Optimal Conditions for Preparation of Perovskite Materials for Optoelectronic Devices

Abstract

Several data on the preparation of perovskite crystals have been obtained because samples/devices were prepared using films of different qualities. Identifying optimal conditions for perovskite material synthesis and thin film preparation as well as optimizing the properties will go a long way in reducing the disparities in the data obtained. The optimal composition management of various elements of perovskite remains an outstanding research. The chapter will pave the way for the optimum design of the synthesis process of perovskite-based devices for better performance. Further still, the study provides basis for explaining the effective optimizations of synthesis conditions and material properties.

1. Introduction

In recent decades, modern technologies in the fields of optical materials and optoelectronics, plus energy storage and up-conversion luminescent applications have received spectacular prominence. They have been instrumental in tackling socio economic needs, including the rising global energy demand, the increasing demand for renewable clean energy, the transition to digitization, the Internet of Things, and the development of multifaceted applications like colorful optoelectronic devices, heat control and agrivoltaics [1, 2, 3].

Various materials have been synthesized by many research groups to fabricate optoelectronic and photovoltaic (PV) devices, but, only a few materials have met the basic requirement to manufacture optoelectronic and PV devices. The majority of electronic and optoelectronic devices are made using GaAs, CdTe, Si, CuInSe₂ and InP semiconductor materials. Of these materials, the Si semiconductor dominates the market currently [4].

Silicon is globally employed in PV devices [5] and likewise for optoelectronic devices, several silicon materials are employed for manufacturing. During synthesis and fabrication silicon materials require sophisticated equipment which operate at high temperatures. Also, fabrication of crystalline silicon is conducted in an extremely controlled environment to prevent oxidation as the combined oxygen resists the movement/alternation of electrons/charge carriers in PV cell, thereby decreasing the required purity [6, 7]. Consequently, to maintain every basic parameter, the total cost rises and the entire procedure becomes complex.

The demand for better and more sustainable material is increasing either to reduce or replace the dominance of silicon materials. More efficient materials will be needed to meet the growing global need. Hence, for manufacturing of efficient PV cells as well as optoelectronic devices, there is an urgent need to search for the useful combinations of potential materials. The arrival of hybrid perovskite material has created enormous excitement in PV and optoelectronic community.

Hybrid perovskite is a semiconductor which may be described as a class of materials that mix organic, inorganic and halides components. Perovskite has the chemical formula of ABX₃, in which A is an organic or metal cation (such as MA+, FA+, Cs+), B is a metal cation (such as Pb2+, Sn²⁺), and X is a halogen (such as Cl-, Br-, I-).

Perovskite materials exhibit excellent optoelectronic properties and lower crystallization activation energies, in contrast to silicon [6, 7] Hybrid perovskites, which blend the advantages of excellent optical and electronic properties together with solution-processed manufacturing, have appeared as a new class of revolutionary and innovative optoelectronic materials with the potential for several practical applications.

Furthermore, from an application perspective, this family of materials is employed initially as PV cell material, where they have escalated rapidly to be a rival with silicon in the space of a few years, and have now proven to be useful in nearly all optoelectronic devices as shown in Figure 1.

Figure 1.

Potential application in various fields of optoelectronic [8].

1.1 Motivation behind the optimal control and point of interest

Organic–inorganic halide perovskites have emerged as high performance optoelectronic materials and show still greater potential due to their unique properties such as tunable band gap, long charge carrier diffusion length, high absorption coefficient, large carrier lifetime and ambipolar charge transport. At the same time, this kind of rapid increase in the efficiency of perovskite-based solar cells, for nearly a decade, has not been a walk in the park. The credit goes to the researchers/scientists working around the planet utilizing different device fabrication and design methods [9]. Interestingly, perovskites blend the properties of inorganic materials like high photoluminescence quantum yield, long carrier diffusion lengths, high color purity together with the properties of organic materials such as solution processability at low temperature, and high production yield [10].

Additionally, there are two considerations that increase the market value of this semiconductor material. First of all, crude materials should be cheap. Luckily, the starting materials of perovskites (like C, H, Pb, Cl, I and Br) are earth abundant. Consequently, the cost of raw materials will not hamper the commercial use of perovskite materials.

In the second place, the manufacturing method need to be based on inexpensive equipment and procedures. Once more, formation of perovskites exhibits the affordable characteristic, as a result of uncomplicated preparation of perovskite films (i.e., low formation energy or simple reaction between metal halide and organic halide), low-cost equipment [6].

Thanks to the epic efforts on perovskite materials from perovskite research community, remarkable achievement has been made for both laboratory and large scale fabrication processes especially for PV cells. But, based on the screening from literature [6] there are several issues and parameters that should be considered and optimized as explained in the following paragraphs.

