Electronic structure of the CsPbBr₃/polytriarylamine (PTAA) system

The inorganic lead halide perovskite CsPbBr₃ promises similar solar cell efficiency to its hybrid organic-inorganic counterpart CH₃NH₃PbBr₃ but shows greater stability. Here, we exploit this stability for the study of band alignment between perovskites and carrier selective interlayers. Using ultraviolet, X-ray, and inverse photoemission spectroscopies, we measure the ionization energy and electron affinities of CsPbBr₃ and the hole transport polymer polytriarylamine (PTAA). We find that undoped PTAA introduces a barrier to hole extraction of 0.2-0.5 eV, due to band bending in the PTAA and/or a dipole at the interface. p-doping the PTAA eliminates this barrier, raising PTAA's highest occupied molecular orbital to 0.2 eV above the CsPbBr₃ valence band maximum and improving hole transport. However, IPES reveals the presence of states below the PTAA lowest unoccupied molecular level. If present at the CsPbBr₃/PTAA interface, these states may limit the polymer's efficacy at blocking electrons in solar cells with wide band gap materials like CsPbBr₃ and CH₃NH₃PbBr₃.