Using microgels to control the morphology and optoelectronic properties of hybrid organic–inorganic perovskite films

Abstract

Microgels (MGs) are crosslinked polymer colloid particles that swell in a good solvent. Although MGs have been studied for over 80 years their ability to control the morphology and optoelectronic properties of composite films containing photoactive materials is in its infancy. Solution processable hybrid organic–inorganic perovskites such as CH₃NH₃PbI_3−zCl_z have attracted enormous fundamental and applied interest because of their outstanding optoelectronic properties. There is considerable interest in establishing methods to control perovskite film morphology, for example, using micropatterning. Here, hydrophilic poly(N-vinylformamide)-based MGs were dispersed in perovskite precursor solution which was then spin coated to deposit CH₃NH₃PbI_3−zCl_z/MG films for the first time. Remarkably, the CH₃NH₃PbI_3−zCl_z/MG composites formed disordered inverse opal (DIO) films. The CH₃NH₃PbI_3−zCl_z/MG composition ranges which gave DIO films are identified using a phase diagram. The pore wall thickness is shown to be controlled by the volume fraction of MGs used and a simple model is presented to explain this behaviour. The MGs not only caused CH₃NH₃PbI_3−zCl_z to be more efficiently deposited but also increased light absorption and photoluminescence intensity. Demonstration solar cells constructed containing the DIO CH₃NH₃PbI_3−zCl_z/MG films had an average conversion efficiency of 6.58 ± 0.81%. A mechanism for DIO film formation is discussed. The principles established in this study wherein MGs control the morphology and properties of CH₃NH₃PbI_3−zCl_z/MG films should also apply to other perovskite/MG composites.