The chemical composition management along with the structure of perovskite with best output need to be broadly investigated, since the crystallization dynamics are highly dependent on the composition of solution, concentration, and solvent type. At the moment, numerous compositions are published [7, 11], and several manufacturing techniques, together with treatment techniques (like types of annealing, and variety of additives) are employed for fabrication of optoelectronic devices. However, there is no one technique that is powerful due to the various compositions and the corresponding numerous physiochemical properties of them. Hence, the role of each technique or its mechanism should be deciphered so as to establish the protocol or the optimal condition for manufacturing optoelectronic devices with greater performance especially in large scale production.

More focus should be on stability, and currently concerted efforts are ongoing to find a lasting solution. But for now, encapsulation is just considered as a complementary technique, and the need for tackling stability concern is to explore perovskite materials with improved intrinsic stability and superb optoelectronic property. Also, the issue of lead toxicity though very minute in quantity can be solved by using effective encapsulation technology to reduce Pb leakage into the surroundings and recycling technique. In conjunction with above issues, the fabrication process of large-scale devices plus fabrication parameters should be broadly studied and meticulously modified to obtain high-quality perovskite films [6, 7, 10].

The main objective in material optimization is to indicate the properties and performance of materials as well as optimal synthesis control prior to fabrication of the devices. This will serve as a reference point/requirement for good repeatability of processing and manufacturing for large-scale production. In addition to optimizing the material composition and fabrication techniques, the sequential development of perovskite film quality such as film coverage, grain size, surface passivation, offer more incremental improvements.

Moreover, the quality of a film for example is controlled by nucleation and crystallization of the material which consequently affects the properties of the film and its stability. Hence, optimizing the perovskite crystals is a noteworthy technique for enhancing the properties of the perovskite film. Controlled formation of perovskite crystal during preparation method is vital in achieving better morphological properties and consequently the material properties. Basically, the morphology and size of the crystals are largely affected by the solvent employed, annealing time and annealing temperature [12, 13].

A lot of data on the preparation of perovskite crystals has been obtained because samples/devices were prepared using films of different qualities. Identifying optimal conditions for perovskite material synthesis and thin film preparation as well as optimization of the properties will go a long way in reducing the disparities in the data obtained. The optimal composition management of various elements of perovskite remains an outstanding research.

The tunability of the hybrid perovskites through their halides and its constituents has many potentials for methodically establishing structure–function relationships to help design novel perovskites. Also, mechanisms of nucleation and growth of these perovskite crystals should be part of these synthetic observation

2. Treatment method: formation of qualitative perovskite film

An extensive various morphologies have already been achieved simply by varying the crystallization parameters thereby attaining uniform nucleation and compact films, especially on smooth substrates. As these films are prepared at low temperature and often by rapid crystallization, it appears that kinetics play a major role in the process.

Treatment technique is the procedure employed to obtain a highly qualitative film with an utmost degree of crystallinity and coverage of a corresponding/associated substrate. The quality of a film in terms of crystallinity, uniformity and coverage is mostly linked to a controlled development of the morphology in the process of film formation.

The morphology of perovskite film is a primal factor in deciding/resolving the charge carrier dynamics such as carrier lifetime, the carrier diffusion lengths, and ultimately the device performance [10, 14].

Morphology control has been improved through careful optimization of processing conditions. Inert processing environments, optimized spin coating conditions and adequate annealing temperature control allowed improvements in film morphology resulting in enhanced efficiency of devices.

For example, poor morphology which is characterized by uneven/irregular grain size, voids/pinholes, low coverage and high roughness leads to low light absorption and ineffective charge transfer. Additionally, it causes easy degradation due to the hydrophilic nature of perovskite. Therefore, from application perspective, it should be stressed that stability is as important as efficiency.

For the energy-efficient PSCs, large grains with interlinked grain boundaries are required, whereas for the high-efficient light-emitting diodes (LEDs), small grains with pinhole-free film morphology are needed. Hence, it is vital to track the crystal growth and control in accordance with the application. Good film morphology implies a smooth, pinhole-free, and compact film as well as favorable interfacial electrical properties [10, 14].

In order to address the issues raised above, the following considerations which are the focus of this chapter must be optimized.

2.1 Heat treatment: nucleation and crystal growth

Perovskite materials show a vast array of film properties such as size of grain, morphology, surface coverage, crystallinity, etc. as a result of different processing techniques. Numerous works have also demonstrated that perovskite films show composition-structure relationships in term of properties [15]. Therefore, in achieving excellent control over the reaction between the inorganic and organic elements, various process parameters have to be included in preparation so as to produce high quality thin film. Chief among these are: solvent engineering, stoichiometry, thermal treatment, and additives.

