(color online) (a) X-ray diffraction (XRD) pattern of solution processed perovskite films on ITO/PEDOT:PSS with the crystal structure pictured in the inset. (b) Cartoon of the four different perovskite photodiode (PPD) structures: Type 1 with thin PC₆₀BM (10 nm) layer, Type 2 with thick PC₆₀BM (50 nm) layer, Type 3 with thick C₆₀ layer, and Type 4 with thick PC₆₀BM (50 nm)/C₆₀ (130 nm) layer. (c) Dark current density–voltage (J–V) characteristics obtained at a scanning rate of 1 mV·s⁻¹. Each point shown for the Type 4 devices are an average of the dark current measured over several seconds after each voltage step. (d) Typical external quantum efficiency spectra (measured at 120 Hz) for each of the four PPD types (from Ref. [22]).