A critical overview of thin films coating technologies for energy applications

Abstract

We report on several state of the art thin films coating technologies including physical vapor deposition (PVD) and solution process deposition techniques. Such techniques have their own significance to develop the energy efficiency devices. Choosing the right technique has become very critical in recent days as the scaling varies significantly with the technology. Thus, it becomes obvious to pick-up the right deposition procedure according to the needs where substrate size, film thickness, and required surface roughness are critically involved. Among all techniques, PVD offers various advantages, which include full control over growth, pristine film quality, large-scale fabrication, stacking of thin films, co-deposition flexibility, and controlling of the deposition temperature. In addition, such techniques can be adapted in small-scale research based labs with capping in terms of budget. Such vapor deposition processes also result in pin-hole free, homogeneous and continuous thin films which are highly desirable parameters for many energy related applications. Solution process techniques such as dip, spin, blade, and roll coating techniques are also discussed to provide an overall view. This report aims to give in-depth cross sectional review of all deposition techniques considering both positive and negative sides. PVD growth processes are very efficient where pattering of films are very significantly required. Hence, in this work, we have captured many deposition techniques where the properties of the films are very crucial to end up with different morphologies. The goal is to compare various techniques as a key parameter to control the fine formation of thin films. We are convinced that our report will show substantial importance to a wider group of materials scientists, chemists, physicists, and to the wider photovoltaic community to pick the best-suited vehicle for thin film growth purposes.

Keywords:

• e-beam evaporation
• thermal evaporation
• sputtering
• molecular-beam epitaxy
• dip coating
• spin coating
• spray coating

1. Introduction

Thin films coating techniques consist of various involved methods to process the growth of conducting, semiconducting, and dielectric materials on various types of substrates. Such phenomenon allow developing transparent, semi-transparent, very durable and insulating materials as required. Such techniques are very important in scaling device production of opto-electronic, solid state and medical devices and products (Bernard et al., 2021). Coating technology processes the implantation of thin films ranging from nanoscale to microscale onto a substrate through vapor phase or fluid phase (Baptista et al., 2018). The processes can be engineered accordingly to achieve the highest quality level of films, required thickness, deposition rate, process temperature, surface roughness, phase structure, tailored morphology, defect engineering, passivation, and device patterning (Rossnagel, 2003). In general, deposition procedure requires a very good vacuum to maintain a clean environment (Dan et al., 2022). Such technique mainly comprises the vapor phase of solid sources as per desired films (Alfihed et al., 2013). In the physical vapor deposition (PVD) process, source materials are transformed into vapor phase from solid phase to be deposited on solid or flexible substrates. A uniform film of high purity cannot be achieved unless the chamber is first cleared of excess atmospheric particles. These particles can be trapped on the substrate surface by the condensing thin film, leaving “pinholes” or other surface distortions (Rashid et al., 2019). During the solution process technique, chemicals are mainly dissolved using a mediator and evaporated films result in dry films from wet films (Hossain et al., 2020). Some of the cross-cutting vapor deposition processes are sputtering (Sergievskaya et al., 2022), e-beam evaporation (Hossain, Al Kubaisi et al., 2022), thermal evaporation (Mostaque et al., 2022), molecular beam epitaxy (MBE; Nunn et al., 2021), chemical vapor deposition (CVD; Yi et al., 2021), atomic layer deposition (Plutnar & Pumera, 2021), plasma assisted deposition (Fievez et al., 2021), and flame hydrolysis deposition (Holmes et al., 2022) to develop thinner metals (Hossain, Khandakar et al., 2022), semiconductors (Hossain, 2012) and dielectric materials (Sellers, 1998) and are vastly involved in producing photovoltaic (PV) devices, sensors, optical filters, passivating layers to produce electricity, light management under high vacuum (Hossain et al., 2021). The layers can be guided to have a high refractive index, low refractive index, better carrier mobility through doping as well as reactive deposition (Hou et al., 2022; Limarga & Clarke, 2009). Hence, it enables full control over growth to develop thin to bulk materials, which eventually allows pursuing defects engineering through shallow or deep level material tuning (Baben et al., 2017). Certainly, the optical properties can be altered accordingly as well as through making metal rich or metal poor films, which correctly reflects the characteristics of a desired semiconducting layer (Hossain, Khandakar et al., 2022). Various deposition methods are well established and have been adapted by the industries to grow optimized coatings with enhanced electrical, morphological, topological, optical, and surface properties. However, no system has been developed yet to fit all requirements, hence, it becomes obvious to design the system as well as process parameters according to the need of the industry.

