In few months ago, a Korean team at UNIST reported ground-breaking results in the production of perovskite materials for use in solar cells. The next power conversion efficiency of 25.8 % for a single-junction perovskite solar cell was reached by building a "coherent interlayer" between electron-transporting and perovskite layers to minimize interfacial defects.
To begin tackling the plot, let's start with the basics. A typical perovskite solar cell
(PSC) compose of six main interfaces, including:
(1) the interface between the transparent conductive oxide and electron transport layer (ETL);
(2) the interface between the ETL and perovskite;
(3) the perovskite grain boundaries or the interface between perovskite particles;
(4) the interface between the perovskite and hole transport layer (HTL);
(5) the interface between the HTL and electrode;
(6) the interface between the electrode material and its surroundings1.
layered stack of the ETL, perovskite, and HTL, with an interlayer between the perovskite absorber and the ETL. The work accredits to the reference2
The work was inspired by the fact that the interfaces between the perovskite and ETL layers in perovskite solar cells have a significant number of interfacial defects (about 100 times that within the perovskite layer). Specifically, “the deep-level” type defects, that seriously affect (reduce) the power conversion efficiency of the PSC.
However, most efforts to reduce these interfacial defects have concentrated on surface passivation. Passivation in PSC mostly relates to either chemical passivation, which decreases defect trap states to maximize charge transfer across multiple surfaces, or physical passivation, which protects functional layers from the external environment to prevent device degradation.
Passivating the perovskite surface that touches the ETL layer is hard because the surface-treatment agents on ETL may break down while covering a thin film of perovskite. Alternatively, if a coherent interface can be created between the ETL and the perovskite layer, these defects may not be a worry. Thus, the article discussed how to create an interlayer between a SnO2 "ETL" and a halide perovskite light-absorbing layer by Cl-bonding SnO2 to a Cl-containing perovskite precursor. There are less interfacial imperfections in this interlayer, which improves charge extraction and transport from the perovskite layer. According to their article, a "coherent interlayer" led researchers to create perovskite solar cells with a power conversion efficiency of 25.8 % under typical illumination. Furthermore, even after 500 hours of continuous light exposure, unencapsulated devices retained approximately 90% of their initial efficiency. While this study provides a framework for creating reduced defect interfaces between perovskites and ETL.
Why perovskite solar cell?
It was first discovered as the calcium titanium oxide mineral perovskite, however any substance with a crystal structure following the formula ABX3 is considered a perovskite (CaTiO3).3 In 1839, Gustav Rose discovered the mineral in Russia's Ural Mountains and named it after Russian mineralogist L. A. Perovski (1792–1856). 'A' and 'B' are two ions with typically differing sizes ('A' atoms are typically larger than 'B' atoms), and X is an ion (often oxide) that forms a bond with both ions. A 6-fold coordinated B cation is surrounded by an octahedron of anions and a 12-fold coordinated A cation in the ideal cubic form. Perovskites possess unique properties that make them a viable choice for solid-state solar cells, such as a broad absorption spectrum, long electron and hole transport routes, fast charge separation, extended carrier separation lifetime, and others. Perovskite PVs offer a significant advantage over standard solar technology in that they can respond to a wide range of light wavelengths. This allows them to convert more of the sunlight that enters them into electricity. Methylammonium lead halides one of perovskite materials has the chemical formula of CH3NH3PbX3, where X is I, Br, or Cl.
Schematic graphed of the perovskite structures
Perovskite solar cells are those in which the light-harvesting active layer is composed of the compound perovskite. Most of the time, this active layer consists of a lead or tin halide hybrid material. They are solid, cheap, and easy to make. They have the potential to be employed in solar cells, photodetectors, radiation detectors, lasers, magneto-optical data storage, hydrogen production, scintillators, and other applications 4.
What is holding perovskite PVs back?
Perovskite solar cell technology, in comparison to more mature solar technologies, is still in its early stages of commercialization despite its enormous potential. This is due to a multitude of challenges.
Perovskite solar cells have a relatively higher total cost due to the use of gold as an electrode material and the fact that less expensive perovskite solar cells have a limited lifespan. Perovskite photovoltaics degrade rapidly in the presence of moisture, and the decay products attack metal electrodes. Encapsulation to protect the perovskite can add to the cell's cost and weight. Another issue is scaling up; exceptional efficiency rates have been achieved using extremely small cells, which are ideal for laboratory testing but are too small to be used in a practical solar panel. But even though of such new materials the accelerated research approaches would expedite in solving these issues and will results in good materials that is practical in the coming few years.
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References
(1) Li, Y.; Zhao, Y.; Chen, Q.; Yang, Y. (Michael); Liu, Y.; Hong, Z.; Liu, Z.; Hsieh, Y.-T.; Meng, L.; Li, Y.; Yang, Y. Multifunctional Fullerene Derivative for Interface Engineering in Perovskite Solar Cells. J. Am. Chem. Soc. 2015, 137 (49), 15540–15547. https://doi.org/10.1021/jacs.5b10614.
(2) Min, H.; Lee, D. Y.; Kim, J.; Kim, G.; Lee, K. S.; Kim, J.; Paik, M. J.; Kim, Y. K.; Kim, K. S.; Kim, M. G.; Shin, T. J.; Il Seok, S. Perovskite Solar Cells with Atomically Coherent Interlayers on SnO2 Electrodes. Nature 2021, 598 (7881), 444–450. https://doi.org/10.1038/s41586-021-03964-8.
(3) Wenk, Hans-Rudolf; Bulakh, Andrei (2004). Minerals: Their Constitution and Origin. New York, NY: Cambridge University Press
(4) Johnsson, Mats; Lemmens, Peter (2007). "Crystallography and Chemistry of Perovskites". Handbook of Magnetism and Advanced Magnetic Materials. arXiv:cond-mat/0506606. doi:10.1002/9780470022184.hmm411. ISBN 978-0470022177. S2CID 96807089.
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