Photoinduced Energy-Level Realignment at Interfaces between Organic Semiconductors and Metal-Halide Perovskites

In contrast to the common conception that the interfacial energy-level alignment is affixed once the interface is formed, we demonstrate that heterojunctions between organic semiconductors and metal-halide perovskites exhibit huge energy-level realignment during photoexcitation. Importantly, the photoinduced level shifts occur in the organic component, including the first molecular layer in direct contact with the perovskite. This is caused by charge-carrier accumulation within the organic semiconductor under illumination and the weak electronic coupling between the junction components.

Figure 1

Ultraviolet photoemission spectra of $\mathrm{PEDOT}:\mathrm{PSS}/\mathrm{CsFAMA}/7\mathrm{nm}$ HATCN measured in dark and under simulated solar irradiation, as indicated for the (a) secondary electron cutoff (SECO), (b) valence, and (c) Pb $4f$ core level regions. (d) Sample surface electrostatic potential (SEP) measured with respect to the conductive substrate Fermi level ($E_F$) as function of irradiation intensity. Schematic energy-level diagrams (e) in the dark and (f) under 1.5 sun irradiation. VL is the vacuum level. The indicated energy values are given with respect to $E_F$ set to zero. The blue dashed lines in (f) represent the quasi-Fermi levels $E_{F,n}$ and $E_{F,p}$ for electrons and holes, respectively; the red arrows and numbers indicate the illumination-induced change of the HATCN energy levels with respect to those of the perovskite.

Figure 2

Ultraviolet photoemission spectra of $\mathrm{PEDOT}:\mathrm{PSS}/\mathrm{CsFAMA}/0.4\mathrm{nm}$ HATCN in dark and under simulated solar irradiation, as indicated, for the (a) SECO, (b) valence, and (c) zoomed valence regions; also perovskite-background (BGS) subtracted spectra are shown to highlight the HATCN HOMO and SOMO features. Schematic energy-level diagrams (d) in the dark and (e) under 1.5 sun irradiation. The indicated energy values are given with respect to $E_F$ set to zero. The blue dashed lines in (e) represent the quasi-Fermi levels $E_{F,n}$ and $E_{F,p}$ for electrons and holes, respectively; the red arrows and numbers indicate the illumination-induced change of the HATCN energy levels with respect to those of the perovskite.

Figure 3

UPS spectra of $\mathrm{PEDOT}:\mathrm{PSS}/\mathrm{CsFAMA}/6\mathrm{nm}$ spiro-OMeTAD in the (a) SECO, and valence regions (b) on linear and (c) on logarithmic intensity scale, measured under varied simulated solar irradiation (as indicated). (d) SEP measured with respect to the conductive substrate $E_F$ as function of irradiation intensity. Schematic energy-level diagrams (e) in dark and (f) under 1.5 sun irradiation. The spiro-OMeTAD HOMO onset was extrapolated on a logarithmic intensity scale. The indicated energy values are given with respect to $E_F$ set to zero. The blue dashed lines in (f) represent the quasi-Fermi levels $E_{F,n}$ and $E_{F,p}$ for electrons and holes, respectively; the red arrows and numbers indicate the illumination-induced change of the HATCN energy levels with respect to those of the perovskite.