with humidity humidity and and in ambientin ambient light. light. The The corresponding corresponding current–voltage current–voltage curves curves are are shown shown in in Figure Figure6 a,6a, where where itit isis possiblepossible toto observeobserve aa signiﬁcantsignificant changechange ininthe the behavior behavior of of the the device. device. BetweenBetween thesethese consecutive measurements, the behavior of the device appears to approach a diode instead of the already observed ohmic behavior.

Photonics 2021, 8, x FOR PEER REVIEW 10 of 15

consecutive measurements, the behavior of the device appears to approach a diode instead of the already observed ohmic behavior. In addition, it was also possible to conclude that changes in the conductivity of these films (due to the presence or absence of moisture and/or oxygen) have a high rate of reversibility, as current values were recorded close to the initial ones, indicating that there are no chemical reactions that can degrade organic compounds, under the tested experimental conditions. To finalize the characterization of this device, its photosensitivity was evaluated by performing two measurements (eighth and ninth) with a relative humidity of about 55%. The first measurement of this study was performed when the sample was exposed to ambient light, while the second was exposed to a light spot from a halogen lamp. The obtained curves are shown in Figure 6b and the results evidence that this device has acquired a diode behavior, after being subjected to successive applied electric fields. This behavior change may be related to the reduction of the graphene oxide due to the applied electric fields [36]. Moreover, the reduction process causes a decrease in the resistance of the sheet of graphene oxide (Rs), making it easier to transport charges [19]. An increase in the current produced by the cell is also noticeable, when it is exposed to the light spot, which Photonics 2021, 8, 75 suggests that this device with the 20 PAH/GO bilayers has photosensitivity, unlike10 those of 16 in which the inorganic layer was deposited.

(a) (b)

FigureFigure 6.6. ((aa)) Current–voltageCurrent–voltage curvescurves ofof aa (PAH/GO)(PAH/GO)20 filmﬁlm in air, inin thethe presencepresence ofof ambientambient light.light. ((bb)) Current–voltageCurrent–voltage 20 curvescurves ofof aa (PAH/GO)(PAH/GO)20 filmﬁlm in in air, air, in the presence of ambient light and a light spot.

3.5.2.In (PAH/GO) addition,20 itonto was IDE also Gold possible Electrodes to conclude Glass Substrates: that changes Impedance in the conductivity Spectra of

theseThe films analysis (due to of the (PAH/GO) presence20 or by absence impedance of moisture spectroscopy and/or involved oxygen) havedifferent a high sets rate of ofmeasurements, reversibility, asperformed current valuesin two weredifferent recorded environments. close to the initial ones, indicating that there are no chemical reactions that can degrade organic compounds, under the tested • Dry environment experimental conditions. ToThese finalize measurements the characterization were carried of this out device,in ambi itsent photosensitivity light, inside a beaker was evaluated with argon, by performingwith the intention two measurements of reducing (eighththe presence and ninth) of oxygen with a and/or relative humidity, humidity elements of about 55%.that Thecould first influence measurement the sample ofthis behavior. study The was corre performedsponding when Nyquist the sample diagram was is presented exposed toin ambientFigure 7a. light, The whileobservation the second of the was diagram exposed allo tows a the light inference spot from that a halogenit corresponds lamp. Theto a obtainedsemicircle, curves in which are neither shown the in Figurereal nor6b imaginary and the results component evidence deviate that from this the device origin. has It acquiredis also observed a diode that behavior, there afteris an beinginverse subjected behavior to between successive the applied resistive electric and reactive fields. Thispart behaviorof the impedance: change may For below related frequencies, to the reduction there is a of decrease the graphene in the oxidereactive due component to the applied and electrican increase fields in [ 36the]. Moreover,resistive component, the reduction whil processe for high causes frequencies, a decrease the in theopposite resistance occurs. of theFigure sheet 7 also of graphene presents the oxide graph (Rs), of making the resist itive easier component to transport of the charges device’s [19 impedance]. An increase (Z’) inas thea function current of produced the angular by thefrequency cell is also (ω), noticeable,which shows when that it up is exposedto 104 rad.s to− the1, the light Z’ value spot, which suggests that this device with the 20 PAH/GO bilayers has photosensitivity, unlike those in which the inorganic layer was deposited.

