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Transparent conductive oxide (TCO) electrodes are key components in the fabrication of optoelectronic devices such as organic photovoltaic cells (OPVs) or organic emitting devices (OLEDs). Pulsed laser deposition (PLD) results in TCO coatings with adequate optical and electrical properties, the preservation of the target chemical composition in the transferred films being the major advantage of this technique. Furthermore, the performance of the optoelectronic devices can be enhanced by patterning the TCO electrodes. Indium tin oxide (ITO) remains the most popular TCO due to its high conductivity and transparency. The scarcity of the indium resources encouraged the efforts to find an alternative to ITO, a promising candidate being Al-doped ZnO (AZO). Therefore, this chapter is focused on PLD deposition of TCO films (ITO and AZO) on patterned glass substrates prepared by ultraviolet nanoimprint lithography (UV-NIL) for obtaining transparent electrodes with improved characteristics, which further can be integrated in optoelectronic applications.
National Institute of Materials Physics, Magurele, Romania
Nicoleta Preda
National Institute of Materials Physics, Magurele, Romania
Carmen Breazu
National Institute of Materials Physics, Magurele, Romania
Oana Rasoga
National Institute of Materials Physics, Magurele, Romania
*Address all correspondence to: marcela.socol@infim.ro
1. Introduction
Organic optoelectronic device such as organic photovoltaics (OPVs) and organic light-emitting devices (OLEDs) focused over the past few decades the attention of both academia and industries due to the possibility to fabricate flexible, transparent devices on large area using low-cost solution processes, leading to cost-effective production [1, 2]. At this stage, in the OPV field, a major concern regards the fabrication of flexible structures with high efficiencies for various applications [3]. Although, OPV with efficiency over 18% has been reported in 2021 [4], further improvements are still needed for making them a real alternative to other photovoltaic cell (PV) technologies (PV based on silicon, PV based on perovskites, etc.). The improvements can be linked to: (i) the type of the organic materials used in the fabrications of the PV structures; (ii) the deposition techniques used to obtain the organic component as films; and (iii) the different approaches used for enhancing the absorption in the PV structure such as antireflection coatings, back-reflectors, or the surfaces patterning (texturing) [5, 6]. In the PV structures, the thickness of the organic active film is limited by the low carrier mobility and the short exciton diffusion length [7]. An increase in the film thickness leads to a lowering in the device efficiency, while a decrease in the film thickness results in a poor absorption. Lately, some studies reported that the nanopatterning of the transparent electrodes increases the optical path length of light inside the active material improving the performances of the devices [6, 8].
Different optical approaches and structures such as microlens, nanostructured electrodes, scattering layers were used in the field of OLEDs to improve the light extraction efficiency of the devices [9, 10]. The light extraction efficiency is one of the most important parameters of OLED, defined as the ratio of the total number of photons emitted by the OLED and the total number of photons generated within the organic emitter [10, 11]. Thus, the majority of the generated photons in the organic layers are confined inside the device due to the total internal reflection, which takes place at the glass/air and organic/layer substrate interfaces owing to the mismatch of the refractive index [12]. In this way, almost 30% of the emitted photons are trapped in the glass substrate (glass mode), while a 50% are trapped at the organic/anode interface (waveguide mode). Therefore, various methods were used to extract more efficiently the light from the OLEDs [9, 13].
Transparent conductive electrodes (TCE) play a key role in the development of optoelectronic devices such as OPVs, OLEDs, touch screens, electrochromic devices, heat mirrors, smart windows, and so on [14, 15, 16]. Over time, various materials such as metal oxides, ultrathin metals, metal nanowires, graphene, carbon nanotubes, conductive polymers, etc., were deposited and investigated as TCE [1, 14]. However, indium tin oxide (ITO) remains the most commonly used TCE due to its remarkable properties such as high transparency (90% at 550 nm wavelength), adequate sheet resistance (10–30 Ω/□), work function (4.7 eV), and reduced roughness (<1 nm) [17, 18]. Besides that, aluminum-doped zinc oxide (AZO) is a suitable metal oxide for replacing ITO since this material met the necessary criteria regarding the high transparency and the electrical resistivity [19, 20].
