**2. Toward scalable fabrication of perovskite solar cells for mobility**

Over the last decade, various support policies for electric vehicles (EVs) have been established in key markets, facilitating a major expansion of EVs models. But the challenge remains enormous such as driving range improvement and electricity production for charging EVs. Therefore, there are many efforts to increase the mileage of EVs. The use of solar energy would theoretically be the ideal fuel for EVs. In addition, significant fiscal incentives were provided for cars equipped with solar roofs (see IRC Section 30D(g)(1)(A)). Thus, major manufacturers expressed great interest in vehicle-intergraded solar cells. Still, the limited coverage area and efficiency of crystalline silicon-based solar cells make it difficult to have been active in a car application. Recently, Sono Motors, Lightyear one, and Mercedes-Benz presented the concept of

the electric concept sedan with a range of 620 miles (1000 km) by increasing the solar panel area such as the roof, hood, and trunk. However, unlike stationary solar cells, mobile one requires higher standards (e.g. climatic tests such as temperature, humidity, and sun, and mechanical tests such as shock, drop, and scratch resistance are conducted for pedestrian safety.) Hyundai (Sonata and IoniQ5) and Toyota (Prius) successfully launched solar-roof-integrated cars from the lamination technique of the panoramic sunroof.

Hyundai Motor Group continues to study solar roofs, hoods, and trunks with different requirements. Although the development of next-generation solar cells has been underway for a long time, unfortunately, silicon is still a primary material for commercial solar cells. As mentioned earlier, the mobile car with a limited area requires a new material solar cell capable of higher efficiency (above 30%) and flexibility for easy application. Here, we deal with perovskite cells (PVCs) for mobility only from the point of view of the fabrication method. Perovskite is regarded as one of the next-generation materials that can overcome or create synergy with c-Si. Many researchers and company have suggested fabrication methods on a lab scale, not in the mass production stage. In this section, we show the scalable fabrication method of PSCs, considering that different integration technologies are required depending on the parts in the car.

#### **2.1 Solution-based fabrication of perovskite solar cells**

There are representative PSCs manufacturing companies. They present various fabrication methods and strategies for the commercialization of perovskite (**Table 1**). The latest trends in development in schools and companies, along with increasing interest in PSCs, are described on website [60]. Solution-based fabrication of PSCs is similar to organic solar cells (OSCs) and organic light-emitting diode (OLED). In the case of OSCs, relatively high efficiency (18.2%) and mass production processes have been successfully researched in Kolon Industry [61]. Still, it has not been expanded due to the poor stability of organic materials. Moreover, the operating life issues of solution-processed OLED panels compared with the vacuum-processed device make it still an uncommon technique. Similar to these two devices, fabrication methods of PSCs have been developed. Similar to these two devices, PSCs are expected to have the same problem. To avoid these problems, we will briefly review the perovskite coating process.


#### **Table 1.**

*Comparison of some companies commercializing perovskite.*

*Lead-Free Perovskite and Improved Processes and Techniques for Creating Future… DOI: http://dx.doi.org/10.5772/intechopen.106256*

#### *2.1.1 Spin coating technique*

It has been widely used to fabricate small cells less than 0.1 cm2 and large module with area of 100 cm2 as well [62]. The main advantage of this technique is simple, has high reproducibility, and is easy to control a nanoscale film thickness. For perovskite application, one-step and two-step of spin coating method are used. For one-step spin coating, the mixed precursor solution (organic halide salts and lead halide salts are dissolved in commonly used solvent such as DMF and DMSO) is spin-coated under layer (charge-transporting layer). In order to prevent incomplete crystallization during on-step process, the two-step method including so-called anti-solvent engineering was introduced [63] This method has been widely used to produce high-quality film because it rapidly induces supersaturation, but the anti-solvent method cannot guarantee uniformity over a large area as the wet film is washed out. Additionally, since this method causes materials loss and stains with large-area coating, it is necessary to develop a method to replace this technique.

#### *2.1.2 Meniscus coating (slot die and blade) coating*

The term "meniscus coating" is the translation of a meniscus across the surface of a substrate as the solution is spread through a coating head or blade. These coating methods include slot die coating and blade coating. These coating can be applied in both sheet-to-sheet (S2S) and roll-to-roll (R2R) (see **Figure 9(c)** and **(d)**). Using R2R process, the material loss rate is less than 1% and high productivity can be expected [64]. The film thickness of the meniscus coating can be governed by the precursor solution concentration, the coating speed, and the gap distance between the blade or coating head and the substrates. A blade or a coating head moves across a surface or vice versa in the case of R2R. The blade spread predispensed ink, and the slot-die spread ink through a microfluidic metal die. They form it into a wet film. Compared with the spin coating, the solvent evaporation rate of meniscus-coated perovskite film is relatively slow, which promotes the growth of larger crystals. Because of the difficulty in controlling crystallization of perovskite film only by the natural drying, additional mechanical (e.g. preheating substrate [65] and gas blowing [66]) and chemical treatment (e.g.. anti-solvent [67]) are required. The former two cases are