3.5.2. (PAH/GO)20 onto IDE Gold Electrodes Glass Substrates: Impedance Spectra

The analysis of (PAH/GO)20 by impedance spectroscopy involved different sets of measurements, performed in two different environments. • Dry environment These measurements were carried out in ambient light, inside a beaker with argon, with the intention of reducing the presence of oxygen and/or humidity, elements that could inﬂuence the sample behavior. The corresponding Nyquist diagram is presented in Figure7a. The observation of the diagram allows the inference that it corresponds to a semicircle, in which neither the real nor imaginary component deviate from the origin. It is also observed that there is an inverse behavior between the resistive and reactive part of the impedance: For low frequencies, there is a decrease in the reactive component and an increase in the resistive component, while for high frequencies, the opposite occurs. Figure7 also presents the graph of the resistive component of the device’s impedance (Z0) as a function of the angular frequency (ω), which shows that up to 104 rad·s−1, the Z0 value Photonics 2021, 8, x FOR PEER REVIEW 11 of 15

Photonics 2021, 8, 75 𝐴 11 of 16 𝑍 (1) 1𝐵ω This equation corresponds to the real impedance part of an RC circuit in parallel (see Figureremains 7c), constant where andthe A decreases parameter for corresponds higher frequencies to the (Figureresistance7a). (R) The value, ﬁtting and performed the B pa- to the obtained data was based on the following equation: rameter corresponds to the product of resistance by capacitance (RC) at the angular frequency ω. The best adjustment to the experimental points was achieved with the A and B 0 A parameters of Equation (1), taking Zthe= values presented in Table 1. (1) 1 + (B × ω)2

(a)

(b)

(c)

20 Figure 7. (a) Nyquist diagramFigure obtained 7. (a )by Nyquist impedance diagram spectroscopy obtained by for impedance a (PAH/GO) spectroscopy film deposited for a (PAH/GO) on a substrate20 ﬁlm with deposited gold interdigitated electrodeson in aan substrate argon atmosphere. with gold ( interdigitatedb) Real impedance electrodes component in an as argon a function atmosphere. of angular (b) frequency. Real impedance In red, the adjustment made to the experimental points is shown. (c) Equivalent circuit of a PAH/GO film with 20 bilayers component as a function of angular frequency. In red, the adjustment made to the experimental analyzed by impedance spectroscopy. R and C are the resistor and capacitor, respectively. points is shown. (c) Equivalent circuit of a PAH/GO ﬁlm with 20 bilayers analyzed by impedance spectroscopy. R and C are the resistor and capacitor, respectively. • Humid environment This equation corresponds to the real impedance part of an RC circuit in parallel (see Figure7c), where the A parameter corresponds to the resistance (R) value, and the B parameter corresponds to the product of resistance by capacitance (RC) at the angular PhotonicsPhotonics 20212021, 8,, 8x, FOR 75 PEER REVIEW 1212 of of15 16

frequencyThe secondω. The set bestof measurements adjustment to was the experimentalalso carried out points under was ambient achieved light, with but the in A air, and withB parameters a relative humidity of Equation between (1), taking 50% theand values 60%. The presented Nyquist in diagram Table1. shown in Figure 8a differs from that shown in Figure 7a, in which the system impedance was equivalent to Table 1. Estimated parameteran RC (A, circuit B, C, D, in E, parallel. and/or F) values through the ﬁtting with Equation (1) or (2) to the real part values of the measured impedanceThus, in differentgiven the environmental differences between conditions. the two diagrams, it is possible that, in the case where the sample was analyzed in the air, several conduction mechanisms contribute to Environmental Equationthe presented A (R1[ behavior,Ω]) Bin (R1C1[s]) addition to the C (R2[ resistΩ])ive and D (R2C2[s]) reactive part E of (R3[ a Ωsingle]) RC F (R3C3[s]) circuit. Conditions Figure 8b shows that the behavior of the resistive part of the impedance is not typical of a −5 Dry (<10% RH)single (1) RC circuit. 98 747 In this1.3 sense,× 10 the curve th————at best fitted the experimental points was ob- −4 −6 Humid (50–60% RH)tained (2) using 30,060 the following 0.05457 equation: 11,642 6.2 × 10 17,513 3.5 × 10 𝐴 𝐶 𝐸 𝑍 = + + (2) • Humid environment1+(𝐵×ω) 1+(𝐷×ω) 1+(𝐹×ω) TheThe equation second presented set of measurements above then was reflects also the carried actual out impedance under ambient component light, butof three in air, RCwith circuits a relative in parallel, humidity as shown between in 50%Figure and 8c. 60%. Thus, The A Nyquist corresponds diagram to R shown1, B to inR1C Figure1, C to8a R2differs, D to R from2C2, thatE to shownR3, and in F Figure to R3C73a,. Again, in which the the parameter system impedance ω represents was the equivalent angular tofre- an quencyRC circuit of the in system. parallel. The obtained results are summarized in Table 1.