Irrespective of the manufacturing route, controlling and understanding the preparation parameters, such as precursor’s concentration, annealing temperature, and used solvents and additives, are crucial. They play fundamental roles in the film surface quality, coverage, conversion of precursors to perovskite, degree of crystallinity, size of the crystal, hence the performance of the layer and the whole device. In addition, perovskite film coverage is a function of annealing temperature during processing by solution technique. Lower annealing temperatures produce film poor convergence while higher annealing temperatures give rise to decomposition of active layer. In sum, final output of the perovskite PV devices is closely dependent on the perovskite film quality.

2.2 Controlling crystallization process: temperature

Temperature treatment is one of the frequently employed methods to aid this fabrication process. The fundamental difference in the crystal structure and morphology is dependent upon annealing temperature. In the synthesis of perovskite by solution processable method, the temperature plays a key role. The optimum temperature for the growth of perovskite is between 70 and 110°C, and the optimum growth temperature is about 110°C [7, 16].

It has been reported that higher processing temperatures close to or greater than the optimal temperature for crystallization of perovskite may cause simultaneous evaporation of solvent and growth of crystal thereby restraining the processing window along with the repeatability/reproducibility of device fabrication. In light of this, preparation of a controlled crystallization protocol that is in line with low-temperature deposition of precursor films is extremely needed for manufacturing of large-scale perovskite thin film. Thus, it is of paramount importance to investigate novel procedure for fabrication of hybrid perovskites with low-temperature and offer a profound understanding of some basic properties of these materials [13, 17].

Crystallization is a complex process which is designed for preparing a crystalline material from liquid, gaseous, or amorphous solid systems. Nucleation and crystal growth are the two basic stages of great significance which usually occur during the formation of a supersaturated state. Specific to polycrystalline perovskite thin-film growth, the one-step fabrication method has been extensively examined and the exploration of the crystallization mechanism would stimulate the large-scale fabrication procedure. The common crystallization technique entails three steps: (1) the supersaturation of solution; (2) the nuclei formation (3) the crystal growth. By means of the anti-solvent together with the evaporation of the solvent, the solution is supersaturated, then the nucleation process begins, accompanied by the consumption of solute, and the beginning of the crystal growth [18, 19].

As indicated, it is noteworthy that the crystallization process runs under ideal conditions when the precursor deposition is achieved with required properties. Based on the details stated above, it is useful to remember that the variation of two basic steps of the crystallization process – nucleation and crystal growth with the aid of optimization of some parameters like selection of solvent, solution concentration, precursor ratio, annealing temperature – is a vital point for formation of a qualitative and uniform film of perovskite without voids and with a full coverage. Furthermore, it is essential to observe the optimum conditions for high perovskite crystallinity, which enables the separation efficiency of charges, their transport and diffusion length [7].

3. Optimal conditions for hybrid perovskite crystallization

Having understood the importance of high-quality perovskite films as an active layer in a device, we now turn to outline the generality of the protocol for preparation of perovskite material. This will increase reproducibility and enhance the stability.

There are various techniques for regulation and determination of optimal conditions for crystallization as listed in Figure 2; they are solvent selection, regulation of solvent evaporation rate, special additions including antisolvents and additives, annealing by solvent and temperature annealing.

Figure 2.

Parameters to optimize for up-scaling [20].

3.1 Solvent selection

In the preparation of perovskite, the choice of solvent is a crucial parameter influencing the crystallization and morphology of perovskite. In the selection of preferred solvent, the key perquisite/condition is that the solvent must be polar for easy dissolution of precursors and its physical properties like boiling point and vapor pressure, must to be taken into consideration with respect to the desired crystallization mechanism, that is, rapid or slow. Polar solvents such as dimethyl sulfoxide (DMSO), dimethylformamide (DMF), γ-butyrolactone (GBL), N-Methyl-2-pyrrolidone (NMP) are the generic solvents used in dissolving precursor and mixing these solvents is a window of opportunity to optimize the crystallization [21].

It is important to carefully control the solvent evaporation as a means to form stepwise crystallization. The dissimilarity in the crystallization process is observed when employing different solvents: (DMF > DMSO > NMP from fast to slow crystallization).

DMF which has high vapor pressure evaporates rapidly during spin coating leading to short drying window for formation of perovskite thin films. While the other solvents like DMSO and NMP have lesser vapor pressure and are tricky to be evaporated, therefore, it highly prolongs the drying window. To that effect, DMF is generally not utilized alone, but it is mixed with other solvents so as to widen/prolong the antisolvent window.

The preferred solvent in these series for formation of qualitative perovskite film is DMSO. Nevertheless, perovskite produced from this solvent has several drawbacks like incomplete conversion (non-reacted PbI₂) and large polydispersity in crystal size. These limitations can be resolved by mixing the solvents [7].

3.2 Anti-solvent treatment

A solution is a mixture of solute dissolved in a solvent. An anti-solvent is a liquid that does not dissolve the solute but is miscible with the solvent. During preparation of perovskite via solution technique, perovskite precursor is dissolved in solvents.

