The Okinawa Institute of Science and Technology Graduate University (OIST) in Japan has recently unveiled a new type of all-inorganic perovskite solar cell that its research team has developed. The design and test results of the prototype solar cell, which has been published in the journal Advanced Energy Materials, indicate that the OIST team has found a solution that allows a perovskite solar cell to maintain its conversion efficiency and operational stability under high temperatures.
The solar photovoltaic industry is currently the most exciting and competitive section within the whole green energy sector. The majority of solar cells for now are made of crystalline silicon, but there are many alternative material technologies on the horizon. Materials based on the perovskite structure (i.e. having the same type of crystal structure as calcium titanium oxide, or CaTiO3) have the potential to knock off crystalline silicon from its position as the market mainstream. Advantages that perovskite materials offer include higher light absorption, lower production cost, and the ability to be dissolved in a solvent. This last mentioned advantage is especially valuable because the solubility of perovskite materials allow them to be directly spray-coated onto a substrate. However, perovskite materials also have two major shortcomings – instability and degradation upon exposure to heat. Devising solutions to these two problems will greatly expand the opportunity of perovskite solar cells in the green energy market.
According to the research that the OIST team published in Advanced Energy Materials, the new prototype cell that the team has created is totally inorganic. This is a crucial difference from the other perovskite cells that are partially organic and thus easily degrade under heat.
The conversion efficiency of solar cells falls inversely to the rising temperature, and this issue is unavoidable since solar cells will heat up after a long period of exposure under the sunlight. Therefore, the solar industry has been pursuing various technologies that can increase the thermo-stability of the materials that make up the cell. In designing the material structure of the prototype perovskite cell, the OIST team replaced organic parts with inorganic ones to improve the overall stability. Zonghao Liu, who works in the Energy Materials and Surface Sciences Unit of OIST and is a member of the research team, said that the conversion efficiency of their prototype cell dropped just 8% after being under the sunlight for 300 hours.
On the other hand, the light absorption rate of an all-inorganic perovskite cell is lower than that of a perovskite cell with an organic-inorganic hybrid material structure. To raise the light absorption capability of the prototype cell, the OIST team doped the inorganic material with manganese, which modifies the crystal structure of the perovskite material. Liu compared the manganese doping process to adding salt to a dish. In the latter case, the addition of salt improves flavors. In the former case, the manganese changes the material property to improve light absorption.
To further mitigate heat degradation, the OIST team replaces the conventional gold electrodes with ones based on carbon. This particular solution also has the additional benefit of bringing down the production cost because gold electrodes have to be fabricated in a vacuum environment using high-temperature tools. Carbon electrodes are much easier and cheaper to make, and they can be integrated into solar cells through the printing method.
The prototype perovskite cell is made up of several layers of materials. The substrate is glass of a few millimeters thick. The second layer above the substrate is a conductive film made of fluorine doped tin oxide (FTO). On top of the FTO film are a photo-catalyst layer of tin dioxide (TiO₂) and a photoactive perovskite layer (CsBX3). The very top layer is composed of carbon.
Going forward, the OIST team will focus on increasing the conversion efficiency (i.e. from the current 6.14% to a commercially viable level) and the durability of their perovskite cell. The highest conversion efficiency rate of the perovskite cells has risen from 3.81% in 2009 to the current 22%, a level comparable to crystalline silicon cells. Considering the rapid progress in the development of this material technology, the solar industry is bullish about its commercialization in the near future.
(The above article is an English translation of a Chinese article written by Daisy Chuang. The credit of the photo at the top of the article goes to the Okinawa Institute of Science and Technology Graduate University.)