Transparent conductive oxide (TCO) films can be deposited by numerous chemical and physical methods such as sol-gel [21], spray pyrolysis [22], magnetron sputtering [23], chemical vapor deposition (CVD) [24], atomic layer deposition [25], pulsed laser deposition (PLD) [20], etc., each of them having both advantages and limitations. PLD is a versatile technique used in the deposition of high-quality films based on ITO, AZO, indium-doped zinc oxide (IZO), Ga-doped ZnO (GZO), indium gallium zinc oxide (GIZO), ZnO-Y2O3 (YZO), the obtained TCO layers having adequate properties for optoelectronic device area [26, 27, 28, 29, 30].
Patterning techniques such as X-ray lithography, electron projection lithography, ion beam projection lithography, multiple e-beam lithography, extreme ultraviolet lithography, or nanoimprint lithography (NIL) are essential in the niche technology that manufactures high-volume and low-cost nanoscale devices [31–34]. The development and improvement of NIL technique have extended the nanoscale fabrication from standard semiconductor devices for electronics and optoelectronics to complex ones for optics, plasmonics, microfluidics, or biomimetic area [35, 36, 37, 38, 39]. Among NIL technologies, ultraviolet nanoimprint lithography (UV-NIL) is an efficient technique because it allows the manufacture of a wide range of pattern sizes and shapes on different rigid or flexible substrates [34, 40].
In this chapter, we present some of our contributions regarding the TCO layers deposited by PLD on flat and UV-NIL nanopatterned glass substrates. Therefore, metal oxides films (ITO and AZO) deposited by PLD were studied for emphasizing their potential applications in the field of optoelectronic devices such as OPVs and OLEDs.
Pulsed laser deposition (PLD) is a well-established method used to grow thin films from a wide range of materials, enabling a stoichiometric transfer of these. Although PLD was introduced in 1965, it was applied intensively in the late 1990s [41, 42]. PLD is a physical vapor deposition technique where an external high-power laser (typically an UV laser source) ablates a target based on a single or a combination of compounds depending on the desired composition of the film [43]. In comparison with other deposition methods such as sputtering, molecular beam epitaxy, chemical vapor deposition, or thermal evaporation, PLD has the following advantages: (i) any type of substrate can be used for depositing thin films; (ii) by using UV laser sources, a wide range of materials can be ablated; (iii) the pressure during the deposition process can be choose from 10−7 mbar up to 1 mbar; (iv) due to progressive growth with each laser pulse, a rigorous control of the thickness is possible; (v) the stoichiometry can be preserved or changed in a controlled manner during the deposition; (vi) the kinetic energy of the evaporated species can be moderated in order to control the film growth properties; (vii) a background gas can be used in order to obtain the adequate reactive atmosphere; (viii) multilayered thin films can be obtained by switching different target materials in the deposition cycle; and (ix) assure the purity of the initial composition because the ablation source is the light [42, 43, 44, 45]. As any deposition technique, the PLD process has also some drawbacks: (i) limited deposition area for standard setups; (ii) the uniformity of the deposition is influenced by energy profile and inhomogeneity of the laser pulse; (iii) macroscopic and microscopic droplets are sometimes ejected from the target [45, 46].
PLD is a versatile method that proved its potential in different research areas considering that a wide class of the materials can be ablated using excimer lasers and deposited as thin films [42, 44, 47, 48, 49, 50, 51, 52, 53]. Thus, metal films, semiconductor films, superconductors, ceramic layers, oxides, insulators can be easily obtained by this laser technique [54, 55]. Moreover, nanostructures with different morphologies such as nanowires, nanoflowers, nanorods, nanotubes, and even quantum dots based on ZnO, ITO, graphene, molybdenum disulfide (MoS2), tungsten disulfide (WS2), cadmium selenide (CdSe) can be deposited by PLD [45, 47, 56, 57, 58]. The thin films or nanostructures fabricated by PLD were integrated in various devices: photovoltaics, environmental sensors, actuators, light emitters, ferroelectrics, photocatalysis, biomaterials, medical implants, etc. [45, 47, 59].
A common PLD deposition setup is depicted in Figure 1. Hence, the growth of the thin film is the result of the interaction between the laser beam and the target. When the laser fluence (the energy delivered per unit area at given pulse duration) reaches the ablation threshold, the vaporization of the material from the target surface takes place, process followed by the generation and expansion of the plasma plume. Further, the plasma species (free electrons, ions, neutral atoms, molecules) with appropriate energy nucleates on the deposition support [45, 59, 60]. In PLD, the film growth and the film quality depend generally on various experimental parameters: laser fluence, laser wavelength, pulse duration, repetition rate, target-substrate distance, background gas and its pressure, quality of the target substrate temperature, etc. Because the influence of each deposition parameter on the properties of films deposited by PLD, from specific materials, was extensively discussed and analyzed in literature, in the following we briefly resumed their importance [42, 44, 47, 48, 49, 50, 51, 52, 53].
