## 1 Introduction  
The ecological environment has deteriorated in recent years, which has resulted in an increasing demand for new clean and renewable energy sources. Solar energy has been recognized as almost inexhaustible and the most abundant of all natural energy resources [1; 2]. Therefore, various solar cells have been developed to convert solar energy to electrical energy directly, including crystal silicon solar cells, dye-sensitized solar cells, quantum-dot solar cells and organic solar cells. However, the sophisticated processing, environmental damage and high cost of those commercial solar cells force scientists to develop new types of solar cells. Currently, the perovskite solar cell is the most promising one, with relatively low cost and high power conversion efficiency (PCE), in which perovskite layer plays the role of absorbing material for producing exciton. There is a long history of research on the synthesis of halide perovskites. CaTiO\({}_{3}\) was discovered in the Ural Mountains of Russia by [PERSON] in 1839, and perovskite was named after Russian mineralogist [PERSON]. There is also a hybrid perovskite with a composition of ABX\({}_{3}\), where A are monovalent cations (MA\({}^{+}\) or FA\({}^{+}\)), B are metal cations (Sn\({}^{2+}\) or Pb\({}^{2+}\)) and X are halide anions (I\({}^{-}\), Br\({}^{-}\), and/or Cl\({}^{-}\)) [3]. Perovskite materials have excellent optical and electrical properties for outstanding performance in solar cell application, such as direct band gap, steep optical absorption edge [4; 5], high charge carrier mobility [6] and long charge carrier diffusion lengths and carrier lifetimes [7; 8]. The PCE of perovskite solar cells (PSCs) has increased from 3.8% in 2009 [9] to 25.2% in this year, becoming the fastest-advancing solar cell.  
There are still some issues impeding the practical application of the perovskite solar cells, such as hysteresis [10], ionic migration [11], heavy metal lead [12] and moisture stability [13]. Tremendous effort has been devoted to overcome those challenges, for example, through interface engineering [14; 15], anti-solvent doping [16; 17], new perovskite materials [18; 19], new device structures [20; 21], using additives [22], etc. With those approaches, additive engineering has been reported to slow down the crystallization rate of the perovskite crystals in solution processing. This review mainly analyzes the mechanism of the additives in perovskite precursor solvent, such as hydrogen bonding or coordination between the additive and perovskite components. In addition, the formed mesophase or the additives reacting with a certain substance during the perovskite crystallization could improve the quality of the perovskite film. The morphology of perovskite film is not only a key factor affecting the efficiency of perovskite solar cell, but is also paramount in other applications, e.g., light emitting diodes (LEDs), potential spintronics applications and so on. The mechanism for ameliorate the morphology of perovskite films with additives are mainly to slow down the crystallization rate of perovskite crystals to achieved large crystal grains, thereby realizing an improvement of PCE. The purpose of this review was to classify the main weak forces involved in the formation of the perovskite films and to provide useful information to readers working in this field.