# Excitation of Wet Perovskite Films by Ultrasonic Vibration Improves the Device Performance
## 1 Introduction  
Perovskite solar cells (PSCs) are currently one of the most favorable and efficient types of emerging solution-processed thin film solar cells, owing to the excellent optoelectronic properties of the perovskite light harvesters [1; 2; 3], although the device instability under normal operation and the process scale-up, using low-cost fabrication routes, are two major challenges against the commercialization of this technology. PSCs may be designed as the p-i-n or n-i-p types, owing to the ambipolar behavior of the perovskite light harvesters. The basic and widely-studied n-i-p PSC is comprised of glass/TCO/c-TiO\({}_{2}\)/m-TiO\({}_{2}\)/perovskite/HTL/metal electrode, where the TCO stands for the transparent conducting oxides, such as fluorine-doped tin oxide (FTO), c-TiO\({}_{2}\) denotes a compact film of TiO\({}_{2}\), m-TiO\({}_{2}\) denotes a mesoporous layer made of the TiO\({}_{2}\) nanoparticles, and the HTL stands for the hole transporting layer. The c-TiO\({}_{2}\) ultrathin layer plays the pivotal role of the electron transporting layer (ETL), whereas the HTL is made of a material that has the ability to transport the holes only, such as spiro-OMeTAD (2,2\({}^{\prime}\),7,7\({}^{\prime}\)-Tetrakis [\(N\),\(N\)-di(4-methoxyphenyl)] amino]-9,9\({}^{\prime}\)-spirobifluorene). The m-TiO\({}_{2}\) layer controls the growth and deposition of the perovskite nanocrystals within the pores of the mesoporous layer, but may be excluded in some designs, so as to simplify the fabrication process and reduce the cost. The heart of a lead halide methylammonium PSC has the formulation of methylammonium lead halide, CH\({}_{3}\)NH\({}_{3}\)PbX\({}_{3}\) (MAPbX\({}_{3}\)), where X = I, Br or Cl or any combination thereof [4]. Beside the MA-based PSCs, formamidinium (FA) and Cs based PSCs have been developed, in order to improve the device performance and stability [5; 6]. As a modelsystem, this fundamental work adopts the basic mesoporous structure (c-TiO\({}_{2}\)/m-TiO\({}_{2}\)) [7] with MA as the cation and both single and mixed halides in the perovskite layer for further investigation and the optimization of the deposition process of the perovskite layer atop the mesoporous layer, using a novel ultrasonic vibration technique to be elaborated upon later on. To prepare the perovskite layer, the precursors such as lead halides and methylammonium halides may be mixed and deposited directly from solution in one-step, or separately deposited in two sequential steps for better coverage, at the expense of additional steps and cost. Therefore, the one-step method is easier to perform and is in line with the scale-up of the technology, and therefore adopted in this work. Regardless of the method of deposition, the perovskite crystallization process is usually controlled by cooling/heating and/or solvent engineering through the application of proper solvents and additives to manipulate the solubility limit and the solvent evaporation rate [8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19]. For improved device performance, many works employ additional interfacial layers, as well. Although effective, solvent, and interfacial engineering processes are expensive, tedious, environmentally disfavored, and generally non-repeatable and non-reproducible. Therefore, in this study, a novel, facile, low-cost, and more repeatable mechanical approach, termed as ultrasonic substrate vibration post treatment (SVPT), is employed to control the crystal growth and to improve the interfacial contacts between the perovskite and m-TiO\({}_{2}\) layers. The application of the SVPT method on organic [20; 21] and perovskite [22] solar cells has shown that imposing low-amplitude ultrasonic vibration on the wet thin films results in an improvement in the film nanostructure and the device reproducibility, because it promotes the evaporation rate [23] and causes micromixing of the solution components [24; 25].