(a)

(b)

Figure 8. Cont.

Photonics 2021, 8, x FOR PEER REVIEW 13 of 15

Photonics 2021, 8, 75 13 of 16

(c)

Figure 8. (a) Nyquist diagram obtainedFigure by 8. impe(a) Nyquistdance spectroscopy diagram obtained for a by (PAH/GO) impedance20 film spectroscopy deposited for on a a (PAH/GO) substrate 20withfilm deposited gold interdigitated electrodes in air.on ( ab) substrate Real impedance with gold component interdigitated as a electrodesfunction of in angular air. (b) frequency. Real impedance In red, component the adjust- as a function ment made to the experimental pointsof angular is shown. frequency. (c) Equivalent In red, the circuit adjustment of a PAH/GO made to the film experimental with 20 bilayers points analyzed is shown. by (c) Equivalent impedance spectroscopy. R and C are the resistor and capacitor, respectively. circuit of a PAH/GO film with 20 bilayers analyzed by impedance spectroscopy. R and C are the resistor and capacitor, respectively. Table 1. Estimated parameter (A, B, C, D, E, and/or F) values through the fitting with Equation (1) or (2) to the real part values of the measured impedance in different environmental conditions. Thus, given the differences between the two diagrams, it is possible that, in the case Environmentalwhere the Condi- sample wasEqua- analyzedA in the air,B severalC conduction D mechanismsE contributeF to thetions presented behavior,tion in addition(R1[Ω]) to(R the1C1 resistive[s]) (R2[Ω and]) reactive(R2C2[s]) part (R3 of[Ω a]) single (R3C3 RC[s]) circuit. DryFigure (<10%8b RH) shows that the(1) behavior 98 747 of 1,3 the × resistive10−5 --- part of the --- impedance --- is not --- typical of a single RC circuit. In this sense, the curve that best fitted the experimental points was Humid (50–60% RH) (2) 30 060 0.05457 11 642 6.2 × 10−4 17 513 3.5 × 10−6 obtained using the following equation: Although this model of three parallel RC circuits in series does not fully fit the exper- A C E imental data, the obtained0 = result suggests+ that, in addition+ to the behavior of polymer Z 2 2 2 (2) molecules and graphene oxide,1 + there(B × areω) at least1 + two(D other× ω) constituents,1 + (F × oxygenω) and water vapor, that decrease the impedance of the film, which is in accordance with the other ob- The equation presented above then reflects the actual impedance component of three tained data for the devices with the inorganic layer. This means that these devices must RC circuits in parallel, as shown in Figure8c. Thus, A corresponds to R , B to R C ,C be encapsulated to work properly as solar cells. 1 1 1 to R2, D to R2C2, E to R3, and F to R3C3. Again, the parameter ω represents the angular frequency of the system. The obtained results are summarized in Table1. 4. Conclusions Although this model of three parallel RC circuits in series does not fully fit the Inexperimental this work, data, two the types obtained of solar result suggestscell devices: that, inFTO/(PAH/GO) addition to thex behavior/TiO2/Al ofand polymer FTO/(PAH/GO)moleculesx and/ZnO/Al graphene (x—number oxide, there of bilayers), are at least were two developed. other constituents, oxygen and water Thevapor, morphological that decrease characterization the impedance evi ofdenced the film, larger which aggregates is in accordance for the TiO with2-based the other devicesobtained in comparison data for with the devicesZnO devices with the with inorganic 20 and 30 layer. PAH/GO This means bilayers, that where these the devices sur- must face morphologybe encapsulated is dependent to work properly on the asnumber solar cells.of bilayers, while for ZnO devices, no change in the aggregates size was detected with the number of bilayers. Based4. Conclusions on the electrical characterization, a resistive behavior was detected for TiO2-