The article has been arranged in a way to provide an in-depth knowledge to the wide range of readers where both theoretical and basic mechanism of PVD methods are considered. Such techniques are the best due to the technological adaptability to fabricate inorganic, hybrid, and nanocomposite thin films. MBE is a significant deposition technique to grow epitaxial, layered structures under ultrahigh vacuum conditions on different substrate materials (Kim et al., 2022; Richards et al., 2022; Yao et al., 2022; Zhao et al., 2022; Zhu et al., 2022). In general, chemical reaction occurs through molecular beam impinging on the surface, where this chemical reaction is the material transition from the gas phase in the molecular beam to the solid state on top of the substrate. MBE tool allows higher qualities of the deposited films through heating and rotation capabilities. In-situ reflection high energy electron diffraction (RHEED) can be used to check the quality of the deposited films, which is an added advantage. In this tool, growth process involves controlling molecular and/or atomic beams via shutters and source temperature, deposited on mono-crystal substrate for epitaxial growth.

Though many deposition techniques are currently available in the market, this paper gives an overview mainly about the PVD tools mainly due to its’ compatibility, adaptability, affordability, flexibility, robustness, efficient, and effective processing capabilities (Lee and Park, 2022, Lee et al., 2022).

Figure 1 shows the variation of different fabrication techniques considering physical as well as chemical mechanisms.

Figure 1. The conventional methods to develop thin films for miscellaneous applications.

2. Basic deposition techniques

Physical vapor deposition involves the plasma-assisted or evaporation coating of a substrate starting from nanoscale to microscale in a vapor phase (Hossain, Khandakar et al., 2022). The principal goals involve the growth of thin films with effectiveness, robustness, lifespan, and quality. Such technique is significantly accredited due to its simplicity, choice of deposition materials, and co-sputtering by many research labs and manufacturing companies. The applications are in large domain, which include bonding of semiconducting materials, metal coatings, large bandgap material deposition, etc., where growth can take place simultaneously or sequentially. Most of the compound materials as source are in solid-state and inert, hence, safe handling becomes obvious (Zakaria et al., 2019). Among solution process techniques, spin coating process has drawn a significant attention to fabricate perovskite solar cells due to the efficient source utilization, sequential growth of transport materials, and accurate thickness within the range of 300–400 nm (Baloch et al., 2018; Guo et al., 2022; Nazir, 2022). However, it involves scalability issue due to the substrate size (He et al., 2002). On the other hand, evaporation involves no scalability issue and allows sequential growth of all transport and metal contact layers to complete a device structure (Amin et al., 2011; Chelvanathan et al., 2017; Hang Li et al., 2022; Vaynzof, 2020).

Knudsen evaporation sources are usually heated through resistance heaters or using electron beam. During resistance heating process, source temperature is limited to 1900 K as measured by thermocouples. Generally, platinum is used to capture the temperature up to 1300 K and hybrid tungsten–rhenium for >1300 K. During e-beam bombardment, source material plays the most dominating factor to cap the highest melting temperature. Various types of Knudsen sources are available to meet the basic evaporation process, which are stainless steel, quartz, nickel, graphite, tungsten or molybdenum. Cross contamination between source materials and inner wall can be controlled through depositing a protective layer inside. In general, laser can be used to heat up the evaporation sources to a certain higher temperature. On the other hand, linear sources for in-line and roll to roll processing for semiconducting devices. Such sources are made of a bulk evaporator to achieve the temperature more than 1500 °C with full control over the growth. Generally, developed manifold helps in resisting spitting of source materials with full control over defect states.

Such technique also offers the adjustable processing temperature and deposition rate with rotation for homogeneity as required. Thus, evaporating perovskite materials becomes certain to deploy large scale manufacturing (Wang et al., 2018). This will eventually replace any other solution process technique for scalability. The capability to utilize various inorganic materials enables developing all inorganic and stable perovskite photonic devices with full control over thickness and roughness. Such advantages restrict the ideas of using highly volatile materials, post annealing, and additional mediators.

