The present invention relates to devices comprising metal halide perovskites and organic passivating agents. In particular, the invention relates to photovoltaic and optoelectronic devices comprising passivated metal halide perovskites. The device according to the invention comprises: (a) a metal halide perovskite; and (b) a passivating agent which is an organic compound; wherein molecules of the passivating agent are chemically bonded to anions or cations in the metal halide perovskite. The invention also provides a process for producing a photovoltaic device, which photovoltaic device comprises: (a) a metal halide perovskite; and (b) a passivating agent which is an organic compound; wherein molecules of the passivating agent are chemically bonded to anions or cations in the metal halide perovskite, wherein the process comprises treating a metal halide perovskite with a passivating agent, which passivating agent is an organic compound and is suitable for chemically bonding to anions or cations in the metal halide perovskite.