What are Perovskites and Perovskite Solar Cells?

The term perovskite describes a material with a specific crystal structure (ABX3). By varying the components of A, B and X used, the crystal properties can change dramatically. These crystals can therefore be used in a wide range of applications.

In 2009, perovskite materials were first used in solar cell devices. Since then, they have gone from strength to strength, due to sky rocketing efficiencies and simple optimization. In just 15 years of research, perovskite solar cells (PSCs) have achieved record power conversion efficiencies (PCEs) of 26.7% (as of Oct 2024) [1], rivalling performances of crystalline silicon (c-Si) solar cells (PCEs of 27.6%) [1]. 

As well as their impressive performances, one of the main benefits of PSCs is that they can be solution processed. This allows for the possibility of fully printed, roll-to-roll (R2R) processible solar panels. Currently, c-Si solar panels are produced through energy intensive and costly processes involving multiple purification and heating phases. This contributes to both their environmental and manufacturing costs. Therefore, the idea of “printable” solar panels is extremely appealing.

Additionally, PSCs can be printed on top of c-Si solar cells to improve their performance, creating perovskite-silicon tandem solar cells. By carefully tuning the properties of the top perovskite solar cell, the tandem device can efficiently use more light, improving efficiencies compared to each cell individually. The current champion perovskite-silicon tandem device has a power conversion efficiency of 34.6% [1].

How to Make Perovskite Solar Cells

Perovskite layers can be deposited through vapor-deposition processes or by solution processing. Vapor deposition is reliable, but costly and highly sensitive. Therefore, most perovskite fabrication is done via solution processing methods.

Creating a highly efficient PSC involves several key steps. Firstly, you must prepare a precursor solution with precise ratios of each precursor component. Even fractional variations in these ratios can dramatically affect device efficiencies. [2] This precursor solution often combines a metal halide salt (usually lead-based such as lead iodide, PbI2, or lead bromide, PbBr2) and an organic salt (such as methylammonium or formamidinium iodide) in a protic, polar solvent. However, sometimes inorganic halide salts (such as cesium iodide) are used as well, as or instead of, an organic salt.

Once the precursor solution is ready, the solution is deposited onto a substrate. Deposition methods vary, with techniques like spin-coating, slot-die coating, or blade coating often used to create uniform thin films.

This layer can be deposited using either a one-step or two-step method. In the one-step process, a thin film is created from a single precursor solution containing all the perovskite precursor material. This layer is evenly deposited using a precise recipe depending on your substrate, perovskite composition and coating method.

In the two-step process, the metal halide (such as PbI₂) and the organic component (like methylammonium iodide) are dissolved separately and are deposited sequentially as separate layers. One step deposition is the most commonly used method and is the easiest to scale, so it is the method we will discuss in this article. The highest quality perovskite films are created within controlled environments; therefore this deposition is often done within a glove box. 

After deposition, the substrate is typically annealed at elevated temperatures (usually around 100°C) to complete the crystallization process. If everything goes right, this will form a stable, dark, shiny perovskite layer. Other layers are deposited before and after the perovskite layer to create a full perovskite solar cell device.

Scaling Up Perovskite Solar Cells

The potential for solution processible PSCs is very exciting as it could open the door to roll-to-roll solar panel creation. However, in order to move from laboratory fabrication to commercial production, a few key challenges must be tackled.

Currently, most PSC devices are made on a small scale in laboratory environments, using methods like spin coating. Spin coating ensures high-quality films and good devices, but only over small areas. Therefore, these methods are not suitable for large-scale applications.

There are still improvements to be made with PSCs that can be achieved on a laboratory scale. However, the question must be asked: what’s the point in investing thousands of dollars of research making perfect small-area devices, only to find they are not viable for large-scale production?

For this reason, all key research groups in the perovskite cell field must pay attention to the scalability of their technology. More and more research is focused on creating high-quality perovskite films using methods like slot-die coating and blade coating, that can be more easily adapted into R2R processing.

Additionally, with the improving performance and stabilities of PSC devices, perovskite-focused companies are emerging rapidly, signaling that the commercialization of perovskite solar cells is closer than ever before.

Essential requirements for a suitable precursor ink include solvent compatibility, stability under reasonable processing conditions, reduced environmental impact, and controlled evaporation and drying kinetics. On top of this, R2R processing and compatible methods impart specific demands on the perovskite precursor inks which must also be addressed if PSCs are to be printed on a large scale.

Choosing the Right Solvents

Choosing the right solvents for a perovskite precursor ink is essential for assuring film quality. To create the consistent uniform films for high performance PSCs, a good solvent should ideally achieve the following things: 
• Dissolve all precursor components.
• Have good wettability and adhesion to other device layers. They must not also degrade any layers below the perovskite layer.
• Enable controlled evaporation and drying of the wet film to control crystal growth.
• Ideally, be green, non-toxic solvents.

Solvent choice plays a particularly important role in the formation of perovskite crystals. There are three stages of perovskite’s crystallization:
1. Precursor solution is deposited, most of the solvent evaporates and crystals begin to form.
2. Some of the solvent must remain in the film to facilitate an intermediate phase which stops unwanted crystal phases forming.
3. Lastly, one the perovskite phases are stable, the remaining solvent must be removed.

To facilitate this process, two-part solvent combinations are often used involving a bulk, volatile solvent with a small amount of highly bonding solvent to facilitate the described intermediate stage. The bulk solvent will leave the wet film during early deposition, and the highly bonding solvent is removed with high processing.

On top of this complicated drying mechanic, the solutions must also meet the requirements of whatever deposition technique you’re using. For example, in slot die coating, solution viscosity affects the uniformity and thickness of the coating layer, as well as flow stability. Solution viscosity is determined in part by the solvent.