In general, vapor deposition techniques are efficient, effective, and attractive to grow thin films on various kinds of substrates such as ITO coated glasses, FTO coated glasses, quartz, Si substrates, flexible ones, and the successful growth of effective layer is to the pristine growth and morphology (Deng et al., 2020; Mahan, 2000; Shi et al., 2022; Wadley et al., 2001). When source materials are heated at certain point, the centripetal force and the solid’s surface energy result in dense, continuous, and uniform films. Such process can grow films starting few nanometers to micron range through atomization of a desired recipe, where both thickness and deposition rate can be fixed. In addition, flexible substrate sizes can be adjusted with the sample holder which results in a convenient process, better quality, and effective material utilization. One of the common vapor deposition techniques is sputtering which is also considered as rapid, handy, cost effective, and excellent coating technique for the usage in lab and production scales (Garah et al., 2022; Hu et al., 2022; Liang et al., 2021; Lundin et al., 2019; Padamata et al., 2022). Such technique is widely used in developing optical filter, anti-reflection coatings, anti-soiling coatings, PV devices, and nano-biosensors. In such technique, deposition occurs due to the kinetic energy of excited atoms resulted from the creation of plasma. It is a physical-based technique where solid phases of a desired layer is deposited onto substrates (Yang et al., 2021). The puttering target materials are conventionally in solid forms and attached to the magnetrons. Few parameters are actively involved in the sputtering process, which may alter the film quality such as cross-contamination, implanted impurities, poor vacuum, substrate roughness, highly energetic bombardment of atoms onto substrates. Figure 2 shows the conventional structure of a CIGS solar cell, which can be prepared using PVD techniques.

Figure 2. The conventional schematic to develop CIGS thin film solar cells using PVD techniques.

2(a). Thermal evaporation

The thermal evaporation process using direct resistive heating is one of the simplest forms of evaporating a material for thin-film deposition. A low-voltage, high-current power supply forces an electrical current through a conductor in contact with the desired source. By adjusting the power of the supply, the evaporation rate can be increased or decreased accordingly. In order to make the most effective use of such deposition system, it is necessary to understand some of the basic parameters involved in creating thin films using a thermal evaporation process.

In general, a source material placed on a metallic boat requires heating until it reaches it is evaporating phase to start deposition. Later, growth of layers occur due to the condensation of the source vapor and the process happens within the low vacuum range between 10⁻⁶ and 10⁻⁵ Torr to re-route any reaction of the produced vapor and ambient atmosphere (Wu et al., 2022). Such pressure helps in the same mean free path to the chamber inner dimension; hence, the atoms evaporate in a straight geometrical dimension. However, shadowing might occur due to the inaccessibility of atoms to some area of the substrates. Due to the low energy particles, such process sometimes results in less adhesion and coverage of films, which require process optimization. Also, various methods have been adapted to heat the source materials (Hsu et al., 2022). The vaporization rate depends on the rate that molecules leave the source surface and the area of the evaporating surface. The rate that molecules leave the surface is related to the vapor pressure of the evaporant. Figure 3 shows the process chamber layout of a thermal evaporation system to grow the thin films and perovskite films and contacts.

Figure 3. Thermal evaporation: (a) process chamber lay out, (b) evaporation process.

The vapor pressure is the equilibrium pressure of the material (the density of molecules in the gas phase) above an evaporating surface. A uniform film cannot be achieved unless the chamber is first cleared of excess atmospheric particles. These particles can be trapped on the substrate surface by the condensing thin film, leaving “pinholes” or other surface distortions. Since the thermal process generally takes places at pressures around 10⁻⁴ Torr, it is desirable to have pumping systems capable of achieving base pressures, which are several orders of magnitude below this range, usually 10⁻⁶ Torr or less. Most metals reach their normal melting point before the vapor pressure is high enough to achieve a significant evaporation rate. Two exceptions are chromium and manganese, which sub-lime; they evaporate rapidly while still a solid. Other materials, called refractory metals and compounds, are difficult to evaporate since they have low vapor pressures even at high temperatures. Some materials that leave the surface can be scattered back. The amount that is scattered depends on the molecular weight of the evaporating atoms and the vapor density above the evaporant surface.

