Theoretical investigation on carbazole-based self-assembled monolayer hole transport materials: the effect of substituent and anchoring groups

Abstract

Perovskite solar cells (PSCs) are a promising green energy technology, although further improvements in efficiency and operational stability are still required. In recent years, carbazole-based self-assembled monolayer (SAM) hole-transport materials (HTMs) have been widely employed as interfacial modifiers in PSCs, effectively reducing the defect density at the perovskite/HTM interface and thereby enhancing device performance. However, significant challenges remain in elucidating the origin of the enhanced hole mobility in these HTMs and its possible correlation with perovskite interfacial properties. In this work, a series of new SAM-type HTMs (BM1–BM7) were designed using carbazole as the core building unit, followed by a comprehensive theoretical investigation. Their optoelectronic properties, as well as the interfacial characteristics of perovskite/SAM heterojunctions, were evaluated using density functional theory (DFT) and compared with those of the widely adopted HTM, (4-(3,6-dimethyl-9H-carbazol-9-yl) butyl) phosphonic acid (Me-4PACz). The scope of the study covers ground-state geometries, crystal packing, charge-transport properties, UV-Vis absorption and fluorescence emission spectra, and interfacial interaction analyses of HTM/perovskite and HTM/ITO systems. Notably, a high hole mobility of up to 2.79 cm² V⁻¹ s⁻¹ was achieved. All molecules, except BM4, exhibit favorable energy-level alignment with the perovskite absorber. Molecules possessing asymmetric π-extended structures show particularly strong adsorption at the perovskite interface. Overall, the results demonstrate that rational modulation of the conjugation degree and anchoring groups can substantially enhance charge-transport characteristics and interfacial performance, offering valuable insights for the development of high-performance HTMs.