|Fig. 1: Fig. 1: Perovskite Crystal Structure (Source: Wikimedia Commons).|
One of the latest and most promising types of solar cell developed in recent memory is the perovskite solar cell. From the very first perovskite solar cell in 2009 to the present day, the power conversion efficiency has increased over fivefold, from 3.8% to 20.1%.  Though the technology is not yet ready for commercialization and widespread use, perovskite solar cells hold incredible potential for high efficiencies and low cost in the next generation of photovoltaics.
A perovskite solar cell is a solar cell with the perovskite crystal structure that usually consists of an organic group, a metal like lead or tin, and a halogen. For example, one of the most prominent types of perovskite cells currently is methylammonium lead iodide. Two different types of perovskite cells exist: sensitized cells and planar thin film cells. In sensitized cells, the perovskite material is coated onto a charge-conducting material. The perovskite simply absorbs light, and afterwards the charge is conducted to the electrodes via the other material. In the planar thin film type, the layer of perovskite both absorbs the light and transfers the charge to the electrodes. 
The first use of perovskite materials in a photovoltaic was by Miyasaka et al. in 2009 using the aforementioned sensitized structure. The cell was fairly unstable and only achieved a 3.8% power conversion efficiency.  In 2013 the planar thin film structure was developed, and since then, numerous developments have increased efficiencies to well over 15%.  The current research cell record efficiency for perovskite was achieved by researchers at Korea University of Science and Technology with 20.1%. 
Perovskite solar cells have many distinct advantages over traditional silicon cells. Firstly, the fabrication of perovskite photovoltaics is much cheaper and simpler than silicon photovoltaics production. The wafer processing and cell fabrication associated with silicon require expensive equipment and facilities. In comparison, perovskite cell fabrication has been accomplished with simpler methods like solution spin coating.  Additionally, perovskite cells have a higher band gap than traditional silicon or thin film cells. Therefore, they are transparent to typical solar absorption wavelengths and can be placed on top of lower band gap cells. This combined "tandem" photovoltaic system is able to absorb a greater part of the solar spectrum and achieves a higher efficiency than either type of cell individually. 
|Fig. 2: Perovskite cell architecture and charge extraction. (Source: Wikimedia Commons)|
Perovskite solar cells still face a number of challenges before they can be implemented on a widespread level. The components of the photovoltaics degrade quickly in the presence of water.  In addition, the efficiency of perovskite solar, though relatively high, is uncertain. To determine the efficiency of a solar cell, an I-V curve showing the current vs. the voltage is measured, and the point of maximum power is found. This power is compared to the incident light to determine the power conversion efficiency. In perovskite cells, however, the I-V curve changes depending on how quickly the voltage is varied and whether the voltage is scanned from high to low or low to high.  Hence, the true performance of a perovskite photovoltaic in a long-term practical scenario unclear.
Over the past five years, perovskite solar cells have seen an astronomical fivefold increase in efficiency rarely seen in photovoltaic technology. Of the latest generation of solar cell technologies, perovskite photovoltaics arguably have the greatest potential to revolutionize the industry.
© Kevin Tran. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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