Oriented Quasi-2D Hybrid Perovskite Grow on 3D-like Perovskite Surface

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By introducing long-chains cations, three-dimensional (3D) organic-inorganic hybrid perovskites (OIHPs) can be sliced into stacked quantum wells (QWs) with sandwiched long-chains organic cations as protective spacers, resulting in quasi-2D OIHPs with superior working stability. But due to the limited charge transport between QWs and grains, out-of-plane (OP) orientation of quasi-2D OIHPs (i.e. the QWs align vertically on substrates) with large grain size become vital. Therefore, knowledge about manipulating the orientation and crystallinity of quasi-2D perovskites is the key for developing solar cells with high power conversion efficiencies. 

Usually, in-plane orientation is more common for two-dimensional (2D) or layered materials due to the interaction between the face of layered materials and substrates, such as transition metal dichalcogenides and 2D oxide perovskites. Interestingly, OP orientation of quasi-2D OIHPs by solution method is conditionally available, such as hot casting method and additive method for quasi-2D OIHPs with layer number of n>1. However the major driving force for the conditional OP orientation was unknown. On the other hand, the nucleation, crystallization and phase distribution of quasi-2D perovskite crystals could be rather complex since it is polynary system. 

In this work, using NH4Cl as additives in the precursor solution was shown to result in quasi-2D perovskite with OP orientation and high crystallinity; and as another benefit, the fabrication of the high quality quasi-2D perovskite film is available with room-temperature solution method. Deep insights of the nucleation behavior of the quasi-2D perovskites and the driving force for the OP orientation has been disclosed in this study: 

1. We proved that the increased crystallinity of the quasi-2D perovskite is resulted from the much reduced nucleation centers during solution thinning process by using NH4Cl additive, which offer larger grain size. For the first time, we demonstrated that NH4Cl additive can suppress the precipitation of PbI2-DMF-contained solvate phase (PDS) in the solution phase, which developed into nucleation centers for 3D-like perovskite and quasi-2D perovskite in the following drying process. Similar effect can be found if replacing NH4Cl with varied additives such as AX salts (A = NH4+ or MA+; X = Cl-, Br- or I-); 

2. We discovered templated growth behaviour of quasi-2D perovskites from 3D-like perovskites (converted from PDS) with particular orientation, which is dominated by the interface lattice matching. The templated growth of quasi-2D perovskites from 3D-like perovskites explains the conditional OP orientation: when the PDS preferably formed on the liquid-air interface, it will trigger downward growth of quasi-2D perovskite with OP orientation; while when the PDS formed inside the liquid phase, it will lead to quasi-2D perovskite with random crystal orientation, as illustrated in Fig 1. 

Fig. 1. Illustration of preformed PDS and 3D-like perovskites defined nucleation and growth of quasi-2D perovskites from precursor solution without (a) or with (b) excessive AX additives.

The thermodynamically available templated growth of quasi-2D perovskites can be used to construct heterojunction structures with bonding interfaces, which would open up avenues for perovskite optoelectronic devices with functional nanostructures.
More details can be found in our article in Nature Communications (https://www.nature.com/articles/s41467-019-13856-1).

Yongbo Yuan

Professor, Central South University