Featured paper: Kinetic model for more efficient organic solar cells.


Back cover of the J. Mater. Chem. C highlights the paper of the Brazilian team.
Back cover of the J. Mater. Chem. C highlights the paper of the Brazilian team.

Unlike other solar cells that have dominated the market for a long time, such as silicon cells, the organic ones are thin, light, flexible and semi-transparent. With these characteristics, they become very attractive for specific segments. In Brazil, for example, which has national production, some of the largest installed surfaces in the world can be seen in business buildings, as well as some installations in shopping centers, trucks and bus stops.

Although the organic version of solar cells also offer advantages in large-scale production (simpler industrial processes with lower carbon footprint, such as the roll-to-roll), conquering big markets largely depends on an ongoing efficiency improvement to convert sunlight into electricity. To overcome this challenge, it is essential to develop materials with suitable properties and to combine different materials within the device.

A scientific team from the Brazilian Federal University of Paraná (UFPR) studied in detail, using experimental and theoretical tools, the charge generation mechanism in organic solar cells – a complex process that is not yet fully understood. In practice, the results of this work help choosing which materials should be used and how they should be synthesized, so that their properties enhance the efficiency in converting light into electricity. The research paper was reported in the Journal of Materials Chemistry C (impact factor 7.059), where it was highlighted on the back cover.

Unraveling the exciton dissociation 

In the sandwich of layers that forms solar cells, the active layer (responsible for absorbing light and generating electric charges) is composed of semiconductor materials that, for organic devices, are polymers or other carbon-based molecules. When excited by light, these materials do not generate free electric charges, as is the case with inorganic semiconductors. They generate excitons, which are electron–hole pairs connected by forces of attraction between the negative charge of the first and the positive charge of the second.

In order to generate free charges, which form the electric current, it is necessary to break this connection, in a phenomenon called exciton dissociation. One way to achieve this is to create, in the active layer, an interface between an electron donating material and an electron acceptor. “Depending on the combination of these two materials, exciton dissociation processes can occur at a very low time scale, resulting in a more efficient charge generation,” explains Leandro Benatto, corresponding author of the paper. “However, this process is still not well understood,” he adds.

In their work, Leandro and the other authors focused specifically on trying to understand the exciton dissociation and the generation of free charges at the interface between the donor and acceptor material. The team carried out photoluminescence experiments, which are generally used to measure the efficiency in generating free charges in systems of this type, and developed a mathematical model that simulates the process. The experimental and theoretical results were very similar, proving the model’s accuracy. “We developed a model that simulates the kinetics of the process, including the several stages of exciton dissociation and considering the main characteristics of the interface,” he says. “Based on the kinetic model, it was possible to reproduce the experimental results in a comprehensible manner and more clearly observe the main factors that influence the efficiency of the free charge generation process in donor/acceptor interfaces,” he adds.

Fullerenes vs. Non Fullerenes

The study that produced the article was coordinated by two professors from the Physics Department of UFPR, Marlus Koehler and Lucimara Stolz Roman, who have a longstanding partnership in the theoretical and experimental study of organic solar cells. “The theoretical part began to be developed in 2019, at the end of my PhD in Physics at UFPR under the guidance of Professor Marlus, and continued in my postdoctoral work at the Nanostructured Devices Laboratory (DINE) under the coordination of Professor Lucimara,” says Leandro. Also participating in the research were Maiara de Jesus Bassi, PhD student in Physics in the group of Professor Lucimara, and Luana Cristina Wouk, PhD in Physics who was also under the supervision of Professor Lucimara Roman, and currently working at CSEM Brazil, a private applied research center, which helped contextualize the problem in the large-scale development scenario.

