(color online) (a) Optical field distributions in the red narrowband photodiodes (fidi thicknesses of 500-nm CH₃NH₃PbI₂Br and 60-nm C₆₀) for four wavelengths: for λ < 600 nm (the Beer–Lambert region) photons cannot penetrate the whole of the film and carriers are surface generated, and for 700 nm > λ > 600 nm (the cavity region) the photo carriers are volume-generated. (b) EQEs of narrowband red CCN photodiodes with various junction thicknesses at a reverse bias of −0.5 V. The thinnest junction delivers an almost broadband response because surface- and volume-generated carriers are collected. By increasing the junction thickness, surface generation and volume generation are distinguished and the EQE at shorter wavelengths (in the Beer–Lambert region) is suppressed (from Ref. [30]).