As shown in Figure 4, to validate the optimized growth of ETM layer (TiO₂) and HTM layer Cu₂O, perovskite absorber layers were grown on the ETM layers (Du et al., 2022). Then, perovskite layer was formed on top of the TiO₂/FTO samples by thermal evaporation technique from the source powder of PbI₂ (Sigma-Aldrich) and CH₃NH₃I (Dyesol) by changing the ratio.

Figure 4. Thermal evaporation to develop perovskite solar cells: (a) schematic of the prepared device, (b) XRD results of evaporated perovskite layer before optimization, (c) XRD results of evaporated perovskite layer after optimization (Du et al., 2022).

2(b). E-beam evaporation

For e-beam evaporation, fixed high-voltage, low-current power supply forces an e-beam in contact with the desired source. In addition, it is an alternative process of thermal evaporation to grow some materials (Hossain, Al Kubaisi et al., 2022; Hossain, Khandakar et al., 2022). In addition, e-beam technique has the capability of higher deposition rate comparing to other vapor deposition processes. E-beam evaporation uses the electron beam to melt and evaporate source materials, as shown in the Figure 1. One of the advantages of the e-beam evaporation is that it can be directed to a confined area, which is also called crucible, and when we need multilayer film deposition sequentially then the crucibles can be rotated with the source material changed. This happens without opening the vacuum system (Hossain, Al Kubaisi et al., 2022; Hossain, Khandakar et al., 2022). As aforementioned, such technique consists of heating until evaporation of the material to be deposited. Before developing any recipe, it is required to calibrate the tool to measure the expected film thickness (Hossain, Al Kubaisi et al., 2022; Hossain, Khandakar et al., 2022). Such calibration follows an iteration process before growing the requested films through tuning tooling factor and thickness measurement using Dektak profilometer. This enables to provide the right service to the customers through homogeneous, right thickness, compact, and pin holes free layers. Figure 5 shows the e-beam evaporation schematic and results as developed for various energy applications such as optical filter, plasmonic solar cells, antireflection and anti-soiling coatings.

Figure 5. E-beam evaporation: (a) process chamber lay out, (b) X-ray diffraction results of evaporated Au/TiO_x samples with dewetting at different temperatures, (c) developed films on flexible substrates using e-beam evaporation, (d) atomic force microscopy of evaporated films, and (e) pristine morphology of dewetted samples as grown by e-beam evaporation (Hossain, Al Kubaisi et al., 2022; Hossain, Khandakar et al., 2022).

E-beam evaporation has been used in developing various thin films such as optical filters, carrier transport materials, optical layers with maximum material utilization and reduced cost (Hossain, Al Kubaisi et al., 2022; Hossain, Khandakar et al., 2022). As shown in Figure 6, evaporation recipe follows iteration process for optimization.

Figure 6. Procedure to develop an optimum recipe for evaporated thin metal or metal oxide films.

The evaporation was performed using Denton^TM Explorer e-beam evaporator under three different oxygen pressures: 2e-4 Torr, 9e-5 Torr and 2e-5 Torr. The deposition was carried at room temperature. Evaporation pellets were Mo, Ni, Ti, Ni, and Al from Kurt J Lesker with a purity of 4N5. The base pressure of the deposition chamber was brought down to 10⁻⁶ Torr using cryo pump. The evaporation parameters are given in Table 1. All the room temperature films were kept within the chamber in vacuum atmosphere for natural cooling process to circumvent oxidation.

Table 1. Typical evaporation conditions of metal oxide thin films

Parameter	Deposition condition	Deposition condition	Deposition condition	Deposition condition	Deposition condition
Pellets	Ti (99.999% pure)	Sn (99.995% pure)	Mo (99.995% pure)	Ni (99.995% pure)	Al (99.995% pure)
Deposition pressure	2e-4 Torr, 9e-5 Torr, 2e-5 Torr	2e-4 Torr, 9e-5 Torr, 2e-5 Torr	2e-4 Torr, 9e-5 Torr, 2e-5 Torr	2e-4 Torr, 9e-5 Torr, 2e-5 Torr	2e-4 Torr, 9e-5 Torr, 2e-5 Torr
Deposition rate	1 A/s	1 A/s	1 A/s	1 A/s	1 A/s

