Fig. 2.

Optical and electronic properties of perovskite QD heterojunction. a Energy band positions for perovskite QD compositions and contact layers considered in this work. The Fermi level positions (denoted by horizontal dotted lines) and valence-band edges (E_v) of all perovskite QD films were determined by UPS and XPS, respectively. The conduction band edges (E_c) were calculated by adding the bandgap energy²⁶ to E_v for the corresponding perovskite QD films. b EQE spectra of solar cells with varying thickness ratio of the Cs_0.25FA_0.75PbI₃ layer to CsPbI₃ layer in the perovskite QD absorber. c EQE spectra of solar cells with different compositions of the bottom layer in the perovskite QD absorber where the thickness ratio of the mixed-cation Cs_xFA_1–xPbI₃ QD layers to CsPbI₃ QD layers is 1:3 in all cases