Residual solvent extraction via chemical displacement for efficient and stable perovskite solar cells

Solvent residue is inevitable to occur in solution processed thin films, but its influence on the thin film quality has not been identified and addressed to date. Methylammonium acetate (MAAc) ionic liquid has recently been realized as an environmentally friendly solvent for solution processed perovskites. The specific high viscosity, low vapor pressure and strong association with perovskite precursor of the MAAc solvent is a double-edged sword, which endowed an advantageously ambient air operational and anti-solvent free perovskite deposition, but the MAAc is likely to be retained within the film and bring in detrimental effects on device performance of the corresponding solar cells. Herein, we reported a novel route to eliminate the residual solvent via a facial hydrochloric acid (HCl) annealing post-treatment (HAAP). In particular, chemical displacement reaction between the incorporated HCl and residual MAAc can be initiated to form volatile MACl and HAc, efficiently extracting MAAc residue. In the meanwhile, the stimulated mass transport via downward penetration and upward escape can trigger secondary perovskite growth with enlarged grain size and smoothened surface, leading to reduced defect state and improved interfacial contact intimacy, and also partial chloride ions are able to enter the crystal lattice to stabilize perovskite phase structure. As a result, a champion efficiency up to 20.78% originating from enhanced V_oc was achieved, and more than 96% of its initial efficiency can be maintained after 1000 h shelf-storage.