The initial idea of the work was to understand the difference between two types of electron acceptor molecules: those derived from fullerene (a carbon allotrope), which have excellent performance in the collection and transport of electrons but have a limited spectrum of light absorption, and compounds not derived from fullerenes, which in recent years have optimized the collection and transport properties. “This is a very interesting topic since, recently, the efficiency of organic solar cells based on non-fullerenes surpassed the efficiency of those based on fullerenes, although, a few years ago, it could not be imagined that fullerenes would be surpassed,” reports Leandro. “Currently, laboratory produced organic solar cells based on non-fullerenes have reached 18% efficiency,” he adds.

This research received funding from Brazilian agencies Capes, CNPq and FAPEMIG, INCT–Nanocarbono and COPEL (Companhia Paranaense de Energia).

The authors of the paper, from the left: Leandro Benatto, Maiara de Jesus Bassi, Luana Cristina Wouk, Lucimara Stolz Roman and Marlus Koehler.
The authors of the paper, from the left: Leandro Benatto, Maiara de Jesus Bassi, Luana Cristina Wouk, Lucimara Stolz Roman and Marlus Koehler.

[Paper: Kinetic model for photoluminescence quenching by selective excitation of D/A blends: implications for charge separation in fullerene and non-fullerene organic solar cells. L. Benatto, M. de Jesus Bassi, L. C. Wouk de Menezes, L. S. Roman and  M. Koehler. J. Mater. Chem. C, 2020,8, 8755-8769].

Featured paper: Nanorods to develop new anti-inflammatory drugs.


[Paper: Characterization of the structural, optical, photocatalytic and in vitro and in vivo anti-inflammatory properties of Mn2+ doped Zn2GeO4 nanorods. Suzuki, V. Y.; Amorin, L. H. C; Lima, N. M; Machado, E. G; Carvalho, P. E.; Castro, S. B. R.; Souza Alves, C. C.; Carli, A. P.; Li, Maximo Siu; Longo, Elson; Felipe La Porta. J. Mater. Chem. C, 2019, 7, 8216. DOI: 10.1039/c9tc01189g]

nanobastoesA team of researchers from Brazilian universities found, in cylindrical nanostructures known as nanorods, an anti-inflammatory effect equivalent to that achieved by commercial drugs. Researchers have also demonstrated the effectiveness of these nanorods as catalysts (accelerators) in the degradation of a pollutant. These applications are even more relevant considering that the scientific team was able to produce large quantities of the material through a simple and fast process. The work carried out shows the potential of these nanorods for the development of new medicines and for the treatment of effluents.

The work originated about three years ago when Professor Felipe de Almeida La Porta, who had recently joined the faculty of the Federal Technological University of Paraná (UTFPR), Londrina campus, was implementing a research group on nanotechnology and computational chemistry at this university. “Our laboratory was investigating some classes of emerging materials, with the perspective of aligning theory and practice, thus driving new discoveries and applications,” says La Porta. One of the materials studied by the group was zinc germanate (Zn2GeO4), a versatile semiconductor with well-known applications in sensors, catalysts, batteries and other devices.

Together with undergraduate researcher Victor Yuudi Suzuki, the professor started a project in which he synthesized pure Zn2GeO4 nanorods at the UTFPR laboratory with very small percentages of manganese ions. To produce this series of nanorods, they used “microwave assisted hydrothermal synthesis.” The method consists, in broad lines, of mixing aqueous solutions containing certain compounds, heating the final solution in a microwave oven and allowing the compounds to react for a certain period of time at controlled pressure and temperature. In this study, the manganese ion-doped Zn2GeO4 was prepared, and the reactions were performed at 140 °C for 10 minutes. The resulting material from these reactions was collected at room temperature, then washed and dried, which generate the nanorods.

Professor La Porta and his research group were able to optimize one of the process steps, the crystallization of materials, thus reducing the synthesis time from hours to a few minutes, but maintaining the quality of the material and the possibility to control its shape.