Figure 1.
Schematic representation of PLD deposition chamber.
The laser fluence is one of the principal parameters because it impacts the kinetic energy of the species presented in the plasma plume and their movement toward the deposition substrate [52]. As was discussed by Schou, the chosen laser fluence must be high enough to induce target ablation but not so high to avoid the re-sputtering and possible implantation of some species in the film [53].
The laser wavelength is connected with the energy absorbed by the target material [61], thinner films being obtained when the target material is transparent to the laser wavelength used during the deposition. Lower threshold fluences and also low ablation rates are obtained when short laser wavelengths are used [48]. Thus, the laser wavelength must be selected depending on the material type intended to be deposited.
The pulse duration parameter can be controlled to prepare films with expected performances. In general, nanosecond pulse lasers are implied in the PLD deposition [48]. When long laser pulses are implied, the absorbed laser energy firstly heats the target surface to the melting point, and afterward at the vaporization temperature, the thermal wave penetrates the target and produces the melting of the material, evaporation appearing from the liquid phase. In the case of the femtosecond-pulse lasers, the vapor and plasma phases appear quickly, therefore the heat conduction is negligible, and as a consequence, the liquid phase is absent [62].
The pulse repetition rate influences the deposition rate, this being related to the duration necessary to get a specific thickness of the film [63]. The number of the particles, which are found as islands, grown firstly on the deposition substrate, subsequently tend to diffuse and aggregate depending on the pulse repetition rate, a higher density of islands being favored by the increase of this parameter. Moreover, it was emphasized that using higher pulse frequencies, a high density of small-size islands can be obtained facilitating the diffusion of some adatoms from islands top to the substrate, in this way films characterized by a smooth surface being obtained. At lower pulse frequencies, a low density of islands is formed resulting in rougher surfaces [64].
Although some PLD films can be fabricated just in ultrahigh vacuum, most of them required a background gas; this parameter affects the plume dynamics and furthers the growth and properties of the films [52, 65]. The background gas decreases the kinetic energy of the species presented in the plasma plume, a high pressure of this can decrease the sputtering of the film, but at the same time can lead to the preferential diffusion of some species to the deposition support [53, 66]. Argon, helium, or nitrogen is frequently used in the PLD deposition, but the most studied gas is still oxygen, due to the possibility of producing films with controlled oxygen content [50].
The target-substrate distance influences the mass ratio of the species that reach the substrate, thus influencing the thickness of the obtained film. A higher distance is equivalent with a reduction of the deposited material while a lower distance has as effect a rebound of the species due to their high kinetic energies [67]. Thus, it is essential to choose an optimal target-substrate distance. Some studies show that TCO layers on flexible substrate characterized by cracks or peeling off are obtained when the deposition is performed at lower target-substrate distance (4 cm) while cracks-free, smoother films are obtained at higher target-substrate distances (6 or 8 cm) [26].
The substrate temperature can influence the film growth and its surface morphology [67]. Even if the deposition can be carried on at room temperature leading usually to amorphous films, it was highlighted that at higher substrate temperatures, the adatom mobility increased resulting in crystalline films [52, 67]. When the temperature of the deposition substrate is increased, even the low kinetic energy species can be capable of constituting uniform layers [47].
Accordingly, the optimal PLD deposition conditions for developing high-quality complex films from a large number of materials can be found by tuning the experimental parameters involved in this laser process [50, 67].
Nowadays, the transition from millimeter to micro and further to nano dimensions, the tendency to pass from rigid to flexible electronics, and also the continuous need of device enhanced efficiencies based on surface patterning using the principles of the plasmonic and photonic theories have forced the industry to search nanopatterning techniques that can be used in volume manufacturing [68]. In order to gain the industrial attention, these patterning techniques need to fulfill at least some key attributes such as: (i) high resolution; (ii) ability to simultaneously pattern different types of structures; (iii) high throughput and low defectivity; and (iv) reduced costs [69].
