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].

SBPMat newsletter. English edition. Year 3, issue 1.


 

Brazilian Materials Research Society (SBPMat) newsletter
News update from Brazil for the Materials community

English edition. Year 3, issue 1. 

SBPMat news: XV Meeting - Campinas (SP), Sept 25-29, 2016 

Simposia: Symposia proposals on subjects of Materials Science and Technology can be submitted up to February 11 (extended deadline).  The submission is open to the international scientific community. Read more.

Venue: Visit the site of event and watch the video about the city of Campinas and the folder on the Expo D. Pedro convention center. 

Organization: This edition of the event is chaired by UNICAMP Professors Ana Flávia Nogueira (Institute of Chemistry) and Mônica Alonso Cotta (Institute of Physics “Gleb Wataghin”). Find the members of the local committee here.

Featured paper 

The performance of organic solar cells, devices that are able to produce electricity from sunlight, can now be assessed more accurately thanks to research entirely conducted at the São Carlos Institute of Physics of the University of São Paulo (IFSC-USP). The study included a series of experiments and the development of an analytical model, and led to the publication of a paper on the journal Solar Energy Materials and Solar Cells. Read our story about the study. 

 

People in the Materials community 

We interviewed Osvaldo Novais de Oliveira Junior, Professor at the São Carlos Institute of Physics (USP), who took office as SBPMat’s president on last Friday. We discussed his life history, career and plans for SBPMat. Osvaldo Novais does not belong to the group of people who discover early a professional vocation, but this has not impede the developing a scientific career with great results, such as an H index of 53. Passionate about knowledge, he made important contributions to the field of Materials, particularly for the study and application of Langmuir films and the development of electronic tongues, in addition to participating in the creation of the first software for grammar revision in Portuguese, as well as studying and promoting how to write good papers in English. In his message to the youth, the scientist stressed the importance of language proficiency (Portuguese, English, Mathematics) as base for lifelong learning. Read our interview. 

Edgar D. Zanotto and Victor C. Pandolfelli, both Full Professors of the Materials Engineering Department at the Federal University of São Carlos (UFSCar), received special honors from the Rector, Professor Targino de Araújo Filho, at the closing ceremony of UFSCar’s 45th anniversary celebrations. 

 

Reading tips

  • Borophene, a two-dimensional metallic, conductive material made of boron atoms (based on Science paper). Here.

  • Strange, but real: structures that expand volumetrically, both when streched and compressed (based on paper on Nature Materials, section “News and Views”). Here.
  • Healthcare materials: responding to UV light stimulus, hydrogel capsules can release RNA “on demand” (based on Advance Healthcare Materials paper). Here.
Opportunities

  • Postdoctoral fellowship in confocal microscopy and cell membrane models. Here.

Events

  • 5th International Conference on Surface Metrology. Póznan (Poland). April, 4 to 7, 2016.  Site.
  • 43rd International Conference on Metallurgical Coatings and Thin Films (ICMCTF). San Diego (USA). April, 25 to 29, 2016. Site.
  • 9th Brazilian-German Workshop on Applied Surface Science. Maresias, SP (Brazil). April, 10 to 15, 2016. Site.
  • 40th WOCSDICE ‐ Workshop on Compound Semiconductor Devices and Integrated Circuits held in Europe & 13th EXMATEC ‐ Expert Evaluation and Control of Compound Semiconductor Materials and Technologies. Aveiro (Portugal). June, 6 to 10, 2016. Site.
  • Photonic Colloidal Nanostructures: Synthesis, Properties, and Applications (PCNSPA Conference 2016). Saint Petersburg (Russia). June, 27 to July, 1, 2016.  Site.
  • XXV International Conference on Raman Spectroscopy (ICORS2016). Fortaleza, CE (Brazil). August, 14 to 19, 2016.  Site.
  • XV Encontro da SBPMat. Campinas, SP (Brazil). September, 25 to 29, 2016. Site.
  • Aerospace Technology 2016. Stockholm (Sweden). October, 11 to 12, 2016. Site.

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Vaga para pós-doutorado em materiais poliméricos.


Vaga para pós-doutorado na área de materiais poliméricos para OLEDs/Células Solares Orgânicas, com bolsa Capes no valor de R$ 3.700,00. O candidato deve ter defendido o doutorado há, no máximo, 5 anos.

Mais informações, com a Profa. Dra. Wang, pelo e-mail wangshui@usp.br ou telefone (11) 3091-5694.

Laboratório de Engenharia de Macromoléculas, Eng. de Materiais da POLI/USP.