**3. Thin film solar cells**

Even though the photovoltaic module market is dominated by crystalline silicon wafer-based technologies, thin film solar cells have the advantages of being lightweight, flexible, transparent and high temperature and radiation tolerant, making them competitive alternatives for c-Si in applications such as wearable devices, space applications, building-integrated PV and vehicle integrated PV. Thin film solar cells also avoid the massive energy consumption in the fabrication of high-purity silicon ingots in c-Si manufacturing. Moreover, thin film solar cells can be fabricated on Si solar cells to form tandem solar cells, enabling higher conversion efficiency.

**Figure 5** shows three different configurations for CdTe, perovskite and CuInGaSe (CIGS) solar cells. The basic structure of CdTe and CIGS (same as Cu2ZnSnS4 (CZTS)) solar cells includes a window layer (TCO), buffer layer, absorber layer and back contacts and substrates, while perovskite solar cells have an n-i-p structure with extra electron/holes transport layers. Different layer stacks are coated on the substrate with thin-film coating methods using either solution-based processes like a chemical bath or an ink-like coating procedure, or using vacuum-based processes like thermal evaporation or sputtering [16].

#### **3.1 Solution processing**

#### *3.1.1 Spin coating*

Since it was first reported in 2009, the power conversion efficiency of perovskite solar cells has increased from 3.8–25% (lab scale) within 13 years. The emerging perovskite solar cells are under massive investigation in both research and industry due to their low-cost and high efficiency, as well as their use in Si-based tandem solar cells with further enhanced efficiency. Solution processing is the traditional fabrication method for perovskite solar cells. Spin coating is the most common fabrication approach used in the laboratory, and it has the advantages of a simple process, with no requirement for expensive and complex vacuum systems. This method could also introduce additives into the perovskite precursor solution to improve device performance [17]. **Figure 6** shows the schematics of the spin coating method for perovskite

#### **Figure 5.**

*(a) Superstrate configuration for CdTe cells; (b) perovskite cells in the n-i-p configuration; c. substrate CIGS cell configuration. Reproduced from [16] with permission.*

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

#### **Figure 6.**

*Schematic illustration of the deposition of a perovskite layer by spin-coating and post annealing. In the one-step method, all perovskite precursors are mixed in a single solution, which is deposited onto the substrate; In the twostep approach, the precursors is deposited first, followed by the deposition of a second precursor and thermal annealing; solvent engineering approach is to deposit all perovskite precursors in a single step, during which an antisolvent is applied triggering the crystallization of the perovskite film. Reproduced from [17] with permission.*

fabrication. The fabrication of perovskite absorber can be achieved by one-step deposition, two-step deposition or solvent engineering methods with spin-coating deposition and post annealing processes, as shown in **Figure 6**. However, the spin-coating method is generally not suitable for mass production.To translate such PV technology from the laboratory to industrial-scale manufacturing, other techniques need to be explored.

#### *3.1.2 Slot-die coating*

One of the most promising techniques for achieving large-scale (roll to roll) perovskite solar cell production is slot-die coating [18–20].

Slot-die coating can achieve highly precise control of material usage and results in very low waste levels of ink compared to other deposition methods such as spin coating or spray and screen printing. **Figure 7** shows the schematics of slot-die coating. By adjusting the ink flow pumped to the die coating head by syringe and the substrate speed, fine control of the deposited film thickness can be achieved, from tens of nanometers to tens of microns. An air knife with nitrogen gas flow would help with drying the perovskite ink, and heating could also be applied for better crystallization [18, 21].

The film formation process can also be controlled by the choice of precursor, solvent and additive. **Figure 8a** shows a schematic of slot-die coating setup with two inks and the influence of the strongly coordinating solvent dimethyl-sulfoxide (DMSO) as an additive in 2-methoxy-ethanol (2-ME) based perovskite ink on film formation. Adding a limited amount of DMSO (11.77 mol%) leads to a denser

**Figure 7.** *Schematic of roll-to-roll slot-die setup. Reproduced from [20] with permission.*

#### **Figure 8.**

*(a) Schematic of slot-die coater setup with 2-ME ink and 2-ME-DMOS ink. (b) SEM cross-sectional images of the coated films without and with DMOS. Reproduced from [22] with permission.*

thin-film without pinholes and large columnar crystallites as shown in the SEM crosssection images in **Figure 8b** [22]. Research on the film morphology and optimization of perovskite ink and chemicals is still ongoing.

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

#### *3.1.3 Inkjet printing*

Inkjet printing is another promising large-scale production method for perovskite solar cells, as it offers several favorable properties. Ink-jet printing of perovskite can achieve high-resolution film formation with customized patterns under ambient conditions.

A schematic of inject printing is shown in **Figure 9**. Inkjet printing of perovskite is carried out by a drop-on-demand (DOD) printing approach, which by definition, enables the generation of a single droplet when required, hence enabling high precision of material usage. The location of film formation can be finely controlled by motions of the inkjet printhead nozzles and the substrate. The ejection of printing materials (perovskite ink) is forced out of the nozzle by regular pressure pulses caused by contractions of ink chamber volume in the nozzles. The pressure pulse can be generated by either the mechanical deformation of a piezoelectric transducer or the collapse of thermal bubbles which involves resistive localized heating in the ink chamber. Due to the technical challenge of generating vapor bubbles in high-vapor pressure fluid, and the fact that a piezoelectric inkjet printing system can control droplet size and velocity by simply adjusting the actuation pulse, piezoelectric DOD inkjet printing would be more suitable for large-scale production of perovskite solar cells. **Figure 9** demonstrates the schematic of an inkjet printing nozzle, which an industry-scale printhead would have several hundreds of, enabling rapid and high-resolution printing at a low cost [23, 24].