Under the name “NIL” can be found the classical thee imprint techniques: micro-contact printing (μ-CP), hot-embossing (also known as thermal NIL), and UV-NIL, but also the newly added roll imprint process, laser-assisted direct imprint, reverse imprint lithography, substrate conformal imprint lithography, ultrasonic NIL [32]. As a general definition, the nanoimprint lithography can be understood as a physical pressing process to replicate the master patterns into a polymer negative resist by thermal or ultraviolet curing [38]. Master is the name of the so called “mother” template that is usually fabricated using electron beam lithography on silicon substrates. From this master, in the case of UV-NIL, rigid or soft stamps (negative copies of the master pattern designs) based on elastomeric materials can be manufactured. Thus, common materials based on silicone polymers (usually modified formulas of polydimethylsiloxane), polyimides, or polyurethanes are applied as free-standing membranes or attached to a flexible or rigid backplane [33, 37, 38, 70]. Actually, these cheaper manufactured stamps are used in the lithography process reducing the production costs and thus prolonging the lifetime of the master, this being fabricated by more time-consuming and expensive methods.
The steps involved usually in the UV-NIL process are presented in Figure 2. Relatively simple, they can be described as follows: (i) spin-coating deposition of both primer and photoresist on the desired substrate, each followed by a heat treatment; (ii) alignment of the stamp with the coated substrate; (iii) adding them in contact, pressing and irradiating them with UV radiation; and (iv) detaching the mask after UV curing.
Figure 2.
Schematic representation of UV-NIL process.
The advantages of using NIL in comparison to other photolithography techniques are arising from the fact that using a direct contact between the stamp and the coated substrate, the resolution is given by the resolution of the patterns existing on the surface of stamp, which can be beyond the diffraction limits or beam scattering. However, exactly this advantage can easily become the disadvantage of the technique due to the resist filling rheology behavior and demolding capabilities [32, 33]. Therefore, one of the common defect mechanisms that appear in the NIL processes is connected with the detachment of the stamp after resist curing, when the polymer may stick on the stamp surface due to the interfacial forces (adhesion and friction forces) that appear between the resist and the stamp material. Interfacial forces are strongly linked to the quality of the stamp (design, roughness, antisticking layer, and material type), to the resist material and to the residual stress that appears during the UV irradiation due to the shrinkage of the resist that makes the stamp to adhere more to the resist surface. Taking into account all these aspects, a special attention must be paid to the selection of the materials and the process parameters that must be optimized in function of the stamp characteristics and pattern design [71, 72].
4. Indium tin oxide (ITO) and aluminum-doped zinc oxide (AZO) films deposited by PLD on flat and nanopatterned glass substrates
ITO is the most widely used TCO due to its exceptional properties, a large number of papers being focused on it [73, 74, 75, 76]. Several works reported on the PLD deposition of ITO films and on the correlation between the experimental parameters and their optical, structural, morphological, and electrical properties, some results being well summarized by Yap and Kim [47, 77, 78]. The best properties achieved for the ITO films deposited by PLD had over 90% transparency and 7.2 × 10−5 Ωcm electrical resistivity [18].
In the last decade, many attempts were made to replace ITO due to the indium sources depletion [79]. An adequate alternative for ITO seems to be AZO, a nontoxic material that can be found at low cost—its precursors being abundant compounds, and already successfully applied in the OPV and OLED areas [80]. AZO transparent films characterized by an adequate electrical resistivity were deposited by different methods on both rigid and flexible substrates, proving its compatibility for wearable electronics [20, 81, 82, 83]. PLD technique was also used in the deposition of AZO layers on either rigid glass or plastic substrates with suitable optical and electrical properties [20, 26, 84].
In the following part, the preparation steps implied in the fabrication of ITO and AZO films by PLD on flat and UV-NIL nanopatterned substrates will be described [85, 86]. The patterns were fabricated on glass by UV-NIL (EVG 620 mask aligner) using the following procedure: (i) preheating of the glass substrate for 2 min at 150°C; (ii) spin coating of a primer to enhance the adherence of the polymeric photoresist film; (iii) deposition by spin coating of the UV-resist film that further is thermally treated for 30 s at 120°C; (iv) pressing the soft stamp (mold) with the pattern model over the photoresist film with an uniform contact pressure (100 mbar); (v) exposure of the photoresist layer at UV light for 90 s; and (vi) removal of the soft mold [87]. As can be seen in the field emission scanning electron microscopy (FESEM) images from Figure 3, a periodic array of pillars having ~350 nm in diameter and ~ 1100 nm distance between pillars were fabricated on glass substrate by this procedure. The height of the pillars was estimated at ~250 nm from the cross-sectional FESEM images given in Figure 4. The quality of the patterns (height, diameter, distance between pillars) imprinted onto photoresist depends on the experimental conditions mentioned above in the UV-NIL process.
