Study on Formation and Decomposition of Perovskite Crystals for High Efficiency and Stability

Abstract

Organic inorganic hybrid perovskite crystals, which ca be easily formed by chemical reaction of metal halide and organic halide, show excellent photo-electronic properties such as band gap, absorption coefficient, and carrier diffusion length by themselves. The properties are dependent on comprehensive quality of perovskite crystal-clustered thin films which finally influences on photovoltaic performance of complete devices. To realize perovskite solar cells with high performance, it is required to understand fundamental principles of crystal formation and develop fabrication process of perovskite crystal-clustered thin films based on basic principles. Additionally, since these perovskites have weak bonding between organic and inorganic parts, the decomposition of perovskites occurs actively in air condition under light irradiation. To achieve stable operation of perovskite solar cells for several years, investigation of fundamental origin and principle of perovskite decomposition should be preferentially performed. First, the general way to fabricate perovskite films is reacting solidified lead halide thin films with organic halide dissolved in solutions. In this crystallization process, which is simply called two-step method, various reaction conditions affect the crystallization growth rate and crystal sizes which consequently determine the quality of perovskite films. To obtain high-quality perovskite films, the effect of reaction conditions such as temperature and concentration of organic halide was theoretically and experimentally investigated. Based on thermodynamic analysis on the crystallization of perovskite, the grain size equation was derived by considering the change in Gibbs free energy. From the equation, the grain sizes of the perovskite can be controlled not only by organic halide concentration but also by the reaction temperature of the crystallization process. By varying the reaction temperature from -10 ℃ to 50 ℃, the grain sizes were dramatically changed from few hundred nanometers to few micrometers under the same concentration. It is found that the concentration and the temperature play a critical role in determining the grain sizes and the performance of perovskite device. In the two-step method, there is the limitation in terms of perovskite film coverage due to crystal growth along vertical direction. Since the perovskite coverage is crucial in photovoltaic performance, it is required to develop a new fabrication method which enables formation of fully-covered, dense and homogeneous perovskite films by controlling the perovskite crystallization. Before the formation of perovskite films, transparent Lewis-base adduct films was previously formed as the intermediate step and heated to be slowly crystallized by removing Lewis-base. Perovskites fabricated from Lewis-base adduct exhibited high charge extraction characteristics and slow recombination rate. Finally, Perovskite solar cells fabricated via Lewis-base adduct approach showed the best PCE of 19.7% and average PCE of 18.3% from 41 cells, which is indicative of high reproducibility. These perovskite materials are vulnerable to the deterioration caused by humidity and light. The most widely used perovskite, CH3NH3PbI3, is structurally distorted tetragonal crystals due to unbalance of ionic radii between ions. The crystal structure of perovskites can become more stable by increasing the Goldschmidt tolerance factor close to unity by incorporating other organic cation and halide anion with different ion sizes. An addition of formamidinium (HC(NH2)2) cation and bromide (Br) anion not only structurally stabilize the perovskite materials by inducing cubic crystals, but also enhance their photovoltaic performance. By comprehensively optimizing mixed cation and/or halide anion system of MAxFA1-xPbIyBr3-y in terms of both stability and performance, structurally-stabilized mixed compositional MA0.6FA0.4PbI2.9Br0.1 perovskite-based devices formed via lewis-base adduct method exhibited the best PCE of 20.2% without photocurrent hysteresis and enhanced stability under one sun illumination. Perovskite solar cells have shown fast degradation under actual operation even with encapsulation and its reason has been still elusive. A novel experimental set-up to deposit charges generated via a corona discharger on the perovskite surface reveals that perovskite materials degrade irreversibly through grain boundaries in the presence of moisture only when charges traps on the perovskite surface. These results indicate that the moisture-induced irreversible decomposition of perovskite materials was triggered by trapped charges. The Kelvin Probe Force Microscopy measurements confirmed that charges are trapped preferentially on the grain boundaries when gas ions are deposited and light is illuminated. Moreover, the synergetic effect of oxygen on the trapped-charge driven degradation was also identified. From these observations, possible scenario on the degradation mechanism is suggested. The deprotonation of organic cation induced by locally-intensified electrostatic force due to trapped charge would be attributed to the irreversible degradation of perovskite materials.