Co-Solvent Effect in the Processing of the Perovskite:Fullerene Blend Films for Electron Transport Layer-Free Solar Cells

An understanding of the improvements achieved in the use of cosolvents for methylammonium lead triiodide (MAPbI₃):C₇₀ blend films (MAPbI₃:C₇₀) processing is presented. A comparative study using aromatic (i.e., <i>o</i>-xylene and <i>o</i>-dichlorobenzene) and aliphatic (i.e., methylcyclohexane and chlorocyclohexane) cosolvents proves the nature of the cosolvent interacting with fullerene to be determining for achieving enhanced devices. UV–vis spectra of the different C₇₀ solutions suggest a major impact of the solute–aromatic solvent interactions on the optoelectronic properties. The effect of aromatic and aliphatic solvents in the electronic structure of C₇₀ crystals, obtained from the different solutions, is indeed demonstrated by electron energy loss spectroscopy. Morphological studies show elimination of pinholes (field emission scanning electron microscopy) and different nanometric features related to fullerene (atomic force microscopy) in MAPbI₃:C₇₀ blend films processed using aromatic cosolvents. A severe quenching of the perovskite emission is observed, suggesting that electron transfer happens from MAPbI₃ to C₇₀ in the MAPbI₃:C₇₀ blend films. Furthermore, a faster charge transfer seems to occur in blend films processed using aromatic cosolvents