Effect of ground state charge transfer and photoinduced charge separation on the energy level alignment at metal halide perovskite/organic charge transport layer interfaces

Abstract

One of the most promising routes for the future fabrication of solution-processable high-performance solar cells is to employ metal halide perovskites as photoactive material combined with organic semiconductors as charge extraction layers. An essential requirement to obtain high device performance is a proper energy level alignment across the device interfaces. Here, we investigate the interface between a triple cation perovskite and a prototypical electron acceptor molecule. Photoemission spectroscopy reveals a ground state charge transfer induced band bending on either side of the junction, which significantly alters the charge extraction barriers as compared to assumed vacuum level alignment and flat-band conditions. In addition, we demonstrate that upon white light illumination, the energy levels of the organic layer exhibit rigid shifts by up to 0.26 eV with respect to those of the perovskite, revealing a non-constant energy offset between the frontier energy levels of the two materials. Such level shifts of the organic transport layer are fully reversible upon switching on/off the light, indicating an electrostatic origin of this phenomenon caused by unbalanced distribution of photogenerated charge carriers. We therefore stress the importance of determining the energy level alignment at perovskite-based interfaces not only in the electronic ground state (dark) but also under device operating conditions (operando) to enable for a reliable correlation with the device performance.