The measurement of thickness and optical parameters of the thin metal oxides thin films was performed with a HORIBA Scientific UVISEL2 Spectroscopic Ellipsometer in the wavelength range 190–2100 nm (0.6–6.5 eV) at 65° angle of incidence (Hossain, Al Kubaisi et al., 2022; Hossain, Khandakar et al., 2022). This ellipsometer is based on a phase modulation technology. The signal is modulated at a high frequency (50 kHz) which allows to average more data, and it is equipped with two photomultipliers for detection in the UV-VIS range which enables a high signal to noise ratio data. The infrared detector is an InGaAs semiconductor detector. Each sample was represented using a two-layer model, with a rough over layer on the top of the main layer to enhance the fitting. This rough layer is described using the Bruggeman Effective Medium Approximation with a mixture of 50% void and 50% metal oxide. The classical formula, containing a Drude term and a single oscillator, was utilized to fit the data and describe dispersions of the optical constants of the films over the entire spectral range (Hossain, Al Kubaisi et al., 2022; Hossain, Khandakar et al., 2022).

The optical properties were studied with UV-Vis spectroscopy of the annealed samples between 350 and 800 nm. Transmission and absorbance measurements (Figures 7 and 8) confirm the difference between metal rich or metal poor films of the clean samples, where all oxide films grown at 2e-4 Torr deposition pressure show the highest transmission and less absorbance in the visible range. Based on the deposition pressures, a clear shift in the spectra can be observed (Hossain, Zakaria et al., 2022). As shown in Figure 9, it can be observed k value is higher for the wavelength of 632 nm for metal rich films, which proves the high absorption due to the formation of metallic droplets. The formation of metallic droplets at the deposition pressure of 2e-5 Torr is the main reason for this effect, where the oxygen flow rate is limited. Beside this, the high value of refractive index (n > 2; Figure 10) is obtained for almost all the films, which is certainly desirable for PVs (Hossain & Alharbi, 2013).

Figure 7. Transmission spectra of five metal oxide films at three different deposition pressures (Hossain, Zakaria et al., 2022).

Figure 8. Absorbance spectra of four metal oxide films at three different deposition pressures.

Figure 9. Refractive index and extinction coefficient of four metal oxide films at three different deposition pressures.

Figure 10. Sputtering system: (a) process chamber lay out, (b) magnetron assembly.

2(c). Sputtering

A uniform film of high purity can be achieved through cleaning the excess atmospheric particles. These particles can be trapped on the substrate surface by the condensing thin film, leaving “pinholes” or other surface distortions. Since sputtering generally takes places at pressures of about 10⁻³ Torr, it is desirable to have pumping systems capable of achieving base pressures, which are several orders of magnitude below this range, usually 10⁻⁵ Torr or less before flushing the system with a gas for sputtering. Permanent magnets are set up behind the cathode – which surrounds the loaded target – such that the magnetic field is normal to the flat surface of the cathode. This creates an electron trap, allowing electrons to enter the area and due to columbic force but keeping ions from escaping. The inter-electrode spacing in the sputtering gun needs to be smaller than the mean-free path of the collision between particles to prevent arcing between the cathode and anode. A gas, generally argon (Ar), is introduced into the chamber and any positive ions are attracted to the cathode at pressure around 10⁻² Torr has a mean free path of collision around 1 mm loaded target. The ions strike the target, ejecting atoms from the target by means of momentum transfer and emitting more electrons to join those in the electron trap. The created electron trap also increases the frequency of collisions between electrons and any neutral gas atoms, creating more positive ions that will strike the target. The atoms ejected from the target then form a vapor that is neutrally charged and can escape the electron trap, which condenses into a thin film on the substrate. With planar magnetron sputtering, high deposition rates with lower voltages are available. This is due to the cyclical, almost self-perpetuating, nature of the process (each striking of the target creates more electrons that will create more positive ions, which will strike the target and start all over again). The system that has been used during the process with both RF and DC sources. Figure 10 shows the sputtering system.