Figure 3.
FESEM images (at different magnifications) of the periodic pillars array obtained by UV-NIL method on glass substrates.
Figure 4.
Cross-sectional FESEM images (at two magnifications) of nanopatterned glass substrates.
Further, TCO layers were deposited on both flat and UV-NIL patterned glass substrates by a PLD system using an excimer laser with KrF (248 nm wavelength, 25 nm pulse duration, COMPex-Pro 205, Coherent Inc.) [85, 86]. The TCO solid targets (SCI Engineered Materials) were formed by In2O3:SnO2 = 90%:10% weight (ITO) and ZnO doped with 2% Al (AZO), the laser beam being directed on the target surface with a MgF2 lens having 300 mm focal length placed outside of the deposition chamber. During the deposition, the solid targets were rotated to avoid their local damage. For comparison, both types of substrates were coated with TCO layers in the same deposition cycle.
The ITO solid target placed at 5 cm distance toward substrate holder was irradiated with 7000 pulses under 45° incidence angle, the laser working at 10 Hz repetition rate into a deposition chamber filled with oxygen 6.0 at 1.5 Pa pressure and working with a low laser fluence of 1.2 J/cm2 [85]. The oxygen pressure was selected in order to obtain a low electrical resistivity, at room temperature (RT), as was mentioned in the reference [78]. The ITO layer thickness was estimated at ~340 nm as average media between the measurements made (with a profilometer) in three different points on the film deposited on flat glass substrate.
The AZO solid target placed at 8 cm distance toward substrate holder was ablated with 8000 laser pulse, a laser fluence of 2 J/cm2, and an oxygen pressure of 1 Pa [86], the values being selected based on other preliminary results where films characterized by a high transmittance were fabricated using these experimental conditions [84]. The AZO layer thickness was estimated at ~300 nm from the interference fringes observed in the UV-VIS spectra considering two consecutive maxima and minima and the refractive index = 1.8 for AZO film with 2% Al content [88].
The TCO layers deposited by PLD were labeled taking into account the substrates type, flat (glass) or nanopatterned (NP-glass), as follows: ITO/glass, AZO/glass and ITO/NP-glass and AZO/NP-glass. The morphology and optical properties of the samples were investigated by field emission scanning electron microscopy (FESEM, Zeiss Merlin Compact field emission scanning electron microscope), atomic force microscopy (AFM, Nanonics Multiview 4000), and UV-VIS spectroscopy (Carry 5000 Spectrophotometer).
The FESEM images from Figure 5 disclose that the ITO/glass (Figure 5left) has a smooth surface while the AZO/glass (Figure 5right) has a granular morphology, some particles being also presented on the surface of this sample. The results are in accordance to data already reported for ITO and AZO layers deposited by PLD [18, 84] or by other deposition techniques [89, 90].
Figure 5.
FESEM images of ITO (left) and AZO (right) films deposited by PLD on flat glass substrates.
The AFM topographic images from Figure 6 were collected on ITO/glass (Figure 6left) and AZO/glass (Figure 6right), interpolated root mean square (RMS) having a low value in both cases, 1 nm and 2.8 nm, respectively. As was expected, in the case of ITO film, the RMS value is lower in comparison with ITO layers deposited by other techniques but in agreement with those calculated for ITO layers previously deposited by PLD [23, 78, 91]. It has to be mentioned that in the PLD, the resultant smooth surface is associated to the low energy density implied in the deposition process [91]. As was already emphasized, such low roughness value is necessary for various applications being known that this parameter has a significant influence on optical and electrical properties of the deposited films. Thus, it was demonstrated that using a low laser fluence, ITO and AZO films characterized by a small roughness can be obtained by PLD at room temperature, making them suitable and compatible with flexible (plastic) substrates.
Figure 6.
AFM topographic images of ITO (left) and AZO (right) films deposited by PLD on flat glass substrates.
