*3.2.1 Evaporation*

Among vacuum-based processing techniques, evaporation is a widely used deposition process for the formation and growth of thin films in PV. The process is beneficial in a contemporary environment and extensively applicable in the laboratory and industrial manufacturing for the deposition of thin films. A schematic of the evaporation coating is shown in **Figure 10a**.

The basic sequential steps for the evaporation process are given below:


The evaporation process has been reported to be performed using different configurations, including molecular beam epitaxy, reactive evaporation and activated reactive evaporation [3].

Snaith and his team first demonstrated that perovskite can also be deposited by thermal evaporation, reaching an efficiency of 15.4% in 2013 [25]. A recent study by

#### **Figure 10.**

*Schematics of (a) evaporation coating methods. Reproduced from [36] with permission. (b) Close space sublimation (CSS) system. Reproduced from [37] with permission.*

#### *Thin Film Deposition Technologies and Application in Photovoltaics DOI: http://dx.doi.org/10.5772/intechopen.108026*

Bruno and co-workers demonstrated thermally evaporated perovskite mini-modules with an active area of 21 cm<sup>2</sup> and an impressive efficiency of 18.13% [26]. These results are superior to all other reports of large-area devices in the literature, among which the highest is a blade-coated perovskite solar cell with a maximum efficiency of 16.4% [27]. Notable progress was made in 2016 by Momblona *et al.,* who demonstrated a fully evaporated planar perovskite solar cell with an efficiency above 20%. It is worth noting that the charge carrier-selective layers, the MAPbI3 absorber and the metal electrode were all sequentially evaporated, demonstrating an all vacuum-based process for the first time [28].

Thermal evaporation offers the unique possibility of depositing multilayers of perovskite materials. Moreover, the fact that perovskite layers can be formed without the need for thermal annealing makes thermal evaporation particularly suitable for flexible optoelectronics application, in which low processing temperatures are required. Finally, this process avoids the use of the toxic solvents and allows the deposition of perovskite films without the risk of damaging the underlying layers of tandem devices [17].

Thermal evaporation is also employed in fabrication processes for commercial CIGS and CdTe solar cells [29–32]. CIGS absorbers are usually fabricated by thermal coevaporation of the constituents, using the so-called 3-stage process. During this process, In, Ga and Se are evaporated in the 1st and 3rd stages, while Cu and Se are deposited in between, leading to the so-called double gradient of the In and Ga concentrations [33].

Close-spaced sublimation (CSS) is the fastest and simplest deposition process for both the CdTe and CdS semiconductors used in CdTe thin film solar cells, permitting high-speed in-line production [34]. The current CdTe thin film solar cell record is 21% on an approximately 1cm<sup>2</sup> cell on glass made by First Solar [35].

**Figure 10b** shows a schematic of a CSS system, in which the CdTe is deposited at a pressure between 1 and 100 mbar in argon or nitrogen. The substrate and crucible are kept a few centimeters apart for vapourization of CdTe granulate and condensation (crystallization) on the substrate. Because the substrate is kept at a temperature range from 450–600°C for high-quality crystallization, a relatively high pressure is applied to suppress re-evaporation of the material [34, 37, 38].

#### *3.2.2 Sputtering*

Sputtering is another widely used PVD technique in PV that can be upscaled. In sputtering processes, a magnetron is positioned near the target. The ionic gas is introduced in an accelerated way into the vacuum chamber, blasting the target, releasing atomic-sized particles to be deposited, which will be violently projected onto the substrate. A schematic of sputtering deposition is shown in **Figure 11a**. Sputtering deposition has become a generic name for a variety of sputtering processes. These processes are named based on their source and the orientation of the process. Variants of sputtering include diode sputtering (cathode or radio frequency), reactive sputtering, bias sputtering, magnetron sputtering and ion-beam sputtering with a DC or RF power source [3]. Because of the mechanism nature of sputtering, it is ideal for the deposition of doped materials with multiple target sources. With gas inlets, the sputtering process could be used for the deposition of thin film metal or nonmetal oxides, and with hydrogen gas, hydrogenated thin films as well. Sputtering enables the production of smooth surfaces using lower temperatures, presenting excellent mechanical and tribological properties and having very good adhesion to the main materials used as substrate.