#### **Figure 9.**

*Schematic of solution-based fabrication and vacuum thermal evaporation (co-evaporation).*

possible through modified coating equipment. The latter, the two-step method with anti-solvent, has been commonly used in spin coating, as discussed before. However, this method is difficult to be transferred from spin coating to meniscus coating due to its narrow time window. In this regard, solvent engineering of perovskite precursor ink is required to enable a one-step method. Depending on the existence of an ink reservoir, the slot die coating is more useful than a blade coating for scale-up [68]. Regardless of substrate type, all slot-die coating is still difficult due to materials and device structure restrictions (see **Table 2**). Still, the current intermediate results of these coating are likely to be the first solid step toward future manufacturing of the PSCs cells.

#### *2.1.3 Spray coating*

As a low-temperature coating technology, spray coating with with low cost, high volume, rapid manufacturing, and low material loss rate is the most widely used technique. Spray coating uses relatively low-concentration "inks." This process can be divided into four stages: *i)* the generation of the ink droplets, *ii)* the transport of the droplets to the substrate, *iii)* the coalescence of the droplets into a wet film, and *iv)* the drying of the thin film (**Figure 9(f)**) [68]. In recent years, 15.5% of mini perovskite module (size40 cm2 ) produced by spraying process are reported [72]. Until now, efficiency of spray-coated PSCs still lags behind a meniscus-coated PSC due to nonuniform crystallization of perovskite and film coverage issue when one-step process is applied.

#### *2.1.4 Inkjet printing coating*

The inkjet printing method can reduce the procedure of the laser etching processing for module fabrication. For inkjet printing of the perovskite films, most inkjet-printed demonstrations utilize piezoelectric MEMs print heads, which provide controllable microfluidic jetting through a silicon-etched nozzle. To improve the reliability and the speed of printing, multiple jet nozzles are also devised [79]. Inkjet printing enables specific cell shapes for particular functions, such as small-scale utility power and building-integrated photovoltaics (BIPVs). Additionally, an essential advantage of continuous and other inkjet printing systems is that they do not require physical contact or critical gaps between the jet and the substrate, making them suitable for printing on uneven, curved, or pressure-sensitive surfaces [68]. It will also enable the printing of solar cells ideal for cars, especially esthetics, in the future. Saule Technologies uses this method for 1-m-wide perovskite in building-integrated photovoltaics (BIPVs) and EV charging ports. Thus, it is a very scalable technique for fabricating PSCs.

#### **2.2 Solvent**

Many papers have explained the growth mechanism of perovskite crystals in solvent [80, 81]. The interaction between solvents and perovskite compositions is significant in the nucleation and crystallization processes during the perovskite film formation. Noted that the choice of solvent in perovskite solution should consider how well solvent can dissolve precursors and crystallize them in a one-step method, as mentioned earlier. Polar aprotic solvents (DMF, DMSO, γ-butyrolactone (GBL), pyrrolidone (NMP), and acetonitrile (ACN)) are commonly used solvents. Organic


**Table 2.** *Comparisons*

 *of some work by fabrication.*

*Lead-Free Perovskite and Improved Processes and Techniques for Creating Future… DOI: http://dx.doi.org/10.5772/intechopen.106256*

halides are relatively well soluble in organic solvents compared to metal halides (see **Table 2**). Many researchers prepare perovskite inks by dissolving organic halides into the DMF-based solvent before lead halides often decrease solvation time. The solvent has physical properties, such as Hansen's solubility parameters, dielectric constant, and Gutmann's donor number (DN). Solvents with dielectric constants more than 30 show generally good solubility for the perovskite precursors. In addition, halide in the solvents with low DN dominates the coordination with Pb2+, and perovskites are quickly crystallized. In contrast, solvents with high DN compete for the coordination of I with Pb2+ slow down the crystallization of perovskites. Then higher DN solvents can be employed as solvent additives to control the crystallization of perovskite [82, 83]. However, by combining volatile non-coordinating solvents, i.e. 2-methoxy ethanol (2ME), and low volatile, coordinating solvent, i.e. N-cyclohexyl-2-pyrrolidone (CHP), Seok groups obtained bar-coated PSCs cells with a PCE of 20% with the area of 31 cm2 [76]. 2ME has no or much weaker coordination capability. Thus, coordination ability and volatility (such as the boiling point and vapor pressure) can be critical parameters for perovskite crystallization. This solvent engineering will be a powerful technology for the scalability of printing. The toxicity of solvents, such as toxic DMF, skin-penetrating DMSO, or carcinogenic NMP, is also an important point, especially for operators who have direct contact with volatile solvents in the printing process [82]. It is difficult to determine whether 2ME is a commercially useful solvent or not due to the different regulations on industrial safety standards in country. On the other hand, DMSO is most environmentally friendly and least harmful to human health by systematically considering solvent production. Therefore, the search "green" solvent systems could also be critical for safe printing production [84].