Analyzing the FESEM images of the ITO/NP-glass and AZO/NP-glass from Figures 7 and 8, respectively, it can be clearly seen that the patterns imprinted onto glass substrate are preserved during the TCO deposition by PLD. Considering that the TCO films are relatively thin (ITO ~ 340 nm and AZO ~300 nm), they tend to copy the topography of the substrate.
Figure 7.
FESEM images (at different magnifications) of ITO films deposited by PLD on nanopatterned glass substrates.
Figure 8.
FESEM images (at different magnifications) of AZO films deposited by PLD on nanopatterned glass substrates.
However, attention must be paid when the TCO layers are deposited on a patterned surface by PLD because the interaction between the ablated species, presented in the plasma plume, characterized by high kinetic energy and the deposition substrate can affect the growth of the film during the laser deposition [35, 92]. Thus, point defects can be formed due to species kinetic energy transfer toward the surface atoms [92]. In the PLD deposition on nanopatterned substrates, the first encountered layer is that based on photoresist (polymer) nanopillars. Nevertheless, the pillars are clearly observed in the FESEM images of the TCO deposited of nanopatterned glass substrates, only a small change in their shape being noted (in the case of ITO/NP-glass from cylindrical into a pyramid trunk-like one). Both TCO films seem similar at lower magnification, some differences due to the film thickness and the specific morphology being visible only at higher magnification. Thus, in comparison to the nanopatterned glass substrates, an enlargement in the pillars width and a narrowing in the distance between pillars are remarked, the TCO films tending to fill the space between pillars. Although the TCO films have thickness appropriate to the pillars’ height, these are not hidden by the deposited layers.
The optical transmittance is an essential criterion for the selection of the TCO films for their use in the field of OPV and OLED. Hence, the UV-VIS spectra of the prepared samples were presented in Figure 9. The TCO layers deposited on flat glass substrates are characterized by a transmittance over 80% for ITO and 75% for AZO in the visible part of the solar spectrum. Interference maxima are visible for both analyzed materials, their presence being associated with the uniformity of the deposited films [23]. This is not surprising, as it is known that high-quality layers can be obtained by PLD [93]. The refractive index (n) value was estimated from the interference maxima and minima with the Swaneopel method [94] as being ~1.9 for ITO/glass, value characteristic for this TCO films deposited by PLD. Kim reported that the n value depends on the Sn concentration in the targets and on the substrate temperature, for example, ITO films with low n were obtained increasing the substrate temperature [78]. Additionally, the band gap value was estimated from the UV-VIS spectra. Thus, for ITO/glass, the band gap was estimated at ~3.65 eV, in agreement with other results reported for ITO layers deposited by PLD or by magnetron sputtering, other technique frequently used to deposit commercially available ITO layers, with the same SnO2 content (10%) [23, 91]. It is well known that this oxide is a semiconductor characterized by a direct wide band gap that can exceed 3.0 eV [95]. According to the Burstein-Moss theory, the increase in the band gap is linked to the increase in the film’s carrier concentration [78].
Figure 9.
UV-VIS spectra of TCO layers (ITO or AZO) deposited by PLD on flat (left) and nanopatterned (right) glass substrates.
Compared with the ZnO band gap value (3.3 eV [96]), the AZO/glass band gap was estimated at ~3.7 eV, similar to the value reported for AZO grown by PLD at room temperature and 1 Pa oxygen pressure [97]. Depending on the experimental conditions, especially by the oxygen pressure and the substrate temperature, the band gap of the AZO films deposited by PLD can take value between 3.32 and 3.77 eV [98].
In the case of the TCO layers deposited on nanopatterned glass substrate, a lowering in the transmittance is noticed in the UV-VIS spectra compared with the ones deposited on flat glass substrates. Moreover, the pillars introduced additional absorptions and reflections at interfaces [35]. The light couples to waveguide modes via diffraction and thus is trapped in the nanostructures, the pattern characteristics (mainly the period) affecting the optical properties of the films deposited on it [99]. Also, a shift of the absorption edge is visible for both transparent electrodes. A possible explanation for the peculiar behavior observed in the absorption edge shift of nanopatterned TCO (ITO/NP-glass to long wavelength region and AZO/NP-glass to short wavelength region) can be linked to the arrangement of the molecules inside the cavities determined by the nanostructuration. Thus, the interaction between the neighboring molecules can modify differently the energy levels of nanopatterned TCO with effect on their band gap.
