Effect of a 2D-Modification of Cs ₂ AgBiBr ₆ on Nucleation and Contact Formation of Subsequently Deposited Hole Transport Layers as Revealed by <i>In Situ</i> Growth Studies

A two-dimensional (2D) perovskite interlayer prepared by modification of a three-dimensional (3D) perovskite absorber with organic ammonium ions such as butylammonium (BA⁺) or phenethylammonium (PEA⁺) between the 3D perovskite and contact layers is widely known to significantly improve the performance of perovskite solar cells. This has also been confirmed previously for the lead-free double perovskite absorber Cs₂AgBiBr₆. In this work, film growth of copper phthalocyanine (CuPc) or pentacene (Pn), used as model hole transport materials (HTM), was investigated. Mimicking solar cell geometry, the HTMs were evaporated onto thin films of 2D perovskites BA₄AgBiBr₈ or PEA₄AgBiBr₈, as well as on 3D Cs₂AgBiBr₆, either in its pristine form or after modification by BA⁺ or PEA⁺. The morphology and work function were inspected intermittently with respect to the evaporation of the HTMs by Kelvin probe force microscopy at different average film thicknesses. By these means, the origin of device improvements following a 2D-modification in contact with HTMs, as established earlier, was revealed by analyzing in detail the interface of the HTM with the respective perovskite starting at monolayer coverage and proceeding toward bulk thickness. On modified Cs₂AgBiBr₆, the energy alignment between the perovskite and the HTM was found to be well confined, and the growth of both HTMs was improved compared to pristine Cs₂AgBiBr₆. HTM growth occurred more homogeneously and led to layer formation, even at early stages of deposition. For CuPc as HTM, these changes were accompanied by preferential formation of needles in a crystal phase different from that formed on pristine Cs₂AgBiBr₆, as also detected on 2D PEA₄AgBiBr₈. Pn formed large dendritic islands on the 2D perovskites as well as on layered terraces formed upon ammonium modification of Cs₂AgBiBr₆, in contrast to the growth of small grains on pristine Cs₂AgBiBr₆. Implications of these observed changes in film growth and energy level alignment on the observed contact characteristics with the HTMs in model solar cells are discussed. Insight into the mechanism of improving perovskite-based devices by use of 2D/3D perovskite heterostructures is, thereby, provided by these measurements using CuPc or Pn as model HTMs.