Thin Film Solution Processable Perovskite Solar Cell

*Mayur Jagdishbhai Patel, Himangshu Baishya, Ritesh Kant Gupta, Rabindranath Garai and Parameswar Krishnan Iyer*

## **Abstract**

Perovskite has emerged as a promising light-harvesting material for solar cells due to its higher absorption coefficient, bandgap tunability, low-exciton binding energy, and long carrier diffusion length. These lead to high power conversion efficiency >25% for thin film-based perovskite solar cells (PSCs). Additionally, PSCs can be fabricated through simple and cost-effective solution processable techniques, which make this technology more advantageous over the current photovoltaic technologies. Several solution-processable methods have been developed for fabrication of PSCs. In this chapter, the advantages and disadvantages of various solution processable techniques and their scope for large-scale commercialization will be discussed.

**Keywords:** perovskite, solar cells, thin film, solution processable, commercialization

### **1. Introduction**

Solar cell technologies have grown in the past few decades across four different generations. The first generation consisted of wafer-based photo active layer which was dominated by silicon wafer and is continuing to conquer the photovoltaic market. However, its high energy and cost of production has allowed the thin film technologies to gain attention among the research community. In the second-generation thin film based inorganic materials are being utilised to develop solar cells, but the efficiency has not reached the first-generation solar technology and the material production cost is also on the higher side. To reduce the cost of production further, organic and hybrid materials are being used in the third generation. The main advantage of this generation is that it allows photoactive layers to be deposited using low-cost solution processable techniques including spin coating, dip coating, etc. [1]. Further, large area fabrication is also facilitated by techniques such as doctor blading, inkjet printing, etc. [2, 3].

This advantage has also been strategically utilised in fabrication of perovskite solar cell which is the most promising and growing solar cell technology [4–6]. These techniques facilitate quick deposition of perovskite on any substrate at low temperature processing. Further, the solution processing techniques offer added advantage over the well-known thermal evaporation methods which requires complex vacuum systems. It has been also observed that the high-performance perovskite solar cells

(PSCs) are generally fabricated using one of the solution processing techniques. This chapter compiles all solution-processing techniques that are being utilised for the fabrication of perovskite solar cell. Each technique has been explained in details covering the advantages and disadvantages as well as its way in controlling the crystallinity of the deposited perovskite film for photovoltaic application.

### **2. Solution processable techniques**

#### **2.1 Anti-solvent dripping: one step deposition technique**

Anti-solvent dripping is sub part of spin coating process. Spin coating is a batch process that spreads a liquid film onto a rotating substrate using centrifugal force. This spin coating technique is categorised into two types: (a) one-step process and (b) two-step process. This method is successfully employed to develop a small area and large area PSCs of 0.1 cm2 and 1 cm2 , respectively. From the above-mentioned techniques, the solution processable one step deposition (OSD) technique is widely accepted to develop PSCs from the laboratory to the industrial level. The formation of perovskite in the one-step process involves two stages: (a) evaporation of excess solvent in the active layer and (b) crystallisation of the active layer [7]. Its popularity stems from its ease of use and low cost of equipment. However, like any other method, this technique suffers a significant drawback. This is primarily due to a lower substrate coverage area and the formation of a rough and porous surface at interface layer of the perovskite solar cell. This method relies on uniformity of film thickness and morphology control to achieve a desirable film quality. This method also faces the challenge of reducing pinholes in the perovskite film. This has a drastic impact on the optoelectronic properties of the PSCs resulting in poor performance [8].

#### *2.1.1 Antisolvent in chemistry*

In chemistry, antisolvent precipitation is a well-known method of crystallising a substance. In **Figure 1a** the antisolvent precipitation is illustrated. The unique part of the antisolvent method is its applicability for the manufacture of PSCs. In this method, a non-dissolving liquid, or anti-solvent, is dropped onto a spinning substrate containing a perovskite solution to quickly remove specific solvents like DMF and GBL (**Figure 1b**) [9]. The treatment causes rapid nucleation in the film and converts it to a homogeneous intermediate film. Annealing the substrate then results in smooth perovskite film.

#### *2.1.2 Antisolvent in perovskite*

To address all of the aforementioned issues in OSD, the Antisolvent dripping (ASD) method is used to control crystal growth kinetics and film quality. A dense layer of larger grain size (100–500 nm) CH3NH3PbI3 crystals is observed after successful ASD treatment. The significant formation of CH3NH3PbI3 crystals by ASD method has catapulted perovskite research into a whole new realm. For the preparation of high-quality Pb-based perovskite crystalline films, anti-solvents such as benzene, toluene, ethanol, methanol, acetonitrile, benzonitrile chloroform, isopropyl alcohol, ethylene glycol, and chlorobenzene have been utilised [10, 11]. This method was first reported by Jeon and his team, who discovered that using an antisolvent in the

*Thin Film Solution Processable Perovskite Solar Cell DOI: http://dx.doi.org/10.5772/intechopen.106056*

#### **Figure 1.**

*(a) Schematic representation of antisolvent assisted precipitation/or crystallisation. (b) Simple representation of anti-solvent treatment for the OSD technique.*

fabrication of perovskite films resulted in better-quality, dense films with big grain size. The comparison of the various perovskite materials performance with different antisolvents have been shown in the **Table 1**.