Electrical properties of the prepared TCO layers are considered key features since, in the field of optoelectronic applications, conductive films are required. Hall measurements were performed on ITO/glass and ITO/NP-glass samples in order to analyze their electrical parameters, the obtained values being presented in Table 1.
Sample
ITO/glass
ITO/NP-glass
Resistivity (Ωcm)
1.8 × 10−4
2.8 × 10−4
Mobility (cm2/Vs)
10.6
15.1
Carrier concentration (cm−3)
3.3 × 1021
1.5 × 1021
Sheet resistance (Ω/sq)
5.3
8
Table 1.
Electrical parameters of ITO films deposited by PLD on flat and nanopatterned glass substrates evaluated from Hall investigations.
In principle, the electrical resistivity values of ITO films deposited on flat and nanopatterned glass substrates are lower than ~4 × 10−4 Ωcm reported for ITO films deposited at room temperature by PLD [100] in the same conditions (laser wavelength, target composition, and repetition rate) with those used in our study. Interesting, the electrical resistivity value of ITO film deposited on flat glass substrate is nearly to that of ITO films deposited by PLD from targets with different SnO2 content (5 or 10%) but with a heated substrate [18, 91, 101]. Kim carried on a comprehensive study regarding the influence of various experimental parameters such as oxygen pressure, SnO2 content, and deposition temperature on the resistivity of ITO films deposited by PLD [78]. Hence, this work shows that the resistivity of ITO film is influenced by the oxygen pressure through the number of the oxygen vacancies presented in the TCO layer. Also, the resistivity of ITO films is sensitive to the SnO2 content, an increase up to 5% leads to the resistivity decrease while an increase above this percent results in the increase of resistivity because the concentration of the electron traps expands due to Sn excess [91].
The carrier concentration values of ITO films deposited on flat and nanopatterned glass substrates are in concordance with those reported usually on ITO films deposited by PLD [78]. The refractive index of ITO films is influenced by the carrier density, a reduction of this parameter being possible by increasing the electron density, which can be achieved by enlarging the Sn content from the deposition target up to a certain value [78].
The extracted Hall mobility values of ITO films deposited on flat and nanopatterned glass substrates are just a little smaller than other value reported for ITO films deposited by PLD [91] utilizing the same deposition target with that implied in our work. The low Hall mobility values of ITO films can be related to the carrier-carrier scattering [44].
In the case of AZO film deposited on flat glass substrates, the resistivity was evaluated to be 2.4 × E−4 Ωcm using a Jandel four-point probe, the value being in the same range with others obtained for the AZO layers deposited by PLD on glass substrates [20, 102] using the same oxygen pressure with that applied in our study. A thoroughgoing study regarding the influence of the oxygen pressure on the optical and electrical properties of some AZO layers deposited by PLD was carried on in Ref. [102] pointing out that the films grown at a low oxygen pressure (under 3 Pa) have a compact structure characterized by a low resistivity.
5. TCO layers deposited by PLD on flat and nanopatterned glass substrates for developing organic heterostructures
The TCO films (ITO and AZO) deposited by PLD on flat and nanopatterned glass substrates were used for developing organic heterostructures for optoelectronic applications. Schematic representation of two organic heterostructures and their I-V characteristics are given in Figure 10: one based on adenine (Ade), the nucleic acid base film being deposited on ITO by vacuum thermal evaporation [103], and another based on N,N′-di(1-naftalenil)-N,N′-diafenil-(1,1′-bifenil)-4,4′-diamina (α-NPD), 1,4-bis [4-(N,N-diphenylamino)phenylvinyl] benzene (P78) and 4,7 diphenyl-1,10-phenanthroline (BPhen), the three stacked organic films being deposited on AZO by matrix-assisted pulsed laser evaporation (MAPLE) [86]. For both organic structures, aluminum electrode (100 nm) was deposited by vacuum thermal evaporation.
Figure 10.
Schematic representation of the organic structures using TCE deposited by PLD on flat and nanopaterned glass substrates and I-V characteristics recorded on representative organic structures (single organic film – blue curve and three stacked organic films – green curve).