The installed crystal sensor uses the impendence of the oscillation frequency on the mass of your crystal oscillator to determine the thickness created during a given deposition process (Garah et al., 2022; Hu et al., 2022; Liang et al., 2021; Lundin et al., 2019; Padamata et al., 2022). As mass increases by any factor, the frequency will decrease by square root of the inverse of that factor. The FTM-2000 re-zeroes itself before each process to utilize relative changes in mass, allowing for numerous deposition processes before changing the quartz sensor. The density of the material is measured by the quartz crystal sensor in grams per cubic centimeter (gm/cc). Z-Factor compensates for the mechanical stress a material causes to the quartz crystal. Tooling factor is for compensation of measured deposition rates that differ from the actual substrate deposition rate based upon relative distance and position of the crystal sensor when compared to position of substrate stage. The tooling factor must be fine-tuned for each crystal sensor at the end user’s facility upon successful installation of the system and anything with the position of the crystal sensor (Garah et al., 2022; Hu et al., 2022; Liang et al., 2021; Lundin et al., 2019; Padamata et al., 2022).

Figure 11 shows the sputtering of gold contact on perovskite solar cells without disturbing surface through controlling the deposition rate at room temperature.

Figure 11. Sputtered gold contacts on perovskite solar cells.

2(d) Molecular beam epitaxy (MBE)

A MBE system is an ultra-flexible tool with a design configurable emerging materials to develop photonic devices including sensors, microelectronics. MBE tool is used to grow elemental, compound and alloy semiconductor epitaxial films on pre-heated substrate materials under a very high vacuum pressure of 10⁻¹⁰ Torr (Kim et al., 2022; Richards et al., 2022; Yao et al., 2022; Zhao et al., 2022; Zhu et al., 2022). It happens mainly due to the reaction of highly energetic beams of atoms or molecules as energized by the mean of electron beam. In general, chemical reaction occurs through molecular beam impinging on the surface, where this chemical reaction is the material transition from the gas phase in the molecular beam to the solid state on top of the substrate. MBE tool allows higher qualities of the deposited films through heating and rotation capabilities. This technique is the upper standard of the evaporation with more precise growth and control over the beam fluxes and deposition conditions.

Figure 12 shows the schematic of a MBE tool. In general, MBE can handle > 8” dia. substrates. One chamber has oxygen plasma as well as e-beam evaporation capability and is used for reactive deposition of oxides. The second chamber is dedicated for semiconductor compounds. In-situ RHEED can be used to check the quality of the deposited films, which is an added advantage. In this tool, growth process involves controlling molecular and/or atomic beams via shutters and source temperature, directed at a single crystal sample (suitably heated) to achieve epitaxial growth.

Figure 12. MBE system: (a) side view and (b) top view (Kim et al., 2022; Richards et al., 2022; Yao et al., 2022; Zhao et al., 2022; Zhu et al., 2022).

It is one of the several methods of depositing single crystals or monocrystals (Kim et al., 2022; Richards et al., 2022; Yao et al., 2022; Zhao et al., 2022; Zhu et al., 2022). MBE can be classified as a physical deposition, more specifically a PVD technique. In most of the cases, monocrystalline layers are expected due to the epitaxial growth in the chamber. The terminology “sticking coefficient” considers the ratio of the number of adsorb atoms and the total number of impinged atoms on the surface area. It is a suitable tool to deploy full-scale fabrication process of electronic devices related to energy and has gained tremendous attention in the recent days. The added advantage is the adaptability of such tool according to the research or manufacturing needs. Source guns can be chosen accordingly. MBE will continue to be source and means of basic research that will open up more horizons never ventured before (Kim et al., 2022; Richards et al., 2022; Yao et al., 2022; Zhao et al., 2022; Zhu et al., 2022).

3. Solution process techniques

Considering solution process techniques, spin, slot die, and dip coating techniques show the advantages of small scale to large applications development, minimal chemical wastage, and better interfaces.

3(a). Dip coating

One of the most efficient sol-gel methods is dip coating, which is very common in the industry as well as research institutes. Such solution process technique allows developing films for a wide range of applications including carrier transport layer for perovskite solar cells (Du et al., 2022). Such technique has a very effective control over growth considering the surface textures of any substrates. Generally, any coated or uncoated substrate needs to be submerged into a solution with an optimal transfer and retracting speed. The dipping numbers as well as the time determine the thickness of the thin layers. Viscosity of the liquid determines the compactness and the quality of the films. Later, the samples are required post-annealing to evaporate the solvents.