Antisolvent is commonly added to the perovskite film after its formation or during its growth in order to avoid decomposition and reaction with perovskite. During spin coating procedure, most of the solvent is removed owing to the centrifugal force produced by spinning. But, there is still leftover solvent in the film made by spin coating due to film wetness. For qualitative crystallization, this residual solvent needs to be removed by thermal annealing. But, these solvents evaporate gradually during annealing which may lead to poor film morphology, hence influencing the overall performance of perovskite based devices. To overcome this issue, the use of antisolvent was introduce during spinning operation so as to quickly lessen the solubility of perovskite precursor and facilitate the rapid crystallization, which enhances the performance of devices.

Various anti-solvents were reported in literature [6, 7, 21] such as chlorobenzene (CB), benzene, xylene (XYL), toluene (TL), ethyl acetate etc. The properties of these anti-solvents particularly the boiling points and polarity, occupy an important place in the quality of the films. For instance, if the anti-solvent polarity is as strong as usual solvent, it will dissolve perovskite. The optimal values for suitable antisolvent polarity fall within 2–4.5 [22], above 4.5 is detrimental to the growth of perovskite film. Also, the antisolvent polarity ascertains the miscibility of antisolvent to solvent, which initiates the effect of removal of antisolvent on solvent. The high polarity increases miscibility of the antisolvent with the solvent, which give rise to the high solvent removal rate. However, antisolvent with too low polarity will result in poor solvent removal effect [6]. Moreso, favorable antisolvent depends also on the boiling point. The boiling point of antisolvent checks the rate of perovskite crystal growth [19]. The drying tempo of high boiling point antisolvent is delay in spin coating procedure, which lengthens the period of crystal growth. The presence of antisolvent in the film offers adequate fluidness, which enables the neighboring nuclei bigger and enhances the size of the grain. If the boiling point is small, the antisolvent will melt away too quick, which give rise to poor removal effect of solvent [23]. In sum, the fundamental role of antisolvent are listed below:

• Improving surface coverage

• Increasing grain size and crystallinity

• Minimizing film roughness

• Enhancing photovoltaic performance

• Increasing stability

3.3 Additive treatment

An additive is a substance added to something in small quantities to improve it in some way. It is another parameter affecting the morphology of perovskite. Additives can be added in two ways: one is to add additives in perovskite precursor and the other is to add them in the antisolvent. In additive treatment, the following considerations must be optimized;

First and foremost, additives must be soluble in perovskite dissolvents. If not, they should be added into the antisolvent [24]. The basic roles of additives are as follows:

• Stabilizing the crystal structure

• Improving the uniformity of the film

• Solving the problem of too fast crystallization of perovskite synthesized by antisolvent technique

• Delaying the crystal growth and aid in formation of dense perovskite films with larger grain size.

• Passivating the grain boundaries and prevent non-radiative recombination

Various anti-solvents were reported in literature, such as hydroiodic acid (HI), sulfobetaine zwitterion, Chlorine ion additives, ionic liquid (methyl formate), dimethyl sulfoxide (DMSO), etc.

For instance, the aim of adding additives like HI and sulfobetaine is to stabilize the crystal structure [24], enhance the film uniformity and addressing the issue of too fast crystallization. Also, Terephthalic acid (TPA) [6, 24] and conjugated polymer [6, 24] and the likes are employed to regulate the nucleation and crystal growth of perovskite, increase the crystallinity and stability of perovskite, and minimize the defects in the film. In short, it was observed that the primary role of the additives is to passivate the grain boundaries (GBs) and prevent non-radiative recombination [24].

4. Summary and outlook

The main point of this review is to establish the optimal conditions/control for increasing the repeatability/reproducibility of perovskite thin films fabrication especially for large scale production. The thin film quality synthesized by convectional solution procedure could be poor, which is characterized by irregular grain size, voids, low coverage and high roughness. The application of optimal conditions will greatly solve these problems and increase the reproducibility of preparation. Proper annealing temperature optimizes the nucleation and perovskite crystal growth. Controlling the crystallization of perovskite by the extraction of solvent using antisolvent treatment is very important. The boiling point and polarity of the preferred antisolvent must be moderated that is, not too high or low, otherwise, it will lead to poor morphology of the film. Mixing antisolvents with different polarities can be employed to neutralize the polarity, and hence the optimal crystallization rate will be achieved.

From a practical perspective, actualization of the essential features in the cells and devices directly depends on perovskite film quality (uniformity, absence of voids, high degree of crystallinity and coverage, optimum sizes of crystallites). The considerations and optimal conditions stated in the content of the write up are applicable to all the available techniques of film fabrication with indication of optimization mechanism of perovskite formation and crystallization.