Hence, in the case of adenine deposited on ITO/glass substrate, the I-V characteristic (recorded in dark between −1 V and 1 V applied voltage) is changed from linear (at small voltage) to nonlinear at higher voltage (>0.5 V) probably due to the different properties shown by the contacts ITO/adenine and adenine/Al [103]. Regarding the electrode patterning, it is expected that this effect induces some changes in the electrical properties of the investigated structures by modifying the electrical field, which in turn can affect the charge carrier transport and their collection [104]. The scattering/recombination processes can be influenced by (i) the enlargement of the contact area between the nanopatterned TCO and the organic film, (ii) the change in the pathway of the charge carriers to the electrodes due to the presence of pillars; and (iii) the morphology of films characterized by grain boundaries. Compared with the structure prepared on ITO/glass electrode, the shape of the I-V characteristic of the structure deposited on ITO/NP-glass electrode was changed into a very close rectifying diode behavior. At small voltage, a slow increase in the current value is noted at the same time with the voltage increase, while a faster increase in the current is obtained after 0.5 V probably due to the growth of the number of electrons that cross the barrier and are more easily collected to the patterned electrode [103].
Concerning N,N′-di(1-naftalenil)-N,N′-diafenil-(1,1′-bifenil)-4,4′-diamina, 1,4-bis [4-(N,N-diphenylamino)phenylvinyl] benzene and 4,7 diphenyl-1,10-phenanthroline, an OLED-type structure was practically obtained using a hole transport layer (α-NPD), an emissive film (P78), and an electron transport layer (BPhen), respectively. Hence, the I-V characteristic plotted for the structure prepared on AZO/glass electrode presents a diode behavior. The structure fabricated on AZO/NP-glass electrode evidenced an improvement in the current value (at 1 V), meaning that the electrode patterning influences positively the electrical properties of the organic structures obtained on it [86], the charge transport being favored by the enlargement of the contact area between the nanopatterned AZO and the organic films [35]. This improvement recorded in the current value could be reflected in the final performances of the organic device fabricated on this type of nanostructured TCO.
Consequently, the optical and electrical properties of the organic structures fabricated on nanopatterned transparent electrodes can be enhanced due to the nanopatternation process. Taking into consideration that the organic heterostructures developed on TCO substrates are already part of our daily life (Heliatek company develops projects based on OPV solar films that can be attached in different locations or building facades or roofs [105], and LG Display produces OLED TV panels offering its OLED panels to other companies such as LG Electronics, Sony, Vizio, and Panasonic [106]), the organic layers deposited on patterned TCO can be also applied in the field of the organic optoelectronic devices.
TCO films (ITO and AZO) were deposited by PLD on flat and UV-NIL nanopatterned glass substrates, further these being used for developing organic heterostructures, which can find applications in optoelectronic device area. Thus, the glass substrates were patterned by UV-NIL technique, nanopillars arrays with suitable dimensions (width ~350 nm, height ~250 nm, and separation step(pitch) ~1100 nm) being fabricated. Although, the magnetron sputtering is preferred as deposition technique on large substrates, PLD is a viable alternative for fabricating high-quality TCO films with reduced roughness and appropriate optical and electrical properties by tuning the experimental deposition parameters such as: substrate temperature, oxygen pressure, target content, and laser fluence. Moreover, because the deposition of TCO films was carried at room temperature and the obtained TCO layers are characterized by low electrical resistivity, this laser technique can be also applied in the TCO deposition on plastic substrates for developing flexible devices.
The investigations prove that AZO is suitable for replacing ITO in TCO domain considering that the deposited AZO layers are featured by similar optical and electrical properties to those revealed by ITO layers.
Organic heterostructures were deposited on the fabricated TCO films (ITO and AZO) by vacuum thermal evaporation or matrix-assisted pulsed laser evaporation. The electrical measurements show that the patterning effect improves the optical and electrical properties of the organic heterostructures obtained on the TCO layers. Consequently, compared with an organic structure developed on a flat TCO electrode, an organic structure fabricated on a nanopatterned TCO electrode can be more efficient in the optoelectronic device area.
This research was funded by the Romanian Ministry of Research, Innovation and Digitization through the National Core Program PN19-03 (contract no. 21 N/2019) and PN-III-P4-IDPCE-2020-1691 (contract no. 66/2021).
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Written By
Marcela Socol, Nicoleta Preda, Carmen Breazu and Oana Rasoga
Submitted: 24 May 2022Reviewed: 10 June 2022Published: 06 July 2022
Open access peer-reviewed chapter
