Halogen anion and organic cation vacancies are the main defects which restrict the power conversion efficiency (PCE) and long-term stability of perovskite solar cells. How to eliminate them simultaneously is a remain question in this field.
Recently, Huanping Zhou’s group in the College of Engineering at Peking University proposes a new elimination mechanism, which introduces fluoride into perovskite layer, to achieve the dual passivation effect by forming strong hydrogen bond with organic cations and strong ionic bond with lead ions, using the extremely high electronegativity of fluorine. Thus, the vacancy defects of organic cations and halogen anions in perovskites are effectively eliminated, and the PCE and long-term stability of corresponding solar cells are greatly improved.
The related research was published in the journal “Nature Energy” on May 13, 2019, entitled “Cation and anion immobilization through chemical bonding enhancement with fluorides for stable halide perovskite solar cells” (DOI: 10.1038/s41560-019-0382-6).
In recent years, organic-inorganic hybrid perovskite solar cells have attracted a lot of attention in academia and industry due to their advantages of high efficiency and low cost, and their PCE has rapidly increased to 24.2% in a few years, which is the most efficient thin-film solar cells in a single junction cell.
However, the poor stability of these solar cells is a major obstacle on their commercial application. Compared with traditional inorganic photovoltaic materials, organic-inorganic hybrid perovskite materials have soft lattices and are the ionic crystals. Ions are easy to migrate under the external environment, resulting in a large number of vacancy defects which can induce lattice collapse and component decomposition, so that it no longer has excellent photovoltaic performance. To solve these problems, Huanping Zhou’s group introduces fluoride into the perovskite layer. By using the extremely high electronegativity of fluorine, fluoride can passivate both the cation and anion vacancy defects in perovskites simultaneously via chemical bonding enhancement, to improve the efficiency and stability of perovskite solar cells. The highest efficiency of devices treated by fluoride is 21.92% (certification value is 21.7%) and there is no obvious hysteresis. At the same time, the device with fluoride exhibits the excellent thermal and irradiation stability: After 1000 hours of continuous irradiation or 85 ℃ heating, the device can still maintain 95% and 90% of the original efficiency, respectively. After 1000 hours of continuous operation at the maximum power point, the device can maintain 90% of the original efficiency.
Stability performance of PSCs under various conditions. Evolution of normalized PCEs of devices a) on continuous one-sun illumination in a nitrogen atmosphere, b) at 85 °C in a nitrogen atmosphere, c) in ambient air with a relative humidity of about 25%–45%, and a temperature of about 25–40 °C, d) under MPP tracking and continuous light irradiation with a white LED lamp in a nitrogen atmosphere.
The first author of this paper is the Ph.D. candidate, Nengxu Li (2017), from Zhou group (College of Engineering). Collaborators include researchers from Shuxia Tao team (Eindhoven University of Technology), Qi Chen team (Beijing Institute of Technology), Jiawang Hong team (Beijing Institute of Technology), Shihe Yang team (Hong Kong University of Science and Technology), Haipeng Xie team (Central South University), Geert Brocks (University of Twente).
The work was jointly supported by National Natural Science Foundation of China (51722201; 51672008; 91733301), National Key Research and Development Program of China grant no. 2017YFA0206701, the Natural Science Foundation of Beijing, China (grant no. 4182026), National Key Research and Development Program of China grant no. 2016YFB0700700, the National Natural Science Foundation of China (51673025) and Beijing Municipal Science and Technology Project no. Z181100005118002.