Enhancing the stability of perovskite solar cells using additives
Perovskite solar cells have received much interest in recent years due to their high efficiency (more than 25 %) and low cost of production. However, perovskite solar cells have low stability compared to other forms of solar cells, which is one of the critical issues. Stability difficulties can cause a decline in the efficiency of the solar cell over time (typically a few days), limiting its practical application. The perovskite material’s instability is a crucial reason for stability difficulties in perovskite solar cells. Perovskite materials are incredibly susceptible to moisture, oxygen, and light, which can lead to material breakdown over time. The perovskite material can experience chemical and physical changes when exposed to certain environmental variables, reducing solar cell efficiency and stability.
Additives play a critical role in improving the stability of perovskite solar cells. During production, additives are typically small molecules such as salts, polymers, or nanoparticles introduced to the perovskite precursor solution. These chemicals can interact with the perovskite material and change its structure, shape, and characteristics, improving solar cell stability and performance. Understanding the interaction of the additives with the perovskite materials is crucial to enhance the stability of the perovskite solar cells.
Tuning the morphology of perovskite with additives
The morphology of the perovskite film for the solar cell is vital for enhanced performance. The perovskite films are mostly polycrystalline, which leads to high roughness of the film, large grain boundaries, and pinhole formation, which are detrimental to cell performance. Controlling the crystallization of perovskite films using the proper additives can tune the morphology of the films and improve their quality. According to a study, the use of acidic additives such as hydroiodic acid or hydrobromic acid during precursor preparation for lead halide formamidinium in Dimethylformamide (DMF) can slow down the crystallization of the perovskite film and result in a better morphology. Other acidic additives such as zwitterionic sulfamic acid, citric acid, hypophosphorous acid, and formic acids reportedly enhanced the perovskite cell performances to reach 18-20 % efficiency. Such additives help improve the surface coverage of the perovskite material to prevent pinhole formation on the film.
Organic halide salts and water can also work as additives for the precursor solution of perovskite. Additives such as acetonitrile, chloride, or iodide of methylammonium can tune the interaction between the anionic part and the solvent required for the perovskite synthesis. Such additives provide higher surface coverage and create larger crystals of perovskite. The objective is often to age the precursor solution to synthesize smaller colloids in the precursor solution that can slowly crystalize and form larger grains of pure perovskite crystals. A few reports suggest using such additives can lead to 19-20 % efficiency of the perovskite solar cells. However, adding such additives alone cannot improve the film morphology, but other factors such as solvent and solute temperature, and stirring speed of the precursor are equally important.
Stabilizing the perovskite Solar Cellsphase using additives
To achieve the best performance of the solar cells, it is desirable to tune the band gap of the photovoltaic material close to 1.37 eV. The most common photovoltaic perovskite materials are methylammonium lead iodide (MAPbI3) and formamidinium lead iodide (FAPbI3) which have 1.57 eV and 1.47 eV bandgap, respectively. FAPbI3 has better thermal stability than the former, making it more suitable for perovskite solar cells. Synthesis of such materials requires drying the coated precursor solution at a higher temperature.
When synthesized above 1500C, FAPbI3 loses the perovskite crystal structure. The ionic size of the FA+ is high and very close to the tolerance factor of the perovskite phase. Hence, retaining the perovskite phase during the thermal treatment for synthesis is vital. A few reports recommend partially replacing FA ions with MA ions or Iodine ions with Bromine ions to create hydrogen bonding for better stability. However, these ions undergo phase segregation during light soaking, drastically decreasing performance with time. A few reports suggested that replacing FAI by 10 % with cesium iodide (CsI) or rubidium iodide can significantly enhance light and moisture stability.
Methylammonium chloride, often utilized as an additive, is employed to recrystallize FAPbI3 perovskite materials in vertical phase orientation, thereby imparting a stable crystalline phase and enhancing stability against ambient conditions. Another approach to stabilize the perovskite crystal phase is by molecular locking using organic halides of bigger ionic size, such as phenylethylammonium iodide and other alkyl or aromatic ammonium iodide molecules. Reducing the surface energy on the particles during heat treatment can similarly stabilize other perovskite materials, such as CsPbI3.
