Atomic Scale Investigation of the CuPc–MAPbX₃ Interface and the Effect of Non-Stoichiometric Perovskite Films on Interfacial Structures

Abstract

Metal halide perovskites (MHPs) have become a major topic of research in thin film photovoltaics due to their advantageous optoelectronic properties. These devices typically have the MHP absorber layer sandwiched between two charge selective layers (CSLs). The interfaces between the perovskite layer and these CSLs are potential areas of higher charge recombination. Understanding the nature of these interfaces is key for device improvement. Additionally, non-stoichiometric perovskite films are expected to strongly impact the interfacial properties. In this study, the interface between CH 3 NH 3 PbI 3 (MAPbI 3 ) and copper phthalocyanine (CuPc), a hole transport layer (HTL), is studied at the atomic scale. We use scanning tunneling microscopy (STM) combined with density functional theory (DFT) predictions to show that CuPc deposited on MAPbX 3 (X = I,Br) forms a self-assembled layer consistent with the α-polymorph of CuPc. Additionally, STM images show a distinctly different adsorption orientation for CuPc on non-perovskite domains of the thin film samples. These findings highlight the effect of non-stoichiometric films on the relative orientation at the MHP/HTL interface, which may affect interfacial charge transport in a device. Our work provides an atomic scale view of the MHP/CuPc interface and underscores the importance of understanding interfacial structures and the effect that the film stoichiometry can have on interfacial properties.

Atomic Level Insights into Metal Halide Perovskite Materials by Scanning Tunneling Microscopy and Spectroscopy

“…Complementary DFT calculations indicated that the bright spots correspond to the iodine anions at the MAI‐terminated (001) surface. Similar results were reported by Qi and co‐workers in their investigation of the CuPc‐MAPbI 3 interface structures, [18k] which will be discussed in more detail in Section 3.6.…”

“…Insight into the fundamental properties of interfaces is essential to guide the improvement and optimization of MHPMs for optoelectronics. Recently, Qi and co‐workers investigated the interface properties of MAPbX 3 (X=I or Br) and copper phthalocyanine (CuPc) at the atomic scale using a combination of STM and DFT calculations; [18k] in this case, CuPc was used as the hole transport layer (HTL) material [64] . As shown in Figure 11 a, the majority of the surface was covered by MAPbI 3 , which exhibited characteristic surface reconstruction, while only a small area did not show this surface corrugation.…”

“…A similar result was obtained for CuPc on the MAPbBr 3 surface, which indicates that changing the type of halide does not significantly affect the adsorption behavior of CuPc on the perovskite surface. The structurally different CuPc on non‐perovskite and perovskite material would have a significant effect on the orbital overlap at the perovskite‐HTL interface [18k] . In addition, the molecular orientation would strongly affect the device performance [66] .…”

“…All panels are reproduced from Ref. [18k] with permission. Copyright 2021 American Chemical Society.…”

“…Qi and co‐workers focused on the preparation of MAPbX 3 (X=Br or I) based perovskite thin films by dual‐source co‐evaporation in an UHV environment [18f,h–k] . In the case of MAPbBr 3 , MABr and PbBr 2 were evaporated at 361 and 498 K, respectively, for 10 min, while the clean Au(111) substrate was kept at 130 K during deposition to ensure the adhesion of the MABr molecules.…”