The ASD method can significantly alter the morphology of the MAPbI3 film's surface. During the deposition of the perovskite layer by the OSD method, many voids and pinholes were observed. On the other hand, the ASD method reveals lesser pinholes with large grains, densely packed MAPbI3 crystals due to its smooth and homogeneous surface morphology [12]. However, the anti-solvent preparation process must be done at the correct time and in the correct quantity as well as with a high level of proficiency. Uncontrolled crystallisation can also result in pinholes and higher defect density, which can reduce device efficiency and stability.

The performance of PSCs and their reproducibility is significantly improved when the MAPbI3 film is of higher quality and covers the entire surface area by using ASD method. The addition of favourable additives has facilitated perovskite crystal growth to improve the morphology, stability, excitonic, and optoelectronic properties of hybrid inorganic-organic perovskite films. Although it has been discovered that the solution-processable technique is capable of producing the ideal perovskite film, the quality of the film may be compromised due to factors such as temperature, precursor solubility, atmosphere, and annealing time [20].

Zhou et al. used an antisolvent-solvent extraction process to study the crystallisation behaviour of mix halide perovskites at room temperature. A small amount of


#### **Table 1.**

*PCE comparison of various PSCs using different antisolvents.*

#### *Thin Film Solution Processable Perovskite Solar Cell DOI: http://dx.doi.org/10.5772/intechopen.106056*

solvent diffuses within a large amount of antisolvent in this strategy. This antisolventsolvent extraction method achieves supersaturation state and nucleation for the crystallisation procedure, and nucleation rate can be improved by magnetic stirring of antisolvent. The antisolvent DEE (anhydrous diethyl ether), and magnetic stirrers were used to introduce advection in the antisolvent bath. The antisolvent-solvent extraction process is a straightforward method for producing high-quality perovskite films with improved morphology [21].

Xiao et al. demonstrated a fast, single-step, solution-based deposition crystallisation technique that allows control over the dynamics of nucleation and grain growth of CH3NH3PbI3, resulting in rapid and repeatable fabrication of high-quality perovskite thin films. In this method, a DMF solution of CH3NH3PbI3 perovskite is spin-coated on a substrate, followed by a second solvent, such as chlorobenzene (CBZ), applied on top of the wet film during the spin coating process to induce fast crystallisation. The second solvent is important for lowering the solubility of CH3NH3PbI3 and promoting crystal nucleation and growth within the thin film [22].

For inverted planar perovskite solar cells, Liu et al. reported effective and stable green mixed anti-solvent engineering. This green mixed anti-solvent technique can improve the surface morphology of perovskite films and passivate the grain boundary of perovskite thin films [23].

#### *2.1.3 Modified antisolvent treatment*

Modification, particularly during the anti-solvent treatment, is absolutely necessary to manage perovskite crystal growth in a humid environment and obtain a highly efficient device. Currently, the anti-solvent is dropped 15 s after the fast-spinning programme begins, with chlorobenzene, toluene, and diethyl ether being the most common anti-solvents used in PSC fabrication [24]. Determining an appropriate time delay for quenching anti-solvents is the most important part of anti-solvent treatment. The turbid point during the spin deposition stage must be identified in order to achieve this goal. During spin coating, the turbid point is the point at which the precursor film turns from transparent to turbid. The time delay increases as the RH level rises. This result is linked to the previously discussed solvent evaporation dynamics. According to Wang et al., the turbid point appears at approximately 9, 15, 19, and 20 s for RH0, RH50, RH70, and RH90%, respectively. In order to develop PSCs under high-humidity conditions, it appears that determining an accurate dripping time is an inconvenient procedure [25].

Thus, there are three possible strategies to avoid the antisolvent dripping time window: (a) creating an anti-solvent mixture by combining traditional non-polar solvents, such as diethyl ether and chlorobenzene, with other polar solvents (e.g., R-OH); (b) finding another anti-solvent that is suitable for high-humidity processing and completely substituting the commonly used anti-solvent; and (c) applying preheating treatment to the solution [26]. Anti-solvent mixes can be divided into two types: those with (1) a small amount of polar solvent and those with (2) a large amount of polar solvent. Because the anti-solvent contains a polar solvent, the adsorbed H2O molecules on the perovskite intermediate layer can be dissolved and removed concurrently with DMF and excess DMSO. As a result of the direct contact between the electron transfer layer and the hole transfer layer, a homogenous perovskite intermediate is generated, which gradually converts into a smooth and pinhole-free film, limiting charge recombination [27].

