π-Conjugation-extended dinaphthocarbazole phosphonic acid as a hole-selective layer for inverted perovskite solar cells

In the rapidly evolving field of photovoltaic technology, self-assembled monolayers (SAMs) have become essential hole-selective layers (HSLs) for inverted perovskite solar cells (PSCs). SAMs not only determine interfacial hole extraction but also significantly influence the film quality of the atop perovskite layers, consequently affecting the efficiency and stability of perovskite solar cells. Among various SAMs, carbazole-based SAMs, exemplified by 4PACZ, have emerged as prominent due to their electron-rich characteristics, making them some of the most prevalent HSLs in modern inverted PSCs. Nevertheless, 4PACZ exhibits significant limitations: one major issue is its limited molecular dipole, which leads to insufficient energy level alignment between the treated substrate and the perovskite, causing substantial interfacial energy loss. Another critical challenge is the flat structure of the carbazole unit, which often promotes molecular stacking, resulting in incomplete substrate coverage and non-uniform film formation, thereby compromising both device performance and stability. In this study, we designed a novel SAM based on a polycyclic aromatic hydrocarbon derivative, (4-(8H-dinaphtho[2,3-c:2',3'-g]carbazol-8-yl)butyl)phosphonic acid (4PADNC), with the aim of optimizing hole transport in inverted PSCs. This SAM incorporates the structurally extended dinaphtho[2,3-c:2',3'-g]carbazole (DNC) as the functional terminal group, replacing the single carbazole unit in the traditional material 4PACZ. The key structural difference is that the DNC group provides a significantly expanded π-conjugated skeleton and enhanced electron-rich characteristics. These features not only greatly enhance hole extraction and transport at the interface but also induce a significant increase in the molecular dipole moment, which is crucial for effectively adjusting the work function of ITO, ensuring proper alignment with the perovskite layer. Additionally, there is an intramolecular dihedral angle of approximately 34.62° in the DNC unit at the core of 4PADNC. This non-planar configuration contrasts sharply with the planar carbazole structure. The larger dihedral angle effectively suppresses excessive π-π stacking between molecules, which not only aids in forming a denser and more ordered molecular layer on the ITO surface but also provides a more favorable and defect-free substrate for the growth of the upper perovskite. With these upgrades, the inverted PSCs based on 4PADNC achieved a PCE as high as 24.32%, compared to 22.89% for the control devices based on 4PACZ. Furthermore, the 4PADNC-based devices also exhibited superior thermal stability and operational stability.