To my knowledge, the rapid nucleation would cause an undesirable grain size, thus creating abundant grain boundaries. With respect to quick growth rate, it is not suitable for single-crystal growth.
while in two papers in top journals (Angew. Chem. Int. Ed. 2014, 53, 9898; Nano Energy, 2014, 10, 10), they presented the improved quality and morphology of perovskite materials, resulted from the quick nucleation and growth rate, which really made me confused.
I really appreciate if who can give me some tip to help advance my understanding into this issues.
First of all, there is nothing like good or bad grain size in the absolute sense. Each application requires its own size range. It is especially important in the photochemical processes which require fine balance of the crystal size and quality. Second, crystallinity is not the only parameter that affects the property of the thin films. In many cases it is possible to lose a bit of crystallinity but improve other characteristics so that overall properties will be better. And third, the direct relationship between the crystallinity and the growth rate exists only if all other parameters remain the same, and this is clearly not the case in the articles that you mention. The task of improving the crystallinity simultaneously with decreasing of the reaction time exists in many industrial applications and quite often it is solved.
Dear Hu:
The answer to your question requires special study. I think that the processes of phase separation consists of two stages.
1.When added to DMF solution of CH3NH3PbI3 сhlorobenzene spinodal decomposition occurs. See Figure 2b,e with cells
2. In each cell there is a rapid crystallization.
The combination of the two types of phase separation leads to a deviation from crystallization theory.
Xu,
I'll add my two cents to what was mentioned before. A major problem with solution fabricated solar cells is avoiding pin-holes, which lead to shunts and therefore reduced efficiency. While what you say is true, having more nucleation sites usually leads to more grains and therefore "less crystalline materials", but it also leads to better surface coverage, less shunts and better light absorption, which leads to better efficiency. As was said above, it depends on the grain boundaries properties to determine which effect will be more detrimental of beneficial for the intended function. I'm adding here links to two papers on the topic.
Article Crystallization of Methyl Ammonium Lead Halide Perovskites: ...
Article Chloride Inclusion and Hole Transport Material Doping to Imp...
Thanks for your helpful explanation, actually what i am focused is merely the relationship among nucleation number, growth rate, grain size and crystallinity, and the overall performance had not been involved at this stage. However, what you mentioned is really helpful. Regarding to the overall performance, I think, only all other parameters are the same can we gain a clear insight into the effect of a specific impact factor.