Ink Engineering for Uniform Metal Halide Perovskite Thin Films

Abstract

Hybrid perovskite materials, specifically methylammonium lead iodide (MAPbI3 or CH3NH3PbI3), are exceptionally attractive for thin film photovoltaic applications due to their tunable optoelectronic properties, high power-conversion efficiencies in the lab, solution processability, and versatility of fabrication. Despite these promising attributes, one notable issue is the tendency for halide perovskites to crystallize rapidly during deposition, making it difficult to create smooth, continuous films, which are necessary to prevent shorting of thin film devices. Rapid crystallization occurs because thin films are spin coated from unstable solutions of methylammonium iodide (MAI): lead iodide (PbI2) in dimethylformamide (DMF). Common prevention efforts include solution processing via additives that increase solubility to slow crystallization. Though there has been some success, the additive selection process and implementation is extremely empirical (trial and error) and often unfit for scaling up in a manufacturing process. Thus the goal of this research was to engineer a simplified solution processing that decouples crystal growth from spin coating that reliably produces high quality solutions and films. Here is presented a novel room temperature flash precipitation nanoparticle synthesis that employs a turbulent mixer. The optimal ligand:precursor ratio for the system’s current materials is DDAI:MAI:PbI2 = 0.5:1.10:1 where there is a 10% excess of MAI and 0.5 mL of 50 mg/ml DDAI stock solution in DMF. Solutions and films are further enhanced through a non-solvent composition of 90% chloroform and 10% dichlorobenzene (10 ml total). These system parameters culminated in the fabrication of an LED. The LED achieved a maximum external quantum efficiency of 0.4% with a turn-on voltage of about 3.2 V and a maximum luminance of 24.8 cd/m2 under the applied voltage of 6 V. It is promising that even without complete optimization, this ink engineering process was able to create working LEDs. With continued exploration, this processing methodology has the potential to make a meaningful contribution to the perovskite and optoelectronics research communities.