The dip-coating process is shown in Figure 13. As studied previously (Du et al., 2022), the coating speed, angle of disposition, and solution concentration play a crucial role to determine the ultimate thickness (Liang et al., 2021; Lundin et al., 2019; Padamata et al., 2022). Generally, it recommended developing the process in clean room environment to develop pristine surfaces. Previous works complemented the perovskite device fabrication, where dip coating technique was used to develop contact electron transport material layers. Compact TiO₂ films were deposited using dip coating on FTO coated glass. The dip coatings of the samples were carried out manually for two, four, and six dips. The chemical solution was the mixture of titanium diisopropoxidebis (acetylacetonate) (Sigma-Aldrich) and isopropanol. All the as deposited samples were annealed in air at 450°C for 60 min. The films were structurally and morphologically characterized by X-ray diffraction and scanning electron microscope (SEM). The results showed crystalline, compact, stoichiometric growth of titanium oxide. The structural analysis of titanium oxide thin films was performed by X-ray diffractometer with different diffraction angle 2θ from 20° to 70°. The analysis shows the preferential growth of the film along the (101) and (004) directions of TiO₂ phase with anatase structure (Du et al., 2022). The surface SEM images of titanium oxide layer on glass as deposited illustrate the nice full coverage of the films with less pinholes. Dip coating is not suitable for all coatings applications. A gradient of thickness can occur due to the variation throughout the entire surface. Hence, it becomes critical to control the immersion rate, viscosity of the source solution.

Figure 13. Dip coating: (a) process schematic and (b) XRD of a dip coated TiO_x films.

3(b). Spin coating

Spin coating technique is well adapted in many places to produce uniform films; however, the process becomes costly due to the loss of chemicals during the spinning process, which can be approximately 90% (Baloch et al., 2018; Guo et al., 2022; Hossain et al., 2020). It is a conventional method to deposit thin films on substrates during rotation at a very high speed. Spinning process and coating need to be parallel as otherwise gradient of films will be grown rather than compact films with homogeneity. Later, thin films are distributed evenly along the substrate edge. The thickness of the desired film depends on solution concentration, viscosity, spinning speed, and volume of solution drops. However, substrate size restricts the development of devices in large scale. Also, material wastage is in big range as almost 95% of the prepared solution needs to be discarded due to its unused proportion. Many works related to perovskite solar cells have been developed using spin coating technique in small scale. TiO₂ mesoporous layer was deposited on the aforementioned prepared 11 samples using a programmable spin coater for 30 s at 4000 rpm. The solution was prepared using 0.886 g of TiO₂ paste (Dyesole, 18NR-T) dissolved in 300 µL of ethanol. This mixture was kept on the sintering machine for 1 h. FTO side with compact layer at the edge was covered with a tape to protect from the mesoporous layer. All the samples were cleaned with dry air before placing on the substrate holder. Both compact and mesoporous layers were done outside of the glove box. A hotplate near to the spin coater was kept at 70°C. Figure 14 shows the spin coating technique and the results.

Figure 14. Spin coating technique: (a) process schematic, (b) IV characterization of the perovskite devices, and (c) device structure.

All the as prepared samples were kept on the hot plate after spin coating with removing the tape. After coating on all substrates, samples were put on a hot plate chamber to be annealed at 450°C for 1 h with compressed air flow inside the chamber using tube. The approximate layer thickness is around 150 nm. Perovskite solution was prepared inside the glove box using CH₃NH₃I (Sigma-Aldrich), PbI₂ (Sigma-Aldrich), and DMSO (Sigma-Aldrich, 99.7%). This solution was kept on hot plate at 80°C for 1 h (Baloch et al., 2018; Guo et al., 2022; Hossain et al., 2020). All the TiO₂ samples were transferred into the glove box to do the spin coating of perovskite layer. The spin coater was programmed for a single run at 2000 and 4000 rpm for 10 and 30 s, respectively. Chlorobenzene needs to be dropped while running the coater between 19 and 22s during the second step. This dropping needs to be very precise as film homogeneity depends on the drop of chlorobenzene. All the samples were annealed at 100°C on hot plate inside the glove box for 1 hour. The thickness of the perovskite layer was around 300 nm. The IV characteristic of the following structure FTO/blocking-TiO₂/mesoporous-TiO₂/perovskite/Spiro-OMETAD/Au shows promising results with the highest efficiency of 16.51% (V_oc = 1.095 V, J_sc = 20.580 mA/cm², and FF = 71.7%). Very high open circuit voltage around 1.095 V indicates the high quality of interfaces (Baloch et al., 2018; Guo et al., 2022; Hossain et al., 2020).