The crystal growth mechanisms and drying stages will be completely different for large-area deposition techniques compared to traditional spin coating. Therefore, traditional solvent combinations like dimethylformamide (DMF) & dimethyl sulfoxide (DMSO) currently used in lab scale PSC devices have shown limitations in R2R processing. These systems often result in defects such as pinholes and inhomogeneous grain sizes.

Recent studies have explored alternatives which might be suitable for large area deposition, such as 2-methoxyethanol (2-Me) combined with 1,3-dimethyl-2-imidazolidinone (DMI), which supports stable intermediate phase formation. This solvent combination gave the perovskite precursor a broader processing window and better coating uniformity than conventional perovskite inks based on DMF/DMSO. [3]

Perovskite Precursor Lifetime

Storage stability is highly important for commercial-scale manufacturing, where inks will need to be made in large quantities and be stored for extended periods. The most treacherous component of perovskite precursors are typically the organic molecules. These organic components are known to be vulnerable in devices with both inherent instabilities and external degradation factors (moisture, high temperatures, etc.)

This also appears to be true in perovskite precursor solutions. Especially in solutions containing both multiple organic components, unwanted by-products form in solution over time which tank device performance. High-performing highly-stable perovskite precursor solutions have a shelf life of less than 2 weeks, even when stored in inert conditions [4].

However, research has shown that storing inks at low temperatures (even at only 4°C in a commercial refrigerator) can increase stability to several months without significant performance loss. This could be one way to extend stable ink lifetime, reducing waste of ink during development stages and allowing for larger quantities of precursor solution to be made and stored.

Additionally, this study showed that the commonly used technique nuclear magnetic resonance (NMR) can be a good scanning tool for precursor inks, effectively telling if the solution has “gone off”. This simple monitoring of precursor inks will be invaluable for large scale solution processing, as you can quickly and non-invasively sample the viability of your ink by quantifying the amount of organic components still present in a sample. 

Toxicity and Environmental Friendliness

Perovskite inks for R2R processing should ideally be low-toxicity and processable in ambient conditions. In order to achieve these goals, researchers have started to focus on creating good PSC devices using greener solvents, such as gamma-valerolactone (GVL), to reduce environmental impact.

GVL-based inks have demonstrated high performance in inkjet printing applications, producing efficient perovskite films under ambient conditions with reduced coffee-ring effects and enhanced long-term stability. [5] Advancements in greener solvents should make large-scale production safer and more environmentally friendly.

Future Challenges for Reliable Perovskite Precursors for R2R Processing

The path to producing printed PSCs commercially presents many challenges extending beyond simply increasing manufacturing volume. Some of the key issues which need to be addressed include:

1. Long-Term Stability of Perovskite Materials
Many perovskite materials will degrade under environmental stresses such as high levels of moisture, oxygen, and ultraviolet light. Although some perovskite compositions and encapsulation methods have increased PSC stability, improvements are still needed to meet the standards required of commercially available PV devices.

2. Process Optimization for High Uniformity and Efficiency
For R2R processes, achieving uniform film thickness and quality across large areas is challenging. However, it will be necessary for achieving for high-efficiency PSCs. More research using scalable techniques like slot-die and blade coating, combined with carefully tuned drying, process optimization and precise ink formulation, is needed before commercialization is possible.

3. Equipment and Infrastructure Requirements
Spin coating is currently the fabrication method of choice for most research groups as it is low cost and easy to optimize. To enable the scalability of new technologies like perovskite solar cells, perovskite labs and manufacturers will need to invest in larger deposition systems which can be costly to install and maintain. In the meantime, there are intermediate systems to conduct developmental research with methods that are compatible with roll-to-roll technologies. These include sheet-to-sheet coating systems, which ease the route to industry for PSCs.

4. Environmental and Health Impacts
Traditional solvents like DMF and DMSO pose environmental and health risks, prompting the need for greener alternatives in the PSC manufacturing process. Furthermore, regulatory standards may evolve, requiring PSC manufacturers to adopt eco-friendly solvents for compliance in global markets.

Printable PSC for Printed Electronics

Perovskite solar cells offer a breakthrough opportunity in renewable energy, combining record-breaking efficiency with the potential for low-cost, scalable production through R2R processing. Unlike conventional solar technologies, PSCs can be solution-processed, paving the way for fully printed, flexible solar panels that could dramatically expand solar accessibility and reduce environmental impact. However, moving from lab success to commercial viability demands advancements in stability, solvent selection, and large-area film uniformity.

With focused research tackling these challenges, PSC technology could redefine solar power, making clean, affordable energy more adaptable and widespread than ever. Advancements in printable PSCs could lead to new commercial applications within printed electronics, such as the development of wearable electronic devices, or building-integrated photovoltaics. 

References
[1] Best Research-Cell Efficiency Chart, National Renewable Energy Laboratory. (Accessed at https://www.nrel.gov/pv/cell-efficiency.html, October 2024)
[2] Fassl, P. et al. (2018) ‘Fractional deviations in precursor stoichiometry dictate the properties, performance and stability of perovskite photovoltaic devices’, Energy & Environmental Science, 11(12).
[3] Chung, J. et al. (2023) ‘Engineering perovskite precursor inks for scalable production of high‐efficiency perovskite photovoltaic modules’, Advanced Energy Materials, 13(22).
[4] O’Kane, M.E. et al. (2021) ‘Perovskites on ice: An additive‐free approach to increase the shelf‐life of triple‐cation perovskite precursor solutions’, ChemSusChem, 14(12).
[5] Chalkias, D.A. et al. (2023) ‘Development of greener and stable inkjet‐printable perovskite precursor inks for all‐printed annealing‐free perovskite solar mini‐modules manufacturing’, Small Methods, 7(10).