Sputtering has been used in the deposition of CIGS and CZTS absorber layers [39, 40]. A record efficiency of 23.35% in CIGSSe thin film solar cells was achieved with a sputtered absorber layer [40]. Sputtering is also commonly used for TCO, buffer layers and ETL/HTL with materials such as ITO [41], FZO [42], MoOx [43], AZO [44], TiOx [45], and other metal oxides as interlayers, passivating thin film solar cells [46–48]. A commercialized all-sputtering system was developed by Midsummer for CIGS production with process sequence completed in different sputtering chambers for diffusion barrier, absorber, buffer, window and TCO layer deposition [49]. The sputtering method could also be employed for the deposition of poly-Si contacts and passivation for heterojunction solar cells [50–53].

#### *3.2.3 MOCVD (MOVPE)*

III–V thin film solar cells are widely used in aerospace applications, due to the high energy conversion rate, wide operating temperature range and high radiation resistance [54]. The record efficiencies for III–V multijunctions are 38% for a five-junction cell (bonded) and 37.9% for InGaP/GaAs/InGaAs solar cells. The record efficiency for a GaAs single-junction cell is 25.1%, held by Alta Devices [35]. One key deposition method used in III–V thin film solar cell fabrication is metalorganic chemical vapor deposition (MOCVD), also called metalorganic vapor-phase epitaxy (MOVPE). Metal–organic CVD (MOCVD) is a CVD process for growing epitaxial films and is done by flowing precursor gases over the substrate. In III–V semiconductors, the metallic element is carried by an organic gas such as Ga(CH3)3) and In(CH3)3 along with AsH3 or phosphine (PH3). The gases are allowed to decompose due to pyrolysis on the heated substrate surfaces to produce the desired film. A schematic is shown in **Figure 12**. Commercially available MOCVD tools are designed to produce traditional III–V semiconductor devices for electronic and optoelectronic applications. Relative to solar cells, these devices have complex layered structures that require extremely precise control of thickness, composition and doping profiles, and each fabrication typically takes multiple hours to complete. The simpler structure and relatively wide process windows of solar cells present an opportunity to use correspondingly simpler

*Thin Film Deposition Technologies and Application in Photovoltaics DOI: http://dx.doi.org/10.5772/intechopen.108026*

#### **Figure 12.**

*MOCVD reactor basic scheme and fundamental working principle: I—Precursor, II—Cracking, III—Deposition and IV—Removal of residual gases.*

and less expensive equipment. With customized MOCVD, combining automated substrate loading and unloading, fast temperature ramping, high growth rate and the elimination of pressure cycling, the fabrication process time could be reduced to 15 min, which makes a significant contribution to fabrication cost reduction [55].

### **4. Solar modules**

Another important application of thin films in PV is the antireflection coating (ARC) on the surface of solar glass where the light first reaches the solar panels. Currently, single-layer antireflection coated solar glass has a dominant market share of 95% compared to glass with other coatings or no coating, for Si PV modules [2]. This ARC results in an efficiency gain of 2–3%; that is, 2–3% more light can enter the solar modules to be

**Figure 13.** *Schematic of roller coating process.*

absorbed by the solar cells and converted into electricity. The most common PV ARC consists of a 100 nm single layer of nano-porous silica deposited onto the solar glass cover via sol–gel roller coating, followed by a high-temperature sintering and tempering process. The roller coating is one type of sol–gel method that is scalable and low cost. A schematic of a roller coating process is shown in **Figure 13**. Other chemical methods such as slot-die coating, dip coating, spin coating, spray coating, as well as PVD methods like sputtering multilayer ARC are also under investigation.