Alignment of Energy-Level by using additives
Energy-level alignment in solar cells significantly impacts charge extraction, charge transfer, and charge recombination. High-efficiency Perovskite solar cells require precise energy-level alignment. Suitable contact materials or the tuning of the energy band level of the perovskite materials can align the energy levels. Doping with suitable materials, such as additives of monovalent or trivalent cations such as iodides of silver, sodium, and copper, can alter the Fermi energy level of the perovskite.
One can also use an additive of antimony or indium to tune the energy level alignment and achieve an efficiency of around 21%. Doping aims to impart more n-type or p-type characteristics in the perovskite. Literature supports the fact that water absorption on the MAPbI3 surface can enhance n-type characteristics and adding tert-butylpyridine (TBP) can induce p-type doping that improves charge collection efficiency. Introducing cyanocarbon additives can also alter the fermi level of perovskite for better alignment with conducting substrates such as ITO and provide a stable 20 % efficiency.
Suppression of defects using additives
Defects present in the perovskite materials considerably lower the voltage and fill factor of the cell. Defects can originate from crystal dislocation, vacancy defects, and grain boundaries. Such defects act as electron carrier traps. By using additives, one can greatly diminish defects in the crystal. People often use chlorine as an additive to passivate such defects. Thiocyanate ions are also used to reduce defects that prolong the electron lifetime in the cell, leading to a significant efficiency of over 21%.
Additives to enhance the operational stability of perovskite solar cells
Additives can be used to improve the operational stability in Perovskite cells. The stability of perovskite deteriorates with heat, UV lights, moisture, air, and external electric fields.
Stability against moisture
Various organic surfactants with ammonium or carbonyl functional groups and hydrophobic functional groups protect the cells from moisture. A report suggests that the use of hygroscopic polyethylene glycol (PEG) scaffolds with perovskite materials can retain performance for up to 300 hours under highly humid (70%) conditions. Other additives, such as aliphatic fluorinated amphiphilic additives, benzylamine, and thiourea, are promising in stabilizing the perovskite cells against moisture.
Stability against heat and light
The long-term stability of perovskites can be improved by functionalizing the grain boundaries of the perovskite crystals. Short-chain polymer molecules such as PEG and PCBM are well known for improving device stability. A study shows that adding PVP polymers to the perovskite film can exhibit 90 days of performance without significant drop in the device efficiency. Other additives, such as triblock copolymers and conjugated polymers, can reduce the cations migration when heated at high temperatures (up to 80 degrees Celsius).
Stability against oxygen
Tin-based perovskites such as SnF2 are very popular due to their low bandgap and display great possibility in tandem solar cells. However, tin easily gets oxidized, leading to significant deterioration in performance. Antioxidants are typically used for such situations. A study reported that hydroxybenzene sulfonic acid or its derivative salts could retain 80 % of the device’s performance for exposure to more than 500 hours of operation without cell encapsulation.
Outlook of the perovskite solar cells
Perovskite solar cells are very promising and could dominate the photovoltaic industry. The efforts to improve the stability of these solar cells are tremendous. Additives are proving to be essential for the long-term stability of such solar cells against environmental and processing conditions and for controlling the morphology, energy level alignment, and defect passivation.
Over recent years, the efficiency of perovskite solar cells has increased by over 25%. However, we should make further efforts to improve the efficiency to its theoretical limit of more than 30% and enhance the stability for commercial applications. It would be promising to develop additive-assisted methods for creating single crystal-like or highly orientated perovskite films by making perovskite precursor solution. Furthermore, researchers need to conduct extensive investigations into the effects of those compounds that persist in perovskite films on perovskite energy level structure and energy level bending at interfaces to understand their impact on efficiency.