Another viable strategy for forming a high-quality perovskite film in high humidity is to replace the standard anti-solvent, as previously described. For high-humidity PSC production (up to RH75%), Troughton et al. utilised ethyl acetate. This was due to the fact that ethyl acetate, unlike other anti-solvents like toluene, chlorobenzene, and diethyl ether can absorb a significant amount of moisture in the air. Because of these properties, ethyl acetate can absorb moisture from the air and prevent it from interacting with the intermediate phase of perovskite. As a result, regardless of the RH level, uniform and smooth films can be created. Another effective strategy for making the anti-solvent treatment humidity insensitive is to speed up the rate of solvent evaporation by pre-annealing the substrate, causing the turbid point to appear earlier. According to Wang et al., the anti-solvent can be applied to the perovskite film 2 s after the spinning protocol starts, regardless of the humidity level, to facilitate the fabrication of highly efficient PSCs when the substrate is pre-heated to 70°C [28].

#### **2.2 Hot-casting**

Hot-casting technology has emerged as an excellent tool for the deposition of high-quality perovskite thin films with notable benefits including rapid crystallisation, a quick film formation process, increased grain size, preferred crystal orientation, and low defect density [29]. Within the framework of nucleation growth theory, a direct formation mechanism is proposed that constitute a driving force for the phase change provided by a high substrate temperature leading to an ultrashort crystallisation process. Meanwhile, a sufficient thermal energy allows atoms to diffuse in a liquid without forming an intermediate phase. Nie et al. were the first to report this method, and they went on to improve the film quality by tweaking deposition parameters like substrate temperature, annealing temperature, and precursor composition. This technology has recently been expanded to include the deposition of organic– inorganic hybrid, all-inorganic, lead-free, and low dimensional perovskite films [30].

#### *2.2.1 Fundamentals of nucleation and crystal growth*

The goal of the hot-casting technology is to spin coat a hot precursor solution over a substrate at higher-temperature. The crystal growth is influenced by factors such as substrate temperature, solution concentration, solvent, and supersaturated environment. In **Figure 2a**, the LaMer diagram represents these deposition parameters correlated with the nucleation and growth processes [31].

#### *2.2.2 Classical nucleation and classical growth*

The Volmer-Weber model, the Frank-van der Merwe model, and the Stranski Krastanov model are three classic thin film nucleation and growth models [32, 33]. The Volmer-Weber model is valid for the nucleation and growth of most polycrystalline thin films if the substrate temperature is sufficiently high and the deposited atoms have a certain diffusion capability.

According to classical nucleation theory, to initiate the crystallisation, the solution must be supersaturated. Nucleation can be either heterogenous or homogenous. Heterogeneous nucleation is defined as nucleation with preferential nucleation sites, which means that new phases form preferentially within certain regions of a liquid phase. The system must overcome an energy barrier known as the maximum free energy of critical nucleation (G) during the thermodynamic nucleation process.

*Thin Film Solution Processable Perovskite Solar Cell DOI: http://dx.doi.org/10.5772/intechopen.106056*

#### **Figure 2.**

*(a) Lar Mer model for nucleation and growth of perovskite thin films, (b) a schematic illustration of nucleation and growth of perovskite films at each stage.*

Heterogeneous nucleation is defined as nucleation with preferential nucleation sites, which indicates that new phases form preferentially within some parts of a liquid phase. The system must surpass an energy barrier known as the maximal free energy of critical nucleation (G) during the thermodynamic nucleation process.

#### **2.3 Temperature and thermal annealing**

#### *2.3.1 Direct formation mechanism*

Nucleation growth is influenced by the substrate temperature and thermal annealing. When the substrate temperature is low (less than 100°C), the perovskite film formation process includes three steps- the initial solution stage, the transition-to-solid film stage, and the transformation stage from intermediates to a perovskite film [34]. However, when the substrate temperature is raised to 100–180°C, enough thermal energy is provided to speed reactant diffusion and contact, hence no intermediate phase development occurs during the hot-casting process leading to the direct formation of perovskite film.

#### *2.3.2 Substrate temperature*

The supersaturation of the solution is affected by the substrate temperature, which changes the nucleation rate and film shape. The high boiling solvent supports a stable

development of the perovskite crystal with big crystal grains when the substrate temperature is higher than the crystallisation temperature of the perovskite phase. A large-grain perovskite layer improves device performance by lowering the defect density and boosting the mobility which suppresses charge trapping [35]. The effect of Substrate temperature on various perovskite material has been provided in the **Table 2.**
