Science | December 19th, 2019
Towards more efficient photovoltaic cells

Research presents nanoscale chemical composition mapping of materials for solar energy production

The search for clean and renewable energy sources has intensified in recent years, including, for example, the conversion of sunlight into electricity through photovoltaic cells. Simply put, sunlight incident on these devices is absorbed by electrons in the material. They are expelled from the atoms or molecules to which they were associated, forming the electric current that will be used to charge a battery or to operate other electric devices.

Silicon ($\rm Si$), an abundant material on Earth’s crust, is the basis of most solar panels installed today. However, despite the continuous reduction of production costs, silicon is not very efficient for the conversion of solar energy. This efficiency depends on intrinsic properties of the materials used to make photovoltaic cells and increases year by year with the discovery of new and better materials.

Figure: Nano-FTIR analysis of OIHP films. (A) Synchrotron broadband IR radiation is focused onto a metallic tip that tightly confines the fields at its apex for further interaction with the sample surface in standard tapping-mode AFM. AFM mapping retrieves nanoscale morphology (B) and IR broadband intensity (C) of individual grains in the $\approx 150 nm$-thick OIHP film. The IR broadband image (C) reveals changes in the vibrational response of specific OIHP grain boundaries that are further associated to early stages of organic depletion in the film. Scale bar, 200 nm. Reproduced from [1].

Among such materials, so-called organic-inorganic hybrid perovskites (OIHP) have profoundly changed the scenario for the future of solar energy, reaching efficiencies of over 20% in only a few years of research, matching the efficiency of silicon-based photovoltaic cells. In addition, this is a low-cost material for production in industrial scale and can be integrated into transparent or flexible products such as windows and clothing.

Despite their rapid development, perovskites have several stability problems that hinder their commercial application. For example, at room temperature, the presence of moisture leads to irreversible degradation, directly impacting the energy conversion efficiency.

For the preparation of these photovoltaic cells, the molecules of the material are deposited in nanometer-thick layers in what is called a thin film. Distributed along this film, the molecules eventually aggregate into nanometer-sized grains. As such, although the degradation is usually expressed as a macroscopic property, it is more likely that it occurs on the scale of these individual grains.

Degradation occurs through the modification of the structural phases of molecules in these grains. Due to humidity or high temperatures, for example, the atoms that make up the perovskite can change from a structure with photovoltaic activity to another form which is inactive. However, it is not well known how these different phases are distributed in the thin film and how it influences the properties and, consequently, the performance of the device.

In this context, Rodrigo Szostak et al. [1] investigated two types of Perovskite thin films, called CsFAMA and FAMA. The group used the facilities of the IR1 Infrared Nanospectroscopy beamline of the Brazilian Synchrotron Light Laboratory (LNLS) to chemically map individual nanometric grains in the OIHP thin films through a technique called nano-FTIR.

This technique allows the identification of different chemical states present in the samples through the response of molecules to the incidence of infrared radiation. Thus, despite the tiny morphological differences between the grains of the perovskite film, the researchers observed that specific grains had stronger vibrational activity. This activity is associated with degraded grains, containing molecules in a chemical state that is inactive for photovoltaic processes.

Finally, in this work, the researchers demonstrated that the nano-FTIR technique provides a unique tool for tracking morphological and chemical properties of isolated nanoparticles and, thus, for investigating the influence of different structural phases on perovskite solar cell performance.

Source: [1] R. Szostak, J. C. Silva, S.-H. Turren-Cruz, M. M. Soares, R. O. Freitas, A. Hagfeldt, H. C. N. Tolentino, A. F. Nogueira. Nanoscale mapping of chemical composition in organic-inorganic hybrid perovskite films, Sci. Adv. 2019; 5: eaaw6619. DOI: 10.1126/sciadv.aaw6619

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