Kinetics Controlled Perovskite Crystallization for High Performance Solar Cells

The power conversion efficiencies (PCEs) of perovskite solar cells have recently developed rapidly compared to crystalline silicon solar cells. To have an effective way to control the crystallization of perovskite thin films is the key for achieving good device performance. However, a paradox in perovskite crystallization is from the mismatch between nucleation and Oswald ripening. Usually, the large numbers of nucleation sites tend to weak Oswald ripening. Here, we proposed a new mechanism to promote the formation of nucleation sites by reducing surface energy from 44.9 mN/m to 36.1 mN/m, to spontaneously accelerate the later Oswald ripening process by improving the grain solubility through the elastic modulus regulation. The ripening rate is increased from 2.37 angstrom m & sdot; s-1 to 4.61 angstrom m & sdot; s-1 during annealing. Finally, the solar cells derived from the optimized films showed significantly improved PCE from 23.14 % to 25.32 %. The long-term stability tests show excellent thermal stability (the optimized device without encapsulation maintaining 82 % of its initial PCE after 800 h aging at 85 degrees C) and an improved light stability under illumination. This work provides a new method, the elastic modulus regulation, to enhance the ripening process. Crystallization control of perovskite thin films is the key in commerciallization of perovskite solar cells. However, the fast crystallization of the thin film including nucleation, growth and Oswald ripening, is still a mystery though some efforts made. According to the theories of nucleation and Oswald ripening, we modified the perovskite surface energy and improved the grain solubility, and quantified the nucleation and ripening rate in experiments. These quantified parameters will facilitate the model building in perovskite crystallization for further industry production.Finally, the optimized films were applied in solar cells to improve efficiency from 23.14 % to 25.32 % with VOC of 1.175 V and FF of 83.53. Furthermore, the excellent ambient stability, light stability and thermal stability are obtained. This work paves the way for controls on overall crystallization kinetics of perovskite films, including nucleation, growth and ripening.+ image