based devicesIn this with work, 20 and two 30 types PAH/GO of solar bilayers cell devices:, without FTO/(PAH/GO) photosensitivity xwhen/TiO2 analyzed/Al and FTO/ in air (PAH/GO)at ambient xlight,/ZnO/Al which (x—number may be due of to bilayers), the high wereresistance developed. between the photoactive layer and theThe electrode morphological or to an characterization inefficient donor–acceptor evidenced larger interface. aggregates Regarding for the the TiO TiO2-based2 devicedevices with 50 in bilayers comparison of PAH/GO, with ZnO analyzed devices inside with 20a desiccator and 30 PAH/GO to reduce bilayers, moisture, where a the characteristicsurface morphologyI–V curve of a is diode dependent was obse onrved, the number which ofwas bilayers, more conductive while for ZnOwhen devices, pos- no itivelychange polarized. in the In aggregates the case of size the wasZnO-ba detectedsed devices with the with number 20 bilayers of bilayers. of PAH/GO, an ohmic behaviorBased was on thedetected, electrical and characterization, with 30 bilayers, a resistivea semiconductor behavior behavior was detected was forre- TiO2- vealed.based A slight devices increase with in 20 current and 30 PAH/GOwas recorded bilayers, with withoutthe latter photosensitivity sample when exposed when analyzed to a lightin spot air atfrom ambient a halogen light, lamp. which In may the device be due with to the 50 high bilayers, resistance a typical between diode the behavior photoactive was detected,layer and although the electrode it was or much to an less inefﬁcient conductive donor–acceptor than all the other interface. samples Regarding analyzed. the TiO2 In fact,device it was with studied 50 bilayers inside a of desiccator, PAH/GO, wi analyzedth a relative inside humidity a desiccator close to to 10%, reduce which moisture, alloweda characteristic the conclusion I–V that curve humidity of a diodeis a key was factor observed, in the conduction which was of more the organic conductive films. when Thepositively study of polarized. the organic In thelayer case performed of the ZnO-based with the devices(PAH/GO) with20 20device bilayers initially of PAH/GO, re- vealedan a typical ohmic resistance behavior wasbehavior; detected, however, and withduring 30 the bilayers, measurements, a semiconductor a diode behavior behavior was was acquired.revealed. When A slight placed increase in an in Ar current atmosphere, was recorded the sample with proved the latter to be sample less conductive. when exposed to The devicea light was spot also from subjected a halogen to a lamp. light Inspot, the where device an with increase 50 bilayers, in current a typical was observed, diode behavior demonstratingwas detected, its photosensiti although itvity. was Regarding much less conductivethe impedance than spectroscopy all the other samplesanalysis analyzed.under ambientIn fact, light it was and studied in Ar atmosphere, inside a desiccator, the Nyquist with diagram a relative showed humidity an closeincrease to 10%,in the which allowed the conclusion that humidity is a key factor in the conduction of the organic ﬁlms.

Photonics 2021, 8, 75 14 of 16

The study of the organic layer performed with the (PAH/GO)20 device initially revealed a typical resistance behavior; however, during the measurements, a diode behavior was acquired. When placed in an Ar atmosphere, the sample proved to be less conductive. The device was also subjected to a light spot, where an increase in current was observed, demonstrating its photosensitivity. Regarding the impedance spectroscopy analysis under ambient light and in Ar atmosphere, the Nyquist diagram showed an increase in the resistive component of the impedance of the sample (real component) for low frequencies, while for high frequencies, it decreased. The reactive component (imaginary component) presented an inverse behavior, thus suggesting that the sample’s equivalent circuit in these conditions is an RC circuit in parallel. In the study of this device under ambient light but performed at air, the Nyquist diagram evidenced a behavior dependent on the contribution of several conduction mechanisms. Therefore, it was possible to conclude that the equivalent circuit of the sample consisted of three parallel RC circuits in series, meaning that these devices should be encapsulated to work properly. This work further evidences that the use of ZnO as an electron acceptor material in a solar cell appears to be more suitable for the performance of such devices than TiO2, and the conduction mechanisms of the devices are inﬂuenced by the surrounding environment and surface morphology of the inorganic layer.

Author Contributions: Conceptualization, D.C., M.R. and S.S.; methodology, D.C., M.R. and S.S.; software, D.C.; validation, D.C., P.A.R., M.R. and S.S.; formal analysis, D.C., P.A.R., M.R. and S.S.; investigation, D.C.; resources, P.A.R., M.R. and S.S.; data curation, D.C.; writing—original draft preparation, S.S.; writing—review and editing, D.C., P.A.R., M.R. and S.S.; visualization, D.C., P.A.R., M.R. and S.S.; supervision, M.R. and S.S.; project administration, S.S.; funding acquisition, P.A.R., M.R. and S.S. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the Centre of Physics and Technological Research CEFITEC, which is financed by national funds from FCT/MEC (UID/FIS/00068/2019). The authors also acknowledge and appreciate the support given to CEFITEC by the Portuguese Foundation for Science and Technology (FCT) through the strategic project UID/FIS/00068/2020. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Acknowledgments: The authors acknowledge the financial support from FEDER, through Programa Operacional Factores de Competitividade—COMPETE and Fundação para a Ciência e a Tecnologia— FCT, for the project UID/FIS/00068/2020. Conflicts of Interest: The authors declare no conflict of interest.

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