3(c). Spray coating

Spray coating technique is also widely been used considering its potential usage for scalability. Spray coating has been used to do perovskite solar cell device fabrication as shown in Figure 15. Compact TiO₂ films were deposited using spray pyrolysis technique on FTO coated glass at 450°C (Baloch et al., 2018; Guo et al., 2022; Hossain et al., 2020).

Figure 15. Spray coating technique: (a) process schematic and (b) spray on FTO substrates.

At the beginning, all the cleaned FTO substrates were put on the hot plate keeping the temperature to ramp up at 450°C. The solution was the mixture of 80 µL of titanium diisopropoxidebis(acetylacetonate) (Sigma-Aldrich), 60 µL of isopropanol, and 800 µL of ethanol. After the temperature reached up to 450°C, spray pyrolysis process was started with the dry air connection. After each round of deposition on all the substrates, there was a break for 10 s to allow proper crystallization on the substrates. Ramping down of the temperature to RT took around 1 h after spraying the whole volume of the solution (Baloch et al., 2018; Guo et al., 2022; Hossain et al., 2020). The approximate thickness of the compact layer is around 50 nm. Later, these samples (1.4 × 2.4 cm) were cut and prepared accordingly to complete the device fabrication.

3(d). Chemical vapor deposition

Chemical vapor deposition (CVD) technique is an essential system to grow crystalline semiconducting materials under vacuum to develop different devices (Mittal et al., 2021). In general, chemical reaction takes places in the presence of a mediator gas between two different materials to produce active and homogeneous thin films on substrates. The significant part of CVD is that it allows multidirectional depositional rather than a linear type like PVD. This tool allows users to grow materials in various forms, including monocrystalline, polycrystalline, amorphous, and epitaxial. The main working principle allows chemical reaction between a blend of gases and the bulk surface of the material, which causes chemical breakdown of some of the specific gas constituents, forming a thin coating on substrates. CVD has been utilized heavily in many industries to develop semiconducting, dielectric films for various applications (Hoang et al., 2021; Huang et al., 2021; Kalita & Umeno, 2022; Konar & Nessim, 2022; Presti et al., 2022). Some CVD techniques are atmospheric pressure CVD, low-pressure CVD, ultrahigh vacuum CVD, plasma-enhanced CVD, microwave plasma-assisted hot filament CVD, metal-organic CVD, and photo-initiated CVD.

3(e). Slot die coating

Slot-die coating is an enormously multipurpose deposition technique to insert solution through a slot, which is close to the surface (Hengyue Li et al., 2022; Khambunkoed et al., 2021; Seo et al., 2022; Xu et al., 2022). The controlled deposition process depends on wet-film coating thickness, the flow rate and the speed of the coated substrate relative to the slot. In addition, this technique is capable of achieving uniform films across large areas. This technique can easily be adapted to develop large area samples. Many research labs have adapted this technology to develop polymer and perovskite PVs, organic light-emitting diodes, quantum dots, and photonic structures with the highest ability of scaling-up process.

4. Conclusions

PVD techniques have shown the potential usage in both industrial and research scale due the scalability, cost effectiveness, reliability, and reproducibility. All PVD processes confirm a quick deposition process to grow from thin films to devices with compact, homogeneous, dense, defect free layers. Moreover, such techniques offer the option of growing films simultaneously or sequentially without breaking the vacuum, which ensures the development of clean devices. As studied for the films grown by PV tools, structural properties such as crystal orientation, grain size and lattice parameters confirm the required parameters. Thus, the results keep the space of developing various energy applications through the refinement of photon reflection and enhancement of photon scattering in such devices. This report has given an in-depth review of all deposition techniques considering both positive and negative sides. The report will have a significant impact to a wider group of materials scientists, chemists, physicists, and to the wider PV community to pick the best-suited vehicle for thin film growth purposes.

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Disclosure statement

No potential conflict of interest was reported by the authors.

Data Availability Statement

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Additional information

Funding

Open Access funding provided by the Qatar National Library.