#### **2.3 Vacuum thermal evaporation-based fabrication of PSCs**

Although vacuum thermal evaporation studies are relatively scarce, the following improvements led to PSCs with a PCE of 20%, not bad compared to the result of spincoated PSCs (see **Table 2**) [77]. It was a significant result since there would be a feasible route to produce PSCs commercially, as proven by the CIGS and OLED industry. Next, we briefly summarize vacuum thermal evaporation-based fabrication methods.

#### *2.3.1 Co-evaporation*

This method is the most suitable vacuum-based process for many applications. The perovskite films are fabricated inside a high vacuum at a pressure of 10<sup>5</sup> <sup>10</sup><sup>6</sup> bar. Each perovskite precursor is loaded into a crucible and heated to an appropriate sublimation temperature (**Figure 9(h)**). Removing the annealing step enables to deposit perovskite on any underlayer material [ref]. Perovskite films prepared in this way are more homogeneous, with better adherence to substrates than those prepared by spin coating. In addition, they are denser and pinhole-free, thus, more compatible with planar solar cells [85]. However, the most critical step is to control volatile organic halide (e.g. MAI) within the chamber, because it causes the compound to condense incompletely outside the evaporation cone region. The process is relatively slow and requires accurate periodic calibration to maintain deposition rates and precise stoichiometry.

*Lead-Free Perovskite and Improved Processes and Techniques for Creating Future… DOI: http://dx.doi.org/10.5772/intechopen.106256*

#### *2.3.2 Sequential evaporation*

The method involves depositing several film layers sequentially on top of each other and then converting these multiple film layers through diffusion and recrystallization. The metal halide layer is typically deposited first and then converted by the organic halides. This method may not be ideal for optimal commercial scaling due to its throughput. By optimizing the system's pressure for each evaporation step, high efficiency of 17.6% was achieved in a small area. [78] This technique can be challenging to commercial scale as alternating evaporation can slow throughput and material utilization. When using a large vacuum chamber, the relatively long distance between the source and the substrate reduces the deposition rate and increases material waste. In addition, the vacuum-based fabrication of PSCs uses a variety of vapors, making control of the precursor stoichiometry challenging. The vacuum-based technology tends to be costly because of the sophisticated infrastructures required. However, major display companies such as Samsung and LG are converting their small- and medium-sized OLED panel manufacturing technology from the sixth generation (1500x1850 mm) to the eighth generation (2200 x 2500 mm). The evaporator and Fink Metal Mask (FMM) in a micrometer are the key to OLED panel production: The evaporator is critical equipment that forms red, green, and blue pixels, and FMM is a thin metal plate with small fine holes to induce the OLED material to be deposited on the substrate's required position. In addition, a curved display has also been developed. The curvature of curved TVs on the market today is roughly between 2000R and 4000R. The radius of curvature (R) is the reciprocal of the curvature. For example, 2000R TV refers to a curved TV radius of 2000 mm, and the lower R, the more pronounced curve, and the higher R, the more subtle curve. For mobile solar cell, the area of the panoramic sunroof in the vehicle is smaller than that of the 6Gen or 8Gen display. For example, the Hyundai YF sonata has a sunroof of 1000 x 1800 mm and a curvature of about 8125R (length) and 2512R (width). The vacuum deposition method applies to flexible substrates, metal foils, and ultrathin glass substrates with a thickness of 0.7 mm, so the range of solar application in a car is also wide

### **3. Summary**

Perovskite solar cells have become one of today's most promising photovoltaic technologies. In this book, as the first section, we have introduced a lead-free CsSnI3 and a stable molecular iodosalt, Cs2SnI6, and demonstrated that it is a vaporprocessable semiconductor. Due to the instability properties of CsSnI3, this research put more effort to develop Cs2SnI6. The use of a new class structural material with intrinsic stability and beneficial optoelectronic properties can be considered as a start of the next chapter in pervoksite photon devices. As the second section, largearea applicable perovskite coating technologies for commercialization were overhauled. PSCs based on the solution process can be manufactured at a much lower cost than conventional silicon cells as well as showing similar efficiency to silicon cells. However, to apply and commercialize this in real life, it needs a largescale production technology while maintaining high efficiency. Until now, spin coating is the main technique. However, in general, this process has cracks or pinholes in the film when the coating area is expanded, resulting in poor density and uniformity. In addition, there is a limitation to reduce the amount of discarded solution, and it is difficult to do continuous (in-line) production. With increasing

demand for mass production, production of M6-sized PSCs module has been many technological advances. Plus, improving stability is a problem that must be solved. For long-term reliability of PSCs, various encapsulation processes are also being studied. By overcoming many issues that perovskite can have, our company plans to integrate the PSCs to vehicles by 2025.