After preparing the samples, they traveled from Londrina (state of Paraná) to São Carlos (São Paulo state) to characterize the materials at the Center for Functional Materials Development (CDMF) at the Federal University of São Carlos (UFSCar) and at the Institute of Physics at the University of São Paulo (USP). Together with the local researchers, they were able to analyze the shape, structure and luminescence of the four types of nanorod compositions produced: manganese-free and with 1, 2 and 4% of this element incorporated into the structure of Zn2GeO4.

Finally, knowing that compounds containing zinc, germanium or manganese exhibit considerable effects on living things, the team contacted some collaborators to investigate these properties in the nanorods. Thus, several experiments were performed at the Departments of Chemistry and Pharmacy of the Federal University of Juiz de Fora and at the Federal University of Vales do Jequitinhonha and Mucuri, both in the state of Minas Gerais.

The authors of the paper. From the left: Victor Suzuki, Luís Amorin, Felipe La Porta, Maximo Si Li, Elson Longo, Sandra de Castro, Paloma de Carvalho, Alessandra Carli, Emanuelle Machado, Caio Alvez, Nerilson Lima.
The authors of the paper. From the left: Victor Suzuki, Luís Amorin, Felipe La Porta, Maximo Si Li, Elson Longo, Sandra de Castro, Paloma de Carvalho, Alessandra Carli, Emanuelle Machado, Caio Alvez, Nerilson Lima.

To study the anti-inflammatory action, the team performed in vitro tests (in contact with cells in laboratory containers) and also in vivo tests (using rats with paw edema, within the norms of the Brazilian code for laboratory animal use). Both types of experiments revealed that nanorods with about 4% manganese were the most effective in controlling inflammation. The in vitro tests showed these nanostructures were able to modulate molecules that regulate inflammation without causing cell death (without cytotoxicity). In the in vivo experiments, the nanorods reduced the induced rat paw edema with results similar to that of the application of dexamethasone, a well-known drug of the corticoid group.

“At first, we thought that combining these elements to form a ternary oxide could somehow potentiate these effects. But we had no idea the results would be so significant. Given that the drugs currently available in therapy are proving to be less effective every day, these results may encourage the use of these nanorods, for example in the production of a new pharmaceutical formulation, especially for cases of inflammation,” says Felipe La Porta, who is the corresponding author of the paper that was recently published by the research team in the Journal of Materials Chemistry C (impact factor 6,641).

In addition to proving the potential of the material for this application in the health area, the authors of the paper have experimentally verified the ability of nanorods to degrade a chemical dye widely found in industrial effluents, known as methylene blue. For this application, 2% manganese nanostructures were the most efficient, completely decomposing the dye in 10 minutes. “Due to the manufacture simplicity of this system, coupled with its excellent properties, this material is also promising for cleaning various environmental pollutants, and can be easily recovered at the end of this process,” adds Prof La Porta.

In the center, a cluster of 4% manganese zinc germanate nanorods. Clockwise: photoluminescence measurements of the samples; representation of the structure of manganese-doped zinc germanate; pollutant degradation mechanism and methylene blue degradation measures; anti-inflammatory action of nanorods and other treatments in induced-edema rat paw.
In the center, a cluster of 4% manganese zinc germanate nanorods. Clockwise: photoluminescence measurements of the samples; representation of the structure of manganese-doped zinc germanate; pollutant degradation mechanism and methylene blue degradation measures; anti-inflammatory action of nanorods and other treatments in induced-edema rat paw.

The superior properties that the Brazilian scientific team found in the nanorods with manganese can be related to the structural defects observed in these samples. In fact, the three-dimensional network of atoms that forms zinc germanate is crystalline, that is, organized in regular patterns. The introduction of manganese generates irregularities, and new properties emerge.

The scientific paper that reports this work was selected to be part of the Materials and Nano Research in Brazil collection, prepared by the Royal Society of Chemistry in celebration of the 18th B-MRS Meeting, and can therefore be accessed free of charge until October 15 of this year, here.

The work was carried out with funding from Brazilian research support agencies: the federal CNPq and Capes, and the state Araucaria Foundation, Fapesp and Fapemig.