1. A photovoltaic device comprising:a hole-transport layer comprising 2,2?,7,7?-tetrakis(N,N-di-p-methoxyphenylamine)-9,9?-spirobifluorene (spiro-OMeTAD) and an acid; and
a perovskite layer, wherein:
the perovskite layer is in physical contact with the first hole-transport layer, and
the acid is selected from the group consisting of citric acid, formic acid, lactic acid, oxalic acid, cinnamic acid, a sulfonic acid, a phosphonic acid, a carboxylic acid, hydrochloric acid, hydrobromic acid, hydroiodic acid, nitric acid, phosphoric acid, phosphorous acid, sulfuric acid, and selenic acid.