A reducing agent improve of 30% the efficiency of all-perovskite solar cells


A team of researchers from China, Canada and Australia has found a way to improve the efficiency of all-perovskite solar cells through use of a reducing agent.

In their paper published in the journal Nature Energy, the group describes their technique and how the resulting solar cells performed.

In the push to improve the efficiency of solar panels, making them more competitive with fossil fuels as an energy source, scientists have turned to new raw materials.

One of these materials is perovskite – a mineral that consists mostly of calcium titanate. Prior research has shown that stacking perovskite cells on top of silicon cells can increase efficiency but not enough to warrant their use.

More recent research has focused on replacing silicon altogether by stacking two kinds of perovskite cells. Study of the material has shown that it could boost the efficiency of solar cells by 30 percent.

To make them, engineers have been adding a metal, in most cases a lead-tin mixture.

However, the tin oxidizes during fabrication, leading to degradation and reduced efficiencies. In this new effort, the researchers have found a way to prevent oxidation and loss of efficiency.

The work by the researchers involved looking for something to add to the tin to keep it from oxidizing. After a great deal of search and testing, they found the zwitterionic antioxidant inhibiter commonly known as the reducing agent FSA.

Adding it to the mix when making lead-tin perovskite cells prevented oxidation and did not interfere in other ways with operation of the solar cells. Without the oxidation and subsequent degradation, the researchers were able to make all-perovskite solar cells with improved efficiency.

In their best effort, they created a 1.05cm² single-junction cell with an efficiency of 21.7 percent. They then created a stacked cell using the reducing agent and had it certified as 24.2 percent efficient by JET Laboratories.

The team also created larger solar cells to confirm that they could be fabricated to industrial standards—though they acknowledge that new production methods are required to produce the new kind of solar cell.

They also note that more testing is required to ensure the cells can withstand real-world conditions.

Biofouling, such as nonspecific protein adsorption, microorganism adhesion, and biofilm formation, is of the most important phenomena and a huge problem from biosensors, implanted medical devices, drug delivery systems, separation membrane to enormous ship hulls.

It causes many problems like haemolysis, thrombosis, anticoagulation- related haemorrhage, immune responses, infection, and tissue over- growth in implanted devices [1,2].

Biofouling can also increase the voyage resistance by 60% and decreases the voyage rate by 10%. One of the crucial factors affecting fouling is the surface properties because they determine the interaction between fouling substance and the surface.

To address the fouling problem, various hydrophilic polymers, including poly(vinyl alcohol), poly(N-vinylpyrrolidone), poly(2-oxazo- line) and poly(ethylene glycol) (PEG), and zwitterionic polymers, have been developed to construct the hydrophilic surface, and these poly- mers can evidently reduce the biofouling on the surface.

It should be noted that PEG is a biocompatible polymer and it becomes a gold standard for unfouling polymers [3] because of its high hydrophilicity, non-toxicity, and anti-protein-fouling [4]. However, PEG has poor sta- bility as PEG macromolecular chains can rapidly autoxidize and degrade during storage and handling at room temperature, especially by transition metal ions, which exists in most biological related solution [5–7].

The autoxidation mechanism is a free radical mechanism in- itiated by minor amounts of free radicals present or catalyzed by metal salts, e.g. copper sulfate [8]. Peroxides and hydroperoxides are the

primary oxidation products followed by formation of carbonyl com- pounds, including ethoxylated formats, formaldehyde, ethoxylated al- dehydes, and acetaldehyde, as secondary oxidation products [7,9,10]. It is shown that oligo(ethylene glycol)-terminated SAMs decompose within a week at 45 °C or after 1 month at 20 °C [11], and the decomposition is greatly accelerated by the direct contact between ethy- lene glycol segments and a catalyst (e.g., gold), oxygen and higher temperatures [12]. PEG brushes lost their antifouling capacity when the temperature rises to 35 °C [13].

Furthermore, it is frequently reported that PEG induced antibodies after repeated injection and PEGylation reduces the bioactivity of conjugated therapeutic proteins, antibodies, and enzymes [14,15]. In addition, PEG cannot be metabolized natu- rally. These drawbacks greatly limit the application of PEG.

