Additive Engineering in 2-Methoxyethaol Based Perovskite Film Formation for Perovskite Solar Cells

Perovskite solar cells (PSCs) have attracted significant attention because of their unique characteristics such as low-cost fabrication through solution processes and excellent photoelectric properties. Solution-processed perovskite films using polar aprotic solvent such as dimethyl sulfoxide (DMSO) generally required auxiliary process like anti-solvent drop. However, the anti-solvent dripping process has strict process window to form a uniform film such as dripping duration, volume, and pressure of the anti-solvent.<br/>Recently, solvent engineering using 2-methoxyethanol (2MOE), which has selective solubility that dissolves methylammonium iodide (MAI) but not PbI₂ alone, have been studied for perovskite film formation for an anti-solvent free process. Although perovskite phase film could be obtained on the 2MOE solvent system without the anti-solvent process, the film generally has many pinholes leading to low power conversion efficiency (PCE) in PSCs. This problem is caused by low coordination with the precursor and high volatile of 2MOE.<br/>To overcome this issue, we applied several additives, which affect to the kinetics of crystallization, in the 2MOE-based anti-solvent free process. First, we used various co-solvents, such as dimethyl sulfoxide (DMSO) and N-methyl-2-pyrrolidone (NMP), that can induce intermediate phases in which highly coordinated system with the precursors and solvents (MAI-PbI₂-DMSO and MAI-PbI₂-NMP). The formation of intermediate phases retarders crystal growth of perovskite and leads to high crystallinity of the perovskite. Through this, we found that NMP is a suitable additive for pinhole free perovskite film. We obtained an efficiency of 20.39% with the maximum efficiency for the device based on the MAPbI_3-xCl_x film fabricated with a 2MOE-based solution containing NMP.<br/>Second, we compared film formation by adding alkyl amines with different chain lengths in 2MOE single solvent system. Through this, we found that the uniform and dense film could be obtained by adding small molecule of oleyamine (OAm) having long 18-carbon chain without anti-solvent dripping. In that study, we measured real-time grazing incidence wide angle X-ray scattering (GIWAXS) in spin cast process to study how the added alkylamine affected to film quality. We found that the longer alkyl chain of alkyl amines induced the faster nucleation and fast crystal growth of perovskite in film formation. We fabricated inverted PCSs using the high-quality film containing OAm as a light absorption layer and obtained PCE of 21.01%.