Therefore, it is urgently to seek alternative non-fouling material other than PEG. Zwitterionic polymers are just perfect alternatives for PEG. As compared with amphiphilicity of PEG, zwitterionic polymers are super hydrophilic due to the presence of abundant ions and subsequent strong
hydration layer.
Zwitterionic polymers refer to a family of materials that have the same number of cations and anions along their polymer chains. Typical cations are quaternized ammonium, and zwitterionic groups can be classified into sulfobetaine (SB), carboxybetaine (CB), phosphorylcho- line (PC) according to anions. Specially, zwitterionic group is SB when anions are sulfonates, CB when anions are carboxylates, and PC when anions are phosphonates. Hydrophilicity of anionic groups decreases with increasing acidity in the order: -COO – > (RO)2 – P(=O) O- > – SO3− [16].

SB based polymers are most promising to beindustrialized because it is easy to prepare SB monomers, and some of them are commercially available. CB-based materials have attracted great interests for their advantages over other zwitterionic materials, including their super antifouling properties [17], good biocompatibility [18] and functionability.

PC based polymers also have excellent bio and blood compatibilities, but their high production cost hinders its wide application.
As compared with other moieties with no charges, these materials are characterized with both high dipole moments and highly charged groups [19].

Due to the presence of so many cations and anions on the macromolecular chains, zwitterionic polymers are super hydrophilicity, while other polymers such as PEG are amphiphilic with both hydro- philic and hydrophobic properties [3]. It is reported that CB based zwitterions have better unfouling properties than PEG, and their self- assembled micelles have greater stability than PEG analogs during lyophilization [20,21].

The superior antifouling properties of zwitter- ionic materials in complicated environment were derived from their strong interaction with water via ionic salvation, while the antifouling capacity of PEG was dependent on hydrogen bond with water [22].

Besides their excellent antifouling capacity, zwitterionic materials have various other merits such as ease of functionalization and design flexibility [23]. It has many choices of zwitterions to be chosen from. Especially, as far as PCB is concerned, it can be functionalized by the eCOOH groups through the 1-ethyl-3-(3-dimethylamino) propyl car- bodiimide (EDC) and N-hydroxysuccinimide (NHS) chemistry. Moreover, PCB have many derivatives and their polymer backbones can be (meth)acrylate or (meth)acrylamide [24].

There are various choices of substitution groups of quaternary ammonium, and number (n) of carbon spacers between carboxylate and quaternary ammonium groups [24–26]. Thus, zwitterionic materials have a wide variety of applica- tions in antifouling blood contacted sensors, drug delivery, etc. Besides these biomedical applications, they are also very useful in chemical separation membrane and marine coating.

Just as any other functional polymers, zwitterionic polymers can be synthesized by two different approaches: (1) direct polymerization of zwitterionic monomers or (2) post zwitterionization of reactive polymers.

Evidently, polymerization from zwitterionic monomers is the most straightforward strategy to obtain zwitterionic polymers, and can achieve polymers with 100% zwitterion functionality. Most frequently, they are prepared by free radical polymerization [27].

But it’s difficult to synthesize well-defined polymers or polymers with complex architectures (e.g., block copolymers and star copolymers) through free ra- dical polymerization. Controlled radical polymerization, such as atom transfer radical polymerization and particularly the reversible addition fragmentation transfer method, is a convenient strategy to obtain polybetaines with defined end groups and molecular weight, and block and star copolymers [28–30].

As compared with direct polymerization, post zwitterionation is easy to perform and it is allowed to prepare reactive polymers with adjustable chemical structures. But the steric hindrance from neighboring group can have some adverse effects on reaction kinetics and yield can seldom be 100%.
This review will be focus on development and progress of the ap- plications of zwitterionic polymers, such as PSB, PCB and PPC, in bio- medicine, separation membrane and marine coating. Examples of the methods to construct the antifouling surfaces or nanoparticles for these

applications will be given and discussed from molecular level. Problems existed in these applications are also discussed and their future is also prospected.


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More information: Ke Xiao et al. All-perovskite tandem solar cells with 24.2% certified efficiency and area over 1 cm2 using surface-anchoring zwitterionic antioxidant, Nature Energy (2020). DOI: 10.1038/s41560-020-00705-5


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